  In this paper, we consider a multiuser MISO downlink cognitive network
coexisting with a primary network. With the purpose of exploiting the spatial
degree of freedom to counteract the inter-network interference and
intra-network (inter-user) interference simultaneously, we propose to perform
zero-forcing beamforming (ZFBF) at the multi-antenna 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 inter-network 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. In order to achieve a balance
between the performance and feedback cost, we take the maximization of feedback
utility function, defined as the difference of 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.
