  In this paper, the large-scale MU-MIMO system is considered where a base
station (BS) with extremely large number of antennas (N) serves relatively less
number of users (K). In order to achieve largest sum rate, it is proven that
the amount of users must be limited such that the number of antennas at the BS
is preponderant over that of the antennas at all the users. In other words, the
antennas at the BS should be excess. The extra antennas at the BS are no longer
just an optional approach to enhance the system performance but the
prerequisite to the largest sum rate. Based on this factor, for a fixed N, the
optimal K that maximizes the sum rate is further obtained. Additionally, it is
also pointed out that the sum rate can be substantially improved by only adding
a few antennas at the BS when the system is N=KM with M denoting the antennas
at each user. The derivations are under the assumption of N and M going to
infinity, and being implemented on different precoders. Numerical simulations
verify the tightness and accuracy of our asymptotic results even for small N
and M.
