On Feasibility Conditions for Linear Interference Alignment over Cognitive Interference Channel: A DoF Analysis with Diversity Tradeoff

In a multiple-antennas cognitive radio network, the rate of the primary link is maximized by water-filling power allocation along the spatial directions associated with the singular values of primary channel matrix. Due to transmitter power constraint, even when the primary user occupies the entire spectrum, some spatial opportunities remain that can be utilized by secondary users. This scheme is called opportunistic interference alignment (OIA). The feasibility issue of OIA in a general symbol-extended system considering the diversity tradeoff, in particular, is the topic that is considered in this paper. An outer bound on the achievable degrees of freedom and also a necessary condition on the feasibility of a multiuser OIA network are presented analytically. In the remaining parts of the paper, the diversity and rate tradeoff is considered. In order to achieve diversity, it is assumed that the paired nodes utilize a space-time coder before the OIA pre-coder. A closed-form relation is presented that relates the minimum achievable diversity to interference alignment quality and code rate.