Coordinated Beamforming for the Multiuser MIMO Broadcast Channel With Limited Feedforward

Linear transmit beamforming and receive combining is a low complexity approach for communications in the multiuser multiple-input multiple-output (MIMO) channel. This paper proposes a full search and two low complexity coordinated algorithms where each receiver uses a quantized combining vector. Since the optimization is performed at the transmitter with channel state information from all the receivers, the receive combining vectors are (either jointly or independently) quantized at the transmitter and sent to the receivers via a low-rate feedforward control channel. Rate bounds are provided to estimate the impact of quantization loss on the achievable rate in Rayleigh channels. Simulations show that the sum rate of the proposed methods using Grassmannian codebooks is close to the sum capacity of the MIMO broadcast channel, especially in the high signal-to-noise ratio (SNR) regime.