Research On Key Technologies Of Cooperative Relay In Wireless Communication Systems

Recently, since the fast growing demands for high-data-rate, high-quality com-munications and high-network-capacity over wireless channels make the design and implementation of wireless communication systems more and more challenging, co-operative communications, which is based on multiple-input multiple-output (MIMO) technique, attracts much attention and extensive research. While one or more relays are equipped with, auxiliary channels can be built between the communication terminals. This technology can combat the wireless fading channels, expand the network cover-age and increase the network capacity. As a special form of distributed MIMO system, cooperative communication technique can be applied to cell networks, sensor network-s and ad hoc networks. Therefore, cooperative communication has already become a research hotshot in both academic and industrial society.In this thesis, recent research activities in multi-point cooperative communications are comprehensively addressed and systematically analyzed. This dissertation studies the key technologies of distributed cooperative communications and multiuser MIMO cooperative communications, the scopes of which cover the enabling technologies of the fourth and future generations of wireless mobile communication systems. The main contributions of the dissertation in physical layer lie in a new distributed space-time block codes in full-duplex cooperation, Orthogonal Frequency Division Multiplexing (OFDM) based STBC transmission schemes in asynchronous cooperative systems, and joint base station/relay station precoder design for multiuser MIMO relay systems.Compared to half-duplex mode, full-duplex mode can provide higher spectral effi-ciency in cooperative systems. In this dissertation, cyclotomic quasi-orthogonal space time block codes (QOSTBC) are proposed for full-duplex cooperative communication scenarios. With simple signal processing at the relays, the received signals at the des-tination node have the STBC structure. Then, the decoding can be performed by a joint method, and full cooperative diversity can be achieved. Besides, compared to that of conventional QOSTBC, the proposed codes achieve a lower decoding com-plexity without any performance degradation, and induces lower end-to-end delay in full-duplex mode. The proposed transmission scheme is evaluated and compared in terms of performance by monte carlo simulation.In ideal case, the different relay nodes are assumed to be synchronous. That is, these relay nodes simultaneously transmit the signals. Besides, not only transmit car-rier frequency but also the timing delay between relay nodes and destination node are same. However, in actual applications, since different relay nodes have different oscil-lators and different locations, multi-relay cooperative communication system is inher-ently time asynchronous. Consequently, it is very important to do the research on the time synchronization for these systems. This dissertation investigates Alamouti space-time transmission scheme for asynchronous cooperative systems, and extends it to the four-relay case by using cyclotomic QOSTBC. The adoption of OFDM technique has been fully taken advantage of, then it is possible to combat timing delay by preceding cyclic prefix with enough length. In addition, the relay nodes adopt advanced signal processing methods such as time-reversion. These methods solve the problem that the conventional STBC schemes are not effective in the asynchronous cooperative scenar-ios.In MIMO multi-user cooperative communications, the base station (BS) can trans-mit the data to the multiple users via relay station (RS) simultaneously. However, there exists inter-stream interference among different users. Then, precoding technique is adopted to make some kind of pre-disturbance to the transmitted signals so that it could combat with the adverse effects, and thus effectively improves system perfor-mance as well as simplifies the design of the receivers. This paper propose a joint BS/RS lattice-reduction-aided Tomlinson-Harashima Precoding (THP). As BS know the perfect channel state information by feedback technique, it is possible to transfer the complicated signal processing from user terminals to transmitters. Lattice reduc-tion technique is capable of transforming channel matrix to nearly orthogonal one. The fading channel and noises matrices between BS to RS and RS to users may cause the data transmission conditions deteriorate. Lattice reduction can reduce this influence, and improve detection performance. THP precoding is able to enhance the control of constellation boundary by decision feedback and improve the accuracy of decision. Simulation results show that the proposed scheme significantly improves system per-formance and achieves full diversity.The summary, research and innovation of the distributed STBC scheme and pre-coding scheme in the new generation of wireless communication systems enrich the theoretical results of cooperative communication, providing insights to the practical system design.