A Low-Complexity SLM PAPR Reduction Scheme for OFDMA

In orthogonal frequency division multiplexing (OFDM) systems, selected mapping (SLM) techniques are widely used to minimize the peak to average power ratio (PAPR). The candidate signals are generated in the time domain by linearly mixing the original time-domain transmitted signal with numerous cyclic shift equivalents to reduce the amount of Inverse Fast Fourier Transform (IFFT) operations in typical SLM systems. The weighting factors and number of cyclic shifts, on the other hand, should be carefully chosen to guarantee that the elements of the appropriate frequency domain phase rotation vectors are of equal magnitude. A low-complexity expression is chosen from among these options to create the proposed low-complexity scheme, which only requires one IFFT. In comparison to the existing SLM technique, the new SLM scheme achieves equivalent PAPR reduction performance with significantly less computing complexity. MATLAB tool is used for simulating the proposed work

저복잡도 후보 OFDM 신호 생성을 이용한 새로운 PTS 방법

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2014. 2. 노종선.This dissertation proposes several research results on the peak-to-average power ratio (PAPR) reduction schemes for the orthogonal frequency division multiplexing (OFDM) systems. The PAPR is the one of major drawback of OFDM system which causes signal distortion when OFDM signal passes through nonlinear high power amplifier (HPA). Various schemes have been proposed to reduce the PAPR of OFDM signals such as clipping, selected mapping (SLM), partial transmit sequence (PTS), active constellation extension (ACE), companding, and tone reservation (TR). Among them, PTS scheme can transmit an OFDM signal vector by generating many alternative OFDM signal vectors using the partitioned subblock signals and selecting the optimal OFDM signal vector with the minimum PAPR. However, the PTS scheme requires large computational complexity, because it needs many inverse fast Fourier transforms (IFFTs) of subblock signals and lots of alternative OFDM signal vectors are generated. In this dissertation, we concentrate on reducing the computational complexity of the PTS scheme. In the first part of this dissertation, we propose a new PTS scheme with low computational complexity using two search steps to find a subset of phase rotating vectors showing good PAPR reduction performance. In the first step, sequences with low correlation are used as phase rotating vectors for PTS scheme, which are called the initial phase vectors. Kasami sequence and quaternary sequence are used in this step as the initial phase vectors. In the second step, local search is performed based on the initial phase vectors to find additional phase rotating vectors which show good PAPR reduction performance. Numerical analysis shows that the proposed PTS scheme can achieve almost the same PAPR reduction performance as the conventional PTS scheme with much lower computational complexity than other low-complexity PTS schemes. In the second part of the dissertation, we propose another low-complexity PTS schemes using the dominant time-domain OFDM signal samples, which are only used to calculate PAPR of each alternative OFDM signal vector. In this PTS scheme, we propose efficient metrics to select the dominant time-domain samples. For further lowering the computational complexity, dominant time-domain samples are sorted in decreasing order by the proposed metric values and then the power of each sample is compared with the minimum PAPR of the previously examined alternative OFDM signal vectors. Numerical results confirm that the proposed PTS schemes using new metrics show large computational complexity reduction compared to other existing low-complexity PTS schemes without PAPR degradation. In the last part of the dissertation, for the reduced-complexity PTS scheme, a new selection method of the dominant time-domain samples is proposed by rotating the IFFTed signal samples to the area on which the IFFTed signal sample of the first subblock is located in the signal space. Moreover, the method of pre-exclusion of the phase rotating vectors using the time-domain sample rotation is proposed to reduce the number of alternative OFDM signal vectors. Further, three proposed PTS schemes are introduced to reduce the computational complexity by using simple OFDM signal rotation and pre-exclusion of the phase rotating vectors. Numerical analysis shows that the proposed PTS schemes achieve the same PAPR reduction performance as that of the conventional PTS scheme with the large computational complexity reduction.Docto