Beamforming with Finite Rate Feedback for LOS MIMO Downlink Channels

We consider a MIMO line of sight (LOS) broadcast channel where each user has perfect knowledge of its own channel and feeds back ´quantized´ channel information to the transmitter with finite number of bits. The rate loss incurred due to imperfect channel information at the transmitter is quantified in terms of the number of feedback bits, assuming that low-complexity zero-forcing beamforming is used by the transmitter. Moreover, in order to maintain a constant rate loss relative to a perfect CSIT system, it is shown that the number of feedback bits per user should grow linearly with the system SNR in dB but only logarithmitcally with the number of transmit antennas. Simulation results comparing zero-forcing, MMSE and conventional beamforming with greedy user selection and finite rate feedback are also presented.