  Full-dimensional (FD) channel state information at transmitter (CSIT) has
always been a major limitation of the spectral efficiency of cellular
multi-input multi-output (MIMO) networks. This letter proposes an
FD-directional spatial channel estimation algorithm for frequency division
duplex massive FD-MIMO systems. The proposed algorithm uses the statistical
spatial correlation between the uplink (UL) and downlink (DL) channels of each
user equipment. It spatially decomposes the UL channel into azimuthal and
elevation dimensions to estimate the array principal receive responses. An FD
spatial rotation matrix is constructed to estimate the corresponding transmit
responses of the DL channel, in terms of the frequency band gap between the UL
and DL channels. The proposed algorithm shows significantly promising
performance, approaching the ideal perfect-CSIT case without UL feedback
overhead.
