Article contributed by :
manoj@itportal.in
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Most
of you might be familiar with the concept of frequency reuse. We often come across this term in
Mobile Computing. Quite a straightforward and simple concept, but still it
requires a detailed explanation. This is one of the most common terms used in
the world of Cellular Telephony (Wireless Communication). Most cellular systems
use frequency reuse scheme to improve capacity and coverage. Let us understand
what exactly a cell mean and how they are related to frequencies.
In
a cellular system, each mobile station
(MS) is connected with its base
station (BS) via a radio link. The BS is responsible for sending the calls
to and from the MS, which lie in the coverage area of that BS. The coverage
area of a base station or a sector of a base station is known as a cell. Each
BS consists of a number of frequency channels, which serve as a link between
the MS and the BS. Every time, a call propagates through a channel which is
currently idle and receiving the best signal. As the coverage area of a BS can
be termed as a cell, we can also say
that a cell uses the frequency channels for call forwarding. These cells are
usually of hexagonal shape (this explanation is certainly not in the scope of
our discussion here). The Fig 1-1
shows a typical structure of a cell.
Fig
1-1. A cell.
A PCS (Personal
Communication System) is a combination of many such cells. So, a cell may be
surrounded by a large number of adjacent cells. This is shown in Fig 1-2.
Fig 1-2. Cells adjacent to each other
(Cluster).
Now,
let us look at a more general term used for the above structure- a cluster. A number of cells are
grouped to form a cluster. So, a cluster
is a collection of various cells. Now, after understanding the concepts of
cells and cluster, let us move into the actual concept of frequency reuse.
As
we have seen, cells use frequencies. But imagine two or more cells in a single
cluster using the same frequency. Obviously, there is a wide scope of
interference. So, it is always a better option to avoid two cells in a cluster
using the same frequencies. That is, inside a cluster, all the cells must use
different frequencies. A 3-cell cluster with all the adjacent cells using
different frequencies (F1, F2 and F3) is shown in Fig 1-3.
Fig 1-3. Cells in a
cluster using different frequencies.
But
this will definitely lead to a new problem. As the network grows, if every cell
in a system uses different frequencies, the frequency spectrum will be heavily
utilized. A large amount of frequencies will be utilized by these cells. A
solution to this problem is the Frequency Reuse.
All
the cells in a cluster must still have different frequencies, but these
frequencies can be reused by the cells in other clusters. This is the concept
of frequency reuse. That is, if
frequencies A, B, C, D, E, F and G are used by the cells in a 7-cell cluster,
these same frequencies A, B, C, D, E, F and G can be used by the cells in other
clusters. See Fig 1-4.
Fig
1-4. Frequency Reuse.
In the above figure, three different clusters
are shown with three different colors. Each of the 7 cells in each clusters use
different frequencies (A through G). But, the same frequencies (A through G)
are reused by the seven cells of each of the other clusters. Thus, the problems
of interfering frequencies as well as over-utilization of frequencies are
overcome using the concept of frequency reuse.
Article contributed by :
manoj@itportal.in
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