Optimal Feedback Bits Allocation for Two-Cell Massive MIMO Downlink

Massive multiple-input multiple-output (MIMO) is an attractive solution to achieve high data rate and tackle interference. In this paper, we study the downlink of a two-cell massive MIMO system employing coordinated zero-forcing beamforming with limited feedback, where each base station (BS) acquires both the direct link channel state information (DCSI) and the cross link CSI (CCSI) from the users through a limited feedback link. The DCSI is used to support multi-user MIMO transmission (control intra-cell interference) at each BS and the CCSI is used to control the inter- cell interference. Since the total number of feedback bits at each user is fixed, there is a tradeoff between the quantization errors of the DCSI and CCSI. We consider the allocation of the feedback bits over the DCSI and CCSI to maximize the sum-rate of the system. We obtain closed-form solution for the asymptotically optimal feedback bits allocation as the number of antennas per BS goes to infinity. The solution reveals insights on how the key system parameters such as path gains, transmit powers and the number of antennas/users affect the optimal feedback bits allocation. Numerical results validate our theoretical analysis.