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Radio Access Network Audit & Optimization in GSM (Radio Access Network Quality Improvement Techniques) Mudassar Ali, Asim Shehzad, Dr. M.Adeel Akram.

Abstract— Network audit is necessary to judge the network performance and maintain QOS standards. The network audit identifies inconsistencies or limitations in current overall network design, helps to improve processes resulting in optimized network and improved quality of service. Radio Access Network audit includes many aspects of network like performance, neighbor, parameter, frequency, competitive benchmark audits. In this paper, real GSM Radio Access network evaluated, and different issues, findings, trials and improvements have been summarized and observations/recommendations have been listed to correlate the practical aspects of RF optimization, which affect the performance, and QoS of an operational cellular network.

Index term— RAN, BSC, MSC, BTS, CSSR, CDR, HSR, TCH, COVERAGE, QUALITY, KPI, DT and QoS.

I. INTRODUCTION The mobile communication aims to offer anytime and anywhere communications between any objects. GSM , One of the fastest growing and most demanding of all telecommunications technologies .

GSM Network usually called as „cellular network‟ (as the whole coverage area is divided into different cells and sectors) is comprised of a mobile Station (MS) which is connected to the Base Transceiver Station (BTS) via air interface. In addition to other hardware, BTS contains the equipment called Transceiver (TRX), which is responsible for the transmission and reception of several radio frequency (RF) signals to/from the end user .BTS is then connected to the base station controller (BSC) via abis interface. BSC usually handles radio resource management and handovers of the calls from one BTS (or cell/sector) to the other BTS (or cell/sector) equipped in it. BSC is then connected to Mobile Switching Centre (MSC).

This paper focuses on audit of radio access part of GSM network, suggestions to optimize the network and post optimization benchmarks. Following is sequence of Objectives.

1. Network Audit and Recommendations 2. Optimization Plan and Cluster optimization 3. Post Optimization Evaluations (Benchmark Improvements) II. RADIO ACCESS NETWORK AUDIT The ultimate purpose of the audit is to establish a baseline that measures overall network design, quality, performance and process, and to identify and characterize the areas where improvement can be achieved. The audit is usually a

comparative process and requires an initial baseline of KPI‟s and/or objectives. These can be derived from the design guidelines, service requirements, customer expectation, market benchmarks and others. The comparative baseline and/or the KPI‟s can be re-adjusted during the audit itself to improve its diagnostic capabilities.

Benefits of Network Audit.

Key benefits of network audit are:  Identify actions to improve network quality in problem areas.  Discover inconsistencies or limitations in current overall network design.  Discover practices or processes that can be improved or optimized  Results in improved Network quality , thereby reduced churn  Optimize overall network design:  Reduction of excess network elements  Postponement future CAPEX, leading to CAPEX savings  It takes only a delay in the CAPEX for few BTS‟s to justify the network audit investment  Streamline processes:  OPEX savings Network Audit Steps The audit has 3 distinct steps: 1. Information gathering (The Diagnosis) 2. Information analysis and results (The Prognosis) 3. Recommendations (The Cure) III. INFORMATION GATHERING (DATA COLLECTION) Following data will be collected for radio network audit 1. BSS Network Design and Performance Data.  All Site Data : sites position, antenna types, height, tilt electric and Mechanical) and azimuth, sector power, number of sectors, Number of TRXs per sector  Macro Site data and Micro Layer data  BSC and transcoder designs  Coverage data  Neighbor List  Handover Parameter  Frequency Plan and Interference Analysis  OMC Statistics.  Drive Test Statistics.

2. Major Complaints from Customer Care.

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IV.

Revenue Churn report from Revenue assurance department

INFORMATION ANALYSIS-RADIO NETWORK AUDIT FLOW CHART Algorithm for audit methodology used for test case of network of city K is given in Fig.1

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The paging success rate measures the percentage of how many paging attempts that have been answered, either as a result of the first or the second repeated page.

PSR 

Time _ of _ Paging _ Re sponses Time _ of _ Paging

Possible reasons for poor Paging Performance could be:  Paging congestion in MSC  Paging congestion in BSC  Paging congestion in Base Transceiver Station (BTS)  Poor paging strategy  Poor parameter setting  Poor coverage  High interference 2. SDCCH Access Success Rate.

SDCCH access success rate is a percentage of all SDCCH accesses received in the BSC. Possible reasons for poor SDCCH Access Performance could be:  Too High Timing Advance (MHT)  Access Burst from another Co-channel, Co-BSIC Cell  Congestion  False Accesses due to High Noise Floor  Unknown Access Cause Code 3. SDCCH Drop Rate The SDCCH DROP RATE statistic compares the total number of RF losses (while using an SDCCH), as a percentage of the total number of call attempts for SDCCH channels. This statistic is intended to give an indication of how good the cell/system is at preserving calls.

SDCCH _ Drop _ Rate 

SDCCH _ Drops SDCCH _ Seizures

Possible reasons for SDCCH RF Loss Rate could be: Fig. 1. Radio Network Audit Flow Chart

V. RADIO NETWORK PERFORMANCE AUDIT All the events being occurred over air interface are triggering different counters in the Base Station Controller (BSC). The KPIs are derived with the help of these counters using different formulations.

A. Accessibility Service accessibility is : “The ability of a service to be obtained, within specified tolerances and other given conditions, when requested by the user.” In other words: Total _ NO _ of _ Successful l _ Calls _ Setup Acessibility  Total _ Calls _ Accesses _ to _ Network Listed below are the KPIs connected to accessibility.

1. Paging Success Rate

   

Low Signal Strength on Down or Uplink Poor Quality on Down or Uplink Too High Timing Advance Congestion on TCH

4. Call Setup Success Rate

The Call Setup success rate measures successful TCH Assignments of total number of TCH assignment attempts.





CSSR  1  SDCCH _ Congestion _ Rate * TCH _ Assignment _ Success _ Rate SDCCH_Overflows   1   * 1  TCH_Congestion_Rate  * 1  TCH_Assignment_failureRate  * 100  SDCCH_Call_Attempts  Reasons for low call setup success rate could be:  TCH congestion  Interference CSSR 

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Poor coverage Faulty HW units

5. Call Setup TCH Congestion Rate The Call Setup TCH Congestion Rate statistic provides the percentage of attempts to allocate a TCH call setup that were blocked in a cell.

Call _ Setup _ TCH _ Congestion_ Rate 

No _ of _ TCH _ Blocks( Excluding _ HO ) No _ of _ TCH _ Attempts

Possible reasons for call setup block could be:  Increasing Traffic Demand  Bad Dimensioning  HW Fault & Installation Fault  High Antenna Position  High Mean Holding Time (MHT)  Low Handover Activity  Congestion in Surrounding Cells B. Retain ability Service retain ability is “The ability of a service, once obtained, to continue to be provided under given conditions for a requested duration.” In other words:

Re tainabilit y 

Total _ Calls _ Completed Total _ Successful _ calls _ setup

Listed below are the KPIs connected to retain ability.

1. Call Drop Rate This KPI gives rate of drop call. Percent of TCH dropped after TCH assignment complete.

       

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Congestion Link Connection or HW Failure Bad Antenna Installation The MS Measures Signal Strength of another Co-orAdjacent Cell than Presumed Incorrect Handover Relations Incorrect Locating Parameter Setting Bad Radio Coverage High Interference, Co-channel or Adjacent

VI. NEIGHBOR, PARAMETERS AND FREQUENCY AUDIT. It is important for the neighbor list to be updated and optimized as it directly affects handover decisions in a mobile network. Wrong or missing neighbor relations may cause an on-going connection to drop or be handed over to the wrong neighbor cell. Excessive neighbor relations in a cell, on the other hand, may cause wrong handover decision on the part of the BSS because of inaccurate measurements.

Neighbor relations and parameters are cross checked to identify and remove  One way neighbors  Illogical neighbor Relations Methods used to assign frequencies in the network are reviewed and the steps are taken to control interference and maximize network capacity through frequency planning.

BCCH/BSIC reuse is crosschecked. MA (Moblile Allocation) List is cross verified.

VII. COMPETITIVE BENCHMARK AUDIT In competitive benchmarking, coverage and quality comparison of network is performed with competitors.

Below is coverage and quality comparison for two operators for a city, Results are taken Drive test tool, TEMS.

Total _ TCH _ Drops CDR  TCH _ Normal _ Assignment _ successes  inco min g _ DR  Inco min g _ HO _ Successes  Outgoing _ HO _ successes

*DR is directed retry Possible reasons for TCH Drop Call Rate could be:           

Low Signal Strength on Down or Uplink Lack of Best Server Congestion in neighboring cells Battery Flaw Poor Quality on Down or Uplink Too High Timing Advance Antenna problems Low BTS Output Power Missing Neighboring Cell Definitions Unsuccessful Outgoing Handover Unsuccessful Incoming Handover

Coverage Comparison : As visible from Plots mentioned in Fig.2 and Fig .3 operator 1 have better coverage than operator 2. which is also visible in KPIs.

2. Handover Success Rate The handover success rate shows the percentage of successful handovers of all handover attempts. A handover attempt is when a handover command is sent to the mobile.

HOSR Possible reasons for poor handover success rate could be: Fig. 2 . Coverage Plot for Operator 1 For City K :

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Fig. 5. Quality Plots for Operator 2 for city K

Legend For Quality Plots is as follows : Fig. 3. Coverage Plot for Operator 2 for City K

Legend For Coverage is given below .

Formula for Quality: ( RX _ Qual0  RX _ Qual1  RX _ Qual2  RX _ Qual3)  ( RX _ Qual4  RX _ Qual5  RX _ Qual6 )  .07 Re ceive _ Quality  Total _ Samples

Results for receive quality for operator 1 and operator 2 taken for city K.

KPI

Formula For Coverage Rate: Covergae_ Rate 

No _ of _ samples _ having _ Signal _ strength  90dbm * 100

Operator 1

Rx QUALITY

88.19%

Operator 2 87.18%

Total _ Samples

Results for Coverage rate for operator 1 and operator 2 taken for city K are given below

KPI

Operator 1 Operator 2

COVERAGE RATE

[1] [2] [3]

94.64%

72.77%

[4]

VIII. REFERENCES Halonen T., Romero J., Melero J.: GSM, GPRS and EDGE Performance. John Wiley & Sons Ltd, 2003.

ITU-T recommendation G.1000 (2001), Communication quality of Service: A framework and definition.

Jens Zander. „Radio Resource Management for Wireless Networks‟. Artech House Inc., 2001 [4] Bilal Haider,M.Zafarrullah Khan, M.K.Islam: Radio Frequency Optimization and QOS in operational GSM network.

Quality Comparison: As visible from plots mentioned in Fig 4 and Fig 5 , Operator 1 has better quality than operator 1 , which is also visible in KPIs

Fig. 4. Quality Plots for Operator 1 For City K.

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