GSM Optimization Guide bookHan Bin Jie / CMPAK

1. Call Drop Analysis 31.1 Problem Description 3

1.2 Call Drop Caused by Coverage 31.2.1 Reason Analysis 31.2.2 Solutions 4

1.3 Call Drop Caused by Handover 41.3.1 Reason Analysis 41.3.2 Observation methods 51.3.3 Solutions 5

1.4 Call drops caused by fault of hardware equipment or system parameter 61.4.1 Parameter fault 61.4.2 Hardware fault 61.4.3 Wrong BTS longitude and latitude 71.4.4 Impact from system capacity expansion, upgrade and patch 7

1.5 Call drop caused by interference 71.5.1 Reason Analysis 71.5.2 Method to figure out interference type by OMC-R 71.5.3 Solutions 8

1.6 Call drop caused by antenna and feeder 91.6.1 Reason Analysis 91.6.2 Analysis and solutions for antenna and feeder problem 10

1.7 Call drop caused by transmission fault 101.8 Call drop caused by utilization of repeater 11

2 Assignment failure rate 112.1 Problem Description 112.2 Common fault analysis 122.3 Solutions 13

3 SDCCH/TCH congestion 143.1 Problem description 143.2 Only SDCCH is congested 15

3.2.1 Common problem analysis 153.2.2 Measures for solving SDCCH congestion and sharing signaling load 16

3.3 TCH congestion 173.3.1 Common problem analysis 173.3.2 Measures for solving TCH congestion and sharing traffic load 19

4 Analysis of handover failure rate 204.1 Problem description 204.2 Common failure analysis and solution 21

5 Handover reason proportion analysis 245.1 Proportion of handover triggered by bad downlink quality is high 24

5.1.1 Problem description 245.1.2 Common fault analysis 25

5.1.3 Solutions 255.2 Proportion of handover triggered by bad uplink quality is high 26

5.2.1 Problem description 265.2.2 Common Fault Analysis 265.2.3 Troubleshooting 26

5.3 Big proportion of handover due to downlink level 275.4 Big proportion of handover due to uplink level 275.5 big proportion of handover due to distance 28

6 RACH access validity 296.1 description 296.2 Common fault 296.3 Common fault analysis 296.4 Troubleshooting 30

7 No traffic or handover in the cell 317.1 description 317.2 Common fault analysis and troubleshooting 31

8. call successful rate 318.1 PAGING TIMEOUT 328.2 Communication link establishment fail 33

9. reason and troubleshooting of coverage lessening of the sites 339.1 reason on sites side and troubleshooting 349.2 Others 35

10 analysis and troubleshooting of hot issues 3510.1 terminating MS has signaling but when it’s terminated, it tells that user is not in theserving area 3510.2 MS has signaling but is implicitly power-off when being paged 3610.3 MS signaling unstable in idle state 3710.4 Signaling fluctuate during MS making calls 3710.5 Echo in MS communication 3710.6 Other 38

11 typical case analysis and optimization measures 3811.1 Case one 3811.2 case two 3911.3 case three 4011.4 case four 4011.5 Case five 4111.6 case six 4111.7 Case seven 4211.8 Case eight 42

1. Call Drop Analysis1.1 Problem Description

Figure 1 Radio link fault signaling flow

1.2 Call Drop Caused by Coverage

1.2.1 Reason Analysis

（1）Call drop occurs in the following situations: the server cell’s coverage is too large due tovarious reasons (such as perfect radio propagation environment or too high power) and the servercell even covers its neighbor cells; the azimuth angle of its neighbor cell’s directional antenna (wesuppose neighbor cell is a directional cell) has problem or neighbor cell’s signal is too weak thatcause MS still occupies the original server cell A’s channel when MS moves out of the coverageof cell B defined as the server cell A’s neighbor cell and moves into cell C which isn’t definedas server cell A’s neighbor cell, so MS fails when it tries to hand over neighbor cell B offered byserver cell A. These situations usually happen in urban areas where BTS are densely distributed.（2）Obvious holes without being covered exist in the boundary between two cells.（3）Too small coverage may be because hardware equipment of a certain cell has problems,such as antenna is blocked or BCCH TRX has problems (in the power amplifier part).（4）Shadow effect caused by some tall buildings may bring fast fading of MS signal, then calldrops when handover can’t be carried out in time.（5）If neighbor cell isn’t defined completely, MS will be in connected status in the server celland call drop occurs when MS moves out of the server cell’s coverage.

1.2.2 Solutions

1．Find out weak covered areasThrough traffic statistic analysis, first we shall make sure whether cell’s call drop rate is high(accompanied by high outgoing handover failure rate) and handovers are mostly rescue Rxlev

handover but other indicators are normal. If so, we shall check whether it is coverage problem,then we ascertain weak covered areas through DT. In addition, we shall analyze whether calldrop is caused by special topography, such as tunnel, supermarket, subway entrance and hollow.Generally, this kind of call drop focuses on a certain direction; we can solve it through addingmicro-beehive.

2．Expand BTS coverageFind out weak covered areas through customer complaints, and then check whether new sitesneed to be added, or site’s coverage needs to be expanded through some means. For instance,increase site’s max transmitting power, change antenna’s azimuth angle, tilt angle, height, etc. (weshall comprehensively take into account frequency planning situation and coverage status in otherdirections).

3．Eliminate the impact from shifted signalThrough regular DT to find out sites with irregular coverage, and eliminate the impact of itsshifted signal to other sites. As to call drops caused by impact from shifted signal, we can solvethem through decreasing site’s tilt angle, or decreasing BSPWR MAX and increasing RXLEVACCESS MIN to shrink the coverage. Of course we shall avoid blind area from appearing duringthe adjustment.

4．Eliminate hardware faultIf call drop rate rises abruptly but other indicators of the site are all normal, then we shall checkwhether its neighbor cells work well (problems may occur in the downlink, such in TRX, diversityunit and antenna; if problems occur in the uplink, then the cell’s outgoing handover failure ratewill be high).

5．Check whether neighbor cell list is defined completelyCheck whether neighbor cells defined in OMC-R database are mutual neighbors and whetherneighbor cell list is incomplete. Different operators shall often refer to neighbor cells’ data.

1.3 Call Drop Caused by Handover

1.3.1 Reason Analysis

1．Unreasonable parametersWhen BTS is carrying out rescue Rxlev handover (when mobile phone’s Rxlev is lower thanIRXLEVULH, IRXLEVDLH), some handover request may fail due to too weak signal strengthof the incoming handover cell, and call usually drops even if the handover is successful due to tooweak signal strength.

2．T3103 overtime

T3103 is overtime: T3103 starts to record when BSC sends message HANDOVER COMMANDto MS, T3103 reset when BSC receives message HANDOVER COMPLETE from target cellfor incoming handover or message HANDOVER FAILURE from the original cell. BSC willsend message HANDOVER COMMAND to BTS, if T3103 receives none of the upper two replymessages, BSC regards that radio link fails in the original cell and then release channel of theoriginal cell. Signaling flow is shown in Figure 4.

Figure 4 call drop due to T3103 overtime

1.3.2 Observation methods

If high call drop rate is caused by handover problem, we can find out the main reason for handoverthrough analyzing OMC-R traffic report. Handover may be caused by downlink RX LEVEL,uplink/downlink RX QUAL, uplink/downlink interference, power budget (PBGT), call directionalretry, traffic, etc.

1.3.3 Solutions

Carry out DT in a large extent, because handover is taken place among cells and BTS, calldrops in one cell may be because of unreasonable handover setting with its neighbor cells. Payhigh emphasis on cells have topological relation with the original cell and have high congestionrate, and check whether blind cell exists around the original cell. If call drop is caused by thesetwo reasons, we shall modify relevant frequency and add new sites, or expand the coverage ofthe original site. If call drop is due to unreasonable handover setting, we can modify handover

parameter according to the field test. As to call drops caused by unbalanced traffic and target BTShas no channel to hand over on busy hour, we can adjust the traffic to solve it.

1.4 Call drops caused by fault of hardware equipmentor system parameter

1.4.1 Parameter fault

We can check whether parameters are reasonable through parameter checking tool, like whetherfrequency planning is reasonable, whether MAIO of TRX in the cell conflicts with each other(in this situation, all indicators will be bad, such as assignment failure rate), whether hoppingfrequency has interference, whether timer of BSC matches that of MSC (if CELL’s T3103 ishigher than BSC’s BSSMAPT8, then call drops when MS is carrying out handover). Whenparameter IRXLEVDLH and RXLEVMINCELL are unmatched in definition, it is easily to makecall drop because when MS reaches IRXLEVDLH or IRXLEVULH but there is no neighbor cellreach the RXLEVMINCELL. Besides, unreasonable HOMAGIN also causes call drop during thehandover. We can observe whether the definitions of T3101 and T3107 are so strict that systemhas no sufficient time to send assignment completed message to BSC, and call drops because timerhas already reset.

1.4.2 Hardware fault

We can check alarm related to hardware in OMC-R if call drop is caused by hardware fault. Ifthere is no hardware alarm in OMC-R, it may be caused by fault of a certain TRX or diversity part,then at the same time assignment failure rate and uplink/downlink quality handover proportionwill be very high. We can find out the fault by ABIS monitoring software or by carrying outCQT to suspicious TRX after turning off other TRX in the cell. We shall replace the faultedhardware in time once discover it; if there is no equipment available for replacing, we shall turn offfaulted equipment to avoid call drops that may impact network operation quality. Generally, whenframe processing unit is faulted, assignment failure rate and uplink/downlink quality handoverproportion will be high; when receiving part is faulted, assignment failure rate and uplink qualityhandover proportion will be high; when transmitting part is faulted, faulted, assignment failurerate and downlink quality handover proportion will be high.

1.4.3 Wrong BTS longitude and latitude

In the field DT, we find that sometimes few BTS’s longitude and latitude are not the same withthose in the planning or even have huge differences with them. It is mainly because difficultiesappeared in site selection make it impossible to locate BTS according to design demand. But the

new location of the site wasn’t updated in planning database, and we still planned its neighborcells and frequencies according to the original plan, then lots of neighbor cells are missed or setwrong and this finally causes high call drop rate.

1.4.4 Impact from system capacity expansion, upgrade andpatch

After system has been adjusted on a large scale, like cutting over newly-added sites for launch,BTS capacity expansion, frequency re-planning, upgrade and patch, we shall carry out an overallchecking and adjustment to its related system parameters. We shall pay special attention toneighbor cell relations, frequency interference as well as hopping parameter, cell parameter, etc.

1.5 Call drop caused by interference

1.5.1 Reason Analysis

Because the frequency of SDCCH that BTS assigns to MS may not be the same with that ofTCH, we need to analyze them separately. Interference includes co-channel interference, adjacentchannel interference and intermodulation interference. Error rate will be deteriorated and mobilephone can’t demodulate BSIC of neighbor cell accurately or can’t receive MS measure reportcorrectly when mobile phone receives strong co-channel or adjacent channel interference signal.Intermodulation interference is produced by transmitter of analog BTS in digital and analog co-sited BTS; the direct impact of this kind of interference is BTS resource waste because time slotcan’t be assigned.

1.5.2 Method to figure out interference type by OMC-R

1）We can observe OMC-R counter related to interference. In some systems, when channelis idle, system will observe the interference to the channel and report it in a certain period.When channels working in interference are too many, then we can judge that the system hasinterference.2）We can judge whether uplink interference exists through observing the absolute value of theaverage level in RACH request that can be decoded by system.3）We can judge through observing handover reason statistic of OMC-R. In normal situation,PBGT handover shall be much more than other handover (accounts for over 60% of the totalhandover times); we can regard it as uplink interference or hardware fault if uplink qualityhandover is too many; we can regard it as downlink interference or hardware fault if downlinkquality handover is too many; we can regard it as hardware fault if both uplink and downlink

quality handovers are too many (it is also possible that both uplink interference and downlinkinterference exist at the same time).

1.5.3 Solutions

1．Eliminate uplink interferenceThis is the main interference at present. Uplink interference often occurs on a period with themaximum traffic. It mainly comes from co-channel interference or exterior interference. Co-channel interference is related to the traffic of intra-frequency cell; the higher traffic, the largerinterference. Exterior interference is intermodulation interference. Uplink interference can besolved by analyzing related reports in DT, modifying co-channel frequency in intra-frequencycell, increasing distance between two intra-frequency cells (statistic shows that signal strengthattenuates along with the 4 power of the distance) or using spectrum analyzer. It can also bedecreased through diversity and effective power control.

2．Eliminate downlink interferenceDownlink interference is not very common. It is mainly co-channel interference and adjacentchannel interference of some BTS caused by improper frequency planning. We can judge itthrough handover test report in OMC, downlink interference usually cause frequent downlinkhandover. If co-channel interference and adjacent channel interference are discovered in testreport and field test, we shall re-optimize and re-adjust frequency planning of the cellular system.As to cells have interference but aren’t in the upper situations, we can find out interference sourceby using spectrum analyzer.

3．Use discontinuous transmission (DTX), hopping technology, power control and diversitytechnologyDTX includes uplink DTX (set by parameter DTXMODE) and downlink DTX (set by parameterCELLDTXDOWNLINK); both of them use VAD technology and stop transmitting when voicesignal isn’t transmitted (only a group of SID frame is sent in each 480ms to meet the measurerequirement of BTS). Then useless information is restrained from sending and effectivetransmitting time is decreased, so that system’s interference level can be depressed and life ofthe battery can be prolonged. Hopping can improve radio signal’s transmitting quality effectively,especially to the transmitting quality of slowly-moving object. This is because hopping makestransmitting TRX hop on each unit length of burst that can obviously decrease co-channelinterference and frequency selectivity fading effect. But DTX has to be adjusted according topractical surrounding radio environment and relations with neighbor cells. If the receiving signalof the mobile phone is not good, call drop may occur when using DTX. It is because that whenDTX is used in downlink and mobile phone is in connected status, BTS transmitting power getsstrong while the subscriber is talking and gets weak on gaps. In this way, on the one hand we candecrease interference to other BTS, but on the other hand if there is interference surrounds theBTS then discontinuous transmission of downlink signal makes call quality worse; and when BTSdecrease transmitting power, call quality deteriorates and even call drop occurs in some placeswhere receiving signal is relatively weak and interference signal is relatively strong.

1.6 Call drop caused by antenna and feeder

1.6.1 Reason Analysis

1．Call drops due to different down/up tilt angles of two pairs of antennasIn BTS installation, each directional cell has two pairs of antennas, then BCCH and SDCCH ofthe cell may be transmitted separately by different antennas. Different down/up tilt angles of twopairs of antennas will cause different coverage of the two pairs of antennas, which means calldrops may occur when subscriber can receive BCCH but can’t occupy SDCCH sent by anotherantenna while setting a call.

2．Call drops due to antenna’s azimuth angleIn BTS installation, each directional cell has two pairs of antennas. If the azimuth angles of thetwo pairs of antennas are different, then call drops occur when subscriber can receive SDCCH andbe assigned to TCH sent by another pair of antenna.

3．Call drops due to antenna feederThe transmitting power and receiving sensitivity will be decreased due to damaged and foldedfeeder, or because feeder is affected by water or loose contact. Consequently, call drop occursseverely. We can ascertain the reason through testing VSWR.

4．Call drops due to too short distance between antennas for diversity or unparallel Rx and TxantennasA certain horizontal distance between two pairs of antennas should be insured to realize diversityreceiving; otherwise receiving sensitivity will be decreased and call drops will occur. When weuse diversity receiving antenna, generally an ideal effect will be obtained and gain is 3dB ifdistance between Rx and Tx antennas is about 3 to 5 meters. But diversity receiving will be hardto realize if the distance between Rx and Tx antennas is shorter than 1 meter. Besides, if Rx andTx antennas are unparallel, or even transmitting antenna directs receiving antenna, or there is avery tall iron pole not far away from Rx and Tx antennas, signals will be obstructed and reflectedeasily and interference will be produced.

5．Call drops due to too strong backward signal strength from directional antennaIf backward signal from a cell’s directional antenna is too strong, call drops when MS occupiesthe signal but can’t find its neighbor cell.

1.6.2 Analysis and solutions for antenna and feeder problem

1．Check antenna’s azimuth angle and down/up tilt angle

If call drops due to incorrect antenna’s azimuth angle and down/up tilt angle, we shall first observethe BTS. We can find out fault reason by carrying out CQT, DT while combining with statisticsfrom OMC in faulted cells, and decrease call drop rate through adjusting antenna azimuth angleand down/up tilt angle. If it is because of too strong backward signal strength from directionalantenna, we shall replace the antenna in time.

2．Eliminate feeder problemIf call drops are due to decreased BTS transmitting power and receiving sensitivity causedby damaged feeder or loose contact, we can ascertain fault reason and faulted place throughmeasuring feeder by feeder test instrument, so as to replace faulted feeders and connectors intime.Notices in feeder system:Avoid feeders from being connected reversely because they are wrong labeled; avoid feeders fromgetting affected by water, pay attention to dust prevention and firmness of feeder’s combinationparts; be familiar with feeder’s working environment and physical nature, check whether feeder isaging according to practical installation and cabling rather than doing analysis simply relying onVSWR tested by instrument; avoid other things from getting inside connectors so that situationssuch as feeder’s short circuit or dust entry won’t happen.

3．Use reasonable antenna typeBecause presently sites distribute more densely and network structure keeps on changing, wesuggest using some small-sized antennas with low gain and high forward and backward isolationin BTS in urban areas or areas with densely-distributed sites. We don’t need to worry about anybad impact that small antenna may impose to signal coverage at all; contrarily, radio space willbe purer and easier to be controlled than before because of antenna’s lower gain and higherforward and backward isolation. According to the practical effect of using small antenna, networkperformance has been improved obviously.

4．Eliminate interference from antenna backward signalAntennas are usually installed on poles, towers or rails on top floor in urban area, thereforethey may be too high and it’s not easy to control signal coverage, even backward signal may beinterference to the network. We suggest installing antennas on a floor’s walls, and then antenna’sbackward signal will be obstructed by buildings.

1.7 Call drop caused by transmission faultFaults in Abis interface include that BSC doesn’t receive measure report from BTS; failedsignaling during handover and some internal reasons; impact of error rate on Abis interface. Thereisn’t many faults occurs on A interface, most of them are handover failure (handover betweenBSCs or MSCs) because handover data are not complete or target BTS isn’t qualified to be handedover in.We shall adjust BTS clock, check transmission synchronization and transmission qualityregularly. Adjusting BTS clock and checking transmission synchronization is for signal

synchronization and improving handover success rate between MSCs or BSCs. Checkingtransmission quality, checking transmitting error rate by instrument, or even checking 2M cableconnectors regularly are very necessary. Because unstable transmission link will cause signalinglose and unstable lower link or even call drops. Stable transmission quality can decrease lots ofAbis call drops.

1.8 Call drop caused by utilization of repeaterSome business and trade centers like shopping mall as well as some small BTS in counties userepeater to amplify signals directly for reducing investment and expanding coverage. Signals aretransmitted by fiber or in microwave. Due to topography, environment and engineering equalityreasons, demanded indicators can’t be satisfied and then call drop occurs.When we use repeater, distance control parameter setting (TA setting) requires special attention.Otherwise distance reason will cause handover failure, consequently call drop or call clearing.We usually transmit signals in microwave when using outdoor repeater. Therefore repeateramplifies interference as well while amplifying required BTS uplink and downlink signals. Thensignal quality gets worse and finally call drops accompanied with obvious assignment failure rateraise.We shall survey repeater’s real surrounding cells according to its neighbor cell definition whileinstalling it, and pay special to whether it will cause mutual interference to its surrounding cells;if so, call drop due to handover failure (T3103 overtime) and interference will occur. Afterrepeater has been installed, we shall observe whether assignment failure rate, call drop rate andhandover proportion due to quality reason are deteriorated suddenly; if so, we can ascertain thereis interference.If the situation mentioned above occurs, we shall re-plan frequency, modify neighbor cell list,adjust handover parameter and power control parameter.Anyway, no matter what kind of reason causes call drop, we shall analyze it through all kinds oftest methods and test reports from OMC, and carry out CQT and DT periodically.

2 Assignment failure rate2.1 Problem DescriptionSignaling flow assigned by TCH is shown in figure7-5.

Figure 5 Signaling flow assigned by TCHThe following reasons cause assignment failure:BTS downlink fault prevents assignment message from sending correctly; MS can’t receive theassignment message; MS can’t identify the assignment message; MS can’t occupy the assignedchannel; BTS can’t receive or can’t identify SABM from MS; BTS uplink fault preventsassignment completion message from sending to BSC. When MS can’t occupy the channelassigned by network, it will send back assignment failure message. Other situations will makeT3107 overtime.

2.2 Common fault analysis1．Faulted transmitter or receiverGenerally assignment failure rate will be very high when TRX is faulted; incoming handoverfailure rate will also be very high because BSC assigns channel for MS during incominghandover. If a cell’s assignment failure rate is over 10%, then it is most probably due to TRXfault; if the rate is over 5%, usually we suspect it is due to TRX fault or interference; if the rateis 3%~5%, then it may be caused by congestion or interference; if the rate is lower than 3%,commonly it is because of radio environment influence.

2．Co-channel interference or adjacent channel interferenceWe shall analyze it according to interference value. High error rate caused by interferenceprevents MS from establishing the second layer link with BTS and then causes failed assignment.

3．Feeder problemIf feeder is broken or get corroded, then VSWR will be so high that it may impact receiving andtransmitting performance.

4．Main diversity antenna is blocked or can’t offer symmetric coverageWhen antenna that only transmits TCH TRX is blocked or its covered area is different fromanother antenna that transmits BCCH TRX or SDCCH TRX, then MS may not occupy the TCH.

5．Unreasonable parameters（HSN, MAIO, T3107 setting）When network uses hopping technology, if HSN or MAIO are set unreasonably, then co-channelinterference or adjacent channel interference in one cell or among cells on the same MA Listwill be severe and then assignment failure rate will be very high. Generally, when this situationhappens, all indicators will not be good.Check whether T3107 is set so strict that network has already released the channel because T3107is overtime before it gets assignment completion message.

6．Transmission fault on A interface or Abis interfaceIf transmitting error rate is too high on A interface or Abis interface, then signaling exchangebetween MS and network can’t be accomplished normally, and then assignment failure happens.

7．Impact from repeaterPlease refer to call drop analysis.

2.3 Solutions1．We shall check cell’s incoming handover execution failure rate for better locating the problemAscertain whether handover failure rate ((handover execution – handover success) / handoverexecution) is related to assignment failure rate. It is because the process that MS establishes thesecond layer link with handover target cell is similar to the channel assignment process when callis set up.

2．We shall check hardware fault when assignment failure rate is high (higher than 5%)If it may be TRX problem, operation and maintenance engineers shall check OMC-R alarmrecord (mostly a receiving module or transmitting module has problem). If there is no problemdiscovered in OMC-R, then use Abis interface monitor to find out the faulted TRX that causesassignment failure (equipment SN is TEI).

3．Find out reason through cell’s link balance testAssignment failure may caused by uplink signal loss or receiving path failure. We can find out thereason through cell’s link balance test by Abis track analyzer or path test statistic.

4．Eliminate interferenceOther situations occur when high assignment failure rate is caused by interference, such as lotsof handovers triggered by quality problem (high BER), or too high RACH decoding level. Wecan find out system interior interference by carrying out DT. If uplink and downlink levels arehigh but signal quality is bad, then interference may exist. Then we check whether the occupiedchannel (BCCH TRX) is hopping or not. If BCCH TRX has interference, we re-plan the frequencyof co-channel cell and adjacent channel cell.If there is no hardware fault but TRX voice quality is bad, then it means frequencies it uses haveinterference. One solution is to optimize cells that have the same MA list with the cell; another

solution is to suppose the cell isn’t in hopping mode and set TRX on each frequency on MA list,so as to find out frequencies get interference. Of course, network quality will be decreased if weuse this solution, so we suggest using it on non-busy hour.

5．Eliminate antenna problemIf there is no interference or hardware problem but handover failure accompanies with assignmentfailure, we can check whether diversity antennas’ down tilt angles are different or antennas getblocked.If the assignment failure is severe but reason can’t be found temporarily, we shall turn off the cellto avoid network quality from getting serious impact.

6．Check cell parameter setting statusModify parameters set improperly.

7．Eliminate transmission problemCheck whether transmission problem exists through testing error rate by instruments; if yes, solvethe problem.

3 SDCCH/TCH congestion3.1 Problem descriptionThere are two kinds of congestions: one is that network has no signaling channel to use inimmediate assignment; another is that network has no TCH to use in voice channel assignment.In this section we mainly analyze congestion problems as well as propose solutions and balancemethod to signaling load and traffic load.In immediate assignment, SDCCH assignment failure counter accumulates once BSC sendsIMMEDIATE ASSIGNMENT REJECT according to command that channel can’t be activated.When there is no available SDCCH or ground resource on Abis interface, T3122 is includedin IMMEDIATE ASSIGNMENT REJECT message sent by BSC to MS. T3122 defines theminimum interval for MS to set up the next call.Signaling flow is shown in Figure 6.

Figure 6 SDCCH congestion signaling flow

When assigning TCH, assignment failure occurs (reason is congestion) if BSC has no resourcewhen receives ASSIGN REQUEST or HO REQUIRED from MSC, or no available ground circuitresource when sends channel activation. If this happens, BSC sends ASSIGN FAILURE to MSC(reason is no radio resource), and TCH assignment failure counter accumulates once. Please referto Figure 5.Through analysis, we can solve these two kinds of congestions separately.

3.2 Only SDCCH is congested

3.2.1 Common problem analysis

1．LAC boundary problemIf a cell locates on the boundary of LAC, then frequent LAC updating program triggers TCHcongestion because the cell’s boundary may cross a high traffic area.When this situation happens, we can check that the cell hands over frequently into which cellswith different LAC and BSC by observing its handoff status, and then optimize LAC (cut the cellover other LAC). If we use this method, we shall also take into account parameters related to C1,C2 algorithm, cell reselection hysteresis parameter and handover margin parameter, etc.

2．Improper parameter configurationToo less signaling channels are configured; relevant parameters of C1, C2 algorithm are setimproperly; T3101, T3122 and T3212 are set improperly.

3．Hardware faultFaulted or unstable hardware such as TRX will trigger that BSC can’t activate the channel’sground resource in immediate assignment.

4．Neighbor cell faultNeighbor cell’s fault will cause server cell congestion because it absorbs some extra traffic.

5．Illegal subscribers’ frequent registrationCheck whether congestion is triggered by frequent registration of some roam-restrictedsubscribers. If a roam-restricted subscriber enters a restricted area and keeps the mobile phoneturning on, then the subscriber will try to register on the local network ceaselessly but alwaysfails due to authentication failure. This will aggravate the signaling load and equal the subscriberoccupies the capacity of a local signaling channel.

3.2.2 Measures for solving SDCCH congestion and sharingsignaling load

1．Increase the value of cell reselection hysteresisIf SDCCH congestion is due to LAC boundary problem and the congestion is not severe, we canincrease the value of cell reselection hysteresis, so as to decrease frequent MS location update asmuch as possible.

2．Reconfigure channel typeIf the method mentioned above doesn’t have much effect, we can reconfigure TDMA accordingto TCH traffic status, like to change TCH time slot as SDCCH/8 while referring to the times thatTCH used as signaling channel.

3．Decrease T3101Adjust T3101 to reduce system resource waste. Generally, immediate assignment can beaccomplished within 3 seconds. Then we set T3101 as 3 seconds to avoid BSC from assigningresource because of resending channel request and to release resources that have been wasted forother requests.

4．Adjust MAXretrans（maximum retransmission times）On the premise that the radio environment is favorable, we try to decrease MAXretrans to reducesystem resource waste as much as possible. Generally we shall try to increase its value if cell’sload is normal.

5．Adjust T3107 and T3103We can reduce TCH waste by adjusting T3107 and T3103. On the premise that the radioenvironment is favorable, it only takes system about 2 seconds to assign TCH, so we can set thelowest T3107 as 2s. Generally we set both T3107 and T3103 as 5s.

6．Adjust T3122

We can prevent subscribers from frequently sending channel request message which will increasenetwork RACH and CCCH load by increasing T3122 when system has no resource (normally weset T3122 as 10s, or 30s when SDCCH traffic is very large). This timer shall be set as shorter aspossible when channel’s traffic isn’t much big.

7．Adjust relevant parameters impact C2We can decrease the cell’s C2 by reducing the value of CELL RESELECT OFFSET (the premiseis that C2 algorithm has to be activated). In this way, we can reduce the cell’s traffic and thedifficulty for MS to select the cell as server cell, so as to make calls be set up in the cell’s neighborcells as much as possible.

8．Increase T3212 properlyT3212 is used for limiting the time of periodical location update. We can alleviate SDCCH loadfrom periodical location update by increasing T3212. But we shall be aware that its value has tobe shorter than MSC connotative turning off time limit while modifying it. Connotative turningoff time limit shall be set a little bit longer than “periodical location update”; otherwise, MS willbe regarded to be turned off by the network if it doesn’t have any contact with the network duringa period of time.

9．Eliminate hardware faultCheck whether cell and its neighbor cells are working normally. Check whether TCH is availableto ascertain unstable equipment. If neighbor cell isn’t working normally, the cell will take theextra traffic.

3.3 TCH congestion

3.3.1 Common problem analysis

1．Traffic density is higher than BTS designed capacityIf TCH keeps congested during a period of time, it means subscriber group has changed thatmakes the BTS traffic density higher than the original designed capacity. As for this situation, theeasiest solution is to expand capacity.

2．Handover problemUnreasonable call model caused by too frequent handovers among cells will decrease systemcapacity. Ping Pong handover caused by boundary problem, coverage problem, interference,hardware fault or parameter problem will trigger TCH congestion. When system has interferenceor hardware problem, MS receiving level will be high but signal quality is bad. If this happens,MS may be handed over out due to bad signal quality and handed over in due to power budgetreason, and then Ping Pong handover occurs.

Some similar situations happen such as MS can also be handed over out due to TA reason andhanded over in due to power budget reason.

3．Transmission problemIf transmission breaks off for an instant or high error rate occurs on Abis interface, because thefault hasn’t been transmitted to BSC and ground circuit resource is unavailable when BSC isactivating channel, it is counted as congestion. It is especially obvious after queuing function hasbeen activated.This problem will also trigger severe incoming handover failure. Because MSC doesn’t knowthe BTS has transmission problem and still sends incoming handover request to the BSC, thenconsequently incoming handover request fails. BSC will also count this event as congestion. Thisproblem will last till MS that try to handover in deletes the cell on handover cell list reported tothe system.

4．Isolated island problemWhen a cell’s coverage is isolated in an area, mostly the cell can’t find pre-defined neighbor cells,and then MS will always stay in the cell where call set up and can’t be handed over no matterhow signal changes till call drops. We can take two measures to avoid this: the first measure is toadjust antenna of the isolated cell and eliminate isolated island phenomenon.We have to do tests many times to eliminate isolated island without influencing covered area,but tall building’s isolated island phenomenon still can’t be eliminated completely because ofcomplexity of wave propagation. So the second measure is to define new neighbor cells forisolated cell. The principle for defining parameters is that isolated cell has the priority in locationupdate and priority to be handed over in normal cells.

5．Neighbor cell problemPlease refer to SDCCH congestion analysis.

6．Neighbor cell definition problemNeighbor cells are not defined completely prevents calls from timely handover out and then causescall drop.

7．Improper parameter settingT3107 and T3103 are set too long, queuing parameter is unreasonable; handover threshold andhandover margin are set unreasonably; the definition of cell’s minimum accessing level and BTSpower are unreasonable.

8．TCH is used as signaling channelIdle TCH is used as signaling channel due to lack of SDCCH. Then there is no resource availablewhen MSC is assigning channels.

9．Coverage problemThese problems exist in urban areas:

Too large coverage caused by too high antenna height or unreasonable antenna down/up tilt angle;BS TXPWR MAX is set too high due to RXLEV ACCESS MIN is set too low; the BTS overlapsits neighbor cells because its coverage is perfect in some directions (such as in square, lake orhighway) that makes some traffic can’t be handed over out normally and causes congestion due totoo concentrated traffic.

10．Hardware is instableInstable BTS equipment (like TRX) will impact channel’s availability and channel can’t beactivated while assigning channel that will trigger congestion.

3.3.2 Measures for solving TCH congestion and sharingtraffic load

1．BTS capacity expansionIf channel congestion is caused by limited capacity, then we shall calculate cell’s trafficrequirement according to cell’s congestion rate and practical loaded traffic, and increasecorresponding radio resource in the planning.

2．Prevent Ping Pong handoverWithin the T3103 time limit, TCH of both server cell and target cell will be occupied at the sametime during the handover, so too many handovers will waste resource. Then possibly we shallreduce handover times especially when resource is tight.If the ratio of handover times to total call times is too high, we can avoid unnecessary Ping Ponghandover by increasing cell’s handover power margin. Possibly algorithm for Ping Pong handovercontrol shall also be activated in proper situation. For instance, if the outgoing handover is becauseof quality reason, then incoming handover due to power budget shall be refused in a period oftime. We shall pay special attention to it in dual band handover.

3．Activate queuing functionQueuing function has obvious effect for solving congestion caused by temporary traffic summit.

4．Adjust T3103 and T3107Decrease time limits of T3107 and T3103 to reduce system resource waste as much as possible.

5．Control SDCCH congestionWhen SDCCH is not enough, BSC will use TCH to transmit signaling. Therefore if TCHcongestion is caused by lack of SDCCH, we shall control SDCCH congestion by the methodmentioned above.

6．Eliminate neighbor cell faultsCheck whether there are faults exist in neighbor cells, find out undefined cells through DT andneighbor cell parameter checking.

7．Shrink BTS coverageIf congestion is caused by too large BTS coverage, then we can increase RXLEV ACCESS MINand decrease BS TXPWR MAX according to practical situation.

9．C2 algorithm optimizationIf a cell is severely congested but its surrounding cells are relatively idle, we can optimize thecell’s traffic load through adjusting parameter PT and CRO (set CRO as minus bias) after C2algorithm has been activated. We can also low the cell’s priority (set CELL BAR QUALITY as1), this is equal to shrink the access range of the cell’s RACH into an area that only the cell cancover, so that part traffic will be shared. Meanwhile, we shall increase its neighbor cell’s C2 andhandover margin to avoid shared traffic from being handed over in again.

10．Adjust antenna’s installation height as well as down tilt angle and up tilt anglePossibly control BTS coverage by adjusting antenna’s installation height as well as down tilt angleand up tilt angle, so as to optimize traffic load. But we shall prevent blind area from appearing.

11．Eliminate equipment faultRepair faulted equipment if it is the reason for incomplete channel utilization.

4 Analysis of handover failure rate4.1 Problem descriptionFrom view of signaling, handover failure could be divided into two aspects: handover selectionfailure and handover execution failure. Let’s make deep analysis. Signaling procedure isillustrated in Figure 9.

Figure 9 handover failure procedure

Handover selection failure（HANDOVER SELECTION FAILURE）is the difference betweenamount of HO COMMAND from BSC to BTS and amount of HO INDICATION received byBSC. The reasons of handover failure sometimes are channel resource absence of target cell,or system parameters or hardware issues (viz. difficult L2 connection between BSC and BTS).Handover execution failure（HANDOVER EXECUTION FAILURE）is the difference betweenamount of HO COMMAND from BSC to BTS and amount of HO COMPLETE received by BSC.This reflects the quality of radio interface in the air.When handover succeeds, MS will send HO COMPLETE message to target cell. After target cellreceives the message, one handover-in success will be counted; if this handover is INTRA MSChandover, after originating BSC receives CLEAR COMMAND message (this message includesclear reason which is handover success), one handover-out will be counted.When MS could not capture target cell channel, which results in handover failure, due to radiomatters, MS will send handover failure message to handover originating cell. After this, MSCsend CLEAR REQUEST to target BSC, the message includes clear reason which is handoverfailure. And then, one handover-in failure will be counted.

4.2 Common failure analysis and solution1．Hardware failure

When handover execution failure rate is very high, hardware failure is most possible (seeallocation failure). Here, we have to locate failure hardware by traffic analysis, and dispose wellin time.

2．Neighbor cell relationship issueIn dedicated mode, MS send measurement report to system by SACCH, and distinguish differentcell RX power level with BCCH and BSIC. If two cells have same BSIC and BCCH, innormal situation, the distance between the two cells should be far enough, so nothing should bebetween them. But due to isolated cell environment, MS often send inveracious report to system,this inveracious neighbor cell report will misguide handover control program send handoverinjunction, it will cause connections in cell attempt sending handover request to cell with lowRX power level, named ping-pong handover, and result in higher handover-out failure rate fromisolated cell to target cell. On the other hand, higher handover-in failure rate will appear at targetcell which has the same BSIC and BCCH with isolated cell.Due to compact frequency reuse in urban, it sometimes results in isolated effectiveness. Theprinciple how to judge isolated point is, that observes whether advantaged radio propagationenvironment exists between isolated point C and Cell A, for example: higher antenna height andsmall down tilt for Cell C, open land, highway and river between Cell A and Cell C, etc.

Figure 10 isolated effectiveness

In the area of city boundary, when more INTRA-MSC handover failure happens, and excludesother possible failures, here check whether frequency change on boundary neighbor cell, whichresults in isolated effectiveness.For a pair of neighbor cells, if higher handover execution failure due to isolated effectiveness,Abis interface test could be done to find isolated cell by checking time ahead (TA). Then biggerdown tilt could abate isolated effectiveness, or change BSIC to eliminate same BCCH/BSIC.Check whether neighbor cell table matches BTS parameters, if not, modification should be donein OMC-R.

3．Neighbor cell channel resource absence or transmission failureIf handover selection failure rate（HO SELECTION FAILURE RATE）is very high, it ispossible that the load of target cell is very heavy, and without available TCH. Here BSC receivesHO INDICATION message, but handover could not be executed. For higher handover selectionfailure rate between a pair of neighbor cells, check whether it is the load problem of target cell.When transmission failure happens in neighbor cell (e.g. higher BER or transmissioninstantaneous break), BSC has available resource, but terrain circuit resource could not be

activated, therefore, results in handover selection failure. If transmission failure happens betweenintra MSC handover or inter MSC handover, the problem will be more serious. Because MSCdoes not know transmission failure between BSC and BTS, if handover request happens, it willcause channel allocation request is sent to target BSC, and results in handover failure. Thisproblem will continue until originating BSC does not receive MS measurement report about targetcell.If neighbor cell channel resource absence results in handover failure, we could refer to solutionof TCH/SDCCH congestion. In addition, configuring handover request priority higher thanallocation request priority is also an effective solution for handover selection failure problem.

4．Terrible radio environmentIf handover execution failure rate（HANDOVER EXECUTION FAILURER RATE）is high, itis possible radio environment problem of target cell or originating cell. Terrible radio environmentwill cause that MS could not receive handover command from originating cell, or capture channelallocated by target cell.Normally, the reason of terrible radio environment is overlap coverage absence between cells.We could check coverage prediction, especially some special places (e.g. tunnel, etc), once havesuspicion, it is better to confirm it by drive test and use relevant radio optimization approachesto improve coverage. Reducing down tilt and adding antenna height could improve cell coverage,but this could also introduce much adjacent interference. Fundamental method is to add sites ormicro cells, but this is out of optimization.

5．Coverage issueIf there is not enough overlap coverage between originating cell and target cell (shown in Figure11), handover could fail because MS is not able to capture TCH of target cell. In the situation,replacement is difficult with lower probability.If make sure terrible coverage results in failure, we have to use relevant approaches to improveoverlap coverage.

Figure 11 terrible coverage results in handover failure

6．Interference issueIf interference exists, it is difficult for MS to capture TCH of target cell. And here, RX power levelis better, but uplink / downlink signal quality is terrible.If handover-in execution failure rate is high, it is necessary to check cell interference. Checkinghandover-in and handover-out execution failure rate of every pair of cells could indicateinterference exists in single cell or in multi-cell, and further estimate interference area andinterference property.If handover-out execution failure rate is high, probably downlink interference exists in originatingcell, MS could not decode HANDOVER COMMAND message from BTS, here T3103 time outresults in call drop. The reason of higher handover-out execution failure rate may be due todownlink / uplink interference in target cell. Therefore, it is necessary to check every indicatorof every pair of cells to help estimating interference, at the same time check correlation between

handover-in execution failure rate and allocation failure rate of target cell. Higher handover failurerate caused by interference always accompanies high allocation failure rate.To confirm cell interference, proportion of handover due to uplink / downlink quality could beestimated.If confirming cell interference, change cell frequency in time or eliminate interference source.

7．Antenna issueDue to obstruction before antenna or two antennae coverage asymmetry of the same cell, forsolution, please see allocation failure analysis.

8．Repeater issuefor analysis and solution, please see call drop analysis.

9．Parameters issueDue to inconsequent or mismatch parameters, e.g. configuring T3103 with a small number resultsin MS could not capture channel if target cell.If parameter description of neighbor cell about LAC and CI of target cell, it will result in higherhandover selection failure rate.

10．Incomplete LAC list of MSCIf the definition of REMOTELAC table of MSC faultiness or error, and there is MSC boundarycell definition on BSS side, it will result in handover selection failure. When handover requestreaches MSC, MSC finds LAC does not belong to itself, MSC will search in remote LAC table.If remote LAC table has no information about LAC, it will result in handover failure. Whenexecuting INTER MSC continuous handover, if there is no definition of LAC of target MSC inoriginating MSC, it will result in handover failure.

11．Higher signaling link load impactDue to congestion caused by heavy load on interface A, there is no available resource whensending handover request and handover command between intra MSC or inter MSC. In thissituation, trunk link capacity should be expanded.

5 Handover reason proportion analysis5.1 Proportion of handover triggered by bad downlinkquality is high

5.1.1 Problem description

It is one of the rescue handovers and it’s different from normal power handover. Rescue handoveris carried out not because a server cell with stronger signal has been discovered, but because the

present server cell’s signal quality is too bad to support normal call. Handover triggered by baddownlink quality is carried out because server cell’s signal quality is too bad (high error rate).If the original BSC decides that voice quality error rate has reach the downlink quality handoverthreshold, then it will choose a cell with the strongest signal as the target cell for incominghandover from well-performed neighbor cells. Therefore the handover is only carried out whenvoice quality error rate is higher than L RXQUAL DL H.

5.1.2 Common fault analysis

1．Unreasonable handover parameterIf L RXQUAL DL H is set too low, then it’s very easy for voice quality error rate to reach qualityhandover threshold. If handover algorithm judging period is too short, then quality handover willtake place even just a few samplings are lost.

2．Hardware problemIt may be downlink hardware problem if handover failure rate is high, such as BTS transmitter,power amplifier, transmitting antenna or feeder has problem.

3．GSM interferenceCheck whether co-channel interference or adjacent channel interference exists. If there is severeinterference inside system, quality handover will also be triggered during the call because of badvoice quality even if subscriber can occupy the assigned channel. But quality handover usuallycauses Ping Pong handover: if quality handover is triggered by interference, then commonly theoriginal cell’s signal strength is high but quality is bad; therefore MS will be handed over backinto the original cell again because of power budget even if it was handed over out due to qualityreason.

5.1.3 Solutions

1．Parameter checkingCheck whether cell handover parameter is set properly.

2．Eliminate hardware problemIf there is downlink hardware problem, for instance, too small transmitting power of some TRX,it will make receiving signal’s strength and quality of MS that uses the TRX is bad. If thissituation happens, high downlink level handover will accompany high downlink quality handover.Therefore, we can check traffic change as well as the ratio of incoming handover to outgoinghandover.Downlink hardware problem will also make downlink quality handover, assignment failure, radiolink failure increase and much higher than those of other cells. Some downlink hardware problemswill be shown in VSWR alarm in OMC-R. Solutions are: track Abis interface, check whether

uplink and downlink are balanced, check downlink level status, measure VSWR of BTS, as wellas check cables in transmitter.

3．Eliminate GSM system internal interference1）Track Abis interface. Abis analysis software can find out downlink TRX with high level andbad quality;2) Observe OMC-R traffic report. If the cell’s traffic is high, then uplink quality handover willalso be very high;3）Analyze cell’s planning figure. Try to find out interfering cell (the interference may notbe caused by neighbor cells, we shall be aware that it is possibly because of isolated islandphenomenon). If the interference is newly-appeared, it may be related to newly-added BTS;4）Ascertain interference by DT;5）Turn off interfering cell and check whether the interference disappears, so as to ascertaininterference source.

5.2 Proportion of handover triggered by bad uplinkquality is high

5.2.1 Problem description

It is one of the emergence handovers, carried out if server cell’s uplink signal is too bad. Thetarget for outgoing handover is a cell with strongest BCCH signal among well-performed neighborcells. Handover trigger threshold is L RXQUAL UL H, which is network uplink quality handoverthreshold.

5.2.2 Common Fault Analysis

1．Too strict parameter settingThis makes MS easily reach the threshold of handover caused by quality and handover becomefrequent.

2．Facility faultIf uplink handover times is great, it may probably caused by the hardware of radio uplinktransmission facilities (TRX, antenna, diversity and feeder).

3．Coverage problemCheck whether it relates to handover caused by uplink and downlink power level. If it’s thecase，it may be result from inadequate coverage.

4．Uplink interferenceSee whether uudecoded RACH level is too great in OMC-R traffic statistics report, if so, the faultmay caused by uplink interference. Take DT to see whether there are interference in the cell.5．GSM interference

When there is interference is GSM system, handover caused by voice quality will be considerablylarge.

5.2.3 Troubleshooting

1．Parameter checkingCheck the parameters of cell handover.2．Check the hardwareIf there are problems concerning uplink hardware, loss of cell or some TRX will be great. At thesame time, we shall check the failure rate of handover in. Because if uplink BER is high, even ifthe downlink is normal, handover can be hard to carry out.If there are problems concerning the uplink, handover due to uplink power level, assignmentfailure and RL failure will be more than normal cell.

3．Eliminate external interferenceTake the following measures to make sure whether it’s caused by BCCH uplink interference:check related RACH demodulation level; analyze and check GSM uplink frequency band usingspectrum analyzer in BTS; track Aibs; check uplink & downlink balance and uplink distributedlevel ; check BTS transmission cable etc.4．Eliminate GSM interior interferencePlease refer to the cause of handover due to downlink quality.

5.3 Big proportion of handover due to downlink level1．description

This handover is an emergency one due to the signaling of serving cell is lower than normal callrequirements. The target is the adjacent cell with strong BCCH signaling.

note：if handover parameter setting is unreasonable, the cell which handover is implemented outto is hard to find. Otherwise PBGT handover will be processed. Thus this kind of handover failurerate is great.Alarm threshold is L RXLEV DL H, that is, network downlink level threshold.2．Common Fault AnalysisThis is not usually seen and it’s caused mainly by the following reasons：1）parameter setting is improper；2）downlink hardware problem3．TroubleshootingIt’s the same with handover due to downlink quality caused by problems of parameter andhardware.

5.4 Big proportion of handover due to uplinklevel1．description

This handover is an emergency type which is invoked when signaling level of the serving cell islower than normal call. The target is the adjacent cell with the strongest BCCH signaling.Note: if handover parameter setting is reasonable, the cell which handover is implemented out tois hard to find. Otherwise PBGT handover will be processed. Thus this kind of handover failurerate is great.Alarm threshold is L RXLEV UL H，that is, network downlink level threshold.2．Phenomenon analysis1）inadequate coverage is accompanied by much handover due to uplink quality;2）cell handover parameter setting is improper；3）hardware problem if uplink & downlink is imbalance；4）Ping-pong handover between two cells. For example, handover in to the adjacent cell withlower level due to quality and back to the previous cell due to PBDG or level3．troubleshooting（1）parameter. check the handover parameters of related cell including power handovertolerance, power level handover threshold, and minimum access level

（2）hardware. check the hardware using the following methods: make Abis track, check celllink budget and special attention shall be given to balance of uplink & downlink.（3）coverage or adjacent cells.Check the definition of adjacent cells of related sites is consistent with coverage predictions.Check whether there are a great proportion of handover between adjacent cells due to quality. Ifthere are ping-pong handover of adjacent cells, interference shall be kept in control. Please referto quality handover& interference chapter;If isolated island or coverage problem is not found via TA gotten by Abis track, DT is needed tosolve the problem.

5.5 big proportion of handover due to distance1．Problem descriptionThis handover is an emergent one due to large distance between site and MS. The threshold isMS RANGE MAX, which is the maximum distance between sites and MS.

Notice: the priority of handover due to distance is lower than handover due to quality and level,as a result, when handover due to distance is triggered, level and quality shall all below thethreshold. We can reduce isolated island effectively by setting the distance threshold.

Notice：great proportion handover due to distance indicated that the coverage scope of this cellis greater than MS RANGE MAX（it may be caused by too high antenna, broad terrain and highbuilding etc.）

2．Common fault analysis1） parameter MS RANGE MAX is too small ( notice: this unit of this parameter is TA(timeadvancement. 1 represents 500m instead of 1km)2）isolated island；3）definition of adjacent cell is wrongPower handover haven’t be implemented when MS is moving away from the site into the regionthat is not covered.

3．Troubleshooting

1）check whether it’s due to the smallness of the parameter of handover due to distance2）check whether there is malfunction cell；3）check other indications such as RL failure or handover failure.；4）make Abis track to see whether there is isolated island.

6 RACH access validity6.1 descriptionValid RACH access is the process from MP sending RACH REQUEST to the completion ofSDCCH establishment. Please refer to Figure 12.

Figure 12 RACH access signaling process

6.2 Common fault1）RACH power level is high and can’t be demodulated (noise RACH level is high) and BTScan’t be decoded (information is wrongly coded);；2）RACH request times is low which is obviously ；3）RACH demodulation rate is low（RACH sends to BSC / all RACH）；4）LapDm establishment success rate is low（LapDm success /SDCCH assignment number）；5）excessive invalid RACH channel request（RACH CHANNEL REQUEST）。

6.3 Common fault analysis（1）The most frequently-seen fault is BCCH uplink interference. When BCCH uplinkinterference is severe, BTS can’t decode RACH access request information(information iswrongly coded) .

（2）hardware of receiving path.（3）uplink & downlink imbalance will affect signaling process after RACH is demodulated.（4）parameter is inappropriately defined. For example, “ minimum access powerlevel”（RXLEV ACCESS MIN）is set to be too high.（5）RACH decoded power is too high. When radio condition is consistent with normalpropagation model, if signaling level is high than BTS sensitivity，BTS can demodulate RACHbursts received. When there is no interference, BTS sensitivity is decided by noise coefficientand thermal noise power and is -115dBm(not taking fading tolerance into consideration). Uplinkinterference and block will disable minimum demodulation level to reach -115dBm.In the cities, because the propagation ambience is complicated, BTS demodulation ability will beaffected by background noise and reduced to -100～-110dBm.（6）SDCCH establishment success rate is low. Only small number of MS SDCCH channelrequests are successful . There are two possibilities:

Main reason: BTS receiving path loss is severe（cable, receiving separator, andconnector），thus receiving bursts from MS can’t reach BTS or is lower than demodulationlevel.（<-110 dBm）

Another reason: RACH short bursts can be demodulated, and level required by LapDm can’t besuccessfully established. It maybe caused by the low uplink receiving level or interference.

6.4 Troubleshooting1．Confirm the time when error occursCheck the time when RACH access failed and whether it still exists and is caused by incident.2．Make sure whether it’s caused by hardware or interferenceCompare the following running status: handover in, establish requests, and assign requests.If it’s caused by BCCH uplink interference, it will not affect handover greatly, and RACHdemodulation level and channel assignment success rate will be reduced. If it’s caused byhardware, access success rate will be very low.

3．Eliminate hardware malfunctionIf there is hardware error in uplink, network will not be able to assign channels responding to MSchannel requests. In this case, check site hardware, especially TRX, antenna and feeder. If theproblem is not sure, do Abis analysis and check link budget.

4．Eliminate interferenceIf the problems don’t lie in hardware and connection, then there is great possibility that it’s causedby interference. Check whether there is newly-added RF equipment around the cell based on thetime that error arises and the duration. Check frequency scheme to make sure that there is no intra-frequency interference and adjacent cell interference. If it’s the case, modify BCCH frequency tosee whether RACH level is changed also. If it’s necessary, do DT, use spectrum analyzer or RFreceptor to check the features of interference signaling. Please refer to interference chapter.5．Remove the effect of repeaterAttention shall be given to wideband repeater. The function of wideband repeater is to enhancenetwork coverage and is mainly used in suburbs and rural areas. In the cities, indoor repeater ismainly used for coverage of large buildings and basements. The usage of repeaters will changethe coverage condition, so coverage range shall be controlled. Interference will not be caused

in the area with no signaling from other sites, but the balance of uplink & downlink shall beguaranteed. In the cities the repeater will cause great interference especially uplink interference.If the equipment is good, most problems of RACH inefficiency is caused by the interference fromthe repeaters. So if repeater is needed, closed circuit repeaters are strongly recommended or installnarrow-beam antenna beside the site to control transmission power.

7 No traffic or handover in the cell7.1 descriptionCompare to normal cells, there are few handover or none, few establishment requests or none, fewassignment requests or none.

If there are no attempt calls, make sure where error occurs using traffic statistics report, such asSDCCH fail, no RACH request, RACH access fail and TCH establishment fail. The first threefaults may make traffic of initial calls and traffic of handover in of the cell low.

7.2 Common fault analysis and troubleshooting1．Parameters are not appropriately setCheck CELL BAR ACCESS to see whether this cell is barred，if it’ s the case, the traffic of thiscell are mostly handover in calls. Check if the setting of CI and LAC of this cell is consistent withthe setting of exchange. If it’s inconsistent, the call path establishment will fail and therefore, thetraffic of the cell are mainly handover in calls.

If there are no handover in traffic, check whether adjacent cell is correctly defined and handoveris allowed in the system; Check the definition of LAC of the malfunction cell coupled with MSCof the adjacent cell.

2．Hardware faultDo Abis track, check the hardware by checking link budget. If exchange matrix control panel,transmission control panel and control panel of corresponding BSC breaks down, MS will fail toinitial calls. If transmission power and receiving sensitivity of the carrier of this cell is low, MSwill be unable to use the channel assigned.It may be caused by the problem in antenna, feeder, coupler, receiving divider and receptor etc.3．Antenna errorThe antenna is blocked and thus calls established in this cell are few. Currently there are manyresidential areas in the sites, antenna is in mast shape and the first sector of many sites is towardrectangle roof, which makes it hard to absorb the traffic even in high-traffic region. When thishappens, antenna parameters like height, direction angle and pitch angle shall be adjusted.

8. call successful ratecall connection rate is the indicator to evaluate network running performance. The following isthe factors to affect long-distance call connection rate.Paging overtime; user busy; no response from the user; telecommunication link establishmentfail(line is busy, TCH channel busy or TCH assignment fail)；the definition of parameters of BSCand MSC causes overtime of the paging; dialing incomplete number.We will make analysis of the above problems in the following report.

8.1 PAGING TIMEOUTPaging timeout indicates that MSC sends paging message successfully and in fixed time MSCdoesn’t get response from MS. Paging overtime is the important factor that affects long-distancecall connection rate and the reasons are many.Paging handling is classified into two levels: one is MSC, and the other BTS. MSC is responsiblefor sending paging messages and to avoid bad radio condition of MS, MSC will resend it again,the interval is 10 seconds. BTS will place paging messages to certain paging group according toCCCH assignment of this cell and will resend it to avoid the relay caused by uncoding of themessage in PCH because of interference.The main factors are as follows：（1）MP subscriber enters blind spots or MP powers off. If the time for exchange’s area periodicupgrading is not reached (MSC will timely inquire the user of IMSI ATTACH and will set MSthat doesn’t communicate with the system to be in power-off status), MSC will continue to sendpaging messages to this subscriber which will result in paging overtime and waste of systemresources.We can make optimization the parameters of T3212 and the parameters of T3212 and exchangehidden power-off in order to enhance connection rate.BSC area periodic update timer T3212 must be later than hidden power-off timer in MSC. Underthis premise, we can make settings of T3212 freely.

In some suburban and rural areas, the blind spots and region with weak signaling are many,therefore we can set T3212 to be small. For example, set T3212 to be 8（which means areaupgrading every 48 minutes）. But in urban area, considering the considerable great averagetraffic and signaling flow, T3212 shall not be small, otherwise signaling flow will be added greatlyand thus adds to MSC load and finally reduces paging success rate. If MSC hidden power-offtime is set to be too short, on one hand, it will improve paging success rate, on the other hand,it will cause some subscribes to be in power-off status because of some subscribers’ long-timedisconnection with the system. If hidden power-off time is set to be too long, user area periodicupdate times will be added and thus improve paging success rate; at the same time, one problemmay occur. Because user stays in blind spots for long time(time is shorter than hidden power-offtime) and is still in ATTACH status in MSC and when the user is being called, VLR will allocatethe user with MSRN, which will result in an effective paging and thus reduce paging success rate.Therefore, the setting of T3212 and hidden power-off time makes difference for paging successrate, and shall be made based on analysis of traffic statistics and BSC & MSC parameters.

We shall also make optimization of TRX maximum transmission power and MS minimum accesspower level（RXLEV ACCESS MIN）etc , adjust antennae and build new sites to enhancenetwork coverage.（2）lack of signaling channel of the cell in which MS being called, RACH access failure ofuser being called, SDCCH assignment failure and SDCCH call drop can all lead to one of thefollowing results: RACH access failure, signaling channel of access cell busy and call drop onSDCCH during signaling link establishment, which will make MSC to fail to receive pagingresponse messages and finally result in paging overtime.In this case we shall use traffic statistics to find cells of which signaling channels are congestedand which severe failure of immediate assignment exists. For the cells with congested SDCCH,PCH, RACH and AGCH，we can adjust related parameters and number of related signalingchannels (refer to the handling of SDCCH congestion) based on requirements in order to reducecongested cell and improve PAGING success rate. As for the cells with many immediateassignment failures, we can check whether it’s caused by interference, equipment hardware error,improper parameters or feeder error etc.

In the process of optimization, we can set T3122 (wait indication duration) to be as shortas possible so as to enhance network average access performance. Raise MAXRETRANS“maximum resend time” as much as possible which is a key parameter to improve accessibilityrate，and can be set according to traffic volume of the cell. Also, we shall make optimizationof BS AG BLKS RES, BS PA MFRMS and TX INTEGER in order to improve the respondingcapability of MS toward the system.

8.2 Communication link establishment failIt refers to the call failure after the RR establishment between MS and the network caused byauthentication failure, radio call loss, TCH congestion, TCH assignment failure, lack of groundresources of A interface or Abis interface .（1）Call loss may occur due to radio problem during the time from MSC sending SETUPmessage to BSC to BSC sending back CALL CONFIRMED message. If the call loss is too great,we can do track of signaling of A interface and find worse cell based on OMC-R report and gethelp from site personnel.（2）During the period from MSC getting CALL CONFIRED message from BSC to MSCsending assign TCH request message to BSC, call establishment may fail caused by TCHcongestion of target cell due to lack of exchange resources. Therefore, we shall pay attention toTCH congestion of the cells in the network and signaling load of A interface in order to avoidthe circumstance.

（3） During the period from MSC sending TCH assignment request to BSC to BSC resendingTCH assignment success message, call establishment may fail due to failure of TCH assignment.（4）During the period from getting message from BSC to BSC sending ANC message toTMSC, call loss may occur caused by one of the following reasons: the calling MS hangs up, noresponse from MP, disconnection due to overtime and the called MS hangs up.

It’s hard to resolve for the user. We shall check whether instability of signaling or transmissionexists in MSC and resolve it immediately.

9. reason and troubleshooting of coveragelessening of the sitesIt’s not only related to system technical indicators such as system frequency, sensitivity andpower, but also has direct relationship with engineering quality, geographical status andelectromagnetic environment. Generally when indicators of the system is stable, the environmentis bad, and engineering quality is not up to the standard, the coverage area of the site may besmall.

9.1 reason on sites side and troubleshootingThe factors that affect the coverage due to the sites are: output power of transmitter reduces,sensitivity of receptors lessens, directional angle of the antenna tilts, pitch angle of the antennachanges; antenna gain changes; antenna height varies; feeder loss, coupler loss, workingfrequency changes, propagation environment (terrain and human environment) changes anddiversity receptor effect etc.To sum up,（1）check whether it is due to the growing of reversing leakout pf directional antenna, whichwill make forward output power become weaker and finally narrow the coverage area of thecell. It’s recommended to change the antenna.（2）check whether there are other antenna or buildings that block the antenna. They will affectreception and transmission of the antenna and thus affect the coverage of the site. In this case,adjust the directional angle or height of the antenna to avoid the bad effect.

（3）check whether the performance of the inclination or direction angle changes. Theenlargement of the inclination or offset of the direction will narrow the coverage of the site, whichshall be handled once being found.（4）Check whether output power of site receptor or the coupler is descending. If so, adjust thepower of the amplifier or change hardware equipment. If not, we shall check whether it’s causedby leakage of the signaling due to bad connection of the downlink hardware units.（5）check whether the loss of the feeder is increasing. We can use apparatus to measurestanding wave ratio. If it’s less than 1.5, it may be caused by feeder connector or leakage of thefeeder which reduces the transmission power and the coverage of the sites.

（6）make sure whether it’s due to capacity enlargement. If it’s the case, check the following：1）whether different types of couplers are used. Different type has different loss which we shallpay special attention to.2）After operation or capacity enlargement of the site, check whether the installation of the newantenna is up to the requirements; If the distance between MS and tower is near, the tower mayblock the antenna; check the height of the antenna. If it’s too high, it may lead to dark-down-tower;if it’s tow low, it may make the coverage narrower; check whether the inclination of the antennais reasonable; check whether the coverage of the major antenna and diversity is consistent. Forinconsistency will affect the coverage effect which leads to call drop, assignment failure and handover failure and finally results in narrow coverage of the sites.

3）check the receiving sensitivity of the sitesIf the sensitivity decreases, the coverage of the site will become weaker.（7）check whether RACH decoding level varies. If it changes greatly, the signaling from MSmay not reach the site.（8）check whether interference exists. Strong interference will reduce receiving sensitivity ofthe site and thus affect the site coverage.（9）eliminate the electromagnetic interference . If the electromagnetic interference is great, itwill affect SNR, which will affect communication of long-distance subscriber.

9.2 Others1．Signaling blockNew buildings block the propagation of the electromagnetic wave and make the signalingbecome weak and users in far area can’t use normally. The high buildings near the sites will havegreatest effect on the propagation.

2．Propagation ambience changesThe change of propagation environment of electromagnetic makes the receiving signaling fade.Especially in mountainous region, the propagation there rely much on the reflection of the hillsand if the plantation changes, the signaling may fade and can’t reach the user.

3．Technical indicator of the terminal descendsThe descending of the MS power and sensitivity may affect the reception of the radio signaling.The feeder which is used to transmit signaling and the connector shall be in good condition.We can seen from the above that the descending of the site coverage is caused by many reasons.Thus we shall make daily maintenance and do troubleshooting timely to guarantee smooth runningof the system.

10 analysis and troubleshooting of hot issues10.1 terminating MS has signaling but when it’sterminated, it tells that user is not in the serving areaMSC paging MS and if in fixed time, it doesn’t get PAGING RESPOSE from the user, it willreplay user is not in serving area recording and handle it in another way. In fact there are manycauses such as the user entering blind spot or MS power off. VMSC hasn’t make periodic query ofthe IMSI attach user and can’t confirm the status of the MS which results in paging failure. Themalfunction are as follows:

1. Signaling channel of the cell which terminating MS is in is very busyAfter MS receiving the paging, it will away from idle state and sends channel request to thestanding cell. BSC receives this request and hasn’t channel to assign and send assignment refusemessage to MS and padding fails. When 当MSC的“寻呼超时”（PAGING TIMEOUT）后，it

will send recording notification(user is not in serving area) to originating user or handle it in otherway（replay busy tone etc）We can see whether the signaling channel of the cell is congested or unavailable. If this happens,MS will receive immediate assignment refuse message. In this case, it’s even hard for MS tomake originating calls.

2．Terminating MS immediate assignment failWhen MS is being paged, if channel assigned by the network is unavailable（radio environmentis bad due to interference）or equipment error（feeder, major equipment or transmitterbecomes unstable or error）makes immediate fail, it will make MSC paging overtime.

When this happens, check whether the difference between RACH request number immediateassignment success number is considerably big（subtract RACH resend time）using trafficstatistics. MS being paged can’t get immediate assignment command after sending channelrequest or can’t get response from the system after sending SABM（PAGING RESPONSE. Inthis case, it’s hard even for MS to be originating.

3．Terminating user RACH access failWhen terminating MS sends channel request on RACH, network can’t decode it due to radiointerference or equipment error(generally one control channel in this cell error) and paging timesout. When this happens, check whether RACH requests is less than TCH assignment times. If it’sthe case, MS can’t enter this cell and can only use TCH of this cell by handover in.In this condition, regardless of MS to be originating or terminating, MS will resume idle status(in1- 2 seconds) because network can’t assign channels which results in T3216 overtime and makecell reselection.

4．Area update of terminating MS resulting in paging failureWhen MS being paged is doing area update(normal location update & periodic locationupdate)，this MS can’t make response（because location update is invoked and other programcan’t be invoked at the same time. Measures may be taken to counteract this situation are:1）increase cell reselection lag value, and the difficulty for location update; 2）optimizinglocation area is the most effective way to handle it.The time for MS to do location update is generally 3.5s. In this case, there are many incidenceshappen along the border.5．Terminating MS CCCH decoding fail

Due to severe downlink interference in BCCH frequency, MS fails to decode CCCH of thepaging group. This case is few.

6．System is overloaded with pagingWhen system is overloaded with paging and paging channel of the cell which MS is in is toofew, MSC may lose MS paging message.

7．NSS malfunctionDue to location updating or VLR database fault, MSC fails to page MS.

10.2 MS has signaling but is implicitly power-off whenbeing pagedThis happens when T3212 (periodical location update timer) is bigger or equals to MSC implicitpower-off time, MSC consider MS is power-off due to no response from it.We shall pay attention to the setting of T3212. When T3212 is less than implicit power-off ,reducing T3212 will deduct system load and enhance the sensitivity of the network to MS status. But improper value will add to user’s complaints (MS has signaling but is implicitly power-offwhen being paged). Please see the following descriptions:Set time for implicit power-off to be 1 h, T3212 of cell A to be 4（0.4h），T3214 of cell B to be8(0.8h) and the two cells are in the same location area. Set MS location update status in A to be3/4，and MS make cell reselection to be B and status becomes 3/8, if MS stays in B for a time,MS status will return to 3/8. In this case, although T3212 of these two cells is less than MSCimplicit power-off time, time to make location update is later than the time of implicit power-off,thus this user is considered to be power-off.

10.3 MS signaling unstable in idle stateWhen we are handling complaints, we find many come from MP instability. Weak signaling isrelated to receiving strength instead of algorithm. Besides unavoidable factors such as multi-patheffect and environment factor etc, the causes may be as follows:1．Equipment faultWhen BCCH transceiver power amplifier works unsteadily, transmission power is instable andthus the signaling is instable.2．Reselection parameter 重选参数影响

Adjusting C2 may bring bad results: during network optimization, C2 needs to be adjusted tobalance the traffic. Cell reselection is based on C2, and MS signaling fluctuates with practicalreceiving level. If two measured C2 of the two cells is close and practical receiving levels arefar, cell reselection will frequently happen and make MS signaling fluctuate in idle state.

Especially in the place where micro-cell and dual-band exist, the problem becomes even severedue to difficulty to define reselection coefficient.In border area, due to the effect brought by cell reselection hysteresis, signaling may fluctuatewhen MS is doing location update.3．Coverage reason

when signaling of cell C drifts to cell A, if cell A defines cell B as its adjacent cell，and thefrequency of cell B and C are the same, MS standing in A may select cell C. At this time, MSwill monitor BA(BCCH）table broadcast by Cell C（frequency of cell A may not in the table）,and signaling of BCCH tends to be weak, therefore, MS may not receive drift signaling of cell Cand will make cell reselection again. This will make signaling fluctuate.

10.4 Signaling fluctuate during MS making calls1．Equipment malfunctionIf TCH and BCCH of MS don’t belong to one carrier, and the transceiver worksunsteadily（signaling is weak），signaling of MS in idle state will appear strong but is weakduring communication.2．Problems brought by handoverWhen MS handover is frequent, signaling of MS may fluctuate.When adjacent cells of handover are too many, frequency samplings in BA table for MS may beless and results in incorrect measurement level and frequent handover.

10.5 Echo in MS communicationCauses are:1．Echo suppressor(EC）errorEC is placed between PLMN and PSTN. So if the subscribers of these network complain aboutecho during communication, it’s probably caused by EC error.2．TCU errorTch is responsible for rate transferring adaptation, so if it error, echo will occur.3．Subscriber MP errorIn the process from MS voice coding to radio wave sending, if relevant hardware error, echo willoccur.

4．Propagation circuitDue to careless maintenance of A interface and Abis interface, loop or cutover may caused whichmay result in echo.

5．BTS hardware errorBTS is responsible for the whole process(demodulation, burst and channel decoding ect) exclusiveof voice decoding, if one hareware error, echo may occur.6．Update and patchIf system update and patch are inconsistent with current system hardware, echo may occur. It’ssuggested to do dialing test to check the compatibility of the system.

10.6 Other1．When MS originates calls and returns to idle state, MS can’t dial in a while. It’s because MSreceives immediate assignment refuse message（due to signaling channel congestion）whichincludes refuse connect timer T3122，thus MS is hard to get access to the network

2．Bad voice quality when MS is making callsWhen MS is making calls with PSTN subscriber，voice quality of MS is bad while the otherparty is fair (receiving level is high), there may be error or interference in downlink of thestanding cell of MS. So, check downlink hardware（transceiver ,frame processor, combiner and

feeder etc） first，then use traffic statistics to check whether there are too many handover ofdownlink handover due to quality.When voice quality of PSTN subscriber is bad, it may be caused by uplink error or interferenceof MS standing cell. Check uplink hardware such as transceiver, frame processor, combiner andfeeder etc

If it happens between two MS, two network situation shall be taken into consideration.

11 typical case analysis and optimizationmeasures11.1 Case one1．ErrorOne suburb site is in S777 model and adopt 1：3 hopping. Call drop of three-sectored voicechannel is high due to radio link error and other indicators are normal.2．Malfunction analysisBecause the sector assignment failure rate and handover in failure rate are normal, it can’t bemajor equipment hardware error. The causes may be: geographic and environment effect（suchas blocking of new buildings）, coverage, interference , feeder error（quality, height, pitchangle and direction angle etc, adjacent cell not defined, and unreasonable relevant parameters.Interference, feeder error, environment and coverage shall pay great attention to.3．Fault findingStanding wave ratio of feeder, feeder connector and antenna performance are in normal state;hardware of major equipment is normal; no decoding in propagation; cell parameter, hoppingparameter, handover parameter and adjacent cell definition are all favorable. By doing DTand ABIS signaling analysis, we find that average receiving level of this sector is comparablyhigh (-77dBm/uplink, -73dBm/uplink) while receiving quality is relatively bad(4.33/uplink,3.25/downlink). When there is only one carrier function in the sector, call drop indicator andreceiving quality are all in the normal range. By above analysis, we can decide that it’s caused byhopping frequency interference.4．troubleshootingUse HP8921A to scanner uplink of the site, and find that spectrum of BCCH of the site isfrequently covered by stronger spectrum.It shows that interference spectrum and simulative system spectrum is similar and HP8921Acan demodulate voice signaling and find it’s due to co-channel interference of co-site simulativesystem. Correct frequency of the hopping being interfered and it will resume normal state.

5．experienceWhen choosing site, we shall analyze the environment and make facility adjustment andparameter adjustment.

11.2 case two1．Error descriptionSite with O3 model uses radio hopping and 1:3 hopping. When the site has finished capacityexpansion( from O2 to O3），voice channel assignment failure rate remains high due to radiolink error accompanied by high call drop and handover in failure rate. RACH decoding successrate , coding level, SDCCH immediate assignment success rate and call drop rate are normal.2．Malfunction analysisAssignment fail rate is accompanied by high call drop rate and handover in failure rate. Thereare two possibilities, one, error occurs when assigning voice channel, two, frequency or timeslotof this call may encounter interference or unsteadiness. Because RACH decoding success rate,decoding level are normal, SDCCH immediate assignment success rate and call drop rate arenormal, we can decide that there are few chances for BCCH TRX and BCCH frequency error.From above, we can see there are great possibilities of error or interference of non-BCCH trx orfrequency error. Therefore, we shall first check the equipment, then decide whether it’s due tointerference by the help of traffic statistics analysis tool, DT coupled with parameter.

3．Fault findingCheck equipment hardware, feeder and propagation steadiness and no problem is found. Bydoing dialing test, we find that dialing success rate is low and voice quality is mostly bad. Whenchecking the parameters we find that new MAIO is the same with MAIO of another carrier whichcaused clash during channel assignment and communication. That cause this result

4．troubleshootingReset MAIO value of new TDMA.5．experienceAfter capacity expansion and update of the site, we shall check all the parameters and related datato avoid the same malfunction

11.3 case three1．Error descriptionThe urban site is in S333 model and use GSM900, frequency hopping and adopt 1:3 hopping.Handover failure rate in one sector of this site remains high, handover in fail rate from cell A tothis cell reached 80%. Call drop rate and voice channel assignment failure rate are normal.2．Malfunction analysisWe find that it’s not due to error and interference in hardware. First because although failurerate is high, there is no TCH assignment failure, which indicates MS can occupy voice channelassigned by BSC; secondly MS has no call drop and voice quality problem which indicates thereare no severe interference. By analysis we find that source cell is far from this cell and it’s maybe due to isolated island.3．Fault finding

We find no problem in equipment hardware, propagation steadiness and interference. Then weinvestigate whether there are the same BSIC/BCCH in the surrounding cells of source cell A.

Finally we find cell B, has favorable radio propagation environment with cell A due to a squarenewly built between , and this leads to isolated island.

4．troubleshootingMake frequency planning of this cell again, adopt a new frequency and set cell B to be adjacentcell of cell A.

5．ExperienceWe shall taken environment into consideration when handling network problem. If environmentchanges ,we shall adjust system software parameters(adding or deleting adjacent cells) andengineering parameter（antenna height, pitch angle and direction angle. Due to limitation of GSMfrequency, isolated island may become a big issue which we shall carefully handle.What’s more, if co-channel interference（co-BCCH）is severe, handover success rate will alsobe affected.

11.4 case four1．malfunctionOne suburban site configuration is S444, use GSM900 and use frequency hopping and adopt 1:3hopping model. The two sectors has some handover failure rate, assignment failure rate and calldrop rate and no congestion.

2．Malfunction analysisWe shall check hardware equipment first. Then check the interference between the two cells andoutside the system through parameter checking, traffic statistics report and DT test. Next checkfeeder fault（see call drop analysis）。

3．Fault checkingCheck equipment hardware and find no error. Check MAIO, HSN, MA, handover parameter, co-channel and adjacent channel and find no inappropriate configuration. In DT report, the coverageof two sectors are found to be improper which is due to the reverse installation of the antenna.4．troubleshootingResume the antennae of the two sectors and the problem is resolved.5．ExperiencePay special attention to the quality when doing acceptance check.

11.5 Case five1．Malfunction descriptionThe site with O4 configuration and adopt frequency hopping, DTX and 1：3 hopping. Call dropof this cell keeps relatively high due to radio link error accompanied by relatively-high handoverfail rate and assignment failure rate.

2．Malfunction analysisSee the analysis of case four.3．Fault finding

Check equipment hardware and parameters and find no problems. OMC-R traffic statisticsreport reveals average receiving level of MS in this cell is relatively high (-85dBm/uplink and

-81dBm/uplink on average) and receiving quality is bad(-3.8/uplink and 3.15/downlink), whichindicates there are probably great interference in the system. By investigation, we find that arepeater amplifies the signaling of this cell as well as the interferer signaling（BCCH frequencyof surrounding cell and frequency of the hopping）and co-channel and adjacent channel areintroduced..

4．troubleshootingClose DTX function of this cell, change hopping mode to non-hopping mode and do frequencyplanning again. Avoid using the same and adjacent frequency. Adjust handover parameter andpower control parameters in order to avoid hopping.5．experienceAvoid using repeater where the same frequency or adjacent frequency may be used. Use micro-cell to resolve coverage issue.

11.6 case six1．Malfunction descriptionThe site with S777configuration uses hopping and DTX. The user around the site find it hard todial both for originating MS and terminating MS, although MS signaling is strong. The originatingMS will resume idle status indicating no signaling and terminating MS will prompt to be not inserving area.2．Malfunction analysisFirst make sure whether malfunction lies in one cell of the site, all cell of the site or no site’sproblem at all. Judging from the malfunction, it’s hard for user to use signaling channel, therefore,check whether SDCCH is congested. Otherwise, check whether there is severe interference. If theabove are fairly good, check hardware, parameters and feeder.3．Malfunction checking and troubleshootingThrough the analysis of OMC-R traffic statistics report, we find there is no congestion in this site.But the report of one sector is abnormal. The report shows that SDCCH immediate assignmenttimes are dozen including area update while TCH assignment times is several hundred with mosthandover in assignment. Close this cell and network resumes its normal working state.

Do DT test and find no interference, and do cell parameter check again no problem is found.Change one control panel and the problem disappears.

11.7 Case seven1．Error descriptionOne subscriber along the border of two provinces complains that although there is no problemconcerning roaming in his dwelling, but once he received roaming signaling from other provinceand can’t away from the signaling. The two provinces are not adjacent to each other.2．Malfunction analysisThrough DT of this region, the network structure is showed in 7-15

Figure 15 network structure along the border

The subscriber usually stand at P and reside at cell A, Cell A and B are adjacent cells and areaffiliated to user network. Cell C and D are adjacent cells and belong to roaming network. CellA has no adjacent relationship with cell C and D.Because cell D and Cell B defined by A uses the same BCCH frequency, MS at P reselects cellD and back to cell C later. But there is no frequency of cell A in adjacent cell table of cell C andD, therefore, user keeps staying in the network of roaming area. When MS powers off in cell Cand powers on again, it keeps stay in cell C and its adjacent cells. This is called roaming.

3．TroubleshootingDefine adjacent cells of the two provinces. If it can’t be realized, change frequency of cell B.

11.8 Case eight1．Malfunction descriptionUrban subscriber find that although MS has signaling, but when being paged, recording promptsit to be not in serving area.

2．Malfunction analysisCheck whether the network has problems such as signaling channel congestion, frequent locationupdate and immediate assignment failure. Check whether T3212 is too small3．Malfunction checkThrough investigation, we find that there are two LAC in the urban area and cell location updatealong the border of LAC is frequent, which leads to signaling channel congestion. One reasonis some terminated MS can’t use SDCCH, thus paging is overtime, another reason is that MS ispaged when doing location update, and can’t respond timely, which results in paging overtime.4．TroubleshootingMake planning of the location area, avoid setting borders to be area with frequent cell reselection,and assign signaling channel to the cells in the boundary area as many as possible.