Bosonic channels describe quantum-mechanically many practical communication
links such as optical, microwave, and radiofrequency. We investigate the
maximum rates for the bosonic multiple access channel (MAC) in the presence of
thermal noise added by the environment and when the transmitters utilize
Gaussian state inputs. We develop an outer bound for the capacity region for
the thermal-noise lossy bosonic MAC. We additionally find that the use of
coherent states at the transmitters is capacity-achieving in the limits of high
and low mean input photon numbers. Furthermore, we verify that coherent states
are capacity-achieving for the sum rate of the channel. In the non-asymptotic
regime, when a global mean photon-number constraint is imposed on the
transmitters, coherent states are the optimal Gaussian state. Surprisingly
however, the use of single-mode squeezed states can increase the capacity over
that afforded by coherent state encoding when each transmitter is photon number
constrained individually.Comment: 8 pages, 3 figure