A cellular repeater is a device capable of rebroadcasting a signal from an area with good reception to an area with poor reception. These devices are commonly used to receive a strong outdoor signal and essentially bring it inside a building. The outdoor antenna may be omnidirectional, though directional antennas can increase the overall signal strength. In order for a cellular repeater to work with any given phone, both devices must operate on the same frequency. These devices will not typically interfere with a provider's cellular network, though some units have more stringent safety precautions built in than others.
The three components that typically make up a cellular repeater are two antennas that are joined by a signal amplifier. One antenna is mounted outside where the signal strength is high, and a second one is installed indoors where there is little or no coverage. The outdoor one can be omnidirectional, though more expensive units tend to have directional antennas. If the exterior antenna is directional, it can be pointed toward the nearest cellular tower and provide the highest possible signal strength. A physical connection of some sort, typically coaxial cable, is then used to connect each antenna to a signal booster.
Together, the components that make up a cellular repeater are known as a bi-directional amplifier (BDA). The term "bi-directional" simply means that the device can receive signals from the cellular network and send them as well. In order to function properly, a minimum distance typically has to be maintained between the two antennas. If the interior antenna has too much overlap with the exterior antenna, most cellular repeaters will shut down or issue an alarm warning. It is equally important that cellular service is available at the location of the exterior antenna, or else the device will fail to function at all.
Each cellular phone network operates within a specific frequency band of the radio spectrum, though some use more than one. Some networks use one band for voice and another for data, or have built their broadband networks on a different frequency than voice and low speed data. In order for a cellular repeater to function with a phone, it must match the same frequency. Some repeaters operate on two or three different frequencies, which can allow them to work with more than one network or a larger range of phones offered by a single provider.