Fast and reliable 5G service on high-speed trains
In 3GPP Rel-15, the initial version of the New Radio (NR) Performance Requirements focused primarily on low-speed scenarios such as pedestrians (about 3 km/h) and automobiles (30-120 km/h ). The 3GPP has also specified some performance requirements for HST conditions, with an assumed speed of 300 km/h in the low/mid frequency bands. As HST technology continues to evolve towards even higher speeds – exemplified by the Maglev train in Japan due to launch in the late 2020s (Chuo Shinkansen), which will have a top speed of 500 km/h – further standardization work was needed to define requirements in HST conditions, including radio base station deployment options.
The objective of HST-specific 3GPP standardization is to ensure good network performance on HSTs by specifying good terminal support and performance requirements for base stations (BS) and user equipment ( EU) in particular. Another reason why this standardization was important is that the EU target speed of 500 km/h is one of the KPIs for the IMT-2020 air interface described by the ITU-R M.2410 report. Although 3GPP designed NR in Rel-15 to meet IMT-2020 requirements, it is also important to specify performance requirements. The HST-related specifications clearly demonstrate that NR is capable of delivering high-quality 5G even for challenging and high-mobility scenarios.
The new standards
The 3GPP specifications related to HST guarantee the minimum data download/upload speed and mobility of terminals under HST conditions for two frequency ranges (FR): FR1 (low/mid frequency bands, up to 3.6 GHz ) and FR2 (high frequency band, up to 30 GHz).
Following the creation of the Rel-15 EU specifications for 300 km/h, dedicated HST work for NR in FR1 has started in Rel-16. Rel-16 guarantees fundamental mobility performance in FR1 bands below 3.6 GHz at train speeds up to 500 km/h, including cell search and handover between NR cells and handover between NR and LTE cells. Version 17 improves performance requirements under HST conditions, especially when the network configures carrier aggregation (CA).
HST work for NR in FR2 started with Rel-17. Rel-17 guarantees fundamental mobility performance in the FR2 bands below 30 GHz at speeds up to 350 km/h. Given the characteristics of radio propagation in the high frequency bands, the FR2 HST scenario assumes the presence of dedicated UEs installed on the roof of the train. Additionally, 3GPP has agreed to improve FR2 HST performance in Release 18, including the CA scenario.
Ericsson’s standard team has made significant contributions to the HST specifications within 3GPP, particularly in the areas of UE RF performance, Radio Resource Management (RRM) and UE/BS demodulation, in order to ensure good EU/BS performance.
HST operation in FR1 – how it works
FR1 is generally capable of providing 5G coverage inside train carriages from outside BSs that are either on the ground or off the ground. A ground BS is defined as being within two meters of the track, while non-ground BS can be up to 150 meters.
The main challenges of operating HST in FR1 are securing performance with increased Doppler shift (especially for mid-band time-division duplex (TDD) frequencies), providing good and consistent coverage, and the guarantee of a transparent transfer between the BS along the way.
To further improve coverage along the track, a Single Frequency Network (SFN) can be deployed. In SFN deployment, all BS antennas – otherwise known as transmit and receive points (TRxPs) have the same cell ID and simultaneously transmit the same data from two or more TRxPs.
To enable fast switching of the serving TRxP, dynamic point selection (DPS) can be used instead of simultaneous transmission in SFN deployment. In this case, all the TRxPs along the path have the same cell ID but only one TRxP transmits data at a time. The serving TRxP is switched with transmission configuration indicator (TCI) switching based on channel state information (CSI) reports from the UE.
HST operation in FR2 – how it works
Unlike FR1, the FR2 case assumes that dedicated UEs are mounted on the roof of the train to avoid loss of penetration, and that this UE provides a link to serve users inside the train as a sort of mobile router. Operation in FR2 typically requires beam sweeping, where the UE/BS switches the transmitter (TX) and receiver (RX) beams to transmit/receive the signal, but this need is greatly reduced in the HST scenario . To reduce the time needed to search for the best TX/RX beam, it is assumed that the roof-mounted UE consists of two antenna panels; one RX beam pointing forward and the other RX beam pointing backward.
Two deployment scenarios were considered for the deployment of BS in FR2. In Scenario A, the BS is on the ground, and in Scenario B, the BS is not on the ground. By covering both of these scenarios, the specification enables FR2 deployments in any mix of deployment scenarios.
For Scenario A, our investigation shows that due to the line-of-sight (LoS) path to the UE, excellent SNR can be achieved with a single TX beam and a single RX beam. There is no need for beam management (i.e. the procedure to keep the best TX/RX beam).
For Scenario B, our investigation shows similar results to Scenario A: the LoS path to the EU gives excellent SNR. It is possible to work with single TX and RX beams, but beam sweeping around 2 TX and 3 RX beams works best.
The deployment of the BS antenna in FR2 can be bidirectional or unidirectional. In unidirectional deployments, the BS (TRxP) antennas are placed along the runway pointing in the same direction, whereas in bidirectional deployments, the TRxPs point in both directions. The bidirectional deployment reduces the distance between the train and the antenna, but the Doppler shift changes rapidly when the UE panel is switched.
In FR2, simultaneous data transmission in SFN deployment does not work because the UE cannot simultaneously receive signals from different directions. 3GPP therefore defines performance requirements only with DPS in Rel-17.
Bi-directional deployment for FR2 is more complex and does not provide an increase in capacity and throughput. This is because in Rel-17 the UE can only have one active panel in one direction at a time. However, bi-directional deployment for FR2 can be used to increase BS separation (at the cost of two panels per BS instead). In Rel-18, the intention is to introduce multi-panel UE operation for bi-directional operation, which would allow for increased throughput.
Deployment and next steps
The 3GPP work that has been completed so far to support the HST use case enables the provision of high quality 5G services on trains traveling at speeds of up to 500 km/h (using FR1 ) or 350 km/h (using FR2). The operation of the HST in FR1 provides connectivity to UEs inside the wagons directly. On the other hand, HST operation in FR2 provides higher data upload/download speed due to wider channel bandwidth and good SNR, although it requires a dedicated UE mounted on the roof of the trains .
Looking ahead, Rel-18 HST related extensions for FR2 include multi-panel operation from a single train-mounted UE, introduction of tunnel deployment scenario and addition of AC scenario .
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