  Pilot contamination, defined as the interference during the channel
estimation process due to reusing the same pilot sequences in neighboring
cells, can severely degrade the performance of massive multiple-input
multiple-output systems. In this paper, we propose a location-based approach to
mitigating the pilot contamination problem for uplink multiple-input
multiple-output systems. Our approach makes use of the approximate locations of
mobile devices to provide good estimates of the channel statistics between the
mobile devices and their corresponding base stations. Specifically, we aim at
avoiding pilot contamination even when the number of base station antennas is
not very large, and when multiple users from different cells, or even in the
same cell, are assigned the same pilot sequence. First, we characterize a
desired angular region of the target user at the serving base station based on
the number of base station antennas and the location of the target user, and
make the observation that in this region the interference is close to zero due
to the spatial separability. Second, based on this observation, we propose
pilot coordination methods for multi-user multi-cell scenarios to avoid pilot
contamination. The numerical results indicate that the proposed pilot
contamination avoidance schemes enhance the quality of the channel estimation
and thereby improve the per-cell sum rate offered by target base stations.
