1 Remote Antenna Unt Selecton Asssted Seamless Handover for Hgh-Speed Ralway Communcatons wth Dstrbuted Antennas Yang Lu,, Ke Xong,, Zhuyan Zhao, Pngy Fan, Zhangdu Zhong, School of Computer and Informaton Technology, Bejng Jaotong Unversty, Bejng, P.R. Chna Natonal Moble Communcatons Research Laboratory, Southeast Unversty, Nanjng, P.R. Chna Department of Electronc Engneerng, Tsnghua Unversty, Bejng, P.R. Chna Noka Solutons and Networks, P.R. Chna Correspondng emal: kxong@bjtu.edu.cn arxv:1603.06461v1 [cs.ni] 11 Mar 2016 Abstract To attan seamless handover and reduce the handover falure probablty for hgh-speed ralway (HSR communcaton systems, ths paper proposes a remote antenna unt (RAU selecton asssted handover scheme where two antennas are nstalled on hgh speed tran (HST and dstrbuted antenna system (DAS cell archtecture on ground s adopted. The RAU selecton s used to provde hgh qualty receved sgnals for trans movng n DAS cells and the two HST antennas are employed on trans to realze seamless handover. Moreover, to effcently evaluate the system performance, a new metrc termed as handover occurrence probablty s defned for descrbng the relaton between handover occurrence poston and handover falure probablty. We then analyze the receved sgnal strength, the handover trgger probablty, the handover occurrence probablty, the handover falure probablty and the communcaton nterrupton probablty. Numercal results are provded to compare our proposed scheme wth the current exstng ones. It s shown that our proposed scheme acheves better performances n terms of handover falure probablty and communcaton nterrupton probablty. Index Terms Hgh-speed ralway communcaton, seamless handover, remote antenna unt selecton, dstrbuted antenna system I. INTRODUCTION Recently, hgh-speed ralway (HSR system has developed very fast owng to ts convenence and safety. Meanwhle, the wreless servce demand durng the trp of hgh-speed tran (HST grows dramatcally but ths demand s hard to be satsfed, snce there are varous challenges n the broadband wreless communcaton for HSR systems [1]. One of the bggest challenges s how to provde seamless handover wth low handover falure probablty for HSR system. Frstly, due to the hgh moblty speed of HST, the handover occurs very frequently whch nterrupts the wreless connecton ntermttently. Secondly, the remote base staton (BS provdng rado sgnals wth poor strength causes a hgh handover falure probablty and degrades the qualty of servce, especally, n the boundary regon of two neghborng cells. Amng at releasng these problems, a lot of research work have been done n the lterature [2] - [6]. To release the frst problem, two-hst antennas were adopted n [2], n whch durng the handover, the front antenna of the tran carres out handover whle the rear antenna keeps connectng wth BS. After the front antenna completes establshng connecton wth the target BS, the rear one swtches to the workng frequency of the target BS. However, due to the near-far sgnal prorogaton effect, the seamless handover of such a confguraton s stll dffcult to be ensured, especally when the base statons are relatvely far way from the tran. To release the second problem, the dstrbuted antenna system (DAS cell archtecture was ntroduced to HSR systems [3], [4], whch also mproves the system supported data rate as the remote antenna unt deduces the dstance between the BS and the tran. To nhert the advantages of both two-hst antennas and DAS cells, [5] and [6] nvestgated them n a sngle HSR system to mprove the handover performance, where however, only tradtonal/blanket transmsson scheme was consdered. In the tradtonal/blanket transmsson scheme, each BS allocates ts avalable power to ts RAUs equally, even durng the handover procedure, whch neglects the dfference of the lnks between the RAUs and the tran antennas. Snce the RAUs are usually deployed along the track, there s always one RAU beng the closest to the tran and other ones are relatvely far away from the tran. Therefore, the closest RAU should consume all power to acheve a better system performance, rather than all RAUs equally dvdng up the power. From ths motvaton, dfferent from [5] and [6], we adopt the remote antenna unt (RAU selecton to enhance the handover performance of HSR system based on two-hst antennas and DAS cells. In our proposed scheme, the avalable power of each BS s always allocated to the RAU wth the mnmum pathloss accordng to the poston of the tran. As a result, the receved sgnal strength (RSS at the tran can be mproved, so does the handover performance. The contrbuton of our work can be summarzed as follows. Frstly, we nvestgate the handover scheme for HST communcaton systems, whch s the frst work on ntroducng the RAU selecton n DAS cells to enhance the handover performance of HSR system wth two-hst antennas. Secondly, we present a new performance metrc named as handover occurrence probablty to descrbe the relaton between handover occurrence poston and handover performance. Based on ths metrc, we obtan one sgnfcant nsght. That s, the approprate handover poston can reduce handover falure
2 probablty greatly. Thrdly, The system performance of our proposed scheme n handover trgger probablty 1, handover falure probablty and communcaton nterrupton probablty are analyzed n theory and numercal results shows ts advantages over the current extng ones. The rest of ths paper s organzed as follows. Secton II descrbes the system model. Secton III presents our proposed handover scheme. The analytcal results and the numercal results are presented are provded n Secton IV and Secton V, respectvely. Fnally, Secton VI summarzes ths paper. d0 du Servng BS dr RAU ds Core Network Network Gate Servng Cell O v Target BS Target Cell A. System Archtecture II. SYSTEM MODEL Consder a HSR system, where a tran s runnng at a constant speed v on a straght ralway track. The passengers n the tran expect a hgh qualty wreless communcaton servce durng the trp. To satsfy ths requrement and provde broadband wreless communcaton for the HST, two confguratons, ( two-hst antennas and ( DAS cells, are nvolved n our system, whch are descrbed as follows. 1 Two-HST antennas: In ths confguraton, two antennas are deployed on the top of the tran whch are characterzed by the front antenna and the rear antenna. The dstance of two antennas s set as the length of HST. These two antennas are connected wth a Tran relay staton (TRS nsde the tran carrage also by optcal fber lnks, whch s used to provde a cooperatve communcaton servce between users and cells along the ralway. It means that all nformaton transmtted from (or sent to the users are aggregated and forwarded by the TRS. By dosng so, all users n the tran can be consdered as a sngle bg user connectng wth TRS, so that the group handover (.e., all actve users smultaneously requrng handover from the servng cell to the target cell can be released by executng the handover of the bg user. When the tran moves wthn the coverage of one DAS cell, both HST antennas communcate wth the servng cells and TRS receves the sgnals from two antennas by usng maxmal-rato combnng (MRC method. When the tran moves nto the overlap area of two neghborng cells, the front antenna executes handover to the target cell (also termed as the next cell whle the rear antenna keeps connectng wth the servng cell (also termed as the current cell, so that a seamless communcaton servce can be provded durng handover procedure. Actually, two- HST antennas can not only mprove the channel capacty by usng space dversty but also help to avod the penetraton loss by usng the two-hop BS-TRS-user lnk transmsson. 2 DAS cells: Dfferent from the tradtonal cell whch has a centralzed antenna array at the base staton (BS, one DAS cell s formed by a master BS and N RAUs whch physcally connect wth the master BS by optcal fber lnks. As shown n Fg. 1, RAUs are dstrbuted along the ralway to provde a hgh-qualty seamless coverage of rado sgnals to HST. The master BS delver data from core network to each RAU va optcal fbers. It s assumed that the optcal fbre lnks are able to provde super hgh data rate, so the transmsson delay 1 In other papers, ths probablty s usually called handover probablty. Fg. 1. Rear Antenna AP L TRS Front Antenna System archtecture: two-hst antennas and DAS cells. between the BS and the RAU s neglected. Wth each DAS cell, the qualty of rado sgnals over both downlnk and uplnk can be greatly enhanced by reducng the transmsson dstance between the servng cell and the antennas of HST. Snce there are usually more data transmsson n downlnk compared wth uplnk, ths paper manly focuses on the downlnk scenaro. As shown n Fg. 1,d s s the dstance between two neghborng (master BSs, d r s the dstance between two neghborng RAUs n one DAS cell, d 0 s the dstance between (master BS and ralway, d u s the dstance between RAUs and ralway, and L s the length of HST. The tran locatonxs measured by the dstance between the front antenna and orgnal pont whch s the cross pont between ralway track and ts vertcal lne through the servng (master BS. For clarty, we use n and to represent the n-th RAU of a DAS cell and the -th antenna of HST respectvely, where 0 n N, and 1,2, where 1 represents the front antenna and 2 represents the tal antenna. B. Receved Sgnal Strength (RSS RSS s a vtal parameter usually adopted n nvestgatng handover scheme. As prevously mentoned, the two antennas perform handover separately. It was proved n [2] that the handover schemes are not senstve to small-scale fadng and the nter-carrer nterference (ICI and nose component could be averaged out of RSS whch has no effect on the fnal average sgnal strength. Therefore, at tme t, the tran locaton x s v t and the RSS at the -th antenna from the n-th RAU 2 n the cell s s ( n, (x = P n P L d (s n, ζ (x (1 where s TC, SC. SC and TC represent the servng cell and the target cell, respectvely. P n s the transmt power of n-th RAU n s-th cell, P n P t, P t s the constrant power of s-th cell, P L (d = A d γ s the path loss, where A s a constant, d s the dstance, γ s the path loss exponent. ζ s 2 We only consder RAUs for transmsson n a DAS cell.
3 Handover Preparaton Handover Executon Handover Completon Fg. 2. TRS!"#Measurement Reports 4 Handover Command 4 Handover Confrm Servng Cell 2 Handover Request 2 Handover Request ACK Packet Data Target Cell 2 Power Allocaton 5 Handover of Rear Antenna 6 Release Resources 3 Dual-castng Request and ACK Packet Data 6"#Dual-castng Complete Request and ACK Handover procedure of proposed scheme Core Network 3 Dual-cast used to descrbe the shadow fadng whch s a normal random varable and ζ CN(0,(σ (s n, 2. III. HANDOVER SCHEME Ths secton frstly presents the handover condton based on the RAU selecton. Then a dual-cast transmsson scheme from the core network to fully utlze the two-hst antennas. At last the detals of handover procedure are presented. A. Handover Condton As there are multple RAUs n each DAS cell, several transmsson schemes, such as the blanket transmsson scheme and RAU selecton transmsson scheme [4], can be adopted for the nformaton transmsson n a DAS cell. The blanket transmsson scheme transmts sgnals through all RAUs and the power s allocated equally for all RAUs. The RAU selecton transmsson scheme selects one RAU wth mnmum propagaton pathloss for transmsson. Compared wth blanket transmsson scheme, RAU selecton transmsson scheme s more sutable for HSR system accordng to [3]. Thus, we adopt the later one n our handover scheme. The RAU selecton s employed for the servng cell to allocate all avalable power to one RAU for transmsson whch s closet to the HST; for the target cell to allocate all power to ts frst RAU durng the handover procedure. In ths case, the RSS at -th antenna from the n-th RAU n s-th cell s (x = max n,. (x Snce every selected RAU consumes P t for transmsson, the transmt power P n n (1 should be modfed by P t. The condton that a handover from the servng cell to the target cell should be trggered by the -th HST antenna s 1, R (SC N, > H (2 where s the RSS at the -th antenna from the cell s and H s the protecton factor, whch s used to avodng the Png-Pang effect durng handover. B. Dual-cast Snce two-hst antennas are employed, durng the handover these two antennas shall connect wth servng cell and target cell respectvely, thus two ndependent transmsson lnks are constructed. As for how to keep two lnks synchronous, we adopt a transmsson scheme named as dual-castng whch s very smlar wth the b-castng [7]. In ths transmsson scheme two copes of data are sent from the core network to both the servng cell and the target cell durng handover by optcal fber lnks whch are regarded as no delay. Then the servng cell and the target cell provde the same nformaton to the rear antenna and the front antenna synchronously by RAUs. C. Handover procedure Fg. 2 shows the procedures of our proposed handover scheme and the detals are gven as follows. a. Handover preparaton 1 HST ssues the Measurement Report ncludng the RSS of two antennas from TRS to the servng cell perodcally. If the servng cell decdes to trgger a handover for the front antenna, t sends a handover request message ncludng the selected RAU ndex to the target cell. 2 The target cell makes an admsson control & confgure and allocates transmsson power accordng to the ndex message. Then t sends a handover request ACK to the servng cell. b. Handover Executon 3 The servng cell contnues transmttng data receved from the core network to the rear antenna of HST. Meanwhle, the target cell requests one duplcated data from the core network by dual-castng. 4 Once TRS receves the Handover Command of the front antenna from the servng cell, the front antenna breaks the connecton wth the servng cell and establshes a new connecton wth the selected RAU n the target cell. 5 When the rear antenna enters the overlap regon and receves the handover command from the servng cell whch decdes to trgger a handover for t based on the Measurement Report, t starts a handover procedure and connects wth the target cell. c. Handover Completon 6 When the rear antenna fnshes the handover, the servng cell requests to fnsh dual-castng and the core network makes a request ACK. Then, the data s only transmtted to the target cell and the servng cell releases the resource assocated to the TRS. In ths procedure, the handover of two antennas occurs at dfferent tme, so t can be ensured that at least one antenna receves data from the core network by the servng cell or target cell. Therefore, the passengers n the tran can enjoy an seamless wreless communcaton servce. IV. PERFORMANCE ANALYSIS To evaluate the performance of the proposed scheme, we analyze four dfferent metrcs,.e., the handover trgger probablty, the handover occurrence probablty, the handover falure
4 probablty and the communcaton nterrupton probablty for t. It s worth notng that n the followng analyss, the poston of tran s front antenna x s regarded as the reference and we consder the perod of [0,d s ]. Before the analyss, we frst drve the CDF and the PDF of the transmsson lnk. As mentoned prevously, for each cell, only one RAU s selected to transmt sgnals, so the CDF of (x can be gven by ( F (s (x,r = f (s = N max n=1 Φ ( r µ (s n, (x < r n, (x σ (s n, ( where µ (s n, (x = P t P L d (s n, (x and σ (s n, are the mean and standard devaton of (x, respectvely. Then PDF of (x can be expressed by f (s (x,r = N n=1 Φ ( r µ (s n, (x σ (s n, A. Handover Trgger Probablty Π Φ j n (r µ (s j, (x σ (s j, (3. (4 Handover trgger probablty s the probablty of that the handover trgger condton s met at poston x. Based on the dstrbuton of RSS and handover condton, the handover trgger probablty of -th antenna of HST can be gven by (5. P (trg (x,h = Pr 1, (x R (SC N, (x > H (5 = Pr 1, (x > r+h Pr R (SC N, (x = r ( = r 1 F (SC (x,r+h f (TC (x, rdr Snce the handover s trggered between the last RAU of the servng cell and the frst RAU of target cell n RAU selecton scheme, P (trg (x,h can be rewrtten as P (trg (x,h = Pr 1, (x R (SC N, (x > H, (6 where 1, (x CN(P t P L (d (TC 1, (x,(σ (TC 1, 2 and R (SC N, (x CN(P t P L (d (SC N, (x,(σ(sc N, 2. As the sum of Gaussan dstrbuton stll follows Gaussan dstrbuton, (6 can be expressed by P (trg (x,h = 1 Φ B. Handover Occurrence Probablty ( ( ( H P L d (SC N, (x P L d (TC 1, (x ( 2+ ( σ (SC N, σ (TC 2 1, Handover occurrence probablty s the probablty of that the handover occurs at postonx, whch means that at poston x the handover condton s met and before t the handover s not trggered (.e., the handover condton s not met before x. The handover occurrence probablty of the -th antennas on HST n the proposed scheme can be gven by (7. C. Handover Falure Probablty Handover falure occurs when the handover condton s met but the average RSS from the target cell s lower than a threshold T whch s the mnmum RSS to mantan a wreless communcaton. So the handover falure probablty of the -th antenna of HST can be expressed as P (fal (x,h,t (8 = Pr 1, (x < T 1, (x R (SC N, (x > H = Pr 1, (x<t,r(tc 1, (x R(SC N, (x>h Pr 1, (x R(SC N, (x>h = Pr R (SC R N, (x<r H (TC 1, (x=r,r(tc 1, (x<t P (trg (x,h T 1 = P (trg (x,h F (SC (x,r Hf (TC (x,rdr and t can be rewrtten as (9 for our proposed handover transmsson scheme. D. Communcaton Interrupton Probablty Communcaton nterrupton of -th antenna means that durng the trp of HST, RSS from the servng cell 3 s lower than T, resultng n wreless lnk falure. It s an mportant metrc to evaluate the qualty of wreless communcaton servce and can be gven by P (out (x,t = Pr max s = mn s Pr (x (x < T < T = mn F (s s (10 (x,t As for our proposed handover scheme, the communcaton nterrupton occurs only when both two antennas fal to connect to the servng cell. As the channels between the two antennas and the BS are ndependent, the communcaton nterrupton probablty can be gven by P (out (x,t = 2 =1 P(out (x,t (11 = 2 mn sf (s =1 (x,t It can be seen from (11 that two-hst antennas s useful n mprovng the communcaton nterrupton performance durng the trp of the HST. V. NUMERICAL RESULTS AND DISCUSSION Some numercal results are provded to show the performance of our proposed handover scheme here. For comparson, three other schemes are also smulated,.e., two-hst antennas and DAS cells wth blanket transmsson scheme (DAS-B, sngle HST antenna and DAS cells wth RAU selecton transmsson scheme (DAS-S and two-hst antennas and tradtonal cells. Partcularly, as for the RSS of DAS-B, t s a sum of lognormal varables whch does not have a closed-form probablty dstrbuton functon, so we use the moment generatng functon (MGF [8] to approxmate t. 3 When -th antenna fnshes the handover, ts former target cell becomes the servng cell.
5 Fg. 3. RSS versus poston of the front antenna x. In the presented smulaton results, each pont of the curves was averaged over 1000 realzatons. The parameters used n smulatons are summarzed n Table I. TABLE I SIMULATION PARAMETER Parameters Value Transmt power of each cell (P T 86dBm Shadow fadng devaton (σ 4dB Path loss model P L (d 31.5+35log 10 (d Dstance between two cells (d s 3000m Number of RAUs n one cell (N 4 Nose densty (N 0 145dBm/Hz Sgnal threshold (T 30dB Hysteress margn (H 2dB Dstance from BS to ralways (d 0 100m Dstance from RAU to ralways (d u 60m Length of tran (L 200m Speed of tran (v 100m/s Fg. 4. Handover trgger probablty durng the overlap. Fg. 3 shows RSS of four dfferent schemes durng the perod of nterest,.e., [0,d s ]. It can be seen that the proposed scheme has the hghest RSS n most tme and the DAS-B has the second-hghest RSS. Compared wth DAS-S, employng two-hst antennas n our proposed scheme yelds a hgher RSS. Compared wth tradtonal cells whch only has a hgher RSS for x = 0 and x = 3000, the proposed scheme can mprove the qualty of RSS by reducng the transmsson dstance wth RAUs. Fg. 4 and Fg. 5 present the handover trgger probablty and handover occurrence probablty versus the poston x. It s known that an effcent handover scheme expects handover occurrence poston around the mddle pont of two neghborng cells. Snce there s a protecton factor, the approprate handover occurrence pont shall shft a lttle to the target Fg. 5. Handover occurrence probablty durng the overlap. cell. The optmal handover trgger probablty should change from 0 to 1 dramatcally when the tran passes the approprate handover occurrence pont. In ths sense, our proposed scheme does obtan a better performance than tradtonal one. It s noted that all the curves ntersect around the pont wth x- coordnate of 1500 n Fg. 4, whch s the mddle pont of the two neghborng BS. In Fg. 6, the proposed scheme acheves a lower handover falure probablty compared wth DAS-B n the boundary regon. Snce the handover trgger probablty of these two schemes s smlar, the dfference n ths performance s caused by the varance of RSS between target cell and servng cell. The proposed scheme can acheve a better performance because the RAU selecton transmsson scheme always allocates P (occ (x,h = Pr = Pr 1, (x R (SC R N, (x > H (TC 1, (x R (SC N, (x > H Pr 1, (τ R (SC N, (τ > H,τ < x 1, (τ R (SC N, (τ > H,τ < x = P (trg (x, H (7 P (trg (τ,hdτ
P (fal (x,h,t = Pr 1, (x < T = 1 P (trg (x, H 1, (x R (SC T ( r H (P t P L (d (SC N, (x Q N, (x > H σ (SC N, = Pr R (SC R N, (x<r H (TC 1, (x=r,r(tc 1, (x<t P (trg (x,h ( 1 exp (TC 2πσ 1, ( r (P t P L (d (TC 1, (x 2σ (TC 1, 6 2 dr (9 Fg. 6. Handover falure probablty durng the overlap. Fg. 7. Communcaton Interrupton Probablty durng the overlap. the power to the RAU closet to HST and makes the transmsson suffer mnmum pathloss. As for DAS-S and tradtonal cell, the former one has a smlar performance n handover falure probablty wth the proposed scheme and the later one always suffers a very hgh handover falure probablty snce the BSs are too far away from HST when the tran s movng n the boundary regon. Based on Fg. 5 and Fg. 6, a very mportant nsght s observed that there exsts an approprate poston wth hgh-qualty receved sgnals for actvatng the handover to get very low handover falure probablty. Such a poston s often near to the mddle pont of the overlap of two neghborng cells and hghly depends on the handover trgger RSS protecton factor. Fg. 7 plots the communcaton nterrupton probablty of three dfferent schemes all adoptng DAS cells durng the overlap. It can be observed that our proposed scheme has the lowest communcaton nterrupton probablty among the four dscussed schemes. Ths certfcates that the RAU selecton transmsson scheme provdes a better RSS. VI. CONCLUSION Ths paper proposed a RAU selecton asssted handover scheme for HSR system, whch adopted two-hst antennas and DAS cells to provde seamless wreless coverage and mprove the qualty of RSS so that the handover falure probablty could be reduced. The expressons of the receved sgnal strength, the handover trgger probablty, the handover occurrence probablty, the handover falure probablty and the communcaton nterrupton probablty of our proposed method were derved. Both the analytcal and smulaton results show that the proposed handover scheme acheves a better handover performance compared wth the current exstng ones. ACKNOWLEDGMENT Ths work was supported by the Natonal Basc Research Program of Chna (973 Program, no. 2012CB316100(2, by the Open Research Fund of Natonal Moble Communcatons Research Laboratory, Southeast Unversty, no. 2014D03 and also by the Bejng Natural Scence Foundaton, no. 4162049. REFERENCES [1] J. Calle-Sanchez, M. Molna-Garca, J. I. Alonso and A. Fernandez- Duran, Long term evoluton n hgh speed ralway envronments: Feasblty and challenges, Bell Labs Techncal Journal, vol. 18, no. 2, pp. 237-253, Sep. 2013. [2] T. Ln, J. L, Y. Huang, J. L. Sh and J. H. Zhou, Seamless Dual-Lnk Handover Scheme n Broadband Wreless Communcaton Systems for Hgh-Speed Ral, IEEE J. Select. Areas Commun., vol. 30, no. 4, pp. 708-718, May 2012. [3] Z. Lu and P. Z. Fan, An effectve handover scheme based on antenna selecton n ground-tran dstrbuted antenna systems, IEEE Trans. Veh. Technol., vol. 63, no. 7, pp. 3342-3350, Sep. 2014. [4] W. Cho and J. Andrews, Downlnk performance and capacty of dstrbuted antenna systems n a multcell envronment, IEEE Trans. Wreless Commun., vol. 6, no. 1, pp. 69-73, Jan. 2007. [5] C. Z. Yang, L. H. Lu, C. D and X. M. Fang, An on-vehcle dual-antenna handover scheme for hgh-speed ralway dstrbuted antenna system, n Proc. IWCMC, pp. 1-5, Sep. 2010. [6] X. Y. Qan, H. Wu and J. Meng, A Dual-Antenna and Moble Relay Staton Based Handover n Dstrbuted Antenna System for Hgh-Speed Ralway, n Proc. IMIS, pp. 585-590, July 2013. [7] 3GPP R3-060741: Inter-RAT Moblty for Real-tme flows, Shangha, Chna, May 2006. [8] N. Mehta, J. Wu, A. Molsch and J. Zhang, Approxmatng a sum of random varables wth a lognormal, IEEE Trans. Wreless Commun., vol. 6, no. 7, pp. 2690-2699, Jul. 2007.