Research On Spectrum Access In Cognitive Radio Networks

With the development of the society and the wireless communication technology, the demand of the wireless spectrum resources is growing. As a result, wireless spectrum shortage becomes an abstacle of sustainable development of applying wireless communication and service. Cognitive radio spectrum access technology is one of the most important technologies to solve the problem.This paper mainly made a in-depth research of cognitive radio dynamic spectrum. The current research is mainly about how to ensure efficient access of secondary users with changing conditions, and avoid secondary users'interference to primary users as well as conflicts between secondary users themselves in the access process. Cognitive radio dynamic spectrum access is generally used in two situations: one is to share the frequencies dynamically in an open spectrum, the other is to find a way to be accessed to the segment which have been distributed to other systems.The CSMA mechanism, considered as a good way to solute the conflicts between users in wireless environment, is applied to cognitive radio system in this paper. To make CSMA mechanism adapt to the characteristics of cognitive system, a lot of improvements also have been done: the business of secondary users is graded and the counter access method of CSMA is improved according to the characteristics of cognitive radio; based on the dynamic change of cognitive radio environment, the dynamic adaptive retreats window algorithm is proposed based on 802.11 CSMA/CA. It is based on the number of secondary users in network, dynamic adjustment retreats window value, making its saturation throughput value of S maintain higher level. This paper make the comparative simulation of proposed algorithm, it is concluded that improved retreats counter access method saved access time and dynamic adaptive retreats window algorithm can effectively improve the system throughput.The fourth part mainly studied chance access algorithm based on partially observable markov decision process (POMDP). Secondary users uses the partially observable markov decision process model under the condition of partially perceiving the available channel. Secondary users firstly assess the probability of the available channel, but different secondary users may want to access the same channel at the same time, which makes it inevitable to grade assessment access. This paper was based on different secondary users to evaluate channel, using collaboration method to make different secondary users to competition access, making secondary users can smoothly finish communications. In order to improve the system throughput, secondary users use double antenna technology and can also access multiple channels based on perception results. According to the results of simulation comparison, it is found that the throughput of the system has been significantly improved.