On Distributed Beamforming Technique For Wireless Cooperative Networks

In recent years, cooperative communications have attracted a lot of attention fromboth academic and industry owing to its strong capability of boosting throughput, en-hancing transmission reliability and etc.. Via sharing antennas with other scattering re-lay nodes, cooperative technology aims to form a virtual multiple-input-multiple output(MIMO) system. In this sense, cooperative relaying is a distributed MIMO in essence.Therefore, many of the key technology in traditional MIMO can be inherited in co-operative communication systems. For cooperative relaying, much interest has beenfocused on the design of relay selection, distributed space-time codes and collabora-tive beamforming. This thesis investigates the distributed beamforming technique inwireless relay networks and the main contributions can be described as followsFirst consider a single-relay network with multiple-antenna base station anddecode-and-forward (DF) protocol. We derive exact expressions for the outage proba-bility and the symbol error rate (SER) in closed-form. Moreover, a simple asymptoticapproximation and a tight lower bound are presented for the outage probability and SERrespectively, based on which we propose two optimum power allocation schemes. Afterthat, we investigate a multiple relay-assisted hybrid cooperative beamforming system.In this system, a tight upper and lower bounds for the outage probability are derived inclosed-form. On the basis, achievable diversity of this hybrid relaying scheme has beenstudied. Numerical results are provided to validate our theoretic analysis.For one-way amplify-and-forward (AF) relay networks with two-hop informationleakage, a novel transmission scheme has been proposed to improve the physical lay-er security. Unlike traditional cooperative jamming (CJ) design, where only artificialnoise is exploited to degrade eavesdroppers’ channels, in our scheme, the jammer in thesame time confuse the eavesdroppers and help forwarding confidential signals to thedestination. The enhanced quality of the received signals at the destination contributesto the improvement of the secrecy rate. By utilizing the idea of two-level optimiza-tion framework, suboptimal solutions can be efficiently obtained. Simulations resultsdemonstrate the merits of the proposed scheme.For two-way secure AF relay network, a joint cooperative beamforming (CB) andartificial noise (AN) design is proposed under individual relay power constraints. Basedon secrecy sum rate maximization criterion, such joint design problem is mathematical-ly hard due to the nonconvex objective function. With the help of the semidefinite re- laxation (SDR) technique and successive convex approximation (SCA), a high-qualityapproximate solution can be efficiently developed after iterations. Proof is also givento show that the rank-relaxation is always tight. Simulations are presented to validatethe efficiency and advantage of the proposed design and algorithm.