The Research On Cooperative Diversity For Cognitive Radio

Nowadays, with the development of high data transmission service such as Internet and multimedia, the demands for high speed wireless access have been rapidly increasing. However, high speed communication has two severe challenges in wireless communication environment: one is how to improve the bandwidth efficiency of limited radio spectrum; and the other is how to cope with more severe channels fading, such as frequency selective fading and time selective fading to improve the transmission quality of signals in the high speed communication, which is also a very difficult task. Cognitive radio is a new paradigm in wireless communications that promises an enhanced utilization of the limited spectral resource. Cooperative diversity can achieve diversity order to improve the performance of capacity and error rate, though we can not input several antennae in one mobile terminal to achieve diversity order because of the size limitation. Because of the scarcity of spectral resources, cognitive radio arises as a hot research issue in wireless communications. Cooperation is increasingly regarded as a key technology for tackling the challenges of a practical implementation of cognitive radio.Two main challenges towards the implementation of the principle of cognitive radio appear to be: 1,Spectrum sensing: Cognitive users need to monitor the available spectrum, which is spectral hole. A typical way to address the problem is to look for primary transmissions by using a signal detector. The most important problem is how to detect the primary user quickly and effectively. In order to enhance the performance of primary detectors, a natural solution that is able to cope with different fading conditions is cooperation. The basic idea is to employ distributed detection at the cognitive nodes: each node measures the local received signal, then local signals are exchanged and finally a global decision on the primary activity is achieved. This approach is clearly robust to possible unbalance of the channel qualities of different secondary users and shows to achieve a drastic improvement of agility gain. 2,Transmission opportunity exploitation: Once a spectral hole has been identified, how do secondary users exploit the transmission opportunity? Firstly they should make their activity transparent to the primary users, and they have to vacate the spectrum if detecting primary users. Secondly they should maximize their own performance. Secondary users might be competing for the resource, or cooperating in order to improve efficiency and fairness of resource sharing. It's obviously the solution of both problems illustrated above can greatly benefit from cooperation.The background and meaning of the research, as well as basic knowledge and concept of cognitive radio and cooperative diversity is introduced in this thesis first. A brief overview of the envisioned applications of cooperative technology to cognitive radio is introduced. After analyzing the development of cognitive radio and cooperative diversity, the basic problem and main target of the paper is confirmed. Then we focus on two aspects: cooperative sensing for detection of the primary activity and cooperative transmission. Through analyzing detailed models, the results of emulator and theory analysis show that the agility gain of the system and the throughput of secondary users are both improved when cooperative diversity applies to cognitive radio.