Rate Loss Analysis of Transmitter Cooperation with Distributed CSIT

We consider in this work1 the problem of finding a limited feedback rate to quantize the channel state information (CSI) which ensures a maximal rate loss in a broadcast channel (BC) with K transmit antennas (or equivalently K transmitters (TXs)) and K single-antenna receivers (RXs). We focus on an extension of the conventional BC where instead of having centralized CSI at the TX (CSIT), i.e., only one version of the channel estimate which is shared by all the TX antennas, each TX receives its own estimate, or quantized version, of the global multi-user channel. This CSIT configuration, denoted as distributed CSIT, is particularly adapted to the joint transmission from TXs which are not colocated. With centralized CSIT, a very important design guideline for the feedback link was provided by Jindal [Trans. Inf. Theory 2006] where a sufficient feedback rate for achieving a given rate loss compared to the perfect CSIT precoding is derived. In the distributed CSIT setting, additional errors occur and the design guidelines for the centralized case are no longer valid. Consequently, we obtain a new relation between the rate loss and the number of feedback bits. Interestingly, the feedback rate derived in the distributed CSIT setting is roughly K log2(K) bits larger than its counterpart in the centralized case. This highlights the critical impact of distributed CSIT on the performance.