Joint beamforming and scheduling scheme for underlay MIMO cognitive radio networks with imperfect channel knowledge

Cognitive radio is a promising technique for efficient spectrum utilization in wireless systems. In this paper, we consider a multiple-input multiple-output (MIMO) underlay cognitive radio system where a large number of secondary users (SUs) can share the spectrum with a primary user (PU). We assume that the cognitive base station (CBS) does not have the full channel state information (CSI) from secondary users (SUs) and has imperfect CSI of the interference channel between the CBS and the primary receiver. We study the problem of secondary users scheduling under the constraint of interference to the PU while maximizing the throughput of the secondary system. In order to minimize the amount of required feedback, we propose a scheduling algorithm based on opportunistic beamforming (OB) and consisting of two steps. In the first step, in order to minimize the interference to the PU, we propose to generate orthogonal beams to the estimate of the interference channel. In the second step, the N_s cognitive users (or antennas) having the highest signal to interference and noise ratios (SINRs) are selected for transmission. Since each SU is equipped with multiple antennas, we consider different possibilities of feeding back the SINRs from the SUs to the CBS and also consider the case where optimal combining based on Minimum Mean Square Error (MMSE) criterion is used.