Troubleshooting & Optimizing GSM Networks

Company Confidential 10/4/99 1


Troubleshooting with HP E74XX

Blocked CallsPoor Quality and Drop callsAbnormal HandoversInterferenceTermination Failures


Blocked CallTroubleshooting


Blocked CallsBlocked Calls can occur due to :

•Access Failures•SDCCH Congestion•SDCCH Drop•TCH Congestion

Trouble shooting cause :•Use Layer 3 messages to analyze the cause•Decode System Information Type 3 messages.•Note the parameter , “max_retransmission” ; “CCCH CONF” and “BS_AG_BLKS_RES”•Analyze the blocked call reason using L3 messages


Blocked Call Analysis - L3 messages

1 3

Channel Request Channel RequestRACH .

: Imm AssignmentRACHmax_retrans Service Request

NO RESPONSE FROM N/WACCESS FAILURE ! Signalling

:Signalling

2NO TCH ASSIGNMENT

Channel Request Mobile Returns To IdleRACH TCH BLOCKED !

Imm Assign Reject

SDCCH BLOCKED !


Timeslot - Testing

BT

TT7 6

1 0

……..

TT

TT……..

7 6

1 0BCH

TCH

•Activate Cell Barring from OMC.•Remove this cell from the neighbor list of other cells.

•Get the cell configuration•ARFCN’s and Timeslots configured for TCH.

•For BCH carrier select the Timeslot and carry out the Testing

•For TCH Carriers: Block the BCH Timeslots from OMC•Carry out Timeslot testing.•If more than 1 TCH Carrier is activated, block all others .


Dropped Call Troubleshooting


Drop Calls Analysis1 2

Channel Request Channel Request

Imm Assignment Imm Assignment

Service Request Service Request

Signalling SDCCH Signalling: :

Signalling SpeechTCH

RLT = 0 ; DROPS RLT = 0 ; DROPSSDCCH DROP ! TCH DROP !

3 SDCCH / TCH

Handover Command

Hand Access

Handover Failure

HANDOVER FAILURE DROP !


Drop Call - Analysis with HP E7475

Setting up Alarms can figure out the type of Drop

SDCCH Drop

Alarm : Value Phone State = Constant Dedicated AND

Value Access Time = Constant 0AND

Field Radio Link Counter = Constant 0 Current Value

TCH Drop

Alarm : Value Access Time > Constant 0OR

Value Phone State = Constant Wait on AnswerAND

Field Radio Link Counter = Constant 0 Current Value


Dropped Call Analysis

• SDCCH Drops - Causes– Coverage– Interference & Multipath– BTS performance

• TCH Drops - Causes– Coverage– Interference & Multipath– BTS performance– Pre-emption

• Handover Failure - Causes– Threshold parameters– Missing neighbors

• Solutions to Dropped Calls– Optimize Coverage– Interference Management– Optimize neighbors– Optimize handover

parameters– Effective Frequency Hopping– Use of DTX & Power control


Drop Call - Troubleshooting

RLT=0

RxQual > 4

YES

YES

YESYES

Troubleshoot Multipath/

Co-Channel/BSIC decode

TCH=BCH Pri/Int <= 9

C/IAa <= -9C/IAb <= -9

Co-Channel /MultipathInterference!!No

YESNo

No Adjacent Channel Interference!!

YES YES

Troubleshoot Source Cell

Using BCH Analyzer

L3 MessageAnalysis

No

Rx_Lev < -95

No **

Coverage Hole !!!

Uplink Problem !!!

No

Troubleshoot Link ImbalanceUL-Interference

TCH -Pri/Int <= 9

Optimize Neighbors/Coverage

YES

No

** If TCH is Hopping, then all causes needs to be verified further, before concluding on the root cause and optimizing.

YES

DROP CALL

Handover Attempted ?Handover Failed ?

Area of concern

No

MultipathNo


Poor Quality

•Poor Speech Quality could be due to •Patchy Coverage ( holes)•No Target cell for Handover•Echo , Audio holes, Voice Clipping•Interference ---:

•Co-channel•Adjacent channel•External•Multipath•Noise


Speech Quality Parameters

RxQUAL : Measured on the midamble.Indicates poor speech quality due to radio interface impairments

FER : Measured on the basis of BFI ( Ping -Pong effect on speech )Preferred under Frequency Hopping situation

Echo and distortion : Generally caused by the Transmission and switching system.

Audio holes : Blank period of speech, due to malfunctioning of Transcoder boards or PCM circuits.

Voice Clipping : Occurs due to improper implementation of DTX.

Mean Opinion Score (MOS) : ITU standard for estimating speech quality.


Speech Quality ParametersMean Opinion Score

Criteria for Voice Quality : A set value “x” at which “y” percent of customersrate the voice quality at Circuit Merits(CM) 4 - 5.

MOS Quality Scale

5 Excellent ( speech perfectly understandable)

4 Good ( speech easily understandable, some noise)

3 Fair ( speech understandable with a slight effort,occasional repetitions needed)

2 Poor ( speech understandable only withconsiderable effort, frequent repetitions needed)

1 Unsatisfactory ( speech not understandable)


Speech Quality - EstimationSpeech RxQual FER FH DTX Reason

BAD HIGH LOW NO NO Air Interface ImpairmentsBAD HIGH HIGH NO NO Severe Air Interface ImpairmentsBAD LOW HIGH NO NO Transmission and Switching system , TranscoderBAD LOW HIGH YES NO Air Interface ImpairmentsBAD GOOD HIGH YES NO Transmission and Switching system , Transcoder

GOOD HIGH LOW YES NO Hopping ImplementationCLIPPING LOW HIGH YES YES Hopping Implementation and VADCLIPPING LOW LOW YES YES VAD

ECHO LOW LOW Y/N Y/N Transmission and Switching system , Transcoder

MOS up to a certain extent can easily estimated by configuring an algorithm using the Alarms in the HP E74XX systems for the following elements, an example of subset of which is illustrated above

- RxQual Full and Sub- RxLev Full and Sub- FER and RLT- L3 Measurement Report- L3 Handover specfic messages


Speech Quality - Troubleshooting

RxQual> 6

FER > 1%

YES

YES

YES

YES

Troubleshoot Multipath/

Co-Channel/BSIC decode

TCH=BCH Pri/Int <= 9

C/IAa <= -9C/IAb <= -9

Co-Channel Interference!!

No

YESNo

No Adjacent Channel Interference!!

YES YES

Troubleshoot Source Cell

Using BCH Analyzer

No

Rx_Lev < -95

No **

Coverage Hole !!!

Uplink Problem !!!

Troubleshoot Link ImbalanceUL-Interference

TCH -Pri/Int <= 9

Optimize Neighbors/Coverage

YES

No

** If TCH is in Hopping, then all interference causes needs to be verified further, before concluding on the root cause and optimizing.

YES

Poor Quality

Area of concern

Hopping ON

YES

RLT going down

DTX_DL ON

FER > 2%YES

No No

NoNo

Transmission/Switching SystemsTranscoder

Patchy Quality Multipath

High Probability with Hopping ON

YES

No A

A

BB

A

MOS < 4

No


Troubleshooting Speech Quality with Post Processing

•Export the Plan Wizards to MAPINFO Compatible Text Files.

•First Open the QualPhoneData table in Mapinfo and create points and build the layer on your city MAP.

•Do the same thing for RcvrData and Cellinfo files

•Close all.

•Open QualPhoneData.tab as Mappable

•Create Thematic Map for Rx_Qual_Full with 2 legend < 5 and => 5

•Create a Query for FER > 1% and Label the MAP with this Query table.

•Select the bad spots ( where FER is very high )


Filtered Spots- RxQual > 4

Bad Spot 1


Creating Queries for Troubleshooting Causes

Create Following Queries

Query for FERTable : QualPhoneData.tabFER < 1Query Name: FER.tab

Query for Poor CoverageTable : QualPhoneData.tabRx_Lev_Full < -97 Query Name : Coverage.tab

Query for Call on BCHTable : QualPhoneDataARFCN_BCH = ARFCN_TCHQuery Name: CALLBCH.tab

Query for Call on TCHTable : QualPhoneDataARFCN_BCH <> ARFCN TCHQuery Name : CAllTCH.tab

Query for CoChannel Table: CALLBCH.tabCo_Channel P/I <=9 Query Name: CoChannel

Query for Adjacent-BCHTable: CALLBCH.tabAdj_A_ N/N-1 <=-9 OR Adj_A_ N/N+1Query Name: ADJCallONBCH

Query for Adjacent-TCHTable CALLTCH.tabAdj_B_ N/N-1 <=-9 OR Adj_B_ N/N+1Query Name: ADJCallONTCH


Add Cause Layers

•MAP --- Layer Control

•ADD the following Query Tables in the following Sequence


Identifying Layers

•Select Display for each Layer.

•Select a common style , but different colour and size for each layer.

Example

RxQual > 4 Size 18

Rxqual < 4 Size 18

Coverage (Rxlev < -97) Size 16

Co-Channel ( on BCH ) Size 12

AdJCallONBCH Size 10

AdjCallONTCH Size 6


Troubleshooting - Coverage and Co-Channel

ZOOM IN -- On the First BAD SPOT

AND

TROUBLESHOOT THE CAUSE.

What can we conclude on BAD SPOT 1 ?????


Troubleshooting - Bad Spot 1


Which is this CELL ??

•Select QualPhoneData Layer.

•Check on Labels

•Select Labels

•Select CellID.

•Display it on the MAP


Which is the cell ? 47450 !!


Did the Call Drop ???

•Create RLT.tab Query for RLT < 8•Import as Label Layer

RLT = 0 , DROP CALL !!


Conclusion

•BAD Spot 1 has poor quality and Call Drop•This spot is covered by Cell 47450•Poor Coverage. Level below -97 dbm•But Call should not Drop•The other Problem is Interference.

•Mobile is Hopping on 99 and 84•99 is also the BCH.•Co-Channel on BCH is very high. •50% of the time quality will be poor.•But Poor Quality is consistent.

•Channel 84 is also suffering from Interference.•No Adjacent Channel on 84 and 99•This means there is Co-Channel on 84 also. •It could also be multipath issue on 84.

•WHY NO HANDOVER


Interference Measurements and

Troubleshooting


Types of Interference

We already know ….

Co-Channel

•BCH - BCH•TCH - BCH•BCH - TCH•TCH - TCH

Adjacent Channel Interference

•BCH - BCH•TCH - BCH•BCH - TCH•TCH - TCH

Multipath


Co-Channel

•Real Time Measurements with Interference Analyzer

•Secondary BSIC decode ( with specific level limitations)

•Primary - Interference includes Multipath also

•Discriminate between Multipath and Co_Channel by using :-- Secondary BSIC -- Measurement Status-- Delay Spread Chart

BCH - BCH

TCH - BCH•Real Time Measurements with Interference Analyzer

•No Secondary BSIC decode

•Identify TCH as the source by usingMeasurement Status - NO FCH


Troubleshooting - TCH-TCH Interference

•TCH -TCH interference will be dominant when two TCH slots in both the re-use cells come in synch and they are ON.•Viper cannot measure interference on any Non-BCH carrier. So we need to take the intrusive approach. •Change the Frequencies and then measure, but this will also not be effective since TCH carrier is active only when call is on.•The most effective way is to measure the delta of the BCH carriers, since propagation will remain almost the same for both the BCH and TCH carriers, so if we measure ratio of F1/F9 in C2 , we can correlate this to the ratio of F5/F5 , there may be some difference with the practical values, since multipath effects are frequency dependent.

C1 C2

F1 F9

F5 F5

F1 & F9 : BCHF5 : TCH


TroubleShooting - TCH-TCH


Uplink Problems


Troubleshooting - Uplink Problems

Create a PLAN WIZARD

•TX LEVEL•RX_LEVEL•CELL_ID•ARFCN•RADIO_LINK COUNTER

•Export the Pland wizard to Excel•Filter out CellId to the cell where troubleshooting is to be done•Filter out Tx Level = 5•Plot the Time-Chart for RxLevel and Tx Level.•Note the spots where the Rxlevel > UL_PCON Threshold and Tx_Level = 5•Plot these spots on the MAP


Estimating Uplink Quality from Downlink

Power ControlThreshold

Tx_Level =5

Poor Uplink Spots


Locating Uplink quality Bad Spots

RX_Level is near P_Con Threshold Ms Tx Level is max ( 5)


Locating Uplink Bad Quality Spots.

Trend shows:High receive levelAnd High Ms Tx Levelsin this area.


Locating Uplink Bad Quality Spots•Processing was done for 1 cell only.•At some areas for low signal level ( < -88), MS at full power•In another area for high signal level ( -75 to -85 ) , MS full power

•MS Tx Level is controlled by the network. •Network will ask MS to increase power if Uplink level is poor or Quality is poor.

•Link Imbalance will result into low uplink level even if DL Level is good.•Then MS will be at high power in all those spots where level is good. BUT THIS IS NOT THE CASE !!! ( as seen from two spots)

•This means this is a problem of Uplink quality being poor at good level.

•This can happen due to UPLINK INTERFERENCE.•This interference seems to be burst in nature !!!!


Troubleshooting - Uplink Interference

•Uplink Interference can be due to:• Mobiles in reuse and adjacent channel cells• Spurs and Intermods from base stations• External sources

•Interference due to Mobiles will be bursty and intermittent.•The behavior and its effect on quality will be time dependent.•More interference during heavy traffic hours.•Interference from external sources can be continous or also time dependent if the source is not continously ON.

•Uplink Interference measurement needs long term monitoring , collection of data, processing the data and estimating the probability of interference , and also estimating the source.

Company Confidential 10/4/99 40 Qos.pre

Uplink Interference Monitoring Setup

BPF

Pre-Amp

Splitter

TRX1

TRX2

1A 1B 2A 2B

BTS

RX

PATH

•Replace the F1 by F2•F2 becomes the Carrier•F1 becomes the Co-channel interferer•Measure F1, F2, •Measure Adjacent Channel Interference also•Use the BCH Analyzer or Channel Power•Create Alarms for Power > Threshold for occurrence of 60secs.•Also configure the Spectrum Analyzer for F2•Record the data at least for 24 hours or in multiple breaks for known trouble hours.


Uplink Interference - Troubleshooting

•Process the collected data and compute the probability charts.•Severity of interference can be estimated by correlating the carrier and interference charts•Time Chart of E7480 can also be used to be more precise on severity. •Time chart will clearly the conditions and instances when F2/F1 was below 9 dB.•Compute the Minumum, Maximum and Mean values for the interferer and troubleshoot the source of interference (next pg)•If Alarms have occurred, then analyze the spectrum analyze to understand the characteristics of this interferer, since alarm was configured for continous interference of 60 secs.

-105 to -95

-95 to -85

-85 to -75

- 75 & >

30% 40% 20% 10%

-105 to -95-95 to -85

-85 to -75

- 75 & >

40% 30% 20% 10%F2 - Carrier F1 - Interferer


Uplink Interference - Troubleshooting

max. = avg. signal is always on

max. > avg. and avg. noise signal is intermittentand rarely on

max. > avg. and avg > noise signal is intermittentbut often on

power statistics

ch 1 ch 2 ch 3 ch 4 ch 5 ch 6 ch 7 ch 8 ch 9 ch 10

maximum power

average power

max. avg. noise no signal in channel

The minimum, maximum, average and noise floor values collected over the measurement period can be used to estimate the behavour and possible source of interference.


Cell Optimization with

HP E74XX


Optimization with HP E7475Cell Loading

C1

C2C3

•Cells C1 and C2 are reporting Blocking•Cell C3 is added to cater for capacity•Still C1 and C2 report blocking. Why ?????


Optimizing - Cell loading

15 12 8 6 4

Reselection

C2: Reselect Offset of 6 dB( + for Cell C3, - for Cell C1 )

Optimizing with C2 Parameters in either C1 or C3

Phone

RCVROptimize by increasing C3 cell power by 6 dB-80 dBm

-97 dBm

-78 dBm

-78 dBm


Optimizing - Cell Reselections

•Mobile Monitors the Neighbor cells BCCH Carriers and decodes SCH of the neighbors in the idle mode.•The list of the BCCH ARFCN’s which the mobiles will monitor in the idle mode is broadcasted in the System Information Type 2 Message .•More the neighbors , less the samples taken by the mobile per neighbor to the calculate the value of C1.•More neighbors of good and near to equal power levels will result into excessive reselections.•Excessive reselections can result into missing of paging messages•Solution to this problem would be to adjust power levels of base stations.•Adjusting power levels most of the time either effects coverage or enhances interference, since the terrain is not flat.•To optimize cell reselections, the best possible solutions are:

• Optimizing the BA list•Implement C2 Reselections


Optimization - Cell Reselection

-75 dBm

-100 dBm

PHONE’s Reselections with this BA List

Geographical BA List for =

Optimized BA List for =

Phone

Phone PHONE’s Reselections with optimized BA List


Optimizing - Neighbor lists

•Mobile reports neighbors as per Type 5 BA list• Number of samples collected depends on numbe of neighbors•Maximum 32 neighbor : for 32 neighbors, 3 - 4 samples per neighbor•Less samples, averaged reported value not authentic•Results in Ping-Pong effect of Handovers•Neighbor list must be optimized •This optimization can be done in a similar way to reselection, by using the BCH Analayzer. •BSIC decoding of the neighbors using the Receiver will also help in understanding the probability that the Phone will measure a re-use ARFCN of the neighbor set in the BA list.


Optimizing -Handover Traffic

A B

C D


Adjacent Channel Interference Reduction

Tight Re-use patterns can permit the use of Adjacent ARFCN in adjacent cell.

This should not pose an problem if Handover margin is set appropriately low.

If Handover Margin is set to 6 dB, Handover will occur from C1 to C2 at spot B itself

C/Ia = -1 db C/Ia = -6 db C/Ia = -9 db

A B C

C1 C2

Optimizing - Handover Margin


Optimizing - Handover Margin• Selecting an appropriate value of handover margin is

essential to ensure good quality communication .• Receiver can be used to decide the value .• BCH Analysis on all BCH’s used in the network.• Plotting the Time Chart and analyzing the delta between the

source to target cells.• Also analyzing the quality at the same time, will help in

estimating the value of Handover margin for a desired quality.

• Handover margin set for a cell is applicable to all calls going on in the cells, to which this cell is a neighbor.

Troubleshooting & Optimizing GSM Networks

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