CDMA 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. It 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 fort he 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 capacity:   One of the chief claims for CDMA is that it gives significant improvements in network capacity. Original expectations for some of the proponents of CDMA technology were for some very significant improvements:

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.

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.

CDMA has been a particularly successful technology. CDMA technology has been used in all the 3G cellular telecommunications systems in one form or another and has enabled significant improvements to be gained over previously technologies used in 2G systems, for example.

CDMA is based around the use of direct sequence spread spectrum techniques. Essentially CDMA is a form of spread spectrum transmission which uses spreading codes to spread the signal out over a wider bandwidth then would normally be required. By using CDMA spread spectrum technology, many users are able to use the same channel and gain access to the system without causing undue interference to each other.

Although as the number of users increases care has to be taken to ensure that interference levels do not rise to the extent that performance falls, it is still possible to provide access to a large number of different users and allow them access.

Drawbacks caused by the CDMA near far problem

While the power control schemes that are adopted by the different cellular telecommunications technologies work very well and allow the CDMA systems to operate over a wide area, there are penalties for using them:

Reduced data capacity:   The power control mechanism requires data to be sent in both directions across the radio interface. This utilises data capacity that could be otherwise used for carrying revenue earning data.

High power handset power consumption at cell edges:   In order to be able to maintain the required signal level at the base station when the handset is close to the edge of the cell, it will be required to transmit at a high power level. This will reduce battery life. Other cellular systems might not require such high signal levels at the base station and may be able to conserve battery power as a result.

The CDMA near far problem is resolved in systems such as cdmaOne, CDMA2000 and W-CDMA by using sophisticated power control schemes to ensure that the power levels at the base station fall within a given band. Although there are some penalties to be paid for these

schemes to overcome the CDMA near far problem, they operate well and enable significant gains to be made by using CDMA over previous technologies.