We consider precoding strategies at the secondary base station (SBS) in a cognitive radio network with interference constraints at the primary users (PUs). Precoding strategies at the SBS which satisfy interference constraints at the PUs in cognitive radio networks have not been adequately addressed in the literature so far. In this paper, we consider two scenarios: i) when the primary base station (PBS) data is not available at SBS, and ii) when it is made available at the SBS. We derive the optimum MMSE and Tomlinson-Harashima precoding (THP) matrix filters at the SBS {\em which satisfy the interference constraints at the PUs} for the former case. For the latter case, we propose a precoding scheme at the SBS which performs {\em pre-cancellation of the PBS data}, followed by THP on the pre-cancelled data. The optimum precoding matrix filters are computed through an iterative search. To illustrate the robustness of the proposed approach against {\em imperfect CSI at the SBS}, we then derive the robust precoding filters under imperfect CSI for the latter case. Simulation results show that the proposed optimum precoders achieve good bit error performance at the secondary users while meeting the interference constraints at the PUs.
['Ranga Prasad', 'Ananthanarayanan Chockalingam']
Precoder Optimization in Cognitive Radio with Interference Constraints