Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1998-08-14
2001-12-04
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
C375S267000, C375S346000, C375S347000
Reexamination Certificate
active
06327310
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to modulation methods for wireless signal transmission. More particularly, the invention relates to modulation methods that reduce the error rates of received signals in fading environments and that enable data rates to be increased without the need to increase bandwidth or transmitted power. Still more particularly, the invention relates to such methods in conjunction with the use of multiple antenna arrays.
BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
We have found a new and useful modulation method. Signals transmitted and received according to our method are robust against fading, as well as against receiver-induced noise, in flat fading environments. Our method does not require knowledge of the propagation coefficients, although in some implementations of our method, such knowledge can be used to further improve performance. Our method will be useful, inter alia, in connection with the use of multiple antenna arrays for improving error rates in transmitted and received signals.
In accordance with our method, each message that is transmitted is made up of a sequence of signals, each selected from a constellation of L such signals, L a positive integer. Thus, each transmitted signal embodies a number of bits given by log L. (Herein, “log” denotes the logarithm to the base 2.)
Each transmitted signal is distributed spatially across the transmitting array, and is also distributed in time. That is, each signal occupies T successive time units, which we refer to as symbol intervals. The length (in, e.g., microseconds) of a symbol interval is determined by the bandwidth of the communication system, which is well known to be a matter of system design.
At each discrete time value t=1, 2, . . . , T, a respective vector of complex amplitudes defines the baseband voltage to be placed on a carrier and transmitted. Letting the positive integer M (M≧1) represent the number of transmitting antennas, this vector has M entries, each representing the complex baseband voltage amplitude at a respective one of the transmitting antennas.
We refer to such a vector as a symbol. Thus, each transmitted signal may be regarded as a sequence of T symbols. Alternatively, each transmitted signal may be regarded as a collection of M vectors in time, each of length T, and each associated with a respective transmitting antenna. Both of these views flow directly from the representation of each transmitted signal as proportional to a T×M matrix &PHgr;
l
, where the index l runs over the signal constellation: l=1, 2, . . . , L. In such a representation, each of the T rows is a respective symbol, and each of the M columns is one of the vectors in time.
In accordance with the invention, all of the columns of each matrix &PHgr;
l
are orthonormal. The baseband signals provided to the transmitting array are represented by respective matrices S
l
, which are related to the matrices &PHgr;
l
by S
l
={square root over (TP)}&PHgr;
l
, where P is the average power fed into each antenna.
Thus, in one aspect, the invention involves a method for wireless communication, comprising transmitting at least one signal from one transmitting antenna or from an array of two or more transmitting antennas. (Thus, M≧1.) (The term “array” will be used herein to collectively designate the transmitting antennas even if there is, in fact, only one transmitting antenna.) Each signal to be transmitted is selected from a constellation of known signals. The baseband amplitude of each of these signals is distributed across the antennas of the transmitting array according to the signal matrix S described above.
The transmitted message may be received by a single receiving antenna, or it may be received by an array of N receiving antennas, in which the integer N may be greater than, equal to, or less than M. The received baseband amplitudes can be tabulated as a T×N matrix X, in which, as above, the row index increases in discrete time, and the column index varies across the antennas of the (receiving) array.
Hochwald Bertrand M.
Marzetta Thomas Louis
Chin Stephen
Finston Martin I.
Liu Shuwang
Lucent Technologies - Inc.
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