Cooperative and non cooperative multi-secondary users cognitive radio system with channel uncertainty

Cognitive radio (CR) permits multiple unlicensed users, secondary users, to share the same channels with the licensed users, primary users, without affecting the latter performance. The main challenge resides in the design of the secondary users´ transceivers (precoding, postcoding, and power allocation) such that both the primary and secondary receivers enjoy satisfactory quality of service (QoS). In this paper, two novel methods are proposed for designing secondary users´ postcoding and power allocation matrices. The proposed methods consider both perfect and imperfect channel state information (CSI) in the design. The proposed techniques align the interference at the primary user´s receiver in a direction orthogonal to the primary desired signal space by using interference alignment (IA). On the other hand, postcoding and power allocation matrices are chosen such that they maximize secondary users´ rates by whitening the interference at secondary user´s receiver, while satisfying the interference constraint on the primary user´s receiver in case of channel imperfection. Numerical results show the ability of the proposed methods to allow multiple secondary users to transmit without degrading the primary user´s performance and to maximize their rates in case of channel uncertainty.