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1 Durham Rsarch Onlin Dpositd in DRO: 28 Octobr 215 Vrsion of attachd l: Accptd Vrsion Pr-rviw status of attachd l: Pr-rviwd Citation for publishd itm: Zhou, Tao and Tao, Chng and Salous, Sana and Liu, Liu and Tan, Zhnhui (216) 'A LTE-basd channl masurmnt mthod for high-spd railway communications.', IEEE transactions on instrumntation and masurmnt., 65 (1). pp Furthr information on publishr's wbsit: Publishr's copyright statmnt: c 215 IEEE. Prsonal us of this matrial is prmittd. Prmission from IEEE must b obtaind for all othr uss, in any currnt or futur mdia, including rprinting/rpublishing this matrial for advrtising or promotional purposs, crating nw collctiv works, for rsal or rdistribution to srvrs or lists, or rus of any copyrightd componnt of this work in othr works. Additional information: Us policy Th full-txt may b usd and/or rproducd, and givn to third partis in any format or mdium, without prior prmission or charg, for prsonal rsarch or study, ducational, or not-for-prot purposs providd that: a full bibliographic rfrnc is mad to th original sourc a link is mad to th mtadata rcord in DRO th full-txt is not changd in any way Th full-txt must not b sold in any format or mdium without th formal prmission of th copyright holdrs. Plas consult th full DRO policy for furthr dtails. Durham Univrsity Library, Stockton Road, Durham DH1 3LY, Unitd Kingdom Tl : +44 () Fax : +44 ()

2 A LTE-Basd Channl asurmnt thod for High-Spd Railway Communications Tao Zhou 1, Chng Tao 1, Sana Salous 2, Liu Liu 1, Zhnhui Tan 1 1 Institut of Broadband Wirlss obil Communications, Bijing Jiaotong Univrsity, Bijing 144, P.R.China 2 School of Enginring and Computing Scincs, Durham Univrsity, Durham DH1 3LE, UK taozhou.china@gmail.com, chtao@bjtu.du.cn, sana.salous@durham.ac.uk, {liuliu, zhhtan}@bjtu.du.cn Abstract Bcaus of th masurmnt rstriction and masurmnt fficincy issus of applying convntional channl soundrs in high-spd railway (HSR) scnarios, railway ntwork basd channl masurmnt mthods hav rcntly attractd much attntion. A mthod that mploys long-trm volution (LTE) railway ntworks to achiv channl masurmnts for HSR communications is prsntd. Th principl of th mthod is dscribd and th channl sounding prformanc is analyzd. Th mthod is implmntd by a novl masurmnt systm which can nabl th collction of tim-frquncy-spac channl data. Basd on th systm, fild masurmnts that considr both dirct and rlay covrag schms ar conductd on Bijing to Tianjin HSR in China. asurmnt data ar partitiond into singl-link cas in which th common channl paramtrs can b xtractd and multi-link cas in which th corrlation btwn diffrnt links can b charactrizd. Finally, statistical rsults, involving path loss (PL), Rican K-factor, root man squar (RS) dlay sprad (DS), singl-link spatial corrlation (SC) and multi-link SC, ar prsntd, which not only confirm th viability of th proposd mthod but also provid ralistic channl charactristics availabl for th study of both xisting HSR LTE systms and futur HSR ddicatd systms. Indx Trms Channl masurmnt, high-spd railway, long-trm volution (LTE), propagation charactrization, multi-link spatial corrlation I. Introduction High spd railway (HSR) communications, consisting of profssional communication and public

3 communication, ar ssntial parts in th whol HSR systm. Th profssional communication aims at guaranting th scurity of railway opration, whras th public communication is to mt th in-journy xprinc of passngrs. Th lading global systm for mobil communications for railway (GS-R) is primarily in charg of th train control data transmission, which can only provid low data rat srvics,. g., dispatching, shunting, and maintnanc. Profssional broadband srvics, involving onboard vido survillanc and track monitoring, as wll as public srvics, such as Intrnt accss, onlin TV, and mobil srvics, far xcd th capability of GS-R. In fact, th nxt-gnration HSR ddicatd communication systm is nvisiond to achiv profssional and public broadband communications simultanously [1]. Sinc th radio channl dtrmins th prformanc of wirlss communication systms, dtaild knowldg and accurat charactrization of its paramtrs in ralistic HSR propagation scnarios is crucial. Th majority of radio channl modls usd for systm simulation ar basd on xtnsiv channl masurmnt data. Thrfor, channl masurmnts ar th prcondition for th dsign of HSR communication systms and th valuation of HSR communication tchnologis. Though masurmnt campaigns on HSR ar xpnsiv, tim-consuming, and difficult to carry out, a fw HSR channl masurmnts hav bn conductd, which can b classifid into two catgoris: 1) Channl soundr basd masurmnts. Thr ar only svral masurmnt campaigns [2-4] takn undr high mobility conditions using commrcial channl soundrs, such as RUSK and Propsound. And non-commrcial channl soundrs [5-6] hav not yt bn usd in th HSR channl masurmnts until now. On of th first rportd HSR masurmnts mployd th RUSK soundr to masur th singl-input multipl-output (SIO) rlay channl in Grmany [2]. Propsound HSR channl masurmnts in rural and hilly scnarios in Taiwan wr rportd in [3]. Th dirct link was considrd to masur th SIO and th multipl-input singl-output (ISO) channl. Furthr, singl-input singl-output (SISO) mobil rlay channl masurmnts using Propsound wr carrid out in viaduct scnarios on HSR in China [4]. Howvr, thr ar still no rportd multipl-input multipl-output (IO) channl masurmnts in a spcific HSR scnario utilizing th standard channl soundrs. 2) Railway ntwork basd masurmnts. Du to th masurmnt rstriction and masurmnt fficincy issus of applying traditional channl soundrs in HSR scnarios [7], sinc 211 som rsarchrs rsortd to railway ntwork basd channl masurmnt mthods. A sris of GS-R channl masurmnts wr conductd in viaduct scnarios on HSR in China [8]. For channl

4 charactrization purposs, th GS-R signal is rgardd as a narrowband continuous wavform (CW) signal and hnc not suitabl for widband masurmnts. To nabl th widband channl charactrization, th common pilot channl (CPICH) signal in WCDA railway ntworks was collctd and analyzd to xtract th multipath proprtis [9]. Unfortunatly, th masurmnt bandwidth of this mthod dos not mt th rquirmnts of th futur HSR ddicatd communication systm, and it lacks th spatial sounding ability. To th bst of our knowldg, xisting HSR channl masurmnt data ar insufficint for th dsign of th nxt-gnration HSR ddicatd communication systm. It is ncssary to carry out mor masurmnts for th furthr HSR channl charactrization. Rportd masurmnt campaigns do not provid a suitabl approach with th potntial of nabling comprhnsiv and fficint HSR channl masurmnts. In rcnt yars, thr hav bn larg-scal dploymnts of long trm volution (LTE) ntworks around th world. In particular, th LTE railway ntworks hav almost covrd all HSRs in China, for a total of 15, km by 214, and as HSRs continu to grow, th LTE railway ntworks that grow with thm will xcd 3, km in 22 [1]. Dpnding on th currnt LTE railway ntworks, w prsnt and implmnt a LTE-basd HSR channl masurmnt mthod. Th contributions and novltis of this papr ar summarizd as follows: 1) A channl sounding mthod that mploys th cll-spcific rfrnc signal (CRS) of LTE as th xcitation signal is proposd to xtract th channl impuls rspons (CIR). Th CIR prformanc is analyzd by considring impacts of carrir frquncy offst (CFO) and timing offst (TO). 2) A channl masurmnt systm composd of th LTE railway ntwork and th LTE soundr is built up for th collction of tim-frquncy-spac channl data. SISO and IO channl masurmnts considring dirct covrag (DC) and rlay covrag (RC) schms ar conductd on Bijing to Tianjin (BT) HSR in China. Th masurmnt data ar furthr partitiond into singl-link cas and multi-link cas, which nabl not only singl-link channl charactrization but also multi-link channl charactrization. 3) Basd on th masurmnt data, in addition to th convntional HSR channl charactristics,. g., path loss (PL), Rican K-factor, and root man squar (RS) dlay sprad (DS), spatial corrlations (SCs), involving singl-link and multi-link SCs, ar drivd and analyzd. Ths rsults confirm th viability of th proposd mthod and provid th usful information for th optimization of th currnt HSR LTE systm and th dsign of th nxt-gnration HSR ddicatd communication systm.

5 Th rmaindr of this papr is outlind as follows. Sction II prsnts th LTE-basd channl sounding mthod. In Sction III, th LTE-basd HSR channl masurmnt campaigns ar dscribd. asurmnt rsults ar drivd and analyzd in Sction IV. Finally, conclusions ar drawn in Sction V. II. LTE-Basd Channl Sounding thod In this sction, th fram structur of th xcitation signal will b dscribd firstly. Thn th principl and mthodology for CIR xtraction will b prsntd. Aftr that, impacts of CFO and TO on th CIR prformanc will b analyzd. A. Excitation Signal LTE supporting frquncy division duplxing (FDD) and tim division duplxing (TDD) adopts orthogonal frquncy division multiplxing (OFD) and IO tchnologis. Typical dployd carrir frquncis ar in th rang of 4 Hz to 4 GHz, with scalabl carrir bandwidths from 1.4 Hz to 2 Hz. Th tim-frquncy-spac FDD LTE fram structur is shown in Fig. 1. On radio fram of 1 ms is subdividd into tn 1 ms subframs, ach of which is split into two.5 ms slots. In th cas of th normal cyclic prfix (CP), on slot compriss svn OFD symbols and th spacing of subcarrirs in on OFD symbol is 15 khz. Th CRS mbddd in th tim-frquncy-spac plan is usd for channl stimation in th LTE downlink. Whn it coms to channl sounding, th CRS can b rgardd as a kind of xcitation wavform, whos signal structur dtrmins th masurmnt capability. As shown in Fig. 1, in th tim dirction, th maximum rptition priod of th CRS is.5 ms, corrsponding to 2 khz channl sampling rat and 1 khz maximum xpctd Dopplr shift. In th frquncy dirction, thr is on rfrnc symbol of th CRS vry six subcarrirs with a spacing of 9 khz on ach OFD symbol. This spacing dtrmins th maximum tim dlay window of 11 s. In th cas of 2 Hz LTE, th total numbr of rfrnc symbols on ach OFD symbol is 2, which corrsponds to 18 Hz masurmnt bandwidth and 56 ns tim dlay rsolution. In th spac dirction, whn a rsourc lmnt (RE) carris th rfrnc symbol on on antnna port, th corrsponding RE on anothr antnna port is unusd. This guarants th orthogonality of th CRSs on th 4 antnna ports, thus nabling th IO channl masurmnt with th full paralll architctur [11] at th transmittr.

6 Fig. 1 Th tim-frquncy-spac FDD LTE fram structur Dnoting th complx-valud LTE signal on th i -th subcarrir of on OFD symbol as Si, th transmittd signal in radio frquncy (RF) is givn by N 1 1 xt Si i N N j2 it/ Ts j2 fct,,1,, 1 (1) i whr N is th numbr of subcarrirs on on OFD symbol, T s dnots th sampling priod, f c indicats th cntr frquncy, and rprsnts th carrir phas. Th mbddd CRS is quadratur phas shift kying (QPSK) modulatd, which can b writtn as [12] 1 1 Sk ( ) 12 c(2 k) j 12 c(2k1), k,1,..., 1, (2) 2 2 whr is th numbr of CRSs on on OFD symbol and cx ( ) is a lngth-31 Gold squnc with diffrnt initialization valus dpnding on th cll idntity (ID). Th CRS is similar to th multi-carrir sprad spctrum signal (CSSS) applid in th RUSK soundr, which not only nsurs prcis concntration of th nrgy in th band of intrst but also nabls corrlation gain basd on th frquncy domain corrlation procssing [13]. Th corrlation gain of th CRS is 1log,.g., in th cas of 2 Hz LTE, 2 and th corrlation gain is about 23 db. In Fig. 2, an xampl of th CRS during on fram priod in th cas of 2 Hz LTE is givn in tim and frquncy domains.

7 agnitud Tim [ms] 1.8 agnitud Frquncy [Hz] Fig. 2 CRS wavform in tim and frquncy domains B. CIR Extraction Th transmittd xcitation signal will xprinc th propagation fading in tim, frquncy and spac. Dnoting th tim-variant multipath channl btwn on transmittr and on rcivr as ht, k, th rcivd signal is th convolution of th transmittd signal and th channl, xprssd as tk, whr rprsnts th convolution opration and y t x t * h t, (3) t is th additiv whit Gaussian nois (AWGN). At th rcivr, th discrt-tim basband signal can b acquird according to down convrsion and A/D sampling, writtn as whr r n y t c c c tnts t j 2 fˆ ˆ ct N 1 1 j2 intst/ NTs j2 fcntst S i hn, k n, N i j f fˆ 2 fˆ ˆ cnts f indicats th CFO, t n. dnots th TO, and n Hr, w tak on OFD symbol as an xampl to xplain th CIR xtraction. Assum that fram and slot synchronizations ar achivd. Aftr th discrt Fourir transform (DFT) opration, th rcivd signal in th frquncy domain can b rprsntd as (4)

8 R i N 1 n 1 N r n j2 ni/ N N1 N1 m n j2 n m i / N fcts j2 mt/ NTs j2 fct H k S m i N 1 j2 mi NfcTs 1 1 j2 mt/ NTs j2fct H ksm i, j2 mi / NfcTs N m 1 (5) i i i N. j2 / whr Th CRS, Rk, can b xtractd from th rcivd frquncy domain signal according to th CRS pattrn in th LTE fram structur. Thn, th frquncy domain corrlation procssing is prformd as Hˆ k R k S k 1 j2 mk f T 1 1 c s j2 mt/ Ts j2fct HkSmS k (6) j2 mk/ fcts m 1 ksms k, whr * dnots th conjugation opration. Finally, th stimation of th CIR can b obtaind from (6) via th invrs DFT (IDFT) as hˆ z 1 1 k k Hˆ k H k j2 kz/ 1 j2fcts j2fcts j2 kt/ Ts j2 fct j2 kz/ k 1 1 j2 mk fcts 1 1 HkSmS k 1 2 j2 mk / fcts k m, mk j2 mt/ Ts j2 fct j2 kz/, (7) and SmS k whr z,1,, j2 kz/ k k. In (7), th first trm rfrs to k th xpctd CIR with th impacts of CFO and th TO, th scond trm stands for th intr-carrir intrfrnc (ICI) ffct, and th third trm is th AWGN. C. CIR Prformanc Analysis Impact of CFO on CIR: In ordr to analyz th indpndnt impact of CFO on th xtractd CIR, stting t and k. Thus, th quation (7) is writtn as

9 hˆ z j2 fcts 11 hz j2fct s j2 mk fcts HkSmS k j2 m k/ fcts k m, mk 1 Th normalizd CIR can b xprssd by j2 kz/. (8) 1 1 j2fcts 1 1 n j2 mk/ fcts k m, mk hˆ z h z H k S m S k 1 h z I z, f, whr I z f, c c j2 kz/ rprsnts th ICI ffct with diffrnt CFOs. Equation (9) mans that th CFO causs an additiv intrfrnc on th normalizd CIR, in othr words, th CFO has an influnc on th nois floor in th CIR masurmnt. Fig. 3 illustrats th simulation rsult of magnitud squar of th normalizd CIR in th cas of diffrnt CFOs using th 2 Hz LTE signal. It can b sn that th CFO raiss th nois floor and dcrass th dynamic rang of multipath idntification. In addition, th CFO yilds an xtra frquncy shift, which will affct th masurmnt of Dopplr charactristics. In practic, to avoid th CFO in th outdoor channl masurmnt, a common way is th us of Rubidium or GPS rfrnc clock, which can guarant th frquncy consistncy at th transcivr. (9) Normalizd powr [db] f c =.1KHz f c =1KHz f c =5KHz -6 Fig. 3 agnitud squar of th normalizd CIR in th cas of diffrnt CFOs using th 2 Hz LTE signal. Impact of TO on CIR: Stting f c highlightd. Equation (7) is simplifid as Path indx and k, th isolatd impact of TO on th xtractd CIR is

10 1 ˆ 1 hz Hk k 1 1 k h zt/ T, j2 kzt/ Ts / Hk (1) s j2 kt/ Ts j2 kz/ whr t t1 t2. t and 1 t2 dnot th intgr TO and fractional TO, rspctivly. Th intgr TO lads to th cyclic shift of th CIR, which has no influnc on th powr and rlativ dlay stimation of th multipath, whras th fractional TO causs th powr lakag of th CIR. Fig. 4 shows th simulation rsult of magnitud squar of th normalizd CIR in th cas of diffrnt fractional TOs using th 2 Hz LTE signal. It is obsrvd that th lakag svrity of th CIR is mor srious with th incras of th fractional TO, and maximum lakag appars at t2.5t s. Th lakd powr xponntially sprads to othr paths, which will affct th powr stimation of th multipath. In [14], a hanning window function with a narrow mainlob and low sidlobs is applid to rduc th powr lakag of th CIR. In this papr, w us th simpl windowing mthod to tackl th fractional TO issu. t 2 =.5T s -1 t 2 =.2T s t 2 =.5T s Normalizd powr [db] Path indx Fig. 4 agnitud squar of th normalizd CIR in th cas of diffrnt fractional TOs using th 2 Hz LTE signal. III. LTE-Basd HSR Channl asurmnt Campaigns This sction will rport on th implmntation of LTE-basd HSR channl masurmnt campaigns. W will first show th stup of th masurmnt systm, and subsquntly will dscrib th fild masurmnts. W will also mak a classification for th masurmnt data.

11 A. asurmnt Systm Stup Th stup of th LTE-basd HSR channl masurmnt systm is shown in Fig. 5. This systm consists of th LTE railway ntwork and th LTE soundr. Th LTE soundr is usd to collct th tim-frquncy-spac channl data in th whol railway lin, which maks continuous masurmnts fasibl and thus improvs th masurmnt fficincy. Unlik th convntional cllular ntwork, th railway ntwork adopts a narrow strip covrag mod and a building basband unit (BBU) plus rmot radio unit (RRU) structur. On bas station (BS) sit has two RRUs which mploy dirctional antnnas to transmit radio frquncy (RF) signals in opposit dirctions along th track. Cll combination tchnology [1] is usd in this ntwork, which combins svral RRUs into a big narrow-strip-shapd cll via optical fibr. Ths RRUs ar controlld by th BBU that is in charg of RF signal procssing. Du to such a ddicatd structur, th ntir ntwork can b classifid into two catgoris: non-ovrlappd covrag rgion and ovrlappd covrag rgion. In th non-ovrlappd covrag ara, only on signal from on BS is rcivd. In th ovrlappd ara, howvr, th two sam signals from nighboring BSs arriv at th rcivr simultanously. From th dlay domain prspctiv, ths two signals can b distinguishd according to diffrnt propagation dlays. BBU ` RRU RRU LTE Signal Non-ovrlappd covrag rgion GPS Antnna LTE Railway Ntwork RF Antnnas Ovrlappd covrag rgion LTE Soundr Fig. 5 LTE-basd channl masurmnt systm

12 Fig. 6 Hardwar diagram of LTE soundr Diffrnt from th convntional channl soundrs, th LTE soundr has only a rcivr as shown in Fig. 6. Th LTE soundr consists of two RF units, a data acquisition card, a data transmission card, a GPS rfrnc clock, a PC, a solid stat disk (SSD) as wll as a GPS rcordr. Th rciv antnnas ar connctd to th RF units that output 7 Hz intrmdiat frquncy (IF) signals. High-frquncy down-convrtr of th RF unit uss a broad frquncy synthsizr in th rang of 2 Hz-3 GHz (which can b configurd with a stp of 1 khz) and a bandwidth of 2 Hz [15]. Automatic gain control (AGC) of th RF unit can b disabld, and th RF gain can b controlld. Th outputs of th RF units ar transfrrd to th data acquisition card whr th IF signals ar sampld in a dual-channl 14-bit A/D convrtor (ADC) with a maximum sampling rat of 15 SPS. Th ADC is configurd oprating at 56 SPS to mt th rquirmnt of band-pass sampling in th cas of 7 Hz IF and 2 Hz bandwidth. In th data transmission card, th digitizd IF signal transmission is controlld by a Virtx-5 FPGA and a DDR2 SDRA is mployd to bridg data strams btwn th PC mmory and th ADC. Th PC stors th collctd data in th high-spd SSD [16] with 1TB capacity storag mmory. Th GPS rfrnc clock [17] with 1-11 frquncy accuracy provids a common 1 Hz rfrnc clock signal for th RF units, data acquisition card and data transmission card. In addition, th GPS rcordr is usd to collct th ral-tim navigation data involving GPS, tim, and spd information. B. Fild asurmnt Dscription Basd on th stablishd systm, fild masurmnts ar prformd on BT HSR in China. Fig. 7 illustrats th LTE ntwork structur and propagation nvironmnt on BT HSR. Th chosn ntwork covrs about 16 km distanc with 15 BSs and 3 RRUs. Th maximum spacing btwn nighboring BSs is around 1.2 km. Thr ar two typs of cll structurs srving for 265 Hz and 189 Hz ntworks, rspctivly. Th 265 Hz ntwork has 6 clls, ach of which contains 3 BSs or 2 BSs, whras th 189 Hz ntwork

13 has 3 biggr clls with 5 BSs ach. Th BS is gnrally lss than 2 m away from th railway which is built on viaduct. Thr ar thr typical BS hights, such as 1 m, 2 m and 35 m, which indicat th vrtical distanc btwn th BS antnna and th viaduct. Along th railway lin, plain is th main propagation nvironmnt whr thr ar usually a larg rang of opn ara, light forstation or a fw buildings with an avrag hight of lss than 1 m. 1.2 Km LTE BS Viaduct Bijing 16 Km Plain Environmnt Cll_265 Cll_189 BS Track Tianjin Fig. 7 LTE ntwork structur and propagation nvironmnt on BT HSR Two HSR wirlss covrag schms, DC and RC, ar considrd in our masurmnts. SISO channl masurmnts ar carrid out using th 265 Hz ntwork for th DC scnario, whras 2 2 IO channl masurmnts ar prformd mploying th 189 Hz ntwork for th RC scnario. Fig. 8 shows th masurmnt quipmnt in th two scnarios and Tab. 1 lists th corrsponding masurmnt paramtrs. At th transmittr, th BS uss ±45 cross-polarizd dirctional antnnas to transmit th CRSs with th powr of 12.2 dbm. At th rcivr, as for th RC cas, th LTE soundr adopts th profssional train-mountd antnnas, HUBER+SUHNER [18], to rciv th xcitation signal. In th tst, antnnas 5 and 6 with th spacing of 1.2 m (7.6 wavlngths at 189 Hz) ar chosn for th IO masurmnt, antnna 3 is connctd to spctrum analyzr to monitor th signal stat, and antnna 1 is usd to rciv th GPS signal. With rgard to th DC cas, a bi-conical antnna with bttr antnna radiation pattrn is utilizd and placd clos to th window of th train carriag.

14 Fig. 8 asurmnt quipmnt in th two scnarios. (a) RC cas. (b) Structur of train-mountd antnnas. Antnnas 1, 3, 5, 6 can b usd for th widband signal tst, whil antnnas 2 and 4 can only b availabl for th spcific GS-R ntwork. (c) DC cas. Tab. 1 asurmnt paramtrs asurmnt scnario DC RC asurmnt frquncy [Hz] asurmnt bandwidth [Hz] CRS Transmittd powr [dbm] Transmit antnna typ ±45 cross-polarizd ±45 cross-polarizd Transmit antnna gain [dbi] Horizontal half-powr bamwidth of transmit antnna [dg] 6 67 Vrtical half-powr bamwidth of transmit antnna [dg] Elctric tiltd angl of transmit antnna [dg] 3 3 Rciv antnna typ Bi-conical HUBER+SUHNER Rciv antnna gain [dbi] 8.5 Rciv antnna numbr 1 2 Rciv antnna spacing [m] (7.6 wavlngths) Train vlocity [km/h] C. asurmnt Data Partitioning Sinc th masurmnt data ar collctd in both th non-ovrlapping and ovrlapping aras of th ntwork, thy can b rasonably partitiond into two cass: singl-link and multi-link masurmnt data. As shown in Fig. 9, w tak an xampl of th powr dlay profil (PDP) and Dopplr powr spctral dnsity (DPSD) in on cll in th RC scnario to xplain how to partition th masurmnt data. In Fig. 9(a), th tim-variant PDP during th first 35 s priod in th cll is plottd. Two obvious PDP transitions rgarding BS1 and BS2 whos positions ar idntifid by th whit circls ar highlightd. Whn th train ntrs into th non-ovrlapping ara, w dfin a singl-link rgion whr th propagation link btwn BS1 and th train, indicatd by PDP (i), occupis th whol tim dlay window. Howvr, onc th train movs into th

15 ovrlapping ara btwn BS1 and BS2, anothr propagation link btwn BS2 and th train, dnotd by PDP (ii), appars in th tim dlay window as wll. W rgard this ara as a multi-link rgion whr th tim dlay window covrs two links simultanously. In th multi-link rgion, th tim dlay window can b dividd into two parts: on is for PDP (i) and anothr is for PDP (ii), as illustratd in Fig. 9(b). As th train is moving away from BS1, th dlay diffrnc btwn PDP (i) and PDP (ii) is gradually shortning. If th train travls at th crossing point markd with a rd circl in Fig. 9(a), th PDP (i) and PDP (ii) would b indistinguishabl,. g.,. Sinc dtrmins th tim dlay window of PDP (i), a thrshold, 1 s, is st to nabl th covrag of most multipath componnts. Fig. 9(c) plots th corrsponding tim-variant DPSD rsult. Whn th train passs through th covrag of BS1 and BS2, two typical Dopplr transitions from th maximum positiv frquncy to th minimum ngativ frquncy ar obsrvd and markd as DPSD (i) and DPSD (ii). In th singl-link rgion only DPSD (i) xists, whil in th multi-link ara DPSD (i) and DPSD (ii) appar at th sam tim, as shown in Fig. 9(d). Th two DPSDs hav th sam maximum Dopplr shift but opposit angls of arrival. In th singl-link rgion w xprss th masurd CIR as H S ij and in th multi-link rgion w xtract th CIRs from BS1 and BS2 according to th abov analysis, rprsntd as,1 H ij and,2 H ij. Not that i and j ar th indics of th antnna lmnts at th BS and th rcivr sids, rspctivly. In addition, w choos 6 groups of DC masurmnt data and 3 groups of RS masurmnt data, ach of which corrsponds to on cll. Thn, w avrag all th rsults drivd from ths masurmnt data to analyz th channl charactristics in th whol ntwork.

16 BS2 PDP (ii) Crossing point PDP (i) BS1 (a) Crossing point BS2 BS1 DPSD (i) ulti-link rgion Singl-link rgion DPSD (ii) Powr [db] Powr [db] Powr [db] PDP (i) Excss tim dlay [s] x PDP (i) PDP (ii) Excss tim dlay [s] x (b) DPSD (i) PDP (ii) DPSD (ii) Dopplr frquncy [Hz] DPSD (ii) DPSD (i) Singl-link rgion ulti-link rgion Powr [db] -1-2 (c) Dopplr frquncy [Hz] (d) Fig. 9 An xampl of th masurd PDP and Dopplr PSD in on cll in th RC scnario. (a) Tim-variant PDP. (b) Two snapshots of th PDP in th multi-link rgion. Th top on is capturd whn th train is clos to BS1, whras th bottom on is rcordd whn th train movs away from BS1. (c) Tim-variant DPSD. (d) Two snapshots of th DPSD in th multi-link rgion. Th top on is acquird whn th train gos away from BS1, whras th bottom on is obtaind whn th train is clos to th crossing point. IV. asurmnt Rsults and Analysis In this sction, w first focus on th traditional channl charactristics, involving PL, Rican K-factor, RS DS, and singl-link SC, which ar xtractd using th masurmnt data in th singl-link rgion. W also analyz th multi-link SC basd on th masurmnt data in th multi-link rgion.

17 A. Path Loss To xtract th PL, th masurd CIR should b avragd within a local ara, whr th condition of wid-sns stationary and uncorrlatd scattring (WSSUS) is satisfid. Th avragd lngth is commonly chosn to b 2 wavlngths in th mobil masurmnt [19]. Th PL is convntionally modld as a function of logarithmic distanc. Th stimatd PL modls in db for th DC and RC scnarios ar givn as and L log1 P d d (11) P d log1 d, (12) L whr d is th distanc btwn th BS and th train. Fig. 1 illustrats th masurd PL rsults and th PL modls in th DC and RC scnarios. It can b sn that th PL in th DC scnario is approximatly 2-3 db highr than that in th RC scnario. This valu corrsponds to th pntration loss from th train body. On th othr hand, th rsulting PL xponnts for th DC and RC cass ar 3.16 and 3.76, rspctivly, which ar all much highr than that of 2. in th fr spac modl. For th DC it is undrstandabl that th outdoor to indoor propagation condition intnsifis th attnuation of th signal powr, howvr, for th RC th main rason which has bn xplaind in [2] [4] is that th train carriag roof would act as a ground plan and thus affct th antnna radiation pattrn by causing a null in a crtain incidnc angl ara of th radiation pattrn Fr spac modl asurd PL in DC scnario PL modl in DC scnario asurd PL in RC scnario PL modl in RC scnario 13 PL [db] Distanc [m] Fig. 1 PLs in th DC and RC scnarios

18 B. Rican K-factor Rican K-factor is a paramtr indicating th tmporal fading svrity of th radio channl, which can b xtractd by th CIRs of rmoving th larg-scal ffct. Th narrowband K-factor [2] is stimatd in th sam st of 1 m windows using th classical momnt basd mthod in [21], as a function of distanc. Fig. 11 prsnts th cumulativ distribution functions (CDFs) of th K-factors and th fitting of th CDFs with a normal distribution in th DC and RC scnarios. A K-factor with a man of 4.27 db and a standard dviation of 2.87 db is obsrvd for th DC cas, whras thy ar 7.47 db and 4.39 db for th RC cas. Th strngth of K-factor in th DC scnario is wakr than that in th RC scnario sinc th rciv antnna placd insid th train carriag ncountrs mor multipath chos. In addition, th K-factor rsult for th RC scnario matchs th K-factor modl with 7 db man valu and 4 db standard dviation for th Ra scnario in ITU-R.2135 modl [22]. In Fig. 12, th K-factors ar plottd as a function of th distanc. Th masurd K-factors appar a linar downward trnd with th incras of distanc. To charactriz th variation of th K-factor, th linar distanc-dpndnt K-factor modls for th DC and RC cass ar givn as and.76d 7.64 K d (13).24d K d (14) It is notd that th K-factor valu in [4] is much largr than that in th RC scnario. On rason is that th K-factor modl in [4] rfrs to th widband K-factor drivd from th data of th main path or th strongst path, and th scattring componnts in othr multipath ar not includd in th widband K-factor stimation.

19 1.9.8 DC masurmnt DC normal distribution RC masurmnt RC normal distribution.7.6 CDF K-factor [db] Fig. 11 CDFs of th K-factors in th DC and RC scnarios 2 15 asurd K-factor in DC scnario K-factor modl in DC scnario asurd K-factor in RC scnario K-factor modl in RC scnario K-factor modl in Rf. [4] K-factor [db] Distanc [m] Fig. 12 K-factors as a function of th distanc in th DC and RC scnarios C. RS Dlay Sprad Th RS DS is dtrmind by th PDP at local ara, which can b calculatd as th standard dviation of th xcss tim dlay wightd with th powr [23]. Fig. 13 shows th CDFs of th RS DSs and th fitting of th CDFs with a lognormal distribution in th DC and RC scnarios. It is obvious that th DC cas has th highr RS DS than th RC cas sinc th indoor antnna rcivs mor multipath wavs. Th fitting paramtrs log1s and log1s of th RS DS for th RC cas ar and.57, whras

20 thos for th DC cas ar -7.6 and.34. It can b found that th RC rsult is clos to th ITU-R.2135 modl [22] in th Ra scnario with th man valu of and th standard dviation of.55. Fig. 14 dpicts th RS DSs against th distanc. It shows that th RS DS xprinc a rising trnd with th incras of distanc. This is bcaus whn th train is moving away BS, incrasing multipath wavs ar likly to b distinguishd. Th similar phnomnon is also obsrvd in [24] whr th RS DS is modld as a simpl linar function of th distanc. Assum that th RS DS is ns at m, th fitting modls for th DC and RC cass ar givn as and rms d.22d (15) rms d.13 d. (16) It can b sn that th cutting scnario in [24] has th largr RS DS than th masurd viaduct scnario no mattr whthr th antnna is placd insid th carriag or on top of th train CDF DC masurmnt DC lognormal distribution.1 RC masurmnt RC lognormal distribution RS DS [ns] Fig. 13 CDFs of th RS DSs in th DC and RC scnarios

21 asurd RS DS in DC scnario RS DS modl in DC scnario asurd RS DS in RC scnario RS DS modl in RC scnario RS DS modl in Rfrnc [24] RS DS [ns] Distanc [m] Fig. 14 RS DSs as a function of th distanc in th DC and RC scnarios D. Spatial Corrlation To invstigat th IO prformanc, w focus on th SC btwn th diffrnt antnna lmnts at both nds of th individual link, namly, singl-link SC. Bsids, w also considr th multi-link SC [25] which xists du to th nvironmnt similarity arising from common scattrrs contributing to diffrnt links and can significantly affct th prformanc of coordinatd multipoint transmission (CoP) tchnology. Basd on th masurmnt data in th singl-link rgion, th singl-link SC btwn two sub-channls, H S 11 and H S 22, is drivd. Similarly, according to th ffctiv masurmnt data in th multi-link rgion, th multi-link SC btwn,1 H 11 and,2 H 11 is stimatd. Fig. 9 illustrats th singl-link SC rsult in th RC scnario and multi-link SC rsults in th DC and RC scnarios. It is obsrvd that almost 65% of singl-link SC valus ar lss than.8. For th multi-link SC, th rsults ar optimistic in both DC and RC scnarios whr th majority of th corrlation cofficints ar blow.8. orovr, sinc rich scattrrs in th in-train nvironmnt lad to a low dgr of nvironmnt similarity, th DC cas has th lowr multi-link SC than th RC cas. From ths rsults, w can infr that in th plain viaduct scnario th IO prformanc can b improvd by mans of th cross-polarizd antnna configuration at th BS sid and th larg antnna spacing at th train sid. orovr, CoP tchnology could hav a good prformanc in trms of micro-divrsity du to th low multi-link SC.

22 CDF CDF Sing-link SC in RC scnario Corrlation cofficint ulti-link SC in DC scnario ulti-link SC in RC scnario Corrlation cofficint Fig. 9. CDFs of th SCs in th DC and RC scnarios V. Conclusion In this papr, th principl and implmntation of th LTE-basd HSR channl masurmnt mthod wr prsntd. Th CIR xtraction using th CRS is dscribd, and th impacts of CFO and TO on th CIR prformanc wr analyzd, which show that th CFO causs th incras of NF and th fractional TO lads to th powr lakag of th CIR. Basd on th stablishd systm, fild masurmnts taking both DC and RC schms into account wr prformd on BT HSR in China. asurmnt data wr collctd and partitiond for th singl-link and multi-link channl charactrizations. Th masurmnt rsults provd th fasibility of th proposd mthod and providd som ralistic HSR channl information in th plain viaduct nvironmnt. At first, th PL xponnts in both th DC and RC scnarios hav much highr valu than that in th fr spac propagation scnario. In addition, thr ar strongr K-factor and smallr RS DS for th RC cas than for th DC cas. Finally, 65% of singl-link SC valus ar lss than.8 undr th condition of ±45 cross-polarizd antnna configuration at th BS sid and 7.6 wavlngths antnna spacing at th train sid, and th multi-link SC is found to b rlativly low. Acknowldgmnt Th authors would lik to thank Huishng Wang and Yuzhng Zhang from China Acadmy of Railway Scincs for thir hlp to prform th channl masurmnts, and Long Sun from Huawi for th usful

23 discussion of LTE railway ntwork structur and configuration. Th rsarch was supportd in part by th NSFC projct undr grant No , No , and Bijing Natural Scinc Foundation (414241). Rfrnc [1] B. Ai, X. Chng, T. Kürnr, Z. D. Zhong, K. Guan, R. S. H, L. Xiong, D. W. atolak, D. G. ichlson, and C. B. Rodriguz, Challngs toward wirlss communications for high-spd railway, IEEE Tran. Intll. Transp., vol. 15, no. 5, pp , Oct [2] P. Kyösti, WINNER II channl modls part II radio channl masurmnt and analysis rsults, 27. [3] R. Parviainn, P. Kyösti, and Y. Hsih, Rsults of high spd train channl masurmnts, Europan Coopration in th Fild of Scintific and Tchnical Rsarch, Tch. Rp., 28. [4] L. Liu, C. Tao, J. H. Qiu, H. J. Chn, L. Yu, W. H. Dong, and Y. Yuan, Position-basd modling for wirlss channl on high-spd railway undr a viaduct at 2.35 GHz, IEEE J. Sl. Aras Commun., vol. 3, no. 4, pp , ay 212. [5] J. Kivinn, T. O. Korhonn, P. Aikio, R. Grubr, P. Vainikainn, and S. G. Haggman, Widband radio channl masurmnt systm at 2 GHz, IEEE Trans. Instrum. as., vol. 48, no. 1, pp , Fb [6] S. Salous, P. Filippidis, R. Lwnz, I. Hawkins, N. Razavi-Ghods, and. Abdallah, Paralll rcivr channl soundr for spatial and IO charactrisation of th mobil radio channl, IEE Proc. on Commun., vol. 152, no. 6, pp , Dc. 25. [7] L. Liu, C. Tao, T. Zhou, Y. P. Zhao, X. F. Yin, and H. J. Chn, A highly fficint channl sounding mthod basd on cllular communications for high-spd railway scnarios, EURASIP J. Wirlss. Commun., vol. 212, Oct doi:1.1186/ [8] R. S. H, Z. D. Zhong, B. Ai, and J. Ding, An mpirical path loss modl and fading analysis for high-spd railway viaduct scnarios, IEEE Antnnas Wirlss Propag. Ltt., vol. 1, pp , Aug [9] J. H. Qiu, C. Tao, L. Liu, and Z. H. Tan, Broadband channl masurmnt for th high-spd railway basd on WCDA, in Proc. IEEE 75th VTC-Spring, Yokohama, Japan, ay 212, pp.1-5. [1] [Onlin]. Availabl: [11] S. Salous, Radio Propagation asurmnt and Channl odlling, John Wily and Sons Ltd., Wily,

24 213. [12] 3GPP TS V9.1., Tchnical Spcification Group Radio Accss Ntwork; Evolvd Univrsal Trrstrial Radio Accss (E-UTRA); Physical Channls and odulation, 21. [13] [Onlin]. Availabl: [14] T. Pdrsn, G. Stinböck, and B. Flury, odling of rvrbrant radio channls using propagation graphs, IEEE Trans. Antnnas Propag., vol. 6, no. 12, pp , Dc [15] [Onlin]. Availabl: [16] [Onlin]. Availabl: [17] [Onlin]. Availabl: [18] Sncity Rail Antnna: HUBER+SUHNER data sht, HUBER+SUHNERAG RF Industrial, 21. [19] X. Zhao, J. Kivinn, P. Vainikainn, and K. Skog, Propagation charactristics for widband outdoor mobil communications at 5.3 GHz, IEEE J. Sl. Aras Commun., vol. 2, no. 3, pp , Apr. 22. [2] T. Zhou, C. Tao, L. Liu, and Z. H. Tan, Rican K-factor masurmnts and analysis for widband high-spd railway channls at 2.35 GHz, Radionginring, vol. 23, no. 2, pp , Jun [21] L. Grnstin, D. ichlson, and V. Ercg, omnt-mthod stimation of th Rican K-factor, IEEE Comm. Ltt., vol. 3, no. 6, pp , [22] ITU-R.2135, Guidlins for valuation of radio intrfac tchnologis for IT-Advancd, 29. [23] J. H. Zhang, D. Dong, Y. P. Liang, X. Ni, X. Y. Gao, Y. Zhang, C. Huang, and G. Y. Liu, Propagation charactristics of widband IO channl in urban micro- and macroclls, in Proc. IEEE 19th PIRC, Canns, Franc, Spt. 28, pp [24] L. Tian, J. H. Zhang, C. Pan, L. Liu, and C. Tao, Small scal fading charactristics of widband radio channl in th U-shap cutting of high-spd railway, in Proc. IEEE 78th VTC-Fall, Las Vgas, USA, Spt. 213, pp.1-6. [25] X. Chng, C.-X. Wang, H. Wang, X. Gao, X.-H. You, D. Yuan, B. Ai, Q. Huo, L. Song, and B. Jiao, Cooprativ IO channl modling and multi-link spatial corrlation proprtis, IEEE J. Sl. Aras Commun., vol. 3, no. 2, Fb. 212, pp