Enforcing coordination in network MIMO with unequal CSIT

In this paper1 we consider the Network MIMO channel under the so-called Distributed Channel State Information at the Transmitters (D-CSIT) configuration where the

precoder is designed in a distributed manner at each Transmitter (TX) on the basis of local versions of Channel State information (CSI) of various quality. In that setting, a novel transmission scheme has been developed in [1] and surprisingly it was shown

that in a so-called regime of weak-CSIT it was possible to achieve a centralized-outerbound obtained when all TXs exchange their local channel estimates. The main novelty of the scheme was the estimation of the interference at the TX having the most accurate CSIT, followed by the quantization and the retransmission of these interference terms using superposition coding. In this work, we develop a new precoding scheme, coined as Hierarchical ZF (HZF), which allows to efficiently exploit the CSI estimates at

all the TXs instead of relying on the estimate at a single TX as in [1]. Combining HZF with the transmission scheme of [1], a novel transmission scheme significantly enlarging the so-called weak CSIT regime has been designed. Finally, HZF precoding leads to a stronger interference attenuation as state-of-the-art precoding from the literature and is expected to be useful in other wireless settings with unequal CSIT.