Cognitive Radio Spectrum Access with Prioritized Secondary Users

Cognitive radio has been widely studied as one of the potential approaches for inefficient usage of frequency resources. In cognitive networks, secondary users not having a license for spectrum usage are allowed to opportunistically occupy an idle spectrum owned by a licensee named primary user. This paper considers multiple primary channels and three types of calls: primary calls, high priority secondary calls, and low priority secondary calls. The primary calls have the highest priority to use the channels and can reclaim any channel used by secondary ones. Therefore, the presence of secondary calls is entirely transparent to the primary ones. In this paper, a channel reservation scheme for high priority secondary calls is considered to reduce their blocking probability. Numerical results indicate that the channel reservation scheme can cause performance degradation such as significant increase of blocking probability and significant decrease of throughput for low priority secondary calls. To compensate the performance degradation due to channel reservation, we introduce a buffer for low priority secondary calls. Analytic model is suggested to characterize the effect of the channel reservation and the buffering on performance of secondary calls. Based on this model, we evaluate the blocking probability, the forced termination probability, and the throughput for both high and low priority secondary calls under various buffer sizes. Mathematical analysis shows that the buffering of low priority secondary calls can significantly decrease their blocking probability and increase their throughput.