Aligning DL paths for scalable CSI feedback in FDD massive MIMO

In frequency-division duplexing (FDD) massive multiple-input multiple-output (MIMO) systems, the downlink (DL) and uplink (UL) channels are not reciprocal anymore. However, some long-term parameters, e.g. the time delays and angles of arrival (AoAs) of the channel paths, still enjoy reciprocity. In this paper, through efficiently exploiting the aforementioned limited reciprocity, we address the DL channel state information (CSI) feedback in a practical wideband massive MIMO system operating in the FDD mode. In particular, the base station (BS) can transmit the FFT-based pilots with carefully-selected phase shifts to align the DL paths. Then the user can rely on the so-called time-domain aggregate channel (TAC) to derive the feedback of reduced dimensionality per the instructions from the serving BS. We further demonstrate that the BS can recover the DL CSIs with the scalable feedback from the users. Numerical simulation results corroborate our designs.