A communications system includes multiple mobile wireless communications devices transmitting at a same time to a fixed receiver assembly. Each mobile wireless communications device transmits a source signal defined by a respective character set comprising a plurality of symbols. At least one of the symbols in the respective character sets is transmitted at a different power level so that the transmitted source signal from each mobile wireless communications device appears with linearly independent power level time periods. The linearly independent power level time periods are used by the fixed receiver assembly to populate a mixing matrix for signal separation processing.