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(Page 12) Cellular Telephone Basics, Appendix: Page 1 of Bell System Overview
Learn the present by looking at the past. Here's some great reading on the transition from mobile telephone service to cellular. It outlines the IMTS system that influenced tone signaling in AMPS, and gives some clear diagrams outlining AMPS' structure. This is from the long out of print A History of Engineering and Science in the Bell System: Communications Sciences (1925 -- 1980), prepared by members of the technical staff, AT&T Bell Laboratories, c. 1984, p.518 et. seq.:

A History of Engineering and Science in the Bell System: Communications Sciences (1925 -- 1980)

Channel Availability

Mobile telephone service began in the late 1940s. By the seventies, it included a total of thirty-three 2-way channels below 500 megahertz MHz), as shown in Table 11-2. The 35-MHz band, which is not well suited to mobile service (because of propagation anomalies), is not heavily used. The other bands are fully utilized in the larger cities. In spite of this, the combination of few available channels per city and large demand has led to excessive blocking. The FCC's recent allocation of 666 channels at 850 MHz for use by cellular systems (described below) should change this situation. This allocation is split equally between wire-line and radio common carriers (each is allocated 333 channels). In many areas, the wire-line carrier will be the local operating company.

Use of conventional systems on the new channels would increase the traffic-handling capacity by a factor of about 10. The cellular approach, however, will increase the capacity by a factor of 100 or more. How this increase is achieved is discussed later in this section. The potential for very efficient use of so valuable and limited a resource as the frequency spectrum was a persuasive factor in the FCC's decision.

Transmission Considerations

Radio propagation over smooth earth can be described by an inverse power law; that is, the received signal varies as an inverse power of the distance. Unlike fixed radio systems (for example, broadcast television or the microwave systems described in Chapter 9), however, transmission to or from a moving user is subject to large, unpredictable, sometimes rapid fluctuations of both amplitude and phase caused by:
Shadowing: This impairment is caused by hills, buildings, dense forests, etc. It is reciprocal, affecting land-to-mobile and mobile-to-land transmission alike, and changes only slowly over tens of feet.

Multipath interference: Because the transmitted signal may travel over multiple paths of differing loss and length, the received signal in mobile communications varies rapidly in both amplitude and phase as the multiple signals reinforce or cancel one another.

Noise: Other vehicles, electric power transmission, industrial processing, etc., create broadband noise that impairs the channel, especially at 150 MHz and below.

Because of these effects, radio channels can be used reliably to communicate at distances of only about 20 miles, and the same channel (frequency) cannot be reused for another talking path less than 75 miles away except by careful planning and design.
In a typical land-based radio system at 15 or 450 MHz, one channel comprises a single frequency-modulation (FM) transmitter with 50- to 2;0-watt output power, plus one or more receivers with 0.3- to 0.5 microvolt sensitivity. This equipment is coupled be receiver selection and voice-processing circuitry into a control terminal that connects one or more of these channels to the telephone network (see Figure 11-34). The control terminal is housed in a local switching office. The radio equipment is housed near the mast and antenna, which are often on very tall buildings or a nearby hilltop.

Conventional System Operation

Originally, all mobile telephone systems operated manually, much as most private radio systems do today. A few of these early systems are still in use but because they are obsolete, they will not be discussed here.
More recent systems (the MJ system at 150 KHz and the MK system at 450 KHz) [Improved Mobile Telephone Service or IMTS, ed.] provide automatic dial operation. Control equipment at the central office continually chooses an idle channel (if there is one) among the locally equipped complement of channels and marks it with an "idle" tone. All idle mobiles scan these channels and lock onto the one marked with the idle tone. All incoming and outgoing calls are then routed over this channel. Signaling in both directions uses low-speed audio tone pulses for user identification and for dialing. Compatibility with manual mobile units is maintained in many areas served be the automatic systems by providing mobile-service operators. Conversely, MJ and MK mobile units can operate in manual areas using manual procedures.

One desirable feature of a mobile telephone system is the ability to roam; that is, subscribers must be able to call and be called in cities other than their home areas. The numbering plan must be compatible with the North American numbering plan. Further, for land-originated calls, a routing plan must allow calls to be forwarded to the current location. In the MJ system, operators do this. Because of the availability of the MJ system to subscribers requiring the roam feature, the MK system need not be arranged for roaming.. .

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