  We introduce novel blind and semi-blind channel estimation methods for
cellular time-division duplexing systems with a large number of antennas at
each base station. The methods are based on the maximum a-posteriori principle
given a prior for the distribution of the channel vectors and the received
signals from the uplink training and data phases. Contrary to the
state-of-the-art massive MIMO channel estimators which either perform linear
estimation based on the pilot symbols or rely on a blind principle, the
proposed semi-blind method efficiently suppresses most of the interference
caused by pilot-contamination. The simulative analysis illustrates that the
semi-blind estimator outperforms state- of-the-art linear and non-linear
approaches to the massive MIMO channel estimation problem.
