  To meet the goal of ten-fold increase in spectral efficiency, multiuser
multiple-input-multiple-output (MU-MIMO) techniques capable of achieving high
spatial multiplexing gains are expected to be an essential component of
fifth-generation (5G) radio access systems. This increase in multiplexing gain,
made possible by equipping base stations with a large number of antennas,
entails a proportional increase in channel state information (CSI) acquisition
overhead. This article addresses the problem of reducing this CSI overhead by
optimizing the amount of time-frequency resources allocated for channel
training purposes while not affecting the quality of the associated channel
estimate. First we show that in MU-MIMO, adapting pilot symbol density in the
time-frequency grid should be performed both on a per resource block (RB) basis
and on the basis of groups of users sharing similar channel conditions. Next,
we propose a practical scheme that can perform grouping based per-RB pilot
pattern adaptation. Finally, we evaluate using both analytical and numerical
results the gain in spectral efficiency that can be achieved using this scheme
as compared to conventional MU-MIMO systems that use fixed pilot patterns.
