Feedback overhead analysis for base station cooperative transmission

In this paper, we analyze the feedback overhead of channel direction information for downlink coherent base station (BS) cooperative transmission. The per-cell codebooks are considered, which are of practice importance. Instead of analyzing the required number of bits for feedback to keep a constant rate loss, we analyze the required overhead to ensure a target average signal-to-interference-plus-noise ratio (SINR) of each user. To this end, we formulate an optimization problem of bit allocation among the codebooks for local and cross channels that minimizes the total number of bits under the constraint of the average SINR, and find the explicit expression of the solution. We proceed to study the impact of various system parameters and channel features on the overall feedback overhead. Analytical and simulation results reveal that the overhead scales linearly with the overall number of transmit antennas, but decreases with the grow of the cell-edge signal-to-noise ratio. Moreover, the overhead can be significantly reduced by exploiting the diverse performance requirements of the users and the heterogeneous channel features through allocating the number of bits for feedback among multiple users and multiple per-cell links. This provides useful insight for the feedback strategy design of BS cooperation systems.