Repeater or booster usually consists of antenna, RF duplexer, low noise amplifier, mixer, RF attenuator, filter, power amplifier and other components or modules, including uplink and downlink amplification links. The basic principle of its work is: the downlink signal of the base station is received into the repeater by a forward antenna (donor antenna), and the useful signal is amplified by a low-noise amplifier to suppress the noise signal in the signal and improve the signal-to-noise ratio (S/N); Then down-converted to an intermediate frequency signal, filtered by a filter, amplified by the intermediate frequency, frequency-shifted and up-converted to a radio frequency, amplified by a power amplifier, and transmitted to a mobile station by a backward antenna (re-transmission antenna); At the same time, the uplink signal of the mobile station is received by the backward antenna, and processed by the uplink amplification link along the opposite path: that is, it is transmitted to the base station through the low noise amplifier, down converter, filter, intermediate amplifier, up converter and power amplifier. So as to achieve bi-directional communication between the base station and the mobile station.

As one of the necessary equipment to achieve the goal of "small capacity and large coverage", repeater or booster is mainly used to ensure network coverage without increasing the number of base stations, and its cost is far lower than that of micro-cellular base-station systems with the same effect. Repeater or booster is an economic way to solve the problem of extended coverage of communication network. Compared with the base station, it has the advantages of simple structure, less investment and convenient installation. It can be widely used in blind-signal areas and weak-signal areas that are difficult to cover, such as shopping malls, hotels, airports, docks, stations, gymnasiums, entertainment halls, subways, tunnels, highways, islands and other places, to improve communication quality and solve the problem of dropped calls.