Reaching the passengers

Even where mobile coverage is perfect at the trackside, Butarelli continued, users inside the train often cannot be reached by an outside signal. The velocity of high-speed trains – up to 300?km/h and more – also complicates matters. As the train moves rapidly to and from base stations along the route, the communications system needs to adapt quickly to the fast-changing signal levels from the various mobile networks involved.

And the onboard wireless systems must manage the rapid hand-off between base stations as the train moves along its route. If this process is not managed correctly, it can result in dropped calls or interrupted data connections.

Nick Barker, senior technology consultant with Nomad, a specialist in on-board wireless systems, said that providing a seamless Wi-Fi experience to passengers, staff or applications on a moving train is a challenging task. The radio conditions experienced by the radio modems used for backhaul off the train can change from urban terrain to open countryside, from dense cellular coverage to little, or in and out of radio black-spots – in the space of a second or two. 

“These radio networks are also inherently noisy, introducing latency and packet loss into the connection”, Barker said. “In turn, these are compounded when the train operates at speed. At the same time, members of the public have come to expect and demand a high bandwidth, high quality broadband-type service wherever they are.”

Nomad’s solution, he said, relies upon a few key underlying technologies and principles to ensure the best possible service:

• excellent radio frequency (RF) engineering, giving the radios the best possible chance to perform;
• multiple bearers, maximizing the coverage provided by all local cellular bearers;
• multiple modems, maximizing the bandwidth available for all users;
• seamless, real-time switching and aggregation – switching modems in and out of use dynamically according to the real time status of the modem, and transparently re-routeing passenger traffic;
• multiple central communications units (CCUs) at different locations on very long trains: interconnecting the CCUs enables the train to be served even when one CCU suffers a complete outage.

Eurostar recently awarded Nomad Digital contracts to supply onboard Wi-Fi connectivity and infotainment aboard its fleet of high speed trains. “For Eurostar in particular”, Barker added, “we also have the challenge that the trains run through several countries in a single journey.

“Rather than carrying multiple modems for each country, Nomad has developed the ability for each modem to take on a different personality as the train enters a different country. This allows the equipment to remain at full performance, whilst allowing the operator to control costs by using local rather than roaming cellular contracts in each of the countries that the train visits.”

More Wi-Fi challenges

CommScope’s Buttarelli added that there are challenges specifically related to Wi-Fi coverage. Wi-Fi connectivity is not brought onboard from an outside network but is generated by routers inside the train, using external connections for backhaul.

“Two methods are typically used”, Butarelli said. “The first method is to use the 3G network as backhaul. In this case, an antenna mounted on the roof of the train picks up the outdoor cellular signal and a SIM-enabled router mounted onboard creates the Wi-Fi connectivity.

“An alternative method is to use a satellite connection. For this, a sophisticated tracking antenna mounted on the roof of the train can provide the necessary backhaul connectivity. This method provides an efficient alternative to the use of a 3G network, especially for rural areas where there are no tunnels along the track.

“In contrast, for 3G backhaul connectivity, special coverage solutions can be installed inside tunnels so that Wi-Fi connectivity remains uninterrupted.”

On-board cellular

For cellular connectivity to support train passengers who want to use their smartphones or USB dongles for 3G data or voice services, he said that the external network signal must be brought into the train carriages, by the use of onboard repeaters.

Depending on the structure of the train, a number of these repeaters may be installed to ensure complete coverage. Using an external donor antenna, the repeater can receive the signal from the outdoor networks (typically from multiple wireless operators) and then rebroadcasts it within the train by means of small indoor antennas or, more often, by using a radiating cable installed inside the carriage.

“3G cellular backhaul is most widely used, due to better availability”, Butarelli said. “However, the main challenge here is network capacity, due to the large number of users sharing the same 3G carrier. This problem is expected to be helped by the deployment of LTE-enabled sites along the trackside to provide higher capacity for users on trains.
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