It is generally desirable to reduce error rates, and to increase channel capacity, in wireless transmission systems. Multiple-antenna arrays can be used to achieve these desirable effects.
Fading is one of several physical phenomena that tend to increase error rates, or to reduce channel capacity, in wireless transmission systems. Fading is the result of destructive interference, at the receiver, between correlated signal portions that because of scattering have arrived over different-length paths.
One technique that tends to mitigate the effects of fading is differential phase modulation, in which phase differences carry transmitted information. Although differential phase modulation is a known technique for single-antenna transmission and reception in fading environments, there are no known adaptations of this technique for use with multiple-antenna arrays.
However, in certain fading environments, the theoretical capacity of a multiple-antenna communication link increases linearly with the size of the transmitter or receiver array, this effect being determined by the array having the lesser number of antennas. This effect has been predicted for rich scattering environments in which fading is "flat." That is, the propagation coefficients that describe the effect of the physical transmission channel on the transmitted signal are approximately independent of frequency over the signal bandwidth. Flat fading can be achieved in practice for a particular environment if the bandwidth is not too great, or if it is restricted appropriately.
Significantly, such a linear increase in capacity occurs only if the propagation coefficients between all pairs of transmitter and receiver antennas are known to the receiver. In practice, this condition can be met only if the receiver is trained, from time to time, by receiving known training signals from the transmitter.
Communication methods that use such a training procedure are described, for example, in the co-pending U.S. patent application Ser. No. 08/938,168, commonly assigned herewith, filed on Sep. 26, 1997 by B. M. Hochwald et al. under the title, "Multiple Antenna Communication System and Method Thereof."
Other co-pending patent applications, commonly assigned herewith, that describe related subject matter are Ser. No. 08/673,981, filed on Jul. 1, 1996 by G. J. Foschini under the title "Wireless Communications System Having a Layered Space-Time Architecture Employing Multi-Element Antennas," Ser. No. 09/060,657, filed on Apr. 15, 1998 by G. J. Foschini and G. D. Golden under the title "Wireless Communications System Having a Space-Time Architecture Employing Multi-Element Antennas at Both the Transmitter and Receiver," and a patent application filed on Jul. 10, 1998 by T. L. Marzetta under the title "Determining Channel Characteristics in a Space-Time Architecture Wireless Communication System Having Multi-Element Antennas."
Unfortunately, training intervals cut into the available time during which data may be transmitted. The length of this interval increases as the number of transmitter antennas is increased. Moreover, the propagation coefficients can be treated as constant only over an average period of time referred to as the fading coherence interval. To be effective, training should be repeated at least once per such interval. However, fading is very rapid in some environments, such as those in which a mobile station is operating within a rapidly moving vehicle. For rapid fading environments, the time between fades may be too short for the communication system to learn the propagation coefficients belonging to even one transmitting antenna, much less those of a multiple antenna array.
Thus, until now, the theoretical benefits of multiple antenna arrays in fading environments have eluded full practical realization. As a consequence, there has remained a need to further improve the channel capacity and error rates achieved with such arrays, without requiring knowledge of the propagation coefficients.