Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-01-18
2001-10-30
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06310704
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to signal processing systems and, more particularly, to apparatus and methods for receiving and processing signals that share a common receiver frequency band at the same time, referred to as cochannel signals. Even two signals transmitted on slightly separated frequency bands may be “cochannel” signals as seen by a receiver operating to receive signals on a bandwidth that overlaps both of the signals. In a variety of signal processing applications, there is a need to recover information contained in such multiple, simultaneously received signals. In the context of this invention, the word “recover” or “recovery” encompasses separation of the received signals, “copying” the signals (i.e., retrieving any information contained in them), and, in some applications, combining signals received over multiple paths from a single source. The “signals” may be electromagnetic signals transmitted in the atmosphere or in space, acoustic signals transmitted through liquids or solids, or other types of signals characterized by a time-varying parameter, such as the amplitude of a wave. In accordance with another aspect of the invention, signal processing includes transmission of cochannel signals.
In the environment of the present invention, signals are received by “sensors.” A sensor is an appropriately selected transducer for converting energy contained in the signal to a more easily manipulated form, such as electrical energy. In a radio communications application, electromagnetic signals are received by antennas and converted to electrical signals for further processing. After separation of the signals, they may be forwarded separately to transducers of a different type, such as loudspeakers, for converting the separated electrical signals into audio signals. In some applications, the signal content may be of less importance than the directions from which the signals were received, and in other applications the received signals may not be amenable to conversion to audible form. Instead, each recovered signal may contain information in digital form, or may contain information that is best understood by displaying it on a chart or electronic display device. Regardless of the environment in which the present invention is employed, it is characterized by multiple signals received by sensors simultaneously at the same or overlapping frequencies, the need to separate, recover, identify or combine the signals and, optionally, some type of output transducer to put the recovered information in a more easily discernible form.
2. Description of Related Art
Separation and recovery of signals of different frequencies is a routine matter and is handled by appropriate filtering of the received signals. It is common knowledge that television and radio signals are transmitted on different frequency bands and that one may select a desired signal by tuning a receiver to a specific channel. Separation and recovery of multiple signals transmitted at different frequencies and received simultaneously may be effected by similar means, using multiple tuned receivers in parallel. A more difficult problem, and the one with which the present invention is concerned, is how to separate and copy signals from multiple sources when the transmitted signals are at the same or overlapping frequencies. A single sensor, such as an antenna, is unable to distinguish between two or more received signals at the same frequency. However, antenna array technology provides for the separation of signals received from different directions. Basically, and as is well understood by antenna designers, an antenna array can be electronically “steered” to transmit or receive signals to or from a desired direction. Moreover, the characteristics of the antenna array can be selectively modified to present “nulls” in the directions of signals other than that of the signal of interest. A further development in the processing of array signals was the addition of a control system to steer the array toward a signal of interest. This feature is called adaptive array processing and has been known for at least two to three decades. See, for example, a paper by B. Widrow, P. E. Mantey, L. J. Griffiths and B. B. Goode, “Adaptive Antenna Systems,” Proceedings of the IEEE, vol. 55, no. 12, pp. 2143-2159, December 1967. The steering characteristics of the antenna can be rapidly switched to receive signals from multiple directions in a “time-sliced” manner. At one instant the antenna array is receiving a signal from one source and at the next instant, from a different source in a different direction, but information from the multiple sources is sampled rapidly enough to provide a complete record of all the received signals. It will be understood that, although steered antenna array technology was developed principally in the communications and radar fields, it is also applicable to the separation of acoustic and other types of signals.
In the communications field, signals take a variety of forms. Stated most generally, a communication signal typically includes a carrier signal at a selected frequency, on which is impressed or modulated an information signal. There are a large number of different modulation schemes, including amplitude modulation, in which the amplitude of the signal is varied in accordance with the value of an information signal, while the frequency stays constant, and frequency or phase modulation, in which the amplitude of the signal stays constant while its frequency or phase is varied to encode the information signal onto the carrier. Various forms of frequency and phase modulation are often referred to as constant modulus modulation methods, because the amplitude or modulus of the signal remains constant, at least in theory. In practice, the modulus is subject to distortion during transmission, and various devices, such as adaptive equalizers, are used to restore the constant-modulus characteristic of the signal at a receiver. The constant modulus algorithm was developed for this purpose and later applied to antenna arrays in a process called adaptive beam forming. The following references are provided by way for further background on the constant modulus algorithm:
B. Agee, “The least-squares CMA: a new technique for rapid correction of constant modulus signals,”
Proc. ICASSP
-86, pp. 953-956, Tokyo, Japan, April 1986.
R. Gooch, and J. Lundell, “The CM array, an adaptive beamformer for constant modulus signals,”
Proc. ICASSP
-86, pp. 2523-2526, Tokyo, Japan, April 1986.
J. Lundell, and B. Widrow, “Applications of the constant modulus adaptive algorithm to constant and non-constant modulus signals,”
Proc. Twenty
-
Second Asilomar Conference on Signals, Systems, and Computers,
pp. 432-436, Pacific Grove, Calif., November 1988.
B. G. Agee, “Blind separation and capture of communication signals using a multi-target constant modulus beamformer,”
Proc.
1989
IEEE Military Communications Conference,
pp. 340-346, Boston, Mass., October 1989.
R. D. Hughes, E. H. Lawrence, and L. P. Withers, Jr., “A robust adaptive array for multiple narrowband sources,”
Proc. Twenty
-
Sixth Asilomar Conference on Signals, Systems, and Computers,
pp. 35-39, Pacific Grove, Calif., November 1992.
J. J. Shynk and R. P. Gooch, “Convergence properties of the multistage CMA adaptive beamformer,”
Proc. Twenty
-
Seventh Asilomar Conference on Signals, Systems, and Computers,
pp. 622-626, Pacific Grove, Calif., November 1993.
The constant modulus algorithm works satisfactorily only for constant modulus signals, such as frequency-modulated (FM) signals or various forms of phase-shift keying (PSK) in which the phase is discretely or continuously varied to represent an information signal, but not for amplitude-modulated (AM) signals or modulation schemes that employ a combination of amplitude and phase modulation. There is a significant class of modulation schemes used known as M-ary quadrature amplitude modulation (QAM), used for transmitting digital data, whereby the instanta
Dogan Mithat Can
Stearns Stephen Deane
Heal Noel F.
Pascal Leslie
Sedighian M. R.
TRW Inc.
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