INTERNATIONAL JOURNAL OF PURE AND APPLIED RESEARCH IN ENGINEERING AND TECHNOLOGY A PATH FOR HORIZING YOUR INNOVATIVE WORK HYBRID TECHNIQUE FOR PAPR REDUCTION IN OFDM USING PARTIAL TRANSMIT SEQUENCES MS. SHRADDHA WAGHMARE 1, MRS. M. R. PATIL 2, MS. ASHWINI REWATKAR 1, DR. S. P. MOHANI 3 1. Assistant Professor, Electronics & Communication Engineering Dept., Dr. Babasaheb Ambedkar College of Engineering & Research, Nagpur, India. 2. H.O.D & Assistant Professor, Electronics & Communication Engineering Dept., Dr. Babasaheb Ambedkar College of Engineering & Research, Nagpur, India. 3. Professor, Electronics & Telecommunication Engineering Dept., College of Engineering Pune, India. Accepted Date: 05/03/2015; Published Date: 01/05/2015 Abstract: This Orthogonal frequency division multiplexing (OFDM) is a robust and effective multicarrier transmission technique for high speed communication in wireless mobile environment and applications. A major challenging issue in application of OFDM is its high peak to average power ratio (PAPR). Partial transmit sequence (PTS) scheme is an attractive technique for PAPR reduction in OFDM systems. An exhaustive search for phase sequences is needed to obtain desirable PAPR performance, which results in high computational complexity. In this work, a hybrid tree PTS (T-PTS) algorithm is proposed, where real and imaginary parts of the candidate are separately multiplied with phase factors, moreover, PAPR is conjointly optimized in real part and imaginary part. The simulation results of the proposed technique show improved PAPR performance with remarkably reduced computational complexity and bit error rate (BER). Keywords: Multicarrier Systems, Orthogonal Frequency Division Multiplexing (OFDM), Peak to average power ratio (PAPR), Partial Transmit sequence (PTS), Quadrature Amplitude Modulation (QAM), IFFT, FFT \ Corresponding Author: MS. SHRADDHA WAGHMARE Access Online On: www.ijpret.com How to Cite This Article: PAPER-QR CODE 965
INTRODUCTION Orthogonal Frequency Division Multiplexing is extensively implemented in various high speed wireless communication standards because of its favorable properties such as high spectral efficiency, robustness to channel fading, capability of handling multipath fading and immunity to impulse interference. Interestingly, OFDM is a combination of modulation and multiplexing. One major limitation of OFDM is its large Peak to Average Power Ratio (PAPR). These large peaks cause saturation in power amplifiers at the transmitting end, leading to inter-modulation among the subcarriers, which causes an increase in the out of band (OOB) energy of the spectrum. Hence, to design a cost effective and robust system, it is highly desirable to reduce the PAPR. In this work Partial Transmit Sequence (PTS) technique for PAPR reduction is modified by introducing new set of phase sequences. The proposed phase sequences show enhanced performance with respect to PAPR reduction. The multicarrier modulation techniques employ several carriers, within the allocated bandwidth, to convey the information from source to destination. Each carrier may utilize one of the several available digital modulation techniques (BPSK, QPSK or QAM) [1]. Generally, an OFDM signal can be represented as: N 1 c t = n=0 s n t sin(2πf n t) (1) Where: c(t) s n t : Ofdm Signal Representation In Time Domain : Symbols Mapped To Chosen Constellation (Bpsk/Qpsk/Qam) f n : Represents The Orthogonal Frequencies Equation (1) can be thought of as an Inverse Fast Fourier Transform (IFFT) where is the size of IFFT. The Fourier transform breaks a signal into different frequency bins by multiplying the signal with a series of sinusoids. Since the OFDM signal is in time domain (refer (1)), IFFT is the appropriate choice to use at the transmitter. This can also be interpreted as converting frequency domain samples to time 966
domain sample. To acquire the original transmitted signal, FFT is performed at the receiver side [2]. I. Peak to average Power ratio (PAPR) OFDM has several features which make it an attractive modulation scheme for high speed transmission links. However, one major limitation is its large Peak to Average Power Ratio (PAPR). These large peaks cause saturation in power amplifiers at the transmitting end, leading to inter-modulation among the subcarriers, which causes an increase in the out of band (OOB) energy of the spectrum. Hence, to design a cost effective and robust system, it is highly desirable to reduce the PAPR. Some of the distortion based techniques for PAPR reduction include clipping and filtering, companding, coding. Selective mapping (SLM), partial transmit sequence (PTS), tone reservation, tone injection, constellation extension are Distortionless Techniques for PAPR reduction. PTS technique improves PAPR statistics of an OFDM signal significantly without any in-band distortion and out-of-band radiation. The selection of proper phase sequences to achieve good PAPR reduction is very important in the PTS technique [3]. II. Partial Transmit Sequence Fig 1. Block Diagram of PTS In a typical OFDM system with PTS approach to reduce the PAPR, the input data block in X is partitioned into disjoint subblocks, which are represented by the vectors. 967
Therefore, we can get {X m, m = 0,1, M 1} X = M 1 m =0 X (m ) (2) where X m = [X m o, X m 1,., X m N 1 ] with X m k = X k or (0 m M 1). In general, for PTS scheme, the known subblock partitioning methods can be classified into three categories adjacent partition, interleaved partition and pseudorandom partition. Then, the subblocks X m are transformed into M time-domain partial transmit sequences x m m = x 0 x m m 1.. x LN 1 = IFFT LNx N [X m ] (3) These partial sequences are independently rotated by phase factorsb = {b m = e j θ m, m = 0,1,., M 1}. The objective is to optimally combine the subblocks to obtain the time domain OFDM signals with the lowest PAPR x = M 1 m=0 b m X (m ) (4) Therefore, there are two important issues should be solved in PTS: high computational complexity for searching the optimal phase factors and the overhead of the optimal phase factors as side information needed to be transmitted to receiver for the correct decoding of the transmitted bit sequence. Following is an example of phase sequences [ 1+0i, -1+0i, 0+1i, 0-1i ] [ -1+0i, -1+0i, 1+0i, 0-1i ] [ 0+1i, 0+1i, 0+1i, 1+0i ] [ 0-1i, 1+0i, -1+0i, -1+0i ] III. Proposed System High computational complexity, due to search of phase vectors through a high-dimensional vector space, is a potential problem for practical implementation of PTS. To address the complexity issue, a hybrid combination of T-PTS along with Real part (RP) Imaginary part (IP) 968
separation is proposed in this work. This technique adaptively generates the real and imaginary phase factors separately by using Tree searching algorithm. The phase vector is chosen such that the real part of the signal is multiplied with real part of the phase vector and similar operations are done for imaginary part of the IFFTed signal. The possible combinations for real phase vector will be: {1, 1} Similarly, for imaginary phase vector possible combinations are given as: {j, j} Four combinations of the phase vectors along with the first subblock are obtained as follows: Combination no. 1: RP * 1 + IP * j Combination no. 2: RP*-1 + IP *j Combination no. 3: RP * 1 + IP * -j Combination no. 4: RP * -1 + IP* -j IV. Results Following are the system specifications used for both unmodified OFDM, conventional PTS, RP- IP separation PTS, T-PTS and proposed Hybrid PTS technique [12]. The simulation is performed using MATLAB. Table I: System Specification Sr. No. Simulation Parameters Parameter Values 1 Number of input bits 1,00,000 2 Modulation for data symbols 16 QAM 3 Modulation for side information 8 QAM 4 No. of bits per symbol 4 5 Oversampling Rate 4 6 No. of sub-blocks (M) 4 7 IFFT/FFT size for OFDM symbols (N) 256 8 IFFT/FFT size for side information 16 During transmission, the OFDM data block is sent first and the side information related to that particular data block follows on a separate set of frequencies. This keeps the PAPR of the data signal unaltered even after addition of side information to the transmitted signal. 969
Bit Error Rate Probability,P(PAPR(db)) >= Z Research Article Impact Factor: 4.226 ISSN: 2319-507X 10 0 10-1 CCDF plots of PAPR 256 IFFT (QAM)-PTS Proposed System Basic ofdm Basic PTS RP-IP-PTS T-PTS Hybrid RP-IP 10-2 1 2 3 4 5 6 7 8 9 10 11 12 papr, Z db Fig 2. CCDF plots of PAPR using PTS For the first 10 symbols, Basic PTS shows reduction in PAPR as compared to the Basic OFDM, whereas, RP-IP shows PAPR reduction more than the other considered techniques. The Proposed PTS technique gives better performance than T-PTS. The advantage of Proposed PTS over RP-IP PTS is the computational complexity for finding the phase sequence is reduced comparatively. 10 0 10-1 Bit Error Probability curve-pts Proposed System Basic ofdm Basic PTS RP-IP-PTS T-PTS Hybrid RP-IP 10-2 10-3 10-4 1 2 3 4 5 6 7 8 9 10 Eb/No, db Fig 3. BER plots of PTS systems 970
The Bit error rate of the system is also a major issue of concern in an OFDM system. The proposed technique shows excellent performance in reception of the data even after it is passed through the AWGN channel. As shown in fig3 significant reduction in the BER is offered by the proposed technique compared to the other techniques considered in this experimentation. Table II. Computational Complexity PTS Technique Execution Time (sec.) Conventional PTS 402.908585 RP-IP PTS 4.886889 Tree PTS 1.747156 Hybrid RP-IP PTS 3.173329 V. CONCLUSION In this work, a modification in the conventional PTS scheme is proposed. The proposed technique of hybrid combination of Tree-PTS and RP-IP separation shows a significant reduction in PAPR as compared to that offered by individual techniques considered in this experimentation. Computational complexity is reduced by a large factor than the conventional PTS. The proposed technique offers better BER performance than that of all variants of PTS techniques under consideration. Considerable reduction in computational complexity is offered by the proposed technique in finding the phase sequence as compared to the conventional PTS and other PTS techniques considered in this experimentation. VI. REFERENCES 1. Richard Van Nee, Ramjee Prasad, OFDM for Wireless Multimedia Communication, Artech House universal personal communication library, Boston, London, pp. 33 37. 2. Charan Langton, Intuitive guide to principles of communication, Orthogonal Frequency Division Multiplexing Tutorial, Copyright 2004. 3. Tao Jiang, and Yiyan Wu An Overview: Peak-to-Average Power Ratio Reduction Techniques for OFDM Signals, IEEE Trans. on Broadcasting. vol.54, No.2, June 2008. 4. Tellambura, Chintha and Jayalath, Dhammika (2001) PAR reduction of an OFDM signal using partial transmit sequences. In Proceedings IEEE VTS 54th Vehicular Technology Conference, 2001 (VTC 2001 Fall). 971
5. Trung Thanh Nguyen and Lutz Lampe, On Partial Transmit Sequences for PAR Reduction in OFDM Systems, Revised as Paper for publication in the IEEE Transactions on Wireless Communications, November, 2006. 6. Chusit Pradabpet and Kobchai Dejhan, A new PAPR reduction in OFDM systems using PTS combined with APPR for TWTA nonlinear HPA,Songklanakairn journal of Science and Technology. Received 4 April 2007; Accepted 1 May 2008. 7. Jayalath, Dhammika and Tellambura, Chintha (2003), Side information in PAR reduced PTS- OFDM signals. In Proceedings 14th IEEE Conference on Personal, Indoor and Mobile Radio Communications, 2003 (PIMRC 2003) 1, pages pp. 226-230, Beijing, China. 8. Xinchun Wu, Zhigang Mao, Jinxiang Wang, Bin Zhou, A Novel PTS Technique with Combinative Optimization in Real Part and Imaginary Part for PAPR Reduction in OFDM Systems, 2009 Third International Conference on Next Generation Mobile Applications, Services and Technologies. 9. Byung Moo Lee and Rui J. P. de Figueiredo, An Enhanced Iterative Flipping PTS Technique for PAPR Reduction of OFDM Signals, Mobile and wireless communication, physical layer development and implementation. 10. Jayalath, Dhammika and Tellambura, Chintha, Adaptive PTS approach for reduction of peak-to-average power ratio of OFDM signa, ELECTRONICS LETTERS 6th July 2000 Vol. 36 No. 14, Copyright 2000 IEEE.. 11. ByungMooLee Rui J. P. defigueiredo YoungokKim, A Computationally Efficient Tree-PTS Technique for PAPR Reduction of OFDM Signals, Springer Science+Business Media, LLC. 2010. 12. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Highspeed Physical Layer in the 5 GHz Band, IEEE Std 802.11a- 1999(R2003) (Supplement to IEEE Std 802.11-1999). 972