  In this paper, we characterize the fundamental limits of a communication
system with three users (i.e., three transmitters) and a single receiver where
communication from two covert users must remain undetectable to an external
warden. Our results show a tradeoff between the highest rates that are
simultaneously achievable for the three users. They further show that the
presence of a non-covert user in the system can enhance the capacities of the
covert users under stringent secret-key constraints. To derive our fundamental
limits, we provide an information-theoretic converse proof and present a coding
scheme that achieves the performance of our converse result. Our coding scheme
is based on multiplexing different code phases, which seems to be essential to
exhaust the entire tradeoff region between the rates at the covert and the two
non-covert users. This property is reminiscent of the setup with multiple
non-covert users, where multiplexing is also required to exhaust the entire
rate-region.
