  In this paper, a novel two-dimensional super-resolution angle-of-departure
(AoD) and angle-of-arrival (AoA) estimation technique is proposed for wideband
millimeter-wave multiple-input multiple-output systems with cross-polarized
antenna elements. The key ingredient of the proposed method is to form custom
designed beam pairs, and devise an invertible function of the AoD/AoA to be
estimated from the corresponding beam pairs. Further, a new multi-layer
reference signal structure is developed for the proposed method to facilitate
angle estimation for wideband channels with cross-polarized antenna elements.
To facilitate feedback in closed-loop frequency division duplexing systems, a
novel differential feedback strategy is proposed aiming at feedback reduction
for the two-dimensional angle estimation. Numerical results demonstrate that by
using the proposed method, good azimuth/elevation AoD and AoA estimation
performance can be achieved under different levels of signal-to-noise ratio,
channel conditions, and antenna array configurations.
