Combination of OFDM and CDMA for high data rate UWB Combinaison des techniques OFDM et CDMA pour l UWB haut débit

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1 Combnaton of OFDM and CDMA for hgh data rate UWB Combnason des technques OFDM et CDMA pour l UWB haut débt Emerc Guéguen *, Nada Madaou, Jean-Franços Hélard, Mattheu Crussère Insttute of Electroncs and Telecommuncatons of Rennes (IETR) INSA, 2 Avenue des Buttes de Coësmes, 3543 Rennes cedex, France Abstract For Wreless Personal Area Networ (WPAN) systems, resource allocaton between several users wthn a pconet and the coexstence of several pconets are very mportant ponts to tae nto consderaton for the optmzaton of hgh data rate Ultra Wde (UWB) systems. To mprove the performance of the Mult OFDM (Orthogonal Frequency Dvson Multplex) soluton proposed by the Mult OFDM Allance (MBOA), the addton of a spreadng component n the frequency doman s a good soluton snce t maes the resource allocaton easer and also offers a better robustness aganst the channel frequency selectvty and narrowband nterferences. The Spread Spectrum - Mult Carrer - Multple Access (SS-MC-MA) system proposed n ths paper offers not only the advantages of Mult Carrer - Coded Dvson Multple Access (MC- CDMA) brought by frequency spreadng but also a more effectve dynamc resource allocaton n a mult-user and mult-pconet context. These mprovements are obtaned wthout ncreasng the complexty of the radofrequency part compared to the classcal MBOA soluton. Résumé Pour les systèmes WPAN (Wreless Personal Area Networs), la geston des ressources entre pluseurs utlsateurs d une même pcocellule ans que la co-exstence de pluseurs pcocellules sont des ponts mportants à prendre en compte lors de l optmsaton d un système Ultra Large e (ULB) haut débt. Afn d amélorer les performances de la soluton Mult OFDM (Orthogonal Frequency Dvson Multplex) proposée par l allance MBOA (Mult OFDM Allance), l ajout d une composante d étalement selon l axe fréquentel s avère une bonne soluton pour faclter la geston des ressources, qu offre en outre une melleure robustesse vs-à-vs de la sélectvté en fréquence du canal et des nterférences à bande étrote. Le système SS-MC-MA (Spread Spectrum - Mult Carrer - Multple Access) que nous proposons, bénéfce non seulement des avantages du MC-CDMA (Mult Carrer - Coded Dvson Multple Access) apportés par l étalement fréquentel mas permet également une allocaton dynamque des ressources plus effcace dans un contexte mult-utlsateurs et mult-pcocellules. Ces améloratons peuvent être obtenues, sans augmenter la complexté du segment radofréquence par rapport à la soluton MBOA. Keywords: SS-MC-MA, UWB, MB-OFDM, Mult-user, WPAN Mots clés : SS-MC-MA, ULB, MB-OFDM, Mult-utlsateurs, WPAN. 1. Introducton Ultra Wde (UWB) rado systems are today acnowledged as hgh potental solutons for Wreless Personal Area Networs (WPAN). Many UWB system approaches are studed and proposed regardng dfferent objectves n terms of servce (localzaton or communcaton), dstance ranges (short or mddle) or data rates (hgh or low) leadng to dfferent waveforms. The novelty of these systems conssts n the possblty of a non regulated access to the spectral resource, leadng to a flexble use of the rado channel for an mportant number of applcatons. The standardzaton process, led by the Tas Group a of the Insttute of Electrcal and Electroncs Engneers (IEEE) to defne a hgh data rate physcal layer for these WPAN, has seen these three last years the emergence and the confrontaton of many solutons desgnated as ultra wde band solutons. Partcularly, the soluton nown as Mult OFDM (Orthogonal Frequency Dvson Multplex), consdered by the Mult OFDM Allance (MBOA) consortum, s currently promoted by the man actors of the general publc and components ndustres [1,2]. It actually presents some advantages but also some drawbac. After a crtcal analyss of the MBOA soluto ths paper studes the nterest of the addton of a Coded Dvson Multple Access (CDMA) component to a Mult OFDM sgnal. Partcularly, t s demonstrated that the * Correspondng author. E-mal address : emerc.gueguen@nsa-rennes.fr (E. Guéguen)

2 proposed soluton based on the combnaton of OFDM and CDMA technologes offers, for the future WPAN, good performance and a great flexblty for the resource allocaton between users of a same pconet. 2. Dffuson constrants and MBOA soluton In 22, the Federal Communcatons Commsson (FCC) regulated UWB systems by mposng the spectral mas, llustrated n Fg. 1, to lmt transmsson power. To be consdered as UWB, a sgnal must have a mnmum bandwdth of 5 MHz or a bandwdth to central frequency rato above,2. The power spectral densty (PSD) must not exceed -41,3 dbm/mhz. The UWB channel, runnng from 3,1 to 1,6 GHz, s frequency selectve and consdered as almost nvarant n tme. Proposed UWB systems must not dsturb exstng narrowband systems, le for example Wreless Local Area Networ 82.11a standard at 5 GHz. Fg. 1. Indoor PSD mas of the FCC Fg. 1. Masque de DSP ndoor de la FCC Nowadays, wthn the IEEE a standardzaton authortes, two man approaches have been proposed for the hgh data rate UWB: a pulse rado soluton usng DS-CDMA (Drect Sequence-CMDA) ternary codes and the mult-carrer mult-band soluton [1]. The latter, supported by the MBOA consortum, proposes to dvde the avalable band nto 14 sub-bands of 528 MHz each, as llustrated n Fg. 2. Channel 1 Channel 2 Channel 3 Channel 4 Channel f (MHz) Fg. 2. Channels dstrbuton for MBOA soluton Fg. 2. Organsaton des modes de la soluton MBOA Each of these sub-bands allows the transmsson of an OFDM sgnal, obtaned from a 128 ponts Inverse Fast Fourer Transform (IFFT). In the frst step, studes are focused on the frst mode whch clusters the frst three sub-bands from 3,1 to 4,8 GHz. The mult-user management wthn a pconet s based on Tme Dvson Multple Access (TDMA) by usng a Tme-Frequency Code (TFC). At a gven moment, each user occupes one of the three sub-bands of mode 1 [2]. The sgnal, sampled durng the analog to dgtal converso has a lmted bandwdth of 5 MHz, leadng to low cost components. However, the TFC applcaton ntroduces frequency hoppng from one sub-band to another at the end of each OFDM symbol, thus allowng each user to beneft from frequency dversty owng to the wdth of three sub-bands n mode 1. In addto consderng that each user occupes a gven sub-band only one thrd of tme, t s possble to optmze the transmtted power whle respectng the PSD mas mposed by the FCC. Lastly, t s also advsed to plan the cohabtaton of 4 pconets n a same envronment. Man parameters of the MBOA soluton Transmtted data rates n each sub-band depend frstly, on the codng rate, as the appled modulaton to the dfferent subcarrers of the OFDM multplex s a quadrature phase-shft eyng (QPSK). These dfferent modes correspondng to data rates from 53,3 to 48 Mbt/s are enumerated n Tab. 1.

3 Data Rate (Mbt/s) Modulaton Codng Rate (R) Conjugate Symmetrc Input to IFFT Tme Spreadng Factor (TSF) Coded bts per OFDM symbol (N CBPS ) 53,3 QPSK 1/3 Yes QPSK 1/2 Yes QPSK 11/32 No QPSK 1/2 No QPSK 5/8 No QPSK 1/2 No 1 (No spreadng) 2 4 QPSK 5/8 No 1 (No spreadng) 2 48 QPSK 3/4 No 1 (No spreadng) 2 Tab. 1. Data rates n dfferent modes - MBOA soluton Tab. 1. Débts transms dans les dfférents modes - soluton MBOA The channel code s a 64 states convolutonal code wth a rate of 1/3 (wth generator polynomals g = 133 8, g 1 = and g 2 = ) whch s punctured to obtan hgher rates. For certan modes, each complex symbol and ts conjugate symmetrc s transmtted nto the same OFDM symbol by one subcarrer and ts mrror subcarrer respectvely. Ths way, the frequency dversty s exploted nto each sub-band at the cost of a dvson by 2 of the useful transmtted data rate. Moreover, for modes correspondng to data rates from 53,3 Mbt/s to 2 Mbt/s, a tme spreadng of 2 s appled. It conssts n the transmsson of the same nformaton durng 2 consecutve OFDM symbols n order to tae advantage of a better frequency dversty, due to the jont applcaton of the TFC. Fnally, an nterleavng on bnary data generated by the encoder s appled n three steps: the symbol bloc nterleaver permutes bnary elements wthn the span of 6 OFDM symbols, the tone bloc nterleaver permutes bnary elements transmtted by the subcarrers of an OFDM symbol and the ntra-symbol cyclc shfts whch conssts of dfferent cyclc shfts of symbol blocs wthn the span of 6 symbols. The man OFDM parameters of the MBOA soluton are summarzed n Tab. 2. Parameter Value IFFT/FFT sze 128 Samplng frequency 528 MHz Transmsson bandwdth 57,37 MHz Number of data subcarrers 1 Number of plot subcarrers 12 Number of guard subcarrers 1 N ST : Total number of used subcarrers 122 F : Sub-carrer frequency spacng 4,125 MHz (=528MHz/128) T FFT : IFFT/FFT perod 242,42 ns (1/ F ) T CP : Zero Paddnd prefx duraton 6,61 ns T GI : Zero Paddnd guard nterval duraton 9,47 ns T SYM : Symbol nterval 312,5 ns (T FFT + T CP + T GI ) Tab. 2. Man OFDM parameters - MBOA soluton Tab. 2. Prncpaux paramètres OFDM - soluton MBOA The IFFT sze s 128 and the total number of used subcarrers s 122. The useful duraton of each OFDM symbol s 242 ns, leadng to subcarrer frequency spacng of Δ F = 4,125 MHz. A zero paddng (ZP) guard nterval of 6,61 ns duraton s added at the end of each OFDM symbol to cope wth nter-symbol nterference. The only dfference of ZP wth the tradtonal cyclc prefx (CP) s that the CP s replaced by D tralng zeros. Ths operato performed on OFDM symbols at the IFFT nput, s presented by equaton (1): szp = FZPsM (1) H F M where FZP =, F M s the M M FFT matrx and ( ) H H. denotes conjugate transposto so F M s the D M IFFT matrx. s M () s the th M 1 OFDM symbol at the IFFT nput and M s the OFDM symbol length at the IFFT nput, n ths case M = 128.

4 The nput recever sgnal s gven by: x = HF s + H F s ( 1) n (2) ZP ZP M IBI ZP M + where H s the P P ( P M + D h K h L L, H IBI s the P P upper trangular Toepltz matrx wth the frst row [ L h L Lh1 ] whch symbolzes nter-bloc nterference (IBI). The frequency-selectve propagaton s modelled as a FIR flter wth channel mpulse response column vector h = [ h ] T L Lh where L s the channel order ( L D M ). np ( ) s the P 1 addtve whte Gaussan nose (AWGN) vector. = ) lower trangular Toepltz matrx wth the frst column [ ] T The all-zero D M matrx D M elmnates the IBI, snce H IBI F ZP P M H = H, H ZP denotes a partton of the P P matrx H where H s the matrx made of the M frst columns of H and H ZP the matrx made of the D last columns. The receved P 1 vector then becomes x P =. Let [ ] H = HF s + n = H F s n ( ) (3) ZP ZP M P M M + P We can splt x ZP n (3) nto ts upper M 1 part x ( ) u = HusM and ts lower D 1 part x ( ) l = HlsM where H u (respectvely H l ) denotes the correspondng M M (resp. D M ) partton of H. So we can form xl xm = xu + ( M L) 1 Hl = H u + s M (4) ( M L) M = C ( h) s M where sm ( ) s the th OFDM symbol at the IFFT output and C M (h) s M M crculant matrx wth the frst row C ( h) Crc ( h L h Lh ) M = M L 1. The used of ZP prefx requres to mae the operaton presented by the equaton (4) and called Overlap and Add (OLA) [3] before the FFT demodulaton n order to restore the orthogonalty between subcarrers of the multplex. Also, the obtaned result,, s the temporal sgnal at the FFT nput.e. after correct wndowng and OLA operaton. The other parts of the recever are strctly dentcal to those used wth the tradtonal CP. ZP allows obtanng a spectrum wth fewer rpples n the useful band than wth a tradtonal CP. Thus the sgnal can tae the exact shape of the PSD mas. The sgnal shape s optmzed but at the cost of an addtonal processng n the recever. Moreover, an addtonal guard nterval, also of ZP type, of 9,47 ns s added for the commutaton from one subband to another. The complete soluton of the MBOA soluton s descrbed n detals n [1], and an accurate analyss of ts performances s gven n [4] n the case of a perfect and real channel estmaton. To summarze, the MBOA soluton offers some advantages for hgh data rate UWB applcatons, such as the sgnal robustness aganst the channel selectvty and the effcent explotaton of the sgnal energy receved wthn the prefx duraton. The man argument of mult-carrer modulaton n general s often quoted n favour of the MBOA soluto when one compares t wth the compettve DS-CDMA soluton. Indeed, the latter can mae use of all the receved energy wth dffculty, the RAKE fnger number beng compulsorly lmted for complexty reasons. However, the freedom degrees of the MBOA soluton are relatvely lmted n a mult-user and mult-pconet context. Partcularly, when only the three frst sub-bands of the frst mode are consdered, conflcts appear mmedately wth the fourth user wthn a pconet, whereas scenaros gong up to 6 smultaneous users have classcally to be consdered. 3. Why addng a CDMA component to the MBOA soluton? Startng from the MBOA soluto some studes have already proposed to add a CDMA component n order to mprove the system robustness or the resource sharng between several users. Indeed, ths spreadng component allows partcularly to organze the access of several users to a common resource. Tang nto account the UWB M x M

5 channel characterstcs, frequency selectvty and slow tme varaton n ndoor envronment, spreadng s generally performed along the frequency axs, leadng to MC-CDMA sgnals [5,6,7,8]. The symbols of all users are then transmtted by all the subcarrers, the spreadng codes length beng lower or equal to the subcarrers number of the OFDM multplex (Fg. 3). By another way, [9] compares an OFDM system wth an MC-CDMA system for UWB applcatons at 6 GHz. Generally, these dfferent contrbutons show that, compared to the tradtonal MBOA soluto beyond a greater faclty n the resource sharng n mult-user case, an MC-CDMA system also presents a better robustness aganst the channel frequency selectvty. Moreover, a spreadng component n the frequency doman mproves the UWB sgnal robustness aganst narrowband nterferences. Ths last pont s fundamental for uncontrolled access to the spectral resource consderng a flexble use of the rado channel for a great number of dfferent applcatons. However, n a partcular case [5], authors suggest to use an MC-CDMA sgnal wth a bandwdth B W = 1,58 GHz, equvalent to 3 sub-bands of the MBOA sgnal, whch leads to hghly ncrease the samplng frequency of the analog-to-dgtal converson. Code User 3 Tme Freq. User 2 User 1 Spread Symbol Fg. 3. MC-CDMA prncple Fg. 3. Illustraton du prncpe du MC-CDMA 4. A new waveform for mult-band UWB: the SS-MC-MA By applyng a spreadng code and a mult-access component, we propose n ths artcle an SS-MC-MA (Spread Spectrum Mult Carrer Multple Access) waveform [1], whch s new for UWB applcatons and offers better performance and more flexblty n the resource management. Whle consderng the WPAN context and UWB channel characterstcs, we choose a frequency doman spreadng soluton. In addto partcularly for technologcal constrants due to the analog-to-dgtal converso the bandwdth of the transmtted sgnal at a gven moment wll be lmted to 528 MHz and not 1,584 GHz (equvalent to 3 sub-bands of the frst mode) as proposed n [5]. By usng a frequency hoppng technque over the 3 sub-bands, as proposed by the MBOA soluto t s possble to beneft from the frequency ndependence due to a bandwdth equal to 1,584 GHz SS-MC-MA prncple Wth MC-CDMA, resource sharng s realzed by the assgnment to each user of one or more codes, whch are transmtted all over the avalable bandwdth. Thus, all subcarrers of the whole allocated spectrum transmt the symbols of all users dfferentated by ther ndvdual code. The MC-CDMA can be vew as a mono-bloc system (Fg. 3). In a mult-bloc system, spectrum s dvded nto blocs of many subcarrers. Among the possble combnatons, the SS-MC-MA soluto llustrated n Fg. 4, conssts n assgnng to each user a specfc bloc of subcarrers accordng to a Frequency Dvson Multple Access (FDMA) scheme. Code dmenson can then be exploted for an adaptve resource optmzaton and sharng (modulaton type, data rate ). Spreadng n the frequency doman leads to dversty gan and, as t s the case for a MC-CDMA sgnal, mproves the UWB sgnal robustness aganst narrowband nterferers. Wth a SS-MC-MA sgnal, symbols are transmtted smultaneously on a specfc subset of subcarrers by the same user and undergo the same dstortons. Self-nterference (SI), whch then replaces the multple access nterference (MAI) obtaned wth MC-CDMA sgnals, can be easly compensated by mono-user detecton wth only one complex coeffcent per subcarrer. Code User 1 User 2 User 3 Tme Freq. Fg. 4. SS-MC-MA system prncple llustraton Fg. 4. Illustraton du prncpe du système SS-MC-MA Spread Symbol

6 4.2. SS-MC-MA advantages compared wth MBOA soluton and MC-CDMA Let us consder the case of the MBOA standard n mode 1 (Fg. 2). Case of three or less users In ths case, the SS-MC-MA system allows the allocaton of a 528 MHz sub-band for each user. Ths system offers the same performance and advantages as MC-CDMA, whle brngng an addtonal freedom degree compared wth the MBOA soluton for the dynamc resource allocato va the allocaton of a gven number of spreadng codes. Another advantage, compared n ths case wth an MC-CDMA system, s the smpler channel estmaton n recepton. Indeed, a gven subcarrer s dstorted by only one channel, the one of the user assocated wth ths subcarrer. Wth an MC-CDMA system, each subcarrer s corrupted by the dfferent channels of dfferent users, whch ncreases n a consderable way the channel estmaton complexty. In that case, each user has to estmate the response of many channels all over the total avalable bandwdth. Case of more than three users In the MBOA soluto conflcts appear from 4 users and could cause nformaton losses. In the SS-MC- MA case, code dmenson could be exploted to share a same 528 MHz sub-band between 2 or even 3 users f necessary. In that case, the generated sgnal wthn a gven bloc corresponds to a MC-CDMA sgnal, but wth a lmted number of users per bloc (2 or even 3). More generally, n a mult-pconet context, flexblty brought by the resource sharng, by modfyng the number of spreadng codes assgned to a gven user n a gven pconet, allows the SS-MC-MA system to offer a more effcent dynamc resource allocaton than the MBOA soluton. 5. Proposed system descrpton 5.1. Studed system The proposed system s very smlar to the MBOA one. Fg. 5 ntroduces the MBOA transmsson chan n contnuous lnes and, n dashed lnes, the added functons to obtan an SS-MC-MA sgnal. These functons are manly the Hadamard Transform ( Fast Hadamard Transform : FHT) at the transmtter and the nverse transform (Inverse FHT) at the recever. In addto Zero Forcng (ZF) or Mnmum Mean Square Error (MMSE) sngle user detecton s appled. Input data Scrambler Convolutonal Encoder Puncturer Bt Interleaver Constellaton Mappng Spreadng (FHT) Inserton plots and guard sub-carrers OFDM Modulaton + Zero Paddng Add (ZP) Channel Output data De-Scrambler Vterb Decoder De-puncturer Bt De-Interleaver Detecton + Despreadng (IFHT) Remove plots and guard sub-carrers Overlap & Add + OFDM Démodulaton Fg. 5. MBOA transmsson chan (SS-MC-MA n broen lnes) Fg. 5. Chaîne de transmsson MBOA (SS-MC-MA en trats pontllés) The spreadng length L c s chosen equal to 16 and the number of data subcarrers s reduced from 1 to 6x16 = 96 for each OFDM symbol. That means that 4 more guard subcarrers are added. Other functons stay unchanged, partcularly the OLA operaton before the FFT demodulaton to restore the orthogonalty between subcarrers Expresson of the sgnals In the MBOA soluton case, the sgnal generated at the output of the nverse FFT s equal to: S OFDM ( t) = + n= N ST / 2 n = n= N ST / 2 X p ( t T c CP ) e j2 nδ F ( t TCP ) π (5)

7 where Δ F, N ST and T SYM represent subcarrers spacng, the number of total used subcarrers and the spacng between two consecutves OFDM symbols respectvely. X n () s a complex symbol, belongng to a QPSK constellaton and s transmtted by subcarrer n durng the th OFDM symbol. It represents a data, a plot or a reference symbol. p c (t) s a rectangular wndow defned by: 1, t TFFT + TCP pc ( t) = (6), TFFT + TCP t TFFT + TCP + TGI In the SS-MC-MA case, complex symbols are converted nto P parallel symbols D l () (wth P L c ), whch are transmtted by the same L c subcarrers. P, representng the load, s for example equal to L c n the full load case and equal to L c /2 n the half load. C l = [c l,1.c l,m.c l, Lc ] s the l th Walsh-Hadamard orthogonal spreadng code. In ths case, the waveform s the same as prevously, but the complex symbol X m () whch s transmtted by the m th subcarrer (m varyng from 1 to L c = 16, wth m = n modulo(16)) of a bloc of L c = 16 carrers bound by the same spreadng codes of length L c can be express by : X m l P = = Dl cl, m (7) l= 1 where D l () represents the P complex symbols, belongng to a QPSK constellaton and whch are transmtted by the bloc of L c subcarrers consdered durng the OFDM symbol. In recepto as n the classcal OFDM system case, mono-user detecton s smply realzed at the output of the FFT by one complex multplcaton per subcarrer. ZF and MMSE detecton technques are consdered leadng to coeffcents respectvely gven by: g 1 = h Zero Forcng g = h 2 * h 1 + γ MMSE (8) where h n, and γ represent the complex channel response and the sgnal to nose rato for the subcarrer n of the symbol respectvely UWB channel modelsaton Wdeband propagaton channels whch are used for UWB systems PHY layer evaluato result from Saleh- Valenzuela model for ndoor applcatons [11]. Ths ray based model taes nto account clusters phenomena hghlghted durng channels measurements. The multpath channel mpulse response for the th user s gven by: h ( t) = M P m= p= α ( m, p) δ ( t T ( m) τ ( m, p)) (9) where T (m) s the cluster m delay, α (m,p) and τ (m,p) are the gan and the delay of the path p of the cluster m respectvely. The mean excess delay τ m, and the root mean square delay spread, τ rms, are gven n Tab. 3 for the 4 channel models CM ( = {1,,4}). Characterstcs CM1 CM2 CM3 CM4 Mean excess delay (ns) : τ m 5,5 1,38 14,18 Root mean square delay spread: τ rms 5,28 8,3 14,28 25 Dstance (m) < 4 < LOS/NLOS LOS NLOS NLOS NLOS Tab. 3. Characterstcs of wdeband channels CM Tab. 3. Caractérstques des canaux large bande CM

8 In the Lne Of Sght (LOS) confgurato transmtter and recever antennas are n drect vsblty, contrary to the Non Lne Of Sght (NLOS) confguraton. Under the assumpton that the prefx duraton s hgher than the channel mpulse response spreadng and that the subcarrers number N ST s suffcently large to guarantee the frequency non-selectvty of the channel for each one of these subcarrers, UWB channel can be modelled n the form of N ST sub-channels. In ths case, the frequency response for the subcarrer n for the th user s gven by: M P m= p= j2πnδ F ( T ( m) + τ ( m, p)) H ( n) = α ( m, p) e (1) where Δ F s the subcarrer spacng. The channel s thus modelled n the frequency doma and for each sub-band, t s normalzed n mean energy for each realzaton. 1 dfferent realzatons are used for each sub-band, one realzaton beng appled along the frame duraton. Typcal realzatons of the frequency responses of CM1 and CM4 models are represented n Fg. 6. As expected, we can note that the frequency selectvty s hgher wth CM4 than wth CM Attenuaton (db) Attenuaton (db) Subcarrer Number (a) Example of frequency response of CM Subcarrer Number (b) Example of frequency response of CM4 Fg. 6. Realzatons of UWB models channels CM1 and CM4 Fg. 6. Réalsatons des models de canaux UWB CM1 et CM4 6. Systems performance Frstly, the performance of the MBOA has been estmated for the dfferent UWB channels. Fg. 7a and 7b exhbt the results obtaned wth both channels CM1 and CM4 n the deal case of perfect channel estmaton and for rates ranged from 48 to 53,3 Mbts/s. These results are gven versus E b /N whch s the rato between energy per useful bt and the nose monolateral spectral densty. Note that ZP has been used n smulatons and that have thus been consdered to compute E b /N. The performance of the MBOA system for a gaussan channel wth a rate of 32 Mbt/s s also mentoned as a reference Mbt/s 48 Mbt/s 4 Mbt/s 4 Mbt/s BER Mbt/s 2 Mbt/s BER Mbt/s 2 Mbt/s 16 Mbt/s 16 Mbt/s 11 Mbt/s 11 Mbt/s Mbt/s Mbt/s 53.3 Mbt/s 53.3 Mbt/s 32 Mbt/s AWGN 32 Mbt/s AWGN Eb/No (db) (a) Channel CM1 Fg. 7. Performance of the MBOA system Fg. 7. Performances du système MBOA Eb/No (db) (b) Channel CM4

9 For each mode, the performances wth the two channels are very close. Results are slghtly better wth CM4 channel, because n that case the receved sgnal benefts from a better frequency dversty. However the frequency doman representaton of the channel does not tae nto account the scarce paths effects that could run over the guard nterval and consequently could brng nter-carrer nterference. Results versus the E b /N rato for the rates of 32 Mbt/s and 16 Mbt/s are almost the same. For the rate of 16 Mbt/s, the spreadng over two consecutve symbols does not brng any dversty, as the channel s the same for the two consdered symbols. Wth the SS-MC-MA system, the number of data subcarrers s reduced from 1 to 96 n order to obtan a multple of the spreadng length equal to 16. The total number N ST of used subcarrers s henceforth 118 and the transmsson bandwdth becomes equal to 49,87 MHz nstead of 57,37 MHz. Consequently, wth a channel code rate equal to ½, the resultng useful data rate s 37 Mbt/s for full load SS-MC-MA system, wth P = L c = 16. The SS-MC-MA waveform brngs an addtonal freedom degree for resource allocaton. Va the attrbuton of a gven number P of spreadng codes, t s very easy to obtan the wanted rate, as shown n Tab. 3 whch gves the dfferent data rates. For example, the 153 Mbt/s rate s obtaned wth a channel code rate equal to ½ and a half load system wth P = 8, and the 76 Mbt/s rate corresponds to P = 4. Furthermore, nether conjugate symmetrc functon nor tme spreadng are appled. Data Rate (Mbt/s) Modulaton Codng Rate (R) Load P Coded bts per OFDM symbol (N CBPS ) 57 QPSK 1/ QPSK 1/ QPSK 1/ QPSK 1/ QPSK 2/ QPSK 3/ Tab. 3. Data rates n dfferent modes SS-MC-MA soluton Tab. 3. Débts transms dans les dfférents modes - soluton SS-MC-MA Mbt/s BER Mbt/s 49 Mbt/s BER Mbt/s 37 Mbt/s 37 Mbt/s 153 Mbt/s Mbt/s 57 Mbt/s Mbt/s 57 Mbt/s 57 Mbt/s AWGN 57 Mbs/s AWGN Eb/No (db) (a) Channel CM Eb/No (db) (b) Channel CM4 Fg. 8. Performance of the SS-MC-MA system Fg. 8. Performances du système SS-MC-MA MBOA SS-MC-MA 8 Mbt/s 32 Mbt/s 76 Mbt/s 37 Mbt/s CM1 8,2 db 1,3 db 7,6 db 1,2 db CM4 8,4 db 9,85 db 7, db 9,4 db Tab. 4. Requred E b /N for BER = 1-4 versus the bt rate for the MBOA and SS-MC-MA systems Tab. 4. Rapport E b /N nécessare pour un TEB = 1-4 en foncton du débt pour les systèmes MBOA et SS-MC-MA The performances of the SS-MC-MA scheme wth MMSE sngle user detecton are shown on Fg. 8a and 8b for the two channels CM1 and CM4. Note that the spreadng codes have not necessarly been chosen optmally n

10 smulatons. As evdent from the results obtaned at 76 Mbt/s and 57 Mbt/s, the used codes are not favourable regardng the SI, whch mples that the 76 Mbt/s scenaro outperforms the 57 Mbt/s one. Moreover, a comparson of the two systems s presented n Tab. 4, whch gves the requred E b /N to obtan a BER equal to 1-4 for two dfferent bt rates of the MBOA and SS-MC-MA systems for the channel CM1 and CM4. For example, the results obtaned wth the SS-MC-MA scheme at 76 Mbt/s (P = 4) are more attractve than those obtaned wth the MBOA system at 8 Mbt/s. These results show that the soluton of the MBOA consortum, tme spreadng and conjugate symetrc, to mprove the performance of the low bt rate modes are not the most effcent. At hgher bt rates, for example at 37 Mbt/s for SS-MC-MA and 32 Mbt/s for MBOA, ths tendency turns out to be less perceptble and both systems perform very close to each other. It s eventually hghlghted that the proposed SS-MC-MA soluton can be worth of nterest, namely for low bt rates. 7. Concluson To mprove the performance of the MBOA soluton and especally for a better resource allocaton n a mult-user context, an addng spreadng component n the frequency doman s a good soluton whch offers a better robustness to cope wth the channel frequency selectvty and narrowband nterferers. The proposed SS-MC-MA scheme has the advantages of MC-CDMA whch are brought by frequency spreadng, and also allows a more effectve dynamc resource allocaton n a mult-user and mult-pconet context. These mprovements could be obtaned wthout ncreasng the system complexty n comparson wth the reference MBOA soluton. Especally, the bandwdth of the receved sgnal beng stll equal to 5 MHz, the rado-frequency part constrants are unchanged compared to the MBOA soluton. Acnowledgements Authors would le to than France Télécom R&D/RESA/BWA whch supports ths study wthn the contract References [1] A. Batra & al., Mult-band OFDM Physcal Layer Proposal for IEEE Tas Group 3a, IEEE document P /493r1, Texas Instruments & al., September 24. [2] A. Batra & al., Desgn of Multband OFDM System for realstc UWB Channel Envronments, IEEE Transactons on Mcrowave Theory and Technques, Vol. 52, No 9, pp , September 24. [3] B. Muquet et al., Cyclc prefx or zero paddng for wreless multcarrer transmsson?, IEEE Transactons on Communcatons, Vol. 5, pp December 22 [4] L. Maret, I. Saud, Ultra-Wdeband MBOA Physcal layer performance analyss and enhanced ssues (IST MAGNET project), n Proc. European Conference on Propagaton and Systems ECPS 5, Brest, France, March 25. [5] Y-B. Par et al., Performance of UWB DS-CDMA/OFDM/MC-CDMA System, n Proc. IEEE 47th Internatonal Mdwest Symposum on Crcut and Systems, Hroshma, Japa Vol. 1, pp. 37-4, July 24. [6] M. Schmdt, F. Jondral, Ultra Wdeband Transmsson based on MC-CDMA, n Proc. IEEE Global Telecommuncatons Conference GLOBECOM 3, San Francsco, USA, Vol. 2, pp , 1-5 December 23. [7] W.T. Tung, J. Wang, MMSE Recever for Multcarrer CDMA Overlay n Ultra-Wde- Communcatons, IEEE Transactons on Vehcular Technology, Vol. 54, No. 2, pp , March 25. [8] J. Wang, L.B. Mlste Multcarrer CDMA overlay for Ultra-Wdeband communcatons, IEEE Transactons on Communcatons, Vol. 52, No 1, pp , October 24. [9] I. Saud, R. Legouable, Ultra Wde MultCarrer Spread Spectrum technques for short range rado communcatons at 6 GHz, Wreless World Research Forum WWRF#14, San Dego, USA, 7-8 July 25. [1] L. Carou, J-F. Hélard, A novel upln transmsson system based on MIMO frequency hoppng SS- MC-MA for the future wreless cellular networs, Telecommuncaton Systems Journal, ISSN (paper) (Onlne), Vol. 3, No 1-3, pp , November 25. [11] A. Saleh, R. Valenzuela, A statstcal model for ndoor multpath propagato IEEE Journal on Selected Areas n Communcatons, Vol. 5, No 2, pp , February 1987.