High speed rail services pose wireless connectivity problems

4 min read

Railway operators around the world are embracing wireless technology to help improve levels of security, raise levels of reliability, boost operating efficiency and enhance the consumer experience, whether that's by providing on board Wi-Fi access, better passenger information services or location based travel announcements.

The spread of high speed rail services across Europe, driven both at the national and cross border level, and around the world brings with it the need for countrywide standards based, interoperable systems and the provision of equipment that is capable of supporting train signalling and control systems (for either train to train or train to track communications) and private network communications for railway personnel, whether that involves deploying a system that uses a commercial public cellular network or one that is based on the European Rail Traffic Management System's (ERTMS) GSM-R standard for private railway infrastructure or public Wi-Fi or mobile services.

GSM-R – where R stands for Railway – is an international wireless communications standard for railway communication and applications. A sub system of the European Rail Traffic Management System (ERTMS), it is used to regulate communication between train and railway regulation control centres.

The system is based on GSM and EIRENE–MORANE (European Integrated Radio Enhanced Network and Mobile Radio for Railway Networks in Europe) specifications, which look to guarantee performance at speeds of up to 500km/hr without any communication loss.

The rapid development of high speed services has seen a growing demand for a range of wireless technologies, whether embedded modules and gateways or cellular handheld devices for use in operational or commercial applications.

Remote monitoring and management are also in demand, including GPS tracking which has become crucial for the successful operation of on board repeater systems and this is being done through the use of network management system tools deployed by many mobile operators.

Increasingly, managed network services including monitoring, corrective and preventive services, spare part stocking and reporting are also being deployed.

Passengers, meanwhile, expect their wireless devices to function reliably and properly wherever they are – whether travelling underground, across the countryside or through dense urban landscapes.

As a result, railway operators have to overcome a range of obstacles that will pose a significant challenge to providing reliable wireless service. Amongst them is the impact of high speed forcing rapid signal handovers between cell sites.

The wireless technologies being deployed have to work alongside existing infrastructure, but the process also involves adding new frequency bands with more efficient wideband radio technology, such as LTE (Long Term Evolution, the primary 4G technology). The moves will also try to bring capacity gains in the radio's efficiency by using techniques such as MIMO (multiple input multiple output) antenna technology. MIMO is still under investigation to evaluate if those capacity gains can be achieved when speeds exceed 250km/h.

Carriage design is also a major concern. "The structure of the next generation of trains and the speed at which these trains will be travelling are challenging when it comes to delivering reliable wireless services," explains Samuel Buttarelli, vice president of distributed coverage and capacity solutions sales for CommScope.

"We're finding that newer carriages typically shield radio signals from outdoor networks, preventing wireless connectivity inside the train. When speeds exceed 250km/hr, it can also create several network challenges. For example, the high speeds impact handover success rates between the base stations providing backhaul and potentially impact the radio channel itself with increased frequency shift (Doppler effect) reducing the efficiency of the radio receivers."

According to Buttarelli, depending on the type of carriage body, the size of the windows and the material for the windows shielding, the signal penetration from outside the train into the carriage can vary considerably.

As a result, there is growing deployment of on board repeaters in order to compensate for these losses. It is now mandatory to carry out a wireless design for each specific car type to determine the best possible integration for the system.

"Different and changing topologies of mobile networks along the tracks (macro, dedicated track coverage, MIMO) can also have an impact in terms of the coverage available on board and the requirements for active signal processing and distribution inside the train carriages," suggests Buttarelli.

While commercial solutions are available to handle in train issues and are capable of optimising the passenger experience during transit for both cellular and Wi-Fi, Buttarelli says the challenge for onboard systems is the ability to repeat the outdoor network inside the train.

"The key challenge is to assure enough isolation to avoid an unwanted 'feedback' effect. In addition special design considerations are necessary as to where and how to deploy radiating cable inside the train carriages, especially if the carriage ceilings are made of metal. Distributed antenna systems may also be required in tunnels, particularly when tunnels are longer than 500m. In this case, special design considerations for the tunnel require using a combination of radiating cable and potentially optical repeaters.

"Space for the integration of repeater and antenna systems remains challenging and impacts the possibilities and efforts for proper thermal, mechanical and electrical integration inside the train cars," he continues. "This is especially valid for new trains, as the train manufacturing industry is always challenged to improve the number of available seats per footprint."

Additionally, system components often have to meet strict requirements for use in harsh railway environments, meaning higher standards for temperature, shock and vibration and fire protection are required.

CommScope is currently working on several projects, collaborating with railways, wireless network operators and train manufacturers. In one of the most notable projects, CommScope is providing systems supporting both cellular and public safety communication inside the 57km long Gotthard Base Tunnel in the Swiss Alps.

For on board systems, the company has been involved in successfully delivering large in train repeater projects in Germany, Switzerland and Italy and for many smaller projects across Europe.
In Italy, for instance, it provided the 2G/3G system for the ETR-500 fleet of Trenitalia. The project was implemented with Telecom Italia as lead operator. Currently, CommScope is testing new systems, including LTE, for the ETR-1000 train. One of the fastest trains in Europe, ETR-1000 is capable of speeds of more than 300km/h and is expected to enter service later in 2015.

Looking to the future, Buttarelli expects to see more frequency bands being deployed on board and in the supporting backhaul macro networks. In particular, 800MHz LTE seems 'quite promising, as it does not require replacement of the existing radiating cable inside the tunnel'.

However, the real challenge remains the backhaul capacity in order to support higher data throughput performance inside the train. The on board architecture may also have to change with approaches that consider a single system per train rather than per car with potentially more functions being transferred to digital systems.