Radio Network Optimization (basic)

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Giving a basic explanation around radio optimization

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Radio Network Optimization: Contents Reasons for the Need of Optimization Performance Data Measurements Drive Tests Optimization Strategies Optimization of Physical Parameters Optimization of Database Parameters Example Drive Tests Example Drive Tests: ExercisesReasons for the Need of OptimizationNetwork optimization is an iterative process which should improve the quality and performance of a network and also run the network more efficiently. As in any optimization problem, also in network optimization, the network will mostly not run optimal from the very beginning. There can be mentioned several reasons: Systematic inaccuracies Statistical nature of the involved processes like e.g. traffic and RF propagation Dynamical nature of the involved processes like e.g. change of the subscriber's telephonebehavior (e.g. SMS) Wrong (or only too rough) planning assumptions, input data and/or planning models Increasing number of subscribers Installation errors (for example a wrong cabling: transmitting into cell A, but receiving fromcell B) Hardware / software troublePerformance Data MeasurementsPerformance data measurements can help the network operator for example to localise problem areas as early as possible and also to verify improvements of the network optimisation.Concerning radio network optimisation there are related performance data measurements foreseen by GSM (see: GSM 12.04) and in addition also vendor specific ones.In general performance data measurements can be run continuously, periodically or sporadically, for a long time or a short time, observing smaller or greater parts of the network.The related counters could in principle be actualised continuously during the observation period, but mostly a scanning method is used. Scanning method means that the system counts the number of events not continuously but only at particular times. This leads to some uncertainty for the measurement results. Nevertheless, the error performed can be estimated using statistical methods. In general, the smaller the scanning interval the higher the precision of the measurement (for constant observation periods). Typical scanning intervals are 100 ms or 500 ms.Performance Data MeasurementsScanner Performance data measurement(s) Counter(s)Condition(s) for the updating of the counter value(s)

Drive TestsDrive tests are performed by the network operator for various reasons: To check the coverage in a certain area To check the quality of service in a certain area To find the answer for customer complaints To realise that the network is not properly running To verify that the network is properly running To verify that certain optimisation steps have been successfulDrive tests must be well prepared. Before, during and after the drive test the following steps should be performed:Drive TestsBefore drive test Plan the route where to drive Plan the time when to drive Determine the MS mode (idle mode/ connected mode) and also thecall strategy (long / short calls) Decide which values to focus on (for example: RXQUAL, RXLEV,SQI, ...) Select an appropriate test equipment and check the test equipment Think of notes which should be inserted later on in the recording fileDuring drive test Monitor the test equipment Reconnect dropped calls Insert notes in the recording file Note interesting events separately (e.g. on a piece of paper)After drive test Make back-up files of the captured data Replay the captured data and analyse them Find out problem areas and problem eventsUse further post-processing tools to display the captured data moreclearly and to graphically display further values

Perform statistics and summarise the results

Optimization Strategies

Optimization of Physical ParametersAltering antenna tilt: to reduce interference to limit coverage area to improve coverage (e.g. coverage weakness below main lobe) to improve in-building penetrationAltering the Antenna tilt must be done very carefully to really improve the situation.Typical down-tilts are between 0 and 10, however even higher values (up to 25) have already been used.Altering antenna azimuth: to overcome coverage weakness between different sectors to reduce interference in certain directionsIncreasing or decreasing antenna height: to reduce or improve coverage to reduce interferenceChange of antenna type to achieve desired ration characteristicsOptimization of Physical ParametersAddition / re-movement of TRXs: Depending on the real measured traffic load either TRXs can be removed (switched off orblocked) or must be added. Not really needed TRXs may interfere other cells. The number of needed TRXs and also the configuration of the different channels depend on theoffered traffic, and the subscriber behaviour.Optimization of Physical ParametersCell sectorization / cell splitting:Can be used for: Coverage enhancements (since the antenna gain of sectorised antennas is higher than that ofomni directional antennas) Interference reduction Capacity enhancements, but only if together with the sectorisation also the number of TRXs isincreased (compare Erlang-B loss formula)Depending on how the splitting is performed: it may be a more or less expensive and difficult (time consuming) solution coverage weakness between the main lobes may appear the capacity will be reduced if the total number of TRXs remains constantOptimization of Physical ParametersImplementation of Antenna near Pre-Amplifiers:Link imbalances are one reason for poor quality, increased call drop rate and increased handover failure rate. In case of an unbalanced link, the uplink and downlink coverage ranges differ. Often the downlink range is bigger than the uplink range. This problem can be overcome by using antenna near preamplifiers which improve the sensitivity and the noise figure of a base station system. Looking to the link budget:The better the sensitivity of the base station, the more fare the possible uplink range. In any case, a proper running network requires a balanced link.Implementation of Repeaters:A repeater (see GSM 11.26 (ETS 300 609-4) and GSM 05.05) is a bi-directional (full duplex) RF amplifier and is used to overcome coverage holes in a base station area. Typical applications of repeaters are the coverage of problem zones like tunnels, valleys, in buildings, ...A repeater receives, amplifies and retransmits the downlink signal from a donor base station into an area with weak or no coverage, and the uplink signal from mobile stations which are located in such an area. Repeaters extend but do not replace base stations.Optimization of Database ParametersFrequency Changes: To overcome e.g. sever cases of downlink interference (therefore it is advisable to have somespare frequencies). May influence other areas. Re-planning may become necessary. In high-density areas often difficult.Strategies: Using spare frequencies in severely interfered regions. TCH - BCCH change as temporary solutions in low TCH traffic load areas. Re-planning of TCH and BCCH frequencies.Optimization of Database ParametersFrequency Hopping: Cyclic or pseudo random hopping?

Example of cyclic hopping: HSN = 0

Time in TDMA framesOptimum frequency diversitydue to averaging of Rayleigh fading=> rural, coverage limited areasOptimization of Database ParametersFrequency Hopping: Example of pseudo random hopping: HSN = 1 -63

Co-channel interference averagingCollision probability: 1/number of hopping frequencies=> interference limited areas (hot spots)Optimization of Database ParametersRadio Link Failure (RLF) / Radio Link Timeout (RLT):(see also GSM 04.08 and GSM 05.08)Calls which fail due to radio coverage problems or which suffer under unacceptable voice or data quality (due to e.g. interference) which cannot be improved by power control or handover are either released or re-established in a defined way.The criterion for the detection of a radio link failure by the MS is the success rate of decoding DL-SACCH messages.The criterion for the determination of a radio link failure by the BS is either the success rate of decoding UL-SACCH messages or it is based on RXLEV / RXQUAL measurements.The MS checks the DL with the help of a radio link (failure) counter running in the MS. The BS checks the UL with the help of a radio link (failure) counter running in the BS.Optimization of Database ParametersRadio Link Failure (RLF) / Radio Link Timeout (RLT):The algorithm for the modification of the radio link failure counter S is the following:Starting value for the Radio Link Failure Counter: Radio_Link_Timeout In case of successful decoding of SACCH messages: Snew=Sold+2 In case of non-successful decoding of SACCH messages: Snew=Sold-1 value range for S: 0< S< Radio_Link_Timeout Radio link failure is detected if: S=0This algorithm is only running after assignment of a dedicated channel (i.e. in connected mode).The starting value Radio_Link_Timeout for the MS counter is sent on the BCCH system information type 3 or on the SACCH system information type 6 in the information element 'Cell Options'.Optimization of Database ParametersHandover and Power Control:Reasons for the optimisation of handover parameters: To reduce the number of call drops To reduce the number of handovers To maximise the time duration the MS spends in the best cell To improve the speech qualityHandover types: intra- / inter- cell, BTS, BSC, MSC handoversHandover causes: (Bad) RXQUAL (Low) RXLEV (far) DISTANCE (Power Budget) PBGTOptimization of Database ParametersThresholds needed for handover evaluation:AbbreviationRemarks

L_RXLEV_UL_HRXLEV threshold on the uplink for handover process to commence (outgoing HO)

L_RXQUAL_UL_HRXQUAL threshold on the uplink for handover process to commence (outgoing HO)

L_RXLEV_DL_HRXLEV threshold on the downlink for handover process to commence (outgoing HO)

L_RXQUAL_DL_HRXQUAL threshold on the downlink for handover process to commence (outgoing HO)

MS_RANGE_MAXThreshold for the maximum allowed distance between MS and current BTS (outgoing HO)

RXLEV_UL_IHRXLEV threshold on the uplink for intracell (interference) handover

RXLEV_DL_IHRXLEV threshold on the downlink for intracell (interference) handover

RXLEV_MIN(n)Minimum RXLEV required for a MS to be allowed to be handovered to neighbour cell (n) (incoming HO)

HO_MARGIN(n)Threshold for power budget process

MS_TXPWR_MAXMaximum transmission power a MS may use in the serving cell

MS_TXPWR_MAX(n)Maximum transmission power a MS may use in the adjacent cell (n)

BS_TXPWR_MAXMaximum transmission power the BTS may use

Optimization of Database ParametersLevel triggered handover:Depending on the measured and averaged RXQUAL_XX and RXLEV_XX values the system (MS and / or BS) may increase or decrease the output power or may handover the call.Remarks to the corresponding handover threshold settings:L_RXLEV_UL_H and L_RXLEV_DL_H should be set some dB (e.g. 5 dB) above the effective (+ diversity gain, + preamplifier) receiver sensitivity limit:Receiver sensitivity levels due to GSM 05.05: For DCS 1800 class 1 or class 2MS: -100 dBm For DCS 1800 class 3 MS: -102 dBm For GSM 900 small MS: -102dBm For other GSM 900 MS: -104 dBm For normal BTS: -104 dBmExample settings: L_RXLEV_DL_H =-95 dBm L_RXLEV_UL_H = -102 dBmOptimization of Database ParametersLevel triggered handover:There should be a level hysteresis between the threshold RXLEV_MIN(n) for incoming handover and the threshold L_RXLEV_XX_H for outgoing handover:RXLEV_MIN > L_RXLEV_XX_H + 4....10 dBThe size of this hysteresis should be related to the standard deviation of the long term fading (typically 4...10 dB) and should be large enough to avoid ping-pong handovers and small enough to allow fast handovers.Example setting: RXLEV_MIN = -90 dBOptimization of Database ParametersLevel triggered Power Control:Depending on the measured and averaged RX_QUAL and RX_LEV values the system (MS and / or BS) may increase or decrease the output power or may handover the call.For the power control and handover threshold settings the following considerations should be taken into account:L_RXLEV_UL_P(Lower) RXLEV threshold on the uplink for power increase

U_RXLEV_UL_P(Upper) RXLEV threshold on the uplink for power reduction

L_RXLEV_DL_P(Lower) RXLEV threshold on the downlink for power increase

U_RXLEV_DL_P(Upper) RXLEV threshold on the downlink for power reduction

Optimization of Database ParametersLevel triggered Power Control:To avoid consecutive power increase or decreases directly after each other the difference between upper and lower power control thresholds should be large enough (e.g. 10 dB).To allow the system to perform power control before handover is executed, the lower power control level thresholds should be about 10 dB above the lower handover level thresholds.Example settings:L_RXLEV_DL_H = -95 dBm, L_RXLEV_DL_P = -85 dBm, U_RXLEV_DL_P = -75 dBmL_RXLEV_UL_H = -102 dBm, L_RXLEV_UL_P = -92 dBm, U_RXLEV_UL_P = -82 dBmOptimization of Database ParametersPower Control Execution:For the power control execution parameter settings the following considerations should be taken into account:Since typically a power increase command is more urgent than a power reduction command, the power increase step size should be greater than the power reduction step size.The power increase and power reduction step sizes should be on the one hand small enough to enable an accurate power control, on the other hand large enough to reduce the number of necessary power control commands and therefore the signalling load.Example settings: POW_INCR_STEP_SIZE = 4 dB POW_RED_STEP_SIZE = 2 dBOptimization of Database ParametersQuality triggered handover:Depending on the measured and averaged RXQUAL_XX and RXLEV_XX values the system (MS and/or BS) may increase or decrease the output power or may handover the call.Remarks to the corresponding handover threshold settings: L_RXQUAL_UL_H, L_RXQUAL_DL_H RXLEV_UL_IH, RXLEV_DL_IHIn case of bad quality (RXQUAL_XX > L_RXQUAL_XX_H) and high signal strength (RXLEV_XX > RXLEV_XX_IH) at the same time, there is a high probability of the presence of:cochannel interference, adjacent channel interference, intermodulation problems, intersystem interference.Temporary solution: intracell handoverIntracell handover doesn't help: if frequency hopping is switched on, or if there is only 1 TRX in the serving cell and the interference is continuous and not bursty.Examples settings: L_RXQUAL_UL_H = 5, L_RXQUAL_DL_H = 5RXLEV_UL_IH = -85 dBm , RXLEV_DL_IH = -78 dBmOptimization of Database ParametersQuality triggered Power Control:Depending on the measured and averaged RXQUAL_XX and RXLEV_XX values the system (MS and/or BS) may increase or decrease the output power or may handover the call.Power is increased if the received quality is bad: RXQUAL_XX > L_RXQAUL_XX_PPower can be decreased if the received quality is very good: RXQUAL_XX < U_RXQAUL_XX_PHowever, often it is more suitable to control the power decrease by the level criteria and to set U_RXQAUL_XX_P = 0 or a small value, i.e. to 'disable' the power decrease due to good quality.To make 'power up before handover' possible, the following relation between power control and handover thresholds should be taken into account:U_RXQUAL_XX_P < L_RXQUAL_XX_P < L_RXQUAL_XX_HExample settings: U_RXQUAL_XX_P = 0 (or 1) L_RXQUAL_XX_P = 4 L_RXQUAL_XX_H = 5Optimization of Database ParametersHandover Thresholds:

Optimization of Database Parameters

POW RED STEP SIZE Example settingsL_RXLEV_DL_P = 25L_RXLEV_UL_P = 10POW_RED_STEP_SIZE = 2 dBU_RXLEV_DL_P = 35U_RXLEV_UL_P = 15POW_INCR_STEP_SIZE = 4 dBL_RXQUAL_DL_P = 4L_RXQUAL_UL_P = 5U_RXQUAL_DL_P = 1U_RXQUAL_UL_P = 4Optimization of Database ParametersHandover triggered by power budget:In an interference limited area (e.g. small cells in cities) most of the handovers should be power budget handovers:For this type of handover not the level, quality, or distance is the handover cause, since all the corresponding thresholds are not exceeded in the serving cell, but a neighbour cell offers a better service (a smaller path loss, see link budget).Since the power budget hanodver looks for the serving cell with the smallest path loss, this kind of handover will:Reduce interference Prolong MS battery timeThe power budget is defined as the difference between the path loss in the serving cell and the path loss in the neighbour cell:PBGT(n) = (BS_TXPWR - RXLEV_DL) - ( BS_TXPWR_MAX(n) - RXLEV_DL_NCELL(n))Optimization of Database ParametersHandover triggered by power budget:Assumption:BS_TXPWR_MAX - BS_TXPWR_MAX(n) = MS_TXPWR_MAX- MS_TXPWR_MAX(n)PBGT(n) = RXLEV_DL_NCELL(n) - RXLEV_DL - PWR_C_D + min (MS_TXPWR_MAX,P) - min (MS_TXPWR_MAX(n),P)Where PWR_C_D is defined as: BS_TXPWR_MAX - BS_TXPWRIf PBGT(n) > HO_MARGIN(n) the path loss in the serving cell is greater than the path loss in the neighbour cell + HO_MARGIN so that the neighbour cell is considered as the much better cell.Optimization of Database ParametersRemarks to the setting of the Handover Margin: The HO_MARGIN setting should be a compromise between ideal power budget handover (whichrequires a small HO_MARGIN value) and a setting to reduce the risk of ping-pong handovers(which requires a greater HO_MARGIN value). A small handover zone increases the risk of ping-pong handovers. Usually HO_MARGIN is set symmetrically. Asymmetrical HO_MARGIN can be used to influence the size of the handover area and/or tomove the handover area, i.e. to move the cell boundaries. Adjusting HO_MARGIN values can therefore also be used to adapt the cell area to the traffic loador to avoid local interference. RXLEV_MIN(n) should be set to such a value that RXLEV_DL_NCELL(n) > RXLEV_MIN(n) inthose areas where PBGT(n) > HO_MARGIN(n) to really allow the power budget handover as soonas the power budget condition is fulfilled.Optimization of Database ParametersHOM: Handover margin foradjacent cell

Optimization of Database ParametersRemarks to the pre-processing (averaging) of the measurements needed for power control and handover decisions:In general: Many measurements should be averaged in case that reliable decisions are necessary (betterstatistics). Only a few measurements should be averaged in case that fast decisions are necessary.For level / quality triggered handover / power control decisions: To allow the system to 'power up before handover' usually the averaging process for the handoverdecisions should include more measurements than for power control decisions. Usually for level triggered handover decisions more measurement values should be averagedthan for quality triggered handover decisions since quality handovers must be executed quickly ifsudden interference appears.Optimization of Database ParametersBCCH allocation:Also neighbor cell list is target of optimization process:Missing neighbor cell => perhaps call dropToo many neighbors => bad statistics, unprecise measurement values, perhaps wrong decisionsIn practice: perhaps paging overload (PCH overload)(MS is paged in the whole location area) Too small => perhaps too many location updates (AGCH overload)(MS has to perform location update if location area is changed)Example Drive Test

Example Drive Test: ExerciseExercise:Discuss the drive test given above:a) Localize the problem area(s)b) Suggest counter measures to solve the problem(s)Example Drive Test

Example Drive Test

Example Drive Test

Example Drive Test: ExerciseExercise:Discuss the drive test given above:a) Localize the problem area(s)b) Suggest counter measures to solve the problem(s)