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Definition: CDMA


(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. In the U.S., Verizon and Sprint are CDMA carriers, and Qualcomm designs the chips for the air interface.

CDMA and GSM
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, 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 4G 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.