  Millimeter-wave (mmWave) communication is considered as an indispensable
technique for the next-generation backhaul/fronthaul network thanks to its
large transmission bandwidth. Especially for heterogeneous network (HetNet),
the mmWave full-dimension (FD)-MIMO is exploited to establish the
backhaul/fronthaul link between phantom-cell base stations (BSs) and macro-cell
BSs, where an efficient precoding is prerequisite. Against this background,
this paper proposes a principle component analysis (PCA)-based hybrid precoding
for wideband mmWave MIMO backhaul/fronthaul channels. We first propose an
optimal hybrid precoder by exploiting principal component analysis (PCA),
whereby the optimal high dimensional frequency-selective precoder are projected
to the low-dimensional frequency-flat precoder. Moreover, the combiner is
designed by leveraging the weighted PCA, where the covariance of received
signal is taken into account as weight to the optimal minimum mean square error
(MMSE) fully-digital combiner for further improved performance. Simulations
have confirmed that the proposed scheme outperforms conventional schemes in
spectral efficiency (SE) and bit-error-rate (BER) performance.
