Saturday, November 30, 2013

Frequency Reuse with Cellular Topology

Start this tutorial from : Introduction

Primitively mobile networks  (that should probably be called the 0g mobile network such as IMTS),were designed like TV networks... one very powerful base station catered to a large area. 

The IMTS by AT&T for instance in New York, had 12 channels only. This means that at a given time only 12 calls could be active. Because of limited channels, AT&T had to limit subscribers to 2000. On an average users had to wait around 20 minutes before there calls could be placed.

This user capacity limitation was overcome by using a Cellular Topology

A land area to be covered is divided into regular shaped cells(as big as 10-15 miles), which can be hexagonal, square, circular or some other regular shapes.Hexagonal cells are conventional because hexagons best resemble a circle. (as a BTS would radiate around itself in more or less a circular radius)
Cluster of Cells
A cluster of cells
Each cell is served by one or more fixed-location transceiver, known as a cell site or base station. A mobile device would communicate with a cell-site most closest to it. Every radio base stations transmits/receives information in its cell at a particular frequency range from f1 - f6 . The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference.

How capacity is improved by cellular topology?

In IMTS : 
we can think the entire area A as one big cell with one BTS. 
Let total number of channels = 14Number of Active calls that can be supported in area A = 14
In Cellular: 
Let A be divided in 10 clusters
No. Of cells in one cluster = 7 Number of channels per cluster = 14/7 = 2
Number of channels in area A = (14*10)/7=20Number of Active calls that can be supported in area A = 20
This was a short description of channel multiplication using frequency reuse. Now, that we know basic concepts behind cellular topology, we shall for now, stop here. There's a lot more to frequency re-use and cell planning which is a major field research and application. Cell planning is a science in itself.

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