Monday, November 5, 2012

Frequency Reuse - Mobile Computing : [BE : IT/Comp]

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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.

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