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Radio is often viewed as having evolved in two principal phases. The first was the pioneer phase, during which the fundamental technologies were developed, initial technical bugs were ironed out, applications were created (or at least conceived), and bureaucratic and governmental haranguing over the roles of various agencies initiated. This phase was characterized by the development and acceptance of FM transmission, the dominance of military and police usage of radio, and the challenge of building a radio that could withstand the rigors of mobile life. The pioneer phase lasted until the early 1940s, when the commercial phase began. During the 1940s, radio technology advanced to the point that FMbased car phones were commercially available. Initially, AT&T provided mobile telephone service under the umbrella of its nationwide monopoly. In 1949, however, the Justice Department filed suit against AT&T, settling the case with the Consent Decree of 1956. Under this Consent Decree, AT&T kept its telephone monopoly, but was forced to give up a number of other lines of business, including the manufacture of mobile radiotelephones. This allowed companies like Motorola to enter the fray and paved the way for the creation of radio common carriers, or RCCs, much to AT&T s chagrin. These RCCs were typically small businesses that offered wireless service to several hundred people in a fixed geographical area.
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Before proceeding further, let s introduce some fundamentals of radio transmission. For radio waves to carry information such as voice, data, music, or images, certain characteristics of the waves are changed or modulated.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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Those characteristics include the amplitude, or loudness, of the wave; the frequency, or pitch, of the wave; and the phase, or angle of inflection, of the wave. By modulating these characteristics, a carrier wave can be made to transport information, whether it is a radio broadcast, a data stream, or a telephone conversation. (Please review this topic in 1 for more information.) Transmission space is allocated based on ranges of available frequency. This range is known as a spectrum, a word that means a range of values. The radio spectrum represents a broad span of electromagnetic frequencies between one kilohertz (1,000 cycles per second) and ten quintillion (that s ten followed by 19 zeroes) cycles per second. Most radiobased applications operate at frequencies between 1 kHz and 300 GHz. One concern that has surfaced repeatedly over the years since the inception of radio is spectrum availability. Different services require differing amounts of spectrum for proper operation. For example, a modern FM radio channel requires 30 kHz of bandwidth, while an AM channel only requires 3 kHz. Television, on the other hand, requires 6 MHz. As you might imagine, battles between the providers of these disparate services over spectrum allocation became quite heated in the years that followed.
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The first FM radiotelephone systems were massively inefficient. They required 120 kHz of channel bandwidth to transport a 3 kHz voice signal. As the technology advanced, though, this requirement was reduced. In the 1950s the FCC mandated that channels be halved to 60 kHz, and by the 1960s things had advanced to the point that the spectrum could be further reduced to 30 kHz, where it remains today in analog systems. Realize that this is still a 10:1 ratio. AT&T introduced the first commercial mobile telephone service in St. Louis in 1946. It relied on a single, high-power transmitter placed atop one of the city s tallest buildings and had an effective service range of about 50 miles. This FM system was fully interconnected with AT&T s wireline telephone network. The service was not cheap. There was a basic $15 monthly charge, and usage was billed at a flat $0.15 per minute. In spite of the cost, the service was quite popular; a fact that led to its undoing. System engineers based their design on existing radio systems, which were primarily designed for dispatch applications. The traffic patterns of
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