As part of the Norwegian Ministry of Transport’s Follo Line project, mobile operators Telia, Telenor, and Bane NOR along with RFS recorded download speeds of 560 megabits per second (Mbit/s) in a 20-kilometer abandoned road tunnel in Holmestrand, Norway. The previous maximum speed in such conditions had been around 145 Mbit/s in a typical 2×2 multiple-input multiple-output (MIMO setup), which is currently implemented in various tunnels globally, according to Andreas Bergmeister, a product manager at RFS. 

Bergmeister added that even if you were to extrapolate out the previous record to a 4×4 MIMO system—which has not yet been implemented anywhere—it would still only achieve theoretically around 300 Mbit/s. “So the 560 Mbit/s is the new world record figure,” said Bergmeister.

The results have not been published in a journal, but Bergmeister described the experimental setup in an interview. Inside the tunnel, RFS deployed its radiating cables connected to a radio transmitter with 4G 4×4 MIMO capability. These cables essentially create a flexible base station that runs along the inside of a tunnel.

The test started by using a 2×2 MIMO setup that utilized a horizontally polarized radiating cable and a vertically polarized radiating cable. The number of cables essentially doubled in the 4×4 MIMO setup. The results achieved approximately 95 percent of theoretical maximum speeds for 4×4 MIMO in the tunnel.

The effort also demonstrated a 100-percent increase in download rates when going from single-input single-output (SISO) to 2×2 MIMO and an additional 100 percent increase in download rates from 2×2 MIMO to 4×4 MIMO.

While it might seem like it would be harder to get signals to propagate to a tunnel that runs underground, it’s typically more difficult to get signals to penetrate through the cars of the train, according to Bergmeister.

“You have a metal block with some small windows, in some cases the windows are even metallic,” said Bergmeister. “So the radiating cable—although invented by RFS in 1972—is still the go-to solution to provide coverage and capacity in such confined areas.”

Bergmeister explains that in order to upgrade MIMO in a tunnel, you essentially need to double the infrastructure placed there. So for 2×2, you have two runs of cables and for 4×4, you need 4 parallel runs. Bermeister notes that conceptually, it’s possible to do MIMO 2×2 with just one cable or 4×4 with 2 cables, but these approaches present limitations to the system.

What this all means for a user is that they will be able to stream videos on a train going through a tunnel.

Bergmeister added: “The Norwegian operators do have a target of 5 Mbit/s per active user, which will allow continuous streaming of videos in very high quality/resolution for all users simultaneously.”