Optimisation GSM

7/31/2019 Optimisation GSM

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GENERAL GSM RADIONETWORK OPTIMIZATION

Abstract

This document will provide with an

overview general GSM radio networkoptimization areas; with regards toanalysis and troubleshooting.


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RADO NETWORK PERFORMANCE

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Table of Contents

1 INTRODUCTION .................................................................................

2 RANDOM ACCESS.............................................................................

2.1 REASONS FOR POOR RANDOM ACCESSPERFORMANCE 5

2.2 USEDFORMULAS............................................................................2.3 ANALYSIS ......................................................................................

3 P AGING AND LOCATION UPDATE .............................................. ......

3.1 REASONS FOR POOR PAGING AN LUPERFORMANCE 9

3.2 USEDFORMULAS............................................................................3.3 ANALYSIS .............................................. ...................................... 1

3.3.1 Paging ............................. ................................................ 13.3.2 Location Update............................ .................................... 1

3.4 TROUBLESHOOTING ................................................................... ... 13.4.1 General .................................................................... ........ 13.4.2 Unsuccessful Location Updating ......................... ............... 1

4 CALL SET-UP ................................................................ .................. 1

4.1 REASONS FOR POOR CALL SET -UP

PERFORMANCE 194.2 USEDFORMULAS................................................. ......................... 24.3 ANALYSIS .............................................. ...................................... 2

4.3.1 Random Access problems .............................................. ... 24.3.2 Cell parameter settings and RN

features 214.4 TROUBLESHOOTING ................................................................... ... 2

4.4.1 General problems .................................................... ......... 24.4.2 Low signal strength .......................................... ................. 24.4.3 SDCCH and TCH congestion.. ........................................... 24.4.4 HW faults and other problems ............................................ 2

5 DROPPED CALLS ...................... ...................................................... 2

5.1 REASONS FOR DROPPED CALLS ....................... ............................... 25.2 USEDFORMULAS................................................. ......................... 25.3 ANALYSIS .............................................. ...................................... 2

5.3.1 SDCCH Results ........................ ........................................ 25.3.2 TCH Results .................................................................. ... 2

5.4 TROUBLESHOOTING ................................................................... ... 25.4.1 Dropped Calls Due To Bad Quality.................................... . 25.4.2 Dropped Calls Due To Low Signal

Strength 305.4.3 Dropped Calls Due To Other

Reasons 31

6 SDCCH & TCH ....................................... .......................................... 3

6.1 REASONS FOR TRAFFIC CAPACITY PROBLEMS.................................... 36.2 USEDFORMULAS................................................. ......................... 36.3 ANALYSIS .............................................. ...................................... 3


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6.3.1 SDCCH/TCH availability......... ........................................... 336.3.2 Cell size and location analysis ........................................... 346.3.3 Feature activation ............................. ................................ 34

6.4 TROUBLESHOOTING ...................................................................... 356.4.1 Congestion, general .......................... ................................ 356.4.2 SDCCH Congestion .......................... ................................ 356.4.3 TCH Congestion ............................................................... 38

7 INTERFERENCE ........................................................ ...................... 40

7.1 REASONS FOR HIGH INTERFERENCE LEVELS ......................... ............ 41

7.2 USEDFORMULAS ......................................................................... 417.3 ANALYSIS .................................................................................... 427.3.1 Bad frequency plan ........................................................... 437.3.2 External interference ........................................... .............. 447.3.3 Congestion ....................................................................... 447.3.4 Missing neighbour cell relations ......................................... 447.3.5 Wrong antenna type or bad

antenna positions 457.3.6 HW/SW Problems and site outages ......................... .......... 457.3.7 Cell parameter settings and RN

features 457.4 TROUBLESHOOTING ...................................................................... 46

7.4.1 Uplink Interference........................... ................................. 467.4.2 Downlink Interference ....................................................... 477.4.3 External Interference.................................................. ....... 48

8 HANDOVER ........................ ............................................................. 49

8.1 REASONS FOR POOR HANDOVERPERFORMANCE 49

8.2 USEDFORMULAS ......................................................................... 508.3 ANALYSIS .................................................................................... 51

8.3.1 Neighbouring cell relation problems ......................... .......... 518.3.2 Cell parameters settings and RN

features 518.3.3 Hardware problems.................. ......................................... 528.3.4 Too many measurement

frequencies in the active BA list 528.3.5 Poor coverage and coverage holes....................... ............. 528.3.6 Congestion problems ....................................... ................. 528.3.7 High interference ....................... ....................................... 538.3.8 Poor inter-MSC handover

performance 53 8.4 TROUBLESHOOTING ...................................................................... 53

8.4.1 Too few Handover attempts or nohandovers 53

8.4.2 Unsuccessful (lost) handovers ........................................... 548.4.3 Handover reversions ......................................................... 558.4.4 Ping-Pong Handovers ....................................................... 57

9 REFERENCES ....................................................... .......................... 57


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Introduction

The purpose of this document is to provide an overview on general

GSM radio network performance areas.

General GSM radio network performance areas may be summarized

as:

Random Access

Paging and Location Update

Call set-up

Dropped Calls

SDCCH & TCH

Interference

Handover

Following chapter will review each of above areas with focus on

possible reasons for poor performance, formulas for STS monitoring,

performance analysis and troubleshooting.


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1 Random Access

A Random Access burst is the first thing that will be sent when an MS

tries to access the network. The Random Access (RA) performance is

important for the accessibility performance and is linked to the BSIC

planning.

Reasons for poor random accessperformance

Areas with possible problems with BSIC planning, too low ACCMIN,

wrong MAXTA, interference or bad link budgets. A very high number of

not approved Random Accesses on BSC level might also indicate

problems with software file congestion in the BSC or MSC.

1.1 Used Formulas

RAACCFA: Total Number of Failed Random Access Attempts.

RA_TOT: Total Number of Random Access Attempts.

CNROCNT: Total Number of Accepted Random Accesses.

RA_FAIL: Failed Random Accesses of Total RA Attempts.

RA_ANSWPAG: Answer to Paging of Total Random Accesses.

RA_SERVICE: Other Services Requested of Total Random Accesses.

RA_EMERG: Emergency Calls of Total Random Accesses.


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RA_CALLREE: Random Accesses with Cause Call

Reestablishments of Total Number of Accepted Random Accesses

RA_OTHER: All Other Cases of Total Random Accesses.

S_EST: Number of SDCCH Establishments of Total Number of

SDCCH Seizure Attempts when No SDCCH Congestion.

1.2 AnalysisA cell can interpret a handover burst (supposed for another cell) as a

Random Access burst, which causes the counter RAACCFA to be

stepped. A necessary condition for this to happen is that the cells have

BSIC and an ARFCN in common. The handover burst is sent by an MS

to the target cell on the new TCH and contains the BSIC for the cell. If

another cell in the vicinity uses the frequency as BCCH and have the

same BSIC, the problem can occur. The general system performance

will not be affected unless any congestion occurs due to this

unnecessary use of RACCH and AGCH (Access Granted channel).

Anyway, a lot of RA failures (RAACCFA) always mean co-channel

interference. A problematic cell has to be checked for neighbours with

identical BSIC and where BCCH for the problem cell is used as ARFCN.

If this neighbour is far away, the co-channel interference will usually not

cause any performance problems (although there are a lot of RAACCFA

detected).

High timing advance can also be a reason for RAACCFA to be stepped.

The parameter MAXTA should be checked in that case.


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ACCMIN controls the access threshold for access to the system and if

set too low it could cause RA failures. Generally, there is no need of any

analysis of ACCMIN as the setting should be rather conservative i.e.

without any noticeable effects on the RA performance. Often the setting

of ACCMIN is determined by non-technical reasons, e.g. it is set to the

lowest value, -110 dBm, in order to catch as much roamers as possible.

When the MS sends repeated RAs without noticing the responses on

the downlink, the system will allocate a new SDCCH for each RA as

there is no identification of the different RA bursts. This will cause

unnecessary use of the SDCCH resources and affects the S_EST

figures for the cells. This can indicate a bad link budget, interference on

the DL or too low ACCMIN. As long as it does not cause any SDCCH

congestion a deliberately low ACCMIN might be excused. The number

of RA retries is controlled by the parameter MAXRET. The time between

two RA is defined by the parameter TX (keep in mind that the time is

chosen randomly; TX just gives the time range).

There might also be external interferers sending signals that could be

misinterpreted as Random Access bursts by a base station. These

problems might be discovered during a site survey.

The number of not approved RAs on the BSC level is high can be

caused by software file congestion. By checking SAACTIONS table

these problems can be detected. SAACTIONS will print those size

alteration events included in the supervision and experienced

congestion.


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Random Access due to other reasons includes location updating. A

high rate of RAs due to other reasons could mean that there are too

many location updates made in the system. This information should be

used in the analysis of the location updating performance.

If there are only a few random accesses in a cell (low traffic), the RA

performance will usually be quite bad. The reason is so called phantom

random accesses, which are generated by the noise of the base

stations receiver.

2 Paging and Location Update

A network with paging and location updating problems will impact

customer perceived quality and performance. A good paging and location

updating performance is necessary to assure that the mobile subscribers

can be reached by incoming calls, thus it is vital for any network to provide

as good paging and location updating performance as possible.


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2.1 Reasons for poor paging and LUperformance

Possible reasons for poor paging and location updating perf ormance

could be:

Paging

Paging congestion in MSC

Paging congestion in BSC

Paging congestion in Base Transceiver Station (BTS)

Poor paging strategy

Poor parameter setting

Poor coverage

High interference

Location Updating

Poor LA dimensioning/planning

Poor SDCCH dimensioning

Poor parameter setting

Poor coverage

High interference

2.2 Used Formulas

Paging LA level

PL_TOT: Total Number of Page Attempts (First and Repeated Pages).


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PL_2-1: Repeated Page Attempts of Total Number of First Page

Attempts.

PL_SUC-1: Successful First and Repeated Page Attempts of Total

Number of First Page Attempts.

Paging MSC level

P_1_TOT: Total Number of First Page Attempts.

P_1_GL: Global First Page Attempts of Total Number of First Page

Attempts.

P_2_TOT-1: Repeated Page Attempts of Total Number of First Page

Attempts.

P_1_SUC-1: Successful First Page Attempts of Total Number of First

Page Attempts.

P_12_SUC-1: Successful First and Repeated Page Attempts of Total

Number of First Page Attempts.

Location Update Location Area level

NLALOCTOT: Total Number of Location Update Attempts on Location

Area Level

LA_LU_SUC: Successful Location Update Attempts of Total Number of

Location Update Attempts on Location Area Level

Location Update MSC level

LU_TOT: Total Number of Location Update Attempts


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LU_R: Location Update Attempts from Already Registered Subscribers

of Total Number of Location Update Attempts

LU_NR: Location Update Attempts from Non-Registered Subscribers of

Total Number of Location Update Attempts

LU_SUC: Successful Location Update Attempts of Total Number of

Location Update Attempts

LU_R_SUC: Successful Location Update Attempts from Already

Registered Subscribers of Total Number of Location Update Attempts

from Already Registered Subscribers

LU_NR_SUC: Successful Location Update Attempts from Non-

Registered Subscribers of Total Number of Location Update Attempts

from Non-Registered Subscribers

LU_NORM: Normal Location Update Attempts of Total Number of

Location Update Attempts from Already Registered Subscribers

LU_PERIOD: Periodic Location Update Attempts of Total Number of

Location Update Attempts from Already Registered Subscribers

LU_IMSI_AT: IMSI Attach Attempts of Total Number of Location Update

Attempts from Already Registered Subscribers

LU_IMSI_DE: IMSI Detach Attempts of Total Number of Received IMSI

Attach Attempts

2.3 Analysis


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It is very important to do the paging analysis together with the LU

analysis. If for example the time between periodic registrations is

decreased the paging success rate will most likely improve but the

overall LU load will increase in the network. If the LU load is increased,

cells with high SDCCH load e.g. cells close to LA borders will get even

higher SDCCH load. Another example on how the LU load can get

higher is if the number of LAs within a BSC is increased for instance

with the purpose to lower the BTS paging load. A high paging- or LU

load will affect the CP load in concerned MSC and BSC nodes.

2.3.1 Paging

2.3.1.1 Page congestion in MSC

In case of congestion the appropriate SAE should be increased.

2.3.1.2 Page congestion in BSC

If page congestion exists in the BSC, the paging strategy should be

redefined and more LAs should probably be introduced. See also the

MSC parameters deciding the paging strategy.

2.3.1.3 Page congestion in BTS

Incoming Paging Commands are buffered in a queue in the BTS. The

BTS distributes the Paging Commands as Paging messages on the

radio path when paging blocks are available. A too high rate of

incoming Paging Commands increases queuing time. If the queue is

full, the incoming pages will be disregarded and the mobile will not be

paged. If the page is queued for a too long time in the BTS, the page


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may also be lost due to the fact that the MSC does not receive the

page response before the timer has expired.

2.3.1.4 Parameters and paging strategies affecting paging performance

Parameters in the MSC decide how paging procedure is done. For

example, the parameter PAGEREP1LA decides if the second page

should be sent only to the LA or as a globally to all LAs in the MSC.

2.3.1.5 Poor Location Area plannin g af fecting paging performance

For each Paging message received in the BSC, Paging Command

messages have to be sent to all cells belonging to the LA where the

target mobile is registered. A too large LA may lead to too high page

load in the BTS, resulting in congestion and lost pages. For these

cases consider a reduction of the LA size. However, smaller LAs

implies a larger location updating load since the rate of the mobile

stations crossing LA borders and performing location updating

increases, leading to a higher SDCCH load.

2.3.1.6 Poor coverage and/or high interference

High interference may cause paging problems. If interference is

suspected, check the frequency plan to see if there are any adjacent or

co-channel frequencies for cells in the area. Areas with poor coverage

can be identified using STS and/or MRR measurements. Poor

coverage might affect the paging performance. Check if the cells in the

area indicating poor paging performance also suffer from poor

coverage.


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2.3.2 Location Update

2.3.2.1 Software files congestion

Check SAE for the software blocks related to location updating.

2.3.2.2 SDCCH congestion at Location Area border

It is important to take into consideration the SDCCH load in the cells

located at the cell area borders when planning LAs. LA border crossing

over high mobility areas, e.g. highways, should be avoided and LAs

should not consist of non-continuous, small areas.

2.3.2.3 Parameters affecting location update performance

For example, the setting of the parameters CRH, T3112 and BTDM

may have a large impact on the location update performance. The CRH

parameter is used in order to prevent ping-pong effects in the LA

borders. If the CRH value is too low, the variations in the signal

strength can give ping-pong effects and this will increase the SDCCH

traffic. If the parameter has a too high value the mobile may camp on a

cell not being the best server for a too long time.

Another example is the BSC parameter T3212, which decides the

periodic registration interval. The parameter must be set together with

the MSC parameter BTDM. If, for example, BTDM is shorter than

T3212 the mobile will be erased from the Visitor Location Register

(VLR) before it has performed a periodic location updating.

2.3.2.4 Poor coverage and high interference

Poor coverage and high interference can also affect the locationupdating performance.


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2.4 Troubleshooting

2.4.1 General

Optimization of the LU performance is closelyrelated to optimization of paging performance.

A too high paging load can be alleviatedthrough a reduced LA size. Smaller LAs

though tends to generate a larger location

updating load since the rate of mobilescrossing LA borders increases as the size of

the LAs decreases. This increased locationupdating load has a restraining effect on thedesire to reduce the size of the LAs. Theincreased location updating load is mainly

manifested in an increased SDCCH traffic inthe LA border cells. The limiting factor will thusbe the SDCCH capacity requirement.

In Table 1 some of the most important BSCparameters for Location Updating and Pagingare given.


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In Table 2 MSC parameters and exchangeproperties relevant for paging and location

updating are given.


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The philosophy of Location area dimensioning

is different depending on if it is in rural areas,medium size cities or ma jor size cities:

Rural areas

The size of the LAs in rural areas,characterised by a low subscriber density, is

not very critical. The possible need for moreSDCCH resources in the LA border cells has amarginal effect on the system since in general

capacity is not a scarce resource in ruralareas. (Cellular systems in rural areas aremost often limited by coverage and not bycapacity.)

LA borders should be drawn up outsidevillages and minor cities and unnecessary

criss-crossing over high ways should beavoided.

Medium size cities

It is preferable to fit a medium sized city (less

than 1 million people) into one LA to reducethe location updating load.

The LA borders should be drawn up in lowsubscriber density areas well outside the city.LA borders crossing high ways should beavoided as much as possible.

Major size cities

LAs in large cities (more than 1 million people)tend to be quite large, in particular where thecity is served by more than one BSC (If more

than one BSC/LA the paging load of the LA is

shared by all the involved BSCs. Hence thetotal paging capacity increases with thenumber of BSCs at least at BSC level. Thecells in the LA are split between the BSCs. Thepaging load in the BTSs is however not

reduced by load sharing in the BSCs and it i sthus important to look out for overloadsituations in the BTSs/paging channels). One

reason to this is that it is often difficult to find agood way to split a city into more than one LA.


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However, provided that it is possible to split acity in two or more LAs, without creatingoverload situations in those cells that therebybecomes LA border cells, this is

recommendable.

The upper bound of the size of an LA dependson the capacity of the equipment used and thescenario. It is thus difficult to give a generallyapplicable recommendation on the size of an

LA. The best approach is to measure theactual paging load and to evaluate theperformance.

In case a city is covered by more than one LA,the LA border should be drawn up in low -

density subscriber areas and it should notcriss-cross over high ways.

The SDCCH capacity of the LA border cellsshould be dimensioned to cater for theexpected location updating load.

2.4.2 Unsuccessful Location Updating

A check list of what can be done if there is alow ratio of successful location update ispresented below.

Border Cell

Check if the cell is a border cell.

? Reconsider the tuning and increase of thehysteresis CRH. The cell could be reallocated

to another location area.

Low CRH Hysterisis

Check the setting of CRH.

? Increase CRH.

Bad Location Area Dimensioning

Check if a high amount of users is moving

along or across the border.

? If for example a highway is going along the

border, the location area should preferably beredone. Cells could be moved to anotherlocation area.

Short Time Interval between PeriodicRegistrations


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Check the periodic registration interval,parameters T3212 and BTDM.

Interference

Check Interference level in the system.

? Decrease interference.

Software File Congestion

Check SAE for software blocks MLUAP,MLCAP and MMMLR.

? Change SAE if incorrect.

Insufficient Number of SDCCH Channels

Check SDCCH configuration.

? Consider increasing the number of SDCCHchannels.

Automatic De -registration Not Used

Check if automatic de-registration not used.

? Activate automatic de-registration.

3 Call Set-upThe call set-up investigation includes analysisand improvement of Random Access success

rate, SDCCH drop rate, SDCCH and TCH

congestion and TCH assignment success rate.

3.1 Reasons for poor call set-upperformancePossible reasons for poor call set-up

performance could be:

TCH Congestion


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3.3.1 Random Access problems

A cell can interpret a handover burst(supposed for another cell) as a Random

Access burst, and this causes the counter forfailed Random Access to be stepped.

Another possibility is that random noise isinterpreted as Random Accesses.

3.3.2 Cell parameter settings and RNfeatures

Incorrect settings of the cell parametersACCMIN, MAXRET, TX and MAXTA couldnegatively influence the Random Access

performance.

The parameter ACCMIN gives the minimumsignal strength that have to be received in

the mobile for permission to access thesystem. A low value of ACCMIN means that

the accessibility to the network in idle modeis increased, at the expense of the risk of

having an increased number of call setupfailures.

Low SDCCH establishment success ratemight indicate problems with the response toImmediate Assignment from mobiles (e.g.because of low signal strength orinterference). It can also depend on a wrongsetting of MAXRET and TX. One example: IfMAXRET=7 and TX=50, there is a possibilitythat more random accesses are sent before

the MS has got the message immediateassignment from the BSC. The BSCallocates a SDCCH for the 2nd RA eventhough the 1st one was successful. Thisgives unnecessary SDCCH load (since the

SDCCH is never used).


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Assignment to another Assignment toanother cell, (ASSOC), is a feature thatallows a mobile to seize a traffic channel inanother cell than the serving one during call

setup. If the feature is used the assignmentsuccess rate might show a too low value ifthe cell in question perform many

assignment handovers. The reason is that incase of an assignment failure, the counter

TASSALL is stepped in the originating cell,although the actual attempt was made in the

destination cell. Therefore, correlate theassignment handover success rate with the

low assignment success rate cells.

3.4 Troubleshooting

3.4.1 General problems

Below follows a check list, but it is not sorted inorder of importance relevance.

Has the site been down during the

measurement period? Sites can during certaintime periods (due to for example transmission

problems) which can give bad statistics that isnot representative in a long perspective.

Congestion on TCH or SDCCH removed?

Too low value of ACCMIN?

Change MAXRET and TX to the default

setting recommended in MAXRET=4, TX=32, ifa high SDCCH load is suspected to dependrandom accesses. One example of a

parameter setting that is recommended

NOT to have: If MAXRET=7 and TX=50, thereis a big possibility (and 3rd up to 7th) randomaccess is sent before the BSC knows wassuccessful or not. The BSC allocates a

SDCCH for the 2nd RA though the 1st onewas successful. This gives a lot of

unnecessary load (since the SDCCH is neverused) and also a lot of failed RA already the

1st one was successful).

Use info from customer complaints and go toa specific address with and try to find andsolve the problem. If possible: halt serving cellfor internal or external interferers. Ask local


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employees if AMPS located close to theproblem area. These can cause interferencewithin distance of at least 100 m. Sometimesfilters can solve the interference AMPS sites.

MRR, CTR, MTR used to point out areas ofproblems?

Frequency change tried?

Antenna down tilt tried? Before doing downtilt it should be verified TEMS that theinterference occurs in the border area of the

cell, coverage from the cell that should be tiltedis unreasonably large.

Check that antenna directions are accordingto the plan.

Poor coverage?

3.4.2 Low signal strength

Is the cell situated in a poor coverage area,

for example on the countryside?

Correlate with the analysis of dropped callsand look especially for drops due to low signal

strength. Highlight in the report how manypercentages (approximately) of the call setup

failures that are due to poor coverage andsuggest areas for new sites.

3.4.3 SDCCH and TCH congestion

Check the SDCCH time congestion.Especially cells close to a location area bordercan be heavily loaded and need additionalSDCCH capacity to be able to set up calls. It

does not matter how many idle TCHs there arein a cell if there at the same time is congestionon the SDCCH.

3.4.4 HW faults and other problems

If a frequency change did not have theexpected effect or if the coverage is far less

than the frequency planning tool shows it candepend on one of the faults listed below. Some

possible faults are:

If a cell is not covering the area that it issupposed to cover according to the frequency-


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planning tool it can depend on that the antennais connected to the wrong feeder.

The site can in reality be lower than in thepredictions in the frequency planning tool,giving less coverage than planned.

There can be alarms indicating HW faults.

Software file congestion

4 Dropped CallsThe retainability performance evaluates thesystems ability to handle established

connections. Dropped calls are probably thesingle most important quality item to control in

the system. The level of dropped calls in thesystem is in high extent depending on theinitial RF planning, optimization and also thesystem growth.

4.1 Reasons for dropped calls

Possible reasons for a high rate of droppedcalls could be:

TCH Congestion

Parameter Settings

HW problems

Interference

Poor signal strength

Missing cell relations and/or missing

measurement frequencies

The reasons for dropped calls can, accordingto STS, be:

- Low signal strength

- Bad quality

- Sudden loss of connection (only TCH)

- Excessive timing advance

- Other


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4.2 Used Formulas

S_DR-C: Dropped SDCCH Connections ofTotal Number of SDCCH Connections.

S_DR_ERLM: Erlang Minutes per DroppedSDCCH Connection.

S_DR_SS: Dropped SDCCH Connectionsdue to Low Signal Strength of Total Number

of Dropped SDCCH Connections.

S_DR_BQ: Dropped SDCCH Connections

due to Bad Quality of Total Number ofDropped SDCCH Connections.

S_DR_TA: Dropped SDCCH Connections

due to Excessive Timing Advance of TotalNumber of Dropped SDCCH Connections.

S_DR_OTH: Dropped SDCCH Connections

due to Other Reasons than Low SignalStrength, Bad Quality or Excessive Timing

Advance of Total Number of DroppedSDCCH Connections.

T_TRAF: Average TCH Traffic Level.

T_CONGT: TCH Time Congestion of TotalMeasurement Interval.

T_AVAIL: Available TCHs (not blocked) of

Total Number of Defined TCHs.

T_DWN: Average Cell downtime for activecells

H_SUC: Successful Handovers of Total

Number of Handover Attempts.

T_DR-S: Dropped TCH Connections of TotalNumber of Calls Terminated in the Cell.

T_DR_ERLM: Erlang Minutes per DroppedTCH Connection.

T_DR_SS_DL: Dropped TCH Connectionsdue to Low Signal Strength on Downlink of

Total Number of Dropped TCH Connections.

T_DR_SS_UL: Dropped TCH Connectionsdue to Low Signal Strength on Uplink of TotalNumber of Dropped TCH Connections.

T_DR_SS_BL: Dropped TCH Connections

due to Low Signal Strength on both links ofTotal Number of Dropped TCH Connections.


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T_DR_SUD: Suddenly lost connections ofTotal Number of Dropped TCH Connections.

T_DR_BQ_DL: Dropped TCH Connectionsat Bad Quality on Downlink of Total Numberof Dropped TCH Connections.

T_DR_BQ_UL: Dropped TCH Connectionsat Bad Quality on Uplink of Total Number ofDropped TCH Connections.

T_DR_BQ_BL: Dropped TCH Connections atBad Quality on both links of Total Number ofDropped TCH Connections.

T_DR_TA: Dropped TCH Connections due toExcessive Timing Advance of Total Number

of Dropped TCH Connections.

T_DR_OTH: Dropped Calls due to OtherReasons than Low Signal Strength, BadQuality or Excessive Timing Advance ofTotal Number of Dropped TCH Connections.

4.3 Analysis

4.3.1 SDCCH Results

If a high drop rate on SDCCH has beennoticed the following actions isrecommended in order to proceed and solvethe problems.

Improvements for dropped calls on TCH willimprove the drop call rate on SDCCH. I.e.recommend to trouble shoot the TCH dropcalls first if there is poor performance on both

TCH and SDCCH.

The drop call rate on SDCCH can beimproved if the congestion on TCH is

decreased. Recommend to use the featureassignment to worse cell or increase thecapacity on TCH.

The reasons for low SS drops could be toofew sites, wrong output power, shadowing,no indoor coverage or network equipmentfailure.


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The reasons for dropped calls due to badquality on the uplink or downlink are relatedto internal or external interference andtrouble shooting is needed to find the

interferers. The situation could be temporaryimproved by means of applicable features.Recommend features that are not activated

or recommend alternative parameter setting.

The reasons for drops on to high timingadvance is related to the site location i.e.close to open water, desert or hilly terrainand the setting of MAXTA and TALIM.

Setting MAXTA to 63 and TALIM to 62 couldsolve the problem and/or tilt the antennas,

reduce antenna height, change antenna orreduce output power.

Miscellaneous problems could for example

be mobile errors which can occur when oldmobiles may cause dropped calls if certain

radio network features are used. Anotherreasons could be that the MS is damagedand not working properly.

4.3.2 TCH Results

If a high drop rate on TCH has been noticed

the following actions is recommended inorder to proceed and solve the problems.

High drop rate due to high outage time orlow availability. Inform the operation and

maintenance department about the problemsor check the reasons for the downtime.Check also the alarm list or BTS error log.

Another way is also to check the resolutiontime for the different alarm categories. Many

problems with dropped calls are often relatedto insufficient O&M routines and not to radio

problems.


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Dropped TCH due to bad quality are oftendue to interference problems and/or lowsignal strength in areas where there is nodominant server. The reason could also be

missing neighbour relations so that themobile is not connected to the strongestserver and therefore perceives bad quality.

The interfered cell should be investigated inorder to find the source for the interference.

In most cases the interference is internal butcan also originate from external sources

such as other networks, radio stations,microwave links etc. The interference could

also be reduced by means of applicablefeatures such as frequency hopping,assignment to better cell, DTX, MS/BTSpower control etc. Recommend applicablefeatures or alternative parameter setting iffounded incorrect.

Dropped TCH due to low signal strengthare in most cases related to coverage gaps

but can also be a result of missing neighbourrelations, hidden antennas, wrong antenna

type (to low gain), antenna or feederproblems, incorrect power settings, etc. The

reason can also be unforeseen subscriberbehaviour i.e. the subscribers use theirmobiles indoor, in elevators, parking lots etc.

This can also be indicated if there is a highpercentage of suddenly lost connections.

Dropped TCH due to excessive timingadvance should in normal cases not occur inthe network. The reasons for timing advance

drops are site location (close to open water,desert or hilly terrain) and the setting of

MAXTA and TALIM. Setting MAXTA to 63and TALIM to 62 could solve the problem. Orreduce the coverage by down tilting theantennas, reduce antenna height, changeantenna or reduce output power. If the site is

located close to open water etc. the

extended range feature could be considered.

TCH drops due to other reasons than lowSS, bad quality, excessive timing advancecould for example be because of BTS HW

problems, transmission problems, serviceaffecting maintenance work, uplink

interference problems (external or internal),mobile station problems etc.


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4.4 Troubleshooting

The trouble shooting of dropped calls isdivided into three areas, dropped calls due to

bad quality, low SS and other reasons.

4.4.1 Dropped Calls Due To Bad Quality

If the cell suffers from dropped calls due to

bad quality the first step is to check that theparameters QLIMDL and QLIMUL are set to

correct values. SAUDI 55

Additional data should be collected in orderto make correct conclusions.

Check if there are any normal disconnections

at bad quality in the cell, this could giveindications on that there really is a bad

quality problem in the cell.

Run MRR on the cell and check the averageRXQUAL value in the cell.

Display C/I and C/A predictions in planning

tool. Check if the cell is located in anyinterference area. Remember that there

might be interference in the cell even if it isnot displayed in the planning tool. Check idle

channel measurements (ICM) for the cell tosee if there exists uplink interference in the

cell.

Check the handover statistics on a neighbourrelation level for the cell. See if there is anyneighbour relation with a high number of badquality handovers. This information might

give you the location for the bad quality areain the cell.

Frequency Allocation Support (FAS) can beused in order to find the interfered frequency.This can be useful especially for frequencyhopping systems. FAS gives however only

the uplink information.

Perform TEMS drive tests in the suspectedarea. Try to locate the interferer, one way for

C/I problems is to halt the affected cell andperform a frequency scanning in order tolocate the interfering cell.


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See if it is possible to change frequency orreduce the signal strength of the interferer.For example down tilting the antenna. Checkthe antenna diagram in order to see the

effect of different tilt angels. A secondalternative is to change the frequency orincrease the signal strength in the interfered

cell. Check if there is any missing neighbourrelations causing low SS and by that bad

quality drops.

4.4.2 Dropped Calls Due To Low SignalStrength

If the cell suffers from TCH drop due to low

signal strength the first step is to check thepower setting, power balance and neighbour

relations in the affected cell. Check forexample on a map with site positions or inthe cell-planning tool for any obviously

missing neighbour relations. Check also theamount of normal disconnections at low SSfor the cell.

Check the alarms on the RBS to verify thatthere is no VSWR alarm causing the

reduction in output power.

MRR can be used for checking the timingadvance and RXLEV distribution in the cell.This can give an indication if the subscribersare close to the base station or in the

outskirts of the cell. If most of the subscribersare on low TA values the low SS drops might

be lack of indoor coverage or if most of thesubscribers are on high TA values theproblem might be a missing site or neighbour

relation. By checking RXLEV for the cellindication and compare with coverage plotsin planning tool indications of obstaclescovering the antenna, feeder problems or

other reasons for low SS might be found.

Use the handover statistics on a neighbourrelation level to get an indication on where in

the cell the problem might be. Check also ifany of the target neighbours suffers fromsevere congestion.


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If there are a lot of suddenly lost connectionsin the cell this could indicate that there is atunnel, underground parking lot, high indoorusage etc. Try to find the most likely position

in the cell where this kind of drops mightoccur.

Check in planning tool to see if any coverage

gaps or areas with low coverage can befound in the cell. Verify that the differentclutter values make sense in the planningtool. Is the site position OK? Is the antennaazimuth correct, is it shooting to the correct

location, road, building etc.

Perform drive tests in the cell and check formissing neighbours, swapped antennas etc.Perform the drive test close to the site andtry to see if it is line of sight or of the

antennas are hidden by any obstacles. Makea site visit and check the antennas if

necessary.

If the low signal strength is not related to anyfaults or missing configuration probably anew site or improved indoor coverage isneeded and the problem should be passed

on to the cell planning department.

4.4.3 Dropped Calls Due To OtherReasons

If the cell suffers from dropped calls besides

the reasons low SS, bad quality andexcessive timing advance the dropped callswill be counted as other reasons. That is that

the counters for SS, quality and timingadvance are not incremented and only theCNDROP, TFNDROP or THNDROP arestepped.

This could for example be the case in cells

with uplink interference.

Check if ICM is indicating uplink interferencein the cell.

Other possible reasons could be problemswith the mobile stations of BTS HWproblems, transmission problems, andservice affecting maintenance work.


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Check with the operation and maintenancedepartment or check the applicable alarmlogs if there have been any HW problems,transmission problems, and service affecting

maintenance work during the time period.The average cell downtime and TCH andSDCCH availability should also be checked.

If mobile station problems are suspected inthe network this needs to be raised with theCustomer and his customer care departmentto investigate the problem further.

5 SDCCH & TCHCongestion on SDCCH makes it impossible toset up a call unless the feature immediate

assignment on TCH or adaptive configurationof logical channels is used. Congestion on

TCH makes it impossible to set up a callunless features such as Assignment to WorseCell or Cell Load Sharing are used. TCHcongestion also means that handover fromanother cell is impossible to perform.

5.1 Reasons for traffic capacityproblemsPossible reasons for traffic capacity problemsare:

High number of blocked devices

HW problems.

Poor dimensioning of SDCCH and TCH

Features impacting the traffic behaviour

Traffic distribution between 900 and 1800cells

Traffic distribution between micro- and macrocells

5.2 Used Formulas

RA_OTHER: Random Accesses with CauseAll Other Cases, e. g. Location Updating,


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Detach, Attach, etc. of Total Number ofAccepted Random Accesses.

S_TRAF: Average SDCCH Traffic Level.

S_CNGT: SDCCH Time Congestion of TotalMeasurement Interval.

S_MHT: SDCCH Mean Holding Time.

S_AV_NR: Average Number of AvailableSDCCHs.

S_AVAIL: Available (not blocked) SDCCHs ofTotal Number of Defined SDCCHs.

S_DR: Dropped SDCCH Connections of TotalNumber of SDCCH Connections.

T_AS_SUC: Successful TCH Assignments ofTotal Number of Assignment Attempts.

T_TRAF: Average TCH Traffic Level.

TF_CNGT_U: TCH/F Time Congestion in

under laid Sub cell of Total MeasurementInterval.

T_MHT: TCH Mean Holding Time.

T_AV_NR: Average Number of AvailableTCHs.

T_AVAIL: Available TCHs (not blocked) ofTotal Number of Defined TCHs.

T_DR_ERLM: Erlang Minutes per DroppedTCH Connection.

5.3 Analysis

5.3.1 SDCCH/TCH availability

From the STS data it can be seen how muchof the hardware that is being used. Normally

the availability for SDCCH and TCH should be100%. For the cells showing low availability,check the BTS error log to make sure that

there are no problems with the hardware errorlogs.


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the congestion is not caused by hardwareproblems or link failures. If no problems can befound, check if it is possible to add extra TRXsto the cell or to add (micro) cells in the area.

It may also be possible to activate the featureCell Load Sharing and/or Assignment to Worst

Cell as short-term solutions.

5.4 Troubleshooting

5.4.1 Congestion, general

Check if the congestion can depend on ashort-term growth or a long-term growth:

Short term growth

If the high traffic related to an occasional

event, like sport event, fairs, conference, atemporary solution might be considered.

Long term growth

If there is a long-term growth the networkcapacity has to grow according to the demand.

Check if there is an expansion planned in thenear future for the TCH congested cells.

Check if the congestion is on SDCCH, TCH orboth.

5.4.2 SDCCH Congestion

The time congestion should be used instead ofcongestion based on access attempts as thereis no way to estimate the number of access

attempts a single mobile does.

Increasing Traffic Demand

The increase in traffic could be related to anoccasional event or due to a long-term growth.

Check if short term traffic growth. Make trendcomparisons.

Check if combined SDCCH is used.

Check SDCCH dimensioning.


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Increase the number of SDCCH channels.Note that an increase may lead to the need fornew transceivers.

If combined SDCCH is used, non-combinedchannel configuration should be introduced.

Long Mean Holding Time

If the mean holding time is long, this generates

a higher traffic load. Check SDCCH Mean Holding Time.

TCH Congestion

TCH congestion may cause the mobiles tostay extra long time on the SDCCH beforebeing allocated TS on a TCH. Check if there

exists TCH congestion and if the SDCCHmean holding time is above 7 seconds. Forimmediate assignment the time is 2-2, 5seconds.

Check TCH Congestion.

Check SDCCH Mean Holding Time.

Check if Assignment to Worse cell is used

and existing parameter setting.

Check if Cell Load Sharing is used.

Increase TCH capacity.

Use the features for traffic distribution suchas Cell Load Sharing and Assignment toWorse Cell.

Low Availability

Check SDCCH Availability.

Check if the unavailable channels are

manual, control or automatically blocked.

Change & repair faulty equipment.

Review the O&M procedures.

Too Frequent Periodic Registration

Check Random Access distribution.

Check the timer T3212 in the BSC and theparameters BTDM and GTDM in the MSC.

Decrease the number of periodicregistration.


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Wrong SDCCH Dimensioning

Check SDCCH dimensioning.

Location Area Border Cell

If a cell is located on a non-optimised LocationArea border, unnecessary normal LUs are

performed.

Check site position and location area border.

Check Location Update performance.

Check the parameter CRH.

If the site is located close to major road orrailway; consider moving the Location Areaborder.

Increase the hysteresis CRH. The CRH is

the hysteresis value used when the MS in idlemode crosses a LA border. The default value

for this parameter is 4. If a high number ofLocation Updates occurs in a Location Areaborder cell, a higher CRH can be set in order

to reduce the number of LUs.

SMS Usage

Extensive SMS usage increases the SDCCHtraffic and could cause congestion if badlydimensioned SDCCH channels.

Check SMS activity.

Re-dimension the SDCCH channels withconsideration taken to SMS usage.

Cell Broadcast Used

Chec k if Cell Broadcast is active.

If active, check if the operator uses it.

Remove Cell Broadcast if not used.

IMSI Attach/Detach in Use

An introduction of IMSI attach/detach will

increase the traffic on SDCCH. However, thebenefits are that the paging success rate willincrease. The recommendation is to use

Attach/Detach.

Software File Congestion

Check SAE setting.


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High Ratio of Random Accesses

Check Random Access performance.

5.4.3 TCH Congestion

The time congestion should be used instead ofcongestion based on access attempts as there

is no way to estimate the number of access

attempts a single mobile does.

Probable reasons for TCH congestion arelisted below:

Increasing Traffic Demand

The increase in traffic could be related to anoccasional event or due to a long-term growth.

Check if short term traffic growth.

Check TCH dimensioning.

Check the use of congestion relievingfeatures such as Assignment to Worse cell,Cell Load Sharing and HCS.

Increase the number of transceivers. This

may lead to problems with floor space,antenna installations, CDU type, expansion

cabinets and combiner type.

If not used, introduce Assignment to Worsecell and Cell Load Sharing. Note that theinterference level will increase if Assignment toWorse is used, as some mobiles will be closer

to a co-channel than what was intended in thefrequency plan. The feature will be moreeffective if the neighbours are not congested.In a tight network with a high reuse andcongestion in a larger area, the feature might

only make the situation worse.

Bad Dimensioning

Bad allocation of TCH in a system may cause

unnecessary congestion. Investigate if it ispossible to move transceivers from non-congested areas to congested areas. Ofcourse, the base station type, CDU-type,

current number of transceivers, floor space,combiner type, etc., should be consideredbefore a recommendation to move transceivercould be made.

Check TCH traffic.


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Re-dimension the TCHs.

Hardware Fault & Installation Fault

Faulty equipment will lead to that not all timeslots can be used for handling traffic thatcauses congestion.

Low availability can happen if the channelshave been manually or automatically blocked

and taken out of service. Availability isdepending on the number of frequenciesdefined per cel l. The parameter NUMREQBPC

can be used.

Check TCH Availability.

Check TCH blocking.

Change and repair faulty equipment. Reviewthe O&M procedures.

High Antenna Position

A high antenna position could mean a toolarge service area. Also antennas placed on

hilltops will cover large areas. A largecoverage area might mean that the cell takes a

lot of traffic.

Check antenna height.

Check antenna type.

Lower antenna if there is no risk for loss ofcoverage (no coverage at all). Tilting of theantenna or changing antenna type may alsodecrease the coverage area.

Long Mean Holding Time

A low handover activity might lead to a longmean holding time. A long mean holding timeis not a problem, but if there is congestion,new capacity is needed.

Check Mean Holding Time.

Check Handover Performance.

Increase the number of TCHs if no faults.

Low Handover Activity

A low handover activity may lead to congestionif the MS is forced to stay on a cell longer thannecessary.


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Check if congestion in neighbouring cell.

Check handover performance

Check neighbouring cell definitions. Missingrelations could cause handover problems.

Correct handover parameters such as toohigh or too low hysteresis values, missingneighbour relations, one-way handovers.

Congestion in neighbouring cell needs to bedecreased.

Congestion in Surrounding Cells

Check congestion in neighbouring cells.

Review neighbour cell list. New relation couldrelieve the congestion.

Check if Assignment to Worse cell is used. Ifassignment handover to worse cel l is used

(directed retry) check the setting of theparameter AWOFFSET.

Add new neighbour cells if appropriate.

6 InterferenceCellular systems are often interference limited

rather than signal strength limited and it willtherefore always exist interference in thesystem. Frequency planning guiding values isC/I 12dB without frequency hopping and 9 dBwith frequency hopping. C/A recommendedplanning value is 3 dB but in the GSMspecification it is stated 9dB. The inter symbol

interference (ISI) or the carrier to reflectionC/R, must be larger that 9 dB according to theGSM specification.

Interference problems could be divided intoservice retainability affecting problems and

service integrity problems. The interferencemight affect the retainability performance of acall and having it to drop before normaltermination. The interference might also affect

the speech quality (integrity performance)during the call as well as the serviceaccessibility performance.


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6.1 Reasons for high interferencelevels

Possible reasons for interference problems ina cellular network could be:

External interference

Bad or too tight frequency plan

Dragged Calls due to missing neighbour

relations and congestion

Antenna positions and/or antenna type

HW problems

Incorrect parameter settings

6.2 Used FormulasT_DR_S: Handover Decisions due to BadDownlink Quality of Total Number of Bad

Quality Urgency Handover Decisions.

T_DR_ERLM: Call Minutes per Dropped TCHConnection.

T_DR_BQ_DL: Dropped TCH Connections atBad Quality Downlink of Total Number of

Dropped TCH Connections.

T_DR_BQ_UL: Dropped TCH Connections atBad Quality Uplink of Total Number of

Dropped TCH Connections.

T_DR_BQ_BL: Dropped TCH Connections at

Bad Quality both links of Total Number ofDropped TCH Connections.

ICH_x_U: Percentage of Idle TCHs in

Interference Band x in under laid Sub cell.

ICH_x_O: Percentage of Idle TCHs in

Interference Band x in Overlaid Sub cell.

IHO_TOT-C: Intra-Cell Handover Decisions ofTotal Number of TCH Connections.

IHO_BQ_DWN: Intra-Cell Handover Decisionsdue to Bad Downlink Quality of Total Number

of Intra-Cell Handover Decisions.

IHO_BQ_UP: Intra-Cell Handover Decisionsdue to Bad Uplink Quality of Total Number ofIntra-Cell Handover Decisions.


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H_BQ_DWN: Handover Decisions due to BadDownlink Quality of Total Number of HandoverDecisions.

H_BQ_UP: Handover Decisions due to BadUplink Quality of Total Number of HandoverDecisions.

H_BQ_DWN-R: Handover Decisions due to

Bad Downlink Quality of Total Number of Bad

Quality Urgency Handover Decisions.

H_BQ_UP-R: Handover Decisions due to Bad

Uplink Quality of Total Number of Bad QualityUrgency Handover Decisions.

H_REV: MS Reversions to Old Channel ofTotal Number of Handover Attempts.

TERM_BQ: Bad Quality at successful

termination of Total Number of successfultermination.

T_DWN: Average Cell Downtime for ActiveCells.

6.3 AnalysisCheck which cells that have high call drop rate(T_DR_ERLM or T_DR_S). Sort out thosecells that have high percentage of drop due to

bad quality. Check later with Idle ChannelMeasurement if these cells have high

percentages in band 3, 4 and 5. In that casethe interfered cells are found. (In case of veryhigh rate of TCH congestion in a cell, ICMresult might not be reliable).

In order to verify the result and locate the

interference for the worst 10-15 cells directionthe following checks should be made:

Check bad quality urgency handover and

handover reversions. A high number of

handovers caused by bad quality directlypoints out interference problems in the cell ifQLIM is set correctly. Check the percentage of

bad quality handovers to the differentneighbour relations in order to point out wherethe interference problem might be, check also

if the bad quality handovers is mainly on uplinkor dow nlink. Check the percentage of

handover reversion, a handover reversionoccurs when the mobile cant receive the


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physical information from the target cell withina specific time. That is when the mobilereceives the handover command message thetimer T3124 is started in the MS if the MS cant

receive the physical information sent by thetarget cell before T3124 expires the MS tries toreactivate the old channel. The problem may

be caused by interference in the target cell.

Check bad quality at successful termination,

this might indicate that the quality is so poorthat the subscriber has to terminate the call.

Check intra cell handover for the found cells. Ahigh number of intra cell handovers normallyindicates bad quality at high signal strength.

Be aware of, that intra cell handover is not anaccurate interference indicator in congestednetworks.

Check the idle channel measurement (ICM)statistics. Check the limit values for theinterference bands 3, 4, 5 in order to see theSS level for the interference band. Check the

percentage of measurements especially in thehigher SS bands 4 and 5. If there is a high

percentage of measurement in these bands,

the cell has uplink interference problems.

Check if the frequencies in the interfered cells

are co channels with the neighbours. Checksalso if the BCCH in the interfered cells is

adjacent with BCCH or TCHs in theneighbours and BCCH in the neighbours areadjacent with TCHs in the interfered cells. Iffrequency hopping is used (synthesiserhopping) check also that the same hopping

sequence number is not used on neighbourswith the same TCH frequencies or that aneighbour BCCH frequency is used in thehopping TCH frequencies.

Another reason for interference problems can

be high-situated cells or cells that shoot very

far e.g. over open water or other open areas.Check therefore which cells that have thesame frequency group as the interfered cells.

6.3.1 Bad frequency plan

Check the frequency plan using the C/I plots

to see that cells close to each other donthave the same or adjacent frequencies.


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6.3.2 External interference

If external interference is suspected in a cell,visit the site or find information if the external

interference may be caused by other cellularsystems. The interference might also becaused by be microwave links, radio stationsor any other radio equipment.

One way to investigate external up-link

interference is to run FAS or FOX, ifavailable, on the cell to check whichfrequencies that are disturbed

Another and more accurate action is to goout to the site and use a spectrum analyzerto find the external interference.

Actions to solve the problems with externalinterference if it can not be stopped, can be

to install or change filters in the Base Station,redirect the antenna or even move the site.

6.3.3 Congestion

If congestion is a problem, the features CellLoad Sharing and Assignment to Another

Cell can be activated and optimized as ashort-term solution. As a long term solution,more capacity should be added, e.g. with

more TRXs in affected cells, micro cells ornew sites to handle the traffic. Before thistype of activities can be recommended, adeeper analysis of the traffic and congestionproblem needs to be made.

6.3.4 Missing neighbour cell relations

Missing neighbour cell definitions, or otherproblems preventing handover, might lead to

dragged calls and thereby interference in co-channel cells.

Check the cell plan and/or run the featureNCS, to see how to use the features to findmissing neighbor cell relations. Consider to

add the identified missing neighbourrelations. It is possible to temporarily insertneighbor relations to evaluate theperformance before finally make the change.


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6.3.5 Wrong antenna type or bad antennapositions

Check site data to see which type ofantennas that are used and if they are the

most appropriate ones. Are low gainantennas used? Should some antennas be

changed?

High antenna positions or bad azimuths can

create interference problems.

Check the coverage and C/I plots, together

with site information and statistics forsuspected antenna position related

problems. Perform site visits to verify theantenna position and make drive tests.

Possible ways of improving the situation may

be a change of azimuth or down tilting. If thatdoes not help, an alternative antennaposition should be considered.

6.3.6 HW/SW Problems and site outages

HW and/ SW problems may also result in,what appears to be, high interference. There

could, for example, be problems with BTSRX, lack of antenna diversity, RX feeder

problems, and BTS HW problems. Badconnectors and similar problems can also

cause Inter-modulation problems.

Check if the interference is related to any siteoutage. Investigate if any cells those

separates co- or adjacent-channel cells havebeen down in the problem area. Check thealarm log, cell down time statistics or contactthe operation and maintenance center for

information.

6.3.7 Cell parameter settings and RN

featuresCheck Cell Design Data for the concernedcells. Check if interference reducing features

are activated and configured according torecommended values. Check also other

parameter settings such as locatingparameters.


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6.4 Troubleshooting

The trouble shooting if interference is dividedinto uplink interference, downlink interference

and external interference. If frequencychanges are necessary remember to alwayscheck the BSIC also since it may benecessary to change that as well.

6.4.1 Uplink Interference

If uplink interference is suspected in the cellcheck ICM measurements for the concernedcell. Use a measurement period for several

days to ensure that the problem was not atemporary problem. Use Mobile TrafficRecording and TEMS drive tests in thesuspected cell. Frequency Allocation Support

(FAS) can also be used to find the interferingfrequency. Investigate if the interference isinternal or external. Check if there are anydifferences in time for the interference levels.If the interference is related to high traffictimes it is likely that the interference isinternal and comes from other mobile

stations in the network.

Probable reasons for uplink interferencemight be:

Non working MS power regulation. Check

the MS power control parameters andchange to recommended values.

Bad antenna positions. High antennapositions or bad azimuths (e.g.

Antennas shooting directly to each other and

has line of sight) etc. can have the impactthat co-channel sites are overheard. Try tolocate the problem antenna and change

azimuth or down tilt it. If that does not helpan alternative antenna position should beconsidered, for example a position protectedby obstacles or below roof level etc.

Internal co-channel or adjacent channel

problems. Try to locate the interferer andchange frequency on it or on the interfered

cell.

Problems with BTS RX. Lack of antennadiversity, RX feeder problems, and BTS HW

problems.


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DTX on uplink not activated.

External interference.

If the problem can not be located try tochange frequency in the cell and see if thesituation improves.

6.4.2 Downlink Interference

If downlink interference is suspected in thecell the first step is to check the C/I or C/A inthe cell-planning tool (EET or TCP). In TCPdisplay only the disturbed frequency andcheck if any closely located sites have thesame frequency (or adjacent). Look also forany high located sites, sites close to open

areas, water etc with the same or adjacentfrequency. Display the coverage for thesesites in order to check the potentialinterference signal strength in the affectedcell. Check that the different clutter codeshave reasonable values.

Check also the cell structure and antennadirections. A good cell pattern is regular with

the sites evenly distributed and the antennasare not pointing to each other.

Trouble shooting with the TEMS isrecommended in order to determine theinterfering source. For co-channel

interference the cell can be halted during lowtraffic times and frequency scanning used (or

just observing if the co- frequency appears inthe TEMS). If the interferer is defined as aneighbour analysing the log files might do ifdriving has been performed in and out of theconcerned cells neighbours. For adjacent

channel interference analysing the log fileswill show the problem if the adjacent

interferer is defined as a neighbour otherwisefrequency scanning is recommended. Check

also if the problem is because of missingneighbours or bad neighbour relations

dragging unwanted frequencies into the celland causing the interference. The problemmay also be caused by swapped antennas

having the effect that the wrong frequency istransmitted in that direction.


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It might also be necessary with site visits inorder to find high located sites that are in lineof sight or buildings that might causereflections etc. In order to solve the problem

changing of frequency on the interferer or theinterfered cell might be necessary. The twointerferers should however be logged in

some way, so that the problem doesnt comeback in the next frequency plan.

Another way to solve the problem is toreduce the signal strength of the interferingfrequency in the cell. This is for example

done by down tilting the interferer orchanging the antenna type to a lower gain,

other beam-with or lowers the position. Theback lobe from a antenna may also causeinterference and also here the interfering SSshould be reduced, for example by mounting

the antenna on the house wall or changingantenna type to a antenna with lower front toback ratio or down tilt the interfered cell in

order to make it stronger in the nearby area.

6.4.3 External Interference

If external interference is suspected in a cell,visit the site or find information if any sitesfrom other system are close to the site. It

might be other base sites, microwave links,radio stations or any other radio equipment.It can also be another national GSM network

that is using the same frequencies; this ishowever normally discovered by the BSIC

(NCC value).

One way to investigate external interferenceis to switch off Frequency hopping and run

Channel Event Recording (CER) or FAS onthe cell. The interfered frequency is then

discovered if the external interference is inuplink frequency band. If the externalfrequency is in the downlink frequency band,switch off frequency hopping and use CER,MTR and/or TEMS drive test and check the

result by STS until the interfered frequency isfound. The above mentioned actions should

preferably be performed during low traffichours.

Another and more accurate action is to go

out to the site and use a spectrum analyserto find the external interference.


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Actions to solve the problem can be to installor change filters on the RBS, contact theowner of the interfering equipment, redirectthe antenna or even move the site.

7 HandoverHandover is a key function in a GSM network.If the handover performance is bad thesubscribers will perceive the quality of thenetwork as bad.

7.1 Reasons for poor handover

performancePossible reasons for handover problems in acellular network could be:

TCH congestion

Parameter settings

HW problems


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Interference

Poor coverage

Missing neighbor cell relations or missingmeasurement frequencies

Incorrect Inter-MSC handover definitions

7.2 Used FormulasH_SUC: Successful Handovers of TotalNumber of Handover Attempts.

H_REV: MS reversion to old channel of TotalNumber of Handover Attempts.

H_LOST: MSs lost at HO of total number ofHO attempts

H_DEC_TOT: Total Number of HandoverDecisions.

H_DEC_SUC: Handover Attempts of TotalNumber of Handover Decisions.

H_BQ_DWN: Handover Decisions due to Bad

Downlink Quality of Total Number of HandoverDecisions.

H_BQ_UP: Handover Decisions due to BadUplink Quality of Total Number of HandoverDecisions.

H_KCELL: Handover Decisions to Better K-Cell of Total Number of Handover Decisions.

H_LCELL: Handover Decisions to Better L-Cellof Total Number of Handover Decisions.

H_10SEC: Successful Handover Back to OldCell within 10 sec.

HO_SUC: Successful Internal OutgoingHandovers of Total Number of InternalOutgoing Handover Attempts.

HO_REV: MS Reversion to Old Channel at

Internal Outgoing Handovers of Total Numberof Internal Outgoing Handover Attempts.

HOE_SUC: Successful External OutgoingHandovers of Total Number of ExternalOutgoing Handover Attempts.


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HOE_REV: MS Reversions to Old Channel atExternal Outgoing Handover of Total Numberof External Outgoing Handover Attempts.

HI_SUC: Successful Internal IncomingHandovers of Total Number of InternalIncoming Handover Attempts.

HI_REV: MS Reversions at Internal Incoming

Handovers of Total Number of Internal

Incoming Handover Attempts.

HI_LOST: MSs lost at Internal Incoming

Handovers of Total Number of InternalIncoming Handover Attempts.

T_DR_HO: Lost Handovers of Total Number ofDropped TCH connections.

7.3 Analysis

7.3.1 Neighbouring cell relation problems

Check from the NCS or NOX measurements ifnew neighbouring cell relations should be

defined.Verify from the consistency check that:

All neighbours are mutually defined

All cells have correct measurement lists.

Two neighbouring cells to a cell do not havethe same BCCH and BSIC

Neighbours may also be defined but have thewrong BSIC/BCCHNO defined in another BSC,

causing inter-BSC handover problems.

7.3.2 Cell parameters settings and RNfeatures

Check that the cell parameters are set torecommend. For example, using L-locatingmay cause problem if not properly optimized.

Furthermore too long locating filter lengthvalues may trigger late handover attempts thatmight lead to many unsuccessful attempts.

Verify which radio network features that areused and how the parameters are set for the

features.


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Check the use of the features Cell LoadSharing and Assignment to Another cell.Incorrect use of the features may also affectthe Handover performance negatively

7.3.3 Hardware problems.

If the handover reversion rate is very high (> 5-6%), for a cell, and there is no strong

indications of interference problem in the targetcell, check the BTS error log. Even if the error

log does not indicate any problems, theantenna or the antenna near parts can befaulty and a site visit could be recommended.

7.3.4 Too many measurement frequenciesin the active BA list

Check the number of measurement

frequencies in the active BA-list. Too manymeasurement frequencies in the active BA listcan result in fewer samples per frequency and

less accurate handover decision.

Make sure that the BA list contains only the

measurement frequencies of the neighboursdefined for the cell. Check also, with STSmeasurements, number of handover attemptsper handover relations. Handover relationswithout handover attempts for a couple of days

of measurements either indicates HW/SWproblems or that the handover relations are not

used and can therefore be removed.

7.3.5 Poor coverage and coverage holes

Check the best server plots for suspected

coverage problems. For areas with poorcoverage, more sites might be needed.However, before suggesting this type ofsolution a more thorough investigation is

needed.

7.3.6 Congestion problems

If congestion problems are identified in cellsclose to the poor performing handover cells,this might at least be one cause for thehandover problems.


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Check if the congested cells are able to use allinstalled transceivers or if the availability islow. The features Cell Load Sharing and

Assignment to Another cell might be used as a

short term solution to handle congestion.

It is also possible to recommend expansionwith more TRXs in affected cells orrecommend cell splits, micro cells or new sitesto handle the traffic congestion. However,

before suggesting this type of solution a morethorough investigation is needed.

7.3.7 High interference

If interference is causing handover problems,check the frequency plan to see if there are

adjacent- or co-channel frequencies near thecell. If this is the case, frequency changes

might solve the problem.

In some cases a cell is covering an area faraway from its site, causing interferenceproblems. Down tilting the antenna orreduction of output power are examples of

actions that could solve the problem.

7.3.8 Poor inter-MSC handoverperformance

If there is a low handover success rate in theinter-BSC or inter-MSC handover relations, aprobable cause might be incorrect definitions

in either MSCs or BSCs.

7.4 TroubleshootingTrouble shooting is divided into three maintopics, low handover attempts, unsuccessful

handovers, handover reversion and Ping-Ponghandovers. If bad inter-MSC HO performance

can be seen a more detailed analysis shouldbe recommended.

7.4.1 Too few Handover attempts or nohandovers

Check for cells and cell relations with lowamounts of handovers compared to other cellsin the same area or according to traffic


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situation. Check also for cell relations withunbalanced handover, i.e. high amount ofhandovers in one direction but a fewhandovers in the opposite direction.

Formulas:

Total number of internal and external outgoingand incoming Handover Attempts

Total number of Handover Attempts per cellrelation

Handover Attempts of total HandoverDecisions

7.4.2 Unsuccessful (lost) handovers

Unsuccessful handovers are divided into two

cases; handover lost which is a dropped calland handover reversion where the mobiles cancontinue the call in old cell. Handover lost arealso registered as drop call in the drop call

formula.

Formulas:

Successful Handovers of Total Number ofHandover Attempts


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MS reversions to old channel of total number

of handover attempts


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7.4.4 Ping-Pong Handovers

Ping-Pong handovers are defined as

handovers that are returned to old cell within10 seconds.

Formula:

Handovers back to old cell within 10 seconds.

8 ReferencesRNI Module Description, General Optimization andTroubleshooting, GSM 00021-FAY 201 693


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NPI Description for the area Random AccessPerformance LVR/P-97:0435

RNI Module Description, Paging and LocationUpdating, GSM, 00021-FAY 201 688

NPI Description for the area Paging LVR/P-97:0463

NPI Description for the area Location UpdatingLVR/P-97:0434

RNI Module Description, Call Set-up, GSM 00021-FAY 201 685

NPI Description for the area Call Set-up LVR/P-97:0476

RNI Module Description, Dropped Calls, GSM00021-FAY 201 691

NPI Description for the area Dropped Calls LVR/P-97:0437

RNI Module Description, Traffic and Capacity, GSM00021-FAY 201 692

NPI Description for the area SDCCH & TCH LVR/P-97:0475

RNI Module Description, Interference, GSM 00021-FAY 201 689

NPI Description for the area Interference LVR/P-97:0438

RNI Module Description, Handover, GSM 00021-FAY 201 690

NPI Description for the area Handover PerformanceLVR/P-97:0472

Optimisation GSM

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