Throughput maximization in cognitive radio with limited feedback in the interference-limited regime

We consider a spectrum sharing scenario in a cognitive radio network where a secondary user (SU) shares a narrowband channel with N primary users (PU). We consider the problem of SU ergodic capacity maximization under individual peak interference constraints at the PU receivers, in the interference limited regime so that the SU transmit power constraint is not active. The novelty in this paper lies in considering quantized channel information at the SU transmitter, as opposed to the full channel state information (especially for the SU transmitter to PU receiver channels) assumption which is prevalent in the existing literature. To this end, we assume the existence of a central controller (termed as the cognitive radio (CR) network manager) which can obtain the full channel information about the SU transmitter to PU receivers (the interference channels g0i, i = 1, 2, . . . , N) from the PU base station and the SU transmitter to SU receiver channel information (g1) from the SU base station if necessary, via (possibly) fibre-optic links. We consider two different schemes: (1) Quantized Power Allocation (QPA) where the SU transmission power is quantized and (2) Quantized Rate Allocation (QRA) where the ratio g1/(maxi g0i) is quantized. The designed power or rate codebooks are then communicated a priori to SU transmitter and receivers. In real time, based on the full channel information of g0i (QPA) and g1, g0i (QRA) at the CR network manager, it forwards the corresponding quantization index to the SU transmitter via a finite-rate feedback link, which is then used to select the transmission power or rate. Approximate expressions for quantization thresholds and SU ergodic capacity with quantized feedback are developed as N -> ? or L -> ?. Numerical results are presented to show that only 4-6 bits of feedback can make the SU ergodic capacity with limited feedback approach the capacity with full channel information at SU transmitter.