Degrees of Freedom Region of the MIMO Z-Interference Channel With Mixed CSIT

The degrees of freedom (DoF) region is established for the two-user multiple-input multiple-output Z-interference channel (IC) under the assumption of mixed channel state information at transmitters (CSIT), in which each transmitter obtains accurate delayed CSIT as well as an imperfect estimate of the current channel state, with the quality of the estimate determined by a parameter, α. A block-Markov achievability scheme is designed that uses transmit beamforming, interference quantization, and multicasting along with backward decoding, and is shown to be DoF optimal. When applied to a two-user IC with mixed CSIT and one cross-link statistically weaker than the rest, this scheme can not only improve over the DoF region otherwise achievable by using the DoF-optimal scheme for the IC (which treats all links as comparable in strength) but also dispenses with the requirement of obtaining delayed feedback or current channel estimate for the weaker link.