ST2115
Scientech Technologies Pvt. Ltd.
9
Theory
Multiple Access Techniques:
A limited amount of bandwidth is allocated for wireless services. A wireless system is
required to accommodate as many users as possible by effectively sharing the limited
bandwidth. Therefore, in the field of communications, the term
multiple access
could
be defined as a means of allowing multiple users to simultaneously share the finite
bandwidth with least possible degradation in the performance of the system. There are
several techniques through which multiple accessing can be achieved. There are three
basic schemes:
1.
frequency Division Multiple Access (FDMA)
2.
Time Division Multiple Access (TDMA)
3.
Code Division Multiple Access (CDMA)
1.
Frequency Division Multiple Accesses (FDMA):
FDMA is one of the earliest multiple-access techniques for cellular systems when
continuous transmission is required for analog services. In this technique the
bandwidth is divided into a number of channels and distributed among users with a
finite portion of bandwidth for permanent use as illustrated in figure 1 the vertical axis
that represents the code is shown here just to make a clear comparison with CDMA.
The channels are assigned only when demanded by the users. Therefore when a
channel is not in use it becomes a wasted resource. FDMA channels have narrow
bandwidth (30 KHz) and therefore they are usually implemented in narrowband
systems. Since the user has his portion of the bandwidth all the time, FDMA does not
require synchronization or timing control, which makes it algorithmically simple.
Channel Uses by FDMA
Figure
1
Even though no two users use the same frequency band at the same time, guard bands
are introduced between frequency bands to minimize adjacent channel interference.
Guard bands are unused frequency slots that separate neighboring channels. This
leads to a waste of bandwidth. When continuous transmission is not required,
bandwidth goes wasted since it is not being utilized for a portion of the time. In
wireless communications, FDMA achieves simultaneous transmission and reception
by using Frequency division duplexing
(FDD). In order for both the transmitter and
the receiver to operate at the same time, FDD requires duplexers.
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