Research On Key Technologies In Multi-User Cooperative Communication Systems

Diversity is a technique which can combat fading and improve transmis-sion reliability in wireless communication. Cooperative communication, which can achieve cooperative diversity in a distributed manner, has become a key technique. With the increase of the users and the decrease of the broadband resources, the classic schemes cannot meet the needs any more. Network cod-ing can improve the processing ability of the nodes and bandwidth utilization. By the combination of network coding and cooperative communication, the performance of the communication system can be further improved. Based on network coding, this dissertation proposes cooperative schemes with better performance.Moreover, with the increasing scarcity of spectrum, cognitive radio, as a emerging technology of spectrum sharing, has been a key point. By employing cooperative communication in cognitive radio systems, the performance can be improved. However, current researches are mainly about the cooperative transmission between secondary users, there are fewer researches about the co-operative transmission between secondary users and primary users. And these researches only focus on one of the systems. In view of this situation, this dissertation studies the cooperative transmission between secondary users and primary users and proposes a novel cooperative transmission scheme which can ensure the performance of two systems.The major contributions of this dissertation are summarized below.Firstly, this dissertation proposes a dynamic bit-xor scheme for a system with two users and one relay. The theoretical analysis and simulation prove that compared with the incremental relaying scheme, the proposed scheme is better. Then, a dynamic bit-xor with opportunistic relay scheme for multi- user multi-relay networks is proposed. During the broadcast phase, the users transmit one’s own data one by one, which can ensure the fairness. During the relay phase, the best relay is selected. The selected relay dynamically applies network coding according to whether the source-destination channel suffers an outage and the source-relay channel is good. Thus, the relay can serve multiple users in a time slot. The outage probability and diversity-multiplexing tradeoff are analyzed. Theoretical and numerical results show that the proposed scheme can provide superior system outage performance to the compared scheme and yields a full diversity order.Secondly, based on liner network coding, a cooperative scheme is pro-posed for multi-user multi-relay networks. Compared with the existing scheme, the proposed scheme add a user selection and take full advantage of the multi-user feature, the outage performance and ergodic capacity are improved signif-icantly.Finally, by employing network coding and cooperative communication in cognitive radio system, this dissertation proposes a bit-xor based relay cooper-ative transmission scheme. The proposed scheme takes advantage of the coop-eration between the primary user and the secondary user. The secondary user decides whether to aid the primary user according to the channel condition from the primary user to the secondary user. In the cooperative mode, the secondary user applies network coding between the received signal and its signal, and for-wards the coded signal. Thus, the secondary user can access to the spectrum licensed to the primary user and serve the primary user, and the performance of the primary system can be ensured. Meanwhile, with the help of the cognitive relay, the performance of the secondary system can be improved. Theoretical analysis and simulation results verify that the proposed scheme can achieve a diversity gain and better outage performance than the existing scheme;...