International Jornal of Ftre Compter and Commniation, Vol. 4, o. 4, Agst 05 Hysteresis Margin and Load Balaning for Handover in Heterogeneos etwork Ranada Prasad Ray and Ln Tang Abstrat Long term evoltion (LTE) heterogeneos network an represent improvement of ell overage and network apaity by adopting advaned physial layer tehniqes. In heterogeneos network, one of e major hallenges is e handover deision between different ebs. The tilization of e pioells reslts to se more network resores and handover proedre. In is paper, we investigate e hysteresis margin and load balaning problem in a 3GPP LTE heterogeneos network. Firstly, we onsider A3 event, Hysteresis Margin (HM) and Cell Individal Offset () objetives are not fixed for every ells. Then we analyze e omplexity of e problem and propose a pratial algorim whih allate adaptive hysteresis margin and load balaning in heterogeneos network. The reslts show at or algorim an lead to signifiantly better performanes, sh as redndant handovers redtion and improvement of e network performane. Index Terms LTE heterogeneos network, hysteresis margin, ell individal offset, load balaning. different base stations. When UE move to new base stations, all onnetion will be reonneted to new base stations. This proess is alled handover. In e same ommniation networks, e proeeded handoff is alled horizontal handover. In heterogeneos ommniation networks e proeeded handoff is alled vertial handover [3]. The streng of signal will be more sseptible to shadowing effet, and be likely to ase handoff between e two base stations bak and for onstantly, whih is known as e ping-pong effet. By sing hysteresis margin vale or ell individal offset vales e pingpong effet an be signifiantly reded. Mobility Load Balaning (MLB) aims to ope wi e neqal traffi load between ells. Alogh e two fntions operate independently wi eah oer, ere is a lose orrelation between em, as ey bo hoose adjsting handover parameters as optimization ations. I. ITRODUCTIO Heterogeneos networks (Hetets) orrespond to a salable hierarhial elllar network model at is deploying to improve effiieny and inrease indoor overage. Two-tier Hetet omprises onventional MeBs in e first tier overlaid wi e seond tier short range, low power and low omplexity base stations (orresponding to pioells or femtoells) in Fig. []. Bease of e smaller overage area, sing same liensed freqeny band an be effiiently resed mltiple times wiin e seond-tier elements of a Hetet, s improving e apaity and e spetral effiieny per nit area of e network. In Hetet pioells are sally deployed to eliate overage holes and improve e apaity of e network. The overage area of pioells sally varies between (40 75 m) []. 3GPP LTE networks an ahieve high spetrm effiieny de to e sage of Mlti-Inpt Mlti-Otpt (MIMO) antenna and orogonal freqeny division mltiple (OFDM) tehnology. However, e network performane is still inflened by several fators, among whih different ells and load imbalane. In mobile ommniation system elllar strtre is e main arrangement. User eqipment (UE) often moves rogh Mansript reeived Febrary 5, 05; revised Jne, 05. This work was spported in part by Key Lab of Mobile Commniation Tehnology, Department of Commniation and Information at Chongqing University of Posts and Teleommniations, Chongqing, 400065, China. Ranada Prasad Ray is wi e Commniation and Information Department, Chongqing University of Posts and Teleommniations, China (e-mail: ran_ete@yahoo.om). Ln Tang is wi e Chongqing University of Posts and Teleommniations, Chongqing, China (e-mail: tangi@qpt.ed.n). Fig.. Hetets wireless network. The rest of is paper is organized as follows: In Setion II e system model and some definitions are proposed. Setion III explains e proposed shemes. Setion IV evalates its performanes via system level simlations. The paper is onlded in Setion V. II. SYSTEM MODEL We assme at serving ell has heavy load bt target ell is lighted loaded. So, Cell will sffer from high handover failre rate and high all bloking rates when UEs belonging to oer ells handover into e ell. However, e ell is less heavy loaded and low radio resore tilization. Then MLB algorims bias e handover region between neighboring ells and handover e UEs to e ell. This initial sene has been shown, bt anoer items need to be desribed frer. DOI: 0.7763/IJFCC.05.V4.39 3
International Jornal of Ftre Compter and Commniation, Vol. 4, o. 4, Agst 05 A. Handover Proess In LTE system, handover an be triggered when a UE detets at a neighboring ell offer better reeived signal streng from e serving ell. We sppose at UEs handover from sore ell to target ell. The A3 event an be expressed as: M M ( HM ) (), where M and M denote e reeived signal streng UE from ell and from ell, HM e hysteresis parameter of ell for A3 event, and, e ell individal offset set by ell for ell and is e key parameter to be adjsted to make load balane between ells [4]. If M and M satisfy e A3 event by Eq.() dring one TTT, en e handover from ell to ell is triggered. Fig. illstrates e handover proess: The point A means at e entering ondition for A3 event has jst been satisfied, e point B means at e UE starts to handover from ell to ell, and e point C means e at handover is jst finished. The time period while e UE moves from point A to point B eqals to TTT. B. Resore Assignment Fig.. A3 event in LTE. In is paper, e fos is on e LTE downlink whih is a loalized OFDMA system. Aording to e LTE system, physial resore bloks (PRBs) are e smallest nit. When a UE beomes ative, e system will assign resores to it. The resore assignment is jst like e sheme in de to e main onsideration on MLB algorim. The nmber of PRBs assigned to e UE is briefly taken into aont below [5]. Ative UE has a bit rate reqirement whih is denoted as D and e reqired nmber of PRBs to meet D an be denoted as. Given e. The reqired resores sing a roghpt mapping depends on e, we an get e data rate per PRB R = R ). The most ( proent example for sh a fntion R() is Shannon s eqation. We an express e Shannon formla: R () log ( ) P L X. x( ) ( ). P. L (3) where P is transmit power for a ell and denotes as ermal noise. The X (), L x() and L denote e rrently serving ell of UE, e pa loss wi shadow fading from e rrently serving ell and oer interferene ell, respetively. The denotes e load of e ell. Wi e definition, e amont of reqired PRBs be obtained as: ˆ an ˆ D (4) R( ) For reality, e atal amont of PRBs opied by UE an be yielded: ˆ ˆ PRB (, ) (, ) Left K K 0 else (5) where K is a onstant ( in simlation) to keep PRBs opied by e sers wi low hannel qality nder reasonable level. PRB denote e left PRBs of ells for Left aessing. From e eqation (5), we an obtain at e sers will be bloked when is zero PRB ( ˆ, K )). ( Left C. Load Measrement Cell load is an important parameter whih inflenes e MLB algorim greatly. The MLB algorim is arried ot based on ell load measrement, so e performane lies on it heavily. We define ell load as e mean tilization of e total amont of physial resore bloks aross e ell. M PRB X ( ) M PRB (6) is e total nmber of resores in e ell. In is paper, we follow e real senario and will not assign resores when ere are not enogh PRBs for new og ative UEs [6], [7]. III. THE PROPOSED SCHEME A. Adaptive Hysteresis Margin Usally e level of e hysteresis is onstant. The adaptive hysteresis margin (HM) is based on e modifiation of atal HM vale aording to e position of e ser in e ell. The HM is dereasing wi UE s moving loser to e ell border. The border of ells are neier reglar irles nor hexagons sine e system is not distane or signal level limited bt it is interferene limited. Therefore, e shape of e ells is strongly inflened also by e interferene [8]. 3
International Jornal of Ftre Compter and Commniation, Vol. 4, o. 4, Agst 05 Hene, is paper frer proposes to implement (Signal to interferene pls noise ratio) instead of (reeived signal streng indiator) RSSI for allation of e atal level of HM. A signal level inflened by e interferene and noise (I) an be desribed aording to e next eqation: TPst PL I RSSI I (7) TPst is transmission power of e station of interest. The level is in different range of vales an RSSI. Immediately it has to be related to e differene between imm and imm in e observed area. Ths, e atal HM level aording to is derived as follows: exp HM HM ( 0 ) ; HM where at and at (8) e atal is measred by a UE; are imm and imm vales in e investigated area respetively, exp represents e exponent eqal to 4 and HM is e imm HM at an be set p eqal to 0 [9]. The atal of UE an be easily measred dring UE s operation. It is sally performed wi e prpose of handover deision and initiation. However, also e imm and imm vales also have to be known for e tilization of e adaptive HM. orresponds to e ell radis and to e level, at whih e UE is able to reeive data. Therefore, it is set p as a fix vale for eah FAP and BS. The an be detered by two ways: ) measrement of by a FAP at e point of its loation; or ) monitoring and reporting of by all UEs onneted to e given FAP and an seleting e highest from all known vales as e. The first way implies to eqip all FAPs wi ability to measre. Hene it is not frermore onsidered in e paper. The seond approah tilizes e knowledge of previos vales in e area reported by UEs. Sine e hannel is time variant, e time interval for seletion of shold be detered. B. Load Balaning We assme at e serving ell named Cell is over loaded. It will hoose e light loaded target ell named Cell to balane e load. To ahieve is prpose, Cell shold adjst e handover parameters to make e UEs handover from Cell to Cell more easily while make e UEs handover from Cell to Cell more diffiltly. This is eqivalent to at e handover position shold be loser to Cell wheer UEs handover from Cell to Cell or handover from Cell to Cell. To make e handover position of UEs handover from Cell to Cell move from point A to point A (Fig. 3), e handover parameters are adjsted as follows: M HM M (9) Load (0) ( ) ( ) Load () where M is e reshold of e reeived signal streng reqired by handover, M e reeived signal streng of e UE from Cell when e reeived signal streng e UE reeives from Cell is eqal to M, a ritial vale at makes e A3 event jst to be satisfied. In order to prevent e too late handover, when UE moves from Cell to Cell,, between e vale of, and shold be adjsted to whih is. The step is deided by e load of Cell and Cell. The Smaller vale of (Load/Load) and e larger e vale is. Then loser point A to Cell. Fig. 3. Adjst point A to A. To make e UEs in Cell handover to Cell more diffilt, we will make e UEs handover positions from Cell to Cell loser to Cell. This means point B will be hanged to point B as illstrated in Fig. 4. So e point A is e new handover position when UEs move from Cell to Cell and point B is e new handover position when UEs move from Cell to Cell. In onsideration of Ping-Pong handover, e point B shold be frer from Cell an e point A, bt loser to Cell an e point B. Fig. 4. Adjst point B to B. Optimization restrition: We an get e adjstment on and is restrited in e allowed range., 33
International Jornal of Ftre Compter and Commniation, Vol. 4, o. 4, Agst 05,, HM HM This proposed sheme is triggered by MLB, so e MLB proedre shold work as follows: ) The MLB fntion in Cell ollets e load statistis of neighbor ells. ) The Cell detets an eligible neighbor (Cell) and reqests its handover parameters, and H. 3) The Cell adjsts e parameter, and, to, and ineqality. 4) The Cell transfers e parameter, aording to eqality and to Cell, and bo of e Cell and Cell synhronize e new parameters. sess rate, RLF rate and ping-pong handover are signifiantly better an e original sheme. De to e fat at MLB fntion in e proposed sheme adjsts e parameter to a reasonable vale aording to e load distribtion of neighboring ells. Fig. 5d) show at e system average roghpt inrease. This is bease after e MLB operation, e handover positions of UEs moving from overloaded ell to its light load neighbor ell and e handover position of UEs moving from e light load neighbor ell to e overload ell all get loser to e overloaded ell. Therefore, e UEs an handover from e overloaded ell to its neighbor ell more easily, and e UEs handover from e neighbor ell to e overload ell more diffiltly. This makes e UE grow rate of e overloaded ell is lower an e UE derement rate. So e load level of e overload ell dereases. IV. PERFORMACE EVALUATIO We evalate e proposed sheme by LTE system level simlator. LTE-Sim is an open sore framework to simlate LTE networks mainly developed by G. Piro and F.Capozzi at "Politenio di Bari"[0]. Dring e simlation time, UEs always onnet to e network and keep alling. When e simlator starts rnning, every UE hooses random diretion and moves straight wi a speifi speed. Compare between e original sheme and e proposed sheme, we an see in original sheme MLB operates wiot restrition, in e proposed sheme e operation of MLB is restrited in e allowed range to avoid e load problem. Oer main simlation parameters smmarized in Table I. TABLE I: SIMULATIO PARAMETER Parameter Vale System Bandwid Freqeny Radis of Miroell Radis of Pioell eb Tx power Pio Tx power Thermals oise Paloss Model 5 MHz GHz 0.5 km 0.05 km 46 dbm 30 dbm -74 dbm/hz As 3GPP 36.84 V.5. Load Threshold 0.8 Cell layot Shedler Traffi Model UE speed Wrap arond, 7 Miro EodeBs PF CBR 30 km/h, 0 km/h Fig. 5a). Handover sess rate. Fig. 5b). Radio link failre rate. A. Simlation Reslts We present e simlation reslt for performane omparison between e original sheme and proposed sheme. The MLB modifies e parameter to a reasonable range. Fig. 5a), 5b) and 5) show at e handover Fig. 5). Ping-pong handover rate. 34
International Jornal of Ftre Compter and Commniation, Vol. 4, o. 4, Agst 05 Fig. 5d). Average roghpt. V. COCLUSIOS In is paper, MLB meods have been proposed for load balaning in LTE Hetets senario. The proposed sheme is based on e load distribtion of neighboring ells to adjst e handover parameters. System level simlations have shown at e proposed shemes an greatly inrease e handover sess rate, e RLF rate, ping-pong handover rate and roghpt. Frermore, it an rede e nmber of overloaded ell and improve e overall system roghpt. REFERECES [] S. Zaidi and G. Talagery, Developing and integrating a high performane HET-ET, 4G Amerias, Otober 0. []. Saqib, E. Hossain, and D. I. Kim, Frational freqeny rese for interferene management in LTE-advane Hetets, IEEE Wireless Commniations, pp. 3-, April 03. [3] M. Ravihandra et al., A srvey on handovers literatre for next generation wireless networks, IJARCCE, vol., isse, Deember 03. [4] 3GPP standardization, Evolved niversal terrestrial radio aess (E-UTRA); Radio resore ontrol (RRC), Release 8, V8.5.0, TS 36.33, 009. [5] 3GPP standardization, Evolved niversal terrestrial radio aess (E-UTRA); Physial layer proedres, Release 0, V0..0, TS 36.3, 0. [6] P. Mnoz, R. Baro, and I. Bandera, On e potential of handover parameter optimization for self-organizing networks, IEEE, vol. 6, no. 5, pp. 895-905, Jne 03. [7] 3GPP standardization, Evolved niversal terrestrial radio aess (E-UTRA); Layer Measrements, Release 0, V0..0, TS 36.34, 0. [8] Z.-Q. Li, P.-L. Hong et al., Conflit avoidane between mobility robstness optimization and mobility load balaning, IEEE, 00. [9] Z. Bevar and P. Mah, Adaptive hysteresis margin for handover in femtoell networks, FP7 projet FREEDOM IST-4889 STP. [0] G. Piro, L. A. Grieo, G. Boggia, F. Capozzi, and P. Camarda, Simlating LTE elllar systems: An open sore framework, IEEE Trans. Veh. Tehnol., Ot. 00. Ranada Prasad Ray reeived his B.S. degree in eletronis and teleommniation engineering from Daffodil International University, Bangladesh, in 00. Crrently he is a M.S. stdent of e Shool of Commniation and Information Engineering at Chongqing University of Posts and Teleommniations, China. His researh interests inlde e load balaning in heterogeneos networks, handover deision for mobility management and qality of servie in wireless networks. Ln Tang reeived e Ph.D. degree in e Chong Qing University, China in 00. Sine 004, he has been wi e College of Commniation Engineering at e Chong Qing University of Posts and Teleommniations, China. He is rrently a professor. In e past few years he has fosed mainly on wireless networks (elllar networks, vehile ad ho networks). 35