Adaptive Mode Selection in Multiuser MISO Cognitive Networks With Limited Cooperation and Feedback

In this paper, we consider a multiuser (MU) multiple-input-single-output (MISO) downlink cognitive network coexisting with a primary network. With the purpose of exploiting the spatial degree of freedom to counteract the internetwork interference and intranetwork (interuser) interference simultaneously, we propose performing zero-forcing beamforming (ZFBF) at the multiantenna cognitive base station (BS) based on the instantaneous channel state information (CSI). The challenge of designing ZFBF in cognitive networks lies in how to obtain the interference CSI. To solve it, we introduce a limited internetwork cooperation protocol, namely, the quantized CSI conveyance from the primary receiver to the cognitive BS via purchase. Clearly, the more the feedback amount, the better the performance, but the higher the feedback cost. To achieve a balance between the performance and feedback cost, we take the maximization of the feedback utility function (FUF), which is defined as the difference of the average sum rate and feedback cost while satisfying the interference constraint, as the optimization objective, and derive the transmission mode and feedback amount joint optimization scheme. Moreover, we quantitatively investigate the impact of CSI feedback delay and obtain the corresponding optimization scheme. Furthermore, through asymptotic analysis, we present some simple schemes. Finally, numerical results confirm the effectiveness of our theoretical claims.