Joint subcarrier and antenna state selection for cognitive heterogeneous networks with reconfigurable antennas

Reconfigurable antennas (RA) offer an emerging technology that allows wireless devices to alter their antenna states determined by different radiation patterns to maximize received signal strength. In this paper, we consider multiuser orthogonal frequency-division multiple access cognitive heterogeneous networks (HetNets) and we study the potential benefits of employing RA in terms of improving the overall network capacity. In cognitive HetNets, a secondary network is allowed to share the spectrum with the primary network under the condition that the interference level experienced by the primary network is below a predetermined threshold. To satisfy this interference constraint, a secondary user (SU) employs a power control mechanism, which typically limits its transmission power and thus reduces substantially its performance. Moreover, the large number of users expected for next-generation networks brings dense interference to the secondary network and, as such, even efficient interference mitigation and resource allocation techniques can fail in maintaining an acceptable performance level for the network. In this work, we consider utilizing RA technology at SUs to act as an additional resource in terms of selecting antenna radiation patterns that improve received signal strength among SUs. This also limits the mutual interference between the secondary and primary networks. We propose a game theoretical framework for jointly selecting the subcarriers as well as the RA antenna state at each SU that maximizes the overall capacity of the network while meeting the interference target in the primary network. Using potential games that guarantee the existence of a Nash equilibrium, our results show that, by selecting the best RA state and subcarriers for each SU, the capacity of the secondary network increases substantially compared to a scenario with conventional omnidirectional antennas.