Exponential transformation and modified nth root exponential transformation for peak to average power ratio reduction in OFDM system

Abstract

Due to the High data rate requirement of modern cellular network, multicarrier modulation (MCM) is now becoming more and more mandatory for Wireless Networks. Orthogonal Frequency Division Multiplexing (OFDM) has proved to a promising MCM technology for application requiring High Data Rate. For instance, in the case of 3GPP and 4G cellular networks, OFDM is used for its huge spectral efficiency and zero Inter Symbol Interference (ISI). However OFDM has a major drawback in terms of Peak to Average Power Ratio (PAPRR) and Inter Carrier Interference (ICI). The resulting peaks from superposition sum, creates high peaks in the signal, which drives the Amplifier saturation region. The saturation region has non-linear characteristics which creates Out of Band (OOB) radiations that are filtered resulting in a data loss. To restrict the amplifier from driving in to the saturation region, high Power Amplifiers (HPAs) must be used to keep the signal In Bound. Thus the power dissipation increases and the battery drains out quickly. Unless the peak power problem of OFDM is not addressed, then the power inefficiency of the system surpasses the advantage of Higher Data Rate. Researches, over the years have proposed versatile methods for reducing the PAPRR of OFDM signals. The methods are categorized using numerous parameters such as computational complexity, necessity of Side Information (SI), noise performance, implementation process and so on. Performance of each method is affected by the parameters considered for the transmission. Because of this, the generality of PAPRR Reduction implementation cannot be defined. Some of the methods involve the scrambling of signal like Partial Transmit Sequence (PTS), Selective Mapping (SLM) and Active Constellation Extension (ACE), while others employ signal distortion technique as Iterative Clipping and Filtering, Tone Injection (TI) and Tone Rejection (TR). In this Thesis PAPRer, we have explored some non-conventional methods for the purpose of PAPRR reduction. Our first proposal involves an exponential conversion the time domain signal in order to achieve lower PAPRR values. The method introduces no further need of SD and any change of phase or frequency whatsoever. The computation of the exponential equivalent is simple and requires less numbers of complex addition and multiplication resulting in low complexity. Next proposed method is a modification of the exponential conversion using the signal mean square average to scale the transformed signal. Combining this modified proposal with n times root, we get even higher PAPRR reduction. The reduction performance is illustrated as Complementary Cumulative Distribution Function (CCDF) vs PAPRR plot using MATLAB. The advantages of the proposed models over the current state of the art technologies are demonstrated by tabulating the reduction results achieved for various methods. Though the better reduction result is achieved, there exists a tradeoff between the PAPRR and Bit Error Rate (BER). The optimization of BER is excluded from the capacity of this Paper, though a representation is drawn. Mathematical expression and Simulation results of the proposals outperform the result of both PTS and SLM.