  Pilot contamination has been regarded as a main limiting factor of time
division duplexing (TDD) massive multiple-input-multiple-output (Massive MIMO)
systems, as it will make the signal-to-interference-plus-noise ratio (SINR)
saturated. However, how pilot contamination will limit the user capacity of
downlink Massive MIMO, i.e., the maximum number of admissible users, has not
been addressed. This paper provides an explicit expression of the Massive MIMO
user capacity in the pilot-contaminated regime where the number of users is
larger than the pilot sequence length. Furthermore, the scheme for achieving
the user capacity, i.e., the uplink pilot training sequence and downlink power
allocation, has been identified. By using this capacity-achieving scheme, the
SINR requirement of each user can be satisfied and energy-efficient
transmission is feasible in the large-antenna-size (LAS) regime. Comparison
with two non-capacity-achieving schemes highlights the superiority of our
proposed scheme in terms of achieving higher user capacity.
