12-06-2012, 12:52 PM
Introduction of CDMA TECHNOLOGY
CDMA TECHNOLOGY.pdf (Size: 354.61 KB / Downloads: 1)
ABSTRACT
Code-Division Multiple Access, a digital cellular technology that
uses spread-spectrum techniques. Unlike competing systems, such
as GSM, that use TDMA, CDMA does not assign a specific frequency
to each user. Instead, every channel uses the full available spectrum.
Individual conversations are encoded with a pseudo-random digital
sequence.
As the term implies, CDMA is a form of multiplexing which allows
numerous signals to occupy a single transmission channel, optimizing
the use of available bandwidth. The technology is used in ultra-highfrequency
(UHF) cellular telephone systems in the 800-M1-Iz and 1.9-
GHz bands.
WHAT IS CDMA?
(Code Division Multiple Access) A method for transmitting realtime
signals over a shared portion of the spectrum. The foremost
application of CDMA is the digital cellular phone technology that
operates in the 800MHz band and 1.9GHz . Unlike GSM and TDMA,
which divides the spectrum into different time slots CDMA uses a
spread spectrum technique to assign a code to each conversation.
After the speech codec converts voice to digital, CDMA spreads the
voice stream over the full 1.25MHz bandwidth of the CDMA channel
coding each stream separately so it can be decoded at the receiving
end. The rate of the spreading signal is known as the “chip rate,” as
each bit in the spreading signal is called a “chip” voice conversations
use the full bandwidth at the same time.
MULTIPLE ACCESS COMPARISON
It is easier to understand CDMA if it is compared with other
multiple access technologies. The following sections describe the
fundamental differences between a Frequency Division Multiple
Access Analog technology (FDMA), a Time Division Multiple Access
Digital technology (TDMA) and a Code Division Multiple Access
Digital technology (CDMA).
The International Cocktail Party
To illustrate the conceptual differences among the multiple
access technologies, the “International Cocktail Party” analogy will be
applied. Picture a large room and a number of people, in pairs, who
would like to hold conversations. The people in each pair only want to
talk and listen to each other, and have no interest in what is being said
by the other pairs. In order for these conversations to take place,
however, it is necessary to define the environment for each
conversation.
