Summary of CDMA: Another transmission technique
Code division multiple access is quite a different way to send information, it's a spread spectrum technique. Instead of concentrating a message in the smallest spectrum possible, say in a radio frequency 10 kHz wide, CDMA spreads that signal out, making it wider. A frequency might be 1.25 or even 5 MHz wide, 10 times or more the width a conventional call might use. Now, why would anyone want to do that?, to go from a seemingly efficient method to a method that seems deliberately inefficient?
The military did much early development on CDMA. They did so because a signal using this transmission technique is diffused or scattered -- difficult to block, listen in on, or even identify. The signal appears more like background noise than a normal, concentrated signal which you can easily target. For the consumer CDMA appeals since a conversation can't be picked up with a scanner like an analog AMPS call. Think of CDMA in another way. Imagine a dinner party with 10 people, 8 of them speaking English and two speaking Spanish. The two Spanish speakers can hear each other talking with out a problem, since their language or 'code' is so specific. All the other conversations, at least to their ears, are disregarded as background noise.
CDMA is a transmission technique, a technology, a way to pass information between the base station and the mobile. Although called 'multiple access', it is really another multiplexing method, a way to put many calls at once on a single channel. As stated before, analog cellular or AMPS uses frequency division multiplexing, in which callers are separated by frequency, TDMA separates callers by time, and CDMA separates calls by code. CDMA traffic includes telephone calls, be they voice or data, as well as signaling and supervisory information. CDMA is a part of an overall operating system that provides cellular radio service. The most widespread CDMA based cellular radio system is called IS-95.
Download this! In these pages from Bluetooth Demystified (McGraw Hill), Nathan Muller presents good information on CDMA, spread spectrum, spreading codes, direct sequence, and frequency hopping. (6 pages, 509K in .pdf)
Bluetooth Demystified ordering information (external link to Amazon)