ETSI TR 103 865 V1.1.1 (2024-09)

Rail Telecommunications (RT); Future Railway Mobile Communication System (FRMCS); Radio performance aspects

DTR/RT-0079

General Information

• Status: Not Published
• Technical Committee: RT - Railway telecommunications
• Current Stage: 12 - Completion
• Due Date: 30-Sep-2024
• Completion Date: 06-Sep-2024

Standards Content (Sample)

TECHNICAL REPORT
Rail Telecommunications (RT);
Future Railway Mobile Communication System (FRMCS);
Radio performance aspects
2 ETSI TR 103 865 V1.1.1 (2024-09)

Reference
DTR/RT-0079
Keywords
FRMCS, radio parameters
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ETSI
3 ETSI TR 103 865 V1.1.1 (2024-09)
Contents
Intellectual Property Rights . 5
Foreword . 5
Modal verbs terminology . 5
Executive summary . 5
Introduction . 5
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definition of terms, symbols and abbreviations . 8
3.1 Terms . 8
3.2 Symbols . 8
3.3 Abbreviations . 9
4 Interference Mitigation . 10
4.1 Frequency Reuse Techniques . 10
4.1.1 Presentation. 10
4.1.1.0 Introduction . 10
4.1.1.1 Hard Frequency Reuse . 10
4.1.1.2 Soft Frequency Reuse . 10
4.1.1.3 Partial Frequency Reuse . 10
4.1.2 Remarks and Potential Implementation of Frequency-Reuse Techniques . 11
4.1.2.1 Introduction . 11
4.1.2.2 Performance of frequency reuse . 11
4.1.2.3 Dynamic Frequency-Reuse implementation . 12
4.1.2.4 Static Frequency-Reuse implementation . 12
4.1.2.5 Frequency-Reuse techniques and RMR operating bands . 12
4.2 Coordinated Multi-Point techniques . 13
4.2.1 Downlink Coordinated Multi-Point techniques . 13
4.2.2 Uplink Coordinated Multi-Point Reception . 15
4.2.3 Backhaul requirements and CoMP zone . 16
4.3 Interference Rejection with MMSE-IRC Receivers . 16
4.3.1 Analysis . 16
4.3.2 Remarks . 17
4.4 Interference Mitigation through beam management . 17
4.5 Interference mitigation techniques comparison . 17
4.5.1 Comparison metrics . 17
4.5.2 Scenarios . 17
4.5.2.0 Introduction . 17
4.5.2.1 Urban scenario . 17
4.5.2.2 Station scenario in urban environment . 18
4.5.2.3 Shunting scenario . 18
4.5.2.4 Rural scenario conventional lines . 18
4.5.2.5 Rural scenario high speed lines . 18
4.5.3 Comparison tables . 18
4.5.4 Conclusion . 21
5 TDD operation . 22
5.1 Introduction . 22
5.2 Synchronization . 22
5.3 Cross-Border Coordination . 23
5.4 Cross-Link Interference . 23
5.5 Remote Interference Management (RIM) . 24
6 Inter-System Interference . 25
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4 ETSI TR 103 865 V1.1.1 (2024-09)
7 Doppler Effect . 25
7.1 DMRS configuration trade-offs . 25
7.2 Potential network lay-out constraints related to maximum change rate of doppler shift . 26
8 Impact of Sub-Carrier Spacing . 26
8.1 Analysis . 26
8.2 Additional Results . 27
9 Impact of Speed on Time Synchronization . 28
10 Observations and Conclusions . 29
Annex A: Interference mitigation using MMSE-IRC . 31
Annex B: Effect of Subcarrier spacing on cell edge performance in 1 900 MHz railway
scenario . 34
Annex C: Link level simulation on subcarrier spacing effects . 35
C.1 Introduction . 35
C.2 Performance summary . 35
C.3 Results . 37
Annex D: Change History . 40
History . 41

ETSI
5 ETSI TR 103 865 V1.1.1 (2024-09)
Intellectual Property Rights
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Foreword
This Technical Report (TR) has been produced by ETSI Technical Committee Railway Telecommunications (RT).
Modal verbs terminology
In the present document "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be
interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Executive summary
The goal of the present document is to help railway infrastructure managers identifying features and techniques
recommended to optimize radio network performance of a Future Railway Mobile Communication System (FRMCS) in
different railway contexts. Several topics have been studied, such as interference mitigation, TDD operation, Doppler
effect or impact on time synchronization of sub-carrier spacing and of speed, and a conclusion has been drawn for each
of them.
Introduction
3GPP 5G NR is the radio access technology to be used in the Future Railway Mobile Communication
System (FRMCS). Some performance evaluations of NR in typical railway deployments have been made in ETSI
TR 103 5542 [i.1]. Furthermore, the need has been identified to clarify some technical aspects and assumptions that
have been made in ETSI TR 103 554-2 [i.1]. Although ETSI TR 103 554-2 [i.1] highlighted the importance of
mitigating interferences to guarantee enough throughput at any location of the train in a cell, especially at cell edge,
further study and clarification of possible mitigation techniques was deemed beneficial.
ETSI
6 ETSI TR 103 865 V1.1.1 (2024-09)
The allocation of a TDD spectrum band for train services in Europe raised some additional questions specific to TDD
deployment, in particular when considering continuous operation across country borders, with each country having
deployed its own FRMCS network.
The present document has several objectives. A first objective is to provide clarifications about some technical aspects
such as Doppler effect impact, impact of sub-carrier spacing and impact of train speed on time synchronization. The
second goal is to provide an analysis and a comparison of some different features and techniques that are included in
3GPP specifications regarding interference mitigation techniques in the context of railway-specific deployment
scenarios. A third point is to provide an analysis of TDD operation implications regarding synchronization and
cross-border coordination.
Overall, the aim of the present document is to help railway infrastructure managers with identifying features and
techniques recommended to optimize radio network performance in different railway contexts.

ETSI
7 ETSI TR 103 865 V1.1.1 (2024-09)
1 Scope
The present document analyses and provides additional information on 5G NR radio performance for FRMCS operation
limited to RMR bands 900 MHz (FDD) and 1 900 MHz (TDD).
Starting from the most representative FRMCS use cases defined in ETSI TR 103 554-2 [i.1], the efficiency of different
interference mitigation techniques is compared. Further, given the availability of a TDD band for RMR, some aspects
related to performance when operating in TDD are explored. Finally, miscellaneous technical aspects affecting radio
performance (Inter System Interference, consideration of Doppler effect, impact of the Sub Carrier Spacing (SCS), etc.)
are studied.
2 References
2.1 Normative references
Normative references are not applicable in the present document.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document, but they assist the
user with regard to a particular subject area.
[i.1] ETSI TR 103 554-2: "Rail Telecommunications (RT); Next Generation Communication System;
Radio performance simulations and evaluations in rail environment; Part 2: New Radio (NR)".
[i.2] Loïc Brunel, Hervé Bonneville, Akl Charaf and Émilie Masson: "System-Level Evaluation of
Next-Generation Radio Communication System for Train Operation Services", Proceedings of
th
7 Transport Research Arena TRA 2018, April 16-19, 2018.
[i.3] G. B., N. H. M., T. B. S., P. M., Fernando M. L. Tavares: "On the Potential of Interference
Rejection Combining in B4G Networks".
[i.4] 3GPP TR 36.884: "Performance requirements of MMSE-IRC receiver for LTE BS".
[i.5] ETSI TS 136 423: "LTE; Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2
Application Protocol (X2AP) (3GPP TS 36.423)".
[i.6] ETSI TS 136 300: "LTE; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved
Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (3GPP
TS 36.300)".
[i.7] Shangbin Wu and Yinan Qi: "Centralized and distributed schedules for non-coherent joint
transmission".
[i.8] ETSI TS 138 104 (V17.6.0): "5G; NR; Base Station (BS) radio transmission and reception (3GPP
TS 38.104 version 17.6.0 Release 17)".
[i.9] ETSI TS 138 101-1 (V17.6.0): "5G; NR; User Equipment (UE) radio transmission and reception;
Part 1: Range 1 Standalone (3GPP TS 38.101-1 version 17.6.0 Release 17)".
[i.10] Bernd Holfeld, Moritz Lossow, Maksym Tyrskyy, Said Mehira, Lourdes Garcia, Simon Biemond,
and Christoph Bach: "Field Study on Multi-Antenna Radio Technologies for Future Railway
Communications at 1.9 GHz", August 2021.
ETSI
8 ETSI TR 103 865 V1.1.1 (2024-09)
[i.11] ETSI TS 138 211: "5G; NR; Physical channels and modulation (3GPP TS 38.211)".
[i.12] ETSI TS 138 214: "5G; NR; Physical layer procedures for data (3GPP TS 38.214)".
® ®
[i.13] Lars Lindbom (Ericsson ), Robert Love, Sandeep Krishnamurthy (Motorola Mobility), Chunhai
® ®
Yao (Nokia Siemens Networks), Nobuhiko Miki (NTT DOCOMO), Vikram Chandrasekhar ®
(Texas Instruments ): "Enhanced Inter-cell Interference Coordination for Heterogeneous
Networks in LTE-Advanced: A Survey", December 7, 2011.
[i.14] Zainab Zaidi, Vasilis Friderikos, Zarrar Yousaf, Simon Fletcher, Mischa Dohler and Hamid
Aghvami: "Will SDN be part of 5G?", February 2018.
[i.15] CEPT ECC Report 353 (16 June 2023): "Cross-border coordination and synchronisation for
Railway Mobile Radio (RMR) networks in the 1900-1910 MHz TDD frequency band".
[i.16] 3GPP TR 38.828: "Cross Link Interference (CLI) handling and Remote Interference Management
(RIM) for NR".
[i.17] ETSI TS 138 300: "5G; NR; NR and NG-RAN Overall description; Stage-2 (3GPP TS 38.300)".
[i.18] 3GPP TR 36.878: "Study on performance enhancements for high speed scenario in LTE".
[i.19] CEPT ECC Recommendation (23)01 (16 June 2023): "Cross-border coordination for Railway
Mobile Radio (RMR) in the 1900-1910 MHz TDD frequency band".
[i.20] IEEE™ Volume 17 - Issue 1: "Interference rejection combining in LTE networks", Y. Léost,
M. Abdi, R. Richter and M. Jeschke. In Bell Labs Technical Journal, pp. 25-49, June 2012,
doi: 10.1002/bltj.21522.
[i.21] Fernando M. L. Tavares, Gilberto Berardinelli, Nurul H. Mahmood, Troels B. Sørensen, and
Preben Mogens: "On the Potential of Interference Rejection Combining in B4G Networks".
[i.22] Haifan Yiny, Haiquan Wang, Yingzhuang Liuy, David Gesbert: "Dealing with the Mobility
Problem of Massive MIMO using Extended Prony's Method".
[i.23] Kien T. Truong and Robert W. Heath Jr: "Effects of Channel Aging in Massive MIMO Systems".
[i.24] ETSI TS 138 133: "5G; NR; Requirements for support of radio resource management (3GPP
TS 38.133)".
[i.25] ETSI TS 138 213: "5G; NR; Physical layer procedures for control (3GPP TS 38.213)".
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
cell centre: geographical area where a terminal experiences a good radio link quality (e.g. high SINR values)
cell edge: geographical area where a terminal experiences a poor radio link quality (e.g. low SINR values)
3.2 Symbols
Void.
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9 ETSI TR 103 865 V1.1.1 (2024-09)
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
ACLR Adjacent Channel Leakage Ratio
BBU Base Band Unit
BS Base Station
CDMA Code-Division Multiple Access
CE Control Element
CJT Coherent Joint Transmission
CLI Cross Link Interference
CLI-RSSI Cross Link Interference - Received Signal Strength Indicator
CoMP Coordinated Multi-Point
CP Cyclic Prefix
CSI Channel State Information
CSI-RS Channel State Information - Reference Signals
DIP Dominant Interferer Portion
DL CoMP DownLink CoMP
NOTE: NCJT, CJT.
DL DownLink
DMRS Demodulation Reference Signal
DPS Dynamic Point Selection
DSRC Dedicated Short-Range Communications
FDD Frequency Division Duplex
F-NCJT Fully overlapped NCJT
gNB 5G Node B
GP Guard Period
GSM General System for Mobile communication
HFR Hard Frequency Reuse
ICI Inter-Carrier Interference
ISD Inter-Site Distance
LOS Line Of Sight
LTE Long Term Evolution
MAC Media Access Control
MAC-CE Medium Access Control - Control Element
MCS Modulation and Coding Scheme
MFCN Mobile/Fixed Communications Network
MIMO Multiple Input Multiple Output
MMSE-IRC Minim Mean Square Error - Interference Rejection Combining
MRC Maximum Ratio Combining
NCJT Non-Coherent Joint Transmission
NF-NCJT Non-Fully overlapped NCJT
NLOS Non-Line Of Sight
NR New Radio
O&M Operation and Maintenance
OFDM Orthogonal Frequency Division Multiplex
PFR Partial Frequency Reuse
PRACH Physical Random Access CHannel
PRB Physical Resource Block
QAM Quadrature Amplitude Modulation
QPSK Quadrature Phase Shift Keying
RB Resource Block
RF Radio Frequency
RIM Remote Interference Management
RMR Railway Mobile Radio
RRC Radio Resource Control
RSSI Radio Signal Strength Indication
SCS Sub-Carrier Spacing
SFN Single Frequency Network
SFR Soft Frequency Reuse
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10 ETSI TR 103 865 V1.1.1 (2024-09)
SINR Signal-to-Noise plus Interference Ratio
SNR Signal-to-Noise Ratio
SRS Sounding Reference Signal
SRS-RSRP SRS - Reference Signal Received Power
SSB Synchronization Signal Block
TDD Time Division Duplex
TP Transmit Power
TRP Transmission Reference Point
UE User Equipment
UL CoMP UpLink CoMP
UL UpLink
4 Interference Mitigation
4.1 Frequency Reuse Techniques
4.1.1 Presentation
4.1.1.0 Introduction
Hard Frequency Reuse, Soft Frequency Reuse and Partial Frequency Reuse are three different frequency reuse
techniques depicted in Figure 1 and described hereafter.
4.1.1.1 Hard Frequency Reuse
Hard frequency reuse consists of assigning different fractions of a frequency band to each cell of a group of cells. This
is described by a hard reuse factor. The geographical separation between cells using the same frequency chunk in two
different groups is larger, hence the cochannel interference is reduced. In hard frequency reuse the frequency band is
divided in a static manner between cells of a group. This provides frequency orthogonality between cells but reduces
cell capacity since the allocation is limited to a fraction of the carrier bandwidth (e.g. half of the carrier bandwidth for a
hard reuse factor of 2), even in low cell load situation or in absence of interference from neighbouring cells.
4.1.1.2 Soft Frequency Reuse
Using soft frequency reuse, neighbouring cells dynamically coordinate their allocation of frequency resources and
transmit power to users based on their knowledge of their radio conditions. This exploits the difference in path loss
between different users in the cell by allocating a given fraction of the band with reduced power in a region close to the
cell centre while the other part is allocated for cell edge users with higher power. A neighbouring cell does the same but
swaps the frequency fractions to maximize the separation and avoid interference. The whole bandwidth is reused in
every cell. Soft frequency reuse scheme avoids interference to some extent. Only a fraction of the band is allocated for a
given user, but from a network perspective, cell spectrum utilization is much higher compared to a hard frequency-reuse
scheme.
4.1.1.3 Partial Frequency Reuse
Using partial frequency reuse, the frequency band is divided into three fractions:
• one fraction used by all cells with relatively low power expected to serve users in cell centre;
• two other equal fractions allocated to neighbouring cells where each fraction is exclusively used by one of the
two cells with a high transmission power.
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11 ETSI TR 103 865 V1.1.1 (2024-09)

Figure 1: Interference mitigation through frequency reuse
4.1.2 Remarks and Potential Implementation of Frequency-Reuse
Techniques
4.1.2.1 Introduction
For clarity, some definitions are provided hereafter. For more details, refers to ETSI TS 138 211 [i.11] and ETSI
TS 138 214 [i.12].
Sounding Reference Signals (SRS) are uplink signals designed to infer the uplink channel state information. Sounding
Reference Signals are UE-specific and scheduled by BS to gather channel quality on specific RBs. UEs are configured
to transmit UL SRS so that the BS can estimate the channel quality in the UL.
Channel State Information Reference Signals (CSI-RS) are DL signal which allows BS to gather Channel State
Information. CSI-RS are UE-specific and can be used and configured to fulfil different purposes. They can be
multiplexed in time or frequency while they may also be periodic, semi-periodic or aperiodic. It should be noted that, in
5G New Radio, all reference signals are scheduled by the BS.
A UE is notified via RRC messages about the resources on which it is scheduled to receive CSI-RS or through
MAC-CE for aperiodic CSI-RS.
Non-Zero Power (NZP-CSI-RS) and Zero Power (ZP-CSI-RS) are configured for a given UE so it can estimate the
CSI and provide feedback to the base station.
CSI Interference Measurement (CSI-IM) are blank resources used for interference measurement. The BS does not
transmit any signals on CSI-IM so that a UE can measure on these configured resources interference signals from other
cells.
Measurements that are performed using the above reference signals can be used to exchange information between BS
for coordination between cells or on a centralized common scheduler so that the interference can be minimized.
4.1.2.2 Performance of frequency reuse
In [i.2] it is shown how partial frequency reuse outperforms hard reuse and soft reuse and combines their respective
advantages. At cell edge partial reuse offers better interference mitigation than soft reuse since the resources used on
cell edge by the neighbouring cell are unused (blanked). On the other hand, with partial reuse all cells keep enough
bandwidth (reused resources with low power) for mid-cell and cell centre so that the cell capacity is not penalized.
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12 ETSI TR 103 865 V1.1.1 (2024-09)
Frequency reuse techniques described above have not been considered in 3GPP specifications for 5G NR. They can
potentially be implemented, including hard reuse, at scheduling or RRC level. They could be negotiated dynamically
between base stations but could also be configured more statically by O&M, and thus be part of a deployment
configuration. However, the detailed implementation impacts (i.e. which components would be impacted and to what
degree) have not been studied within the present document.
4.1.2.3 Dynamic Frequency-Reuse implementation
For LTE, 3GPP has defined standardized messages allowing base stations from different vendors to coordinate
dynamically their resource allocations through frequency-reuse technique. This can be done:
• in frequency domain:
- High Interference Indicator (HII);
- Overload Indicator (OI);
- Relative Narrowband Transmit Power (RNTP); and also
• in time domain:
- Almost Blank Subframes (ABS) (ETSI TS 136 423 [i.5] and ETSI TS 136 300 [i.6]).
The standardized signalling is transported between base stations through X2 interface, which has a latency typically of
the order of a tens of milliseconds. Hence, the dynamic of a frequency-reuse technique could be expected to be in the
order of a few hundred of milliseconds [i.13].
The coordination interface described above for LTE has not been defined in NR specifications but left up to vendor
implementation. This means that dynamic frequency reuse could be feasible between NR base stations provided by a
single vendor and implementing a vendor-specific coordination messages/interface. Hence, a dynamic frequency-reuse
technique is not guaranteed to be inter-vendor inter-operable by the 3GPP NR specifications as of today.
It is also worth noting that for deployment options where one Base Band Unit (BBU) controls several
transmission/reception points, a dynamic frequency-reuse technique could be implemented at BBU level for the whole
set of transmission/reception points. However, the interference problem remains between zones controlled by different
BBUs.
4.1.2.4 Static Frequency-Reuse implementation
Frequency-reuse techniques could also be implemented without the need of inter-base stations communication by
providing each of them (e.g. through O&M) some allocation rules for cell-edge UEs. Hard-reuse is typically one
possibility, but partial and soft-reuse techniques could also be configured.
For example, one possible implementation of the soft frequency reuse technique is that schedulers in base stations
serving two adjacent cells can be configured to allocate the resource blocks for cell-edge users starting from a different
edge of the carrier bandwidth. In low load situation and a low data traffic (moderate payload size) condition, this would
avoid interference between the two adjacent cells. However, when load and/or data traffic become greater, the two
allocations may overlap since they are not aware of each other, and the resulting interference may significantly degrade
the performance on both sides.
4.1.2.5 Frequency-Reuse techniques and RMR operating bands
For both 900 MHz and 1 900 MHz bands, soft reuse and partial reuse techniques rely on a single carrier, hence with one
Synchronization Signal Bloc (SSB). They could be implemented in a vendor-specific manner, with possible impacts on
scheduler and/or other internal functions.
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13 ETSI TR 103 865 V1.1.1 (2024-09)
Hard reuse can be understood as a split of the bandwidth into separated RF carriers. In 1 900 MHz where the available
bandwidth is 10 MHz, this is possible while in 900 MHz band where the available bandwidth is limited to 5,6 MHz this
raises issues as the SSB does not fit in a chunk smaller than 3,6 MHz, and has been considered in ETSI
TS 138 211 [i.11], clause 7.4.3.1. However, this issue in 900 MHz band hard reuse can be overcome by implementing
hard reuse as a one single carrier via a static splitting of the resources between neighbouring cells for data channels
while SSB remains in reuse 1 (see Figure 2). Indeed, SSB and control channels are designed to be robust enough to
tolerate cochannel interference among other impairments. ETSI TR 103 554-2 [i.1] includes simulations with reuse
factors 1 to 3.
Figure 2: Example of hard frequency reuse implementation as
a single carrier with static resource splitting
To be noted that the above frequency reuse techniques go in a different direction compared to 3GPP RAN4's studies on
High Speed Train (HST) enhancements for Single Frequency Network (SFN). Indeed, RAN4 study on HST is based on
a SFN deployment and targets a different use case compared to FRMCS (typically supporting non-critical data for
passengers and not train control applications). Moreover, 3GPP SFN study considers a completely different ISD (below
2 km).
4.2 Coordinated Multi-Point techniques
4.2.1 Downlink Coordinated Multi-Point techniques
CoMP-like techniques can be implemented in 5G New Radio as part of the Multi-Transmission Reception
Point (Multi-TRP) framework which includes Coherent and Non-Coherent Joint Transmission (CJT and NCJT) [i.7], as
well as Dynamic Point Selection (DPS).
The cooperative set (CoMP set) is the set of TRPs that are participating in transmission to/from a given UE:
• CJT (see Figure 3) performs joint beamforming and assumes accurately synchronized DL transmission with
shared data transmitted from all TRPs (same MIMO layer S) and highly accurate CSI reporting for coherent
add up at UE. It requires backhaul links with high capacity and low latency as well as synchronization among
the CoMP set. CJT requires a central scheduler which collects CSI reports from TRPs then selects the
precoding codebooks and schedules the resources accordingly.
ETSI
14 ETSI TR 103 865 V1.1.1 (2024-09)
f
S
S
t
S
BBU
TRP
Figure 3: Coherent Joint Transmission (CJT)
• NCJT (see Figure 4) assumes synchronized (lower accuracy with respect to CJT) DL transmission with
different data transmitted from TRPs (different MIMO layers Si), frequency resources may be overlapping or
not. NCJT requires less data exchange among TRPs. It handles each transmission from a TRP to UE
individually (MCS selection, precoding and scheduling are performed on each TRP separately). Scheduling for
NCJT is implemented in a distributed manner (per TRPs schedulers). This distributed approach is more
suitable for operation with non-ideal backhauls. NCJT can be categorized into Fully overlapped
NCJT (F-NCJT) and Non-Fully overlapped NCJT (NF-NCJT).
f
S1
S3
t
S2
BBU
TRP
Figure 4: Non-Coherent Joint Transmission (NCJT)
• F-NCJT (Fully overlapped NCJT) assumes the same resources (RBs) are used by all TRPs in a CoMP set to
a given UE to transmit different codewords/streams. TRPs need to synchronize their resource scheduling but
no exchange of data inside the CoMP set is required since the TRPs are transmitting different layers. This
solution can be interesting when scheduling geographically separated UEs in different cells, one train close to
a cell center while the other one is on the cell edge of the neighbouring cell.
• NF-NCJT (Non-Fully overlapped NCJT) is a more distributed scheme and leaves more freedom to each
TRP (local scheduler) to choose the resources to be used. There is no need to exchange CSI reports or user
data between TRPs.
• Dynamic Point Selection (DPS)/muting: data transmission is performed from one point (within the CoMP
set) in a time-frequency resource. The transmitting/muting point may change from one subframe to another
and may also vary within a subframe. Data are to be available simultaneously at the multiple points of the
CoMP set.
If the network can make data available on the different TRPs, DPS can be feasible. However DPS in case of high ISD
may not bring gain since the Path Loss difference makes a train stay connected mostly to one cell until reaching the cell
edge. [i.7] provides a summary of some requirements of the different DL CoMP techniques.
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4.2.2 Uplink Coordinated Multi-Point Reception
Several site sectors from different cells in an Uplink Coordinated Multi-Point set could use multi-TRP features and
adaptively select two site sectors (one primary and one secondary) to receive UL signals from a UE (Figure 5). The
received signals are sent via backhaul link to central BBU that combines the two signals. When a UE is on the border
between two cells, the combination of UL signals received on the two different sites is performed at a central node, it
may result in an increase of the SINR and consequently an increase of the cell edge UE uplink average throughput (2 %
to 5% increase if the secondary cell load is 100 % (worst case scenario), and 7 % to 13 % if the secondary cell load is
33 %) with linear deployment and ISD around 1 km, see [i.10]). This UL CoMP consists in performing
MRC-Combining from multiple base stations.

Figure 5: Two sites sharing the same frequency resources
In FRMCS, UL CoMP is practically limited to two sites since any other additional site would be very far from actual
train therefore it would receive a very low uplink power and no additional gain can be expected.
Uplink Coordinated Multi-Point groups are disjoint so a site sector can belong to only one Uplink Coordinated
Multi-Point group. This implies that UL intercell interference may be observed when the UE hands over from one
CoMP group to another. Indeed, the interference problem remains between zones controlled by different BBUs
(Figure 6).
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Figure 6: Residual interference between CoMP groups
4.2.3 Backhaul requirements and CoMP zone
The different CoMP techniques put different requirements on the backhaul links connecting the transmission points of a
CoMP set and the BBU, especially in terms of transmission delays. For DL techniques, CJT, DPS and F-NCJT require
having the data available at all transmission points within a short time window. For UL techniques, data from the
different transmission points have to be available at the BBU also within a short time window.
According to [i.14], the one-way delay between radio unit and BBU should be under 75 µs. As the latency of light in
the fiber is approximately 5 µs/km, the length of the fiber link between transmission points and BBU should not be
more than 15 km. Therefore, CoMP zone could span up to 30 km, i.e. 15 km away along the track from both sides of the
combining BBU assuming it located near the track.
4.3 Interference Rejection with MMSE-IRC Receivers
4.3.1 Analysis
Minimum Mean Square Error-Interference Rejection Combining (MMSE-IRC) is a linear receiver based on
Interference rejection mostly used in uplink. The linear receiver is designed in such a way to perform a receive
beamforming based on the knowledge on the channel between the victim and the aggressor. The beamforming target is
to create nulls in the directions of the interferers. In Annex A an analysis of MMSE-IRC in FRMCS is provided.
MMSE-IRC requires knowledge of channel characteristics between the victim and the aggressor (i.e. between a train
and the interfering BS in DL or between a BS and an interfering train in UL). More details can be found in [i.3] and
3GPP TR 36.884 [i.4].
As MMSE-IRC performance relies on low correlation between wanted and interfering signals. This may be the case
especially in urban environment where victims and aggressors are unlikely to be aligned, the speed is relatively low and
scattering environment is rich. In such conditions, a low correlation between the channels encountered by the wanted
and interfering signals is observed. However, in other scenarios (e.g. rural environment) a higher correlation may be
observed, and MMSE-IRC performance might be degraded. Another important aspect is channel information aging
especially at high speed, where the CSI varies faster than the MMSE-IRC reconfiguration.
It should be noted that the performance of MMSE-IRC improves with the number of antennas on the reception side.
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4.3.2 Remarks
Interference mitigation by frequency reuse is implemented via resource scheduling policies on BS side and during
network planning. Interference rejection is performed on receiving side at signal level. Frequency reuse and interference
rejection are compatible and can be combined for a better mitigation depending on the scenario as well as other channel
information. Residual interference for MMSE-IRC depends on the frequency reuse scheme used on the transmitter side.
4.4 Interference Mitigation through beam manage
...