Code division multiple access

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General information

Generically (as a multiplexing scheme), code division multiple access (CDMA) is any use of any form of spread spectrum by multiple transmitters to send to the same receiver on the same frequency channel at the same time without harmful interference. Other widely used multiple access techniques are Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). In these three schemes, receivers discriminate among various signals by the use of different codes, time slots and frequency channels, respectively.

The term CDMA is also widely (but perhaps too liberally) used to refer to a family of specific implementations of CDMA pioneered by Qualcomm for use in digital cellular telephony. These include IS-95 (aka cdmaOne) and IS-2000 (aka CDMA2000). The two different uses of this term can be confusing.

To lessen confusion, the Qualcomm brand name cdmaOne may be used to refer to the 2G CDMA standard, instead of using more confusing generic term CDMA, or the technical term IS-95. The Qualcomm CDMA system includes highly accurate time signals (usually referenced to a GPS receiver in the cell base station), so cellphone CDMA-based clocks are an increasingly popular type of Radio clock for use in computer networks. The main advantage of using CDMA cell-phone signals for reference clock purposes is that they work better inside buildings, thus often eliminating the need to mount the GPS antenna on the outside of a building.

Also frequently confused with CDMA is W-CDMA. Here are a few quick facts:

CDMA (the multiplexing technique) is used as the principle of the W-CDMA air interface.
The W-CDMA air interface is used in the global 3G standard, UMTS, and Japanese 3G standards, FOMA by NTT DoCoMo and Vodafone.
The CDMA family of standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of standards.

Another important application of CDMA — predating and entirely distinct from CDMA cellular — is the Global Positioning System, GPS.

Technical details

All forms of CDMA use spread spectrum process gain to allow receivers to partially discriminate against unwanted signals. Signals with the desired spreading code and timing are received, while signals with different spreading codes (or the same spreading code but a different timing offset) appear as wideband noise reduced by the process gain.

The way this works is that each station is assigned a spreading code or chip sequence. Such chip sequences are expressed as a sequence of −1 and +1 values. The normalized dot product (i.e. divide the computed dot product by the number of elements in the respective vectors) of each chip sequence with itself is +1 (and the normalized dot product with its complement is −1), whereas the normalized dot product of two different chip sequences is 0.

E.g. if C1 = (−1, −1, −1, −1) and C2 = (+1,−1,+1,−1)

C1 .  C1 = (−1, −1, −1, −1) . (−1, −1, −1, −1) = +4/(4 elements) = +1
C2 .  C2 = (+1, −1, +1, −1) . (+1, −1, +1, −1) = +4/(4 elements) = +1
C1 . −C1 = (−1, −1, −1, −1) . (+1, +1, +1, +1) = −4/(4 elements) = −1
C2 . −C2 = (+1, −1, +1, −1) . (−1, +1, −1, +1) = −4/(4 elements) = −1
C1 .  C2 = (−1, −1, −1, −1) . (+1, −1, +1, −1) =  0
C1 . −C2 = (−1, −1, −1, −1) . (−1, +1, −1, +1) =  0

This property is called orthogonality. These sequences are Walsh codes and can be derived from a binary Walsh matrix.

A station sends out its chip sequence to send a 1, and its inverse to send a 0 (or +1 and a −1; zero being silence).

When multiple chip codes are sent by multiple stations, the signals add up in the air. For example the chip sequences (−1, −1, −1, −1) and (+1,−1,+1,−1) add up to (0,−2,0,−2). The receiver merely needs to calculate the normalized dot product of the station it's interested in with the signal in the air. E.g. (−1, −1, −1, −1) · (0,−2,0,−2) = +1. Had −1 been sent the signal in the air would have been (+2,0,+2,0) and the normalized dot product would have been (−1, −1, −1, −1) · (+2,0,+2,0) = −1.

A TDMA or FDMA receiver can in theory completely reject arbitrarily strong signals on other time slots or frequency channels. This is not true for 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 CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power control scheme to tightly control each mobile's transmit power.

The need for power control can be deduced neatly from the above calculations; if some stations would broadcast +0.8 and −0.8 and others +1.2 and −1.2, this would wreak havoc with the calculations.

Forward error correction (FEC) coding is also vital in any CDMA scheme to reduce the required signal-to-interference ratio (by tolerating a certain rate of bit errors) and thereby maximize channel capacity.

CDMA's main advantage over TDMA and FDMA is that the number of available CDMA codes is essentially infinite. This makes CDMA ideally suited to large numbers of transmitters each generating a relatively small amount of traffic at irregular intervals, as it avoids the overhead of continually allocating and deallocating a limited number of orthogonal time slots or frequency channels to individual transmitters. CDMA transmitters simply send when they have something to say, and go off the air when they don't.

External link

The Third Generation Partnership Project 2 (3GPP2) (http://www.3gpp2.org/)
The Third Generation Partnership Project (3GPP) (http://www.3gpp.org/)
CDMA Development Group (CDG) (http://www.cdg.org/)
Radio-Electronics.Com (http://www.radio-electronics.com/info/cellulartelecomms/)

Code division multiple access

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