General Gsm Radio

8/4/2019 General Gsm Radio

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20/05/2008 Page 1 of 5 8

GENERAL GSM RADIO

NETWORK OPTIMIZATION

Abstract

This document will provide with an overview general GSM radio network optimization areas; with

regards to analysis and troubleshooting.

RADO NETWORK PERFORMANCE

20/05/2008 Page 2 of 58

Table of Contents

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

. . . . . . . . . . 4

2 RANDOM ACCESS............................................................................. 5

2.1 REASONS FOR POOR RANDOM ACCESS PERFORMANCE 5

2.2 USED FORMULAS................................ ............................................ 5

2.3 ANALYSIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . 6

3 P AGING AND LOCATION U PDATE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 REASONS FOR POOR PAGING AN LU PERFORMANCE 9

3.2 USED FORMULAS................................ ............................................ 9


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3.3 ANALYSIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 11

3.3.1 Paging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

3.3.2 Location Update................................................................ 14

3.4 TROUBLESHOOTING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . 15

3.4.1 General. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.4.2 Unsuccessful Location Updating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 CALL SE T-UP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . 19

4.1 REASONS FOR POOR CALL SET -UP PERFORMANCE 19

4.2 USED FORMULAS................................ .......................................... 20

4.3 ANALYSIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 20

4.3.1 Random Access problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.3.2 Cell parameter settings and RN features 21

4.4 TROUBLESHOOTING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . 22

4.4.1 General problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.4.2 Low signal strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.4.3 SDCCH and TCH congestion............................................. 23

4.4.4 HW faults and other problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5 DROPPED CALLS............................................................................ 24

5.1 REASONS FOR DROPPED CALLS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.2 USED FORMULAS................................ .......................................... 25

5.3 ANALYSIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 26

5.3.1 SDCCH Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

5.3.2 TCH Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27


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5.4 TROUBLESHOOTING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . 29

5.4. 1 Dropped Calls Due To Bad Quality..................................... 29

5.4.2 Dropped Calls Due To Low Signal Strength 30

5.4.3 Dropped Calls Due To Other Reasons 31

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

. . . . . . . . 32

6.1 REASONS FOR TRAFFIC C APACITY PROBLEMS.................................... 32

6.2 USED FORMULAS................................ .......................................... 32

6.3 ANALYSIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 33

6.3.1 SDCCH/TCH availability.................................................... 33

6.3.2 Cell size and location analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.3.3 Feature activation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.4 TROUBLESHOOTING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . 35

6.4.1 Congestion, general. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 35

6.4.2 SDCCH Congestion.......................................................... 35

6.4.3 TCH Congestion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

7 INTERFERENCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .

. . . . . . 40

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

7.2 USED FORMULAS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . 41

7.3 ANALYSIS.................................................................................... 42

7.3.1 Bad frequency plan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7.3.2 External interference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

7.3.3 Congestion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44

7.3.4 Missing neighbour cell relations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44


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7.3.5 Wrong antenna type or bad antenna positions 45

7.3.6 HW/SW Problems and site outages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 45

7.3.7 Cell parameter settings and RN features 45


. . . . 46

7.4.1 Uplink Interference............................................................ 46

7.4.2 Downlink Interference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

7.4.3 External Interference................................ ......................... 48

8 HANDOVER. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . 49

8.1 REASONS FOR POOR HANDOVER PER FORMANCE ..49

8.2 USED FORMULAS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . 50

8.3 ANALYSIS.................................................................................... 51

8.3.1 Neighbouring cell relation problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 51

8.3.2 Cell parameters settings and RN features ..51

8.3.3 Hardware problems........................................................... 52

8.3.4 Too many measurement frequencies in the active BA list 52

8.3.5 Poor coverage and coverage holes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 52

8.3.6 Congestion problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.3.7 High interference.............................................................. 53

8.3.8 Poor inter-MSC handover performance .53


. . . . 53

8.4.1 Too few Handover attempts or no handovers .53

8.4.2 Unsuccessful (lost) handovers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

8.4.3 Handover reversions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

8.4.4 Ping-Pong Handovers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57


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9 REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . 57

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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RADO NETWORK PERFORMANCE

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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 access

performance

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 Ra ndom 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.

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 Analysis


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A 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.

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. Th is 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 t w o 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.

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.


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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.

2.1 Reasons for poor paging and LU

performance

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).

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

Attempts.


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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

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


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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 Subscr ibers

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

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

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

page response before the timer has expired.


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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 planning affecting 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.

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


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Poor coverage and high interference can also affect the location

updating performance.


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2.4

Troubleshooting

2.4.1

General

Optimization of the LU performance is closely

related to optimization of paging performance.

A too high paging load can be alleviated

through a reduced LA size. Smaller LAs

though ten ds to generate a larger location

updating load since the rate of mobiles

crossing LA borders increases as the size of

the LAs decreases. This increased location

updating load has a restraining effect on the

desire to reduce the size of the LAs. The

increas ed location updating load is mainly

manifested in an increased SDCCH traffic in

the LA border cells. The limiting factor will thus

be the SDCCH capacity requirement.

In Table 1 some of the most important BSC

parameters for Location Updating and Paging

are given.


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In Table 2 MSC parameters and exchange

properties relevant for paging and location

updating are given.

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 more

SDCCH resources in the LA border cells has a

marginal effect on the system since in general

capacity is not a scarce resource in rural


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areas. (Cellular systems in rural areas are

most often limited by coverage and not by

capacity.)

LA borders should be drawn up outside

villages and minor cities and unnecessary

criss-crossing over high ways should be

avoided.

Medium size cities

It is preferable to fit a medium sized city (less

than 1 million people) into one LA to reduce

the location updating load.

The LA borders should be drawn up in low

subscriber density areas well outside the city.

LA borders crossing high ways should be

avoided as much as possible.

Major size cities

LAs in large cities (more than 1 million people)

tend to be quite large, in particular where the city 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 the

total paging capacity increases with the

number of BSCs at least at BSC level. The

cells in the LA are split between the BSCs. The

paging load in the BTSs is however not

reduced by load sharing in the BSCs and it i s

thus important to look out for overload

situations in the BTSs/paging channels). One

reason to this is that it is often difficult to find a

good way to split a city into more than one LA.


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However, provided that it is possible to split a

city in two or more LAs, without creating

overload situations in those cells that thereby

becomes LA border cells, this is

recommendable.

The upper bound of the size of an LA depends on the capacity of the equipment used and the

scenario. It is thus difficult to give a generally applicable recommendation on the size of an LA. The

best approach is to measure the

actual paging load and to evaluate the

performance.

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 not

criss-cross over high ways.

The SDCCH capacity of the LA border cells

should be dimensioned to cater for the

expected location updating load.

2.4.2

Unsuccessful Location Updating

A check list of what can be done if there is a

low ratio of successful location update is

presented below.

Border Cell

Check if the cell is a border cell.

? Reconsider the tuning and increase of the

hysteresis CRH. The cell could be reallocated

to another location area.

Low CRH Hysterisis

Check the setting of CRH.

? Increase CRH.


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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 be redone.

Cells could be moved to another

location area.

Short Time Interval between Periodic

Registrations

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 SDCCH

channels.

Automatic De -registration Not Used

Check if automatic de-registration not used.

? Activate automatic de-registration.

3

Call Set-up

The call set-up investigation includes analysis

and improvement of Random Access success


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rate, SDCCH drop rate, SDCCH and TCH

congestion and TCH assignment success rate.

3.1

Reasons for poor call set-up

performance

Possible reasons for poor call set-up

performance could be:

TCH Congestion

SDCCH Congestion

Incorrect Parameter Settings

HW Problems

Interference

Poor coverage

3.2

Used Formulas

S_EST: Number of SDCCH Establishments of

Total Number of SDCCH Seizure Attempts

when No SDCCH Congestion.

S_CNGT: SDCCH Time Congestion of Total

Measurement Interval.

T_AS_SUC: Successful TCH Assignments of

Total Number of Assignment Attempts.

Tx_CNGT_U: TCH/x (x=F, H) Time

Congestion in Underlaid Subcell of Total


HA_BE_SUC: Successful Assignment

Handovers to Better Cell of Total Number of


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Assignment Handover Decisions to Better Cell.

HA_WO_SUC: Successful Assignments

Handovers to Worse Cell of Total Number of

Assignment Handover Decisions to WorseCell.

3.3

Analysis

SDCCH is used for the signalling during the

call setup phase, at location updating and for

SMS. A TCH assignment occurs during a call

setup when changing from the SDCCH to the

TCH. Congestion, hardware problems,insufficient coverage or interference can cause

low assignment success rate.

Some useful guidelines on how to find possible

explanations for bad call setup performance

can be found below. Make sure that there are at least some SDCCH and TCH call attempts in a cell

before judging a cell to have bad call setup performance.

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 for

failed Random Access to be stepped.

Another possibility is that random noise is

interpreted as Random Accesses.

3.3.2

Cell parameter settings and RN

features

Incorrect settings of the cell parameters

ACCMIN, MAXRET, TX and MAXTA could


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negatively influence the Random Access

performance.

The parameter ACCMIN gives the minimum

signal strength that have to be received in

the mobile for permission to access the

system. A low value of ACCMIN means that the accessibility to the network in idle mode is increased,

at the expense of the risk of

having an increased number of call setup

failures.

Low SDCCH establishment success rate

might indicate problems with the response to

Immediate Assignment from mobiles (e.g.

because of low signal strength or

interference). It can also depend on a wrong setting of MAXRET and TX. One example: If MAXRET=7

and TX=50, there is a possibility that more random accesses are sent before the MS has got the

message immediate

assignment from the BSC. The BSC

allocates a SDCCH for the 2nd RA even

though the 1st one was successful. This

gives unnec essary SDCCH load (since the

SDCCH is never used).

Assignment to another Assignment to

another cell, (ASSOC), is a feature that

allows a mobile to seize a traffic channel inanother cell than the serving one during call

setup. If the feature is used the assignment

success rate might show a too low value if

the cell in question perform many

assignment handovers. The reason is that in

case 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 the


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assignment 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 in

order of importance relevance.

Has the site been down during the

measurement period? Sites can during certain

time periods (due to for example transmission problems) which can give bad statistics that is not

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, if

a high SDCCH load is suspected to depend

random accesses. One example of a

parameter setting that is recommended

NOT to have: If MAXRET=7 and TX=50, there

is a big possibility (and 3rd up to 7th) random

access is sent before the BSC knows was

successful or not. The BSC allocates a

SDCCH for the 2nd RA though the 1st one

was successful. This gives a lot of

unnecessary load (since the SDCCH is never

used) and also a lot of failed RA already the

1st one was successful).

Use info from customer complaints and go to

a specific address with and try to find and

solve the problem. If possible: halt serving cell


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for internal or external interferers. Ask local

employees if AMPS located close to the

problem area. These can cause interference within distance of at least 100 m. Sometimes filters can

solve the interference AMPS sites.

MRR, CTR, MTR used to point out areas of

problems?

Frequency change tried?

Antenna down tilt tried? Before doing down

tilt it should be verified TEMS that the

interference occurs in the border area of the

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

Check that antenna directions are according

to 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 calls

and look especially for drops due to low signal

strength. Highlight in the report how many

percentages (approximately) of the call setup

failures that are due to poor coverage and

suggest areas for new sites.

3.4.3

SDCCH and TCH congestion

Check the SDCCH time congestion.

Especially cells close to a location area border

can be heavily loaded and need additional

SDCCH capacity to be able to set up calls. It


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does not matter how many idle TCHs there are in a cell if there at the same time is congestion on the

SDCCH.

3.4.4

HW faults and other problems

If a frequency change did not have the

expected effect or if the cov erage is far less

than the frequency planning tool shows it can

depend on one of the faults listed below. Some

possible faults are:

If a cell is not covering the area that it is

supposed to cover according to the frequency-

planning tool it can depend on that the antenna

is connected to the wrong feeder.

The site can in reality be lower than in the

predictions in the frequency planning tool,

giving less coverage than planned.

There can be alarms indicating HW faults.

Software file congestion

4 Dropped Calls

The retainability performance evaluates the

systems ability to handle established

connections. Dropped calls are probably the

single most important quality item to control in

the system. The level of dropped calls in the

system is in high extent depending on the

initial RF planning, optimization and also the

system growth.

4.1

Reasons for dropped calls


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Possible reasons for a high rate of dropped

calls 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, according

to STS, be:

- Low signal strength

- Bad quality

- Sudden loss of connection (only TCH)

- Excessive timing advance

- Other

4.2

Used Formulas

S_DR- C: Dropped SDCCH Connections of

Total Number of SDCCH Connections.

S_DR_ERLM: Erlang Minutes per Dropped

SDCCH Connection.

S_DR_SS: Dropped SDCCH Connections

due to Low Signal Strength of Total Number

of Dropped SDCCH Connections.

S_DR_BQ: Dropped SDCCH Connections

due to Bad Quality of Total Number of

Dropped SDCCH Connections.


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S_DR_TA: Dropped SDCCH Connections

due to Excessive Timing Advance of Total

Number of Dropped SDCCH Connections.

S_DR_OTH: Dropped SDCCH Connections

due to Other Reasons than Low Signal

Strength, Bad Quality or Excessive Timing

Advance of Total Number of Dropped

SDCCH Connections.

T_TRAF: Average TCH Traffic Level.

T_CONGT: TCH Time Congestion of Total


T_AVAIL: Available TCHs (not blocked) of

Total Number of Defined TCHs .

T_DWN: Average Cell downtime for active

cells

H_SUC: Successful Handovers of Total

Number of Handover Attempts.

T_DR-S: Dropped TCH Connections of Total

Number of Calls Terminated in the Cell.

T_DR_ERLM: Erlang Minutes per Dropped

TCH Connection.

T_DR_SS_DL: Dropped TCH Connections

due to Low Signal Strength on Downlink of

Total Number of Dropped TCH Connections.

T_DR_SS_UL: Dropped TCH Connections

due to Low Signal Strength on Uplink of Total

Number of Dropped TCH Connections.


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T_DR_SS_BL: Dropped TCH Connections

due to Low Signal Strength on both links of


T_DR_SUD: Suddenly lost connections of


T_DR_BQ_DL: Dropped TCH Connections at Bad Quality on Downlink of Total Number of Dropped

TCH Connections.

T_DR_BQ_UL: Dropped TCH Connections at Bad Quality on Uplink of Total Number of Dropped TCH

Connections.

T_DR_BQ_BL: Dropped TCH Connections at

Bad Quality on both links of Total Number of

Dropped TCH Connections.

T_DR_TA: Dropped TCH Connections due to

Excessive Timing Advance of Total Number

of Dropped TCH Connections.

T_DR_OTH: Dropped Calls due to Other Reasons than Low Signal Strength, Bad Quality or Excessive

Timing Advance of


4.3

Analysis

4.3.1

SDCCH Results

If a high drop rate on SDCCH has been

noticed the following actions is

recommended in order to proceed and solve

the problems.

Improvements for dropped calls on TCH will

improve the drop call rate on SDCCH. I.e.

recommend to trouble shoot the TCH drop


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calls first if there is poor performance on both

TCH and SDCCH.

The drop call rate on SDCCH can be

improved if the congestion on TCH is

decreased. Recommend to use the feature

assignment to worse cell or increase the

capacity on TCH.

The reasons for low SS drops could be too

few sites, wrong output power, shadowing,

no indoor coverage or network equipment

failure.

The reasons for dropped calls due to bad quality on the uplink or downlink are related to internal

or external interference and

trouble shooting is needed to find the

interferers. The situation could be temporary

improved by means of applicable features.

Recommend features that are not activated

or recommend alternative parameter setting.

The reasons for drops on to high timing advance is related to the site location i.e. close to open

water, desert or hilly terrain and the setting of MAXTA and TALIM.

Setting MAXTA to 63 and TALIM to 62 could

solve the problem and/or tilt the antennas, reduce antenna height, change antenna or reduce output

power.

Miscellaneous problems could for example

be mobile errors which can occur when old mobiles may cause dropped calls if certain radio network

features are used. Another reasons could be that the MS is damaged and not working properly.

4.3.2 TCH Results

If a high drop rate on TCH has been noticed the following actions is recommended in order to

proceed and solve the problems.

High drop rate due to high outage time or

low availability. Inform the operation and maintenance department about the problems or check thereasons for the downtime.


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Check also the alarm list or BTS error log. Another way is also to check the resolution

time for the different alarm categories. Many problems with dropped calls are often related to

insufficient O&M routines and not to radio problems.

Dropped TCH due to bad quality are often due to interference problems and/or low signal strengthin areas where there is no dominant server. The reason could also be missing neighbour relations so

that the mobile is not connected to the strongest server and therefore perceives bad quality. The

interfered cell should be investigated in order to find the source for the interference.In most cases

the interference is internal but can also originate from external sourcessuch as other networks, radio

stations,microwave links etc. The interference couldalso be reduced by means of applicable features

such as frequency hopping,

assignment to better cell, DTX, MS/BTS

power control etc. Recommend applicable

features or alternative parame ter setting if

founded incorrect.

Dropped TCH due to low signal strength

are in most cases related to coverage gaps

but can also be a result of missing neighbour

relations, hidden antennas, wrong antenna

type (to low gain), antenna or feeder

problems, incorrect power settings, etc. The

reason can also be unforeseen subscriber

behaviour i.e. the subscribers use their

mobiles indoor, in elevators, parking lots etc.

This can also be indicated if there is a high

percentage of suddenly lost connections.

Dropped TCH due to excessive timing

advance should in normal cases not occur in the 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 63

and TALIM to 62 could solve the problem. Or

reduce the coverage by down tilting the

antennas, reduce antenna height, change

antenna or reduce output power. If the site is

located close to open water etc. the

extended range feature could be considered.


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TCH drops due to other reasons than low

SS, bad quality, excessive timing advance could for example be because of BTS HW problems,

transmission problems, service

affecting maintenance work, uplink

interference problems (external or internal),

mobile station problems etc.

4.4

Troubleshooting

The trouble shooting of dropped calls is

divided 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 the parameters

QLIMDL and QLIMUL are set to correct values. SAUDI 55

Additional data should be collected in order

to make correct conclusions.

Check if there are any normal disconnections

at bad quality in the cell, this could give

indications on that there really is a bad

quality problem in the cell.

Run MRR on the cell and check the average

RXQUAL value in the cell.

Display C/I and C/A predictions in planning

tool. Check if the cell is located in any

interference area. Remember that there

might be interference in the cell even if it is

not displayed in the planning tool. Check idle

channel measurements (ICM) for the cell to see if there exists uplink interference in the cell.


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Check the handover statistics on a neighbour

relation level for the cell. See if there is any

neighbour relation with a high number of bad

quality handovers. This information might

give you the location for the bad quality area

in the cell.

Frequency Allocation Support (FAS) can be

used in order to find the interfered frequency.

This can be useful especially for frequency hopping systems. FAS gives however only the uplink

information.

Perform TEMS drive tests in the suspected

area. Try to locate the interferer, one way for

C/I problems is to halt the affected cell and

perform a frequency scanning in order to

locate the interfering cell.

See if it is possible to change frequency or

reduce the signal strength of the interferer.

For example down tilting the antenna. Check

the antenna diagram in order to see the

effect of diff erent tilt angels. A second

alternative is to change the frequency or

increase the signal strength in the interfered

cell. Check if there is any missing neighbour

relations causing low SS and by that bad

quality drops.

4.4.2

Dropped Calls Due To Low Signal

Strength

If the cell suffers from TCH drop due to low

signal strength the first step is to check the

power setting, power balance and neighbour

relations in the affected cell. Check for

example on a map with site positions or in


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the cell-planning tool for any obviously

missing neighbour relations. Check also the

amount of normal disconnections at low SS

for 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 timing

advance and RXLEV distribution in the cell.

This can give an indication if the subscribers

are close to the base station or in the

outskirts of the cell. If most of the subscribers

are on low TA values the low SS drops might

be lack of indoor coverage or if most of the

subscribers are on high TA values the

problem might be a missing site or neighbour

relation. By checking RXLEV for the cell

indication and compare with coverage plots

in planning tool indications of obstacles

covering the antenna, feeder proble ms or

other reasons for low SS might be found.

Use the handover statistics on a neighbour

relation level to get an indication on where in

the cell the problem might be. Check also if

any of the target neighbours suffers from

severe congestion.

If there are a lot of suddenly lost connections

in the cell this could indicate that there is a

tunnel, underground parking lot, high indoor

usage etc. Try to find the most likely position

in the cell where this kind of drops might


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occur.

Check in planning tool to see if any coverage

gaps or areas with low coverage can be

found in the cell. Verify that the different

clutter values make sense in the planning

tool. Is the site position OK? Is the antenna

azimuth correct, is it shooting to the correct

location, road, building etc.

Perform drive tests in the cell and check for missing neighbours, swapped antennas etc. Perform the

drive test close to the site and

try to see if it is line of sight or of the

antennas are hidden by any obstacles. Make

a site visit and check the antennas if

necessary.

If the low signal strength is not related to any

faults or missing configuration probably a

new site or improved indoor coverage is

needed and the problem should be passed

on to the cell planning department.

4.4.3

Dropped Calls Due To Other

Reasons

If the cell suffers from dropped calls besides

the reasons low SS, bad quality and

excessive timing advance the dropped calls

will be counted as other reasons. That is that

the counters for SS, quality and timing

advance are not incremented and only the

CNDROP, TFNDROP or THNDROP are

stepped.

This could for example be the case in cells

with uplink interference.


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Check if ICM is indicating uplink interference

in the cell.

Other possible reasons could be problems

with the mobile stations of BTS HW

problems, transmission problems, and

service affecting maintenance work.

Check with the operation and maintenance

department or check the applicable alarm

logs if there have been any HW problems,

transmission problems, and service affecting

maintenance work during the time period.

The average cell downtime and TCH and

SDCCH availability should also be checked.

If mobile station problems are suspected in

the network this needs to be raised with the Customer and his customer care department to

investigate the problem further.

5

SDCCH & TCH

Congestion on SDCCH makes it impossible to

set up a call unless the feature immediate

assignment on TCH or adaptive configuration

of logical channels is used. Congestion on

TCH makes it impossible to set up a call

unless features such as Assignment to Worse

Cell or Cell Load Sharing are used. TCH

congestion also means that handover from

another cell is impossible to perform.

5.1

Reasons for traffic capacity

problems


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Possible reasons for traffic capacity problems

are:

High number of blocked devices

HW problems.

Poor dimensioning ofSDCCH and TCH

Features impacting the traffic behaviour

Traffic distribution between 900 and 1800

cells

Traffic distribution between micro- and macro

cells

5.2

Used Formulas

RA_OTHER: Random Acc esses with Cause

All Other Cases, e. g. Location Updating,

Detach, Attach, etc. of Total Number of

Accepted Random Accesses.

S_TRAF: Average SDCCH Traffic Level.

S_CNGT: SDCCH Time Congestion of Total


S_MHT: SDCCH Mean Holding Time.

S_AV_NR: Average Number of Available

SDCCHs.

S_AVAIL: Available (not blocked) SDCCHs of

Total Number of Defined SDCCHs.

S_DR: Dropped SDCCH Connections of Total

Number of SDCCH Connections.

T_AS_SUC: Successful TCH Assignments of


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Total Number of Assignment Attempts.

T_TRAF: Average TCH Traffic Level.

TF_CNGT_U: TCH/F Time Congestion in

under laid Sub cell of Total Measurement

Interval.

T_MHT: TCH Mean Holding Time.

T_AV_NR: Average Number of Available

TCHs.

T_AVAIL: Available TCHs (not blocked) of

Total Nu mber of Defined TCHs.

T_DR_ERLM: Erlang Minutes per Dropped

TCH Connection.

5.3

Analysis

5.3.1

SDCCH/TCH availability

From the STS data it can be seen how much

of the hardware that is being used. Normally

the availability for SDCCH and TCH should be

100%. For the cells showing low availability,

check the BTS error log to make sure that

there are no problems with the hardware error

logs.

5.3.2

Cell size and location analysis

Check where different high traffic cells are

located to verify if they are located in the same

areas. Look for cells with high traffic loads

surrounded by cells with low traffic loads.


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Check the congestion (together with the

number of TRXs) in the small cells. The reason

to the low traffic loads could be that the cells

have been given too small dimension s. Check

if the coverage from the small cells could be

improved. A larger coverage area might

capture more traffic and off -load the

neighbouring cell.

Check if the large cell actually is too big and

consequently captures more traffic than it

should.

5.3.3

Feature activation

In this chapter it is described how short-term actions can be taken to decrease congestion and

improve the capacity of the system.

5.3.3.1

SDCCH Congestion

Selecting the number of time slots in a cell that

are going to be used for signalling is a critical part of network optimization. Increased use of

subscriber services such as Short Message

Service can make the demand for SDCCHs

more unpredictable.

The feature Immediate Assignment on TCH

can be used to lower the load on SDCCH.

Note, however, that the SDCCH first strategy

is recommended, i.e. an idle SDCCH is always

allocated and in case of SDCCH congestion,

the signalling is performed on a TCH instead.

If the optional feature "Adaptive Configuration

of Logical Channels" is available and activated

in the BSC, the system can automatically

assign SDCCHs. The way the feature should

be used depends on the channel dimensioning


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strategy and therefore great care should be

taken when implementing this feature. No

recommended parameter values are given in

the User Descriptions.

5.3.3.2

TCH Congestion

Make sure that the congestion is not caused

by hardware problems or link Make sure that

the congestion is not caused by hardware

problems or link failures. If no problems can be

found, 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 feature

Cell 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 a

short-term growth or a long-term growth:

Short term growth

If the high traffic related to an occasional

event, like sport event, fairs, conference, a

temporary solution might be considered.

Long term growth

If there is a long-term growth the network

capacity has to grow according to the demand.

Check if there is an expansion planned in the

near future for the TCH congested cells.

Check if the congestion is on SDCCH, TCH or


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both.

5.4.2

SDCCH Congestion

The time congestion should be used instead of

congestion based on access attempts as there

is no way to estimate the number of access

attempts a single mobile does.

Increasing Traffic Demand

The increase in traffic could be related to an

occasional event or due to a long-term growth.

Check if short term traffic growth. Make trend

comparisons.

Check if combined SDCCH is used.

Check SDCCH dimensioning.

Increase the number of SDCCH channels.

Note that an increase may lead to the need for

new transceivers.

If combined SDCCH is used, non-combined

channel 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 to

stay extra long time on the SDCCH before

being allocated TS on a TCH. Check if there

exists TCH congestion and if the SDCCH

mean holding time is above 7 seconds. For

immediate assignment the time is 2-2, 5

seconds.


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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 such

as Cell Load Sharing and Assignment to

Worse 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 B SC and the

parameters BTDM and GTDM in the MSC.

Decrease the number of periodic

registration.

Wrong SDCCH Dimensioning

Check SDCCH dimensioning.

Location Area Border Cell

If a cell is located on a non-optimised Location

Area border, unnecessary normal LUs are

performed.


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Check site position and location area border.

Check Location Update performance.

Check the parameter CRH.

If the site is located close to major road or railway; consider moving the Location Area border.

Increase the hysteresis CRH. The CRH is

the hysteresis value used when the MS in idle

mode crosses a LA border. The default value

for this parameter is 4. If a high number of

Location Updates occurs in a Location Area

border cell, a higher CRH can be set in order

to reduce the nu mber of LUs.

SMS Usage

Extensive SMS usage increases the SDCCH

traffic and could cause congestion if badly

dimensioned SDCCH channels.

Check SMS activity.

Re-dimension the SDCCH channels with

consideration 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, the benefits are that the paging success rate will increase.

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 of

congestion 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 are

listed below:

Increasing Traffic Demand

The increase in traffic could be related to an

occasional event or due to a long-term growth.

Check if short term traffic growth.

Check TCH dimensioning.

Check the use of congestion relieving

features 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 Worse

cell and Cell Load Sharing. Note that the

interference level will increase if Assignment to

Worse is used, as some mobiles will be closer

to a co-channel than what was intended in the

frequency plan. The feature will be more

effective if the neighbours are not congested.

In a tight network with a high reuse and

congestion in a larger area, the feature might


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only make the situation worse.

Bad Dimensioning

Bad allocation of TCH in a system may cause

unnecessary congestion. Investigate if it is

possible to move transceivers from non- congested areas to congested areas. Of course, the base

station type, CDU-type,

current number of transceivers, floor space,

combiner type, etc., should be considered

before a recommendation to move transceiver

could be made.

Check TCH traffic.

Re-dimension the TCHs.

Hardware Fault & Installation Fault

Faulty equipment will lead to that not all time

slots can be used for handling traffic that

causes congestion.

Low availability can happen if the channels

have been manually or automatically blocked

and taken out of service. Availability is

depending on the number of frequencies

defined per cel l. The parameter NUMREQBPC

can be used.

Check TCH Availability.

Check TCH blocking.

Change and repair faulty equipment. Review

the O&M procedures.

High Antenna Position

A high antenna position could mean a too

large service area. Also antennas placed on


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hilltops will cover large areas. A large

coverage 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 of

coverage (no coverage at all). Tilting of the

antenna or changing antenna type may also

decrease the coverage area.

Long Mean Holding Time

A low handover activity might lead to a long

mean holding time. A long mean holding time

is 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 congestion if the MS is forced to stay on a cell longer than

necessary.

Check if congestion in neighbouring cell.

Check handover performance

Check neighbouring cell definitions. Missing

relations could cause handover problems.

Correct handover parameters such as too

high or too low hysteresis values, missing

neighbour relations, one-way handovers.

Congestion in neighbouring cell needs to be


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decreased.

Congestion in Surrounding Cells

Check congestion in neighbouring cells.

Review neighbour cell list. New relation could

relieve the congestion.

Check if Assignment to Worse cell is used. If

assignment handover to worse cell is used

(directed retry) check the setting of the

parameter AWOFFSET.

Add new neighbour cells if appropriate.

6

Interference

Cellular systems are often interference limited

rather than signal strength limited and it will

therefore always exist interference in the

system. Frequency planning guiding values is

C/I 12dB without frequency hopping and 9 dB

with frequency hopping. C/A recommended

planning value is 3 dB but in the GSM

specification it is stated9dB. The inter symbol

interf erence (ISI) or the carrier to reflection

C/R, must be larger that 9 dB according to the

GSM specification.

Interference problems could be divided into

service retainability affecting problems and

service integrity problems. The interference

might affect the retainability performance of a

call and having it to drop before normal

termination. The interference might also affect

the speech quality (integrity performance)


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during the call as well as the service

accessibility performance.

6.1

Reasons for high interference

levels

Possible reasons for interference problems in

a 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 Formulas

T_DR_S: Handover Decisions due to Bad

Downlink Quality of Total Number of Bad

Quality Urgency Handover Decisions.

T_DR_ERLM: Call Minutes per Dropped TCH

Connection.

T_DR_BQ_DL: Dropped TCH Connections at

Bad Quality Downlink of Total Number of


T_DR_BQ_UL: Dropped TCH Connections at

Bad Quality Uplink of Total Number of



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T_DR_BQ_BL: Dropped TCH Connections at

Bad Quality both links of Total Number of


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 of

Total Number of TCH Connections.

IHO_BQ_DWN: Intra-Cell Handover Decisions

due to Bad Downlink Quality of Total Number

of Intra-Cell Handover Decisions.

IHO_BQ_UP: Intra-Cell Handover Decisions due to Bad Uplink Quality of Total Number of Intra-Cell

Handover Decisions.

H_BQ_DWN: Handover Decisions due to Bad Downlink Quality of Total Number of Handover

Decisions.

H_BQ_UP: Handover Decisions due to Bad Uplink Quality of Total Number of Handover Decisions.

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 Quality Urgency

Handover Decisions.

H_REV: MS Reversions to Old Channel of

Total Number of Handover Attempts.

TERM_BQ: Bad Quality at successful

termination of Total Number of successful

termination.

T_DWN: Average Cell Downtime for Active

Cells.


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6.3

Analysis

Check which cells that have high call drop rate

(T_DR_ERLM or T_DR_S). Sort out those

cells that have high percentage of drop due to

bad quality. Check later with Idle Channel

Measurement if these cells have high

percentages in band 3, 4 and 5. In that case

the interfered cells are found. (In case of very

high rate of TCH congestion in a cell, ICM

result might not be reliable).

In order to verify the result and locate the

interference for the worst 10-15 cells direction

the following checks should be made:

Check bad quality urgency handover and

handover reversions. A high number of

handovers caused by bad quality directly

points out interference problems in the cell if

QLIM is set correctly. Check the percentage of

bad quality handovers to the different

neighbour relations in order to point out where

the interference problem might be, check also

if the bad quality handovers is mainly on uplink

or dow nlink. Check the percentage of

handover reversion, a handover reversion

occurs when the mobile cant receive the

physical information from the target cell within

a specific time. That is when the mobile

receives the handover command message the

timer T3124 is started in the MS if the MS cant

receive the physical information sent by the

target cell before T3124 expires the MS tries to

reactivate 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 poor that thesubscriber has to terminate the call.


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Check intra cell handover for the found cells. A

high number of intra cell handovers normally

indicates bad quality at high signal strength.

Be aware of, that intra cell handover is not an

accurate interference indicator in congested

networks.

Check the idle channel measurement (ICM)

statistics. Check the limit values for the

interference bands 3, 4, 5 in order to see the

SS level for the interference band. Check the

percentage of measurements especially in the

higher 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 wi

th the neighbours. Checks

also if the BCCH in the interfered cells is

adjacent with BCCH or TCHs in the

neighbours and BCCH in the neighbours are

adjacent with TCHs in the interfered cells. If

frequency hopping is used (synthesiser

hopping) check also that the same hopping

sequence number is not used on neighbours

with the same TCH frequencies or that a

neighbour BCCH frequency is used in the

hopping 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 the

same frequency group as the interfered cells.

6.3.1

Bad frequency plan


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Check the frequency plan using the C/I plots

to see that cells close to each other dont

have the same or adjacent frequencies.

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 cellular systems. The interference might also be

caused by be microwav e links, radio stations

or any other radio equipment.

One way to investigate external up-link

interference is to run FAS or FOX, if available, on the cell to check which frequencies that are

disturbed

Another and more accurate action is to go

out to the site and use a spectrum analyzer

to find the external interference.

Actions to solve the problems with external

interference 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 Cell

Load Sharing and Assignment to Another

Cell can be activated and optimized as a

short-term solution. As a long term solution,

more capacity should be added, e.g. with

more TRXs in affected cells, micro cells or new sites to handle the traffic. Before this type of activities

can be recommended, a

deeper analysis of the traffic and congestion

problem needs to be made.

6.3.4

Missing neighbour cell relations

Missing neighbour cell definitions, or other


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problems preventing handover, might lead to dragged calls and thereby interference in co- channel

cells.

Check the cell plan and/or run the feature

NCS, to see how to use the features to find

missing neighbor cell relations. Consider to

add the identified missing neighbour

relations. It is possible to temporarily insert

neighbor relations to evaluate the

performance before finally make the change.

6.3.5

Wrong antenna type or bad antenna

positions

Check site data to see which type of

antennas that are used and if they are the

most appropriate ones. Are low gain

antennas 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 for

suspected antenna position related

problems. Perform site visits to verify the

antenna position and make drive tests.

Possible ways of improving the situation may be a change of azimuth or down tilting. If that does not

help, an alternative antenna

position should be cons idered.

6.3.6

HW/SW Problems and site outages

HW and/ SW problems may also result in,

what appears to be, high interference. There


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could, for example, be problems with BTS

RX, lack of antenna diversity, RX feeder

problems, and BTS HW problems. Bad

connectors and similar problems can also

cause Inter- modulation problems.

Check if the interference is related to any site

outage. Investigate if any cells those

separates co- or adjacent-channel cells have

been down in the problem area. Check the

alarm log, cell down time statistics or contact

the operation and maintenance center for

information.

6.3.7

Cell parameter settings and RN

features

Check Cell Design Data for the concerned

cells. Check if interference reducing features

are activated and configured according to

recommended values. Check also other

parameter settings such as locating

parameters.

6.4

Troubleshooting

The trouble shooting if interference is divided into uplink interference, downlink interference and

external interference. If frequency

changes are necessary remember to always

check the BSIC also since it may be

necessary to change that as well.

6.4.1

Uplink Interference


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If uplink interference is suspected in the cell check ICM measurements for the concerned cell. Use a

measurement period for several days to ensure that the problem was not a

temporary problem. Use Mobile Traffic

Recording and TEMS drive tests in thesuspected cell. Frequency Allocation Support

(FAS) can also be used to find the interfering

frequency. Investigate if the interference is

internal or external. Check if there are any

differences in time for the interference levels.

If the interference is related to high traffic

times it is likely that the interference is

internal and comes from other mobile

stations in the network.

Probable reasons for uplink interference

might be:

Non working MS power regulation. Check

the MS power control parameters and

change to recommended values.

Bad antenna positions. High antenna

positions or bad azimuths (e.g.

Antennas shooting directly to each other and

has line of sight) etc. can have the impact

that co-channel sites are overheard. Try to

locate the problem antenna and change

azimuth or down tilt it. If that does not help

an alternative antenna position should be

considered, for example a position protected

by obs tacles or below roof level etc.

Internal co-channel or adjacent channel

problems. Try to locate the interferer and

change frequency on it or on the interfered


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cell.

Problems with BTS RX. Lack of antenna

diversity, RX feeder problems, and BTS HW

problems.

DTX on uplink not activated.

External interference.

If the problem can not be located try to

change frequency in the cell and see if the

situation improves.

6.4.2

Downlink Interference

If downlink interference is suspected in the

cell the first step is to check the C/I or C/A in

the cell-planning tool (EET or TCP). In TCP

display only the disturbed frequency and

check if any closely located sites have the

same frequency (or adjacent). Look also for

any high located sites, sites close to open

areas, water etc with the same or adjacent frequency. Display the coverage for these sites in order to

check the potential

interference signal strength in the affected

cell. Check that the different clutter codes

have reasonable values.

Check also the cell structure and antenna

directions. A good cell pattern is regular with the sites evenly distributed and the antennas are not

pointing to each other.

Trouble shooting with the TEMS is

recommended in order to determine the

interfering source. For co-channel

interference the cell can be halted during low traffic times and frequency scanning used (or just

observing if the co- frequency appears in the TEMS). If the interferer is defined as a


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neighbour analysing the log files might do if driving has been performed in and out of the concerned

cells neighbours. For adjacent

channel interference analysing the log files

will show the problem if the adjacent

interferer is defined as a neighbour otherwise

frequency scanning is recommended. Check

also if the problem is because of missing

neighbours or bad neighbour relations

dragging unwanted frequencies into the cell

and causing the interference. The problem

may also be caused by swapped antennas

having the effect that the wrong frequency is

transmitted in that direction.

It might also be necessary with site visits in

order to find high located sites that are in line

of sight or buildings that might cause

reflections etc. In order to solve the problem

changing of frequency on the interferer or the

interfered cell might be necessary. The two

interferers 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 to

reduce the signal strength of the interfering

frequency in the cell. This is for example

done by down tilting the interferer or

changing the antenna type to a lower gain,

other beam-with or lowers the position. The

back lobe from a antenna may also cause

interference and also here the interfering SSshould be reduced, for example by mounting

the antenna on the house wall or changing

antenna type to a antenna with lower front to

back 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,


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visit the site or find information if any sites

from 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 is

however normally discovered by the BSIC

(NCC value).

One way to investigate external interference

is to switch off Frequency hopping and runChannel Event Recording (CER) or FAS on

the cell. The interfered frequency is then

discovered if the external interference is in

uplink frequency band. If the external

frequency 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 is

found. The above mentioned actions should

preferably be performed during low traffic

hours.

Another and more accurate action is to go

out to the site and use a spectrum analyser

to find the external interference.

Actions to solve the problem can be to install

or change filters on the RBS, contact the

owner of the interfering equipment, redirect

the antenna or even move the site.

7

Handover

Handover is a key function in a GSM network.

If the handover performance is bad the


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subscribers will perceive the quality of the

network as bad.

7.1

Reasons for poor handover

performance

Possible reasons for handover problems in a

cellular network could be:

TCH congestion

Parameter settings

HW problems

Interference

Poor coverage

Missing neighbor cell relations or missing

measurement frequencies

Incorrect Inter-MSC handover definitions

7.2

Used Formulas

H_SUC: Successful Handovers of Total


H_REV: MS reversion to old channel of Total


H_LOST: MSs lost at HO of total number of

HO attempts

H_DEC_TOT: Total Number of Handover

Decisions.

H_DEC_SUC: Handover Attempts of Total

Number of Handover Decisions.


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H_BQ_DWN: Handover Decisions due to Bad Downlink Quality of Total Number of Handover

Decisions.

H_BQ_UP: Handover Decisions due to Bad Uplink Quality of Total Number of Handover Decisions.

H_KCELL: Handover Decisions to Better K-

Cell of Total Number of Handover Decisions.

H_LCELL: Handover Decisions to Better L -Cell

of Total Number of Handover Decisions.

H_10SEC: Successful Handover Back to Old

Cell within 10 sec.

HO_SUC: Successful Internal Outgoing

Handovers of Total Number of Internal

Outgoing Handover Attempts.

HO_REV: MS Reversion to Old Channel at

Internal Outgoing Handovers of Total Number

of Internal Outgoing Handover Attempts.

HOE_SUC: Successful External Outgoing

Handovers of Total Number of External


HOE_REV: MS Reversions to Old Channel at External Outgoing Handover of Total Number of External


HI_SUC: Success ful Internal Incoming Handovers of Total Number of Internal Incoming Handover

Attempts.

HI_REV: MS Reversions at Internal Incoming


Incoming Handover Attempts.

HI_LOST: MSs lost at Internal Incoming


Incoming Handover Attempts.

T_DR_HO: Lost Handovers of Total Number of


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Dropped TCH connections.

7.3

Analysis

7.3.1

Neighbouring cell relation problems

Check from the NCS or NOX measurements if

new 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 have

the same BCCH and BSIC

Neighbours may also be defined but have the

wrong BSIC/BCCHNO defined in another BSC,

causing inter-BSC handover problems.

7.3.2

Cell parameters settings and RN

features

Check that the cell parameters are set to

recommend. For example, using L-locating

may cause problem if not properly optimized.

Furthermore too long locating filter length

values may trigger late handover attempts that

might lead to many unsuccessful attempts.

Verify which radio network features that are

used and how the parameters are set for the

features.

Check if the congested cells are able to use all

installed transceivers or if the availability is

low. The features Cell Load Sharing and


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Assignment to Another cell might be used as a

short term solution to handle congestion.

It is also possible to recommend expansion

with more TRXs in affected cells or

recommend cell splits, micro cells or new sitesto handle the traffic congestion. However,

before suggesting this type of solution a more

thorough 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 the

cell. If this is the case, frequency changes

might solve the problem.

In some cases a cell is covering an area far

away from its site, causing interference

problems. Down tilting the antenna or

reduction of output power are examples of

actions that could solve the problem.

7.3.8

Poor inter-MSC handover

performance

If there is a low handover success rate in the inter-BSC or inter- MSC handover relations, a probable

cause might be incorrect definitions in either MSCs or BSCs.

7.4

Troubleshooting

Trouble shooting is divided into three main

topics, low handover attempts, unsuccessful

handovers, handover reversion and Ping- Pong

handovers. If bad inter- MSC HO performance can be seen a more detailed analysis should be

recommended.

7.4.1


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Too few Handover attempts or no

handovers

Check for cells and cell relations with low

amounts of handovers compared to other cells

in the same area or according to traffic


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situation. Check also for cell relations with unbalanced handover, i.e. high amount of handovers in

one direction but a few

handovers in the opposite direction.

Formulas:

Total number of internal and external outgoing

and incoming Handover Attempts

Total number of Handover Attempts per cell

relation

Handover Attempts of total Handover

Decisions

7.4.2

Unsuccessful (lost) handovers

Unsuccessful handovers are divided into two

cases; handover lost which is a dropped call


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and handover reversion where the mobiles can

continue the call in old cell. Han dover lost are

also registered as drop call in the drop call

formula.

Formulas:

Successful Handovers of Total Number of

Handover Attempts


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MS Lost at Handover of total number of

Handover Attempts

7.4.3

Handover reversions

A handover reversion is when the MS is going

back to the old channel. This happens when

the MS fails to establish itself on the new traffic

channel but succeeds to return to the old traffic

channel. If the mobile does not succeed to

return it will be lost.

Documents

General Gsm Radio