CDMA Technology
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Welcome to the World of CDMA
Introduction to CDMA

Code Division Multiple Access (CDMA) is a radically new concept in wireless communications. It has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the service quality. It has likewise been chosen for deployment by the majority of the winners of the United States Personal Communications System spectrum auctions. It may seem, however, mysterious for those who aren't familiar with it. This site is provided in an effort to dispel some of the mystery and to disseminate at least a basic level of knowledge about the technology.

CDMA is a form of spread-spectrum, a family of digital communication techniques that have been used in military applications for many years. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI). It has a history that goes back to the early days of World War II.

The use of CDMA for civilian mobile radio applications is novel. It was proposed theoretically in the late 1940's, but the practical application in the civilian marketplace did not take place until 40 years later. Commercial applications became possible because of two evolutionary developments. One was the availability of very low cost, high density digital integrated circuits, which reduce the size, weight, and cost of the subscriber stations to an acceptably low level. The other was the realization that optimal multiple access communication requires that all user stations regulate their transmitter powers to the lowest that will achieve adequate signal quality.

CDMA changes the nature of the subscriber station from a predominately analog device to a predominately digital device. Old-fashioned radio receivers separate stations or channels by filtering in the frequency domain. CDMA receivers do not eliminate analog processing entirely, but they separate communication channels by means of a pseudo-random modulation that is applied and removed in the digital domain, not on the basis of frequency. Multiple users occupy the same frequency band. This universal frequency reuse is not fortuitous. On the contrary, it is crucial to the very high spectral efficiency that is the hallmark of CDMA. Other discussions in these pages show why this is true.

CDMA is altering the face of cellular and PCS communication by:

  • Dramatically improving the telephone traffic (Erlang) capacity
  • Dramatically improving the voice quality and eliminating the audible effects of multipath fading
  • Reducing the incidence of dropped calls due to handoff failures
  • Providing reliable transport mechanism for data communications, such as facsimile and internet traffic
  • Reducing the number of sites needed to support any given amount of traffic
  • Simplifying site selection
  • Reducing deployment and operating costs because fewer cell sites are needed
  • Reducing average transmitted power
  • Reducing interference to other electronic devices
  • Reducing potential health risks

Subsequent pages in this site address these questions, provide facts, and show where tradeoffs are necessary.


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Copyright © 1996-1999 Arthur H. M. Ross, Ph.D., Limited