guideline for network design and optimization

5/23/2018 Guideline for Network Design and Optimization

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Guidelines forNetwork Design and

Optimization



Guideline for Network Design and Optimization

Page 2

CONTENTS

1 OBJECTIVE............................................................................................................................6

2 SUMMARY..............................................................................................................................7

2.1 REVIEW STRUCTURE..........................................................................................................7 2.1.1 Performance Review.................................................................................................72.1.2 Network Design and Dimensioning Review.............................................................7

2.2 NETWORK PERFORMANCE AND DESIGN REVIEW PHILOSOPHY ........................................7 2.2.1 Network Performance Audit.....................................................................................72.2.2 Network Design and Dimensioning Review.............................................................8

3 NETWORK PERFORMANCE REVIEW .......................................................................... 93.1 OMCSTATISTICS REVIEW ................................................................................................9

3.1.1 Call Success Rate......................................................................................................93.1.2 Call Setup Success Rate..........................................................................................113.1.3 SDCCH RF Loss .................................................................................... .................143.1.4 TCH Blocking..........................................................................................................153.1.5 TCH Assignment Failure (RF) ...............................................................................163.1.6 SDCCH Access Performance ................................................................................. 17

3.1.6.1 SDCCH Blocking .............................................................................................................173.1.6.2 SDCCH Access Success Rate ..........................................................................................18

3.1.7 Dropped Calls.........................................................................................................193.1.7.1 Call Drop Rate .................................................................................................................. 193.1.7.2 Mean Time Between Drops (MTBD) ..............................................................................213.1.7.3 Breakdown of Drop Call Reasons....................................................................................22

3.1.8 Handovers...............................................................................................................233.1.8.1 Intra-BSS Handover Failures ...........................................................................................233.1.8.2 Inter-BSS Handover Failures ...........................................................................................243.1.8.3 Handover Causes .............................................................................................................. 25

3.2 A-INTERFACE ANALYSIS .................................................................................................27 3.2.1 Call Setup Failures .......................................................................................... .......273.2.2 Location Update Success Rate ...............................................................................293.2.3 Handover Causes....................................................................................................31

3.3 CALL TRACE ANALYSIS...................................................................................................32 3.3.1 Downlink Receive Level and BTS Power ...............................................................323.3.2 Uplink Receive Level and Mobile Transmit Power ...............................................343.3.3 Uplink and Downlink RxQual Distributions ..........................................................35

4 DRIVE TEST ANALYSIS................................................................................................... 37

4.1 DRIVE TEST PROCESS ......................................................................................................38 4.2 GSMDRIVE TEST METRICS ............................................................................................38

4.2.1 Graphical Presentation ..........................................................................................384.2.1.1 Route Plots........................................................................................................................ 384.2.1.2 Events ...............................................................................................................................39

4.2.2 Statistical Analysis..................................................................................................404.2.2.1 RxLev Distribution: ..........................................................................................................404.2.2.2 RxQual Distribution: ........................................................................................................ 404.2.2.3 FER Distribution: ............................................................................................................. 414.2.2.4 MS TX Power:.................................................................................................................. 414.2.2.5 Access Failure Rate (1-Call Setup Success Rate):...........................................................42




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4.2.2.6 Blocked Call Rate:............................................................................................................424.2.2.7 Call Drop Rate:................................................................................................................. 434.2.2.8 Handover Failure Rate: ....................................................................................................434.2.2.9 Average SQI: .................................................................................................................... 44

4.3 GPRSDRIVE TEST ..........................................................................................................44 4.3.1 Graphical Presentation ..........................................................................................44

4.3.1.1 Route Plots........................................................................................................................ 444.3.1.2 Events ...............................................................................................................................45

4.4 NETWORK PERFORMANCE REVIEW -SUMMARY.............................................................45

5 NETWORK DESIGN AND DIMENSIONING REVIEW...............................................46

5.1 NETWORK DESIGN SUMMARY.........................................................................................46 5.1.1 Size ..........................................................................................................................465.1.2 Subscribers..............................................................................................................46 5.1.3 Description of the environment ..............................................................................465.1.4 Available Spectrum.................................................................................................46

5.2 RFDESIGN DETAILED ANALYSIS ....................................................................................47 5.2.1 Site Design ............................................................................... ...............................47

5.2.1.1 Network Growth Pattern ..................................................................................................475.2.1.2 High Sites Replacement ...................................................................................................475.2.1.3 RF Design Strategy ..........................................................................................................48

5.2.2 Traffic Distribution.................................................................................................485.2.3 Frequency Plan.......................................................................................................49

5.2.3.1 Site design.........................................................................................................................495.2.3.2 Terrain and Topography...................................................................................................495.2.3.3 External Interference ........................................................................................................ 495.2.3.4 BCCH Plan .......................................................................................................................495.2.3.5 Non-BCCH Plan............................................................................................................... 50

5.3 OPTIMISING FOR GROWTH...............................................................................................51 5.3.1 Synthesizer Frequency Hopping (SFH)..................................................................52

5.3.1.1 Hopping spectrum allocation ........................................................................................... 525.3.1.2 Choice of SFH Design...................................................................................................... 525.3.1.3 Hopping System Parameters ............................................................................................ 52

5.3.2

Baseband Frequency Hopping and Multiple Re-use Patterns (MRP) ..................535.3.3 Downlink Power Control and DTX........................................................................53

5.3.4 Microcell Traffic Management Algorithms............................................................535.3.5 Dual Band Traffic Management Algorithms..........................................................54

5.4 THENETWORK GROWTH PLANNING PROCESS................................................................54 5.5 BSSDATABASE REVIEW .................................................................................................55

5.5.1 Radio Resource Timers...........................................................................................555.5.1.1 rr_t3111 (layer 2 channel release guard timer) =>1200ms.............................................. 555.5.1.2 rr_t3212 (Periodic Location Update Timer) => Align With MSC Implicit Detach Timer 555.5.1.3 link_fail => 16 SACCH....................................................................................................565.5.1.4 radio_link_timeout => 16 SACCH .................................................................................. 565.5.1.5 rr_t3109 (TCH Reallocation Timer) => 8000ms.............................................................565.5.1.6 rr_t3103 (Intra-BSS Handover Guard Timer) => 15000ms ............................................ 565.5.1.7 bssmap_t10 (Assignment Guard Timer) => 14000 ......................................................... 575.5.1.8

bssmap_t8 ( Handover Guard Timer) => 14000.............................................................. 57

5.5.2 Handover and Power Control Parameters ............................................................575.5.2.1 RxQual Handovers: ..........................................................................................................575.5.2.2 RxLev Handovers:............................................................................................................ 585.5.2.3 Uplink Power Control: ..................................................................................................... 585.5.2.4 MS Fast Power Down: ..................................................................................................... 585.5.2.5 Downlink Power Control: ................................................................................................595.5.2.6 Adaptive Handover:.......................................................................................................... 595.5.2.7 Adaptive Power Control:.................................................................................................. 595.5.2.8 Directed Retry and Intelligent Directed Retry (Handover on Congestion):.................... 59

5.6 LOCATION AREA PLANNING AND PAGING PERFORMANCE .............................................60




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5.7 SYSTEM PROCESSOR PERFORMANCE...............................................................................62 5.8 MTLPERFORMANCE .......................................................................................................62 5.9 ADDITIONAL BSSDESIGN ISSUES ...................................................................................63

5.9.1 Hardware configurations........................................................................................635.9.2 Transmit Combining Options ..................................................................... ............635.9.3 Antenna Selection ...................................................................................................635.9.4 Diversity Choice......................................................................................................63

5.10 BSSOPERATIONS REVIEW ..............................................................................................64 5.10.1 Frequently Occurring Alarms ................................................................................645.10.2 Frequency of Outages.............................................................................................645.10.3 Transmit Power Calibration...................................................................................645.10.4 External Alarms ......................................................................................................645.10.5 Maintenance Schedules ..........................................................................................64

6 RECOMMENDATIONS......................................................................................................65




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REVISION HISTORY

Revision Date Name Comments




Page 6

1 OBJECTIVE

The purpose of this document is to describe the process of conducting a NetworkPerformance Audit and Design Review. The process is biased towards GSM networks,

including GPRS, but can also be applied to other technologies. The purpose of such an

audit is to assess the performance of a network using the full range of available data,and identify aspects of the design and operation of the network that can be improved.

An audit will typically result in a series of recommendations and an action plan fornetwork design and performance improvements, along with a process for ongoing

performance review and analysis.

Operators of GSM/GPRS networks have access to enormous amounts of performancedata from a wide range of tools and reporting mechanisms available to them. The aim of

a performance audit is to focus on those key metrics which are most useful in measuring

system performance and to make efficient use of the tools and large quantities of data

available.




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2 SUMMARY

The Network Audit process will be described according to the following outlinestructure:

2.1 Review Structure

2.1.1 Performance Review

Network Performance statistics review (OMC)

Call Trace Analysis

A-Interface performance statistical analysis

Alarms and Events

GSM Performance Drive Test

GPRS Performance Drive Test

Competitive Drive Test Benchmarking

2.1.2 Network Design and Dimensioning Review

RF Planning Tools, map data and model calibration

Link Budgets

Design strategy and spectrum utilisation (dual band, multi-layer, etc.)

System Dimensioning and Expansion Strategy

Frequency planning, including frequency hopping

GPRS Design Strategy

2.2 Network Performance and Design ReviewPhilosophy

The Network Audit and design review is intended to be the starting point for a network

improvement programme. The purpose of the audit is to identify as many network

design, optimisation and maintenance issues as possible and to allow a logical andmethodical action plan to be generated from the results and recommendations.

2.2.1 Network Performance Audi t

The performance Review is not intended to provide all the answers to all the problems,

but to highlight the major issues and provide all the necessary background for furtheranalysis, investigation and in-depth troubleshooting of the major performance-




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impacting problems in the network. It is important that any network performance audit

should follow a methodical process and should be systematic in its approach to datacollection. For each of the performance category headings in the outline structure, the

following logical process is applied:

Objective:What parameter are we trying to measure?

Description: Why are we measuring it and what is the relevance of themeasurement to network performance?

Report Format:How should the measurement be presented, in what kind of graphand what format?

Interpretation:What are the possible conclusions we can draw from the results?

Recommendations: Based on our observations and conclusions, what

recommendations can we make for solving the problem or for further investigation?

2.2.2 Network Design and Dimensioning Review

The Network Design Review draws on the conclusions and findings from the Network

Performance Audit. These findings help to guide the auditor towards the aspects of thenetwork design requiring the most attention. Similarly to the Network Performance

Audit, the following logical process is then applied:

Objective:What design parameter (or set of parameters) are we reviewing?

Description:Why are we reviewing it and what is the relevance of the parameter

(or set of parameters) to network functionality and performance?

Format:How should the design data be presented to allow us to effectively reviewit?

Conclusions: How does the observed design practice compare with known bestpractices, and what conclusions can we draw?

Recommendations: Based on our observations and conclusions, whatrecommendations can we make for improvements to the network design and/or

design process?




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3 NETWORK PERFORMANCE REVIEW

The network Performance Review aims to make use of all the commonly available datasources, presented in such a way as to extract as much useful information as possible

and quickly identify network performance problems. The choice of tools used to create

the required reports is not critical, and may vary according to the network operatorand/or network equipment vendor. The format of the reports presented in this document

is generic.

AIRCOM International Performance and Benchmarking tools can be used for many

elements of the performance review. Application notes covering the use of AIRCOMtools for this purpose are available separately.

3.1 OMC Statist ics Review

Key performance metrics required to assess network performance are presented in thefollowing sections.

3.1.1 Call Success Rate

Objective:

To determine the percentage of calls which are successfully set up and which are

terminated normally (ie. do not drop).

Description:

Call Success Rate is a good overall indicator of network health. It combines call setup

success rate and drop call rate into one single figure, and is generally calculated fromthe following formula:

Call Success Rate = Call Setup Success Rate x (1- Call Drop Rate)

Format 1:

Call Success Rate is usually studied for the whole network, to give an overall indicationof network health. It is useful to observe changes in Call Success Rate over time, and

also to display along with traffic data to observe the relationship of Call Success Ratewith network loading.




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Conclusions:

Call Success Rate in itself does not allow any detailed conclusions to be drawn. Poor

Call Success Rate requires further investigation of Call Setup Success Rate and CallDrop Rate as described in the following sections.

Format 2:

Calculate Call Failure Rate (1-Call Success Rate), and show the separate components ofcall failure rate

Objective:

To determine the contribution of dropped calls and call setup failures to the total callfailures figure. It is useful to observe Call Failure Rate on a per-BSC basis, and to see

the separate contribution of call drops and call setup failures to the total figure.




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3.1.2 Call Setup Success Rate

Objective:

To determine the proportion of call attempts that result in a successful call completion

(ie. successful call setup), and to identify and quantify the individual reasons for call

setup failure.

Description:

Call setup failures can occur for a number of reasons. It is important to identify the

causes and determine the origin of call setup failures. There are various ways toaccomplish this through statistical analysis as described below.

Call setup failures can be categorised as follows:

Failure before assignment (SDCCH RF loss, MSC service rejection, user clearing,MSC clearing)

Blocked TCH Assignment (Insufficient TCH resources)

Failed Assignment (Failure to assign to TCH due to RF reasons, eg. Interference)

Format 1 (Failures per BSC):

Calculate Call Setup Failure Rate per BSC, and show individual failure categories ascomponents of the overall figure, as described above. In generic terms, the individual

failure categories are calculated as follows:




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Failed Assignments (Blocking):

TCH Blocking statistic (TCH allocation commands blocked due to lack of Radio

Resources).Failed Assignments (RF):

Allocation requests from MSC Allocation commands blocked Allocations

completed

Failed Call Setups before Assignment:

Total Call Setup Failures Failed Assignments (Blocking) Failed Assignments (RF)

Note: Call setup failures before assignment further analysis

The category of call setup failures before assignment can be further subdivided into its

component failure reasons. To do this accurately requires access to MSC statistics, orthe collection of A-Interface logs using a protocol analyser such as K1103/K1205. This

is described in a later section.




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Format 2: Worst Ten Cells

Having identified the worst BSCs, call setup failures can be presented for the worst 10

cells per BSC. This helps to focus on the cells causing the greatest impact to the callsetup success rate. Cells known to carry very low traffic should be discounted, for

example cells inside conference centres while not in use, cells on remote highways, etc.

This analysis should also be performed for cells whose performance is known to becritical (eg. Those cells covering important VIP areas, or important routes).

Causes of call setup failure for each poorly performing cell can then be identified and

analysed. Failure causes that may be easily analysed from BSS statistics are:

SDCCH RF Loss (call setup failure before assignment)

TCH Assignment Failure (Blocking)

TCH Assignment Failure (RF)




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3.1.3 SDCCH RF Loss

Objective:

To determine the proportion of allocated SDCCHs which are dropped due to RF

reasons.

Description:

SDCCHs are used in a large number of transactions, including call setup, location

update, SMS, and so on. High SDCCH RF loss is not only a cause of poor call setup

success rate, but also poor location area update success rate, IMSI Attach/Detachsuccess rate, etc.

Format:

Display the worst 10-20 cells with highest SDCCH RF Loss Rate.

Interpretation:

High SDCCH RF Loss is generally caused by one of the following problems:

Interference on SDCCH carriers, poor frequency plan or external interference.

Poor coverage, many mobiles at the coverage boundary.

Hardware problems (Poor link balance, poor calibration, radio failure)

Recommendations:

Each cell identified with high SDCCH RF Loss should be investigated according to thepossible problems shown above.




Page 15

3.1.4 TCH Blocking

Objective:

To determine the proportion of attempts by the BSS to allocate a TCH that are blocked

due to lack of available TCH resources.

Description:

TCH blocking impacts call setup success rate, and also handover success rate since

TCH resources are required to accept incoming handovers. High TCH Blocking is

generally an indication of insufficient capacity in the network (or part of the network).

Format:

Display the worst 10-20 cells with the highest TCH blocking figures.

Interpretation:

High TCH Blocking is usually caused by one of the following conditions:

Cell requires expansion (sometimes not possible due to frequency plan constraints)

Unusual traffic conditions (traffic jam, exhibition, holiday traffic, etc.)

Cell coverage area too large (coverage optimisation required)

Poor traffic management between cell layers (eg. Between macro and micro layers,or between 900 and 1800 carrier layers in dual band systems)

Surrounding cells temporarily off-air

Failure of one or more radio carriers in the cell, causing remaining carrier(s) tobecome overloaded.




Page 16

Recommendations:

Cells with high TCH blocking should be investigated according to the above possible

causes. Often there will be a combination of issues resulting in TCH blocking in anetwork, all of which must be tackled for a complete solution. Optimisation of network

design for maximum capacity is a complex process requiring the input of many more

design parameters. This process will be discussed in the Network Design and

Dimensioning Review section.

3.1.5 TCH Ass ignment Failure (RF)

Objective:

To quantify the proportion of allocated TCH channels that are unable to be successfully

accessed by a mobile.

Description:

TCH assignment failure refers to the case in which the BSS has allocated a controlchannel (SDCCH), MSC has assigned a circuit, and the BSS has allocated a traffic

channel (TCH). However for some reason the mobile has been unable to complete the

call setup on the allocated traffic channel. This is generally caused by interference-related problems on the traffic channel carriers.

Format:

Display the worst 10-20 cells with highest TCH Assignment Failure Rate. As discussed,this can be calculated generically as follows:

TCH Assignment Failures (RF) = Allocation requests from MSC Allocation

commands blocked Successful Allocations

TOP 20 TCH ASSIGNMENT FAILURE RATE

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

5010

8-17

01-R

epm

5010

8-17

27-R

epm2

1110

5-67

18-T

rim4

1310

4-65

82-P

vg1

1110

5-16

36-Ja

n1

1010

4-64

35-B

ur1

1110

5-16

173-A

sta2

1010

4-64

53-A

vu2

1310

4-60

59-P

org1

5010

8-67

37-A

lamm

1110

5-11

73-E

st1

1110

5-65

27-M

mo2

5010

8-16

102-C

du2

5010

8-11

56-P

es1

1110

5-16

37-Ja

n2

5010

8-68

93-Lb

t3

1110

5-16

33-F

ot2

1310

4-16

351-A

bej2

1310

4-11

18-A

lv

5010

8-91

82-A

vl3D

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

TCH_assignment_

failure_rate

Call Vol




Page 17

Interpretation:

High TCH Assignment Failure Rate (RF) is generally caused by the following

conditions:

Interference on TCH carriers due to poor frequency plan, or external interference.

Antennas too high, resulting in excessive uplink interference.

Poor coverage (many mobiles on coverage boundary)

Hardware problem (poor link balance, poor calibration)

Recommendations:

Cells with high TCH Assignment Failure Rate (RF) should be investigated according to

the possible causes shown above.

3.1.6 SDCCH Access Performance

3.1.6.1 SDCCH Blocking

Objective:

To determine the proportion of SDCCH allocation attempts that are blocked due to a

lack of available SDCCH resources.

Description:

Some equipment vendors consider blocking on the SDCCH channels to be a componentof Call Setup Failure Rate, while others do not. Regardless of this, SDCCH Blocking

results in the failure of mobiles to access the network for a number of actions, such as

call setup, location update, IMSI attach/detach, etc.

Format:

Display the worst 10-20 cells with highest SDCCH Blocking Rate. This is usually

available as a statistic from the OMC.




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Interpretation:

Increase in traffic requires expansion of SDCCH Resources Cell coverage area too large, too many Phantom RACHs (also related to poor

SDCCH Access Success Rate Section 2.3.3.2)

Poor Location Area border planning (too many location updates)

Inappropriate timer settings in BSS database (eg. Periodic location update timer tooshort)

Interference, causing SDCCH holding time to increase

Recommendations:

Each cell with high SDCCH Blocking should be analysed according to the above

possible causes. Poor location area border planning is frequently a cause of SDCCH

resource problems, especially in difficult RF environments such as coastlines, bays,cities built on rivers, and so on. SDCCH resources can simply be increased to carry

excessive SDCCH traffic due to poor planning, but this in turn reduces available TCH

resources and may result in TCH blocking, and is an inefficient use of network

infrastructure.

3.1.6.2 SDCCH Access Success Rate

Objective:

To determine the proportion of allocated RACHs (Random Access Channels)

successfully accessed by mobiles.

Description:

Some RACHs received and decoded by the BSS are from distant mobiles, spurious

emissions resembling RACHs, and so on (sometimes referred to as phantom

RACHs), and will result in a SDCCH assignment which cannot be successfully

accessed by any mobile. After the expiry of BSS timers the SDCCH resources are de-

allocated and returned to the radio resource pool, but excessive allocation of SDCCHresources to Phantom RACHs results in a waste of SDCCH resources and contributes

to SDCCH blocking.

Format:

Display the worst 10-20 cells with lowest SDCCH Access Success Rate. This is

gererally available as a statistic reported in the OMC, but can also be calculated from

raw statistics.




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Interpretation:

Cell coverage area too large, receiving uplink interference from distant mobiles.

External uplink interference (eg. 900MHz cordless telephones in GSM uplinkchannels 50-55)

Antennas too high and/or inappropriate vertical beamwidth, and/or not properlyoriented.

Hardware problem (eg. poor link balance)

Recommendations:

Cells with poor SDCCH access success rate should be analysed according to the abovepossible causes. Experience shows that all of these causes occur frequently, although

the most fundamental cause is poor RF planning and poor antenna location. This can

generally be remedied by antenna optimisation of some kind, such as relocating into a

less prominent place or making use of building structures to shield the antenna fromunwanted interference.

3.1.7 Dropped Calls

A call that suffers abnormal termination is termed a dropped call. Dropped calls occur

for a multitude of reasons, many of which can be quantified through statistical analysis.

3.1.7.1 Call Drop Rate

Objective:

To quantify the proportion of successful call set-ups that subsequently suffer abnormal

termination.




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Description:

Calls usually drop as a result of a failure to maintain communication over the air

interface. This can be due to interference, mobile moving out of range of the cell,mobile moving indoors, handover failure, mobile battery failure, mobile hardware

problem, BSS hardware problem, and so on. Call Drop Rate is usually a good indication

of overall network performance, speech quality and data throughput.

Drop calls can also arise due to a failure in communication on any of the interfaces (and

subsequent expiry of timers on the air interface), although experience suggests airinterface failure is the most usual cause.

Drop Call Rate is calculated with the following generic formula:

TCH RF Losses + Handover Failures (RF Loss)

Total call setups + Incoming Handovers

Format:

Display the worst 10-20 cells with highest Drop Call Rate. This is generally available asa statistic reported in the OMC, but can also be calculated from raw statistics. Drop Call

Rate is also sometimes calculated per BSC to help identify the worst performing BSCs

or worst performing regions of a network.

Cells with very low call volume should normally be discounted or treated with a lower

priority.

Interpretation:

Interference due to poor frequency plan

Interference due to poor site design, high sites, inappropriate antenna selection, etc.

Poor quality and call drops due to overloaded frequency hopping carriers




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Insufficient coverage (indoor or outdoor)

Poorly optimised coverage areas causing handover problems

Poorly optimised neighbour lists

Traffic congestion leading to cell dragging (handover delayed due to lack of TCHresources at target cell) and call drops.

Hardware problem (eg. Poor link balance, radio failure)

Recommendations:

Cells suffering from bad call drop rate should be analysed according to the above

possible causes. The problems causing high drop call rate are many and varied, and aregenerally related to a number of other symptoms of poor performance, eg. Poor call

setup success rate, TCH blocking, hardware problems etc.

Action plans to address poor call drop performance will probably be developed in

conjunction with other performance initiatives for improving call setup, TCH blockingand so on.

3.1.7.2 Mean Time Between Drops (MTBD)

Objective:

To determine the average time duration between call drops.

Description:

This is usually calculated as the ratio of number of call drops to total TCH usage timeduring a given interval. This is a useful measure often preferred by network operators as

it gives a better indication of actual user perception compared to Drop Call Rate. The

Drop Call Rate figure can be influenced by other factors such as incoming handovers

(eg. If the number of incoming handovers to a cell increases, the drop call rate ratiodecreases, while MTBD remains the same).

Format:

Show the worst 10-20 cells for highest MTBD.




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Interpretation:

Same as for Call Drop Rate.Recommendations:

Same as for Call Drop Rate.

3.1.7.3 Breakdown of Drop Call Reasons

Objective:

To break down and quantify the different reasons for dropped calls.

Description

Generally speaking, dropped calls can be divided into 2 distinct categories; TCH RF

Losses and Handover Failures. It is useful to understand the contribution of these two

categories to the total drop call rate as this assists troubleshooting.Format:

Display the worst 10-20 cells with highest Drop Call Rate, showing contributions of

TCH RF Loss and Handover separately.

Note: Handover Failure in this case specifically means handover failures that result in a

dropped call (Handover_Fail_DROP). Some equipment manufacturers count handover

failures that do not drop but in fact re-establish again on the originating cell(Handover_Fail_RETURN). Make the distinction between Handover_Fail_DROP and

Handover_Fail_RETURN, and count only Handover_Fail_DROP.

Interpretation

Reasons for high TCH RF Loss rate are the same as for Call Drop Rate. High handover

failure rate can also be attributed to other handover-specific reasons:

Insufficient coverage at handover boundary

TCH RF Loss and Handover Failures Combined

0

2

4

6

8

10

12

cell01 cell02 cell03 cell04 cell05 cell06 cell07 cell08 cell09 cell10

Cell ID

Failure%

HO_FA IL_LOST_M S

TCH RF Loss




Page 23

Handover parameters incorrectly set

Neighbours incorrectly defined

Recommendations:The same as for Drop Call Rate. Also examine handover boundaries between cells with

high Handover Failure Rate. Especially inter-BSC and inter-MSC handover boundariesneed larger overlaps as the handover process takes longer than the intra-BSC case.

3.1.8 Handovers

Failures can often occur in GSM during the handover process. There are several types

of handovers (intra-cell, intra-BSS, inter-BSS, inter-MSC). It is helpful to consider

these different handover types separately, especially intra-BSS and inter-BSS whichcombine to make up the majority of all handovers.

3.1.8.1 Intra-BSS Handover Failures

Objective:

To determine the proportion of Intra-BSS handover attempts that are successfullycompleted.

Description:

Intra-BSS handovers are managed by the BSC without MSC involvement. Intra-BSS

handovers taking place between cells of the same BTS site are usually synchronised,and their success rate is generally better than intra-BSS handovers between cells of

different sites.

Format:

Display the worst 10-20 cells with lowest intra-BSS Handover Success Rate.




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Interpretation:

Interference at handover boundary

Hardware problem at target cell (eg. Poor link balance, poor calibration etc.)

Traffic congestion at target cell causing delayed handover

Insufficient coverage at handover boundary



Recommendations:

Cells with poor intra-BSS handover success rate should be examined for the possible

causes as described above. Most equipment manufacturers provide per-neighbour

statistics at the OMC. These show for each of the poorly performing cells which

neighbour relationships are suffering the worst failure rate. Having established this,

individual neighbour relationships can be analysed for failure causes.

3.1.8.2 Inter-BSS Handover Failures

Objective:

To determine the proportion of Inter-BSS handover attempts that are successfully

completed.

Description:

The Inter-BSS handover process involves the MSC, and therefore requires more

complex signalling and takes more time compared to intra-BSS handovers. This tends to

result in a greater chance of the handover failing, especially for fast moving mobiles,

unless specific steps are taken in the design process to allow for larger coverageoverlaps at inter-BSS boundaries.

Format:

Display the worst 10-20 cells with lowest inter-BSS Handover Success Rate.




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Interpretation:

Insufficient coverage at handover boundary, especially for inter-BSS neighbours indifficult RF conditions (highways, hilly terrain, etc.)

Poorly defined inter-BSS boundaries causing high inter-BSS handover traffic.



Problems on inter-MSC links, in case inter-BSS handover is across a MSC border

Recommendations:

Cells with poor inter-BSS handover success rate should be examined for the possible

causes as described above. Most equipment manufacturers provide per-neighbour

statistics at the OMC. These show for each of the poorly performing cells which

neighbour relationships are suffering the worst failure rate. Having established this,individual neighbour relationships can be analysed for possible failure causes.

3.1.8.3 Handover Causes

Objective:

To determine the distribution of handover attempts according to their cause values.

Description:

As an input into the audit process, it is helpful to understand the numbers of handoverstaking place according to the different causes. This may reveal an abnormally large

proportion of handovers due to a specific handover cause, and consequently a design

problem that needs to be addressed.




Page 26

The main handover causes are:

Uplink Quality

Uplink Level

Uplink Interference

Downlink Quality

Downlink Level

Downlink Interference

Power Budget (Better Cell)

Distance (timing advance)

Congestion

Format:

The pie-chart below shows a typical distribution of handover causes, with the majority

of handovers caused by Power Budget decision.

Interpretation:

The majority of handovers taking place in a properly configured GSM system will bedue to Power Budget (Better Cell) decision. A Large proportion of quality handovers

would indicate interference problems and/or incorrect settings of quality handover

thresholds. A large proportion of level handovers would indicate coverage problems

and/or incorrect settings of level handover thresholds.

It is particularly useful to monitor changes in the distribution of handover causes while

monitoring the progress of optimisation action plans.




Page 27

3.2 A-Interface Analysis

The BSS performance statistics only refer to radio-related information, hence do not

include signalling issues between the Mobile and the MSC, which are transparent to theBSS. In order to achieve a complete understanding of call set-up failures, which must

include DTAP signalling, the A interface data must be investigated. This is typicallyachieved by taking a sample 20 Megabytes of data, using a K1205 Protocol Analyser,

from each of the BSCs underinvestigation.

Analysis of the A-Interface logs requires a post-processing tool of some kind. The

following reports can be generated from the collected data:

3.2.1 Call Setup Failures

Objective:

A-Interface analysis allows us to accurately quantify the causes of call setup failure forboth mobile-originating and mobile-terminating calls. This is more accurate than the

previous call setup analysis using BSS statistics.

Description:

It is possible to quantify the following call setup failure causes:

CM Service Reject

SDCCH RF Loss

User Initiated CM Service Abort

Set Up / Call Proceeding Losses

Blocked TCH Assignment TCH Assignment Failure

Format:

A-Interface analysis can be presented per-BSC, showing the different causes for call

setup failure. The following example shows 2 charts for the same group of BSCs, the

first showing a simplified breakdown (pre-assignment and post-assignment), and thesecond showing a more detailed breakdown of the pre-assignment failures.

The exercise should be repeated for Mobile Originating and Mobile Terminating calls.




Page 28

Interpretation:

Analysis of Mobile Originated call setup failures immediately shows that around 50%of all failures are caused by user initiated CM service abort. This is due to mobile

users dialling wrong numbers and then quickly clearing the call, accidentally pressingthe call button twice, and other such unintentional mistakes. Clearly it is not possible to

address this problem through network optimisation.




Page 29

The other failure causes give an indication of Network Health as follows:

Pre-assignment Failures:

CM Service Reject

Set Up / Call Proceeding Losses

These failures indicate problems outside the control of the BSS, such as MSC circuit

problems, routing errors, PSTN interface problems, etc.

Radio Failures:

SDCCH RF Losses

TCH Assignment Failures

Blocked TCH Assignments

These failures occur as a result of radio-related problems, as discussed in detail insection 2.3.2, such as interference, congestion, hardware failure and so on.

Recommendations:

Having established any call setup problems on a per-BSC basis, further analysis should

focus on two main areas:

Non-BSS issues affecting whole BSCs or the whole network

BSS-related issues probably due to specific cell performance issues.

An action plan addressing the main issues should be made.

3.2.2 Location Update Success Rate

Objective:

To determine the success rates of the different types of location updates.

Description:

Location updates can be categorised as follows:

Normal (moving between Location Areas)

Periodic (set by timer, usually every 4-8 hours)

IMSI Attach (Location Update when switching on and registering)

The success rates of different types of location update can be helpful in identifying

network problems.

Format:

Show location update success rates per LU type and per BSC. It may also be useful toknow the number of location updates according to LU type, as an input into the design

review process.




Page 30




Page 31

Interpretation:

Poor location update success rate is often an indication of poor RF conditions, including

interference problems and poor coverage. However a poor success rate of one particular

type of location update suggests there may be a MSC-related problem requiring furtherinvestigation.

Very large numbers of normal location updates compared to periodic and IMSI Attach

location updates could be due to small location areas with heavy traffic, or couldindicate excessive location updates due to poor location area planning.

Recommendations:

The reasons for poor location update should be investigated further, according to theabove guidelines.

3.2.3 Handover Causes

Objective:

To determine the causes of all handovers, from analysis of A-Interface logs.

Description:

The handover cause value is contained within the Handover Required message on the

A-Interface. Analysis of these messages provides a breakdown of all the handovers bycause value.

Format:

Show Handover causes per BSC in percentage terms:




Page 32

Interpretation:

Typically 70-80% of handovers will be due to Power Budget (Better Cell) decision. Alarge proportion of quality handovers would indicate interference problems and/or

incorrect settings of quality handover thresholds. A large proportion of level handovers

would indicate coverage problems and/or incorrect settings of level handover

thresholds.

A BSC covering predominantly rural areas with low cell density and large areas ofmarginal coverage will typically have a greater proportion of U/L and D/L Level

handovers (if enabled), compared to a busy BSC in an urban area with high site density.

Recommendations:

BSCs with abnormally high proportions of Level, Quality or Interference handovers

should be investigated further.

3.3 Call Trace Analysis

Analysis of call trace files provides additional information not available from BSS

statistics and A-Interface logs. Call trace data collection procedures are vendor-specific,and require vendor-specific tools for analysis and post-processing.

Call Trace refers to the collection of Measurement Reports (MRs) generated forUplink and Downlink and made available at the BSC for collection. While in Dedicated

Mode (ie. during a call), mobiles generate one MR per SACCH multiframe (approx450ms). UL and DL measurement information is then compiled at the BTS and sent to

the BSC on the A-Bis link.

Reports available from Call Trace Analysis include UL/DL RxQual distribution, UL/DL

RxLev distribution, timing advance, neighbour analysis, and so on. These may be used

to troubleshoot individual cells or carriers, or may be monitored on a per-BSC level formore general performance observations.

3.3.1 Downl ink Receive Level and BTS Power

Objective:

To observe the distribution of Downlink receive measurements on per-BSC basis and

per-Cell basis, along with BTS Transmit Power measurements

Description:The Downlink Receive level distribution gives an indication of the coverage in a cell

with respect to the distribution of actual mobiles in the cell. On a BSC level it provides

a more general indication of coverage level. The BTS Transmit Power distribution is

also related to this.

Format:

Show cumulative distributions of Downlink RxLev per BSC and per Cell, and show

BTS Transmit Power distribution per BSC and per Cell.




Page 33

Example 3.3.1a - Downlink RxLev distribution shown per BSC

Example 3.3.1b - BTS Power Distribution shown for one BSC.

Interpretation:

A large proportion of MRs reported at a very low Downlink RxLev indicates many

mobiles are operating in areas of poor coverage.




Page 34

In a similar way, a large proportion of BTS transmit power measurements at full or

nearly-full power indicates that coverage is weak and consequently the BTS istransmitting at or near full power all the time. This would be typical of a cell covering a

rural area.Recommendations:

If poor coverage is suspected, the analysis could be repeated per cell for all cells in the

BSC to establish those with the weakest coverage. This could then be an input into acoverage improvement plan.

3.3.2 Uplink Receive Level and Mobile Transmit Power

Objective:

To observe the distribution of Uplink receive measurements on per-BSC basis and per-Cell basis, along with Mobile Transmit Power measurements

Description:

The Uplink Receive level distribution gives an indication of the coverage in a cell withrespect to the distribution of actual mobiles in the cell. On a BSC level it provides a

more general indication of coverage level. The Mobile Transmit Power distribution is

also related to this.

Format:

Show cumulative distributions of Uplink RxLev per BSC and per Cell, and show

Mobile Transmit Power distribution per BSC and per Cell.

Example 3.3.2a - Uplink RxLev distribution shown per BSC




Page 35

Example 3.3.2b - BTS Power Distribution shown for three BSCs.

Interpretation:

Mobile Transmit Power should always be minimised so as to minimise uplink

interference levels and maximise system capacity. This is especially true of FrequencyHopping Systems. If the ms power distribution shows a high proportion of mobile

transmit powers at or near full power, the following conclusions could be considered:

Most of the mobiles in the cell are operating at or near the cell boundary, and hence

need to transmit full power to maintain the uplink (eg. large rural cell) There is excessive loss in the receive antennas/feeders causing a loss in sensitivity

of the base station, in turn causing the mobiles to transmit full power.

Incorrect settings of power control parameters (power window)

Poor frequency plan, excessive interference causing the mobiles to transmit higherpower.

Recommendations:

Mobile transmit power should always be minimised as far as possible. Based on theobservations, all possibilities to reduce mobile transmit power should be considered,

including any vendor-specific enhanced power control algorithms.

3.3.3 Uplink and Downlink RxQual Distribut ions

Objective:

To observe the distribution of RxQual measurements and identify cells or BSCs with

poor RxQual.




Page 36

Description:

RxQual distributions simply give an indication of the BER (Bit Error Rate) of the

received signal at the BTS and Mobile. On a per-cell basis they help to identify cellswith particular quality problems.

Format:

RxQual cumulative distributions can be shown per cell or per BSC, as follows:

Cumulative RxQual Distributions per BSC and per cell (example of poor quality cell)




Page 37

Interpretation:

A poor RxQual distribution indicates a quality problem of some sort:

Interference, poor frequency plan Poor antenna location, uplink/downlink interference problem

Poor coverage, mobiles at cell boundary

Hardware problem (poor calibration, poor link balance, radio failure)

Recommendations:

Any cell found with a poor RxQual distribution should be investigated according to the

above possible causes.

There are many other possible applications of Call Trace data, limited only by theavailability of suitable functionality in the tools provided by the vendor. Call Trace is

especially useful for fast and efficient trouble-shooting on a cell and carrier level.

4 DRIVE TEST ANALYSIS

Drive Test Performance Analysis can be carried out in addition to, or as an alternativeto, OMC statistical analysis. Drive testing has the following advantages over OMC

statistical analysis:

Drive test data is representative of the actual experience of subscribers.

Drive test allows the measurement of speech quality from the subscribersperspective

It is easy to collect drive test data for several networks simultaneously for

competitive benchmarking purposes.

Many operators do not fully trust OMC statistics as they may understand or agreewith the formulas used to derive key performance metrics. Drive test data is much

easier to understand and much harder to dispute.

The main advantages of OMC statistical data are as follows:

It is comprehensive and includes data from all subscribers, not only those onesdriving along certain pre-defined drive test routes.

It is readily available and easy to manipulate into the required report formats.

Therefore it is recommended to conduct analysis based on both drive test and OMCstatistics, and combine the results for a more complete understanding of the

performance issues in the network.




Page 38

4.1 Drive Test Process

A detailed description of the drive test process is outside the scope of this document. A

complete GPRS drive test process document is also available separately. Below is a

brief summary of the main points to be considered:

Drive test routes should be chosen to be representative of the part of the networkunder study, covering a range of different coverage areas (urban, suburban, etc.).

If possible simultaneously collect GSM speech and GPRS drive test data.

Drive test to be carried out during normal daylight hours to reflect normal networkload conditions.

Calls to be made preferably mobile-to-PSTN.

Call duration to be equal to the average call duration for the network, as derivedfrom OMC statistics. Allow 10s idle time between calls. GPRS data calls to be set

according to the average data call length for the network.

At least 1000 calls required for good statistical confidence.

4.2 GSM Drive Test Metrics

GSM drive test data can be presented in a number of ways. A combination of graphicalpresentation and statistical analysis is recommended.

The examples below show measurements for 2 networks for comparison purposes.

4.2.1 Graphical Presentation

The following parameters can be displayed on a map, allowing the visualisation of

specific problems by location:

4.2.1.1 Route Plots

RxLev Full: Route Coverage Plot

RxLev Sub: Route Coverage Plot (excluding dummy bursts during DTX operation)

RxQual Full: Route Quality Plot

RxQual Sub: Route Quality Plot (excluding dummy bursts during DTX operation)

FER: Route Frame Erasure Rate Plot

MS TX Power: Route plot of Mobile Transmit Power

SQI Plot: Route Plot of Speech Quality Index (or equivalent, if available)




Page 39

4.2.1.2 Events

Events plots may be superimposed on one of the above route plots, eg. RxLev orRxQual.

Call Drops: Plot of dropped call events

Setup Failures:Plot of call setup failure events

HO Failures: Plot of Handover Failure events

HO Success: Plot of Successful Handover events (if required)




Page 40

4.2.2 Statistical Analysis

The following set of statistics should be calculated from the collected drive test data:

4.2.2.1 RxLev Distribution:

The proportion of RxLev Measurements falling within defined ranges.

4.2.2.2 RxQual Distribution:

The proportion of RxQual Measurements falling within defined ranges.

RxLev Distribution

0

500000

1000000

1500000

2000000

2500000

3000000

3500000

4000000

4500000

0-10 11-20 21-30 31-40 41-50 51-60 61-70

RxLev

NumberofMe

asurements

Network A

Network B

RxQual Distribution

0

2000000

4000000

6000000

8000000

10000000

12000000

RxQu

al0

RxQu

al1

RxQu

al2

RxQu

al3

RxQu

al4

RxQu

al5

RxQu

al6

RxQu

al7

Number

ofMeasurements

Network A

Network B




Page 41

4.2.2.3 FER Distribution:

The proportion of Frame Erasure Rate Measurements falling within defined ranges.

4.2.2.4 MS TX Power:

The proportion of Mobile Transmit Power Measurements falling within defined ranges.

Frame Erasure Rate Distribution

0

2000000

4000000

6000000

8000000

10000000

12000000

0-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100

FER

NumberofMeasurements

Network A

Network B

Mobile Transmit Power Distribu tion

0

1000000

2000000

3000000

4000000

5000000

6000000

7000000

8000000

9000000

10000000

33 31 29 27 25 23 21 19 17 15 13

MS TX Power

Numberofmeasurements

Network A

Network B




Page 42

4.2.2.5 Access Failure Rate (1-Call Setup Success Rate):

The proportion of call setup attempts that fail.

Access Fai lure Rate %

9.8%

8.2%

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

12.0%

Network A Network B

%ofAccessAttem

pts

4.2.2.6 Blocked Call Rate:

The proportion of call attempts that fail due to lack of resources.

Blocked Calls and No Service [%]

0.3% 0.2%

9.5%

8.0%

0%

2%

4%

6%

8%

10%

12%

Network A Network B

No Service Attempts Blocked Calls




Page 43

4.2.2.7 Call Drop Rate:

The proportion of calls terminated abnormally before the end of the call.

Dropped Call Rate %

2.9%

1.3%

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

3.5%

Network A Network B

%ofCompletedCal

ls

4.2.2.8 Handover Failure Rate:

The proportion of handover attempts that fail.

Handover Summary

1176

1444

35

12

0

200

400

600

800

1000

1200

1400

1600

Network A Network B

NumberofHandover

Handover completed Handover failed




Page 44

4.2.2.9 Average SQI:

The average Speech Quality Index measured over the combined drive test route.

Average Speech Qual it y Index (SQI)

19.3 19.1

0.0

3

6

9

12

15

18

21

24

Network A Network B

SQI

4.3 GPRS Drive Test

GPRS drive test data can be presented in a number of ways, much the same as GSM

drive test data. A combination of graphical presentation and statistical analysis isrecommended.

4.3.1 Graphical Presentation

The following parameters can be displayed on a map, allowing the visualisation of

specific problems by location:

4.3.1.1 Route Plots

UL/DL RLC Throughput: Radio Link Layer data throughput

UL/DL LLC Throughput: Logical Link Layer throughput (user data)

UL/DL RLC Block Error Rate (BLER): Radio Link Block Error Rate

UL/DL RLC Retransmission Rate: Radio Link Retransmission Rate

UL/DL Coding scheme used (CS1-4): Allocated Coding Scheme

UL/DL Number of timeslots used: Allocated timeslots




Page 45

4.3.1.2 Events

Events plots may be superimposed on one of the available route plots, eg. RxLev,

RxQual, RLC throughput, etc.PDP Context Activation Failure: Failure to activate PDP Context (Packet

Data Protocol)

PDP Context Loss: Loss of PDP Context (GPRS Call Drop)

4.4 Network Performance Review - Summary

The summary of the Network Performance Review should aim to highlight the specific

performance problems identified in the network, on Network level, BSC level and Cell

level. The following headings should be included here: Network Performance Summary Data

Key Network Performance Observations

List of worst performing cells and BSCs

Detailed conclusions can be made only after completing the Network Design and

Dimensioning Review, at which time all the required information will be available

to allow detailed recommendations to be made.

Network

Name CallSu

ccessRate

CallSe

tupSuccessRate

DropC

allRate

TCHCongestion

SDCCH

AssignmentSuccessRat

CallVo

lume/traffic

HandoverSuccessRate

XYZ-net 91.70% 93.40% 1.85% 0.73% 92.10% 1244300 95.60%




Page 46

5 NETWORK DESIGN AND DIMENSIONING REVIEW

5.1 Network Design Summary

Before making any recommendations based on network performance reports it isimportant to know more about the network, and the constraints inside which the

network has been designed and is being operated.

5.1.1 Size

How big is the network? Plots from network planning tools are useful as a visual aid,

along with numerical information in spreadsheets:

MSCs

BSCs

BTSs

Cells

OMCs

HLR/VLR,

SMS Centres

5.1.2 Subscribers

Subscriber Distribution, usage and growth information:

Roughly how many subscribers distributed over the network, by area or by clutter.

Projected subscriber growth, pre-paid and fixed contract.

Traffic generated by subscriber, current and projected (typically in the range of 20-25mE per subscriber in the busy hour)

5.1.3 Descript ion of the environment

It is helpful to know about environmental factors that influence network design and

performance, such as:

Type of urban environment (typical building heights, building density, etc.)

Type of terrain (mountainous, hilly, flat, etc.)

Presence of water bodies (coastline, estuaries, rivers, lakes)

5.1.4 Available Spectrum

What spectrum is available, and how is it split between the different layers?




Page 47

The following example shows a typical allocation of GSM channels given an available

spectrum of 10MHz (50 Channels).

Dual Band (900/1800) spectrum should also be shown.

5.2 RF Design Detailed Analysis

The high level design summary provides an overview of the relevant information. Next

a more detailed analysis is required.

5.2.1 Site Design

5.2.1.1 Network Growth Pattern

Networks in urban areas (especially older networks) tend to follow a set growth pattern:

Launch rollout with minimum sites for maximum coverage.

Fill in coverage holes and add capacity by cell splitting

Add increasing numbers of microcells, in-building cells and street-level cells toincrease capacity focused on high subscriber density areas.

In terms of RF design, the problem with this approach is that the legacy sites from thelaunch rollout phase tend to be high and prominent, and increasingly contribute uplink

and downlink interference into the network as the number of lower sites around them

increases. The net effect of this is to minimise frequency re-use efficiency and limit thecapacity of the network. Therefore a process is required to identify and eliminate these

interferers to allow network growth to continue and high quality to be maintained.

5.2.1.2 High Sites Replacement

A typical process for replacing or modifying high sites would be as follows:

From BSS performance statistics and call trace logs, identify those cells whichcontribute the most interference to the largest number of other cells.

BCCH TCH Hopping MICRO

Guard Band Guard Band

14 ch

1 ch

26 ch 8 ch

1 ch




Page 48

Develop a plan for de-commissioning the site, or lowering the antennas to a positionconsistent with surrounding sites if possible. Include in the rollout plan therequirements for additional in-fill sites due to the loss of coverage from the high

site.

As new low sites are integrated, de-commission or modify the high site in such away as to cause minimum disruption to coverage. Prioritise the integration of the

required new sites to target high sites in order of severity.

The network design review will include a study of high sites in urban areas of the

network where growth is limited by frequency re-use problems. An action plan will bedeveloped according to this outline process, and will be provided as an input into the

network expansion and rollout process.

5.2.1.3 RF Design Strategy

Although not strictly part of a performance and optimisation review, it is important toconsider the design strategy in place in the network, and to provide input into theexpansion process to account for performance-related issues. This includes a review of

the following design techniques:

Microcellular and Picocellular underlay

Dual Band (Dual-BCCH and Single-BCCH)

In-building cell deployment

5.2.2 Traffic Distribut ion

In most networks it is found that the distribution of traffic between cells is not even, and

that a small number of cells may be heavily congested while most others are under-utilised. The key to the efficient utilisation of network infrastructure is to attempt to

distribute traffic evenly between BTSs and achieve maximum frequency re-use

efficiency. There are various techniques available to achieve this, including:

Removal of high or prominent sites which tend to suck in disproportionate levelsof traffic owing to their high coverage level compared to surrounding sites.

Downtilting antennas to reduce levels of unwanted coverage outside the intendedcoverage area.

Hotspot detection: Using Call Trace logs, it is possible to determine roughly thelocation of traffic hotspots, helping the RF designer to plan new sites in exactly the

right locations to serve high traffic areas. This also has the effect of reducing the

average path loss between BTS and mobile (because on average the BTSs are

closer to the mobiles), and therefore the interference levels in the network arereduced.

Traffic Management Algorithms: Many BSS vendors provide advanced trafficmanagement algorithms, allowing traffic distribution to be controlled to a greaterextent by the optimiser. These included microcell handover algorithms, congestion-

based handover algorithms and so on.




Page 49

The design review will include a study of the traffic distribution across the network, and

for the most congested cells recommendations will be made for ways to re-distributetraffic. In many cases these inputs are directly relevant to the ongoing network

expansion and rollout process. This will also include a review of GPRS trafficprojections, and how this will impact the combined traffic distribution carried by thenetwork.

5.2.3 Frequency Plan

Frequency Planning is a complex subject. The quality of a frequency plan (re-useefficiency, interference levels) is directly related to the quality of the RF design. A poor

frequency plan is usually the result of a poor RF design, resulting in turn in an inability

to produce a good frequency plan. This section attempts to highlight the main

considerations behind creating an efficient frequency plan.

5.2.3.1 Site design

As mentioned in previous sections, frequency reuse efficiency is affected by site design.Inconsistent site heights (mixture of high and low sites) reduce re-use efficiency.

5.2.3.2 Terrain and Topography

Hilly terrain presents more frequency planning problems compared to flat terrain, as cellcoverage areas are harder to control and unwanted splashes of coverage are hard to

avoid. Site design and antenna location can be critical in minimising these effects.

5.2.3.3 External Interference

Sometimes the performance of radio channels is affected by external interference (ie.

interference originating from outside of the network). This could be due to unauthorised

users occupying radio spectrum for other communications purposes. An example of thisis the 900MHz cordless telephone standard used in the USA, that use part of the GSM

Uplink spectrum (between channels 70 and 75). This is allowed in the USA but causes

problems to mobile networks in other countries where these channels are licensed andallocated to GSM operation. Although these phones are generally not licensed to be

used outside the USA, they are widely available in most countries of the world and

result in strong uplink interference.

Another example could be interference in coastal or port areas from radio

communications systems offshore (such as shipping, drilling platforms, etc.). Finally, in

border regions of neighbouring countries there may be spectrum re-use issues. Thesecan generally be resolved by agreements between operators in the neighbouring

networks.

5.2.3.4 BCCH Plan

The number of channels required to make a good BCCH plan will vary according to a

number of factors:

Site Design (high sites etc.)

Terrain and topography

Subscriber distribution




Page 50

Regularity of cell plan

In a well optimised network, it is generally possible to produce a high quality BCCH

plan within 14-15 channels.

5.2.3.5 Non-BCCH Plan

The same issues with the BCCH plan also affect frequency planning of the non-BCCH

(TCH) carriers. However there are additional techniques available for the TCH layer to

improve re-use efficiency and increase capacity, such as:

Synthesizer Frequency Hopping

Baseband Frequency Hopping

MRP (Multiple Reuse Pattern)

Concentric Cell

These are described in detail in the Optimising for Growth section.

The network design review will include a study of the frequency plan, and will suggest

optimisation steps required in order to produce a more efficient plan and hence a better

quality and higher capacity network.




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5.3 Optimising for Growth

The need for optimisation generally arises out of a need for growth and expansion of anetwork to serve a growing number of subscribers, and to support an increasing range ofservices. This section attempts to describe the optimisation techniques available for

maximising network capacity while maintaining high network quality. The availability

and effectiveness of these features and optimisation techniques varies between

infrastructure suppliers. The network optimisation process can be represented in adiagram, as shown below:

The effectiveness of all of these features also largely depends on the network design,

and how the feature parameters are optimised. A careful examination of all design

factors affecting the use of these features should be undertaken, and recommendationsmade as to the suitability of the features and/or improvements in performance through

optimisation.

Quality-Driven Network Design

Review, Expansion and

Optimisation Process

QOS Metrics

DatabaseParameters

RF DesignParameters

Core NetworkDesign Parameters

Drive Test Data

A-Interface Data

Network Planningand Optimisation

Call Trace Data

OMC Stats Data

OMC Management

Network Operations

- Rollout- Change Control

PerformanceReporting

Field Operations

Marketing Strategy

Expansion Plans

Optimisation Plans

Performance Requirements




Page 52

5.3.1 Synthesizer Frequency Hopping (SFH)

SFH is a widely accepted technique in GSM for providing capacity and quality

improvements. These benefits are as a consequence of the following features of SFH:

Increased immunity to fading due to frequency diversity.

Better frame erasure rate through interference averaging

Greatly simplified frequency planning allowing faster rollout and better quality.

The effectiveness of SFH in achieving capacity and/or quality gains is dependent on a

number of optimisation-related factors:

5.3.1.1 Hopping spectrum allocation

Since the benefits of SFH arise as a consequence of the nature of spread-spectrum

operation, the amount of benefit is related to the degree of spreading. In SFH this is

determined by the spread of channels allocated in the MA list (hopping sequence).

Simulations show that up to around 2MHz spread (10 channels) there is an appreciableincrease in hopping gain, but above 2MHz spread the additional gain reduces.

5.3.1.2 Choice of SFH Design

SFH can be deployed in a number of ways according to the network design. For

example:

1x3 SFH: In this scheme, the hopping band is divided into 3 equal groups andplanned according to a regular re-use pattern. This is suited to networks

with regular cell plan and 3-sector sites

1x1 SFH: In this scheme, the whole hopping band is allocated to a single hopping

group, which is re-used in every cell and every site. This technique isbetter suited to irregular networks.

1x1 Split SFH:This is similar to the 1x1 SFH scheme, except that it allows for

operation with different cell layers (for example high sites and lowsites). The hopping band is divided into two groups, and each group is

applied according to the 1x1 scheme on a per-layer basis.

Other variations are also possible, depending on the particular implementation of the

technique in the suppliers BSS software.

5.3.1.3 Hopping System Parameters

A full review of the use of hopping system parameters is required, to ensure compliance

with recommended SFH planning rules.

MA List: Frequencies allocated to the hopping sequence

HSN: Hopping Sequence Number (0 = cyclic, 1-63 = pseudo-random)

MAIO: Mobile Allocation Index Offset. Sometimes set automatically, however

manual definition of MAIO is essential for the correct implementationof certain hopping techniques (eg. 1x1 SFH).

These parameters also apply to a baseband hopping system, although their use issomewhat different.




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5.3.2 Baseband Frequency Hopping and Mult iple Re-use Patterns (MRP)

This is a technique preferred by a few suppliers, notably Ericsson, although SFH is a

more commonly used technique.MRP is a variation of baseband hopping in which frequencies are allocated to carriers

hierarchically with an increasingly aggressive re-use pattern. In other words, the BCCH

carrier would be planned with a 5x3 pattern, TCH1 with 4x3, TCH2 with 3x3, and soon. TCH channels are then allocated in priority order, starting with the BCCH.

One feature of MRP is that since interference increases on the higher carriers due tothe increasingly aggressive re-use patterns, the area in which an acceptable C/I can be

achieved those carriers is correspondingly smaller. This requires careful optimisation to

maximise traffic capacity.

5

guideline for network design and optimization

Documents

network performance

design review philosophy

network performance

dimensioning review

review structure

omcstatistics review

sdcch access performance

sdcch access success