Because of the shade of the mountains, most tunnels are blind zones. To cover the signal uninterruptedly of the road, the railway and the highway, the main problem is that the tunnel is in a closed structure, narrow space, and the zonal distribution of the network signal, when the mobile station switch and reselect fast and frequently and the terminals move in a high speed , it will bring the Doppler effects.
The normal solution for tunnel as below:
1) Passive distributed antenna coaxial feed
Using passive distributed antenna coaxial feed to cover is an optional way. This coverage solution has a more flexible design, costs relatively less and is more convenient to install. The attenuation of the coaxial cable is relatively low. The selection of the gain of the antenna depends mainly on the restrictions of the installation conditions. When the conditions permit, the higher gain of the antenna, the larger coverage area will be. The simplicity of this solution is to use a single antenna to cover the tunnel, which is the cheapest solution to cover the relatively short tunnel.
2) Optical fiber feed active distributed antenna system
In some complex tunnel coverage environment, optical fiber feed active distributed antenna system can be used to replace passive distributed antenna coaxial feed system. It is more suitable for covering the underground tunnels (subway tunnels) and the platform.
Advantages: reduce the quantity of cables for indoor installation; adopt much thinner cables; reduce electromagnetic interference by adopting optical fiber; more flexible design in complex networks.
Disadvantage: High cost