Degrees of Freedom of the Bursty MIMO X Channel with Instantaneous Topological Information

We study the effects of instantaneous feedback of channel topology on the degrees of freedom (DoF) of the bursty MIMO X channel, where the four transmitter-receiver links are intermittently on-and-off, governed by four independent Bernoulli (p) random sequences, and each transmitter and receiver are equipped with M and N antennas, respectively. We partially characterize this channel: The sum DoF is characterized when p ≤ [1/2] or when [min(M,N)/(mm(M,N))] ≤ [2/3]. In the remaining regime, the lower bound is within 5.2% of the upper bound. Strictly higher DoF is achieved by coding across channel topologies. In particular, codes over as many as 5 topologies are proposed to achieve the sum DoF of the channel when p ≤ [1/2]. A transfer function view of the network is employed to simplify the code design and to elucidate the fact that these are space-time codes, obtained by interference alignment over space and time. A full version of this paper is accessible at: https://arxiv.org/abs/1805.02527.