design features

1. Advanced Course on Wireless Communications
ELT-45306 Summer 2021 implementation
Essay (peer reviewed)
Submitted by
NCHANG TITA MARTIN (# 050497919)
Topic:
High Speed Train Communications in 5G: Design Elements to Mitigate the Impact of Very High
Mobility
Introduction
High speed mobile scenarios pose wireless access problems that mobile operators need to handle.
Such problems include frequent handovers, causing frequent cell-reselection, call quality decline, service
unavailability, Doppler shifts, and fast fading, etc. In this essay, we discuss the 5G design aspects to
mitigate the impact of very high mobility. We begin with an evolutionary review, and then describe the
5G NR high speed train scenario before examining the design elements [1].
The first standardized train communication system was based on the Global System for Mobile
(GSM) technology, namely GSM-Railway (GSM-R). It of course had limited capability, offering a peak
rate of 172 kb/s only, with a latency on the order of 400 ms [2]. Then came the IEEE 802.16 based
Mobile Worldwide Interoperability for Microwave Access (WiMAX) technology. Compared to GSM-R,
Mobile WiMAX could provide much higher data rate on the order of a few hundred Mb/s and much
lower latency up to 50 ms at a speed of about 300km/h [3].
The next technology LTE-Railway (LTE-R) enabled diverse railway services including control
information transmission, real-time monitoring, multimedia dispatching, railway emergency
communication, etc. [2]. The field trial revealed that the LTE-R system can achieve the data rate of a few
tens of Mb/s at the speed of around 200 km/h [4].
5G High Speed Train Scenario.
The 5G NR high speed train scenario aims to provide broadband access to train passengers with
continuous coverage along train tracks. Performance requirements include an experienced data rate of 50
Mb/s (downlink) and 25 Mb/s (uplink), area traffic capacity of 15 Gb/s/train (downlink) and 7.5
Gb/s/train (uplink), overall user density of 1,000 users/train and a speed up to 500 km/h.
The 5G high speed train scenario can be deployed in two layout options, namely macro-only and
macro-and-relay layouts, as depicted in figure 1 below:

2. FIGURE 1. Layout options for the high-speed train deployment scenario: a) macro-only layout; b)
macroand-relay layout [1]
The macro-only layout establishes direct links between the gNodeB and user equipment (UEs) inside the
train. The advantage of this is that no additional infrastructure is required in the train. However, this
layout leads to severe signal penetration losses due to the metal train carriages, higher UE transmission
power to overcome the penetration loss and handover failures.
The macro-and-relay layout introduces an onboard relay to avoid the penetration loss, but it may be
challenged by the increased latency and additional resource consumption for the relaying operation.
Body
Physical Layer System Design Elements to Support the High-Speed Scenario.
Several design aspects interplay in the 5G high speed train scenario. These includes numerology, frame
structure, reference signals, multi-antenna schemes, cell search, random access, and mobility support.
Numerology and Frame structure:
In 5G NR, orthogonal frequency-division multiplexing (OFDM) is chosen as a basic transmission scheme
both in the downlink and uplink. In order to cover a wide range of carrier frequency bands (from sub-1
GHz to mmWave bands) for various use cases and deployment scenarios having diverse requirements, the
NR supports flexible and scalable numerology with the subcarrier spacing of 2m
· 15 kHz where m is an
integer from 0 to 4. Hence, the supported subcarrier spacing values are {15, 30, 60, 120} kHz for the data
transmission and are {15, 30, 120, 240} kHz for initial access operations. Figure 2 shows the timedomain
NR frame structure. Each radio frame of 10 ms consists of 10 subframes of 1 ms each. Each subframe is
then divided into a number of slots, determined by the subcarrier spacing. [1]
For very high mobility scenarios such as the high speed train scenario, large subcarrier spacing (and
consequently short OFDM symbol duration) is also desirable to avoid Doppler-induced interference.

3. Reference Signal Design
Several reference signals are defined for diverse purposes: demodulation reference signal (DM-RS) for
the downlink/uplink channel estimation; channel state information reference signal (CSI-RS) for the
downlink channel state measurement; sounding reference signal (SRS) for the uplink channel state
measurement; and phase-tracking reference signal (PT-RS) for the downlink/uplink phase noise
compensation. Reference signals in NR are highly configurable. They can be turned on and off as
necessary, and their specific pattern, density, and multiplexing schemes are adjustable.
Multi-antenna Schemes:
NR specification supports up to eight-layer downlink transmission and up to four-layer uplink
transmission. This higher layer transmission is unfortunately not practically feasible for a very high
Doppler scenario like the high-speed train scenario, for two reasons; difficulty in obtaining accurate CSI
at the transmitter through feedback; and dominance of line-of-sight components over multipath
components because high speed train tracks tend to be linear. Consequently, supporting up to two-layer
transmission (preferably across cross-polarization components) seems reasonable for the high-speed train
scenario.
Initial Access and Mobility Procedures to Support High Speed Scenario
As in LTE, a 5G UE does cell search to acquire downlink synchronization and determine cell ID. After
this, the UE is then able to decode the PBCH to obtain some necessary system information. Unlike in
LTE, 5G NR supports multibeam operations by repetitive transmission of SS/PBCH blocks. Each
SS/PBCH block consisting of primary and secondary synchronization signals, and PBCH located within
four OFDM symbols may correspond to a physical beam generated at the gNodeB. Regarding random
access procedure, LTE-like four-step procedure consisting of random-access preamble transmission
(Msg1), random access response (Msg2), connection request (Msg3), and contention resolution (Msg4) is

4. also employed in NR. In addition, NR has also developed a two-step random access procedure in Rel-16
[7], where the uplink messages Msg1 and Msg3 are aggregated into MsgA. Similarly, the downlink
messages Msg2 and Msg4 are combined to form MsgB. Two-step random access will be efficient in
terms of latency and overhead reduction especially for the high-speed train scenario where there are
typically fewer UEs, thereby leading to lower contention probability [1].
Efficient mobility support is also of a main concern in the high-speed train scenario. Due to very high
speed, handover will be quite frequent but can be avoided by intra-cell handover between RRHs
connected to the same baseband unit (BBU).
Conclusion
We have seen various design aspects like Numerology, reference signal design, multiantenna
schemes that help mitigate the problems encountered with very high mobility such as frequent handovers,
Doppler shifts and fast fading.
References:
1. G. Noh, B. Hui and I. Kim, "High Speed Train Communications in 5G: Design Elements to Mitigate
the Impact of Very High Mobility," in IEEE Wireless Communications, vol. 27, no. 6, pp. 98-106,
December 2020, doi: 10.1109/MWC.001.2000034.
2. R. He et al., “High-Speed Railway Communications: From GSM-R to LTE-R,” IEEE Veh. Technol.
Mag., vol. 11, no. 3, Aug. 2016, pp. 49–58.
3. [2] H.-W. Chang et al., “Field Trial Results for Integrated WiMAX and Radio-Over-Fiber Systems on
High Speed Rail,” Proc. IEEE Int’l. Symp. Personal Indoor Mobile Radio Commun. (PIMRC 2011),
June 2011, pp. 2111–15.
4. Y.-S. Song et al., “Long Term Evolution for Wireless Railway Communications: Testbed
Deployment and Performance Evaluation,” IEEE Commun. Mag., vol. 54, no. 2, Feb. 2016, pp. 138–
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