What is CDMA: code division multiple access tutorial

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CDMA Includes:
CDMA basics     Orthogonal spreading codes     CDMA near far problem    

See also:     DSSS basics    

Code Division Multiple Access, CDMA was an innovative use of direct sequence spread spectrum technology used to provide a multiple access scheme for mobile telecommunications and other wireless systems.

CDMA used the property of DSSS that unless the transmitter and receiver used the same spreading code for both ends of the process, the signal could not be decoded and in this way it was able to provide a means of enabling a variety of different users to use the same channel to access a base station without mutual interference.

In this way, using CDMA different users were allocated different codes rather than different slots, channels, etc.

CDMA history

CDMA is based around a form of transmission known as Direct Sequence Spread Spectrum. The CDMA history can be directly linked back to the 1940s when this form of transmission was first envisaged. As electronics technology improved, it started to be used for covert military transmissions in view of the facts that the transmissions look like noise, it is difficult to decipher without the knowledge of the right codes, and furthermore it is difficult to jam.

With the revolution in cellular telecommunications that occurred in the 1980s a then little know company named Qualcomm working on DSSS transmissions started to look at this as the basis for a cellular telecommunications multiple access scheme - CDMA - code division multiple access.

The concept of CDMA had to proved in the field and accordingly Qualcomm was joined by US network operators Nynex and Ameritech to develop the first experimental CDMA system. Later the team was expanded as Motorola and AT&T (now Lucent) joined to bring their resources to speed development.

As a result this it was possible to start writing a specification for CDMA in 1990. With the support of the Cellular Telecommunications Industry Association (CTIA) and the Telecommunications Industry Association (TIA) a standards group was set up. This group then published the standard for the first CDMA system in the form of IS-95, resulting in the formal publication of IS-95-A in 1995.

The first CDMA system was launched in September 1995 by Hutchison Telephone Co. Ltd. in Hong Kong and SK Telecom in Korea soon followed along with networks in the USA.

This was only one cellular telecommunications system, although it was the first. Its development lead on to the CDMA2000 series of standards.

The use of CDMA did not stop with CDMA2000 as it became necessary to evolve the GSM standard so that it could carry data and provide significant improvements in terms of spectrum use efficiency. Accordingly CDMA, in the form of Wideband CDMA (WCDMA) was adopted for this standard.

Key elements of CDMA

CDMA is a form of spread spectrum transmission technology based around a scheme called direct sequence spread spectrum.

By proving different users with different spreading codes, the different users are able to utilise the same frequency channel, whilst still being able to individually communicate with the base station.

The use of CDMA has been likened to being in a room where there are many people speaking different languages, and being able to understand someone speaking in your language despite the high level of noise.

A CDMA signal will be able to be decoded when the receiver uses the same code as used for the transmission despite the presence of other signals with different codes being used on the same channel.

CDMA has a number of distinguishing features that are key to spread spectrum transmission technologies:

• Use of wide bandwidth:  CDMA, like other spread spectrum technologies uses a wider bandwidth than would otherwise be needed for the transmission of the data. This results in a number of advantages including an increased immunity to interference or jamming, and multiple user access.
• Spreading codes used:   In order to achieve the increased bandwidth, the data is spread by use of a code which is independent of the data.
• Level of security:   In order to receive the data, the receiver must have a knowledge of the spreading code, without this it is not possible to decipher the transmitted data, and this gives a measure of security.
• Multiple access:   The use of the spreading codes which are independent for each user along with synchronous reception allow multiple users to access the same channel simultaneously.

CDMA technology advantages

The use of CDMA offers several advantages and it is for this reason that CDMA technology has been adopted for many 3G cellular telecommunications systems.

• Improvement in handover / handoff:   Using CDMA it is possible for a terminal to communicate with two base stations at once. As a result, the old link only needs to be broken when the new one is firmly established. This provides significant improvements in terms of the reliability of handover / handoff from one base station to another.
• Improvement in capacity:   One of the chief claims for CDMA is that it gave significant improvements in network capacity. Original expectations for some of the proponents of CDMA technology were for some very significant improvements, although in reality these were somewhat exaggerated over what real world experience found:
  
  • 18 fold increase in capacity when compared to AMPS (1G technology used in USA)
  • 6 fold increase in capacity when compared to US TDMA (2G technology used in USA) - similar increases were also claimed over GSM.
  In reality the original expectations were not fulfilled although increases of a factor of about two were seen when compared to US TDMA and GSM. This in itself was a significant improvement.

CDMA< code division multiple access provided the multiple access technology for all the major 3G cellular telecommunications systems. As such it gave a significant step forwards over the TDMA / FDMA schemes used for 2G, enabling data transmission in a way that had not been previously possible.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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