PAPR Reduction of Localized Single Carrier FDMA using Partial Transmit Sequence in LTE Systems

Transcription

1 Iteratioal Joural of Computig ad etwor Techology ISS (0-59) It. J. Com. et. Tech. 5, o. (Ja.-07) PAPR Reductio of Localized Sigle Carrier FDMA usig Partial Trasmit Sequece i LTE Systems Ahmed J. Jameel Departmet of Telecommuicatio Egieerig, Ahlia Uiersity, Maama, Kigdom of Bahrai Receied: 3 Sep. 06, Reised: Dec. 06, Accepted: 0 Dec. 06, Published: ( Jauary 07) Abstract: I this paper, the partial trasmit sequece (PTS) techique is used to decrease the pea-to-aerage power ratio (PAPR) of the localized sigle-carrier frequecy diisio multiple access (LFDMA) i the upli of LTE systems. For a clippig rate of 0.% with 64 user subcarriers ad 56 system subcarriers with four differet phase sequeces; the gai of PAPR for the suggested PTS based LFDMA system is 6.56 db ad 3.06 db, respectiely whe compared with the orthogoal frequecy diisio multiple access (OFDMA) ad the LFDMA upli systems with QPSK modulatio. Keywords: Partial trasmit sequece (PTS), Pea-to-Aerage Power Ratio (PAPR), Localized Frequecy Diisio Multiple Access (LFDMA).. ITRODUCTIO The requiremets of high data rate for wireless ad mobile multimedia serices hae orieted the iterest to high speed broadbad wireless mobile techologies. Orthogoal frequecy diisio multipleig (OFDM) as a multi-carrier commuicatio techique, has bee adopted i may applicatios because of its performace agaist the broadbad mobile wireless commuicatios frequecy selectie fadig effect []. Orthogoal frequecy diisio multiple access (OFDMA) represets the atural etesio of OFDM to support may simultaeous users. The mai disadatage of OFDM ad OFDMA is their high peato-aerage power ratio (PAPR) []. There are may solutios already proposed to decrease the PAPR alues for the multicarrier systems. These solutios may be categorized ito sigal scramblig, such as selectie mappig (SLM) ad partial trasmit sequece (PTS), ad sigal distortio methods such as clippig. If clippig based PAPR reductio method is used, the the sigal with high pea will be clipped which results i loss of useful iformatio. Because of this loss of useful iformatio, the clippig based method produces i-bad distortio as well as out-of-bad radiatio. Hece, the clippig techique is ot suitable for PAPR reductio. O the other had, the PTS based PAPR reductio techique does ot create ay i-bad distortio or out-of-bad radiatio. Therefore, the PTS based techique is always better tha the clippig based techique [3]. Sigle-carrier frequecy diisio multiple access (SC-FDMA) represets a techique that is deried from OFDMA with similar throughput, performace, ad with comparable compleity. The mai adatage of the SC-FDMA techique oer OFDMA is that it has lower PAPR because of its sigle carrier structure [4]. SC-FDMA is a good alteratie to OFDMA particularly i the upli commuicatios, where lower PAPR oticeably improes the performace of the mobile deice i terms of the trasmit power efficiecy. For this reaso, it has bee used for upli multiple access scheme i Log Term Eolutio (LTE) of the 3rd Geeratio Partership Project (3GPP) [5]. LTE has bee deeloped by 3GPP to meet the requiremets of the Iteratioal Telecommuicatio Uio (ITU), which iclude: high data rate, icreased coerage ad capacity, ad better Quality of Serice (QoS). There are two differet techiques for subcarriers selectio of SC-OFDMA sigals trasmissio: the distributed subcarrier mappig method, at which the DFT outputs of the required iput data are assiged oer the total badwidth while the zeros are padded i uused sub-carriers. Howeer, successie sub-carriers are filled with the DFT outputs of the iput data i the localized sub-carrier mappig method of SC-FDMA, which is called localized FDMA (LFDMA). The case for the distributed method with equal distaces betwee filled sub-carriers is defied as iterleaed frequecy diisio multiple access (IFDMA) [4]. Howeer, usig sigle carrier frequecy diisio multiple access (SC-FDMA) ad frequecy domai equalizatio at receier, the trasmitted localized SC- FDMA sigals still hae higher (PAPR) tha their releat iterleaed sigals []. Although, IFDMA adulaimi@ahlia.edu.bh

2 Ahmed J. Jameel: PAPR Reductio of Localized Sigle Carrier PAPR is less tha that of LFDMA, the LFDMA is used for practical applicatio because subcarriers assigmet with equidistace oer the total badwidth is ery hard to be implemeted, as IFDMA eeds additioal requiremets such as guard-bad betwee adjacet subcarriers ad pilots [6]. It seems that there is still some space for PTS implemetatio for the Localized SC-FDMA upli i the LTE systems to reduce the PAPR. Therefore, i this paper, Partial Trasmit Sequeces (PTS) method will be itroduced, which diides the ormal OFDM stream ito seeral sub-streams ad multiplyig each sub-stream by differet weights, the the optimum alue will be selected to be trasmitted with low PAPR [7]. The rest of this paper is arraged as follows: sectio presets the multicarrier sigals characteristics, i sectio 3, the pea-to-aerage power ratio techiques are described usig selectie mappig ad partial trasmit sequece techiques, while the PAPR distributio is preseted i sectio 4, the proposed PTS based LFDMA upli system model for PAPR reductio is aalyzed i sectio 5, sectio 6 presets the computer simulatio results ad sectio 7 cocludes the paper.. MULTICARRIER SIGALS CHARACTERISTICS A multi-carrier sigal is geerated by addig may idepedetly modulated sub-carriers. Let s cosider a bloc of symbols, X X, 0,,, is made with eery symbol modulatig oe of the set of subcarriers f, 0,,,, where represets the umber of subcarriers. The subcarriers are selected to be orthogoal, i.e., f, where f ad T represets the origial symbol T period. The, a cotiuous-time basebad represetatio of a multicarrier sigal is gie by: 0 ( t) X ep( jf t), 0 t T () The base-bad discrete-time multicarrier sigal sampled by the yquist rate t T may be represeted by the followig equatio: Let 0 X ep( j ), 0,,, L T [ 0, L, L L, L f () ], be the oersampled discrete-time multicarrier sigal ector, where, L represets the oersampled discrete-time multicarrier T sigal sampled with t described as follows: L where, L 0 X ep( j ), 0,,, L L X, X 0, 0 L The base-bad cotiuous-time multicarrier sigals may be approimately described with L-times oersampled base-bad discrete-time sigals. It is proed i [5], that selectig L 4 is eough for represetatio of the pea alue of cotiuous-time multicarrier sigals. 3. PEAK-TO-AERAGE POWER RATIO TECHIQUES (3) I this paper, the scramblig techique will be used to scramble the iput data bloc of the OFDM symbols ad trasmit oe of them with the miimum PAPR so that the probability of obtaiig high PAPR ca be reduced. Two techiques will be preseted, the Selectie Mappig (SLM), ad Partial Trasmit Sequece (PTS). A. Selectie Mappig I this method, at the begiig, idepedet sequeces are geerated from the iput OFDM sigal, each sequece will be multiplied by differet phase sequece to geerate aother idepedet sequeces X [ X, X,, X ] ad these sequeces are the forwarded ito operatio simultaeously to geerate the ectors: [,,, ]. The ector d with miimum PAPR will be selected for trasmissio as show i Figure. Serial to Parallel P P P X X X Select d with Miimum PAPR Figure. Bloc diagram of SLM techique B. Partial Trasmit Sequece (PTS) Partial Trasmit Sequece (PTS) algorithm is a approach used to improe the basic stats of a multicarrier sigal. The mai fuctio of the PTS method is that it diides the iput OFDM sigal ito

3 It. J. Com. et. Tech. 5, o., -6 (Ja. 07) 3 may sub-sigals, each sub-sigal is the multiplied by a specific, deferet weight, ad this fuctio is repeated util the eeded alue is reached. Figure displays the bloc diagram of the OFDM Dowli system based o the PTS algorithm. The iput sigal of M symbols is diided ito sub-sigals as X [ X, X,, X ] where X i sub-sigals are successiely located ad are of equal sizes. S/P & Partitio ito Blocs X X X b b Pea Power Optimizatio b Adder Figure. Pre-codig based PTS-OFDM dowli system. The sigal is obtaied through applyig the operatios o X i the time domai, ad that s defied by: ( X) b ( X ) b (4) A suitable factor combiatio eeds to be selected where b [ b, b,, b ] ad the results i the output reachig perfectio, this could be defied through: b [ b, b,, b ] arg mi( b,,, )(ma b b b (5) I the aboe equatio, arg mi ( ) equals the output of the least alue. Usig this method, the best possible b ca be reached to improe the PAPR efficiecy. The ew costs ca be calculated usig multiplied by the umber of the s operatio. 4. DISTRIBUTIO OF PAPR The pea-to-aerage power ratio (PAPR) of the basebad discrete-time multi-carrier sigal ca be represeted as the ratio of the maimum pea power oer the aerage power of the multicarrier sigal [8], i.e.: ma PAPR ( ) (6) 0 E 0 ) where, E {} represets the epected alue. Whe all of the subcarriers i a OFDM system hae the same phases, it s possible to reach the maimum power which results i mea s( t), this maes the PAPR which meas the maimum power equals times the mea power. At the same time, the probability that the maimum power happeig becomes smaller whe the umber of subcarriers is icreased. Accordig to the cetral limit theorem ad if the umber of subcarriers is large eough, the real ad imagiary parts of the time domai sigal samples follow the Gaussia distributio fuctio with zero mea ad ariace. The cumulatie distributio fuctio (CDF) of the amplitude of a sigal is gie by [9]: F( z) ep( z) (7) The complemetary CDF (CCDF) for the PAPR is oe with the most frequetly used performace measure for PAPR reductio methods. The CCDF of the PAPR deotes the probability that the PAPR of a data bloc surpasses a specified threshold. The CCDF of the PAPR for a data bloc with yquist rate samplig is defied as follows: P( PAPR z) P( PAPR z) F ( z) 5. PTS BASED LFDMA UPLIK SYSTEM ( ep( z)) (8) Partial Trasmit Sequece (PTS) algorithm is a method used to improe the statistics of a multi-carrier sigal. The mai idea of PTS method is to partitio the mai OFDM sequece ito may sub-sequeces, ad the each sub-sequece is multiplied with a differet weights util the optimum alue with lowest PAPR is achieed. Figure 3 displays the bloc diagram of the suggested PTS based LFDMA for upli system. The iput data bloc of M symbols is diided ito disjoit subblocs as [,,, ], where i are the subblocs that are sequetially located ad also are of equal size ad each sub-bloc cosists of M ozero elemets ad set the rest part to zero. The, each partitioed sub-bloc is iserted through DFT precoder T bloc to yield X [X,X,,X ]. Assumig that the sub-blocs hae eactly the same size ad there is o gap betwee them, the sub-bloc ector ca be defied as follows: T Xˆ b X (9)

4 4 Ahmed J. Jameel: PAPR Reductio of Localized Sigle Carrier j where b e ( [0, ]) {,,, } represets a weightig factor used for phase rotatio. The, the sub-carrier mappig of the precoded sigal is doe i the localized mode. The sub-carrier mappig of the localized-mode for PTS-LFDMA upli system would be defied as: M X, 0,, M M X 0,,, (0) M X,,, where is the umber of subcarriers i the system, M is the umber of user subcarriers. S/P & Partitio ito Blocs FFT FFT FFT X X X Subcarrier Mappig Subcarrier Mappig Subcarrier Mappig X X X b b Pea Power Optimizatio Figure 3. Pre-codig based PTS-LFDMA upli system. b Adder The time domai represetatio of the sigal is obtaied by applyig operatio o X, i.e., ˆ ( X) b ( X ) b () It has bee show i [] that usig pulse shapig does ot improe the localized FDMA system. Therefore, PAPR without pulse shapig ca be represeted by: ma PAPR 6. SIMULATIO RESULTS 0,,, 0 (4) Simulatios are carried out to chec the performace of the ew suggested PTS-LFDMA system. The ew system is ealuated with 4. The data is geerated radomly, after that, it is modulated with QPSK, 6QAM ad 64QAM, respectiely. The, the PAPR is calculated by usig the complemetary cumulatie distributio (CCDF) techique. The CCDF for the PAPR of the LFDMA upli sigal has bee used to represet the probability of beig larger a specific alue of threshold PAPR0 (CCDF = Prob(PAPR> PAPR0)). The simulatio results of the suggested system are compared with the OFDMA, the SLM based LFDMA system, ad the localized SC- FDMA (LFDMA) upli systems, respectiely. Figure 4 displays the CCDF comparisos for PAPR of the OFDMA, the LFDMA, the SLM based LFDMA, the PTS based LFDMA upli systems with 4, for M 64 ad 56 with QPSK modulatio. For a CCDF of 0.%, the PAPR is 0.7, 7.5, 5.9 ad 4.7 db for the OFDMA, the LFDMA, the SLM based LFDMA, ad the PTS based LFDMA upli systems, respectiely. Select oe appropriate combiatio factor b [ b, b,, b ] which lets the result achiee optimum. The combiatio ca be represeted by: b [ b, b,, b ] arg mi( b,,, ) (ma b b b () ) where arg mi ( ) is the coditio that output the miimum alue of fuctio. Accordigly, we ca select the best b so as to optimize the PAPR performace. The additioal cost we hae to pay ca be calculated ia times s operatio. The output sequece as follows: 0 j ca be represeted X e (3) Figure 4. CCDF compariso of PAPR for the OFDMA upli system, the LFDMA upli system, the SLM based LFDMA, ad the PTS based LFDMA upli system with 4, respectiely. Figure 5 displays the CCDF comparisos for PAPR of the OFDMA, the LFDMA, the SLM based LFDMA, the PTS based LFDMA upli systems with 4, for M 64 ad 56 with 6QAM modulatio. For

5 It. J. Com. et. Tech. 5, o., -6 (Ja. 07) 3 a CCDF of 0.%, the PAPR is 0.7, 8.5, 6.6 ad 5.6 db for the OFDMA, the LFDMA, the SLM based LFDMA, ad the PTS based LFDMA upli systems, respectiely. tha the SLM based LFDMA, the LFDMA ad the OFDMA upli systems. Table I At CCDF of 0.%, the PAPR comparisos of the OFDMA, the LFDMA, the SLM based LFDMA, the PTS based LFDMA with 4. Upli Trasmissio PAPR [db] Scheme 4QAM 6QAM 64QAM OFDMA LFDMA SLM-LFDMA PTS-LFDMA Figure 5. CCDF compariso for PAPR of the OFDMA upli system, the localized LFDMA upli system, the SLM based LFDMA, the PTS based LFDMA upli system. Figure 6, displays the CCDF comparisos for PAPR of the OFDMA, the LFDMA, the SLM based LFDMA, the PTS based LFDMA upli systems with 4, for M 64 ad 56 with 64QAM modulatio. For a CCDF of 0.%, the PAPR is 0.7, 8.7, 6.6 ad 5.8 db for the OFDMA, the LFDMA, the SLM based LFDMA, ad the PTS based LFDMA upli systems, respectiely. Figure 6. CCDF compariso for PAPR for the localized OFDMA, the localized LFDMA, the SLM based LFDMA, the PTS based LFDMA upli systems. I Table I, we summarize the PAPR aalysis of the suggested PTS based LFDMA, the SLM based LFDMA, the OFDMA, ad the LFDMA upli systems, respectiely. For a CCDF of 0.%, it is clear from Table that the suggested system has less PAPR 7. COCLUTIOS I this paper, the PTS based LFDMA upli system is preseted for PAPR reductio. Simulatio results hae show that, the suggested system has less PAPR tha the OFDMA, the LFDMA ad the SLM based LFDMA upli systems, respectiely. The PAPR for the ew system may be reduced more if the size of is icreased. Howeer, if we icrease of size, it will icrease the suggested system compleity. Therefore, the alues of must be selected carefully. Because of the low PAPR alues, the ew PTS based LFDMA upli system is ery suitable for the LTE mobile stadard release 0 LTE-Adaced tha the LFDMA upli system which is already used i the release 8 LTE. Refereces [] Myug, H. G., Lim, J., Goodma, D. J., (006) Pea- To-Aerage Power Ratio of Sigle Carrier FDMA Sigals with Pulse Shapig, IEEE 7th Iteratioal Symposium o Persoal, Idoor ad Mobile Radio Commuicatios (PIMRC 06), pp. 5. [] H. Sari, G. Karam ad I. Jea Claude, (994) Frequecy-Domai Equalizatio of Mobile Radio ad Terrestrial Broadcast Chaels, Proc. IEEE GLOBECOM, pp. 5, Sa Fracisco, CA. [3] Muller, St. H., Huber, J. B., (997) OFDM with Reduced Pea-to-Aerage Power Ratio by Optimum Combiatio of Partial Trasmit Sequeces, Electroics Letters, ol. 33, o. 5, pp [4] Hyug G. Myug, Jusug Lim, ad Daid J. Goodma, Sigle Carrier FDMA for Upli Wireless Trasmissio. IEEE ehicular Techology, Sept 006. [5] 3rd Geeratio Partership Project (3GPP); Techical Specificatio Group Radio Access etwor; Physical Layer Aspects for Eoled UTRA, [6] ChoY. Soo, Kim, Jaewo, Yag W. Youg, Kag, Chug G. (00) MIMO-OFDM Wireless Commuicatios with MATLAB, Wiley, Hoboe. [7] I. Baig, M. Ayaz,. Jeoti, (03) A SLM based localized SC-FDMA upli system with reduced PAPR for LTE-A, Joural of Kig Saud Uiersity Egieerig Scieces, 5, 9-3.

6 6 Ahmed J. Jameel: PAPR Reductio of Localized Sigle Carrier [8] Muller, St. H., Bauml, R.W., Fischer, R.F.H., Huber, J.B., (997) OFDM with Reduced Pea-to-Aerage Power Ratio by Multiple Sigal Represetatio, Aals of Telecommuicatios, ol. 5, o. -, pp [9] S. H. Ha ad J. H. Lee, (005) A Oeriew of Pea-to-Aerage Power Ratio Reductio Techiques for Multicarrier Trasmissio, IEEE Wireless Commuicatio, ol., o., pp Ahmed J. Jameel was bor i 963 i Baghdad, Iraq. He receied his B.Sc. ad M.Sc. degrees i Electrical Egieerig ad Electroics ad Commuicatios Egieerig from Uiersity of Baghdad, College of Egieerig i 985 ad 994, respectiely ad his Ph.D. degree i Commuicatio ad Iformatio Systems, from Southeast Uiersity, ajig, Chia i 003. He joied Ahlia Uiersity i 008 as a Assistat Professor of Telecommuicatio Egieerig. From , he wored as a Assistat Professor i the Departmet of Electroics ad Commuicatio Egieerig of Al-Ahliyya Amma Uiersity, Jorda. From , he was a Post- Doctoral Research Associate at the Departmet of Commuicatio Egieerig, Southeast Uiersity, ajig, Chia. His research iterests iclude digital commuicatios, error-correctig codes, Coded Multi-Iput Multi-Output Systems for the B3G/4G Mobile Commuicatios, ad speech codig for mobile commuicatios.