Transmit beamforming and interference shaping in cellular cognitive radio networks

Optimising the quality-of-service for secondary users and minimising interference to the primary users are two important objectives of cognitive radio networks. In this study, the authors address these issues by introducing interference shaping criteria into orthogonal transmit beamforming (OTBF) strategies. Then, the authors formulate this problem using semi-definite programming (SDP) and find the optimal beamforming weight vectors at the secondary base station. The authors also introduce a resource allocation strategy which enables the base station to ensure a weighted fairness among the secondary users, hence, optimising the network for heterogeneous traffic patterns. The authors show that in terms of satisfying the signal-to-interference-plus-noise-ratio (SINR) requirements at the secondary users, their modified maximin SINR and modified minimum transmit power strategies produce fairly close results at the same transmit powers. However, in terms of minimising interference seen by the primary users, the former outperforms the latter. Finally, the authors study the impact of various interference shaping margins (ISMs) on the OTBF strategies. The authors show that the allocated power by the modified minimum transmit power strategy and the SINRs achieved by the modified maximin SINR strategy are inversely and directly proportional with the ISMs, respectively.