An Opportunistic Relaying Protocol Exploiting Distributed Beamforming and Token Passing in Cognitive Radios

Cognitive radio (CR) is a powerful solution that can significantly improve the utilization of the precious limited radio spectrum. It allows secondary users (SUs) to opportunistically access spectral holes of the licensed spectrum without causing harmful interference to primary users (PUs). However, waiting for idle timeslots may induce very poor quality of service (QoS) for SUs. To alleviate this, an opportunistic relaying protocol exploiting distributed beamforming and token passing is proposed in this paper. We consider a cognitive radio network (CRN), where SUs constitute a two-hop relaying network. Specifically, a distributed beamforming method is applied to enable concurrent transmissions of PUs and SUs, thereby improving the opportunistic spectrum access. Our protocol applies a token passing mechanism in the MAC layer to dynamically balance transmission opportunities between two hops, and hence adapts to the random packet arrival and PUs' presence. We shall formulate a Markov chain to analyze the performance of this protocol. Numerical results show that our proposed protocol can significantly improve QoS of SUs in terms of the packet-loss rate and average delay, compared to conventional relaying protocols that utilize only silent timeslots.