Improving PAPR Reduction for OFDM using Hybrid Techniques Kashif Sultan, Hazrat Ali, Zhongshan Zhang, School of Computer and Communication Engineering University of Science and Technology Beijing, Beijing, PR China. kashif_rao@outlook.com, engr.hazratali@yahoo.com, zhangzs@ustb.edu.cn Fakhar Abbas College of Information and Communication Engineering Harbin Engineering University, Harbin, China. PR China. fakkhar.14@gmail.com Abstract— The Peak-to-Average Power Ratio (PAPR) reduction in Orthogonal Frequency Division Multiplexing (OFDM) system has gained widespread attention during the last decade, leading to the development of several techniques for PAPR reduction. To benefit from these techniques while overcoming their inherent shortcomings, more recent work shows the use of hybrid variants of these techniques. In this paper, we present a brief overview of the hybrid techniques adapted for PAPR reduction. We also comment on the computational complexity and practicability of these approaches. Furthermore the paper covers a variety of hybrid techniques combining simple schemes e.g. clipping, companding and more complex schemes such as partial transmit sequence. Keywords-Orthogonal frequency division multiplexing; peak-to- average power ratio; hybrid techniques. I. BACKGROUND Orthogonal Frequency Division Multiplexing (OFDM) is the backbone of modern communication systems. OFDM offers the advantage over traditional frequency division multiple access. The sub-carriers in OFDM do not overlap as they are orthogonal to each other. OFDM also has the advantage of robustness against inter-symbol interference (ISI) and inter-carrier interference (ICI), the formal being addressed through the insertion of guard interval and the later due to orthogonality of the sub-carriers. An OFDM signal is generated using multiple subcarriers transmitted in parallel. Simultaneous transmission of multiple subcarriers is a challenging task as this directly increases the system complexity. The issue is primarily addressed by using the Inverse Discrete Fourier Transformation (IDFT), also represented as Inverse Fast Fourier Transform (IFFT) as the name given to it for the fast calculation of the transformation. The block diagram of OFDM system is shown in Figure 1. A major issue of concern in OFDM is the high peak-to-average power ratio (PAPR), a direct effect of the high peaks in the transmitted signal. Modern requirements of higher data rates push the technology to increase the number of sub-carriers which then leads to high peaks in the transmitted signal. The peaks in an OFDM signal cause challenges as they create non- uniform envelope. Assuming an OFDM signal x(t) with N number of sub-carriers. For sub-carriers to be high in number, the resulting signal has a Gaussian distribution, as given by central limit theorem [1] and the power distribution becomes Rayleigh distributions. The PAPR of the signal can be expressed as, ሺ[]ሻ = max ଴≤≤−ଵ |[]| 2 [|[]| 2 ] (1) Where E[.] is the expectation. Due to high peaks in OFDM signal, the power amplifier goes to its non-linear region, resulting in non-linear distortion and spectral spreading [2]. Figure 1Block diagram of OFDM system PAPR is a widely-used metric for envelope fluctuation measures. Typically, PAPR is characterized by Complementary Cumulative Distribution Function (CCDF), Bit error ratio (BER) and spectral spreading. Several techniques have been proposed to address the PAPR issue and can be categorized into Signal Distortion Techniques, Multiple Signaling & Probabilistic Techniques, Coding Techniques and Hybrid Techniques. A very useful survey of the techniques which fall under the first three categories is available from [3]. In this paper, we present an overview of the hybrid techniques that are based on the combination of more than one technique. The rest of paper is organized as follows; In Section II, we discuss the various available hybrid techniques. We comment on the computational complexity of the major techniques in Section III. In Section IV, we summarize key observations made from the study on PAPR reduction. Finally, we conclude the paper in Section V. II. HYBRID TECHNIQUES Hybrid techniques refer to those schemes in which the joint use of more than one PAPR reduction techniques has been proposed. As pointed out by [3], not a single technique can be designated as the best, and choices are usually made based on trade-off between BER, PAPR and overall system complexity, e.g. in [4][5]. The use of hybrid techniques is usually inspired by