Delay Optimal Scheduling for Cognitive Radio Networks with Cooperative Beamforming

In this paper, we propose an opportunistic scheduling scheme to serve bursty traffics in cognitive radios, where cooperative beamforming is exploited to access busy timeslots or spatial spectrum holes to forward messages without causing interference to primary users. Specifically, based on cooperative beamforming in the physical layer and automatic repeat request for error recovery in the link layer, our proposed scheme strives to balance available spectrum resources, namely temporal and spatial spectrum holes, between the source and the relays so as to greatly reduce the average delay. To analyze the proposed scheme, we then develop a tandem queueing analytical framework, which captures bursty traffic arrival, dynamic availability of spectrum holes, and time-varying channel fading. By modelling it with a multi-dimensional Markov chain, the average delay is derived using a structured matrix-analytical method. Finally, we obtain delay optimal scheduling schemes by jointly optimizing the scheduling parameters. Simulation results reveal that the proposed scheme provides significant quality of service gains over conventional scheduling schemes that access only temporal spectrum holes.