The cellular concept

The cellular concept was a major breakthrough in mobile communications, and it initiated the era of modern wireless communications. It helped to solve the problem of spectral congestion and user capacity. For wireless communications, the antennas are typically required to be omnidirectional. The cellular structure divides the geographical area into many cells. A BS equipped with an omnidirectional antenna is installed at the center of each cell. Neighboring cells use different frequency bands to avoid co-channel interference (CCI).

The same frequency bands can be used by different cells that are sufficiently far away. This leads to frequency reuse. For a distance D between two cells that use the same frequency bands and a radius R of the cell, the relative distance D/R between the two cells is the reuse distance. There is no CCI within a cluster of cells, where each cell uses a different frequency spectrum. The number of cells in a cluster is called the cluster size. The cluster size determines the capacity of the cellular system: a smaller cluster size leads to a large capacity.

The cell shape usually takes the form of hexagon, and the overall division of space is like a beehive pattern. This is illustrated in Fig. 4.1 for cluster size 4 and reuse distance D/R ∼ 4. This hexagonal cell shape is suitable when the antennas of the BSs are placed on top of buildings with a coverage radius of a few miles, such as in the 1G mobile systems.