Coding and Modulation for Space-Time Systems

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

Wireless communication systems must inevitably deal with the severe effects of fading, the phenomenon whereby received signals fluctuate over very wide ranges as the propagation environment changes in time. Various forms of diversity are commonly employed to combat the effects of fading. In various scenarios, it is possible to achieve diversity by transmitting a signal that is redundant in time or frequency, by using multiple transmit antennas and/or multiple receive antennas. While techniques for achieving time diversity, frequency diversity, and space diversity through multiple receive antennas are well known, developing techniques for achieving diversity through the use of multiple transmit antennas is a relatively new area which is currently of great practical and theoretical interest. This research involves the design of improved modulation and coding techniques for the so-called space-time systems, i.e., those with multiple transmit antennas. One main goal of this project is to maximize improvement in what is in some sense the worst case scenario where the number of receive antennas is either one or a very small number and also where the channel is fading in a very slow (i.e., quasi-static) manner. In this environment, transmitter diversity and possibly frequency selectivity may be the only sources of potential diversity. This research studies the potential of improving on existing techniques by overcoming two flaws that plague many existing or proposed space-time systems: 1) Space time codes have difficulty providing both diversity and significant coding gain, Many space-time codes must use up all their redundancy to provide diversity and actually provide little or no coding gain. By splitting the jobs of providing diversity and coding gain between either two separate codes or between the coding and modulation formats, this research produces a more effective space-time communication system. 2) Space time codes which promise significant coding gain often do not produce it in a quasi-static fading channel (QSFC). The currently accepted design rules for space-time codes are in fact shaky when applied to a QSFC. By forming a more solid foundation for designing codes for QSFCs, this research produces better design criteria and ultimately better space-time coding techniques.