Telecommunications – Transmitter – Plural separate transmitters or channels
Reexamination Certificate
1999-12-20
2001-05-15
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter
Plural separate transmitters or channels
C455S060000, C455S063300, C455S272000, C455S273000, C455S276100
Reexamination Certificate
active
06233435
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for communications with electromagnetic waves. The system preferably operates at millimeter-wave frequencies and uses polarization diversity.
2. Description of Prior Art
The information transmission capacity of a communications system can be substantially increased with the use of polarization diversity. This is true for both one-way and two-way communications systems. Vertical and horizontal polarizations are often used in satellite communications and other point-to-point microwave links to isolate both transmit and receive signals, or to increase information capacity.
For local communications systems using a millimeter-wave carrier, polarization cross-talk caused by precipitation is a commonly encountered problem for systems using dual linearly polarized signal transmission. Further, where a transmission link involves successive reflections by buildings and other objects, as in an urban environment, considerable variations in the polarization state of the signals occur, making signal isolation by orthogonal polarization less effective.
For a given frequency, a circularly polarized plane or quasi-plane electromagnetic wave propagating in open space may have its field vectors revolving clockwise (CP), or counterclockwise (CCP). Two such waves, rotating oppositely with respect to each other, are orthogonal to each other and may be isolated with proper antenna feeds and electronic circuitry. However, precipitation and/or reflection/diffraction from buildings and other obstacles can distort the waves and cause elliptical polarization. If the waves become excessively elliptically polarized, the information carried by the waves cannot be retrieved.
U.S. Pat. No. 4,747,160 teaches a low power multi-function cellular television system capable of two-way communication services. An omnidirectional transmitter transmits vertical and horizontal linear polarized waves. The system taught by the '160 patent preferably operates in the 27.5 GHz to 29.5 GHz millimeter wave band.
U.S. Pat. No. 4,264,908 teaches a polarization correction network that automatically compensates for cross polarization caused by, for example, precipitation. The network transmits vertical and horizontal linearly polarized waves.
U.S. Pat. No. 4,106,015 discloses a radar system that eliminates rain echo signals. Pulsed, polarized waves are transmitted, and two separate receiving channels receive orthogonal components of a rain echo signal. The rain echo signal is eliminated by adjusting an amplitude of the orthogonal components of the rain echo signal, and then adjusting the phase of the signals to be opposite with respect to each other.
U.S. Pat. No. 4,737,793 discloses a dual-polarized microstrip antenna capable of simultaneously transmitting mutually orthogonal polarizations, including clockwise and counterclockwise circularly polarized waves, to double the capacity of a given frequency band.
U.S. Pat. No. 4,146,893 teaches a satellite communications system which compensates for polarization distortion caused by precipitation and incomplete polarization characteristics of antennas by predistorting a circularly polarized wave to an elliptically polarized wave. As the elliptically polarized wave encounters the depolarizing medium, a circular wave forms and is received by the satellite.
U.S. Pat. No. 3,956,699 discloses an electromagnetic wave communications system which transmits and receives waves having mutually orthogonal polarizations. The system provides polarization control prior to power amplification when transmitting, and subsequent to amplification when receiving.
U.S. Pat. No. 5,337,058 teaches a fast switching lens which is positioned in front of a radar antenna to manipulate the polarization of a transmitted wave to various polarizations. The lens can also receive waves of various polarizations.
U.S. Pat. No. 4,329,687 discloses a radar system which alternately radiates clockwise and counterclockwise circularly or elliptically polarized waves. A relatively high signal-to-clutter ratio is achieved by analyzing the phase differences between the two orthogonal components of the transmitted wave and the phase differences of the two orthogonal components of the received wave.
The prior art references discussed above fail to disclose a method or apparatus for a communications system that can restore circular polarization to a distorted wave, and can operate in an urban environment at the millimeter-wave frequencies. Thus, it is apparent that a communications system that operates in the millimeter-wave frequencies, provides dual polarization, and achieves relatively high signal restoration and isolation is needed.
SUMMARY OF THE INVENTION
It is one object of this invention to provide a method and apparatus for communications with electromagnetic waves that eliminates or greatly reduces the fading effects caused by precipitation.
It is another object of this invention to provide a method and apparatus that uses dual polarization to increase channel capacity and in which the effects of cross-polarization are negligible.
It is yet another object of this invention to provide a method and apparatus for a dual-polarization two-way communications system that can provide communications in millimeter-wave frequencies in urban environments despite negative effects of reflection and/or diffraction of waves due to obstacles.
These and other objects are achieved with a communications system in which dual polarization is utilized to effectively double the capacity of a given frequency band. According to one preferred embodiment of this invention, elliptically and/or circularly polarized waves are simultaneously broadcasted from a transmitting antenna. A first wave rotates in a direction opposite with respect to a second rotating wave. At millimeter-wavelength frequencies, such as those generally over 18 GHz, precipitation such as rain, snow, or fog, and diffraction/reflection due to urban obstacles, such as buildings, can attenuate and depolarize such waves. Consequently, circularly polarized waves can become elliptical, and the axes of the elliptical waves can rotate. Without proper signal isolation features, the information carried by such distorted waves can be irretrievable.
The communications system according to this invention includes an adaptive receiver capable of restoring circular polarization to such elliptically polarized waves. According to one preferred embodiment of this invention, the adaptive receiver includes an electromechanically driven antenna with an orthogonal-mode feed that receives the dual rotating waves. Components of each of the waves enter each of two channels. The frequency of the signals in the channels can be down-converted to an intermediate frequency (IF). If the received waves are elliptical, at any given time, the signals in the channel corresponding to the feed aligned with the major axis of the counter-rotating waves will have a greater magnitude than the signals in the channel corresponding to the feed aligned with the minor axis of the waves. Each channel preferably has automatic gain control circuitry to equalize the magnitude of the signals in the channels. Portions of the signals in each channel undergo a phase shift of ±90° and are combined with the signals of the other channel to isolate one rotating signal from the other rotating signal. A phase detector can detect the loss of quadrature between the two channels and emit a signal to a servomotor that rotates the orthogonal-mode feed of the antenna to align the feed with the major and minor axes of the signals.
According to one preferred embodiment of this invention, an antenna diversity control switch controls a plurality of antennas to track the incident signal. The diversity control switch samples the signal strength in each channel during the IF stage, and selects an antenna with sufficient signal intensity. Because the feeds of the selected antenna may not be precisely aligned with the major and minor axes of th
Hunter Daniel
Pauley Petersen Kinne & Fejer
Telecommunications Equipment Corporation
Woldetatios Yemane
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