Study Of Wireless Cooperative Transmission Strategies With Performance Analysis

Efficiency, reliability and security are usualy used to measure the performance of a communication system. Compared to wire communications, it is more chanllegning to guarantee efficiency, reliability and security in wireless communications. On the one hand, multipath fading may lead to distorted signals and low signal-to-noise ratio at the receivers, and hence affects efficiency and reliability of the transmissions. On the other hand, due to the open nature of wireless channels, secure messages may be intercepted by eavesdroppers, so the transmission security is offen at high risk. In recent years, MIMO technique and cooperative communication technique are recognized as effective approaches to enhance the performance of wireless communications, which attract a lot of attention in both academic and industrial areas. The crucial content of the technique of wireless cooperative communications is to design effective cooperative transmission strategies, which can improve efficiency, reliability and security in wireless communications.It is well known that multiplexing gain and diversity gain are usualy used to evaluate efficiency and reliability in MIMO wireless systems, respectively. Existing works have shown that a higher multiplexing gain will lead to a lower diversity gain, vice versa; and Diversity-Multiplexing Tradeoff (DMT) reflects the tradeoff relationship between diversity gain and multiplexing gain. Therefore, DMT is a suitable information-theoretic criterion to evaluate the efficiency and reliability in wireless transmissions. In addition,"secrecy rate", the rate of a secret message, is usually used to measure the security level of wireless transmissions. Here a message is said to be "secret" if it can be correctly decode by logical users, but cannot be intercepted by eavesdroppers.For several typical wireless cooperative channel models, this dissertation is devoted to proposing wireless cooperative strategies which can enhance the DMT performance or enlarge the secrecy rates. For combating multipath fading, more effective multi-relay cooperative transmission strategies will be proposed, which can enhance the DMT performance; for combating eavesdropping, more effective hybrid cooperative transmission strategies will be proposed, which can enlarge the secrecy rates. The main works and contributions are listed as follows: 1) For the multi-relay point-to-point channel model, this dissertation proposes a cooperative transmission strategy which is based on the concept of "relay reuse". The basic idea is to use two half-duplex relays to mimic a full-duplex relay, which can overcome the half-duplex constraint. Moreover, this strategy combines the layered relay selection and the linear zero-forcing detection, where the layered relay selection is to ensure the optimal diversity gain, and the zero-forcing detection with low complexity is to cancel inter-relay interference. The theoretic analysis and Monte Carlo simulations show that, the proposed strategy can enhance the DMT performance and approach the MISO upper bound of the system.2) For the multi-relay multi-access channel model, this dissertation proposes a cooperative transmission strategy which is based on a superposition code and the "relay reuse" concept. The key feature of this cooperative strategy is that, it ensures each source message can be transmitted by each source and each relay. So this cooperative strategy realizes the "full cooperation" which is formed by the cooperation between sources and the cooperation from relays, and can achieve the optimal diversity gain of the considered system. The theoretic analysis and Monte Carlo simulations have shown that, the proposed strategy can enhance the DMT performance and approximately achieve the MISO upper bound of the system.3) For the relay-eavesdropper channel, this dissertation proposes three hybrid cooperative transmission strategies:the one based on noisy network coding (NNC), the one based on partial decode-forward (PaDF) and the one based on artificial state (AS). The first cooperative strategy combines the NNC compress-forward technique and the interference-assisted technique, where NNC is to enhance the communication capability between logical users, and the interference-assisted technique is to confuse the eavesdropper. So it can enhance the main channel and suppress the eavesdropper channel at the same time. The second cooperative strategy utilizes some basic encoding and decoding methods, such as superposition coding and backward decoding, in order to combine the partial decode-forward technique and the noise forward technique, which can enhance the communication capability between logical users and suppress the eavesdropper channel. The third cooperative strategy combines artificial state and the noise forward technique. Here artificial state is generated at the source, which can associate with the interference generated at the relay to effectively confuse the eavesdropper, but does not affect the destination. The key feature of this strategy is that the relay does not receive any signals from the source, and hence is not affected by the source-relay channel condition.In addition, for the Gaussian relay-eavesdropper channel, the above three cooperative transmission strategies are specified according to Gaussian characteristic. Furthermore, their achievable secrecy rates are analyzed based on information-theoretic security. Analysis and numerical results have shown that the proposed cooperative strategies can enhance the system’s secrecy level.4) For the conferencing multiple access channel with an eavesdropper (CMAC-E), this dissertation proposeds a hybrid cooperative transmission strategy, which is based on Willems’coding. The basic idea is to combine Willems’coding and Wyner’s random binning; the key feature is that it can enhance the communication capability from sources to the destination via the "conference" links, and suppress the eavesdropper’s capability via the multiple access channel. Moreover, the inner bound (i.e., achievable secrecy rate region) and outer bound on the secrecy capacity of the addressed channel is analyzed based on information-theoretic security.For the Gaussian CMAC-E channel, two particular transmission schemes are proposed:the multiplexed scheme and the cooperative time division multiple-access (TDMA) scheme. Analysis and numerical results have shown that the "conference" links between sources can effectively enhance the system’s secrecy level and each source’s secrecy rate.