Non-Orthogonal Multiple Access in Cognitive Relay Networks

Abstract

In this paper, we present the optimal power allocation and relay precoder design for a cognitive relay network employing non-orthogonal multiple access (NOMA) technique. The secondary user (SU) nodes share the spectrum with the licensed primary user (PU) while keeping the interference at PU nodes below a permissible threshold. The cognitive nodes communicate with each other with the assistance of a cognitive amplify and forward (AF) relay equipped with multiple antennas. The proposed source power allocation and relay precoder design aim at maximizing the sum-rate of the cognitive destination nodes while maintaining the interference to the PU node below the specified threshold. Given that the SU transmit node has to restrict its transmit power in order to reduce the interference to the PU, use of NOMA can significantly improve the SU performance. The resulting optimization problem is non-convex in its original form. In order to circumvent the difficulties associated with the non-convexity, we employ minorization-maximization technique to obtain convex approximations to some of the non- convex functions appearing in the problem. We then solve this problem to obtain the optimal resource allocation policy at source and precoder design at the relay. Numerical results are provided to illustrate the performance of the proposed design for various operating conditions and parameters.