BDA (Bi-Directional Amplifier) is used to provide bi-directional communication between radio terminals and the base station in order to fit in any kind of environment. It can be used outdoors to extend the coverage range of base stations, making communication services available in areas with no or weak signals. Additionally, it can be used indoors as the signal source to improve signal strength. With a multichannel selecting unit, BDA can selectively amplify the signals, ensuring the frequency purity of repeated signals and the stability of output power. It has been widely used in scenarios such as tunnels, along roads and railways, indoor areas, outskirt areas and congested residential areas. It has excellent compatibility, widely used in trunking communication systems such as DMR, TETRA, APCO25, MPT-1327, as well as conventional communication system.