International Journal of Electrical Engineering & Technology (IJEET) Volume 6, Issue 8, Sep-Oct, 2015, pp.53-62, Article ID: IJEET_06_08_006 Available online at http://www.iaeme.com/ijeetissues.asp?jtype=ijeet&vtype=6&itype=8 ISSN Print: 0976-6545 and ISSN Online: 0976-6553 IAEME Publication RF OPTIMIZATION FOR QUALITY IMPROVEMENT IN GSM NETWORK Dheepak. M Asst. Professor-EEE, AMET University, Chennai Dr. S. V. Saravanan HOD-EEE, AMET University, Chennai ABSTRACT Quality Of Service and Performance of GSM Network can be determined from Key performance Indicators (KPIs) such as Call setup success rate (CSSR), Call Drop Rate (CDR), Handoff Success rate (HSR), Traffic Channel (TCH) Congestion rate, SDCCH and TCH Blocking and Radio Resource Utilization etc.hsr is an important parameter in key performance indicators (KPI) of the wireless network. In this paper we have discussed regarding handoff fundamentals, types, procedure and analysis which can improve the quality of services. In addition the RF drive test simulation result is attached which can clearly show the handoff success rate contributing to the quality of network after during suitable changes. Key word: Handoff Success rate (HSR), Quality of Service (QOS). Cite this Article: Dheepak. M and Dr. S. V. Saravanan, RF Optimization for Quality Improvement in GSM Network. International Journal of Electrical Engineering & Technology, 6(8), 2015, pp. 53-62. http://www.iaeme.com/ijeet/issues.asp?jtype=ijeet&vtype=6&itype=8 1. INTRODUCTION Handover refers to a process of transferring an ongoing call or data session from one channel connected to the core network to another. The quality of service of wireless network can be improved by improving handoff success rate (HSR). If the handover performance is bad then the subscriber will perceive the quality of the network as bad. The major reason of bad handover performance is due to following factors. If the neighbours are not properly defined or missing. Unnecessary handover occurs. During congestion in the network. If there in interference in Co-channel or adjacent channel. Coverage gap exist in the network. http://www.iaeme.com/ijeet/index.asp 53 editor@iaeme.com
Dheepak. M and Dr. S. V. Saravanan If radio resource utilization is high. A main reason for handover in cellular networks is to overcome call drops due to handoff failure. If the mobile device moves out of the range of one cell (base station) and a different base station can provide it with a stronger signal. If all channels of one base station are busy then a nearby base station can provide service to the device. Handover process is an important one in any cellular network, must be completed efficiently and without inconvenience to the user. 2. GSM NETWORK OVERVIEW The GSM network is divided into various subsystems. Each of these systems is comprised of a number of functional units which are individual components of the mobile network. Mobile Station (MS) It consists of Mobile Equipment (ME) and Subscriber identity module (SIM). Base Station Subsystem It consists of Base transceiver station (BTS), Base station controller (BSC) Network Switching Subsystem (NSS): It consists of Mobile switching center (MSC), Home location register (HLR), Visitor location register (VLR), Authentication centre (AuC) and Equipment Identity register (EIR). Public Switched Telephone Network (PSTN): It is owned and deployed by mobile phone operators and allows mobile devices to communicate with each other and telephones in the wider Public Switched Telephone Network (PSTN) used for processing and switching. Figure 1 GSM Cellular Network Architecture Home Location Register (HLR) and Visitor Location Register (VLR) maintains and manages the home and visiting subscriber datas,it is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations. http://www.iaeme.com/ijeet/index.asp 54 editor@iaeme.com
RF Optimization For Quality Improvement In GSM Network The NSS originally consisted of the circuit-switched core network, used for traditional GSM services such as voice calls, SMS, and circuit switched data calls. It was extended with overlay architecture to provide packet-switched data services known as the GPRS core network. This allows mobile phones to have access to services such as WAP, MMS, and the Internet. Base station subsystem (BSS) Base station subsystem (BSS) consists of Base Transceiver station (BTS) and Base station controller (BSC). BSS manages the radio link between MS and CN. This access network provides access to both Circuit switch (CS) and Packet switched (PS) core networks. It is the section of a cellular network which is responsible for handling traffic and signaling between a mobile station and network switching subsystem. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile station, paging, transmission and reception of information in form of voice and data over the air interface. The Network Switching Sub-system (NSS) comprising an MSC and associated registers. Several interfaces are defined between different parts of the system. 'A' interface between MSC and BSC 'Abis' interface between BSC and BTS 'Um' air interface between the BTS (antenna) and the MS 3. KEY PERFORMANCE INDICATORS Key Performance Indicators (KPI) is a measure of successful network performance and its quality. With growing customer base and continuous addition of capacity and coverage sites, operators need to continuously monitor the KPIs of their network to assess service quality. To improve the performance of a network detection of problem and symptoms in early stage is very important and it s far too late when a customer complaint arises. 3.1 Performance Evaluation The most reliable KPIs to evaluate the network performance with NMS are: SDCCH and TCH Block Blocking percentage [%] CDR (Call Drop Rate). HSR (Handover Success Rate). CSSR (Call Set up Success Rate). TCH (Traffic Channel) Congestion Rate. Average quality DL and UL 3.2 Quality of Service Objective Healthy Network provides Healthy Subscribers and Healthy Business Quality of Service (QOS) targets as per the Operators specifications: Efficient network utilization Coverage of Service Area: Urban, Suburban, Roads or the Rural areas Call Success Rate Minimizing Handover failures Rx quality http://www.iaeme.com/ijeet/index.asp 55 editor@iaeme.com
Dheepak. M and Dr. S. V. Saravanan Improve Speech Quality Index for higher Network quality Satisfied customers resulting in low churn Reduced investment costs and increased revenue for the customers 4. TYPES OF HANDOVER Two main types of handover Hard hand over Soft hand over Figure 2 Hard and Soft handover in a wireless network 4.1 Hard Handover Hard handover: break before make connection Existing radio link must be dropped for a small period of time, and then taken over by another base station. A call in progress redirected not only from a base station to another base station but also from its current transmit receive frequency pair to another frequency pair. An ongoing call can not exchange data or voice for this duration 4.2 Soft Handover Soft handover: make-before-break connection. Mobile station at the boundary of two adjacent cells does not suffer call drops due to handover in the boundary region Gives seamless connectivity to a Mobile station An offset to pseudo noise code method of soft handover Soft handover does not require http://www.iaeme.com/ijeet/index.asp 56 editor@iaeme.com
RF Optimization For Quality Improvement In GSM Network breaking of the radio link for cell-to-cell transfer of a call. A mobile device can be simultaneously connected to several base stations 4.3 Frequency related Hand Over In network following handover can occur: 4.3.1 Intra-frequency hard handover Due to interference of certain frequencies, the signal quality poor. The BSC can handover the call to another Frequency of the cell in such cases 4.3.2 Inter-frequency hard handover When the signal strength goes weak due to several reasons (for example, the mobile moving away from the cell in which it is presently localized to the boundary region of another cell), there is handover from a cell to another 4.4 Call drop in hard handover Figure 3 Handoff success and Handoff failure When an MS moves from one sector/cell to another, it breaks connection of original sector/cell and makes new connection with the target sector/cell. In wireless communications, during HO process, there is a high probability of call drop, which directly affects the system performance (90% of radio call drop occurs during handoff). Proper handoff algorithm can reduce system call drop and increase network capacity. Break in call transmission Handover takes place in a few ms (at best in 60 ms) Interruption is hardly discernible by the user Handover to another cell is required when the signal strength is low and error rate is high. GSM systems perform hard handovers In network, one user needs to switch within Same frequency Different frequency Soft handoff is possible only within the same frequency i.e. intra-frequency handoff. Hard handoff is required for inter-frequency handoffs Softer handoff is possible only within the same frequency and BTS. Not all the intra-frequency handoffs are soft handoff. http://www.iaeme.com/ijeet/index.asp 57 editor@iaeme.com
Dheepak. M and Dr. S. V. Saravanan Figure 4 Connect to Base Station 1(BS1) and start conversation. Figure 5 Moving out of Base Station 1 s coverage connect to Base Station 2(BS2). Figure 6 Moving out of BS2 s coverage-connect to Base station 3(BS3). http://www.iaeme.com/ijeet/index.asp 58 editor@iaeme.com
RF Optimization For Quality Improvement In GSM Network Figure 7 Conversation ended Still within BS3 s Coverage Figure 8 Handover process in GSM Network The procedure for analyzing the handoff Check the successful handover per cell Check the handover activity from the number of handover performed Check the site location, whether the site to site distance or co-sited are properly located or not. Check whether the cell is isolated or not Check the handover flow whether is balance between incoming and outgoing. Check if many Ping-Pong handover. Check if assignment handover are used. Check if cell has Hardware problem. If problem found, swap or repair hardware. Check if congested target cell. If the target cell is congested then solve the congestion by expanding the capacity of the cell. Perform measures to improve HO performance. This is the assumption comes from the above analysis. http://www.iaeme.com/ijeet/index.asp 59 editor@iaeme.com
Dheepak. M and Dr. S. V. Saravanan 5. NETWORK OPTIMIZATION PROCESS 5.1. Young Network Case In a young network the primary target is normally the coverage In this phase usually there is a massive use of drive test measurement check the signal and the performance of the competitors 5.2 Mature Network Case In a mature network the primary targets are quality indicators drop call rate, average quality, handover failures Important to use the information from Network Management System (NMS) a general view of the network performance Drive test measurements are still used but not in a massive way in areas where new sites are on air where interference and similar problems are pointed out by NMS data analysis 6. DRIVE TEST ANALYSIS 6.1 Network Data Collection & Analysis Poor Performance Sites and regions are identified based on the NMS data collection and Analysis and Network indicating parameters are monitored. 6.2 Drive Test Drive Test is conducted for checking coverage criteria of a cell site with RF drive test tool. The data collected by drive test tool as Log files is analyzed to evaluate various RF parameters of the network. Drive Route and Site Data for the sites to be driven is taken from customer Drive Test LOG File of the specified Drive Route / Site is saved and the same will be handed over for analysis at the end of the day Figure 9 Pre drive test status before correcting Handoff failure showing poor signal strength and high call drop. http://www.iaeme.com/ijeet/index.asp 60 editor@iaeme.com
RF Optimization For Quality Improvement In GSM Network Figure 10 Post drive test status after correcting Handoff failure showing good signal strength and high call drop. RECOMMENDATIONS FOR IMPROVEMENT OF QOS Defining missing neighbor relations Proposing antenna azimuth changes Proposing antenna tilt changes Re tuning of interfered frequencies Proposing antenna type changes RF Parameters should be monitored and updated on periodic basis Adjusting Handover Margins Re-tuning of interfered frequencies Avoid voltage standing wave ratio (VSWR) Analysis of Pre and Post drive test results Identify Coverage Holes and carryout optimization or new site proposal Physical Audit and Optimization Avoid Site Outage and Maintain Transmission Hardwares Good Condition Identifying vital locations and understanding the geographical terrain. Minimize the call drop and RF interference in the network Increase the indoor & outdoor coverage and better speech quality Daily KPI Report analysis 7. CONCLUSION In this paper, we have recommended some of the techniques which can improve the handoff success rate and reduce call drop which plays a vital role in maintaining the quality of services (QoS) of GSM cellular network. These recommendations might be helpful for telecom engineers during there operation and maintenance activity. A network Operator can maintain a Healthy competition and Business Model only when he fulfills customer s requirements and reliable network in maintained. http://www.iaeme.com/ijeet/index.asp 61 editor@iaeme.com
Dheepak. M and Dr. S. V. Saravanan REFERENCES [1] Sharma.A, Konai.S, Bhattacharya - New Call and handoff call management scheme for reuse partitioning based cellular systems, IEEE, 9-11 May 2014, Pg 1-7. [2] S. Kyriazakos, G. Karetsos, E. Gkroustiotis, C. Kechagias, P.Fournogerakis Congestion Study and Resource Management in Cellular Networks of present and Future Generation, IST Mobile Summit 2001, Barcelona, Spain, 9-12 September 2001. [3] Kechagias, S.Papaoulakis, N.Nikitopoulos, D. Karambalis: A Comprehensive Study on Performance Evaluation of Operational GSM and GPRS Systems under Varying Traffic Conditions, Oct 2002,pg 1-5. [4] QINGAN ZENG and DHARMA P. AGRAWAL 'Handoff in Wireless Mobile Networks' Department of Electrical Engineering and Computer Science, University of Cincinnati, pg 1-12. [5] Nasif Ekiz, Tara Salih, Sibel and Kemal Fidanboylu "An Overview of handover Techniques in Cellular Networks"-, International Journal of Information Technology, Volume 2 Number 3 2005 ISSN 1305-239x. http://www.iaeme.com/ijeet/index.asp 62 editor@iaeme.com