Radio transmitters within a wireless network only have a specific coverage area beyond which the signal level falls so low that it can't be used. At this point, it will also not cause significant interference to a mobile device connected to another radio transmitter. This makes it possible to re-use a channel once it is outside the range of a specific radio transmitter. The same holds true for signals that can only be received over a certain range.
A network operator will typically split up their coverage area into smaller regions called cells. Each cell is then covered by a different transmitter. This is why it is called "cellular" technology. In a drawing, the cells are often shown as hexagonal shapes that fit together like a honeycomb. This is not really the case. Their boundaries are often irregular because of the terrain over which signals travel. Buildings, hills and any other object will cause the signal to attenuate differently in different directions.
When a cellular network is planned, operators normally use different frequency channels or bands for adjacent cells. This reduces interference even when coverage areas overlap. When cells are grouped together in this way, it is called a cluster. Clusters are often made up of seven cells, but they can be configured differently. When the number of cells in a cluster is decided on, a number of conflicting requirements need to be balanced. These include the number of frequency channels or bands that can be used by each cell site, and limiting interference levels.
It is critical to limit the interference between cells using the same frequencies. The cell configuration topology impacts on this in a big way. The more cells in a cluster, the bigger the distance between cells sharing the same frequencies needs to be.
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