Metros' evolving communications requirements

TETRA and Wi-Fi are the dominant radio standards for metro and light rail communications and signalling systems, but LTE is starting to make inroads in the market, as James Atkinson finds out

The number of metro and light rail systems is growing at a phenomenal rate as populations flock to the cities, placing increasing demands on urban transport systems. As a result, urban authorities are building new metro lines, particularly in the Asia Pacific region (see "Strong global growth in metro lines" below).

Modern metros rely heavily on wireless radio systems for voice, data, signalling and safety systems, and this reliance is increasing as levels of automation rise and more driverless metros come on-stream. Metros require mission-critical communication standards with high levels of resilience, availability and security. The TETRA two-way radio standard is unsurprisingly, therefore, a popular choice.

“TETRA systems dominate the radio communications market for metro networks today,” says Eric Davalo, head of strategic development, Secure Land Communications at Airbus. “There is some LTE use starting to come in, largely driven by the Chinese market.”

How did this dominance come to pass? “TETRA was attractive to metro operators looking to move to digital,” recalls Mark Skinner, solutions architect at Motorola Solutions. “TETRA is an open European standard, there are lots of suppliers and the equipment is all interoperable between vendors.”

Marta Fontecha, product marketing director at Hytera subsidiary Teltronic, notes that TETRA was also attractive to metro operators because it is relatively easy to update and expand as the metro system grows. “TETRA’s greatest value though is that a single network supports voice and data communications.”

Voice systems
TETRA enables instant push-to-talk (PTT) individual and group communications with priority and pre-emption services between the driver, control centre/dispatchers, depots, station operations and platform staff, security, maintenance and engineering crews. Direct mode operation allows crews on the same train to talk to each other and to platform staff. The metro TETRA system can also be integrated with external TETRA networks used by the emergency services.

TETRA can be integrated with the metro intercom system and public address solutions, so the driver or the control room can communicate with passengers on the trains. It can also connect with other train subsystems both locally and in the control centre including PABX and PSTN telephony networks, computer aided dispatch (CAD), the passenger information system and, very importantly – recording systems. Metros are equipped with Automatic Train Stop (ATS) systems in case they pass a red light, exceed speed limits, or an obstruction is detected on the track ahead. “ATS is very important for both safety and security, as there must be a way to automatically stop the train,” says Davalo. “When this happens, the TETRA system can be used to share this information among the relevant professionals.”

Davalo also points out that in metros, instead of using telephone numbers they often allocate functional numbers relating to the train number or the metro line number based on the TETRA Dynamic Group Number Assignment (DGNA) feature.

“A DGNA is allocated to each train departure and released as soon as the train arrives at [its] destination. That way, the control centre can call the train or line number and get hold of the right driver, rather than trying to find a particular driver via his individual handheld radio subscriber number. It’s a role-based numbering system,” explains Davalo.

He adds that the DGNA application also automatically changes the talk group every time a train enters a new station, so the driver can talk to the station staff. “That kind of thing is designed into TETRA,” he notes.

Data systems
Skinner reveals that some metro customers just use TETRA for voice with no integration with other systems. “But other metro customers are using TETRA in a much more integrated way with other systems. TETRA provides short data services (SDS), packet data and TEDS (TETRA Enhanced Data Services), so there is a data pipe available with TETRA, albeit it is still a fairly narrowband one,” he points out.

Nonetheless, TETRA can support a wide range of data applications within metros. TETRA equipment is integrated with the onboard control and signalling systems, which enables the system to report back on what is going on in the metro. Fontecha cites three levels of data applications. “There’s critical data like rolling stock monitoring, emergency/fire detection alarms and event management, vehicle diagnostics, location information [from trackside and GPS location systems], door opening, air-conditioning and braking systems.

“Then there is vital data such as the communication system to support data rail signalling applications, and then there is non-critical data for online video
surveillance and other applications to support the metro operation and improve the passenger experience,” says Fontecha.

“If there has been an incident on a metro line and you need to quickly change the passenger information systems, you can send a message via the TETRA system,” says Skinner. “You can also change the passenger announcement structure remotely from the control room, thereby allowing the driver to concentrate on driving the train. If a station ahead is closed, you can automate the PA announcements to tell passengers to alight at the station before or after, for example.

“By sending timely relevant information around the train you provide a better service as an operator. Likewise telemetry data can provide early fault notification, which allows you to respond faster, keep the service and trains running with less downtime, and that improves revenue.”

Fault detection and remote diagnostics telemetry data can be sent over the TETRA IP backbone, allowing remote monitoring of the location and condition of all carriages in real time. Some operators are using this kind of data for preventative maintenance.

The operator builds up a historical database of types of faults and the frequency of failure rates of particular assets to try to identify the main factors causing the fault. Algorithms analyse the data and identify anomalous patterns of events, which may indicate something is about to fail. The potential problem can then be fixed before the component fails.

Building the network
A typical TETRA network in a metro line will comprise an operations control centre with CAD systems, base stations in each metro station, mobile radios in each train, handheld radios for station and maintenance staff and drivers/conductors and connections to the passenger PA and information systems.

Teltronic’s Fontecha says: “The main objective must be ‘no single point of failure’. Our experience in metro projects shows that proper RF planning and system design well ahead of the deployment is a key factor to optimise installation.

“Co-ordination with civil works for cable routing, leaky cable location and indoor antennas installation is critical to make sure that the right information is delivered and received on time by the civil works contractors and radio system designers to reach an efficient radio design co-ordination and to avoid a costly design and implementation process.”

When a metro goes down it is enormously disruptive, so it is important the radio equipment is robust and reliable. “We put resilient cores in for metro systems,” says Skinner. “These solutions will maintain communications in degraded modes with geographical redundant switches and bring the system back in minutes. There is a lot of resilience built into TETRA anyway and also now on the cyber security side too.”

Signal distribution
One of the main challenges is providing a consistent signal throughout the miles of tunnels. Distributed antenna system (DAS) connected to radiating (leaky feeder) cable is the normal way to distribute the signals evenly down the tunnels, according to Ingo Flomer, business development director at Cobham Wireless.

“In developed countries I’d say 90 per cent are covered by DAS with multiple radios and operators. Metros have long tunnels, so a multi-radio, multi-operator approach is the only efficient way to do it. The other advantage of DAS is that you can continue to distribute the signal coverage outside the tunnel.”

Flomer explains that when metro trains exit a tunnel there can be handover difficulties with the transmission from inside the tunnel to the outside world. “Suppose there is a traffic jam at the exit of the tunnel. That can occupy the entire cell capacity and you can’t make a call.

“If the train is using this cell, all calls will drop, but if you continue the DAS strategy outside the tunnel with a dedicated cell for the metro, you will get continuity of coverage and adequate capacity for the metro train and its passengers,” he says.

In the Middle East, Cobham’s idDAS system is being used to provide public safety communications on a metro system. “They are using LTE on two bands dedicated for public safety use, along with a UHF one for TETRA and one for P25. It is the first mixed-use system of its kind. There are 500 radio units, making it the biggest public safety system for metros as far as we know,” says Flomer.

Looking ahead, Flomer says Cobham’s idDAS system is 5G-ready, but he points out that 4G still has to be deployed in any significant way. Where he does see a big change happening is in the way DAS is fed, moving from power-hungry full base stations to just using the base band unit connected to a multi-sector digital hub, connecting in turn to digital remote units.

“Our virtual RAN idDAS solution will offer massively reduced power consumption,” says Flomer. “We have done feasibility tests and the metro industry is asking for it, as it makes the communications systems more efficient for both the opex and the capex side.”

Signalling systems
Traditionally, metro signalling was controlled by fixed-block, track-circuit-based systems, but many metros have now switched to Communication-Based Train Control (CBTC) moving-block signalling systems. Wireless technology is used to pinpoint the exact locations of all the trains and ensures they maintain the correct safe operating distance between each other.

CBTC enables real-time train control information to allow the distance, or headway, between trains to be safely reduced to as little as 60 seconds, meaning more trains can be run, thereby boosting capacity on the line.

Wi-Fi is the dominant technology used for CBTC, which is made up of three integrated networks: the train onboard network; the train-to-trackside radio network, which generally uses Wi-Fi; and the trackside backbone network situated along the tracks.

The onboard control unit or computer sends train control information to the trackside network at regular intervals and works together with the Automatic Train Protection (ATP) system, which controls the safety functions such as emergency braking, and the Automatic Train Operating (ATO) system, which controls the train driving functions such as doors, starting, stopping, accelerating and braking.

PMR manufacturers have worked with signalling companies to develop a CBTC system using TETRA, but the solution is not being adopted. “It is technically possible to use TETRA for CBTC, but I am not aware of any metro line using TETRA as the primary CBTC channel today,” says Davalo. “There have been trials and I know of at least one case where TETRA is used operationally as a back-up solution, but TETRA has its limitations in terms of bandwidth.”

LTE on metros
Broadband LTE has the necessary bandwidth and is being looked at for CBTC systems, as well as for supporting other functions such as onboard CCTV, passenger internet and information (and entertainment) systems.

Broadband technology is also required to support driverless, fully automated metros known as GoA4 (Grade of Automation 4) systems, which are becoming increasingly popular, especially in China.

The International Association of Public Transport (UITP) reports that in March 2018 the total line length of operational fully automated metros reached the milestone of 1,000km with the opening of the Pujiang Line in Shanghai. While this only represents seven per cent of the total length of installed metro assets, 32 GoA4 lines are due to enter revenue service in 16 Chinese cities by 2022.

GoA4 metros require even more specialist equipment, including remote surveillance of tunnels, tracks and platforms and automated platform barrier doors. The trains need extra safety precautions including automated emergency braking, smoke detectors, fire extinguishers, emergency lighting, audible signals and door monitoring. An obstacle detection feature and carriage derailment detector that will automatically brake the train are also required.

As there are no onboard crew, the control centre must be able to address passengers and vice versa to provide journey updates or guidance if there is an emergency. “In full GoA4 mode the control room needs to be aware of things happening on the train. You need CCTV in the cabs, so if there is an incident you can see who has requested help or if they are just messing around,” says Motorola’s Skinner.

All this takes a lot of bandwidth, but if LTE is to replace Wi-Fi for CBTC and be deployed for GoA4 metros then spectrum has to be found. China is forging ahead as it has allocated some 1.8GHz spectrum for critical industries, including transportation, for private LTE networks.

“But lack of availability of LTE spectrum in many parts of the world is the big issue for private LTE networks,” says Airbus’s Davalo. “In France, there are discussions around using 2.6GHz spectrum for train and metro networks, but no decision has been taken and applications are ongoing. For an industry that needs a solution that can span many countries, there is the question of whether it is economically viable to develop it if only one country adopts 2.6GHz.”

He adds that the 700MHz band is being looked at as an alternative, but so far all the licensed spectrum has gone to mobile network operators with a few exceptions where some guard bands have been allocated to public safety (with the exception of the USA) as public protection disaster relief (PPDR) spectrum.

Those countries are discussing how this PPDR band could also be used for critical industries such as transportation. It is complicated as the 700MHz band comes with a problem. Both public safety and transportation are mission-critical sectors, so which will have priority on the network?

“There is not much spectrum that could be used. What happens when public safety personnel come into a railway or metro station and both are using the 700MHz spectrum? The rules need to be defined,” says Davalo.

Fontecha feels that in the short term, the most likely scenario is that TETRA will continue to be used with LTE as an overlay for some features. “TETRA part manages the voice communications and critical data, while LTE supports CBTC signalling applications, video, passenger information systems and the like.”

Skinner agrees. “I can see TETRA being continued for voice services for quite some time yet. When 3GPP MCPTT LTE has been around for long enough and the metro industry is confident about it and sees a clear way of migrating between the two, they will move to LTE for both voice and data. Metro operators are not agile users. They like to know the technology they use is mature and safe.”

Strong global growth in metro lines
According to the International Association of Public Transport (UITP): “At the end of 2017, there were metros in 178 cities in 56 countries, carrying on average a total of 168 million passengers per day; 75 new metros have opened since the year 2000 (+70 per cent). This massive growth is to be credited largely to developments in a few countries in Asia.

“As of 31 December 2017, the 178 metro systems together made up an installed asset base of 642 lines for a total length of 13,903km and 11,084 stations. 1,901km of new infrastructure was put in revenue service between the start of 2015 and the end of 2017. This includes the new lines that opened in the 19 new metro cities in China, India and Iran (577km), but also new lines in already established metro cities (820km) as well as line extensions (504km).”

This hectic growth shows no signs of slowing down. UITP estimates that in the next five years, more than 200 new lines and even more extensions are expected to open in most regions, including in Sub-Saharan Africa. In the summer of 2018, some 5,400km were reported to be under construction or at testing stages, with another 1,700km in design and tender stages.

Source: UITP Statistics Brief: World Metro Figures (September 2018)