Degrees of Freedom for the 4 User SIMO Interference Channel

The 4 user single input multiple output (SIMO) Gaussian interference channel where each transmitter has a single antenna and each receiver has two antennas is studied. We show that if the channel coefficients are time-varying and drawn from a continuous distribution, the maximum number of spatial degrees of freedom per orthogonal time dimension for this channel is 8/3 almost surely. Achievability is based on the idea of interference alignment, i.e., signal spaces are aligned at receiver where they constitute interference while they are separable at receivers where they are desired.