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Code division multiple access

Code division multiple access (CDMA) is a channel access method used by

various radio communication technologies. It should not be confused with the mobile

phone standards called cdmaOne, CDMA2000 (the 3G evolution of cdmaOne) and

WCDMA (the 3G standard used by GSM carriers), which are often referred to as

simply CDMA, and use CDMA as an underlying channel access method.

One of the concepts in data communication is the idea of allowing several

transmitters to send information simultaneously over a single communication channel.

This allows several users to share a band of frequencies (see bandwidth). This concept

is called multiple access. CDMA employs spread-spectrum technology and a special

coding scheme (where each transmitter is assigned a code) to allow multiple users to

be multiplexed over the same physical channel. By contrast, time division multiple

access (TDMA) divides access by time, while frequency-division multiple access

(FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling,

since the modulated coded signal has a much higher data bandwidth than the data

being communicated.

Steps in CDMA Modulation

Each user in a CDMA system uses a different code to modulate their signal.

Choosing the codes used to modulate the signal is very important in the performance

of CDMA systems. The best performance will occur when there is good separation

between the signal of a desired user and the signals of other users. The separation of

the signals is made by correlating the received signal with the locally generated code

of the desired user. If the signal matches the desired user's code then the correlation

function will be high and the system can extract that signal. If the desired user's code

has nothing in common with the signal the correlation should be as close to zero as

arbitrarily random starting points and which make full use of the code space, unique

"pseudo-random" or "pseudo-noise" (PN) sequences are used in asynchronous CDMA

systems. A PN code is a binary sequence that appears random but can be reproduced

in a deterministic manner by intended receivers. These PN codes are used to encode

and decode a user's signal in Asynchronous CDMA in the same manner as the

orthogonal codes in synchronous CDMA (shown in the example above). These PN

sequences are statistically uncorrelated, and the sum of a large number of PN

sequences results in multiple access interference (MAI) that is approximated by a

Gaussian noise process (following the central limit theorem in statistics). Gold codes

are an example of a PN suitable for this purpose, as there is low correlation between

the codes. If all of the users are received with the same power level, then the variance

(e.g., the noise power) of the MAI increases in direct proportion to the number of

users. In other words, unlike synchronous CDMA, the signals of other users will

appear as noise to the signal of interest and interfere slightly with the desired signal in

proportion to number of users.

All forms of CDMA use spread spectrum process gain to allow receivers to

partially discriminate against unwanted signals. Signals encoded with the specified

PN sequence (code) are received, while signals with different codes (or the same code

but a different timing offset) appear as wideband noise reduced by the process gain.

Since each user generates MAI, controlling the signal strength is an important

issue with CDMA transmitters. A CDM (synchronous CDMA), TDMA, or FDMA

receiver can in theory completely reject arbitrarily strong signals using different codes,

time slots or frequency channels due to the orthogonality of these systems. This is not

true for Asynchronous CDMA; rejection of unwanted signals is only partial. If any or

all of the unwanted signals are much stronger than the desired signal, they will

overwhelm it. This leads to a general requirement in any asynchronous CDMA

system to approximately match the various signal power levels as seen at the receiver.

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