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McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


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(Code Division Multiple Access) A method for transmitting digital signals simultaneously over the same carrier frequency (channel). CDMA's basic principles were developed to secure radio signals at the beginning of World War II; however, the most widely known application of CDMA came much later as a cellular transmission method. Qualcomm is the designer of CDMA chips.

In the U.S., Verizon and Sprint are CDMA carriers. However, T-Mobile acquired Sprint in 2020, and although T-Mobile is based on GSM, both CDMA and GSM are being phased out as LTE has become the dominant technology with 5G coming on strong. See LTE, GSM and cellular generations.

For years, CDMA and GSM have been the two primary cellular, but incompatible, 2G and 3G transmission systems. Old cellphones support either CDMA or GSM, while newer models handle both. Although all current phones support 4G LTE, if they do not have both CDMA and GSM built in, users cannot switch carriers and be guaranteed service. The problem is that in rural or highly congested areas, transmission may throttle down from LTE to 3G. See LTE, GSM and cellular generations.

CDMA Was a Major Advance
CDMA provides up to 10 times the calling capacity of earlier analog networks (AMPS) and up to five times the capacity of GSM systems. CDMA is also the basis for the WCDMA and HSPA 3G technologies used by GSM carriers (see WCDMA and HSPA). See IS-95 and CDMA2000.

Spread Spectrum
Unlike GSM, which uses TDMA to divide the channel into time slots, CDMA's spread spectrum assigns a unique code to each conversation and uses the full bandwidth of the channel. An often-used analogy is the ability to detect one's own language in a room full of people speaking multiple languages.

Each bit is multiplied into 128 coded bits, giving the receiving circuit an enormous amount of data it can average just to determine the value of a single bit when decoding the signal. See chip rate, BREW, FDMA, TDMA and spread spectrum.

The following illustrations were created with the assistance of Klein Gilhousen, co-founder of Qualcomm and co-inventor of CDMA. They show the path of a single bit being transmitted and received.

Transmitting the Bit
The vocoder compresses the conversation, and the convolutional encoder adds error checking bits. Each bit is replicated 64 times and exclusive OR'd with a Walsh code that uniquely identifies the call. The pseudo-random number generator identifes all calls in one cell sector.

Receiving the Bit
The analog-to-digital converter (ADC) quantizes the call into bits. The Walsh code and PN sequence correlation receivers recover the original signal. Using the convolutional code, the Viterbi decoder corrects errors every 20 milliseconds. The vocoder decompresses the bits, and the digital-to-analog converter (DAC) turns them back into waveforms.
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References in periodicals archive ?
CDMA markets the Quality Choice private label program and offers a variety of other programs and services for its retail and wholesale members.
The operator was said to be looking for a buyer for its CDMA business.
Besides, it wants DoT to clarify if CDMA spectrum auctioned in March can also be allowed for GSM mobile phone services.
CDMA created a Product Showcase to give associate members (suppliers) an additional opportunity to show off their new and unique products.
Endeavoring to expand the Chinese market for CDMA-based mobile phones, China Telecom has included over 100 handset IC chip designers, handset manufacturers and brand owners in its CDMA2000 Alliance, in hopes of enhancing development of the CDMA phone industry.
"Looking to the next decade, we will continue growing, enhancing and optimizing 3G CDMA while providing interworking with technologies such as LTE and Mobile WiMAX."
Since CDMA mobile services were introduced in December 2002, the cellular landscape in India changed rapidly.
Keywords: Guard time, MMSE equalizer, Multi-carrier CDMA, Multipath fading channels.
NORDIC BUSINESS REPORT-23 June 2006-Nokia and SANYO will not proceed on joint CDMA company(C)1994-2006 M2 COMMUNICATIONS LTD
Nortel has announced a CDMA WLAN Converged Mobility solution designed to bridge CDMA wireless networks and wireless local area networks (WLANs) to provide a seamless communication experience to end users.
Our strategy to adopt CDMA technology ahead of our competitors and commercialize it before others turned out to be very successful.