Back to the CDMA Discussion

Qualcomm's CDMA system uses some very advanced speech compression techniques, utilizing a variable rate vocoder, a speech synthesiser and voice processor in one. Vocoders are in every digital handset or phone; they digitize your voice and compress it. Phil Karn, KA9Q, one of the principal engineers behind Qualcomm, wrote about an early vocoder like this:

"It [o]perates at data rates of 1200, 2400, 4800 and 9600 bps. When a user talks, the 9600 bps data rate is generally used. When the user stops talking, the vocoder generally idles at 1200 bps so you still hear background noise; the phone doesn't just 'go dead'. The vocoder works with 20 millisecond frames, so each frame can be 3, 6, 12 or 24 bytes long, including overhead. The rate can be changed arbitrarily from frame to frame under control of the vocoder."

This is really sophisticated technology, eerily called VAD, for voice activity detection. Changing data rates allows more calls per cell, since each conversation occupies bandwidth only when needed, letting others in during the idle times. Some say VAD is the 'trick' in CDMA that allows greater capacity, and not anything in spread spectrum itself. These data rate changes help with battery life, too, since the mobile can power down in those moments when not transmitting as much information.

Several years ago CDMA was in its infancy. Some wondered if it would work. I was not among the doubters. In May, 1995 I wrote in my magazine private line that I felt the future was with this technology. I still think so and Mark van der Hoek agrees. Click here if you want to read his comments or continue on this page if you want to learn more about this technology.

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[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. 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
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