  In this paper, we propose to use Brownian motions to model families of
continuous-time multiuser Gaussian channels without bandwidth limit. It turns
out that such a formulation allows parallel translation of many fundamental
notions and techniques from the discrete-time setting to the continuous-time
regime, which enables us to derive the capacity regions of a continuous-time
white Gaussian multiple access channel with/without feedback, a continuous-time
white Gaussian interference channel without feedback and a continuous-time
white Gaussian broadcast channel without feedback. In theory, these capacity
results give the fundamental transmission limit modulation/coding schemes can
achieve for families of continuous-time Gaussian one-hop channels without
bandwidth limit; in practice, the explicit capacity regions derived and
capacity achieving modulation/coding scheme proposed may provide engineering
insights on designing multi-user communication systems operating on an
ultra-wideband regime.
