A Multipath Extraction-Based CSI Acquisition Method for FDD Cellular Networks with Massive Antenna Arrays

Unlocking the true potential of massive antenna arrays for cellular networks operating in frequency division duplex (FDD) mode is a challenging task as inadequate channel state information (CSI) by conventional limited-size UE feedback forces crude beamforming in downlink. This paper develops a new technique for the CSI acquisition problem of FDD massive multiple-input multiple-output (MIMO) networks based on estimating the reciprocal characteristics of the multipath channel directly from uplink, while merely the nonreciprocal properties of the dominant propagation paths are estimated and fed back from UE. The main benefits of this technique over compressive-sensing methods include allowing single-beam pilot transmission during downlink training and relaxed UE-side computational complexity, as the full knowledge of CSI vector is no longer required at UE. The design can be accomplished by measuring the direction-of-arrival information and average powers of the dominant paths from uplink, indicating the target dominant paths to corresponding UEs, estimating and subsequently signaling the random phase information of the target paths from each UE, and reconstructing the aggregate CSI vector at eNB. The numerical evaluation results demonstrate that the proposed mechanism offers promising potential for reliable FDD operation in next generation cellular networks equipped with massive antenna arrays.