Frequency reuse
The heart and soul, the inner core, the sine qua non of cellular radio is frequency reuse. The same frequency sets are used and reused systematically throughout a carrier's coverage area. If you have frequency reuse you have cellular. If you don't, well, you don't have cellular. Frequency reuse distinguishes cellular from conventional mobile telephone service, where only a few frequencies are used over a large area, with many customer's competing to use the same channels. Much like a taxi dispatch operation, older style radio telephone service depended on a high powered, centrally located transmitter which paged or called mobiles on just a few frequencies.
Cellular instead relies on a distributed network of cells, each cell site with its own antenna and radio equipment, using low power to communicate with the mobile. In each cell the same frequency sets are used as in other cells. But the cells with those same frequencies are spaced many miles apart to reduce interference. Thus, in a 21 cell system a single frequency may be used several times. The lone, important exception to this are CDMA systems which we will cover later. In those, the same frequencies are used by every cell.
Each base station, in addition, controls a mobile's power output, keeping it low enough to complete a circuit while not high enough to skip over to another cell. (back to Cell Basics article)
The frequency reuse concept. Each honeycomb represents a cell. Each number represents a different set of channels or paired frequencies. A cellular system separates each cell that shares the same channel set. This minimizes interference while letting the same frequencies be used in another part of the system. This is frequency reuse. Note, though, that CDMA based systems can use, in theory, all frequencies in all cells, substantially increasing capacity . For review, a channel is a pair of frequencies, one for transmitting on and one for receiving. Frequencies are described by their place in the radio spectrum, such as 900mHZ, whereas channels are described by numbers, such as channels 334 through 666. Illustration from the CDC.
Click here to go to another frequency resuse explanation in my Cellular Baiscs Article -- it contains a large graphic from an early AT&T journal.