Study On Cooperative Diversity Technology Based On Cognitive Radio

Nowadays, with the fast growth of high data transmission service such as broad band Internet and multimedia,the demands for high speed wireless access have been rapidly increasing. Due to the bad environment in wireless communication, there are many technical problems in broad band communication. Aiming at the limited wireless frequency resource, it is one of technical problems to fully improve usage efficiency. As a smart spectrum sharing technology, cognitive radio can sense the wireless communications environment with the support of artificial intelligence; changing system parameters automatically according to certain learning and decision making algorithms; testing idle spectrums dynamically and make efficiently use of them. Theoretically, it allows spectrum to multiplex and share in time, space and frequency domain. Spectrum sensing is one of the bases of constructing a cognitive radio system. Collecting statistical information about wireless environment can not only provide assistance for higher level spectrum management but also some data to support the power control of the system. Cooperative diversity is one of the effective methods to sense spectrum, which can improve agility and reduce detection time. Cooperative detection can restrain multipath fading and shadow effect to a great extent, also it can improve the detection efficiency even in sever shadow environment. In this dissertation the main contents are:(1) By analyzing and summarizing the current researches of cooperative diversity schemes in cognitive radio, we have established the mathematical models of AF (Amplify - and -Forward),DF (Detect - and - Forward) and CC (Code Cooperation), focusing on the situation that there is only one relay between source node and destination node. At the same time, we have analyzed and compared the transmission performance of the three cooperative diversity schemes in Rayleigh channel, proving that they could provide cooperative diversity to combat fading.(2) Power allocation method is one of key technologies to put cooperative diversity into practice. It can efficiently improve the useage of resource by optimizing the transmit power for each user according to channel conditions. Taking the condition of one relay between source node and destination node for example, when the total transmission power keeps the same, we have put forward and analyzed the optimistic power allocation method for AF and DF by minimizing average bit error rate. (3) We have expanded the condition of one relay to multi-relays, and all the relays arranged in parallel. Then we have established the mathematical models of AF,DF and CC, analyzed and compared the transmission performance and application of the three cooperative diversity schemes in Rayleigh channel.(4) People have paid more attention to the transmission of multi-hop in wireless system. Therefore, when all the relays are in series, we have established the mathematical models of AF,DF and CC, analyzed and compared the transmission performance of the three cooperative diversity schemes in Rayleigh channel.