Tunnel radio system
Tunnel radio systems , also known as tunnel transmitters , are technical devices that enable wireless radio communication in tunnels or mines .
Due to the strong attenuation of radio waves in the ground, it is not possible to supply a tunnel with radio technology from outside. The only exceptions are short tunnels (typically tunnel lengths of less than 500 m) or tunnel entrance areas that can be served by directional antennas installed on the tunnel front portal.
Tunnel systems can be used to supply tunnel systems with radio , public mobile radio ( GSM , UMTS ), company radio ( e.g. for road service ) and BOS radio for the police , fire brigade and rescue services. Special tunnel radio systems are also used for commercial radio in mines.
construction
Construction of tunnel radio systems for road tunnels
High-level roads (i.e. expressways and motorways ) with a tunnel length of more than 500 meters are usually equipped with tunnel radio systems.
The frequency range typically required for road tunnels is in the 70 cm, 2 m, 4 m bands and in the VHF radio band. In addition, tunnels are partially supplied with mobile radio frequencies ( GSM or UMTS ).
State-of-the-art systems consist of the following components:
- Free field antennas (for coupling and decoupling the radio signals from the tunnel into the free field)
- Tunnel radio head-end station (central unit that forwards the HF signal of the free field to the tunnel stations or receives it from the individual tunnel stations)
- Tunnel stations (HF receiver / amplifier units arranged at a distance of 500 to 1000 m, which supply the tunnel in sections)
- Antenna system in the tunnel (radiator cable (also called leak cable or slotted cable ) for frequencies up to about 1 G Hz or pendulum antennas for higher frequencies)
Systems of this type are divided into sectors, each of which is supplied by a tunnel station. The tunnel stations are usually connected to the head station in a star shape via fiber optic cables. Signals received in a sector from the tunnel station there are converted to a fiber optic signal by means of electro-optical converters and transported to the head station. In the opposite direction (head station → tunnel station), those signals are transmitted that are to be transmitted into the tunnel.
The tunnel is usually supplied by radiator cable antennas. For frequencies> 1 GHz, directional antennas (e.g. parabolic antennas ) are used instead of radiator cables, since radio waves with frequencies> 1 GHz propagate well in tunnels. Radiator cables are special coaxial cables that are usually laid on the tunnel ceiling parallel to the tunnel and can emit or receive a signal over the entire length (typical diameter of the radiator cables: 0.5-1.5 inches ).
Modern tunnel radio systems allow the mobile radio signal to be fed in via fiber optic cable directly from the base station in the head end of the tunnel radio system . In a longer tunnel, a handover of the mobile radio is often implemented. In addition, modern tunnel radio systems allow the feed of the mobile radio signal from two base stations via fiber optic cable in the head station.
In the handover zone, the mobile radio signal can be easily received by two radio cells . Before entering the handover zone, the mobile station uses the radio signal from the old radio cell. In the handover zone, the signal strength of the old radio cell decreases continuously. At the same time, the signal strength of the new radio cell increases continuously. As soon as the radio signal from the new radio cell is significantly stronger than the radio signal from the old radio cell, the mobile station switches to the new radio cell and uses the radio signal from the new radio cell after the handover. So that the handover can be carried out successfully, the mobile station must be in the handover zone for a minimum period of time determined by the radio technology used. The higher the driving speed, the longer the handover zone must be. So that the required length of stay in the handover zone is observed.
The handover zone can also be relocated to the area in front of the tunnel portal with the external antennas mounted on the tunnel portal. Then the handover takes place outside the tunnel. That is why one often sees cell phone antennas with directional beam characteristics mounted on tunnel portals , which are aligned with the roadway. These external antennas are not used to receive radio signals for the tunnel radio system, but rather to implement the handover zone outside the tunnel.
Construction of tunnel radio systems for underground and train tunnels
Modern tunnel radio systems for railway tunnels do not differ in their technical structure or hardly differ from those for road tunnels.
The only difference to road tunnels is the usable frequency range. Broadcasting signals are generally not fed into underground and railroad tunnels. Intrain repeaters are sometimes used on the vehicle side .
Underground tunnels or underground underground stations are usually supplied with mobile radio signals to enable passengers to receive mobile communications with conventional mobile telephones, as on the surface of the earth. These mobile radio systems do not always function as a relay station , but are designed as an independent base station.
GSM-R is increasingly used on railway lines ( e.g. for ETCS Level 2 ), which makes radio coverage in the 876-925 MHz frequency range necessary.
Other embodiments
Earlier tunnel radio systems (installed before around 1995) for road and rail tunnels were mostly built using the so-called cascade technology. With this technique, the transmission signal is coupled into a slotted ribbon cable at one end and refreshed by RF amplifiers at intervals of typically 250-500 m. The disadvantage of this variant is a poorer radio quality caused by repeated amplification and an increased interference range in the event of a fault (interference in the cable or in one of the amplifiers affects the entire subsequent tunnel section).
Notes on the radio signal emitted
Cellular
Older tunnel radio systems do not support MIMO for modern mobile communications. These older tunnel radio systems only support SISO . Some newer tunnel radio systems support MIMO. MIMO-capable tunnel radio systems allow faster data transmission rates than SISO-capable tunnel radio systems.
Specialties
Tunnel transmitters for radio transmitters are in rare cases also designed for long and medium wave . An example of this can be found in the Dartford Tunnel in London, where the program “ Virgin Radio ” in the medium wave range and the long wave program of the BBC are made available to motorists inside the tunnel.
In the area of non-public company radio in mines and mines, radio systems are used in the course of operations.
Literature sources
- Shuqi Wang, Xiaobing Han: Influence of Transmitter Position and Dielectric Constant on Electromagnetic Waves Propagation in Mine Tunnel . 2009, doi : 10.1109 / MMIT.2008.40 .
Individual evidence
- ↑ Page no longer available , search in web archives: Guideline: radio systems in road tunnels V3.02. ASTRA 2007.
- ↑ https://www.comlab.ch/dam/jcr:7b23e904-b7c6-4d84-86c2-da11601d09b5/MIMI%20LTE.pdf Comlab - 4G LTE MIMO Repeater Systems
- ↑ Cell phone reception for London Underground to Olympia. In: Der Standard , September 22, 2010
- ↑ Cell phone reception in the Munich subway from summer 2009. In: golem.de , March 31, 2009
- ↑ https://www.comlab.ch/dam/jcr:7b23e904-b7c6-4d84-86c2-da11601d09b5/MIMI%20LTE.pdf Comlab - 4G LTE MIMO Repeater Systems