The capacity region of a two-transmitter Gaussian multiple access channel
(MAC) under average input power constraints is studied, when the receiver
employs a zero-threshold one-bit analog-to-digital converter (ADC). It is
proved that the input distributions of the two transmitters that achieve the
boundary points of the capacity region are discrete. Based on the position of a
boundary point, upper bounds on the number of the mass points of the
corresponding distributions are derived.