A method for operating a transmitting device using semi-orthogonal multiple access (SOMA) includes determining power allocations and sub-quadrature amplitude modulation (sub-QAM) allocations for a first receiving device and a second receiving device in accordance with channel information associated with the first receiving device and the second receiving device, and transmitting information about a first power allocation for the first receiving device, and a first sub-QAM allocation for the first receiving device to the first receiving device.