Electrosmart.Net

Code Division Multiple Access [CDMA]

Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. It should not be confused with cdmaOne (often referred to as simply "CDMA"), which is a mobile phone standard that uses CDMA as its underlying channel access method.

CDMA Telecommunications

CDMA networks employ a 2G Mobile Telecommunications Standard that uses CDMA, a multiple access scheme for digital radio, to send voice, data and signaling data between mobile telephones and cell sites.

CDMA or "code division multiple access" is a digital radio system that transmits streams of bits (PN Sequences). CDMA permits several radios to share the same frequencies.

Unlike TDMA "time division multiple access", a competing system used in 2G GSM, all radios can be active all the time, because network capacity does not directly limit the number of active radios. Since larger numbers of phones can be served by smaller numbers of cell-sites, CDMA-based standards have a significant economic advantage over TDMA-based standards, or the oldest cellular standards that used frequency-division multiplexing.

In North America, the technology competed with Digital AMPS (IS-136, a TDMA technology). It is now being supplanted by IS-2000 (CDMA2000), a later CDMA-based standard. It is used in the USA, South Korea, Canada, Mexico, India, Israel, Australia, New Zealand, Sri Lanka, Venezuela, Brazil and China. In Q1 2007, around 15 % of the global subscribers used CDMA, while about 85 % used GSM or 3GSM[1].

Spread Spectrum Technology

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" signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated.

An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.

Capacity

IS-95 CDMA techniques have limited throughput due to fixed bandwidth, however it takes active steps to improve SNR.

With CDMA, signals not correlated with the channel of interest appear as noise, and signals carried on other properly time aligned Walsh codes are essentially removed in the de-spreading process.

The variable-rate nature of traffic channels provide lower-rate frames to be transmitted at lower power causing less noise for other signals still to be correctly received.

These factors provide an inherent lower noise level than other cellular technologies allowing the IS-95 network to squeeze more users into the same radio spectrum.

Active (slow) power control is also used on the forward traffic channels, where during a call, the mobile sends signaling messages to the network indicating the quality of the signal. The network will control the transmitted power of the traffic channel to keep the signal quality just good enough, thereby keeping the noise level seen by all other users to a minimum.

The receiver also uses the techniques of the rake receiver to improve SNR as well as perform soft handoff.

Layer 2

Once a call is established, a mobile is restricted to using the traffic channel. A frame format is defined in the MAC for the traffic channel that allows the regular voice (vocoder) or data (RLP) bits to be multiplexed with signaling message fragments. The signaling message fragments are pieced together in the LAC, where complete signaling messages are passed on to Layer 3.

CDMA has been used in many communications and navigation systems, including the Global Positioning System and the OmniTRACS satellite system for transportation logistics.