EMR Bi-Directional Amplifiers (BDAs) provide two way (uplink and downlink) filtering and amplification of RF signals in buildings, tunnels or areas that are shaded from adequate RF signal coverage. In addition to the BDA, other devices needed for a distribution system include transmission line, power splitters, hybrid & directional couplers and indoor antennas. The use of radiating cable can also be used, particularly in tunnels and long corridors. The choice of distribution method depends on the nature of the structure in which signal enhancement is required.