Tunnels are diverse. Divided into highway tunnels and subway tunnels according to their purposes.  Different functional tunnels have different requirements for signal coverage. According to the tunnel's structure, it can be divided into curved, straight, long, and short. Tunnels of different structures also have different signal coverage.

In  general, the use of highway or rail tunnels for calling or the Internet accessing is not very high.Outdoor base station signals can be extended to achieve signal coverage.Take the example of a 5km straight road tunnel.a set of 10W air-coupled fiber optic repeater with 1 pcs local unit and 5 pcs of remote units is adopted.A high-gain outdoor plate antenna is installed at the mouth of the tunnel to receive the signal from the base station and connect it to the fibre optic repeater via a 1 / 2 coaxial cable. local units are installed in tunnels as close as possible to outdoor reception antennas, and five remote units are installed in tunnels of 0.5km, 1.5km, 2.5km, 3.5km and 4.5km. Five remote units are connected to local unit via fibre-optic cable.The radio frequency output port of each remote unit needs to be connected to a two-way splitter and then to 2 directional antennas ( installed in reverse), each covering a distance of 500m.

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