Precoding and Downlink Beamforming in Multiuser MIMO-OFDM Cognitive Radio Systems With Spatial Interference Constraints

This paper presents optimal sum-rate precoding and beamforming strategies, to enable serving multiple and single data streams per user, respectively, for multi-user MIMO-OFDM based interweave cognitive radio (CR) networks. The proposed schemes maximize the rate of the CR users subject to spatial interference constraints. Initially, a beamforming scheme is derived for a multiuser CR scenario. A framework is also presented for regularized secondary user (SU) beamforming followed by an iterative procedure to compute the optimal value of the regularization parameter based on decomposition into convex subproblems. For a multiuser CR scenario with multiple receive antennas at each user, a scheme is proposed for the downlink interference limited block diagonalization and power allocation considering both the absence and presence of channel state information (CSI) of the primary user. SU rate maximization for MIMO-OFDM scenarios is also considered with CSI uncertainty, for both stochastic and worst case CSI uncertainty models. Finally, in order to multiplex several CR users with a limited number of base-station antennas, a paradigm is also introduced for coordinated secondary user beamforming. Simulation results demonstrate the improved performance of the various proposed multiuser MIMO-OFDM CR transmission schemes.