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CDMA
CDMA
takes an entirely different approach from
TDMA. CDMA, after digitizing data,
spreads it out
over the entire available bandwidth.
Multiple calls are
overlaid
on each other on the channel, with each
assigned a
unique sequence code.
CDMA is a form of
spread spectrum,
which simply means that data is sent in
small pieces over a number of the discrete
frequencies available for use at any time in
the specified range.
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In CDMA, each phone's data has a
unique code. |
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2G is a cell phone
network protocol. Click here to
learn about network protocols for
Smartphones. |
All of the users transmit in the same
wide-band
chunk of spectrum. Each user's signal is
spread over the entire bandwidth by a
unique spreading code.
At the receiver, that same unique code is
used to recover the signal. Because
CDMA
systems need to put an accurate time-stamp
on each piece of a signal, it references the
GPS
system for this information. Between eight
and 10 separate calls can be carried in the
same channel space as one analog AMPS call.
CDMA technology is the basis for
Interim Standard 95
(IS-95) and operates in both the 800-MHz and
1900-MHz frequency bands.
Ideally, TDMA and CDMA are transparent to
each other. In practice, high-power CDMA
signals raise the noise floor for TDMA
receivers, and high-power TDMA signals can
cause overloading and jamming of CDMA
receivers.
2G is a cell phone network protocol. Click
here to learn about
network
protocols for Smartphones.
Now let's look at the distinction between
multiple-band and multiple-mode technologies
To read about any of the technologies click on the blue links
2G cell-phone networks
for transmitting information (we'll discuss
3G technologies in the 3G section):
·
Frequency division multiple access
(FDMA)
·
Time division multiple access
(TDMA)
·
Code division multiple access
(CDMA)
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