A Comparison of Peak to Average Power Reduction Schemes for OFDM

Mr. Yogeshver Khandagre

PG scholar in Department of E.C.,NIIST Bhopal,India

yogesh.khandagre@gmail.com

Mr. Anoop Tiwari PG scholar in Department of

E.C.,NIIST Bhopal,India tiwarianoop84@gmail.com

Abstract – In this paper we are introduced newly but powerful and less noisy peak to average power reduction techniques for Orthogonal Frequency Division Multiplexing (OFDM). These two techniques are comparing and show its results. One investigated technique is Threshold selected mapping (TSLM).In this scheme the transmitted signal is minimum PAPR by thresholding a selected signal from a set of signals and the another investigated technique is modernized selected mapping (MSLM). In this scheme the transmit signal is separate in two part real and imaginary then selected from minimum PAR of each set of signals. This real and imaginary combined. They both techniques introduce some computational complexity. The comparisons of TSLM and MSLM schemes are by simulation results.

Index Terms- OFDM, PAPR, SLM,TSLM and MSLM.

I. INTRODUCTION Orthogonal frequency division multiplexing (OFDM) [1] is considered as a root technology for broadband mobile communications [2] due to its high spectral efficiency. OFDM can be easily implemented by the IFFT (inverse fast Fourier transform) and FFT (fast Fourier Transform) process in digital domain, and has the property of high-speed Broadband transmission [3]. The channel bandwidth is divided into many channels so that in a multi-user environment each channel is allocated to a single user. However the difference lies in the fact that the carriers chosen in OFDM are much more closely spaced than in FDMA. The orthogonality principle [4] [10] essentially implies that each carrier has a null at the centre frequency of each of the other carriers in the system while also maintaining an integer number of cycles over a symbol period. OFDM signal has high PAPR [5] (peak to average power ratio) because of the over lapping of multi-carrier signals with large number of sub- carriers.

Mr. Sudesh Gupta

PG scholar in Department of E.C.,NIIST Bhopal,India

sudesh.gupta75@gmail.com

Prof. Ravi Shankar Mishra Dept. of Electronics & Comm.

NIIST, Bhopal, India ravishankarmishra@rediffmail.com

To transmit the high PAPR signal without distortion requires more expensive power amplifier with high linear characteristics. OFDM has been adopted for various wireless communication Systems [6][7] such as wireless local area networks (WLANs) wireless metropolitan area networks (WMANs) digital audio broadcasting (DAB) and digital video broadcasting (DVB).OFDM is an very attractive technique for achieving high data rate in the wireless communication systems. However the OFDM signal can have very high peak-to-average power ratio (PAPR) [7][3][12] in the transmitter side due to nonlinear characteristics of amplifier then the PAPR reduction is one of the most important research interests for the OFDM systems.

II. SYSTEM DESCRIPTION

In an OFDM system shown in fig.1 [5] data is modulated in the frequency domain to N adjacent subcarriers. These N subcarriers span a bandwidth of W Hz and are separated by a spacing of Δf =W/N. The baseband signal representation of this is.

Where T = 1/∆f is the symbol period and N is Number of sub carriers.

12 /

0

1x(t)= [ ] , [0, ]N

j fkt T

kX k e t T

Nπ

−Δ

=

∈∑

2011 International Conference on Communication Systems and Network Technologies

978-0-7695-4437-3/11 $26.00 © 2011 IEEE

DOI 10.1109/CSNT.2011.82

366

Orthogonal Frequency Division Multiplexing is based on a parallel data transmission technique that decreases the effect of multipath fading. Orthogonal Frequency Division Multiplexing (OFDM) is come from the fact that the digital data is sent using different carriers each of a different frequency and these carriers are orthogonal to each other.

III. PEAK-TO-AVERAGE RATIO Basically peak-to-average power ratio (PAPR) is the most popular parameter used to find out the dynamic range of the time-domain OFDM signal. The very simplest definition of the PAPR for the OFDM symbol in the time domain x may be expressed as

Peak Amplitute of the signalPAPR = Average value of the signal

( )( )

2

2

max xPAR {x} =

E x⎡ ⎤⎣ ⎦

Usually, the continuous time PAPR of x (t) is approximated using the discrete time PAPR, which is obtained from the samples of the OFDM signal. There have been several attempts to determine the distribution of x (t). The first came from where is claimed that

2.8 Pr [PAR{x[t]}> ] 1-(1- ) Ne γγ −≈ The above equation is a very good approximation of the PAR.

IV. SELECTED MAPPING In a Selected mapping (SLM) shown in fig.2 [4] is a specific scheme for PAPR reduction. The SLM takes advantage of the fact that the PAPR of an OFDM signal is very sensitive to phase shifts in the frequency-domain data. PAPR reduction is achieved by multiplying independent phase sequences to the original data sequence and determining the PAPR of each phase sequence combination. The combination with the lowest PAPR is transmitted. In other words, the data sequence X is element-wise phased by D N-length phase sequences. PAPR reduction is achieved by multiplying independent phase sequences to the original data and determining the PAPR of each phase sequence combination. The combination with the lowest PAPR is transmitted.

fig.2 SLM

The CCDF of the PAPR in SLM OFDM symbol is

Pr [PAR{x[n]}> ] = 1-(1- )Ne γγ −

V. THRESHOLD SELECTED MAPPING The threshold selected mapping (TSLM) is extended version of selected mapping (SLM).In TSLM the comparison of phase sequence till threshold level. After this threshold level phase mapping is not useful for reducing PAPR. The idea of thresholding the PAR of a SLM system was first mentioned in [4]. However that work dealt with an interleaving SLM method and thresholding the PAR was only mentioned in [7][9]passing without any analysis. Threshold PAR was as it is presented here was originally developed in the context of reducing the computational complexity of SLM. The CCDF of the PAPR in TSLM OFDM symbol is

(d ) Pr [PAPR{x }> ] = [1-(1- ) ]N De γγ

≈ −

VI. MODERNIZED SLM SCHEME In Modernized SLM Scheme [6] firstly real and imaginary part of the complex modulating signal Am is separated as Am = ARm + jAIm (1≤m≤M) Where ARm is the real part and jAIm is the imaginary part of the frequency domain vectors. In the modernized SLM separate scheme the complex Baseband signal separate into real and imaginary part. These real parts converted into serial to parallel and individual phase sequence is multiply into every parallel data and selected minimum PAPR. Similarly imaginary parts converted into serial to parallel and Individual phase sequence is multiply into every parallel data which selected minimum PAPR. Both real and imaginary parts are again combining and

367

transmit. Since the signal are real valued. The phase vector DR and DI have to be real. These candidates are transformed into time domain using IFFTs.then each combination of one real and one imaginary from these the best candidate with minimum PAR is selected. The CCDF of the PAR in Modernized SLM OFDM symbol is

Fig.3 MSLM

VII. SIMULATION RESULTS We are compare performence of the PAPR of some techniques SLM, TSLM and MSLM using the MATLAB simulator show in fig 4.We use similar parameter and reduce some PAPR by MSLM techniques. We are also analyze TSLM and SLM for various conditions show in fig. 5 and fig. 6.

Fig.4 Simulation results SLM, TSLM and MSLM

Fig.5 PAR CCDF of TSLM for N = 128, L = 1 and

varies clipping threshold.

Fig.6 PAR CCDF of SLM for D = 10, L = 4 and N

= 128, 256, 1024, and 4096.

VIII. CONCLUSION In this paper we compared some recently proposed techniques which allow powerful but distortion less PAPR reduction for OFDM transmission. We show the some powerful techniques for reducing the PAPR of the OFDM system. We have presented here SLM, TSLM and MSLM. As a result the MSLM technique reduces more PAPR as compared to TSLM and SLM technique. The MSLM technique is best method to reduce OFDM peak power without introduce nonlinear distortion.

(d ) R IProb [PAPR{x }> ] = [1-(1- ) ] [1-(1- ) ]N D N De eγ γγ

≈ − −+

368

REFERENCES

[1] P. V. Eetvelt, G. Wade, and M. Tomlinson, “Peak to average power reducing for OFDM schemes by selective scrambling,” IEEE Electron. Lett., vol. 32, no. 21, pp. 1963–1964, Oct. 1996. [2] Y.Wu, and W.Y.Zou, “Orthogonal frequency division multiplexing a multi-carrier modulation scheme,” IEEE Trans. on Consumer Electronics, vol. 41, no. 3, pp. 392-399, Aug. 1995. [3] J. A. Davis and J. Jedwab, “Peak-to-mean power control in OFDM, Golay complementary sequences, and Reed-Muller codes,” IEEE Trans. Inform. Theory, vol. 45, no. 7, pp. 2397–2417, Nov. 1999. [4] M. Breiling, S. H. Müller, and J. B. Huber, SLM peak power reduction without explicit side information,” IEEE Commun. Lett., vol. 5, no. 6, pp. 239–241, June 2001. [5] X. Wang, T.T. Tjhung, and C.S. Ng, “Reduction of peak-to-average power ratio of OFDM system using a companding technique,” IEEE Trans .on Broadcastaing, Vol. 45, No. 3, pp. 303-307, Sep. 1999. [6] Yogeshver khandagre “Modernized SLM (MSLM) Scheme for PAPR Reduction of OFDM Systems" ICCA Trans.on Telecommunications, Pondicherry, pp. 365-369, December 2010 [7] Bauml, R., Fischer, R., and Huber, J., “Reducing the peak-to-average power ratio of multicarrier modulation by selected mapping," IEE Electronics Letters, vol. 32, pp. . [8] A. D. S. Jayalath and C. Tellambura, “A blind SLM receiver for PAR reduced OFDM,” in Proc. IEEE Vehicular technology Conf., Sep. 2002,pp. 219–222. [9] R. J. Baxely, G. T. Zhou, “ MAP Metric for Blind Phase Sequence Detection in Selected Mapping”, IEEE Trans. on Broadcast., vol. 51, no.4, Dec. 2005, pp 565-570. [10] N. Chen, G. T. Zhou, “Peak-to-Average Power Ratio Reduction in OFDM with Blind Selected Pilot Tone Modulation”, IEEE Trans. On Wireless Comm., vol. 5, no. 8, Aug. 2006, pp.1536-1542. [11] Ravi Shankar Mishra “A comparison of peak to average power reduction schemes for ofdm" IJJCET Trans.on Telecommunications, Chennai, pp. 165-169,april 2011.

369