DoF-robust strategies for the K-user distributed broadcast channel with weak CSI

In this paper, we consider the Network MIMO channel under the so-called Distributed Channel State Information at the Transmitters (D-CSIT) configuration. In this setting, the precoder is designed in a distributed manner at each Transmitter (TX) on the basis of the locally available multi-user channel estimate. Although the use of simple Zero-Forcing (ZF) was recently shown to reach the optimal DoF for a Broadcast Channel (BC) under noisy, yet centralized, CSIT, it can turn very inefficient in the distributed setting as the achieved number of Degrees-of-Freedom (DoF) is then limited by the worst CSI accuracy across all TXs. To circumvent this effect, we develop a new robust transmission scheme improving the DoF. A surprising result is uncovered by which, in the regime of so-called weak CSIT, the proposed scheme is shown to achieve the centralized outerbound obtained under a genie-aided centralized setting in which the CSI versions available at all TXs are shared among them. Building upon the insight obtained in the weak CSIT regime, we develop a general D-CSI robust scheme which improves over the DoF obtained by conventional ZF approach in an arbitrary CSI quality regime and is shown to achieve the centralized outerbound in some other practically relevant CSIT configurations.