Research On Relay Selection And Power Allocation In AF Cooperative Communication System

With the development of wireless communication technology, the requirements of wireless communication services to people are increasing. Wireless communication is moving towards the high speed, high reliability, low power consumption, wide coverage. Diversity that can combat multipath fading effect and channel interference becomes a very important technology to improve communication transmission quality. MIMO (multiple input multiple output) can obtain space diversity by setting multiple antennas at sender or receiver and receives wide attention. However, in many situations, due to the limited size, cost, power and other constraints, it is not practice to equip multiple antennas at some terminals. This limitation motivates the concept of cooperative communication which means that single antenna terminals share antennas of each other to generate a virtual MIMO and could achieve space diversity. Cooperative communication can extend system coverage, improve communication quality and reduce transmission power. Cooperation communication also has wide application prospect, such as Cellular Network, Ad hoc, WLAN and WSN.This thesis first overviews the diversity and combing techniques and introduces the basic concepts and principle of cooperative diversity. Next we analysis and compare three cooperative protocols and introduce the general solution of relay selection and power allocation problems in cooperative communication. Then based on Amplify-and-Forward protocol, we focus on searching the relay selection and power allocation problems in cooperative communication in which the instantaneous CSI (channel state information) is known. For different system models we propose relay selection algorithms and power allocation schemes. Simulation results demonstrate the efficiency of our proposed strategies to improve system performance, which has certain conductive significance for designing cooperative communication system.According to the number of selected relay nodes, we divide the relay selection problem into single relay selection and multiple relay selection. We start from researching the single relay selection problem and give three selection schemes. And taking the system BER performance as the evaluative standard we illustrate the improvement on system performance by comparing these schemes. Then we propose a simple iterative selection algorithm for multiple relay selection. This algorithm uses the features of received SNR and chooses the best relay nodes to participate in cooperative communication. It has lower complexity and can effectively improve the system BER performance. Then we propose an AF cooperative scheme that is combined space time coding with relay selection. It takes advantage of space time coding and relay selection and effectively improves the system performance.According to the number of users and relay nodes, we give three AF cooperative communication models, single user single relay, single user multiple relays and multiple user multiple relays. We start from the single user single relay model and discuss the design of power allocation scheme. Then we put more efforts into the study of power allocation problem for the other two models. A power allocation scheme based relay selection is proposed for single user multiple relay nodes system. This algorithm has low complexity and positive effect. For multiple users multiple relay nodes system we consider the total relay power consumption and the fairness among users and propose two relay power allocation schemes based on convex optimization theory. One is min total relay power based power allocation and the other is max-min SNR based power allocation. The idea of schemes is to improve utilizations of relay's power, reduce relay power consumption under the guarantee of the transmission quality for each user and balance the difference to each other under the total relay power limited constraints.