  This work concerns wireless cellular networks applying massive multiple-input
multiple-output (MIMO) technology. In such a system, the base station in a
given cell is equipped with a very large number (hundreds or even thousands) of
antennas and serves multiple users. Estimation of the channel from the base
station to each user is performed at the base station using an uplink pilot
sequence. Such a channel estimation procedure suffers from pilot contamination.
Orthogonal pilot sequences are used in a given cell but, due to the shortage of
orthogonal sequences, the same pilot sequences must be reused in neighboring
cells, causing pilot contamination. The solution presented in this paper
suppresses pilot contamination, without the need for coordination among cells.
Pilot sequence hopping is performed at each transmission slot, which provides a
randomization of the pilot contamination. Using a modified Kalman filter, it is
shown that such randomized contamination can be significantly suppressed.
Comparisons with conventional estimation methods show that the mean squared
error can be lowered as much as an order of magnitude at low mobility.
