CDMA

CDMA

(Code Division Multiple Access) A method for transmitting simultaneous signals over a shared portion of the spCDMA is the digital cellular phone technology from QUALCOMM that operates in the 800MHz band and 1.9GHz PCS band. CDMA phones are noted for their excellent call quality and long battery life.

CDMA is less costly to implement, requiring fewer cell sites than the GSM and TDMA digital cellphone systems and providing three to five times the calling capacity. It provides more than 10 times the capacity of the analog cellphone system (AMPS). CDMA has become widely used in North America and is also expected to become the third-generation (3G) technology for GSM.

Unlike the other digital systems that use TDMA, which divides 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 the ability for you to discern your own speaking language in a room full of people speaking many other languages.

After the speech codec converts voice to digital, CDMA spreads the voice stream over the full 1.25MHz 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.

CDMA transmission has been used by the military for secure phone calls. Unlike FDMA and TDMA methods, 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.

How the Technology Works

CDMA is a fascinating technology, and the illustration below shows you how calls from a base station are encoded and transmitted to a cellphone.

At the base station, each voice conversation is converted into digital code and compressed with a vocoder. The vocoder 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.

At the receiving side, the received signals are quantized (turned into bits) and run through the Walsh code and PN sequence correlation receiver to recover the transmitted bits of the original signal. When 20ms of voice data is received, a Viterbi decoder corrects the errors using the convolutional code, and that all goes to the vocoder which turns the bits back into waveforms (sound).

The following illustration shows how bits move from base station to cellphone and a single bit example takes you through the Boolean math. The example bit is a 1, and the Walsh and PN codes are 0.

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Follow the Single Bit Example This exclusive OR truth table shows you the Boolean algebraic rules if you would like to prove the single bit example in the CDMA illustration above. The example bit is a 1, and the Walsh and PN codes are 0.

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