Research On Key Technologies In Physical Layer For OFDM Based Space-time Cooperative Communication Systems

The space-time (ST) cooperative orthogonal frequency division multiplexing (OFDM), denoted as ST Co-OFDM, is the integration of the cooperative communication, OFDM and distributed ST coding, which has many advantages to the traditional OFDM system, such as high frequency and power efficiency, improved reliability and large covering range. The ST Co-OFDM system, however, has two drawbacks, that is, high peak-to-average power ratio (PAPR) and sensitivity to carrier frequency offset (CFO), which become the focuses in the research field of cooperative communication.According to the characteristics of the ST Co-OFDM system, the main idea of the dissertation is to study some key technologies in the physical layer for ST Co-OFDM systems, and provide improved schemes. The contributions of the dissertation are as follows:Firstly, based on the existing protocols in Co-OFDM systems, the decision-feedback decode-and-forward protocol, named as the two-relay space-time coding (TR-STC), is proposed, where the single-relay (SR) protocol and Alamouti orthogonal ST coding are integrated. The performances of the average outage probability, pairwise error probability (PEP) and bit error rate (BER) are analyzed, and simulation studies are concluded in Rayleigh channels. It is shown that the proposed protocol achieves full diversity order, and significantly outperforms the SR protocol in outage probability, with the same bandwidth. Furthermore, the proposed TR-STC is more practical than the SR in energy-efficient networks, due to its transmission diversity.Secondly, like the OFDM system, the ST Co-OFDM system also suffers from the high PAPR, since the OFDM symbols are employed in data transmission. The common PAPR reduction schemes are investigated, where the clipping and filtering (CF) scheme and its improved algorithm are considered to be suited for the Co-OFDM system. An improved repeated clipping and filtering (RCF) scheme for PAPR reduction, named simplified repeated clipping and filtering with spectrum mask (SRCF-SM), is proposed for Co-OFDM systems, based on the idea of the in-band and out-of-band independent processing. The comparison of the computational complexity analyses is completed among different schemes, and simulation results demonstrate that the proposed scheme significantly outperforms the traditional RCF and SRCF ones in PAPR reduction, with very little increase of the computational complexity. It can also be concluded that the proposed scheme is especially useful in the circumstance that requires more PAPR reduction, and practical in implementation on digital signal processor (DSP) or field programmable gate array (FPGA), due to only three FFT/IFFT units required.Thirdly, since the signal distortion, caused by the clipping at the transmitter in the proposed SRCF-SM scheme, greatly degrades the system performance, the amplitude reconstruction scheme for the clipping signal at the receiver should be adopted. After investigating the traditional amplitude reconstruction schemes, the low-complexity improved iterative amplitude reconstruction (IIAR) scheme is proposed, based on the classical decision-aided reconstruction (DAR) scheme. The aim of the proposed IIAR is improving the BER performance of the DAR, with almost the same computational complexity. Simulation results show that the BER performance of the proposed scheme is much better than that of the DAR, and closed to that of the IAR in low modulation order.Finally, different from the multi-input multi-output (MIMO) system, the nodes in the ST Co-OFDM system are not only distributed in space, but also have their own oscillators, leading to the multiple timing and frequency offsets in the cooperative system. The ST Co-OFDM system can combat the timing offsets, since the OFDM with the long enough cyclic prefix (CP) insertion is robust to the time offsets; however, it is very sensitive to the multiple carrier frequency offsets (CFOs), resulting in serious inter-carrier interference (ICI) and inter-symbol interference (ISI), which is impossible for the destination to compensate simultaneously. To solve this problem, the data detection algorithms for ST Co-OFDM systems are discussed. According to the error propogation in maximum likelihood (ML) and iterative ML (IML) schemes, two simplified IML schemes are proposed, based on the sorting idea of the signal to noise ratio (SNR) and signal to interference plus noise ratio (SINR), respectively, whose main idea is that the signal is ordered in the decreasing way of SNR or SINR before iterative process begins. It is shown that the proposed schemes greatly outperform the existent ML and IML schemes, especially for the higher modulation order and larger number of subcarriers, which is practical in complex communication systems.