WCDMA Radio Network ion Guidelines_Orange

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Confidential WCDMA RADIO NETWORK OPTIMISATION GUIDELINES MARCH 2003 Doc No: Spec1267 Date Issued: 11/4/2003 Copy No: Version: 1 Version Status: Current Category: Specification Sub- Category: Design Comments: Document Owner: Robert Joyce Page 1 of 40

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WCDMA RADIO NETWORK OPTIMISATIONGUIDELINESMARCH 2003

Doc No: Spec1267 Date Issued: 11/4/2003 Copy No:

Version: 1 Version Status: Current

Category: Specification Sub-Category: Design

Comments:

Statement of Confidentiality

Copyright in this document is the property of Orange Personal Communications Services Limited and its contents shall be held in strict confidence by the recipient hereof and shall be used solely for the purposes of Orange Personal Communications Services Limited. Neither this document nor its contents shall be disclosed to any other person or used for any other purpose without prior written permission of Orange Personal Communications Services Limited.

© Orange Personal Communications Services Limited 2000. All rights reserved.

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AMENDMENT HISTORY

Version Date Issued Originator/ Modified by Reason(s) For Issue/ Re-issue

1A 19/03/03 Robert Joyce First draft, pre-approval review

1 10/04/03 Benoit Graves First Issue

1.1 26/08/03 Robert Joyce Minor ammendment, coverage levels brought in line with SPEC1337.

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TABLE OF CONTENTS

1 INTRODUCTION............................................................................................................51.1 Purpose.............................................................................................................................................. 51.2 Scope................................................................................................................................................. 51.3 Responsibilities.................................................................................................................................. 51.4 Reference / Related Documentation..................................................................................................51.5 Definition of Terms............................................................................................................................. 5

2 BACKGROUND.............................................................................................................6

3 WCDMA BASICS...........................................................................................................73.1 Frequency Reuse............................................................................................................................... 73.2 Scrambling Codes in WCDMA...........................................................................................................8

3.2.1 Scrambling Code Planning..........................................................................................................83.2.2 Scrambling Code Measurements................................................................................................9

3.2.2.1 Total Received Power Io..........................................................................................................93.2.2.2 Received Power of a CPICH Ec.............................................................................................103.2.2.3 The CPICH Quality Ec/Io.........................................................................................................103.2.2.4 Ec/Io Calculation Example......................................................................................................103.2.2.5 Ec, Io and Ec/Io Measurement..................................................................................................11

3.3 Handovers in WCDMA.....................................................................................................................113.3.1 Softer Handover........................................................................................................................ 113.3.2 Soft Handover........................................................................................................................... 12

3.4 Basic WCDMA Optimisation.............................................................................................................12

4 COVERAGE OPTIMISATION......................................................................................134.1 Target Measured Coverage Levels..................................................................................................134.2 Target Measured Ec Levels – In Train.............................................................................................144.3 Link between coverage and capacity...............................................................................................14

5 MISSING NEIGHBOURS OPTIMISATION..................................................................155.1 Definition of missing neighbours in WCDMA....................................................................................155.2 Problems of missing neighbours......................................................................................................15

5.2.1 Downlink quality........................................................................................................................ 155.2.2 Uplink quality............................................................................................................................. 15

5.3 Solution............................................................................................................................................ 155.3.1.1 Missing Neighbour Detection with a scanner.........................................................................155.3.1.2 Missing Neighbour Detection with a trace mobile..................................................................165.3.1.3 Missing Neighbour Detection with a scanner and a trace mobile..........................................16

6 SOFT HANDOVER AREA OPTIMISATION................................................................176.1 Definition of Soft Handover Area......................................................................................................176.2 Aim of SHO Area Optimisation.........................................................................................................17

6.2.1 Soft HO..................................................................................................................................... 176.2.2 Softer HO.................................................................................................................................. 18

6.3 SHO Area Target.............................................................................................................................. 186.4 Mechanisms to achieve SHO area target.........................................................................................18

6.4.1 Aim of SHO Area Optimisation..................................................................................................186.4.2 SHO Area Optimisation.............................................................................................................18

6.4.2.1 Parameter changes...............................................................................................................196.5 Methods of SHO Analysis.................................................................................................................20

6.5.1 SHO Analysing with a scanner..................................................................................................206.5.1.1 Display of SHO area..............................................................................................................206.5.1.2 Identification of cells to optimise............................................................................................20

6.5.2 Method with a trace mobile.......................................................................................................216.6 SHO Area Optimisation Summary....................................................................................................21

7 PILOT POLLUTION OPTIMISATION..........................................................................227.1 Definition of Pilot Pollution................................................................................................................227.2 Problems caused by Pilot Pollution..................................................................................................227.3 Distinction between coverage and interference problems................................................................227.4 Practical thresholds for pilot pollution...............................................................................................23

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7.5 Pilot Pollution Optimisation...............................................................................................................237.5.1 Pilot Pollution Optimisation Target............................................................................................237.5.2 Generic method......................................................................................................................... 23

7.5.2.1 Pre-check.............................................................................................................................. 237.5.2.2 Improvement of Ec level........................................................................................................237.5.2.3 Reduction of interferer Ec levels............................................................................................24

7.5.3 Optimisation Actions.................................................................................................................. 247.6 Practical Pilot Pollution Analysis.......................................................................................................25

7.6.1 Analysis with a scanner.............................................................................................................257.6.1.1 Identify pilot pollution areas...................................................................................................257.6.1.2 List all cells involved in the pilot pollution area......................................................................257.6.1.3 Determine the dedicated best serving cells...........................................................................257.6.1.4 Determine the worst interferers.............................................................................................26

7.6.2 Analysis with a trace mobile......................................................................................................267.7 Pilot Pollution Optimisation Summary..............................................................................................27

8 OVERALL OPTIMISATION PROCESS.......................................................................288.1 Site Verification and Cluster Validation............................................................................................288.2 Step 1 – Site Verification.................................................................................................................. 298.3 Step 2 – Cluster Optimisation...........................................................................................................298.4 WCDMA vs. GSM............................................................................................................................. 30

9 SUMMARY...................................................................................................................31

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1 INTRODUCTION

1.1 Purpose

The purpose of this document is to document guidelines for the optimisation of Orange’s WCDMA radio network.

1.2 Scope

The document covers only radio aspects of optimisation and does not cover RRM or core network optimisation.

1.3 Responsibilities

Access Network Design will be responsible for updating this document as and when re-quired.

1.4 Reference / Related Documentation

[1] SPEC 1337 – “Shadow Fading Margins & Drive Survey Levels for 2G & 3G”

1.5 Definition of Terms

(Overtype term here) (Overtype definitions here)

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2 BACKGROUNDThis document is aimed at the Network Planning and Network Optimisation teams and gives guidelines on how to optimise a WCDMA network. These guidelines are the result of the work per-formed on the Bristol Experimental Network and armed with these guidelines it should be possible for the reader to optimise a WCDMA network so that it delivers a reliable quality of service similar if not better than Orange’s current 2G network.

The document begins by covering the basic metrics of WCDMA radio measurements and intro-duces new concepts not previously required for 2G optimisation. It then goes on to explain how op-timisation can be achieved in stages, looking at the various consideration to be made when optim-ising WCDMA such as coverage level, coverage quality, missing neighbours and so on. Finally the document summarises the steps required to optimise a WCDMA radio network together with the need to maintain the performance of the Orange GSM network.

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3 WCDMA BASICSThis section outlines some WCDMA basics.

3.1 Frequency Reuse

The table and diagram below show Orange’s 3G spectrum allocation alongside the alloca-tions of the other 3G operators.

F r o m ( M H z ) T o ( M H z ) F r o m ( M H z ) T o ( M H z ) F r o m ( M H z ) T o ( M H z )A H u t c h i s o n 3 G 1 9 2 0 . 0 1 9 3 4 . 9 2 1 1 0 . 3 2 1 2 4 . 9 1 9 1 4 . 9 1 9 2 0 . 0 2 x 1 5 + 5 = 3 5B V o d a f o n e 1 9 4 4 . 9 1 9 5 9 . 7 2 1 3 4 . 9 2 1 4 9 . 7 N / A N / A 2 x 1 4 . 8 = 2 9 . 6C O 2 1 9 3 4 . 9 1 9 4 4 . 9 2 1 2 4 . 9 2 1 3 4 . 9 1 9 0 9 . 9 1 9 1 4 . 9 2 x 1 0 + 5 = 2 5D T - M o b i l e 1 9 5 9 . 7 1 9 6 9 . 7 2 1 4 9 . 7 2 1 5 9 . 7 1 8 9 9 . 9 1 9 0 4 . 9 2 x 1 0 + 5 = 2 5E O r a n g e 1 9 6 9 . 7 1 9 7 9 . 7 2 1 5 9 . 7 2 1 6 9 . 7 1 9 0 4 . 9 1 9 0 9 . 9 2 x 1 0 + 5 = 2 5

T o t a l ( M H z )T D D

L i c e n c e O p e r a t o rF D D D o w n l i n kF D D U p l i n k

1 9 0 51 9 1 0 1 9 6 0

F D D U p l i n k

L i c e n c e A - H u t c h i s o n 3 G

L i c e n c e B - V o d a f o n e

1 9 3 5

L i c e n c e C - O 2

L i c e n c e D - T - M o b i l e

F D D D o w n l i n k

2 1 5 02 1 1 0

L i c e n c e E - O r a n g e

1 9 1 51 9 2 0 1 9 4 5 1 9 7 0 1 9 8 0 2 1 2 5 2 1 3 5 2 1 6 0 2 1 7 0

T D D

Figure 3.1 – UK 3G Spectrum Allocations

As can be seen Orange has 2 FDD carriers and 1 TDD carrier. Orange will initially launch 3G using only the lower FDD carrier (F1). All launch cells will use F1 and therefore Orange’s ini-tial 3G network will have a frequency reuse of 1. The table below shows the frequencies of Orange’s uplink and downlink carriers and their associated UARFCN (UTRA Absolute Radio Frequency Channel Number.

UARFCNFrequency

(MHz)TDD 9537 1907.4

FDD Uplink F1 9861 1972.2F2 9886 1977.2

FDD Downlink F1 10811 2162.2F2 10836 2167.2

Figure 3.2 – OrangeUK 3G Carriers (Orange will initially launch using FDD carrier F1).

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3.2 Scrambling Codes in WCDMA

Since every cell in a WCDMA network can transmit on the same frequency some means of differentiation is required. This differentiation is achieved through the use of Scrambling Codes. In the downlink there are 512 DL Scrambling Codes, these 512 codes are grouped into 64 groups of 8 codes.

Code Group 0 0 1 2 3 4 5 6 7Code Group 1 8 9 10 11 12 13 14 15Code Group 2 16 17 18 19 20 21 22 23

Codes

Figure 3.3 – First three downlink code groups of WCDMA.

3.2.1 Scrambling Code Planning

In WCDMA Scrambling Codes (SCs) require planning rather than frequencies - as in the case of GSM. Orange’s 3G scrambling code planning will be performed by Access Design and a full scrambling code plan can be found on the Access Design Doc Cab-inet for all planned 3G sites.

The Orange scrambling code plan follows a code group reuse approach leading to a reuse of 64. This means each site is assigned a code group and each sector of each site is assigned codes from the group in numerical order.

For example a site assigned a code group of 2 will have the following scrambling codes.

Sector A 16

Sector B 17

Sector C 18

Sector E 19

Sector F 20

Sector G 21

This scrambling code group planning approach by site rather than cell has many ad-vantages.

It simplifies the planning without degrading performance (a reuse of 64 provides same co-code protection as a reuse of 512). In WCDMA every code has the same ability to in-terfere with every other code therefore there is no concept of adjacent code interference.

It means a field op. Engineer only needs to know the code group for the site and from this they can derive the scrambling codes of each sector of the site

It increases the probability of a UE camping on to the network.

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3.2.2 Scrambling Code Measurements

The Common Pilot Indication Channel (CPICH) is a common channel broadcast from each and every cell within a WCDMA network. It carries no information and can be thought of as a “beacon” constantly transmitting the Scrambling Code of the cell. It is this “beacon” that is used by the phone for its cell measurements for network acquisi-tion and handover purposes.

CPICH

Figure 3.4 – Each cell broadcasts a CPICH (Common Pilot Indication Channel)

The majority of 3G coverage measurements are based upon measurements of the CPICH.

Golden Rule: If the UE can’t see the CPICH the UE can’t see the cell.

Initial 3G network optimisation will be performed purely from CPICH measurements. Three key related measurements for 3G optimisation are;

Ec - The Received Signal Level of a particular CPICH (dBm)

Io - The Total Received Power (dBm)

Ec/Io - The CPICH Quality (The ratio of the above two values)

3.2.2.1 Total Received Power Io

Figure 3.5 – UE receives its total power from a number of sources

In a WCDMA network the User Equipment (UE) may receive signals from many cells whether in handover or not

Io = The sum total of all of these signals + any background noise (dBm)*

*Note: Sometimes Io is referred to as No, RSSI or ISSI.

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3.2.2.2 Received Power of a CPICH Ec

Ec1 Ec2

Figure 3.6 – UE can differentiate between signals from different cells using the property of scrambling codes.

Using the properties of the WCDMA downlink scrambling codes the UE is able to extract the respective CPICH levels from the sites received.

Ec = The Received Power of a Particular CPICH (dBm)*

*Note: Sometimes Ec is referred to as RSCP

3.2.2.3 The CPICH Quality Ec/Io

Ec

Figure 3.7 – For each received CPICH Scrambling Code the UE is able to make an Ec/Io quality measurement

From the previous two measures we can calculate a signal quality for each CPICH (Scrambling Code) received

Ec/Io = Ec - Io (dB)*

*Note: Sometimes Ec/Io is referred to as Ec/No

3.2.2.4 Ec/Io Calculation Example

Ec1=-95dBm Ec2=-90dBm

Io=-80dBm

Figure 3.8 – Ec/Io Calculation Example

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From the above three measurements we can calculate for each pilot the Ec/Io for that particular pilot

(Ec/Io)1 = -95 - -80 = -15dB

(Ec/Io)2 = -90 - -80 = -10dB

Clearly in this example the second pilot is seen by the UE as the stronger.

3.2.2.5 Ec, Io and Ec/Io Measurement

All commercial scanners and test UEs are capable of Ec, Io and Ec/Io measure-ments. It is these measurements that are used for cover analysis and basic op-timisation. A screen shot from the Anritsu scanner is shown below.

Figure 3.9 – Anritsu scanner screen shot showing a number of pilots being meas-ured.

3.3 Handovers in WCDMA

Various types of handover (HO) exist in WCDMA,

Those between WCDMA sites (intra-system HO)

Those between WCDMA and GSM (inter-system HO)

For initial WCDMA optimisation we will concentrate primarily on intra-system HO.

3.3.1 Softer Handover

Figure 3.10 – Softer Handover in WCDMA

Softer handover occurs between sectors of the same site. A UE is said to be in Intra-system handover when it is communicating with more than one BTS. Another way of saying this is that its active set (the set of cells the UE is communicating with) is greater than 1.

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3.3.2 Soft Handover

Figure 3.11 – Soft Handover in WCDMA

Soft handover occurs between sectors of the different sites. For both softer and soft it is the Ec/Io levels used to determine whether a cell should be added or re-moved from the active set.

3.4 Basic WCDMA Optimisation

The remainder of this document will now show that using the basics introduced in this section it is possible to optimise a WCDMA network using mainly Ec and Ec/Io measurements.

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4 COVERAGE OPTIMISATIONThe aim of this section is to provide details on how to analyse 3G coverage from drive test meas-urements to determine whether or not sufficient coverage has been provided to a particular loca-tion.

Spec 490 – Summary of Orange 2G & 3G Coverage Planning Levels details the required coverage levels to be achieved by network planning when planning both 2G and 3G networks. The planning levels have been derived using appropriate link budgets with appropriate shadow fading margins. However when actually measuring coverage these shadow fading margins are no longer required in the target measured coverage levels since the levels measured are actually after some or all (depends on environment and where measurement is made – indoor/outdoor) of the shadow fad-ing.

Therefore using this approach it is possible to derive target measured Ec levels to guarantee a par-ticular service. For the time being this document concentrates on the launch services of voice and PS 64kbps, other service types are included for completeness

4.1 Target Measured Coverage Levels

Based on the February 2003 WCDMA link budgets the following target measured Ec levels have been derived. All values assume Orange Default CPICH transmit power of 33dBm.

Dense Urban Urban/Suburban Rail Road Rural IndoorDeep Indoor - 95% Deep Indoor - 95% Indoor Window - 95% Indoor Window - 95% Outdoor - 90% 95%

12.2k Speech -71.0 -81.0 -88.0 -95.0 -102.0 -102.064k CSD -72.0 -82.0 -90.0 -96.0 -103.0 -102.064k PSD -73.0 -82.0 -90.0 -97.0 -103.0 -103.064k CSD Videophone -71.0 -80.0 -88.0 -94.0 -101.0 -101.0144k CSD -69.0 -79.0 -86.0 -93.0 -100.0 -99.0144k PSD -70.0 -79.0 -87.0 -94.0 -100.0 -100.0384k CSD -65.0 -75.0 -84.0 -90.0 -95.0 -95.0384k PSD -66.0 -76.0 -84.0 -90.0 -96.0 -96.0

ServiceEnvironment

Table 4.1 – Target Measured Ec Levels for WCDMA (dBm)

Notes on Target Ec Levels (these target levels should be check against those found in SPEC1337)

All levels are in the above table are specified for outdoor measurement (except in-door). It is expected that survey vehicles with carkit measurements will be used however care should be taken to ensure such equipment is calibrated such that the feeder and the an-tenna system has a gain of 0dB or any such gain/loss is removed during analysis.

The most onerous Ec level is used. For example if a voice and 64 PS data were the target services then clearly in this case voice has the higher signal level requirement because of its associated body losses.

The correct type of environment is assumed for the drive run. Obviously drive routes may cross many types of environments, therefore it is important that during the analysis the drive route is classified into the different types of environments so that the correct measured Ec levels can be used.

It is possible to make voice calls below the values given in the table. However below these levels it is impossible to offer any guaranteed level of service quality.

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4.2 Target Measured Ec Levels – In Train

It is now common practice to take rail coverage measurements from within trains. If this is the case the following target Ec levels should be used.

In TrainIndoor Window 95%

12.2k Speech -100.064k CSD -102.064k PSD -102.064k CSD Videophone -100.0144k CSD -98.0144k PSD -99.0384k CSD -96.0384k PSD -96.0

Service

Table 4.2 – In Train Target Measured Ec Levels for WCDMA (dBm)

4.3 Link between coverage and capacity

In UMTS coverage and capacity are closely related. When the load of the network increases, the cell will shrink and its coverage will be reduced.

A full representation of the coverage with load from drive test would be impossible, therefore we shall not expect to get a cell capacity figure out of a drive test.

A simplified approach has been taken, basically the Ec coverage levels given above are defined for a loaded network and simply include a fixed margin to represent the shrunken cell. A margin of 4dB (60% UL load) is taken for Dense Urban, Urban/Suburban and Indoor, whereas a margin of 3dB (50% UL load) is applied for In Car, In Train and Rural.

Whilst the network is unloaded coverage will be provided at locations with signal levels a few dBs below these targets, however this coverage will disappear as the network becomes loaded. Therefore be warned of using the load margin to achieve coverage … one day your coverage will disappear!!

If more coverage is required then just as with GSM either downtilts or new sites are the an-swer.

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5 MISSING NEIGHBOURS OPTIMISATIONThe aim of this section is to give details on how to analyse and solve missing neighbour is-sues using drive test measurements.

5.1 Definition of missing neighbours in WCDMA

By a missing neighbour we mean a cell (MISSING_N) not declared as neighbour of the best active cell (BEST) although it could be included in the active set.

In other words:

Orange default: Addition_window = 1 dB.

However for drive test analysis a larger margin is recommend such that:

Where Margin_missing_n = 5 dB.

5.2 Problems of missing neighbours

In WCDMA a missing neighbour leads to both downlink and uplink quality problems.

5.2.1 Downlink quality

In the downlink two effects can occur

1: Increased DL interference

2: Call drop because of excessive DL interference

5.2.2 Uplink quality

In the uplink the missing neighbour cell can experience an UL noise rise, hence cell shrinkage can occur leading to the possible deterioration of all calls on the cell.

5.3 Solution

The way to solve missing neighbour is straightforward. Any cells found to be missing neigh-bours need to be added in the neighbouring list of the source cell at the OMC.

The difficult part is to identify the missing neighbour. RF scanners are the best tool for this job and therefore it is recommended that they are actively used for WCDMA missing neigh-bour optimisation.

5.3.1.1 Missing Neighbour Detection with a scanner

When used with “Top N” functionality a scanner will report all decoded scrambling codes seen within the band.

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With a post-processing tool we can list all required neighbour pairs fulfilling the condition:

A comparison can then be made between this list and the declared neighbour list and this will give the missing neighbour list for the corresponding drive.

5.3.1.2 Missing Neighbour Detection with a trace mobile

When only a trace mobile is available it is far more difficult to detect missing neighbours. This is why scanners are really required for this purpose. The method with a trace mobile is similar to that used to detect missing neighbours in GSM.

Basically the trace mobile will obviously not report the missing neighbour in its monitored set because this neighbour is not declared. The problem is identical in both connected and idle mode. But a few events may help identifying missing neighbours:

If the missing neighbour causes the call to drop, the UE will “lose the network” and revert to scanning mode in order to select a new suitable cell. In this case the UE will most probably select the missing neighbour cell. Correlating the last con-nected cell and the newly selected cell is likely to give a missing neighbour pair.

If cell_C is a missing neighbour to cell_A but declared as neighbour of cell_B, a handover from cell_A to cell_B will suddenly trigger the reporting of cell_C’s level. If when first reported, cell_C appears to be the best server then it was most probably a missing neighbour of cell_A.

5.3.1.3 Missing Neighbour Detection with a scanner and a trace mobile

A comparison between neighbours detected by the trace mobile and those seen by the scanner will immediately point out any missing neighbours.

The process is as follows:

Step 1: Merge two sources on a time basis

Step 2: For each timestamp, edit all neighbours reported within Margin_missing_n (5 dB) for both sources. The scanner source will give all necessary neighbour declarations, whereas the trace mobile source will only reports neighbours if declared.

Step 3: The comparison gives the list of missing neighbours

Note that this method does not require any comparison with a list of previously declared neighbours although this can also be done for completeness.

As can be seen from the above method without a WCDMA scanner missing neighbour detec-tion is very difficult. Missing neighbours will be the biggest cause of call drop and quality de-gradation in a WCDMA network it is therefore highly recommended that OrangeUK uses WCDMA scanners and either method 1 or 3 above to detect missing neighbours within its WCDMA network.

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6 SOFT HANDOVER AREA OPTIMISATIONThe aim of this section is to give details on how to analyse and optimise soft and softer HO areas from drive survey measurements.

6.1 Definition of Soft Handover Area

By SHO area we mean the area where a UE has several cells in its active set. For conveni-ence of analysis we will not distinguish soft from softer HO, that is to say SHO in this section refers both soft and softer HO.

As a reminder soft HO means HO between a 2 cells from different NodeBs, whereas softer HO means HO between 2 cells from the same NodeB.

The definition of SHO is slightly different depending on the type of measurements used:

1) Trace mobile in connected mode (call up):

SHO <-> nb_cells_in_active_set > 1

2) Trace mobile in idle mode or scanner:

In this case, we need to calculate the SHO area from Ec/Io measurements. Based upon the parameter settings for SHO. The soft HO area is defined by the equation:

Where

SHO_window is the average of Addition_window and Drop_window (Current Orange Default parameters add_window=1dB, drop_window=3dB, recommended SHO window=2dB)

Ec/Io_best is the CPICH Ec/Io value of the best serving cell

Ec/Io_2nd_best is the CPICH Ec/Io value of the 2nd best serving cell

6.2 Aim of SHO Area Optimisation

The aim of SHO area optimisation is to limit the SHO area within the network to a reasonable area. An excessively large SHO area generates a loss of capacity on the network since each UE in SHO uses 2 links or more for its connection.

Note that there is no quality problem linked to large SHO areas when the network is un-loaded. Only when the load increases will excessive SHO areas generate measurable quality degradation, and not only for the users in SHO but also other users of the network who may not be in SHO.

6.2.1 Soft HO

A UE in Soft HO means:-

Twice as many channel elements in the NodeB (hard resources, in the WSP Card) are required for the call

Twice as many backhaul resources are required up to the RNC.

2 cells transmitting dedicated DL power, yet slightly less power per cell than if only 1 cell was serving the UE thanks to the combination gain in DL at the UE.

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No change in UL (no combination in UL for soft HO, the RNC selects the best signal re-ceived from the UE by the cells in soft HO)

Globally if a Soft HO area increases, it means a loss of hard NodeB processing capa-city, backhaul capacity and DL capacity, and an unchanged UL capacity.

6.2.2 Softer HO

A UE in Softer HO means:-

The same number of channel elements in the NodeB (hard resources)

2 cells transmitting dedicated DL power, yet slightly less power per cell than if only 1 cell was serving the UE thanks to combination gain in DL at the UE.

UE is transmitting less UL power thanks to the combination at the NodeB for softer HO.

Globally if a Softer HO area increases, it means a loss of DL capacity, an unchanged hard capacity and a gain of UL capacity. However as it is expected that in downlink will be the capacity limiting link, softer HO reduces overall system capacity.

6.3 SHO Area Target

The target for SHO is SHO_area < 30%

6.4 Mechanisms to achieve SHO area target

The reduction of SHO areas is achieved through the reduction of the overlaps between cells. See also “Pilot pollution module”. In this section we shall focus on purely on SHO area optim-isation.

6.4.1 Aim of SHO Area Optimisation

The aim is to achieve a network where the overall percentage of the area in SHO is around the SHO area target of 30%. In a non-optimised network the SHO areas will be much higher than this. In practice, the way to achieve this target is to reduce the Ec level from some of cells in SHO which are not best server. More precisely we should reduce the Ec level of the 2nd best (or up to nth best if there are n cells in the active set) so that the optimised cell is no longer eligible for the active set, i.e.,

6.4.2 SHO Area Optimisation

After having displayed the SHO area and sorted the cells in terms of priority (refer to the later subsections of this section for details) optimisation shall be car-ried out.

SHO area optimisation will be achieved by either;

1. Modifying the tilt of the antenna, preferably the electrical tilt if adjustable, otherwise the mechanical tilt

2. Changing the antenna, in order to obtain a narrower beamwidth

Note that adding tilt to an antenna will not help reducing the Soft HO area if the SHO area is located close to the site. What will most certainly happen is just a shift of the Soft HO area, but in terms of surface area the area will not be smaller.

Tilt will be most useful in the case when the Soft HO area in which a cell is in-volved is remote from the site.

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The beamwidth of the antenna also has a great effect on the SHO area.

If we simplify the analysis by focusing on softer HO between 2 sectors of a same site, an H65 antenna will give better (less SHO area) results than an H85.

The following table is a theoretical calculations of the softer handover areas for each of the 3G antenna types currently employed by Orange.

Antenna Type Theoretical SHO area (SHO_window=3dB)

H65 Cross Polar 7%

H85 Cross Polar 13%

H85 Plane Polar 11%

Table 1 Softer handover area

Caution: above values are with only 2 cells and are not comparable to the SHO figure of 30%.

We can see that a H65 antenna gives better results than a H85. Therefore repla-cing a H85 antenna by a H65 can be a useful optimisation technique to reduce the SHO area.

6.4.2.1 RAN Parameter changes

No parameter changes at the time are being recommended.

For the same reason detailed in the section on pilot pollution document, we do not recommend any change in CPICH power.

Some specific SHO parameter may prove useful to be optimised in the future, however with the current Orange default parameter set, it is unnecessary and not recommended.

Apart from the physical antenna downtilts, two main RAN parameters will impact the size of the SHO area, these are “addition_window” (Aw) and “drop_window” (Dw).

Reducing the values of Aw or Dw will effectively reduce the SHO area, however the current Orange default parameter setting are:

Addition_window = 1 dB

Drop_window = 3 dB

And therefore the addition window cannot be reasonably reduced, and bearing in mind that the drop window should always be greater than the addition window (required hysteresis) to avoid ping-pong, we cannot reduce the drop window either. A different type of default parameter (Aw = 4 dB, Dw = 6 dB) would have left room for parameter optimisation, but not the current one.

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6.5 Methods of SHO Analysis

6.5.1 SHO Analysing with a scanner

6.5.1.1 Display of SHO area

With a scanner, no call can be made so no HO will be performed.

We need to extrapolate from the scrambling code measurements where a UE would be in SHO or not.

Quite simply we compare the Ec/Io levels of the best and the 2nd best cell, based on the following formula:

Note that the timing issues are not considered with this expression, but they would not really matter to get a global figure of SHO area.

6.5.1.2 Identification of cells to optimise

Unlike in the case of pilot pollution we cannot define an area as being of “excess-ive SHO” at a bin level. The issue requires a broader picture where percentage of SHO is assessed on a larger scale than bin level.

Therefore as it is difficult to decide that a particular area requires optimisation, it will be handy to calculate figures on a cell basis, in order to assess the “cell’s per-formance”.

Ideally a cell should have a large best server area and a small 2nd or 3rd best server area. If all cells follow that rule we will indirectly get good SHO percentage figures for the network.

We shall then introduce some cell based statistics:

Defining the expression “In_SHO_not_best_server(i)” as the area where the cell (cell_i) is not best server but may be eligible for inclusion in the active set:

A performance indicator will be the ratio of this area over its best server coverage area:

this a similar metrics will be implemented in the post-processing tool.

The worst cells will be those to be found to have the highest SHO_perf(i) and will be the top priority cells to be investigated. After double-checking on a map dis-play where the cell is best server and “In_SHO_not_best_server”, we can trigger optimisation action.

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6.5.2 Method with a trace mobile

A good analysis of SHO areas with a trace mobile first requires that all missing neigh-bour issues have been cleared.

Then displaying all areas in SHO is straightforward, simply logging in-call where

SHO <-> nb_cells_in_active_set > 1

The rest of the analysis is exactly similar to the method using a scanner.

6.6 SHO Area Optimisation Summary

Both a scanner or trace mobile can provide relevant results in terms of SHO areas. Note however that a planning tool will be required for simulating possible optimisation actions.

Optimised SHO areas will provide a higher capacity from WCDMA network. SHO area optim-isation should not be an issue before the network reaches a significant load. However SHO optimisation may require antenna changes and should therefore be considered before quality degradation is experienced on the network.

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7 PILOT POLLUTION OPTIMISATIONThe aim of this section is to provide details of how to analyse and optimise pilot pollution from drive test measurements.

7.1 Definition of Pilot Pollution

Confusion is often made between pilot pollution and missing neighbours or number of cells in the active set.

Generally the meaning of pilot pollution is that an excessive number of scrambling codes are received within a certain area leading to degradation of downlink quality on the best serving cell.

Pilot pollution is a form of downlink interference and we shall define pilot pollution by the fol-lowing condition:

Best server CPICH_Ec is “good”

And

Best server CPICH EcIo is “bad”

Caution: this formula can be applied as it stands for measurements from a scanner, how-ever if measurements from a trace mobile are used, first make sure that no neighbours are missing otherwise a bad CPICH EcIo value could be due to a missing neighbour rather than pilot pollution.

7.2 Problems caused by Pilot Pollution

Pilot pollution leads mainly to downlink quality problems, similar to those experienced from missing neighbours but not for the same reason.

In the downlink two effects can occur

1: Increased DL interference and cell capacity loss

2: Call drop because of excessive DL interference

Note however that in the case of an unloaded network, only level 1 should be experienced.

Call drops should only occur when the load increases.

7.3 Distinction between coverage and interference problems

Before trying to optimise areas with poor quality (Ec/Io) it is important to clearly distinguish problems of coverage from problems of interference.

Bad quality can be directly related to CPICH Ec/Io levels. However when the coverage (Ec) is poor the CPICH Ec/Io value naturally decreases, even for the single cell case. The reason being, that at cell edge the CPICH levels is less than the noise floor of the UE. In this case coverage rather than interference may be blamed for the poor quality.

This is why a single Ec/Io threshold is not enough to identify the problem as being interfer -ence. An extra Ec threshold is added so that we focus only on areas where coverage is not the major issue.

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7.4 Practical thresholds for pilot pollution

The current recommended thresholds for an unloaded network are:

A pilot pollution area is defined by:

Best server CPICH_Ec > -100 dBm

And

Best server CPICH Ec/Io < -10 dB

Below –10 dB Ec/Io, call quality is not guaranteed.

Note that these levels are defined for an unloaded network and therefore include a load factor so that the network is optimised for load. When the load increases less aggressive thresholds will be preferred (lower CPICH_Ec/Io thresholds depending on the load).

7.5 Pilot Pollution Optimisation

7.5.1 Pilot Pollution Optimisation Target

In order to tackle pilot pollution, we need to improve CPICH Ec/Io of the best server to –10dB or above.

This can be done be either:

Improving the best server Ec level,

OR

Reducing the Ec level (a fraction of Io) from some of the interferers.

7.5.2 Generic method

As defined previously we should either improve the best server or reduce the interfer-ers.

7.5.2.1 Pre-check

Before any kind of optimisation on pilot pollution areas, we need to check that:

1. All sites in the area are up and running properly, in other words discard any maintenance issue from fine tuning problems

2. There are no extra sites planned to come on air in the near future

3. The area is worth optimising, if the potential traffic is very low, no optim-isation maybe required

7.5.2.2 Improvement of Ec level

It may be surprising to recommend signal enhancement when tackling interfer-ence, but in some cases it is the most straightforward and easiest solution to im-plement. This method is recommended when the best server cell has an excess-ive tilt - if reducing the tilt does not create other new interference areas. Due to the risk of creating interference areas elsewhere, this solution should be handled with care and simulation should first be performed using the plan-ning tool.

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An example where this method might be useful is the case where an antenna is pointing towards a hill with downtilt of 6 degrees.

Hilly areas have a tendency for interference since line of sight is possible with many sites. In these areas pilot pollution is quite likely. Trying to tilt the antennas of all the interferers received on the hill can be an impossible task since too many cells may be involved. In this case it is much easier to clear the problem by in-creasing the Ec level from a specific site close to the hill by uptilting the antennas towards the hill.

7.5.2.3 Reduction of interferer Ec levels

The first difficulty when trying to reduce interferers is to identify the interferers in an area of pilot pollution.

A few questions need to be asked:

1. Which cells are received in the area?

2. Which cells are the dedicated serving cells of the area?

3. Which cells are the worst interferers?

The answer is not straightforward. Details on this analysis will be found in the fol-lowing sections.

7.5.3 Optimisation Actions

When the worst interferers are identified, optimisation action should be carried out in order to reduce their interference.

In order to achieve an increase or a reduction of Ec level, we need to clarify what changes we allow on the network:

Ideally we should be able to:

1) Modify the tilt of the antennas, preferably the electrical tilt if adjustable, otherwise the mechanical tilt

2) Modify the azimuth of the antenna

3) Change the antenna, in order to get a more narrow beamwidth (H65 instead of H85 for instance), to increase the fixed electrical tilt (T6 antenna instead of T0), or to get an ad-justable tilt device.

We do not recommend the modification of the CPICH power at this stage.

We could argue that reducing the CPICH power helps to reduce the interference gener-ated by a cell, however reducing the CPICH power creates other issues: a straight power reduction means that the whole cells coverage is reduced (including indoor areas close to the site), which is not advisable when coverage is an essential target.

The preferred option is to tilt the interfering antenna(s), such that the interference area from the remote site can be greatly reduced while the main coverage area of that site can remain unspoilt.

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7.6 Practical Pilot Pollution Analysis

7.6.1 Analysis with a scanner

7.6.1.1 Identify pilot pollution areas

When used with “Top N” functionality a scanner will report all decoded scrambling codes on the carrier.

Pilot pollution areas can be displayed by means of a post-processing tool based on the condition described in 7.4.

7.6.1.2 List all cells involved in the pilot pollution area

All decoded scrambling codes in the pilot pollution area should be listed.

Beware however that all the interferers will not be decoded. Since all cells are transmitting on the same frequency, the interferer themselves are interfered until their own Ec/Io gets so weak that they will not be decoded and will not be repor-ted by the scanner.

How can we then identify the interferers?

In normal conditions most important interferers should be decoded, even for a short period of time.

Extra drive dedicated to the pre-identified pilot pollution area is also an option to get more information on the interferer list. Indeed when pilot pollution is experi-enced on a short stretch of road – leading a UE to drop when the network is loaded, very few samples may be recorded. Driving side streets or repeating the drive at slower speed if possible will help decoding more scrambling codes.

Support from a planning tool is also an option, however caution should be given on the reliability of the simulation, especially on Ec/Io values, which involve differ-ences between signals. We may end up optimising a site that is not actually inter-fering in the field.

7.6.1.3 Determine the dedicated best serving cells

From a list of decoded scrambling codes we need to identify cells which are “nat-ural” best servers in the area from other cells. It might seem obvious but the issue is real.

Firstly we need to clarify how many best servers are relevant. Bearing in mind that the maximum number of cells in the active set (in soft(er) handover) is 3, we shall most of the time consider that the 3 best serving cells are not interferers. Between 2 sites it is natural to get at a certain point 3 cells at equal level, mean-ing that the CPICH Ec/Io will be at most –9 dB. This is not pilot pollution in itself.

Secondly within an area Ec levels will fluctuate and the 3 best cells will not obvi-ously remain the same.

The determination of the best servers then requires a pragmatic approach when there is a doubt on the best servers, namely:

1. First choice is to keep as best servers the 3 reported cells which are the closest to the area of pilot pollution

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2. If a remote cell appears consistently as best server, ask yourself whether this cell is absolutely necessary in the area. If not it shall be considered as an interferer rather than as a wished best server.

7.6.1.4 Determine the worst interferers

When all best servers have been identified, the remaining cells on the decoded list are the interferers. All of them should not be optimised.

Priorities can be given to the degradation they induce. Sorting the list by des-cending Ec/Io will give a priority order.

Then when selecting cells to optimise we need to look at the broad picture rather than at a local analysis.

Worst cells will be those involved with a high priority on the highest number of pi-lot pollution areas. A global analysis of a city may show that a few cells pollute large areas and create numerous pilot pollution areas. Those will be the worst in-terferers.

A useful condition will help display the interference effect of a single cell (cell_i) on a whole area:

If this condition is fulfilled it means that cell_i is not the best server but is poten-tially interfering the best server. The larger the surface the worst the interference generated by cell_i.

Such analysis can be done by means of a post-processing tool.

Finally when the worst interferers of an area have been identified, their Ec level shall be reduced as explained in 7.5.3. The main optimisation action is to perform a tilt of the interfering antenna.

The most important issue when tilting antennas will be not to reduce the useful coverage. A peculiarity of UMTS will be that by tilting cell_i we may paradoxically end up creating new pilot pollution areas. This is why special care shall be given when tilting antennas not to reduce useful coverage.

7.6.2 Analysis with a trace mobile

The general method with a trace mobile is the same as with a scanner, yet some further issues will arise.

The main difference is that the trace mobile is affected by missing neighbours. A degraded Ec/Io value may be caused by a missing neighbour rather than pilot pollution. It is therefore necessary to sort out any missing neighbour problems be-fore studying pilot pollution effects.

Note that with a scanner such a doubt will not occur: when used in “Top N” mode the scanner decodes all possible scrambling codes regardless of neighbour de-clarations.

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The trace mobile only reports Ec/Io levels for cells within the monitored set. This set is the list of declared neighbours for the cells within the active set. If the strongest cell has been omitted in the neighbour declaration, the best reported cell will not be the strongest. For more details refer to the missing neighbour module.

We can stress then that the dedicated tool for pilot pollution tracking is the scan-ner, not the trace mobile. If only the latter is available though, make sure before analysing that no missing neighbours remain.

7.7 Pilot Pollution Optimisation Summary

The scanner is the required tool for pilot pollution analysis.

Pilot pollution areas can be easily displayed but may be quite difficult to optimise, and will re-quired antenna modifications - mainly tilts.

Note that as long as the network load is low, the main source of downlink interference will be missing neighbours and only few quality problems are expected due to pilot pollution.

However, interferences will arise quickly with load, therefore pilot pollution areas will require pre-emptive actions before the network quality rapidly degrades.

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8 OVERALL OPTIMISATION PROCESSGiven the above considerations to be made when optimising a WCDMA network, it is now possible to specify the steps to be taken to optimise the network and in what order of priority the above con-siderations should be made.

8.1 Site Verification and Cluster Validation

WCDMA optimisation should be a two stage process

1) Site Verification

2) Cluster Optimisation

As can be seen from the table below, very little network optimisation can be done at the Site Verification stage and because of the nature of WCDMA the majority of optimisation must be performed at a “cluster” level.

Analysis Single cell Cluster of cells

Coverage Yes Yes

Missing Neighbour No Yes

Pilot Pollution No Yes

SHO Area No Yes

The problem with cluster validation is that it is just that, it optimises the cluster, add a new cell and we have a new cluster and therefore further cluster optimisation is required. Unlike GSM, in WCDMA a adding a new un-optimised cell to an already optimised cluster could have a negative effect not only to calls on the new cell, but also to calls in all surrounding cells. Therefore cluster validation should be performed ideally when all cells in that cluster have been built and are operational.

Obviously because of build/integration problems, for some clusters it may not be possible for all sites to be on air for cluster validation. In this case it is up to IE/Network Optimisation to make a judgement on how to proceed. The options would be

1) Optimise the initial cluster and then re-optimise the cluster every time a new site is integ-rated into the cluster. (Labour Intensive)

2) Optimise the initial cluster and then integrate other sites, keeping the new sites locked down (no RF transmission) until the cluster is complete and then bring all the remaining sites on air and perform the final cluster validation. (Delay to overall cluster completion, reduction in number of cluster optimisations)

It must be remembered at all time that a new site transmitting in an already optimised cluster can cause massive problems. Once new sites are integrated and on air in “live” areas cluster optimisation should be performed as soon as possible to avoid significant network degrada-tion.

Regarding the typical number of sites to be included in a cluster, around 20 is the current re-commendation by Access Network Design, but this number will be reviewed as feedback comes in from cluster optimisations across the network.

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8.2 Step 1 – Site Verification

The process to be followed for site verification is given in Plan 413. This verification should allow the Engineer to verify that the cell is transmitting, takes calls. Whilst the document sug-gests that the Engineer attempts SHO, it is unlikely at this stage that the necessary adjacen-cies have been created at the OMC. Therefore the only realistic optimisation that can be per-formed at this stage is Coverage Verification/Optimisation.

8.3 Step 2 – Cluster Optimisation

The overall aim of cluster optimisation is as follows;

1) Make sure the network provides the coverage required by marketing to users within the cluster

2) Make sure that the users experience a high quality of service within the cluster (call setup, call success, handover success etc.)

3) Make sure that the network is operating optimally and that the maximum achievable capacity is obtained from the network

Points 1&2 are obvious and as we know from our GSM experience these are benchmarks that will be used to compare us to the competition.

Point 3 is new, unlike in GSM, in WCDMA it is not just the number of channel elements (WCDMA equivalent of TRXs) that dictate the site capacity but also how the network is op-timised. Adding more and more channel elements to a WCDMA site will not improve the ca-pacity of the site if the bottleneck is the radio interface because of a poorly optimised net-work. Therefore whilst point 3 may not be important for launch in the medium to longer term it will be point 3 which goes a long way towards improving the profitability of Orange’s WCDMA network.

Therefore we can rank our optimisation aims in the following order

1) Provide the user with the coverage required by marketing (there’s no point optimising further if there’s no coverage).

2) Provide the user the required quality of service

3) Extract maximum capacity from the network

It therefore follows that cluster optimisation should be carried out in the following order of pri -ority

1) Coverage Optimisation

2) Missing Neighbour Optimisation

3) Pilot Pollution Optimisation

4) Soft Handover Optimisation

Steps 1&2 provide the coverage and quality of service, whilst steps 3&4 provide the capacity optimisation. Clearly for launch steps 1&2 are the most critical and these must be performed for all areas before launch. Steps 3&4 optimise the network to avoid dropped calls when loaded and to improve capacity and are therefore not necessarily required for launch but should be performed at some stage after launch to delay the time when the network be-comes capacity limited.

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8.4 WCDMA vs. GSM

The optimisation of the GSM network is still Orange’s no.1 priority and will be for the foresee-able future. Fortunately steps 1 & 2 of cluster optimisation should not require too much downtilt adjustment (3&4 are where downtilt becomes key and by this point the WCDMA net-work should be taking enough traffic to justify optimisation). Obviously there will be areas where 3G coverage could be improved by downtilt adjustment, however this should only be done when it can be shown that this will not degrade GSM coverage/performance.

Therefore we can state that the initial optimisation set out in this document for launch of Or-ange’s WCDMA network should have not degrade, and may improve the GSM network. In fact to back this up, call and handover success rates on the experimental network are now >90% without a single antenna downtilt being adjusted. Missing Neighbour optimisation was where the majority of the optimisation effort in Bristol was spent and it is expected that this will also be the case across the rest of the network.

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9 SUMMARYThis document has summarised the key optimisation principle for WCDMA. It began by looking at WCDMA basics and then considered Coverage Optimisation, Missing Neighbour Optimisation, Soft Handover Area Optimisation and finally Pilot Pollution Optimisation. The document then introduced the concept of cluster optimisation and stated the order in which these different types of optimisa-tions should be performed on the clusters. Finally the document clarified the position of WCDMA optimisation with respect to GSM.

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