Opportunistic interference alignment in cognitive multi-user MIMO relay networks

In this paper, we present an opportunistic interference alignment (IA) scheme for cognitive multi-user (MU) multiple-input multiple-output (MIMO) relay networks. We consider a secondary relay network with 2 K communicating nodes without direct link between any two nodes, and hence, nodes use a common relay for communicating with each other. Using the proposed scheme secondary user opportunistically accesses the primary users' resources (i.e. frequency band) without causing any interference to the primary user. The rate of the primary user is maximized by using the well known water-filling algorithm for optimal power allocation on channel's singular modes. The unused singular modes can then be utilized by secondary users. The secondary users can align their transmission in these unused singular modes which results in spectrally efficient communication. We assume that the number of antennas available are not sufficient to perform transmit and receive zero-forcing (ZF) at the secondary transmitter, receiver and the relay node. We propose a closed form solution for pre-coding and post-coding matrices at the secondary nodes. The proposed solution nullifies the interference on the primary user caused due to the transmission from secondary transmitter and secondary networks' relay node whilst achieving a non-zero transmission rate. Simulation result shows the achievability of significant sum rate for the secondary network.