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KG70C
CHAPTER 1. System Introduction
1. System Introduction
1.1 CDMA Abstract
The cellular system has a channel hand-off function that is used for collecting the information on the
locations and movements of radio mobile telephones from the cell site by automatically controlling
several cell site through the setup of data transmission routes and thus, enabling one switching
system to carry out the automatic remote adjustment. This is to maintain continuously the call state
through the automatic location confirmation and automatic radio channel conversion when the busy
subscriber moves from the service area of one cell site to that of another by using automatic location
confirmation and automatic radio channel conversion functions. The call state can be maintained
continuously by the information exchange between switching systems when the busy subscriber
moves from one cellular system area to the other cellular system area.
In the cellular system, the cell site is a small-sized low output type and utilizes a frequency allocation
system that considers mutual interference, in an effort to enable the re-use of corresponding
frequency from a cell site separated more than a certain distance. The analog cellular systems are
classified further into an AMPS system, E-AMPS System, NMT system, ETACS system, and JTACS
system depending on technologies used.
Unlike the Time Division Multiple Access (TDMA) or the Frequency Division Multiple Access (FDMA)
used in the band limited environment, the Code Division Multiple Access(CDMA) system which is one
of digital cellular systems is a multi-access technology under the interference limited environment. It
can process more number of subscribers compared to other systems (TDMA system has the
processing capacity three times greater than the existing FDMA system whereas CDMA system,
about 12~15 times of that of the existing system).
CDMA system can be explained as follows: TDMA or SDMA can be used to enable each person to
talk alternately or provide a separate room for each person when two persons desire to talk with each
other at the same time, whereas FDMA can be used to enable one person to talk in soprano, whereas
the other in bass (one of the two talkers can carry out synchronization for hearing in case there is a
bandpass filter function in the area of the hearer).
Another method available is to make two persons to sing in different languages at the same time,
space, and frequency when wishing to let the audience hear the singing without being confused. This
is the characteristics of CDMA.
On the other hand, when employing the CDMA technology, each signal has a different
pseudo-random binary sequence used to spread the spectrum of carrier. A great number of CDMA
signals share the same frequency spectrum. In the perspective of frequency area or time area, several
CDMA signals are overlapped. Among these types of signals, only desired signal energy is selected
and received through the use of pre-determined binary sequence; desired signals can be separated
and then, received with the correlator used for recovering the spectrum into its original state. At this
time, the spectrums of other signals that have different codes are not recovered into its original state
and instead, processed as noise and appears as the self-interference of the system.
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