Rail transportation plays a more important role in our modern lives, not only due to the statistical data showing the fast-growing kilometers/miles of the bullet train or metropolitan subway systems globally, but also because people more like the comfortable commuting experience and the extra personal time for leisure and working than being absorbed in self-driving and traffic jam. But what is better than a cozy and speedy train service? Yes, the answer is a cozy and speedy train service with the ultra-wideband, high-speed and reliable internet connection, which will be available soon from 3GPP.

On March 23rd 2022, 3rd Generation Partnership Project (3GPP), the leading international standardization body in the wireless industry, announced the successful completion of Release-17 work item on 5G New Radio (NR) support for high-speed train scenario in frequency range 2 (FR2) led by Samsung, in which FR2 is the 3GPP terminology for milli-meter wave (mmWave) bands ranging from 24.25 GHz to 52.6 GHz.

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5G NR operating in mmWave bands is recognized as a breakthrough technology capable of providing ultra-high data-rate transmission, thanks to the availability of enormous amount of bandwidth in FR2 and also the beamforming-based operation in the advanced 5G NR design. Inspired by the commercial mmWave deployments world widely, more potential application scenarios to be enabled by 5G NR mmWave have drawn industrial attentions, among which high speed train (HST) has the special importance. To match the great demands of high-speed internet connections from passengers and HST special services, the empowering technology, i.e., mmWave for HST, is strongly requested by the leading cellular operators globally.

However, in the current 5G NR standards and 5G-compatible products, mmWave for HST is not possible. In the past years, 3GPP Technical Specification Group (TSG) Radio Access Network (RAN) Working Group 4 (abbreviated as RAN4), as the technical working group responsible for the spectrum-specific requirements and the high-speed train system performance in 3GPP, has endeavoured to lift the operating frequency for both 4G and 5G HST systems, which nevertheless can only support up to 3.5 GHz lacking any study on the more challenging mmWave for HST.

In the mmWave spectrum, the frequency shift due to high-mobility is severer (e.g., for 240 km/h speed using 28 GHz carrier frequency, the frequency shift can be up to 6.22 kHz) and more challenging to manage radio resource due to the high mobility. The current procedure to support FR2 mobility and the existing baseband demodulation performance specified in 3GPP have not yet taken into account the impact of high speed in the above-mentioned scenario.

In order to meet the growing data connection demands for HST and to extend the mmWave application scenarios, Samsung has been leading this 3GPP Release-17 work item, targeting the support of train-roof mounted high-power devices in the HST scenario, which communicate with track-side deployed base stations (referred to as gNBs in 5G NR) for backhaul link and provides on-board broadband connections to customers.

Figure 1.  Illustration of mmWave High-Speed Train scenario of 3GPP

In this standardization work, the possible FR2 HST deployment scenarios were studied, based upon which FR2 HST channel models are provided accordingly. From the radio resource management and baseband demodulation perspectives, the FR2 HST scenario has been focused and evaluated, with the feasibility of FR2 HST scenario being technically validated and confirmed.

Figure 2.  Illustration of uni- and bi-directional RRH deployment in FR2 HST scenario

In the 5G NR standard, FR2 user equipment (UE) power classes (i.e., the UE’s capability for maximum transmission power) are specified based on the assumption of certain UE types with the specific device architectures. To enable the FR2 HST deployment scenario, the new UE power class, referred to as FR2 power class 6, is desired for the UE type of train-roof mounted high power devices, under the assumption of two directional antenna panels oriented to track-side remote radio heads (RRHs) of cellular network. FR2 power class 6 UE is characterized by the higher maximum output power, i.e., 30.0 dBm minimum peak effective isotropic radiated power (EIRP) at 28 GHz band, compared to 22.4 dBm achieved by the power class 3 for the handheld UEs.

It is worth noting that mmWave devices suffer from the high attenuation during the signal transmission, and need directional beamforming antennas to compensate the excessive propagation loss. By adjusting the directional beamforming antennas to the selected directions, the device’s mmWave beambook can be constructed. Different from the existing UE power classes, the newly introduced FR2 power class 6 for HST has the special mmWave beambook design, which covers the expected spherical coverage regions respectively located in the forward and backward directions along the track, as required by the typical deployment case. By guaranteeing that with a new spherical coverage requirement framework, the RF transmission and reception performance dedicated for the FR2 HST UE has been introduced.

Figure 3.  Illustration of spherical coverage of FR2 Power Class 6 UE for HST

High mobility is another key challenge in this FR2 HST scenario. To cope with this challenge, Samsung Research and Samsung Network colleagues worked together on improving the system’s mobility performance. However, the enhanced mobility performance comes with a cost. By reducing the mmWave beambook size adaptively to expedite the procedures for HST mobility, such as cell re-selection, connection mobility control, beam failure detection, link quality measurement, etc., the trade-off was carefully balanced with the system performance impairment due to a smaller beambook design.

When UE is moving across neighboring RRHs, to maintain the wireless link, the network needs to switch the working RRH from one to another, where the excessive propagation delay difference could be experienced at the UE side, as a particular technical problem not experienced by non-HST systems. To solve this problem, we introduced a novel network-controlled one-shot timing adjustment mechanism in this Release-17 work, which is capable of enabling UE to perform the cross-RRH switch with much reduced data package losses.

Figure 4.  Illustration of one-shot timing adjustment mechanism for FR2 HST UE

After completing the Release-17 standard on the mmWave HST application scenario, 3GPP starts to further enhance the FR2 HST system in the next release, i.e., Release-18 for 5G-Advanced from March, 2022. By planning to study the practical tunnel deployment scenario, the future Release-18 standardization topics such as the multi-panel operation at UE side, the advanced timing adjustment solution with network signalling assistance, etc., have already attracted great interests from major cellular operators and vendors as co-signers and supporters. Samsung is excited to lead the follow-up Release-18 standardization work item again for FR2 HST scenario by leveraging the further enhanced mmWave operation, for your cozy and speedy train experience with an even better internet connection.