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Optimisation process guide Optimisation process guide Document Transcript

  • RADIO ENGINEERING SOLUTIONS AIRCOM International Technology Consulting System Optimisation Guide CS-SG-TPP-0042-01
  • CONTENTS 1.1.1 Optimisation Tips: ................................................................................................. 7 1.2 LOW SIGNAL LEVEL ....................................................................................................... 8 1.2.1 Optimisation tips: .................................................................................................. 8 1.3 ADJACENT AND CO CHANNEL INTERFERENCE ................................................................ 8 1.3.1 Optimisation tips: .................................................................................................. 8 1.4 EXTRANEOUS INTERFERENCE ......................................................................................... 9 1.5 LINK IMBALANCE ........................................................................................................... 9 1.6 HIGH HANDOVER FAILURE RATE:................................................................................. 10 1.7 HIGH NEIGHBOUR INTERFERENCE ................................................................................ 10 1.7.1 Optimisation tips.................................................................................................. 10 1.8 NO DOMINANT SERVER ................................................................................................ 10 1.8.1 Optimisation tips.................................................................................................. 10 1.9 DATABASE PARAMETERS .............................................................................................. 10 1.9.1 Optimisation Tips ................................................................................................ 11 1.10 SDCCH BLOCKING ...................................................................................................... 12 1.11 HIGH PAGING LOAD ...................................................................................................... 12 1.11.1 Optimisation Tips: ............................................................................................... 12 1.12 INCORRECT OR INAPPROPRIATE TIMER VALUES ............................................................ 12 1.12.1 Optimisation Tips: ............................................................................................... 12 1.13 TCH BLOCKING: .......................................................................................................... 13 1.14 HANDOVER MARGIN .................................................................................................... 13 1.14.1 Optimisation tips.................................................................................................. 13 1.15 CELLS TOO LARGE ........................................................................................................ 13 1.16 IMPROPER TIMER .......................................................................................................... 13 1.16.1 Optimisation Tips ................................................................................................ 14 1.17 SDCCH MEAN HOLDING TIME ...................................................................................... 14 1.18 ERLANGS, TOTAL CALLS .............................................................................................. 14 1.19 PROCESSOR LOAD ........................................................................................................ 14CS-SG-TPP-0042-01 Page 1
  • 1 INTRODUCTIONThis document describes in the process of optimisation in GSM900/1800 mobiletelephone systems. The document covers the traditional methods of optimising acellular system in the basic optimisation section. It introduces the improvementsbeing made to Aircom’s Optimisation process with the introduction of new tools andtechniques in the advanced optimisation section. Aircom has a tool namely “ optima ”to monitor the system performance. A series of counters are monitored on daily,weekly and monthly basis to check the network health. The document will specify therecommended metrices and the standard performance management procedures toidentify and rectify the problems.Optimisation is an invaluable element of service required to maintain and improve thequality and capacity of a network. It is essential if an operator wants to implementchanges to the network to maintain the high quality of service levels expected bysubscribers in GSM900/1800 networks. Without optimisation the network willdegrade from the commissioned state, due to the network changing radically as thetraffic on the GSM system grows, and snapshot optimisation will not keep pace withthese changes. Without optimisation the system will suffer poor call quality, manydropped calls due to interference and inaccurate parameters resulting in poorhandover performance. These together with other problems, have the same result,Subscriber Dissatisfaction.Setting the parameters that control mobility has equal importance to the frequencyplan. In GSM900/1800 networks there is a series of parameters that control mobility.Tuning these parameters for improved GSM900/1800 operations, in terms ofmaximising calls carried, improved handover performance and increased callsuccess rate, is termed ‘Optimisation’.The aim of optimisation is to maximise the Quality of Service (QoS) of the GSMnetwork. In order to do this you need to measure the QoS, compare the measuredvalue with the desired value, and then take steps to correct the causes of anydeviations from the desired value.CS-SG-TPP-0042-01 Page 2
  • INPUTS PROCESS OUTPUTS RF DESIGN PROCESS QUALITY OF INTERGRATION OF SERVICE METRICS SYSTEM OR NEW FREQ. PLAN DATABASE PARAMETERS OPTIMISATION OPTMISATION REPORT RF DESIGN PARAMETERS OMC DRIVE TEST ROUTESAPPROX 2 - 4 WEEK PROCESS PERFORMANCE PERFORMANCE ENGINEERING REPORT SYSTEM ACCEPTANCE FIGURE 2.1 Basic Optimisation Process The flowchart shown in figure 2.1 summarises the basic optimisationprocess.CS-SG-TPP-0042-01 Page 3
  • 2 D AILY C OUNTERS :The following metrics can be used to measure the performance of the network.These counters should be monitored daily on a per cell basis. These counters aredefined in Optima under the heading of “ network level ”.S.NO Daily Counters to Monitor Standard Thresholds1 Call set up success Rate Above 90 %2 Tch Congestion Less than 2%3 Sdcch Congestion Less than 2 %4 Handover Success rate Above 85 %5 TCH Drop Rate Less than 1 %6 TCH blocking Less than 1 %7 SDCCH blocking Less than 1 %8 Erlangs per hour -9 Average no of TCH’s - available10 Sdcch Mean Holding Time 3 seconds11 Total Calls -12 Processor Load -2.1 Call Setup Success Rate:The call setup rate should be above 90 % for a healthy network. However a CCSR of85%-90% is satisfactory. There could be so many reasons for a poor CSSR. Someare described as follows.No Access to SdcchCM service rejectTch Failure assignmentHardware Problems2.1.1 No Access to SDCCH:BSS detects Channel Request (in the form of RACH) from a source, requestingresources for network transactions. After validation of the RACH, BSS will attempt toallocate a dedicated channel (SDCCH) for the source. Once the availability ofSDCCH channel is confirmed, the BSS will send Immediate Assignment to MSindicating the dedicated SDCCH sub-channel (via AGCH), whereby subsequentmessage exchange will be performed over the dedicated SDCCH.CS-SG-TPP-0042-01 Page 4
  • 2.1.2 Examples of Abnormality :2.1.2.1 Valid RACH (SDCCH Congestion)Due to the unavailability of SDCCH, BSS will response to MS with ImmediateAssignment Reject, terminating the transactions. In which case, call setup is termedas unsuccessful due to SDCCH congestionInvalid RACH (Invalid Establishment Cause detected in the received RACH )2.1.2.2 Phantom RACHsThe received RACH is in fact generated from an “unknown source”, whereby it failsto continue the transactions after SDCCH has been allocated by the BSS. Forinstances, cases of Channel Request detected by over-shooting cells, HandoverAccess burst from distanced MS, hardware deficiency, UL/DL imbalance path, MSmoving out of range would carry the Phantom RACHs symptoms.2.1.2.2.1 Optimisation tips:Within the optima there are certain stats which can be monitored before coming tothe conclusion that there is SDCCH problem1-) SDCCH Blocking2-) SDCCH CongestionIf the SDCCH blocking is greater than 1 % or the SDCCH congestion is greater than2% than that means that it is a capacity related issue and more slots should beassigned for SDCCH.A Tch can be allocated bypassing Sdcch. A parameter namelyImmediate_assign_mode when enabled allocates Tch bypassing SDCCH.2.1.3 CM Service Reject:CM Service Request [MOC] or Paging Response [MTC] to BSS/MSC. Inside the CMService Request message (MS initiated service request), MS informs the network thetypes of services it requires (i.e. Mobile Origination Call, Emergency Call, ShortMessage Transfer or Supplementary Services Activity), whereby Paging Response isspecific to MTC. Subsequently, BSS embraces the information with its own initiatedConnection Request BSSMAP Message, sends to MSC for approval. MSC willresponse with either Connection Confirmed, confirming the success in linkestablishment between MS-BSS-MSC, or Connection Refused, indicating thetermination of the specific network transaction.2.1.3.1 Examples of AbnormalityBasically, the generation of excessive Connection Refused messages could relate toMSC internal issue.CS-SG-TPP-0042-01 Page 5
  • Upon successful link establishment, a number of signalling activities will proceed.Firstly, Authentication Request will be sent to MS followed by cipher mode command.Thirdly, MSC will send Identity Request to MS, questioning the IMSI. MS shouldresponse with its IMSI in the Identity Response message to MSC for validation. MS-BSS-MSC connection will be cleared down if any of the above three signallingactivities fail. In general, Authentication Request, Cipher Mode Command or IdentityRequest does not always appear during call setup, the occurrences of the threesignalling activities is controlled by MSC. (e.g. Authentication Request is onlyperformed on every 10th call in the network).Examples of AbnormalityMS related (e.g. incompatibility in ciphering algorithm, identity, etc)2.1.4 TCH Failure Assignment:Upon completion of MS/BSS/MSC link establishment, MSC issues AssignmentRequest to BSS, requesting TCH assignment to the dedicated MS. Subsequently,BSS will attempt to allocate free TCH for MS voice messaging. Once AssignmentCommand is received by MS, stating the availability of TCH for the MS, it will move tothe dedicated TCH and responds with Assignment Complete. In turns, BSS willsubmit Assignment Complete to MSC as to complete the signalling activity.2.1.4.1 Examples of Abnormality - TCH CongestionBSS responses to MSC via Assignment Failure, indicating the unavailability of TCHresources. BSS will subsequently terminate all transactions, in which case, call setupis termed as unsuccessful due to TCH congestion.Interference on TCHThere could be co-channel or adjacent channel interference on a particular TCHcausing TCH failure assignment.2.1.4.1.1 Optimisation tips:For TCH congestion certain features can be enabled like TCH queuing, DirectedRetry and Congestion Relief.In the case of TCH Queuing feature is enabled, MS willqueue in the original SDCCH, awaiting for the next available TCH. It is to bereminded that once Queuing Timer expires, BSS will also terminates all transactions,in which case, call setup is termed as unsuccessful due to TCH congestion. Thesame situation also applies in situation where Congestion Relief feature is enabled.In the case of Directed Retry feature is enabled, MS will perform handover to TCH ofanother cell if a valid handover neighbour is detected. The best thing to do is to addmore radios in the cell to remove congestion.Interference analysis on a particular carrier can be done through an optimisation toollike Neptune. Once interfering frequencies are determined, the frequency plan canbe cleaned from such frequencies.CS-SG-TPP-0042-01 Page 6
  • 2.1.5 Hardware Problems:Hardware failures also play the major role for Poor CSSR. Improper functionality ofany BTS hardware can affect the overall performance of site2.1.5.1.1 Optimisation Tips:If there are no capacity or RF issues then equipment needs to be checked. Beforestarting the drive test make sure that the cell sites are free of any hardware alarms.The important parameter to check is the Path balance. If Path balances are not finethen start checking the power from radio to connected antennas. If we take theexample of GSM 900 scenario, the link budget defines that the radio should transmit40 watts power and at the top of the cabinet, 20 watts are received (considering the 3dB loss of combiner). while checking the power, if any components seems toproduce more losses than expected, change that component. Similarly check thepower at antenna feeder ports. Some time due to the water ingress, connectors getrusty and needs to be replaced.2.1.6 High Dropped Call Rate:For a healthy network the drop call rate should be less than 1 %. There are againnumber of reasons, which could contribute towards higher dropped call rate.a-) Drop on Handoverb-) low signal levelc-) adjacent channel interferenced-) Co-channel interferencee-) Extraneous interferencef-) link imbalancea-) Drop on HandoverThe call may drop on handover. It’s mostly high neighbor interference on the target cell, which causesthe main problem. Sometime the mobile is on the wrong source cell (not planned for that area butserves due to the antenna overshoot) which may result in the call drop.2.1.7 Optimisation Tips:Within optima, monitor the following statistics. Theses statistics are defined underthe category of BSC level statistics.1-) Total and successful handovers on up/dl quality.2-) Total and successful handovers on up/dl signal strength.3-) Total and successful power budget handovers.From the above statistics , quality or level issues can be estimated.CS-SG-TPP-0042-01 Page 7
  • 2.2 Low Signal LevelSignal level below –95 dbm is considered to be poor. If the mobile is unable tohandoff to a better cell on level basis, the call will possibly be dropped. Topology orMorphology issues may also be there like if Mobile enters into a tunnel or a building,higher RF losses will be developed.2.2.1 Optimisation tips:First of all Path balances should be checked. If Path balances are deviating form thestandard value than Check the BTS transmitted power with the help of wattmeter.BTS may transmit low power because of the malfunctioning of radio or highercombiner losses. Also check the feeder losses, antenna connectors. Enable downlinkPower Control. Power control is bi directional. The lower and upper receive leveldownlink power control values should be properly defined.1-) l_ rxlev_ dl_pdefines the lower value for receive level for the power control to be triggered .Range = 0 to 63Where 0 = -110 dbm 1 = -109 dbm 63 = -47 dbme.g if the value of 20 is set it means that the BTS will start transmitting more if itsenses that downlink receive level is below –90 dbm.2-) u_rxlev_dl_pdefines the upper threshold value for receive level for the power control to betriggered. (Range is same as described above)e.g on setting the value of 50 ( equivalent to –60 dbm ) BTS will lower down thepower .2.3 Adjacent and Co Channel InterferenceFrequency planning plays a major role to combat adjacent channel and Co channelinterference. Co channel is observed mostly when mobile is elevated and receivessignals from cell far away but using the same frequencies.2.3.1 Optimisation tips:An optimisation tool like Neptune could be helpful in identifying the interference on aparticular area. Such frequencies can be cleaned form existing frequency plan. TheCS-SG-TPP-0042-01 Page 8
  • following statistics can also be monitored to confirm that there are interference issuesin the cell. These stats are defined in optima under the category of BSC stats.1-) TCH interference at level 12-) TCH interference at level 23-) TCH interference at level 34-) TCH interference at level 4When a tch timeslot is idle it is constantly monitored for an uplink ambient noise.During a SACCH multiframe an idle time slot is monitored 104 times. These samplesare then processed to produce a noise level average per 480 ms. An interferenceband is allocated to an idle slot depending upon the interference level. Thethresholds for these levels can be set in the system parameters, interference level 1being the least ambient and interference level 4 being the most ambient. Whileplanning the network care should be taken that the cells do have the properfrequency spacing.2.4 Extraneous InterferenceExtraneous interference might be fromOther mobile networksMilitary communicationCordless telephonesIllegal radio Communication equipmentOptimisation tips:External interference is always measured through spectrum analyser which can scanthe whole band. Some Spectrum analyser can even decode voice from AMPScircuits or Cordless Phones.2.5 Link ImbalanceSome time the malfunctionality of vendor hardware becomes responsible for highCDR. One of the possible scenarios could beTransmit and Receiving antenna facing different directionTransmit and Receiving antennas with different tiltsAntenna feeders damage, corrosion or water ingressPhysical obstructionCS-SG-TPP-0042-01 Page 9
  • 2.6 High Handover Failure rate:High Handover failures rate will probably be due to one or more of the followingreason.a-) High Neighbour Interferenceb-) No Dominant Serverc-) Database Parameters2.7 High Neighbour InterferenceWhile handing off to the best neighbour the interference on the target cell frequencymay result in the hand off failure.2.7.1 Optimisation tipsWhen designing the cell frequencies care should be taken that there is properfrequency spacing between the cells to avoid neighbour interference. In most of thecases Ping pong Handover starts i.e the mobile hand off to a cell for better level anddue to interference (quality issues) hand off again to original cell. A thorough drivetest can determine the “interfering frequencies” which should be eliminated from thefrequency plan.2.8 No Dominant ServerIf cell sites are designed poorly there might be areas where neighbour being receivedat the same level and some neighbours randomly look good for handoff for a certainamount of time. Such situation is disastrous because handoff decision will be hardand mostly it will end up in unsuccessful handovers.2.8.1 Optimisation tipsAntenna tilts provide the good way to reduce the footprint of the sites. Efforts shouldbe made that a single dominant server should serve the specific area. Timingadvance limitation ( ms_max_range) is applied to cell areas where there is multipleservers.2.9 Database parametersCS-SG-TPP-0042-01 Page 10
  • Received level, receive quality and Power budget algorithm are set in the systeminformation to define the criteria for Handover. Improper values for these criteria mayresult in poor handoff.2.9.1 Optimisation TipsEnable the “ Per neighbour ” feature which displays the successful andunsuccessful handovers on a per cell basis. In “optima ” monitor the following stats,which comes under “ cell statistic category ”.HOCNT (handover count from source cell to destination cell over readingperiod)HOSUCC (Success Count from source cell to destination cell over readingperiod)HORET (Returned Count from source cell to destination cell over readingperiod)HODROP (Dropped count from source cell to destination cell over readingperiod)All those cells can be identified which are problematic in terms of hand off so one canfocus only specific cells causing the major contribution towards poor HSSR. Ensurethat handover margins are optimised. Rule of thumb is 4 dB for adjacent frequenciesand 6 dB for cell without adjacent frequencies. The following parameters can beplayed for defining the thresholds for imperative and non-imperative handovers.1-) l_rxqual_up_h (defines the lower threshold for uplink quality handover)Range: 0 to 1800Step size: 0.01e.g. a value of 500 defines the lower threshold value of 5( BER ) for a qualityhandover to be triggered for uplink. The optimum value for this threshold is 5002-) l_rxqual_dl_h (defines the lower threshold for downlink quality handover)3-) l_rxlev_up_h (defines the lower threshold for received level uplink handover)e.g. a value of 20 defines the threshold value of -90 dbm for a level handover to betriggered for uplink.Range: 0 to 63Where 0= -110 dbm 1= -109 dbm 47= -63 dbmThe optimum value for this threshold is 15 i.e. 95 dbm. If the signal level goes belowthat, a level handover is initiated.4-) l_rxlev_dl_h (defines the lower threshold for received level downlink handover)CS-SG-TPP-0042-01 Page 11
  • 5-) u_rxlev_dl_ih (defines the upper threshold for downlink interference hand over)6-) u_rxlev_dl_ih ( defines the upper threshold for uplink interference Handover )2.10 SDCCH BlockingSDCCH blocking is probably due to one or more reasons.a-) No access to SDCCHb-) Failure before assignment of TCHc-) High Paging loadd-) Incorrect or inappropriate timer valuesThe first two reasons has already been discussed.2.11 High Paging loadIrregular paging distribution in location areas results in SDCCH blocking. Higherpaging load in certain location area means higher location updates on SDCCHresulting in SDCCH blocking.2.11.1 Optimisation Tips:A location area with a high paging load needs to be reduced in size to relieveSDCCH blocking. A location area with a low paging load needs to be enlarged in sizeto reduce the overall number of location areas.2.12 Incorrect or inappropriate timer valuesTimer rr_t3 111 sets the amount of time allowed to delay the deactivation of a trafficchannel (TCH) after the disconnection of the main signalling link.2.12.1 Optimisation Tips:The suitable value for this timer is 1200 ms (max being 1500 ms). The timer willcause the BSS to wait before the channel in question is allocated anotherCS-SG-TPP-0042-01 Page 12
  • connection. A lower value of timer will result in higher capacity since the channel isheld for less time before being released.2.13 TCH Blocking:Tch blocking may be due to the following reasonsa-) Handover and Power budget marginsc-) Cells too larged-) Capacity Limitations ( congestion )e-) Incorrect or appropriate timer2.14 Handover MarginHandover margins should be properly optimised to move the traffic to neighbouringcell. Strict handover margins can result in lower handovers and ultimately congestionin cell2.14.1 Optimisation tips6 db handover margin is considered to be an appropriate margin for handover. Astrict handover margin results in the strict criteria for Power budget handovers also.Setting a lower value of handover margin will initiate ping-pong handovers, which arenot considered good for network health. ( hand over margins have already beendiscussed )2.15 Cells too largeIf cells are too large meaning antenna too high or antenna too shallow, it will pull inout of area traffic again causing congestion in the cell2.15.1 Optimisation tipsConsider reducing antenna height to reduce the footprint of the site. Also increasethe antenna tilt ( the max tilt being 12 )2.16 Improper timerTimers are important in terms of allocating TCH resources. Timer rr_t3111 sets theamount of time allowed to delay the deactivation of a traffic channel TCH after thedisconnection of the main signalling link. Parameter link_fail and radio_link_timeoutCS-SG-TPP-0042-01 Page 13
  • sets the thresholds for the number of lost SAACH messages before a loss of SAACHis reported on abis.2.16.1 Optimisation TipsSetting the lower value rr_t3111 will increase capacity as the channel is held forsome time before being released. The best value for this timer is 1200 ms. The lowervalues for radio link timeout and link_fail will result in early disconnection of Tch’s.The optimised value for these timers is 3 which means that BSS will wait for 12 sacchmessages before it declares that the link has been broken.2.17 Sdcch Mean Holding TimeSdcch performs number of tasks like authentication, ciphering, periodic updates,location update, IMSI attach/detach, and SMS. Holding time for different services isdifferent. The average holding time for call set-up should be 2.8 seconds and forperiodic location updating it is 3.6 seconds. An “ increase ” in “SDCCH mean holdingtime ”can result in longer call set-ups or may end up in unsuccessful call attempts.2.18 Erlangs, Total callsTotal calls and Erlangs stats on a per cell basis gives the insight on amount of traffica cell can cater. These statistics are useful in terms of future planning of sites. A highvalue of Erlangs for a specific cell recommends that new load sharing sites bedesigned to cater the increased demand for capacity.2.19 Processor LoadThe statistic provide the distribution of processor load on a per BSC basis. Processoroverloading can affect the overall performance of the cell sites like call set-ups,handovers and call drops. This stat should be monitored on daily basis.CS-SG-TPP-0042-01 Page 14