RF OPTIMISATION IN GSM NETWORKS Regional Telecom Training Centre, Trivandrum : 695 040.

RF OPTIMISATION IN GSM NETWORKS

Regional Telecom Training Centre, Trivandrum : 695 040

Regional Telecom Training Centre, Trivandrum : 695 040.

OPTIMISATION

• Optimisation involves monitoring, verifying and improving the performance of the radio network.


OPTIMISATION


Why Optimisation?


Optimisation Process


REASON FOR OPTIMISATION

• Radio network planners first focus on three main areas: coverage, capacity and frequency planning. Then follows site selection, parameter planning,


Objective of Optimisation

The objective of optimisation procedure is:

To check whether the network meets the customer’s given requirements, on the basis of which network was designed.

To check whether the parameters and configurations are defined correctly or not.

• To find out and suggest changes in the defined parameters and configurations to achieve best possible quality of service.


Optimisation using DT

• Following steps are taken to fulfil the objective of network optimisation using a drive test tool.

• Collection of Data and extraction of relevant information from it.

b) Analysis of the extracted data.

c)Suggesting changes in the network configurations based on the analysis.


Stages of Optimisation


Cell Selection


Criteria for Cell selection

C1 Criteria• Used for cell selection and re-selectionC2 Criteria• Used in a hierarchical cell structure for re-selection only


Parameters checked during DT

1) Rx Level.

2) Rx Quality

3) Timing Advance

4) Handover parameters

5) Data of six best neighbour cells.

6) Layer 2 and layer 3 messages.

6) Handover details.


KEY PERFORMANCE INDICATORS

Which KPIs the operator decides to use will depend on:

• The nature of the network• Operator assessment of priorities for their business• Counters implemented by vendors• KPIs are generally based on two sets of data:• Network statistics from the OMC• Drive test statistics using test mobile equipment


GENERAL KEY PERFORMANCE INDICATORS

• KPI’s vary from operator to operator but the following are generally common to all KPI statistics:

1.TCH Usage

2.SDCCH Usage

3.Handover Statistics

4.Connection Establishment


KPIs –TCH/SDCCH Usage

KPI’S commonly monitored for TCH and SDCCH usage include:

• Traffic Flow• Mean holding time• Congestion• Busy time• RF Losses


KPIs –Handover Statistics

KPI’s commonly monitored for Handover performance monitoring include:

• Success rate• Failure rate• Handovers due to poor quality• Handovers due to poor level• Power budget handovers• Handover due to interference• Inter-cell handovers• Inter-BSS handovers• Intra-BSS handovers


KPIs – Connection Establishment

KPI’S commonly monitored for connection establishment include:

• Successful immediate assignment procedure

• Attempted immediate assignment procedure

• Cell TCH assignments


Speech Quality Measurements

• • Speech QoS represented by:• • Received Signal Level – RxLev• • Received Signal Quality – RxQual• • Factors affecting these values include:• • BTS power output• • MS power output• • Current BTS loading


BSS KPI Definitions

CALL SETUP SUCCESS RATE (CSSR)

• CSSR measures the proportion of call setup attempts resulting in a successful TCH assignment, excluding those setup attempts for reasons that do not require a TCH (eg. Location Update, IMSI Detach etc.)


BSS KPI Definitions

DROP CALL RATE (DCR)• DCR measures the proportion of successfully

established calls that are terminated abnormally.

• DCR is logically defined as the ratio between the TCH drops occurring during the conversation phase to the number of successful seizures on the cell or area.


BSS KPI Definitions

DROPPED CALL RATIO


BSS KPI Definitions

CALL SUCCESS RATE (CSR)/CALL SETUP FAILURE

• CSR measures the proportion of call setup attempts that are successfully established and terminated normally (ie. not dropped)



• CSR measures the ability of a cell/network to provide traffic channel to call originating mobiles.

• Any failure in the assignment phase is charged to the CSR. Here also results may vary according to what the operator wants to track.



• Some operators start considering CSF after the SDCCH has been assigned not to consider SDCCH blocking.

• Others want to model the user’s perception and to consider all types of failures.


HANDOVER SUCCESS RATE (HOSR)

• HOSR measures the proportion of total handover attempts resulting in a successfully completed handover, including inter-cell, Intra-BSS and Inter-BSS Handovers


HANDOVER SUCCESS RATE (HOSR)


HANDOVER SUCCESS RATE (HOSR

• HOSR is defined as the percentage of successful outgoing handover attempts.

• The total number of attempts, failures and blockings on incoming and outgoing handovers are available either to cell-to-cell or BSC-to-BSC levels.

• High rates of outgoing HO failures explain high values of the DCR.


BSS KPI Definitions

HANDOVER FAILURE RATE (HOFR)• HOFR measures the proportion of handover

attempts that result in handover failure with dropped call (ie. excluding handover attempts that fail and recover to the source cell)


HANDOVER FAILURE RATE (HOFR)


TRAFFIC CHANNEL BLOCKING (TCH_Cong)

TCH_Cong measures the proportion of attempts to allocate a TCH that are blocked due to lack of TCH resources.

• This should exclude blocked TCH assignment attempts that subsequently succeed due to queuing procedures.


TRAFFIC CHANNEL BLOCKING

• TCH_Cong measures the proportion of attempts to allocate a TCH that are blocked due to lack of TCH resources.

• This should exclude blocked TCH assignment attempts that subsequently succeed due to queuing procedures.


TRAFFIC CHANNEL BLOCKING


CALL DROPPED – RF FAILURE (TCH_RF_Loss)

TCH_RF_Loss measures the proportion of successfully established calls that drop due to TCH RF loss (as distinct from HO_Fail_Drop


CALL DROPPED – RF FAILURE (TCH_RF_Loss)


TRAFFIC CHANNEL ASSIGNMENT FAILURE RATE (TCH_Ass_Flr)

• TCH_Ass_Flr measures the proportion of TCH assignment attempts that fail after TCH allocation and before completion of the call setup procedure.


TRAFFIC CHANNEL ASSIGNMENT FAILURE RATE (TCH_Ass_Flr)


SDCCH ACCESS SUCCESS RATE (SD_Acc_Suc)

• SD_Acc_Suc measures the proportion of Immediate Assignment Attempts that result in successful assignment to SDCCH, for all call setup reasons including location update, IMSI detach etc


SDCCH ACCESS SUCCESS RATE


NON-BSS CALL SETUP FAILURE RATE (Non-BSS_CSFR

• Non-BSS_CSFR measures the proportion of Call Setup Attempts that fail due to Non-BSS reasons (eg. MSC rejections, subscriber error, etc.)


NON-BSS CALL SETUP FAILURE


UTILISATION FACTOR

• Utilisation Factor is the ratio of actual traffic carried to the theoretical traffic that can be carried for a 2% Grade of Service, based on the number of available TCH’s and Erlang-B formula


UTILISATION FACTOR


TIMING ADVANCE

• Cells with high timing advance are those taking communications far from the site. Due to weak coverage and path unbalance, it is normal to have high DCR and CSF values on such cells.

• In this case a solution can be obtained by changing and/or down-tilting the antenna, reducing power or limiting access by specific parameters.


PATH BALANCE

Generally, due to high power and gain from the BTS side, downlink power is greater than that reached by uplink.

Such path unbalance impacts an uplink quality, which indicates a greater probability of a higher drop call and lack of setup success.


TRX Quality

• TRX quality counters give an idea of the percentage of samples having quality 0 to 7 in the uplink and downlink directions. Quality scale 0 is the best.

• The more samples are in Q-0 the better is the performance of the cell.

• Low samples in Q-0 to Q-4 indicate an interference or hardware problem on the TRX


DRIVE TESTING

• The quality of the network is ultimately determined by the satisfaction of the users of the network, the subscribers.

• Drive tests give the 'feel' of the designed network as it is experienced in the field.

• The testing process starts with selection of the 'live' region of the network where the tests need to be performed, and the drive testing path.


DRIVE TESTING

• Before starting the tests the engineer should have the appropriate kits that include mobile equipment (usually three mobiles), drive testing software (on a laptop), and a GPS (global positioning system) unit.


DRIVE TESTING

• When the drive testing starts, two mobiles are used to generate calls with a gap of few seconds (usually 15-20 s).

• The third mobile is usually used for testing the coverage. It makes one continuous call, and if this call drops it will attempt another call.

• .


DRIVE TESTING

• The purpose of this testing to collect enough samples at a reasonable speed and in a reasonable time.

• If there are lots of dropped calls, the problem is analysed to find a solution for it and to propose changes


DRIVE TESTING

• After the launch of the network, drive tests are performed periodically. In contrast, the statistics are monitored on the NMS daily with the help of counters.

• The NMS usually measures the functionalities such as call setup failures, dropped calls, and handovers (successes and failures).

• It also gives data related to traffic and blocking in the radio network, apart from giving data related to quality issues such as frequency hopping, PER and BER, field strength, etc.


Optimisation Drive test based


NETWORK PERFORMANCE ASSESSMENT

There are different performance indicators are listed below:

• Amount of traffic and blocking• Resource availability and access• Handovers (same cell/adjacent cell, success

and failure)• Receiver level and quality• Power control.


COVERAGE

• Drive test results will give the penetration level of signals in different regions of the network.

• These results can then be compared with the plans made before the network launch.

• In urban areas, coverage is generally found to be less at the farthest parts of the network, in the areas behind high buildings and inside buildings.


COVERAGE

• These issues become serious when important areas and buildings are not having the desired level of signal even when care has been taken during the network planning phase.

• This leads to an immediate scrutiny of the antenna locations, heights and tilt. The problems are usually sorted out by moving the antenna locations


COVERAGE

• and altering the tilting of the antennas. If optimisation is being done after a long time, new sites can also be added.

• Coverage also becomes critical in rural areas, where the capacity of the cell sites is already low.

• Populated areas and highways usually constitute the regions that should have the desired level of coverage.


COVERAGE

• A factor that may lower the signal level could be propagation conditions, so study of link budget calculations along with the terrain profile becomes a critical part of the rural optimisation.

• For highway coverage, additions of new sites may be one of the solutions.


CAPACITY

• Data collected from the network management system is usually used to assess the capacityof the network.

• As coverage and capacity are interrelated, data collected from drive tests is also used for capacity assessment.

• The two aspects of this assessment are dropped calls and congestion.

• Generally, capacity-related problems arise when the network optimization is taking place after a long period of time.


CAPACITY

• Radio network optimisation also includes providing new capacity to new hot-spots, or enhancing indoor coverage.

• Once the regional/area coverage is planned and executed in the normal planning phase, optimisation should take into consideration the provision of as much coverage as possible to the places that would expect high traffic, such as inside office buildings, inside shopping malls, tunnels, etc.

•


QUALITY

• The quality of the radio network is dependent on its coverage, capacity and frequency allocation.

• Most of the severe problems in a radio network can be attributed to signal interference.

• For uplink quality, BER statistics are used, and for downlink FER statistics are used.


QUALITY

• When interference exists in the network; the source needs to be found.

• The entire frequency plan is checked again to determine whether the source is internal or external.

• The problems may be caused by flaws in the frequency plan, in the configuration plans (e.g. antenna tilts), inaccurate correction factors used in propagation models, etc.


PARAMETER TUNING

• The ending of the assessment process sees the beginning of the complex process of finetuning of parameters.

• The main parameters that are fine-tuned are signalling.parameters,radio resource parameters, handover parameters and power control parameters.


PARAMETER TUNING

• The concepts that are discussed in the radio planning process and the KPI values should be achieved after the process is complete.

• The major complexity of this process is the inhomogeneity of the radio network.

• Network planning will have used standard propagation models and correction factors based on some trial and error methods that may be valid for some parts of the network and invalid for other parts.


PARAMETER TUNING

• However, as the network is inhomogeneous,these default parameters may not be sufficiently accurate in all regions, thereby bringing down the overall network quality - and leading to a reduction in revenue for the network operator.

• Radio network optimization must be a continuous process that begins during the prelaunch phase and continues throughout the existence of the network.


NETWORK OPTIMIZATION TOOLS

Test MS• Test MS is a daily must for engineers to perform

network test. The test MS can display the service cell of a mobile telecommunication network and the six neighbor cells.

• It can also be used to test network parameters. • The test MS can be connected to a computer, so it

can collect and analyze data with the help of drive test software.


Test Mobile station

The functions of a test MS are listed below:

View the IMSI of the SIM card

Scan BCCH• The test MS can scan the BCCH, and it will provides the

Rxlev and BSIC of the scanned BCCH in each cell.View network parameters• The test MS can display network parameters of the service

cell of and the six neighbor cells when it is idle or in conversation state.


Test Mobile station

Forced cell selection• The test MS can be forced to reselect the designated

cell for dialing test and handover test.

Forced handover• The test MS can be forced to hand over to the

designated cell to analyze whether the handover is normal during conversation.

Frequency selection

GPRS function test (The MSs of OT96 support this function)


Drive Test Software

GSM drive test software• ANT• TEMS• SAFCO • AGILIENT in common use.

Generally, the drive test software consists of two parts:

• Foreground data collection software • Background data analysis software.


Drive Test Software

The functions of background data analysis software are listed below:

Radio coverage evaluation and analysis

Interference analysis

Neighbor cell analysis

Handover analysis

Signaling analysis• Speech record and analysis


Types of Optimisation


Thankyou

RF OPTIMISATION IN GSM NETWORKS Regional Telecom Training Centre, Trivandrum : 695 040.

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