The caller pushed the portable phone’s off-hook button. For a split second, the telephone—a new type of computerized, walkie-talkie-size portable—”chatted” inaudibly with a minicomputer in another building. Then I heard a familiar dial tone, and the caller tapped the pushbutton keyboard, placing a call around the world to Australia.
Motorola’s Communications Division was demonstrating its Dynatac phone system in a New York Hilton penthouse suite. For each call, the portable was tied directly into a telephone exchange several blocks away over an ultra-high-frequency (uhf) radio signal. Dynatac bypasses the mobile-telephone operators required to place calls with conventional mobile and portable phones.With Federal Communications Commission approval, the first Dynatac system may be operating in New York by 1976. Using untapped frequencies above 900 mHz, it would use a complex computer-controlled transmitter and receiver network (as in photo and drawings) for thousands of simultaneous on-the-go calls. Today’s badly overcrowded radio-telephone bands (450 mHz and below) are limited to a few dozen calls at one time in major cities.
“In a city where the Dynatac system is installed,” says John Mitchell, division general manager, “it will be possible to make telephone calls while riding in a taxi, walking down the city’s streets, sitting in a restaurant, or anywhere else a radio signal can reach.
“We expect there’ll be heavy usage by widely diverse groups—businessmen, journalists, doctors, housewives—virtually anyone who needs or wants telephone communications in areas where conventional telephones are unavailable,” Mitchell said. You would not need a license to operate a portable, but would lease it from a common carrier—Motorola, a phone or radio company.
To find out how Dynatac works, I visited Motorola’s sprawling plant near Chicago where the company produces a variety of communications gear. Martin Cooper, director of systems operations, first described the new solid-state technology required for the 45-ounce portable, then explained how the computer-controlled transmitters and receivers would function.
New uhf circuits. “Getting up to 1000 mHz is pushing everyone’s state of the art,” Cooper said. So far, the 900-1000-mHz uhf region has been used for point-to-point applications such as traffic-light control. Opening the band for land mobile communications, first proposed by the FCC in 1970, now requires new uhf semiconductors that will operate efficiently at low battery voltages. Manufacturers are just introducing integrated circuits with the ultrafine chip structures needed for uhf.
Using these new ICs, the portable’s 14-volt battery powers the one-watt uhf transmitter for 12 three-minute calls, and provides 12 hours of standby reception for incoming calls.
FM communication over Dynatac is duplex with VOX (voice-operated transmission). Duplex means you can talk and listen at the same time, in contrast to press-to-talk mikes on private-dispatch and military radios. When you transmit, an LED (light-emitting diode) on the portable glows.
Most of the portable’s complex circuitry is used to generate 380 transmission channels and “talk” in binary computer language with central-processor computers. Fourteen LSI (large-scale integration) circuits, containing some 3000 transistors, are used for these functions.
Urban areas where Dynatac will be used are notorious for severe RF interference from vehicles, fluorescent lighting, and other sources. “We have to be concerned about an ignition pulse knocking out a bunch of data,” says Cooper. “What you’re looking for is the optimum way of plowing through this interference.”
The electronic chitchat between the portable and central processor, which goes on inaudibly even while you’re talking, is relatively immune to interference. The key is redundancy—extra bits of digital information beamed over the air. Losing some bits won’t destroy a message.
Here’s how the transmitter and receiver network would be set up: Transmitters, each surrounded by several receivers, would be strategically located throughout a city. The range of each 16-watt transmitter will correspond to the area’s population density and expected phone use by subscribers.
Coverage and population. In super-dense Manhattan, for example, a transmitter and its antenna may be designed to cover a 15-block area. Another transmitter in a residential Brooklyn area may cover several miles. As the number of subscribers grows, more transmitters would be added. The range of existing transmitters would then have to be reduced. Numerous receivers are required because the portable’s output is low (one watt).
Although Dynatac would use only 380 channels, through geographic reuse thousands of simultaneous calls and some 200,000 subscribers are possible (not everyone telephones at once). This means a caller in one part of the city would be using one channel while others use the same channel in nearby areas.
To avoid interference, the central processor would assign the same frequency to every third transmitter. A channel might be used simultaneously by 40 callers in later, high-density stages of Dynatac.
To understand how the portable operates with the central processor, let’s trace a call from start to finish: Making your call. When you push the off-hook button the portable is sitting on a signaling channel. The portable’s supervisory section (see photos) sends a stream of digital data from the transmitter.
Several nearby receivers pick up the data, and send it by telephone wire, microwave, or cable to the central-processor station. There, minicomputers sense which signal is strongest and select the best receiver and transmitter for your call. A burst of data from the central processor is beamed from a nearby transmitter to your portable.
This data tells the supervisory section which talking channel the synthesizer section should electronically generate. Now the central processor ties you into a telephone exchange and you hear a dial tone. The portable is then used like a cradle pushbutton telephone.
“We’ll probably have several sites near telephone exchanges with a trunk going in; one pair of wires for every customer,” says Cooper.
If you travel through the city while telephoning, the central processor will sense changes in your ¦ signal and switch you to other transmitters and receivers. It may also change your channel if you approach another portable on the same frequency. The inaudible switching takes place in a few thousandths of a second.
Adjusting power output. Also as you move around, Dynatac will use automatic output control (AOC) to adjust your portable’s power output, minimizing interference and conserving battery power. When you’re close to a receiver you won’t need much power. Each base transmitter sends signals to your unit’s supervisory section, telling it to crank transmitter power up or down.
When you’ve completed your call, you push the on-hook button. This sends digital data to the processor, telling it to “hang up”—break the connection—at the phone exchange.
What about incoming calls? Say you’re in a taxi with your portable inside an attache case. A call to your portable’s number comes into the telephone exchange and is sensed by the central processor. A computer sends out a digital code unique to your portable over every transmitter in the city. Your portable, on standby, picks up the signal and sends back data saying, “I’m here.”
Now that the central processor has located your portable, it sends out another signal—now from one transmitter. This data switches your portable to a talking channel and tells it, “ring your bell.” You push the off-hook button to answer.
Pending FCC approval, Motorola hopes to install a 10-to-20-transmitter system in New York at a cost of $5 million. Motorola, however, expects to supply hardware to phone companies here and abroad rather than operate Dynatac systems itself.
Crowded spectrum. The company faces stiff competition for the new block of radio spectrum. Two years ago, AT&T proposed a mobile or car system. Other users of the overcrowded lower-frequency bands —fire, police, taxis—would like to expand, too. Dynatac would use a small slice of the new spectrum.
Costs to subscribers would be similar to current mobile-phone rates initially: $60 to $100 a month. Later, with high-density use, monthly rates could drop to $30 or less.
The portable’s size will undoubtedly shrink, also. Eventually, the unit could fit your shirt pocket. Dynatac’s computer-controlled network may well be a forerunner to a Dick Tracy-type wrist-phone system you may wear one day.
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