A Handover Scheme for Mobile WiMAX Using Signal Strength and Distance

Transcription

1 A Handover Scheme for Mobile WiMAX Uing Signal Strength and Ditance Mary Alatie, Mjumo Mzyece and Anih Kurien Department of Electrical Engineering/French South African Intitute of Technology (F SATI) Thwane Univerity of Technology, Private Bag X68, Pretoria 1, South Africa Tel: , Fax: {MzyeceM, Abtract- The addition to the WiMAX tandard through 82.16e ha made the tandard more flexible and a potential choice a a wirele broadband technology. To upport mobility in any wirele cheme, the application of an efficient handover control cheme i crucial. Any inefficiency experienced during data tranmiion in hard handover in mobile network could caue handover delay with potential connection drop. In order to olve the problem of handover delay, variou type of handover cheme have been adopted. Thi paper propoe the combining of the Relative Signal Strength with Threhold and Hyterei and Ditance (RSTH-D) to initiate handover. Ditance i conidered a a criterion to initiate fat handover. Thi cheme eek to reduce handover delay, which contribute to effective radio reource management. It i hown through the imulation reult obtained that the propoed cheme ha ignificant potential performance benefit compared to tandard handover cheme. implement, but caue HO delay and ervice interruption. Thi ha a negative impact on many application, epecially delay-enitive application. SHO i characteried by make before break. In thi approach, new reource are allocated before releaing the old reource. SHO can be ubdivided into two type: Macro Diverity HO (MDHO) and Fat Bae Station Switching (FBSS). For MDHO and FBSS, the i connected to multiple BS, collectively termed a the Diverity Set (DS). Thi i hown in Figure 2 and 3. For FBSS, the communicate with only one BS which i called the Anchor BS while for MDHO, the communicate with two or more BS in the DS. Continuou canning and updating procee demand more bandwidth and ubequently wate radio reource [3]. WiMAX Backbone Index Term Mobile WiMAX, handover, ignal trength I. INTRODUCTION Mobile WiMAX (Worldwide Interoperability for Microwave Acce) i a promiing emerging broadband wirele network tandard. The IEEE82.16e tandard wa developed from the previou IEEE releae of WiMAX [1, 2]. Mobile WiMAX uer have the characteritic of mobility which allow uer to move anywhere at any time and be erved a long a there i network coverage within the area. Conequently, an important amendment in IEEE 82.16e i the upport for Handover (HO). The HO proce i neceary to maintain required ignal trength to upport continuou data tranmiion for all type of application when the Mobile Station () i witching from one Bae Station (BS) to another. In mobile WiMAX, two major type of HO are defined: Hard Handover (HHO) and Soft Handover (SHO). HHO i mandatory while SHO i optional [3, 4]. HHO i characteried by break before make. Thi implie that current radio reource are releaed before new reource are ued. The releae it connection from the Serving Bae Station (SBS) before etablihing a new connection to the Target Bae Station (TBS). The can only be connected to one BS at a time from which HO mut take place a hown in Figure 1. HHO i imple to WiMAX BS NBS Figure 1: HHO [3] ABS Diverity et (DS) monitoring the ignal trength to update DS WiMAX BS NBS tranmiion of UL and DL traffic imultaneouly to all active BS in the DS Figure 2: MDHO [3]

2 NBS ABS Diverity et (DS) NBS monitoring the ignal trength to update DS monitoring the ignal trength to update the Anchor BS. tranmiion of UL and DL traffic Figure 3: FBSS [3] Previou work on HO cheme implemented on IEEE 82.16e reveal that ditance need to be conidered a an important factor to initiate fat handover. The author of [5] and [6] conidered Global Poitioning Sytem (GPS) to determine the poition and calculate the ditance between the BS. The ue of GPS provide good location accuracy, but it i quite complex to implement. It require additional hardware in the which increae the cot of device. In [7], the author introduced Handover Delay Timer (HDT) focuing on multiple BS during the HO proce. Qi et al. in [8] only conidered the location to improve HO proce. In [9], ditance wa conidered to preallocate reource and not to reduce handover delay. In [1] a technique wa propoed that demand adding new Downlink (DL) management meage in order to minimie the ervice interruption. Thi implie a modification to the exiting tandard. Velocity i another factor that can be conidered for HO. In [11], the author propoed a velocity adaptive HO cheme to minimie handover delay and enhance the network reource utiliation. Thi method ha merit ince it balance the HO delay and channel reource watage. However, the author did not conider the influence of ditance a a criterion for handover. None of the above technique conidered the combined influence of ignal trength and ditance on the HO proce. In thi paper, a combined cheme of ignal trength and ditance i propoed to reduce HO delay and it effect on real-time application. Typically in WiMAX, the Received Signal Strength Indicator (RSSI) i ued to meaure the ignal trength of each BS. When the RSSI of the Serving Bae Station (SBS) i lower than the ignal trength of the Target Bae Station (TBS) by a certain threhold, HO i executed. The can the multiple BS and elect the mot uitable BS a the TBS. The TBS elected i the mot appropriate becaue of it tronger ignal trength when compared to other adjacent BS a hown in Figure 2. Here, multiple BS are conidered intead of two BS which may not be practical. The propoed cheme conider both ignal trength received by the and the ditance between the and the BS. The remainder of thi paper i organied a follow: Section II decribe the HO proce in IEEE 82.16e. Section III explain the propoed cheme in detail. The imulation cenario and performance analyi i preented in Section IV, and the concluion i provided in Section V. II. HANDOVER PROCESS 2.1 Principle of Handover proce Handover (HO) in mobile WiMAX can be divided into two major phae: the Network Topology Acquiition phae and the actual HO proce [3]. The Network Topology phae precede the actual HO proce which include handover deciion and initiation, ynchroniation and ranging proce, cell reelection and termination context. The breakdown i hown in Figure 4. The HO procedure are explained further a follow. The Bae Station (BS) broadcat information about the network uing the Mobile Neighbour Advertiement (MOB_NBR_ADV) meage. The meage provide channel information for Neighbour Bae Station (NBS) which i provided by each BS own Uplink Channel Decriptor (UCD)/Downlink Channel Decriptor (DCD) meage tranmiion. The BS keep on gathering information with the help of the backhaul network. The tart canning to chooe the mot uitable BS for HO. The proce i carried out with the help of Mobile Scanning interval allocation REQuet, Repone and REPort meage (MOB_SCN-REQ, MOB_SCN-RSP and MOB_SCN-REP), repectively. In the proce of cell reelection, the can or aociate with one or more BS to determine a uitable final election of the TBS. The deciion of HO may arie either from the or SBS. When the decide HO, a MOB_HO REQ meage i ent. HO deciion and initiation meage from the are alway given preference [3]. Once a final deciion i made on the TBS, the inform the SBS about the HO activity by ending a Mobile HO Indication (MOB_HO-IND) meage. At thi point, the break it aociation with the SBS and begin a new aociation with the TBS. To minimie handover delay, a good handover cheme mut be employed at thi tage. The next phae after electing the TBS i the network reentry. It include the authoriation and the regitration with the TBS. After the ucceful regitration with the TBS, the end MOB_HO_IND meage and inform the SBS that the HO proce i completed and the i fully connected to the TBS. Network Topology Advertiement Scanning of NBS Cell Reelection HO Deciion and Initiation Network Re-entry Sychroniation and Ranging Authoriation and Regitration Normal Operation Figure 4: IEEE 82.16e HO procedure in tage

3 2.2 Handover initiation cheme The HO proce i initiated baed on receiving the Relative Signal Strength (RSS) of the Serving Bae Station (SBS) and the Target Bae Station (TBS), meaured and reported by the Mobile Station () Conventional handover initiation algorithm Thi ection outline a number of conventional HO deciion initiation algorithm propoed for variou wirele communication tandard. Relative ignal trength: In thi algorithm, the HO deciion i baed on the average meaurement of the received ignal. i allowed to handover if the RSS of a TBS exceed that of SBS. The cheme can be expreed a: RSS TBS >RSS SBS Relative ignal trength with threhold: The i allowed to handover only if the RSS of the TBS exceed that of the SBS and the ignal trength of the SBS i below a threhold, TH. The performance of thi method i dependent on the value of threhold choen. The cheme can be expreed a: RSS TBS >RSS SBS and RSS SBS < TH or RSS TBS > RSS SBS and RSS TBS > TH Relative ignal trength with hyterei: The i allowed to handover if the RSS of the TBS i tronger than that of the SBS by a hyterei margin, HYS. The cheme can be expreed a: RSS TBS >RSS SBS and RSS SBS + HYS Relative ignal trength with hyterei and threhold: The i allowed to handover to the TBS, except if the current ignal level of the SBS drop below a threhold and the ignal level of the TBS i capable of upporting the current BS by a given hyterei margin. The cheme can be expreed a: RSS TBS >RSS SBS + HYS and RSS SBS < TH or RSS TBS > RSS SBS +HYS and RSS SBS > TH The relative ignal trength with hyterei and threhold cheme i conidered the bet algorithm to initiate the HO proce becaue it incorporate the advantage of the other algorithm. The conventional HO initiation algorithm mentioned above are baed on four variable: the length and hape of the averaging window, the threhold level, hyterei margin and the delay time [12]. Rapid fluctuation in RSS could trigger handover which at time may not be neceary. Thi ituation could degrade the performance of the ytem reulting in unneceary handover, handover delay and radio reource watage. To minimie thi handover delay, a control meaure cheme called ditance i introduced. Ditance i a factor that improve the proce of handover delay. If the handover delay i reduced, the radio reource will be better managed. The ditance can be meaured uing Equation (2). D BS V T (2) where D -BS i the ditance between the and the BS, V and T are the peed and the travel time of the, repectively. T i defined a the amount of time required for the to move from one BS to the cell boundary of another BS. Thi ditance i calculated at imple ditance interval of 1m. To initiate handover, the propoed cheme follow the condition given below: When the averaged RSS from the SBS fall below the threhold value and the averaged RSS from the TBS exceed the RSS from the SBS by a given hyterei (HYS) in db. If the calculated ditance from the SBS exceed that of the TBS by a threhold ditance given by: DSBS DTBS thd (3) In Equation (3), TH i the abolute threhold level and th d i the threhold ditance. Thi threhold ditance i included to initiate fat handover. Figure 9 how the reult for the average number of handover and average handover delay for the propoed cheme. IV. SIMULATION SCENARIO AND PERFORMANCE ANALYSIS A. Simulation cenario A cenario of multiple BS i ued in thi tudy a hown in Figure 5. The ame figure aume that the move in a traight direction. The can and elect the uitable BS that guarantee the poibility of maintaining the connection among the adjacent BS. Therefore, the releae it connection from BS1, which i the erving BS and etablihe a new connection with BS2, which i the TBS. The i aumed to be moving at vehicular peed. III. PROPOSED SCHEME In thi ection, the Relative Signal Strength with Hyterei and Threhold and Ditance (RSHT-D) cheme i propoed to initiate handover. The exiting tandard only conider the RSS when deciding handover [11]. With regard to the ignal trength algorithm, the hand over from one BS to another BS a depicted in Figure 5. Hyterei i conidered here to reduce the number of unneceary handover, although thi introduce delay. Therefore, a fixed hyterei margin i adopted to control handover delay to a certain level. RSS TH RSS RSS HYS (1) SBS TBS SBS WiMAX BS4 WiMAX BS1 WiMAX BS2 WiMAX BS6 WiMAX BS3 WiMAX BS7 WiMAX BS5 Figure 5: Network Topology [13]

4 The three major factor that affect the propagation in a wirele mobile network are hadow fading, path lo and fat or multipath fading. Multipath fading i found to follow a Raleigh ditribution. The received ignal trength meaurement i averaged by uing exponential averaging window to alleviate the impact of multipath fading. Therefore, the received ignal by the can be conidered to contain only two component: hadow fading and path lo. Path lo i dependent on the ditance between two location and hadow fading i caued by large cale variation in the terrain profile along the path from the to the BS [12, 14, 15]. In thi paper, the path lo propagation i modelled uing COST 231 [16] for a medium-ized city and i given a: Pl log( fc) 13.82log( hte) a( hre) ( log( hte)) log d CM (4) where Pl i the 5 th percentile (median) value of propagation path lo, fc i the carrier frequency (GHz), hte i the height of the BS, a i the area where the BS i located, hre i the height of the, d i the radio path ditance, and CM i the correction factor ( db for medium-ized city and uburban area and 3dB for metropolitan center). The ignal trength received by the from the SBS and the adjacent BS can be expreed a: RSS ( k) Pt Pl L (5) c c c RSS ( k) Pt Pl L i 1,2,...,7 (6) i i i where Pt repreent the tranmitting power of the BS in dbm, Pl repreent the path lo in db, L c and L i repreent the hadowing variable. It i characteried by a log-normal ditribution with zero mean and σ for the BS. (1 exp( 2 Vt / d))1/ 2 (7) where repreent the tandard deviation of low fading, V i the averaged velocity of the, t i the ampling rate and d i the correlation ditance. The relative averaged RSS i the difference of averaged ignal trength from the SBS and the adjacent BS. Thi i given a: R( k) RSS ( k) RSS ( k) i 1,2,...,7 (8) where k 1,..., D / d c i. D i the ditance between adjacent BS and d i the ampling interval, i i the number of BS, RSS i the ignal trength received by the. i The correponding handover probability of Equation (1), (5) and (6) can be written a: 7 Pho P RSSc T RSSi RSSc h i 1 For more information on Equation (9) refer to[13]. In thi paper, a contant velocity of of 2m/ (72km/h) i aumed, the ignal trength i ampled at every 5m and ampling ditance d=1m. In normal condition, d=3m, a random value i generated with tandard deviation =8dB (9) and zero-mean [17-19]. In our imulation, it i aumed that the move from BS1 toward BS2 at a contant peed of 2m/. With thi peed, the handover hould be performed in le than 5m [2]. Any delay greater than 5m may affect delay-enitive application (uch a VoIP and IPTV). Other imulation parameter ued are given in Table 1. The imulation are performed in a imulator developed in MATLAB (21a). To achieve conitency and reliability in output data, each imulation wa run for 1 hour and carried out 15 time. Therefore, the total time for the imulation i 15 hour. Table 1: Simulation parameter Simulation parameter Handover type BS to BS ditance Fixed threhold ditance, th d Cell radiu, R Frequency Value HHO 2m 6m D/ 3 2.5GHz height 1.5m BS height 32m BS tranmitting power (dbm) 43 Propagation Model Hyterei margin Threhold value Mobile velocity Standard deviation Sampling time, t B. Performance analyi COST231-Hata 6 db -94dB 2m/ 8dB 5m The handover parameter in the propoed cheme are the threhold ditance, the hyterei margin and the abolute threhold value. Figure 6 and 7 how the actual ignal trength meaured by the and the relative difference between the two BS. Figure 8 illutrate the comparion of handover probability for the conventional algorithm and the propoed cheme. The figure further how that the handover probability of RSTH-D i mall within the area covered by the SBS and increae fat from to 1 near the cell boundary while other algorithm initiate handover before approaching the cell boundary. Thi could affect the connection and the quality of the network. With RSTH-D cheme handover can be initiated near the cell boundary (9m) and executed a few meter away from the cell boundary (16m). Therefore, the propoed cheme i better than the conventional algorithm. From Figure 9, it can be tated that the average handover delay did not exceed the handover delay propoed by the WiMAX tandard [2] and the average number of handover did not exceed one. For the propoed cheme handover i executed when the ditance of the SBS i greater than the ditance of the TBS by a threhold ditance (D SBS > D TBS + th d ). The threhold ditance pecifie the exact ditance where HO hould take place. The

5 Number of handover Handover probability Difference in ignal trength (db) RSSI(dBm) performance of the propoed RSTH-D cheme how that the handover delay and the average number of handover can be minimied Path lo Received Signal Strength Path lo plu hadow fading Shadow fading Ditance [m] Figure 6: Received ignal trength V. CONCLUSION In thi paper, a combined method of ignal trength and ditance to initiate fat handover wa propoed. Performance i evaluated in term of the average number of handover and handover delay. Since handover initiation i baed on ditance, the propoed cheme can adequately reduce handover delay and maintain quality of ervice. Simulation reult how that the propoed cheme improve performance in term of handover delay. Therefore, the propoed approach can reduce the handover delay a well a provide efficient radio reource utiliation. Future work will conider the impact of different peed for the a upported by the WiMAX tandard. REFERENCES Relative Ditance between BS1 and BS Ditance from BS1 to BS2 (m) Figure 7: Difference in ignal trength RSS Algorithm RSS Algorithm with TH and HYS RSS Algorithm with HYS RSS Algorithm with TH Propoed cheme (RSTH-D) Ditance (m) Figure 8: Comparion of handover probability Propoed cheme (RSTH-D) RSS Algorithm with TH and HYS RSS Algorithm with HYS RSS Algorith with TH RSS algorithm Handover delay (m) Figure 9: Average handover number veru handover delay [1] "IEEE Standard for Local and Metropolitan Area Network Part 16: Air Interface for Fixed Broadband Wirele Acce Sytem," IEEE Std (Reviion of IEEE Std ), pp. _1-857, 24. [2] "IEEE P82.16e/D12, Air interface for fixed and mobile broadband wirele acce ytem: amendment for phyical and medium acce control layer for combined fixed and mobile operation in licened band, 25.." [3] S. K. Ray, et al., "Handover in Mobile WiMAX Network: The State of Art and Reearch Iue," Communication Survey & Tutorial, IEEE, vol. 12, pp , 21. [4] "WiMAX Forum, Mobile-Part I: A technical overview and performance evaluation, 26.." [5] H. Pirkomaji and V. T. T. Vakily, "Improved Handover Interruption Time in WiMAX, Uing GPS," in Next Generation Mobile Application, Service and Technologie (NGMAST), 21 Fourth International Conference on, 21, pp [6] L. Xujie, "A Fat Handover Scheme for Wimax Sytem," in Wirele Communication Networking and Mobile Computing (WiCOM), 21 6th International Conference on, 21, pp [7] J. Z. Zdenek Becvar, "Implemetation of Handover Delay Timer into WiMAX." [8] L. Qi and M. Maode, "A Location-Aware fat handover cheme for mobile WiMax network," in Information, Communication and Signal Proceing, 29. ICICS 29. 7th International Conference on, 29, pp [9] C. Jenhui, et al., "Pre-Coordination Mechanim for Fat Handover in WiMAX Network," in Wirele Broadband and Ultra Wideband Communication, 27. AuWirele 27. The 2nd International Conference on, 27, pp [1] G. H. S. Choi, T. Kwon, A. Lim, and D. Cho,, ""Fat handover cheme for real-time downlink ervice in IEEE 82.16e BWA yyem," IEEE Vehicular Technology Conference (VTC 25) " vol. 3, pp , May 25. [11] Caiyong HAO, "A Velocity-Adaptive Handover Scheme for mobile WiMAX" ( pp , September 12,

6 [12] G. P. Pollini, "Trend in handover deign," Communication Magazine, IEEE, vol. 34, pp. 82-9, [13] Z. Huamin and K. Kyung-up, "Adaptive Handoff Uing Ditance Information," in Vehicular Technology Conference, 26. VTC 26-Spring. IEEE 63rd, 26, pp [14] M. Gudmundon, "Correlation model for hadow fading in mobile radio ytem," Electronic Letter, vol. 27, pp , [15] G. S. Tomar and S. Verma, "Analyi of handoff initiation uing different path lo model in mobile communication ytem," in Wirele and Optical Communication Network, 26 IFIP International Conference on, 26, pp. 5 pp.-5. [16] T. S. Rappaport "Wirele communication principle and practice" 22. [17] B. Singh, "Hard Handover Performance Evaluation through Link Drop," in Signal Proceing, Communication and Networking, 27. ICSCN '7. International Conference on, 27, pp [18] K. I. Itoh, et al., "Performance of handoff algorithm baed on ditance and RSSI meaurement," Vehicular Technology, IEEE Tranaction on, vol. 51, pp , 22. [19] S. Dhar Roy, "Handover initiation algorithm baed on combined ignal trength meaurement and ditance," in Information and Communication Technology in Electrical Science (ICTES 27), 27. ICTES. IET-UK International Conference on, 27, pp [2] IEEE 82.16e-25, " IEEE tandard for local and metropolitan area network-part16;air interface for fixed and mobile broadband wirele acce ytem-amendent2:phyical and medium acce control layer for combined fixed and mobile operation in licened band,." Mary Alatie received her undergraduate degree in 28 from the Federal Univerity of Technology, Akure and i preently tudying toward her Mater of Technology degree at Thwane Univerity of Technology, South Africa. Her reearch interet include performance analyi, mobility management of real-time application and handover management in broadband wirele network.