ST2115
Scientech Technologies Pvt. Ltd.
10
2.
Time Division Multiple Access (TDMA):
In digital systems, continuous transmission is not required because users do not use
the allotted bandwidth all the time. In such systems, TDMA is a complimentary
access technique to FDMA. Global Systems for Mobile communications (GSM) uses
the TDMA technique. In TDMA, the entire bandwidth is available to the user but only
for a finite period of time. In most cases the available bandwidth is divided into fewer
channels compared to FDMA and the users are allotted time slots during which they
have the entire channel bandwidth at their disposal. This is illustrated in figure2
TDMA requires careful time synchronization since users share the bandwidth in the
frequency domain. Since the number of channels is less, inter channel interference is
almost negligible; hence the guard time between the channels is considerably smaller.
Guard time is spacing in time between the TDMA bursts. In cellular communications,
when a user moves from one cell to another there is a chance that user could
experience a call loss if there are no free time slots available. TDMA uses different
time slots for transmission and reception. This type of duplexing is referred to as
Time division duplexing
(TDD). TDD does not require duplexers.
Channel usage by TDMA
Figure 2
3.
Code Division Multiple Access:
The world is demanding more from wireless communication technologies than ever
before as more people around the world are subscribing to wireless. Add in exciting
Third-Generation (3G) wireless data services and applications - such as wireless
email, web, digital picture taking/sending, assisted-GPS position location
applications, video and audio streaming and TV broadcasting - and wireless networks
are doing much more than just a few years ago.
This is where
CDMA
technology fits in.
CDMA
consistently provides better capacity
for voice and data communications than other commercial mobile technologies,
allowing more subscribers to connect at any given time, and it is the common
platform on which 3G technologies are built.
Co
de-
D
ivision
M
ultiple
A
ccess
,
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, but give the right to use both to all users simultaneously. To
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