for FM and TETRA coverage in confined areas in tunnels in Europe and Asia
Radio coverage in confined areas like tunnels is an application of radio technology that is particularly challenging. Nevertheless there is good reason for regulation that ensures that fire brigades, police and emergency forces can rely on a radio network even in road or railway tunnels. In a case of emergency caused e.g. by traffic accidents or natural disasters the radio communication allows to organize release or evacuation activities across multiple groups and well-coordinated with external emergency centers.
In Europe the radio in confined areas is typically a combination of FM (Frequency Modulation) and TETRA or TETRAPOL. FM is considered part of the safety infrastructure, as it is the preferred way to insert emergency messages for the public into the broadcast programs typically listened to on the car radio. As an example when driving through the Gotthard tunnel and listening to a radio station, a message is inserted regularly that informs the car passengers about safety regulations in the tunnel.
In some countries the requirement for the radio infrastructure becomes even more challenging as multiple FM channels corresponding to multiple languages officially imposed by regulation need to be broadcasted. The combination of multiple FM channels puts tougher requirements on the radio amplification, as linearity requirements and dynamic range increases drastically with the number of channels transmitted simultaneously. Similarly the number of TETRA/TETRAPOL channels available in the tunnel will influence the requirements on the radio infrastructure. TETRA employs digital modulation techniques which naturally require higher linearity from the radio infrastructure than earlier analog channels.
The radio propagation in tunnel lends itself to radiating cables as the antenna of choice for safety communication. A coaxial leaking cable is fixed to the tunnel ceiling or at the sides, releasing its radio waves gradually along its way and ensuring a very homogeneous coverage inside the tunnel. The length of such a leaking cable is limited by the attenuation of the cable so that typically after a few 100m a new relay station is required to feed the next segment of radiating cable. Longer segments are more cost-efficient both from installation and maintenance point of view, thus the radio amplifier must have high output power on top of the linearity requirements in order to achieve the lowest number of required segments inside the tunnel.
In any event, maintenance of radio equipment in tunnels is often related to blocking the tunnel from traffic (specifically for railway tunnels) and therefore must be as infrequent as possible. Radio infrastructure in tunnels thus requires high reliability to avoid regular intervention. Still failures can happen and it is of utmost importance that the tunnel control centers are informed about radio infrastructure failing. Failure indication becomes an additional requirement on components for coverage in tunnels. Furthermore in the period between a failure indication and the next scheduled maintenance, it must be ensured that a minimum radio performance can still be guaranteed. The HW radio infrastructure is built with redundancy duplicating circuits and keeping up communication even in case of partial failure.
Becker Nachrichtentechnik GmbH has developed an optimized amplifier for this application that fulfills all the above requirements. Due its compact form factor and very high power efficiency it has already be deployed in 100s of sites across Europe and Asia.
Latest modification: 20.03.2019