Study on Protocol Design and Performance Analysis for Cognitive Relay Networks

This paper investigates the cooperative transmission in the downlink of a large-coverage secondary system which opportunistically accesses the licensed spectrum of some small-coverage primary systems scattered inside. We propose a cognitive-relay-based selection cooperation protocol under which cognitive relays help forward information in an opportunistic fashion by acquiring spectrum holes. We also present two cognitive relay selection strategies and analyze the performance in terms of the outage probability and the system throughput. Compared to the optimal instantaneous channel state information (ICSI) based strategy, the partial CSI (PCSI) based one can provide a close-to-optimal performance with no additional channel estimation. Simulation results indicate that the scenario where relays have to opportunistically access spectrum holes incurs a cooperative diversity order penalty, and the performance advantage of our proposed protocol depends on conditions of spectrum acquisition and spectrum quality of cognitive relays.