An adaptive transmission scheme for multiuser MIMO broadcast channel in Cognitive Radio

Cognitive Radio (CR) technology provides various options of spectral efficiency for unlicensed wireless communication devices to adapt their spectrum use in response to their operating environment. However, there is an obvious room for improvement regarding the data transmission efficiency and co-existence of secondary users (SUs) in underlay CR. In this paper, we introduce a novel adaptive transmission scheme for multiuser broadcast channel in underlay CR. The proposed scheme applies, on the one hand, a modified water filling algorithm to the primary link ensuring a steady transmission for SUs and, on the other hand, a precoding approach based on the interference alignment and null-steering algorithms to the secondary link enabling coexistence in multiuser underlay CR without constraints on the secondary transmit power. Simulation results show that utilizing this scheme both primary and secondary links can adapt their transmission to commonly improve the spectrum efficiency. Although this comes at the cost of capacity loss in the primary link at high SNRs, it is tolerable as long as a given quality of service for the primary link remains unaffected. In addition, the proposed scheme provides a good performance in terms of BER.