The advent of the cellular concept was a crucial contribution in the development of mobile communication. A cell is viewed as the (approximate) coverage area of a particular land site. Each cell within a cluster is allocated a distinct set of frequencies (channels) and cells labeled with a given number -- i.e. co-channels reuse the same channel set.
As the cell size decreases traffic carrier capacity increases and, thus, cells start big and split as system grows.
With the shift parameters i and j defined in the figure, we see that the number of cells in a cluster is given by
and the frequency reuse distance is given by
To further reduce intercluster interference, each cell is quite ofter sectored -- i.e.directional antennas are used at the mobile base stations.