Multi-access Control And Carrier Allocation In OFDMA-based Cognitive Radio System

Cognitive radio is the key technology to solve the problems of spectrum shortage and low spectrum usage. Cognitive radio is a wireless technology based on software defined radio to achieve the spectrum sharing of licensed system and cognitive radio system. Now, 802.22 and 802.16j have been proposed with cognitive radio technology in line. And furthermore, more protocols will use this technology. The article firstly introduces the concept of cognitive radio, then analyzes the characteristics of Distributed Cognitive Radio System and provides the test platform. Finally, multi-access control and carrier allocation have been researched.The multi-access control is the key point technology in the system. Two problems are presented in the multi-access control in the cognitive system. One is that the sender and the receiver may don't the same available channels set and they can't exchange control information at the same channel at first. The other is the traditional hidden terminal problem. With the above problems, a time-frequency control information exchanging method is proposed. The transceiver-receiver coordinates both available channel information and transmission time to have a more successfully access process.Meanwhile, the article analyzes the carrier allocation in multi-access process and lists some proposed methods of resource allocation in the literature. In this cognitive system, I mainly concern on two parts: choosing carrier number and deciding the access carriers and propose two algorithms to solve the problems. The algorithms aim at the maximizing system successfully access probability to guarantee users' equality. One algorithm simulation shows that the performance has an obvious improvement compared to MMAC algorithm introduced in the 4.2 section. Eventually, four factors that impact on the performance of the proposed algorithm are analyzed and evaluated through simulations, i.e. second user numbers, second users' activity, probability of the idle carriers and statistical transmission time of primary users. The simulation results demonstrate that the proposed algorithm could achieve fast system convergence and stability while suffering from the variation of the environment.In the distributed cognitive radio system, multiuser will share multicarrier, and cognitive users will have a more dynamic carrier usage with OFDMA modulation. This article analyzes the new problems presented in distributed cognitive radio system, and then mainly discusses the method of exchanging control information while the transceiver and receiver don't know exactly each other's carrier availability. Meanwhile cognitive users may make collision when they choose the same carrier to access at the same time. The system is based on software defined radio to implement the link-level transmission and signal processing on the hardware platform which exhibit a very good processing ability and transmission ability.