  In practical propagation environments, different massive MIMO users can have
correlated angles in spatial paths. In this paper, we study the effect of angle
correlation on inter-user channel correlation via a combination of measurement
and analysis. We show three key results. First, we collect a massive MIMO
channel dataset for examining the inter-user channel correlation in a
real-world propagation environment; the dataset is now open-access. We observed
channel correlation higher than $0.48$ for all close-by users. Additionally,
over $30$ % of far-away users, even when they are tens of wavelengths apart,
have inter-user channel correlation that is at least twice higher than the
correlation in the i.i.d. Rayleigh fading channel. Second, we compute the
inter-user channel correlation in closed-form as a function of inter-user angle
correlation, the number of base-station antennas, and base-station
inter-antenna spacing. Our analysis shows that inter-user angle correlation
increases the inter-user channel correlation. Inter-user channel correlation
reduces with a larger base-station array aperture, i.e., more antennas and
larger inter-antenna spacing. Third, we explain the measurements with numerical
experiments to show that inter-user angle correlation can result in significant
inter-user channel correlation in practical massive MIMO channels.
