Multipath phase indication (MPI) feedback for CSI acquisition in FDD massive MIMO

We propose a new type of UE feedback report for future frequency division duplex (FDD) systems that relaxes the acquisition problem of downlink channel state information (CSI) at eNB with massive antenna arrays. The CSI acquisition is based on estimating the reciprocal characteristics of the multipath propagation channel at eNB during uplink transmission, whereas the non-reciprocal fast-fading characteristics of the downlink channel is estimated at UE and subsequently fed back in the form of multipath phase indication (MPI) feedback report. Such an approach offers several critical benefits over the compression-based feedback schemes, such as its overhead being immune to the number of antenna elements and avoiding the time consuming training-based channel estimation at UE; hence this new CSI acquisition should be considered a viable candidate for future massive multi-input multi-output (MIMO) systems, particularly when operating at higher frequency bands that will allow densely packed antenna structures.