Achieving the degrees of freedom for the two-cell MIMO interfering broadcast channels with limited feedback

We consider the two-cell MIMO interfering broadcast channel (IBC) with K users assigned to each base station, and we develop two closely-related Interference Alignment schemes that are well suited to a limited feedback implementation. The schemes are developed for BSs with K + 1 antennas and receivers with K or K + 1 antennas, respectively. The schemes achieve the maximum degrees of freedom (DoF), 2K, and are shown to have a smaller power offset than some competing schemes. We provide SNR scaling laws for the number of feedback bits that our scheme require to maintain the optimal DoF. Our analysis suggests that there are tradeoffs between the feedback requirements, spatial dimensions and user cooperation. In particular, having one more antenna at each user can greatly reduce the number of feedback bits. Interestingly, the same reduction can be realized without additional receive antennas if direct user cooperation is allowed.