The Influence of Doppler Effects in HAP-OFDMA Systems

Transcription

1 Th Influnc of Dopplr Effcts in HAP-OFDMA Systs Michal Ban, S. Tapuchi 2 M. Haridi, Miria Ban 3, B. Hill 4,, U. Mahlab J.Gavan 2 HIT-Holon Institut of Tchnology, 2 Sai Shaoon Enginr Collg, 3 Hbrw Univrsity Jrusal, 4 ss Co. banichal@gail.co Abstract w considr th ipact of Dopplr frquncy shift on th Orthogonal Frquncy Division Multipl Accss (OFDMA) signal and Frquncy Ban Signal (FBS) transittd ovr High Altitud Platfors ( HAPs). Thortical analysis and siulations show that OFDMA systs prfor wll whn th Dopplr frquncy shift is sallr than 2% of carrir frquncy sparation. Ths conditions corrspond to opration frquncy of 2GHz and train vlocitis up to 2 /h. FBS thod, on th othr hand, allows woring undr or svr Dopplr ffcts, naly a Dopplr shift of up to 6-8 %, hnc nabling highr carrir frquncy bands and highr vhicl spds. Kywords OFDMA, Dopplr shift, Pilot signals, FBS, HAPs, Mobil counications.. ITRODUCTIO Trrstrial obil radio Counication systs ar xpanding incrasingly and fast. Howvr, lac of Lin of Sight (LOS) propagation conditions rsults in shadowing and ulti-path fading which liit th quality and th opration rang of ths systs.. Th LOS conditions and opration rangs can b xtndd significantly using costly Gostationary (GEO) satllits at an altitud of about 36 [,2] A nw tchniqu for fficint obil radio counication systs is th High Altitud Platfors (HAPs) locatd in th stratosphr at an altitud of th ordr of 7 to 24,whr wind vlocity is inial, for rlaying radio stations [3,4]. Ths HAP stations will b stabilizd rlativ to th ground as Stratosphric Quasi Stationary Platfors (SQ-SP) du to significant progrss in tchnology [5,6]. Th prforancs of HAPs ar in svral aspcts bttr than GEO and LEO satllits and trrstrial radio syst altrnativs [ ] On iportant futur HAPS application is rlatd to radio counications with vry high spd trains or aircrafts. In ths cass HAPs ar significantly bttr than trrstrial radio du to th iprovd LOS propagation conditions,uch lss Dopplr frquncy shift ffcts and fwr hand ovr vnts with rlay stations, as shown in th scnario of figur.th rduction in Dopplr frquncy shifts is du to th rlativly low transission angls bcaus of th HAPS altitud[7,8]. HAPs ar also bttr than GEO satllits du to th significantly lss disprsion losss and ti dlay. Howvr, vn for HAPs with vry high spd vhicls and high carrirs frquncis, rquird for transission of broad band signals and fast intrnt, th significant Dopplr frquncy shift ffcts can dtriorat th quality of th radio lin [,7 ]. A ultiplxing tchniqu rcondd also for th HAP radio systs is th Orthogonal Frquncy Division Multipl Accss (OFDMA which provids iprovd frquncy fficincy and rducs th Multi-Path Propagation (MPP) influnc [9], Howvr OFDMA basd systs ar vulnrabl to poor frquncy synchronization du to Dopplr ffcts. Thus, OFDMA fails to cobat th Dopplr shift influnc. In addition, OFDMA rquirs various additional signals such as high powr pilot signals, which dcras th spctral fficincy spcially with liitd powr systs li HAPs. In OFDMA, pilot signals ay b usd on th sa additional carrirs as in Trrstrial Digital Vido Broadcasting (DVB-T and DVB-H) systs. Bsids pilot signals, th OFDMA signals includ additional guard intrvals. For chcing Dopplr Effct influnc on OFDMA syst an xprintal assssnt of rcivd DVD-T signal quality during otion 6

2 was prford. Exprints wr prford in collaboration with th Masurnt Laboratory of th Israli Counication co. - Bzq. Th Bzq obil laboratory allows asurnt of lctroagntic fild dnsity and Bit Error Rat (BER) aftr Vitrbi dcodr. In addition, th quipnt nabls th obsrvation of iag quality including synchronization disturbancs. A fairly straight sction of road and fr of havy traffic was chosn for conducting xprints (s Figur ) Figur 2b, Spd 2 /h Figurs 2. Dopplr Effct influnc asurnts Figur. Rout and transittr (Tx) location for Dopplr Effct influnc chcing. Two sts of asurnts wr rcordd for two discrt spds: V 4/h and V 2/h. Th rsults of ths xprints on can s on Figur 2. Th nubr of synchronization losss incrass proportionally according to th incras in vhicl spd du to th Dopplr Effct Figur 2a, Spd 4/h Th nhancd OFDMA thod, calld th Frquncy Ban Signal (FBS) thod, is capabl of itigating th Dopplr ffcts, and yild s high spctral fficincy by roving th nd for pilot signals, and hnc achivs highr throughputs. Th basic principls of FBS hav bn publishd in [9]. In this papr w apply th FBS thod to th HAP syst, It is shown that using th FBS thod th throughput of th HAP systs can b gratly nhancd in coparison with coon OFDMA thods. 2. THE EFFECT OF DOPPLER SHIFT O THE OFDM SIGALS In a ulti-path channl th duration of a rcivd sybol is subct to changs du to th variations in th lngth of th signal propagation path. Th chang of th signal duration will b dnotd hraftr by factor d. Howvr, sinc th frquncy changs by th sa factor d th nubr of carrir cycls in ach sybol dos not chang, rgardlss of how fast th Transittrs (Tx) or Rcivrs ( Rx) ov. So in cas of Dopplr Shift, th frquncy f and sybol ti T ar varid by a factor of d [2] V d + cosϕ, () c Whr V is th oving obct vlocity, c is th light vlocity and φ is th angl btwn th dirctions of signal propagation and th oving obcts.ot that undr ths circustancs, th orthogonal condition prvails: 62

3 [( f f ) ] [ T / ] i+ i d d In ral situations, howvr, th orthogonal proprty of th subcarrirs will b violatd to so xtnt. Sinc th rcivr s synchronization syst cannot ract instantly to changs in th sybol priod or in th FFT paratrs, w ay assu that th Rx sybol ti st by th sybol synchronization syst rains at a fixd valu. A Dopplr shift of ω introducd an a phas shift of ωt in th rcivd signal, and an additional phas shift du to incopatibility btwn carrir frquncis and th spctral coponnts aftr FFT,. Furthrs influncs includ ICI (Intr-carrir intrfrncs) du to partial loss of th orthogonal, changs of th pilot signals paratrs, and a ti dlay (or forstalling) btwn adacnt sybols du to incopatibility btwn of th rcivd sybol duration and Rx cloc synchronization syst [, 2]. Fig. 3 dpicts an xapl of a typical HAP syst using th OFDMA tchnology syst [5,] with sybol duration.s, frquncy diffrnc btwn carrirs ( F) Hz, cntral frquncy in th 2GHz L band rang and fro 5.85 to 7.o75 GHz allocatd by th ITU for th futur HAPS. Ths highr frquncy bands ar rquird spcially for broadband obil radio, vhicls at spd of up to 3 /h, and vn or for aircrafts. Th,angl shift chang as function of th distancbtwn th HAP and th vhicl.? Using Equation. () th Dopplr shift is stiatd as ±2.8 Hz or 3% of F. It should b notd that also th rflctd signals undrgo a Dopplr shift. To itigat th ill ffcts of a randoly changing Dopplr frquncy shift, OFDMA uss pilot signals, which lads to a significant incras in powr, and syst rdundancy [9,]. Th HAP systs, on th othr hand, ar charactrizd by th favorabl Rician distribution as th highr altitud and th rsulting transission angls produc a lin of sight.this lads to a lowr lvl of fading and Dopplr ffcts. In addition, th highr opration rangs of a fw hundrds rsults in a significant rduction of hand ovr stps. For siilar scnarios HAPS will b or advantagous than Satllits du to thir significantly lss disprsion losss and lowr cost. Figur 3. Scnario of th radio counication lin btwn HAP and a obil high spd train W now procd with th athatical dscription of th OFDMA signal variations du to Dopplr shift. Th discrt Fourir x is givn by : transfor (DFT) of { } X x i,,..., And th invrs discrt Fourir transfor: i (2) x X,,..., (3). Hr x,,..., ar sapls in ti doain, and X,,..., ar T spctral coponnts. Dfining: t, ω, Eq.s (2) and (3) can b rwrittn T as ( ) X X( ω ) x t, iω t,..., iωt x x( t) X ( ω),,..., (4). In gnral, absolut valu X and angl aplitud and phas of th th (5) X is a coplx nubr whos X ar th frquncy 63

4 coponnt. Sinc { } x is a ral signal, its * * DFT is sytric, i.. X X (whr X dnots th coplx conugat of X ). As statd abov, th Dopplr shift lads to spctru variations [9,],. Du to th Dopplr frquncy shifts, th frquncy of ach coponnt is shiftd to s ~ ω ω + ω ω, whr T δ andδ dnots th rlativ shift of th th coponnt. In th ti doain th nw signal x% x% t X ( ) i ( ) ( ω ) + ω t X ω i ( + δ ) X i ( + δ ) X ~ sinc { } ( ω ) i % ω t (6) x is a ral signal, ω ω, i.. δ δ. Th spctru of th nw signal is X% x% i i i ( + δ ) X i i ( + δ ) n X i ( + δ ) X Thrfor ~ X a ( δ ) X (7) Whr a i ( + δ ) ( δ ) (8) In othr words, w can obtain th nw spctru vctor X ~ of lngth by ultiplying th atrix a by th original spctru vctor X of lngth. Matrix lnt a ( δ ) shows how th original th spctral coponnt affcts th nw th spctral coponnt. Th atrix lnts can b coputd using Eq. (8). If δ, thn, a ( ) (9.)., δ ( δ < For sall for a q q q q,, ( + δ ) i ( δ ) < q,, w obtain i ( + δ ) i i iδ i i i iδ i { }, ( ) a δ ), using (9.2) is a x atrix whos, ntris ar coplx nubrs dpnding on { δ }. If δ for all,...,, thn (9.) iplis that { ( )} a δ,, is th idntity x atrix ~ X X and in this cas for all 64

5 ,...,. In othr words, if thr ar no frquncy shifts, thn th spctru is not changd, as xpctd [8]. Th rlationδ δ iplis a ( a δ ) * ( δ ) ( ),,. ot that th spctral coponnts affctd by th Dopplr ffct can b coputd by Eq.s (7), (8). Rarably, ths nurical siulation and calculations do not rquir FFT siz incrasing. W bliv that this can b a significant contribution to siulation dsign. 3. Th FBS- METHOD FBS cobins th OFDMA principls with th phas shift copnsation usd in th PAL-TV systs, and sprad spctru concpt using Walsh functions basd on th Walsh-Hadaard atrix. In PAL TV systs, prior to transitting a color signal, th syst s phas sign is changd vry scond lin. In dcodrs, th phas sign is rturnd and sud togthr with phass of nighboring lins. As a rsult, phas dviations in th channl φ ar copnsatd for [9]. In th first vrsion of th FBS thod (FBS-) [ 9,], an OFDMA syst with ight singlcarrir MPSK odulation signals is transford to an FBS- syst with th sa ight signals on th sa ight carrirs (s Fig4). In this thod, ach signal phas is transittd ight tis with a varying phas sign corrsponding to on of th ight Walsh- Hadaard atrix rows. s ( ), l FBS El [f t+ ( ) ( θ + β )] whr E l - coponnt agnitud, l,, - or Walsh-Hadaard atrix lins θ l - initial phas, chosn for a crtain signal. For xapl, it is ithr 45 or othr β l - inforation sybol of th l th FBS signal (BPSK or QPSK rprsntation), f f + f - FBS carrir frquncis,,, - or Walsh-Hadaard atrix coluns W l - squnc of phass of th l th FBS carrir pattrn. To rciv on of ths signals, for xapl signal nubr 2, th following algorith ust b iplntd: th S 2 rcivr rcivs all signals togthr, as FFT, obtains ight spctral coponnts (aplituds and phass), changs th phas signs of coponnts 5, 6, 7 and 8 (corrsponding to th scond row of th Walsh-Hadaard atrix in Fig.2.). Th rsult will b qual to th phas su dividd by ight. Th phas su of th othr svn signals should b qual to zro [9 ]. W calculat arithtical (not vctor) su ϕ of all signals on ach carrir and us aplitud corrsponding to: Fig. 5 illustrat th ain advantag of using th FBS- thod. Wl l l, ()?Figur 4. Transition fro OFDMA (a) to FBS- (b) For transitting signals on carrirs, th FBS signals ar: Figur 5. Phas shift copnsation aftr dcoding FBS- In th cas of sytrical Walsh functions, for xapl,, th Dopplr Shift and Dlay hav no influnc on FBS- 65

6 rcption. On can s that in Fig. 5 whn phas shift du to dlay is proportional to frquncy. If th Walsh function is nonsytrical, (for xapl ), thr will b an additional phas shift, which is indpndnt of th phas inforation and can b copnsatd for. 4. Th FBS-2 METHOD During th phas copnsation procss of th FBS- thod, thr is usually a crtain lvl of data loss in th aplitud. Thrfor, it is possibl to iplnt only phas odulations.g. MPSK [9,]. To ovrco ths difficultis w prsnt th FBS-2 syst [,2]. In this upgradd vrsion, on can iplnt othr odulation thods, such as MQAM. For transitting /2 sub-signals in th FBS-2 syst w us orthogonal subcarrirs f, f2,.., f and Walsh-Hadaard atrix of ordr. For ach sybol of ach sub-signal: 2 I A I + Q arctg x( t) Asin( 2 ; ϕ ω t + ϕ) Q w prsnt I and Q coponnts I Asin ϕ and Q Acosϕ and ( ) A, ϕ < + Th I and Q valus ar transittd sparatly on sub-carrirs corrspond to on of th pairs of Walsh functions slctd fro an Walsh-Hadaard atrix. For xapl, in th cas of 8, using th pair: th following I and Q valus ar transittd: I I I I I I I I Q Q Q Q Q Q Q Q Whn rciving a transittd sub-signal, it is ncssary to iplnt th opposit procss with th hlp of th sa pair of Walsh functions. Th su of all valus in th first lin is 8I and th su of all valus in th scond lin is 8Q. By nowing th valus for I and Q, w can find A and ϕ. Th I and Q sus of th othr signals will b qual to zro. Each slctd pair of Walsh functions ay b addd to a a sytric for. For xapl, th four pairs of Walsh functions listd in Tab giv a sytrical for. Tabl.; Walsh functions choosing xapls for gtting a sytrical for. On of th FBS-2 signals with row l for I and row of th Walsh-Hadaard atrix for Q, can b prsntd as follows [2] Sin Wl [ f + ( ) ( θl, + βl, ) ] + W [ f + ( ) ( θ + β )] ( l, ), FBS 2 El, + Cos l, l, s For coparison purposs, w shall us a typical FBS-2 syst, that uss carrirs for transitting signals. Sinc FBS-2 utilizs only th I and Q carrirs, /2 signals ay b transittd on th sa allocatd channls. vrthlss, th QAM odulation tchniqu (usd in FBS-2) allows transitting twic as uch data as th PSK odulation tchniqu (usd in FBS-), for th sa allocatd channls. It is wll nown that signal to nois ratio (SR) for transitting signal using 6QAM should b gratr by at last 4dB than th corrsponding ratio using QPSK with th sa BER. In th cas of FBS-2, th rquird SR for 6QAM should b only by 2.7 db rlativ to QPSK. This is du to th fact that in FBS-2, M signals ar sud up cohrntly, but M uncorrlatd nois coponnts ar sud in th sns of nrgis. Howvr if w ta into considration PS influnc in OFDMA, w will s that th SRs in OFDM and in FBS-2 ar approxiatly qual. 5. SIMULATIO RESULTS A typical obil counication syst consisting of a HAP and various cllular cobinations, was iplntd in ordr to a a coparativ study btwn OFDMA and th FBS systs [,2]. W considr a syst with four rcivrs and sybol duration of T µs, such that, th frquncy diffrnc btwn sub carrirs will b F /T Hz. W assu th Dopplr shift is 3% of F, and th phas shift pr ach sybol ti du to dlay 3.9. Four signals ar transittd. In cas of OFDMA, ach sybol is transittd on two sub-carrirs using QPSK, () 66

7 and in cas of FBS-2 on ight sub-carrirs using 6QAM. Siulation rsults ar shown in Fig. 6 (OFDMA) and Fig. 7 (FBS-2). In th cas of OFDMA with pilot ratio of : (uppr curvs in Fig.4) th rsulting BER is ~ - is in th rang of uncorrctabl rror floor. Accptabl BERs (5-3 ) ar obtaind only by incrasing th pilot ratio up to :2. Taing into account th 3 db highr powrs of th pilots, th rsulting rdundancy is or thn %. Fig. 6. BER calculations in cas of OFDMA for diffrnt nubrs of pilot signals Fig. 7 shows th siulation rsults for th cas of th FBS-2 thod on th conditions li on Fig. 6. It is sn that in this cas th BER is not affctd by phas distortions du to th Dopplr Effct. Th rlativly highr valus of th forth signal ar attributd to th fact that for this signal th all zro Walsh function W was applid. Fig. 7. BER calculations in cas of FBS-2 with no pilot signals Two ain rstrictions xist qually for both FBS and FDMA syst dsign: T > τ ax and F < B c. Whr Bc is th cohrnc bandwidth []. 6. COCLUSIOS A futur HAP broadband obil radio syst basd on th FBS thod is prsntd for high spd vhicls., and copard to OFDMA basd HAP systs.. It is shown in this papr that th FBS-HAP syst allows transission of th sa quantity of inforation on th sa frquncy band and with th sa powr, as th OFDMA- HAP syst.. Th ain FBS advantags ar: A significant dcras of th Dopplr frquncy shift ffcts, as wll as th ti dlays and ulti-paths influncs. FBS rovs th nd for conducting tsts or using pilot signals and/or qualizing procsssng prolongd slctiv fading by ans of randoly shifting sub-carrirs frquncis. Th FBS thod can b iplntd in diffrnt wirlss counication systs including HAPS using various odulation tchniqus.[3,8] FBS allows for iproving th radio prforancs and quality of HAPS vn in broadband counication with vry high spd vhicls [4, ] All ths advantags ar achivd without any xtra cost in frquncy band and without an incras in th syst powr supply. Iplntation of th FBS syst will rsult in a significant incras in th HAP throughput affording vry fast intrnt counication also for fast TGV or airplan passngrs.. Rfrncs 2. J. Gavan, S.Tapuchi and D. Grac Concpts and Main Applications of High Altitud Platfors Radio Rlays Th Radio Scinc Bulltin.. 33 Sptbr 29.pp(2-3) 3. J. Gavan and S. Tapuchi, "Low Intrfrnc Widband Wirlss Systs Using High Altitud Platfors," Invitd Papr, URSI Gnral Assbly,Chicago., August 28, pp.(-4). 4. R. van, and R. Prasad,. OFDM for wirlss 67

8 ultidia counications, Artch Hous, C. W.L, Mobil cllular tlcounications systs, McGraw-Hill, Inc., R.Struza, Mobil tlcounications via stratosphr, Intrcos intrnational counication proct,23, pp(-2). 7. Grac and M. H. Capstic, Intgrating Usrs into th Widr Broadband twor via High Altitud Platfors, IEEE Wirlss Counications, Octobr G.M. Dunic, J. Fridnflds and Y. Ounv, Establishing Wirlss Counications Srvics via High-Altitud Aronautical Platfors: A Concpt Whos Ti Has Co?, IEEE Coun. Mag., pp.( 28-35), Spt., D. Grac, C. Spillard, T. C. Tozr, High Altitud Platfor Rsourc Managnt Stratgis with Iprovd Connction Adission Control, IEEE Wirlss Prsonal Mobil Confrnc, Octobr, 23. pp.. T.C Tozr, D.Grac, High altitud platfors for wirlss counication,elctronics & counication nginring ournal. Jun2 pp (27-37).. J.Gavan, M.Haridi, Stratosphric Quasi- Stationary Platfors (SQSP) Can thy Rplac Counication Satllit Systs, Tlcounications and Spac Journal. Volu 4 997, pp( ) 2. M. Ban, On incrasing OFDM thod frquncy fficincy opportunity, IEEE Transactions on Broadcasting, 5(2), 24 pp (65-7) ITU-R woring docunt 5C/76-E Intrfrnc analysis odling for sharing btwn HAPS gatway linsin th rang MHz and othr srvics 2..8 Short biography: Michal Ban rcivd th BA and M.Sc. dgrs in counicational nginring fro th Lningrad Institut of Counications in 96, rcivd th Ph.D. dgr in 969 in th fild of FM signal dtction. H rcivd Doctor of Scinc dgr (Russian quivalnt of profssor) in 99. Sinc 992 h is a consultant of Isral counicational copany Bzq and a lcturr in th Holon Acadic Institut of Tchnology. His rsarch intrsts includ digital odulation, digital signal procssing and RF rciving thory. Saad Tapuchi arnd th B.Sc. dgr fro th Bn-Gurion Univrsity of th gv, Br-Shva 973, th M.Sc fro th Hbrw Univrsity of Jrusal School of Applid Scinc and Tchnology 982 and Ph.D. fro Bn-Gurion Univrsity of th gv, Br-Shva 988. Dr. Saad Tapuchi srvd as th Had of th Tlcounication dpartnt in th Tlcounication school for practical nginring of th Bz Corporation in Jrusal fro 973 to 986. In 986 h was noinatd as dirctor of th Tlcounication School till 998. Fro 998 Dr. Saad Tapuchi bgin woring at Sai Shaoon Collg of nginring at two capuss in Br- Shva & Ashdod. Sinc 22 h was lctd as Dan of th EEC Faculty of th Sai Shaoon Collg of nginring. Motti Haridi rcivd his M.Sc. dgr in lctrical nginring fro th Univrsity of Washington in 986 and th Ph.D. dgr in lctrical nginring fro Tchnion of Isral in 992. Sinc 994 h has oind Holon Institut of Tchnology (HIT). During Prof. Haridi was th had of th Dpt. of Counication Enginring at HIT. His rsarch activitis focus ainly on th physical layr of counication systs, including optical systs, icrowav photonics, RF counications, and antnnas. H has publishd ovr 6 paprs. Prof. Haridi acts as a consultant in RF counication systs and antnnas to svral larg Israli copanis. Miria Ban, Ph.D in athatics (Hbrw Univrsity of Jrusal). Intrsts: applid athatics, including spctral transfor thods, partial diffrntial quations. Lcturr in th Hbrw Univrsity and in Jrusal Collg of Enginring. Jacob Gavan, was lctd as IEEE fllow in 995 and foundd a dpartnt of Counication Enginring at th Holon Institut of Tchnology (HIT) in Isral. H was slctd as Dan of th HIT faculty of Enginring and as Full Profssor fro Dcbr 2. ow Jacob is had of a a nw counication nginring dpartnt in th SAMI Shaoon Collg of Enginring (SCE). Jacob has publishd ovr 5 paprs on thortical and Applid EMC, Satllit Counications, and Radar Systs. Profssor Jacob Gavan is activ in IEEE as x associatd ditor of th Trans.on EMC and DL of th Counication socity. In URSI Jacob is cochairan of E8 woring group on Elctroagntic Copatibility in Wir and Wirlss Counication Systs and of th SPS coitt. Fro 28 h srvd as RSB gust ditor on a spcial issu on High Altitud Platfors (HAPS). Jacob arnd th bst papr award fro th IEEE Intrnational Syposiu on EMC in two intrnational confrncs Boris Hill arnd th M.Sc. dgr in coputr nginring fro th Polytchnic Institut of Riga in 97. Sinc 999 h is a syst nginr in th ss Ltd. Intrsts in rsarch of frquncy affiliation probl and Counication ntwors dsign 68