Bit Error Rate of the Primary User in a Cognitive Relay Network over Rayleigh-Fading Channels

Cooperation is one of the key technologies for solving the challenges in cognitive radio networks. The bit error rate (BER) and the probability of missed detection of the primary user (PU) can be decreased if the secondary user (SU) relays its signals. In this paper, the cognitive relay networks with and without a threshold are presented, where the SU acts as a relay in an amplify-and-forward fashion for the PU. In the scheme with a threshold, the SU relays the signals only when the instantaneous SNR of the primary-secondary link is above a given threshold, and the optimum thresholds that minimize the PU´s BER are derived from the simulations. For the optimum threshold, the performance of the PU is better than both that in the cognitive relay network with no threshold and that in the cognitive radio network with no relay even in a low signal-to-noise ratio (SNR) environment. And the corresponding idle probability of the SU is computed according to the optimum threshold, which is very low at high SNR. But it has little effect on the secondary network because of the low spectrum efficiency of the primary network.