Unitary beamforming under constant modulus constraint in MIMO broadcast channels

In this paper we analyse unitary beamforming in MIMO broadcast channels where the entries of the beamforming matrix are of constant modulus (CUBF). We provide a general formal description for the beamforming matrices. We show that this description encompasses currently applied constructions such as those based on the Householder transformation. Among other properties the CUBF proves to be particularly robust to channel feedback errors. We propose an iterative construction of the CUBF which maximizes the sum-rate of the system. Furthermore we provide numerical results that show significant gains of the CUBF compared to existing techniques.