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20/05/2008 Page 1 of 5 8
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
20/05/2008 Page 2 of 58
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
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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 SE T-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
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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 C APACITY PROBLEMS.................................... 32
6.2 USED FORMULAS................................ .......................................... 32
6.3 ANALYSIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 33
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
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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
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9 REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 57
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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RADO NETWORK PERFORMANCE
20/05/2008 Page 5 of 5 8
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 Ra ndom 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.
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
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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.
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. Th is 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 t w o 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.
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.
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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.
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).
PL_2-1: Repeated Page Attempts of Total Number of First Page
Attempts.
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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
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
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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 Subscr ibers
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
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
may also be lost due to the fact that the MSC does not receive the
page response before the timer has expired.
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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.
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
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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
increas ed 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.
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
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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.
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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
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
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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
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
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Assignment Handover Decisions to Better Cell.
HA_WO_SUC: Successful Assignments
Handovers to Worse Cell of Total Number of
Assignment Handover Decisions to WorseCell.
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.
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
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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).
Assignment to another Assignment to
another cell, (ASSOC), is a feature that
allows a mobile to seize a traffic channel inanother 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
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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
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for internal or external interferers. Ask local
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 tiltedis 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
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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-
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
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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
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.
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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.
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T_DR_SS_BL: Dropped TCH Connections
due to Low Signal Strength on both links of
Total Number of Dropped TCH Connections.
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
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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.
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 thereasons for the downtime.
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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.
Dropped TCH due to bad quality are often due to interference problems and/or low signal strengthin 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 sourcessuch as other networks, radio
stations,microwave links etc. The interference couldalso 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 parame ter 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.
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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.
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.
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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.
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 diff erent 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
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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 thatthere 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 proble ms 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.
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
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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.
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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.
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
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Possible reasons for traffic capacity problems
are:
High number of blocked devices
HW problems.
Poor dimensioning ofSDCCH 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 Acc esses with Cause
All Other Cases, e. g. Location Updating,
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
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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.
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.
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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 dimension s. 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
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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
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 TRXsto 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
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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.
Increase the number of SDCCH channels.
Note that an increase may lead to the need for
new transceivers.
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.
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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.
Increase TCH capacity.
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.
Change & repair faulty equipment.
Review the O&M procedures.
Too Frequent Periodic Registration
Check Random Access distribution.
Check the timer T3212 in the B SC and the
parameters BTDM and GTDM in the MSC.
Decrease the number of periodic
registration.
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.
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Check site position and location area border.
Check Location Update performance.
Check the parameter CRH.
If the site is located close to major road or railway; consider moving the Location Area border.
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 nu mber of LUs.
SMS Usage
Extensive SMS usage increases the SDCCH
traffic and could cause congestion if badly
dimensioned SDCCH channels.
Check SMS activity.
Re-dimension the SDCCH channels with
consideration taken to SMS usage.
Cell Broadcast Used
Chec k if Cell Broadcast is active.
If active, check if the operator uses it.
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.
Increase the number of transceivers. This
may lead to problems with floor space,
antenna installations, CDU type, expansion
cabinets and combiner type.
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
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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.
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.
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
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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.
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.
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.
Check if congestion in neighbouring cell.
Check handover performance
Check neighbouring cell definitions. Missing
relations could cause handover problems.
Correct handover parameters such as too
high or too low hysteresis values, missing
neighbour relations, one-way handovers.
Congestion in neighbouring cell needs to be
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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.
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 stated9dB. The inter symbol
interf erence (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)
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during the call as well as the service
accessibility performance.
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.
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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.
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.
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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 cant receive the
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 cant
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 thesubscriber has to terminate the call.
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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 wi
th 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
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Check the frequency plan using the C/I plots
to see that cells close to each other dont
have the same or adjacent frequencies.
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
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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.
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
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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.
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
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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 thesuspected 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 obs tacles 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
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cell.
Problems with BTS RX. Lack of antenna
diversity, RX feeder problems, and BTS HW
problems.
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
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neighbour analysing the log files might do if driving has been performed in and out of the concerned
cells 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.
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 doesnt comeback 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 SSshould 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,
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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 runChannel 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.
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
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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
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.
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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_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.
HOE_REV: MS Reversions to Old Channel at External Outgoing Handover of Total Number of External
Outgoing Handover Attempts.
HI_SUC: Success ful 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
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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.
Check if the congested cells are able to use all
installed transceivers or if the availability is
low. The features Cell Load Sharing and
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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 sitesto 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
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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
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and handover reversion where the mobiles can
continue the call in old cell. Han dover 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.
Recommended