Adaptive subband selection in OFDM-based cognitive radios for better system coexistence

Transcription

1 Univrsity of Wollongong Rsarch Onlin Faculty of Informatics - Paprs (Archiv) Faculty of Enginring and Information Scincs 28 Adaptiv subband slction in OFDM-basd cognitiv radios for bttr systm coxistnc Pingzhou Tu Univrsity of Wollongong, pt15@uow.du.au Xiaojing Huang Univrsity of Wollongong, huang@uow.du.au Eryk Dutkiwicz Univrsity of Wollongong, ryk@uow.du.au Publication Dtails P. Tu, X. Huang & E. Dutkiwicz, "Adaptiv subband slction in OFDM-basd cognitiv radios for bttr systm coxistnc," in 3rd Intrnational Confrnc on Cognitiv Radio Orintd Wirlss Ntworks and Communications, 28, pp Rsarch Onlin is th opn accss institutional rpository for th Univrsity of Wollongong. For furthr information contact th UOW Library: rsarch-pubs@uow.du.au

2 Adaptiv subband slction in OFDM-basd cognitiv radios for bttr systm coxistnc Abstract In an nvironmnt of shard radio spctrum, multipl systms may intrfr with ach othr and caus significant impacts on systm coxistnc. In this papr w propos an adaptiv subband slction tchniqu basd on orthogonal frquncy division multiplxing (OFDM) to avoid intrfrnc for bttr systm coxistnc whn multipl systms ar oprating in th sam unlicnsd industrial, scintific and mdical (ISM) bands. Undr th assumption that th intrfrnc powr lvl and th intrfrd frquncy bands ar idntifid at th rcivr, intrfrnc thrsholds, dtrmind ovr both Gaussian and multipath fading channls, ar applid to adaptivly slct th transmission subbands so that intrfrnc is avoidd and th systm coxistnc issus ar rlaxd. To vrify th ffctivnss of th proposd adaptiv subband slction mthod, th systm bit rror rats (BERs) undr diffrnt intrfrnc lvls ar simulatd and compard. Disciplins Physical Scincs and Mathmatics Publication Dtails P. Tu, X. Huang & E. Dutkiwicz, "Adaptiv subband slction in OFDM-basd cognitiv radios for bttr systm coxistnc," in 3rd Intrnational Confrnc on Cognitiv Radio Orintd Wirlss Ntworks and Communications, 28, pp This confrnc papr is availabl at Rsarch Onlin:

3 Adaptiv Subband Slction in OFDM-Basd Cognitiv Radios for Bttr Systm Coxistnc Pingzhou Tu, Xiaojing Huang, Eryk Dutkiwicz {pt15, huang, School of Elctrical, Computr and Tlcommunications Enginring Univrsity of Wollongong, Australia Abstract In an nvironmnt of shard radio spctrum, multipl systms may intrfr with ach othr and caus significant impacts on systm coxistnc. In this papr w propos an adaptiv subband slction tchniqu basd on orthogonal frquncy division multiplxing (OFDM) to avoid intrfrnc for bttr systm coxistnc whn multipl systms ar oprating in th sam unlicnsd industrial, scintific and mdical (ISM) bands. Undr th assumption that th intrfrnc powr lvl and th intrfrd frquncy bands ar idntifid at th rcivr, intrfrnc thrsholds, dtrmind ovr both Gaussian and multipath fading channls, ar applid to adaptivly slct th transmission subbands so that intrfrnc is avoidd and th systm coxistnc issus ar rlaxd. To vrify th ffctivnss of th proposd adaptiv subband slction mthod, th systm bit rror rats (BERs) undr diffrnt intrfrnc lvls ar simulatd and compard. I. INTRODUCTION Coxistnc is th ability of two or mor nods or ntitis to shar a common frquncy band [1]. On of th main factors to limit th coxistnc btwn systms sharing a common frquncy band is th intrfrnc from all sourcs. To avoid intrfrnc btwn systms, many tchniqus hav bn proposd. Gnrally, ths tchniqus can b classifid into two catgoris. On is th collaborativ mchanism for wirlss systms capabl of sharing dtaild information in ral-tim about thir prdfind occupancy of tim, frquncy, spac and powr, in which th coxistnc is ralizd by dynamically managing transmission frquncy, controlling mission powr and rschduling in tim. Th othr is th noncollaborativ on, in which on or mor systms must sns th nvironmnt and tak a corrsponding action [2]. With this mthod radios scan th channls in a srvic band and slct th channl with th lowst rcivd signal strngth indicator (RSSI) for data transmission [3]. Sinc th slctd signal band with th lowst RSSI may hav intrfrnc abov an accptabl thrshold, this intrfrnc will lad to a larg rror probability. For thos mthods of managing frquncy, controlling powr and tim, th most advancd systms ar approaching Shannon s channl capacity limit. Furthr incras in capacity would rquir additional systm bandwidth [4]. Cognitiv radio (CR), as on of th noncollaborativ tchniqus for improving coxistnc, can sns a wirlss radio scn and idntify intrfrnc. It aims at promoting th fficincy of th spctrum utilization and th coxistnc btwn currnt usrs and primary usrs. Basd on CR and orthogonal frquncy division multiplxing (OFDM) tchniqus, on attmpt to avoid intrfrnc is to xclud som sub-bands xprincing dp-fading by sculpting th corrsponding subcarrir group [5]. Howvr, th rallocation of th transmission signal powr for th xcludd subbands to th rmaining sub-bands incrass th powr of rmaining subbands and introducs intrfrncs to th primary usrs. Morovr, it rquirs mor advancd tchniqus to supprss th sidlobs from rctangular window [6], which is somtims known as a Dirichlt window. In this papr w propos an adaptiv subband slction tchniqu to avoid intrfrnc for th coxistnc in an OFDM-basd systm oprating in unlicnsd ISM bands. Undr th assumption that th intrfrnc powr lvl and frquncy bands can b idntifid basd on cognitiv radio (CR) tchniqus, th subbands of th transmittd signal ar adaptivly slctd so that intrfrnc is avoidd and th systm coxistnc is improvd. Th proposd tchniqu provids a gratly simplifid solution to th systm coxistnc issu. Th rmaindr of this papr is organizd as follow. In Sction II th systm modl is introducd. In Sction III, th mthods of multipl subband signal gnration and adaptiv subband slction ar dscribd. In Sction IV w dtrmin th intrfrnc thrsholds for systms ovr Gaussian and multipath fading channls using analytical and numrical mans rspctivly. In Sction V, th systm bit rror rats (BERs) in diffrnt channl conditions ar simulatd and discussd. Finally, w draw our conclusions in Sction VI. II. SYSTEM MODEL Th systm is an OFDM-basd cognitiv radio (CR) and consists of a CR transmittr and a CR rcivr and channls. Th CR transmittr is composd of a sprad spctrum modulator and a filtr. Th modulator sprads th basband signal into a signal with multipl subbands. Each subband contains th whol transmittd information. Th gnration of th multipl subband signal will b invstigatd in th nxt sction. Th rcivr consists of a radio scn snsor, an adaptiv filtr and a dmodulator. Th function of th radio scn snsor is to scan th spctrum, dtct intrfrnc powr lvl, and idntify intrfrd signal subbands. Aftr th intrfrnc signal powr lvl and th intrfrd subbands

4 ar idntifid, th rcivr can adaptivly slct transmission subbands in trms of prdtrmind intrfrnc thrsholds. Whn signals propagat in fading multipath channls, th prsnc of rflcting objcts and scattrrs may rsult in signal fading. Lt h(m) b th impuls rspons of th multipath channl, and y(m) th transmittd signal passing through th fading channl, th rcivd quivalnt low-pass signal r(m) is viwd as th convolution btwn h(m) and y(m) plus nois v(m) and intrfrnc signal j(m), i.., r(m) =h(m) y(m) +v(m) +j(m), (1) whr v(m) rprsnts th complx-valud whit Gaussian nois corrupting th signal, j(m) rprsnts th intrfrnc signal. In th frquncy domain (1) is quivalnt to th output of a fast Fourir Transform (FFT) dmodulator, i.., R(k) =H(k) Y (k)+v (k)+j(k), (2) whr R(k),Y(k),H(k),V(k) and J(k) dnot th frquncy rprsntations of th rcivd signal, transmittd signal, channl impuls rspons, nois and intrfrnc signal r(m),y(m),h(m),v(m) and j(m) rspctivly. In th following discussion w assum that th intrfrnc signal typs, intrfrnc powr lvls and frquncy bands hav bn dtctd by th CR systm. III. MULTIPLE SUBBAND SIGNAL A. Gnration of th Multipl Subband Signal To nabl adaptiv subband slction, th first challng is how to gnrat a signal that can adaptivly chang th occupid bandwidth without losing transmittd information. Fig.1 indicats th gnration procss of such a multipl subband signal. Lt a i (i =, 1,,N) dnots th i th complx-valud symbol of th N quadratur phas shift kying (QPSK) symbols to b transmittd. Th N QPSK symbols ar modulatd by th modulator, and thn intrlavd by th intrlavr to form a multipl subband OFDM signal. Aftr srial to paralll convrsion th N 1 paralll QPSK symbols modulat th corrsponding Nsubcarrirs. If th OFDM symbol priod is T s, f i = i T s dnots th i th subcarrir frquncy of th N orthogonal subcarrirs, and a i modulats th i th subcarrir at tim t = n N T s, n =, 1,,N 1, whrn is th sampling tim indx. Th modulatd symbol on th i th subcarrir and th n th tim instant is writtn as y i (n) =a i j2πfit = a i j2πni/n. (3) In th OFDM symbol duration T s, ach lmnt of th N 1 data symbol vctor modulats th sam subcarrir N tims, so that N lmnts in th vctor gnrat an N N sampl matrix aftr modulation. In consqunc, N rplicas of ach data symbol ar producd in symbol duration T s by th modulator as displayd in Fig.1. Aftr th modulation, th N N sampls ar shiftd in tim and placd on diffrnt tim slots to form a srial squnc of lngth N 2 in th tim domain. This opration is a procss of intrlaving. Th intrlaving can b ralizd by shifting N modulatd subcarrirs on diffrnt tim slots and adding thm togthr. For instanc, th i th subcarrir of th N subcarrirs is shift by i τ, whrτ is th tim intrval, τ = Ts N, thn, th 2 N shiftd subcarrirs ar addd togthr to form on OFDM symbol with N 2 sampls. Th m th sampl in th OFDM symbol, dnotd by y(m), m = nn + i =, 1,,N 2 1, can b mathmatically writtn as y(m) = N i=1 N y i (n)δ [m i nn], n { 1, for m = nn + i whr δ [m nn i] =, for m nn + i. Dirac impuls. It can b simplifid as is th y(m) =y(nn + i) =y i (n), (4) whr m = nn + i and n, i =, 1,,N 1. Th mathmatical quations abov can b intrprtd as follows. All N sampls in a column of th N N matrix ar takn out, shiftd in tim, and thn placd in diffrnt tim slots, instad of bing suprimposd togthr as in convntional OFDM systms. Th frquncy domain rprsntation of th multipl subband OFDM signal can b obtaind by prforming a fast Fourir Transform (FFT) opration on th signal y(m) in tim domain, i., Y (k) =FFT(y(m)) = N N 1 i= 2π j a i N 2 ki 1 N N 1 n= j 2π N (k i)n } {{ } δ((k i) N ) 2π j or Y (pn + i) =Y p (i) =Na i N 2 (pn+i)i, k = pn + i, whr i, p =, 1,,N 1. Equation abov indicats that th OFDM symbol consists of N subbands. Each data symbol a i is modulatd on th i th subcarrir of ach subband, and th signal spctrum is sprad N tims. Fig. 2 indicats th signal spctrum whn N= 4. It is sn from Fig.2 that th multipl subband OFDM symbol contains 4 subbands and th modulatd data symbol on th i th (i=1, 2, 3, 4) subcarrir appars in all th 4 subbands. Th signal spctrum is xpandd to 4 subbands, ach of which contains 4 orthogonal subcarrirs modulatd by th sam transmittd data symbols. Du to th channl frquncy-slctiv fading, ths powr spctrum dnsitisof th subbands hav diffrnt attnuations. B. Intrfrnc Avoidanc Whn th multipl subband OFDM signal is passing through a multipath fading channl, sinc th fading channl dissipats th signal nrgy, th rcivd signals ar fadd in diffrnt subbands. Manwhil, an intrfrnc signal will b imposd on th transmittd signals. To avoid intrfrnc at th rcivr so that th systm prformanc can b improvd, th intrfrd subbands should b rmovd from th rcivd signal onc th intrfrnc powr lvr is abov a prdtrmind thrshold. Th mchanism is xplaind in mor dtail blow.

5 Assum th valu of th thrshold is γ. Th drivation mthod of th thrshold will b discussd in th nxt sction. If th intrfrnc suprimposd on a subband is gratr than th thrshold valu γ, thn th intrfrnc would caus svr advrs ffcts on th dsird signal. In ordr to achiv bttr systm BER prformanc, th intrfrd subband should b xcludd. If th intrfrnc is lowr than th thrshold valu γ, th intrfrnc can b tolratd and th intrfrd subbands ar kpt in th transmission bands. Othrwis, if w rmov th intrfrd subbands with intrfrnc lowr than th thrshold, th systm prformanc will b wors than that of th systm with th intrfrd subbands, sinc whn w rmov th intrfrd subbands, th signal nrgy is rmovd at th sam tim. Fig. 3 intrprts th fundamntal principl of intrfrnc avoidanc, in which a 4 subband OFDM signal in th unlicnsd 2.4 GHz ISM frquncy band is fadd and intrfrd, but only two of th subbands with intrfrnc ovr th thrshold ar rmovd. Fig.3a displays th OFDM signal of 4 subbands intrfrd with diffrnt powr lvls. As displayd in Fig.3b th subbands with intrfrnc ovr th intrfrnc thrshold γ is sculptd by using an adaptiv filtr. Th othr two intrfrd subbands rmain in th transmission bands sinc thir intrfrnc lvls ar lowr than th thrshold. IV. DETERMINATION OF THRESHOLDS A. Thrsholds ovr Gaussian Channls Assum that M of th N subbands ar rcivd and usd for dmodulation, ach of which has signal powrp S. If th nois powr spctral dnsity is N, and th total intrfrnc signal powr in th obsrvd frquncy band is P J with bandwidth B J, which is distributd in l subbands of th transmittd M subband OFDM signal. Thn, th signal powr of th M subbands is M P S, and th signal-to-intrfrnc nois ratio (SINR) is xprssd as SINR (1) M = M P S N M B + P J = SNR 1+INR, (5) whr SNR = MPS MN, INR = PJ B N MB is intrfrnc nois ratio, B is th bandwidth of ach subband. Lt G p dnot th procssing gain for ach subband, th total SINR, aftr considring th procssing gain is th sum of M subbands SINR (1) M, dnotd by SINR (1) = M G p SINR (1) M = M G p SNR 1+INR. (6) Similarly, if th intrfrd subbands ar cut off, th intrfrnc nrgy P J is corrspondingly compltly rmovd. Thus, th total signal-to-intrfrnc nois ratio (SINR), aftr th intrfrd subbands ar rmovd, can b xprssd as SINR (2) M l = (M l) P S = SNR. (7) N (M l) B Similarly, th total SINR, aftr th l intrfrd subbands ar rmovd and considring th procssing gain, is th sum of a i QPSK Complx Symbols Srial/Paralll Convrtr Fig. 1. a a 1 a N-1 j 2πf t j 2 πf1 t j2πf N 1 t NxN Matrix [ y ( ) i n ] Multipl subband signal modulation. M l subbands SINR (2) M l and dnotd by Intrlavr y ( m) SINR (2) =(M l) G p SINR (2) M l =(M l) G p SNR. (8) Sinc th signal powr is distributd in th whol bands, aftr th l intrfrd subbands ar rmovd, th signal nrgy distributd in th l subbands is also rmovd, which causs th loss of som portions of th signal powr. Thus, it is ssntial to dcid how many of th l subbands should b rmovd for intrfrnc supprssion. This compromis can b ralizd in trms of th adaptivly computd intrfrnc thrshold. In Gaussian channls, th BER prformanc is dtrmind by th normalizd signal-to-nois ratio (SNR) or signal-tointrfrnc nois ratio (SINR). Systm prformanc with highr SINR will b bttr. Thus, th intrfrnc thrsholds ar obtaind by ltting th SINR (2) gratr than SINR (1), i., SINR (2) >SINR (1). (9) Substituting SINR (2) in (8) and SINR (1) in (6) into (9), w hav th following xprssion INR > l M l = γ, (1) whr γ is th intrfrnc thrshold. It can b sn from (1) that th thrshold γ of th INR is rlatd to th numbr of transmittd subbands M and th numbr of intrfrd subbands l ovr Gaussian channls. Givn th intrfrd subband numbr l (l=1, 2, 4, 8), th INR thrshold dcrass with th incras of th numbr M of th transmittd subbands. If th numbr M of th transmittd subbands is givn, th INR thrshold incrass with th incras of th intrfrd subband numbr l. B. Thrsholds ovr Multipath Fading Channls In a multipath fading channl, sinc th channl fading is random, w cannot obtain th sam thrsholds as thos in Gaussian channl. Howvr, it is possibl for us to gt an stimation of th thrsholds by statistically analyzing systm BER prformanc ovr multipath fading channls. Rcalling (6) and considring th multipath fading ffcts, systm BER prformanc bfor th intrfrd subbands ar

6 Tabl 1: Thrshold γ Comparison btwn Gaussian channls and Multipath Channls. Intrfrd Thrsholds (db) M=2 Thrsholds (db) M=4 Thrsholds (db) M=8 Thrsholds (db) M=16 Thrsholds (db) M=32 Subbands Gauss. Multi. Gauss. Multi. Gauss. Multi. Gauss. Multi Gauss. Multi. l= l= l= l= Discrpancy Aftr channl fading subcarrir 1 subcarrir 2 subcarrir 3 subcarrir BER whn M=16 Intrfrd subband l=1 Intrfrd subband l=2 Intrfrd subband l=4 Intrfrd subband l=8 Thrsholds ovr fading channl at EbNo=1dB Powr Spctrum.4 2. BER Subband 1 Subband 2 Subband 3 Subband Frquncy (GHz) INR (db) Fig. 2. Channl fading ffcts on subbands whn subcarrir numbr N=4. Fig. 4. INR thrsholds ovr multipath fading channl. Powr spctral dnsity Powr spctral dnsity a) Spctrum of transmittd signal and intrfrnc Frquncy (GHz) b) Spctrum of filtrd signal Frquncy (GHz) Fig. 3. Subband slction by using adaptiv filtr. rmovd can b xprssd by using Q function as = E C Q M 1 H p 2 SINR (1) P (1) = E C Q M 1 H p 2 SNR, (11) 1+INR whr E C ( ) dnots th statistical xpctation of th con- ditional rror probability ovr multipath fading channls, M 1 H p 2 is th sum of th M multipath fading channl cofficints on th Msubcarrirs. Du to th QPSK modulation, SNR = 2 M E b/n whr E b /N is th signal bit nrgy to nois spctrum dnsity ratio [7]. Th (11) can b rwrittn as P (1) = E C Q 1 M M 1 H p 2 2 E b /N. (12) 1+INR Similarly, rcalling (8), th BER prformanc of th systm ovr multipath fading channls, aftr th intrfrd subbands ar rmovd, can b drivd as P (2) = E C Q M l 1 H p 2 2 E b /N M, (13) whr E C ( ), H p, E b /N hav th sam manings as dfind in (11) and (12). According to (12), w simulat th systm BER prformanc ovr multipath fading channls with th incras of th INR whn th transmission subband numbr is 16 and E b /N =1dB, which is displayd in Fig.4. Aftr th intrfrd subbands (th numbr of th subbands is l=1,2,4,8 rspctivly) ar rmovd, according to (13) and with th sam E b /N, th BER prformanc of th systm ovr multipath fading channls ar shown as flat lins in Fig.4. Th cross points ar th intrfrnc thrsholds γ corrsponding to l=1, 2, 4 and l=8. It can b sn from Fig.4 that th intrfrnc

7 Without Intrfrnc 2 subbands 4 subbands 8 subbands 16 subbands 32 subbands Aftr intrfrd subbands rmovd 2 subbands 4 subbands 8 subbands 16 subbands 32 subbands BER 1-3 BER E / (db) b N Eb / N (db) Fig. 5. BER prformanc without intrfrncs in fading channl. Fig. 7. BER prformanc aftr intrfrd subbands rmovd. BER With intrfrnc at INR=3dB Fig. 6. Eb / N (db) 2 subbands 4 subbands 8 subbands 16 subbands 32 subbands BER prformanc with intrfrncs in fading channl. thrsholds ar -11dB, -7.5dB, -4.dB and 1dB. Th corrsponding thrsholds at th sam conditions ovr a Gaussian channlcan bcalculatdfrom (1). Similarly, th intrfrnc thrsholds in a multipath fading channl whn transmission subbands M=2, 4, 8, 16 and 32 can b also dtrmind. Th thrsholds ovr both multipath fading channls and Gaussian channls ar prsntd in Tabl 1. It is obsrvd from Tabl 1 that th thrshold discrpancis btwn a multipath fading channls and a corrsponding Gaussian channl dcras with th incras of th transmission subband numbr and th discrpancy is approximatd zro whn th transmission subband numbr M=32. That is to say, w can us th thrshold ovr a Gaussian channl to rplac th thrshold ovr th corrsponding multipath fading channl if th numbr of transmittd subbands is gratr than 32. V. SYSTEM PERFORMANCE Onc th systm INR thrsholds ar obtaind, th intrfrd subbands with INR ovr th thrsholds can b adaptivly cut off to avoid intrfrnc. In ordr to vrify th ffctivnss of th subband adaptation in a multipath fading channl, th BERs of th systms without intrfrnc, with intrfrnc, and with intrfrnc rmovd ar simulatd. Ths ar displayd in Fig.5, Fig.6 and Fig.7, rspctivly. Fig.5 indicats th systm BER prformanc without intrfrnc whn systm transmission subband numbrs M=2, 4, 8, 16 and 32. It is obsrvd that with th incras of M, th systm BER prformanc is improvd considrably. Aftr M=16, a furthr incras of transmission subbands is not obviously bnficial to th improvmnt of th BER prformanc, but is hlpful for th intrfrnc avoidanc. Undr th sam conditions, th systm BER with on subband intrfrd is displayd in Fig.6. Th INR is 3 db. It is sn that th impact from th intrfrnc rsults in 5 db -6 db dgradation at th BER prformanc 1 6 compard with th systm prformanc without intrfrnc. Aftr th intrfrd subband is rmovd as displayd in Fig.7, th systm BERs ar improvd.5 db - 5 db compard with th systm prformanc with intrfrnc whn M=4, 8, 16 and 32. In this cas th prformanc is vry clos to that without intrfrnc. Howvr, whn M=2 th systm prformanc bcoms wors, sinc th subband with INR lowr than th thrshold γ =4.9 db is rmovd, which agrs with th prvious analysis. VI. CONCLUSIONS In this papr w hav invstigatd an adaptiv subband slction mthod to supprss intrfrnc for systm coxistnc, and dmonstratd th improvmnt on systm prformanc. W hav also shown that th intrfrnc thrsholds ovr multipath fading channls approximat to th intrfrnc

8 thrsholds ovr Gaussian channls whn th numbr of transmission subbands is sufficint nough. It is dmonstratd that th drivation of intrfrnc thrsholds ovr multipath fading channls can b rplacd by intrfrnc thrsholds ovr Gaussian channls aftr considring a known discrpancy valu. This approximating mthod gratly simplifid drivation of th intrfrnc thrsholds ovr multipath fading channls. Th proposd algorithms can b applid to support th advancmnt of cognitiv radio and futur gnration communication applications. REFERENCES [1] K. E. Nolan, P. D. Sutton, L. E. Doyl, T. W. Rondau, B. L, and C. W. Bostian, Dynamic Spctrum Accss and Coxistnc Expricncs Involving Two Indpndntly Dvlopd Cognitiv Radio Tstbds, in 2nd IEEE Intrnational Symposium on Nw Frontirs in Dynamic Spctrum Accss Ntworks, pp , 27. [2] J. Lansford, UWB Coxistnc and Cognitiv Radio, in Joint UWBST & IWUWBS.24 Intrnational Workshop on Ultra Widband Systms, pp , 24. [3] X. Jing, S. Mau, D. Raychaudhuri, and R. Matyas, Ractiv Cognitiv Radio Algorithms for Co-Existnc btwn IEEE82.11b AND 82.16A Ntworks, in Global Tlcommunications Confrnc, vol. 5, pp , 25. [4] D. Cabric and R. W. Brodrsn, Physical Layr Dsign Issus Uniqu to Cognitiv Radio Systms, in IEEE 16th Intrnational Symposium on Prsonal, Indoor and Mobil Radio Communications, vol. 2, pp , Nov.25. [5] B. Jung, Y. Hong, D. Sung, and S. Chung, Adaptiv Sub-band Nulling for OFDM-Basd Wirlss Communication Systms, in Wirlss Communications and Ntworking Confrnc, pp , 27. [6] J. Chiang and J. Lansford, Us of Cognitiv Radio Tchniqus for OFDM Ultra Widband Coxistnc with Wimax, in Txas Wirlss Symposium, pp , cpb21/shard/pdfs/91.pdf. [7] P. Tu, X. Huang, and E. Dutkiwiz, Divrsity Prformanc of an Intrlavd Sprad Spctrum OFDM Systm ovr Frquncy Slctiv Multipath Fading Channls, in 7th Intrnational Symposium on Communications and Information Tchnologis 27, Sydny, pp , 27.