  We propose a comprehensive scheme for realizing a massive multiple-input
multiple-output (MIMO) system with dual-polarized antennas in frequency
division duplexing (FDD) mode. Employing dual-polarized elements in a massive
MIMO array has been common practice recently and can, in principle, double the
number of spatial degrees of freedom with a less-than-proportional increase in
array size. However, processing a dual-polarized channel is demanding due to
the high channel dimension and the lack of Uplink-Downlink (UL-DL) channel
reciprocity in FDD mode. In particular, the difficulty arises in channel
covariance acquisition for both UL and DL transmissions and in common training
of DL channels in a multi-user setup. To overcome these challenges, we develop
a unified framework consisting of three steps: (1) a covariance estimation
method to efficiently estimate the UL covariance from noisy, orthogonal UL
pilots; (2) a UL-DL covariance transformation method that obtains the DL
covariance from the estimated UL covariance in the previous step; (3) a
multi-user common DL channel training with limited DL pilot dimension method,
which enables the BS to estimate effective user DL channels and use them for
interference-free DL beamforming and data transmission. We provide extensive
empirical results to prove the applicability and merits of our scheme.
