The Degrees of Freedom Region of the MIMO Interference Channel with Hybrid CSIT

The degrees of freedom (DoF) region of the two-user MIMO (multiple-input multiple-output) interference channel is established under a new model termed as hybrid CSIT. In this model, one transmitter has delayed channel state information (CSI) and the other transmitter has instantaneous CSIT, of incoming channel matrices at the respective unpaired receivers, and neither transmitter has any knowledge of the incoming channel matrices of its respective paired receiver. The DoF region for hybrid CSIT, and consequently that of $2\times2\times3^{5}$ CSIT models, is completely characterized, and a new achievable scheme based on a combination of transmit beamforming and retrospective interference alignment is developed. Conditions are obtained on the numbers of antennas at each of the four terminals such that the DoF region under hybrid CSIT is equal to that under (a) global and instantaneous CSIT and (b) global and delayed CSIT, with the remaining cases resulting in a DoF region with hybrid CSIT that lies somewhere in between the DoF regions under the instantaneous and delayed CSIT settings. Further synergistic benefits accruing from switching between the two hybrid CSIT models are also explored.