Ensuring fast and reliable 5G service delivery on high-speed trains

In 3GPP Rel-15, the initial release of New Radio (NR) performance requirements focused mainly on lower-speed scenarios such as pedestrians (about 3 km/h) and automobiles (30-120 km/h). 3GPP also specified a few performance requirements for HST conditions, with an assumed speed of 300km/h in low/mid frequency bands. As HST technology continues to evolve toward even higher speeds – well exemplified by the Maglev train in Japan that is planned to be launched in the late 2020s (Chuo Shinkansen), which will have a maximum speed of 500km/h – additional standardization work was required to define the requirements under HST conditions, including the radio base station deployment options.

The purpose of HST-specific 3GPP standardization is to ensure strong network performance on HSTs by specifying good terminal support and performance requirements for base stations (BS) and user equipment (UE) in particular. Another reason why this standardization was important is because the target UE speed of 500km/h is one of the KPIs for the IMT-2020 radio interface described by the ITU-R report M.2410. Although 3GPP has designed NR in Rel-15 to satisfy the IMT-2020 requirements, it is also important to specify the performance requirements. The HST-related specifications clearly demonstrate that NR is capable of delivering high-quality 5G even for challenging, high-mobility scenarios. 

The new standards

The HST-related 3GPP specifications guarantee the minimum data download/upload speed and mobility of terminals in HST conditions for two frequency ranges (FRs): FR1 (low/mid frequency bands, up to 3.6GHz) and FR2 (high frequency band, up to 30GHz).

Following the creation of Rel-15 UE specifications for 300km/h, dedicated HST work for NR in FR1 started in Rel-16. Rel-16 guarantees the fundamental mobility performance in FR1 bands below 3.6GHz at train speeds up to 500 km/h, including cell search and handover among NR cells and handover between NR and LTE cells. Rel-17 enhances the performance requirements in HST conditions, especially when the network configures carrier aggregation (CA).

The HST work for NR in FR2 started with Rel-17. Rel-17 guarantees the fundamental mobility performance in FR2 bands below 30GHz at speeds up to 350 km/h. Considering the radio propagation characteristics in high frequency bands, the FR2 HST scenario assumes the presence of dedicated UE installed on the rooftop of the train. Moreover, 3GPP has agreed to enhance the FR2 HST performance in Rel-18, including the CA scenario.

Ericsson’s standard team made significant contributions to the HST specifications within 3GPP, particularly with respect to UE RF, radio resource management (RRM), and UE/BS demodulation performance areas, to ensure good UE/BS performance.

HST operation in FR1 – how it works

FR1 is generally capable of providing 5G coverage inside train carriages from outdoor BSs that are either trackside or non-trackside. A trackside BS is defined as being within two meters of the track, while non-trackside BSs can be up to 150 meters away.

The main challenges to HST operation in FR1 are securing performance with increased Doppler shift (particularly for mid-band time division duplex (TDD) frequencies), delivering good and consistent coverage, and ensuring seamless handover between BSs along the track.

To further improve coverage along the track, Single Frequency Network (SFN) can be deployed. In the SFN deployment, all the BS antennas – otherwise referred to as transmission and reception points (TRxP) have the same cell ID and transmit the same data from two or more TRxP simultaneously.

To enable rapid switching of the serving TRxP, Dynamic Point Selection (DPS) can be used instead of simultaneous transmission in the SFN deployment. In this case, all TRxP along the track have the same cell ID but only one TRxP transmits the 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 UE are mounted on the rooftop of the train to avoid penetration loss, and that this UE provides a link to serve users inside the train as a kind of mobile router. Operation in FR2 usually requires beam sweeping, where the UE/BS switches the transmitter (TX) and receiver (RX) beams to transmit/receive the signal, but this need is significantly reduced in the HST scenario. To reduce the amount of time it takes 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 have been considered for BS deployment in FR2. In scenario A, the BS is trackside, and in scenario B, the BS is non-trackside. By covering these two scenarios, the specification enables FR2 deployments in any mixture 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 obtained with a single TX and single RX beam. There is no need for beam management (that is, 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 UE results in excellent SNR. It is possible to operate with single TX and RX beams, but the beam sweeping around 2 TX and 3 RX beams work best.

BS antenna deployment in FR2 can be either bi-directional or uni-directional. In uni-directional deployments, the BS antennas (TRxP) are placed along the track pointing in the same direction, whereas in bi-directional deployments the TRxP point both ways. Bi-directional deployment reduces the distance between train and antenna, but the Doppler shift changes rapidly when the UE panel is switched.

In FR2, the simultaneous data transmission in the SFN deployment does not work since UE cannot receive signals from different directions simultaneously. 3GPP therefore defines the performance requirements only with DPS in Rel-17.

Bi-directional deployment for FR2 is more complex and does not bring increases in capacity and throughput. This is because in Rel-17, the UE can only have one panel in one direction active at a time. Nonetheless, bi-directional deployment for FR2 may be used to increase the 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 enable an increase in throughput.

Deployment and next steps

The 3GPP work that has been completed so far to support the HST use case makes it possible to ensure the delivery of high-quality 5G services on trains travelling at speeds up to 500 km/hr (using FR1) or 350 km/hr (using FR2). HST operation in FR1 ensures the connectivity to the UEs inside train carriages directly. On the other hand, HST operation in FR2 provides higher data download/upload speed due to the wider channel bandwidth and good SNR although it requires dedicated UE mounted on the rooftop of the trains.

Looking ahead, Rel-18 HST-related extensions for FR2 include multi-panel operation from a single train-mounted UE, introducing the tunnel deployment scenario, and adding the CA scenario.

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