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GROUP REPORT
5G Wireless Backhaul/X-Haul
Disclaimer
The present document has been produced and approved by the millimetre Wave Transmission (mWT) ETSI Industry
Specification Group (ISG) and represents the views of those members who participated in this ISG.
It does not necessarily represent the views of the entire ETSI membership.
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2 ETSI GR mWT 012 V1.1.1 (2018-11)
Reference
DGR/mWT-0012
Keywords
5G, application, backhaul, fronthaul, microwave,
midhaul, millimetre wave, mWT, scenarios,
transmission, use case, X-Haul
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Contents
Intellectual Property Rights . 4
Foreword . 4
Modal verbs terminology . 4
Executive summary . 4
Introduction . 5
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definition of terms and abbreviations . 8
3.1 Terms . 8
3.2 Abbreviations . 8
4 5G Access Network Overview . 9
4.1 Moving From 4G To 5G Technology . 9
4.2 5G Radio Access Configurations . 10
4.3 5G RAN Architecture Options . 11
5 Wireless Backhaul/X-Haul Network Overview . 13
5.1 Backhaul/X-Haul Network Topologies . 13
5.2 Backhaul/X-Haul Capacity Requirements . 14
5.3 Backhaul Spectrum. 15
6 Wireless Backhaul/X-Haul Technologies . 17
6.1 Backhaul/X-Haul Technologies Landscape . 17
6.2 Backhaul/X-Haul Technologies Evolution . 17
6.3 5G Wireless Backhaul/X-Haul Capacity Potential . 18
6.4 5G Wireless Backhaul/X-Haul Latency Potential . 19
6.5 Wireless Backhaul/X-Haul Technologies Fit into 5G Scenarios . 20
7 Considerations on Regulation and Licensing . 21
8 Conclusions . 22
Annex A: Authors & contributors . 23
History . 24
ETSI
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Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (https://ipr.etsi.org/).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
Foreword
This Group Report (GR) has been produced by ETSI Industry Specification Group (ISG) millimetre Wave
Transmission (mWT).
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
Mobile communication technology is evolving at rapid pace towards its fifth generation, namely 5G, deployment phase,
which aims to develop new business opportunities related to enhanced Mobile Broadband, Ultra-Reliable and Low
Latency Communications and massive Machine-Type Communications. New radio access network architecture trends,
aiming at higher network efficiency and improved service delivery, are also discussed within the scope of 5G. In
parallel, it is expected that 5G deployments will be characterized by increased network density, mainly driven by small-
cells implementations.
In order to support the aforementioned 5G targets, there is a need to push the envelope of performance in various
network segments, including backhaul, as it is part of the end-to-end 5G network architecture. Figure 1 depicts how 5G
will impact wireless backhaul/X-Haul networks (X-Haul is a newly-coined term to describe the evolved transport
interface resulting from different RAN split options, hence it will be used throughout the present document). For
instance, higher capacity (even more Mbps at existing hop lengths), lower latency, improved spectral efficiency, highly-
accurate synchronization, advanced networking functionalities and network automation, are discussed for 5G wireless
backhaul/X-Haul networks.
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Figure 1: 5G Impact on W-BH/XH Network
The present document presents the important 5G wireless backhaul/X-Haul scenarios focusing on their critical
requirements. Specifically, areas of deployment and network topologies, data rates, latency, hop lengths and networking
functionalities are discussed. The report draws attention to the fact that next-generation backhaul/X-Haul applications
will be more relevant within a mid/long-term time horizon, namely from around 2020 and beyond, additionally
innovations in microwave and millimetre wave technologies that are anticipated over the next few years have also been
taken into consideration. Topics related to regulation and licensing are discussed.
The present document shows that microwave and millimetre wave transmission technologies are going to continue to
play a pivotal role in the 5G era as they will be fundamental pillars of service providers' network development strategy
to address the future 5G demands. This view is also strengthened due to the inherent benefits of wireless backhaul/X-
Haul with regard to:
• Performance
• Ease of deployment
• Fast time-to-market
• Cost efficiency
Consequently, key recommendations discussed include:
• Technology Innovations: More backhaul spectrum and bandwidth, band and carrier aggregation, LOS
MIMO/OAM (Orbital Angular Momentum) plus XPIC, advanced interference cancellation techniques, higher
directivity and smart antennas solutions, SDN automation, etc. (see clauses 6.2, 6.3, 6.4 and 6.5).
• Regulation and Licensing: Apply regulatory policies and costs in-line with the use cases' requirements (see
clause 7).
Introduction
Today wireless backhaul technologies serve more than 50 % of the total mobile backhaul connections worldwide and
they are apparently key solutions to address demands of mobile access networks at fast pace and in an economical way.
In fact, the key benefits of wireless transmission (backhaul) technologies as enablers for LTE-A as well as for other
fixed broadband applications and use cases have been analysed and provided in ETSI GS mWT 002 [i.1]. However,
significant changes and progress has been observed since then in different areas, including applications and network
requirements, technology capabilities, regulatory and standardization matters.
The 5G use cases categories (eMBB, URLLC, mMTC) that are under discussion present extremely diverse
requirements and from network design perspective all of them will be served by the same network infrastructure.
Consequently, future backhaul/X-Haul architecture will need to ensure that all "conflicting" targets are satisfied and
blend well with each other.
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Further on, 5G RAN architecture will need to be highly flexible, which could be enabled by splitting RAN functions.
As a result, new X-Haul interfaces are being discussed in SDOs and in related industry activities (e.g. in 3GPP, xRAN,
IEEE 1914, TIP, etc.) to allow less stringent transport demands, in terms of capacity and latency, when compared to
typical fronthaul (e.g. CPRI), whilst still accomplishing high performance and optimum use of radio access resources.
Moreover, it is foreseen that ultra-dense radio access networks will be built to increase end customers QoE and this is
depicted in the 5G area traffic capacity target of reaching 10 Mbps per m2. By this regard, a new backhaul layer,
namely small-cell backhaul, is expected to grow massively over the next few years. Naturally, such a network
development will impose additional challenges, so efficient technologies, supporting easy roll-out approach, are
targeted.
Considering the above points, it is of paramount importance for industry's stakeholders to prepare the ground for the
new backhaul/X-Haul architecture. To this end, wireless backhaul/X-Haul technologies, namely covering V-band
(60 GHz), E-band (70/80 GHz), in future W-band (100 GHz) and D-band (150 GHz) as well as critical traditional
microwave bands, will be solution enablers to achieve radical enhancement of existing use cases and to develop
emerging use cases that today are not possible on existing networks.
The present document represents an evolution of the ETSI mWT ISG "Microwave and Millimetre-wave for 5G
Transport" white paper [i.2].
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1 Scope
The present document provides information about the prominent 5G wireless backhaul/X-Haul scenarios. In addition, it
shows how current microwave and millimetre wave transmission technologies as well as their foreseen evolution, in the
pertinent areas of innovation, are going to satisfy upcoming 5G access requirements. Moreover, it points out the
importance of the appropriate regulation and licensing to ease 5G wireless backhaul/X-Haul deployments.
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 GS mWT 002 (V1.1.1) (08-2015): "millimetre Wave Transmission (mWT); Applications
and use cases of millimetre wave transmission".
[i.2] ETSI ISG mWT White Paper No. 25 (First edition, 02-2018): "Microwave and Millimetre-wave
for 5G Transport".
NOTE: Available at http://www.etsi.org/images/files/ETSIWhitePapers/etsi_wp25_mwt_and_5g_FINAL.pdf.
[i.3] Recommendation ITU-R M.2083-0 (09-2015): "IMT Vision - Framework and overall objectives of
the future development of IMT for 2020 and beyond".
[i.4] ETSI TR 138 913 (V14.3.0) (10-2017): "5G; Study on scenarios and requirements for next
generation access technologies (3GPP TR 38.913 version 14.3.0 Release 14)".
th
[i.5] NGMN (V1.0) (24 February 2018): "NGMN Overview on 5G RAN Functional Decomposition".
rd
[i.6] 3GPP TR 38.801 (V14.0.0) (03-2017): "3 Generation Partnership Project; Technical
Specification Group Radio Access Network; Study on new radio access technology: Radio access
architecture and interfaces (Release 14)".
[i.7] ETSI GR mWT 015 (V1.1.1) (11-2017): "Frequency Bands and Carrier Aggregation Systems;
Band and Carrier Aggregation".
[i.8] ETSI GR mWT 008 (V1.1.1) (08-2018): "millimetre Wave Transmission (mWT); Analysis of
Spectrum, License Schemes and Network Scenarios in the D-band".
[i.9] ETSI GR mWT 016 (V1.1.1) (07-2017): "Applications and use cases of Software Defined
Networking (SDN) as related to microwave and millimetre wave transmission".
[i.10] IEEE Std 802.11ad-2012 (Amendment to IEEE Std 802.11-2012, as amended by IEEE
Std 802.11ae-2012 and IEEE Std 802.11aa-2012): "IEEE Standard for Information technology--
Telecommunications and information exchange between systems--Local and metropolitan area
networks--Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications Amendment 3: Enhancements for Very High Throughput in
the 60 GHz Band".
ETSI
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3 Definition of terms and abbreviations
3.1 Terms
For the purposes of the present document, the following terms apply:
DU-bb: Baseband of Distribution Unit
gNB: radio access node which supports the NR
3.2 Abbreviations
For the purposes of the present document, the following abbreviations apply:
rd
3GPP 3 Generation Partnership Project
th
4G 4 Generation of Mobile Networks
th
5G 5 Generation of Mobile Networks
BBU Baseband Unit
BCA Band and Carrier Aggregation
BH Backhaul
BW Bandwidth
CPRI Common Public Radio Interface
C-RAN Centralized RAN
CS Circuit-Switched
CU Centralized Unit
DL Downlink
D-RAN Distributed RAN
DU Distributed Unit
eCPRI enhanced CPRI
eMBB enhanced Mobile Broadband
EPC Evolved Packet Core
FDD Frequency Division Duplex
GSM Global System for Mobile communications
HLS High Layer Split
IEEE Institute of Electrical and Electronics Engineers
ISG Industry Specification Group
ITU International Telecommunication Union
LLS Low Layer Split
LOS Line Of Sight
LTE Long Term Evolution
LTE-A LTE-Advanced
MEC Multi-access Edge Computing
MIMO Multiple Input Multiple Output
mmW millimetre Wave
mMTC massive Machine-Type Communications
MPLS Multi-Protocol Label Switching
MW Microwave
NG Next Generation
NGC Next Generation Core
nLOS near LOS
NLOS Non-LOS
nRT non Real-Time
NR New Radio
OAM Orbital Angular Momentum
OPS Operations
PoP Point of Presence
PtP Point-to-Point
PtMP Point-to-MultiPoint
QoE Quality of Experience
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RAN Radio Access Network
RF Radio Frequency
RRU Remote Radio Unit
RT Real-Time
RTT Round-Trip Time
RU Radio Unit
SDN Software-Defined Networking
SDO Standard Defining Organization
SLA Service-Level Agreement
TDD Time Division Duplex
TIP Telecom Infrastructure Project
UE User Equipment
UL Uplink
UMTS Universal Mobile Telecommunications System
UPF User Plane Functions
URLLC Ultra-Reliable and Low Latency Communications
W-BH/XH Wireless Backhaul/X-Haul
XH X-Haul
XPIC Cross Polarization Interference Cancellation
4 5G Access Network Overview
4.1 Moving From 4G To 5G Technology
Nowadays, the deployed LTE macro cell access sites are typically rooftop (or tower) sites consisting of three (3)
sectors, each one having at least two frequency layers with 20 MHz BW channels and 2x2 MIMO configuration. GSM
and UMTS radio access technologies complete the site configuration, in order to support CS voice services and the
typical backhaul capacity provisioning can reach up to 1 Gbps per site.
The advent of 5G technologies will improve user experience and it will increase network performance significantly.
Figure 2 illustrates the enhancement of key capabilities from IMT-Advanced towards IMT-2020 based on
Recommendation ITU-R M.2083-0 [i.3].
Figure 2: From IMT-Advanced to IMT-2020
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In 5G a new radio interface (NR) will be introduced, where the use of more bandwidth at higher frequency bands (e.g.
around 4 GHz, around 30 GHz, etc.) can be available, whilst a higher number of antenna ports and MIMO layers can be
supported by the future radio access technology. Specifically, according to ETSI TR 138 913 [i.4], the 5G NR can
support up to 1 GHz system bandwidth and up to 256 Tx and Rx antenna elements.
Typically, different configurations of mobile sites are implemented per area of deployment. This aspect relates to the
most suitable access spectrum, the number of MIMO layers, even to the type of installed radio access nodes; for
instance, outdoor small cells are more likely to be implemented as hot-spots in highly populated areas. More,
transmission distances to be addressed and the number of hops to the nearest fiber-enabled PoP differ per area. In the
following clauses, scenarios with regard to dense urban, urban, sub-urban, semi-rural and rural areas are examined.
As with the previous 3GPP generations, it is predicted that service providers will start with an early stage of deployment
and they will progressively move towards long-term maturity, hence mobile access sites are going to be gradually
upgraded to 5G configurations that will appear in different flavours and iterations of standards. This view refers not
only to the number of installed 5G sites, but also how advanced each 5G configuration will be over time. For example,
more capacity demanding configurations could be part of later deployments due to higher bands (> 6 GHz) spectrum
availability. For this reason, the terms of 5G "Early Stage" and "Mature Stage" are used later in the document.
4.2 5G Radio Access Configurations
Apart from the foreseen benefits, the future 5G radio access configurations will obviously have major impact on the 5G
backhaul/X-Haul networks. Radio access configuration for 5G is dependent on a number of critical parameters,
including:
• market requirements and use cases
• technology capabilities and availability
• standardization development
• regulatory and spectrum issues
• areas of deployment and network infrastructure readiness
Table 1 presents indicative sites configurations based on ETSI TR 138 913 [i.4] and as per ETSI mWT ISG view.
Table 1: 5G Access Sites Configurations
Sites Configurations
Area Type Cell Type
(indicative)
• LTE 50-100 MHz (Macro-cell)
• NR 100 MHz 16L MIMO ~4 GHz (Macro-cell) • Macro-cell
Dense Urban ('DU')
• NR 100 MHz 4L MIMO ~4 GHz (Small-cell)
• Small-cell
• NR ≤ 800 MHz 4L MIMO ~30 GHz
• LTE 50-100 MHz
Urban ('U')
• NR 100 MHz 16L MIMO ~4 GHz • Macro-cell
• NR ≤ 800 MHz 4L MIMO ~30 GHz
Sub-Urban ('SU') • LTE 50-100 MHz
• Macro-cell
Semi-Rural ('SR') • NR 100 MHz 8L MIMO ~4 GHz
• LTE 50-100 MHz
Rural ('R')
• NR 50 MHz 4L MIMO ~2 GHz • Macro-cell
• NR 20 MHz 4L MIMO ~700 MHz
It is assumed that each macro-cell site consists of three (3) sectors, serving 5G and 4G services, whilst small-cells,
namely, outdoor pico-cell sites, are assumed as 5G NR only single-sector radio access nodes. It is also worthwhile to
indicate the 5G bandwidth as being mostly unpaired for TDD operation as opposed to the LTE case, where spectrum is
usually paired (FDD operation).
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4.3 5G RAN Architecture Options
Development of 5G RAN network architecture can follow different paths in future as a "cloud concept" gains increasing
traction in the standardization organizations and in the industry [i.5]. Today, the vast majority of macro mobile access
deployments is based on distributed architecture, nonetheless, as per 3GPP TR 38.801 [i.6], the functional split between
CU and DU (figure 3) is discussed.
Specifically:
• Option 2: HLS, assumed to be in the F1 interface between CU and DU
• Option 7: LLS, assumed to be in the F2 interface between DU-bb and (Remote)RU and eCPRI (inside of
which three possible inner splits are considered, two for downlink and one for uplink)
High- Low- High- Low- High-
RRC PDCP Low-PHY RF
RLC RLC MAC MAC PHY
Data Option 1 Option 2 Option 4 Option 5 Option 6 Option 7 Option 8
Option 3
High- Low- High- Low- High-
RRC PDCP Low-PHY RF
RLC RLC MAC MAC PHY
Data
Figure 3: 3GPP Function Split Options
Figure 4: eCPRI Inner Splits
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Furthermore, figure 5 shows a high-level overview of backhaul and Centralized RAN (High-Layer and Low-Layer
Split) architectural options that can co-exist network-wide. In the typical D-RAN architecture, each gNB is located at
the RF site and it is connected to the radio core (EPC, NGC) via S1/NG interfaces. Alternatively, C-RAN architecture
discusses the decomposition of conventional RAN functions. For instance, by disaggregating gNB functions, two new
RAN entities appear, CU and DU. In order to enable optimal radio network coordination and to realize the benefits of
virtualisation, CU is targeted to be placed in a (more) central location, whilst DU could remain at the RF site (HLS
option) or could be also moved to a more central location (LLS option), e.g. co-located with CU (DU-bb). As stated
above, new X-Haul interfaces between CU and DU (i.e. F1 HLS) and between DU-bb and (R)RU (i.e. F2 LLS) are
under discussion, whilst S1/NG interfaces are still employed for the connection between CU and core network.
Figure 5: Distributed and Centralized RAN Architectures
Depending on the layer of the functional split, the new X-Haul interfaces have different requirements in terms of
capacity and latency. As the splits move away from the RF split as per the legacy CPRI (i.e. option 8 per 3GPP, E in
eCPRI), the requirements become less stringent. As an example, a split approach could be HLS, where the Layer 3
functions and the nRT Layer 2 functions of the BBU are placed in the CU entity and separated from the lower layer
functions (RT Layer L2, Layer 1) that are kept within the DU entity.
The present document concentrates on HLS interface Option 2 in-line with 3GPP primary focus (Release 15). LLS
requirements are left for later releases of this report due to the absence of consensus on the specific functional split by
the industry. Note that traditional backhaul and F1-based X-Haul (namely, Option 2 HLS) present equivalent capacity
and latency requirements.
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5 Wireless Backhaul/X-Haul Network Overview
5.1 Backhaul/X-Haul Network Topologies
Mobile backhaul network topologies generally evolve over time with the aim of optimizing overall network capacity
and latency performance. The densification of radio access network and increased fiber penetration, which reaches
closer to the access network are the on-going trends. Such factors result in a growth in the number of radio links and a
transformation from daisy-chain (relay) connections to star network topologies with shortened backhaul chains, as
illustrated in figure 6.
Figure 6: Macro-Cell Backhaul Network Topologies Shift
The distance of the wireless backhaul link from its nearest fiber PoP site differs per area of deployment and this
situation usually varies across different networks. For the sake of simplicity, the following practical assumptions are
taken into account when classifying deployment scenarios:
• Dense Urban/Urban Areas: A single-hop distance from the closest fiber PoP site.
• Sub-Urban/Semi-Rural Areas: Up to two-hop distance from the closest fiber PoP site.
• Rural Areas: Up to three-hop distance from the closest fiber PoP site.
Since each hop carries traffic closer to the fiber site, it is conventionally assumed that each hop enters into an area, which
is denser (in terms of mobile sites) than the previous one.
Moreover, the radio propagation conditions are different between macro-cell and small-cell deployment layers. Whilst
the LOS connectivity is generally preferred which can be largely achieved for macro-cell backhaul, this cannot be
guaranteed for small cell backhaul use cases. Small cells, namely outdoor operator-managed pico-cells, could be installed
at a relatively low height above ground (for example, up to 6 m hei
...