On combined beamforming and OSTBC over the cognitive radio Z-channel with partial CSI

We consider a pair of secondary nodes (SU) coupled, in Z-topology, with multiple pairs of primary nodes (PU). The secondary (cognitive) transmitter is combining beamforming with orthogonal space-time block coding (BOSTBC) and operates under Quality-of-Service (QoS) constraints that must be guaranteed for the primary receivers (PU_Rx). The cognitive link is designed assuming imperfect channel state information (CSI) for all links, available at the SU transmitter (SU_Tx). Under this premise we characterize the optimal design in terms of CSI quality and interference and evaluate their impact on the performance of BOSTBC transmission in underlay cognitive networks.