Cellular channels areradio frequency (RF) channels. In an RF-based system, any receiver within range of a transmitter, i.e., within its coverage area, can tune to the frequency in use by the transmitter and, with the proper demodulation and decoding, capture the information transmitted.
However, two or more nearby transmitters simultaneously using a common frequency or channel will interfere with one another, unless perfectly synchronized in both timing and content. A receiver within range of both transmitters will most likely receive a garbled message, which is undecipherable.
This potential for interference limits the capacity of any RF-based system. Like any other system employing a shared medium, such as an Ethernet LAN, only a single device can transmit at a time. The greater the number of devices sharing the physical channel, the less often each can transmit. The only difference between an RF-based system and a LAN is the scope of the shared medium; the RF-based system is not as physically bound, thus the opportunity for interference is greater.
This RF capacity constraint increased the expense to users of early mobile phone systems such as Mobile Telephone System (MTS) and Improved Mobile Telephone System (IMTS). These systems typically broadcast all channels from a single antenna location. Within a city covered by one of these systems, the number of simultaneous users was limited by the number of RF channels available to the system; each RF channel could be used by only one transmitter at a time.