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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
NG-RAN and 5G-NR describes the deployment of 5G NSA (non standalone 5G) and 5G-SA (standalone 5G). 5G-NSA deals with radio access entities. For the 5G-NSA mode, dual MR DC connectivity is based on radio measurements, allowing the master 4G base station MeNB to add or remove a secondary 5G node SgNB. This book describes the architecture of the NG radio access network and the 5G-NR radio interface according to the 3GPP (3rd Generation Partnership Project) specifications. The overall architecture of the NG-RAN, including the NG, Xn and F1 interfaces and their interaction with the radio interface, are also described. The 5G-NR physical layer is mainly connected by implementing antennas, which improves transmission capacity. 5G-SA deals with the 5G Core network. In the 5G-SA model, the mobile is attached to the 5G Core network through NG-RAN. The book explains radio procedure, from switching on a device to establishing a data connection, and how this connection is maintained even if mobility is involved for both 5G-SA and 5G-NSA deployment. NG-RAN and 5G-NR is devoted to the radio access network, but mobile registration, establishment procedures and re-establishment procedures are also explained.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 NG-RAN Network – Functional Architecture
1.1. Functional architecture NSA/SA
1.2. Description of the NG-RAN network
1.3. Functional separation between the NG-RAN radio interface and the 5G core network
1.4. Scheduling and QoS
1.5. Security architecture
1.6. Network slicing
1.7. References
2 NG-RAN Network – Protocol Architecture
2.1. The protocol architecture of the radio interface
2.2. Procedures on the radio network access
2.3. Identities of the XnAP and NG-AP application protocols
2.4. References
3 NG-RAN Network – Procedures
3.1. General procedure of the 5G-NSA mode
3.2. General procedures of the 5G-SA
3.3. References
4 5G-NR Radio Interface – The Physical Layer
4.1. 5G-NR radio interface
4.2. TDD mode configurations
4.3. Physical resource
4.4. Physical channels and physical signals
4.5. Downlink transmission
4.6. Transmission in uplink
4.7. References
5 5G-NR Radio Interface – Operations on the Frequency Bands
5.1. Operations on the frequency bands
5.2. Carrier aggregation
5.3. Supplementary UpLink (SUL)
5.4. Synchronization on the secondary cell
5.5. References
6 5G-NR Radio Interface – MIMO and Beamforming
6.1. Multiplexing techniques
6.2. Antenna port
6.3. Uplink Control Information (UCI)
6.4. PDSCH transmission
6.5. PUSCH transmission
6.6. Beamforming management
6.7. References
7 5G-NR Radio Interface – Bandwidth Part
7.1. Bandwidth part
7.2. CORESET
7.3. BWP switching procedure
7.4. References
8 5G-NR Radio Interface – Data Link Layer
8.1. SDAP protocol
8.2. PDCP
8.3. RLC protocol
8.4. MAC protocol
8.5. References
9 5G-NR Radio Interface – Radio Access Procedure
9.1. System information
9.2. Connection management
9.3. Measurement configuration
9.4. References
Index
End User License Agreement
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1.
Deployment in the SA mode
Figure 1.2.
NSA configuration options
Figure 1.3.
Secondary node addition – option 3
Figure 1.4.
NE-DC architecture – option 4
Figure 1.5.
NE-DC architecture – option 7
Figure 1.6.
NG-RAN general architecture
Figure 1.7.
The functional separation between NG-RAN and 5GC
Figure 1.8.
The fields of the SUCI identifier
Figure 1.9.
The fields of the 5G-GUTI identifier
Figure 1.10.
QFI management in the user’s plane
Figure 1.11.
Security architecture
Figure 1.12.
Ciphering and integrity
Chapter 2
Figure 2.1.
Protocols on the 5G interface
Figure 2.2.
Processing of IP packet in the DataLink layer
Figure 2.3.
The structure of the radio interface
Figure 2.4.
The protocol stack of the Xn interface
Figure 2.5.
Interfaces between gNB entities and gNB-CU/gNB-DUs
Figure 2.6.
Virtualization of radio access
Figure 2.7.
NG-RAN architecture: distributed and centralized
Figure 2.8.
gNB-CU and gNB-DU functional decomposition options
Figure 2.9.
Functions of the physical layer
Figure 2.10. Protocol stack of the control plane between the mobile and SMF func...
Chapter 3
Figure 3.1.
Random access with contention
Figure 3.2.
Random access without contention, in case of handover
Figure 3.3.
Procedure for establishing a DRB
Figure 3.4. Procedure to add a secondary node. For a color version of this figur...
Figure 3.5. Procedure initiated by the eNB to change the secondary node. For a c...
Figure 3.6.
Removing a secondary node initiated by the eNB entity
Figure 3.7. Access radio procedure and beam management. For a color version of t...
Figure 3.8.
Establishment procedure for the RRC connection
Figure 3.9. Registration procedure: authentication and the NAS secure mode. For ...
Figure 3.10. Registration procedure – service access and registration. For a col...
Figure 3.11.
The NG-C interface configuration procedure
Figure 3.12.
Procedure for updating the AMF function
Figure 3.13. The registration procedure option A: selection and reallocation to ...
Figure 3.14. The registration procedure – option B: selection and reallocation o...
Figure 3.15.
The procedure for establishing a PDU session
Figure 3.16. Service continuity. For a color version of this figure, see www.ist...
Chapter 4
Figure 4.1.
OFDM modulation on three subcarriers
Figure 4.2.
5G-NR multiplexing techniques
Figure 4.3.
Time structure of the 5G-NR frame
Figure 4.4.
TDD configuration example
Figure 4.5.
Resource grid
Figure 4.6.
Common resource block and physical resource block
Figure 4.7.
PSS signal generation
Figure 4.8.
SSB block
Figure 4.9.
CSI-RS multiplexing multi-ports
Figure 4.10.
Three examples of the DM-RS signal and mapping on antenna port 0
Figure 4.11.
Additional DM-RS mapping on antenna port 0
Figure 4.12.
Mapping of DM-RS in the frequency domain
Figure 4.13. Multi-port mode (4 DM-RS) in the case of type 1 single-symbol confi...
Figure 4.14.
Multi-port double-symbol (Table 7.4.1.1.2-1/2 TS 38.211)
Figure 4.15.
The occasion of the PRS signal
Figure 4.16.
The mapping of PRS
Figure 4.17.
The PDDCH channel in a slot and at PRB
Figure 4.18.
The allocation of the PDSCH message in the time domain (SLIV)
Figure 4.19.
SRS signal mapping (one or two symbols)
Figure 4.20. The DM-RS signal mapping associated with the short and long PUCCH c...
Figure 4.21. The mapping of the DM-RS reference signal associated with short and...
Figure 4.22.
The NR PUCCH format associated with DM-RS
Chapter 5
Figure 5.1. 5G coverage with DC, CA and SUL (Ericsson). For a color version of t...
Figure 5.2. The bandwidth of CA channels (extract from TS 38.101 Figure 5.3A.2-1...
Figure 5.3. Definition of the sub-block bandwidth for a non-contiguous intra-ban...
Figure 5.4.
Aggregation of component carriers
Figure 5.5. SUL mode. For a color version of this figure, see www.iste.co.uk/lau...
Figure 5.6. L1/L2 control signaling. For a color version of this figure, see www...
Figure 5.7. Carrier aggregation procedure. For a color version of this figure, s...
Figure 5.8. The transmission of acknowledgments on two PUCCH channels. For a col...
Figure 5.9. SUL mode with coexistence LTE and without coexistence. For a color v...
Chapter 6
Figure 6.1.
SU-MIMO mechanism
Figure 6.2.
MU-MIMO mechanism
Figure 6.3.
Beamforming
Figure 6.4. Beamforming in horizontal and vertical planes (source: NTT Docomo Te...
Figure 6.5.
Active antennas
Figure 6.6.
Mapping models between TXU/TRU units and the antenna elements
Figure 6.7.
Beamforming architecture
Figure 6.8.
Antenna configuration
Figure 6.9.
Single panel and sub-array panel
Figure 6.10.
The structure of multi-panels
Figure 6.11.
MIMO transmitter scheme
Figure 6.12.
Massive-MIMO transmission chain
Figure 6.13.
Single CSI-RS and multiple CSI-RS methods
Figure 6.14. The relationship between the number of lobes, the beamwidth and the...
Figure 6.15.
SSB block on the temporal domain
Figure 6.16.
SSB position relative to PRB
Figure 6.17.
Beam selection procedure
Chapter 7
Figure 7.1.
Bandwidth part
Figure 7.2. Remaining minimum information system. For a color version of this fi...
Figure 7.3. Data structure of MIB. MSB: Most Significant Bit, LSB: Least Signifi...
Figure 7.4. CORESET#0 configuration with index 14. For a color version of this f...
Figure 7.5. SIB1 data structure. For a color version of this figure, see www.ist...
Figure 7.6. BWP switching. For a color version of this figure, see www.iste.co.u...
Chapter 8
Figure 8.1. Data link layer protocol. For a color version of this figure, see ww...
Figure 8.2. PDU session. For a color version of this figure, see www.iste.co.uk/...
Figure 8.3. PDU session and QoS flow. For a color version of this figure, see ww...
Figure 8.4.
SDAP frame structure
Figure 8.5.
Header compression. AMR: Adaptive Multi-Rate
Figure 8.6.
PDCP operations relating to the SRB bearer
1
Figure 8.7.
PDCP operations relating to the DRB bearer
Figure 8.8. NR-PDCP frame structure. For a color version of this figure, see www...
Figure 8.9.
Operating mode of the RLC protocol. MBMS: Multicast Broadcast
Figure 8.10.
TM mode operations
Figure 8.11.
UM mode operations
Figure 8.12.
AM mode operation
Figure 8.13.
RLC frame structure – UM mode
Figure 8.14.
RLC structure of frame – AM mode
Figure 8.15.
Structure of an RLC frame – AM mode
Figure 8.16.
RLC protocol control message
Figure 8.17.
MAC operation gNB side
Figure 8.18.
MAC Operation: UE side
Figure 8.19.
Structure of MAC frame (L in octet)
Figure 8.20.
MAC RAR structure of frame
Chapter 9
Figure 9.1. PBCH physical channel. For a color version of this figure, see www.i...
List of Tables
Chapter 1
Table 1.1.
RRC mobile states
Table 1.2.
5G QoS characteristics
Chapter 2
Table 2.1.
XnAP elementary procedures – Class 1
Table 2.2.
XnAP elementary procedures – Class 2
Table 2.3.
F1 elementary procedures – Class 1
Table 2.4.
F1 elementary procedures – Class 2
Table 2.5.
NG-AP elementary procedures – Class 1
Table 2.6.
NG-AP elementary procedures – Class 2
Chapter 4
Table 4.1.
Frequency band FR1 5G-NR
Table 4.2.
Frequency band FR2 5G-NR
Table 4.3.
FFT length according to the 5G bandwidth and SCS
Table 4.4.
Structure of the NR frame in the time domain
Table 4.5.
Prefix cyclic per numerology
Table 4.6.
The structure of the resource grid in the temporal domain
Table 4.7.
SSB mapping
Table 4.8.
CDM multiplexing for multi-port CSI-RS
Table 4.9.
Frequency offset
Table 4.10.
Radio identifier associated with the DCI format
Table 4.11. The allocation of PDSCH in the time domain (TS 38.214, Table 5.1.2.1...
Table 4.12. The length of RBG in function of the bandwidth (TS 38.214, Table 5.1...
Table 4.13.
The structure of a long PRACH sequence
Table 4.14.
The structure of short PRACH sequences
Table 4.15.
PUCCH format
Chapter 5
Table 5.1.
Carrier aggregation class on band FR1
Table 5.2.
List of contiguous intra-bands for carrier aggregation FR1
Table 5.3.
Configuration for contiguous intra-band carrier aggregation
Table 5.4.
Non-contiguous intra-band aggregation carrier in the FR1 band
Table 5.5.
Configuration of non-contiguous carrier aggregation intra-band
Table 5.6.
Inter-band carrier aggregation
Table 5.7.
Carrier aggregation of the FR2 band
Table 5.8.
Carrier aggregation intra-band in FR2
Table 5.9.
Contiguous intra-band carrier aggregation in the FR2 band
Table 5.10.
Maximum spacing of CC radio channels
Chapter 6
Table 6.1.
Comparison between MIMO and massive-MIMO mechanisms
Table 6.2. Mapping of antenna ports and reference signals in downlink transmissi...
Table 6.3.
Numbering of the antenna port in uplink transmission
Table 6.4.
Codebook for type 1/type 2 single-panel
Table 6.5.
Codebook for type 1 multi-panel
Table 6.6.
SSB characteristics
Table 6.7.
Frequency search configuration
Chapter 7
Table 7.1.
Extract from Table 13.4 TS 38.213
Table 7.2.
Radio configurations
Chapter 8
Table 8.1.
Compression protocols
Table 8.2.
LCID field values for the DL-SCH transport channel
Table 8.3.
LCID field values for the UL-SCH transport channel
Table 8.4.
LCID field values for the MCH transport channel
Chapter 9
Table 9.1.
Support of message relating to system information
Table 9.2.
MIB and SIB information
Table 9.3.
The support of the message relating to paging
Table 9.4.
Support for messages relating to the connection establishment
Table 9.5.
Support for messages relating to the activation of security
Table 9.6. Support for messages relating to the reconfiguration of the connectio...
Table 9.7.
Support for messages relating to connection re-establishment
Table 9.8.
Support for messages relating to the connection release
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NG-RAN and 5G-NR
5G Radio Access Network and Radio Interface
Frédéric Launay
First published 2021 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd
27-37 St George’s Road
London SW19 4EU
UK
www.iste.co.uk
John Wiley & Sons, Inc.
111 River Street
Hoboken, NJ 07030
USA
www.wiley.com
© ISTE Ltd 2021
The rights of Frédéric Launay to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
Library of Congress Control Number: 2021935849
British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78630-628-9
Preface
This book aims to describe the deployment of 5G-NSA and 5G-SA.
For the 5G-NSA mode, dual MR-DC connectivity is based on radio measurements allowing the master 4G base station MeNB to add or to remove a 5G secondary node SgNB.
For the 5G-SA mode, the mobile is attached to the 5G core network. Even though this book is devoted to the radio access network, the mobile registration, establishment procedures and re-establishment procedures are also explained.
This book describes the architecture of the NG radio access network and the 5G-NR radio interface according to 3GPP (3rd Generation Partnership Project) specifications.
The 5G-NR radio interface was introduced in Release 15 of the 3GPP standards.
The 5G-NR radio interface is the interface between the UE mobile (User Equipment) and the 5GC mobile network (5G Core).
Chapter 1 presents the 5G-NSA and the 5G-SA deployment architecture. It describes the radio access entities and functions of the 5G-SA core network and presents the functions supported by the radio access and the 5GC core network.
In order to prepare RAN virtualization, the radio protocols of the gNB entity are supported by two units: a centralized unit and a distributed unit. This separation creates new interfaces called F1 on the radio access.
Chapter 2 presents the interfaces between the entities of NG-RAN radio access. It describes the protocol architecture on the Xn interface, the NG interface and the F1 interface, as well as the procedures for managing the interfaces of the NG-RAN network.
Similar to 4G CUPS (Control User Plane Separation) evolution, the 5GC network architecture separates the control plane to the user plane. The 5G-NSA and the 5G-SA architectures will be described by detailing interfaces and applications between the radio access entities and between the radio access and the network core (NG interfaces).
Chapter 3 presents general procedures for 5G-NSA and 5G-SA modes. It describes the procedures on the radio access network concerning the cell search, the bearer establishment, the mobility management, the management of the secondary node, the beam management, as well as the procedures to the core network such as attachment and session establishment.
5G-NR spectrum allocation is more flexible compared to 4G LTE. In the time domain, frame flexibility is improved with the introduction of new numerologies. In the frequency domain, flexibility is achieved with bandwidth partitioning allocated to the terminals.
In order to avoid future development constraints, the management of the radio band is refined. Chapter 3 describes the reference signals, physical signals, physical channels and their allocation on the 5G bandwidth.
5G improves the user’s throughput with active antenna technologies. Spatial multiplexing improvements are presented in both FR1 and FR2 bands. This chapter describes single- and multi-panel antennas, which require new measurements of the CSI radio channel.
To achieve 20 Gbps in the downlink direction and 10 Gbps in the uplink direction, the 5G-NR radio interface needs the following characteristics:
– the maximum radio channel bandwidth is 100 MHz for the FR1 band and 400 MHz for the FR2 band;
– carrier aggregation allows us to extend the channel bandwidth to 1 GHz;
– 256-QAM (Quadrature Amplitude Modulation);
– MIMO (Multiple Input Multiple Output) 8x8 transmission mode is used in downlink.
Chapter 4 presents the 5G-NR physical layer. It describes uplink and downlink waveform formats, multiplexing modes, the 5G frequency bands and radio frames. Physical signals and physical channels on the uplink and downlink are detailed (uplink physical channels like PRACH, PUSCH, PUCCH and downlink like PBCH, PDSCH, PDCCH). The reference signals are also explained: CSI-RS, DM-RS, PT-RS, RIM-RS. Chapter 4 also details the mapping of physical channels and physical signals on resource elements.
Chapter 5 presents the physical layer mechanisms to improve throughput and coverage. It describes the SUL mode, the carrier aggregation mechanism, dual connectivity and spectrum sharing for 4G/5G coexistence.
Chapter 6 presents MIMO mechanisms used to improve the robustness of the transmission (diversity), the bit rate (SU-MIMO, MU-MIMO) and the reduction of interference (beamforming). It describes the evolution of antenna capabilities from MIMO to massive-MIMO and single-panel or multi-panel transmission mechanisms. The procedure for managing the beams and improving data rate is described with the use of the code book for the uplink and downlink transmissions.
Chapter 7 presents the partitioning of the 5G band. It describes initial band partitioning and BWP band switching, allowing the mobile to camp on a cell and, under the control of the base station, to adapt its radio capabilities to the traffic load. It also describes the search area and CORESET control resource elements.
Chapter 8 introduces the data link layer. It describes the different SDAP, PDCP, RLC and MAC sublayers by presenting their functional role and the services between the different sublayers. For each of the sublayers, the structure of the protocol is detailed.
Chapter 9 presents the messages sent by the RRC layer: broadcasting information, connection management, measurement configuration. It details the MIB and SIB messages, the procedures for establishing the radio link and the mobility of the mobile, as well as the elements to be measured.
A summary of the content of these nine chapters is provided in the following table.
Chapter
Designation
Content
1
NG-RAN network architecture
Functional architecture NSA/SAIdentitiesUE contextgNB-CU/DUQoS and network slicing
2
NG-RAN protocol architecture
Protocol architectureeCPRI functional splittingInterface protocols (Xn, NG, F1)NG-AP and Xn procedure
3
Radio access procedure
Search cellAccess procedureData transmission procedureSession managementSecondary node establishmentBeam managementAttachment
4
Physical layer of the 5G-NR interface
Frequency bandMultiplexing structureReference signals: PSS, SSS, CSI-RS, PT-RS, RIM-RSPhysical signals: PSS, SSSPhysical channels: PBCH, PDCCH, PDSCH, PMCH, PUSCH, PUCCH, PRACH
5
Operations on frequency bands
Carrier aggregationSUL modeDual connectivity
6
Multi-antenna structure
SU-MIMOMU-MIMOBeamformingMassive-MIMOAntenna portChannel measurement reportsBeam management
7
Bandwidth partition
BWP initialCORESET#0BWP switching
8
Data link layer
SDAP protocolPDCP protocolRLC protocolMAC protocolStructure protocol
9
RRC protocol
NG-RAN procedureInformation systemsConnection controlMeasurementsBroadcasting control
April 2021
1NG-RAN Network – Functional Architecture
1.1. Functional architecture NSA/SA
Unlike previous generations of mobile networks, the deployment of 5G does not require the simultaneous implementation of the 5G core network (5GC) and the NG-RAN (Next-Generation Radio Access Network).
NSA (Non-Standalone Access) and SA (Standalone Access) are two 5G network models:
– SA is a completely new core service-based architecture: each radio node is autonomously controlled by the 5G core network. A service-based architecture delivers services as a set of NFs (Network Functions). NFs in the 5G core network are cloud native;
– NSA relies either on the 4G core network or on the 5G core network. NSA anchors the control signaling to the core network through a radio MN (Master Node). The MN is either a 4G radio node or a 5G radio node. The MN controls an SN (Secondary Node) (4G radio node or 5G radio node) according to the DC (Dual Connectivity) mechanism.
The 5G-SA architecture requires the deployment of a 5G core network connected to the NG-RAN.
The 5G-NSA architecture and the 5G-SA architecture were introduced in Release 15 of the 3GPP standard.
The 5G-NSA configuration implements the MR-DC (Multi Radio Dual Connectivity) architecture.
Figure 1.1.Deployment in the SA mode
Dual connectivity involves two RAN nodes, i.e. master node (MN) and secondary nodes (SN) which has the following features:
– the MN is connected to the core network for the control plan (signaling) and for the user plane;
– the SN is controlled by the MN. It is connected to the MN for the control plane (C-plane). The user plane (U-plane) is either connected to the MN or connected to the core network;
– the master radio access node controls the secondary radio access node and establishes a bearer, if necessary, for the exchange of data between the two radio nodes.
Dual connectivity defines the “Master Cell Group (MCG) bearer” and the “Secondary Cell Group (SCG) bearer”.
The MCG carries data that will be transmitted on the radio resources allocated by the MN. In the case of carrier aggregation, the MN supports data on the PCell (Primary Cell) and SCells (Secondary Cells).
The SCG carries data that will be transmitted on the radio resources allocated by the SN. In the case of carrier aggregation, the SN supports data on the PCell and SCell.
The split bearer consists of routing the traffic between the MN and the SN. According to the U-plane termination, the split bearer consists of splitting either the MCG bearer or the SCG bearer.
For E-RABs configured as “MCG bearers”, the U-plane termination point is located at the MN.
For E-RABs configured as “SCG bearers”, the U-plane termination point is located at the SN.
For the core network configuration, each support (MCG, SCG, split bearer) can end on the MN and/or on the SN. The split bearer is transparent for the core network entities.
Several deployment scenarios (Figure 1.2) have been defined for the 5G-NSA:
– option 3: the E-UTRAN access network is connected to the 4G core network. The master node is the 4G radio node (
eNB – evolved Node B
). The secondary node is the 5G radio node (en-gNB). The MR-DC architecture is called
EN-DC
(
E-UTRAN NR Dual Connectivity
);
– option 4: the NG-RAN access network is connected to the 5G core network. The master node is a 5G radio node (
gNB – next generation Node Base Station
). The secondary node is a 4G radio node (ng-eNB). The MR-DC architecture is called
NE-DC
(
NR – E-UTRAN Dual Connectivity
);
– option 7: the NG-RAN access network is connected to the 5G core network. The master node is a 4G radio node (
ng-eNB – next generation eNB
). The secondary node is a 5G radio node. The MR-DC architecture is called
NGEN-DC
(
NG-RAN E-UTRAN NR Dual Connectivity
).
Figure 1.2.NSA configuration options
1.1.1. Option 3
Option 3 is the non-standalone EN-DC configuration.
Option 3 uses the MN (Master Node) terminated MCG (Master Cell Group) bearer for signaling: the eNB is the master node, and the gNB (gNodeB) acts as the secondary node. The radio access network is connected to EPC.
The 4G base station (eNB) controls the 5G base station (en-gNB) through the X2 interface.
The eNB supports signaling with the MME (Mobile Management Entity) through the S1-MME interface and supports the user plane traffic (MCG bearer) with the SGW (Serving Gateway) entity through the S1-U interface.
The en-gNB base station supports signaling with the eNB. The 5G-NR interface is activated by the eNB over the X2 interface. Once activated, en-gNB controls its own radio resource allocation. The user traffic is either transmitted from the eNB to en-gNB or transmitted from the 4G core network (SGW) over the S1-U interface to en-gNB.
The master node eNB exchanges data in both directions, uplink and downlink, with the mobile.
The secondary node en-gNB allows us to increase both uplink and downlink data rates.
With a DC mechanism, data is transmitted to the mobile according to one of the following variations (Figure 1.3):
– option 3: in plain option 3, all uplink and downlink data flows to and from the LTE part (
MCG split bearer
) of the LTE/NR base station, i.e. to and from the eNB. The eNB then decides which part of the data it wants to forward to the 5G gNB part of the base station over the Xx interface;
– option 3a: both LTE eNG (
MCG bearer
) and 5G-NR en-gNB (
SCG bearer
) exchange traffic to the 4G core network directly. This means that a data bearer allocated to a node cannot share its load over the second node. This option does not suit the case of mobile use;
– option 3x: user data traffic will directly flow to the 5G gNB part of the base station (
SCG split bearer
). The traffic is delivered over the 5G-NR interface to the device, and part of the data can be forwarded over the X2 interface to the 4G eNB.
Figure 1.3.Secondary node addition – option 3
Option 3 uses the MN terminated MCG bearer for user traffic. The eNB entity splits the S1 bearer into:
– LTE radio support;
– NR support.
Option 3x uses the SN terminated SCG bearer for user traffic. The gNB entity splits the S1 bearer into:
– LTE radio support;
– NR support.
1.1.2. Option 4
Option 4 relies on the 5G core (5GC).
The gNB acts as an MN; it supports signaling exchange (MCG signaling bearer) with the 5G core network’s transport plane through the NG-C interface. The LTE user plane connections go via the 5G-NR through the NG-U interface.
The ng-eNB base station acts as a secondary node. It is controlled by the gNB base station through the Xn-C interface. The ng-eNB is a new generation of the 4G base station.
The gNB controls the ng-eNB through the Xn interface.
The data is transmitted to the ng-eNB entity via one of the following options (Figure 1.4):
– from the master node gNB, which performs the split bearer (option 4,
MN terminated split bearer
);
– from the 5GC network (option 4a,
SCG bearer
).
Figure 1.4.NE-DC architecture – option 4
1.1.3. Option 7
Option 7 relies on the 5G core (5GC).
The ng-eNB acts as an MN; it supports signaling (MCG signaling bearer) with the 5GC core network’s transport plane through the NG-C interface and exchanges data to the 5G core network’s user plane through the NG-U interface.
The gNB base station acts as an SN. It is controlled by the ng-eNB base station via the Xn-C interface.
The ng-eNB (4G base station) controls the gNB through the Xn interface.
The data is transmitted to the gNB entity via one of the following options (Figure 1.5):
– from the ng-eNB base station, which performs a split bearer. This is option 7 (
MN terminated split bearer
);
– from the 5G core network. This is option 7a (
SCG bearer
).
Figure 1.5.NE-DC architecture – option 7
1.2. Description of the NG-RAN network
The NG-RAN provides both NR and LTE radio access.
An NG-RAN node is either a gNB (5G base station), providing NR user plane and control plane services, or an ng-eNB (new generation 4G base station) providing the LTE/E-UTRAN services towards the UE (control plane and user plane).
The NG-RAN ensures the connection of mobiles and the reservation of radio resources between:
– the mobile and the ng-eNB base station on a single 4G carrier (LTE) or on several 4G frequency carriers (LTE-Advanced);
– the mobile and the gNB base station on one or more 5G frequency bands (5G-NR).
The gNBs and ng-eNBs are interconnected through the Xn interface. The gNBs and ng-eNBs are also connected, via NG interfaces, to the 5G core (5GC).
The NG interface is the point of reference between the NG-RAN and the 5G core network:
– the NG-C interface is the interface between the radio node and the AMF (Access and Mobility Management Function). It supports signaling via NG-AP (Next Generation Application Protocol);
– the NG-U interface is the interface between the radio node and the UPF (User Plane Function) for tunneling traffic (the IP packet) via GTP-U (GPRS Tunneling Protocol).
The UPF is configured by the SMF (Session Management Function) under the control of the AMF.
Figure 1.6.NG-RAN general architecture
The mobile exchanges data with the DN (Data Network) through logical connections called PDU (Protocol Data Unit) sessions. This logical connection is divided into two parts:
– the NG-RAN ensures the connection of the mobiles with the base station and interconnects the control plane and user plane (traffic) of the mobile UE with the core network;
– the 5G core network interconnects the NG-RAN, provides the interface to the DN, ensures the registration of mobiles, the monitoring of their mobility and the establishment of data sessions with the quality of the corresponding QoS (Quality of Service).
1.2.1. The NG-RAN
The NG-RAN provides both an LTE radio interface and a 5G-NR radio interface.
An NG-RAN node is:
– a 5G base station (gNB), which provides the control plane services and the transmission of user plane data through the 5G-NR radio interface;
– an advanced 4G base station (ng-eNB), providing control plane services and data transmission from the user plane to mobiles via the LTE radio interface.
The NG-RAN node is responsible for managing radio resources, controlling the radio bearer establishment of the user plane and managing mobility during the session (handover). The mobile connects to one of the radio nodes.
The NG-RAN node transfers the traffic data from the mobile to the UPF and data from the UPF to the mobile.
When the NG-RAN node receives data from the mobile or from the UPF, it refers to the QFI (QoS Flow Identifier) for the implementation of the data scheduling mechanism.
For outgoing data to the UPF entity, the NG-RAN node performs the marking of the DSCP (DiffServ Code Point) field of the IP (Internet Protocol) header, based on the assigned QFI.
The NG-RAN node performs compression and encryption of traffic data on the radio interface. It can also optionally perform the integrity control of the traffic data exchanged with the mobile.
The NG-RAN node performs the encryption and integrity control of the signaling data exchanged with the mobile on the radio interface.
The NG-RAN node performs the selection of the AMF. The AMF is the function of the core network to which the mobile UE is attached.
The NG-RAN broadcasts the RRC paging received from the AMF.
The NG-RAN node also broadcasts the cell’s system information, containing the radio interface characteristics. The devices use these parameters for cell selection and for radio bearer establishment requests.
When a mobile is connected, the NG-RAN uses the measurements made by the mobile to decide on the initiation of a cell change during a session (handover).
In order to manage the services for each connected mobile, the NG-RAN node maintains a UE context information block relating to each mobile. The information saved by the radio node may depend on the mobile usage.
The mobile is either in the RRC connected state (RRC_CONNECTED), the RRC inactive state (RRC_INACTIVE) or the standby RRC state (RRC_IDLE).
When the mobile enters the standby state, the base station is not aware of its presence. Each mobile in the standby state listens to the information broadcasts by the radio node.
There is no UE context at the radio node for the mobile in the RRC_IDLE state.
When the mobile enters the RRC_CONNECTED state or the RRC_INACTIVE state, a mobile radio identifier C-RNTI or I-RNTI, respectively, (Connected/Inactive Radio Network Temporary Identifier) is saved at the radio node. The context of the UE is saved in relation to the RNTI. The context is recorded at the level of the NG-RAN node, which manages the mobile (source node), and it is transmitted to the target node in the event of a handover. The UE context is also created at the level of the MN and at the level of the SN in the event of dual connectivity.
When a mobile is in the connected mode, the NG-RAN node uses measurements made by the mobile to decide whether to trigger a change of node during the session (handover) or to activate or deactivate secondary cells.
1.2.2. AMF (Access management and Mobility Function)
The AMF (Access management and Mobility Function) supports:
– the registration of the mobile;
– the access control and the management of mobility on both the NG-RAN and Wi-Fi network access (non-3GPP access);
– network slicing.
The mobile and the AMF exchange data using the NAS (Non-Access Stratum) protocol.
The registration function allows the attachment of the mobile, the detachment of the mobile and the update of its location.
During the attachment, the AMF records the TAI (Tracking Area Identity) location and private identity of the mobile and assigns a 5G-GUTI (5G Globally Unique Temporary Identifier) to the mobile.
5G-GUTI replaces the encrypted private identifier SUCI (Subscription Concealed Identifier) and the private identifier SUPI (Subscription Private Identifier).
Once the attachment procedure is completed, the AMF selects the SMF, according to the DNN (Data Network Name) and the network slice indicator NSSAI (Network Slice Selection Assistance Information).
A load balancing procedure is applied when different SMF can be selected.
The DNN is either communicated by the mobile to the AMF during attachment, or retrieved from the subscriber’s profile from the UDR (Unified Data Repository).
The AMF manages a list of TAIs allocated to mobiles, in which the mobile, in the standby state, can move without contacting the AMF to update its location.
The AMF manages the addition and removal of the TNL (Transport Network Layer) association with the entities of the NG-RAN node. In the event of a handover, the source AMF will release the TNL association with the source NG-RAN node and redirect the TNL association to the target NG-RAN node.
1.2.3. SMF (Session Management Function)
The SMF (Session Management Function) is responsible for creating, updating and removing PDU (Protocol Data Unit) sessions and managing session context with the UPF (User Plane Function). The SMF injects routing rules to the selected UPFs.
A routing rule corresponds to an entry in the context table of the UPF. This context table contains four fields:
– a correspondence field (PDR (Packet Detection Rule));
– a routing field NH (next hop: IP address, tunnel number TEID (Tunnel End Identifier) or SR (Segment Routing)) to find the next node;
– the quality of service to be applied to the flow (QER (QoS Enhancement Rules));
– the measurement reports to be applied to the flow (URR (Usage Reporting Rules)).
The SMF is responsible for the session management for each DNN and by network slice (S-NSSAI), based on the user profile stored at the UDR.
When requesting a session to be established, the SMF selects a UPF or queries the NRF (Network Repository Function) to obtain the address of the UPF.
The SMF grants an IPv4 or IPv6 address to the mobile. An IP address is provided for each PDU session, based on the address range of the PSA (PDU Session Anchor) selected to join the IP data network. The address range is obtained by either directly querying the selected UPF or by querying the NRF. If the assigned IPv4 address is a private address, the UPF entity performs NAPT (Network Address and Port Translation) in order to translate the IP address and TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) port numbers.
At the end of the IP session, when the mobile enters the standby state, the SMF releases the session by removing the context at the UPF.
In the event of incoming packets, if the mobile is in the idle state, the SMF sends a notification to the AMF (Downlink Data Notification).
1.2.4. UPF (User Plane Function)
The UPF (User Plane Function) manages the routing of user traffic and implements traffic filtering functions.
The PSA UPF is the traffic gateway connecting the 5GC network to the DN (Data Network). The PSA constitutes the anchor point for inter-UPF mobility.
