Hyperloop Transportation Technologies (HTT), one of the American start-up which develops a concept of super fast train moving on electromagnetic suspension at 1,200 km/h has recently signed an MoU with the region of Occitania in France to set-up its European R&D center. This made me wonder how it will be possible to communicate from an Hyperloop train with our smartphone.

Mobile communications from vehicles and in particular from trains has been a field in which specific norms have been developed for this specific use case. GSM-R (or Global System for Mobile Communications – Railway) which allows mobile communications at a speed up to 500 km/h and mainly dedicated to system communication and safety as a replacement of legacy analog communications.

What are the problems?

Doppler effect

When two communicating devices are in a relative movement, the frequency Ft of signal sent by the transmitting device will be seen at a slightly different frequency Fr by the receiving device, the shift is linked to the relative velocity of the transmitter and receiver.

This is the Doppler frequency shift:

Fd = (Ft/c)·|v|·cos(θ)

Assuming a transmitting frequency of 1800 MHz, at 300 km/h which is the average commercial speed of the French High Speed Train “TGV”, that gives a maximum Doppler frequency shift of (+/-) 500 Hz when the train is very far. At 1,200 km/h, the maximum frequency shift becomes 2 kHz for the same frequency.

Download the Excel file to calculate the Doppler frequency shift.


Credit: http://www.selecom.fr/en/coverage-railway-onboard

At this speed, the train will not stay long in the same cell and there will be a lot of handovers required to maintain the connection alive during the travel.

Assuming an ideal scenario where cells of 10km could be created along the track, the train would not stay longer than 30 seconds inside each cell. And assuming an hypothetical inter-cells overlap of 1km, the handover would need to happen in less than 3 seconds.

LTE-R for 4G communications in the train

Huawei solution based on LTE proposes: “High-bandwidth train-ground network: Ensures a stable bandwidth of 50 Mbit/s on a fast-moving maglev train at the speed of 431 km/h.“.

What about 5G?

3GPP plans for the first release of 5G specifications (Release-15) include the following requirement for the use case of high-speed links for trains: “support for connecting to a user travelling at a maximum speed of 500km/h. “.

So no connection at 1,200 km/h?

It seems this will be another challenge, there is little information available on what are the radio implications to have a UE moving at 1,200 km/h relatively to the base stations.

I am wondering if the techniques applied at 500 km/h would still be useablee at 1,200 km/h and what would be the limitations created in terms of throughput and frequency efficiency.