The degrees of freedom region of the two-user MIMO broadcast channel with delayed CSIT

The degrees of freedom (DoF) region of the K-user MIMO (multiple-input multiple-output) Gaussian broadcast channel (BC) is studied under i.i.d. fading when there is delayed channel state information at the transmitter (CSIT). The general case of the MIMO BC is considered where each terminal has an arbitrary number of antennas. The delayed CSIT assumption is that the transmitter has perfect knowledge of `stale' channel states (i.e., with some delay) but no knowledge of current CSI. An outer-bound to the DoF region is derived. This bound is shown to be tight in the 2-user case via an interference alignment scheme that optimally accounts for multiple and possibly distinct number of antennas at the two receivers.