Hi friend
Each geographic area is divided into hexagonal cells that are nested to form a honeycomb pattern. The hexagon shape was chosen because it provides the most effective transmission approximate to a circular pattern, while eliminating spaces present between adjacent circles. A cell is defined by its physical size and, most importantly, by the size of its population and traffic patterns. The number of cells per system is defined by the provider, according to anticipated traffic patterns. Each geographical area of the mobile service is distributed in cellular radio channels. Each transmitter / receiver with a surround area has a fixed subset of available radio channels, based on the anticipated traffic flow.

The hexagonal shape is the geometric shape that allows you to occupy all the space, which would not happen if they were circumferences.
Thanks.