TETRA coverage assured in tunnel network

Wireless profiles a project in Helsinki’s underground energy production tunnels to create a resilient system in a network of underground tunnels

TETRA coverage assured in tunnel network

When Finnish energy production company Helsingin Energia Oy sought to enhance TETRA network coverage in its underground network of tunnels beneath Helsinki it turned to Creowave to provide coverage with TETRA Indoor and Hybrid Repeaters.

The project, which started in 2009, involved enhancing TETRA network coverage inside service tunnels beneath the city, which include plants such as district heating pipes. The tunnels are used by Helsingin Energia personnel as well as various service people.

The challenge of being able to communicate in the multi-kilometre tunnel network was just one element of the tasks to be addressed. By their nature, tunnels are a challenging working environment but, when you add to that the potential safety threatening situations the district heating network can cause if there is a leak in a pipe or valve, the challenge is magnified. Faults in heating pipes could lead to large amounts of hot water or steam getting into the tunnel system and workers need reliable communications for vital safety if such a problem occurs.

The network, called HelenNet, was built to cover every tunnel of Helsingin Energia’s tunnel system to assure the safety of workers and help personnel communicate in the course of their everyday work, says Markus Weinström, who is responsible for the maintenance and installations of the TETRA network.

‘We wanted to build the network to redundant status by bringing the signals to the tunnels from different base stations,’ he explains. ‘This increases the reliability of the network and these were the reasons why Creowave and its solutions were chosen.’

Creowave, which specialises in radio frequency and microwave solutions, was selected for the project based on its experience in providing solutions to enhance TETRA network coverage. In addition, the company’s Professional Radio business area offers a variety of TETRA repeaters that allow a cost-effective approach to be taken to improving network coverage.

Before the project began, the tunnels did not have TETRA coverage and only a few entry ramps and service shafts had adequate coverage from the 16 base stations that surround the tunnels. TETRA coverage was therefore required for the complete tunnel system and also the service areas. This was achieved using Creowave’s TETRA repeaters and the network was built to a redundant status to the extent that in the event of a power cut or breakdown of one of the base stations, entire radio coverage is not compromised.

Creowave’s solution involves five repeaters composed of three band selective indoor repeaters and two band and/or channel selective hybrid repeaters. In addition to the repeaters, leaky feeders were used because, as well as TETRA, a GSM signal can be carried.

The architecture
Within the deployment, the redundant nature of the network structure defines the positions of the repeaters so that there has to be a repeater at the edges of the coverage area. Because of the quite complex structure of the tunnels there have to be four edge repeaters. These edge repeaters feed just one leaky feeder in one direction.

In addition to the edge repeaters, one repeater is also needed for the middle of the coverage area. This repeater feeds two feeders heading in different directions. In order to provide coverage in the service shafts that rise upwards from the tunnel, several couplers were used.

Couplers “steal” a small amount of the repeater power and with an antenna the signal is carried to the shafts to split the signal into two different directions.

The five repeaters were installed in five different installation sites. Typically the sites were service shafts, enabling easy access for service and maintenance. In three of five installation sites, good separation between base stations was achieved using directive antennas. This enabled the use of band selective repeaters, the most cost-effective repeater type.

Two other installation sites were more challenging. At one site, two strong base station signals were discovered, but these unwanted signals could not be separated using reasonably sized directional antenna and an acceptable height mast. Other sites had similar problems, so the Creowave Hybrid Repeater was chosen for these particular locations.

Due to it’s filtering challenges, the installation site 3 solution, (see Installation Sites diagram), merits further explanation. The frequency separation between wanted and unwanted signals was as small as 50kHz. The hybrid repeater is equipped with standard set of filters. This set of filters is optimised for both indoor coverage and tunnel usage.

In a typical indoor, multi-floor building, the filters have to have a relatively small delay to remove any problems in the overlapping areas where a direct signal from base station and the repeater meet.

When it comes to tunnel coverage, however, the small filter delay of standard filters is of less importance because of the isolated structure of an underground tunnel, where a direct signal from a base station does not penetrate into the tunnel.

Enhancing the TETRA network coverage in the service tunnels of Helsingin Energia has proceeded as planned. The deployment was completed in the autumn of 2010 when the final repeater was installed on completion of the tunnel. However, four of the repeaters had been in operation well before that point and according to measurements taken in April 2010, the coverage for the completed areas performed according to plan. The workers of Helsingin Energia, as well as other servicemen, are now able to communicate in the challenging environment of underground tunnels, and the customer is satisfied with Creowave’s installation.

‘During the tests made in April 2010,’ comments Weiström, ‘we discovered that the coverage enhancement solutions provided by Creowave had succeeded as planned.’

Even in the most inhospitable environments, with an intelligent and creative approach to repeater placement and network design, along with the selection of the right solutions, reliable, resilient and effective TETRA signal can be assured, bringing peace of mind to the tunnels’ workforce.

Written by Wireless magazine
Wireless magazine

Leave a Comment