Downlink multiuser MIMO systems with adaptive feedback rate

In this paper, an efficient channel feedback scheme for downlink multiuser multiple-input-multiple-output (MU-MIMO) systems is proposed. The objective of the new feedback design is to adaptively control the feedback rate of each user, depending on the individual channel status, while the overall sum feedback rate is kept the same. The proposed channel feedback scheme is composed of two stages: 1) the common channel feedback stage of all users and 2) the user-specific channel feedback stage of the selected users. We present a low-complexity algorithm to determine the individual feedback rates of users, which utilizes convex optimization, followed by iterative steepest descent direction searching. Numerical results show that zero-forcing beamforming (ZFBF), combined with the proposed channel feedback scheme, considerably outperforms ZFBF with conventional channel feedback schemes.