Optimization of Transmission Strategies for Opportunistic Access in Cognitive Radio Networks

Cognitive radio is a promising technology to mitigate spectrum shortage in wireless communications. It enables secondary users (SUs) to opportunistically access low-occupancy primary spectral bands as long as the primary user (PU) access is protected. Such a protection requirement is particularly challenging for multiple SUs over a potentially wide geographical area. In this paper, we study the fundamental limit on the throughput performance of cognitive networks under the constraint of packet collision probability with the PU. With perfect sensing, we develop an optimum spectrum access strategy suitable under generic PU traffic. Without perfect sensing, we quantify the impact of missed detection and false alarm, and propose a modified threshold-based spectrum access strategy that achieves close-to-optimal performance. Moreover, we develop and evaluate a distributed access scheme that enables multiple SUs to collectively protect the PU while adapting to changes in PU activity patterns. Our results in this paper provide useful insight on the trade-off between the protection of the primary user and the throughput performance of cognitive radios.