Shared Angles-of-Departure in Massive MIMO Channels: Correlation Analysis and Performance Impact

by   Xu Du, et al.

In practical environments, recent massive MIMO measurements demonstrate that user channels can be correlated. In this paper, we study the user channel correlation induced by shared angles-of-departure. We first derive the user correlation distribution in the large array regime, and then examine the user correlation using actual measurements from a large array. As a data-driven observation, we discover that the correlation of all close-by users is higher than 0.4 and barely reduces as the number of base-station antennas M increases beyond 36 antennas. Furthermore, nearly one-third of users, even when they are tens of wavelengths apart, have a correlation that is more than twice the correlation of an i.i.d. Rayleigh fading model. Lastly, we characterize the impact of user correlation on system performance. As M increases, conjugate beamforming systems suffer a linearly growing inter-user interference due to correlated channels. However, for zero-forcing beamforming systems, the inter-user interference is a constant that does not increase with M. In particular, zero-forcing beamforming systems can serve a linearly increasing number of correlated users and achieve a linear growth in the system achievable rate as M increases. Hence, spatial multiplexing correlated users can be an attractive massive MIMO design.



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