Universidade de Aveiro Departamento de Eletrónica,Telecomunicações e Informática 2019
José Miguel
Duarte Domingues
Implementação em FPGA da Cadeia Tx Downlink
de uma DU 5G-NR
FPGA Implementation of a 5G-NR DU Downlink Tx
Chain
Universidade de Aveiro Departamento de Eletrónica,Telecomunicações e Informática 2019
José Miguel
Duarte Domingues
Implementação em FPGA da Cadeia Tx Downlink
de uma DU 5G-NR
FPGA Implementation of a 5G-NR DU Downlink Tx
Chain
Dissertação apresentada à Universidade de Aveiro para cumprimento dos re-quisitos necessários à obtenção do grau de Mestre em Engenharia de Ele-trónica e Telecomunicações, realizada sob a orientação científica do Doutor Arnaldo Silva de Rodrigues Oliveira, Professor auxiliar do Departamento de Eletrónica, Telecomunicações e Informática da Universidade de Aveiro.
o júri / the jury
presidente / president Prof. Doutor Adão Paulo Soares da Silva Professor Auxiliar da Universidade de Aveiro
vogais / examiners committee Prof. Doutor João Paulo de Castro Canas Ferreira
Professor Associado da Faculdade de Engenharia da Universidade do Porto (Arguente)
Prof. Doutor Arnaldo Silva Rodrigues de Oliveira Professor Auxiliar da Universidade de Aveiro (Orientador)
agradecimentos /
acknowledgements Deixo um profundo agradecimento aos meus pais, à minha irmã e à minhafamília por terem estado sempre a meu lado ao longo do meu percurso académico.
A todos os amigos e colegas que fui conhecendo ao longo deste mesmo percurso, com especial menção ao Coutinho, Pedro e Bernardo que durante esta dissertação estiveram sempre a meu lado.
Um especial obrigado ao meu orientador, Professor Doutor Arnaldo Silva Rodrigues de Oliveira, pela confiança depositada e por ter estado sempre disponível para me ajudar.
À Universidade de Aveiro, ao Departamento de Eletrónica, Telecomunica-ções e Informática e ao Instituto de TelecomunicaTelecomunica-ções por fornecerem as condições necessárias de trabalho e aprendizagem.
Este trabalho foi financiado pelo Fundo Europeu de Desenvolvimento Regional (FEDER), através do Programa Operacional Regional de Lisboa (POR LISBOA 2020) e do Programa Operacional Competitividade e Inter-nacionalização (COMPETE 2020) do Portugal 2020 [Projeto 5G com o no
Palavras Chave FPGA, PHY, RAN, 5G, NR, CP-OFDM, DU.
Resumo
A atual geração de comunicações móveis (4G) possibilitou grandes altera-ções no nosso quotidiano, permitindo um enorme crescimento do número de terminais e conexões celulares assim como um grande aumento de tráfego de dados. Este crescimento antecipa o surgimento das redes 5G com uma panóplia crescente de serviços e um elevado número de dispositivos hetero-géneos conectados. Essa nova geração irá proporcionar novos cenários de comunicação, mais desafiantes e complexos que os atuais, designados por URLLC (Ultra Reliable Low Latency Communications), eMBB (enhanced Mo-bile BroadBand) e mMTC (massive Machine Type Communications), em que características como latências muito baixas, débitos extremamente elevados e altas densidades de conexões farão parte desses casos de utilização, que abrirão portas a novas aplicações, como por exemplo a Indústria 4.0.
Devido à necessidade de uma rede cada vez mais eficiente e mais capaz surge assim um novo paradigma em relação às redes de acesso, havendo a necessidade de abandonar a arquitetura tradicional da RAN e alcançar uma solução mais versátil que se possa adaptar a cenários variados. Do 4G (LTE) para o 5G (NR) há diferenças fundamentais ao nível da arquitetura da rede de acesso rádio em que o eNB LTE se dividiu em CU, DU e RU e surgem assim ao longo das cadeias de processamento da RAN a possibilidade de dividir funções entre a CU e a DU, podendo criar assim uma solução diferente de acordo com as necessidades.
Esta dissertação irá focar-se na modelação das funções realizadas pela DU na cadeia de downlink Tx, tendo em conta uma decomposição funcional (split option) 7.1, englobando as funções de modulação de CP-OFDM. Esta modelação será implementada em linguagem de alto nível e posteriormente convertida em RTL para implementação num dispositivo de lógica programá-vel. Para tal irá proceder-se ao estudo de conceitos básicos, arquiteturas e especificações do 5G/NR, especialmente respetivas à camada física. A mo-delação será realizada em Matlab com uso da toolbox 5G para geração da Resource Grid e para comparação de resultados. Posteriormente será utili-zado Simulink para criar uma arquitetura a partir do modelo desenvolvido, ar-quitetura essa que será sintetizada com a ferramenta HDL Workflow Advisor diretamente para RTL de forma a poder ser validado numa FPGA recorrendo ao uso de cores do Integrated Logic Analyzer.
Keywords FPGA, PHY, RAN, 5G, NR, CP-OFDM, DU.
Abstract
The current mobile telecommunications generation (4G) has brought major changes to the society, allowing for a rise both in the cellular connections and in the data traffic. This growth anticipates the emergence of 5G networks with a wide range of services and a greatly increased number of connected devi-ces. This new generation will allow for new scenarios of communication, more challenging and complex than the current current ones, designed as URLLC (Ultra Reliable Low Latency Communications), eMBB (enhanced Mobile Bro-adBand) and mMTC (massive Machine Type Communications), in which spe-cifications such as ultra low latencies, extremely high data rates and high con-nection densities will be part of these scenarios, which will pave the way for new applications like Industry 4.0. The growing need for a more capable and efficient network leads to a new paradigm of Radio Access Network, leaving behind the traditional architecture of the RAN and move towards a more fle-xible architecture capable of being adapted to different scenarios. The eNB from the 4G (LTE) split up into CU, DU and RU in 5G (NR) and along the RAN processing chain there is now the possibility to split functions between the CU and DU, creating this way different solutions according to the needs.
This dissertation focus will be on the modelation of the DU functios on a downlink Tx chain, based on a split option 7.1, embodying the CP-OFDM mo-dulation functions. This modelation will be implemented on an high level lan-guage and will posteriorly be converted into RTL in order to be implemented in a programmable logic device. The major concepts will be studied, along-side the architectures and specifications of 5g/NR, with a major focus on the physical layer. The modelation will be developed in Matlab alongside the 5G Toolbox. Simulink will be used after to develop an architecture of the created Matlab model, which will be synthesized with HDL Workflow Advisor directly onto RTL to be posteriorly validated with Integrated Logic Analyser cores on a FPGA.
Contents
Contents i
List of Figures vii
List of Tables xi
Glossary xiii
1 Introduction 1
1.1 Scope . . . 1
1.1.1 Fourth Generation - Long Term Evolution . . . 2
1.2 Motivation . . . 4
1.2.1 Use Case Scenarios . . . 4
Mission Critical Services . . . . 5
Connecting massive Internet of Things (IoT) . . . . 5
Mobile Broadband Enhanced . . . . 5
1.2.2 Radio Access Network (RAN) . . . 6
1.2.3 Field Progammable Gate Array (FPGA) Implementation . . . 7
1.2.4 High Level Modeling . . . 8
1.3 Objectives . . . 8
1.4 Dissertation Structure . . . 9
2 5G New Radio Overview 11 2.1 Introduction . . . 11
3GPP . . . 11
2.2 Design Principles . . . 11
2.2.1 Spectrum Flexibility . . . 11
2.2.2 Lean Design . . . 12
2.2.3 Interworking and Long Term Evolution (LTE) coexistence . . . 12
2.3 Fifth Generation - New Radio (5G-NR) Key Technology Components . . . 13
2.3.1 Bandwidth parts . . . 13
2.3.2 Duplex Schemes and Dynamic TDD . . . 14
2.3.3 Low Latency Support . . . 15
2.3.4 Scheduling and Data Transmission . . . 15
2.3.5 Control Channels . . . 15
2.3.6 Control resource set (CORESET) . . . 15
2.3.7 Massive Multiple-Input Multiple-Output (MIMO) . . . 15
2.4 Third Generation Partnership Project (3GPP)’s 5G Architecture . . . 16
2.4.1 Deployment options . . . 17
2.4.2 Core Network (CN) . . . 17
2.5 Radio Access Network (RAN) . . . 18
2.5.1 Splitting Architecture . . . 18
2.5.2 Deployment Scenarios . . . 20
2.6 Functional Splits . . . 21
2.6.1 Split option 1 - PDCP/RRC . . . 22
2.6.2 Split option 2 - RLC/PDCP . . . 22
2.6.3 Split option 3 - Intra RLC . . . 22
2.6.4 Split option 4 - RLC/MAC . . . 22
2.6.5 Split option 5 - Intra MAC . . . 23
2.6.6 Split option 6 - MAC-PHY . . . 23
2.6.7 Split option 7.x - Intra PHY . . . 23
2.6.8 Split option 7.3 - (Downlink Only) . . . 24
2.6.9 Split option 7.2 . . . 24
2.6.10 Split option 7.1 . . . 24
2.6.11 Split option 8 . . . 24
2.6.12 Datarates throughout the Different split options . . . 24
2.7 Radio Protocol Architecture . . . 25
2.7.1 Layer 3 Functions . . . 26 2.7.2 Layer 2 Functions . . . 26 2.8 Layer 1 Functions . . . 27 3 Physical Layer 29 3.1 Introduction . . . 29 3.2 PHY Challenges . . . 29
3.3 PHY Key Technologies . . . 30
3.3.1 Modulation . . . 30
3.3.3 Channel Coding . . . 31
3.4 Physical Layer Channels . . . 31
3.4.1 Uplink . . . 31
3.4.2 Downlink . . . 31
Physical Downlink Control Channel (PDCCH) . . . 31
Physical Downlink Shared Channel (PDSCH) . . . 32
Physical Broadcast Channel (PBCH) . . . 32
3.5 Physical Layer Signals . . . 32
CSI-RS . . . 32
DM-RS . . . 32
PT-RS . . . 32
3.6 Waveform, Use of Cyclic Prefix - Orthogonal Frequency Division Multiplexing (CP-OFDM) and Frame Structure . . . 33
3.6.1 Waveform . . . 33
3.6.2 Suitability of Orthogonal Frequency Division Multiplexing (OFDM) . . . 33
3.6.3 Numerology and Scalability . . . 33
3.6.4 SubCarrier Spacing . . . 33 3.6.5 Frame Structure . . . 34 3.6.6 Mini Slots . . . 35 3.6.7 Slot format . . . 35 3.6.8 DC subcarrier . . . 36 3.6.9 Cyclic Prefix . . . 36
3.6.10 Resource Grid, Resource Block and Resource Element . . . 37
Resource Element . . . 37
Resource Block . . . 37
Resource Grid . . . 38
3.7 Channel Bandwidth and Utilization of Spectrum . . . 38
3.8 Operating bands and Transmission Bandwidth Configuration NRB . . . 39
3.9 Initial Access . . . 40
3.9.1 Cell Search . . . 40
3.9.2 SS/PBCH Block . . . 40
3.9.3 Location of a SS/PBCH Block . . . 41
3.9.4 Beam Sweeping . . . 42
3.9.5 Types SS Block transmission . . . 42
3.9.6 Random Access . . . 42
4 Matlab Modeling And Simulation 45 4.1 Introduction . . . 45
4.2 5G Toolbox - Matlab . . . 46
4.2.1 Channels and Signals . . . 46
4.2.2 Waveform and Carrier Configuration . . . 46
4.2.3 SSBurst . . . 47
4.2.4 CORESET and Search Space Configuration . . . 47
4.2.5 Generated Waveform . . . 47
4.2.6 Bandwidth Parts . . . 48
4.2.7 Physical Resource Blocks (PRB) Allocation . . . 48
4.2.8 Resource Grid Generation . . . 49
4.3 Processing Chain . . . 49
4.3.1 Padding and IFFTShifting . . . 50
4.3.2 Inverse Fast Fourier Transfom (IFFT) . . . 51
4.3.3 Cyclic Prefix (CP) addition . . . 52
4.3.4 SSBurst Support . . . 52
4.3.5 Solution for single numerology . . . 53
4.3.6 Multiple Bandwidth part (BWP)s . . . 54
4.3.7 Hardware Description Language (HDL) Coder support for Matlab . . . . 56
5 Simulink Modeling and Simulation 59 5.1 Introduction . . . 59
5.2 Simulink Overview . . . 60
5.2.1 Simulink persistent variables . . . 60
5.2.2 Time Samples . . . 61
5.2.3 Proprietary Library . . . 61
5.2.4 Fixed point data type . . . 61
5.3 Run-time configurable architecture . . . 62
5.4 Processing chain . . . 62
5.4.1 Padding and shifting block . . . 63
5.4.2 Results . . . 64
5.4.3 Variable size IFFT block . . . 65
IFFT supported lengths . . . 66
Results . . . 67
5.4.4 CP addition blocks . . . 67
Results . . . 68
5.5 Pipelining the architecture . . . 69
5.5.1 Continuous output to Radio Frequency (RF) . . . 69
5.6 Block In . . . 69
5.8 Variable Size IFFT Block - Pipelined . . . 71
5.9 CP Addition Block - Pipelined . . . 71
5.10 Disposition of the Units within the processing blocks . . . 72
5.11 SSBurst Addition . . . 73
5.12 Architecture overview . . . 73
6 Synthesis and Functional Simulation 75 6.1 Introduction . . . 75
6.2 HDL workflow advisor . . . 75
6.2.1 Set target device and synthesis tool . . . . 75
6.2.2 Prepare model for HDL code generation . . . 76
6.2.3 HDL code generation . . . . 76
6.2.4 FPGA synthesis and implementation . . . . 77
6.3 Summary of the inputs and outputs . . . 78
6.4 FPGA testbench simulation . . . 78
6.4.1 Testbench the architecture model elaborated in simulink in a standalone scenario 79 6.4.2 Read-only memory (ROM) integration in block design . . . 80
Intellectual Property (IP)s . . . . 80
6.4.3 Block design . . . 81
7 Implementation and Results 83 7.1 Introduction . . . 83
7.2 Integrated Logic Analyzer . . . 84
7.3 Synthesized Design . . . 87
7.4 Implemented Design . . . 89
7.4.1 Full support for FR2 . . . 90
7.5 Results . . . 91
7.6 Supported Bandwidths and Numerologies . . . 92
8 Conclusion and Future Work 95 8.1 Conclusion . . . 95
8.2 Future Work . . . 96
A Length For Normal and Extended CP, in FR1 and FR2 97
APPENDICES 97
List of Figures
1.1 Evolution of telecommunication’s technology [2]. . . 2
1.2 Cellular IoT connections per region, in Billion [4]. . . 2
1.3 Global mobile data traffic per month, extracted from [4]. . . 3
1.4 Traffic measurements in Mobile Data, extracted from [4]. . . 4
1.5 5G diverse case scenarios, extracted from [1]. . . 5
1.6 Split options, based on [10]. . . 7
1.7 Dissertation Workflow. . . 9
2.1 Different services available with BWP, based on [14]. . . 13
2.2 Duplex schemes, extracted from [12]. . . 14
2.3 MIMO exemplification, extracted from [16]. . . 16
2.4 Coordinated MultiPoint (CoMP) exemplification, extracted from [16]. . . 16
2.5 Standalone (SA) and Non-Standalone (NSA) different deployments, extracted from [18]. . 17
2.6 "Monolithic" RAN vs Centralized RAN, based from [24]. . . 19
2.7 Evolution to a split function architecture from LTE to New Radio (NR), extracted from [25]. 19 2.8 New Generation (NG) RAN architecture, extracted from [27] . . . 20
2.9 Different Ran Deployment Scenarios. . . 21
2.10 Split options, extracted from [10]. . . 22
2.11 Intra PHY layer split options, extracted from [31]. . . 23
2.12 Uplink and downlink datarates accross different split options. These datarates require-ments are examples provided when taking the following assumptions: 100MHz(Downlink (DL)/Uplink (UL)) for the channel bandwidth, modulation of 256 Quadrature Amplitude Modulation (QAM) (DL) and 64 QAM (DL) and 8 MIMO layers for all options, the values were extracted from [10] and [31]. . . 25
2.13 Protocol Stack - User-Plane and Control-Plane. . . 26
3.1 Radio interface protocol architecture around the physical layer, extracted from [35]. . . . 29
3.2 4096 IFFT length example accross diferent numerologies, based on [37]. . . 34
3.4 Slot format, extracted from [12]. . . 36
3.5 Cyclic Prefix Insertion. . . 36
3.6 Resource Grid (RG) description in Time-Frequency domain. . . 38
3.7 The channel bandwidth for one RF carrier and the corresponding transmission bandwidth configuration, extracted from [12]. . . 39
3.8 Synchronization Signal-Physical Broadcast Channel (SS/PBCH) Block [37]. . . 41
3.9 SS Burst Set, extracted from [37]. . . 41
3.10 SS/PBCH blocks beam sweeping, extracted from [39]. . . 42
3.11 Four-step random-access procedure, extracted from [12]. . . 43
4.1 Split option 7.1 functions. . . 45
4.2 Channels and signals generated by the 5G Toolbox for downlink. . . . 46
4.3 BWP Parameters, extracted from [41] . . . 47
4.4 RG showing the location of the PDCCH, PDSCH, CORESET and SSBurst. . . 48
4.5 Carrier RGs aligned to the overall channel bandwidth. . . 48
4.6 PRB allocation and spectrum impact. . . 49
4.7 Distributed Unit (DU) functions in split option 7.1. . . 50
4.8 Subcarriers zero padding and IFFTshifting process. . . 50
4.9 IFFTShifting Process. . . 51
4.10 IFFT Process. . . 51
4.11 CP addition process. . . 52
4.12 SSBurst will be added to the final final waveform. . . 52
4.13 Inputs and outputs for single numerology support. . . 53
4.14 Comparison between waveforms, upper is generated by Matlab Toolbox, lower one is generated by the proposed function. . . 54
4.15 Information display about Waveform Parameters. . . 54
4.16 Inputs and outputs for multiple numerology support. . . 55
4.17 Comparison between waveforms, upper is generated by Matlab Toolbox, lower one is generated by the proposed function. . . 56
4.18 Information display about waveform parameters. . . 56
5.1 From a Simulink model it is going to be generated Very High Speed Integrated Circuits Hardware Description Language (VHDL) code. . . 60
5.2 Simulink proprietary library . . . 61
5.3 Workflow in Simulink and use of Matlab references to model the architecture. . . 63
5.4 Processing block: Zero padding and IFFTShifting. . . 64
5.5 Results after the padding and shifting processing block. . . 65
5.7 Processing block: Variable size IFFT. . . 66
5.8 Results after the IFFT processing block. . . 67
5.9 Processing block CP addition. . . 68
5.10 Results after the CP addition processing block. . . 69
5.11 First processing block controller and processing units. . . 70
5.12 Valid signal after the first procssing block. . . 71
5.13 First processing block controller and processing units. . . 72
5.14 Valid signal after the last procssing block. . . 72
5.15 SSBurst will be added to the final final waveform. . . 73
5.16 Full overview of the created architecture model. . . 74
6.1 HDL workflow advisor - First step. . . 76
6.2 Example of an algebraic loop. . . 76
6.3 HDL workflow advisor - Third step. . . 77
6.4 Synthesis summary. . . 77
6.5 Implementation resources utilization. . . 78
6.6 Architecture for standalone scenario testbench. . . 79
6.7 IP core of the implemented ROM. . . 80
6.8 IP core of the implemented counter. . . 81
6.9 Block design for the DU connected to ROMs with the subcarrier data. . . 81
6.10 Testbench simulation results tested against the reference ones from 5G Toolbox. . . . 82
7.1 ZCU102 evaluation kit, extracted from [42]. . . 84
7.2 Finite state machine for the trigger IP core. . . 85
7.3 ILA module on synthesized design. . . 87
7.4 Block design for the DU connected to ROMs with the subcarrier data and a trigger for ILA implementation. . . 89
7.5 Resources used after the FPGA implementation, upper in absolute terms and the graph shows the percentages. . . 90
7.6 Clock wizard parameters. . . 90
7.7 Negative slack inside the clock wizard IP. . . 91
List of Tables
3.1 Mapping of Control Information. . . 30 3.2 Mapping of Transport Channels. . . 30 3.3 Different Subcarrier Spacing (SCS), CP and Frequency Ranges according to Numerology 34 3.4 FR1 - NRB per SCS and Bandwidth. . . 39
3.5 FR1 - NRB per SCS and Bandwidth. . . 40
3.6 FR2 - NRB per SCS and Bandwidth. . . 40
3.7 SS/PBCH blocks in each SSBurst Set depending on the Frequency Range. . . 42
6.1 Model Inputs . . . 78 6.2 Model Outputs . . . 78
7.1 Datarates achieved for each case (5 MHz to 40 MHz) - FR1 . . . 92 7.2 Datarates achieved for each case (50 MHz to 100 MHz) - FR1 . . . 92 7.3 Datarates achieved for each case in MHz(50 MHz to 400 MHz) - FR2 . . . 93
A.1 Length for normal CP, FR1, SCS 15 kHz [44]. . . 97 A.2 Length for normal CP, FR1, SCS 30 kHz [44]. . . 98 A.3 Length for normal CP, FR1, SCS 60 kHz [44]. . . 98 A.4 Length for extended CP, FR1, SCS 60 kHz [44]. . . 99 A.5 Length for normal CP, FR2, SCS 60 kHz [44]. . . 99 A.6 Length for normal CP, FR2, SCS 120 kHz [44]. . . 99 A.7 Length for extended CP, FR2, SCS 60 kHz [44]. . . 99
Glossary
16QAM 16-Quadrature Amplitude Modulation
256QAM 256-Quadrature Amplitude Modulation
3GPP Third Generation Partnership Project
4G Fourth Generation of Mobile Communications
5G Fifth Generation of Mobile Communications
5G-NR Fifth Generation - New Radio
64QAM 64-Quadrature Amplitude Modulation
AMF Access and Mobility Management Function
AR Augmented Reality
ARC Automated Repeat Request
ASIC Application Specific Integrated Circuit
BBU Baseband Unit
BPSK Binary Phase Shift Keying
BS Base Station
BWP Bandwidth part
CAPEX Capital Expenditures
CDMA Code-Division Multiple Access
CN Core Network
CoMP Coordinated MultiPoint
CORESET Control resource set
CP Cyclic Prefix
CP-OFDM Cyclic Prefix - Orthogonal Frequency Division Multiplexing C-RAN Centralized RAN
CRS Cell Specific Reference Signal
CSI-RS Channel State Information - Reference Signal
CSI-RS CSI Reference Signals
CU Central Unit
DAC Digital to Analog Converter
DCI Downlink Control Information
DFT-OFDM Discrete Fourier Transform OFDM
DL Downlink
DM-RS Demodulation Reference Signal
DU Distributed Unit
eNB eNodeB
FDD Frequency Division Duplex
FFT Fast Fourier Transfom
FIFO First In First Out
FPGA Field Progammable Gate Array
FR1 Frequency Range 1
FR2 Frequency Range 2
FSM Finite State Machine
gNB gNodeB
GSM Global System for Mobile communication
HARQ Hybrid ARQ
HDL Hardware Description Language
HSPA High Speed Packet Access
IFFT Inverse Fast Fourier Transfom
ILA Integrated Logic Analyzer
IoT Internet of Things
IP Intellectual Property
ISI Intersymbol Interference
LDPC Low Density Parity Check
LTE Long Term Evolution
MAC Medium Access Control
MBB Mobile BroadBand
MIMO Multiple-Input Multiple-Output
MMS Multimedia Message Service
mmWave milimitter Wave
MTC Machine Type Communications
NAS Non-Access Stratum
NG New Generation
NG-RAN New Generation - Radio Access Network
NR New Radio
NSA Non-Standalone
OFDM Orthogonal Frequency Division Multiplexing
OPEX Operational Expenditures
PBCH Physical Broadcast Channel
PDCCH Physical Downlink Control Channel
PDCP Packet Data Convergence Control
PDSCH Physical Downlink Shared Channel
PHY Physical Layer
PRACH Physical Random-Access Channel
PRB Physical Resource Blocks
PSS Primary Synchronization Signal
PTRS Phase Tracking Reference Signal
PUCCH Physical Uplink Control Channel
PUSCH Physical Uplink Shared Channel
QAM Quadrature Amplitude Modulation
QoS Quality of Service
RAN Radio Access Network
RAR Random Access Response
RB Resource Blocks
RE Resource Element
RF Radio Frequency
RF SoC RF System-On-Chip
RG Resource Grid
RLC Radio Link Control
ROM Read-only memory
RRC Radio Resource Control
RU Remote Unit
SA Standalone
SCS Subcarrier Spacing
SDAP Service Data Adaptation Control
SMS Short Message Service
SRS Sounds Reference Signal
SS/PBCH Synchronization Signal-Physical Broadcast Channel SSS Secondary Synchronization Signal
RTL Register Transfer Level
TDD Time Division Duplex
UCI Uplink Control Information
UE User Equipment
UHD Ultra High Definition
UL Uplink
UPF User Plane Function
V2X Vehicle To Everything
VHDL Very High Speed Integrated Circuits Hardware Description Language
VR Virtual Reality
CHAPTER
1
Introduction
1.1 Scope
Amongst the large number of technologies that were developed in the last forty years, the one with most impact could be considered to be the mobile communication’s. The first generation of mobile communications started in the 1980s and since then there has been a continuous evolution from generation to generation, Fig.1.1. This first mobile network system used analogue signals, the cellphones were bulky and the spectrum was expensive and rarely available [1]. The digital communication era started with 2G, in the mid 1990s with Global System for Mobile communication (GSM), which supported sufficient data rates in order for Short Message Service (SMS) and e-mails to be transmitted, and when it was enhanced to 2.5G with the forthcoming of Code-Division Multiple Access (CDMA), the mobile data communications started to be widely used. Still, these two generations were mainly voice-centric and mobile broadband was yet to be developed, which came with the next generation. With 3G, based on wideband Code-Division Multiple Access (wCDMA), the data mobile services started to be truly used, enabled by high-quality mobile broadband, which started in the 2000s. This allowed the sharing of Multimedia Message Service (MMS) and also streaming of multimedia, and with 3.5G, and the High Speed Packet Access (HSPA) the data rates reached several Mbit/s, which made way for the use of applications requiring internet and that would never had evolved without this rise in the data rates. The current generation we live in is the 4G, Long Term Evolution (LTE), using Orthogonal Frequency Division Multiplexing (Orthogonal Frequency Division Multiplexing) to achieve wider transmission bandwidths and was launched around 10 years ago, by the end of 2009.
Figure 1.1: Evolution of telecommunication’s technology [2].
1.1.1 Fourth Generation - Long Term Evolution
Fourth Generation of Mobile Communications (4G) - (LTE) has made major alterations in the plane of mobile broadband and in the quotidian. Unlike 3G, it was developed from the start as a mobile broadband system [3], and with the evolution new enhancements were made to LTE allowing new use cases like Machine Type Communications (MTC) which led to the increase of Internet of Things (IoT) connections. According to [4], cellular IoT connections will rise each year in about 27% reaching an expected number of 4.1 billion in the year of 2024, see Fig.1.2. This increase in connections has grown in parallel with the traffic generated by each device.
Figure 1.2: Cellular IoT connections per region, in Billion [4].
The number of mobile broadband connections is expected to rise in the near future, being that in 2020 according to [5] there are going to be 6.2 billion and that number will ascend to 8.5 billion by 2025.
to make use of them not only at work and home but also on the move. Far behind are the days people used to wait to reach their TV’s or their PC in order to see their favorite show or video. As mobile traffic increases the most predominant category growth is attributed to video, as depicted in Fig.1.3, due to higher resolutions like Ultra High Definition (UHD), increasing viewing time and the rise of streaming services like YouTube, Twitch and other similar applications.
Figure 1.3: Global mobile data traffic per month, extracted from [4].
Nowadays one does not wait to get home to enjoy their multimedia content leading to an increase in the average time spent on mobile devices per day is increasing, which causes continuous growth in the used spectrum. The greatly increased consumption of data and the rise of mobile subscriptions has made such that the mobile data traffic has increased in about 88% from the fourth quarter of 2017 to the last quarter of 2018 [6], as seen in Fig.1.4.
Figure 1.4: Traffic measurements in Mobile Data, extracted from [4].
1.2 Motivation
The above graphs show a tendency to an exponential rise in data traffic and with all the requirements it will not be feasible within the current generation. The need for great increases in data capacity, speeds and low latencies make way for the next generation, Fifth Generation - New Radio. According to [1], Fifth Generation of Mobile Communications (5G) will
differentiate itself by showcasing new improvements such as: • 10x throughput velocities;
• 10x lower latencies; • 10x density connections; • 3x spectrum efficiency.
It is predicted by [7] that in 5 years 5G networks will carry 25% of all global mobile data traffic as traffic is expected to reach 136 Exabytes per month.
1.2.1 Use Case Scenarios
5G will open the door for new use cases capable of improving the user experience through better latencies, capacities and coverage. It will have to be flexible in order to adapt to the great range of services envisioned for this new generation, having to overcome certain requirements according to different services, Fig.1.5.
Mission Critical Services
Services like robotics, industrial automation and autonomous vehicles, will arise with 5G. The communications inside these systems will have the need to achieve ultra-low latencies, which will require the 5G frame structure to become more flexible and faster than the current generation one. Alongside ultra-low latencies, requirements like high reliability and availability will also be crucial [8].
Connecting massive IoT
Characterized by requiring power efficiency, good coverage and low complexity, available concepts like Smart Cities will require remote sensors and actuators to track objects, manage lighting stops, traffic flow or parking lots. There is going to increase the need for bandwidth in order for the whole data created through all the sensors which include cameras to be able to flow.
Mobile Broadband Enhanced
Having started to see new concepts like Virtual Reality (VR), Augmented Reality (AR) and UHD video it is possible to predict that in the near future, services using these already implemented concepts will be greatly enhanced. Streaming of UHD videos from a cellphone will be also normal. These services, amongst others to be introduced, will require very high throughputs and ultra-low latencies while maintaining a more uniform experience, demanding this way the need to increase network capacity.
Figure 1.5: 5G diverse case scenarios, extracted from [1].
This fifth generation is envisioned to transform the role that telecommunications have and will redefine the way society is connected. Unlike the four generations so far, whose major concern was the achievement of better and faster services, 5G is envisioned to perform these
and much more. It will greatly increase the role that telecommunications plays within the people’s world, by connecting everything and everyone, from people to smart devices creating space for further economic growth in telecommunications. Examples of 5G-ready use-case examples are [2]:
V2X. Vehicle To Everything (V2X) will allow for communication from vehicles to other vehi-cles and everything around them, which has profound applications ranging from autonomous driving to saving people’s lives. The Connected Car market is expeted to reach 141 Billion dollars by 2020 [2].
Smart Cities. Citizens life will be improved by Smart Cities. Sustainability, energy-efficiency, less traffic congestion and security are some of the advantages of this evolution towards Smart Cities. 5G will be the portal to surpass the communication challenges faced towards this evolution. Being such a vast area to cover the market is rising and is expected to reach 1.45 Trillion dollars by 2020 [2].
Augmented Reality/VR. It is very demanding for a device to support AR and VR, and with Cloud VR and AR this factor could be eliminated. Lower latencies and more capacity of the network is essential to allow Augmented Reality and Virtual Reality, whose markets are expected to value at 151 Billion dollars by 2022 [2].
1.2.2 Radio Access Network (RAN)
This continuous increase in the demands for a more bandwidth capable and more efficient mobile network lead to a new paradigm of Radio Access Network (RAN). The traditional "monolithic" RAN architecture consisted in a Base Station (BS) which connected to a fixed number of antennas that would only cover the reception and transmission on a small area, and the utilization rate of these BS tends to be low due to the fact that the peak load in the network is usually much higher than the average load, which leads to waste of processing power since this cannot be shared between other BSs. The way the traditional RAN is structured leads to an unscalable mode of operation since there is an ever growing need of better network capacity and coverage, leading to unfeasible numbers of needed BS to be implemented, which would lead to a great increase both in Capital Expenditures (CAPEX) and Operational Expenditures (OPEX).
The costs and the lack of efficiency make this solution of a "monolithic" RAN not feasible for New Radio (NR). An idea of a more centralized RAN comes then forward, separating the functions of the BS between two elements, the Central Unit (CU) and Distributed Unit (DU), being the first a centralized processing unit which may have a plurality of distributed processing units connected to it. The main reason for the new NR Base Station architecture was that splitting the 5G Base Station, denominated gNodeB (gNB), in CU and DU would, according to [9], bring benefits like:
• A flexible hardware implementation which would allow for scalable cost-effective solu-tions.
• A split architecture which would bring coordination of performance features, load management and real-time performance optimization. It also would enable virtualised deployments.
• Configurable functional splits that would enable adaptation to various use cases, such as variable transport latency.
In Fig.1.6 the processing chain for both upstream and downstream directions is shown. Within this processing chain several potential split options were devised, with each one proposing a different way to split the functions between the CU and DU. These split options lead to some tradeoffs and depending on the required needs the split option to choose will be different. NR functions will be this way split in the architecture according to different deployment scenarios, leading to different conjugations in the way the CU and DU are connected.
Figure 1.6: Split options, based on [10].
1.2.3 FPGA Implementation
The more centralized RANs lead to new requirements such as reconfigurability and low latency operation flexible Hardware accelerators, which make Field Progammable Gate Array (FPGA)s a natural tool to be used in the new RAN architectures [11]. Another factor leading implementation to be chosen on FPGAs is the ever increasing of determinism necessity as more and more data is being provided from the Medium Access Control (MAC) layer to the Physical Layer (PHY) layer which leads to the necessity of FPGAs to handle the timing requirement. The wide bandwidth comes also in play because when dealing with these bandwidths one PC can not simply reach the required datarates needed, the high length of Inverse Fast Fourier Transfom (IFFT)/Fast Fourier Transfom (FFT) becomes too computational heavy to be supported on software and the implementation on a FPGA is more energetic efficient.
1.2.4 High Level Modeling
When modeling a given DU, for any given split option, it is going to have different processing blocks within it. These will have to be modelated and validated before implementing the DU architecture on FPGA. This process traditionally was made in Register Transfer Level (RTL) level, but the option to create a model and validate it through a High Level description language is also possible. When compared to the regular implementation of RTL which uses hardware description languages such as VHDL or Verilog, the second option has some major advantages, such as the time to develop the architecture, since which any complex algorithm, when implemented on, for this dissertation, in Simulink it is easier and faster to test and validate it than it would be by RTL Implementation. Simulink in conjunction with HDL
Coder will then provide a High Level Synthesis tool which integrates algorithms and hardware
design in one single environment, and allows for a simulation at the system level, which can be directly converted to synthesizable HDL code.
1.3 Objectives
The main objective of this dissertation is to model in an High Level language the Physical Layer (PHY) of 5G-NR, after split 7.1, on the downlink, in the transmitter DU of the base station to posterior synthesis and direct implementation into logic programmable devices FPGA. This way it will be possible, from a high level description, to obtain an implementation that supports 5G-NR real time signals. In order to achieve this goal the following stages were adopted:
1. Study of the base concepts, architectures e proposed specifications to 5G-NR, in special attention to the ones on the PHY layer, downlink.
2. Model and simulation in Matlab of the split 7.1 PHY layer on the transmitter DU of the base station.
3. Use of the framework Simulink to create an architecture of the model previously created and simulate it.
4. Direct synthesis from the Simulink project to a complete HDL DU architecture using HDL Workflow Advisor and testbench simulation of the synthesized project.
5. Integration of the generated HDL DU into a project and posterior implementation on FPGA and validation of the project in real time with Integrated Logic Analyzer (ILA).
The dissertation workflow can be seen in Fig.1.7, where the different steps are presented in order. Values from the 5G Toolbox from Matlab will be generated in order to serve as reference for the posterior models to be created in Simulink, in the synthesized project and after implementation in FPGA, as depicted in Fig.1.7.
Figure 1.7: Dissertation Workflow.
1.4 Dissertation Structure
The thesis dissertation contains, besides this introduction, the following chapters:
• Chapter 2 - 5G New Radio Overview - an overview of 5G-NR is presented, including details about NR use case scenarios, design principles and key technologies. The new radio interface architecture is showcased and conclusions are made about split options. Lastly, the NR radio protocol architecture is explained with the different layers.
• Chapter 3 - Physical Layer - this chapter contextualizes the focus of this dissertation - the PHY layer. Its signals, channels and frame structure are presented as well as the
Waveform and the CP-OFDM.
• Chapter 4 - Matlab Modeling And Simulation - the DU functions to be imple-mented are first described in specific, and a Software model is made using Matlab for this purpose. Results about this implementation are made and compared to a the ones provided in 5G Toolbox from Matlab.
• Chapter 5 - Simulink Modeling And Simulation - after validating the Matlab model, a Simulink one is made in order to better emulate an Hardware architecture, obtaining results which are compared to the previous reference ones in order to validate
the Simulink architecture.
• Chapter 6 - Synthesis and Simulation - in this chapter, the HDL workflow advisor is used to create, based on a Simulink Model, synthesizable VHDL code. An IP with the previous model is generated in Vivado Design Suite and testbenches are applied to the proposed IP.
• Chapter 7 - Implementation and Results - using Integrated Logic Analyzer a validation on a Block Design using the IP previously created is implemented on a FPGA. The results are transferred to Matlab where a comparison with the 5G Toolbox results is made.
• Chapter 8 - Conclusion and Future Work - the dissertation is concluded in this chapter with a summary of the obtained results and an enumeration of proposed work for the future is made.
CHAPTER
2
5G New Radio Overview
2.1 Introduction
The use cases mentioned in the first chapter will be made possible due to the NR newly provided benefits, given not only by the new design principles but also by the new major technologies, both of which are being applied to this new generation.
3GPP
The formation of the Third Generation Partnership Project (3GPP) had the purpose to develop the Technology Specifications for 3G. When 3G was to be surpassed by 4G, 3GPP developed LTE which later became the 4G standard and now for 5G, 3GPP is developing the New Radio architecture, providing specifications to define this technology, having three different Technical Specification Groups, which are:
• Radio Access Networks; • Services & Systems Aspects; • Core Network & Terminals.
3GPP Release 14 was the first time the NR technical work was introduced, where the first technical decisions were taken, and the work continued into Release 15, at which time the first version of NR was released.
2.2 Design Principles
An overview of NR will be made in this chapter, regarding the design principles and the major technology components announced on the mentioned 3GPP Release 15.
2.2.1 Spectrum Flexibility
5G-NR will lead to a more networked society, leading to a massive growth not only in the number of connected devices but also in the amount of data traffic produced by each one. It will allow the use of operating bands from 450 MHz that will extend up to 52.6 GHz, which might be complemented or extended with new ranges in future 3GPP releases. This operation
at such high frequencies will allow to accomplish the requirements of high network capacity and extreme data rates because at those frequencies there is more spectrum available and the transmission bandwidths associated to that spectrum are much wider. Nonetheless, the lower frequency bands are still going to be of major importance since the attenuation suffered at the higher frequencies limits the network coverage. In 5G-NR there will be two ranges for frequency bands, according to 3GPP Release 15:
• Frequency Range 1 (FR1) - From 450MHz to 6GHz;
• Frequency Range 2 (FR2) - From 24.25GHz up to 52.6GHz.
2.2.2 Lean Design
In order to make NR more efficient in terms of energy and to reduce the interference in between cells that lead to lower data rates, some changes were made in relation to the "always-on" signals [12]. These signals are present in LTE and are constantly being carried in the network, whether there is user traffic or not. These, in LTE, are vital for BS detection, reference signalling and the broadcast of system information. Having these factors into account, in NR changes were made to reduce these signals. For example, the demodulation reference-signal (DM-RS) will be only present when there is data transmission [12].
2.2.3 Interworking and LTE coexistence
As NR will use high frequency bands, these bands will be able to complement systems with low frequency, being these either LTE or NR. As the downlink has more available power on the transmission from the part of the base station that the User Equipment (UE), it will be beneficial to operate in wider bandwidths that are located in higher spectrum bands, while the uplink, being limited by its power from the UE can transmit in the lower band frequencies since it needs less power and achieves higher data rates this way, due to the smaller radio-channel attenuation, despite the fact that there is less bandwidth [12]. Since the majority of the sub 6 GHz frequency bands are already crowded with majorly LTE (but also previous technologies), it is attractive to have a possibility to use the same spectrum for NR and LTE.
2.2.4 Extensibility
It is crucial to exist compatibility with future evolutions of the technology and services which have their specifications yet unknown. To try to achieve this forward compatibility, according to [13] the principles taken will be:
• Reducing the transmission of the "always-on" signals;
• Allow for the configuration of time allocation and frequency resources in which the channels and signals of the PHY layer will be confined;
• Increase the resources both in time and frequency that can be flexibly used or left in blank not affecting the backward compatibility in the future.
2.3 5G-NR Key Technology Components
In the following subsections a review of the major 5G-NR key technologies is going to be enumerated and explained.
2.3.1 Bandwidth parts
Because NR has such large bandwidths, it would not be feasible for the devices to support each and all of the bandwidths. Unlike in LTE where the devices only had to support 20 MHz of bandwidth, in NR the maximum component carrier bandwidth can reach 400 MHz. On top of that, in NR it is possible for UE to operate with smaller bandwidth than the one configured for medium data rates, which leads to more efficiency in energy when not needing the whole wideband.
The BWPs are a new acquisition of NR, and their main purpose is to indicate to a given UE the bandwidth over which they will receive data. Each BWP has certain individual SCS, having its own signal characteristics allowing for greater spectral and power efficiency. Fig.2.1 presents some different services that can be provided due to this new technology component, according to [14]:
1. Reduction of energy consumption for each user by using smaller parts of one given carrier;
2. Non-contiguous BWPs with different SCS in a given carrier; 3. Non-contiguous spectrum allowing for new services to be inserted.
Figure 2.1: Different services available with BWP, based on [14].
A BWP is composed by contiguous common Resource Blocks (RB) on a given carrier, which start at a certain common RB, and may contain the SS/PBCH block or not. The initial BWP will be signalled by PBCH. According to the specifications mentioned in [15] the device will only receive and operate in the bandwidth part that is active at that moment. While in a given time it can be configured to up to 4 different bandwidth parts, only one of them can be
active, being that it only receives PDSCH, PDCCH or CSI Reference Signals (CSI-RS) in that bandwidth part. In the downlink the initial bandwidth part will be set by the CORESET, in turn, the CORESET is retrieved from the PBCH. In the uplink, a given device also can not transmit in physical channels if not on an active BWP.
2.3.2 Duplex Schemes and Dynamic TDD
Both LTE and NR had in its development the objective of being flexible in the usage of the spectrum, achieving this goal by implementing Frequency Division Duplex (FDD) and Time Division Duplex (TDD) to support paired and unpaired spectrum, and having this way several carrier bandwidths reaching the top bandwidth at 20MHz. FDD and TDD are used in different scenarios. At higher bandwidths TDD is usually the choice with the unpaired spectrum while FDD will be predominantly used at lower frequencies given that allocations tend to be paired in this case. TDD uses a single carrier frequency, which makes it an half-duplex operation where a device can only transmit or receive data, and so the downlink and uplink transmissions stay separated in time, Fig.2.2. On the other hand, FDD has the downlink and uplink separated in frequency which means they both can occur at the same time.
While in LTE the two different schemes make way to different frame structures, in NR this will not need to happen. NR will be able to use only one common scheme working either on paired or unpaired spectrum. As mentioned, TDD is usually more important in the higher frequency bands. These scenarios are affected when covering large areas, where there is more interference than in local areas with small cells.
Figure 2.2: Duplex schemes, extracted from [12].
In NR, contrary to LTE, there is the possibility of dynamic TDD, which leads to a much greater flexibility in which a slot can be allocated dynamically either to uplink or downlink. This enables the dynamic reassignment of the resources either in downlink and uplink directions, making possible fast traffic variations, depending on the current needs at a given time. One use case scenario is for example, when a cell resources are available and one user is trying to upload a given file, the available resources will be allocated majorly for
uplink in order to speed up the process while this upload is being performed, changing again when the situation differs.
2.3.3 Low Latency Support
NR has deployment scenarios needing low latencies, which will be possible due to details in the NR design, such as the use of "front-loaded" reference signals, [12]. A given device is able to start processing data that is received without pre-buffering by locating in the beginning of the transmission the reference signals which carry scheduling information.
2.3.4 Scheduling and Data Transmission
The shared-channel transmission, where resources in time-frequency are dynamically shared is one basic principle of NR. The gNB makes the dynamic scheduling and informs the devices connected to it. In NR the PDCCH are monitored by the devices once per slot per default, in order for it to follow the defined scheduling and to be able to change information. The transmission of the channel coding data uses Low Density Parity Check (LDPC) codes which are less complex at higher rates than the LTE Turbo Codes. [12].
2.3.5 Control Channels
In the downlink the scheduling decisions are carried by the PDCCH. These PDCCH are transmitted in one or more CORESET, which has one advantage when compared to LTE, because instead of using the full bandwidth of the carrier, it can occupy only a part of it. This, again, goes with the previous information, seeing that devices in NR do not need to access the whole spectrum.
2.3.6 CORESET
The CORESET is a time-frequency resource in which the UEs will try to decode candidate control channels, PDCCH transmissions. It is semistatically configured by the network being thus able to be smaller than the carrier bandwidth which is of vital importance since the carriers in NR can be of up to 400 MHz, not being possible for all devices to receive such bandwidths, [12]. The CORESET is not necessarily at the beginning of a slot, being possible to occur at any position in the slot, although it is not required for a given UE to handle the CORESET if this is outside its active bandwidth part. In order for a device to be able to receive the system information when connecting, a CORESET is provided as part of the configuration of the initial bandwidth. The CORESET is used for a device to find the control channels. It spans in 1, 2 or 3 OFDM symbols in the time domain and in multiples of 6 RB in frequency. They also can be reutilised by a given device for transmission of data.
2.3.7 Massive MIMO
As mentioned before, NR will achieve much higher frequencies than LTE. While working at such high frequencies is advantageous in terms of data rates and more allowed traffic, it arises some challenges. The losses get higher as the the frequency increases, and the whole spectrum will be allocated, which means there is going to exist more interference in between systems.
As NR the frequency increases, the propagation loss will be significantly higher. Also, in the lower frequencies, because NR and LTE coexist in this spectrum, spectral efficiency needs to be high. The large number of antenna elements that send and receive directional beams will reduce the propagation loss at the higher frequencies while at the lower, massive MIMO will be used in order to increase spectral efficiency, Fig.2.3.
Figure 2.3: MIMO exemplification, extracted from [16].
CoMP. CoMP will complement MIMO. It characterizes itself by a large number of antennas which are distributed and creates in this way multiple spatial dimensions that lead to an increased capacity and more spatial diversity improving reliability [16], as seen in Fig.2.4.
Figure 2.4: CoMP exemplification, extracted from [16].
2.4 3GPP’s 5G Architecture
In NR there were changes both in the system architecture in respect to the CN and RAN. While the radio-related functions are responsibility of the RAN, the functions who fall out of this category will be part of the responsibility of CN, such as authentication and the
connection setup. The separation of functions between the RAN and the CN is advantageous for it allows to one single CN to serve multitudes of radio-access technologies.
2.4.1 Deployment options
3GPP defined a new core network for this new generation, and one major change made to 5G-NR was to allow for different generation elements to be integrated in different configurations. This changes the current paradigm where each generation needed the access network and the core network of the same generation in order to be deployed, [17]. The two different scenarios available will be either Non-Standalone (NSA) or Standalone (SA). While for the first case the core will be connected to both radio cells from NR and LTE, being the core either a Evolved Packet Core (EPC) or CN, in the SA deployment case, the core network will operate only with a given generation type of cells, either LTE or NR, as depicted in Fig. 2.5. The NSA scenario will be, according to 3GPP Release 15, the first to come out, allowing for 5G devices to connect to NR frequencies, which will allow for improvements on the throughput, while still using LTE for the control-plane services. In the SA deployment the major advantages are the efficiency and simplicity, which will in turn increase the performance while reducing the costs. The New Generation - Radio Access Network (NG-RAN) will be able to be connected not only to the CN but also in non-standalone mode when connected to an EPC.
Figure 2.5: SA and NSA different deployments, extracted from [18].
2.4.2 CN
The new core network, CN, will present some improvements over the EPC. The first one is the service-based architecture, which makes such that it is the services provided by the core that are the focus of the specification. It is able to operate with different Access Networks giving the generalised design of the functionalities and a forward compatible Access Network according to [19]. Network slicing is also present allowing to create, on a common shared physical infrastructure, various virtual networks, each one of them can meet a specific criteria in order to fulfill the client or the business needs. The last area of enhancement was in the split between control-plane and user-plane where each of these will be able to scale in capacity independently of the other plane not affecting it.
2.5 RAN
The RAN is a major asset that provides high data rates and services full-time to the devices in the network.
2.5.1 Splitting Architecture
The traditional architecture of the LTE RAN faces many challenges due to the way it is built upon, on a "monolithic" way. While on one hand this resulted in a very simple RAN architecture, on the other hand this lead to challenges such as the evergrowing need of more BS, which needs investment, and continuous support and the fact that the BS has a low rate of utilization because the peak loads are far higher than the average ones, and so for most of the time one isolated BS is not used as much as it could, and this processing power can not be lended to other BS’s, making it difficult to improve capacity on the spectrum, [20]. With the ever increasing need for wider coverage and capacity of the network, the number of BSs has been increasing at a rapid pace. This increase has brought some unwanted factors, like increase in the total power consumption, which leads to conclude that it is impracticable to keep up with the current and future demands just by implementing new BS. The idea of a Centralized RAN (C-RAN) came then forward, yet in LTE era, as a solution that enabled new technologies, and reduced the consumption without affecting negatively the network. C-RAN would do this by having multitudes of simpler Remote Unit (RU) connected to one central BS in which the whole number of functions would be centrally performed.
C-RAN was one of the major enablers in 5G-NR due to the advantages that it brought, such as simpler implementation of the RUs and easier maintenance, [21], improved efficiency in the spectrum and better conditions for interference coordination techniques [22], faster handovers between cells for the same CU[23], ability to redirect processing powers, avoiding this way the problem of the past mentioned above, and the possibility to scale and add or remove services depending on requirements. Below, in Fig.2.6 is possible to see the difference between a "monolithic" and a centralized implementation of a RAN.
Figure 2.6: "Monolithic" RAN vs Centralized RAN, based from [24].
The eNB from LTE evolved into NR to a gNB, being that the Base Band Unit from LTE was split into CU, DU and RU, according to [25], to note that 3GPP considers a division only into CU and DU, [26]. Each CU may have plural DU connected to it, and the CU is connected to the CN. In the user plane part of the functions are moved from EPC to CU and DU [25]. From the Baseband Unit (BBU) in LTE some functions are handled in the NR by the CU and others in the DU. The EPC distributes functions through the CN, CU and DU, as can be seen in Fig.2.7, [25].
Figure 2.7: Evolution to a split function architecture from LTE to NR, extracted from [25]. As seen in Fig.2.8, a NG-RAN will be a set of gNB connected to the CN. These connections
are made via the NG interface. The Xn interface allows a gNB to be connected to another gNB. The gNB can be split into DU and CU and if so, each DU will be connected to a CU which will dictate the operations of this DU and any more DUs which might be connected to it. This connection between the CU and the DU is made by interface F1.
Figure 2.8: NG RAN architecture, extracted from [27]
2.5.2 Deployment Scenarios
There are different networks connecting the RAN units. Starting from the CN, the network which links it to CU and links CUs between themselves is the backhaul. Between the CU and DU is the midhaul and from the DU to the RU the network is the fronthaul. In 5G-NR four deployments scenarios have been specified for the RAN [25]. In Fig.2.9 it is possible to identify the four different deployment scenarios. The first being composed of co-located CU, DU and RU, which will only have backhaul in this case, connecting the base station to the CN, according to [25], these deployments will majorly be used for small cell and hot-spot scenarios. 2) portraits a scenario where the CU and DU can also be co-located with the RU being isolated, which leads to a solution where there is no mid-haul. In 3) the CU is isolated and the DU and RU co-located, eliminating the midhaul link, being only backhaul and fronthaul present. Lastly, in 4) the scenario is one of independent locations between CU, DU and RU.
Figure 2.9: Different Ran Deployment Scenarios.
2.6 Functional Splits
While the idea of a C-RAN was beneficial in many ways it came at the expense of a severe fronthaul latency and bandwidth requirements, [25], which leads to new solutions to combat this, concretely a possibility which brings most of the benefits of a conceptual C-RAN while lowering the datarates to an affordable one.
Both the uplink and downlink are composed by a chain of processing blocks, which can be split in different points. These split options will determine from which point on the chain the functions will be centralized. The Protocol Stack is split up into CU and DU, leaving, as depicted in Fig.2.10, the sublayers to the left of the split in the CU, centralized, and the ones to the right in the DU.
A tradeoff must be made when choosing which functions are left in the DU and which ones are centralized. Below a discussion will be taken about each split option, their advantages and disadvantages.
Figure 2.10: Split options, extracted from [10].
2.6.1 Split option 1 - PDCP/RRC
The entire User-Plane will be located in the DU given this Split, which gives the advantage of bringing the user data closer to the transmission point. Nonetheless, this split has a limit in the number of supported features such as providing inter-cell coordination which makes it unfeasible for deployments with a CU connected to a large number of cells, [28]. On top of that, the level of complexity in this kind of Split is very challenging.
2.6.2 Split option 2 - RLC/PDCP
The sublayers above this split, PDCP and RRC, stay in the CU, which makes all real-time aspects to be operated in the DU, decreasing the requirements for this split in the link. When compared to a fully integrated eNodeB (eNB) this split multiplexing gains will be only marginal, since only the PDCP and RRC are centralized[29].
2.6.3 Split option 3 - Intra RLC
The Intra RLC split separates the User-Plane functions of the PDCP and the assynchronous processing in the RLC from the remaining User-Plane Functions, [30].This option has de advantage of reducing the latency on the fronthaul link, and being robust over non-ideal conditions of transmission and mobility, since the ARQ is centralized [10].
2.6.4 Split option 4 - RLC/MAC
Separating the RLC sublayer from the MAC on one hand brings the advantage of resource sharing benefits but has severe drawbacks when it comes to 5G and the shorter duration of slots as this leads to more frequent decisions by the scheduler.
2.6.5 Split option 5 - Intra MAC
This split option is characterized by a centralized scheduler in the CU and a sublayer of MAC in each DU which leads to the real-time communication functions being located in each DU. The CU communicates through scheduling and Hybrid ARQ (HARQ) reports with each of the DUs. The major disadvantages with this implementation are the fact of having a complex interface between the DU and CU and being hard to define scheduling operations between these two logical entities [10].
2.6.6 Split option 6 - MAC-PHY
The PHY layer will be located in the DU while the MAC scheduling and every function above is centralized. This split will, similarly to the splits Intra PHY layer, have very strict delay requirements as the HARQ and other time critical procedures are in the CU [30]. While the centralized scheduling and the load dependent datarates on the fronthaul link are advantageous, this split option has potential issues with the shorter slots of 5G due to the fronthaul delay.
2.6.7 Split option 7.x - Intra PHY
In the Fig.2.11 it is possible to locate all the functions located within the PHY Layer and the possible split options made within.
From the transport block received from the upper layer, the PHY chain will pass through a series of processing blocks, ending in IFFT and CP addition, sending the IQ symbols to the RF.
2.6.8 Split option 7.3 - (Downlink Only)
This split will separate the functions higher than the Scrambling are centralized, while the rest of the lower PHY is included in the DU. This split makes such that the fronthaul link is cell load dependent, not having constant datarates when not needed and according to [10] the scheduling can be centralized. Despite this advantages this split option has a complex DU including the local modulation[30].
2.6.9 Split option 7.2
In this split option, the processing blocks above precoding are centralized while precoding and functions below are localized in the DU. When compared to split 7.3 this DU will be less complex and able to centralize more functions, nonetheless the datarates on the fronthaul link will be increased. While this split option supports full implementationMIMO and CoMP for uplink, it only supports partial implementation of MIMO and CoMP in the downlink.
2.6.10 Split option 7.1
This split option will lead to one of the simplest DU, being surpassed only by the Option 8. The iFFT/FFT will reside in the DU alongside with the addition/remove of CP. The data entering the DU will be composed of SubCarriers, and inside the DU will be also added the Guard SubCarriers. The fronthaul datarates will be constant despite the amount of user data traffic because of the fact the Resource Element (RE) mapping will be executed in the CU, being this fct vital to detect subcarriers not being used in order to achieve a variable datarates [30]. Despite having a constant load in the fronthaul link despite the traffic generated by the UE [30], it this datarates has a drop of around 40% when compared to to Option 8 [32], the major disadvantage being that uplink bandwidth is significantly higher than the downlink one. This split has a significant advantage since it allows for full implementation of CoMP and MIMO on both the uplink and downlink without degradating the performance.
2.6.11 Split option 8
This split option was the initial idea of a fully centralized RAN, obtaining this way the shared processing power to functions and leaving only the RF in the DU making it as simple as possible and so the implementation of DUs will become lower in costs and complexity. But as mentioned above it came at the expense of severe datarates and latency requirements on the fronthaul, being this datarates continuous and it gets worsen with MIMO scenarios scaling with the number of antennas.
2.6.12 Datarates throughout the Different split options
The datarates are expressed in Fig.2.12, we can see that, as expected, the datarates will decrease along the higher chain splits, and on top of that they become data traffic dependent.
Figure 2.12: Uplink and downlink datarates accross different split options. These datarates require-ments are examples provided when taking the following assumptions: 100MHz(DL/UL) for the channel bandwidth, modulation of 256 QAM (DL) and 64 QAM (DL) and 8 MIMO layers for all options, the values were extracted from [10] and [31].
As the split option gets higher in the chain the centralization becomes lesser and this increases the complexity and cost of the deployment of the DUs, which is one of the major problems as mentioned above. So the choice has to be within the lower split options (lower split options will be the options with higher number). By the plot presented in Fig.2.12 one can conclude that the major difference gap in datarates goes from the split option 8 to the Option 7.1, bringing, as mentioned above, a drop of 40% in the fronthaul link datarate. Split option 7.1 has also advantages when compared to Split 7.2, since the first supports full implementation of CoMP and MIMO on both the uplink and downlink without affecting performance [33], and the DU implementation will be simpler allowing a higher level of flexibility.
2.7 Radio Protocol Architecture
From LTE and the eNB in NR the network nodes are called gNB. This gNB is not a physical implementation but a logical one, as it can be realized in different ways. There are two different planes in the architecture of the radio protocol, being the first, control-plane, used for setup of the connections and security, while the user-plane is vital for delivering data.
Depending on the type of data one or other stack will be used, if user Data is to be sent, the User-Plane will be used, otherwise if the Data is a Signaling Message it will be used the Control-Plane.
The two planes have many similarities between them, sharing a common structure com-posed of the Physical Layer, Medium Access Control, Radio Link Control and Packet Data Convergence Control. The main differences stand on the top of these layers, while on the User-Plane there is the Service Data Adaptation Control, in the Control-Plane sitting on
top of these layers are the Radio Resource Control and Non-Access Stratum. The top of the User-Plane will connect to the User Plane Function and the Non-Access Stratum (NAS) layer of the Control-Plane will be connected to the Access and Mobility Management Function.
Figure 2.13: Protocol Stack - User-Plane and Control-Plane.
2.7.1 Layer 3 Functions
Radio Resource Control (RRC). RRC main services are the of System Information, paging messages transmission, detection and recovery from a radio link failure, QoS manage-ment functions and securing managemanage-ment. All the messages from RRC use the same sublayers of the User-Plane as depicted in Fig.2.13.
2.7.2 Layer 2 Functions
The Layer 2 of NR has the following sublayers:
SDAP. Service Data Adaptation Control (SDAP) is responsible for the mapping between Quality of Service (QoS) flow and a data radio bearer, which can be seen as a pipe carrying IP packets through a network, getting prioritized according to the QoS requirement [34].
