Optimal Power Allocation for Sensing-Based Spectrum Sharing in MIMO Cognitive Relay Networks

With most of the radio spectrum already allocated, cognitive radio acts as a promising solution to the spectrum scarcity problem. Increasing the secondary throughput without making interference to the primary user (PU) is a major challenge. In this paper, the secondary throughput is maximized under power constraints by obtaining an optimum value of the relay amplification factor. Comparison between using single antenna, double antennas at all the nodes of the cognitive relay network (CRN), and double antennas at the relay node only is performed. The mathematical analysis of the system is investigated for two cases: (1) The opportunistic access case, where the secondary user (SU) can transmit data only in the vacant bands of the allocated spectrum to the PU; and (2) The sensing-based spectrum sharing case, where the SU can transmit data all the time but with different transmit powers depending on the sensing information. Simulation results show that the achieved secondary throughput can be maximized at a given value of the relay amplification factor. Moreover, using double antennas at all the nodes of the CRN increases the maximum achievable throughput and improves the detection capabilities compared with using single antenna or using double antennas at the relay node only. Finally, results show that the SU can achieve more throughput under the sensing-based spectrum sharing case compared with that achieved under the opportunistic access case.