Peak to Average Power Ratio Reduction in OFDM System by Clipping and Filtering

Transcription

1 International Journal o Electronics Communication and Comuter echnology (IJECC) Volume Issue 3 (May 0) Pea to Average Power Ratio Reduction in OFDM System by Cliing and Filtering Sanjeev Saini Deartment o Electronics Communication Engineering, APII SD INDIA, Paniat, India, sanjeev0saini@gmail.com Dr. O.P. Sahu Deartment o Electronics & Communication Engineering National institute echnology, urushetra, India, os_nit@yahoo.co.in Abstract his aer results cliing and iltering technique or ea to average ower ratio (PAPR) reduction in orthogonal requency division multilexing (OFDM). Our simulation results show that this technique removes inband and out o band distortion along with db reduction in ea to average ower ratio when crest ratio changes rom.4 to.. Keywords- Cliing, Crest ratio, Orthogonal requency division multelixing (OFDM), signal to Cliing noise ratio (SCNR) I. INRODUCION Single carrier modulation techniques which are used in 3G are not suitable or high data rate because they suer rom roblems such as inter symbol intererence, time disersion, selective ading etc. are aced []. o overcome these roblems multicarrier modulation technique OFDM is used. Orthogonal requency division multilexing technique (OFDM) is most oular one which suort arallel data transmission and overcome the roblems aced in single carrier modulation technique[3]. Due to advancements in DSP and in VLSI ast Fourier transorm becomes ossible to imlement in OFDM system. OFDM is otential candidate or 4G, where seed o data u to GBPS can be achieved. It is an eicient technology or wireless communications and used in many wireless standards lie IEEE 80., Broadcast radio Access networ (BRAN), Digital audio broadcasting, digital video broadcasting, WLAN, WIMAX and in military alications. Major disadvantage o OFDM system are high ea to average ower ratio and intercarrier intererence. High PAPR occur when signal is converted into time domain through IFF the resulting signal is sum o n number o subcarrier and the ea o resulted signal becomes n times higher than normal averaged signal[4]. Power amliier becomes saturate when this large signal ea is given to it. hus, causing intermodulation distortion in the transmitted signal. Intermodulation distortion can be reduced by Oerating ower amliier at the transmitter in linear range. Such ower reduction or ower bac o results in ineicient oeration o the OFDM system [6]. Other major roblem in OFDM system is intercarrier intererence which arises due to lose o orthogonality among the subcarrier in the system. Due to Doler sread ICI come into existence. PAPR can be reduced by many techniques among them some are categorized as [7]. A. Signal Distortion echnique his technique includes cliing, Pea windowing, Pea cancellation, comanding etc. Cliing technique is the simlest technique to reduce PAPR but this method may aect Bit Error Rate o transmitted signal so it exhibit nonlinear behavior [4]. B. Coding echniques hese techniques are based on selection o code words or the transmitted signal and the code word o smallest PAPR is selected or transmission. Reed mullar codes, Goley comlementary sequence etc are the examle o these techniques [3]. C. Symbol Scrambling echniques hese techniques are based on scrambling sequence and ollow linear rocesses. OFDM signals are scrambled with dierent scrambling sequence and lowest PAPR sequence is selected or transmission. At the receiver side descrambling rocess is carried out. Due to increase in eiciency as extra circuitry is required and circuit becomes more comlex. So comutational comlexity increases in these techniques SLM, PS are the examle o these techniques. [3-9]. he bloc diagram which is used or OFDM system is shown in igure [3]. Figure. OFDM signal transmission bloc diagram ISSN: IJECC 05

2 International Journal o Electronics Communication and Comuter echnology (IJECC) Volume Issue 3 (May 0) II. OFDM SYSEM DESCRIPION he simulation o OFDM system is erormed in which subcarriers are reresented by N. he value o N is to be taen as 8. he guard interval is taen o length 3. he modulation technique is chosen as QAM. Oversamling rate taen as L=8. o remove the out o band distortion in samles, L should be equal to or greater than 4. he bandwidth w is divided into N smaller narrow sub band having requency sacing =w/n. Modulation technique QAM is used having symbol rate o /. All the subcarriers are orthogonal to each other having searation between adjacent carriers equal to n/. he OFDM signal can be created by alying IFF to modulated signal. he inut inormation bits are reresented by S ( ) reerred as data oint and modulated symbol th reresented or subcarrier []. he OFDM samles can be reresented by N equation j n / JN S( ) c( n) e 0 JN N n 0 Oversamling can be achieved by adding the original inut with zeroes.he cliing signal having threshold value is A. Active carriers o same comlex valued is taen as signal set A with same variance. he inactive carrier can be made zero or better ower density sectrum o the transmitted signal []. III. CLIPPING ECHNIQUES It is simlest aroach or reduction o PAPR in OFDM system. he amlitude o signal can be clied uto desired level beore amliication.hat can be achieved at baseband using sot enveloe limiter. Amlitude Cliing is nonlinear rocess and it may create roblems lie inband distortion and outband distortion. In band distortion aect the odm system by degrading the bit error rate and outband distortion may deteriorate sectral eiciency. Filtering ater cliing can reduce out o band raditation to maximum extent, but also roduces some ea regrowth in the iltered signal. Aliasing roblem is aced in cliing ater iltering which can be reduced by adding zeros in original inut called zero adding. o imrove the bit error rate and sectral eiciency some techniques can be used lie Forward Error Correcting codes and band ass iltering with cliing []. A. Cliing and Filtering o Oversamled Signals Cliing and iltering is erormed digitally on an oversamled odm Signal S( ) N N n 0 c( n) e j n/ JN 0 JN Let J >, J= oversamling actor S() is the samles o s(t) corresonding to a given bloc o data c=[x(0),x() x(n-)] he oversamled data sequence S=s(0),s(), s(jn-) can be generated c is as the IFF o the zero added data bloc obtained by extending C with(j-)n zeros. c z c z =[c(0),c() c(n-), 0,0,0 0], which is Each samle s () is clied by a sot enveloe limiter. j e be the reresentation o s() in olar coordinates, the outut rom the limiter is given s( ) i A by S j Ae i A Normalization o clied signal can be reresented as A Cliing level Wher Root mean squarevalueo inut signal in e in is average ower o the unclied samles reresented as in E S() OFDM signal can be modeled as a zero mean circularly symmetric comlex Gaussian rocess, the amlitude is Rayleigh distributed and average ower o the clied samles turns out to be P out ( e ) P I =,it behaves as in P (without cliing) out P in he oversamled aroach has the advantage o reducing inband distortion and ea regrowth to some extent, but generates out o band radiation. Out o band radiation can be removed as ollows: Clied samles are to be assed through a low ass ilter S [ s(0), s(), s()... s( jn )] It roduces vector Y = [y(0),y().y(n-)] o time domain samles, which can be extended by cylic reix(c) and ed to D/A converter. he sequence S is transormed in the requency domain through FF oeration which roduces the ollowing vector o length JN C [ c(0), c(), c()... c( N ), c( N), c( N )... c( JN )] Cliing is erormed with an oversamle rate J> C n S e n JN JN j n / JN ( ) ( ) 0 JN 0 Out o band radiation can be removed by discarding the last (J-) N elements o C while leaving the irst N elements. C [ c(0), c(), c()... c( N ), c( N), c( N )... c( jn )] I N=56 and j= ISSN: IJECC 06

3 International Journal o Electronics Communication and Comuter echnology (IJECC) Volume Issue 3 (May 0) C [ c(0), c(), c()... c(55), c(56), c(57)... c(5)] Cliing is erormed with an oversamle rate J > C N S e n JN JN j n / JN ( ) ( ) 0 J N 0 Out o band radiation can be removed by discarding the last (J-) N elements o C, while leaving the irst N elements, C is transormed bac in the time domain through an N oint IDF, which gives sequence yt () o N modiied time domain samles y t C n e t N j ncst ( ) ( ) 0 N n 0 So it eliminates out o band radiation with oversamling actor J.As cliing is a non linear rocess so it introduces in band distortion also called cliing noise and out o band radiation and intercarrier intererence, which degrades the system erormance and the sectral eiciency [8]. he cliing noise is related to the dierence between the original signal S ( ) andc the clied signal. he signal sent to the receiver is the clied signal, which is dierent rom the signal which has actually sent. his dierence can be measured by the signal to cliing noise ratio(scnr) SCNR C S( ) C It shows the dierence between the clied and unclied signal. he more the signal is clied the larger the dierence between the clied and unclied signals and so SCNR can be decreased. Smaller the dierence mae SCNR will increase. here are urther more cliing techniques which are used in OFDM system are as ollows: B. Reeated Cliing he cliing technique is the simler one which is used to cut the signal ea uto desired threshold level. But reeated cliing and iltering technique roved to be worthy one as it gives better result comared to earlier one. In this technique the ea regrowth which is generated in iltering can be minimized. So the reeated cli and ilter rocess reduces these regrowths in odm system [4]. C. Reconstruction o Lost Clied Signal o remove the ea regrowth o signal oversamled sequence cliing is used which can reconstruct the clied samles and mitigate the cliing distortion in resence o channel noise at the cost o bandwidth exansion. It is observed that by increasing small bandwidth, the erormance o OFDM system can be imroved. PAPR is the biggest roblem in OFDM system. Many techniques are roosed or it. Cliing and iltering technique is considered to be the simlest one [9]. D. Reed Solomon (rs) Method his method is also used in ar reduction. his is decoding method, used or reconstruction o clied signal in multicarrier systems or high value o SNR. his method is very sensitive to additive noise. E. Least Square Method his is decoding method which is used or reconstruction o clied signal in multicarrier systems. It is more robust method against channel noise. F. Iterative Cliing & Filtering echnique his technique is used to eliminate the ea regrowth due to CF technique. In each iteration ea regrowth decreases signiicantly. he rocess o iteration undergoes FF/IFF and one extra IFF is required or conversion into time domain in OFDM [0]. G. Recursive Cliing and Filtering with Bounded Distortion (rcbd) In RCF the signal is clied by reeating rocess many times beore eeding to ower amliier. When the rocess o reetition exhibit on the signal the out o band sectral density and the robability o the occurance o PAPR decreases but error rate increases due to increase in number o reetitions. he bit error rate increases due to increase in inband distortion. So to remove this increased error rate another imroved technique is roosed called recursive cliing and iltering with bounded distortion (RCFBD) to achieve PAPR reduction. he idea o this technique is same as oversamled digital cliing in time domain and removing out o band comonents in requency domains are used. But additional barrier on in band distortion o each subcarrier is alied during the recursive rocess. In this way PAPR can be reduced without roducing any aect on the error rate [9]. RCFBD minimize PAPR and ees the control on the distortion o data carried by each subcarrier. So by using this technique side inormation can be eliminated and receiver art becomes less comlex and BER erormance can be increased more. It is also more robust against AWGN noise []. H. Adative Baseband CFR echnique: CFR done searately on each carrier beore uconversion measurement taen at interolated baseband to determine which delayed baseband symbols to modiy [3]. IV. SIMULAION RESUL In this section, the PAPR erormance is evaluated by simulations. In this simulation QAM symbols are ed as inut to the OFDM system. In roosed cliing technique using crest ratio.4 the PAPR is reduced u to 4dB comared to unclied signal. he requency resonse is shown in igure 3, which shows reduction o amlitude o signal u to 4.Further it is shown in igure 5 that when crest ratio is reduced by actor., PAPR urther can go down uto db. ISSN: IJECC 07

4 Amlitude Amlitude International Journal o Electronics Communication and Comuter echnology (IJECC) Volume Issue 3 (May 0) he algorithm or cliing is as ollows: ) Convert the OFDM symbol to time domain as IFF. Note that oversamling is needed. ) Cli to the threshold 3) he clied OFDM signal then iltered using butterworth ilter. 4) Convert to time domain and transmit it. 6 odm Signal 4 0 Figure 4. PAPR CCDF o clied and unclied signal with crest ratio ime Figure. Frequency resonse OFDM signal without cliing 4 clied Signal ime Figure 3. Frequency resonse Clied signal Figure 5. PAPR CCDF o clied and unclied signal with crest ratio. V. CONCLUSIONS Multicarrier communication technique is owerul and eicient which suort high seed o data in disersive communication channel. he PAPR roblem which is major issue in OFDM system aects the erormance o system. Many techniques have been suggested and in roosed technique it has been shown that PAPR is reduced uto 5dB when crest ratio changes to. REFERENCES [] X.Li and L. J. Cimini, Jr., Senior Member, IEEE, Eects o Cliing and Filtering on the erormance o OFDM IEEE communications letters, vol., no. 5, may 998. [] X. Li and L. J. Cimini, Jr., Eects o cliing and iltering on the erormance o OFDM, in Proc. VC 97, May 997, [3] S. H. Han and J. H. Lee, An Overview o Pea-to-Average Power Ratio Reduction echniques or Multicarrier ransmission, IEEE Wireless Communications, vol., , Aril 005. ISSN: IJECC 08

5 International Journal o Electronics Communication and Comuter echnology (IJECC) Volume Issue 3 (May 0) [4] J. Armstrong, Pea-to-average ower reduction or OFDM by reeated cliing and requency domain iltering, Electronics Letters, vol. 38,.46-47, Feb. 00. [5] J. ellado and J. M. Cio, Pea ower reduction or multicarrier transmission," IEEE GLOBECOM '99, Rio de Janeiro, Brazil. [6] D. Guel, J. Palicot, Cliing ormulated as an adding signal technique or OFDM Pea Power Reduction, IEEE, 009. [7] J. Armstrong, New OFDM Pea-to-Average Power Reduction scheme, IEEE, 00, [8] H. Saeedi, M. Shari, and F. Marvasti, Senior Member, IEEE, Cliing Noise Cancellation in OFDM Systems Using Oversamled Signal Reconstruction, IEEE communications letters, vol. 6, no., ebruary 00. [9] L. Wang and C. ellambura, Senior Member, IEEE, A Simliied Cliing and Filtering echnique or PAR Reduction in OFDM Systems, IEEE signal rocessing letters, vol., no. 6, june 005. [0] S.K. Deng, M.C. Lin, OFDM PAPR Reduction Using Cliing with Distortion Control, his wor was suorted by National Science Council o R.O.C under grant NSC 9-3-E , 005 IEEE [] H. Chen and M. Haimovish, Iterative Estimation and Cancellation o Cliing Noise or OFDM Signals, IEEE Commun. Letters., vol. 7, no. 7, July 003, ISSN: IJECC 09