Joint Sensing and Transmission for AF Relay Assisted PU Transmission in Cognitive Radio Networks

Recent measurements show that there are abundant spectrum opportunities across the licensed cellular bands, over which the relay stations (RSs) are widely employed for both coverage extension and throughput improvement. However, current efforts in cognitive radios focus on exploiting spectrum holes over the licensed direct transmission links. To exploit new spectrum opportunities over the licensed relay assisted transmission links, we propose a novel two-phase joint sensing and transmission scheme (TP-JSTS) for the dual-hop Amplify-and-Forward (AF) relay assisted primary user (PU) transmission in cognitive radio networks. The TP-JSTS takes into account the relay behaviors of the PU system. In the first phase, the secondary user (SU) senses the signal transmitted by the PU base station (BS). In the second phase, the SU detects the signals sent by the PU-Relays. To protect the PU from harmful interference, the SU transmits within the current band in the rest of each phase only when the PU is detected to be absent. We investigate the sensing performance, throughput performance and delay performance of our proposed TP-JSTS when all the PU-Relays adopt the AF relay protocol. We obtain the optimal sensing-time allocation strategy that maximizes the achievable throughput of the SU and the optimal sensing-time allocation strategy that minimizes the average transmission delay of the SU. Simulation results show that there exists an optimal pair of sensing-time durations that maximizes the achievable throughput for the SU, while there exists another optimal pair of sensing-time durations that minimizes the average transmission delay for the SU.