A Modified PTS Combined with Interleaving and Pulse Shaping Method Based on PAPR Reduction for STBC MIMO-OFDM System

Transcription

1 A Modified PTS Combied with Iterleavig ad Pulse Shapig Method Based o PAPR Reductio for STBC MIMO-OFDM System P.MUKUNTHAN ad P.DANANJAYAN Departmet of Electroics ad Commuicatio Egieerig Podicherry Uiversity Podicherry Egieerig College, Podicherry-65 4 INDIA. pdaajaya@pec.edu Abstract: - Multiple iput multiple output orthogoal frequecy divisio multiplexig (MIMO-OFDM) system have bee proposed i the recet past for providig high data-rate services over wireless chaels. Whe combied with space time codig it provides the advatages of space-time codig ad OFDM, resultig i a spectrally efficiet widebad commuicatio system. However, MIMO-OFDM system suffer with the problem of iheret high pea-to-average power ratio (PAPR) due to the itersymbol iterferece betwee the subcarriers. To overcome this problem, the partial trasmit sequece (PTS) based o PAPR reductio by optimally combiig sigal subblocs ad the phase rotatio factors is cosidered. As the umber of subblocs ad rotatio factors icreases, PAPR reductio improves. The umber of calculatio icreases as the umber of subblocs icreases, such that complexity icreases expoetially ad the process delay occur simultaeously. I this paper, PAPR reductio schemes based o a modified PTS combied with iterleavig ad pulse shapig method for STBC MIMO-OFDM system has bee preseted. The paper aalyses the ifluece of the umber of the detected peas o PAPR reductio performace ad o complexity, ad the obtai the optimal parameter to achieve better PAPR reductio performace ad lower complexity. Simulatio results show that the proposed modified PTS with iterleavig ad the pulse shapig method ca obviously improve PAPR performace i the MIMO-OFDM system. Key-Words: - MIMO-OFDM, PAPR, STBC, Partial Trasmit Sequeces, Iterleaved Subbloc Partitio Scheme, Raised-Cosie pulse shape Itroductio Future mobile commuicatio systems demads for high data rates ad high-quality multimedia services to mobile users. Orthogoal frequecy divisio multiplexig (OFDM) is oe of the may multicarrier modulatio techiques, which provides high spectral efficiecy, low implemetatio complexity, less vulerability to echoes ad o-liear distortio. Due to these advatages of the OFDM system, it is vastly used i various commuicatio systems [-4]. Multiple-atea commuicatios systems have geerated a great deal of iterest sice they are capable of cosiderably icreasig the capacity of a wireless li [5]. A space-timefrequecy coded OFDM system which achieves maximum diversity is proposed i [6, 7]. I [8] space-time codes have bee desiged for use with OFDM over frequecy selective chaels, which ca achieve spatial diversity by usig multiple ateas at the trasmitter ad receiver ad it is promisig, sice it does ot icrease the trasmit power ad the sigal badwidth. Therefore, it has bee adopted by may high-speed data trasmissio stadards, such as digital audio broadcastig (DAB), digital video broadcastig (DB), asymmetric digital subscriber lie (ADSL), WLAN (IEEE 8.a/g) ad WiMAX (IEEE 8.6) applicatios. The MIMO-OFDM system has several advatages over the sigle carrier systems such as its robustess agaist multipath fadig ad high power spectral efficiecy. But the major problem oe faces while implemetig this system is the high pea-to-average power ratio (PAPR). A large PAPR icreases the complexity of the aalog-todigital ad digital-to-aalog coverter ad reduces the efficiecy of the radio-frequecy (RF) power amplifier. For short-rage trasmissios, ad i particular for battery powered devices, it is essetial to trasmit waveforms with low dyamic rage. E-ISSN: Issue 3, olume, March 3

2 This esures a low PAPR ad therefore allows the power amplifier to operate i its liear rage without a excessive bacoff [9]. There are a umber of techiques to deal with the problem of PAPR i MIMO-OFDM system. Some of them are amplitude clippig [], pre-distortio method [], iterleaved method [], pulse shapig [3], selective mappig (SLM) [4], ad partial trasmit sequece (PTS) [5]. Amplitude clippig [] is the simplest PAPR reductio techique. Clippig reduces the PAPR by limitig the pea power to a predetermied threshold. Iterleavig techique has bee proposed for reductio of PAPR of a OFDM trasmissio. The iterleaved partitioed ordiary PTS scheme has the lowest computatioal complexity but it has the worst PAPR performace because the geerated cadidates are ot fully idepedet [6]. The reductio i PAPR achieved by the pulse shapig techique [3] is obtaied at the expese of a icrease i the error probabilities of the system, ad differet pulse shapig waveforms result i differet probabilities of errors. SLM ad PTS belog to the probabilistic class because several cadidate sigals are geerated ad the oe with the miimum PAPR is selected for trasmissio. I SLM [4], oe OFDM sigal of the lowest PAPR is selected from a set of several sigals cotaiig the same iformatio data. SLM is a very flexible scheme ad has a effective performace of the PAPR reductio without ay degradatio. However, SLM techique requires high computatioal complexity ad reduced badwidth efficiecy due to the trasmissio of side iformatio. These techiques achieve PAPR reductio at the expese of trasmit sigal power icrease, bit error rate (BER) icrease, data rate loss, computatioal complexity icrease, ad so o. The ordiary PTS scheme is simple ad distortioless, sometimes its computatioal complexity is burde-some. I the PTS [5] approach, the iput OFDM bloc is partitioed ito disjoit subblocs. Each subbloc is multiplied by a phase weightig factors (±, ±j), which is obtaied by the optimizatio algorithm to reduce the PAPR value. However, ordiary PTS requires a exhaustive search over all the phase factor combiatios, which results i the search complexity icreasig expoetially with the umber of subblocs. Hece the modified PTS scheme [7] is proposed to lower the computatioal complexity which maitais the similar PAPR reductio performace compared with the covetioal PTS scheme. Aother possible alterative solutio is the to exploit other parameters of the OFDM sigal. To alleviate the problem of high complexity further a approach [8] has bee proposed, i which real ad imagiary parts are separately multiplied with phase factors, moreover PAPR is cojoitly optimized i real ad imagiary parts. I the previous wor, PAPR reductio is joitly optimized i both the real ad imagiary parts separately multiplied with phase factors whe differet subcarriers ad subblocs [9] some of the existig sigle-atea PAPR reductio based modified PTS with iterleavig techique is exteded to MIMO-OFDM systems[]. I MIMO-OFDM system [9], a straightforward way for PAPR reductio is to apply the existig algorithms separately o each trasmit atea. It is effective to reduce PAPR performace, but requires high computatioal complexity, ad large size of side iformatio. Therefore, i this paper, a low complexity PAPR reductio scheme for STBC MIMO-OFDM system based o modified PTS combied with iterleavig ad pulse shapig method is proposed. As the umber of subcarriers are icreased, reducig the computatioal complexity with better PAPR reductio performace whe the umber of subblocs are icreased is the aim of this wor. The rest of this paper is orgaized as follows. I Sectio, the PAPR i SISO-OFDM ad MIMO- OFDM systems, the PTS scheme ad pulse shapig are discussed. The detailed descriptio of the iterleavig techique is show i Sectio 3. The simulatio results ad discussio are preseted i Sectio 4. Fially, the coclusio is draw i Sectio 5. MIMO-OFDM System ad PTS Scheme. PAPR i SISO-OFDM System I the OFDM modulatio techique, a bloc of N data symbols, X = ( X, X,... XN ), is formed with each symbol modulatig the correspodig subcarrier from a set of subcarriers. The N subcarriers are chose to be orthogoal, with a pulse shape waveform of duratio T, ad f =/T is the frequecy spacig betwee adjacet subcarriers. leave two bla lies betwee successive sectios as here. I SISO-OFDM system, the time-domai OFDM sigal for N subcarriers ca be writte as N j f t = π ( ) = Xe, t T xt N () E-ISSN: Issue 3, olume, March 3

3 where X deotes the modulated symbol i the th subbloc. Replacig t=t b, where T b =T/N, gives the discrete time versio deoted by N j π / LN x ( ) = Xe, =,,,NL- () N = where, x() is the data symbol at th subcarrier ad L is the oversamplig factor. The PAPR of the trasmitted OFDM sigal, x(t), is the give as the ratio of the maximum to the average power, writte as max xt t T ( ) PAPR = (3) E xt ( ) where E[ ] deotes expectatio, or time-averaged expectatio if x(t) is ostatioary. The Cumulative Distributio Fuctio (CDF) is oe of the most regularly used parameters, which is used to measure the efficiecy of ay PAPR techique. Normally, the Complemetary CDF (CCDF) is used istead of CDF, which is used to measure the probability that the PAPR of a certai data bloc exceeds the give threshold. By implemetig the cetral limit theorem for a multicarrier sigal with a large umber of subcarriers, the real ad imagiary part of the time domai sigals have a mea of zero ad a variace of.5 ad follow a Gaussia distributio. The CCDF deotes the probability of OFDM sigal exceedig a give threshold value PAPR is give by CCDF( PAPR( x( ))) = P r ( PAPR( x( ))) > PAPR (4) Sice the iput to the high power amplifier (HPA) must be a cotiuous-time sigal, the CCDF of the PAPR of x(t) is of iterest. Oversamplig has bee used to approximate the CCDF of the PAPR of the cotiuous-time OFDM sigal by PAPR N P = P r ( PAPR( x( )) > PAPR ) = ( e ) (5) This expressio assumes that the Nyquist-rate time domai OFDM sigal samples are mutually idepedet ad ucorrelated. This is ot true, however, whe oversamplig is applied. Also, this expressio is ot accurate for a small umber of subcarriers sice a Gaussia assumptio does ot hold i this case. Therefore, the CCDF of PAPR computed of the L-time oversampled OFDM sigal ca be rewritte as PAPR LN P = P ( PAPR( x( )) > PAPR ) = ( e ) (6) r. PAPR i MIMO-OFDM System A multicarrier system ca be efficietly implemeted i discrete time usig a iverse fast Fourier trasform (IFFT) to act as a modulator ad a FFT to act a demodulator. Cosider the MIMO- OFDM system with M t trasmit ateas that use N subcarriers. With OFDM modulatio, a bloc of N data symbols (oe OFDM symbol), { X,,,..., = N } will be trasmitted i parallel such that each modulates a differet subcarrier from a set { f,,,..., = N }. The N subcarriers are orthogoal, ie., f = f, where f ==NT ad T is the period. The resultig basebad OFDM sigal xm, of a bloc ca be expressed as N jπ f xm, = Xe, =,,... N (7) N = where, X is the trasmitted OFDM sigal at the subcarriers of the m th trasmit ateas. The PAPR is a characteristic of the iput sigal ad is defied as follows for trasmit sigal xt (). max xt ( ) t MT PAPR= (8) MT x() t dt MT where T is the bloc duratio for bloc trasmissio The PAPR of the cotiuous-time OFDM sigal caot be precisely computed at the Nyquist samplig rate, which correspods to N samples per OFDM symbol. Whe this is ot the case, the achieved PAPR reductio might be lower ad suboptimal []. To achieve effective PAPR reductio for this case, sigal peas may be sipped ad PAPR estimates are ot precise. Usually, the oversamplig factor L larger tha 4 is used for a PAPR reductio scheme to icrease the resolutio of discrete-time OFDM sigals. However, such a oversamplig process would sigificatly icrease the computatioal complexity. PTS schemes for SISO systems ca be directly applied to each trasmit atea i MIMO-OFDM system achievig almost the same PAPR reductio performace with sigle trasmit atea. For the etire system, the PAPR is defied as the maximum of PAPRs amog all trasmit ateas [], i.e., PAPR = max PAPR (9) MIMO OFDM i Mt where PAPR i deotes the PAPR at the i th trasmit atea. Specifically, sice i MIMO-OFDM, M t N time domai samples are cosidered compared to N i SISO-OFDM, the CCDF of the PAPR i MIMO- OFDM ca be writte as PAPR M ( ) ( ) t LN Pr PAPRMIMO OFDM PAPR e > = () Comparig () with (6), it is evidet that MIMO- OFDM results i eve worse PAPR performace i E-ISSN: Issue 3, olume, March 3

4 tha SISO-OFDM. I practice, a HPA is expected to provide a certai level of power efficiecy, which meas that for a give HPA ad biasig coditios, the average iput power has to be above a certai amout. This also requires the iput sigal PAPR to be less tha a threshold PAPR..3 PTS scheme The partial trasmit sequece scheme [3-5] is a attractive solutio to reduce PAPR i MIMO- OFDM system without ay distortio of trasmitted sigals. I the PTS scheme, the iput data bloc is partitioed ito disjoited subblocs. Ad each subbloc is multiplied by phase weightig factors, which obtaied with optimizatio algorithm. If the subblocs are optimally phase shifted, they exhibit miimum PAPR ad cosequetly reduce the PAPR of the merged sigal. The umber of subblocs () ad the subbloc partitio scheme determie the PAPR reductio. The mai drawbac of PTS arises from the computatio of multiple IFFTs, resultig i a high computatioal complexity with the factorial of the product of trasmit ateas umber ad subblocs umber. I geeral, Subbloc partitioig types ca be classified ito 3 categories; iterleaved partitio, adjacet partitio, ad pseudo-radom partitio. For the iterleaved method [], every subcarrier sigal spaced apart is allocated at the same subbloc. I the adjacet scheme, successive subcarriers are assiged ito the same subbloc sequetially. Lastly, each subcarrier sigal is assiged ito ay oe of the subblocs radomly i the pseudoradom scheme. It ca be oted that the computatioal complexity of the iterleaved subbloc partitioig scheme is reduced extesively as compared to that of the adjacet ad pseudoradom partitio scheme. This subbloc partitioig scheme reduces cosiderably the evelope fluctuatios i the trasmitted waveform. A simplified bloc diagram of STBC MIMO- OFDM system with M t = trasmit ateas is illustrated i Fig.. A data symbol vector S = [ X, X,..., X ] T N is ecoded with space-time ecoder taes a sigle stream of biary iput data ad trasforms it ito two vectors S ad S, * * T S = [ X, X,... XN, X N ], * * T S = [ X, X,... XN, X N ]. which are fed to the IFFT blocs ad set simultaeously from ateas T X ad T X, respectively. Alterative trasmit vectors represetig the same data ca ow be costructed by mutually iterchagig the correspodig PTSs betwee the differet trasmitter braches, provided that the same subcarrier groupig is applied to all braches. Symbol S ad S represet the two eighborig OFDM sigals i time domai. Fig. A typical structure of trasmitter for modified PTS combied with iterleavig ad pulse shapig method i STBC MIMO-OFDM system The objective is to optimally combie the clusters, which i frequecy domai is give by ' S= bs () m m m= where, {b m, m=,,, } are weightig factors ad are assumed to be perfect rotatios. I other words, the time domai is give by s= bs () m m m= where, s m cosist of a set of subblocs with equal size ad b m is the phase rotatio factor, which are required to iform the receiver as the side iformatio. I order to icrease PTS performace, the size of side iformatio is drastically icreased. It meas that the total throughput cosiderably decreases. The set of weightig factor for clusters or subblocs are optimised i the time domai so as to achieve the better PAPR performace. PTS geerates a sigal with a low PAPR through the additio of appropriately phase rotated sigal parts. The code word to be trasmitted are divided ito several subblocs,, of legth N/ [8]. Mathematically, expressed by A = A, v=,,, (3) v= ( v) All subcarriers positios i A ( v) which are occupied i aother subbloc are set to zero. Each of the blocs, v, has a IFFT performed o it, E-ISSN: Issue 3, olume, March 3

5 a v { } = IFFT A (4) ( v) ( ) The output of each bloc except for first bloc which is ept costat, is phase rotated by the rotatio factor as give by j ( v) e θ [, π ] (5) The blocs are the added together to produce alterate trasmit sigals cotaiig the same iformatio as give by ( v) j ( v) a = a. e θ (6) v= Each alterate trasmit sigal is stored i memory ad the process is repeated agai with a differet phase rotatio value. After a set umber of phase rotatio values, b, the MIMO- OFDM symbol with the lowest PAPR is trasmitted as give by [8],,... arg mi(max ) (7) 3 v φ φ φ = a The weightig rotatio parameter set is chose to miimise the PAPR. The computatioal complexity of PTS method depeds o the umber of phase rotatio factors allowed. The phase rotatio factors ca be selected from a ifiite umber of phases ( v) φ (, π). But fidig the best weightig factors is ideed a complex problem. To icrease the potetial capability of PAPR reductio performace for the PTS method, these phase factors combiatio correctly maitais the orthogoality betwee the differet modulated carriers. However, the PTS PAPR reductio scheme s performace improvemet is achieved at the expese of high complexity ad difficult parameter settig problems. Therefore, modified PTS ideed use the potetial of MIMO trasmissio for PAPR reductio. I MIMO commuicatio, data rate or diversity order ca be improved by exploitig the spatial dimesio [6, 7]. I the same spirit, treatig the parallel trasmit sigals joitly, PAPR reductio may be improved..4 Raised-Cosie pulse shape A set of time waveforms that reduces the PAPR of OFDM sigals was proposed i [8, 9]. However, the reductio obtaied was ot cosiderable. Cosider a time waveform with costat eergy equals to eergy sigal (Es=) ad ucorrelated symbols withi each OFDM bloc, the maximum PAPR is obtaied as follows: N max = max m ( ) N t T = PAPR PAPR P t (8) which is a fuctio of the umber of subcarriers N ad Pm () t is a pulse shape (time waveform) used at each subcarrier. With large umber of subcarriers, the maximum of the PAPR occurs with very low probability. The cross-correlatio fuctio of the OFDM sigal is obtaied as: N N * * s(, ) =, m, ( ) m( ) j c ( t mt) (9) = m= R t t S S p t p t e ω where ω c is the carrier frequecy of the system ad the cross-correlatio coefficiet is zero for all samples separated by multiples of T. A possible solutio to reduce the PAPR of the OFDM sigals is the to create some correlatio betwee the differet OFDM samples of the same bloc. The ew set of pulse shapes idicates that each subcarrier pulse of the OFDM scheme has a differet shape ad all these pulse shapes are derived from the same pulse (cyclic shifts of the same pulse).this will also reduce the PAPR of the OFDM trasmitted sigal sice the pea amplitude of the differet pulse shapes will ever occur at the same time istat uless time waveform is a rectagular pulse. The impulse respose of a raised cosie filter is, παt cos( ) t ( ) si ( ) T rt = cπ () T 4α t T where the parameter α is the roll-off factor which rages betwee ad. Lower values of α itroduce more pulse shapig ad more suppressio of out-of-bad-sigal compoets. Pulse shapes are very flexible ad ca cotrol the correlatio betwee the OFDM bloc samples without destroyig the orthogoality property betwee the subcarriers of the OFDM modulated sigal. 3 Iterleaved MIMO-OFDM Highly correlated data frames of OFDM sigals have large PAPRs, which could thus be reduced, if the log correlatio patters are broe dow. A set of fixed permutatios (iterleavig) is used i this techique to brea these correlatio patters [3-3]. I this approach K- iterleavers are used at the trasmitter. These iterleavers produce K- permuted frames of the iput data. The miimum PAPR frame of all the K frames is selected for trasmissio. The idetity of the correspodig iterleaver is also set to the receiver as side E-ISSN: Issue 3, olume, March 3

6 iformatio. If all the K, PAPR computatios are doe simultaeously ad lowest PAPR sequece is selected i oe step, the processig delay at the trasmitter is sigificatly reduced. Therefore, it ca also be used with high speed data trasmissios. Iterleaved MIMO-OFDM is also feasible for spectrum moitorig. Sice subcarriers of oe subbloc are equally spaced, their frequecy locatios ca be determied by capturig oe subcarrier with the owledge of system parameters. Users ca moitor the radio activity o oe subbloc by sesig oly oe or two subcarriers of the subbloc istead of all the subcarriers across the whole frequecy bad. Iterleavig ca be used to combat the effect of oise bursts ad fadig i error correctio systems. By iterleavig a data frame, the peas i the associated OFDM sigal ca be compressed. For Iterleaved MIMO-OFDM, the N subcarriers are partitioed ito groups with each group havig Q cotiguous subcarriers. The the th subcarrier of each group is assiged to the th user. ( ) ( ) j( π / N )( mq+ ) ( ) m m= x = X e () i which =,,.Q- is the idex of users Let m=n(q+), where q Q- ad N- The, m m j π l N jπ x m ( = x Nq+ ) = Xe l =. Xe l= QN = N Nq+ jπ N =. Xe Q N = N jπ N = Xe () Q N = =. x Q where l deotes a ormalized discrete time istace, q is the sub-chael idex of the th user ad N is the total umber of subcarriers. The resultig time symbols { } m x are simply a repetitio of the origial iput symbols { x } i the time domai [33]. Therefore, a iterleaved MIMO- OFDM system with N subcarriers ca be scaled from a OFDM system. 4 Results ad Discussio The aalysis of PAPR reductio performace for modified PTS combied with iterleavig ad pulse shapig method has bee carried out usig MATLAB 7.. The simulatio parameters cosidered for this aalysis is summarized i Table. Table.Simulatio parameters Simulatio parameters Type/alues Number of OFDM blocs Number of subcarriers (N) 64,8,56,5,4 Number of subblocs (), 4,8,6 Oversamplig factor (L) 4 Roll-off factor (α).6 Subbloc partitioig scheme Iterleavig Number of ateas(m t) Modulatio scheme QPSK Phase weightig factor (b), -, j, -j I the MIMO-OFDM system uder cosideratio, modified PTS techique is applied to the subblocs of iput iformatio, which is modulated by QPSK modulatio, ad the phase rotatio factors are trasmitted directly to receiver through subbloc. The performace evaluatio is doe i terms of complemetary cumulative distributio fuctio (CCDF) of the PAPR, which idicates the probability that the PAPR is larger tha PAPR. It is show i Fig. that the performace of the modified PTS techique with iterleavig ad pulse shapig method for STBC MIMO-OFDM system with differet umber of subcarriers N= 64,8, 56, 5, ad 4. From this figure it is observed that the values of PAPR for N= 64,8, 56, 5, ad 4 become 6.9dB, 7.6dB, 8.dB, 8.6dB ad 9dB respectively whe CCDF = -. The PAPR value icreases sigificatly as umber of subcarriers used i the MIMO-OFDM trasmissio icrease. This improvemet i PAPR is valid for E-ISSN: Issue 3, olume, March 3

7 ay umber of subcarriers of the OFDM sigal by usig pulse shapig techique. values decreases as the umber of subblocs icreases. Fig. CCDF of PAPR for differet subcarriers N = 64, 8, 56, 5, 4 whe =4, L=4, α=.6 ad M t = Fig. 3 CCDF of PAPR for differet subcarriers N = 64, 8, 56, 5, 4 whe =8, L=4, α=.6 ad M t = The impact of the subbloc =8 o the performace of PAPR reductio for differet umber of subcarriers N= 64,8, 56, 5, 4 with trasmit ateas is show i Fig. 3 at CCDF = -. From this Fig. 3 it is observed that the values of PAPR for trasmit ateas for differet subcarriers N= 64, 8, 56, 5, ad 4 become 5, 6, 6.8, 7.4 ad 7.9 db whe CCDF = -. By comparig the Figures ad 3 it is evidet that PAPR is decreased from 8. db to 6.8 db for trasmit ateas whe N=56 subcarriers with umber of subblocs icreased from =4 to 8 at CCDF of -. The PAPR value icreases sigificatly as umber of subcarriers used i the MIMO-OFDM trasmissio icrease, but PAPR Fig. 4 CCDF of PAPR for differet subcarriers N = 64, 8, 56, 5, 4 whe =6, L=4, α=.6 ad M t = The impact of the subbloc =6 o the performace of PAPR reductio for differet umber of subcarriers N= 64, 8, 56, 5, 4 with trasmit ateas is show i Fig. 4 at CCDF = -. From this Fig. 4 it is observed that the values of PAPR for trasmit ateas for differet subcarriers N= 64, 8, 56, 5, ad 4 become.8, 3.4, 4.6, 5.6 ad 6.3dB respectively whe CCDF = -. By comparig the Figures, 3 ad 4 it is evidet that PAPR is decreased from 8. db to 6.8 db ad from 6.8 db to 4.6 db for trasmit ateas whe N=56 subcarriers with umber of subblocs icreased from =4 to 8 ad from =8 to 6 at CCDF of -. The PAPR value icreases sigificatly as umber of subcarriers used i the MIMO-OFDM trasmissio icrease, but PAPR values decreases as the umber of subblocs icreases. Fig.5 shows the CCDFs of PAPR performace of the modified PTS based o iterleavig ad pulse shapig method i MIMO-OFDM system with differet umber of subblocs =, 4, 8, ad 6 for a radom data of bloc size with N=56 subcarriers, spreadig factor β=4, oversamplig factor L=4 ad roll-off factor α=.6. It ca be see from the figure that as the subbloc size is icreased from to 4, 8, ad 6, the PAPR is reduced to 8.8 db, 8.dB, 6.8 db ad 4.6 db respectively whe CCDF= -, resultig i PAPR performace improvemet as the umber of subblocs icreases. E-ISSN: Issue 3, olume, March 3

8 with trasmit ateas. From this figure it is observed that the values of PAPR for trasmit ateas with differet subblocs =, 4, 8, ad 6 become 9.4, 9, 7.9, ad 6.3 db respectively whe CCDF = -. By comparig the Figures 5, 6, ad 7 it is evidet that PAPR is icreased from 8. db to 8.6 db, ad from 8.6 db to 9 db for trasmit ateas whe =4 subblocs with umber of subcarriers icreased from N=56 to 5 ad from N=5 to 4 respectively at CCDF of -. From this figure it is show that PAPR reductio performace icreases with icrease of subblocs. Fig.5 CCDF of PAPR for differet subblocs =, 4, 8, 6 whe N=56, L=4, α=.6 ad M t = Fig.7 CCDF of PAPR for differet subblocs =, 4, 8, 6 whe N=4, L=4, α=.6 ad M t = Fig.6 CCDF of PAPR for differet subblocs =, 4, 8, 6 whe N=5, L=4, α=.6 ad M t = Fig. 6 illustrate the CCDFs of PAPR of the modified PTS with iterleavig ad pulse shapig method for differet subblocs =, 4, 8, 6 whe subcarriers N=5 with trasmit ateas. From this figure it is observed that the values of PAPR for trasmit ateas with differet subblocs =, 4, 8, ad 6 become 9. db, 8.6 db, 7.4 db, ad 5.6 db respectively whe CCDF = -. By comparig the Figures 5, ad 6 it is evidet that PAPR is icreased from 8. db to 8.6 db for trasmit ateas whe =4 subblocs with umber of subcarriers icreased from N=56 to 5 at CCDF of -. From this figure it is show that PAPR reductio performace icreases with icrease of subblocs. Fig.7 shows the CCDFs of PAPR of the modified PTS with iterleavig techique for differet subblocs =, 4, 8, 6 whe subcarriers N=4 Fig.8 CCDF of PAPR for differet oversamplig factor L=, 4, 8, 6 whe N=56, =4,α=.6 ad M t = Fig. 8 illustrates the complemetary cumulative distributio fuctio of the PAPR of the MIMO- OFDM sigal for the case of N=56 subcarriers ad =4 subblocs ad for differet oversamplig factor E-ISSN: Issue 3, olume, March 3

9 (L) icreased from to 4, 8, ad 6. If L is icreased, improved performace ca be obtaied. Of course, this occurs at a icreasig level of complexity. Icreasig L beyod 4 seems to brig very little improvemet i performace. of.6, respectively. From this figure it is cocluded that combied modified PTS with iterleavig ad pulse shapig method results i a sigificat PAPR reductio compared to the other schemes. Fig. 9 Aalysis of PAPR by alterig roll-off factor α=,.,.4,.6,.8, ad for N= 56 whe =4, L=4 ad M t = Fig. 9 illustrates the complemetary cumulative distributio fuctio of the PAPR of the OFDM sigal for the case of N=56 subcarriers ad =4 subblocs ad for differet roll-off factors. As the roll-off factor α icreases from to, PAPR reduces sigificatly for MIMO- OFDM system. This implies that there is a tradeoff betwee PAPR performace ad out-of-bad radiatio sice out-ofbad radiatio icreases with icreasig roll-off factor. It is observed that the proposed broadbad pulse shapes provide cosiderable gai i PAPR for the OFDM sigal whe compared to that of ordiary MIMO-OFDM sigal. By usig pulse shapig filter with high roll off factor sigificat reductio of PAPR occurs but price paid is icreased badwidth requiremet. So a optimal value of roll-off factor is chose accordig to applicatio. Fig. shows the compariso betwee the modified PTS without iterleavig ad modified PTS with iterleavig for MIMO-OFDM system. Whe the parameters are set with N=56 subcarriers, =4 subblocs, oversamplig factor L=4 ad roll-off factor α=.6, iterleaved subbloc partitioig method to reduce hardware complexity of IFFT with little performace degradatio. It ca be see that the PAPR of modified PTS without iterleavig method is 9. db, modified PTS with iterleavig method is 8.9 db ad modified PTS with iterleavig ad pulse shapig is 8. db at CCDF Fig. Compariso of Origial, MPTS with ad without iterleavig, ad MPTS with iterleavig ad pulse shapig method for MIMO-OFDM system with N= 56,=4, L=4, α=.6 ad M t = 5 Coclusio This paper proposes a modified PTS with iterleavig ad pulse shapig method to reduce the pea-to-average power ratio for MIMO-OFDM trasmissio. The method avoids the use of ay extra iverse fast Fourier trasformatios (IFFTs) as was doe i PAPR reductio by ordiary PTS techique but istead is based o a proper selectio of the differet subcarriers ad subblocs. It has bee show that the PAPR performace ca be improved up to.8 db by usig a modified PTS combied with iterleavig ad pulse shapig method compared with the origial sigal. Simulatio results to illustrate the performace of the modified PTS combied with iterleavig ad pulse shapig method for STBC MIMO-OFDM system is a effective scheme to achieve a better tradeoff betwee PAPR reductio ad computatioal complexity. Sice the computatioal complexity reductio ratio icreases as the umber of subcarriers icreases, the proposed scheme becomes more suitable for the high data rate MIMO-OFDM system such as a digital multimedia wireless broadbad mobile commuicatio system. Refereces: [] R.. Nee ad R. Prasad, OFDM for Wireless Multimedia Commuicatios, Lodo, Artech House, Bosto,. E-ISSN: Issue 3, olume, March 3

10 [] S. Hara ad R.Prasad, Multicarrier Techiques for 4G Mobile commuicatios, Artech House, Bosto, 3. [3] Haitham J.Taha ad M.F.M. Salleh, Multicarrier trasmissio techiques for wireless commuicatio systems: A survey, WSEAS Trasactios o Commuicatios, ol. 8, No. 5, May 9, pp [4] S.H. Muller ad J.B. Huber, A ovel pea power reductio scheme for OFDM, i Proceedigs of Iteratioal Symposium o Persoal, Idoor, ad Mobile Radio Commuicatio, Filad, ol. 3, -4 September 997, pp [5] Xu Ya, Wag Chuli Jr., ad Wag Qi, Research of Pea-to-average power ratio reductio improved algorithm for MIMO- OFDM system, i Proceedigs of World Cogress o Computer Sciece ad Iformatio Egieerig, Los Ageles, USA, ol., March 3-April, 9, pp [6] Jig Gao, Jiua Wag ad Yu Wag, A low complexity PAPR reductio techique for STBC MIMO-OFDM system, i Proceedigs of Iteratioal Coferece o Wireless Commuicatios, Networig ad Mobile Computig, Shaghai, -5 September 7, pp. 9-. [7] A.S.Hiwale ad A.A.Ghatol, Capacity ad performace aalysis of space-time bloc codes i raylaigh fadig chaals, WSEAS Trasactios o Commuicatios, ol. 6, No., December 7, pp [8] Zhiqiag Liu, Ya Xi ad Georgios B.Giaais, Space-time-frequecy coded OFDM over frequecy-selective fadig chaels, IEEE Trasactios o Sigal Processig, ol. 5, No., October, pp [9] P.Muutha ad P.Daajaya, PAPR reductio of a OFDM sigal usig modified PTS combied with iterleavig ad pulse shapig method, Europea Joural of Scietific Research, ol. 74, No. 4, May, pp [] Shag-Kag Deg, Mao-Chao Li, Recursive Clippig ad Filterig with bouded distortio for PAPR reductio, IEEE Trasactios o Commuicatio, ol. 55, No., Jauary 7, pp [] Chhavi Sharma, Shiv Kumar Tomar, ad A.K.Gupta, PAPR reductio i OFDM system usig adaptig codig techique with pre distortio method, WSEAS Trasactios o Commuicatios, ol., No. 9, September, pp [] S.G.Kag, J.G.Kim, A ovel subbloc partitio scheme for partial trasmit sequece OFDM, IEEE Trasactios o broadcastig, ol. 45, No. 3, September 999, pp [3] S. Be Slimae, Pea-to-average power ratio reductio of OFDM sigals usig pulse shapig, i Proceedigs of ehicular Techology Coferece, USA, ol. 4, 4-8 September, pp [4] Yug-Lyul Lee, Youg-Hwa You, Wo-Gi Jog-Ho Pai ad Hyoug-Kyu Sog Pea-toaverage power ratio reductio i MIMO OFDM system usig Selective mappig, IEEE Commuicatios Letters, ol. 7, No., December 3, pp [5] D.Phetsomphou, S.Yoshizawa, ad Y.Miyaaga, A partial trasmit sequece techique for PAPR reductio i MIMO- OFDM systems, i Proceedigs of th Iteratioal Symposium o Commuicatios ad Iformatio Techologies, Toyo, Japa, 6-9 October, pp [6] G.Lu, P. Wu ad C. Carlemalm-Logothetis, Ehaced Iterleavig partitioig PTS for pea-to-average power ratio reductio i OFDM systems, IEEE Electroics Letters, ol.4, No.7, August 6, pp [7] Z.Xiuya ad T.Guobi, The research of improved PTS method for pea-to-average power ratio reductio, i Proceedigs of 3rd iteratioal coferece o wireless, mobile ad multimedia etwors, Beijig, Chia, 6-9 September, pp [8] P.Muutha ad P.Daajaya, Modified PTS with FECs for PAPR reductio of OFDM sigals, Iteratioal Joural of Computer Applicatios, ol., No. 3, December, pp [9] S. M. Alamouti, A simple trasmit diversity techique for wireless commuicatios, IEEE Select Areas i Commuicatios, ol. 6, No. 8, October 998, pp [] P.Muutha ad P.Daajaya, PAPR reductio based o a modified PTS with iterleavig ad pulse shapig method for STBC MIMO-OFDM system, i Proceedigs of Third Iteratioal Coferece o Computig, Commuicatio ad Networig Techologies, SNS College of Egieerig, Coimbatore, Tamiladu, Idia, 6 th to 8 th July. (Accepted) [] M.Sharif, M.Gharavi-Alhasari, ad B.H. Khalaj, O the pea-to-average power of E-ISSN: Issue 3, olume, March 3

11 OFDM sigals based o oversamplig, IEEE Trasactios o Commuicatios, ol. 5, No., Jauary 3, pp [] Harish Reddy ad Tolga M.Duma, Spacetime coded OFDM with low PAPR, i Proceedigs of Global Telecommuicatios Coferece, Sa Fracisco, USA, -5 December 3, pp [3] L.J. Cimii, ad N.R. Solleberger, Pea-toaverage power ratio reductio of a OFDM sigal usig Partial Trasmit Sequeces, IEEE Commuicatios Letters, ol. 4, No. 3, March, pp [4] Chitha Tellambura, Improved phase factor computatio for the PAP reductio of a OFDM sigal usig PTS, IEEE Commuicatios Letters, ol. 5, No. 4, April, pp [5] Xiuya Zhag, Yubo Dua, ad Guobi Tao, The research of pea-to-average power ratio performace by optimum combiatio of partial trasmit sequeces i MIMO OFDM system, i proceedigs of the 3 rd Iteratioal Cogress o Image ad Sigal Processig, Yatai, Chia, ol. 9, 6-8 October, pp [6] M.Haridim, H.Matzer,.Neder, D.Ezri, Aalysis of a fixed-complexity sphere decodig method for spatial multiplexig MIMO, WSEAS Trasactios o Commuicatios, ol. 8, No. 3, March 9, pp [7] D. Tse, P.iswaath, Fudametals of Wireless Commuicatio, Cambridge Uiversity Press, Cambridge, UK, 5. [8] H.G.Myug, Jusug Lim ad D.J. Goodma, Pea-to-average power ratio of sigle carrier FDMA sigals with pulse shapig, i Proceedigs of 7 th Iteratioal Symposium o Persoal, Idoor ad Mobile Radio Commuicatio, Helsii, Filad, -4 September 6, pp.-5. [9] Mohamed Ada Ladolsi ad Gaiy B. Hussai, Pulse shapig i No-coheret DLL tracig of CDMA sigals, WSEAS Trasactios o Commuicatios, ol.7, No., December 8, pp [3] S. Zid ad R. Bouallegue, Low-complexity PAPR reductio schemes usig SLM ad PTS approaches for Iterleaved OFDMA, i Proceedigs of Iteratioal Coferece o Ultra Moder Telecommuicatios &Worshops, Sait Petersburg, Russia, -4 October 9, pp.-5. [3] A.D.S, Jayalath ad C.Tellambura, The use of Iterleavig to reduce the pea-to-average power ratio of a OFDM sigal, i Proceedigs of Global Tele commuicatios Coferece, Sa Fracisco, CA, ol., 7 November- December, pp [3] J.Sarawog, T.Mata, P.Boosrimuag ad H.Kobayashi, Iterleaved partitioig PTS with ew phase factors for PAPR reductio i OFDM systems, i Proceedigs of 8 th Iteratioal Coferece o Electrical Egieerig / Electroics, computer, Telecommuicatios ad Iformatio Techology, Kho Kae, Thailad, 7- May, pp [33] U.Sorger, I. De broec ad M.Schell, Iterleaved FDMA - A ew Spread-Spectrum Multiple-Access scheme, i Proceedigs of Iteratioal Coferece o Commuicatios, Atlata, Georgia, USA, ol., 7- Jue 998, pp.3-7. E-ISSN: Issue 3, olume, March 3