How high-capacity 5G could transform subway travel

Sourcing enough bandwidth to deliver reliable high-speed connectivity to passengers travelling on busy trains and buses can be difficult. New high capacity 5G networks operating in mmWave spectrum could be the answer.

South Korea’s Ministry of Science and ICT (MSIT) is looking to dramatically improve the Wi-Fi available on Seoul’s subway trains by using mmWave 5G networks to connect access points to the Internet. MSIT believes this solution will enable subway passengers to watch videos in 4K resolution on their mobile devices – something that isn’t possible with the in-train Wi-Fi coverage today. The existing Wi-Fi service can support less than 1 Mbps passenger at busy times, curbing usage.

High-capacity Wi-Fi could make travelling by subway more appealing, potentially reducing vehicle usage, congestion, greenhouse gas emissions and pollution in South Korea’s cities. In fact, public transport is one of the most promising deployment cases for 5G connectivity. Urban buses and trains generally condense large numbers of people into small spaces for significant periods of the day. That makes public transport a good fit with mmWave 5G, which supports much higher capacity, but a much smaller range, than mid-band or low-band 5G.

Following a proof of concept in 2021, South Korea’s three main mobile operators- KT Corp., SK Telecom, and LG U+ - plan to install mmWave 5G across Seoul subway lines 2, 5, 6, 7 and 8, which have more underground sections than the other lines. These underground lines are well placed to harness a key radio characteristic of millimetre waves, which travel further through tunnels than in the open air. “The radio characteristics of the tunnel create a beneficial wave guide effect, which improves the radio propagation in that space, so we are leveraging that benefit,” explains the MSIT official in charge.

In the proof of concept, mmWave 5G connectivity delivered an average aggregate bandwidth of 700 Mbps to Wi-Fi users inside the subway cars. The trial employed 800MHz in the 5G backhaul.

The operators plan to complete the installation of new dedicated base stations during 2022, with services starting at the end of the year. According to Seoul Metro, there are 2,443 subway carriages (cars) running on Seoul subway lines 2, 5, 6, 7, and 8.

For the trial, Samsung equipped five stations with 26 gNodeB’s (gNB s) – mmWave compact macro units (which combine a baseband unit, radio and antenna in a single lightweight form factor). Five trains were equipped with customer premise equipment (CPE s) from Hucom Wireless and 20 Wi-Fi access points from Linksys. Each train had 2 CPE s installed at the engine room in front and rear car, while an access point was installed in each of the four passenger cars in each train.

Samsung reported² that the mmWave 5G connectivity backhaul boosted public Wi-Fi speeds by as much as 25 times. It said users benefitted from Wi-Fi download speeds of 1.8 Gbps on a “fast-moving subway train” underground.

Following the successful trial, MSIT’s ultimate ambition is to bring 5G mmWave connectivity to all of Seoul’s subway lines. In the first phase, subway lines 2, 5, 6, 7 and 8 will be covered by a shared non standalone (NSA) 5G network built by the three mobile operators. The operators will divide the network construction between them, with each carrier taking responsibility for a different line. The cost of the project hasn’t been made public to respect commercial confidentiality.

If the resulting Wi-Fi service is able to support at least 700 Mbps in each car, 100 passengers could each simultaneously have a 7 Mbps connection. “We have been using CPE designed for a fixed environment,” says the MSIT official. “When we move to the mobile version, we expect to see further improvements. The actual bandwidth coming through the 5G backhaul is 3-4 Gbps.”

In the pilot, the use of a NSA 5G network resulted in some handover issues between base stations and the LTE radio network had to be employed as a fall back. However, SK Telecom has been able to address these issues by changing some of the network parameters.

The Wi-Fi connectivity itself is also set to improve. Once the service deploys Wi-Fi 6E, the latest Wi-Fi standard, the available bandwidth could improve further. Wi-Fi 6E can make use of more channels – each carrier will be able to use three channels of 160 MHz - which will mean a major improvement inside the train, says the MSIT official. “That will create a virtuous circle – as the service improves, more people will use it.”

If the deployment in Seoul is successful, other subway systems in South Korea are likely to follow suit. “The subways outside Seoul are operated by different companies and they will be monitoring the progress of this project closely and they will be interested to see the results,” says the MSIT official.

Operators of stadiums, festivals, airports and other facilities catering to large numbers of people will also be watching the Seoul subway deployment closely. For them, 5G mmWave could be the key to bringing robust and reliable high-speed connectivity to their customers.