Papr reduction of OFDM signal through convex Optimization technique

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is a widely used technique nowadays, both in wireless and wired technology due to its immunity to multipath fading, spectral efficiency and robustness to Inter-Symbol Interference (ISI). This high profile has enabled this technique to be adopted in the upcoming fourth generation (4G) Long Term Evolution (LTE) system. Despite these advantages, this multicarrier modulation technique comes with the disadvantage of high Peak to Average Power Ratio (PAPR) which comes as a result of number of subcarriers used to formulate OFDM symbol transmission. This high PAPR causes distortion to non-linear devices e.g. power amplifiers (PA). Enhancing the dynamic range of amplifier may create other devastating issues; hence it becomes necessary to reduce the PAPR before transmitting the signal. The focus of this thesis is to address this problem of high PAPR and its reduction, and eventually formulate a PAPR reduction technique for LTE. For this purpose, firstly, mathematical computations are carried out that why it is necessary to reduce PAPR and then various factors at transmitter are figured out which can reduce PAPR during the formation of OFDM symbols before actually applying any PAPR reduction technique. Its’ advantage will be to lower the processing – cost of any further applied PAPR reduction technique. In the second phase of the thesis, a classical technique chosen from already existing techniques is evaluated and analyzed for its’ various parameters i.e. iterations, filter etc. The technique is then further flourished by proposing optimization in one of it’s’ stages and then results are shown by comparing the outputs of classical & proposed technique. The results are drawn on the basis of improved performance when CCDF curves are compared for both the techniques. Finally, in order to evaluate systems’ performance BER curves are implemented both for AWGN and Rayleigh channel. In the final stage of the thesis, after showing the proof of concept in the second phase, the proposed technique has been applied on LTE environment. This has been done by using the LTE simulator of Vienna University of Technology (TU Wien, Austria). The classical PAPR reduction technique and its proposed optimized version have been applied on the downlink LTE signal (since OFDM is used as the communication technique in the downlink transmission). Comparison is carried out through CCDF curves drawn for the probability of PAPR of any OFDM symbol. The comparison of the two techniques has been carried out in different scenarios e.g. transmission through different channel models, decoding by different receivers etc.