On Robust Precoder Design for Energy-Efficient MIMO-Cognitive Relay Networks

This paper considers the robust relay precoder design problem for multi-input multi-output cognitive relay networks (MIMO-CRNs), where a pair of secondary user (SU) communicates using an amplify-and-forward (AF) half-duplex (HD) relay in the presence of a primary user (PU). The channel state information (CSI) of the relay-PU channel at the relay node is assumed to be imperfect. The channel imperfection is considered in the form of channel uncertainties, which is modelled by using the ellipsoidal model. Taking into account the imperfect CSI, we design the robust relay precoder to maximize the energy efficiency (EE) of the SU subject to the interference and transmit power constraints. An efficient solution, based on the constraint relaxation and the fractional programming theory, is proposed to tackle the robust precoding problem. Numerical results demonstrate that the proposed robust relay precoder scheme maximizes the EE of the SU while securing the transmission of the PU by satisfying the interference constraint for different channel uncertainties.