Research On Relay Cooperation In Distributed Beamforming System

Cooperative communication can utilize wireless networks resource to achieve high-speed and reliable communication. MIMO technology provides the method for cooperative communication to efficiently utilize network resource. Cooperative communication can utilize relay nodes to form a virtual MIMO, which provides a condition of using MIMO technology in wireless networks. Since beamforming can achieve full diversity and high power- efficiency in MIMO system, it can be utilized in cooperative communication known as distributed beamforming for meeting users'traffic needs. Therefore, it is important to understand distributed beamforming, which can develop the application of MIMO technology in cooperative networks.The main idea of this dissertation is to find the difference between distributed beamforming and MIMO beamforming, and to propose main problems of relay cooperation in distributed beamforming, then to provide useful solutions.Firstly, since the received data is corrupted by channel fading and noise and the transmitted data contains noise or error at relay nodes, the beamforming design should consider the affection of relay processing. In this dissertation, we consider distributed beamforming base on Demodulate-and-Forward, and propose a design rule for the beamforming vector. Simulations show that the proposed scheme can mitigate the error propagation and achieve the tradeoff between the system complexity and performance. Opportunistic collaborative beamforming based on thresholds is proposed to reduce the complexity of relay nodes and the difficulty of obtaining global channel state information (CSI) at the destination. One the basis of the computed thresholds, the received SNR is obtained by theoretical analysis and the optimal cooperation region is also discussed in the proposed scheme. Simulations indicate that the proposed scheme can reduce the computation complexity of the beamforming vector and difficulty of obtaining global CSI.Secondly, since feedback link of distributed beamforming is broadcast channel, feedback error more frequently happens than MIMO beamforming. Due to independently decoding feedback information, relays may obtain the different feedback information each other. An analytical framework is proposed to obtain the affection of feedback error on system performance, and then the methods to restrain feedback error are discussed. In single-relay selection system, is achieved. Power allocation between data transition and information feedback phase is proposed to maximize the upper bound of data transmission rate. Simulations show that the proposed scheme can improve data rate and relay number is limited by feedback error. In more-relay selection system, we discuss index mapping in feedback phase and propose a low-complexity designing method to maximize the received SNR at the destination. Theoretical analysis and simulations show that the proposed index mapping can obtain higher SNR than random index mapping and SNR loss induced by feedback error grows linearly with feedback error probability. In distributed beamforming system, beamforming codebook is designed to maximize the received SNR. Due to high complexity of the optimal codebook, a low-complexity method to design codebook is proposed based on vector quantization (VQ) codebook. Simulations show that the proposed codebook outperforms VQ codebook.Finally, in multiple-user cooperative networks, interference between users is the major problem of influencing data transmission rate. Distributed beamforming is utilized to cancel the interference in this dissertation. Compared to MIMO systems, due to limited resource and independently decoding the received signals at relays, relays cannot completely cancel the interference in Multiple Access channel-Broadcast channel (MAC-BC) scheme. Therefore, we focus on interference alignment with distributed beamforming at relays. Condition on users and relay antennas, the least relay number is given to achieve degree of freedom (DoF) of interference channel. The beamforming vectors are also obtained and optimized to improve system performance. Simulation shows the proposed scheme can achieve DoF of interference channel with the least relays and efficiently improve data transmission rate.