A cell is an area where a Mobile Station (MS) receives a signal strength that is high enough for setting up and maintaining a radio connection on a dedicated channel, that is, SDCCH or TCH. The size of a cell (or the size of the coverage area) is mainly determined by four parameters:
- The output power (BSTXPWR) at the antenna of the BTS: the Effective Radiated Power (ERP)
- The minimum received level at the MS (this is MSRXMIN for MS in “busy” mode)
- The minimum received level at the BTS (this is BSRXMIN)
- The Timing Advance (TA): the TA is a measure of the traveling time of the bursts between the MS and the BTS; the maximum value of TA in a cell is defined by the parameter MAXTA (MAXimum Timing Advance).
A second cell criterion is the existence of a BCCH. A cell must have exactly one BCCH because the BCCH carries essential information which must be known to the MS before call setup.
The previous example presented a cell of circular shape, an omni-cell. Generally, the shape of a cell depends on the antenna, connected to the cell. The antenna can also focus its power on a certain sector of a circle. This is called a sector-cell. It is up to the cell planner to select a suitable antenna. The cell shape can also depend on the geographic conditions. Each sector can have its own output power assigned. The BCCH frequencies must be different in all the sectors.