A method of modulating and demodulating superposed signals for MUST scheme is proposed. A transmitter takes bit sequences intended for multiple receivers under MUST scheme to go through a “bit sequence to constellation points” mapper before entering the modulators to satisfy the Gray coding rule and to achieve high demodulation performance for the receivers. In a first method, each bit sequence is assigned for each constellation point on the constellation map to satisfy one or more conditions under different power split factors. In a second method, the constellation map is divided into sub-regions according to the clustering of the constellation points for bit sequence assignment. A near-UE may use an ML receiver for demodulation and decoding the superposed signal. A far-UE may use an ML receiver or an MMSE receiver for demodulation and decoding the superposed signal.