gsm p&o training material for special subject-handover problems & solutions v1.1

GSM P&O Handover Problems & Solutions----Special Subject Training V1.1

Suitable for staff with junior P&O skill certificates (lower than certificate III)

Issued by GSM Network Planning & Optimization Dept.

Internal Use Only▲

Version introduction

Version Date Writer Assessor Translator Amendment

recordsV1.0 2007-07-

01Ma Junhua Yang Jiazhi None

V1.1 2009-03-16

Fei Aiping Ma Junhua Feng Xiaoyin 1. Handover statistics are added;

2. Document format standardization


Training goals

To 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 handover

Flow of handover signaling

Handover statistics

Handover 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

Inter-MSC

Inter-BSC

Intra-BSC

Intra-cell

Handover classification

Contents



Handover statistics



Intra-cell handover

Air A

TCBTS BSC

New Channel

Old Channel


Signaling flow of intra-cell handover

MS BTS BSC MSC

1、Measurement Report(SACCH)

2、Measurement Report

3、Channel Activation

4、Channel Activation Ack

5、Assigment Command （FACCH)

6、SABM(FACCH)

8、UA(FACCH)

7、Establish Indication

9、Assigment Complete(FACCH)

10、Receiver Ready(FACCH)11、HO Performed

12、RF Channel Release

13、RF Channel Release Ack


A ir A

T CB T S

B T S

B S C

O ld C e ll / B T S N ew C e ll / B T S

Inter-cell handover within one BSC


Signaling flow of inter-cell handover within one BSC

MS Old BTS BSC MSC

1、Measurement Report(SACCH)2、Measurement Report

5、HO Command

7、HO Access(FACCH)

12、UA(FACCH)

13、HO Complete(FACCH)

14、Receiver Ready(FACCH)

16、HO Performed17、RF Channel Release

18、RF Channel Release Ack

New BTS



6、HO Command(FACCH)

8、HO Detect

9、Physical info(FACCH)

10、SABM(FACCH)

11、Establish Indication

15、HO Complete


A ir A

B T S

O ld C ell / B T S

N ew C ell / B T S

B T S

B S C T C

B S C T C

VL RM SC

Inter-BSC handover


Signaling flow of inter-BSC handover

MS Old BTS Old BSC MSC

14、HO ommand

6、HO Command

13、UA(FACCH)

New BTS



10、HO Detect

11、Physical info(FACCH)

12、SABM(FACCH)

New BSC

1、HO_REQ

2、HO_REQ

5、HO_REQ_ACK

7、HO Command8、HO Command

9、HO Access(FACCH)

15、HO Command16、HO Command

17、HO Command


A ir A

B T S

O ld C ell / B T S

N ew C ell / B T S

B T S

B S C T C

B S C T C

VLRM SC

VLRM SC

Inter-MSC handover


Basic signaling flow of Inter-MSC handover

MS/BSS-A

MSC-A MSC-B

MAP-Prep-Handover req. MAP-Allocate-Handover-Number req.

A-HO-REQUEST

A-HO-REQUIRED

BSS-B/MS

VLR-B

A-HO-REQUEST-ACK

MAP-Send-Handover-Report req.

MAP-Prep-Handover resp.

IAM

MAP-Send-Handover-Report resp.

ACM A-HO-COMMAND

A-HO-DETECT

A-HO-COMPLETE

MAP-Process-Access-Sig req.

MAP-Send-End-Signal req. A-CLR-CMD/COM

ANSWER

RELEASE End of call

MAP-Send-End-Signal resp.


MS/BSS-B

MSC-A MSC-B

MAP-Prep-Sub-Handover req. A-HO-REQUIRED

BSS-A/MS

VLR-B

A-HO-COMMAND MAP-Prep-Sub-Handover resp. A-HO-REQUEST-ACK

A-HO-DETECT

A-HO-COMPLETE MAP-Send-End-Signal resp. A-CLR-CMD/COM

A-HO-REQUEST

Release

Signaling flow of inter-MSC back-handover


MSC-B

A-HO-REQUIRED

VLR-B

A-HO-COMMAND

MAP-Prep-Sub-Handover req.

A-HO-DETECT

A-HO-COMPLETE

MSC-A

MS/BSS MSC-B’ VLR-B’

MAP-Prepare-Handover req.

MAP-Prepare-Handover resp.

MAP-Allocate-Handover-Number req.

MAP-Send-Handover-Report req.

IAM

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

MAP-Prep-Sub-Ho resp.

MAP-Process-Access-Signalling req.

MAP-Send-End-Signal req.

ACM

Answer

Release



Release (end of call)

A-CLR-CMD/COM

Signaling flow of inter-MSC handover to a third MSC


Basic flow of handover signaling

Inter-cell handover within BSC

There 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 MSC

BSC 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 signaling

Inter-MSC handover

MSC inquires “REMOTLAC sheet” (including LAC and ro

ute 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 sen

ds （ Prepare-HO ACK ） message to the original MSC, a

nd executes the following flow.”

Internal Use Only▲Main differences between intra-BSC handover and inter-BSC handover

MSC participates or not

MSC participates or not

CGI is carried or not

CGI is carried or not

Inter-BSC

handover

Inter-BSC

handover

Intra-BSC

handover

Intra-BSC

handover

MSC 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 doesn’t participate

before that;

For intra-BSC handover, CGI isn’t

carried in any message, it’s handled

within BSC.


Flow of handover algorithm

MS BTS BSC MSC

BCCH frequency

point, BSIC and level values of

the six adjacent

cells (with strongest level) and

serving cell;

UL MR

Process of MR

Confirmation ofadjacent cell CGI

Execution of handover decision

Selection of target cell

Channel activation

External cell?

HO

requ

est

Intra-MSC handover

Target MSCTarget BSC

BA2 sheet

List of cells under one LAC

HO

requ

est

HO

requ

est

No

Yes


Common timers at BSC

T3107Suitable for: intra-cell handoverStart-up: BSC sends “assignment command”Stop counting: when “assignment completed” or “assignment failure” is received;

T3103

Suitable for: inter-cell handover

Start-up: BSC sends “handover command”

Stop counting: when “handover completed” or “handover failure” is received;

T7

Suitable for: inter-BSC handover

Start-up: original BSC sends “HO-Request” to MSC

Stop counting: when “handover command” is received from target BSC (or when there’s no idle channel available in target cell, a “HO-Request rejected” message is returned.)

T8

Suitable for: inter-BSC handover

Start-up: original BSC receives “HO command” from MSC

Stop counting: when “clear instruction” command is received from MSC

Contents



Handover statistics



Information contained in MR

DL MR1. UL DTX performance2. DL receive level/quality3. Adjacent cell level4. UL dynamic power control

UL MR1. DL DTX performance2. UL receive level/quality3. BS Power4. MS power level5. TA6. DL dynamic power control


MR cycle

MR 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.

12TCH12TCH 12TCH12TCH1SACCH1SACCH 1 idle1 idle

480ms480ms 26 multi-frame26 multi-frames of 4 TCHss of 4 TCHs


Indicator definition of handover success rate

KPI name Handover success rate

Indicator definition

（ busy hour number of handover success times /busy hour total number of handover request times ） *100%

Counter formula

V2.97 ((C11618+C11669-C11675)/(C11617+C11668-C11675))*100%

V6.20 (C900060098+C900060102+C900060120+C900060094+C900060096)*100/(C900060097+C900060213+C900060214+C900060215+C900060099+C900060100+C900060101+C900060216+C900060119+C900060093+C900060095)


Signaling statistical point of handover success

C900060098 C900060102

C900060120

A

BTSBSC

HO_ COM

BSC-controlled inter-cell incoming handover success

A

BSCMSC BTS

HO_COM

HO_COM

MSC-controlled incoming handover success

A

BSC BTS

ASS_COM

ASS_CMD

Intra-cell handover success

C900060097

A

MSCBSC

CLEAR_CMD

No. of MSC-controlled outgoing handover success times


Signaling statistical point of handover success

C900060094

MS

HO_CMD

BTS(Src)

CHL_ACT

BSC

HO_CMD

MEAS_RESMEAS_RES

SABM

UA

HO_COM

MSC

HO_COM

EST_IND

HO_PERFORM

HO_ACCESS

BTS(Target)

CHL_ACT_ACK

HO DETECT

Phy Info

A

BSC-controlled inter-cell outgoing handover success


Signaling statistical point of handover request

C900060097

A

BTSBSC

CHL_ACTIV_ACK

BSC-controlled inter-cell incoming handover execution

C900060213

C900060214

A

BTS( Target) BSC

CHANNEL ACT

CHANNEL ACT ACK

Forced rel ease attempt

，Resource Avai l abl e

Execution of forced release

A

BTS( Target) BSC

CHANNEL ACT

CHANNEL ACT ACK

Cel l queui ng


Execution of cell queuing

C900060215

A

BTS( Target) BSC

CHANNEL ACT

CHANNEL ACT ACK

Force handover attempt


Execution of force handover



C900060099 C900060100

C900060101

A

BSC

HO_REQ

MSC BTS

HO_REQ_ACK

CHL_ACTIV_ACK

CHL_ACTIV

MSC BSC-controlled incoming handover execution

A

BSC

HO_REQ

MSC BTS

HO_REQ_ACKCHL_ACTIV_ACK

CHL_ACTIV

Forced rel ease attempt , resource avai l abl e

Execution of forced release

A

BSC

HO_REQ

MSC BTS

HO_REQ_ACKCHL_ACTIV_ACK

CHL_ACTIV

Cel l queui ng, resource avai l abl e

Execution of queuing

A

BSCBTS

ASSIGN_ CMD

CHL_ ACTIV_ACK

Execution of intra-cell handover

C900060119



C900060216 C900060095

C900060093

BTS

A

MSC

HO_CMD

BSC

HO_CMD

No. of MSC-controlled outgoing handover execution times

A

BTS( Target) BSC

CHANNEL ACT

CHANNEL ACT ACK

Force handover attempt

，Resource avai l abl e

Execution of force handover

MS

HO_CMD

BTS(Src)

CHL_ACT

A

BSC

HO_CMD

MEAS_RESMEAS_RES

SABM

UA

HO_COM

MSC

HO_COM

EST_IND

HO_PERFORM

HO_ACCESS

BTS(Target)

CHL_ACT_ACK

HO DETECT

Phy Info

No. of BSC-controlled inter-cell outgoing handover execution times


Handover-related measurement tasks

Handover causes measurement

Measure the frequency of MS handovers caused by various kinds of reasons, so as to examine radio environment of a cell;

Common handover measurement

Measure the process of MS handover to inspect handover success or failure and abnormal situations causing failures, so as to improve the cell’s radio configuration and observe traffic dispersion, etc.;

Measurement of adjacent cell handover

Measure 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 measurement

Focus on traffic load of the second subcell.

Contents



Handover statistics



Analysis handover problems

Analysis of handover problems Location method of handover problems


Common handover problems

Common handover problemsCommon handover problems Possible influencesPossible influences

Handover nonoccuranceHandover nonoccurance

• Result in call drop;

Handover failureHandover failure • Affect call quality and result in call drop;

Frequent handoverFrequent handover • Affect call quality, and increase system load;

Handover hysteresisHandover hysteresis • Affect call quality and result in call drop;


Discovery of handover problems

Meters at A interfaceMeters at A interface

Traffic statistics analysis

Traffic statistics analysis

Customer complaintsCustomer complaints

DT/CQT testsDT/CQT tests

TOPN analysisTOPN analysis

Abnormal number of handover timesAbnormal number of handover times

Call dropCall drop

Poor speech qualityPoor speech quality

Bad coverageBad coverage

Handover problemHandover problemSlow handoverSlow handover

Handover to best cell inhibited

Handover to best cell inhibited

No handoverNo handover

Handover failureHandover failure

Frequent handoverFrequent handover


Flow of handover problem checkingToo high TCH

handover failure rate of a cell

Complete

Any antenna problems?

Solve antenna

problems

Eliminate equipment

faults

Check & eliminate

interference

Is radio parameter setting

reasonable?

Interference exists?

Any equipment faults?

No

Yes

Adjust parameters

Yes

Yes

Coverage problem exists?

Improve coverage

Yes


Location methods of handover problems

Analyze traffic statistics Conduct 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 reasonably

If 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 problems

Interference checking DT analysis Signaling analysis: Um interface 、 Abis interface 、 A interfa

ce; Hardware checking: like DCU, transceiver, clock generator, R

F connection lines between boards; Antenna system checking


Analysis of handover problems

Coverage & interference Antenna system BTS software & hardware transmission BSC software & hardware A interface malfunction Busy target cell Connection & adaptation to equipment from different suppliers


Coverage & interference

Coverage: 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.

Internal Use Only▲Handover nonoccurance due to isolated-island effect

Adjacent cell N3

adjacent cell N2

adjacent cell N1

Non-adjacent cell

Non-adjacentcell

Non-adjacentcell

Serving cell

Handover can’t Handover can’t happen due to happen due to lack of adjacent lack of adjacent cellscells..

Skip-zone Skip-zone coverage leads to coverage leads to isolated island.isolated island.


Antenna system problems

Too large VSWR Reversed installation of antenna Non-standard antenna installation Unreasonable azimuth, down-tilt Below-standard antenna insulation Twisted cables, loosened connectors and wrong

connections;


BTS software/hardware

Problems about CDU 、 TRM, etc. Clock generator malfunction Internal communication cable malfunction BTS software malfunction


Transmission and BSC problems

Transmission fault Unstable transmission Too high transmission error rate

BSC hardware/software malfunctions Clock generator malfunction: unconformity among clocks in differe

nt BTSs due to clock generator malfunction; Problem about single board Wrong data configuration Unreasonable setting of handover threshold CGI, BCCH and BSIC values in “external cell data sheet” do not m

atch up to those in the corresponding BSC; Wrong BSC signaling point in “list of cell under a LAC” in MSC; c

o-channel& co-BSIC adjacent cells exist.

Internal Use Only▲A interface malfunction, busy target cell, handover between equipment from different suppliers

A interface malfunction Abnormal handover due to lack of link resource, abnormal

calls; Busy target cell

Abnormal handover due to lack of link resource, abnormal calls;

handover between equipment from different suppliers Difference in signaling at interface A and interface E betw

een 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 malfunction

malfunction 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.



Problem description: Through analysis of the cell’s signaling trace data at OMCR,

it’s discovered that the cell’s BCCH carrier suffers from frequent Assignment failure, while TCH carrier is in normal condition, as shown in the figure:



Problem analysis From inspection of the calls with “Assignment Failure”

problem, it’s 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.



Problem checking Exchange BCCH frequency point with that of TCH; Through observation of signaling statistics, it’s 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 handling

Change the carrier with fault, number of times of handover caused by DL interference reduces greatly, and the cell’s handover success rate increases a lot.


Typical case 2- frequency interference

Problem description: The data in performance report shows that Cell 1 under a

BTS suffers from low handover success rate. Problem analysis

Examine 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 handling After adjustment of frequency point, handover success

rate obviously increases, and number of handover times reduces.


Typical case 2- frequency interference

Changes of HO i ndi cators before & after Frequency poi nt adj ustment

0

30

60

90

120

150

180

9- 4 9- 5 9- 6 9- 7 9- 8 9- 9 9- 10 9- 11

Numb

er o

f HO

Req

./nu

mber

of

HO s

ucce

ss

0%

20%

40%

60%

80%

100%

120%

HO s

ucce

ss r

ate

切换请求总次数切换成功总次数 (%)切换成功率No. of HOReq. HO successrate

No. of HOsuccess


Typical case 3- clock malfunction

Problem description For a newly-commissioned BTS, handover nonoccurance appear

s during DT: the MS occupies a channel in cell A; during DT from cell A to cell B, cell B can’t be observed in the adjacent cell list, and it doesn’t start normal handovers.

Problem analysis It’s a common network problem that handover nonoccurance app

ears in many cells; It’s a newly-commissioned BTS; handover parameters are as def

ault in the system; Check adjacent cells relation, no problem found; Observe from test MS, find out that adjacent cell frequency appe

ars in the adjacent cell, but BSIC can’t 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 cell’s BSIC is unobtainable, BSC is unable to locate it, thus handover won’t be started.


Typical case 3- clock malfunction

Problem analysis Process of MS deciphering on DL channel

Decipher FCCH decipher SCH （ SCH comprises MS frame synchronous information and BSIC.

MS can show adjacent cell frequency point, but not BSIC. It’s suspected that adjacent cell’s SCH information can’t be deciphered by MS due to clock or transmission fault.

Check clock and transmission BTS adopts network clock BSC traces superior clock MSC traces superior GPS clock through long-distance

satellite link The long-distance satellite link is found unstable, which le

ads to high error rate on the meter, and warning of clock deterioration appears on MSC.


Typical case 3- clock malfunction Problem handling

Decide that it’s 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 conclusion If no high accuracy clock a

vailable, 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.

Internal Use Only▲Typical case 4-problem about other suppliers’parameter setting

Problem description At 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.

Internal Use Only▲Typical case 4-problem about other suppliers’parameter setting

Problem analysis Observe 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 handling

The 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 problem

Problem description During 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 problem

Problem analysis and handling Slow 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 network’s handover window to be 2, so as to

accelerate handover speed; Adjust the whole network’s handover preprocess to 2, so as to

accelerate handover speed.

Parameter Before adjustment

After adjustment

Level threshold (HOMARGINRXLEV)

30 28

Quality threshold (HOMARGINRXQUAL)

30 26

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 thinking

Please simply illustrate effects on handover due to changing T3103 、 T3107.

Suggestions on parameter settings of handovers on highway.

gsm p&o training material for special subject-handover problems & solutions v1.1

Documents

handover conditions

handover cau

intrabsc handover

subhandover req

subhandover resp

handovernumber req

common handover problems

horequest message