Retrospective Interference Alignment Over Interference Networks

Maddah-Ali and Tse recently introduced the idea of retrospective interference alignment, i.e., achieving interference alignment with only outdated (stale) channel state information at the transmitter (CSIT), in the context of the vector broadcast channel. Since the scheme relies on the centralized transmitter's ability to reconstruct all the interference seen in previous symbols, it is not clear if retrospective interference alignment can be applied in interference networks consisting of distributed transmitters and receivers, where interference is contributed by multiple transmitters, each of whom can reconstruct only the part of the interference caused by themselves. In this work, we prove that even in such settings, retrospective interference alignment is feasible. Specifically, we show that it is possible to achieve more than 1 degrees of freedom (DoF) based on only delayed CSIT in the 3-user interference channel and the 2-user X channel consisting of only single antenna nodes. Retrospective interference alignment is also shown to be possible in other settings, such as the 2-user multiple-input and multiple-output (MIMO) interference channel and with delayed channel output feedback.