gsm handover problems & solutions

GSM Handover Problems & SolutionsZTE university

Training goalsTo master different types of handover and their signaling flows;To master handover statistical signaling point and MR tasks;To know common handover problems and the handling procedures.

Contents Overview of handoverFlow of handover signalingHandover statisticsHandover problem analysis

Aims of handovers Why there are handovers?To keep calls going on during movement;To improve network service quality;To decrease call drop rate;To decrease congestion rate.

Handover classification

Intra-cell handover

Signaling flow of intra-cell handover

MS

BTS

BSC

MSC

1Measurement Report(SACCH)

2Measurement Report

3Channel Activation

4Channel Activation Ack

5Assigment Command FACCH)

6SABM(FACCH)

8UA(FACCH)

7Establish Indication

9Assigment Complete(FACCH)

10Receiver Ready(FACCH)

11HO Performed

12RF Channel Release

13RF Channel Release Ack

Inter-cell handover within one BSC

Signaling flow of inter-cell handover within one BSC

MS

Old BTS

BSC

MSC

1Measurement Report(SACCH)

2Measurement Report


6HO Command(FACCH)

5HO Command

7HO Access(FACCH)

12UA(FACCH)

13HO Complete(FACCH)

14Receiver Ready(FACCH)

16HO Performed

17RF Channel Release

18RF Channel Release Ack

New BTS

3Channel Activation

8HO Detect

9Physical info(FACCH)

10SABM(FACCH)

11Establish Indication

15HO Complete

Inter-BSC handover

Signaling flow of inter-BSC handover

MS

Old BTS

Old BSC

MSC

14HO ommand


6HO Command

13UA(FACCH)

New BTS

3Channel Activation

10HO Detect

11Physical info(FACCH)

12SABM(FACCH)

New BSC

1HO_REQ

2HO_REQ

5HO_REQ_ACK

7HO Command

8HO Command

9HO Access(FACCH)

15HO Command

16HO Command

17HO Command

Inter-MSC handover

Basic signaling flow of Inter-MSC handover

Signaling flow of inter-MSC back-handover

MS/BSS-B

VLR-B

MSC-A

MSC-B

BSS-A/MS

MAP-Prep-Sub-Handover req.

MAP-Prep-Sub-Handover resp.

A-HO-REQUIRED

A-HO-REQUEST-ACK

A-HO-COMPLETE

A-HO-DETECT

A-HO-COMMAND

Release

MAP-Send-End-Signal resp.

A-CLR-CMD/COM

A-HO-REQUEST

Signaling flow of inter-MSC handover to a third MSC

VLR-B

VLR-B

MSC-B

MSC-B

MAP-Prepare-Handover resp.

MAP-Prepare-Handover req.

Release

MAP-Send-Handover-Report req.

MAP-Prep-Sub-Handover req.

A-HO-REQUIRED


ACM

Answer


A-HO-COMPLETE

A-HO-DETECT

A-HO-COMMAND

Release

MSC-A

IAM

MAP-Send-Handover-Rep. resp. (1)

MAP-Send-End-Signal req.

MAP-Process-Access-Signalling req.

MAP-Allocate-Handover-Number req.

MS/BSS

MAP-Prep-Sub-Ho resp.

(end of call)

A-CLR-CMD/COM

Basic flow of handover signalingInter-cell handover within BSCThere is no HO-Request message for intra-BSC handover; all information is analyzed within BSC; Once a target cell in the BSC fulfilling handover conditions is found, send Channel activation message directly;Inter-BSC handover within MSCBSC reports CGI and handover cause of original cell and target cell to MSC through HO-Request;After MSC finds target cell LAC, it sends HO-Request to the BSC which the target cell belongs to;Target BSC activates channel in target cell, and executes the following flow.

Basic flow of handover signalingInter-MSC handoverMSC inquires REMOTLAC sheet (including LAC and route address of adjacent MSC);MSC sends Prepare-HO message to the target MSC-B according to the route address;According to the Prepare-HO message, target MSC-B requests for Handover number from VLR-B, then sends HO-Request message to BSC-B;After the target BSC-B receives HO-Request ACK, it sends Prepare-HO ACKmessage to the original MSC, and executes the following flow.

Main differences between intra-BSC handover and inter-BSC handoverMSC participates or notCGI is carried or notInter-BSC handoverIntra-BSC handoverMSC transmits HO-REQ message, and CGI of original cell and target cell is carried in the message;As for inter-BSC handover, MSC participates in it since HO-Request;As for intra-BSC handover, HO-Performed message is sent to MSC only after the handover is completed; MSC doesnt participate before that;For intra-BSC handover, CGI isnt carried in any message, its handled within BSC.

Flow of handover algorithmMSBTSBSCMSCBCCH frequency point, BSIC and level values of the six adjacent cells (with strongest level) and serving cell;UL MRProcess of MRConfirmation ofadjacent cell CGIExecution of handover decisionSelection of target cellChannel activationExternal cell?HO requestIntra-MSC handoverTarget MSCTarget BSCBA2 sheetList of cells under one LACHO requestHO requestNoYes

Common timers at BSCT3107Suitable for: intra-cell handoverStart-up: BSC sends assignment commandStop counting: when assignment completed or assignment failure is received;

T3103Suitable for: inter-cell handoverStart-up: BSC sends handover commandStop counting: when handover completed or handover failure is received;T7Suitable for: inter-BSC handoverStart-up: original BSC sends HO-Request to MSCStop counting: when handover command is received from target BSC (or when theres no idle channel available in target cell, a HO-Request rejected message is returned.)T8Suitable for: inter-BSC handoverStart-up: original BSC receives HO command from MSCStop counting: when clear instruction command is received from MSC

Information contained in MRDL MRUL DTX performanceDL receive level/qualityAdjacent cell levelUL dynamic power controlUL MR DL DTX performance UL receive level/quality BS Power MS power level TA DL dynamic power control

MR cycleMR is sent to BTS in SACCH UL direction;When MS is in SDCCH, MR cycle is 470ms/time;When MS is in TCH, MR cycle is 480ms/time.

Indicator definition of handover success rate

Signaling statistical point of handover successC900060098C900060102 C900060120 BSC-controlled inter-cell incoming handover success MSC-controlled incoming handover success Intra-cell handover success C900060097No. of MSC-controlled outgoing handover success times

Signaling statistical point of handover successC900060094 BSC-controlled inter-cell outgoing handover success

MS

HO_CMD

BTS(Src)

CHL_ACT

CHL_ACT_ACK

A

BSC

HO_CMD

MEAS_RES

SABM

MEAS_RES

UA

HO_COM

MSC

HO_COM

EST_IND

HO_PERFORM

HO_ACCESS

BTS(Target)

HO DETECT

Phy Info

Signaling statistical point of handover requestC900060097 BSC-controlled inter-cell incoming handover executionC900060213 C900060214 Execution of forced releaseExecution of cell queuing C900060215 Execution of force handover

Signaling statistical point of handover requestC900060099 C900060100 C900060101 MSC BSC-controlled incoming handover executionExecution of forced release Execution of queuingExecution of intra-cell handoverC900060119

Signaling statistical point of handover requestC900060093C900060216 C900060095 No. of MSC-controlled outgoing handover execution timesExecution of force handover No. of BSC-controlled inter-cell outgoing handover execution times

MS

HO_CMD

BTS(Src)

CHL_ACT

CHL_ACT_ACK

A

BSC

HO_CMD

MEAS_RES

SABM

MEAS_RES

UA

HO_COM

MSC

HO_COM

EST_IND

HO_PERFORM

HO_ACCESS

BTS(Target)

HO DETECT

Phy Info

Handover-related measurement tasksHandover causes measurementMeasure the frequency of MS handovers caused by various kinds of reasons, so as to examine radio environment of a cell;Common handover measurementMeasure the process of MS handover to inspect handover success or failure and abnormal situations causing failures, so as to improve the cells radio configuration and observe traffic dispersion, etc.;Measurement of adjacent cell handoverMeasure the number of times of incoming/outgoing handover attempt/success/failure from/to certain cells, and number of times of handover caused by different reasons, so as to get the handover situations of the serving cell and its adjacent cells and to optimize their radio configurations correspondingly;Subcell statistical measurementFocus on traffic load of the second subcell.

Analysis handover problemsAnalysis of handover problemsLocation method of handover problems

Common handover problemsCommon handover problemsPossible influencesHandover nonoccuranceResult in call drop;Handover failureAffect call quality and result in call drop;Frequent handoverAffect call quality, and increase system load;Handover hysteresisAffect call quality and result in call drop;

Discovery of handover problemsMeters at A interfaceTraffic statistics analysisCustomer complaintsDT/CQT testsTOPN analysisAbnormal number of handover timesCall dropPoor speech qualityBad coverageHandover problemSlow handoverHandover to best cell inhibitedNo handoverHandover failureFrequent handover

Flow of handover problem checking

Too high TCH handover failure rate of a cell

Adjust parameters

Coverage problem exists?

Improve coverage

Yes

Yes

Is radio parameter setting reasonable?

Interference exists?

Any equipment faults?

Yes

Any antenna problems?

Solve antenna problems

Complete

Eliminate equipment faults

Check & eliminate interference

No

Yes

Location methods of handover problemsAnalyze traffic statisticsConduct handover statistics measurement, identify problem range:If just some cells fail to make handovers to the cell, check handover data, check if co-channel and co-BSIC exist; If the cell fails to take handovers from all other cells, check its data.Check warnings: single board malfunction, transmission and clock malfunctions, etc.;Check if radio parameters are set reasonablyIf co-channel or co-BSIC exist among adjacent cells;If handover parameters are set reasonably;If data configuration of external cells is correct.

Location methods of handover problemsInterference checkingDT analysisSignaling analysis: Um interfaceAbis interface A interface;Hardware checking: like DCU, transceiver, clock generator, RF connection lines between boards; Antenna system checking

Analysis of handover problemsCoverage & interferenceAntenna systemBTS software & hardwaretransmissionBSC software & hardwareA interface malfunctionBusy target cellConnection & adaptation to equipment from different suppliers

Coverage & interferenceCoverage:Poor coverage: due to influence from forest, complex landforms, houses, indoor coverage, etc.;Isolated site: no adjacent cells around;Skip-zone coverage: no adjacent cells available due to isolated-island effect;

Interference: It makes MS unable to access in UL, or DL signal receiving problem will be resulted.

Handover nonoccurance due to isolated-island effect Adjacent cell N3 adjacent cell N2 adjacent cell N1Non-adjacent cell

Non-adjacentcell

Non-adjacentcellServing cellHandover cant happen due to lack of adjacent cells.Skip-zone coverage leads to isolated island.

Antenna system problemsToo large VSWRReversed installation of antennaNon-standard antenna installationUnreasonable azimuth, down-tiltBelow-standard antenna insulationTwisted cables, loosened connectors and wrong connections;

BTS software/hardware Problems about CDUTRM, etc.Clock generator malfunctionInternal communication cable malfunctionBTS software malfunction

Transmission and BSC problems Transmission faultUnstable transmissionToo high transmission error rate

BSC hardware/software malfunctionsClock generator malfunction: unconformity among clocks in different BTSs due to clock generator malfunction;Problem about single boardWrong data configurationUnreasonable setting of handover threshold CGI, BCCH and BSIC values in external cell data sheet do not match up to those in the corresponding BSC; Wrong BSC signaling point in list of cell under a LAC in MSC; co-channel& co-BSIC adjacent cells exist.

A interface malfunction, busy target cell, handover between equipment from different suppliers A interface malfunctionAbnormal handover due to lack of link resource, abnormal calls;Busy target cellAbnormal handover due to lack of link resource, abnormal calls;handover between equipment from different suppliers Difference in signaling at interface A and interface E between ZTE and other suppliers equipment, causing non-recognition or non-support problem, including speech version, handover code and addressing mode (CGI or LAI) etc., which will result in handover failure.

Typical case 1- carrier malfunctionmalfunction description: Performance indicators at OMCR show that handover success rate of Cell 2 (3carriers) in the flood prevention center is lower, and number of handover cause of DL interference is higher.

Typical case 1- carrier malfunctionProblem description: Through analysis of the cells signaling trace data at OMCR, its discovered that the cells BCCH carrier suffers from frequent Assignment failure, while TCH carrier is in normal condition, as shown in the figure:

Typical case 1- carrier malfunctionProblem analysisFrom inspection of the calls with Assignment Failure problem, its discovered that the problem is caused by intra-cell handover (assignment flow is adopted in intra-cell handover), as shown in the figure: Signaling statistics show strong DL level, but RX quality is low, which results in a lot failed intra-cell handovers on the same carrier due to DL interference. Check TCH carrier and it is confirmed normal.

Typical case 1- carrier malfunctionProblem checkingExchange BCCH frequency point with that of TCH;Through observation of signaling statistics, its discovered that the original BCCH carrier still suffers from problem, which the original TCH carrier is still in normal condition;Eliminate frequency interference;Confirm that the problem is caused by high DL error rate due to malfunction of BCCH carrier.Problem handlingChange the carrier with fault, number of times of handover caused by DL interference reduces greatly, and the cells handover success rate increases a lot.

Typical case 2- frequency interferenceProblem description: The data in performance report shows that Cell 1 under a BTS suffers from low handover success rate.Problem analysisExamine the problem cell, discover that 2 cells under a BTS co-channel and co-BSIC, and close to each other, which results in low handover success rate in the cell.Problem handlingAfter adjustment of frequency point, handover success rate obviously increases, and number of handover times reduces.

Typical case 2- frequency interference

Chart2

139140.1007

124130.1048

166110.0662

12470.0564

11111

10101

12121

No. of HOReq.

HO success rate

No. of HOsuccess

(%)

Number of HO Req./number of HO success

HO success rate

Changes of HO indicators before & after Frequency point adjustment

Sheet1

(%)

9/41391410%

9/51241310%

9/6166117%

9/712476%

9/91111100%

9/101010100%

9/111212100%

Sheet1

(%)

/

Sheet2

Sheet3

Typical case 3- clock malfunctionProblem descriptionFor a newly-commissioned BTS, handover nonoccurance appears during DT: the MS occupies a channel in cell A; during DT from cell A to cell B, cell B cant be observed in the adjacent cell list, and it doesnt start normal handovers.Problem analysisIts a common network problem that handover nonoccurance appears in many cells;Its a newly-commissioned BTS; handover parameters are as default in the system;Check adjacent cells relation, no problem found;Observe from test MS, find out that adjacent cell frequency appears in the adjacent cell, but BSIC cant be decoded. Since adjacent cell is searched through BA2 table during a call, and BA2 relies on BCCH and BSIC to confirm an adjacent cell, when the adjacent cells BSIC is unobtainable, BSC is unable to locate it, thus handover wont be started.

Typical case 3- clock malfunctionProblem analysisProcess of MS deciphering on DL channelDecipher FCCH decipher SCHSCH comprises MS frame synchronous information and BSIC.MS can show adjacent cell frequency point, but not BSIC. Its suspected that adjacent cells SCH information cant be deciphered by MS due to clock or transmission fault.Check clock and transmission BTS adopts network clockBSC traces superior clockMSC traces superior GPS clock through long-distance satellite link The long-distance satellite link is found unstable, which leads to high error rate on the meter, and warning of clock deterioration appears on MSC.

Typical case 3- clock malfunctionProblem handlingDecide that its handover problem caused by poor clock quality.Bring new GPS clock device and adopt the local one, thoroughly solve clock malfunction.Problem of handover nonoccurance is solved.Experience conclusionIf no high accuracy clock available, clock in BTS can be used; calibration of each BTS must be made by using frequency meter and LMT to ensure that frequency deviation meets precision requirement.

Typical case 4-problem about other suppliersparameter settingProblem descriptionAt a marginal network, success rate of handovers between our equipment and that of other suppliers is rather low.From the OMCR statistics, there are more inter-BSC handovers, but success rate is low. After DT, we find that there are unnecessary handovers started from other suppliers equipment during normal calls.

Typical case 4-problem about other suppliersparameter settingProblem analysisObserve settings of other suppliers OMCR parameters, we discover that each adjacent cell has its own property;The max transmission power of the cell with problem is set as 5dBm.Problem handlingThe problem is solved after the value is changed to 33dBm.Handover success rates of ZTE and other suppliers equipment are all improve to normal level.

Typical case 5-HO parameter setting problemProblem descriptionDuring DT at a BTS, we find slow handover problem is common (>10S), which affects speech quality and even causes call drops.Problem: level of cell 2 is higher than that of cell 3 by 20dB, total handover time is 15s.

Typical case 5-HO parameter setting problemProblem analysis and handlingSlow handover seriously affects network quality. Make adjustment of handover parameters accordingly:Change adjacent cell handover threshold to improve timeliness of handover trigger;Adjust the whole networks handover window to be 2, so as to accelerate handover speed;Adjust the whole networks handover preprocess to 2, so as to accelerate handover speed.Result Test after adjustment shows that handover time is reduced to 5s; the slow handover problem is solved and speech quality is improve.

Questions for thinkingPlease simply illustrate effects on handover due to changing T3103T3107. Suggestions on parameter settings of handovers on highway.

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gsm handover problems & solutions

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