Key Technologies Of Cooperative Communication

Due to its advantages in improving transmission rate, combatting multipath fading and enhancing the quality of communications, cooperative communication can obtain the diversity gain without increasing the hardware cost, which helps to enhance the effectiveness and reliability of the wireless communication system, and meet users’ growing demand for real-time multimedia services. Hence, cooperative communication has attracted many attentions. Recenlty, related issues in cooperative communication, such as wireless resource allocation, relay selection, physical-layer security, energy harvesting and the relationship between nodes have triggered a great deal of rearch interests. Based on these rearch topics, we further investigate some key technologies of cooperative communication in this dissertation, and the main research works and innovations are summarized as follows:1. We consider the sparse beamforming design in multiple multi-antenna amplify-and-forward relay networks. Based on the concept of group sparsity from compressed sensor, a group-sparse penalty with a mixed 2 1?/ ? regularization is introduced into the objective function to design beamformers for balancing the size of active relays and the signal-to-noise ratioat the destination, and thus reduce cooperation overhead. To deal with the non-convexity and non-smoothness of the problem, the proximal gradient algorithm is proposed, which can converge to a local optimal solution in theory. The simulation results show that the proposed algorithm can yield the high level of SNR performance with a small set of cooperative relays to reduce cooperation overhead and economize power.2. The physical-layer security issue in the multiple non-regenerative wireless-powered relay(WPR) networks is investigated in this dissertation. In this network, WPRs are supposed to harvest energy to assist the source-destination transmission when active. However, when the relay is idle, it may become a potential eavesdropper in the network, and maximize its decoding signal-to-noise ratio, rather than split power for cooperative transmission. To improve the security of the source-destination transmission, the destination-based artificial noise is applied to our scenario to degrade the receptions of idle relays, which also becomes a new source of energy powering relays to forward the information with power splitting technique. We propose an efficient algorithm ground on block-wise penalty function method to jointly optimize power-splitting ratio and beamforming to maximize the secrecy rate. Despite the nonconvexity of the considered problem, the proposed algorithm is numerically efficient and is proved to converge to the local optimal solution. Simulation results demonstrate that the proposed algorithm outperforms the benchmark method.3. We further investigate the physical-layer security issue in a wireless-powered untrusted relay network. Source and destination are powered by converntional energy supplies, while the relay is wireless-powered by harvesting energy. Different from our previous works, the considered relay is untrusted, which can potentially wiretap the legitimate information while forwarding. Similarly, to guarantee secure transmission, the artificial noise is sent by destionation to degrade the channel condition of relay, which can be cancelled in the second hop at the destination. According to Semidefinite Programming, global optimal algorithm(GOA) and local optimal algorithm(LOA) are proposed to maximize destination’s secrecy rate by jointly designing power-splitting ratio and relay beamforming. Simulation results show that both GOA and LOA significantly outperform the benchmark(Naive amplify-and-forward algorithm, NAF), and LOA can yield satisfactory secrecy rate performance compared with GOA in the whole SNR region, but with less computation time.4. We investigate the influence of trust degree between users on the cooperative strategies. Traditionally, cooperative users are often selected mainly based on their traffic demands and their corresponding quality of physical channels. The selected user may be willing to sacrifice some of its channel because sometime in the future it may have an unreliable link and require assistance. Although numerous benefits can be obtained through user cooperation, trust should be further taken into consideration when we design transmission strategies since each hand-held device is carried by human being. In this dissertation, we investigate user cooperation scheme with respect to the trust degree for a four-node scenario: User1 transmitter(Tu1), User2 transmitter(Tu2) and their corresponding receivers Ru1 and Ru2, and assume that Tu2 has certain probability to help Tu1’s transmission if and only if the Quality of Service(QoS) requirement of Ru2 can be guaranteed. Based on the different number of antennas at Tu1 and Tu2, we investigate the influence of trust degree on the strategies of user cooperation for three cases(SISO, MISO, MIMO). Simulation results show that with the increasing trust degree, Tu1 can get more help from Tu2 when the traffic demand of Ru2 is satisfied, thus Ru1 can obtain higher expected achievable rate.