Tunnels vary widely in type and structure. Functionally, they can be classified as highway tunnels and metro/railway tunnels, each with different requirements for wireless coverage. Structurally, tunnels may be straight or curved, long or short, and these differences also affect the design of the coverage solution.

In most highway and rail tunnels, the demand for high-capacity voice and data services is moderate, so extending the coverage of existing outdoor base stations is usually sufficient.

Taking a 5 km straight road tunnel as an example, a 10 W air-coupled fibre-optic repeater system with one local unit and five remote units can be deployed. A high-gain outdoor panel antenna is installed at the tunnel entrance to receive signals from the base station and is connected to the local unit of the fibre-optic repeater via a 1/2″ coaxial feeder cable. The local unit is installed inside the tunnel as close as possible to the donor antenna, and the five remote units are positioned at approximately 0.5 km, 1.5 km, 2.5 km, 3.5 km and 4.5 km along the tunnel.

Each remote unit is connected to the local unit by fibre-optic cable. The RF output port of every remote unit is then connected to a two-way splitter feeding two directional antennas mounted back-to-back, each covering a section of about 500 m.