Reducing channel state information feedback for downlink distributed antenna system

This paper addresses the channel state information (CSI) feedback problem for downlink transmission in a distributed antenna system (DAS). One feedback-and-transmission scheme is proposed, where channel response ratios of the nondominant paths (e.g. paths other than that from the nearest site) to the dominant path (e.g. that from the nearest site) are quantized and fed back, from each user, to the coordination center. Vector quantization with log squared error as a distortion measure is introduced to design codebook for quantizing these ratios. Zero-forcing transmitter or zero-forcing dirty-paper-coding transmitter can be designed based on the feedback information and be used for downlink joint transmission. Performance of the proposed scheme in a system with three single-antenna sites and three single-antenna users is evaluated. Simulation results indicate that to achieve the same performance, the proposed scheme requires feedback amount less than half of that needed with a straightforward scheme, where all channel elements are fed back directly.