On Beamforming and Orthogonal Space-Time Coding in Cognitive Networks with Partial CSI

We consider a pair of secondary users that coexist, in a cognitive network, with multiple primary user pairs. The secondary link is supplied with partial network side information (NSI), which comprises message side information and partial channel side information (CSI), available in different levels at both the transmitter and the receiver of the cognitive link. The cognitive transceiver design has to obey predefined quality-of-service (QoS) criteria, that need to be maintained at the primary receivers, and at the same time properly handle the incoming interference from each primary transmitter in order to establish reliable communication. In this framework, we investigate the design and performance of the combined beamforming and orthogonal space-time block coding (BOSTBC) strategy, whose merits are well-documented, as a candidate transmission scheme for the secondary link. We study both aspects, QoS and interference, of the composite problem and characterize how they affect the beamformer design and the cognitive link performance, in the presence of partial NSI. Further, we propose a CSI quality-dependent model for the QoS criteria which yields an interesting trade-off between the cognitive link design and the primary QoS. Numerical results illustrate the system performance in this framework.