Improve peak to average power ratio (papr) reduction techniques

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME

International Conference on Communication Systems (ICCS-2013) October 18-20, 2013 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 28

Improve Peak to Average Power Ratio (PAPR) Reduction Techniques

in OFDM Systems

Ashok Kumar kajla1, Rupesh Sharma2, Yash Walia3, Sukoon Mishra4

ARYA Institute of Engineering & Technology, Jaipur, India

[email protected]. [email protected]

ABSTRACT: Although OFDM seems to be a solution to keep up with the demand of increasing data rates, it has some drawbacks. Sensitivity to high PAPR is the most significant of these drawbacks. The main objective of this paper was to investigate and document the effects of PAPR on the performance of OFDM based digital communications under different channel conditions. A step-by-step approach was adopted in order to achieve the objective of this paper. The first step is to provide a basic background on the principles of OFDM. The reasons for the PAPR and a theoretical analysis of these effects on OFDM systems are documented. The OFDM system has a high peak-to-average power ratio (PAPR) that can cause unwanted saturation in the power amplifiers, leading to in-band distortion and out-of-band radiation. To be able to observe the system behavior, the simulation results for different channel models are presented in graphical form. Next, the simulation results obtained in this work are compared to the simulation results reported in related studies. KEYWORDS: Orthogonal frequency division multiplexing, PEAK TO AVERAGE POWER RATIO (PAPR), Selected mapping (SLM), Partial transmit sequence (PTS)

I. INTRODUCTION OFDM was introduced in the 1950s but was first implemented in the 1960s. It was originally developed from the multi-carrier modulation techniques used in high frequency military radios. A patent for OFDM was granted in the 1970s. However, when OFDM was first introduced, it was not very popular because of the cost and complexity of large arrays of sinusoidal generators and coherence demodulators. The actual widespread use of OFDM started after the inverse discrete Fourier transform (IDFT) and discrete Fourier transform (IDFT) made the OFDM implementation possible without the use of large number of sinusoidal generators. OFDM was accepted as a wireless local area network (WLAN) standard in September 1999, but actually it was not the first IEEE physical standard for WLANs. The first standard was approved in June 1997 and specified one medium access control (MAC) and three physical layers (IEEE 802.11 FHSS, IEEE 802.11 DSSS and IEEE 802.11 IR). The IEEE 802.11 direct sequence spread spectrum (DSSS) originally supported both 1 Mbps and 2 Mbps of data rates while the other two supported 1 Mbps or 2 Mbps optionally. Due to the increasing

INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

ISSN 0976 – 6464(Print) ISSN 0976 – 6472(Online) Special Issue (November, 2013), pp. 28-35 © IAEME: www.iaeme.com/ijecet.asp Journal Impact Factor (2013): 5.8896 (Calculated by GISI) www.jifactor.com

IJECET © I A E M E



demand for higher throughput, a high-data-rate DSSS (IEEE 802.11b) was selected for standardization in July 1998, and the data rate increased to 11 Mbps. The IEEE 802.11a and IEEE 802.11b standards were developed simultaneously. The federal communications commission (FCC) released 300 MHz of spectrum in the 5.2-GHz band in January 1997 for WLAN applications, and IEEE 802.11a was targeted to use this spectral band. OFDM is being used in a number of wired and wireless voice and data applications due to its flexible system architecture. There are several publications on OFDM synchronization, covering many different scenarios. Most of them rely on the insertion of pilot symbols known to the receiver , while some are based only on the redundancy present in the cyclic prefix of each symbol and a few are so-called fully blind and rely solely on the OFDM symbol structure . It can be seen, and other, examples that the attainable performance is crucially dependent on efficient use of the available information. In this paper, we propose a simple technique for the reduction of high Peak to Average Power Ratio (PAPR), based on Clipping and Differential Scaling Selected mapping (SLM) is a promising peak-to-average power ratio (PAPR) reduction technique for orthogonal frequency division multiplexing (OFDM) system .Partial transmit sequence (PTS), is one of the most attractive method to reduce the PAPR in OFDM systems. It achieves considerable PAPR reduction without distortion. The Block diagram of transmitter and receiver in an OFDM system is shown in Fig. 1. II. DESIGN ISSUE

The OFDM technique divides the total bandwidth into many narrow sub-channels and sends data in parallel. It has various advantages, such as high spectral efficiency, immunity to impulse interference and, frequency selective fading without having powerful channel equalizer. But one of the major drawbacks of the OFDM system is high PAPR. OFDM signal consists of lot of independent modulated subcarriers, which are created the problem of PAPR. It is impossible to send this high peak amplitude signals to the transmitter without reducing peaks. So we have to reduce high peak amplitude of the signals before transmitting. PAPR Reduction Techniques PAPR reduction techniques are classified into the different approaches: There have been many new approaches developed during the last few years. Several PAPR reduction techniques have been proposed in the literature. These techniques are divided into two groups. These are signal scrambling techniques and signal distortion techniques. The signal scrambling techniques are Signal scrambling techniques work with side information which minimized the effective throughput since they commence redundancy. Signal distortion techniques introduce band interference and system complexity also. Signal distortion techniques minimize high peak dramatically by distorting signal before amplification. III. AMPLITUDE CLIPPING AND FILTERING

The simplest technique for PAPR reduction may be amplitude clipping. Amplitude clipping limits the peak envelop of the input signal to a predetermined value from equation (1), where φ(x) is the phase of x and A is the cutting threshold. The noise caused by amplitude clipping falls both in-band and out-of-band. In-band distortion cannot be reduced by filtering and results in an error performance degradation, while out-of-band radiation reduces the spectral efficiency. Filtering the out of- band distortion after clipping cause some peak re growth and to avoid this undesirable effect, repeated. Fig. 2 shows the block diagram of a PAPR reduction scheme using clipping and filtering.



IV. PROPOSED SELECTED MAPPING TECHNIQUE Selective Mapping (SLM) approaches have been proposed by Bauml in 1965 [13]. This method is used for minimization of peak to average transmit power of multicarrier transmission system with selected mapping. SLM is the method for reducing the PAPR problem. The good side of selected mapping method is that it doesn’t eliminate the peaks, and can handle any number of subcarriers. Fig. 3 shows the block diagram of selective mapping (SLM) technique for PAPR reduction. Here, the input data block X =[X[0], X[1], X[2]...... X[N-1]] is multiplied with U different phase sequences pu = [p , p .......p ] where p = e ∅ and ∅ ∈ [0,2π] for v=0,1,......N-1 and u= 1,2,......U , which produce a modified data block Xu= [Xu[1], Xu[2].....Xu[N-1]]2 IFFT of U independent sequences{Xu[v]} are taken to produce the sequences Xu= [Xu[0], Xu[1], Xu[2]..... Xu[N-1]]T among which the one xv= xu with the lowest PAPR is selected for transmission , as shown as in equation (2). Fig. 3 shows the block diagram of selective mapping (SLM) technique for PAPR reduction. In order for the receiver to be able to recover the original data block, the information (index u) about the selected phase sequence Pu should be transmitted as side information. The implementation of SLM technique requires U IFFT operations. Furthermore, it requires [log2 U] bits of side information for each data block where [x] denotes the greatest integer less than x. V. PROPOSED PARTIAL TRANSMIT SEQUENCE TECHNIQUE

The partial transmit sequence (PTS) technique partitions an input data block of N symbols into V disjoint sub blocks as shown in equation (3), where Xi are the sub blocks that are consecutively located and also are of equal size. Unlike the SLM technique in which scrambling is applied to all subcarriers, scrambling (rotating its phase independently) is applied to each sub block in the PTS technique (see Fig. 4). Then each partitioned sub block is multiplied by a corresponding complex phase factor bv=e ∅ , v= 1,2, . . . ; V, subsequently taking its IFFT to yield from equation (4). Fig. 4 shows the block diagram of partial transmit sequence (PTS) technique for PAPR reduction. Where {xv} is referred to as a partial transmit sequence (PTS). The phase vector is chosen so that the PAPR can be minimized, which is shown in equation (5). Then, the corresponding time-domain signal with the lowest PAPR vector can be expressed as equation (6). In general, the selection of the phase factors {b } is limited to a set of

elements to reduce the search complexity. As the set of allowed phase factors is b= {eπ/ i=0;

1; . . . ; W-1}, Wv-1 sets of phase factors should be searched to find the optimum set of phase vectors. Therefore, the search complexity increases exponentially with the number of sub blocks. The PTS technique requires V IFFT operations for each data block and log2WV b c bits of side information. In fact, there are three different kinds of the sub block partitioning schemes: adjacent, interleaved, and pseudo-random. Among these, the pseudo-random one has been known to provide the best performance [8]. As discussed above, the PTS technique suffers from the complexity of searching for the optimum set of phase vector, especially when the number of sub blocks increases. In the literature, various schemes have been proposed to reduce this complexity. VI. SIMULATION STUDIES In this part we explore Mat lab simulation results of discussed methods in the previous chapters, here most results represented as CCDF of PAPR which studied in We consider 1024 OFDM with 84 useful data subcarriers and 16 QAM modulations which is oversampled by 4 times. Pilot subcarriers are set as known value 1 in the whole simulations. The performance of



the proposed modified PAPR Reduction techniques is evaluated by Complementary Cumulative Distribution Function (CCDF) of PAPR with respect to threshold PAPR0. The CCDF or Pr [PAPR > PAPR0] denotes the probability of the signals having a PAPR greater than threshold PAPR0. The CCDF of PAPR performance of the proposed technique is investigated by 16 subcarriers OFDM- system as shown in Fig. 5 to 12. Fig. 5 describes the successive results pertain to transmit symbols before IFFT using symbol rate sampling. Fig. 6 explains the successive results pertain to receiver symbols after FFT using symbol rate sampling. Fig. 7 shows the successive peak results pertain to Squared Absolute transmit symbols after IFFT showing peaks without PAPR suppression. Fig. 8 explains the successive peak Results pertain to transmit symbols after IFFT showing peaks without PAPR suppression. The performance of OFDM PAPR with Without Reduction Amplitude Clipping is shown Fig. 9. Fig. 10 shows the OFDM PAPR with without Reduction Partial transmit sequence (PTS) in combined form. BER performance with clipping Simulations support the use of the variance of the posterior red color as an indicator of when the OFDM system in With Clipping BER vs. SNR. It is here compared with the actual error in the simulations blue color. OFDM without Clipping BER vs. SNR is shown in Fig. 11. Fig. 12 describes the OFDM PAPR with and without Reduction Partial Transmit Sequence (SLM).

VII. EQUATIONS

퐵(푥) =푥 |푥| ≤ 퐴

퐴푒 ∅( ) |푥| > 퐴 (1)

푢 = argmin , ,… max , … |푥 [푛]| (2) X = {X , X X … … … X } (3) 푥 = 퐼퐹퐹푇{∑ 푏 푥 } = ∑ 푏 . 퐼퐹퐹푇{푋 } = ∑ 푏 푥 (4) (푏~ , … … 푏~ ), = arg푚푖푛 [푚푎푥|∑ 푏 푥 [푛]| (5) 푥 = ∑ 푏~ 푥 (6)

VIII. FIGURES AND TABLES

Fig. 1



Fig. 2

Fig. 3

Fig. 4

Fig. 5



Fig. 6

Fig. 7

Fig. 8

Fig. 9



Fig. 10

Fig. 11

Fig. 12

IX. CONCLUSION The high throughput OFDM signal high PAPR problems are solved by the proposed methods of Modified Interleaving technique, Modified Selected Mapping technique, Modified Partial Transmit Sequences technique and Modified Tone Reservation technique. The analysis based on varying the number of subcarriers, transmit antennas and users indicated that the proposed technique has the high PAPR reduction capability compared with the conventional techniques. This grade is achieved at the cost of slight decrease in the data rate and a negligible degradation in the bit error performance of the system. With the help of proposed Modified Interleaving, Selected Mapping, Partial Transmit Sequences techniques BER degradations performance is improved. Based on PAPR reduction performance comparison, Modified Tone Reservation provides the best result.



REFERENCES [1] R.W.Bauml, R.F.H. Fischer,J.B.Huber (1996), “Reducing the Peak –to-Average Power Ratio of multiuser modulation by selected Mapping,” Electronics Letters, pp 414-424. [2] M.Breiling, S.H.Muller-Weinfurtner, J.B Huber (2001),“ SLM Peak power reduction without explicit sie information,” IEEE Communication Letter, pp 239-241. [3] Jayalath D. and Tellambura C. (2000), ‘Adaptive PTS approach for reduction of peak-to-average power ratio of OFDM signal’, IEEE Electronics Letters, Vol. 36, No. 14, pp. 1226-1228. [4] Lim D.W., Heo S.J., No J.S. and Chung Habong (2005), ‘A new PTS OFDM scheme with low complexity for PAPR reduction’, IEEE Transactions on Broadcasting, Vol. 52, No. 1, pp. 77-82. [5] H.H.Seung, and H.L.Jae (2004),“PAPR reduction of OFDM signals using a reduced complexity PTS technique,”IEEE Signal Processing Letters, Vol.11, No.11, pp. 887-890. [6] K.Yang and S.I.Chang (2003), "Peak-to-average power control in OFDM using standard arrays of linear block codes”, IEEE Communication Letters, Vol. 7, No. 4, pp. 174-176. [7] Trung T.N. and Lampe L. (2008), ‘On partial transmit sequences for PAR reduction in OFDM systems’, IEEE Transactions on Wireless Communications, Vol. 7, No. 2, pp. 746-755. [8] J.Gao, J.K.Xie (2009), “On non-linear Iterative Partial Transmit Sequence for PAPR reduction in OFDM system,” Proceedings of Electromagnetics Research Symposium, Beijing, China, March 23-27, pp. 431-435. BIOGRAPHY

Ashok Kajla received the B.E. degree from M.B.M. College, Jodhpur in Electronics & communication and M.E. in digital communication from Arya College of Engineering & Technology, Jaipur and Ph.D. (Pursuing) from JNIT University, Jaipur, Rajasthan. He is currently working as a Professor and Head with the Electronics & Communication Engineering Department in Arya Institute of Engineering Technology, Jaipur, Rajasthan. He has been published more than 8

papers in International Journals. He is reviewer of various IEEE conferences.

Rupesh Sharma received his B. E Degree in Electronics and Communication Engineering from Rajasthan University, Jaipur in year 2008. Presently, he is a research scholar (M.Tech) in the field of Digital communication. He is a member of IEEE and ISTE society. Where he was involved in designing and implementing real-time DSP systems. He is researching on the OFDM SYSTEM. He is currently working as an Assistant Professor with the Electronics & Communication

Engineering Department in Arya Institute of Engineering Technology, Jaipur, Rajasthan. He has been published 6 papers in national conferences, 3 International Journals and 2 International conferences. He is reviewer of various IEEE conferences