Degrees of Freedom of the Bursty MIMO X Channel with Instantaneous Topological Information
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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 (M,N)/(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.
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