Relay Selection In Cooperative Communication

With the rapid development of wireless communications, people propose the increasing service demanding. A major factor is multi-fading which can limit the improvement of transmission rate and the quality of service in wireless communications. Space diversity technology can solve this problem without increasing the transmission power or bandwidth. MIMO (Multiple Input Multiple Output) technique can make full use of multi-path transmission to increase the performance of the systems, however, it is hardly implemented to real system directly. Cooperative communication is a new kind of space diversity, multiple mobile terminals with single antenna relay information for each other, thus the space diversity is formed, and the capacity of the system will be increase. But in the process of cooperative communication, there are several candidate relay nodes for the source node, how to choose appropriate relay node becomes particularly important.Firstly, this paper introduces the diversity techniques which are widely used in wireless networks, and then educes the concept of the cooperative communication. This paper describes the basic principles of cooperative communication and the kinds of the technology, and analyzes their advantages and disadvantages. Describe the specific types of relay selection algorithm, and analyzes the outage probability of cooperative communication system.Then, we focus on the multi-node relay communication system model, making use of moment generating function, we get the expression of the average symbol error rate in multi-node relay communication systems. Based on it, we conclude the relationship among the system SNR, the number of relay nodes and the average symbol error rate. Then we propose a new relay selection algorithm based on the statistical properties of the channels in AF cooperation system, in which we adopt the average symbol error rate as evaluation criteria, on the equal power condition, the number of relay nodes making the average of symbol error rate minimum depend on the current SNR of the system. The simulation shows that, at any SNR, the new algorithm in the performance of average symbol error rate is superior to the case all the relay nodes are involved or only the best relay node is involved. This paper also proposes a new simple algorithm which can determine the minimum SNR threshold when the system begin to cooperate with relay nodes, that is, when SNR is below a certain threshold, the performance of symbol error rate in direct forward system is lower than in cooperative communication system. Finally, on the condition that the optimal numbers of cooperative relays are determined, taking the limit of total power into account, we propose an algorithm that can improve the performance of average symbol error rate using power allocation.