From the archives: When 1970s cellular technology made ‘traveling telephones’ more accessible

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Until Heinrich Hertz discovered radio waves in 1887, the vast and invisible electromagnetic spectrum was a silent wilderness, punctuated by nature’s static bursts. But Hertz set in motion a new era that would quickly fill that void with low-end radio waves, mid-range microwaves, and high-end gamma rays (medical imaging). Despite the breadth of the wireless spectrum, a small slice (roughly 500 MHz–3.5 GHz) has been staked out like no other: As Popular Science reported in January 1978, it turns out to be the optimal range to propagate signals to and from mobile devices, like traveling telephones. 

Scarcity has driven mobile network innovation ever since the first frequencies were set aside by the FCC in the 1940s. Even as the size of mobile phones shrank from crate-sized in the 1950s and ‘60s to brick-sized in the ‘70s, the pinch was less phone form factor than it was inefficient use of spectrum. Until the late ‘70s, each new mobile phone that went live in a city required its own dedicated frequency, the way a local radio station requires its own channel. As John Mason reported, “because radio channels are so limited, there are long waiting lists in most cities for mobile-phone service.” 

In 1974, to address pent-up demand, the FCC released more spectrum but insisted that companies find a better way to use it. As Mason explains with geeky precision, cellular technology got its name from its design, deploying short-range transmission towers to divide large regions, like cities, into honeycomb-shaped cells, enabling frequency reuse. More than any other technology, cellular (first conceived in 1948 but not computationally practical until the 1970s) paved the way for the mobile era. 

Since the ‘70s, the FCC has continued to release spectrum. A mere fraction of the mobile-device slice sold for more than $20 billion in a November 2021 auction (the 3.45 GHz band). Cell networks continue to aim higher on the spectrum, shrinking cells to overcome propagation limitations and deliver more data. Today’s 5G technology will be capable of reaching all the way to 40 GHz to achieve blazing data-delivery speeds.

“Traveling Telephone–new technology expands mobile/portable service” (John Mason, January 1978)

There’s a button labeled SND on Motorola’s futuristic-looking Pulsar II radiotelephone. I pushed it, and a number stored in its microcomputer memory began stepping, digit by digit, across the red LED handset display. This amazing car telephone not only remembers 10 often-used phone numbers, but calls any of them at the press of one button.

Earlier, at Motorola’s Communications Group plant outside Chicago, I had picked up a portable Dynatac (Dynamic adaptive total area coverage) phone, tapped out a number on its Touch-Tone keypad, and called my New York office. Electronic gear at the plant patched my call directly into the phone network. A mobile/portable telephone operator wasn’t needed. Advanced mobile and portable telephones are already in use throughout the country. Motorola markets its $890 Pulsar II (less transceiver) for 150- and 450-MHz systems; its Dynatac portables aren’t available yet, although other compact portable phones are sold and leased.

But while fancy hardware for on-the-go telephone calls is readily available, the radio frequencies needed to carry today’s heavy volume of mobile/portable calls are not available. Because radio channels are so limited, there are long waiting lists in most cities for mobile phone service.

One cure for this congestion, according to communications experts I’ve talked with, is a blend of the latest in computer and RF technology and the concept of radiotelephone cells and frequency reuse. The two phones I used are examples of this new technology.

The concept is simple. A conventional mobile phone system uses one high powered central transmitter and a sensitive receiver serving all mobile units in the area. Thus a single frequency can be used by only one mobile unit at a time. In a cellular arrangement, the high-powered central transmitter/receiver is not used. Instead, many smaller transmitter/receivers that each cover only a few square miles are installed. Now, a given frequency can be used simultaneously by mobile units in several different areas or “cells” without interfering with each other. The result: A lot more calls can be placed on a given frequency band, and a lot of those people on the waiting list for portable phones can get service.

While the system is simple in principle, it is enormously complicated in practice. How do you decide which mobile unit gets which frequency at which time? And how do you make sure that two adjacent cells aren’t using the same frequency simultaneously—a situation that could possibly cause interference? The answer is a complicated system of computer control. While details vary from system to system and even within systems—more about that later—here’s how one typical setup might operate.

In the system now planned by Motorola for the Washington-Baltimore area, the entire region would be broken into five hexagonal cells, each with an 11-mile radius (see diagram).

The base antenna serving some hexagonal cells has six V-shaped sector transmit-and-receive antennas, breaking each of these areas into six smaller cells.

What happens when you’re driving around and somebody calls your number? “The system first has to locate a mobile unit in order to assign a proper channel in the proper cell,” says Motorola group product manager Andrew Daskalakis.

Pinpointing and monitoring your location is accomplished with computers nine will ultimately be used in the Washington-Baltimore developmental system.

Computer in your trunk

For an incoming call, computer data on special signaling channels are beamed over all cell transmitters. A powerful microcomputer built into the bread-box-size transceiver in your car trunk recognizes your mobile code. Your computer then transmits a signal that instantly tells a base-station computer what cell you’re in.

Next, to determine how far you are from the cell antenna, the main computer sends a six-kHz tone to your mobile. This tone triggers a transponder that retransmits the tone back to the base-station receiver. By comparing phase differences between the transmitted and received tone, the distance from the base to your car is computed.

Using this distance information, and the strength of your computer’s signal, the base computer can crank the power output of your mobile transmitter up or down. “It takes care of the portable-in-the-high-building problem, and the mobile-on-a-high hill problem,” says Daskalakis. Traveling telephones at those elevations can transmit much farther than normal, interfering with other cells.

The inaudible chit chat between computers—redundantly coded to prevent errors from static or fading—takes only a split second. Your mobile is automatically tuned to a voice channel. Once you’re “linked up,” the main computer actuates the “ringer” in your mobile.

After you answer your call, and while you’re driving, the main computer periodically scans your mobile to monitor your location. If you move to where another cell transceiver would provide better reception, the base computer switches you instantly. A similar “handshake” between computers occurs just before the dial tone when you place a call.

The system just described would go into operation in stages. In less heavily populated areas, a less complicated system would be adequate. There, the receiver section of the base station would use multiple antennas to split the cell into six pie-shaped sectors. These high-gain receive antennas can pick up signals from low-power (one watt) portable telephones. But a single omnidirectional transmit antenna could cover the whole cell. Another cellular system is now beginning experimental operation in Chicago. It is operated by Illinois Bell, and, while the principles are the same, it differs somewhat in operating details from the Motorola system. The Chicago system, for example, will initially have 10 cells, each with an eight-mile radius from its central transmitter. Cell coverage will blanket a 2100-square-mile region in Chicago.

This eight-mile cell system, however, is less sophisticated than the setup originally proposed by American Telephone & Telegraph Co. That system, presented in 1971 to the FCC, specified four-mile radius cells, with directional antennas at alternate corners of each hexagonal cell (see diagram). With four-mile cells, frequency reuse would be possible in cells 18 miles apart.

In the system being built, frequency reuse is only possible in two cells, about 48 miles apart. Conventional mobile systems usually have a reuse distance greater than 100 miles.

“We’re authorized to serve 2500 customers,” says James Troe of Bell Laboratories’ telephone service trial department, which is setting up the new system. “We’re sizing the system and channel capacity to accommodate that level,” he said, to explain why AT&T is building a less costly system.

Smaller cells would come later to meet growing demand. Companies can expand cellular systems to serve tens or hundreds of thousands—simply by adding more transmitters, shrinking cell sizes, and reusing the frequencies more often.

While the Illinois Bell system is basically compatible with the Washington-Baltimore setup, there are some differences. A Motorola Dynatac portable, for example, would not function adequately in the initial Chicago system, which lacks sectorized high-gain receive antennas, although a Dynatac car telephone would.

The drive to develop systems that use scarce radio-spectrum space efficiently goes back to 1968, when the FCC began considering what to do about the tremendous demand for mobile telephone service and the lack of frequency space to satisfy that demand. At that time, mobile phones operated in the 35-, 150-, 450-MHz bands.

In 1974, after extensive hearings and delays the FCC set aside a slice of UHF frequencies from 806 MHz to 947 MHz Parts of this so-called 900-MHz band were allocated for private land mobile companies, public service use, and utilities such as telephone companies and Radio Common Carriers (RCC’s) that now operate phone and pocket pager service [PS, July ’77] in the conventional 35-, 150-, and 450-MHz frequency bands.

When the FCC allocated part of the 900-MHz band for mobile telephone use, it also specified that companies interested in using the band would have to design systems to meet growing service demands.

But though the basic decision was made in 1974 and the equipment is ready, no such system is at present operational (the Chicago system is now under limited test, but is not yet available for use by the public). One principal factor blocking final authorization: The 700 small RCC’s that operate a lot of the country’s radiotelephone and paging service don’t want the competition. Almost anybody can go into business and serve a local area as long as he needs only one central transmit and receive location. But the new systems, which would require many base stations plus complex computer control networks, would cost more than most RCC’s could afford. Thus the RCC’s have been protesting vigorously at FCC hearings, filing court cases, and otherwise obstructing movement.

Bell is now operating in Chicago under an experimental license. Motorola, which has signed a contract with a Baltimore RCC, American Radio Telephone Service, has received FCC approval to go ahead and build its proposed Washington-Baltimore system.

Meanwhile, some experts—and some RCC’s—are arguing that cells aren’t really the most efficient way to expand traveling-telephone service. They recommend several alternative concepts-such as the use of digitized voice signals.

Other technologies

A consortium of three RCC’s has filed an application to try this technique in the Washington, D.C. area. This application, of course, is competing before the FCC with the Motorola application. The RCC noncellular concept, which was developed on paper by Harris Corp., requires one extremely powerful (375 kw) transmitter site. Voice signals would be digitized and beamed out as bursts of pulses in packets; each packet is coded for separate mobiles.

Yet another concept known as spread-spectrum is receiving attention among communications experts. Used extensively by the military, spread-spectrum signals are highly immune to jamming and interception. Imagine that each FM station spread its signal across the entire FM band from 88 MHz to108 MHz. Each station, however, would encode its output so that a special filter in your FM set could decode its signal.

For mobile telephone communications, recently developed semiconductor and electronic filter technologies might make it possible for everyone in the country to have a unique spread-spectrum decoding circuit for a traveling phone.

While you can expect to hear about various technologies for mobile systems in coming years, AT&T executive vice-president Thomas Nurnberger thinks expansion of the Chicago cellular system concept “will make it possible in the future for virtually anyone on the move to have a telephone in cars or temporary locations.” Nurnberger cites the boom in CB radios as evidence of a pent-up national need for two-way communication that AT&T thinks can be satisfied with cellular technology.

From the archives: When 1970s cellular technology made ‘traveling telephones’ more accessible
January 1787 cover of Popular Science featuring a cover article on mobile phone technology, waterless toilets, and microelectronics.

Some text has been edited to match contemporary standards and style.

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