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(IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 7, 2011 An Analysis of GSM Handover based On Real Data Isha Thakur Bhavana Jharia Gopal Chandra Manna ME Student, Communication System Associate Professor, Depptt. Of EC Sr. General Manager Engineering Branch Jabalpur Engineering College BSNL, Jabalpur Jabalpur Engineering College, M.P., India M.P, India M.P, India firstname.lastname@example.org email@example.com firstname.lastname@example.org the RxLev and RxQual ,alone, is not sufficient to provide the Abstract—Handover decisions in GSM networks are based on the accurate result for optimum handover solution .So, there is a difference in received signal strength, between the serving cell need of a new handover scheme which not only consider and the neighboring cells. But in a practical scenario, RxQual and RxLev ,but also some other important parameters, particularly in city area ,considering difference in signal level for a better handover process . strength alone , is an inferior criteria to decide handover issue ,because the towers are in close proximity & the absolute signal strength is quite good to continue the communication ,without In the present work ,we focus our attention on much difficulty .Also, in these environments, multipath incorporating some more decision criterion in the handover reflections, scattering due to moving vehicles & diffraction from algorithm .After an extensive study of the GSM measurement multiple building edges ,contributes to poor signal quality, hence reports obtained from a telecom company, it has been forcing the mobile to transmit more power to continue the validated that the transmit power (TxPower) ,aggregate C/I & communication. the FER ,should be given due importance in the handover decision ,along with RxLev and RxQual. The reason behind Continuation of an active call is one of the most incorporating these parameters is explained ahead. important quality measures in the cellular systems. Handover process enables a cellular system to provide such a facility by In the dedicated mode, TxPower consumes the battery transferring an active call from one cell to another. Different power of the mobile handset .Normally the acceptable range approaches have been proposed and applied in order to achieve better handover service, by various researchers. The principal of the TxPower is between 5 and 15, where 5 is the desired parameters considered in the present work, which are used to value. .Hence, TxPower has been proposed to be an important evaluate handover techniques are: Received signal quality parameter in the handover decision process. The FER may (RxQual), FER, Received signal level, MS-BS distance, transmit increase in two cases (1) If the complete frame is lost or power (TxPower) & aggregate C/I. destroyed in transmission and (2) Frame could not be obtained because error correcting code is destroyed .Hence, FER is a In the present work, thorough analysis has been done for considerable parameter in handover decision. Similarly the the received signal strength difference threshold, along with interference level received from all the interference sources in other RF quality parameters. To ensure best performance to all the system should be given due importance in the criterion list mobile users at all times and all locations an active set of for the handover decision .The desired carrier level and the parameters has been calculated for critical values along with signal strength difference threshold. interfering carrier level are calculated and measured in dBm.For convenience, we normally use the C/I ratio to Keywords: Received signal quality (RxQual), FER, Received determine whether an interference case is acceptable or not. signal level (RxLev on uplink and downlink), MS-BS distance, transmit power (TxPower) & aggregate C/I. Since in the real time cellular systems, handover failure may occur due to a number of practical issues, by introducing additional criteria for handover decision making, spurious handover can be avoided to a large extent. Conventional I. INTRODUCTION techniques suffer from inefficiencies caused by the fact that in Traditional handover algorithms are based on relative the practical scenario, particularly in city area, difference in signal strength, relative signal strength with threshold, relative signal level strength has proved to be an inferior criteria to signal strength with hysteresis, relative signal strength with decide handover issue .To overcome these limitations, the hysteresis and threshold  . Handover analysis uses fuzzy authors has proposed an active set of parameters along with logic based prediction techniques also  .Later an their optimum values which can be used to provide better extensive study found that the received signal strength handover decision efficiency. (RxLev) & the received signal quality (RxQual), are the prime parameters in the handover decision. However ,considering 68 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 7, 2011 The rest of the article is organized as follows. Section II III. HANDOVER ANALYSIS reviews related works. Section III gives analysis of the handover algorithm. In Section IV, results are discussed, as Handover initiation criteria analyzed in the present obtained from the model. Finally, conclusions and future paper is based essentially on five variables: the received signal perspectives are discussed in Section V. level (RxLev), received signal quality (RxQual), FER, transmit power (Txpower) & aggregate C/I value. In order to study the effect of the above mentioned variables on the II. LITERATURE REVIEW handover decision, extensive analysis of the GSM measurement data has been carried out .Out of total 21 Several aspects of the analytical handover model have been handovers, few handovers were like transit entry into cell, few investigated in the previous works. An analytic model of were false handover triggers and only in 10 cases, the call handover algorithm has been presented in [5, 6, 7] based on continued for appreciable time i.e. the handover was stable the level crossings of the difference between the signal .The data collected before and after handover were reliable strengths received from two base stations in a log normal and taken into consideration for detailed study. fading environment. The basic model has the route of the mobile chosen to be the straight line between two BSs. Two A. Parameter Evaluation important performance indicators of a handover algorithm are the mean number of handoffs for this route and the delay in We start the analysis by studying the behavior of handing off, both of which need to be minimized. The tradeoff various performance metrics with respect to the distance curve between these two conflicting indicators was drawn in between serving base station & the mobile unit. The line of order to determine the amount of hysteresis and averaging to sight distance is calculated for a number of cells using the be used in the algorithm. distance Haversine formula as under In , the validity of the Poisson model has been Dist_Los= SQRT (POWER (F, 2) +POWER (K, 2) demonstrated for the specific case where the signal strength is Where, stationary. The handover process was studied in terms of F=height of the BTS antenna in meter certain level crossings of the difference between the received K=non-line of sight distance in meter signal strength from two BSs; the model works well where it is most needed, in the range of optimal parameters. This work Where, has been extended in [6, 7] for the no stationary case, in which K= (ACOS (SIN(C)*SIN (H) +COS(C)*COS (H)*COS (J- the level crossings are modeled as Poisson process with time- E)))*6371*1000 varying rate functions. Further, theoretical analysis using level crossings is given in . In , the model was applied to Where, obtain certain criteria for designing practical handoff C=latitude of BTS antenna algorithm, especially for designing algorithms that are robust E=longitude of BTS antenna with respect to variations in the radio propagation H=latitude of mobile station environment. This includes extensions of the model to take J=longitude of mobile station into account the absolute value of the signal strength from the Radius of earth 6371 Km current BS to avoid handoffs when the weaker signal is strong Once the distance values are obtained, the plots between enough, has been shown in . this distance & the respective parameter is plotted. It has been observed that the analysis done in the previous 1) RxLev vs. Distance work on handover ,has been validated by simulation results only .None of the work has been done on the practical data to Ideal plot of the RxLevel verses distance ,will be the one validate their findings .However in the present paper ,the in which the RxLevel value should exhibit a downtrend with effect of several performance metrics (RxLev,RxQual,FER, an increasing distance .One of the plots shown in figure.1 Aggregate C/I &TxPower ) on the handover decision has been ,exhibits this behavior .As the distance between the mobile validated by analyzing the measurement data ,as obtained station & the serving base station increases ,the received from the drive test results in GSM network, from Katni town signal level decreases .The entry to this cell occurred at a of Madhya Pradesh state ,India. distance about 320m, from 320 m to ~340m, the signal strength varies heavily from -60 dam to -77dbm which indicates that the recording are done at shade coupled with heavy transient reflections form neighboring moving vehicles . As the distance is increased, line of sight is available, signal strength was stable and there were gradual fall with distance .The points where the handover situation occurs, are identified by observing the sudden downtrend in the RxLev .Once these 69 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 7, 2011 points are known, the behavior of the RxLevel over these points is recorded for observation. 3) FER vs. Distance In the cellular communication, not only the continuation of the call is necessary but also quality of speech is an essential parameter in analyzing the performance of handover algorithms. The Frame Error Rate (FER) measurement is used by the mobile to detect bad frames. The mobile starts the substitution and muting process, and within 300 ms of bad frame reception it completely mutes the speech. Out of 104, FER measurements are done over 100 frames, which correspond to ~2 s of speech. The variation of %FER with the distance between the serving BTS & the MS is as shown in figure 3.In contrast to observations in case of RxLevel and RxQual ,FER shows better performance at the near region and even at the far region with exception in the middle. This shows strong immunity of GSM system from frame errors. But, the overall trend was in upward direction indicating contribution of this parameter for handover decision. Figure 1. RxLev vs. Distance 2) RxQual vs. Distance RxQual is a value between 0 and 7, where each value corresponds to an estimated number of bit errors in a number of bursts. Each RxQual value corresponds to the estimated bit- error rate according, which varies from BER <0.2% for RxQual 0, 0.8 %< BER<1.6% for RxQual 3 and BER >12.8% for RxQual 7. The RxQual value showing an increase contributes to the handover decision making .The variation of this parameter with the distance ,one of which is shown as under in Figure.2 .At the entry into the cell, RxQual had wide variation which shows presence of strong interference .Slowly this situation improves as the vehicle goes slightly away where a dominant part in line of sight signal .At the end ,few observations show Figure 3. FER vs. Distance RxQual>4 which indicated requirement of handover .Overall positive slope indicated healthy situation for handover prediction . 4) Aggregate C/I vs. Distance The aggregate carrier-to-Interference (C/I) ratio is the ratio, expressed in dB, between a desired carrier (C) and an interfering carrier (I) received by the same receiver. The variation of the aggregate C/I with the distance (Figure .4),better than 15dBm in most cases shows that it has only a minor effect on the handover decision, but considered here as it has positive slope. Figure 2. RxQual vs. Distance 70 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 7, 2011 value – initial value) are taken into account . The distance of the mobile unit from the serving station & the target station is also examined .This approach is adopted to identify the role of each parameter in the handover, more clearly & accurately. The handover cases which exhibit the near ideal situation are identified & are taken into consideration for deriving the optimized handover situation. Calculations: The handover position is identified in the excel sheet of the drive test report ,by looking at the „event type‟ column .After selecting the location ,a set of about 50 observations ,before & after the handover event are considered for evaluating the average values of the before & after values of each parameter .A sample sheet is shown in APPENDIX to demonstrate method Figure 4. Aggregate C/I vs. Distance of calculation has been done (sheet 1).The transition (handover) had taken place from cell id 509 to 619 at the position highlighted in the sheet .Before values pertains to 5) TxPower vs. Distance values (and average thereafter) before transition in old cell and after values after transition to new cell .This method is carried Transmit power plays a very important role in sustaining out for each parameter. The calculations & the respective higher battery life of the mobile handset .In the dedicated sheets are obtained by performing the calculations in the mode, TxPower is monitored constantly by the serving station similar manner .The delta (∆) values of the five parameters are .Normally the acceptable range of the TxPower is between 5 obtained by performing the subtraction of the final & initial & 15, where 5 is the desired value..Higher TxPower is values respectively. The two other distance calculations (from unacceptable not only because it consumes the battery power serving cell to target cell & from serving cell to mobile of the mobile, but also because it may adversely affect the station) is performed by the distance haversine formula as mobile user‟s health. The variation of the TxPower with the mentioned in previous section. distance is given in figure 5. C. Optimization The optimum situation for handover is identified by comparative analysis. The comparison of the parameter values at the time of handover is done with respect to the recommended range of values & the ideal values respectively .The tabulated form of the values obtained is given in sheet2. It has been found that the handover situation in the 5, 8 & 9 th cell case is exhibiting „near ideal‟ situation. These 3 cases are then scrutinized to obtain the optimum condition for handover. Comparisons: (a)RxLev and ∆RxLev: The 4.3dbm increase in the RxLev after the handover has taken place, averaged for all 3 cells, is a sure sign of a successful handover. Figure 5. Txpower vs. Distance (b)RxQual and ∆RxLev: The performance of the RxQual value ~=1.46 is most appropriate in the cell after averaging. (c)FER & ∆FER: The optimum performance of the FER is B. Handover Cell Analysis fulfilled by the cell of the serial number 8. Once the relationship between the distance & the respective (d)TxPower: The cell of the serial number 5 is exhibiting the parameters is drawn, the analysis of handover cells is studied best case of the TxPower based decision criteria. exclusively. The cells in which the handover has occurred are first identified & then the parameter values are studied for (e)Aggregate C/I: The aggregate C/I criteria is fulfilled by the each cell separately .Not only the absolute values of the cell of the serial number 5. various parameters is studied but also the relative values (final 71 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 7, 2011 IV. RESULT & DISCUSSION REFERENCES We have validated the role of various parameters on the handover decision making in this paper. It has been found that  Gregory P. Pollioni, “Trends in Handover Design”, IEEE the behavior of the respective parameters & the role of each Communications Magazine, vol. 34, March 1996, pp. 82-90 chosen parameter on the handover decision making, is satisfying the ideal cases to a close extent .Also the  P. Marichamy, S. Chakrabati and S. L. Maskara, optimization has contributed to obtain a set of values of the “Overview of handoff schemes in cellular mobile networks respective parameters, which serve as the best case to decide and their comparative performance evaluation”, IEEE the handover ,as given table1 . VTC‟99, vol. 3, 1999, pp. 1486-1490 TABLE I RESULT  M. Chiu, M. Bassiouni, “Predictive Schemes for HandoN Prioritization in Cellular Networks Based an Mobile Positioning”, EEE Joumal on selected amas in RxLev RxQual FER Tx Power(dBm) communications, Vol. 18, No. 3, March 2000 (dBm) Aggregate C/I 2 -72 5 15 15.82  M.S.Dang, A Prakash, D.K. Anvekar, D. Kapoor, ∆RxLev ∆Tx ∆Aggregate R.Shorey, "Fuzzy Logic Based Handoff in Wireless ∆RxQual ∆FER Networks", in Proceedings of the 51st Vehicular Technology (dBm) Power(dBm) C/I -0.3794 Conference (VTC 2000 Spring), Tokyo, 4.3 -1.5 -6.537 1.1066 15-18 May 2000, pp.2375-2379  R. Vijayan and J. M. Holtzman, “The dynamic behavior of V. CONCLUSION handoff algorithms,” in Proc. Ist Internat. Con$ UniversG 1 Personal Commun., Dallas, TX, Sept. 1992. The aim of this investigation was first to define some appropriate performance measures for inter-cell handovers.  “Analysis of handoff algorithm using nonstationary signal The obtained results showed the outperformance of handover strength measurements,” in Proc. GLOBECOM ‟92, Orlando, algorithm based on multiple parameters (i.e. RSS, BER etc). In FL., Dec. 1992. this paper, we have extended the model for analyzing the performance of the handoff algorithm based on signal strength  “A model for analyzing handoff algorithms,” IEEE measurements. This model enables us to achieve good Trans. Veh. Technol., vol. 42, no. 3, Aug. 1993. analytical approximations easily and fast. Therefore, this model can be used by the network designer to help optimize  R. Vijayan and J. M. Holtzman, “Foundations for level the behavior of the handover strategy by setting appropriate crossing analysis of handoff algorithm,” in Proc. ICC ‟93, hysteresis, absolute threshold, and other parameters such as Geneia, Switzerland, May 1993. the averaging length for different propagation environments. Handover condition for at least 3 of the 5 parameters should  R. Vijayan and J. M. Holtzman, “Sensitivity of handoff be met to take handover decision while 4 conditions meeting algorithms to variations in the Drooagation environment.” in will be sufficient. Prcc 2nd Internat. Conf I s -Universal Personal Commun., As a future course of work, more importance can be Ottawa, Canada, Oct. 1993. given to the QoS issues where in more number of radio and network parameters are taken into consideration for averaging  R. Beck, F. W. Ho, “Evaluation and performance of field the threshold values. This ensures that a handover can be strength relatedhandover strategies for micro-cellular systems,” hastened or delayed as the situation requires and also prevent in Proc. 3rd Nordic Sem. Digital Land Mobile Radio Commun., unnecessary handover that may take place due to momentary Copenhagen, Denmark, 1988. fading of any one of the parameter. Hastening the handover ensures that a call is not dropped due to non availability of resources. Handover delayed ensures that unnecessary handover does not take place leading to loading of the base station. 72 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 7, 2011 AUTHORS PROFILE Isha Thakur received her B.E. (Hons.) From 1997 to 2002, Dr. Manna has worked as Deputy General degree in Electronics and Manager in a Telecommunication Training Centre of DoT. He was Telecommunication Engineering from first to install live training node for Internet Service Provider (ISP), Takshshila Institute of Engineering and designed training schedules and prepared handbook and lab practice Technology Jabalpur (M. P.) in 2008. schedules. He had conducted training programs for 5 batches of Currently, she is pursuing her M. E. participants deputed by Asia Pacific Telecomm unity (APT) and 3 from the Department of Electronics and more exclusive batches for Sri Lankan Telecom. He had also Telecommunication Engineering, conducted several seminars with international experts through Government Engineering College UNDP/ITU projects. In 2000, he had delivered distinguished speech Jabalpur. Her research interest includes on ADSL in a seminar organized by ITU. During 1995 and 1996, Dr. Computer networks and Future Manna was posted in Telecommunication Engineering Centre (TEC) generation in mobile communication System. and developed Artificial Intelligence (AI) based software for E10B telephone exchanges named E10B Maintenance Advisor (E10BMAD). Dr. Manna had worked as Development Officer in WEBEL Bhavana Jharia received her B.E. (erstwhile PHILLIPS) Telecommunication Industries during 1983- degree in Electronics and 1984 after which he joined DoT and worked in different executive Telecommunication Engineering from capacities up to 1994.He was awarded National Scholarship in 1973 Govt. Engineering College Jabalpur (M. based on school level examination and silver medal for performance P.) in 1987. She did her M.E. (Solid in college. He had both graduated and post graduated in Radio State Electronics) from University of Physics and Electronics Engineering from University of Calcutta and Roorkee, Roorkee in 1998 and Ph.D. undergone trainings at Beijing University of Post and Telecom China (VLSI Technology) from I.I.T. Roorkee in 1990 and DARTEC, Montreal, Canada in 1999. in 2005. She joined the Department of Electronics and Telecommunication Engineering, Govt. Engineering College Jabalpur (M. P.) as faculty in 1990, where at present she is working as an Associate Professor. She has 25 publications in National, International referred Journals and Conferences. Her research interests are in Electronics Design and Simulation and Low Power VLSI Technology. She is a member of IE (I), CSI, VLSI Society of India, senior member of IACSIT and Life Member of ISTE. Dr. Gopal Chandra Manna is working as Senior General Manager (Head Quarters), Inspection Circle, BSNL, a wholly owned Company under Department of Telecommunications (DoT), Govt. of India. Dr. Manna has carried out extensive research on coverage issues of GSM, CDMA, WCDMA and WiMAX radio access. Study of Wireless Traffic and QoS estimation of Cognitive Radio are his current areas of research. In Addition, he has written several articles on advanced telecommunications which has been published in national and international journals and symposiums. Dr. Manna is regularly invited as a panel expert, invited speaker, session chair etc. in seminars and conferences. Dr. Manna has developed and conducted one week course on Quality of Service Monitoring at Information and Communication Technologies Authority, Mauritius as International Expert through Commonwealth Telecom Organisation London during August 2010. He had also delivered a speech on WiMAX coverage Evaluation at International Conference on Advanced Communications Technology 2011 at Seoul, Korea and chaired a session on Network Management. He had also delivered speech on ADSL at International Telecommunication Union seminar in 2000 at Bangalore, India. 73 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 7, 2011 APPENDIX  SHEET 1 SHEET2 74 http://sites.google.com/site/ijcsis/ ISSN 1947-5500
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