Helsinki metro gets a TETRA boost

Creowave enhanced the TETRA network coverage in a large metro depot right in the middle of Helsinki

Helsinki metro gets a TETRA boost

HKL is responsible for running the trams and the metro as well as construction and maintenance of the track, stations and depots in the Helsinki region.

The firm employs over 1,000 public transport professionals who drive trams and metro trains, control traffic, maintain equipment, track and property, plan and implement new projects or perform other managerial tasks. HKL’s most important partner is Helsinki Region Transport (HSL), which is responsible for public transport planning, commissioning, passenger information and ticket inspection.

Modern metropolitan cities are built with large public transport networks to keep the city and its citizens moving. As far as emergency communications are concerned, TETRA is the leading technology in Asia and Europe. The Finnish authorities use the VIRVE network, which is the oldest nationwide TETRA network in the world.

‘Our main goal and requirement is to have the coverage working even when the depot is full of trains,’ commented Tommi Luotamo, HKL’s electricity supervisor.

In spring 2011, HKL contacted Creowave’s partner Bandercom Oy about enhancing the coverage at a metro depot close to Helsinki city centre. The surroundings are very challenging for TETRA coverage, as themetro depot is a huge metal hall, which at times is completely filled up with metallic/aluminium metro trains.

The starting point for the project was that the signal level and quality at the depot was poor and there were areas that had no coverage at all. The customer’s main goal for the project was to boost the coverage inside the metro depot to the point where the signal quality and coverage is strong enough to use even when the depot is full of trains.

Indoor coverage

With a length of 283m and width of 180m, the storage hall required indoor coverage for an area covering 25,000m². The TETRA signal also needed to cover a one kilometre long test rail tunnel. The test rail tunnel is used for technical and usability testing for metros and it was therefore crucial to have coverage there as well. The site will also be enlarged in the future, so that had to be taken into consideration when planning the TETRA installation.

After a careful site survey and planning, it was clear that the site needed a channel selective high power repeater. A band selective repeater was out of the question since 21 base stations reached the depot and three of them were extremely strong and unwanted.

In addition to the large area coverage requirement, the higher power repeater comes in handy to provide one single point signal transmission to the antenna network.

The same antenna network provides legacy UHF system signals in roughly the same power levels as an HP2 TETRA repeater. The antenna network is also built with the capability to distribute WLAN/GSM as a future expansion. Creowave’s Highpower Repeater with its +36dBm output power (CWR 380 HP 2 EG) was chosen and the installations started in autumn 2011.

The whole site consists of one Highpower Repeater, 40 antennas and a coaxial cable of over 1,000m in length. As there was already an existing, working leaky cable in the test rail tunnel, it was connected to  the newly installed cables. Around 2,000m of new leaky cable also needed to be installed. 

Capacity transfer

At the city area, most of the nearest and biggest base stations are already loaded with users. Usage rate of capacity is already at a high level. Choosing a smaller base station further away makes it possible to transfer available capacity easily to where it is needed.

In this case, because of the need for capacity transfer, a repeater was configured to choose the base station from the neighboring cell of the one chosen first. Because the indoor coverage was done with carriers, which are non-existent outside the depot, there is no overlapping of signals in the area.

Overlapping may happen when both of these two conditions are met: base station power against the repeater power is less than 16dB; and delay of the repeater is more than 15us.

The base station and repeater cause an overlapping area around the building when channels at the nearest cell to the building are chosen.

To prevent overlapping areas, the usage of an existing cell for indoor coverage should be avoided where possible. When choosing the channels at the neighboring cell (marked in light and dark blue), one gets rid of the overlapping area.

In the end, a Creowave TETRA Highpower Repeater was configured to enhance four different channels from three different base stations.

Congested areas

All of these selected channels are within 2MHz from each other. From one of these base stations two channels are enhanced – control channel A and secondary control channel B. Secondary control channel B is only used when the system capacity requires it, otherwise B is not active. From the other two base stations, the carrier with the main control channel is chosen and used.

When making these kinds of configurations and choosing base stations further away, it is always important to ensure the neighboring channel list in hand is up to date. With a channel selective repeater it is possible to transfer base station capacity from less used areas to more congested areas and in this way increase the capacity where it is needed.

With careful planning, a TETRA network can be enhanced even in the most difficult areas if the network planning rules and possibilities are kept in mind.

Emergency situation

Following the installation work, the end measurements show that the signal level is -60dBm – -80dBm everywhere at the depot, even when the depot is full of metro trains. The customer is extremely pleased with the results as well as the promptness of the installations. Now the TETRA system is in everyday use across the whole metro depot area, as well as the long test tunnel, to cover any possible emergency situation.

HKL’s Luotamo said: ‘Our main goal was achieved and answered tremendously. We are very happy with the end results and can now be sure there is no lack of capacity when the network is needed.’

Written by Wireless magazine
Wireless magazine

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