Can Cellular Phone Companies Agree On a New Standard for Transmission?
By Keith Bradsher
The New York Times
September 16, 1990
Cellular industry executives now find themselves in a quandary. They agreed last year on a technique to transmit calls that offers a desperately needed threefold increase in the number that can be handled simultaneously in one city. Japan and many European countries have already adopted this technique as their standard.
But a small company in San Diego, Qualcomm Inc., is strenuously advocating another technique that could allow up to a 20-fold increase in capacity, and is urging cellular companies to choose the new method in upgrading their networks even though it may not be available as soon.
Some of the largest cellular companies are actually preparing two sets of plans for upgrading their systems, so as to postpone the decision on which technique to use. ''It is a duplication of effort in some ways; it really is,'' said Keith W. Kaczmarek, the director of advanced technologies at PacTel Cellular, a subsidiary of the Pacific Telesis Group. Both McCaw Cellular Communications Inc., the nation's largest cellular service provider, and the Nynex Mobile Communications Company, which serves New York City, say they are postponing the decision until early next year for systems expected to go into operation in 1992.
Nynex Mobile and the Ameritech Corporation, one of the seven regional Bell companies, announced on Aug. 2 that they are buying some equipment from the American Telephone and Telegraph Company and Qualcomm to test the new technique.
Cellular phones took on that name because the service area is divided into cells, each with a low-power radio transmitter and receiver known as a base station. A driver or pedestrian using a cellular phone and crossing the service area is automatically transfered from one base station to the next, usually without interruption.
Cellular phones now in use transmit and receive analog radio signals, which mimic the wave pattern of the human voice to convey conversations and data. Both of the new transmission techniques involve digital radio signals, which use the 1's and 0's of computer language instead. Digital cellular telephones can only be built to use one or the other of the competing digital techniques, but not both. The digital handsets will also be able to transmit analog signals.
The better established form of digital transmission is called Time Division Multiple Access, or T.D.M.A. It assigns three conversations to each cellular frequency, instead of one, which is the case for analog transmission. The three handsets take turns lasting 6.7 thousandths of a second to transmit signals and then take turns lasting just as long to receive signals. Each handset stores part of the signal it receives and plays it during the 33.3 thousandths of a second when it is transmitting and when the other handsets on the same frequency take their turns.
The two biggest companies in the $4 billion cellular equipment industry, Motorola Inc. and L.M. Ericsson of Sweden, are gearing up to produce large quantities of T.D.M.A. equipment. ''We believe that we should stand firmly behind the T.D.M.A. standard,'' said Manfred M. Buchmayer, president of Ericsson Radio Systems in Richardson, Tex. ''Our focus is getting the T.D.M.A. standard to market as fast as possible.''
The alternative technique is Code Division Multiple Access, or C.D.M.A.. Forms of C.D.M.A. have been used in some military communications since World War II.
Much as a person in a crowded room hears many voices but only distinguishes one of them, the version of C.D.M.A. now proposed for cellular telephones involves transmitting and receiving many coded signals on the same broad band of frequencies while each handset listens just to its own code.
Each cellular telephone would be given its own code to encrypt signals. About 40 billion 42-digit codes are usable, said Allen Salmasi, general manager of digital cellular at Qualcomm. The handset converts the voice of its user into many 1's and 0's, then multiplies them by two small numbers to give a uniformity that makes them easier to receive when transmitted by radio.
Then the handset multiplies the product by its 42-digit unique code and beams it to the central radio transmitter, which decodes the information and sends it to a regular wire-line phone or another cellular phone.
The main question facing C.D.M.A. advocates is whether, in a city with a million or more cellular telephone subscribers, the different signals would drown out one another.
Current cellular phones transmit at seven different levels of power and automatically reduce power when they are near a cellular radio base station to avoid drowning out other cellular phones farther away. But C.D.M.A. phones will require 50 or more levels.
Qualcomm is developing handsets with as many as 160 power levels. ''The idea is always to be able to drop your power to the lowest level'' that still allows communications to take place, Mr. Salmasi said.
Computer simulations have indicated that such a system would prevent cellular phones from interfering with each other, Mr. Salmasi said. But some cellular industry experts are skeptical and say that interference will reduce the capacity of C.D.M.A. systems.
''Recent claims are, I think, optimistic by 50 percent,'' said Theodore S. Rappaport, the director of the mobile and portable radio research group at the Virginia Polytechnic Institute and State University.
The big advantage of C.D.M.A. is that it would allow cellular companies to set up new cells without sending technicians to reset radio base stations by hand.
By contrast, analog and T.D.M.A. transmission require that no two adjoining cells use the same frequencies. ''It's not slapping something in and turning it on, you have to fit it into a mosaic'' of cells using various frequencies, said Robert G. Keller, the chief operating officer of Nynex Mobile.
But C.D.M.A. transmission avoids this problem because all cells are operating on the same frequencies.
If some cellular companies choose C.D.M.A. while the rest use T.D.M.A., the result would be two incompatible systems, said Robert W. Maher, president of the Cellular Telecommunications Industry Association, a trade group in Washington.
If that happens, people taking one type of digital phone to an area with the other digital transmission technique may be forced to rely on the alternate analog setting, he said, adding ''you still have analog everywhere.''
Diagram: how code division multiple access works