Mobile telephone, also called mobile phone, portable device for connecting to a telecommunications network in order to transmit and receive voice, video, or other data. Mobile phones typically connect to the public switched telephone network (PSTN) through one of two categories:
- Cellular telephone systems
- Global satellite-based telephony
Earlier days, the first mobile and portable subscriber units for cellular systems were large and heavy. With significant advances in component technology, though, the weight and size of portable transceivers have been significantly reduced. In this section, the concept of cell phones and the development of cellular systems are discussed.
All cellular telephone systems exhibit several fundamental characteristics, as summarized in the following:
- The geographic area served by a cellular system is broken up into smaller geographic areas, or cells. Uniform hexagons most frequently are employed to represent these cells on maps and diagrams; in practice, though, radio waves do not confine themselves to hexagonal areas, so the actual cells have irregular shapes.
- All communication with a mobile or portable instrument within a given cell is made to a base station that serves the cell.
- Because of the low transmitting power of battery-operated portable instruments, specific sending and receiving frequencies assigned to a cell may be reused in other cells within the larger geographic area. Thus, the spectral efficiency of a cellular system (that is, the uses to which it can put its portion of the radio spectrum) is increased by a factor equal to the number of times a frequency may be reused within its service area.
- As a mobile instrument proceeds from one cell to another during the course of a call, a central controller automatically reroutes the call from the old cell to the new cell without a noticeable interruption in the signal reception. This process is known as handoff. The central controller, or mobile telephone switching office (MTSO), thus acts as an intelligent central office switch that keeps track of the movement of the mobile subscriber.
- As demand for the radio channels within a given cell increases beyond the capacity of that cell (as measured by the number of calls that may be supported simultaneously), the overloaded cell is “split” into smaller cells, each with its own base station and central controller. The radio-frequency allocations of the original cellular system are then rearranged to account for the greater number of smaller cells.
In addition to the terrestrial cellular phone systems described above, there also exists several systems that permit the placement of telephone calls to the PSTN by passengers on commercial aircraft. These in-flight telephones, known by the generic name aeronautical public correspondence (APC) systems, are of two types:
- Terrestrial-based, where telephone calls are placed directly from an aircraft to an en-route ground station and
- Satellite-based, in which telephone calls are relayed via satellite to a ground station. In the United States the North American terrestrial system (NATS) was introduced by GTE Corporation in 1984. Within a decade the system was installed in more than 1,700 aircraft, with ground stations in the United States providing coverage over most of the United States and southern Canada. A second-generation system, GTE Airfone GenStar, employed digital modulation. In Europe the European Telecommunications Standards Institute (ETSI) adopted a terrestrial APC system known as the terrestrial flight telephone system (TFTS) in 1992. This system employs digital modulation methods and operates in the 1,670–1,675- and 1,800–1,805-megahertz bands. In order to cover most of Europe, the ground stations must be spaced every 50 to 700 km (30 to 435 miles).