Guia de Otimização - RN PERFORMANCE

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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

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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 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 SET-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

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 CAPACITY PROBLEMS .................................... 32 6.2 USED FORMULAS................................ .......................................... 32 6.3 ANALYSIS .................................................................................... 33


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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 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 7.4 TROUBLESHOOTING ...................................................................... 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 8.4 TROUBLESHOOTING ...................................................................... 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

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 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 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 Analysis 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.


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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 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).


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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 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.


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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 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 increased 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.


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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 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 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.

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


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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 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 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


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• 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 Measurement Interval.

HA_BE_SUC: Successful Assignment Handovers to Better Cell of Total Number of Assignment Handover Decisions to Better Cell.

HA_WO_SUC: Successful Assignments Handovers to Worse Cell of Total Number of Assignment Handover Decisions to Worse Cell.

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.


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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 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 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).


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• Assignment to another Assignment to another cell, (ASSOC), is a feature that allows a mobile to seize a traffic channel in another 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 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 for internal or external interferers. Ask local


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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 tilted is 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 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-


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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 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


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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.

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 Measurement Interval.

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.

T_DR_SS_BL: Dropped TCH Connections due to Low Signal Strength on both links of Total Number of Dropped TCH Connections.


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

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 Total Number of Dropped TCH Connections.

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 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.


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• 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 the reasons for the downtime. 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.


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• Dropped TCH due to bad quality are often due to interference problems and/or low signal strength in 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 sources such as other networks, radio stations, microwave links etc. The interference could also 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 parameter 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.

• 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.


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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.

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.


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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 different 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 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 that there 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 problems 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.


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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 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.

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.


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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 Possible reasons for traffic capacity problems are:

• High number of blocked devices

• HW problems.

• Poor dimensioning of SDCCH 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 Accesses with Cause “All Other Cases”, e. g. Location Updating,


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

S_TRAF: Average SDCCH Traffic Level.

S_CNGT: SDCCH Time Congestion of Total Measurement Interval.

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 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.


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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. 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 dimensions. 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 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


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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 TRXs to 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 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.


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

Þ 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.

· 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.

Þ Increase TCH capacity.

Þ 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.

Þ Change & repair faulty equipment.

Þ Review the O&M procedures.

Too Frequent Periodic Registration

· Check Random Access distribution.

· Check the timer T3212 in the BSC and the parameters BTDM and GTDM in the MSC.

Þ Decrease the number of periodic registration.


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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.

· Check site position and location area border.

· Check Location Update performance.

· Check the parameter CRH.

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

Þ 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 number of LUs.

SMS Usage

Extensive SMS usage increases the SDCCH traffic and could cause congestion if badly dimensioned SDCCH channels.

· Check SMS activity.

Þ Re-dimension the SDCCH channels with consideration taken to SMS usage.

Cell Broadcast Used

· Check if Cell Broadcast is active.

· If active, check if the operator uses it.

Þ 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.

Þ Increase the number of transceivers. This may lead to problems with floor space, antenna installations, CDU type, expansion cabinets and combiner type.

Þ 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 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.


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Þ 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.

Þ 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 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.

Þ 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.

Þ 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.


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· Check if congestion in neighbouring cell.

· Check handover performance

· Check neighbouring cell definitions. Missing relations could cause handover problems.

Þ Correct handover parameters such as too high or too low hysteresis values, missing neighbour relations, one-way handovers.

Þ Congestion in neighbouring cell needs to be 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.

Þ 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 stated –9dB. The inter symbol interference (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) during the call as well as the service accessibility performance.


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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 Dropped TCH Connections.

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

T_DR_BQ_BL: Dropped TCH Connections at Bad Quality both links of Total Number of Dropped 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 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.


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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_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.

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 can’t receive the


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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 can’t 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 the subscriber has to terminate the call.

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 with 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

Check the frequency plan using the C/I plots to see that cells close to each other don’t have 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 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 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.


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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 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.


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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

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 the suspected 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 obstacles 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 cell.

· Problems with BTS RX. Lack of antenna diversity, 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 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 neighbour analysing the log files might do if driving has been performed in and out of the concerned cell’s 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.


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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 doesn’t come back 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 SS should 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, 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 run Channel 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.


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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 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


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• 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 Number of Handover Attempts.

H_REV: MS reversion to old channel of Total Number of Handover Attempts.

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.

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 Outgoing Handover Attempts.


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

HI_SUC: Successful Internal Incoming Handovers of Total Number of Internal Incoming 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 Internal Incoming Handover Attempts.

T_DR_HO: Lost Handovers of Total Number of 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.


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Check the use of the features Cell Load Sharing and Assignment to Another cell. Incorrect use of the features may also affect the 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 target cell, check the BTS error log. Even if the error log does not indicate any problems, the antenna or the antenna near parts can be faulty and a site visit could be recommended.

7.3.4 Too many measurement frequencies in the active BA list Check the number of measurement frequencies in the active BA-list. Too many measurement frequencies in the active BA list can result in fewer samples per frequency and less accurate handover decision.

Make sure that the BA list contains only the measurement frequencies of the neighbours defined for the cell. Check also, with STS measurements, number of handover attempts per handover relations. Handover relations without handover attempts for a couple of days of measurements either indicates HW/SW problems 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 poor coverage, more sites might be needed. However, before suggesting this type of solution a more thorough investigation is needed.

7.3.6 Congestion problems

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


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Check if the congested cells are able to use all installed transceivers or if the availability is low. 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 expansion with more TRXs in affected cells or recommend cell splits, micro cells or new sites to 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 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 and handover reversion where the mobiles can continue the call in old cell. Handover 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.

Formula:


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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 within 10 seconds.

Formula:

Handovers back to old cell within 10 seconds.

8 References RNI Module Description, General Optimization and Troubleshooting, GSM 00021-FAY 201 693


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

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

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

NPI Description for the area Location Updating LVR/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, GSM 00021-FAY 201 691

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

RNI Module Description, Traffic and Capacity, GSM 00021-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 Performance LVR/P-97:0472

Guia de Otimização - RN PERFORMANCE

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