Mobile network coverage allows the simultaneous use of millions of wireless phones, still or moving. To achieve this objective, all radio access technologies must solve the same problem: distribute a single radio spectrum among many users as efficiently as possible. For this, various techniques are used for multiplexing and separating the coexistence of users and radio cells: time division multiplexing, frequency multiplexing and code multiplexing, or more often a combination of these techniques.
A mobile network has a cell structure that allows reuse of frequencies. It also allows users on the move to change base stations through seamless handovers. At peak times, hundreds of thousands or even millions of devices are in service with only 500 available channels (in the case of GSM)
The mobile networks use electromagnetic waves, just like networks for radio, television, satellite and other communication networks such as those reserved for police, ambulance and others. To transmit information, binary or analog channels are used. In GSM networks, the channel is carried by a specific frequency at which the wavelength is modulated. There is need to keep some space between the channels because if their frequencies are too close, they impinge on each other, producing interference.
Groups of frequencies employed vary by continent, sometimes even from country to country. In Europe, GSM mobile networks use two groups of frequencies around 900 MHz and 1800 megahertz (MHz). It was first necessary to divide each of these into two groups: a band of frequencies to send signals to mobile phones, one for emissions from phones to the network. One to be called and heard, the other to call and talk.
Then comes the subdivision into channels. GSM (Global System for Mobiles), the frequency difference between two adjacent channels was set at 200 kHz or 0.2 MHz. Group 1 defined above can contain 125 channels (25 MHz / 0.2 MHz) and group 2 contains 375, making 500 in total. A large number of short-range transmitters are installed, each covering only a well-defined geographical area called a cell (the area covered by an antenna). The average cell size varies from 1 to 30 km.
Thus, mobile radio waves travel from a base station, if a connection is established, it retains the identity of the device number (IMSI). It reserves a channel and resumes contact when a call is made, this allows reuse thousands of times of the same radio frequencies in a single country. However, this solution poses a problem if all adjacent antennas use the same frequencies for their users. If one of them is located at the border between the two areas of scope, they may receive two signals that disrupt each other.
Base Transceiver Station (BTS) antennas are often grouped in threes on a tower or by equivalent configuration, each emitting at an angle of 120 degrees or 360 degrees in total ( 3×120 degrees). Each of these antennas thus supports multi-tier communications. They create hexagonal checkerboard cells.
This checkerboard is primarily made up of overlapping areas and their capacity varies with the weather, the number of users, etc. Each antenna uses a bunch of different radio frequencies to that of its neighbors, and the same frequencies are therefore reused at a distance so as not to cause interference.