DAB+ Signal Propagation in Tunnels

Tech Review 2017

Radiotelevisione italiana S.p.A. (RAI) has been performing tests to assure service continuity of DAB+ in tunnels, both short and straight and long and bendy.

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In November 2015, preliminary tests on the A5 motorway (between Turin and Aosta) confirmed the difficulty of achieving DAB+ service continuity in tunnels from HPHT installations located nearby.

Radio service transmission inside tunnels requires methods such as the installation of radiating cables (“leaky-feeders”) along the tunnel ceiling. This approach is costly, both in the case where a new radiating cable has to be installed and in the case of sharing cables already in use for other radio services (e.g. FM radio, mobile telephony, radio alerts, police, …).

The RAI Research Centre has investigated the feasibility of an alternative approach to providing DAB+ coverage inside tunnels, using “direct RF radiation” by means of antennas positioned outside the tunnel (not far from the entrance) or internally.

Technical literature regarding 200 MHz radiowave propagation inside tunnels is lacking and so in 2016 the RAI conducted a further DAB+ measurement campaign with the support of “Autostrade per l’Italia” (ASPI).
The results of all the measurements in tunnels (with and without traffic) are summarised in this article, together with a proposal for simplified design criteria for the “direct RF radiation” network and a comparison between the “direct RF radiation” and “leaky-feeder” approaches in terms of performance and implementation complexity and costs.