Cellular Theory

Cellular Theory

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Cellular theory is based on the idea of using low powered antennas which span small areas with their signal in order to re-use frequencies and thereby provide service for many people on a relatively narrow band width. These small cells are arranged in a pattern (like the one shown below such that no similar channels are adjacent). Key factors that are involved in this arrangement of antennas with repeating channels are signal handoff, interference, and the density of callers in a given area.

Figure 1: An illustration of channel reuse and cell splitting in a cellular system. In this example each cell is assigned one-seventh of the total number of channels assigned to the system. Note that two smaller cell clusters have been integrated into the system to accommodate increased local demand in a densely populated region.

The word "Cellular" in "Cellular Network" or "Cellular phones" comes from the design of these networks from cells or units of non-matching frequency range. Each cell is served by an antenna which covers a uniform area with adequate signal strength. These cells are arranged in groups called cell clusters of 4, 7, or 12. The bigger the number of cells in a cluster, the less chance of interference from cells of similar frequency range, but also the fewer calls you can squeeze into an allocated frequency range.

Some Beginning Definitions:

Bandwidth = a designated range of frequencies (like 88.5 to 108 MHz FM on the radio)

Channel = a smaller section of frequencies, a sub-set of the band

SNR = "signal to noise ratio", signal strength / non-signal strength.

The Advantages of a Cellular Network:

frequency reuse allows you to pack more calls in a given bandwidth-> cover large area with small bandwidth because reusing channels at a distance apart

handoff moves call to new cell when signal strength (measured by SNR) gets too weak

Cellular Network Design Constraints:

booming-- If one antenna is very strong, then even though it is on a different channel, it adds noise and therefore lowers the SNR of its neighboring antenna. This explains the uniform size of cells.

Interference-- If cells of the same channel are too close together, they lower each others SNR and cause noisy communications.

uncovered regions-- If cells are not close enough together, then there are areas where there is no network coverage.

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