Optimal Beamforming and Scheduling for MIMO-OFDM Uplink Transmissions in Hierarchical Cognitive Radio Systems

Cognitive radio (CR) technology can be been applied in hierarchical cellular systems to allow concurrent transmissions for licensed (primary) and unlicensed (secondary) users to improve spectrum utilization. The major challenge of hierarchical CR systems is to manage the inter-cell interference between the primary and secondary systems. In the paper, we first present an optimal multiple-input multiple- output (MIMO) orthogonal frequency division multiplexing (OFDM) uplink transmission scheme for hierarchical CR systems that can maximize the sum rate of the underlay secondary CR users, and prevent the primary systems from the severe interference of the secondary systems. The main contribution of this work is to propose a methodology to transform the signal to interference plus noise ratio (SINR) maximization problem of the secondary system into a quasi-convex form, thereby obtaining the optimal beamforming solution using a simple bisection method. Furthermore, we proposed a sub-optimal user scheduling algorithm in the secondary system to effectively mitigate the interference to the primary system.