Comparative Analysis of Different Techniques of PAPR Reduction in OFDM System

Transcription

1 Natioal Coferece o Recet Advaces i Computer Sciece ad IT (NCRACIT) Iteratioal Joural of Scietific Research i Computer Sciece, Egieerig ad Iformatio Techology 18 IJSRCSEIT olume 4 Issue 1 ISSN : Comparative Aalysis of Differet Techiques of PAPR Reductio i OFDM System Sahil Hamid 1, Malik Mubashar Hassa 1 Departmet of Uiversity Polytechic BGSBU, Departmet of ITE BGSBU, Jammu & Kashmir, Idia ABSTRACT Orthogoal frequecy divisio multiplexig (OFDM) is a well-orgaized frequetly used multicarrier modulatio method for high speed data commuicatio over multipath fadig chaels. OFDM modulatio techique offer may beefits for multicarrier trasmissio at high iformatio rates. Oe of the major limitatios of OFDM is high peak to-average power ratio (PAPR) of the trasmitted sigal that badly iflueces the complexity of high power amplifier. arious promisig procedures have bee aticipated ad implemeted to decrease PAPR of OFDM sigal at the outlay of trasmitted sigal power, bit error rate (BER), complexity ad so o. I this paper, Clippig ad Filterig, Selective Mappig (SLM), Partial Trasmits Sequece (PTS), toe isertio ad toe reservatio techiques are discussed briefly for PAPR reductio OFDM sigal at trasmitter. A brief compariso of these PAPR reductio techiques is doe i light of CCDF ad BER Keywords : OFDM, PTS, BER, PAPR I. INTRODUCTION Mobile radio commuicatio systems are gradually required to offer a variety of brilliat multi-media services to mobile users. To meet these demads i far-flug broadbad commuicatio systems must have the capacity to support high limit, variable bit rate data trasmissio ad high data trasfer capacity effectiveess. I the mobile radio coditio, sigals are ormally disabled by fadig ad what's more, multi-path delay pheomeo. I these chaels, severe fadig of the sigal amplitude ad iter symbol iterferece (ISI) due to frequecy selectivity of the chael brig about a icompatible degradatio of the error performace. Orthogoal frequecy divisio multiplexig (OFDM) is a multicarrier modulatio scheme for high data rate mobile commuicatio system because of its vigor to frequecy selective fadig, high spectral efficiecy ad low computatioal complexity. Recetly OFDM system has bee gettig across the board cosideratio. Orthogoal frequecy divisio multiplexig ca be used i cojuctio with multiple-iput multiple-output (MIMO) method to ehace diversity gai ad/or the system capacity by exploitig spatial domai. MIMO-OFDM is viewed as a key iovatio i risig high data-rate commuicatio system, icludig digital subscriber lies (DSL), IEEE 8. l1, IEEE 8.16 ad IEEE a, give a course to 4G techology that support high mobile data rates. The primary advatages of utilizig MIMO-OFDM techique, which ca dimiish the recipiet upredictability ad maage the multipath fadig effectively, icorporate high spectral efficiecy ad robustess agaist arrowbad. The fudametal poits of CSEIT41185 Published - 5 April 18 March-April-18 [ (4 ) 1 : ] 314

2 iterest of utilizig MIMO-OFDM procedures, which ca dimiish the recipiet itricacy ad maage the multipath fadig effectively, icorporate high spectral efficiecy ad stregth agaist arrowbad iterferece, uiform average spectral desity, capacity of dealig with exceptioally solid echoes, less o-liear distortio ad efficiet implemetatio. Notwithstadig, the priciple limitatio of utilizig MIMO-OFDM edures with the problem of high PAPR ad carrier frequecy offset sesitivity [6]. Subsequetly, it is imperative to decrease the PAPR; otherwise, high power amplifier (HP A) i the trasmitter eed to have a liear regio that is substatially bigger tha the average power, which makes them costly ad wasteful. This is because if HPA with a liear regio somewhat more oteworthy tha the average power is utilized, the saturatio brought about by the vast peak will result i itermodulatio distortio, which expads the bit error rate (BER) ad causes spectral wideig, brigig about adjacet chael iterferece. A umber of PAPR reductio techiques have bee proposed to hadle this problem. Oe of the most widely used strategies is Clippig ad Filterig, DFT Spreadig ad Partial Trasmit sequece (PTS), utilizig probabilistic techiques [3]. The guidelie of probabilistic strategy relies o upo decreasig the likelihood of high PAPR by creatig several OFDM symbols (multiple cadidates) coveyig a similar iformatio ad choosig the oe havig the most miimal PAPR []. The probabilistic techique ca likewise be classified ito two systems: sub block partitioig methodology ad etire block procedure. The sub block partitioig procedure, for example, partial trasmit sequece (PTS) [8-9], separates frequecy domai sigals ito several sub blocks. o the other had, the etire block Strategy, for example, Clippig ad Filterig, DFT Spreadig, [5 8] cosider the whole block for creatig umerous cadidates. To begi with, the etire block procedure of the probabilistic strategies to create umerous cadidates is viewed as, ad the the probability Distributio Fuctio (PDF) for the multiple cadidate system is ivestigated. Whe the cadidate havig the reduced APR is chose, the PDF of the amplitude of a selected OFDM symbol becomes a fuctio of the umber of cadidates [1-4]. All through this paper we portray the fudametal rule of every oe of these procedures. The determiatio of ay of the PAPR reductio techique may be at the cost of PAPR reductio ability, sychroizatio betwee the trasmitter ad the receiver. The absece of the PAPR reductio techiques will cause the icrease i the trasmit power, bit error rate at the receiver, the data rate loss, ad the computatioal complexity. Here we have studied through simulatio result the performace of PTS ad Clippig ad Filterig, DFT Spreadig based PAPR reductio techiques for these techiques i view of differet parameters. II. PRINCIPLE ALGORITHM FOR REDUCING PAPR A OFDM sigal icludes various idepedetly modulated sub-carriers, which ca give a cosiderable PAPR whe added up coheretly. At the poit whe N sigals are added with the similar phase, they geerate a peak power that is N times the average power of the sigal. So OFDM sigal has a substatial PAPR, which is sesitive to o-liearity of the high power amplifier. PAPR is a historic issue i the advacemet of the Wireless commuicatio, the more PAPR of OFDM the more prerequisites ad difficulties for executig the HPA. However the PAPR is computed from the peak amplitude of the waveform divided by the average value of waveform as follows: PAPR = ma t T E{ } Where E{ x(t) } is average power of the waveform. The amplitude of the waveform has Rayleigh distributio. The distributio of PAPR states i term complemetary cumulative distributio fuctio (CCDF). CCDF = Pr(PAPR<PAPR = A) Assume PAPR = A olume 4 Issue 1 March-April

3 ma t T E PAPR F(PAPR) = Pr =1-exp (-PAPR) CCDF = Pr(PAPR>PAPR) =1F(PAPR) N = 1 (1-exp(-PAPR)) N The decibel form of above equatio is as followig: PAPR db =1log1 (PAPR) Despite the fact that the probability of the largest PAPR to happe is ot high, but rather to sed the high PAPR of OFDM sigal with o distortio, the probability icreases. All the liearity i High Power Amplifier (HPA) ad A/D coverter ought to meet the ecessities said above [1-3]. Sice the hardware that meets these ecessities is exceptioally costly, therefore, it is extremely requestig ad essetial to reduce PAPR i OFDM system [9-1]. III. PAPR AND ITS REDUCTION IN OFDM SYSTEM A block diagram of OFDM system is delieated i Fig.1. The OFDM flag comprises of a sum of subcarriers that are modulated by phase shift keyig (PSK) or Quadrature Amplitude modulatio (QAM). To start with, the serial iput data stream is arraged ito N group of bits, where Nc is the umber of subcarriers. The umber of bits i each of the Nc group decides the costellatio size mea for that specific subcarrier. The mapped complex symbols are the serial-to-parallel (S/P) coverter ad oversampled by a factor L resultig i a block Nc L complex symbols [11]. The complex discrete time base bad OFDM sigal ca be expressed as: NT ( t) 1 N 1 e j Ft, t T N Where NT trasmit a atea that make use of N subcarrier. By usig OFDM modulatio techique, a block of data symbols (oe OFDM symbol) {, =,1.-1} will be trasmitted i parallel such that each symbol modulate a differet carrier such that each modulates a differet subcarrier from a set {fn, N =, 1 N-1}. The N subcarriers are orthogoal, that is fn= NΔf where Δf = 1/T ad T is the symbol time period. The above power attributes ca likewise be portrayed regardig their magitude (ot power) by defiig the crest factor (CF) as: CF = PAPR I the PSK/OFDM system with N subcarriers, the most extreme power happes whe the greater part of the N subcarrier compoets happe to be added with same phase. Assumig that E{ } = 1. It result i PAPR = N, that is the maximum power equivalet to N times the average power. We have oticed that whe M-QAM is greater tha 4 times M-PSK, the PAPR will be large i this case. The Probability distributio of will follow after the Gaussia distributio. The amplitude of OFDM sigal has a Rayleigh distributio with zero mea ad a variace of N times the variace of oe complex siusoid. let {Z} be chace to be the extets of complex samples. Acceptig that the average power of complex pass bad OFDM sigal is equivalet to oe, the {Z} are the stadardized Rayleigh arbitrary variable with its ow average power, which has the probability desity fuctio [4-9] as show below: Z Z Z Fz ( Z) e Ze 8 Where E { Z }=. I. PAPR REDUCTION TECHNIQUES Figure1. Block diagram of OFDM system. A. Partial Trasmit Sequece olume 4 Issue 1 March-April

4 The Partial Trasmit Sequece has bee preseted by Muller ad Hubber i 1997 [11]. The PTS techique is based o phase shiftig of sub-block of data ad the multiplicatio of data structure by radom vectors. This techique is flexible ad efficiet for OFDM framework. A block diagram of PTS techique is show i figure. I partial trasmit sequece [1] the iput data block of N symbols is further divided ito disjoit sub-blocks. The iput data block ca be writte i form of as: = 1 Each partitioed sub-block is multiplied by a correspodig complex phase factor b v = e jφv, v = 1,,. the takig its IFFT to yield = IFFT{ b v1 b v v v1 v v } = b v v IFFT { } = v1 Where v represet the partial, trasmit sequece. The value of phase vector is selected i such a maer that the value of resulted PAPR is miimized [1]. This is show as arg mi ~ ~ b,..... b v 1 v b,... b max v v,1,... N 1 b x v1 The the correspodig time domai sigal with the lowest PAPR ca be expressed as { b v1 v v} I geeral the reductio i search complexity largely depeds upo the selectio of phase factor which is limited to a set of elemets. As the set of allowed phase factors is { b e j i w i,1,,... w 1}, w v-1 Therefore search complexity is directly proportioal to the umber of sub-block. Figure. A Block Diagram of PTS Techique. CLIPPING AND FILTERING High PAPR is a stadout amogst the most widely recogized issues i OFDM. A high PAPR brigs about limitatios like icreased complexity of the ADC ad DAC ad furthermore decreased efficiecy of radio frequecy (RF) power amplifier. Oe of the straightforward ad, viable PAPR reductio strategies is clippig, which cacels out the sigal compoet that exceed some costat amplitude called clip level. I ay case, clippig yields distortio power, which is called clippig oise, ad expad the trasmitted sigal spectrum, which causes iterferece. Clippig is oliear process ad causes i-bad oise distortio, which causes degradatio i the performace of bit error rate (BER) ad out-ofbad oise, which decrease the spectral efficiecy [1]. Clippig ad filterig techique is powerful i elimiatig compoets of the expaded spectrum. Although filterig ca dimiish the spectrum developmet, filterig subsequet to clippig ca decrease the out-ofbad radiatio, yet may likewise brig about some peak re-growth, which the peak sigal exceed i the clip level. The strategy of iterative clippig ad filterig dimiishes the PAPR without spectrum expasio. However, the iterative sigal takes log time ad it will icrease the computatioal complexity of a OFDM trasmitter. A block diagram of PAPR reductio scheme usig clippig ad filterig is show i figure 3 [1]. Here N is the umber of subcarrier. olume 4 Issue 1 March-April

5 Figure 3. (a) OFDM system (b) Clippig ad Filterig Block diagram. I. DFT SPREADING DFT Spreadig stads for Discrete Fourier Trasform spreadig, it ivolves the spreadig of iput sigal with the help of DFT (Discrete Fourier Trasform) operatio which ca be take ito IDFT. DFT Spreadig results i reductio of PAPR of OFDM sigal to the extet of sigle carrier trasmissio. Mobile termial i uplik trasmissio make use of this techique. It is called as sigle carrier FDMA. The sigle carrier FDMA (SC-FDMA) is used for uplik trasmissio i 3GPP LTE stadard [1]. DFT of idetical size as IDFT is used as (spreadig code). The OFDMA system becomes equivalet to SC- FDMA system. The reasos beig DFT ad IDFT operatios cacel each other. I this case the PAPR of trasmitted sigal will be same as sigle carrier system. I OFDMA framework, subcarrier are partitioed ad allocated to multiple mobile users. I this case each termial i uplik uses a subset of subcarrier to trasmit its ow data whereas it does ot happe i case of dowlik. The remaiig subcarriers which are ot used for their ow trasmissio will be filled with zeros. The effect of PAPR reductio depeds o the way of assumig the subcarrier to each termial [6]. Here the iput data x[m] is DFT spread to produce [i] ad the allocated as ~ xk k / s k s. m1, m1,1,... M otherwise Figure 4: Equivalet of OFDMA system with DFT spreadig code to a sigle-carrier system [1] II. SELECTIE MAPPING I this scheme, firstly M statistically idepedet iput data symbols which represet the similar iformatio are geerated ad the each sequece are processed by M parallel N-poit complex IFFT to geerate M differet timedomai OFDM symbols. The symbol with the miimum PAPR is chose for trasmissio. The key poit of SLM method lies i how to geerate multiple diverget time-domai OFDM symbols whe the iput data for trasmissio is the same. For this purpose, M pseudo-radom phase rotatio sequeces [ ] with are defied; where ad is uiformly distributed i [ ]. This method ca be see as performig a dot product operatio o the iput toes ( ) with rotatio factor. I practice, all the elemets of the phase sequece are set to 1 to as to make this brach sequece the origial OFDM symbol. A block diagram of SLM scheme is show i figure 5 [13]. The selected phase sequece should be trasmitted to receiver as side iformatio to allow the recovery of origial symbol sequece at the receiver which decreases the data trasmissio rate. The phase sequece pm, <=m<=m-1 should be stored at both the trasmitter 1 ad receiver.a erroeous detectio of side iformatio will cause the whole system to be damaged. Therefore a strog protectio of side iformatio is very essetial. olume 4 Issue 1 March-April

6 II. REFERENCES Figure 5: Block diagram SLM techique. III. COMPARISON A brief compariso of various PAPR reductio techiques has bee depicted i the table 1. It is clear from the table 1, that oe of the prior-art techiques have the etire desired feature: very large CCDF (PAPR) reductio, very low implemetatio complexity, o loss of capacity, o icrease of trasmit power, ad so forth. I.CONCLUSION OFDM is a remarkably attractive strategy for multicarrier trasmissio ad has tured out to be oe of the stadard decisios for rapid speed trasmissio over a commuicatio chael. It has various advatages; but has oe oteworthy drawback, it has a high PAPR. I this paper, a few umbers of the techiques for decreasig the high PAPR of the OFDM system were aalyzed ad compared. Amog the techiques that were aalyzed, it was foud that, o particular PAPR reductio strategy is the best solutio for the OFDM framework. Differet parameters like loss i rate, trasmit sigal power icremet, BER icrease, computatioal complexity icremet should to be take ito cosideratio before choosig the appropriate PAPR techique. [1] a Nee, R.D.J. (Nov. 1966) OFDM codes for peak to average power ratio reductio ad error correctio. IEEE GTC Lodo, ol., pp [] Davis, J.A. ad Jedwab,J. (1999) Peak-to mea power cotrol i OFDM, Golay complemetary sequeces, ad Reed muller codes. IEEE Tras. Ifo. Theory, 45(7), [3] Davis, J.A. ad Jedwab,J. (1997) Peak-to mea power cotrol i OFDM, Golay complemetary sequeces, ad Reed muller codes. Electro. Lett,, 33(4),67-68 [4] Urbake,R ad Krishakumar, A.S (Oct.1996) Compact descriptio of Golay sequeces ad their extesios. Proc. Of Thirty Fourth Aual Allerto Coferece o Commuicatio, Cotol ad Computig Pagiatio,Urbaa, IL,pp [5] Li,. ad Ritcey, J.A. (1977) M-sequeces for OFDM peak to average power ratio reductio ad error correctio. Electro. Lett, 33(7), [6] Tellambura, C. (1997) use of m-sequeces for OFDM peak to average power reductio.electro. Lett, 33(15), [7] Park, M.H. et al. () PAPR reductio i OFDM trasmissio usig Hadamard trasform. IEEE ICC, vol.1,pp [8] Bauml, R.M.,Fischer, R.F.H., ad Huber, J.B.(1996) Reducig the PAPR of multicarrier modulatio by selective mappig. Electro.Lett, 3(), [9] Hkubo, N. ad Ohtsuki, T. (Apr. 3) Desig criteria for phase sequeces i selective mappig. IEEE TC 3, vol.1,pp [1] Muller, S.H. et al. (1997) OFDM with reduced PAPR by multiple sigal represetatio. I Aals of Telecommuicatio, 5(1-), [11] S.H. Muller ad J.B. Hubber, A compariso of peak power reductio schemes for OFDM, i Proc. IEEE Global Commuicatios Coferece, Phoeix, AZ, November 1997, pp [1] Yuvraj Sigh Raawat, Mahedra Kumar, Performace Improvemet Of MIMO-OFDM System PAPR Reductio usig Pulse Shapig. IJSR,ol.4,Issue 7, July 15. [13] Yasir Rahmatullah, Seshardri Moha, Peak- To- Average Power Ratio Reductio i OFDM System: A survey ad Texoomy. IEEE. 13. olume 4 Issue 1 March-April