Cell- And Frequency Planning

1/31/20081 May 2004 Cellular Network Planning – CE at UP 1

Cell- and Frequency Planning

Magdaleen Snyman

References

�GSM, GPRS and EDGE Performance: evolution towards 3G/UMTS o T.Halonen, J. Romero, J. Melero

o Second Edition

o John Wiley & Sons

o ISBN 0-470-86694-2

�The Mobile Radio Propagation Channelo J.David Parsons,

o Second Edition,

o John Wiley & Sons

o ISBN 0 471 98857 X

Course Overview

�Conventional Cell and Frequency Planning

o You work, I watch ;-)

�Radio Network Features and their impact

�Investigating the principles

o We all think a bit ;-)

�“Real” Cell and Frequency Planning

�Setting up an AFP

�Site selection – discussion

The inputs to Cell Planning

Traffic: (T

raffic distribution maps)

Cost / Money

Quality

Coverage

Speech Quality

System Choice - C/I

4/12 Cell Pattern

Frequency Groups

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

Channels 1 2 3 4 13 14 15 16

5 6 7 8 17 18 19 20

9 10 11 12 21 22 23 24

D1 D2 C1

Prediction algorithms

�Lee’s model and other empirical models

o Ploss = PR1 + γ10log(d / d1) + n10 log( f / f0) - α0

o PR1 is the reference loss at d1(normally 1 mile)

� (e.g. -84dBm in a city like Tokyo and -49dBm for open areas)

o γ depends on the type of terrain

� (value between 2 and 4)

o n is between 2 and 3

Co-channel interference

# Info

# Coding

Max data rate

(kbs) /TSRequired C/I (dB)

(BLER <10%; TU3 FH)

GSM 260 196 0.5 13.3 9 GMSK

CS-1 181 275 0.45 9.05 9 GMSK

CS-2 268 188 0.65 13.4 13 GMSK

CS-3 312 144 0.75 15.6 15 GMSK

CS-4 428 28 21.4 23 GMSK

MCS-1 176 0.53 8.4 9 GMSK

MCS-2 224 0.69 11.2 13 GMSK

MCS-3 296 0.89 14.8 15 GMSK

MCS-4 352 1 16.8 23 GMSK

MCS-5 448 0.38 22.4 14.5 8PSK

MCS-6 592 0.5 29.6 17 8PSK

MCS-7 896 0.78 44.8 23.5 8PSK

MCS-8 1088 0.92 54.4 29 8PSK

MCS-9 1184 1 59.2 32 8PSK

Adjacent Channel interference

�for co-channel interference C/Ic=9 dB

�for adjacent (200 kHz) interference C/Ia1=-9 dB

Adjacent channel interferenceRelativepower(dB)

0 200 400 600Frequency from the carrier (kHz)

measurement bandwidth 30 kHz measurement bandwidth 100k Hz

1200 1800 60003000

Relative

0 200 400 600 1200 1800 6000

Frequency from the carrier (kHz)

measurement bandwidth 30 kHz

measurement bandwidth 100 kHz

Edge of TX

band + 2 MHz3000

Co-channel interference

�The total co-channel

interference experienced at the yellow spot is the sum of

interference of all six cells with the same frequency

�The interference from one co-channel interferer can be

written as I =KD-γ

�The carrier level is

C= KR-γ

C/I = (D/R)γ /6

Re-use distance

D = (i2 + ij + j2)½2Rcos 30°

D = (i2 + ij + j2)½ (3) ½ R

Number of cells in the

re-use pattern

N = i2 + ij + j2

i in (1,2,3,4 …..)

j in (0,1,2,3,4 …..)

D/R = (3N)½

The Hexagon

Area of a hexagon:

A = 3 (3)½R2/2

Distance between centers

of two adjacent cells:

d = (3)½R

Traffic calculations revision

�An Erlang

�Erlang B Table

�Examples of Traffic channels

Problem

�The average traffic generated by one user is

10milliErlang/Subscriber

�The population density is 50 people/km2

�Assume a phone penetration of 80%

�You are implementing a GSM system.

�You have 48 (1-48)channels available

�Assume free-space propagation … i.e. γ = 2

�Draw the re-use pattern and assign frequencies to

the cells.

�Calculate the site to site distance that you will

need to implement.

C/I = (D/R)γγγγ /2

Sectorisation

Effect of γ and C/I

gamma 9 12 13 17 36

2 18 33 42 102 7965

2.5 12 18 21 42 1323

3 9 12 12 24 399

3.5 6 9 9 15 171

4 6 6 9 12 90

C/I (dB

Minimum

frequencies

Assuming 3 sectored sites

Spectral Efficiency

�Erlang/Hz/km2

�Using the previous problem as starting point – calculate the spectrum density that could be achieved if the sites were sectorised. Compare with the omni-cells

Benefits of sectorisation

�Higher gain antennas are available – better penetration

�Less cost for same traffic density

Underlay / Overlay - MRP

Cell Splitting

Hierarchical Cells

�Umbrella Cell:

�Macro Cell: Antenna above average rooftop height

�Micro Cell: Antenna below average rooftop height

�Pico Cell: Indoors

DTX- Discontinuous

Transmission

�Average Voice activity is around 50%

�DTX is a feature that allows to be transmitted only when there is something to be transmitted

o Uses VAD (Voice Activity Detector)

�It safes on battery power

�Improves the overall network quality by reducing unnecessary interference

Dynamic Power Control

�This enable the BTS and the Mobile to transmit only the power necessary for effective communications

�Power Control Commands are via the SACCH

�This improves the battery live of Mobile Phones

�And it improve the overall network quality by reducing unnecessary interference

Effect of DTX and PC on Quality

10.00%

0 10 20 30 40Time (hours)

DTX + PC Off

PC Off

Base Band Frequency Hopping

Controller

CALL 2 Tx and Rx on f1

Controller

CALL 3

Controller

CALL 4

Controller

CALL 1

Tx and Rx on f2

Tx and Rx on f3

Tx and Rx on f0

“Baseband Bus”

for routing burstsC

er f1 f2 f3 f0

f0 f1 f2 f3

f2 f3 f0 f1

f3 f0 f1 f2

Number of frequencies equal to number of transceiversNumber of frequencies equal to number of transceivers

Synthesised Hopping

Controller

CALL 2Tx and Rx hopping

Controller

CALL 3

Controller

CALL 4

Controller

CALL 1

Tx and Rx hopping

f1 f2 f3 f0

f0 f1 f2 f3

f2 f3 f0 f1

f3 f0 f1 f2

Number of frequencies more or equal Number of frequencies more or equal

to number of transceiversto number of transceivers

Frequency Diversity

�Raleigh fading is frequency dependant

Position

nal le

Frequency Diversity

�Diversity: combining two or more uncorrelated versions of the same signal

� For “conventional” frequency diversity the info is sent on two different frequencies at the same time.

�To be uncorrelated the two frequencies should be more than 1/(multi-path spread), where the multi-path spread is dependant on the environment.

�For urban areas the frequencies should be more than 600kHz apart

Why does hopping work?

�Review interleaving

�If one timeslot gets completely lost during

transmission 1/8 of two speech frames are lost.

�At the receiver the speech frames are de-interleaved

�The channel coding can recover from the 12.5%

�Interleaving and Channel Coding is part and parcel

of the GSM standard - it works even without hopping.

Frequency Diversity Gain

Frequency Diversity Gain vs Number of Hopping Channels

1 2 3 4 5 6 7 8

Number of Carriers

Cyclic Random Poly. (Cyclic) Poly. (Random)

Interference Diversity

�Extent of Interference diversity depends on:

o Interference load (DTX and Power Control)

o Frequency reuse: low re-use -> low gain;

Dependant on area type.

o Number of Frequencies (less -> less gain)

o Cyclic or Random

�Interference diversity gain reached with 25% load, 12 frequencies in Urban area with random hopping is 2.5dB - mostly it is less.

Planning for FH network

�Use separate frequency blocks for TCH and BCCH

o BCCH frequency channel must be Always On

o No hopping over BCCH.

�Plan TCH layer:

o MAL : Mobile radio frequency channel

Allocation List

o HSN: Hopping sequence number

o MAIO: Mobile Allocation Index Offset

o MAI: Mobile Allocation Index

Selecting a BCCH block

�Why a BCCH block?

o Identifying the source of interference

o Re-evaluation of the neighbour list

o For collecting data for a measurement based

�Optimum size?

o Where a change in a BCCH carrier will on

average make the same difference as a change

in a TCH carrier in the optimised plan

Selecting a BCCH block

BlockSize

Total Number of Carriers Available

AverageTraffic TCHlayer Scaling

perCell DTX PCon

_ _ _ _

( / ) ( , )8 1

TCH layer

MAI 0 2 1A 2A 3A 1B 2B 3B 1C 2C 3C

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

2 2 4 10 11 12 13 14 15 16 17 18

3 3 1 19 20 21 22 23 24 25 26 27

4 4 2 28 29 30 31 32 33 34 35 36

4 1 2 3 2 4 3 1

28 1 10 19 10 28 19 1

10 19 28 1 28 10 1 19

HSN =x

TRX1 on 1A has MAIO = 0

TRX2 on 1A has MAIO = 2

MAMAIO

Automatic Frequency

Planning Tools

Coverage

AnalysisInterference

Matrix

Propagation

Predictions

SeparationConstraints,

Frequency

Requirements

AFP Tool

Automatic Frequency Planning

Model of Network

Model effect of particular assignment on quality

�Propagation Predictions �Drive Test Data

�Handover Statistics

�Live Measurements

Cost Function:Sum of remaining interference and other penalties.Quality

Change:�Frequency�BSIC�HSN, MAIO

Interference Matrix

�The “conventional” interference matrix represent:o The Traffic that will be interfered on if two

“radios” were assigned the same frequency;

o The area that will be interfered on if two “radios”were assigned the same frequency –

o pixel by pixel.

o Need ACCURATE propagation predictions and traffic distribution maps.

o What is the cost of accurate enough predictions?

Generating the

Interference Matrix

2.5 km

2.0 km

2 m Resolution

2.5 km

2.0 km

50 m Resolution

Microcell Service Area ≈ 1 pixel

Probability of C/I>9dB

Cummulative Probability Distribution

for C/I exceeding 9dB

-20 -15 -10 -5 0 5 10 15 20 25 30

Calculated C/I (dB)

�Implements a mathematical optimisationmethod or Artificial Intelligence method to minimise

�Cost = � �Cijδij +� � Aijδij

o δij = 1 if radios i and j are assigned the same(adjacent) frequency,

o δij = 0 else

�By changing the frequency assignments to the different cells

What are the true aims in

Cell and Frequency Planning

�What will really give optimum quality?

The inputs to Cell Planning

Traffic: (T

raffic distribution maps)

Cost / Money

Quality

Coverage

Speech Quality

System Choice - C/I

Quality

�Voice Quality

o Impacted by the FER (Frame Erasure Rate /

Probability

o And to some extent by the BER (Bit Error Rate /

probability)

�Dropped Calls

o Radio Link Timeout based on unsuccessful

SACCH frame - FER

FER and SQI vs.RxQual

0 1 2 3 4 5 6 7

RxQual

Non-Hopping

Hopping

C/I to FER

Frame Erasure Rate

-5 0 5 10 15 20C/I(dB)

Frequency Hopping

on 8 freqquencies,

Random Hopping

Non-Hopping

Measurement Based

Frequency Planning�Using Mobile Measurement Reports how

will you go about generating the optimal Interference Matrix?

The first Measurement

Based Plan�Johannesburg’s Central Business District

�12km×12km

�65 sites (≈350 cells)

�477 carriers

�Despite questioned cluttered data and propagation

prediction models

�very low dropped call rate of about 1.4% was very

often achieved

�partly due to dedicated optimisation

�Cell Traffic Recordings was used to collect Mobile

Measurement Reports on all the cells

�With the mobiles measuring on all BCCH channels

�The process took about a month.

�The signal strength of the serving cell and the

reported neighbours was used to calculated the

C/I and eventually the FER.

�The average FER for each server-interferer

relation was calculate.

�and multiplied with the traffic on the serving cell

Measurement Based

Frequency Planning

Sanity

Using MMRs in

Frequency Planning

Dropped Call Rate

0 10 20 30 40

Traffic

DayAvg

Results:

Quality

Intra-cell Hand-over and TCH Dropped

due to Bad Quality

0 10 20 30 40

) %HoBUQ

%HoBDQ

Traffic

%TBQDis*50

Data Sources for the

Interference Matrix (1)

�Propagation Predictionso Well established conventional method

o Based on Predicted Carrier to Interference

ratios that is often translated with a “C/I weights”

o Integration with AFP tools eases use

o Suited for new networks with many new cells

o Dependant on elevation and clutter data that

often has limited accuracy

�Neighbour relations statisticso Well suited for very tight plan

o Too little information for a less tight plan

o Hand-over statistics not directly related to C/I

o Can not model interference from non-

neighbours

�Drive Test Datao Measurements done with network set on measure on all

BCCH channels

o Independent of accuracy of elevation and clutter data

o Extensive measurements necessary for interference matrix

o Difficult to deduce interfered traffic from data

o Drives are limited to roads and does not include high rise buildings

o Effort in importing into an AFP

o Often used to supplement propagation predictions

�Live Data: Mobile Measurement Reportso Mobile Measurement Reports are collected with the cell

set to measure on all BCCHs

o Data reflect the actual traffic distribution as well as the actual C/I. (“as the customer sees it”)

o No additional neighbour relations or exceptions required

o Extensive data collection - slow process. Requires the

network to be fairly mature and stable.

o Difficult to model new sites

o Takes some effort to import into an AFP.

Prediction vs. MMRP

�LIMITED accuracy

o Propagation predictions

o Clutter and Height data

o In building

o Traffic distribution

�Cannot represent new

�MMR limitations:

o RxLev: -110 -> -48dBm

o Only integers

o Only six neighbours

o BSIC decoding

problems

Combining Data Sources

�….one of the remaining challenges. E.g:

o How to complement the shortcomings of the

mobile measurements reports with the

propagation predictions to include new cells.

o How to combine limited measurements with

predictions.

�without

o Spoiling good data with bad data.

o Skewing the matrix, e.g. when drive test data is

available for only part of the network.

Penalties for AFP

�A “bare necessity” approach i.e. set penalties only when

o it is required by law or

o It is required for feasibility – e.g. filter combiner

separation

o it will assist in the improvement of network

quality

o Is penalties to avoid adjacencies required?

�The size of the penalties must reflect their importance and effect on network quality

Examples of Scaling Factors

�Difference in interference introduced

o Traffic load on TCH channels

o Power Control

o Discontinuous Transmission (DTX)

o Over-laid Under-laid - depend on effectiveness

of implementation

o Synthesizer Hopping - dependant on fractional

�Difference in immunity to interference

o Frequency Diversity Gain of Hopping Networks

Interference Load

�The core questions:o How much interference will assigning the same

frequency to a carrier in Cell A and Cell B cause ?

o How much less will that be after DTX?

o How much less will that be after Power Control?

�Interference Loado How much signal or potential interference is

carried on a particular carrier

o Interference Load = Traffic on Cell8 * #Carriers

Interference Load Reduction

�For BCCH

o Interference Load = 1

�For Non-Hopping TCH without DTX and PC

o Interference Load = Traffic on TCH Carriers

o 8 * Number of TCH Carriers

�After DTX

o Voice Activity Factor 40% on TCH channels

o Interference Load = 0.4 * Traffic on TCH Carriers

Interference Load Reduction

�After Power Control ?

o Consider a very simplified model:

� C/I = Server SS / (6* Interferers SS)

� Reducing the signal level of the server and of the interferers by approximately 10dB:

� C/I = 0.1* Server SS / (6*0.1* Interferers SS)

� Approximately unchanged.

o Practical implementation suggest a definite interference reduction - by 60%

o Interference Load = 0.6 * Traffic on TCH Carriers

Inter-modulation Products

�Harmonics or Inter-modulation products results from non-linearity in the system

�May cause a problem if one of these products fall on a receiving frequency.

�IM originate from frequencies in the transmit band and cause interference in the receive band

Inter-modulation Products

GSM1800 Downlink -BaseTxGSM1800 Uplink - MobileTx

GSM900 Uplink - MobileTx GSM900 Downlink - BaseTx

915MHz

1785MHz 1805MHz 1880MHz1710MHz

935MHz 960MHz890MHz

Dualband

GSM1800

GSM900

A few terms

�Frequency Allocation Re-use

o FAR = Total Number of Frequency Channels

Number of Frequencies per Cell

�Effective Re-use

Reff= Total Number of Frequency Channels

Average number of TRX per Cell

�Fractional Load

o Lfrac= Number of TRX per Cell .

Number of Frequencies per Cell

�Hardware Load

o LHW= (Busy Hour Traffic) / (TN /TRX)

A few terms

�Frequency Load

o Lfreq= LHW Lfrac

�Effective Frequency Load

o EFL =. Busy Hour Traffic per Cell .

(TN per TRX for Traffic).(Total # FreqCH)

Optimum # carriers to

Hop over = 24/6

Optimum frequency Re-use

1 2 3 4 5 6 7 8 9

Frequency Reuse = #TCH carriers / #TCH per cell

Erlang p

6MHz available for TCH

Quality vs Capacity

6 7 8 9 10 11 12 13 14 15 16

Average Erlang per Cell (Capacity)

(deduced from Spectrum Utilisation)

ty) The challenge: To maximize Quality * Capacity

Major Interferers

Effect of reducing major interferers

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0.00% 10.00% 20.00% 30.00% 40.00% 50.00% 60.00% 70.00% 80.00% 90.00% 100.00%

P er cent age of Cel l s cont r i but i ng t o I nt er f er ence

Cummulative Contribution

With 5 sites' interference removed

What criteria would you

use for site selection?

�Close to traffic – most effective Power Control

�Contained (high γ )

o In building

o In valleys rather than on top of mountains

�What effect will an unbalanced link have?

What criteria will you provide an

Automatic Cell Planning tool with?

Interference Matrix

Frequency Allocation

Propagation Predictions

Traffic distribution - GIS

Possible sites

Equipment used

Effective Frequency load

Hand over areas

Income: Coverage

of potential traffic

Cost: cost of

changes / sites

Evaluating automatic tools...

�Automatic Frequency Planning Toolso Must Allow various data sources to be imported

o Must model the network accurately (e.g. Model hopping accurately)

o Must be simple to use, hence most of the modelling should be integrated

�Automatic Network Optimisationo Must be reliable and accurate enough to allow it to

run free with very little manual input

�Automatic Cell Planningo Cost function is so complex it should come with

the tool... and allow manual changes

Cell- And Frequency Planning

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