Reception and Measurement of MIMO-OFDM Signals with a Single Receiver

Transcription

1 Rption and Masmnt of MIMO-OFDM Signals with a Singl Riv Mstafa E. Şahin,Hüsyin Aslan, Daljt Singh Dpatmnt of Eltial Eng., Univsity of Soth Floida, Tampa, FL msahin,aslan@ng.sf.d Anits Company, Mogan Hill, CA daljt.singh@anits.om Abstat OFDM mlti-ai modlation is xptd to b th nabling thnology fo 4G wilss systms. On of th fats that mak OFDM th pimay hoi fo 4G is its MIMO ompatibility, bas MIMO has a vy signifiant potntial of nhaning wilss systms fo apaity, data at, and ovag aspts. In this pap, th hallngs of MIMO- OFDM masmnts a addssd in ompaison to SISO, with a spial mphasis on WiMAX systms, whih mploy MIMO- OFDM thnology. It is poposd to pfom th ption and masmnt of MIMO-OFDM signals sing a singl iv banh ath than mltipl ivs. A omplt gid to pfom impaimnt stimation fo WiMAX MIMO signals with a singl iv aoding to th standads is also povidd. I. INTRODUCTION Th bathtaking impovmnts in th wilss ommniations as wll as mltimdia appliations and hadwa hav alatd th mgn of foth gnation 4G) wilss systms. Th pimay xptation fom all ft 4G systms is that thy povid nomosly high data ats to an xssiv nmb of ss at th sam tim. In th wilss ommnity, th is a stong blif that 4G will b basd on OFDM. Basd on that, WiMAX is onsidd a sitabl andidat fo 4G with its potntial ability to satisfy vy hallnging thoghpt and apaity nds [1]. On of th signifiant fatos lading to this opinion is that th apaity of WiMAX systms an b impovd fth by adding mltipl-inpt mltipl-otpt MIMO) fat [2]. Sin MIMO an onsidably nhan th potntial of WiMAX systms, it has bn mad pat of th IEEE and standads [3], [4]. Systms with MIMO apability opat on a nmb of paalll hannls whih lads to a mltiplxing gain [5]. In MIMO systms, at th xpns of inasd hadwa and omptational omplxity, a high sptal ffiiny an b ahivd. This sptal ffiiny, whih an b tilizd as data at, apaity, o ovag impovmnt aoding to th nds, maks th MIMO thnology attativ fo bandwidthgdy wilss appliations. Rliabl MIMO-OFDM systm dsign qis pfoming tain MIMO masmnts on th systm. Optimally, ths masmnts wold b pfomd ith by sing mltipl vto signal analyzs VSAs) o a singl VSA with mltipl inpt banhs. Howv, it is la that this kind of a masmnt stp wold b xtmly ostly. Hn, in this pap, a mh mo fasibl soltion that mploys a singl iv is poposd. Rgading this soltion, th pimay RF font-nd impaimnts a analyzd, and a gid to stimat ah of thm is povidd. Th possibl asons fo diffnt impaimnts in diffnt tansmitt banhs a addssd. It also xplaind how to xamin th onstllation diagam of th ombind) ivd signal in od to hav an ndstanding of th impaimnt diffns btwn th tansmitts. Also, a pod that xplains how to handl WiMAX MIMO signals is povidd. Th flow of th pap is as follows. Stion II dissss th pimay RF font-nd impaimnts. Stion III povids a gid to stimat and mov th ffts of ths impaimnts. In Stion IV, th diffns of MIMO masmnts fom singl hannl masmnts a analyzd in dtail. In Stion V, it is dsibd how to handl WiMAX MIMO signals. Stion VI onlds th pap. II. SIGNAL MODEL AND THE PRIMARY RF FRONT-END IMPAIRMENTS In a MIMO-OFDM systm, th ivd signal ontains th ffts of vaios RF font-nd impaimnts. Ths ffts hav to b dtmind and movd bfo making th symbol disions. Th dtaild fats of RF impaimnts hav bn addssd in [6], [7], h, th ssntial impat of ah of thm will b smmaizd so that th ad an follow th MIMO masmnt soltions that will b povidd. Th ivd fqny domain signal in a MIMO-OFDM systm an b modld as Y m k) = X m k mγ mω) 1) H m k)f k) jkρ jkω jψm µ m k)+n m k), wh m is th OFDM symbol indx and k is th sbai indx. Th maind of th paamts a as follows: [ρ] : Th tim offst btwn th tansmitt and th iv. It ass a phas shift that inass linaly ov th sbais, bt dos not hang fom on symbol to anoth; [γ] :Th fqny offst btwn th loal osillatos in both sids. It slts in a phas shift that hangs ov symbols

2 ath than sbais. Hn, all th sbais in th sam symbol xpin th sam amont of phas shift d to th fqny offst; [ω] :Th inaay btwn th sampling loks of th tansmitt and th iv. Th sampling lok o also ass a phas shift, bt, as opposd to th oth impaimnts, this phas shift gows ov both sbais and symbols; [ψ] : Th andom phas nois that is asd by th instability of th loal osillato in th tansmitt. Th andom phas nois has a simila fft to th fqny offst. Th phas shift it ass is th sam fo all sbais in th sam symbol, bt th amont of this shift vais btwn symbols bas of th andomnss of th phas o; [F k)] : Th fqny spons of th analog filts mployd in both th tansmitt and th iv; [H m k)] : Th fqny sltivity and tim dpndny of th hannl. Bas of its fqny sltivity, th wilss hannl may afft th sbais diffntly. It may also vay ov tim, spially if th hannl is mobil. A tim vaying hannl lads to hangs on th sam sbais fom symbol to symbol; [X m k)] : Th tansmittd data; [N m k)] : Additiv nois tm; [µ m k)] : Th IQ impaimnts; its fft on X m k) an b xpssd as X m k) 2 Is + Q s jα) + X m k) Is Q s jα), 2) 2 wh [I s ] and [Q s ]:Th in-phas and qadat omponnts of th tansmittd signals. Thi atio yilds th IQ imbalan; and [α] :Th qadat o. It indiats any dviation fom othogonality btwn th I and Q omponnts. III. ESTIMATION AND REMOVAL OF IMPAIRMENTS IN THE SISO CASE Aft having intodd th main fatos that lad to impaimnts in th ivd signal, in this stion it will b addssd how to poss th ivd signal fo th singl-inpt singlotpt SISO) as. A stp-by-stp gid that povids th od and shot xplanations of th nssay impaimnt stimations is givn blow. As it will b la, th od of th stimations is impotant bas ah stimation assms that th oth os that afft th sbais in th sam way hav alady bn movd. So, aft ah stimation, th osponding fft has to b movd fom th ivd signal bfo poding to th nxt stp. Pakt Dttion: Th bginning and th nd of th signal pakt is dtmind by tilizing a simpl ngy dttion mthod. Th thshold may hav to b modifid adaptivly aoding to th ivd nois pow. This initial stp svs as a ogh timing stimation. Fqny Offst Estimation Tim Domain): Th tim domain signal is olatd with itslf. Owing to th pilot sbais that a patd glaly in tim, this olation an b tilizd to stimat th fqny offst. Dividing this olation val by 2πD, wh D is th dation btwn th patd pilots, yilds th fqny offst stimat. Fin Fqny Offst Estimation Fqny Domain): Aft onvting th ivd tim signal into th fqny domain, th vals of all sbais inlding th pilots bom availabl. Sin th fqny offst hangs fom symbol to symbol, olating th pilot sbais in diffnt symbols yilds a fin stimat fo th fqny offst. Fin Timing Offst Estimation: If th ivd signal ontains a pambl o a midambl) pat that has bn addd to th signal to failitat synhonization, a fin timing stimation an b don. Sin th tansmitt gnats th pambl aoding to a tain standad, th sam pambl an b gnatd in th iv pat, as wll. Colating th pambl with th tim domain signal yilds a vy aat timing stimation. Sampling Clok Eo Estimation: Eo in th sampling lok at adds a phas shift that inass both ov symbols and sbais. Sin th fft of fqny offst on th symbols) has alady bn movd, th lok o shold b liably dtmind by olating pilots in diffnt symbols. Slop Estimation: A tim offst may still xists at this point, spially if no pambl was snt, sin th pakt stimation dos not vy aatly dtmin th signal stating point. This tim offst will indiat itslf as an inasing phas slop ov sbais. Sin th impat of th sampling lok o was alady anld in th pvios stp, it is xptd that th phas slop an b stimatd by ompaing th phass of th sbais in th sam symbol. Channl Estimation: Channl stimation is don sing again th pilots, whih shold b now f fom all th pviosly mntiond impaimnts xpt th andom phas o. Th hannl stimation an b simply don by intpolating th pilot vals in a asonabl way aft moving th pilot pow boosting in th downlink DL)). Random Phas Eo Estimation: Th hannl stimation is xptd to patially tak a of th andom phas o. Fo th phas o that still xists, pilots in diffnt symbols hav to b olatd. This olation yilds spifially th phas o btwn th two olatd symbols. Sin th amont of o hangs andomly fom on symbol to anoth, it has to b dtmind spaatly fo ah symbol. Aft finding th phas os fo all symbols, th hannl stimation shold b pdatd aodingly bfo making th symbol disions. IV. MEASUREMENT CHALLENGES IN MIMO COMPARED TO SISO As opposd to systms with a singl inpt, in MIMO systms, th ivd signal will inld th simltanos tansmission of data fom mltipl tansmitt antnnas. Thfo, th masd o vto magnitd EVM) will b basd on a ombind o vto, whih annot b spaatd

3 into ontibtions fom spaat antnnas/tansmitt banhs. Howv, nd som imstans, th impaimnts asd by diffnt banhs diff sbstantially, and a ommon EVM stimation fails to flt th o magnitd fo all of th banhs aatly. In what follows, th possibl fatos that lad to diffnt impaimnt vals will b disssd. Th MIMO-OFDM systm onsidd h has two tansmitt banhs and a iv banh, whih an b adily gnalizd fo a high nmb of tansmitt banhs. 1) Tim offst btwn th banhs: In od to kp th masmnts simpl, it wold b dsid to b abl to assm that th signals fom th two diffnt tansmitt banhs a ivd simltanosly. Howv, th may b a tim offst btwn th ivd signals if th tansmitts a not wll synhonizd with ah oth, o if th distans fom ah tansmitt to th iv a onsidably diffnt fom ah oth. In as of a tim offst btwn th tansmitt banhs, th timing stimation don by th iv will not b aat fo at last on of th banhs. 2) Employing spaat loks: Th osillato that is ndd to gnat th sampling instants of th DAC an b ommon to both banhs, o ah banh an s a spaat osillato. If two spaat osillatos a mployd by th tansmitt banhs, sving as sampling loks, th will b an navoidabl inaay btwn th sampling piods. This fat will lad to diffnt sampling lok os fo ah banh. Althogh it is mo asonabl to mploy a singl lok fo th nti tansiv, in som ass, th diffnt tansmitt banhs may n spaat loks. This will b th as if th signals a gnatd by diffnt sos sh as two vto signal gnatos VSGs), o two ollaboativly opating mobil dvis ah with a singl antnna. Evn if th is a singl nit with mltipl otpt banhs, sin ah banh will hav its own DAC, th will b still two diffnt sampling lok os, nlss th DACs a n by a ommon xtnal lok inpt. 3) Using spaat IQ modlatos: Th sag of spaat IQ modlatos in ah tansmitt banh has vaios impats. On is that it ass th IQ impaimnts of ah banh to b diffnt. Anoth on is obsvd on th fqny offst. Sin it is tain that th otpt fqnis of th osillatos in ah IQ modlato an nv b xatly th sam, th signals fom ah banh hav a diffnt fqny offst in th iv pat. Th sond impat of spaat IQ modlatos is sn on th andom phas o. Most loal osillatos display an inonsistnt bhavio in tim in tms of th otpt fqny, i.. thi fqny maks slight vaiations in tim. This impaimnt slts in phas os that a andom in nat. Thfo, mploying two spaat loal osillatos will lad to two indpndnt phas os. 4) Using spaat RF omponnts: Sin ah tansmitt banh mploys its own mix, analog RF filts, pow amplifi, and antnna, th signals fom ah banh will b modifid diffntly bfo bing adiatd into th ai. Th good thing abot th diffnt RF stions is that thi ffts an b foldd into th hannl. Thfo, hannl stimations an b onsidd to inld th infln of th RF stions on th ivd signals. Dtting th Impaimnt Diffns by Examining th Constllation Diagam Whn daling with MIMO signals, having an ida abot th potntial impaimnt diffns btwn th tansmitt banhs an failitat th masmnt of ths impaimnts onsidably. Som of ths diffns an b ognizd by invstigating th IQ onstllation diagam aft th moval of offsts fom th ivd ombind signal. How th onstllation looks fo a QPSK modlatd spa-tim tansmit divsity STTD) MIMO signal aft th moval of offsts and bfo th moval of th hannl ffts, i.. qalization) is shown in Fig. 1. Not that th a 16 onstllation points as wll as 2 olltions of pilot symbols and th point in th middl asd by th non-alloatd sbais) in th diagam. Th a 16 points bas ah of th 4 onstllation points in QPSK a smmd vtoially with anoth 4 points. Two of ths qadpls a xpliitly indiatd with tangls in Fig. 1. Simlations hav bn n aiming to s what kind of ffts a obsvd in th onstllation whn th a tain diffns btwn th two tansmittd signals. Fo this ppos, vaios diffns w intntionally st btwn th two signals. Whn pfoming th osponding simlations, it was assmd that on of th signals is not optd dos not hav an impaimnt) bt th oth on dos. That mans, on of th signals has no IQ imbalan, bt th oth on has 30% IQ imbalan, t. It shold b also notd that only on typ of impaimnt diffn is assmd to xist at a tim; thy hav bn xamind on by on, bas it may not b possibl to mak a liabl gss by simply looking at th onstllation if mltipl sh impaimnt diffns xist at th sam tim. In Fig. 2, th fft of π/12 phas diffn btwn th two tansmittd signals is obsvd. Th otation of th qadpls aond thi nt is appant. Th sam fft an b vifid by hking th position of th pilots. Fig. 3 shows two signals with 30% IQ imbalan diffn. In Fig. 4, th fft of π/12 qadat o diffn btwn th signals is shown. Finally, in Fig. 5 two signals with adian fqny offst diffn a shown. Fqny offst has a simila fft to phas diffn in tms of otation of qadpls. Howv, sin th phas shift asd by th fqny offst inass ov symbols, a la shift is sn in th onstllation points. Appantly, ah of ths impaimnt diffns has a diffnt fft on th onstllation diagam, and stdying ths visal ffts, on an mak a stong gss abot th possibl poblm with th ivd MIMO signal by jst xamining th onstllation diagam.

4 Fig. 1. Th onstllation fo two QPSK modlatd STTD signals bfo qalization. Fig. 4. π/12 qadat o diffn btwn th two tansmittd signals. Fig adian fqny offst diffn btwn th two tansmittd signals. Fig. 2. π/12 phas diffn btwn th two tansmittd signals. Fig % IQ imbalan diffn btwn th two tansmittd signals. V. PROCEDURE TO HANDLE WIMAX MIMO SIGNALS It is a fndamntal iss how to spaat th onstllations and th ontibtions to EVM fom ah tansmitt banh whn possing MIMO-OFDM signals with a singl iv. In this stion, a WiMAX systm, whih is a nt MIMO- OFDM thnology, will b onsidd. Th soltion that will b invstigatd h is basd on th s of pilot sqns. To b mo spifi, spa tim odd downlink DL) and plink UL) WiMAX signals with PUSC pmtation will b analyzd mo losly. Th fqny domain) alloations of pilot sbais in DL-PUSC and UL-PUSC a shown in Fig. 6 and Fig. 7. In ths alloation maps, it is sn laly that ah banh is tansmitting a spaat st of pilots that a othogonal to ah oth ith in fqny o in tim. Basially, this is th fat that nabls spaating th impaimnt ontibtions fom spaat banhs. Th ivd tim domain signal ontains pilot sbais fom both banhs, howv, it is not possibl to poss ths pilots spaatly in tim. Thfo, th pakt dttion

5 F ) s i a b S y n q ) s i a b S y n q F Fig. 6. Alloation of sbais in downlink PUSC WiMAX. Implmntation of Alamoti spa-tim) oding is shown. Fig. 7. Alloation of sbais in plink PUSC WiMAX. Implmntation of spa-fqny oding is shown. and th tim domain basd fqny offst stimation an b appliabl only if th timing offsts and th fqny offsts fom th two banhs a los to ah oth. Othwis, only aft onvting th signal to th fqny domain, sin pilots fom diffnt banhs gt spaatd fom ah oth, on an apply th offst stimations xplaind in dtail fo a singl hannl) to pilots fom ah banh spaatly. In th singl hannl as, aft ah stimat th osponding offst was bing movd fom th signal. In th MIMO as, howv, sin ah banh has diffnt offsts, it is not sfl no valid) to mov th offsts fom th nti signal. Instad, th stimatd offsts a movd fom th osponding st of pilots, only. Aft dtmining and moving all th offsts on by on, th hannl stimations an b obtaind fom th pilot sts. Bfo poding to th symbol dision stp, th offst stimations obtaind fo both hannls shold b inopoatd into th osponding hannl stimations. VI. CONCLUSION Paalll to th gowing intst towads OFDM and MIMO thnologis, th nssity fo MIMO-OFDM masmnts is inasing. This pap aims at sving as an insttional gid to MIMO masmnts sing a singl iv instad of mltipl iv banhs. In th pap, th main fatos slting in IQ impaimnts and th way of liminating thi ffts a addssd. Th masmnt hallngs that a spifi to MIMO snaios a analyzd in ompaison to th singl tansmitt as. It is xplaind how to mak intllignt gsss abot th potntial poblms in ivd MIMO signals by xamining th onstllation diagams. Also, an insttional pod to poss WiMAX MIMO signals with a singl iv is povidd. REFERENCES [1] S. Fattasi, E. Ciana, and R. Pasad, An Intgatd AP fo Samlss Intwoking of Existing WMAN and WLAN Standads, Wilss Psonal Commniations, vol. 36, no. 4, pp , [2] A. Ghosh, D. Wolt, J. Andws, and R. Chn, Boadband wilss ass with WiMAX/ : nt pfoman bnhmaks and ft potntial, IEEE Commniations Magazin, vol. 43, no. 2, pp , [3] IEEE Standad fo Loal and Mtopolitan aa ntwoks Pat 16, IEEE Std. IEEE , [4] IEEE Standad fo Loal and mtopolitan aa ntwoks Pat 16 IEEE Std ), Std., [5] D. Gsbt, H. Bolski, D. Go, and A. Palaj, MIMO wilss hannls: apaity and pfoman pdition, in IEEE Global Tlommn Conf. GLOBECOM 00), vol. 2, [6] H. Aslan, Channl Fqny Rspons Estimation Und th Efft of RF Impaimnts in OFDM Basd Wilss Systms, in Po. 64th Vhila Thnol. Conf. VTC-Fall 2006), Montal, Canada, Sp. 2006, to appa. [7] H. Aslan and D. Singh, Establish tst pods Fo WiMAX Tansivs, Miowavs & RF, vol. 45, no. 7, 2006.