Space-Time Modulation for Unknown Fading


Bertrand M. Hochwald       Thomas L. Marzetta


Abstract: Recent theoretical and experimental work at Bell Laboratories has demonstrated that some widely accepted views about our ability to communicate across a wireless fading channel appear to be incorrect. It now seems that the effects of channel fading, rather than being harmful, can actually be beneficial to wireless communications. BLAST (Bell Labs Layered Space Time) is an example of a multiple-antenna communication link whose capacity in a fading environment grows linearly with the minimum of the number of antennas at the transmitter and receiver, with no increase in bandwidth or transmitted power. BLAST is intended to work in a quasi-stationary environment since it expects the fading coefficients between pairs of transmitter and receiver antennas to change slowly enough for the receiver to track them.

With mobile wireless in mind, we have taken a radically different approach where we assume that nobody knows the propagation coefficients. Our principal results are that capacity cannot be improved by making the number of transmit antennas greater than the symbol-duration of the fading coherence interval, and that the capacity-attaining signals have considerable structure that leads directly to a new signaling scheme: Unitary Space-Time Modulation.

Unitary Space-Time Modulation is ideally suited for fast fading mobile environments, and it does not require the receiver to know or to learn the propagation coefficients. The signals are oblong complex matrices whose columns are orthonormal, each column representing the signal that is fed to one of the transmit antennas during an interval where the fading is approximately constant. Large constellations of these signals are designed systematically according to a special criterion that attempts to minimize the singular values associated with pair-wise products of the signals. Our simulations demonstrate diversity gains for multiple transmit antennas compared with one transmit antenna - all without knowing or learning the propagation coefficients.

Status: Appears in 1999 Proceedings of SPIE Aerosense Conference.

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Bert Hochwald <hochwald@bell-labs.com>