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Radio coverage under the city streets

A network of ‘leaky feeder’ cables and underground repeaters provides Helsinki’s electricity workers with reliable radio communications for efficiency and safety

ning beneath it. One of these tunnel networks belongs to Helsingin Energia, a large energy company which supplies electricity to about 400000 customers in Finland and meets more than 90 per cent of the capital’s heat demand with its district heating pipes. “We have many kilometres long under-


ground tunnels under the city of Helsinki”, says Markus Weiström, who has been respon- sible for the installation and maintenance of a Tetra relay network which serves the tunnels. Te system is part of Helsingin Energia’s own Tetra network, known as HelenNet. “Te tunnel itself creates a challenging work-

ing environment”, he emphasizes, “and, in addi- tion to that, the district heat network can cause threatening situations if there occurs a leak in the pipe or valve. Tat way a load of hot water and steam gets to the tunnels. In these situations it is important the communication works.”

Repeater network Te tunnel radio network, which is used for or- dinary underground work as well as emergency communications, has been built for Helsingin Energia by the Finnish company Creowave. Previously only slight coverage of the tun- nels was available, filtering through from the surface base station network via vehicle access ramps and service shafts. But a network of Tetra repeaters from Creowave now provides full cov- erage of the tunnel system and the shafts too. Redundant coverage was planned to ensure

that events such as a loss of power or one base station would not compromise coverage. “We wanted to build the network to redundant sta- tus by bringing the signals to the tunnels from different base stations”, Mr Weiström adds. “Tis increases the reliability of the network.” Te tunnel bore to be covered is typically

about 10 metres in diameter. Creowave was set the target of achieving a signal level of –85 dBm in 95 per cent of the tunnel area. In addition to

30 Y Energy under Helsinki: inside one of the tunnels

the tunnel itself, service was also required in 12 shafts running upward from the tunnel and in certain open halls.

Leaky feeder To achieve a cost-effective solution, five repeaters were needed, plus leaky feeders (radiating cables) and antennas. Tree of the repeaters were band- selective indoor types and two were band- and/ or channel-selective hybrid repeaters. Te use of leaky feeder enabled GSM to be carried too. To achieve redundant coverage, it was neces-

sary to place repeaters at the edges of the cover- age area. Because of the complex layout of the tunnels, four edge repeaters were required. Tese drive a leaky feeder running in just one direc- tion, into the tunnel. In addition, one repeater is needed at the middle of the coverage area. Here, several couplers were added to provide coverage of the service shafts. Te couplers ‘steal’ a small amount of the repeater power, which is directed into the shafts by an antenna. Also, signal split- ters were utilized to divide the signal between the leaky feeders running in either direction. Site chosen for the repeaters were typically

in service shafts, for ease of access. At three of the sites, good separation between base stations was achieved using directional antennas. Tis allowed the use of band-selective repeaters, the most economical type.

ike other modern, metropolitan cities, Finland’s capital Helsinki has a huge variety of underground tunnels run-

Tunnels – and the five repeater installations However, the two other sites proved more

challenging. At one, two strong base station sig- nals were picked up – the same signals as were received at the middle repeater – and they could not be separated using a directional antenna of reasonable size with a mast of acceptable height. At the remaining site, the same problem was found, except that the unwanted base station sig- nals were very close in frequency to the wanted signal, making the use of a channel-selective repeater unavoidable. A hybrid repeater was therefore chosen for these locations. With the completion of the network last

year, the planned coverage levels have been achieved, and the workforce of Helsingin Ener- gia, as well as other service personnel, can now communicate in the challenging environment of these underground tunnels.

LAND mobile November 2011

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