CDMA
(Code Division Multiple Access) A method for transmitting simultaneous digital signals over a shared portion of the spectrum. Although used in various radio communications systems, the most widely known application of CDMA is for cellular systems that operate in the 800 MHz and 1.9 GHz PCS bands. As of 2008, there were more than 450 million CDMA cellphone users in the world, with more than half in Asia. Verizon and Sprint are CDMA carriers in the U.S., while TELUS uses CDMA in Canada. QUALCOMM is the developer of the CDMA air interface used in cellular systems. See IS-95 and CDMA2000.
Compared to GSM cellular systems, CDMA requires fewer cell towers and provides up to five times the calling capacity. Providing more than 10 times the voice traffic of earlier analog system (AMPS), CDMA is also the basis for 3G data transmission for GSM carriers such as AT&T and T-Mobile (see WCDMA).
Spread Spectrum
Unlike the other digital systems that divide the spectrum into different time slots, CDMA's spread spectrum technique overlaps every transmission on the same carrier frequency by assigning a unique code to each conversation. The often-used analogy for this is your ability to detect your own language in a room full of people speaking other languages. See TDMA.
A Different Kind of Chip
After the speech codec converts voice to digital, CDMA spreads the voice stream over the full 1.25 MHz bandwidth of the CDMA channel, coding each stream separately so it can be decoded at the receiving end. The rate of the spreading signal is known as the "chip rate," as each bit in the spreading signal is called a "chip" (no relation to an integrated circuit). All voice conversations use the full bandwidth at the same time. One bit from each conversation is multiplied into 128 coded bits by the spreading techniques, giving the receiving side an enormous amount of data it can average just to determine the value of one bit.
More Secure
CDMA transmission has been used by the military for secure phone calls. Unlike FDMA and TDMA, CDMA's wide spreading signal makes it difficult to detect and jam. For more information, contact the CDMA Development Group (CDG) at www.cdg.org. See BREW, cellular generations, IS-95, CDMA2000, WCDMA, GSM, FDMA, TDMA, CDPD, CDG and spread spectrum.
Transmitting from the Base Station
Each voice conversation is compressed with a vocoder. The output is doubled by a convolutional encoder that adds redundancy for error checking. Each bit from the encoder is replicated 64 times and exclusive OR'd with a Walsh code that is used to identify that call from the rest.
The output of the Walsh code is exclusive OR'd with the next string of bits (PN sequence) from a pseudo-random number generator, which is used to identify all the calls in a particular cell's sector. At this point, there is 128 times as many bits as there were from the vocoder's output. All the calls are combined and modulated onto a carrier frequency in the 800 MHz range.
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Receiving at the Cellphone
The received frequencies are quantized into bits ("chips") by the analog-to-digital converter (ADC). The output is run through the Walsh code and PN sequence correlation receiver to recover the transmitted bits of the original signal. When 20 milliseconds of voice data are received, a Viterbi decoder corrects the errors using the convolutional code. The Viterbi output goes to the vocoder and digital-to-analog converter (DAC), which decompresses the bits and turns them back into waveforms (sound).
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| Follow the Single Bit Example |
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| This exclusive OR truth table shows you the Boolean algebraic rules to prove the single bit example in the illustrations above. The example bit is a 1, and the Walsh and PN codes are 0. |
| CDMA - Code Division Multiple Access |
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