UMTS Network Post Luanch Optimization and Evolution Training

Copyright 2003 AIRCOM International LtdAll rights reserved

AIRCOM Training is committed to providing our customers with quality instructor led Telecommunications Training.

This documentation is protected by copyright. No part of the contents of this documentation may be reproduced in any form, or by any means, without the prior written consent of AIRCOM International.

Document Number: P/TR/005/O056/v1

This manual prepared by: AIRCOM InternationalGrosvenor House65-71 London RoadRedhill, Surrey RH1 1LQENGLAND

Telephone: +44 (0) 1737 775700 Fax: +44 (0) 1737 775770 Web: http://www.aircom.co.uk

UMTS Post-launch Optimisation and Network Evolution

O056

Contents

1 Introduction 7

1.1 Course Overview 7

2 Factors Limiting Capacity 11

2.1 Cell Throughput 112.1.1 Influence of Loading factor 122.1.2 Influence of External Interference 132.1.3 Influence of Eb/No value 162.1.4 The influence of orthogonality 19

2.2 Hard Blocking 242.3 Methods of increasing capacity of a cell 252.4 Network Throughput 262.5 Prioritising Actions 27

3 Reducing Mutual Interference 29

3.1 Introduction 293.2 Pilot Pollution 30

3.2.1 The High Site 303.2.2 Considerations when Down-Tilting 32

3.3 The Optimum Value of Down-tilt 35

4 Optimising Network Parameters 37

4.1 Introduction 374.2 RNC Parameters 38

4.2.1 BLER target 384.2.2 Time to trigger 414.2.3 Cell reselection times 424.2.4 Reselection hysterisis 434.2.5 RACH power offset 44

4.3 Cell Parameters 454.3.1 Pilot power 454.3.2 DL power per bearer 464.3.3 Soft Hand Over Margin 484.3.4 Exercise 1 514.3.5 Exercise 2 524.3.6 Further Issues regarding soft hand over. 584.3.7 Noise Rise (UL Loading Factor) Limit 60

5 Providing Additional Hard Capacity 61

5.1 Introduction 615.2 Channel Elements 615.3 Fixed network capacity 635.4 High Speed Downlink Packet Access (HSDPA) 66

UMTS Post-Launch Optimisation and Network Evolution 3AIRCOM International Ltd 2003

6 In-building Solutions 69

6.1 Introduction 696.2 The interference loop 706.3 The Dead-zone effect 726.4 Hand over from indoor to outdoor 74

6.4.1 Engineering the border region 746.5 Implementing the in-building cell 76

6.5.1 Choice of Node B 776.5.2 Distribution methods 806.5.3 Radiating cables 816.5.4 Field measurements to verify the implementation 83

7 Using Micro-cells to Service Hot Spots 87

7.1 Introduction 877.2 Micro-cell and In-building cells compared 887.3 The Theory behind the Micro-cell 89

7.3.1 Pilot Power Settings 917.3.2 Engineering the Micro-cell 927.3.3 What can go wrong? 947.3.4 Detecting Problem Areas. 95

7.4 Hotspots straddling macro cell boundaries 967.5 Propagation modelling for micro cells 977.6 Multiple micro cells 1077.7 Limiting factors 107

8 The Effect of Further Site Sectorisation 109

8.1 The sectored antenna 1098.2 Increasing the level of sectorisation 1108.3 Using simulations to assess the effectiveness 112

8.3.1 Vital statistics: 1128.3.2 Estimates of capacity 112

8.4 Neighbour Planning 117

9 Using Additional Carriers – Hierarchical Cell Structures 119

9.1 Spectrum Allocation 1199.2 Deploying extra carriers in the macro cell layer 120

9.2.1 A test case 1209.3 Fixed network provisioning. 1229.4 Carrier loading strategy. 1239.5 Hierarchical cell structures. 124

9.5.1 Capacity of micro-cells using separate carriers 1269.5.2 Pilot and common channel powers in micro-cells 1279.5.3 Link budgets for micro-cells 1289.5.4 Multi-layer strategies for dense urban environments 1299.5.5 Hand over between carriers 131

10 Implementing Diversity Systems 133

10.1 Introduction 13310.2 Definition of Fading 13410.3 Receive Diversity 13410.4 Transmit Diversity 137


10.5 Multi-User Detection MUD 14310.6 Predicting the Effect of Different Coverage and Capacity Enhancement Devices 14610.7 Multiple-beam antennas 152

10.7.1 Beam forming principles 15310.7.2 Implementation in a UMTS network 15710.7.3 Improvement from use of multiple-beam antennas. 157

10.8 Smart (beam-forming) antennas 159

11 Integrating Extra Sites into the Macro-Cell Layer 161

11.1 Introduction 16111.2 Planning the new site 16211.3 Action after activation of the new site 163

11.3.1 Making further drive-tests 16311.3.2 Assessing network capacity 16411.3.3 Interpreting measurements made under unknown loading conditions.

174


1 Introduction

1.1 Course OverviewThe objective of this two-day course is to provide delegates with knowledge of methods, procedures and techniques that will enable them to optimise the performance of an existing UMTS 3g network. This optimisation can take the form of:

Increasing the network capacity

Increasing the provision of high-resource services

Improve the quality of service offered to users

The starting point of the course is the expectation that a UMTS network has been planned, built and “optimised” to a state where it could be launched. Coverage can be described as “OK” (but perhaps coverage for higher services may be “patchy”). Further, for launch, network coverage was prioritised over capacity and therefore the capacity of the network will not be optimised.

We can examine methods that will improve both capacity and coverage and look at implementing them in an optimum way. These methods will start from a “zero resource” (simply using the existing equipment to better effect) point. Optimisation will then involve

Re-configuring the antenna system

Adjusting cell and network parameters

Next, an investigation into ways of utilising extra resources to further enhance the network capacity and quality is carried out. These resources will include:

Provision of micro/pico-cells

Further sectorisation

Additional carriers

Diversity and Multi-user Detection

Additional macro-cell sites.

Aims of CourseAims of Course

• Assuming the network has beenlaunched and:

• Coverage is “OK”.

• Higher services coverage is “patchy”.

• Coverage prioritised over capacity

• Aim is that, at the end of the course you will beable to:

• Increase Network Capacity

• Increase Coverage for high resource services

• Improve quality of service offered to users.

Introductory Session

Aims of CourseAims of Course

• This will include:

• “zero resource” methods

• re-configuring antenna system

• adjusting cell and network parameters

• Adding to the network infrastructure

• provision of micro/pico-cells

• further sectorisation

• use of extra carriers

• diversity and multi-user detection

• addition of sites in the macro-cell layer

Introductory Session

2 Factors Limiting Capacity

2.1 Cell ThroughputIf we consider the factors influencing the capacity of a cell, we find that this is affected by:

The noise bandwidth (that we cannot change)

The loading factor (that we can set within constraints)

The external interference (that we strive to minimise)

The Eb/No required on the service (that affects BLER and relies on functionality such as power control operating well).

The orthogonality on the downlink (that may be possible to influence)

Factors Limiting CapacityFactors Limiting Capacity

• Cell Throughput is given by the simplified expressions for polecapacity in kbps multiplied by the loading factor

• Crucial parameters are Eb/No, inter-cell interference i, orthogonality and loading factor (which is affected by the

Noise Rise limit).

Capacity Limiting Factors

iNE

iNE

b

b

1

3840

1

3840

0

0

•Uplink

•Downlink

2.1.1 Influence of Loading factor

On the downlink, the downlink power limits the loading factor. In most situations, it will be possible to drive the downlink to a loading level of about 85%. On the uplink it is limited in the form of a noise rise limit. As the noise rise curve becomes steeper as the loading factor gets larger, there are concerns regarding the stability of the network if the loading level is allowed to become high. There is a general feeling that the noise rise should be limited to about 4 dB (corresponding to a loading factor of 60%). However, where the site density is very high so that path loss is not a limiting factor, the noise rise limit could be raised.

Factors Limiting Capacity: NR limitFactors Limiting Capacity: NR limit

• NR limit on uplink is directly linked to loading factor:

• NR limit appears in link budget and hence affects coverage prediction.

• If a network is planned so that continuous coverage would be provided

with all cells simultaneously at NR limit, then probability suggests that

coverage is over-dimensioned.

• Coverage could be planned for a NR value 1 to 2 dB below the limit –

but this is often used as a “comfort factor” margin.

• Failures will then be split between Eb/No and NR.


10101 );1log(10NR

NR

N o is e R is e v s . T h r o u g h p u t

0 .0 0

5 .0 0

1 0 .0 0

1 5 .0 0

2 0 .0 0

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2

T h r o u g h p u t (x 1 0 0 k b p s )

No

ise

Ris

e

S e r ie s 1

F a c to rs L im it in g C a p a c ity : N R lim itF a c to rs L im it in g C a p a c ity : N R lim itC a p a c ity L im it in g F a c to rs

S te e p s lo p e -u n s ta b le

S h a llo w s lo p e -s ta b le

• H o w e v e r, if N R is a llo w e d to re a c h v e ry h ig h v a lu e s (e .g .> 7 d B )th e re is c o n c e rn th a t th e n e tw o rk c o u ld b e c o m e u n s ta b le .

• In it ia lly , it is e x p e c te d th a t N R w ill b e lim ite d to a m a x im u m o f, s a y , 6d B u n til c o n f id e n c e in th is a p p ro a c h is g a in e d .

2.1.2 Influence of External Interference

External interference levels are probably the most tempting target. Reduce interference and you increase capacity. However, if your main weapon (as it probably is) in reducing interference is to down

tilt the antenna, you can make hand over regions so small that hand over failures result. This effect will be more noticeable if the UE is travelling at considerable speed. The optimum level is a compromise. Note that, on the uplink, there is one value for i for the whole cell whereas, on the downlink, each UE experiences a different value of i.

F a c t o r s L i m i t i n g C a p a c i t y : F R EF a c t o r s L i m i t i n g C a p a c i t y : F R E

• F r e q u e n c y r e - u s e e f f i c i e n c y i s t h e n a m e g i v e n t o t h ep r o p o r t i o n o f r e c e i v e d p o w e r t h a t c o m e s f r o m a c e l l ’ s o w nu s e r s r a t h e r t h a n f r o m a l l u s e r s i n c l u d i n g o t h e r c e l l s .

C a p a c i t y L i m i t i n g F a c t o r s

11

1

1

cell intracellinter

1

1

cellinter cell intra

cell intra

FREi

iFRE

• F r e q u e n c y r e - u s e e f f i c i e n c y i s a u s e f u l t e r m a s i t v a r i e sb e t w e e n z e r o a n d 1 a s i d r o p s f r o m i n f i n i t y t o z e r o .

Factors Limiting Capacity: FREFactors Limiting Capacity: FRE

• The ideal situation is where the receiving antenna can only“see” its own users but not those of other cells. i.e. FRE = 1

• The power from neighbouring mobiles close to the cell bordercause the biggest problems.


High power mobiles close to

Cell border cause FRE reduction


• A large cell serving a low subscriber density surrounded byseveral smaller cells serving high subscriber densities willexperience a low value of FRE.


A Large cell will experience low FRE

Because it is surrounded by

many users of other cells


• Hotspots near the cell border will cause more problems thatevenly distributed neighbouring cells

• A quantitative analysis is not always possible. A simulatoris extremely valuable in helping to develop a feel for theseriousness of potential problems.


Hot spots near cell border causeFRE reduction


• Increasing FRE: the main weapon is to down-tiltantennas.

• Overlap of coverage cannot be too small otherwise handover will fail. However, large overlaps will lead to lowFRE.

• This is most effective when there is a large anglebetween the line from the antenna to the cell edge andthe horizontal.

• In the case of large cells, planning to avoid hotspots nearthe cell border will reduce the incidence of low FRE.


2.1.3 Influence of Eb/No value

The Eb/No value indicates the air interface resource required by a bearer. The higher the value of Eb/No, the lower the capacity. Eb/No is directly linked to the BLER and so there will be a recommended value for a particular service. However, it relies upon the fast power control loop operating well. This is compromised if the mobile is moving at speed through a multi-path environment and the target Eb/No can rise by as much as 5 dB in such cases. This is something to look out for when drive testing. The major benefit from employing diversity techniques is the resulting reduction in the required Eb/No over the air interface. Multi-user detection (MUD) has a similar effect on the downlink. Optimising of network parameters, such as pilot power and soft hand over margin, can result in a lowering of required power levels overall.

F a c to rs L im itin g C a p a c ity : EF a c to rs L im itin g C a p a c ity : E bb /N/N 00

• H ig h ca p a c ity le ve ls d e p e n d o n lo w le ve ls o f E b /N o b e in gu se d . ( N o te B E R m u s t b e a cce p ta b le ).

• A ch ie v in g th is re lie s o n a ccu ra te , fa s t p o w e r co n tro l toco m p e n sa te fo r fa s t fa d in g .

• F a s t fa d in g o ccu rs a s a m o b ile m o ve s th ro u g h a nin te rfe re n ce p a tte rn .

• In te rfe re n ce p a tte rn s d e ve lo p d u e to re fle c tio n s .

• R e p e titio n d is ta n ce d e p e n d s o n a n g le b e tw e e n in c id e n ta n d re fle c te d w a ve s .

C ap ac ity L im itin g F ac to rs

cos

2

F a c t o r s L i m i t i n g C a p a c i t y : EF a c t o r s L i m i t i n g C a p a c i t y : E bb / N/ N 00

• T h i s i s d i f f i c u l t t o e s t i m a t e , f o r a 6 d Br e f l e c t i o n l o s s t h e n o t c h d e p t h w i l l b ea p p r o x i m a t e l y 1 0 d B .

• F a s t p o w e r c o n t r o l i s i n t e n d e d t oc o m p e n s a t e f o r t h e f a s t e s t f a d i n g i n c i d e n t sa t t h e s t e e p e s t s l o p e .


E 1

21

21log20

EE

EE

E 2

cos2

20/

20/

101

101log20

dBdiff

dBdiff

dbNotch

Factors Limiting Capacity: EFactors Limiting Capacity: Ebb/N/N00

• In situations where the reflected wave is strong, the slope ofthe standing wave pattern can be in excess of 100 dB/m.

• UMTS allows for a power up command to be given at a rateof 1500 Hz. Thus 1.5 dB/ms is the maximum rate that cannormally be accommodated.

• Speeds of greater than 15 m/s (54 km/h) can causeproblems.



• If the mobile cannot respond to power control commands,the UE will notice a variation in the received signal.

• This will lead to BER variations that will cause the networkto require a higher target Eb/No (a “fast fading margin” or“power control margin” will be required).

• The effect can be to increase the target Eb/No from anormal value of perhaps 4 dB to 10 dB or more for fastmoving mobiles.

• This will reduce the capacity of a cell from typically 32simultaneous connections to only 8 – a dramatic reduction.

• Lesson: the multipath environment and user mobility canaffect the target Eb/No and hence cell capacity.



• Reducing the required Eb/No:• Diversity systems provide an Eb/No improvement.

• That means that the Eb/No over the air interfacecan be reduced and hence the air interfacecapacity increases.

• Multi-user detection (MUD) reduces the effect ofmutual interference between users on the uplink.

• This reduces the required transmit power per userand hence reduces the noise rise caused by agiven number of users.

• As a result the pole capacity increases.


2.1.4 The influence of orthogonality

Orthogonality allows the effect of own-cell interference to be reduced by the signal processing in the receiver. Typically, this sort of interference is reduced by about 4 dB. The effect on cell capacity depends on the value of this orthogonality factor and the relative contribution of out of cell interference to the total level. Most importantly, there is no guidance on positioning cells to maximise the benefit from orthogonality and, even if there was, relocation of cells (which means re-doing the network plan) is not at the top of our list.

F a c t o r s L i m i t i n g C a p a c i t y : O r t h o g o n a l i t yF a c t o r s L i m i t i n g C a p a c i t y : O r t h o g o n a l i t y

• D r a m a t i c e f f e c t o n d o w n l i n k c a p a c i t y .


i

1NE

3840 Capacity Pole

0

b

Factors Limiting Capacity: OrthogonalityFactors Limiting Capacity: Orthogonality

• Example: Eb/No = 4 dB, i = 0.6, 12200bps


2548

1.0

1914

0.8

153412801100963Pole Capacity

0.60.40.20Orthogonality

Pole Capacity(kbps)

1000

2000

Orthogonality0.5 10

F a c t o r s L i m i t i n g C a p a c i t y : O r t h o g o n a l i t yF a c t o r s L i m i t i n g C a p a c i t y : O r t h o g o n a l i t y

• T h e L o a d i n g f a c t o r d e l i v e r a b l e o n t h e d o w n l i n k d e p e n d s u p o nt h e l i n k l o s s , m a x i m u m t r a n s m i t p o w e r a n d n o i s e p e r f o r m a n c eo f t h e m o b i l e .

• E x a m p l e : T x P o w e r 4 3 d B m ; N o i s e F l o o r o f M o b i l e - 1 0 0 d B m .

• D e l i v e r a b l e l o a d i n g f a c t o r c a n b e e x p e c t e d t o e x c e e d 7 5 % .

• P o l e c a p a c i t y i s c r u c i a l .


1log10110

11

110log1010

1010log10NR

dBm 1010log10Power Rx Mobile

10143

1014310/100

10/1001043

10/1001043

orth

orthLL

orthLLorthLL

LL


• Question:

• Suppose a group of users of a 64kbps service in an isolatedcell experiencing a link loss of 138.4 dB are demanding a totaldata throughput of 1.024 Mbps at an Eb/No of 4 dB.

• What is the downlink loading factor at this throughput if theorthogonality is i) 0.4 and ii) 0.8?

• Further, what is the traffic channel power demanded and whatis the maximum throughput possible at that path loss if themaximum traffic channel power is 42.7 dBm?

• Assume a noise level at the mobile of -102 dBm before noiserise.



• Answer:

• At an orthogonality of 0.4, the pole capacity is 2548 kbps.

• 1024 kbps represents a loading factor of 39%.

• Hence the Noise Rise would be approximately 2.23 dB.

• The effective received traffic power would be -103.7 dBm

• Actual received traffic power is 2.2 dB higher (-101.5 dBm)

indicating a transmit power of 36.89 dBm (link loss 138.4 dB).

• 42.7 dBm would be able to deliver almost 72% loading factor

and hence the throughput possible should be approximately

1832 kbps.



• Answer (continued):

• At an orthogonality of 0.8, the pole capacity is 7644 kbps.

• 1024 kbps represents a loading factor of 13%.

• Hence the Noise Rise would be approximately 0.6 dB.

• The effective received traffic power would be -110.1 dBm

• Actual received traffic power is 7.0 dB higher (-103.1 dBm)

indicating a transmit power of 35.3 dBm (link loss 138.4 dB).

• 42.7 dBm would be able to deliver almost 46% loading factor

and hence the throughput possible should be approximately

3519 kbps.



• Orthogonality degradation is caused by a multipath radio

propagation environment.

• Typically, it is of the order of 0.6 in an urban environment, higher

in rural environments.

• In an isolated cell, an indication of the orthogonality can be

obtained by measuring the pilot SIR when the transmit powers of

all channels are known.

• At low values of path loss, all interference power will be due to

interference from other channels.



• What can be done to improve orthogonality?

• Currently, very little.

• No guidance regarding placing of sites to

maximise orthogonality known about.

• In future there may well be but:- the only outcome

would be the recommendation to move cells (not a

welcome recommendation as it means start

planning the network from the beginning).


2.2 Hard BlockingUMTS optimisation engineers often concentrate on the air interface or “soft” capacity of a network. However, there is always a need to ensure that there is sufficient “hard” capacity in the form of channel elements and fixed transmission network capacity. It is pointless increasing the capacity of the air interface to above the “hard” capacity of the network. It is tempting to launch a network with a low level of hard capacity. In such cases, increasing the hard capacity of the network should be the first thing to be considered as subscriber demand grows.

Factors Limiting Capacity: Hard BlockingFactors Limiting Capacity: Hard Blocking

• So far we have

discussed air

interface capacity or

“soft” capacity.

• We could suffer also

from “hard” blocking

due to hardware and

fixed network

constraints.


Channel elements?

E1 links?

Factors Limiting Capacity: Hard BlockingFactors Limiting Capacity: Hard Blocking

• There is no value in

increasing the “soft”

capacity of the air

interface above the

network’s “hard”

capacity.

• Often the network will

be launched with a low

level of “hard”

capacity.


Channel elements?

E1 links?

2.3 Methods of increasing capacity of a cellThese can be divided into two categories

1. “zero resource”:

Adjusting the network configuration in order to minimise mutual interference

Adjusting network and cell parameters in order to optimise performance

2. “new resource”:

Install additional channel elements and/or increase the capacity of the fixed transmission network.

Implement diversity and multi-user detection as required.

Methods of Increasing Cell CapacityMethods of Increasing Cell Capacity

• “Zero” resource:

• Adjusting configuration to reduce mutual interference

• Adjusting network and cell parameters in order to optimise

performance.

• New resource requirements:

• Adding channel elements

• Increasing capacity of fixed network

• Implement diversity and/or multi-user detection.


2.4 Network ThroughputThe above equations and discussions are focused on the throughput per cell. Network capacity can be approximated as the sum of the capacities of the individual cells. It is therefore tempting to simply add cells as required in order to increase capacity. But, not only is this a very expensive option, it generally leads to a reduction in the capacities of the individual cells. Therefore the return on investment can diminish. This is largely because the mutual interference between cells tends to increase as cell density increases. Nevertheless, increasing the number of cells in the network has to be considered as the demand for capacity grows. However, it is important that the increase is managed in an efficient manner, implementing the solutions with maximum effect and lowest cost first.

Network CapacityNetwork Capacity

• Capacity calculations have been “per cell”.

• Network is of many cells.

• Can we just multiply the capacity per cell by thenumber of cells?

• Do we just add more cells to increase networkcapacity?

• Very expensive option

• Diminishing returns set in: higher site density results inincreasing interference.

• Procedure needs to be structured for maximumbenefit.


2.5 Prioritising ActionsA possible order of events is:

1. Address hard capacity issues

2. Use pico cells to provide in-building solutions

3. Deploy micro cells to service hot spots

4. Further sectorise (e.g. 6 cells per site) the macro cell layer

5. Provision extra carriers to selected sites (in UMTS a sector with 2 carriers is, logically, 2 cells).

6. Deploy extra sites into the macro cell layer.

Further, attention must be given to where cell activities fit in with the above list. For example, what priority will implementing diversity and MUD be given?

Network CapacityNetwork Capacity

• Possible procedure

• Address hard capacity issues.

• Use pico-cells to provide an “in-building solution”

• Deploy micro-cells to service hot spots

• Further sectorise (e.g. six cells per sector)

• Provision extra carriers on some sites (a sectorwith 2 carriers is, logically, 2 cells in UMTS).

• Deploy extra sites in the macro-cell layer.

• Note: Priority of deployment of diversity/MUDis a topic of discussion.


3 Reducing Mutual Interference

3.1 IntroductionWhilst coverage may be described as very good, it is very probable that the network capacity will be well below its limit. It is useful to undertake a “thought experiment” whereby you imagine a very high user demand is present throughout your network. You can then attempt to devise ways of maximising the number of subscribers who receive a service. Of course, a Monte Carlo simulator on a planning tool will assist in this. But, generally, the lower the mutual interference, the higher the network capacity. However, an overlap region is necessary for successful hand over to occur and it is natural to be somewhat cautious regarding this, initially. Nevertheless, occurrences of many pilots at a high level (pilot pollution) must be addressed.

“Reducing Mutual “Reducing Mutual InteferenceInteference””

• The lower the interference the higher the capacity.

• Because of the single frequency used in a UMTS layer, there isan “Interference feedback loop”.

• This means that interference, rather than just adding to thebackground noise level, consumes a proportion of the networkresource (power on the DL, noise rise on the UL).

Reducing Mutual Interference

3.2 Pilot PollutionEven though interference may not be high enough to produce a noticeable effect on the service at network launch, the coverage area should be investigated in order to identify areas of interference that indicate that capacity will be limited in the near future as demand grows. Taking action will cure the problem before it affects customer service. Drive test data can be analysed to check for the presence of many pilots. It must be remembered that, in some areas, three pilots of near-equal level will inevitably occur. More than three is in principle avoidable. It is sensible to start with areas where the highest number of pilots is noted. These areas should be examined and compared with the planning tool prediction. From this it should be possible to reveal which pilots are “wanted” in that area and which are “unwanted”. The task is then to reduce the level of the unwanted pilots so that they pose a lower interference threat. Identifying unwanted pilots is the easy part. Deciding what to do about them is a harder matter. A few examples are given.

3.2.1 The High Site

The classic text book example of a high site, where one site is much higher than all the surrounding sites, is not very common. However, terrain features do lead to a similar effect. Consider the

situation illustrated in profile here. The distant site produces significant interference in the service region indicated.

“High Sites”“High Sites”

• Often, what is apparently sensible planning can lead to theemergence of high sites.

• In the situation shown a distant site posed an interference threatin the area of interest.


Area of interest

InterferingCell Intended

serving cell


• The first action to be taken would be to increase the down tilt ofthe interfering cell.

• Care must be taken to ensure that it still provides coveragewhere it is intended.


Area of interest


serving cell

CoverageArea


• Other possible solutions include reducing the cell power of theinterfering cell.

• This should be done with great care as it will affect the downlinkcoverage and capacity in its wanted coverage area.


Area of interest


serving cell

CoverageArea

3.2.2 Considerations when Down-Tilting

When down-tilting it is important to realise the effect on the radiation pattern. Antennas will generally have a fixed electrical down tilt and flexibility is in the form of mechanical down tilt. It should be remembered that the effect of mechanical down tilt reduces as you move in azimuth away from the principal direction. Indeed, at 90, mechanical down tilting has no effect at all. As the reduction in radiation strength is surprisingly small at 90 (perhaps 12 dB), the situation can arise where, at distance, there is a stronger signal away from the principal direction than there is in the principal direction. This effectively places a limit on the effectiveness of mechanical down tilt in reducing interference. Electrical down tilt is, however, effective at reducing the radiation in all directions. A typical solution to attempt would be to employ an antenna with a fixed electrical down tilt of perhaps 6 to which can be added a few degrees of mechanical down tilt. Care must be taken to ensure that the cell’s wanted signal is maintained at sufficient levels over its required coverage area and that there is sufficient overlap to allow hand over to occur as necessary.

Exam ples of Antenna TiltExam ples of Antenna Tilt

-35

-30

-25

-20

-15

-10

-5

0

-35

-30

-25

-20

-15

-10

-5

0

-35

-30

-25

-20

-15

-10

-5

0

-35

-30

-25

-20

-15

-10

-5

0

N o Tilt M echanical

D ow ntilt

E lectrical

D ow ntilt

E lectrical D ow ntilt +

M echanical U ptilt

R educing M utual Interference

Limitations on Limitations on DowntiltDowntilt

• If the antennas aremounted centrally on aroof,

• The amount of down-tiltachievable can be limitedby the site geometry

Plan

Block Image


Rooftop Main Lobe ClearanceRooftop Main Lobe Clearance

• Main lobe is typically required to clear the roof

• Parapets around roof edge should be considered

• Spurious reflections off roof may occur if this is notachieved

d = distance from roof edge (m)

h = antennaheight above roof

(m) p = parapet height (m)

= tan-1((h-p)/d)

min * dB_vertical + safety_margin +electrical_tilt + mechanical_tilt


Rooftop Plinth MountingRooftop Plinth Mounting

• Poles mounted on plinths/pads onedge of roof

• Azimuth Restrictions• Three different ranges:

•Within range as for wallmounting of little concern•Until azimuth parallel to wall‘probably’ retain same mounting•Should azimuth face ‘into’structure would want to changemounting location

• Height Flexibility• None with current mounting• Reduction in height would require

wall mounting

• Downtilt Flexibility• No real limit except design

guidelines• Mounting bracket limitations

Plan

Block Image


Rooftop Plinth MountingRooftop Plinth Mounting

• Height and position of adjacent buildings will also affect the downtiltpossible

• A general rule: The principal direction of an antenna shouldexperience clearance of 5 metres for a distance of 100 metres from thecell antenna.


3.3 The Optimum Value of Down-tiltInterference can be said to be the limiting factor in a UMTS network and, yet, it is what makes UMTS “work”: the only way to get zero interference is to have zero activity. The main victim of having an over-zealous approach to reducing interference will be the operation of hand over. The UE must be able to receive at least two good cells for the duration of the hand over procedure. Drive tests must be carried out to ensure the successful operation of hand over in areas where the interference levels have been reduced by down tilting antennas.

The optimum value will depend on likely UE speed. The amount of overlap required by a UE moving at 10 kph will be much less than for a UE at 100 kph. The UE must be in the soft hand over region for sufficient time to allow a hand over to take place. This time can be considered to be up to a few seconds. Thus, a 10-metre region may be sufficient for a slow-moving UE whereas a region of 100 metres would be required by a fast-moving UE.

The Optimum Value of Down TiltThe Optimum Value of Down Tilt

• Although a lot of interference will reduce network capacity, toolittle overlap can lead to hand over failures.


Too much interference:network capacityreduced.

Too little overlap:hand over failures.

The Optimum Value of Down TiltThe Optimum Value of Down Tilt

• Likely speed of UE will be significant.

• If 3 seconds is seen as a typical hand over time:

• UE moving at 10 kph will move approximately 10 metres

• UE moving at 120 kph will move approximately 120 metres.

• SHO region must be sufficiently large.


Too muchinterference:networkcapacityreduced.

Too littleoverlap: handover failures.

4 Optimising Network Parameters

4.1 IntroductionThere are thousands of parameters that can be set, either at RNC, Node B or cell level. Some of these parameters have a more noticeable, immediate effect on the performance of the network than others. The equipment manufacturer will recommend certain default values for initial settings. But, it must be borne in mind that the technology is very new and the recommended values are not necessarily the optimum values. Naturally, the initial focus is on those parameters that have an immediate effect on cell performance. These will now be considered.

Optimising Network ParametersOptimising Network Parameters

• Parameters that can be changed:• RNC - “global”

• Node B

• Cell

• There are thousands of parameters that eachinfluence network functions.

• We will look at some that are among the mostsignificant.

Network Parameters

4.2 RNC ParametersCertain parameters are “global” in nature, rather than being specific to a certain Node B, or cell. Some of these have a very significant effect on network performance. Some of these are discussed below.

4.2.1 BLER target

The network will strive to ensure that signal to noise ratios delivered are sufficient to ensure that the BLER is low enough to satisfy the service users. This value forms part of the outer power control loop in which the target Eb/No value is adjusted in order to meet the BLER target. Again, the value of this parameter is a compromise: a very high level of BLER will result in dissatisfied service users; a very low level of BLER will require a very high target Eb/No to be imposed on the bearer with the resulting reduction in capacity. Optimisation can be thought of as maximising the number of service users who assess that service as being of “toll quality”; that is, they are willing to pay for the service. Figures for circuit-switched traffic typically vary between 0.1% and 1%. BLER targets for packet switched traffic can be much higher as it is possible to allow re-transmissions of packets. The network will be operating with greatest efficiency if the energy used in successfully transmitting a frame of data is kept to a minimum. An

expression that indicates this is where is a ratio and

FER is the frame error rate. 1 – FER can be thought of as the “frame success rate”. If this value is a minimum then the value of Eb per successful frame is also a minimum. Experiments can be made by adjusting Eb/No and monitoring the FER. The value at which the expression becomes a minimum depends on the propagation conditions but is, typically, at an FER of between 20% and 30%. This indicates that packet traffic can be sent most efficiently if there is a considerable re-transmission overhead but a low signal to noise ratio. It should be noted that the above expression does not impose a penalty for the effect of delay.

BLER TargetBLER Target

• Setting a low value of BLER (e.g. 0.3%) will producea high quality digital communication channel

• But: this will in turn require a high Eb/No so eachchannel will use a lot of network resource.

• Optimising a network is supporting as many satisfiedcustomers as possible; this means providing aservice that is “just good enough”.

• Increasing the BLER target (to, say, 1%) willincrease capacity.

• Assessment on impact is often subjective.

Network Parameters

BLER Target - Packet ServicesBLER Target - Packet Services

• Values of 0.3% and 1% are typical for CircuitSwitched (CS) services (e.g. voice, VT).

• For packet switched (PS) services, delays and re-transmissions can be tolerated.

• BLER can be much higher.

• Crucial parameter:

• This indicates the energy required to successfullytransfer a frame of data.

• Network is “optimised” if this is a minimum.

Network Parameters

FER

NEb

10

B L E R T a r g e t - P a c k e t S e r v i c e sB L E R T a r g e t - P a c k e t S e r v i c e s

• V a lu e o f F E R fo r m in im u m d e p e n d s o n r a d io c h a n n e l .

• T y p ic a l l y 2 0 % - > 3 0 % .

N e t w o r k P a r a m e t e r s

FER

NE b

10

FER2 0 % 3 0 %

R a d i o C h a n 1R a d i o C h a n 2

BLER Target - Packet ServicesBLER Target - Packet Services

• For packet services, it appears that low Eb/No, high FERis the most efficient situation.

• Re-transmission strategy/protocol required.

• Method does not impose a penalty for delay: assumesthat information is perfectly “delay tolerant”.

Network Parameters

4.2.2 Time to trigger

In order to avoid an excessive amount of network management activity, the network does not react instantaneously to measurements indicating that event thresholds have been crossed. There is a built in “time to trigger” (TTT) that is a delay time to ensure that anomalous readings do not cause unwanted updating of the active set, for example. Again, the time is a compromise: too short and the network will experience very frequent attempts to enter or leave soft hand over; too long and soft hand over may be delayed by long enough to cause interference problems. One significant question related to this is: “How far will the UE move during the triggering time?”...If the UE moves a long way into another cell’s coverage area before that cell becomes a member of the active set, then interference will result. Soft hand over exists to allow the new cell to control the UE power levels. Typical values of TTT are from 100 ms to 1000 ms.

Time to Trigger (TTT)Time to Trigger (TTT)

• Network functions (such as active set updates) aretriggered by “events”.

• For example: Event 1C:- “A non-active Primary CPICHbecomes better than an active Primary CPICH”.

• This event would normally trigger an active set update.

• However, it is important incorporate a TTT before theupdate is executed.

Network Parameters

Time to Trigger (TTT)Time to Trigger (TTT)Network Parameters

•If TTT is toosmall, too manyhand overrequests will bemade (HO is a“risky time”)

time

RSCP

Active pilot

Non-active pilot

•If TTT is toolong, non-activecell will cause,and suffer from,interference.

• Typical TTTs: 100 ms to 1000 ms

4.2.3 Cell reselection times

When in idle mode, the mobile keeps track of its “best server”. Once it has detected that a new cell is a better server than its existing cell, a re-selection procedure will be instigated. There is, however, a reselection delay before this happens. This delay is typically a few seconds. If it is too small, there will be frequent

reselections. If it is too long, the UE may be in an inappropriate location when it makes a call attempt. This will lead to higher than necessary interference.

Cell Re-selection DelayCell Re-selection DelayNetwork Parameters

• Similar to TTT, but refers to idle mode operation.

• Only one cell is “camped on” at a time.

• If delay is too short, frequent re-selections will occur.

• If delay is too long, UE will attempt to make a connectionon a link with high path loss. This will result in interference.

• Usually longer than TTT - typically two or three seconds.

4.2.4 Reselection hysterisis

In order to avoid a “ping pong” effect in idle mode, a hysterisis margin is used to force the UE to tend of stay with its serving cell. This is similar to the approach used in GSM networks. The value of the margin is typically a few dB. If it is too small, there will be an excessive amount of reselection attempts; if it is too large, there is a danger that the UE will find itself in a poor quality radio channel when it attempts to make/receive a call.

Cell Re-selection HysterisisCell Re-selection HysterisisNetwork Parameters

• Desirable to have cell camping onto “best server” but:

• need to avoid “ping pong” effect.

• Hysterisis is inserted to make UE tend to “hang on” toselected cell (similar concept to that employed in GSM).

• Effect of getting it wrong:

• too small:- “ping pong” re-selections

• too large:- UE can be in poor radio environment when it attempts acall.

4.2.5 RACH power offset

When establishing a call the UE sends a RACH request on the uplink (UL). The power level of the RACH request is determined by assessing the power of the pilot RSCP on the downlink (DL). It is possible to set a parameter that modifies the UL transmit power. Again an optimum value needs to be found. If the RACH power is too high, the uplink traffic channels will suffer from interference and capacity will be reduced; if the power is too low then call set up may fail. The offset is typically from zero to –5 dB.

RACH Power OffsetRACH Power OffsetNetwork Parameters

• Call set up is a vulnerable time.

• Fast power control not established.

• UE “guesses” appropriate power level for initial RACH

• Too low - not heard by cell, set up takes a long time or fails

• Too high - UL interference results

• RACH power offset adjusts initial guess.

•UE measures CPICH RSCP

•Estimates path loss

•RACH power leveldetermined usingestimate of path loss

4.3 Cell ParametersCertain parameters need to be optimised for the particular cell and its environment. Some will depend on quite straightforward issues, such as the feeder loss. The optimum value of other parameters will depend on less definable inputs such as average UE speed. Some examples are given.

4.3.1 Pilot power

The pilot power is set at the “rack” output. However, if masthead amplifiers (MHAs) are used, then soft hand over gain is maximised if the pilot strength at the masthead is equal. Therefore, it is common in such circumstances to, initially, adjust output powers so as to make them equal at the masthead. Later in the optimisation process, it is common to allow different cells to accept different levels of loading. Tailoring of the pilot powers is necessary to ensure that this is effected without problem.

Cell Pilot PowerCell Pilot PowerNetwork Parameters

• Pilot power dictates:

• cell coverage

• soft hand over regions

• UL soft hand over gain is maximised if UL path loss isequal.

• If MHAs are employed, pilot power should be equal at themast head, not at PA output.

4.3.2 DL power per bearer

As well as allocating a maximum total power to traffic channels, it is possible to specify a limit to the power that can be allocated to any one bearer. The most appropriate value of this parameter is dependent on cell activity. If the cell is very quiet then a large allocation will reduce the probability of the downlink failing. As the cell is quiet, capacity issues will not arise. If, however, there is a lot of demand for services, lowering the maximum allocation will ensure that those with the lower demand (i.e. UEs in areas of low interference) will get priority. This will lead to a maximisation of throughput, and hence revenue. It is possible to specify a different maximum level for active bearers compared with the level for call set up.

Maximum DL Power per BearerMaximum DL Power per BearerNetwork Parameters

• DL users share the power available to traffic channels(typically 16 Watts or 42 dBm).

• Allowing one user to use all this power would mean the cellis blocked to other users; a limit is imposed.


• If demand is low, it is best to have a high limit so thatcoverage is maximised.


• If demand is high, a lower limit will maximise throughput(and hence revenue) at the expense of coverage in areasof high path loss or interference.

4.3.3 Soft Hand Over Margin

4.3.3.1 Macro-diversity & Maximal Combining Gain

Soft Hand over is a necessity in any single-frequency cellular network. In a multi-frequency technology, such as GSM, the possibility exists to ensure that the “new” connection has a significantly lower path loss than the “old” connection before hand over takes place. In a single frequency network, the resulting interference on the “new” cell would drastically reduce the capacity of the network. Soft hand over entails the mobile simultaneously connecting with more than one cell. Although the most significant purpose of introducing Soft Hand over was to reduce uplink interference, there are other beneficial effects. Firstly, when more than one path is provided for the radio link, a diversity gain is obtained. There is a low probability of both channels suffering a bad fade simultaneously. Thus there is a reduced need for a margin to accommodate such fades. In this way, the target Eb/N0 value can be reduced when in soft hand over. This is true of both the uplink and the downlink.

In addition to the diversity (or “macro-diversity”) gain afforded, the receiver in the mobile (and the receiver at a Node B that is used when two cells from the same Node B are in soft, or rather “softer” hand over) processes the multiple received signal to produce and output that is of higher quality than any individual signal. The result on the uplink is that the transmit power of the mobile can be

substantially reduced when in soft hand over – having beneficial effects for coverage and interference. On the downlink, providing additional hand over channels places a power burden on the cell. This is partially (but usually not fully) offset by reduction in the target Eb/N0 value. The general conclusion is the Soft Hand over assists the uplink but places an additional burden on the downlink. The amount of use made of soft hand over affects the relative capacities of the two directions.

Soft HandoverSoft Handover

• As well as providing vital power control functionality, Soft Handoverimproves the quality of the channel by means of two methods.

• Macro-diversity Gain

• Maximal Combining Gain

Soft Handover

Macro-Diversity GainMacro-Diversity Gain

• If the mobile communicates withmore than one cell, protectionagainst failure is provided as thisfailure would have to occur on alllinks to cause a call to drop.

• As the better quality link can beselected, there is less variation inoverall channel quality.

• This leads to a reduction in PowerRise – the increase in averagetransmit power that occurs as amobile responds to power controlcommands.

-5

0

5

10

15

20

25

M obile Tx Pwr Average Non-fading

Power Rise

Soft Handover

Macro-Diversity GainMacro-Diversity Gain

• The reduction in Power Risehelps to increase uplinkcapacity as the average Txpower is reduced.

-5

0

5

10

15

20

25

M obile Tx Pwr Average Non-fading

Reduced Power Rise following Macro Diversity Gain

Soft Handover

Soft Handover – Combining the SignalsSoft Handover – Combining the Signals

• On the Uplink there are two possible methods of combining the two(or more) signals.

• When the two cells are on separate sites (conventional “soft”handover), the RNC simply selects the better of the two signals.

• When the two cells are on the same site (“softer” handover),maximal combining of the two signals can be implemented.

• Maximal combining leads to an output that is of better quality ( lessnoisy ) than either of the individual signals.

• Maximal combining is implemented in the mobile to combine thedownlink signals.

• Macro-diversity gain and Maximal combining gain combine toproduce Soft Handover Gain.

Soft Handover

Soft Handover – Maximal CombiningSoft Handover – Maximal Combining

• Consider the case where two signals arrive at the inputs to acombiner. One is “good” (e.g. Eb/No = 8 dB) and the other is “poor”(Eb/No 1 dB).

• It is possible to combine the signals such that the output has anEb/No greater than 8 dB. This requires correct (“maximal”) weightingof the two signals.

•Eb/No 8dB

•Eb/No 1dB

•??

Soft Handover

S o f t H a n d o v e r – M a x i m a l C o m b i n i n gS o f t H a n d o v e r – M a x i m a l C o m b i n i n g

• T h e E b / N o a t t h e o u t p u t w h e n t h e i n p u t s a r e m a x i m a l l yc o m b i n e d i s g i v e n b y t h e s i m p l e f o r m u l a .

• I t m u s t b e n o t e d t h a t E b / N o i s q u o t e d a s a r a t i o ( n o t i n d B ) .

• 8 d B c o r r e s p o n d s t o 6 . 3 a s a r a t i o .

• 1 d B i s a r a t i o o f 1 . 2 6 .

• T h e s e s u m t o 7 . 5 6 w h i c h i s 8 . 8 d B .

20100

N

E

N

E

N

E bb

out

b

S o f t H a n d o v e r

4.3.4 Exercise 1

What Eb/No improvement is offered when two signals of equal quality are combined ?

Answer :-

4.3.5 Exercise 2

What is the Eb/No at the output of a combiner if the input is composed of two signals one with an Eb/No of 6 dB and the other with and Eb/No of -2 dB?

Answer:

Maximal Combining - questionsMaximal Combining - questions

Question 1:

• What Eb/No improvement is offered when two signals ofequal quality are combined?

Soft Handover

Maximal Combining - solutionMaximal Combining - solution

Answer 1:

• As the Eb/No of the two inputs are equal, the Eb/No at theoutput will be “double” that at the input.

• This is an improvement of 3 dB

Question 2:

• What is the Eb/No at the output of a combiner if the inputis composed of two signals: one with Eb/No of 6 dB andthe other with an Eb/No of -2 dB.

Soft Handover

Maximal Combining - solutionMaximal Combining - solution

Answer 2:

• 6 dB is a ratio of 3.98. -2 dB is a ratio of 0.63.

• These sum to 4.61.

• This is equal to an Eb/No of 6.64 dB

Soft Handover

Optimising Soft Hand Over Parameters

Optimising Soft Handover ParametersOptimising Soft Handover Parameters

• The parameter of most significance is the Soft Handover“Add” and “Remove” Windows.

• They influence the number of terminals in soft handover.

• Generally, the larger the window is made, the lower theloading on the uplink and the higher the loading on thedownlink.

• The path loss at the cell edge will influence the optimumvalue of the SHO window.

• The lower the path loss the larger the value can be (as thedownlink will probably have plenty of spare power available).

Soft Handover


• The amount of improvement on the uplink and loading on thedownlink depends on the amount of soft handover gainachieved.

2 dBwindow

4 dBwindow

Soft Handover


• Suppose each terminal shown above represents a 64 kbps4 dB Eb/No connection.

2 dBwindow

4 dBwindow

Soft Handover

Estimating FREEstimating FRE

• Suppose the terminals are arranged in groups of4 with the path loss to the two Node Bs changingin 1 dB increments.

• The red terminals will each cause an interferencelevel 1 dB less than the wanted signals:equivalent to the load of 3 terminals.

• The orange terminals will each cause aninterference level 3 dB less than the wantedsignals: equivalent to 2 terminals.

• ( 1dB less to their Node B, and 2 dB less to ours )

• Total interference load: 5 equivalent terminals.FRE = 62.5% (5/8)

2 dBwindow

4 dBwindow

Soft Handover

Estimating FRE and LoadingEstimating FRE and Loading

• Eb/No is 4 dB

• Pole Capacity = 995 kbps

• Loading = 54% (NR=3.4 dB)

2 dBwindow

4 dBwindow

Soft Handover

Estimating The Effect of SHOEstimating The Effect of SHO

• Assumptions:

• Window set to 4 dB.

• SHO allows the UL Tx power to reduce by 1.5 dB

(Effectively making the Eb/No 2.5 dB).

• SHO allows the target Eb/No on the DL to be reduced.

This is assumed to be 2 dB (maximal combining on

downlink).

• BUT downlinks must service twice the number of

terminals (a 3 dB extra burden).

• Summarising the effect: UL loading factor will reduce

from 54% to 38%. NR will reduce from 3.4 dB to 2.1

dB. Downlink Tx Power will increase by approximately

1 dB.

2 dBwindow

4 dBwindow

Soft Handover

Estimating The Effect of SHOEstimating The Effect of SHO

• If the window is set to 2 dB.

• The DL will only have to suffer an increase of 50% in

the number of terminals (to 12) and 8 of these will

benefit from SHO gain. Overall increase in burden

estimated to be 0.5 dB.

• UL split between users with a target Eb/No of 2.5 dB

and those with 4 dB. Combined loading estimated to

be 27% + 19% = 46%

• Summarising the effect: UL NR will reduce from

3.4 dB to 2.7 dB.

• Downlink Tx Power will increase by 0.5 dB.

2 dBwindow

4 dBwindow

Soft Handover

Estimating The Effect of SHO: ConclusionEstimating The Effect of SHO: Conclusion

• Setting the window to the optimum size can

balance the uplink and downlink in a network.

• Note that example here is with symmetrical

loading. Excessive SHO reduces the ability for

the DL to serve asymmetric users.

• Note also that SHO requires additional

hardware in the Node B to provide the

necessary bearers.

2 dBwindow

4 dBwindow

Soft Handover

4.3.6 Further Issues regarding soft hand over.

Although soft hand over produces a macro-diversity and a processing gain, in order to be successful the receiver must synchronise using the incoming pilot signal. If a UE is to benefit , all active set pilots must be detectable. Detectable is usually taken to mean an Ec/Io better than -15 dB. If we put ourselves in the position of a UE that is attempting to synchronise with a cell that is just strong enough to be in the active set, the results are interesting. Suppose that the primary server pilot is detected with an Ec/Io of -12 dB at the cell edge. Clearly a large soft hand over window will be of little use. This would lead to attempts to admit cells with such a weak pilot that no use could be made of the signals. Unless there are exceptional circumstances, soft hand over windows/margins should be no larger than 6 dB.

SHO: further issuesSHO: further issues

• Even at the edge of the SHO region. All cell pilots must be of a “useful” strength (typically, >-15 dB)

• Large SHO windows not effective as weaker pilot is not detectable.

• In exceptional circumstances, larger SHO windows may be effective.

2 dB window

4 dB window

Soft Handover

SHO: further issues SHO: further issues –– pilot powerpilot power

• We have seen that SHO provides diversity and combining gain, particularly on the uplink. This gain helps with uplink coverage.

• The maximum benefit is obtained if the two signals are at near-equal strength. Thus the hand over region should be where the uplink path loss is the same to cells in the active set.

Maximum UL SHO gain

where UL path loss is equal

Soft Handover

SHO: further issues SHO: further issues –– pilot powerpilot power

• Hand over region is controlled by pilot power.

• If pilot powers are not equal, soft hand over region will not bewhere maximum gain is provided.

Maximum UL SHO gain


Soft Handover

Pilot power controls location of SHO

region

SHO: further issues SHO: further issues –– pilot power: pilot power: MHAsMHAs

• If MHAs are deployed the UL “ends at the mast head”.

• Pilot power should be equal at the mast head.

• If no MHAs, pilot power should be equal at TRx output (rack output).

Maximum UL SHO gain


Soft Handover

Pilot power controls location of SHO

region

4.3.7 Noise Rise (UL Loading Factor) Limit

On the downlink, the downlink power limits the loading factor. In most situations, it will be possible to drive the downlink to a loading level of about 85%. On the uplink it is limited in the form of a noise rise limit. As the noise rise curve becomes steeper as the loading factor gets larger, there are concerns regarding the stability of the network if the loading level is allowed to become high. There is a general feeling that the noise rise should be limited to about 4 dB (corresponding to a loading factor of 60%). However, where the site density is very high so that path loss is not a limiting factor, the noise rise limit could be raised.

5 Providing Additional Hard Capacity

5.1 IntroductionBecause the air interface of a UMTS network is, perhaps, the most challenging area to optimise, the need to ensure that there is adequate “hard” capacity to support the “soft” capacity of the air interface is sometimes overlooked. However, the way in which bearer form require channel elements and the loading cell users put onto the fixed network must be considered. Further, the modulation scheme used in W-CDMA imposes a hard limit on the maximum throughput. New schemes are being put forward that will increase possible throughputs.

5.2 Channel ElementsEvery radio bearer requires an associated bit of hardware within the Node B. This hardware is referred to as a channel element (CE). The different types of radio bearer serviced by the Node B will require a different level of resource. Typically, the requirement could be:

Voice: 1 channel element per connection

64 kbps data: 3 channel elements per connection



The allocation of CEs is to a Node B. This allocation would be shared between cells and between carriers (if more than one carrier is deployed). Further, it should be noted that some of the capacity

will be required to service soft hand over connections (but not softer hand over).

“Hard” Capacity“Hard” CapacityHard Capacity

• Because the air interface in UMTS networks is new, mostattention is paid to maximising the interference-limitedcapacity of the air interface itself (the “soft” capacity).

• However, there must be sufficient capacity in the hardwareof the fixed network to support the demand generated bythe cells.

• This includes considering:

• Channel Elements at the Node B

• Capacity of the interfaces

• Capacity of the RNCs

“Hard” Capacity - Channel Elements“Hard” Capacity - Channel ElementsHard Capacity

• Each bearer requireshardware in the form ofchannel elements (Ces) inthe Node B. The number ofCEs required depends onthe data rate, for example:

• Voice: 1 CE

• 64 kbps: 3 CEs

• 128 kbps: 5 CEs

• 384 kbps: 8 CEs

“Hard” Capacity - Channel Elements“Hard” Capacity - Channel ElementsHard Capacity

• Allocation is shared acrosscells and carriers.

• Provision must be made toaccommodate soft handover.

• Softer hand over imposesno additional burden.

5.3 Fixed network capacityIt is crucial that there is sufficient capacity in the fixed network to service the data flow. This data flow consists of control and signalling channels as well as user data. In particular the following questions should be asked:

Is there sufficient capacity between the Node Bs and the RNC (the Iub capacity)?

Is there sufficient capacity within the RNC itself?

Does the interface between the RNC and the MSC/SGSN (Iu) have sufficient capacity?

Is there sufficient capacity on the RNC-RNC (Iur) links?

Does the core network have sufficient capacity?

The capacity on the interfaces is measured in terms of “E1 links” (approximately 2 Mbps). The fact that a Node B will generate more traffic than a GSM site means that more fixed network capacity will be required. In GSM networks, a single E1 link was sufficient to serve the traffic from 15 TRXs. A site having more than 15 TRXs is very rare. In UMTS networks, a single E1 link will not be sufficient to service the traffic from a Node B. In areas where the demand is low, it may be possible to get away with a single E1 link but it must be borne in mind that hard blocking, rather than air-interface “soft” blocking will be the limiting factor. The fact that the highest data rate, packet switched, services will be available in the downlink only means that it may well be necessary to have a greater

provision through the fixed network in the downlink. This breaks with the tradition of having the same capacity in both directions.

The Iu interface between the RNC and the MSC or SGSN will have to carry all the traffic from users of its Node Bs (with the exception of that traffic that is for a user on the same RNC). Its required capacity will depend on the number of Node Bs that it serves.

The Iur interface allows RNCs to communicate with each other. This is done, for example, when a UE is in soft hand over between cells that are controlled by different RNCs. The “drift RNC” passes data through to the “serving RNC” to allow decisions to be made regarding active set membership and also to allow macro-diversity gains to be realised. Provision of such a link is vital but the capacity requirements are not expected to be large.

“Hard” Capacity - Fixed Network“Hard” Capacity - Fixed NetworkHard Capacity

• Interfaces must besufficiently provisioned.

• To Node Bs

• To the MSC/SGSN

• To other RNCs

RNC •Iu to MSC/SGSN•Iub to Node B

•Iur to other RNCs


• The Iub interface

• The “E1” link is standard (2Mbit/s).

• This may cause hardblocking as a Node B airinterface (3 cells) cansupport a total greater than2 Mbit/s. RNC •Iu to MSC/SGSN•Iub to Node B



• The Iur interface

• Provides a link to otherRNCs where a UE is in softhand over with a cell in adifferent RNC area.

• Low capacity.




• The Iu interface

• Handles almost all userdata from all Node Bs.

• High capacityrequirements.



5.4 High Speed Downlink Packet Access (HSDPA)GSM networks did not use spectrum in an efficient manner because the throughput was confined to a rigid structure based on timeslots and raw bit rates. No matter how good the radio channel, the throughput was limited to a raw rate of 270 kbps. The introduction of EDGE allowed this theoretical rate to be trebled if the radio environment could support it (i.e. C/I was high – there was sufficient “soft” capacity) by utilising an 8PSK modulation scheme.

The flexible way in which UMTS cells allow users to share air interface capacity makes it more efficient. However, there is still the limitation imposed by the chip rate of 3840 kcps and the QPSK modulation scheme. Suppose a cell has one 384 kbps bearer provisioned on the downlink. This bearer will have a maximum power allocated to it that will allow service only if the radio environment is of sufficient quality. But, even if the radio environment is of superb quality, it will not be possible to increase the throughput. However if HSDPA 16QAM is implemented, it will be possible, in areas where the radio environment is very good, to download at 768 kbps. This will increase the overall network capacity by enabling a faster download in areas of high quality radio environment.

“Hard” Capacity - HSDPA“Hard” Capacity - HSDPAHard Capacity

• Ultimate constraint isthe 3840 kcps chiprate.

• QPSK allows 2 bits persymbol.

• High Speed DownlinkPacket Access(HSDPA) uses 16 QAMwhich allows 4 bits persymbol.

• Hard limit is doubled.

QPSK

16 QAM

“Hard” Capacity - HSDPA“Hard” Capacity - HSDPAHard Capacity

• HSDPA is not a“magic” solution.

• Interference limitationsstill exist.

• HSDPA only possiblein areas of lowinterference.

• Aggregate networkcapacity should beincreased.

HSDPAPossible

NoHSDPA

6 In-building Solutions

6.1 IntroductionThe deployment of node Bs with cell antenna located inside buildings will probably be implemented at the initial launch stage. Although these help to ensure that the coverage within these buildings is acceptable, the main reason for implementing such in-building cells is to provide extra network capacity when it is predicted that the macro-cell layer will not be able to serve the offered traffic. As an example, consider an office building of 20 storeys with 200 people on each floor, a total of 4000 people. A particular operator may have 800 subscribers within such a building. If the average traffic offered by a subscriber is 25 mE of voice, that represents an average loading of 20 E. When video telephony (VT) and data traffic are considered, it is clear that the offered traffic justifies at least one cell in its own right. As a single macro-cell’s coverage area may contain several such office buildings, it is clear that the capacity of the macro-cell layer will be insufficient to accommodate the offered traffic in such areas.

Deploying such indoor cells using the same frequency as the macro-cell layer will improve the spectrum efficiency of the network and also make network operations (in particular, hand over) much more straightforward. The indoor cells must be implemented so as to provide coverage within the building whilst minimising interference between the indoor cell and the macro-cell.

InIn--building Solutionsbuilding SolutionsIn-building solutions

• Cells with indoor antennas can help with coverage problems.

• Most importantly, they add to the network capacity and serve an indoor “hotspot”.

• Eg; 20 floor, 200 people per floor (4000 people): 800 subscribers, 20 Erlangs offered.

• If this is VT, this would be typical for a sectored Node B. A macro-cell may contain several such buildings.

• In-building solutions can alleviate macro-cell capacity problems.

Frequency allocationFrequency allocationIn-building solutions

• Advantageous if same frequency as macro-cell layer can be used.

• Spectral efficiency maximised

• Hand over between indoor and outdoor environment simplified.

• Mutual interference must be minimised whilst engineering soft hand over region.

6.2 The interference loopMutual interference reduces capacity in a network due to the fact that a form of feedback loop is established. If a particular cell suffers external interference, it will send a “power up” message to its UEs. This will cause uplink interference to increase at the

neighbouring cells that, in turn, send “power up” messages to their UEs and thus the process continues. The result is that each cell suffers extra noise rise through interference, thus limiting capacity.

The fact that some of the cells are indoors and the macro cell is out of doors, there is a level of isolation afforded that helps to break the feedback loop. Within a cell layer, the “problem areas” are those at the cell border where the path loss to two or more cells is nearly equal. It is in these areas where the UEs suffer from downlink interference and also generate uplink interference, thus reducing frequency re-use efficiency. The physical barrier presented by the outside walls of the building in question allows this region to become negligibly small. For example, from just inside the building, the path loss to the indoor cell can be 10 dB lower than to the macro-cell. Just outside the building, the reverse can be true. Thus mutual interference is low and frequency re-use efficiency high. Effectively, it is a similar situation to that where no users are located at the border region between cells.

““Reducing Mutual Reducing Mutual InteferenceInteference””

• The lower the interference the higher the capacity.

• Because of the single frequency used in a UMTS layer, there is an “Interference feedback loop”.

• This means that interference, rather than just adding to the background noise level, consumes a proportion of the network resource (power on the DL, noise rise on the UL).

In-building solutions

Reducing Mutual Reducing Mutual InteferenceInteference

• The walls of the building will help provide isolation between the indoor and outdoor cells, thus improving capacity.

• “Problem areas” are those where the path loss to both cells is similar.

• Presence of walls makes it possible to make this region negligibly small.

• Similar, in principle, to a macro-cell structure with gaps in coverage –low interference (but HO failures –but people don’t walk through walls).


6.3 The Dead-zone effectInterference will occur between operators, particularly where the carriers are adjacent to each other. An in-building solution provides very low path losses to the serving cell. This means that there is the potential for very high interference values. Consider a user who is a subscriber to an operator that does not have a pico cell inside a particular building. The pilot from the macro cell that serves the UE may be as low as -105 dBm. Protection for adjacent carrier interference by approximately 33 dB can be assumed. If a total interfering level of -57 dBm was received at the adjacent carrier, that would be reduced by the isolation filters to -90 dBm giving an Ec/Io of -15 dB: the lowest level useable. Any larger interference levels than -57 dBm would prevent the UE from synchronising to its serving pilot. If the transmitting power of the pico cell was 33 dBm with a 7 dBi antenna gain, a path loss of anything less than 97 dB would cause problems. Areas where the path loss is less than 97 dB are expected to occur readily within pico cell coverage areas. Two possible solutions to this problem may be considered:

All operators provide a pico cell solution within the building in question (this would lead to all operators having identical strategies for in-building deployment).

Operators agree to share pico cell capacity and allow each others’ users to hand over to the pico cell.

The Dead Zone EffectThe Dead Zone Effect

• Adjacent carrier interference may be particularly noticeable within pico cell environments.

• 33 dB is typical of the isolation between adjacent carriers.

• Case shown illustrates the limiting case where Ec/Io = -15 dB.

• Max EIRP from pico cell is typically 40 dBm (33 dBm Tx Power; 7 dBiantenna).

• Problem cases will occur.


•Macro cell pilot:

-105 dBm

Pico cell

interference: -57 dBm

The Dead Zone Effect: solutions?The Dead Zone Effect: solutions?

• Unlikely to provide a solution by engineering the radio environment: low path loss to best server is generally a “good thing”

• Possibilities

• All affected operators deploy a picocell within a particular building.

• Operators allow hand over to picocell carriers from affected cells.


•Macro cell pilot:

-105 dBm

Pico cell

interference: -57 dBm

6.4 Hand over from indoor to outdoorThe sudden difference in path loss between cells, although useful in increasing frequency re-use efficiency, can cause problems with hand over. It is important that there is a “soft hand over” region within which a UE will have a connection to two or more cells. This is the “border region”. If a UE suddenly receives a lot of power from a cell that is not part of the active set, a call can drop. The UE needs to be able to monitor this neighbour and execute the hand over. If the power from the neighbour increases too rapidly, there could be insufficient time to do this. Thus, the situation where the border region coincides with the outside building wall is probably going to result in problems with hand over. The indoor cell must be engineered so that the border region obeys two conditions:

It must be of sufficient size to allow soft hand over to take place

The subscriber density within this border region should be low

Hand OverHand Over

• A hand over region needs to be provided.

• Sudden changes in signal level from a cell can lead to calls being dropped.

• Required hand over region is near the door.

• Hand over region:• Large enough to allow hand over.• Should be where subscriber

density is low, as hand over region is area where mutual interference is highest.

• Preventing sudden changes in signal strength at the HO region requires appropriate siting of pico-cell and macro-cell antennas.


Required hand over region

6.4.1 Engineering the border region

Taking the above two constraints into consideration it is best if the border region is designed to be just outside the building. This entails designing the indoor antenna arrangement so as to make the contour of equal path loss (to macro-cell and in-building cell) enclose the building. In this way, all the users within the building will connect to the in-building cell only. Further, at all points within the building, the path loss to the in-building cell will be many dB

(typically 15 dB) lower than the path loss to the macro-cell. This will ensure that the users within the building will neither suffer from, nor contribute to, interference with the macro-cell. In the first instance, it is more straightforward if the configuration of the in-building cell (pilot power, maximum total power etc.) is the same as that of the macro-cell. However, it is acknowledged that there are cost and convenience advantages to be obtained if the in building cell is of a lower power rating than the macro-cell.

The result of reducing the in-building cell power by, say, 6 dB is to displace the hand over region from the region of equal path loss. At the equal pilot strength region, the path loss to the macro-cell will be greater than that to the in-building cell. Just outside the soft hand over region, in the area served by the macro-cell, the path loss to the in-building cell could be lower than to the macro-cell. Thus the in-building cell will suffer from high levels of uplink interference because the UE power is controlled by the macro-cell. The seriousness of this problem will depend on the user density within the affected areas. A less serious problem resulting from the use of different pilot powers is that the gain from soft hand over on the uplink will be less than if the hand over region was located where the path losses were equal.

Hand OverHand Over

• Contour of equal pilot strength (between macro-cells and pico-cell) should be engineered to be just outside, rather than just inside the building.

• This is because subscriber density is likely to be much higher inside the building.

• This needs to be checked by measurements.

• This will lead to the path loss to the pico-cell, for subscribers within the building, being much less than that to the macro-cell.

• This is a good thing as it means the pico-cell will have a negligible impact on macro-cell capacity.


Contour of equal pilot strength.

PicoPico--cell Parameterscell Parameters

• Planning and engineering is simplified by using the same major parameters (maximum power, pilot power, noise rise limit) are used for the macro-cell.

• However, cost savings are to be made if the maximum power is reduced. The pilot power would then have to be reduced.

• This will shift the hand over region away from the area of maximum HO gain.

• Disaster scenario is if the UE hands over to the macro cell while in the building.

• Path loss to macro-cell could be higher than to pico-cell and hence UE Txpower will increase. This has severe implications for the pico-cell capacity


UE could be forced onto macro-cell. UE power would increase and pico-cell would suffer.

PicoPico--cell Parameterscell Parameters

• The problem of the pico-cell suffering from uplink interference does not disappear if the hand over region is designed to be outside the building.

• It is the subscriber density at the hand over region that is important.

• Pico-cell capacity can be improved by increasing its noise rise limit. This could be considered a sensible step in conjunction with a reduction in pilot power.


Hand over region in area of low subscriber density

6.5 Implementing the in-building cellThe successful implementation of an “in-building solution” is an area of expertise in its own right. The best approach will depend upon the particular characteristics of the building. The number and nature of the internal walls and floors is a major factor. The

different methods commonly deployed are summarised and compared below.

Implementing the inImplementing the in--building solutionbuilding solution

• Design and implementation of in-building solutions is an area of expertise in its own right.

• The following provides an outline of the decisions and choices regarding the design of the pico-cell.

• A high level overview of the relative advantages of the different options is provided.


6.5.1 Choice of Node B

The major competing options here are:

A central Node B

Multiple compact node Bs

Utilisation of repeaters

The Choices The Choices –– The Node BThe Node B

• Single Node B


• Multiple Compact Node Bs

• Repeater to external Node B

6.5.1.1 Central Node B

In this case, a similar Node-B to those used for outdoor sites supplies the UMTS signal to the whole building. There are the following advantages and disadvantages of this solution.

ADVANTAGES DISADVANTAGESAllows easy capacity expansion (new racks can be added to the existing cabinet)

Being a centralised system, this solution can be affected by faults

No need to have extra training for the technical personnel.

Space is required to locate the base station

6.5.1.2 Multiple compact indoor Node Bs

This solution consists in installing a certain number of compact indoor Node-B’s in the area to be covered.

ADVANTAGES DISADVANTAGESBeing a distributed system, this solution is robust to faults.

Capacity expansion can be problematic due to the installation of extra base stations.

Space is not a critic requirement due to the small dimensions of these indoor Node-B’s.

Extra training needs to be done for the technical personnel.

Better Coverage can be provided.

6.5.1.3 Repeaters

Repeaters are very useful for enhancing coverage where no extra capacity is required. They can be used to extend coverage from either an outdoor cell site or to distribute coverage from an indoor Node-B’s throughout the all area of interest. Repeaters are two port devices for direct connection to the base station and to an antenna or a leaky cable. They can be connected to the base station by means of a radio link (usually a Yagi directed at the base station) or a cable in case a radio link cannot be established. Further, it is possible to establish a fibre optic connection from the base station to the repeater site and send the RF signal over optical fibre.

The Choices The Choices –– The Node BThe Node B

• Single Node B


• Easy capacity expansion (just add more cards).

• Staff familiar with equipment, as Node B can be the same as for the macro-cell.

• Centralised system could be prone to faults.

• Node B could be physically large to accommodate.

The Choices The Choices –– The Node BThe Node BIn-building solutions

• Multiple Compact Node Bs

• More robust to faults as it is distributed.

• Can lead to superior coverage.

• Smaller physical size.

• Capacity expansion can be harder.

• Staff may need extra training on new type of Node B.

The Choices The Choices –– The Node BThe Node BIn-building solutions

• Repeater to external Node B

• Cheap – but no solution to network capacity problems.

6.5.2 Distribution methods

Even if multiple Node Bs are used to provide the indoor solution, each node B will usually be required to serve several floors. Only rarely will a single, centralised antenna be capable of providing coverage for an entire Node B area. Usually, some kind of

distributed system needs to be implemented. There are a variety of solutions put forward by different vendors.

6.5.2.1 A distributed antenna system

This is a system comprising two or more antennas together with associated power dividers and cabling. Each antenna would be responsible for providing coverage in a particular zone.

ADVANTAGES DISADVANTAGES

Allow use of low radiated power. High total system losses in larger buildings.

Allow a flexible network design – different antenna models can be installed in different building areas.

Complex cable installations

Well suited for several building structures

Visual impact

6.5.2.2 Distributed active antenna systems

Distributed active antenna systems represent an alternative to systems using passive antennas. The antennas will contain their own power amplifiers. Different solutions of this type distribute the signal from the central source by means of either optical fibre or CAT-5 cable. The radiated power is usually low (e.g. 100 mW), thus limiting the range. However, as the antenna amplifies the signal itself, internal cable losses are not a significant factor.

ADVANTAGES DISADVANTAGESVery easy installation for the optical fibre/CAT5 cable.

Low radiated power.

Reduced EMC problems. Reduced reliability and need of specialised personnel for equipment installation and maintenance.

Less attenuation losses than coaxial cables.

Power supply may be required at each active antenna site.

6.5.3 Radiating cables

An alternative signal-distribution method is represented by radiating cables or “Leaky Feeders”. The radiating cable is acting as continuous longitudinal antenna distributing the signal along its path. This solution is particularly suitable for long structures such as tunnels. The need for precision installation and the cost makes them less suitable for office-type buildings.

ADVANTAGES DISADVANTAGESWell suited for longitudinal structures. Not well suited for general shape

structures (squared or circular).Uniform signal distribution along its path.

Not cost effective in every situation.

Low visual impact (can be hidden behind a suspended ceiling).

Precision installation required.

The Choices The Choices –– Antenna SystemsAntenna Systems

• Distributed Passive Antennas


• Distributed Active Antenna System

• Radiating Cable

The Choices The Choices –– Antenna SystemsAntenna Systems

• Distributed Passive Antennas


• Closeness of antenna to UE allows low radiated power.

• Most suitable antenna can be chosen for each location allowing good control of radiation.

• Cables can be lossy and expensive.

• Installation of heavy cable can be difficult.

The Choices The Choices –– Antenna SystemsAntenna SystemsIn-building solutions

• Low grade CAT 5 cable can be used instead of feeder making installation easier.

• Lower feeder losses

• Restricted to low transmit power.

• Power supply required at each antenna location.• Distributed Active

Antenna System

The Choices The Choices –– Antenna SystemsAntenna SystemsIn-building solutions

• Easiest to model/predict link loss.

• Produces even coverage.

• Can be hidden from view.

• Often the most expensive solution.

• Not suited for all shapes (better for longitudinal shapes).

• Precision installation required to maximise benefits.

• Radiating Cable

6.5.4 Field measurements to verify the implementation

The requirement of the indoor cell is to provide coverage and capacity inside the building whilst minimising the impact on the macro-cell. Further, hand over between the indoor and outdoor environments should be possible.

The load on the macro-cell layer air interface will be minimal if the path loss to the indoor cell is at least 10 dB less than the path loss to the macro-cell at points of interest. The “points of interest” are the areas within the building where the user density is high – this may be the entire building space. This can be ascertained by measuring and comparing the pilot strengths of the macro-cell and indoor cell at these points. If the pilot powers are the same (a useful starting point), then the pilot strength of the indoor cell should be at least 10 dB above that of the macro cell pilots.

Successful hand over depends on the existence of a soft hand over region existing between the coverage areas. This region must be large enough so as to ensure that the signal strength changes are not too rapid for the hand over to be executed. These regions must be established particularly at the entrances to the building. Fortunately, users are not expected to be travelling at great speed in these areas and a region of 10 metres width should be sufficient. This can be either just inside or just outside the building, or both. If antenna adjustment is necessary to engineer the soft hand over region, it is usually easier to adjust the in-building configuration so that its signal leaks slightly out of the entrance. It must be remembered however, that too much leakage will lead to mutual interference between the indoor and outdoor cells.

Field Measurements to Check on ImplementationField Measurements to Check on Implementation


• Scanner measurements can be used to assess:• Coverage

• Hand over region

• Isolation from macro-cell. Ideally the macro-cell and the pico-cell should not interfere with each other.



• Coverage• Pico-cell pilot should be better than approximately -100

dBm at all locations (note that there is no need for an “in-building allowance” such as for outdoor drive test measurements).

• Hand Over• Hand over region is near the building entrance.

• UEs not expected to be moving at high speed.

• 10 m soft hand over region should suffice.

• This can be just inside or just outside the building (or both).

• Easier to adjust in-building antennas rather than macro-cell antennas.

• Too much leakage outside of the building will result in interference.



• Isolation from macro-cell• Loss to pico-cell should be 10 dB less than that to

macro-cell at all “significant locations” within the building.

• This can be checked by comparing pilot strengths: remembering to consider any difference in pilot transmit power and cell configurations (MHA on macro-cell would need to be considered).

7 Using Micro-cells to Service Hot Spots

7.1 IntroductionThe Macro-cell layer is designed to provide continuous coverage to a specified level (such as VT indoor). The initial result will be a UMTS carrier layer that will service a particular traffic density. Providing an indoor solution as described in the previous section can accommodate extra traffic from office environments. Simulations suggest that, if 20 dB of penetration loss must be accommodated, a cell range of 500 metres would be typical. This would result in a site density of approximately 2 sites per square kilometre and offered traffic of approximately 22 Erlangs per site would be accommodated. If outdoor environments covered by a particular cell generate a high level of offered traffic, that traffic will experience high levels of blocking. A micro-cell may be an attractive solution in such a case. In this instance, the possibility of deploying a micro-cell re-using the carrier that is used by the macro-cell layer is investigated.

MicroMicro--cell planningcell planning• Typical range for macro-cell for VT is 500 metres in dense urban

environment.

• Site Density for coverage approximately 2 sites/km2.

• Capacity then approximately 22 Erlangs of VT per site (44 Erlangs per

km2).

• An area of, say, 100 by 150 metres would be expected to generate only

one Erlang of traffic.

• If an area of this size, or smaller, generates 15 Erlangs of traffic, a micro-

cell can help to accommodate this offered traffic.

Macro cell layer providing continuous coverage Micro cells

serving hot spots.

Micro-cell Planning

MicroMicro--cell planning: carrier recell planning: carrier re--useuse

• If the macro cell carrier can be re-used:

• Spectral efficiency is improved.

• Hand over between micro and macro cells is easier.

Macro cell layer providing continuous coverage Micro cells

serving hot spots.

Micro-cell Planning

7.2 Micro-cell and In-building cells compared

In-building solutions using the same carrier as the macro-cell have been shown to be a viable method of serving areas of high subscriber density. A significant feature is allowing straightforward integration into the network is the isolation between the inside and outside of the building that is provided by the walls of the building itself. This isolation results in low levels of mutual interference and, hence, higher levels of frequency re-use efficiency and cell capacity. When a micro-cell is deployed in an outdoor environment, such as a pedestrian shopping area, no such natural isolation exists. The risk of high levels of mutual interference, leading to reduced capacity, is therefore significant.

MicroMicro--cell planning: mutual interferencecell planning: mutual interference

• In the case of in-building solutions, the building walls formed a

barrier against interference.

• This made re-use of the macro cell carrier more straightforward.

• Outdoor micro cells have no such barrier.

• Potential for more serious interference issues, reducing

capacity gains

In-building solution: walls form barrier against interference.

Micro cells – no barrier against interference..

Micro-cell Planning

7.3 The Theory behind the Micro-cellConsider a macro-cell that serves an area of perhaps 200000 m2. If video telephony is seen as the benchmark service, such a cell could support up to approximately 12 simultaneous connections. Suppose that a hotspot exists that covers about 15000 m2. If this area had a normal traffic loading, the area of the hotspot would be expected to contain only 1 active user. The situation where this area needs to support many active users is envisaged. A hotspot should be deployed such that the effect of these many users, as far as the macro-cell is concerned, is equivalent to one user of the macro-cell. Effectively, this means that the total power of the many users must equal that of the one macro-cell user. If the micro-cell is to support 12 users, then the power of each of these 12 users must be, on average, 11 dB less than the power required for the

macro-cell. This can be taken to mean that the path loss to the micro-cell must be 11 dB less than the path loss to the macro-cell.

MicroMicro--cell planning: theorycell planning: theory

• Suppose an area within a macro cell could accommodate only 1

Erlang of offered traffic if the macro cell capacity was divided

equally on an area basis.

• Now consider the situation if this area was expected to generate 12

Erlangs of offered traffic.

Area generates 12 times the “expected” traffic.

Micro-cell Planning


• If carrier frequency is to be shared and no extra loading to be

placed on macro cell:

• Each UE should operate at 1/12th of the power that it would if it

connected to the macro cell.

• Path loss to micro cell should be 11 dB less than that to macro cell.

Area generates 12 times the “expected” traffic.

Micro-cell Planning


• This can be arranged with pilot power settings but there are

problems:

• If pilot powers are left equal, then border region will be where path loss

is equal between micro cell and macro cell UL interference on macro

cell results.

• If micro cell pilot is 11 dB less than that for macro cell, UEs just outside

the micro cell border will cause a lot of UL interference on the micro

cell.

Micro-cell Planning

Pilot powers equal: macro-cell affected by UEs on border.

Micro cell pilot reduced by 11 dB. Micro cell affected by UEs just outside border.

7.3.1 Pilot Power Settings

This sounds perfectly achievable but there is a complication that becomes apparent when the cell-selection procedure is considered. If pilot powers of the micro-cell and macro-cell are made equal then, at the border, the path loss to the macro-cell and the micro-cell would be equal and the UE would transmit with the same power no matter which cell is selected. In this situation the only benefit would be the macro-diversity gain from soft hand over. An option would be to reduce the pilot power of the micro-cell so that users would only connect to the micro-cell if the UE transmit power was going to be significantly lower than if a connection was made to the macro-cell. This, however, leaves the possibility that a UE connected to the macro-cell would interfere significantly with the micro-cell thus reducing its capacity.


• This can be arranged with pilot power settings but there are

problems:

• If pilot powers are left equal, then border region will be where path loss

is equal between micro cell and macro cell UL interference on macro

cell results.

• If micro cell pilot is 11 dB less than that for macro cell, UEs just outside

the micro cell border will cause a lot of UL interference on the micro

cell.

Micro-cell Planning

Pilot powers equal: macro-cell affected by UEs on border.

Micro cell pilot reduced by 11 dB. Micro cell affected by UEs just outside border.

7.3.2 Engineering the Micro-cell

The problems described above become significant only if there are a significant number of users in the transition regions. Engineering of the micro-cell can avoid this situation occurring. In order to effectively deploy a micro-cell at the same frequency as the macro-cell layer, the following guidelines should prove to be useful.

The capture area of the micro-cell should extend beyond the hotspot. In that way, the number of users in the border/transition region should be small.

The radiation pattern, height and tilt of the micro-cell antenna should be such that the path loss to the micro-cell rapidly increases with distance once the UE leaves the hotspot area. This makes the border/transition region physically small which again reduces the probability of users occurring within the region of high mutual interference.

If the above recommendations are adhered to, then the noise rise limit and pilot power of the micro-cell could, in the first instance, be set to the same as the macro-cell. In this way, the micro-cell could be hoped to provide another ten VT connections whilst producing a negligible effect on the macro-cell.

Engineering the Micro cellEngineering the Micro cell

• The success of any strategy depends on the user behaviour.

• Areas of high mutual interference are only problematic if there

are lots of users.

• Need to engineer the micro cell accordingly

• Micro cell dominance area should exceed the hotspot area.

Micro-cell Planning

Area of dominance of micro cell should exceed the hotspot area.

Engineering the Micro cellEngineering the Micro cell

• Radiation pattern of micro cell antenna should ensure that

path loss rapidly increases once outside the dominance area.

Micro-cell Planning

• Initial setting of pilot power and NR limit can be the same as

for macro cell. Ideally, should be possible to reduce DL power

to 37 dBm (pilot power would then have to be reduced

proportionately).

Engineering the Micro cell: field Engineering the Micro cell: field measurementsmeasurements

• If the pilot powers are equal, the border area is at locations of

equal path loss.

• Pilot strength of micro cell should be 10 dB greater than that

from macro cell at all areas of high subscriber density.

• If transmit pilot power of micro cell is 6 dB less than macro cell,

then micro cell pilot can be just 4 dB greater.

• When micro cell pilot is reduced, problem comes from potential

uplink interference from macro cell UEs just outside the border.

• Should be possible to raise the NR limit to help with this. (E.g.

macro cell NR limit: 4 dB; micro cell NR limit: 8 dB).

Micro-cell Planning

7.3.3 What can go wrong?

The biggest problem is mutual interference. The macro cell base station will still “see” the UEs served by the micro cell. It is vital that they transmit at a lower power than the UEs served by the macro cell, otherwise they would produce a high level of uplink interference on the uplink. Thus the nature of the hot spot should be that the users would experience a much lower path loss to the micro cell than to the macro cell. This is to protect the macro cell from uplink interference. It is tempting to attempt to achieve this by scaling the pilot power (making the pilot power of the micro cell lower than the macro cell). However, this could produce the situation where a UE served by the macro cell caused a lot of uplink interference on the micro cell due to the fact that the path loss to the micro cell was much lower. There is inevitably going to be an area (just as there is in the macro cell layer) where UEs are subjected to a lot of downlink interference and generate a lot of uplink interference: the border area. In planning a micro cell, user behaviour must be considered. The border area should not contain a high density of users.

Possible ProblemsPossible Problems

• If there are lots of users in the border area, this will cause

interference problems.

• If the micro-cell pilot power is reduced it may suffer from

interference as the UEs connected to the macro-cell will be

transmitting with relatively high power.

Micro-cell Planning

Users in the border area will cause and experience interference problems.

7.3.4 Detecting Problem Areas.

If the pilot strength of the macro cell and micro cell are kept equal, the border area is the area where the path losses are equal. The high subscriber density should be restricted to areas where the pilot strength from the micro cell is 10 dB or more greater than the pilot from the macro cell. This can be checked by making field measurements with a scanner.

Using Scanner MeasurementsUsing Scanner Measurements

• With pilot powers of the micro-cell and macro-cell set to equal

levels, the strength of the micro-cell pilot should be at least 10

dB greater than the macro-cell pilot throughout the area of

high expected user density (the “hotspot”).

Micro-cell Planning

Area of expected high user density.

Engineering the Micro cell: field measurementsEngineering the Micro cell: field measurements

• If the pilot powers are equal, the border area is at locations of

equal path loss.

• Pilot strength of micro cell should be 10 dB greater than that

from macro cell at all areas of high subscriber density.

• If transmit pilot power of micro cell is 6 dB less than macro cell,

then micro cell pilot can be just 4 dB greater.

• When micro cell pilot is reduced, problem comes from potential

uplink interference from macro cell UEs just outside the border.

• Should be possible to raise the NR limit to help with this. (E.g.

macro cell NR limit: 4 dB; micro cell NR limit: 8 dB).

Micro-cell Planning

7.4 Hotspots straddling macro cell boundariesSo far we have considered the case where the hotspot lies totally within an area served by a single macro cell. This is not necessarily the case. It may be common for a hotspot to lie on the border area. In fact, hotspots in such locations are likely to cause greater problems with regard to macro cell layer capacity as the border

areas are those where mutual interference is highest. A micro cell can provide a great benefit in such circumstances. The principle of engineering the micro cell is the same as in the case where the hotspot lies within a macro cell coverage area. Namely, the area of high user density should experience a much lower path loss to the micro cell than to the macro cell.

Hotspots straddling cell boundariesHotspots straddling cell boundaries

• A hotspot straddling a cell boundary will benefit greatly from a

micro cell as it is in an area that will cause significant

interference.

• Similar engineering considerations apply: the micro-cell

coverage area should exceed the area of high subscriber

density.

Micro-cell Planning

7.5 Propagation modelling for micro cellsThe initial planning of a UMTS network is often conducted with map data with a resolution of perhaps 50 metres. This is not a fine enough resolution to simulate the effect of a micro cell. Map data at a fine resolution should be used together with an appropriate propagation model. Models used for macro cell planning rely on empirical models that do not consider building reflections explicitly. Building reflections, penetration and diffraction form the dominant propagation mechanisms in micro cells. Accordingly, a “ray tracing” model that considers these mechanisms should be used.

Prediction ResultsPrediction Results• Using Okumura-Hata, you will get predictions that are largely based on the

distance from the Cell.

• This has a certain validity when the antenna is above the building but not when it is down below building height.

Micro-cell Planning

Prediction ResultsPrediction Results• When diffraction and scatter are the main mechanisms, the field strength will

change in a much less straightforward manner.

Low signal strength at some locations close to BTS.

“Canyon Effect”

Micro-cell Planning

Adaptations to MacroAdaptations to Macro--cell Modelcell Model

• Most significant differences between Micro-cell and

Macro-cell predictions:

• Canyon Effect: Low reduction in field strength with

increasing distance (“exponent” approximately 2.0).

• Modelled by: adjusting appropriate parameters to

give low exponent.

• Coverage holes close to BTS.

• Modelled by: more sophisticated macro-cell models.

Micro-cell Planning

Enhancement to MacroEnhancement to Macro--Cell ModelsCell Models• Clutter Offset in dB

• Height and Separation: Used for Diffraction calculations (time consuming).

• Through loss in dB/km: Maximum distance specified. Loss is weighted from zero to 1 over this distance. Faster than diffraction calculations.

Separation

Through-loss distance

Micro-cell Planning

Enhancement to MacroEnhancement to Macro--Cell ModelsCell Models

• Plausible results (statistically quite good regarding s.d. of error) can be obtained by enhancing a macro-cell model. However:

• “Always struggling”: model has to be carefully adapted for each situation. Parameter tuning becomes something of a “fiddle factor”. Would “tuned model” be appropriate for general use?

• No “set up and go” capability.

• Greater accuracy is obtainable from deterministic models.

Micro-cell Planning

3G Network configurations3G Network configurations

Macro-cell: antenna well above rooftops

Micro-cell: antenna below rooftops

Mini-cell: antenna on small buildingrooftops (below higher buildings)

Micro-cell Planning

Radio configurations: MiniRadio configurations: Mini

• Intermediate configurations:• The emitting antenna is located on a roof top but not higher than the

surrounding roof level

• The emitting antenna is against a building façade but not lower than the surrounding roof level

• Mini-cellular configuration

• The radio energy is partly propagated above

obstacles and partly along the streets

• Choice must be the Volcano Mini model

So, 3D channel modelling is required

Micro-cell Planning

Operational casesOperational cases

Accurate coverage in rural hilly area

CDMA network coverage (USA)Multi-band GSM Macro-Micro Cell

Mini-cell model for 3G

Micro-cell Planning

Propagation ModelsPropagation Models

Pure deterministicmodels

Semi-statistical models

Co

st-Ha

taw

ithD

iffractio

n corre

ctions

Deterministic models

Vo

lcano

-De

ygou

t

Statistical models Co

st-Ha

ta

Volcano-RTD

Micro-cell Planning

Which topographic databaseWhich topographic database??• The right database for the right environment: trade-off calculation time vs.

accuracy

• Database quality: adapted to telecom problems

• Importance of the clutter description

• The good choices:

• Deterministic vs. statistical

• High vs. low resolution databases

Required layers

Environment

3 rasters (2 – 5 m) +

1 vector layer

Or 3D vectors

3 rasters / vectors

(5 – 10 m)

2/3 rasters

(10 – 50 m)

UrbanSuburbanRural

1 m

20 m

Micro-cell Planning

DeygoutDeygout modelmodel

E

D

P

d 1 d 2

h

v h d

d

2 1 2

1 2

d + d

if v

A v u u

u v

> -0.7 (cas NLOS)

( ) . log

6 9 20 1

1

A(v)

v 0

-6dB

Terrain profile

Micro-cell Planning

RayRay--tracingtracing

Ray contributions constructed for an emitting site located against a building wall

Micro-cell Planning

A deterministic propagation tool: A deterministic propagation tool: ex. Volcano RTD Modelex. Volcano RTD Model

Only Deygout

Only RT/UTD

Volcano

EE

RR

++

Horizontal planeRay Tracing

Micro-cell Planning

Calibration / measurementsCalibration / measurements

(distance) Log

Measures

Micro-cell Planning

• Not mandatory but recommended

• Environment not entirely described by DTM (stores, balconies)

• Automatic calibration

• Simple and quick calibration

• «Free Space» Correction

• Weighting

• Full calibration

• Non linear parameters (building heights)

Standard deviation= 5 to 8 dB

MacroMacro--cellular cellular coveragecoverage

• Shadowing prediction

• Up to long distance realistic prediction

COST – Hata Simulation (tuned) Volcano Simulation

Higher terrain

Street

Hill

Losses (dB)

Micro-cell Planning

MicroMicro--cellular cellular coveragecoverage

• Shadowing and wave-guiding effects

• Up to long distance realistic prediction

Statistic simulation (Harley model) Volcano Simulation

Receivedpower (dBm)

Micro-cell Planning

New important featuresNew important features

Coverage Map at 1.5m



Multifloor coverage for mini-cellular site

Micro-cell Planning

An important parameter An important parameter Downlink orthogonality factorDownlink orthogonality factor

How is this DOF has been taken into account so far?Globally, Per Area, Per Clutter

A new solution: by Volcano deterministic simulations

Outdoor only Outdoor and indoor

Micro-cell Planning

7.6 Multiple micro cellsIt is possible that a hotspot is physically too large to be served by a single micro cell without border problems arising. In such cases, a small cluster of micro cells can be established. Throughout the hotspot served by this cluster, the path loss to the micro cell should be much lower than to the macro cell. Again, this means that the hotspot should be geographically distant from the macro cell. It is not possible to establish an additional continuous cell layer, consisting of micro cells, that shares the frequency of the macro cell layer. A micro cell layer requires the use of a second carrier frequency.

7.7 Limiting factorsDeploying micro-cells that use the same frequency as the macro-cell layer is clearly a powerful solution to capacity problems. It is not, however, without adverse consequences. If we consider a problem that has resulted in multiple micro cells being deployed, it must be remembered that the reason for deployment was the high subscriber density in that area. Nearby macro-cells will inevitably suffer from greater uplink interference produced by these users, thus reducing the capacity of these cells. This can be limited by optimal engineering of the micro-cells but there is a limit. Much greater isolation between macro-cells and micro-cells is achieved if they are allocated separate frequencies. This is discussed later in the section on hierarchical cell structures.

8 The Effect of Further Site Sectorisation

8.1 The sectored antennaOmni-directional sites are now almost unheard of in an urban environment. The fact that sectored sites will service a greater subscriber density is well established. The standard level of sectorisation is three sectors per site. Antennas have been developed with this level of sectorisation in mind. Although each sector will have to control an angle of 120, the arrangement of sites means that the required range reduces as you move away from the principal direction. This leads to antennas with 85 degrees or, more recently, 65 degrees beamwidth being adopted. It is found that the 65-degree beamwidth antenna is more effective at limiting mutual interference.

Further Sectorisation of SitesFurther Sectorisation of SitesFurther Sectorisation

• Three-sectored siteshave evolved to be thenorm in urban andsuburban areas.

• Each antenna controls a120º sector.

• Antenna beamwidth acompromise betweencoverage andinterference.

• 65º is the most common.

• 18 dBi is typical gain.

8.2 Increasing the level of sectorisationAdding further sectors to a site could allow a greater user density to be served. However, any increase can be limited by mutual interference. Generally, the greater the cell density, the higher the interference levels.

Remembering that throughput , the value of i is

significant.

A specific antenna, with a 35 beamwidth, has been developed for sites that are configured with six sectors, at 60 intervals.

F u r t h e r S e c t o r i s a t i o n o f S i t e sF u r t h e r S e c t o r i s a t i o n o f S i t e sF u r t h e r S e c t o r i s a t i o n

• S i x - s e c t o r e d s i t e s c o u l d , i nt h e o r y , d o u b l e t h e c a p a c i t y .

• I f m u t u a l i n t e r f e r e n c ei n c r e a s e s , c a p a c i t y m a y n o ti n c r e a s e a s e x p e c t e d .

• B u t , i f a n t e n n a s a r e h i g h e rg a i n , c a n i n c r e a s e .

iNE b

1

3840Capacity

0

Further Sectorisation of SitesFurther Sectorisation of SitesFurther Sectorisation

• Antenna beamwidth ishighly significant in arrivingat the optimum betweencoverage and interference.

• 35º is seen as the mostappropriate.

• 21 dBi is typical gain.

• Monte Carlo simulationscan quantify the likelyimprovement.

8.3 Using simulations to assess the effectivenessSix-sector sites are almost certainly going to be an upgrade in UMTS networks. As a test of their effectiveness, the same area is assessed for capacity before and after the upgrade.

8.3.1 Vital statistics:

Parameter 3 sector site 6 sector site

Antenna Beamwidth

65 degrees 35 degrees

Antenna Gain 18.3 dBi 21.0 dBi

NR limit 4 dB 6 dB

Benchmark Service 64 kbit/s VT 64 kbit/s VT

Target Eb/No 3 dB 3 dB

Sites were placed such that the maximum coverage range was 500 metres. This was sufficient to provide continuous indoor coverage with a building penetration loss of 20 dB. An area of 5 km by 3 km was covered using 33 sites.

8.3.2 Estimates of capacity

The pole capacity of a cell with no interference with an Eb/No of 3 dB is approximately 1920 kbit/s. In the three-sector case, the noise rise limit of 4 dB, represents a 60% loading factor: 1152 kbit/s. This represents approximately 18 simultaneous connections that will serve 11.5 Erlangs of offered traffic. An out of cell interference ratio of 0.6 will reduce this to 7.2 Erlangs per cell. The 5 by 3 km area was spread with 720 Erlangs of offered traffic and a simulation was conducted with the UEs being placed indoors. As expected, satisfactory results were obtained. The network was seen to be operating near its limits, however. The offered traffic was then doubled to 1440 Erlangs. The result was that the network reached saturation at a level of approximately 950 terminals being served (66% success rate) before blocking occurred. In this situation the major cause of blocking was downlink capacity (the required Eb/No on the downlink is expected to be approximately 7 dB) but there were a significant number of failures on the uplink with the noise rise limit being hit frequently. At this stage, the effect of further sectorisation was simulated. Adding new cells with new power amplifiers (so each site had six cells of similar configuration to the

original three cells) resulted in the 1440 Erlangs of traffic being well served. An average blocking ratio of 3% and a success rate of 94% of all attempts (note that a failure due to the path loss being too high does not constitute blocking) were reported with the failures split between the uplink and downlink.

In order for a comparison to be made with equal power provided to each site, the simulation was re-run with the power for each of the six-sector cells being halved. This revealed a slight degradation so that 90.8% of all attempts were successful. It was noticed that the majority of failures were due to the limit of power per connection on the downlink being reached. This was set at 10% of maximum total power. Increasing this limit to 16% of total power resulted in an improvement to the success rate to 92.2%. This meant that the six-sector area was serving an average of 1330 terminals compared with 950 in the three-sector situation. This is an increase of 40% but with a much higher grade of service offered to the customer (approximately 10% blocking compared with over 80% blocking).

If the offered traffic in the six-sectored area was increased again to cause saturation of the network, the six-sector area then served an average of 1830 terminals, an increase of more than 90% on the three-sector situation.

As an indication of the levels of mutual interference, the frequency re-use efficiency (the percentage of UE power received by a cell that comes from users of that cell) was approximately 63% for both the 3-sector and the 6-sector networks.

Thus it can be concluded that further sectorisation of sites represents a powerful method of increasing the capacity of a network.

Further Sectorisation of Sites - ComparisonFurther Sectorisation of Sites - ComparisonFurther Sectorisation

• An area of 15 km2 wasplanned to give 64 kbit/s VTservice indoor.

• 33 sites, 99 cells with arange of 500 metres wereneeded.

• Target Eb/No taken to be 3dB on UL

• NR limit set to 4 dB (60%loading).

F u r t h e r S e c t o r i s a t i o n o f S i t e s - C o m p a r i s o nF u r t h e r S e c t o r i s a t i o n o f S i t e s - C o m p a r i s o nF u r t h e r S e c t o r i s a t i o n

• C a p a c i t y e s t i m a t i o n :

• T h i s w o u l d a c c o m m o d a t e1 8 s i m u l t a n e o u sc o n n e c t i o n s .

• E r l a n g B t a b l e ( 2 %b l o c k i n g ) s u g g e s t s t h i ss h o u l d s e r v e 1 1 . 5 E r l a n g s .

• I n t e r f e r e n c e r a t i o o f 0 . 6 w i l lr e d u c e t h i s t o 7 . 2 E r l a n g sp e r c e l l .

kbps 115010

6.038403.0


• 99 cell network shouldsupport 720 Erlangs.

• Simulation confirms this:

• 96% call attempt success.

• Failures distributed betweenUL and DL (both capacity andpath loss failures noted).


• Next; the offered traffic wasdoubled to 1440 Erlangs.

• Network is now saturated.

• 65% call attempt success(950 connections on average).

• 85% blocking.

• DL is particularlyoverloaded (note: higherEb/No required ondownlink).


• Each site then had a furtherthree sectors added.

• 35 degree antennasdeployed throughout.

• Noise Rise limit increasedto 6 dB.

• Result:

• 94% call attempt success.

• 3% blocking.


• Next: an “equal power”comparison.

• DL power halved to eachcell.

• Result:

• 90.8% call attempt success(c.f. 94%).

• Failures tend to be on DL dueto bearer power limit beingreached.


• Next: adjust bearer powerlimit.

• Power per connectionincreased from 10% ofmaximum power to 16% ofmaximum power.

• Result:

• 92.2% call attempt success.

• Failures evenly distributed.

• Average connectionsserved: 1320 (c.f. 950 with3-sectored sites).


• Finally: double offered trafficto saturate network.

• 1830 terminals served onaverage.

• 90% more than for 3-sectored network.

• Conclusion is that furthersectorisation of sites is apotentially powerful methodof increasing networkcapacity.

8.4 Neighbour Planning Additional sectorisation can cause further problems with neighbour planning. The list of neighbours will necessarily become longer considering the following:

All co-sited cells should be declared as neighbours.

There are more cells within a given distance.

However, the fact that the energy radiated from a cell is confined to a narrow beam means that the area over which a cell causes interference will be less. The overall effect of this is that the neighbour list will probably become longer in the 6-sector case than it was for the 3-sector situation but not unbearably so. If a neighbour list of 12 neighbours were seen as typical for a 3-sector area then perhaps 16 neighbours would be typical for the 6-sector area.

Further Sectorisation of Sites - NeighboursFurther Sectorisation of Sites - NeighboursFurther Sectorisation

• Keeping the neighbour(Ncell) list short is seen asgood practice.

• Each cell will have moreneighbours if furthersectorisation is employed.

• Co-sited cells should be madeneighbours.

• There will be more cells withina given range.

• However, energy from eachcell is confined to a narrowbeam and will effect asmaller area.

Further Sectorisation of Sites - NeighboursFurther Sectorisation of Sites - NeighboursFurther Sectorisation

• Result is that the Ncell listwill have to be longer iffurther sectorisation isemployed, but notunbearable so.

• If Ncell list is typically 12 foran area where 3-sector cellsare used, a list of 16neighbours should besufficient where 6-sectorcells are the norm.

9 Using Additional Carriers – Hierarchical Cell Structures

9.1 Spectrum AllocationOperators are not allocated a single carrier. Rather, a block of two or three adjacent carriers is normally allocated. This allows for a flexible approach to be taken to providing additional network capacity. Essentially, this is equivalent to two or more networks operating in parallel. Two issues require addressing:

Hand over between the two carriers is a “hard” hand over. Hard hand overs involve the UE entering compressed mode and putting an extra burden on the network.

There will be interference between the adjacent carriers. In most cases, this is not expected to cause significant problems for a particular network (where rival networks operate at adjacent frequencies, problems can exist. The section on Indoor Solutions goes into detail on this issue).

Using Additional CarriersUsing Additional CarriersMulti-carrier deployment

• Issues to consider:

• Hard hand over between carriers: compressed mode operation.

• Adjacent carrier interference to be considered.

9.2 Deploying extra carriers in the macro cell layerIt is possible to increase the capacity of the macro-cell layer by deploying an additional carrier. This is particularly attractive if an operator has 3 carriers as it leaves the third carrier for use with micro-cells and pico-cells.

9.2.1 A test case

In order to evaluate the likely improvement, the standard 15 km2 area with 33 sites was considered (as in the case where further sectorisation was evaluated – see previous section). A single carrier, 3-sectored network can support 720 Erlangs of VT. The offered traffic was doubled to 1440 Erlangs and an extra carrier was deployed. The simulation assumed that the two frequencies shared an antenna and that the power was shared between the two frequencies. Nevertheless, it was found that the capacity of the network was very nearly doubled as a result of the deployment of an extra carrier. Further, interference between the carriers did not have a noticeable effect on the predicted performance.

To evaluate the capability of a macro-cell layer to accommodate very high traffic densities, the exercise was repeated with each site divided into six, dual-frequency, sectors. Thus, each Node B contained 12 UMTS cells. This was found to support a further doubling of traffic to 2700 Erlangs, a density of almost 200 Erlangs

of video telephony per km2. This is an estimate for a site density of 2.1 sites per km2 (thus each site supports almost 100 Erlangs of video telephony traffic). If this is to be increased further, extra diversity could be used. As the simulations suggest that the network is reasonably balanced (failures were evenly distributed between UL and DL) any enhancement would have to be implemented on the uplink and the downlink. Thus, perhaps, four-component diversity could be implemented on the uplink (instead of the standard, single cross-polar diversity deployed as standard) together with space diversity on the downlink. The downlink would be the limiting factor in determining the likely capacity increase that would result. Estimates vary regarding the likely increase, with 30% being a typical figure.

Higher subscriber densities could be accommodated by increasing the site density. Not only is this an expensive option, it is difficult to implement retrospectively. The original plan was to provide indoor coverage at the cell edge. This determined the site density of the macro-cell layer.

Comparison using the simulatorComparison using the simulatorMulti-carrier deployment

• Initial situation:

• Single carrier

• 33 sites (99 cells) in 15 km2

• 720 Erlangs of VT carried

• 2 carrier situation:

• Two carriers deployed per cell

• Power split between two carriers

• Capacity increased to 1350 Erlangs of VT

Comparison using the simulatorComparison using the simulatorMulti-carrier deployment

• 2 carrier, 6 sector situation:

• Capacity further increased to 2700 Erlangs VT

• Density almost 200 Erlangs/km2 with a site density of 2.1 sites/km2

• Failures distributed evenly between uplink and downlink.

• Any further increase (through diversity etc.) would have to be applied to both uplink and downlink. Downlink diversity improvement expected to be approximately 30% (this would form the limiting factor).

9.3 Fixed network provisioning.If each site can handle 100 Erlangs of VT, the peak loading would be estimated at 114 connections. The aggregate user data rate would be 7.3 Mbit/s. With an overhead for signalling and control, this would increase to approximately 9 Mbit/s. Provisioning of this through E1 links would be a significant consideration.

This is four the case where each Node B controls 12 cells. The requirement would be less if the “6-cell” or “3-cell” option was chosen.

Number of cells per site

Erlangs per site

Peak connections per site

Aggregate user bit rate per site

Estimated requirement

3 25 34 2176 kbit/s

2700 kbit/s

6 50 61 3904 kbit/s

4900 kbit/s

12 100 114 7296 9000 kbit/s

kbit/s

The basic “building block” of fixed network transmission is the “E1 link” that carries 2048 kbit/s. It is tempting to provision a 3-cell site with a single E1 link, at least initially. If this option is implemented, it is the fixed network that is likely to become the first factor limiting network capacity. Fixed network capacity must be addressed before the air interface capacity is enhanced.

Fixed network implicationsFixed network implications

• Capacity of 100E per site would necessitate an upgrade to the fixed network.

Multi-carrier deployment

Number of cells per site

Erlangs per site

Peak connections per site

Aggregate user bit rate per site

Estimated requirement

3 25 34 2176 kbit/ s 2700 kbit/ s

6 50 61 3904 kbit/ s 4900 kbit/ s

12 100 114 7296 kbit/ s 9000 kbit/ s

9.4 Carrier loading strategy.When multiple carriers are used on a network, it is possible to decide on a priority of loading. When the two carriers operate in parallel, as in the case above where the extra carrier is used to add further capacity to the macro cell layer, it is best to load them equally. If one carrier is more heavily loaded than the other, the Ec/No on that carrier will be worse than it is on the lightly-used carrier. This will lead to hard hand over attempts being made between the two carriers. Avoiding these is a good thing. Additionally, the power levels required would be higher on the carrier that had higher levels of activity. Generally, keeping power levels to a minimum is seen as good practice in a UMTS network because, for example, it leads to the maximum coverage for higher

data rate services. Carrier loading strategies will be re-visited later in this section following consideration of hierarchical cell structures.

CarrierCarrier--loading Strategyloading Strategy

• If two carriers are applied to the macro-cell layer, it is beneficial to load them equally.

• If one carrier is more heavily loaded, Ec/Io will be worse leading to hard hand over occurring.

• Required bearer powers will be higher in more heavily loaded carrier – minimising power is a generally desirable aim.


9.5 Hierarchical cell structures. When offered traffic grows beyond approximately 100 Erlangs per km2, it is usually concentrated at “hotspots” rather than evenly distributed across the coverage area. One obvious case is where offices generate a lot of traffic. Indoor solutions usually alleviate any problem associated with this. Other areas that generate very high subscriber densities include pedestrianised shopping areas and open-air sports venues, such as racetracks.

The isolation provided by the walls of the building itself makes it possible to implement an indoor solution using the same frequency as the macro-cell layer. With an outdoor micro-cell, there is not so much isolation. This will limit the effectiveness of a micro-cell solution, as shown in section 7. If a separate carrier is used for the micro-cell, this will provide isolation (in the frequency domain, rather than physical isolation). That, in turn, should lead to greater throughput being possible.

Hierarchical Cell StructuresHierarchical Cell Structures

• The subscriber density that can be accommodated by a macro-cell layer will have a limit (100 Erlangs/km2).

• Higher densities is usually due to “hotspots” that can be treated separately:

• Office buildings: in-building solution required

• Football stadiums, railway stations etc.: micro-cell can be deployed.


Hierarchical Cell StructuresHierarchical Cell Structures• The effectiveness with

which a micro-cell can be deployed sharing the same frequency and the macro-cell layer depends upon the ability to isolate the micro-cell from the macro-cell.

• This is easier if the micro-cell is at a considerable distance from the macro-cell.

• The use of a separate carrier for the micro-cell must be considered.


•Possible to serve hotspot with micro-cell that re-uses macro-cell frequency

•Difficult to serve hotspot with micro-cell that re-uses macro-cell frequency

Hierarchical Cell StructuresHierarchical Cell StructuresMulti-carrier deployment

• The deployment of micro-cell can be extended to provide a second layer.

• A separate carrier is essential.

• Capacity of micro-cell layer can be double that of the macro-cell layer.

9.5.1 Capacity of micro-cells using separate carriers

The major factor that limits the capacity of any UMTS cell is intra-frequency interference. This is particularly the case in the downlink direction as it counteracts the beneficial effects of orthogonality. If micro-cells are deployed to service hotspots, and are allocated a separate carrier frequency, it is possible to effectively isolate them from such interference. Further, if any multipath is over a short extra path distance then the orthogonality of a micro-cell should be much better than that of macro-cells. Values of orthogonality around 0.9 are expected to be typical for a micro-cell whereas 0.6 is typical for a macro-cell. This makes the air interface capacity several megabits a second in the downlink direction.

Capacity of microCapacity of micro--cellscells

• Micro-cells can enjoy lower inter-cell interference (relying on macro-cell layer to provide continuous coverage where necessary).

• Micro-cells at below building height will be more isolated from each other.

• Micro-cells enjoy higher levels of orthogonality.

• Orthogonality is reduced by multipath with path length difference of more than approximately 50 metres.

• Both these factors increase capacity (particularly in the downlink direction).


iNEb

1

3840Throughput

0

9.5.2 Pilot and common channel powers in micro-cells

Pilot and common channel default settings are decided upon as appropriate when cells are experiencing significant levels of external interference, such as in a macro-cell layer. When the levels of interference are lower, the powers allocated to pilot and common channels can be reduced. This in turn can further increase the downlink capacity.

Common and Pilot Powers in MicroCommon and Pilot Powers in Micro--cellscells

• Pilot powers of approximately 10% of maximum power ensure that the pilot can be used for synchronisation in places of maximum interference considering interference levels and orthogonality factors present in a macro-cell layer.

• If levels of interference are lower and orthogonality is higher, the pilot and common channel powers can be reduced, perhaps to 5% of maximum power.


9.5.3 Link budgets for micro-cells

Orthogonality is improved in micro-cell environments. This is because the path length differences are usually very small. However, the result of reflections with small path length differences is to increase the probability of flat fading. In certain circumstances this will lead to such rapid changes in field strength that user mobility will lead to higher target Eb/No values. Increases in target Eb/No of perhaps 2 dB can be expected but this will be environmentally- and mobility-dependent.

Link budget for microLink budget for micro--cellscells

• Due to high levels of multi-path with small path length differences, there will be considerable flat fading.

• A flat fading margin may be required.

• 2 dB is suggested as an appropriate value.

• This will be environmentally and mobility dependent.


9.5.4 Multi-layer strategies for dense urban environments

The initial roll-out of a macro-cell layer will be dominated by coverage concerns. In order to provide VT coverage indoors, this will lead to a site density of approximately two sites per square kilometre. Although some buildings will be serviced by indoor pico-cells, it is expected that this site density will not provide sufficient capacity for dense urban areas. A general procedure to increase capacity would be:

Deploy micro-cells using the same carrier as the macro-cell layer where possible to accommodate hotspots.

Implement inter-carrier hand over and move micro-cells to separate carrier.

Deploy further micro-cells until a micro-cell layer has been established in the dense urban area.

Identify micro-cells that could beneficially re-use the macro-cell carrier.

The micro-cells that can re-use the macro-cell carrier beneficially are those that enjoy geographic isolation from the macro-cell sites as was the case in deploying micro-cells in a single carrier network. However, it is important to note that the conceptually simple process of deploying a micro-cell layer at a separate carrier frequency results in a great capacity jump as the capacity of the micro-cell layer in both uplink and downlink directions is significantly greater than that for the macro-cell layer. Individual cells will have a greater capacity (due to higher loading factor on the uplink and higher orthogonality and lower interference on the

downlink) and the cell density will be typically double that of the macro-cell layer. Thus, implementing a separate micro-cell layer will lead to increasing the capacity of the network by a factor of approximately 3. Additional features such as the optimal re-use of the macro-cell carrier frequency can lead to a further increase of perhaps 15%. It is important that the macro-cell carrier is not simply re-used at all micro-cells. The resulting mutual interference can lead to a reduction in network capacity if it is not undertaken carefully.


• The subscriber density that can be accommodated by a macro-cell layer will have a limit (100 Erlangs/km2).

• Higher densities is usually due to “hotspots” that can be treated separately:

• Office buildings: in-building solution required

• Football stadiums, railway stations etc.: micro-cell can be deployed.



• Strategy• Deploy micro-cells using the same carrier as the

macro-cell layer where possible to accommodate hotspots.

• Implement inter-carrier hand over and move micro-cells to separate carrier.

• Deploy further micro-cells until a micro-cell layer has been established in the dense urban area.

• Identify micro-cells that could beneficially re-use the macro-cell carrier.



• Micro-cells re-using the macro-cell frequency.

• Micro-cells that are suitably physically isolated from the macro-cell layer can beneficially re-use the macro-cell frequency.

• This should be done only if the micro-cell using its own frequency becomes overloaded.

• Deploying an unnecessary extra frequency will – Reduce power available to highly-used carrier

– Introduce extra downlink interference due to pilot and common channels.


Capacity IncreasesCapacity Increases

• Macro-cell layer: 100 Erlangs VT per km2 (single carrier).

• Macro-cell layer plus continuous micro-cell layer at separate frequency: 300 Erlangs VT per km2.

• Macro-cell layer, micro-cell layer at separate frequency plus re-use of macro-cell frequency at selected cells: 350 Erlangs VT per km2.

• In-building solutions will provide additional capacity.


9.5.5 Hand over between carriers

The process of hand over between carriers is similar in principle to that of hand over to GSM except that:

It is a “multi-service”; for example VT calls can hand over.

It is “two-way”; an active call can hand over in both directions.

The main indicator is pilot quality, with Ec/Io levels triggering, firstly, compressed mode operation (Event 2d) and then (if the other carrier pilot is of acceptable quality) hand over (Event 3a).

InterInter--frequency hand overfrequency hand over

• Hand over is “hard”.

• Based on Ec/Io comparison

• Event 2d: enter compressed mode.

• Event 3a: attempt hard hand over.

• Can be “two-way” on an active call.


10Implementing Diversity Systems

10.1 IntroductionDiversity is a well-established method of improving the quality of a communication channel. It traditionally means employing more than one receive antenna and then combining the signal (sometimes merely selecting the one with the larger amplitude) so that the outcome is superior to that which would be obtained without diversity. Combining has usually taken place at RF. In UMTS networks receive diversity actually employs multiple receivers allowing the signals to be combined at base band. This gives an improvement in the value of Eb/N0 which, in turn gives an improvement in both coverage and capacity.

Another innovative feature of UMTS networks is the ability to utilise transmit diversity. This is not so effective as receive diversity but, nevertheless, can provide Eb/N0 improvements of greater than 1 dB (compared to 4 dB improvements possible for uplink diversity).

UMTS Advanced Cell Planning and Optimisation 133AIRCOM International Ltd 2003

10.2 Definition of Fading

FadingFading

• Electromagnetic signals will interact, causing addition and subtraction of their field strengths

• Fast fading signal strength changes are due to relative motion and local scattering objects such as buildings, foliage, etc. and change rapidly over short distances.

• Typically Multipath interference results from fast fading

• Fading of the signal follows a Rayleigh distribution

• Slow fading is the change in the local mean signal strength as larger distances are covered.

• Fading of the signal is a log-normal distribution

• The resultant signal at the Node B and UE antenna will be subject to rapid and deep fading

Diversity

DiversityDiversity

• Signals from multiple antennas (spatial diversity), can be used to reduce the effects of fast fading and improve received signal strength.

• Three common combining schemes used for Rayleigh fading channels (Fast fading) are

• Selection diversity

• chooses the strongest signal power,

• Equal gain

• combines the co-phased signal voltages with equal weights,

• Maximal ratio combining

• weights the co-phased signal voltages relative to their signal to noise ratio.

Diversity

10.3 Receive Diversity

Receive DiversityReceive Diversity

• Basic idea is that, if two or more independent samples of a signal are taken, these samples fade in an uncorrelated manner.

• Each path can then be thought of as separate and worked on in isolation

• Increases the signal to interference ratio, SIR

• Allows a system to reduce the target uplink Eb/No of a channel

• Saves UE & Node B Power

• Standard configuration for WCDMA may be two-branch Rx diversity

• Using a single cross polar antenna or two vertically polarised ones.

• Separation of the vertically polarised antenna is typically a few wavelengths

Diversity

cmtoseparation

cmf

cfc

40 30

15102

1039

8

Uplink Receive Space DiversityUplink Receive Space Diversity

• Even if signal is highly correlated, coherent combination should yield about 3 dB improvement.

• In practice a gain of 4 dB or more is expected from antennas

• Typical dimension 1.5m

Receive antenna 1

Receive antenna 2

Diversity


Uplink Receive Space DiversityUplink Receive Space Diversity

• This is not “conventional” space diversity.

• Each antenna is connected to a separate finger of the Rake

receiver.

• This is possible due to the synchronisation and channel estimation

derived from the Pilot bits on the DPCCH channel.

• Eb/No is improved, rather than simply an effective power gain.

Diversity

10.4 Transmit Diversity

Downlink Transmit DiversityDownlink Transmit Diversity

• UMTS explicitly allows the use of transmit diversity from the base station

• However it is not possible to simply transmit simultaneously from two close antennas as this would cause an interference pattern

• Mobile terminals must have the capability of implementing downlink transmit diversity .

Transmit antenna 1

Transmit antenna 2

Diversity


• UMTS FDD mode does not allow for an accurate measure of the downlink channel using uplink estimations

• The UE can measure the downlink channel and return estimates to the Node B – closed loop

• The alternative is coding the downlink to allow for the UE to correlate the two signals – open loop

• The P-CPICH is transmitted from each antenna differently

• Orthogonal signals

• Antenna 1 { 0,0,0,0,0,0,0,0,0, …… } normal operation

• Antenna 2 { 0,0,1,1,1,1,0,0,0,0,1,1,1,1,0,0,0,0,1,1,

Diversity



Transmit Diversity Method

Description

Open Loop TSTD Time Switched Transmit antenna Diversity for SCH only

Open Loop STTD Space Time block coding Transmit antenna Diversity

Closed Loop Mode 1 Different Orthogonal Pilots CPICH + S-CPICH

Closed Loop Mode 2 Same Pilot

• The following methods are suggested in the UMTS standards to avoid the problem of the interference

Diversity

Time Switched Transmit Diversity (TSTD) for SCHTime Switched Transmit Diversity (TSTD) for SCH

• Even numbered slots transmitted on Antenna 1, odd numbered slots on Antenna 2

Antenna 1

Antenna 2

P-SCH

Slot #0 Slot #1 Slot #14Slot #2

P-SCH

P-SCH P-SCH

S-SCH

S-SCH

S-SCH S-SCH

Diversity

Space Time Transmit Diversity (STTD)Space Time Transmit Diversity (STTD)

• STTD encoding is optional in UTRAN. STTD

support is mandatory at the UE

• Channel coding, rate matching and

interleaving is done as in the non-diversity

mode.

• STTD encoding is applied on blocks of 4

consecutive channel bits

• h is the impulse channel response of each

antenna

Diversity

*211

*22

*221

*11

22*11

*22

122111

ˆˆˆ

ˆˆˆ

)(

)(

rhrhS

rhrhS

nhShSrTtr

nhShSrtr

Analysis of STTDAnalysis of STTD

• STTD encoding effectively spreads a data bit across more than one bit period.

• This leads to a general improvement in noise performance.

• Further, it allows a stronger signal from one antenna to dominate.

b0 b1 b2 b3

-b2 b3 b0 -b1

b0-b2 b1+b3 b0+b2 b3-b1

Processing alternate bits will extract the data

Diversity

Antenna 1

Antenna 2

Combination


Analysis of STTDAnalysis of STTD

• The Space-time combining generates symbols that are proportional to the sum of the powers from both antennas

Diversity

Closed Loop ModeClosed Loop Mode

• Channel coding, interleaving and spreading are done as in non-diversity mode

• The spread complex valued signal is fed to both TX antenna branches, and weighted with antenna specific weight factors w1 and w2.

• The weight factors are determined by the UE, and signalled using the FBI field of uplink DPCCH (Dedicated Physical Control Channel).

1 radio frame: T = 10 ms

PilotNpilotbits

TPCNTPC bits

Slot #0 Slot #1 Slot #i Slot #14

Tslot = 2560 chips, 10 bits

f

DPCCHFBI

NFBI bitsTFCI

NTFCI bits

Diversity

Closed Loop ModeClosed Loop Mode

Spread/scramble

w1

w2

DPCHDPCCH

DPDCH

Rx

Rx

CPICH1

Tx

CPICH2

Ant1

Ant2

Tx

Weight Generation

w1 w2

Determine FBI messagefrom Uplink DPCCH

Diversity

Closed Loop ModeClosed Loop ModeDiversity

• Closed Loop mode 1

• The phase of one antenna is adjusted relative to the other

• Using 1 bit accuracy per slot

• Feedback rate is 1500 Hz

• Closed Loop mode 2

• Relative phase adjusted using 3 bit accuracy

• Amplitude adjusted using 1 bit

• Feedback rate is 1500 Hz


Downlink Downlink EbEb/No reduction/No reductionDiversity

3.5 dB0.0 dB1.0 dB1.5 dBClosed Loop 1

3.0 dB0.5 dB0.5 dB1.0 dBOpen Loop

Pedestrian A …… 3km/h

…………………..120 km/h

Vehicular A …..50km/h

Modified 3km/h…….

Diversity Mode

Source Radio Network Planning and Optimisation for UMTS, Jaana Laiho et al

• Slower speeds and lower multipath interference produce the best results

Transmit Diversity Transmit Diversity -- ConclusionsConclusionsDiversity

• Depends on UE performance

• Estimate of channel impulse and SIR

• Main benefit is reduction in downlink Eb/No

• No advantage in problematic time and multipath environments

• 50km/h -- Eb/No only 0.5dB better in open-loop mode

• 120km/h -- Eb/No no real improvement

• Microcell’s will benefit from TxDiversity

• Beam forming problems associated with location

10.5 Multi-User Detection MUD

One major advantage that the downlink has in a UMTS network is the use of orthogonal codes to reduce the interference effect of other traffic and control channels. This relies on the fact that the downlink channels can be easily synchronised as they originate from the same point. The same sort of cancellation is not possible on the uplink as the transmission delay is different for each user. MUD helps to provide some interference cancellation by performing an inverse transform on the message contained in interfering channels and then removing that from the input of the wanted signal. It is a highly sophisticated method and its potential is yet to be fully realised. However, a 1 dB improvement in uplink performance can be recorded (which can lead to useful coverage and capacity increases). Note that MUD is only effective at a serving cell, the interference effect on neighbouring cells is not reduced.

MultiMulti--User DetectionUser Detection

• Multi-User detection (MUD) is a method used to improve the performance of the receiver by reducing the noise contributions from other CDMA users.

• The concept is based on the fact that noise from CDMA users, although usually approximated with AWGN characteristics, inherently consists of coherent signals.

• MUD reception decodes a number of users simultaneously and subtracts their noise contributions from the others

• Essentially this results in a more sensitive receiver

Diversity



• Mid 1980s research showed that joint, optimal, maximum-likelihood decoding of all users out performed matched filter alternatives.

• The problem was the exponential increase in processing as the number of simultaneous users went up. ( Viterbi trellis techniques )

• Current research interests

• Suboptimal linear receivers

• Data-aided minimum mean squared (MMSE) linear receivers

• Blind ( nondata-aided ) MMSE receiver

• Non-linear multiuser detection

• Multistage interference cancellation, parallel and serial, PIC & SIC

Diversity


• Viterbi decoding uses past symbol knowledge to weight present and future choices

• Multiuser decoding has the added complexity of having present ‘other user’ interfering symbols

• Therefore some decision as to the interfering symbols must be made

• Due to the complexity, multiuser detection is more likely to exist in the Node B

Diversity


• Multiuser detection reduces the need for tight power control

• Power control is still important to the performance of the MUD system

• Best performance used with short spreading codes, repeating every symbol. ( Downlink )

• Can be used with long spreading codes, pseudorandom sequences which are much longer than the symbol duration. (Uplink)

Diversity

VisualisingVisualising the Processing Gain w/o MUDthe Processing Gain w/o MUD

W/Hz W/Hz W/HzEc

Io

Signal

Intra-cell Noise

Inter-cell Noise

Before Spreading

After Spreading With Noise

f f f

W/HzAfter Despreading/Correlation

f

W/Hz Eb

No

Post Filtering(No MUD)

f

dBW/Hz

Eb

No

Eb/No

f

Diversity


VisualisingVisualising the Processing Gain with MUDthe Processing Gain with MUD

W/Hz

Signal

Inter-cell Noise

After Despreading/Correlation

Post Filtering

f

Other Users

Eb

No

W/Hz

f

Eb

No

W/Hz

f

Eb

No

W/Hz

f

Eb

No

W/Hz

f

Because of MUD the contribution of the other users to the Noise is Reduced.

It is not completely eliminated because of the inaccuracies of the Multiple access interference estimation.

Diversity

10.6 Predicting the Effect of Different Coverage and Capacity Enhancement Devices

It is clear that adding certain devices, such as mast head amplifiers or diversity receivers will improve network performance. However, we need to be able to quantify any likely improvement in order to undertake a cost-benefit analysis. As a starting point we shall consider an isolated cell that is serving voice users delivering a bit rate of 12200 bps at an Eb/N0 of 4 dB on the uplink and the downlink. With an uplink Noise Rise of 3 dB the cell can accommodate a link loss of 133 dB.

This information alone is sufficient to suggest that the pole capacity is 1530 kbps on the uplink and 3822 kbps on the downlink (assuming an orthogonality value of 0.6). An uplink Noise Rise of 3 dB would suggest that 63 voice users are seen as a full load for the cell. The loading factor on the downlink would be estimated to be only 20% suggesting a Noise Rise figure of 1 dB. If 36 dBm of common channel and pilot power is transmitted, the effect at the mobile receiver would be that of a -94 dBm interference power if the mobiles are at a path loss of 126 dB. If the noise floor of the receiver is -101 dBm then the overall “noise plus interference” level would be -93.2 dBm. If a Noise Rise of 1 dB must be produced, then an effective traffic channel power of -99.2 dBm (actual receive

power -95.2 dBm) must be received. This would necessitate a transmit power of 30.8 dBm if all users were at a path loss 7 dB less than the cell edge (which is defined by a link loss of 133 dB).

Quick check downlink analysis. 30.8 dBm corresponds to 12.8 dBm per user (if there are 63 users). Received power per user is -113.2 dBm. Effective Noise Power is -92.2 dBm (given a NR of 1 dB). Thus wideband SNR is -21.0 dB. Processing gain of 25 dB will restore the required Eb/N0 value of 4 dB.

Having carried out and understood the mechanism of this calculation it is possible to predict the effect of capacity enhancement devices such as uplink diversity. When considering whether or not to use such devices it is important that their purpose is made clear. For example, is maximising capacity or maximising coverage range our goal (or is it a combination of the two aims)? Additionally, the affect on the downlink must be assessed.

Consider, as an example, the effect of implementing uplink diversity on this cell. The effect is to reduce the target Eb/N0 value by 3 dB. If maximising capacity (whilst keeping the coverage range fixed) is taken to be our goal then it is possible to increase the NR limit by 3 dB to 6 dB and then note that the pole capacity on the uplink has doubled to 3060 kbps. The loading factor of 75% means that a throughput of 2290 kbps is possible, equivalent to 188 voice users. This represents a dramatic increase on the previous value of 62 users. However, there has been no help offered on the downlink. The pole capacity in this direction remains unchanged at 3822 kbps. Thus a loading factor of 60% will be imposed causing a Noise Rise of 4 dB. The effective Traffic Channel Power required to cause this Noise Rise will be -91.5 dBm, an actual received power of -87.5 dBm. The total traffic channel transmit power would have to be 38.5 dBm (15.8 dBm per user). This is a significant increase over the previous value of 30.8 dBm. Notice that the amount of power required by each user has increased significantly.

Alternatively, if may be that uplink diversity has been introduced with the goal of increasing the range of the cell keeping its capacity constant. If that is the case the new pole capacity of 3060 kbps can be used to calculate a reduced loading factor of 25%, which represents a noise rise of 1.2 dB. Thus the cell coverage range can be increased by 4.8 dB. Thus a typical user can be thought of as having a path loss of 131.8 dB to the cell. The result of this is that the interference effect of the pilot and common channels is reduced. However, the fact that users are at a greater distance


means that the power requirements will be greater, although not 4 dB greater. Calculations show that the Traffic Channel power requirement will rise from the initial value of 30.8 dBm to 32.0 dBm.

It is possible to use similar techniques to predict the effect of using mast head amplifiers and of implementing downlink diversity.

Predicting the EffectsPredicting the EffectsDiversity

• It is important to be able to predict the coverage and capacity effects of introducing a feature such as uplink diversity into a cell.

• As a starting point we will consider an isolated cell that is serving voice users delivering a bitrate of 12200 bps in both directions at an Eb/No of 4 dB.

• We shall assume that the orthogonality factor is 0.6.

• Maximum link loss is taken to be 133 dB with the “average user” on the downlink having a link loss of 126 dB.

• Common Channel and Pilot Power taken to be 33 dBm each (total 36 dBm).

• Mobile noise floor is -101 dBm.

Downlink CalculationsDownlink CalculationsDiversity

• Noise Floor of Mobile is -101 dBm

• Common and Pilot Channels received at a level of 36 – 126 = -90 dBm.

• Orthogonality reduces this by 4 dB (10log[1-0.6]=-4). Thus equivalent is -94 dBm.

• -94 dBm + (-101 dBm) = -93.2 dBm

• The pole capacity of the DL has been calculated as 3822 kbps. Throughput of 785 kbps would be a loading factor of 20% and a NR of 1 dB.

• Traffic channel power has to produce this Noise Rise.

Noise plus interference = -93.2 dBm

Downlink CalculationsDownlink CalculationsDiversity

• Noise plus interference plus traffic channel power must be -92.2 dBm.

• Effective traffic channel power must be -92.2 dBm – (-93.2 dBm)=-99.1 dBm.

• But traffic channel power will benefit from orthogonality. Actual received traffic channel power must be -95.1 dBm.

• Transmitted traffic channel power must total -95.1+126=30.9 dBm

• Confidence check: 63 users: 12.8 dBm per user: Rx power per user is -113.2 dBm. Noise plus interference = -92.2 dBm. SNR = -21 dB. Processing Gain = 25 dB. Eb/No = 4 dB as required.

Noise plus interference plus traffic channel power = -92.2 dBm

Actual received traffic channel power = -95.1 dBm

Required transmit traffic channel power = 30.9 dBm.

Introducing UL DiversityIntroducing UL DiversityDiversity

• Now we will introduce UL diversity and prioritise capacity, keeping the range the same.

• UL Eb/No improvement assumed to be 3 dB.

• Pole capacity on UL is now 3060 kbps; on DL it remains at 3822 kbps.

• NR limit can be increased on UL from 3 dB to 6 dB. Throughput on UL increased to 2290 kbps (188 voice users).

• Loading factor on DL is now 60%: a NR of 4 dB.

• Effective Traffic Channel power is now required to be -89.2 dBm –(-93.2 dBm)=-91.5 dBm.

• Actual Traffic Channel Power Received = -87.5 dBm.


Capacity on UL is trebled.

• Required Traffic Channel transmit power = 38.5 dBm (15.8 dBm per user)

Required TCH power = 38.5 dBm.



• Now we will introduce UL diversity and prioritise range increase, keeping the capacity the same.

• UL Eb/No improvement assumed to be 3 dB.

• Pole capacity on UL is now 3060 kbps; on DL it remains at 3822 kbps.

• UL loading factor is now 25%

• NR limit can be reduced on UL from 3 dB to 1.2 dB.

• Path loss can be increased by 4.8 dB so typical user now has link loss of 130.8 dB.

• DL interference from pilot and common channel = -98.7 dBm

• Adding thermal noise gives -98.7 dBm + (-101 dBm) =-96.7 dBm

UL path loss increased by 4.8 dB.


• To give 1 dB NR on downlink, the Effective TCH power must be -95.7 dBm –(-96.7 dBm) = -102.7 dBm.

• Actual Received TCH power must be -98.7 dBm.

• Required Transmit TCH power must be 32 dBm.

• Note: this has risen from 30.9 dBm. The 1.1 dB rise in power is less than the 4.8 dB rise in path loss due to the fact that the majority of “noise plus interference” at the mobile is pilot and common channel power from the cell.

• One conclusion is that it is the loading that most influences requirements on the downlink power level.

UL path loss increased by 4.8 dB.



Introducing MHAIntroducing MHADiversity

UL NR increased by 2 dB.

Capacity increased by 37%


• Now we will now consider the effect of introducing a MHA and prioritisingcapacity, keeping the range the same.

• The Noise Performance improvement is assumed to be 2 dB.

• Pole capacity on UL remains unchanged at 1530 kbps.

• NR limit can be increased on UL from 3 dB to 5 dB. Throughput on UL increased to 1045 kbps (86 voice users).

• Loading factor on DL is now 27%: a NR of 1.4 dB.


• Actual Traffic Channel Power Received = -93.4 dBm.

• Required Traffic Channel transmit power = 32.6 dBm (13.3 dBm per user)

Introducing MHA Introducing MHA –– prioritiseprioritise coveragecoverageDiversity

Max PL increased by 2 dB

Capacity stays the same


• Now we will now consider the effect of introducing a MHA and prioritisingcoverage, keeping the capacity the same.

• The Noise Performance improvement is assumed to be 2 dB.

• Pole capacity on UL remains unchanged at 1530 kbps.

• NR limit is unchanged: maximum link loss now increased by 2 dB to 135 dB.

• Loading factor on DL is unchanged.


• Actual Traffic Channel Power Received = -96.7 dBm. • Required Traffic Channel transmit

power = 31.3 dBm (13.3 dBm per user)


10.7 Multiple-beam antennasThe fact that the target Eb/No is likely to be higher on the downlink than on the uplink (typical values: voice, 5 dB UL, 7 dB DL; 64 kbps cs data, 3 dB UL, 7 dB DL) together with the fact that most common enhancement devices, such as MHAs and diversity (and soft hand over gain), benefit the UL more than the DL suggests that the downlink will become the limiting factor in a UMTS network, particularly as internet services are likely to generate more offered traffic in the downlink direction than in the uplink. A more sophisticated antenna that produces multiple beams is seen as a powerful aid of the downlink.

MultiMulti--beam antennasbeam antennasDiversity

• The downlink direction is likely to be the limiting factor in air-interface capacity considerations.

• This is largely due to the higher Eb/No requirement in this direction owing to the lack of diversity receiver at the UE.

• Voice: UL Eb/No 5 dB; DL 7 dB

• VT: UL Eb/No 3 dB; DL 7 dB

• Multi-beam antennas offer the most powerful method of increasing capacity in the downlink direction.

• They are also beneficial in the uplink direction.


• A typical antenna for use in a 3-sector site will have a beamwidth of approximately 70 degrees.

• A multi-beam antenna will typically have 4 beams with 20 degrees.

• In its simplest form it can be thought of as consisting of four separate antennas, each with a narrow beam (this is unlikely to be the actual configuration).

• The power for each user can be directed to the best antenna.

10.7.1 Beam forming principles

Consider two antenna elements placed side by side and fed from a common source. Suppose that the elements are fed in phase. The electric field contributions from each element along a line perpendicular to a line joining the antennas will add in phase to produce a maximum. However, if we move away from the line the distance to one element is different from the distance to the other and the electric fields will not add in phase. At a point where this difference is half a wavelength (λ/2) the two contributions will be in antiphase and will cancel each other, producing a null.

At the first null, . So, if , the first null occurs where

sinθ = 0.1, an angle of 5.7 degrees. Thus the beam width between the two nulls on either side of the main lobe would be 11.4 ˚.

Where the path length difference was λ the two signals would add in phase again, producing a peak. There would be nulls at dsinθ = 0.5 λ, 1.5λ, 2.5λ etc. And peaks where dsinθ=λ, 2λ, 3λ etc..

If each antenna has a directional pattern itself, then the radiation pattern of the antenna is modified by the “array factor”.

If instead of feeding the two elements in phase, a delay is placed in one of the feeders, this has the effect of moving the main beam. The peak direction will be where the delay in the feeder is compensated for by an short distance from the antenna. For example, if the delay represents a phase-shift of 90˚, the peak will


occur where the path length difference is λ/4. By adjusting the phase difference it is possible to steer the beam.

Antenna arrays commonly consist of more than two elements. This will lead to a narrower main beam for a given spacing. Further, the more antennas, the higher the gain of the array. This leads to the possibility of reducing the gain of the individual elements. This would usually be achieved by increasing the vertical beam width, thus the antennas vertical height could be substantially reduced (in the case of a 4 element array, it could be reduced to a quarter of its height). One antenna array that shows promise for use in UMTS cells is a four-element array (or four separate four-element arrays). The typical arrangement is of an element spacing of approximately 0.6λ. There would be four different phase differences between successive elements: +135˚, +45˚,-45˚,-135˚. These form the “Butler matrix” such that each of the beams is orthogonal to the other three. That is, where one has a peak, the other three have a null. This leads to minimising interference between the beams. This is the “four fixed-beam” arrangement that is adopted in some UMTS networks.


• A typical antenna for use in a 3-sector site will have a beamwidth of approximately 70 degrees.

• A multi-beam antenna will typically have 4 beams with 20 degrees.

• In its simplest form it can be thought of as consisting of four separate antennas, each with a narrow beam (this is unlikely to be the actual configuration).

• The power for each user can be directed to the best antenna.


• The multi-beam antenna will act in a “smart” manner with the UE controlling the power weighting for its traffic channel.

• In order for the UE to identify the appropriate weighting, a different secondary scrambling code is added to each beam.

• The result is effectively a unique, optimised beam for each user.

• On the uplink, the Rake receiver automatically adjusts the weightings to their optimum value.

Pilot1

Pilot2

Pilot3

Pilot4

BeamBeam--forming Principlesforming PrinciplesDiversity

• In practice, we do not use four separate 20 degree antennas (they would be physically quite large).

• Instead a single unit comprising a four-element array of 70 degree antennas is used.

• To understand beam-forming principles, it is best to start with two elements.

• If the two elements shown are fed in phase, there will be a null wherever

?

d

....2

7,

2

5,

2

3,

2sin

d



• If instead of feeding the elements in phase, we introduce a delay in one of the feeders, the direction of the main beam will not be perpendicular to the line joining the antennas.

• The beam can be steered by adjusting the delay.

?


• A four-element array will produce a narrower main beam for a given element spacing and also more nulls.

• A particular group of four element arrays can form “orthogonal beams”.

• A set of orthogonal beams have nulls where the others have peaks.

• The set form a “Butler matrix”.

• Offsets in the feeders correspond to phase shifts of ±45° and ±135°.

Four-element array

-30

-20

-10

0

-80 -60 -40 -20 0 20 40 60 80

Angle (Degrees)

Rel

ativ

e G

ain

(d

B)

The fourThe four--element arrayelement arrayDiversity

• A multi-beam antenna can be produced from a single array with four different feed arrangements. This is much more compact than having four separate narrow-beam antennas.

Radiation Patterns for different phase shifts

-35

-30

-25

-20

-15

-10

-5

0

-80 -60 -40 -20 0 20 40 60 80

Angle (degrees)

Re

lati

ve

Ga

in (

dB

)

45

-45

-135

135

10.7.2 Implementation in a UMTS network

Its implementation is deceptively simple yet effective. On the uplink the combination of the signal from each UE is achieved by a Rake receiver with four fingers (eight if cross-polar diversity is used). In this way, the optimum weighting is applied to each of the elements. The situation on the downlink is not as straightforward as the diversity is at the transmitting end. The UE has to identify the relative strength of the signals from the individual elements. This is achieved by allocating a different secondary scrambling code (each cell has 1 primary scrambling codes and 15 secondary codes available) to each of the elements in order that the UE can differentiate between them. This information is relayed back to the cell on the uplink control channel and the cell can then adjust the weightings of the transmit power so that the most power is delivered to the element that provides the strongest signal.

10.7.3 Improvement from use of multiple-beam antennas.

The improvement in air-interface performance is environment-dependent but it is the most significant method of improving down link performance. As the down link is likely to become the major factor limiting network capacity, it represents a very powerful solution. On the uplink, the gain provided if cross polar diversity is included (requiring eight RAKE fingers in the receiver) is typically 5 dB using a single 2-branch receiver (single antenna with cross-polar


diversity) as a reference. The downlink gain is between 4 and 5 dB, depending on the characteristics of the propagation path. When compared with the 1 to 1.5 dB gain achieved by two-antenna downlink diversity, it is clear that beam-forming represents a hugely significant capacity enhancement technique.

The typical horizontal spacing of 10 cm and the reduction in vertical height possible without reducing coverage means that the four elements can be housed in a single radome and form a physically compact module. However, it must be remembered that the vertical beamwidth of the shorter antenna will be greater than for an antenna of standard height.

Estimates of ImprovementEstimates of ImprovementDiversity

• A multi-beam antenna helps to isolate individual UEs from interference from other users.

• This increases capacity.

• On the downlink an improvement of between 4 dB and 5 dB is expected. This suggests an increase in the air-interface capacity of between 160% and 220%.

• On the uplink an improvement of 5 dB compared with a cross-polar diversity receiver can be expected, providing an increase of 220% in the air-interface capacity.

• Multi-beam antennas provide by far the most effective method of increasing the capacity of a cell, particularly in the downlink direction.

Physical ImplementationPhysical ImplementationDiversity

• The vertical separation of the antennas is 10 cm at frequencies of 2 GHz.

• The four-element array should be contained in a unit approximately 50 cm in width.

• The height of the unit will depend on requirements for the vertical beamwidth.

• Note the UEs are required by the specifications to be capable of implementing the techniques described.

• They monitor the different secondary scrambling codes and feed back information on the “FBI” bits on the uplink control channel.

10.8 Smart (beam-forming) antennasThe antenna described in the previous section is of the “fixed-beam” type. The fact that antenna weighting can be varied for each user does lead to the beam effectively being steered for a user. However, the phase relationship between the elements is fixed. It is possible to deploy “smart” antennas that produce user-specific beams utilising variable phase shifts between elements. This technology in not mature, however and requires some non-standard functionality. Further, it provides little improvement on the performance of the fixed-beam antenna. The adoption of the fixed-beam approach is generally favoured.


11Integrating Extra Sites into the Macro-Cell Layer

11.1 IntroductionA requirement to improve coverage and/or capacity of the macro-cell layer will inevitably arise at times. Sometimes the only viable solution is to introduce an extra site into this layer. It may be at the edge of the current coverage area or embedded within this area. Although the new cell will be using the same frequency as the other cells within the macro cell layer, care must be taken that its introduction does not disrupt the network. This section explains some of the issues that must be considered.


Integrating New SitesIntegrating New SitesAdditional Sites

• The configuration not only of the new site but also all other sites in the region must be considered.

• Quite possible that on “switch on” the new site makes network performance worse than before.

• We must quickly go from a “non-optimised” to an “optimised” situation.

• Careful use of a planning tool can lead to rapid implementation of corrective action:• Down-tilting antennas from old sites.

• Implementing new neighbour lists.

11.2 Planning the new siteWhether the new site is to be used to fill a coverage gap, extend the coverage region or increase capacity, the general principles that must be adopted are similar. At the moment of switch-on, the region around the new site will move from an “optimised” situation to “non-optimised”. Before the new site is made active, the following issues must be planned carefully.

The coverage area of the new site. The new site will provide coverage but also introduce interference. This can be minimised by appropriate site configuration, in particular ensuring the antenna type and orientation are appropriate. Careful use of the planning tool can achieve a near-optimum configuration as switch-on.

The coverage areas of existing sites in the region. The previously-existing sites will interfere with the new site. Careful use of the planning tool will ensure that the down-tilts of antennas that will inevitably be required can be planned in advance. The change to the down-tilts must be implemented as soon as possible after the new site becomes active.

Changes to the neighbour list. After the planning tool has been used to decide the configuration of the new site and those in its vicinity, the neighbour list can be planned, again using the planning tool. This will lead to the creation of neighbour lists to be allocated to the cells in the new site as well as new neighbour lists for the cells in the region. Again

the changes to the neighbour list should be implemented as soon as possible after the new site becomes active.

It is important to realise that, until the region is optimised once more, it is possible for the introduction of the new site to make the network performance worse than before.

11.3 Action after activation of the new site

11.3.1 Making further drive-tests

Following activation of the new site, it is important that drive tests are carried out to investigate the radio environment in the region of the new site. The procedure adopted is similar to that for the pre-launch optimisation phase of the network as a whole. Drive tests should allow the optimisation engineer to:

Confirm that coverage meet requirements

Check that the interference levels are within acceptable limits

Assess the capacity of a network.

Coverage and interference checks would be in accordance with “standard procedures”. That is, incidents where too many pilots were recorded would be dealt with by re-configuration as necessary.

Drive Testing the RegionDrive Testing the RegionAdditional Sites

• Drive Tests can be used to:• Assess coverage

• Assess interference

• Fine-tune the neighbour lists

• Estimate improvement in capacity

• This last point is vital: if we have not significantly increased the capacity of the network then we have wasted our money.


Estimating Capacity ImprovementEstimating Capacity ImprovementAdditional Sites

• Areas in the vicinity of the new site should show improvements in Ec/Io and pilot strength measurements.

• These can be used to estimate a capacity improvement.

11.3.2 Assessing network capacity

The capacity assessment involves an interpretation of the pilot and Ec/Io measurements made before and after the introduction of the new site. The value of Ec/Io should show a definite improvement in the area close to the new site. This can be interpreted as a capacity improvement.

For example, suppose that in a sample of locations the following Ec/Io measurements were made before and after deployment of the new site. Typical measurements made on a quiet network are shown below.

Location Ref

Ec/Io previously Ec/Io new

A -9 dB -5 dB

B -8 dB -6 dB

C -7 dB -7 dB

D -5 dB -6 dB

Notice that the Ec/Io will not improve in all locations. The presence of the new site will inevitably cause some interference within the coverage area of adjacent cells. This means that the throughput possible from a cell will actually drop at some areas. It is important that these areas are not the areas where high subscriber densities

are expected. In order to perform a quantitative prediction on capacity, the pilot SIR needs to be estimated at each location. This will typically be 1 – 4 dB better than the value of Ec/Io. The SIR of a 2 watt bearer (assuming the pilot power is 2 watts) would be the same as that of the pilot. If the Eb/No of the service is known, the bit rate possible for a 2 watt bearer can be calculated. It is then a simple extension to determine the full cell capacity if of the cell if all 16 watts available was used for traffic. The absolute values (2 watts and 16 watts in this case) are not highly significant. It is the ratio of the two powers (8:1 or 9 dB) that affects the capacity in an interference-limited environment.

As an example, consider the Ec/Io in an unloaded cell in an area where there is no significant out-of-cell interference. If the common channel power is the same as the pilot power then:

Pilot SIR is calculated by separating the pilot power from the common channel power and then considering the effect of orthogonality in reducing the effective interference caused by common channels. If the common channel power equals the pilot channel power then:

dB, where α is the orthogonality factor.

This is the situation where there is no interference. Values of Ec/Io worse than -3 dB indicate that out-of-cell power is being received at that location. The ratio, D, of out-of-cell power to in-cell power is given by

. Note that the constant (-3 in this case) in this

equation is related to the interference-free value of Ec/Io. It must be remembered that this value is appropriate only if the network is unloaded. The pilot SIR in an unloaded network (SIRzero) can then be determined as

dB. At full load, the value of cell power will be

approximately 7 dB (a factor of 5) above the unloaded level (36 dBm to 43 dBm) but the non-pilot power (which is what we are interested in) in the own-cell increases by a factor of 9 (typically from 2 W to 18 W, which equates to 9.5 dB). Thus the SIR at full load (SIRFL) can be determined from

dB.


This SIR is for a 2 watt pilot, a 2 watt traffic bearer would experience the same SIR. The capacity for a 2 watt bearer would depend on the Eb/No. The capacity is given by

If the full power of 16 watts is allocated to traffic channels, eight times the throughput would be possible.

. Remember that

the important parameter is the ratio of traffic channel power to pilot power. A value of 8:1 is thought to be appropriate. In summary, the process involved is

Measure Ec/Io in an unloaded network

Determine the ratio of out-of-cell power to in-cell power, D

from

Estimate the pilot SIR that would be experienced if the downlink of the network was fully loaded.

dB.

Estimate the capacity of the downlink for a service based on

the target Eb/No, .

In this way the impact of changes to the network can be assessed on a capacity basis.

Estimating Capacity ImprovementEstimating Capacity ImprovementAdditional Sites

• Not all areas will be improved.

• Ec/Io in adjacent cells can be made worse by presence of new site. These areas should not be “hotspots”.

-6 dB-5 dBD

-7 dB-7 dBC

-6 dB-8 dBB

-5 dB-9 dBA

Ec/ Io newEc/ Io previouslyLocation Ref

Using Using EcEc/Io to Estimate Capacity/Io to Estimate CapacityAdditional Sites

• In an unloaded network, if there is no interference:• Ec/Io should equal approximately -3 dB.

• Assumptions are that thermal noise is not significant and common channel power equals pilot power.



• In this case:

• This is the same SIR as a 2 W (33 dBm) traffic channel.

dB 1log103SIRPilot 0

IEc


• If the network is fully loaded, Io will increase by 7 dB:

dB 1log104SIRPilot 0

IEc


• Throughput from 33 dBm bearer depends on Eb/No target. For a 7 dB target :

kbit/s 103840putThrough 10

71log1040

IEc


• Throughput from 33 dBm bearer depends on Eb/No target. For a 7 dB target :

kbit/s 103840putThrough 10

71log1040

IEc


Considering InterferenceConsidering InterferenceAdditional Sites

• If there is external interference, Ec/Io will drop.

• Out of cell power as a fraction of in-cell power:

110 10

3 0

IEc

D


• Pilot SIR (at zero load) can then be estimated as:

dB 1

1log10

DSIRzero

110 10

3 0

IEc

D


• Pilot SIR at full load can then be estimated as:

dB 1

1log10

DSIRzero

195

1log10

DSIRFL

• Note: other cell power will increase by a factor of 5.non-pilot power from own cell will increase by a factor of 9.


• The cell capacity if 16 watts was allocated to traffic power can then be estimated as:

kbit/s

10

3840Capacity

10

2W0

FLb SIRNE

kbit/s

10

30720Capacity

10

fullpower0

FLb SIRNE

• The capacity from a 2 watt bearer can then be estimated as:


Summary of Capacity PredictionSummary of Capacity PredictionAdditional Sites

110 10

3 0

IEc

D

dB 195

1log10

D

SIRFL

kbit/s

10

30720Capacity

10

fullpower0

FLb SIRNE

Measure Ec/ Io in an unloaded network

Determine the ratio of out-of-cell power to in-cell power, D from

Estimate the pilot SIR that would be experienced if the downlink of the network was fully loaded.

Estimate the capacity of the downlink for a service based on thetarget Eb/ No, .

11.3.2.1Example

As an example, if Ec/Io was measured at -7 dB and orthogonality factor, α, is assumed to be 0.6 and the Eb/No of the required service was assumed to be 7 dB:.

The graph shows the predicted throughput at various locations for a target Eb/No of 7 dB and assuming that all the cell power is devoted to users at the location in question.

Throughput vs. unloaded Ec/Io

0.00

500.00

1000.00

1500.00

2000.00

-14 -12 -10 -8 -6 -4 -2 0

Unloaded Ec/Io (dB)

Cap

acit

y (k

bit

/s)

Eb/No = 7 dB

11.3.2.2Effect of network loading on cell capacity

It is important to remember that the network is assumed to be at full load. If the interfering cells are transmitting at less than full power then a higher capacity can be achieved. A more general equation for the loaded SIR (instead of full load) is

where η is the fractional power loading

of the interfering cells on the downlink. For a value for η of 0.6, the throughput prediction in the above case would be increased from 546 kbit/s to 753 kbit/s.

Example of Capacity PredictionExample of Capacity PredictionAdditional Sites

As an example, if Ec/ Io was measured at -7 dB and orthogonality factor, α, is assumed to be 0.6 and the Eb/ No of the required service was assumed to be 7 dB:.

kbit/s 546

10

30720Capacity

dB 5.106.01951.15

1log10

51.1110

10

5.107fullpower

4.0

FLSIR

D


Variation of Capacity with unloaded Variation of Capacity with unloaded EcEc/Io/IoAdditional Sites

The graph shows how the capacity at full load varies with the unloaded Ec/ Io values.

Throughput vs. unloaded Ec/Io

0.00

500.00

1000.00

1500.00

2000.00

-14 -12 -10 -8 -6 -4 -2 0

Unloaded Ec/Io (dB)

Cap

acit

y (k

bit

/s)

Eb/No = 7 dB

The Effect of Neighbour Cell LoadingThe Effect of Neighbour Cell LoadingAdditional Sites

If the neighbouring (interfering) cells are not fully loaded then the interference will be less and the capacity will be higher.

195

1log10

DSIRLOADED

Where η is the fractional loading of the interfering cells. If fractional loading is 0.6, the previous example would give a

predicted capacity of 753 kbit/ s instead of 546 kbit/ s.

11.3.3 Interpreting measurements made under unknown loading conditions.

The starting point for the analysis described above is obtaining measurements of Ec/Io on an unloaded network. If the network is loaded to an unknown level, the values of Ec/Io recorded will not be of any use. It is possible, by making certain assumptions, to use the pilot signals measured as an indicator of the level of Ec/Io in an

unloaded situation. Suppose, for example, that the following pilots levels were measured at a particular point:

Cell Pilot Strength

Cell 1 -80 dBm

Cell 2 -84 dBm

Cell 3 -86 dBm

Cell 4 -91 dBm

If the pilot powers are added together the sum is . From this level

it can be deduced that, under conditions of zero traffic load, the downlink power received would be approximately -74.6 dBm. Thus the best server could be estimated to have an unloaded Ec/Io of -5.6 dB and the capacity could be calculated in the manner described. The assumptions in this analysis are:

Thermal noise is negligible. The value of thermal noise depends upon the UE and thus any assumption as to its level would be inexact. However, a value of -100 dBm is seen as reasonable. This can usually be deemed “negligible” if the network power is greater than -90 dBm. Under conditions of heavy load, this network power would be received if the link loss was less than approximately 136 dB. The pilot strength would be -103 dB. This pilot strength is seen as a sensible minimum for there to be any meaningful coverage and therefore the assumption that thermal noise is negligible is valid over the coverage area.

Network downlink power is the only significant power. This assumes that there are no other significant interference sources. This should be the situation. Indeed, as the maximum variation of Ec/Io should be 7 dB, the measured value of Ec/Io can be compared with the estimate for an unloaded Ec/Io and, if the difference is bigger than, say, 5 dB an investigation into the conditions at the location under question can be conducted.


Interpreting Measurements made under Interpreting Measurements made under unknown loading conditionsunknown loading conditions

Additional Sites

The analysis uses Ec/ Io results for an unloaded network. This

can be deduced from pilot measurements when the level of

network loading is unknown.

In the above situation, the unloaded total power will be 3 dB

higher than the pilot power from each cell.

-91 dBmCell 4

-86 dBmCell 3

-84 dBmCell 2

-80 dBmCell 1

Pilot StrengthCell

dBm 6.7410101010log10 10/8810/8310/8110/77


Additional Sites

In this case the Ec/ Io of the best pilot (pilot strength – 80 dBm) would be predicted to be -5.6 dB.

Prediction of capacity would be undertaken as before.

The assumptions behind the deduction of unloaded Ec/ Io must be acknowledged.

dBm 6.7410101010log10 10/8810/8310/8110/77


Additional Sites

Assumptions:Thermal noise is negligible.

Network power at least -90 dBm. Generally an acceptable assumption. A pilot power of -103 dBm would lead to a network power of about -90 dBm if network was heavily loaded.

Only power is network power. External interference can cause problems. Ec/ Io is measured on drive test. Events where the measured Ec/ Io is more than 5 dB greater than predicted, unloaded Ec/ Io should be investigated.


UMTS Network Post Luanch Optimization and Evolution Training

Documents

Transcript of UMTS Network Post Luanch Optimization and Evolution Training