Electromagnetic wave communication systems

#There has been much recent interest in the application of electromagnetic wave techniques to the solution of a variety of problems such as civil aviation security, crash sensors, automatic braking devices, vehicular location systems and others. Many of these applications, such as, for example, crash sensors for use in automobiles, involve a high density of users in relatively close proximity. In such cases, it becomes readily apparent that means must be provided to minimize crosstalk in order to avoid spurious indications of a crash situation when, in fact, none exists. The obvious solution to this problem is to provide a separate channel for each user. However, with millions of vehicles on the road, this approach would consume an undue amount of frequency spectrum and, as such, is not an attractive solution to the problem. Nor would it be economically feasible to incorporate sophisticated circuitry into such devices as a means of avoiding the problem of spurious indications if, indeed, it could be done at all. Obviously, if these problems are to be resolved it must be done in an economic manner, economic in both cost and in the use of the available frequency spectrum.

#In accordance with the present invention, the above referred to economies are realized by operating solely at a transmission frequency that is highly attenuated by the atmosphere. For purposes of the present invention, highly attenuated shall be understood to mean that the absorption loss through the atmosphere is about 10 db per kilometer or greater. This would include, for example, the band of frequencies between 55 to 63 gigahertz. In particular, by transmitting at a carrier frequency of 60 gigahertz, maximum attenuation by the oxygen in the atmosphere is realized, significantly reducing the opportunity for crosstalk. More generally, an electromagnetic communication system, in accordance with the present invention, comprises one or more transmitters and receivers communicating through the atmosphere, and operating solely at a transmitted carrier frequency that is highly attenuated by at least one of the constant components of the atmosphere.

#It is an advantage of the invention that it conserves spectrum by permitting the reuse of the same frequency in situations which would normally require a plurality of different frequencies.

#FIG. 1 shows the attenuation of radiant energy by oxygen as a function of frequency;

#FIG. 2 illustrates a first application of the principles of the present invention in connection with vehicular control systems;

#FIG. 3 illustrates a second application of the principles of the present invention in connection with CATV systems; and

#In the design of an electromagnetic wave transmission system of any type, one of the most important considerations is the operating frequency. This is typically chosen with great care to have the lowest loss in the transmission medium, and for obvious reasons. A high loss requires the use of many repeaters and, ultimately, very low noise, sensitive receivers. There are, however, a variety of situations where efficient transmission can be a disadvantage. These typically involve situations where transmission is through the atmosphere, distances are relatively short, and there is a high density of users. For example, in a high density system with efficient transmission, the signal of any one user would have the potential of covering a much larger area than the user requires. Thus, to avoid crosstalk, each user would require a different operating frequency. To conserve spectrum, the frequencies would have to be closely spaced, requiring highly selective receivers, which, in turn, would be relatively expensive.

#The present invention avoids all of these related complications by operating solely within a band of frequencies for which transmission through the atmosphere is very lossy. For example, FIG. 1 derived from an article entitled "Millimeter-Wave Communication through the Atmosphere", by D. C. Hogg, published in the Jan. 5, 1958 issue of Science, pages 39-46, shows the attenuation in decibels per kilometer as a function of frequency for transmission through the atmosphere. It will be noted that the attenuation peaks to approximately 20 db per kilometer at about 60 gigahertz due to absorption by oxygen. Since oxygen is a relatively constant component of the atmosphere, the range of radiation system operating at or near 60 gigahertz would be severely restricted. This, in turn, would permit multiple use of this same band of frequencies with great conservation of spectrum. In addition, the opportunity for crosstalk and multiple path interference would be greatly reduced. The following discussion is illustrative of some of the variety of applications to which the principles of the present invention are applicable.

#Referring again to the drawings, FIG. 2 shows a first application of the principles of the present invention to motor vehicular control. Specifically, the object is to provide each motor vehicle operator with information as to the movements of the motor vehicles immediately ahead and/or behind him. In particular, a change in the speed of any one of these vehicles is of importance on a high speed, highly congested parkway. Accordingly, each of the vehicles 1, 2 and 3, is equipped with a forward looking radar 11, 12 and 13. While not shown, each can also be equipped with backward looking radars as well. Except for the frequency of operation, these can be any of the well-known Doppler radars currently used by the police to detect speeding violations, or devices of the type described in U.S. Pat. Nos. 2,841,782; 2,584,509; 2,891,243 and 3,383,678. Also see, "Radar Car Control Systems Point Way to Automated Travel," published in the Jan. 17, 1972 issue of Electronics, page 54.