Information or tutorial about what is CDMA-Code Division Multiple Access, and the CDMA technology basics.
CDMA or Code Division Multiple Access is a form of access scheme that
has been widely used within 3G cellular telecommunications systems as
well as being used in a number of other technologies as well. CDMA
technology gave some significant advantages when compared to the
technologies used for previous in terms of overall performance and
specifically in terms of spectrum efficiency.
CDMA uses spread spectrum technology with the use of different codes
to separate between different stations or users rather than different
frequencies of time slots as in the case of previous access
technologies. In this way, CDMA is different to the previous schemes
used to provide different cellular users with access to the radio
network.
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. 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.
Summary
CDMA has been a particularly successful technology. CDMA technology
ahs 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.
