PAPR Reduction of an OFDM Signal using Partial Transmit Sequences (PTS) Technique

Owing to the high spectral efficiency and the immunity to multipath channels, orthogonal frequency-division multiplexing (OFDM) is a promising technique for high-rate data transmission. But the high Peak-to-Average Power Ratio (PAPR) is one of the main obstacles to limit wide applications. In this paper, we propose a PAPR (peak to average power ratio) reduction method using Partial Transmit Sequences (PTS) Technique to lower the high PAPR of OFDM (orthogonal frequency division multiplexing) system. The merit of this proposed method is efficient PAPR reduction performance. In the paper, the PAPR reduction process is made more efficient by varying the number of phase sequences and the length of block size. Introduction Orthogonal Frequency Division Multiplexing (OFDM) is an attractive multicarrier technique for high-bit-rate transmission. In OFDM system, data is transmitted simultaneously through multiple frequency bands. It is robust to frequency selective fading and narrow band interference. So, OFDM has been adopted as wireless communication. However it is susceptible to high peak-to-average power due to an unstable envelope. This, in turn, leads to poor power efficiency. Furthermore, when it passes through nonlinear device such as HPA, high peak signals may be clipped. The distortions caused by this clipping effect will affect orthogonality of sub-carriers[1]. In addition to this, large PAPR also demands ADCs (Analog-to-Digital Converters) with large dynamic range [2]. In order to reduce the PAPR of an OFDM signal, many techniques are proposed, which can be organized into three classes: signal distortion, block coding, and signal scrambling. The simplest class of techniques to reduce the PAPR is signal distortion, including clipping and peak windows [3]. To clip the signal, the peak amplitude is limited to some desired maximum level. It can give a good PAPR. But the BER performance becomes very worse due to many defected signals [4].Another method 44 Anand Agrawal and Ashok Kumar Gill for PAPR reduction is based on the use of coding schemes, where the original data sequence is mapped onto a longer sequence with a lower PAPR in the corresponding OFDM signal. Basically, a coding scheme would involve a large look-up table and is more suitable for those OFDM systems with a small number of sub carriers[5]. Signal scrambling includes SLM, DSI and PTS techniques. SLM [6] may be classified into the phase control scheme to escape the high peak. One signal of the lowest PAPR is selected in a set of several signals containing the same information data. Both techniques require much system complexity and computational burden by using of many IFFT block. However, this is very flexible scheme and has an effective performance of the PAPR reduction without any signal distortion [8]. In DSI, each different dummy sequences are added into the same input data, and, after IFFT, the signal has a minimum PAPR is selected for output signal [7]. This method also requires much system complexity computational burden. However, the side information is not necessary because inserted dummy sequence is not used for data demodulation in the receiver. In this paper, an effective scheme is proposed, which constructs a favorable OFDM transmit signal with reduced PAPR in section 2. Partial Transmit Sequences (PTS) Technique is described in section 3 and simulation results for different number of phase sequences and variable block sizes are presented in section 4. OFDM and Distribution of the PAPR Let us denote the collection of all data symbols X[k], k = 0, 1, ..., N – 1, as a vector X = [X0, X1, ..., XN-1] that will be termed a data block. The complex base band representation of a multicarrier signal consisting of N sub carriers is given by where j =√–1 , Δf is the sub carrier spacing, and NT denotes the useful data block period. In OFDM the sub carriers are chosen to be orthogonal (i.e. Δf = 1/NT). The PAPR of the transmit signal is defined as PAPR In particular, a base band OFDM signal with N sub channels has PAPRmax =10log10 N. From the central limit theorem, it follows that for large values of N, the real and imaginary values of x(t) become Gaussian distributed. Therefore the amplitude of the OFDM signal has a Rayleigh distribution, with a cumulative distribution given by F(z)= (1− e) .The probability that the PAPR is below some threshold level can be written as P(PAPR≤ z) =(1−e) . In fact, the complementary cumulative distribution function of PAPR of an OFDM is usually used, and can be expressed as P(PAPR > z) = 1−(1− e). PAPR Reduction of an OFDM Signal 45 Figure 1: Plot of CCDF of PAPR for various values of N. The CCDFs are usually compared in a graph such as Fig. 1, which shows the CCDFs of the PAPR of an OFDM signal with 256 and 1024 sub carriers (N =16,128, 256,1024) for quaternary phase shift keying (QPSK) modulation. The horizontal and vertical axes represent the threshold for the PAPR and the probability that the PAPR of a data block exceeds the threshold, respectively. It is shown that the unmodified OFDM signal has a PAPR that exceeds 10 dB for less than 1 percent of the data blocks for N = 256. With a certain PAPR value, the occurrence probability of OFDM signals which exceed this value augment, as the number N increases. Proposed PAPR reduction method (PTS) In the PTS technique, an input data block of N symbols is partitioned into disjoint sub blocks. The sub carriers in each sub block are weighted by a phase factor for that sub block. The phase factors are selected such that the PAPR of the signal is minimized. Figure 2: A block diagram of the PTS technique. 0 2 4 6 8 10 12 14 10 -5 10 -4 10 -3 10 -2 10 10 0 clipping level in dB P rP A R x (n )> cl ip pi ng le ve l plot of PAPR of x(n)for various values of N