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In cellular telephone communications, the air interface is the radio-frequency portion of the circuit between the cellular phone set or wireless modem (usually portable or mobile) and the active base station. As a subscriber moves from one cell to another in the system, the active base station changes periodically. Each changeover is known as a handoff.
A cellular connection is only as good as its weakest link, which is almost always the air interface. Radio-frequency (RF)circuits are subject to many variables that affect signal quality. Factors that can cause problems include:
Use of the handheld phone set or portable wireless modem inside buildings, cars, buses, trucks, or trains
Proximity to human-made, steel-frame obstructions, especially large buildings and freeway overpasses
Abundance of utility wires that can reflect radio signals and/or generate noise that interferes with reception
Irregular terrain, particularly canyons and ravines
Inadequate transmitter power in phone set or wireless modem
Poorly designed antenna in phone set or wireless modem
In addition to these variables, some cellular networks have inadequate coverage in certain geographic areas. Usually this is because there are not enough base stations to ensure continuous communications for subscribers using portable (handheld) phone sets. As a network evolves, more base stations may be installed in a given region, and in that case, this problem will diminish with time. Conversion of a network from analog to digital can result in dramatic improvement.