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)

Permalink

Post a comment (If you haven't left a comment here before, you may need to be approved by the site owner before your comment will appear. Until then, it won't appear on the entry. Thanks for waiting.)

Name:

Email Address:

URL:

Remember personal info?

Comments: (you may use HTML tags for style)

Human Verification:

Search the site | by Google:
Subscribe to Private Line's feed
[What is this?] Article Index I. Introduction II. Cellular History III. Cell Sector Terminology IV. Basic Theory and Operation IX. Code Division Multiple Access: IS-95 A. Before We Begin: A Cellular Radio Review B. Back to the CDMA Discussion C. Summary of CDMA D. A different way to share a channel E. Synchronization F. What Every Radio System Must Consider G. CDMA Benefits H. Call Processing: A Few Details V. Cellular frequency and channel discussion VI. Channel Names and Functions VII. AMPS Call Processing A. Registration B. Pages: Getting a call C. The SAT, Dial Tone, and Blank and Burst D. Origination: Making a call E. Precall Validation VIII. AMPS and Digital Systems compared X. Appendix A. AMPS Call Processing Diagram B. Land Mobile or IMTS C. Early Bell System Overview of Amps D. Link to Professor R.C. Levine's article XI. Additional Assistance A. Q&A: Cell Tower Capacity Recent Posts What Every Radio System Must Consider CDMA Benefits Call Processing: A Few Details Appendix AMPS Call Processing Land Mobile or IMTS Early Bell System Overview of Amps Link to Professor R.C. Levine's article Cell Tower Lease Expert Q&A: Cell Tower Capacity Powered by
Movable Type 3.2 HomeNewsContentCellular Telephone BasicsDigital Wireless BasicsGSM HistoryTelecommunications HistoryTelephone HistorySwitching & TransmissionContact