Combined beamforming design for underlay spectrum sharing

The beamforming has been extensively studied as possible enabler of the underlay spectrum sharing and recently has been tightly related to the concept of cognitive small cells. This work proposes a more generic underlay sharing system model, which takes into consideration the interference caused to the secondary system and develops a combined (transmit and receive) beamforming design that is applicable in both uplink and downlink use cases. The paper presents a successive transmit/receive beamforming optimization algorithm, which substantially decreases the computational complexity of the combined beamforming design. The performance analysis results show that the proposed combined beamforming achieves satisfying performances (i.e. mitigating the interference from the primary to the secondary system, satisfying the interference constraints to the primary system etc.). The results show that the proposed successive optimization algorithm decreases the complexity of the combined beamforming without degrading the underlying system performances, which makes it applicable for scenarios such as HetNets and cognitive small cells, which require agile and real-time operation.