Abstract
Due to the resistance to multipath channel effects, orthogonal frequency-division multiplexing (OFDM) schemes have been adopted for a number of high-bit-rate transmission systems. One major drawback of OFDM is the high peak-to-average power ratio (PAPR) of the output signal. Selected mapping (SLM) and partial transmit sequences (PTS) methods can provide good performance on PAPR reduction and have no adverse effects on the signal spectrum. However, these methods require a bank of inverse fast Fourier transforms (IFFT’s) to generate a set of candidate signals and thus involve high computational complexity. In this thesis, we focus on reducing the computational complexity of the SLM method and present several new low-complexity schemes for PAPR reduction. In this thesis, we first propose two low-complexity multiplication-free conversion processes to replace the IFFT’s in the SLM method, where each conversion process for an N-point IFFT involves only 3N complex additions. The basic idea of these proposed conversions is that we can utilize one IFFT output to generate another IFFT output in the SLM method. With these proposed conversions, we then develop several new SLM schemes and a combined SLM & PTS method, in which half of the IFFT blocks are reduced at least. Computer simulation results show that, as compared to the conventional SLM method, these new SLM schemes have approximately the same PAPR reduction performance under the same number of candidate signals for transmission selection. To further reduce the complexity of the SLM scheme, we extend the first-part results to form a new kind of low-complexity conversions. By using these conversions to replace the IFFT’s in the conventional SLM method, we develop two novel SLM schemes with much lower complexity than the conventional one; the first method uses only one IFFT block to generate the set of candidate signals, while the second one uses two IFFT blocks. Computer simulation results show that, as compared to the conventional SLM scheme, the first proposed approach has slightly worse PAPR reduction performance and the second proposed one reaches almost the same PAPR reduction performance. Although these two proposed SLM schemes can provide pretty good PAPR performance with very low computational complexity, the conversions they use may attenuate the transmitted signal power on some subcarriers, and would degrade the bit-error-rate (BER) performance of the OFDM systems. To alleviate the above BER degradation problem, we modify the two proposed SLM schemes by inserting a Hadamard transform before the IFFT operation. The purpose of the Hadamard transform used here is to spread the transmitted data to all subcarriers such that the signal space diversity of the transmitted data is increased. The increased computational complexity of the modified SLM schemes is not significant, because the Hadamard transform can be implemented efficiently by a fast algorithm. Computer simulation results show that the modified SLM approaches not only gain BER improvements over the original proposed SLM schemes, but also have better PAPR reduction performance than the conventional SLM scheme.