Commercial cellular phone service got off the ground in Chicago 13 years ago for two reasons: The development of new computer-chip technology and a new way of thinking about radio frequencies.
Before cellular, wireless phones were expensive, difficult to use and rare. They worked pretty much like a taxicab dispatcher’s two-way radio. A limited number of radio channels were available for an urban area like Chicago, and when one channel was engaged in a conversation, it was unavailable for anyone else in the entire region.
Thus if a dozen or so wireless conversations were in progress around Chicago, all other customers would just have to wait to make a call until someone hung up.
The new way of thinking was the basic architecture of the new service, which gave cellular its name. The radio spectrum available for phone calls was used over and over within small portions of the region, perhaps a square mile or two each, instead of once in the entire metropolitan area.
By dividing Chicagoland into 20 or 100 or 200 cells instead of just one huge calling area, you could reuse available radio capacity in each cell. Capacity could be doubled by building a new cell site. As a side effect, the amount of power needed to send and receive signals diminished as geographic size shrank, so demand on batteries fell.
As a customer moved from one part of town to another, his call was handed off electronically from one cell antenna to another.
The technological foundation for all this is a computer system sophisticated enough to keep track of which equipment is handling which call, and where the customers are as they move through the system.
Without cheap and powerful computer chips, cellular systems would have been impossible, but as the cost of computing continues to fall, wireless phone systems become less expensive to operate and, perhaps, inevitable.
In Chicago today, nearly 20 percent of potential customers have wireless phones, and market growth shows no signs of slowing. Many analysts believe the mainstream of telephony’s future is wireless.
“Wireless is ushering in a new era of communications,” said Damon Vickers, an investment strategist with Janssen/Meyers Associates based in New York City. “Two-thirds of the world’s people have never talked on a telephone. You can bet that when they do, it will be wireless.”
And digital, too. Using the zeros and ones common to computer systems gives phone systems more clarity and flexibility in adding features. But the primary allure of digitizing phone signals within the industry comes from the increased capacity this technology offers. Compared with analog, 3 to 10 times as many digital conversations can be carried on the same piece of radio spectrum.
Just how much more conversational bang you get for your radio-wave buck depends upon how you process those digitized zeros and ones. CDMA and TDMA are the two leading and competing technologies designed to get more calls into the same bandwidth.
TDMA is short for Time Division Multiple Access and refers to carrying multiple conversations on a single radio channel by dividing each conversation into packets of data that are dropped into time slots and put back together at the other end. This is a well established, straightforward way to process digital information; a form of it called GSM for Global System for Mobile has been used in Europe for years. With this technology, three conversations can occur simultaneously on a single channel.
A more complex way to process digital data is called CDMA, for Code Division Multiple Access, and refers to scattering the information packets across several channels at once in a way that follows an intricate code that appears random. CDMA is much more difficult to execute than TDMA, but it enables systems to carry two or three times as many calls as TDMA does in a given amount of radio spectrum. CDMA also provides maximum protection against electronic eavesdropping. In fact, the technology was developed by researchers to provide the military with extremely secure communications.
A new consumer report on wireless technology has been prepared by the Telecommunications Research & Action Center, a Washington-based consumer group that finances its activities by selling reports to consumers. (TRAC didn’t do field tests to assure that CDMA’s potential advantages are actually available in the market, said Geoff Mordock, a TRAC research associate.)
TRAC compares digital telephony to a busload of people trying to have a conversation. In a TDMA bus, 50 people hold 25 conversations with each other by taking turns. One pair talks briefly, then stops to let the next pair speak, and so on.
This is a only a rough approximation of how TDMA works because by operating at lightning speed, the electronic system processes conversations without the annoying delays that would plague a busload of people trying this.
Following the analogy, a CDMA bus carries 50 people who hold 25 conversations with each other at the same time in 25 different languages. Everyone talks at the same level, with the two Spanish speakers concentrating on each other and ignoring the German, English, Italian and French spouted by their neighbors, much like ignoring background noise at a cocktail party.
CDMA’s potential benefits are many because it packs so much information into available radio space, and the technology fares very well in the TRAC report.
Because CDMA systems spread information throughout available spectrum, they use the air waves more efficiently and require fewer cell sites and towers to recycle capacity. Another advantage of CDMA is that its signals are recombined after transmission based on a code, so the problem of interference created by signals arriving at different times is eliminated.
And because of the way it spreads signals around, there is a smoother transition when a call is handed off from one base station to another. Two stations can share the signal at the same time, resulting in a so-called “soft hand-off” of a call from one tower to another as a caller drives down the highway.
In contrast, TDMA callers get an abrupt hand-off from one station to another, which may cause a disconnection.
All the chips aren’t on CDMA’s side of the table, however. While CDMA requires fewer sites and reduces the problem of building more towers in a community, the CDMA sites tend to cost more to install than TDMA and pose more engineering and operating challenges.
