Unitary Space-Time Modulation for Multiple-Antenna Communications in Rayleigh Flat Fading

Motivated by information-theoretic considerations, we propose a signalling scheme, unitary space-time modulation, for multiple-antenna communications links. This modulation is ideally suited for Rayleigh fast-fading environments, since it does not require the receiver to know or learn the propagation coefficients. Unitary space-time modulation uses constellations of T x M space-time signals {PHI sub t, l = 1,....L}, where T represents the coherence interval during which the fading is approximately constant, and M < T is the number of transmitter antennas. The columns of each PHI sub l are orthonormal. When the receiver does not know the propagation coefficients, which between pairs of transmitter and receiver antennas are modeled as statistically independent, this modulation can theoretically attain capacity either when the SNR is high or when T > M. We design some multiple-antenna signal constellations and simulate their effectiveness as measured by bit error probability with maximum likelihood decoding. We demonstrate that two antennas have a 6 dB diversity gain over one antenna at 15 dB SNR.