The Optimal Relay Selection And Power Allocation Under Multi-Path Fading Channels

Currently, the mobile telephone services, data services, and multimedia services in wireless communications are developing very fast. The future wireless network will provide more high quality services, and this requires a more fast information transmission rate. In the wireless communication system, the multi-path fading effect greatly influences the quality of the wireless communications. The system which uses the multi-input and multi-output antennas (MIMO) greatly enhances the capacity of the system. The virtual MIMO system is thus proposed and is referred to as the cooperative communications. In this system, it employs the cooperative method to construct the distributed MIMO system and achieve the performance of a MIMO system. The problems of low signal to noise detection channel coding, and service outage in wireless communications can be effectively and relatively solved by using the cooperative communications. The objective of the cooperative communications is to effectively utilize the node resources in a network to help the node which has transmission needs to communicate more robustly and in a higher rate. The transmission process in the cooperative communication systems consist of the source node and the relay nodes. The transmission of information needs to consume energy of the source node and the relay nodes, and the diversity gain of this system is achieved at this cost. Thus, a key problem in the cooperative communications is how to select the optimal relay node and allocate the power between the source and the relay nodes.This thesis consists of the following sections: ●It first introduces the basic knowledge in the wireless communications, such as the wireless channel, its model and the diversity technique at the receiver. It introduces some common diversity methods and the corresponding combing methods at the receiver and compares the performance of these methods through computer simulations.●It then describes the basic model of cooperative communications and the system model of binary receiver under the multi-path fading channels and these knowledge is needed when understanding the optimal relay selection in multi-path channels.●In cooperative communications, the relay selection is an important research direction and a lot of experts have conducted a lot of research on it. However, in the previous research, they only consider the relay selection in the single path channel. In reality, the communication channels are multi-path as discussed above. For this reason, this thesis proposes a new optimal relay selection algorithm under multi-path fading channels to reduce the symbol error rate in this amplify-and-forward system. Simulation results demonstrate that by utilizing the proposed algorithm, the optimal relay node is not the optimal relay node when only considering the single-path channel. Thus, the proposed algorithm is closer to the reality and has better performance in real cooperative communication systems.●It reviews and describes the particle swarm optimization (PSO). Then it analyzes the system model of cooperative communications. Under the total power restriction, the lower bound or upper bound of bit error rate is set as an objective function of PSO to decide the power allocation between the source node and the relay node. The computer simulation shows that, in multi-path channels, the bit error rate of the optimal power allocation scheme through PSO is obviously lower than that of the average power allocation scheme, and it shows the superiority of the proposed method.In summary, this thesis has conducted the research of optimal relay node selection and allocated the power in the cooperative communication systems under multi-path channels. The proposed algorithm is closer to reality and greatly improves the system performance, i.e. under the same symbol error rate requirement, it greatly reduces the total transmission power.