Abstract
Accessibility to instantaneous channel state information (CSI) is crucial for a base station equipped with a massive multiple-input multiple-output (MIMO) feature to optimize its downlink transmission performance via precoding. For cellular systems operating in frequency division duplex (FDD) mode, CSI feedback from the users is usually required, which could be challenging in a massive MIMO setting. This chapter discusses two different approaches of CSI acquisition for massive MIMO in FDD, as well as a downlink precoding method that does not require CSI feedback. First, by noting that a spatially correlating MIMO channel can have a sparse representation via certain linear transformations, a feedback load reduction method based on compressive sensing theory is introduced. Second, we expound a practical and efficient CSI feedback mechanism utilizing multistage beamforming, wherein the pilot symbols are transmitted on angular-domain beams. Last, a downlink precoding technique based on angles-of-arrival knowledge of propagation paths instead of CSI is investigated in detail.
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