  This paper considers a single-cell massive MIMO (multiple-input
multiple-output) system with dual-polarized antennas at both the base station
and users. We study a channel model that includes the key practical aspects
that arise when utilizing dual-polarization: channel cross-polar discrimination
(XPD) and cross-polar correlations (XPC) at the transmitter and receiver. We
derive the achievable uplink and downlink spectral efficiencies (SE) with and
without successive interference cancellation (SIC) when using the linear
minimum mean squared error (MMSE), zero-forcing (ZF), and maximum ratio (MR)
combining/precoding schemes. The expressions depend on the statistical
properties of the MMSE channel estimator obtained for the dual-polarized
channel model. Closed-form uplink and downlink SE expressions for MR
combining/precoding are derived. Using these expressions, we propose
power-control algorithms that maximize the uplink and downlink sum SEs under
uncorrelated fading but can be used to enhance performance also with correlated
fading. We compare the SEs achieved in dual-polarized and uni-polarized setups
numerically and evaluate the impact of XPD and XPC conditions. The simulations
reveal that dual-polarized setups achieve 40-60\% higher SEs and the gains
remain also under severe XPD and XPC. Dual-polarized also systems benefit more
from advanced signal processing that compensates for imperfections.
