It is important to ensure reliable radio communication in enclosed areas like roads and railway tunnels for the convenience as well as for ensuring safety. There is a need of reliable communication systems for the prevention of mishaps, to manage maintenance operations, handling emergency situations etc.

From the communication engineering point of view there are a numbers of things that are required to be kept in mind before designing communication systems for tunnels

1. System for making announcements during emergency situation – provision for having a wireless networks 4G, Wi-Fi etc.
2. Complete coverage of enclosed areas. Combinations of different solutions are required like mixing of base stations, radiating cables, boosters etc.
3. If some equipment gets damaged due to some accidents then redundancy requirements are also required to be taken care of.

There are certain innovations in radio technology which can improve the transport tunnel safety.

Optical Master Unit
Due to very less space inside the tunnel it is required to keep passive equipments inside the tunnel and active equipments at the places which are little more accessible. Now a days, Optical Master Unit (OMU) is employed, which is an optical fiber distribution network. It converts RF signals into modified light and back again which enables the remote antenna to exchange signals throughout the tunnel.

Distributed Antenna System
Another need is to distribute the signal through the tunnel. Some accident may destroy the base station of active equipment. For such conditions we can use DAS (Distributed antenna system) for both the transmitter and receiver ends because if one looses the connection other will still be available. It is possible that train travelling through the tunnel may itself block the signal propagation so DAS are capable to work with on-board equipments i.e. the signal could be received by the running train and then radiated back. A distributed antenna system, or DAS, is a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. DAS antenna elevations are generally at or below the clutter level and node installations are compact. A distributed antenna system may be deployed indoors (an iDAS) or outdoors (an oDAS).

To prevent any interference or disturbance like in the occurrence of any tunnel fire or any emergency, it is required to carefully lay the radio system cables. Many tunnels are driven with a round bore which gives the cables enough space to be laid beneath the road.

Picocell
Another option is to use Picocell. Picocell coverage is recommended for underground railway tunnels. There is no need to use RF cables while using picocell. Ethernet cables or gigabyte fiber are used for the connection of picocell. Picocell is used for covering small areas. It is used in areas where it is difficult to cover using external signals i.e. picocells provide coverage to indoor areas. It is considered to be much smaller then microcell. Picocell can be used for areas which are much smaller in size like areas which are less than 250 yards and can support up to 100 users at a time or more.

By – Sandhya Sharma – Asst. Prof. ECE, Chitkara University H.P.

References:
https://en.wikipedia.org/wiki/Distributed_antenna_system
http://www.newelectronics.co.uk/
https://en.wikipedia.org/wiki/Picocell#References