  We propose an iterative mode-dropping algorithm that optimizes input signals
to achieve the sum capacity of the MIMO-MAC with per-antenna power constraint.
The algorithm successively optimizes each user's input covariance matrix by
applying mode-dropping to the equivalent single-user MIMO rate maximization
problem. Both analysis and simulation show fast convergence. We then use the
algorithm to briefly highlight the difference in MIMO-MAC capacities under sum
and per-antenna power constraints.
