IFFT-based techniques for peak power reduction in OFDM communication systems




Ghassemi, Abolfazl

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Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission technique which provides efficient bandwidth utilization and robustness against time dis¬persive channels. A major problem in the RF portion of a multicarrier transmitter is Gaussian-like time-domain signals with relatively high peak-to-average power ratios (PA¬PRs). These peaks can lead to saturation in the power amplifier (PA) which in turn distorts the signal and reduces the PA efficiency. To address this problem, numerous techniques have appeared in the literature based on signal and/or data modification. In the class of distortionless techniques, partial transmit sequences (PTS), selective mapping (SLM), and tone reservation (TR) have received a great deal of attention as they are proven techniques that achieve significant PAPR reduction. However, high compu¬tational complexity is a problem in practical systems. In PTS and SLM, this complexity arises from the computation of multiple inverse fast Fourier transforms (IFFTs), resulting in a complexity proportional to the number of PTS subblocks or SLM sequences. TR has also a high computational complexity related to the computation of the IFFT as it must search for the optimal subsets of reserved subcarriers and generate the peak reduction signal. In addition, most research in the direction of analyzing and improving the above techniques has employed direct computation of the inverse discrete Fourier transform (IDFT), which is not practical for implementation. This thesis focuses on the development and performance analysis of the major distortionless techniques in conjunction with the common IFFT algorithms to reduce the peak-to-power average (PAPR) of the original OFDM signal at the transmitter side. The structure of the IFFT common algorithms is used to propose a class of IFFT-based PAPR reduction techniques to reduce the computational complexity and improve PAPR performance. For IFFT based PTS, two techniques are proposed. A low complexity scheme based on decimation in frequency (DIF) and high radix IFFT algorithm is proposed. Then, a new PTS subblocking technique is proposed to improve PAPR performance. The periodic auto-correlation function (ACF) of time-domain IFFT-based PTS subblocks is derived. To improve the PAPR, we use error-correcting codes (ECCs) in the subblocking. Our approach significantly decreases the computational complexity while providing comparable PAPR reduction to ordinary PTS (O-PTS). With IFFT-based SLM, a technique for reducing computational complexity is proposed. This technique is based on multiplying the phase sequences with a subset of the inputs to identical inverse discrete Fourier transform (IDFTs). These subsets generate the partial SLM sequences using repetition codes. It is also shown how the partial time-domain sub-sets can be combined to generate new SLM sequences. These sequences do not requires any IFFT operations. The proposed scheme outperforms the existing techniques while pro¬viding comparable PAPR reduction to original SLM (O-SLM). Finally, a gradient-based algorithm is proposed for IFFT-based TR. Unlike previous work, non-static channels are considered where the peak reduction tones (PRTs) locations and consequently the peak reduction kernels should be adjusted dynamically for best per¬formance. Two low complexity algorithms with different degrees of computational com¬plexity and PAPR performance are proposed. To generate the peak reduction kernels, the transform matrices of identical IFFTs are used. This provides low complexity solutions to determining the PRTs and computing the peak reduction kernels.



Orthogonal frequency division multiplexing, Inverse fast Fourier transforms, Communications