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Comparison of different Multiplexing Techniques
Figure 4
Code Division Multiple Access (CDMA) – An Overview:
Cellular systems divide a geographic region into cells where a mobile unit in each cell
communicates with a base station. The goal in the design of cellular systems is to be
able to handle as many calls as possible (this is called capacity in cellular
terminology) in a given bandwidth with some reliability. For the cellular system,
Code division multiple-access techniques allow many users to simultaneously access
a given frequency allocation.
Code division multiple access (CDMA) is a form of multiplexing (not a modulation
scheme) and a method of multiple access that does not divide up the channel by time
(as in TDMA), or frequency (as in FDMA), but instead encodes data with a special
code associated with each channel and uses the constructive interference properties of
the special codes to perform the multiplexing. CDMA also refers to digital cellular
telephony systems that make use of this multiple access scheme, such as those
pioneered by Qualcomm, and W-CDMA by the International Telecommunication
Union or ITU.
A number of different terms are used to refer to CDMA implementations. The original
U.S. standard defined by QUALCOMM was known as IS-95, the IS referring to an
Interim Standard of the Telecommunications Industry Association (TIA). IS-95 is
often referred to as 2G or second generation cellular. The CDMA has been submitted
for approval as a mobile air interface standard to the ITU International
Telecommunication Union.
The size of a given cell depends on the power of the signal transmitted by the handset,
the terrain, and the radio frequency being used. Various algorithms can reduce the
noise introduced by variations in terrain, but require extra information be sent to
validate the transfer. Hence, the radio frequency and power of the handset effectively
determine the cell size. Long wavelengths need less energy to travel a given distance
vs. short wavelengths, so lower frequencies generally result in greater coverage while
higher frequencies result in shorter coverage. These characteristics are used by mobile
network planners in determining the size and placement of the cells in the network. In
cities, many small cells are needed; the use of high frequencies allows sites to be
placed more-closely together, with more subscribers provided service. In rural areas
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