How much feedback overhead is required for base station cooperative transmission to outperform non-cooperative transmission?

Coherent base station (BS) cooperative transmission provides high spectrum efficiency for cellular systems when channel state information (CSI) is perfectly known at the BSs. For frequency division duplexing systems, the CSI is fed back with limited number of bits, which linearly increases with the number of cooperating BSs. In this paper, we study the impact of the quantized CSI on cooperative transmission when zero-forcing beamforming is used. By deriving an optimal bit allocation among local and cross channels according to the locations of users, we show the minimal feedback bits required by cooperative transmission for providing performance gain over non-cooperative transmission.