  The acquisition of channel state information (CSI) in Frequency Division
Duplex (FDD) massive MIMO has been a formidable challenge. In this paper, we
address this problem with a novel CSI feedback framework enabled by the partial
reciprocity of uplink and downlink channels in the wideband regime. We first
derive the closed-form expression of the rank of the wideband massive MIMO
channel covariance matrix for a given angle-delay distribution. A low-rankness
property is identified, which generalizes the well-known result of the
narrow-band uniform linear array setting. Then we propose a partial channel
reciprocity (PCR) codebook, inspired by the low-rankness behavior and the fact
that the uplink and downlink channels have similar angle-delay distributions.
Compared to the latest codebook in 5G, the proposed PCR codebook scheme
achieves higher performance, lower complexity at the user side, and requires a
smaller amount of feedback. We derive the feedback overhead necessary to
achieve asymptotically error-free CSI feedback. Two low-complexity alternatives
are also proposed to further reduce the complexity at the base station side.
Simulations with the practical 3GPP channel model show the significant gains
over the latest 5G codebook, which prove that our proposed methods are
practical solutions for 5G and beyond.
