GSM Handover Problems & Solutions
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
Aims of handovers
Why there are handovers?
To keep calls going on during movement;
To improve network service quality;
To decrease call drop rate;
Handover classification
Inter-MSC Inter-BSC Intra-BSC Intra-cell Handover classificationContents
Overview of handover
Flow of handover signaling
Handover statistics
Intra-cell handover
Air
A
TC
A i r
A
T C B T S B T S B S C O l d C e l l / B T S N e w C e l l / B T SA i r
A
B T S O ld C e l l / B T S N e w C e l l / B T S B T S B S C T C B S C T C V L R M S CInter-BSC handover
A i r
A
B T S O l d C e l l / B T S N e w C e l l / B T S B T S B S C T C B S C T C V L R M S C V L R M S CInter-MSC handover
Basic signaling flow of Inter-MSC handover
MS/BSS-AMSC-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
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 MAP-Send-End-Signal resp. MAP-Send-End-Signal resp. Release (end of call) A-CLR-CMD/COM
Basic flow of handover signaling
Inter-cell handoverwithin 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 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-MSC-B, then sends “HO-Request” message to BSC-B;
After the target BSC-B receives “HO-Request ACK”, it sends （ Prepare-HO ACK ） message to the original MSC, and executes the following flow.”
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 handove r Inter-BSC handove r Intra-BSC handove r Intra-BSC handove rMSC 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 of adjacent cell CGIExecution of handover decision Selection of target cell Channel activation External cell? H O r eq u es t Intra-MSC handover Target MSC Target BSC BA2 sheet List of cells under one LAC
H O r eq u es t H O r eq u es t No Yes
Common timers at BSC
T3107
Suitable for: intra-cell handover Start-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”
Contents
Overview of handover
Flow of handover signaling
Handover statistics
Information contained in MR
DL MR
1. UL DTX performance 2. DL receive level/quality 3. Adjacent cell level
4. UL dynamic power control
UL MR 1. DL DTX performance 2. UL receive level/quality 3. BS Power 4. MS power level 5. TA
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.
12TCH
12TCH 1SACCH1SACCH 12TCH12TCH 1 idle1 idle 480ms
480ms frames of 4 frames of 4 26 multi-26 multi-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%
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
BSC-controlled inter-cell incoming handover success MSC-controlled incoming handover success
Intra-cell handover success
C900060097
Signaling statistical point of handover success
C900060094
Signaling statistical point of handover request
C900060097
BSC-controlled inter-cell incoming handover execution
C900060213
C900060214
Execution of forced release
Execution of cell queuing
C900060215
Signaling statistical point of handover request
C900060099 C900060100
C900060101
MSC BSC-controlled incoming handover execution Execution of forced release
Execution of queuing Execution of intra-cell handover
Signaling statistical point of handover request
C900060216 C900060095
C900060093
No. of MSC-controlled outgoing handover execution times
Execution of force handover
Handover-related measurement tasks
Handovercauses
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
Contents
Overview of handover
Flow of handover signaling
Handover statistics
Analysis handover problems
Analysis of handover problems
Common handover problems
Common handover problems
Common handover problems Possible influencesPossible influences
Handover nonoccurance
Handover
nonoccurance • Result in call drop;
Handover failure
Handover failure • Affect call quality and result in call drop;
Frequent handover
Frequent handover • Affect call quality, and increase system load;
Handover hysteresis
Handover hysteresis • Affect call quality and result in call drop;
Discovery of handover problems
Meters at A interface Meters at A interface Traffic statistics analysis Traffic statistics analysis Customer complaints Customer complaints DT/CQT tests DT/CQT tests TOPN analysis TOPN analysisAbnormal number of handover times
Abnormal number of handover times
Call drop
Call drop
Poor speech quality
Poor speech quality
Bad coverage
Bad coverage
Handover problem
Handover problem Slow handoverSlow handover
Handover to best cell inhibited
Handover to best cell inhibited No handover No handover Handover failure Handover failure Frequent handover Frequent handover
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;
Location methods of handover problems
Interference checking
DT analysis
Signaling analysis: Um interface 、 Abis interface
、
A
interface;
Hardware checking: like DCU, transceiver, clock generator,
RF connection lines between boards;
Analysis of handover problems
Coverage & interference
Antenna system
BTS software & hardware
transmission
BSC software & hardware
A interface malfunction
Busy target cell
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
Handover nonoccurance due to isolated-island
effect
Adjacent cell N3adjacent cell N2
adjacent cell N1 Non-adjacent cell Non-adjacent cell Non-adjacent cell Serving cell Handover can’t Handover can’t happen due to happen due to lack of adjacent lack of adjacent cells cells.. 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
BTS software/hardware
Problems about CDU 、 TRM, etc.
Clock generator malfunction
Internal communication cable 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
different 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
match up to those in the corresponding BSC;
Wrong BSC signaling point in “list of cell under a LAC” in MSC;
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
between ZTE and other suppliers’ equipment, causing
non-recognition or non-support problem, including speech
version, handover code and addressing mode (CGI or
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.
Typical case 1- carrier malfunction
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:
Typical case 1- carrier malfunction
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-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 malfunction
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 3- clock malfunction
Problem description
For 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 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
appears in many cells;
It’s 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 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
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
leads 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
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
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.
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’
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,
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.
