Cyclic Interference Alignment and Cancellation in 3-User X-Networks with Minimal Backhaul

We consider the problem of cyclic interference alignment (IA) on the 3 - user X -network and show that it is infeasible to exactly achieve the upper bound of K-2/2K-1 = 9/5 degrees of freedom for the lower bound of n = 5 signalling dimensions and K = 3 user-pairs. We observe that this infeasibility goes beyond the problem of common eigenvectors in invariant subspaces within spatial IA. In order to gain non-asymptotic feasibility with minimal intervention, we first investigate a communication strategy that enables IA by feedforwarding a subset of messages with minimal rate. In a second step, we replace the proposed feedforward strategy by an analogous cyclic interference alignment and cancellation scheme with a backhaul network on the receiver side and also by its dual cyclic interference neutralization scheme with a backhaul network on the transmitter side.