16-05-2012, 12:47 PM
Code Division Multiple Access
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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. One bit from each
conversation is multiplied into 128 coded bits by the spreading
techniques, giving the receiving side an enormous amount of data it
can average just to determine the value of one bit.
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).
FDMA - Frequency Division Multiple Access
FDMA is used for standard analog cellular. Each user is
assigned a discrete slice of the RF spectrum. FDMA permits only one
user per channel since it allows the user to use the channel 100% of
the time. Therefore, only the frequency “dimension” is used to define
channels.
TDMA - Time Division Multiole Access
The key point to make about TDMA is that users are still
assigned a discrete slice of RF spectrum, but multiple users now
share that RF carrier on a time slot basis. Each of the users alternate
their use of the RF channel. Frequency division is still employed, but
these carriers are now further sub-divided into some number of time
slots per carrier.
A user is assigned a particular time slot in a carrier and can
only send or receive information at those times. This is true whether or
not the other time slots are being used. Information flow is not
continuous for any user, but rather is sent and received in “bursts.”
The bursts are re-assembled at the receiving end, and appear to
provide continuous sound because the process is very fast.