Cooperative Relaying Technique In Wireless Communications Systems

As a new technique in wireless communications, cooperative relaying is able to coordinate multiple single-antenna device to form “virtual antenna array” by distributed signal transmission and processing, which can achieve a new form of space diversity, called “cooperative diversity”. Since this technique can effectively improve many key performance specifications of wireless transmission, such as throughput, link reliability, coverage, etc, it has attracted considerable attentions and has been extensively studied in the past decade.The existing works on cooperative relaying can be divided into two aspects: wireless transmission reliability and wireless transmission security. In wireless transmission reliability,the research works mainly focus on opportunistic relaying strategy design in multiple-relay systems in order to improve the reliability of wireless unicast. On one hand, these works generally select one relay out of all available relays to assist the transmission, which may not be able to maximally enhance the end-to-end performance of wireless transmission.Hence, how to design the relay selection that can appropriately choose a group of relays to further improve the end-to-end performance of wireless transmission is an important issue in cooperative relaying. On the other hand, as an emerging wireless service, wireless multicast needs to deliver messages to a group of users simultaneously, which make its transmission reliability more vulnerable than wireless unicast. Therefore, it is necessary to design new cooperation strategy for wireless multicast to ameliorate its reliability. In wireless security,most existing works consider to use cooperative relaying technique to combat against the passive eavesdropping attack and thereby improve its performance in physical-layer security. However, in practical wireless scenario, artificial jamming may be another attacking way to degrade the secrecy performance of legitimate systems. Thus, designing the cooperative scheme with considering both eavesdropping and jamming is one of key problems in physical-layer security.To tackle the aforementioned problems, cooperative relaying technique is investigated in this dissertation to improve both reliability and security of wireless communications. The main contributions of this dessertation is outlined as follows.1. Relay-aided cooperative unicastConsider that the cooperative unicast is assisted by multiple relays, the relay cooperation factor is defined based on the first order derivative of end-to-end signal-to-noise ratio. With this factor, an iterative multiple relay selection scheme is designed. By taking advantage of the relay cooperation factor is only related to the local information of relays, a distributed implementation of this scheme is proposed, where the distributed multiple relay selection is realized by the local timer of each node and signaling exchange among all nodes. Theoretical and simulation results suggest that the proposed scheme can achieve quasi-optimal performance at a complexity of O(K2)(K is the number of relays), much lower than the one of O(2K) of optimal multiple relay selection.2. Relay-aided cooperative multicastThe relay-aided cooperative multicast is studied in underlay spectrum-sharing scenario. A best-relay-selection based cooperative multicast scheme is proposed, where the end-to-end signal-to-noise ratio of multicast transmission is maximized by relay selection. Then, the outage performance of proposed scheme is studied over independently but non-identically distributed Rayleigh fading channels. Both exact outage probability and asymptotic outage probability in high signal-to-noise ratio are derived in closed-form. Theoretical analysis results show that 1) the diversity order of proposed cooperative multicast scheme equals the number of relays plus one, i.e., the full diversity gain is achieved; 2) the number of multicast users, the number and of primary users interference tolerance only have impact on coding gain of proposed scheme; 3) The number of relays also effects the coding gain of proposed scheme. The complexity analysis reveals that, the complexity of proposed scheme is merely O(K + N) with K being the number of relays and N being the number of multicast users.Overall, the proposed scheme is a low-complexity and full-diversity cooperative multicast scheme.3. User-aided cooperative multicastThe user-aided cooperative multicast is investigated. A best-user-selection based cooperative multicast scheme is proposed, where the best user is selected out of all successful decoding multicast users to serve as a relay and forward its received messages. Compared with relay-aided cooperative multicast, this scheme does not use dedicate relays and thereby saves the cost in infrastructure deployment. The outage performance analysis shows that the diversity order of proposed best-user-selection based cooperative multicast scheme equals the number of multicast users. Thus, its outage probability decreases minus-exponentially reduces with the number of multicast users. It means that the proposed scheme can overcome the drawback of the conventional ones, where the reliability is degrading with number of multicast users increasing. To further reduce the outage probability of proposed scheme,time allocation is also studied to determine the durations used by source and selected user. A near-optimal time allocation strategy is derived, where the time allocation factor is obtained by simple root-finding algorithm, such as bisection search.4. Cooperative relaying in wireless physical-layer securityBy considering the adaptive eavesdropping/jamming attack from eavesdropper, three best relay selection rules, called BRS-F-ECSI, BRS-P-ECSI and BRS-S-ECSI, are proposed to minimize the secrecy outage probability in the cases that F-ECSI, P-ECSI and S-ECSI are known by legitimate system, respectively. The secrecy outage performance of proposed rules are investigated in Rayleigh fading. The closed-form expressions of secrecy outage probability are derived for BRS-F-ECSI and BRS-S-ECSI, while an approximation of secrecy outage probability are derived for BRS-P-ECSI. Furthermore, the asymptotic expressions for lower bound of secrecy outage probability are also obtained in high main-to-eavesdropper ratio and then the secrecy diversity order of proposed relay selection rules are analyzed. Theoretical results show that, all the three proposed rules can achieve full secrecy diversity, i.e.,their secrecy diversity order is equal to the number of relays.