Frequency-division multiplexing and demultiplexing systems and methods for use with M transmit and N receive antennas, M equaling at least two. In one embodiment, a frequency-division multiplexing system includes: (1) a differential modulator that generates M.times.M unitary space-frequency signals from incoming message bits and (2) a time-frequency transformer, coupled to the differential modulator, that transforms the M.times.M space-frequency signals into space-time transmit signals for the M transmit antennas.