Three-User Miso Broadcast Channel: How Much Can Csit Heterogeneity Help?

We study the impact of heterogeneity of channel state information available to the transmitters (CSIT) on the performance of multi-antenna multi-user wireless networks. More specifically, we consider the 3-user multiple-input single-output (MISO) broadcast channel, where the available CSIT with respect to each receiver can be instantaneous (P), delayed (D), or none (N); and we characterize the extent to which such heterogeneity of CSIT impacts its linear degrees of freedom (LDoF). In particular, we completely characterize the DoF region for all possible CSIT configurations, assuming linear encoding strategies at the transmitters. The converse, which is the main contribution of the paper, is based on a novel lemma, called Interference Decomposition Bound, which provides a lower bound on the interference dimension at a receiver with delayed CSIT, based on the dimension of constituents of that interference, thereby decomposing the interference into its individual components.