Effect of OFDM Signal Structure and Subcarrier Modulation on the Reduction of the Signal Peak Power
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The problem of high ratio of peak-to-average power experienced in orthogonal-frequency-division-multiplexed signals remains a major challenge when it comes to practical realization of transmitters for such signals. This is mainly due to the requirement on high power amplifiers to operate with large input power back-offs and thus at undesirable low power efficiency region in order to avoid clipping of such signals and the subsequent degradation of bit error rate and spectral interference to adjacent channels. In this paper, an efficient peak power reducing technique is first proposed, its performance verified, and then used to investigate the influence of OFDM signal structure and subcarrier modulation on the peak power reduction capability of peak power reduction techniques. The proposed technique involves carefully selecting some subcarrier signals that when added to information-modulated subcarriers yield a combined signal with an acceptable lower ratio of peak-to-average power. This technique is based on the application of second order cone convex programming to the minimization of maximum of norms. Simulation results show that the proposed technique reduces peak-to-average power ratio (PAPR) by 4.5 dB and 7.8 dB while utilizing 5% and 20% of subcarriers for peak power reduction. Real OFDM signals are found to exhibit a PAPR that is higher than that of complex signals by 2.4 dB. In addition, the OFDM signal structure and the type of subcarrier modulation are found to have minimal impact on the peak power reduction capability since the variation in PAPR reductions for the modulation schemes used in the research is within ±0.75 dB, and the difference in PAPR reduction for both real and complex signals is on average within ±0.22 dB.
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