In this paper, we propose a new retrospective interference alignment fortwo-cell multiple-input multiple-output (MIMO) interfering multiple accesschannels (IMAC) with the delayed channel state information at the transmitters(CSIT). It is shown that having delayed CSIT can strictly increase the sum-DoFcompared to the case of no CSIT. The key idea is to align multiple interferingsignals from adjacent cells onto a small dimensional subspace over time byfully exploiting the previously received signals as side information withoutdated CSIT in a distributed manner. Remarkably, we show that theretrospective interference alignment can achieve the optimal sum-DoF in thecontext of two-cell two-user scenario by providing a new outer bound.
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