Network MIMO: Transmitters with no CSI can still be very useful

In this paper Channel State Information at the Transmitters (D-CSIT). 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 estimates. Although the use of simple Zero-Forcing (ZF) was recently shown to reach the optimal DoF for a BC under noisy, yet centralized CSIT, it can turn very inefficient when faced with D-CSIT: The number of Degrees-of-Freedom (DoF) achieved is then limited by the worst CSI accuracy across 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 DoF of a genie aided network that centralizes the CSI available at the best informed TX. Hence, even TXs without CSIT can be useful in lifting the DoF. Building upon the insight obtained in the weak CSIT regime, we develop a general DCSI-robust scheme which improves over the DoF obtained by conventional ZF approach in an arbitrary CSI quality regime.