Universidade de Aveiro Departamento de Eletrónica,Telecomunicações e Informática 2019
Fábio Daniel
Lopes Coutinho
Implementação em FPGA da Cadeia Rx Uplink de
uma DU 5G-NR
FPGA Implementation of a 5G-NR DU Rx Uplink
Chain
Universidade de Aveiro Departamento de Eletrónica,Telecomunicações e Informática 2019
Fábio Daniel
Lopes Coutinho
Implementação em FPGA da Cadeia Rx Uplink de
uma DU 5G-NR
FPGA Implementation of a 5G-NR DU Rx Uplink
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 Rodrigues de Oliveira, Professor Auxiliar do Departamento de Eletrónica, Telecomunicações e Informática da Universidade de Aveiro.
o júri / the jury
presidente / president Prof. Doutor José Carlos Esteves Duarte Pedro
Professor Catedrático da Universidade de Aveiro
vogais / examiners committee Prof. Doutor Marco Alexandre Cravo Gomes
Professor Auxiliar da Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Prof. Doutor Arnaldo Silva Rodrigues de Oliveira
agradecimentos /
acknowledgements Aos meus pais e à minha família pelo apoio ao longo destes anos.
A todos os meus amigos que me acompanharam ao longo destes 5 anos, especialmente ao Bernardo, ao Zé e ao Pedro, assim como à Tuna Universitária de Aveiro e à Mariana.
Aos novos amigos e colegas que fiz, ao Samuel, ao Milheiro, ao Pe-dro, ao Luís e à Bea.
Agradecimento especial ao meu orientador, Professor Doutor Arnaldo Silva Rodrigues de Oliveira, pela ajuda e confiança ao longo destes anos. À Universidade de Aveiro, ao Departamento de Eletrónica, Telecomuni-cações e Informática e ao Instituto de TelecomuniTelecomuni-cações por fornecerem as condições necessárias de trabalho e aprendizagem.
Este trabalho foi financiado pelo Fundo Europeu de Desenvolvi-mento Regional (FEDER), através do Programa Operacional Regi-onal de Lisboa (POR LISBOA 2020) e do Programa OperaciRegi-onal Competitividade e Internacionalização (COMPETE 2020) do Portugal 2020 [Projeto 5G com o no 024539 (POCI-01-0247-FEDER-024539)]
Palavras Chave 5G, 5G NR, eMBB, URLLC, mMTC, RAN, OFDM, PHY, FPGA.
Resumo Com a constante evolução dos sistemas de comunicação, o 5G está rapida-mente a tornar-se numa necessidade e uma realidade. Nesse sentido, três cenários de comunicação foram estabelecidos, Enhanced Mobile Broadband (eMBB), Ultra Reliable Low Latency Communications (URLLC) e massive Ma-chine Type Communications (mMTC), sendo a base do estudo e desenvolvi-mento deste novo sistema de comunicação. Para tal, a 3GPP especificou a divisão da arquitetura rádio de acesso à rede (RAN), passando do tradi-cional eNB (Long Term Evolution) para Central Unit (CU) e Distributed Unit (DU) (New Radio), tornando-a assim mais flexível. Convencionalmente, as funções de processamento (L1, L2 e L3) estão co-localizadas na Base Band Unit (BBU), fazendo com que as taxas de transferências no fronthaul sejam demasiado elevadas para cenários 5G.
Esta dissertação foca-se na implementação da cadeia de receção Uplink de uma DU 5th Generation NR, sendo que a DU será composta por funções de processamento provenientes da BBU, mais concretamente por uma cadeia de receção Cyclic Prefix Orthogonal Frequency Division Multiplexing (split 7.1), de modo a obter taxas de transferência mais baixas no fronthaul. Para tal, um estudo prévio sobre a camada física (PHY) foi realizado assim como os procedimentos de sincronização, seguidos da modulação em Matlab. Para implementação da cadeia de receção, foi usada uma ferramenta que sintetiza modelos Simulink em código VHSIC Hardware Description Language (VHDL), o HDL Coder. Após simulação em testbench, foi feita a validação numa Field Programmable Gate Array (FPGA).
Keywords 5G, 5G NR, eMBB, URLLC, mMTC, RAN, OFDM, PHY, FPGA.
Abstract With the constant evolution of the communication systems, the 5G is rapidly becoming a necessity and a reality. In this sense, three scenarios were established, Enhanced Mobile Broadband (eMBB), Ultra Reliable Low La-tency Communications (URLLC) and massive Machine Type Communications (mMTC), being this scenarios the base for the study and development of this new communication system. For that, 3GPP specify the radio access net-work (RAN) division, passing from the traditional eNB (Long Term Evolution) to the Central Unit (CU) and the Distributed Unit (DU) (New Radio), making it more flexible. Conventionally, the processing functions (L1,L2 and L3) are co-located in the Base Band Unit (BBU), making the data rates in the fronthaul too high for 5G scenarios.
This dissertation focus on the Uplink chain implementation of the 5th Gener-ation NR DU, being that the DU will be composed by processing functions from the BBU, more concretely by a reception chain of the Cyclic Prefix Orthogo-nal Frequency Division Multiplexing (split 7.1), in order to decrease the data rates in the fronthaul. For that, a previous study about the physical layer (PHY) and the synchronization procedures was done, followed by the Matlab imple-mentation. For reception chain implementation, a tool used that synthesizes Simulink models in VHSIC Hardware Description Language (VHDL) code, the HDL Coder. After testbench simulation, it was performed the validation in the Field Programmable Gate Array (FPGA).
Contents
Contents i List of Figures v List of Tables ix Glossary xi 1 Introduction 1 1.1 Scope . . . 1 1.2 Motivation . . . 3 1.3 Objectives . . . 4 1.4 Document Structure . . . 52 5G New Radio Overview 7 2.1 Introduction . . . 7
2.2 Frequency Bands . . . 8
2.3 Key Technology in NR . . . 9
2.3.1 Higher-frequency operation . . . 9
2.3.2 Duplex schemes . . . 9
2.3.3 Scheduling and Data Transmission . . . 9
2.3.4 Control Channels . . . 10
2.3.5 Ultra-lean design . . . 10
2.3.6 Forward compatibility and LTE coexistence . . . 10
2.3.7 Transmission scheme, bandwidth parts and frame structure . . . 11
2.3.8 Low-latency Support . . . 11
2.4 Overall System Architecture . . . 11
2.4.1 Core Network . . . 11
2.4.2 Radio Access Network . . . 12
2.6 Physical Layer in gNB . . . 15
3 Physical Layer 17 3.1 Physical Time-Frequency Resources . . . 17
3.2 Frame Structure . . . 18
3.3 Downlink (DL) Overall View . . . 21
3.3.1 DL Physical Channels . . . 22
3.3.2 DL Physical Signals . . . 22
3.4 Uplink (UL) Overall View . . . 25
3.4.1 UL Physical Channels . . . 25
3.4.2 UL Physical Signals . . . 26
3.5 Modulation and Channel Coding . . . 26
3.6 CP OFDM Waveform . . . 27
3.6.1 OFDM transmission chain . . . 31
3.7 OFDM Reception Chain with Synchronization . . . 32
3.7.1 Synchronization Errors . . . 32
3.7.2 Effects of Synchronization Errors . . . 33
3.8 Physical Layer Challenges . . . 37
3.8.1 Multiple Antennas . . . 37
3.8.2 Fronthaul Data Rates . . . 37
3.9 New Functional Split Options in 5G . . . 38
3.9.1 Functional Splits Overview . . . 40
3.9.2 Split Options Summary . . . 42
3.9.3 RAN deployment scenarios . . . 43
3.10 Random Access . . . 45
3.11 Summary . . . 45
4 Matlab Modeling and Simulation 47 4.1 Introduction . . . 47
4.2 Frame Parameterization . . . 47
4.2.1 PUSCH and PUCCH . . . 48
4.2.2 PRACH . . . 49 4.3 5G Matlab Toolbox . . . 50 4.4 Laboratory Setup . . . 50 4.4.1 Equipment . . . 51 4.5 Signal generation . . . 55 4.5.1 5G Toolbox . . . 55 4.5.2 Laboratory Equipment . . . 57
4.6 Signal reception . . . 57
4.6.1 Frame reception . . . 58
4.6.2 PRACH detection . . . 60
4.7 Synchronization Algorithms . . . 61
4.7.1 Symbol Timing Detection . . . 62
4.7.2 Carrier Frequency Offset Estimation . . . 66
4.7.3 Sampling Clock Offset Estimation and Residual CFO . . . 69
4.7.4 Carrier Phase Error . . . 69
4.8 Low Latency CFO estimation . . . 70
4.9 Complete Receiver Architecture . . . 72
5 Simulink Modeling and Simulation 75 5.1 Introduction . . . 75
5.2 Symbol Timing Detection . . . 76
5.2.1 Autocorrelator . . . 76
5.2.2 Energy Calculator . . . 77
5.2.3 Auto Correlation Normalized . . . 78
5.2.4 Orthogonal Frequency Division Multiplexing (OFDM) symbol start detection 79 5.3 Cyclic Prefix (CP) removal . . . 80
5.4 Carrier Frequency Offset (CFO) estimation . . . 81
5.4.1 CFO correction . . . 83 5.5 FFT and FFT shift . . . 83 5.6 Simulink Optimization . . . 85 5.6.1 Fixed point . . . 85 5.6.2 Pipeline architecture . . . 86 5.6.3 HDL Coder compatibility . . . 86
5.7 Simulation and results . . . 87
6 Synthesis and Functional Simulation 91 6.1 Introduction . . . 91
6.2 VHDL Code Generation and Synthesis . . . 91
6.3 IP generation . . . 94
6.4 Simulation . . . 94
6.4.1 Comparison with Matlab model . . . 96
7 Implementation and Results 97 7.1 FPGA . . . 97
7.2 Vivado Design . . . 98
7.3 Real Time Validation with ILA . . . 100
7.3.1 Implementation metrics . . . 100
7.3.2 ILA Validation Results . . . 101
7.3.3 Comparison with Matlab Model . . . 102
8 Conclusion and Future Work 105 8.1 Conclusions . . . 105
8.2 Future Work . . . 106
A FFT size per Channel Bandwidth 107 B Fine Symbol Timing Detection Algorithms 109 B.1 Cross-Correlation . . . 109
B.2 Frequency Response Estimate . . . 109
B.3 Frequency-Domain Phase Shift . . . 109
C Sampling Clock Offset Estimation Algorithms 111 C.1 LLS Estimator . . . 111
C.2 Joint WLS Estimator . . . 111
List of Figures
1.1 Evolution of mobile communications (based on [1]). . . 1
1.2 Fifth Generation (5G) use cases classification (retrieved from [5]). . . 2
1.3 Evolution from 4G/LTE to 5G New Radio transport architecture (based on [7]). . . 3
1.4 Optional split points (retrieved from [9]). . . 4
2.1 Standalone and non-standalone approach (based on [11]). . . 7
2.2 New frequency bands (retrieved from [13]). . . 8
2.3 High-level core network architecture (retrieved from [19]). . . 12
2.4 Radio Access Network (RAN) Interfaces (based on [5]). . . 13
2.5 NR user-plane protocol stack (based on [5]). . . 14
2.6 Physical Layer (PHY) Interfaces (based on [17]). . . 15
2.7 Description of New Radio (NR) PHY processing chain (retrieved from [23]). . . 16
3.1 Resource grid (based on [4]). . . 18
3.2 Frame and Subframe coexistence. . . 20
3.3 Slots in different numerologies. . . 20
3.4 Channels and signals for downlink. . . 21
3.5 SS/PBCH block (retrieved from [4]). . . 23
3.6 An example of DM-RS and PT-RS in a time-frequency structure (based on [21]). . . 24
3.7 Channels and signals for uplink. . . 25
3.8 OFDM symbols and slots for different numerologies (CP normal) (based on [21]). . . 30
3.9 OFDM symbols and slots for numerology 2 (CP extended). . . 30
3.10 Typical OFDM architecture (based on [26]). . . 32
3.11 OFDM synchronization errors (retrieved from [26]). . . 33
3.12 CFO synchronization error effect. . . 34
3.13 Scenarios of DFT window location (based on [26]). . . 35
3.14 First scenario of DFT window location. . . 35
3.15 Second scenario of DFT window location. . . 36
3.17 Fourth scenario of DFT window location. . . 36
3.18 Functional Splits options. . . 39
3.19 Splits presented in the PHY layer (retrieved from [23]). . . 39
3.20 Bandwidth and latency by Functional Split Options (based on [23]). . . 43
3.21 RAN architecture with independent RU, DU and CU locations. . . 44
3.22 RAN architecture with co-located DU and CU. . . 44
3.23 RAN architecture with RU and DU integration. . . 44
3.24 RAN architecture with RU, DU and CU integration. . . 45
3.25 Random Access procedure (retrieved from [5]). . . 45
4.1 Frame Structure function. . . 48
4.2 PRACH structure. . . 49
4.3 Laboratory setup. . . 51
4.4 M8190 AWG. . . 51
4.5 Global view of Waveform configurations. . . 52
4.6 Waveform Setup, with the Carrier Configuration Table and a part of Waveform Attributes Table. . . 52
4.7 BWP configuration. . . 53
4.8 Channel configuration. . . 53
4.9 Keysight E8267D VSG. . . 53
4.10 N9041B SA. . . 54
4.11 Graphical interface with signal example PUSCH with its DM-RS with 20 MHz channel bandwidth and 15 kHz subcarrier spacing. . . 54
4.12 PUSCH Spectrum example. . . 55
4.13 Resource Grid example. . . 56
4.14 Signal 5G NR Uplink example. . . 56
4.15 OFDM general view. . . 57
4.16 Reception without synchronization. . . 58
4.17 Resource Grid obtained. . . 59
4.18 Symbol obtained. . . 59
4.19 Symbol obtained with laboratory equipments signal. . . 60
4.20 Detect a Physical Random Access Channel (PRACH) start. . . 60
4.21 Preamble format B1. . . 61
4.22 Reception with the symbol timing detection. . . 65
4.23 Reception with the CFO estimation. . . 68
4.24 Difference between both approaches. . . 70
4.25 Difference between average and theoretical approaches. . . 71
4.27 Receiver Architecture. . . 72
5.1 5G Uplink Simulink, a global view. . . 76
5.2 Autocorrelator for symbol timing detection. . . 77
5.3 Comparison between Simulink and Matlab autocorrelation model. . . 77
5.4 Energy Calculator for symbol timing detection. . . 78
5.5 Comparison between Simulink and Matlab energy model. . . 78
5.6 Normalized autocorrelator for symbol timing detection. . . 79
5.7 Comparison between Simulink and Matlab autocorrelation model. . . 79
5.8 Maximum normalized autocorrelation detection. . . 80
5.9 CP remove. . . 81
5.10 An example of CP remove. . . 81
5.11 CFO estimation state diagram. . . 82
5.12 CFO estimation blocks diagram. . . 82
5.13 CFO correction blocks diagram. . . 83
5.14 Fast Fourier Transform (FFT) operation output with the assignment of Finite State Machine (FSM) states. . . 84
5.15 FFT Shift state diagram. . . 84
5.16 FFT Shift output. . . 85
5.17 Fixed point data type (retrieved from [37]). . . 86
5.18 Error between Matlab model and Simulink optimized model. . . 87
5.19 Visualization of constellation for two symbols. . . 87
5.20 5G NR receiver architecture final. . . 89
6.1 Workflow Hardware Description Language (HDL) Coder. . . 92
6.2 HDL Coder resources summary. . . 92
6.3 HDL Coder timing summary. . . 93
6.4 VHSIC Hardware Description Language (VHDL) generated files. . . 93
6.5 Summary of resources post-Synthesis (Table). . . 93
6.6 Summary of resources post-Synthesis (Graphic). . . 94
6.7 Intellectual Property (IP) generated. . . 94
6.8 Testbench simulation. . . 95
6.9 Testbench simulation. . . 95
6.10 Error between testbench and Matlab model. . . 96
7.1 Resources results (retrieved from [43]). . . 97
7.2 Clock Wizard of Xilinx library. . . 99
7.3 Block design final. . . 99
7.5 Summary of resources post-Implementation (Graphic). . . 101
7.6 Integrated Logic Analyzer (ILA) global settings. . . 101
7.7 ILA trigger configuration. . . 101
7.8 Validation results. . . 102
7.9 Comparison with Matlab model and ILA results. . . 102
7.10 Error between Matlab model and ILA validation in the first subframe. . . 103
7.11 Block design final with ILA integration. . . 104
A.1 FFT length per Channel Bandwidth. (retrieved from [6]) . . . 107
A.2 FFT length per Channel Bandwidth. (retrieved from [6]) . . . 108
A.3 FFT length per Channel Bandwidth. (retrieved from [6]) . . . 108
A.4 FFT length per Channel Bandwidth, numerology 3 and CP extended in Frequency Range 2 (FR2). (retrieved from [6]) . . . 108
List of Tables
2.1 Main functionalities of layers. . . 14
3.1 Supported numerologies (Release 15). . . 19
3.2 Number of slots per subcarrier and consequences. . . 20
3.3 A brief description about the DL physical channels. . . 22
3.4 Physical signals used in the Downlink. . . 22
3.5 A brief description about the UL physical channels. . . 25
3.6 Physical signals used in the Uplink. . . 26
3.7 Number of Resource Block (RB) per subcarrier spacing per channel bandwidth for (Frequency Range 1 (FR1)). . . 28
3.8 Number of RB per subcarrier spacing per channel bandwidth for (FR1). . . 28
3.9 Number of RB per subcarrier spacing per channel bandwidth for (FR2). . . 29
3.10 Required data rates in 5G wireless network. . . 38
4.1 Parameters for reception Fifth Generation New Radio (5G NR). . . 48
4.2 PRACH preamble formats for LRA= 839. . . 49
4.3 PRACH preamble formats for LRA= 139. . . 50
4.4 Relation between the symbol detection and the Autocorrelators. . . 62
4.5 Comparison between the different Symbol Timing Detection algorithms. . . 66
4.6 Comparison between the Carrier Frequency Offset implementation algorithms. . . 69
4.7 Result depending on µ, BWchannel, CP configuration and diverse channels and signals. . 73
7.1 Sampling Rate depending on numerology and channel bandwidth . . . 98
Glossary
3GPP 3rd Generation Partnership Project
5G Fifth Generation
5G CN Fifth Generation Core Network 5G NR Fifth Generation New Radio ADC Analog to Digital Converter
ARQ Automatic Repeat Request
AWG Arbitrary Waveform Generator
BBU Baseband Unit
BPSK Binary Phase Shift Keying BRAM Block Random Access Memory
BS Base Station
BWP Bandwidth Part
CFO Carrier Frequency Offset
CN Core Network
CoMP Coordinated Multipoint CORESET Control Resource Sets
CPE Common Phase Error
CP-OFDM Cyclic Prefix Orthogonal Frequency Division Multiplexing CPRI Common Public Radio Interface
CSI Channel State Information
CSI-RS Channel State Information Reference Signal
CU Central Unit
CP Cyclic Prefix
C-RAN Cloud Radio Access Network DFT Discrete Fourier Transform
DFT-OFDM Discrete Fourier Transform OFDM
DL Downlink
DM-RS Demodulation Reference Signal DSP Digital Signal Processor
DU Distributed Unit
eMBB Enhanced mobile broadband
eNB Evolved Node B
EPC Evolve Packet Core
EVM Error Vector Magnitude
FF Flip Flop
FFT Fast Fourier Transform
FPGA Field Programmable Gate Array FR1 Frequency Range 1
FR2 Frequency Range 2
FSM Finite State Machine
gNB next generation NodeB
HARQ hybrid Automatic Repeat Request HDL Hardware Description Language
ICI Inter-carrier interference
IDFT Inverse Discrete Fourier Transform
ILA Integrated Logic Analyzer
IP Intellectual Property
IP Internet Protocol
ISI Inter-symbol interference
LDPC Low Density Parity Check LTE Long Term Evolution
LO Local Oscillator
LUT Lookup Table
MAC Medium Access Control
MIMO Multiple Input Multiple Output mMTC Massive machine-type communication mmWave Millimeter Waves
ng-eNB next generation eNodeB
NG-RAN New Generation Radio Access Network
NR New Radio
OBSAI Open Base Station Architecture Initiative OFDM Orthogonal Frequency Division Multiplexing PAPR Peak-to-Average-Power-Ratio
PBCH Physical Broadcast Channel
PDCCH Physical Downlink Control Channel PDCP Packet Data Convergence Protocol PDSCH Physical Downlink Shared Channel PHY Physical Layer
PRACH Physical Random Access Channel PRB Physical Resource Block
PSS Primary Synchronization Signal
PT-RS Phase Tracking Reference Signal PUCCH Physical Uplink Control Channel PUSCH Physical Uplink Shared Channel QAM Quadrature Amplitude Modulation
QPSK Quadrature Phase Shift Keying
RAM Random Access Memory
RAN Radio Access Network
RB Resource Block
RLC Radio Link Control
ROM Read-Only Memory
RU Remote Unit
RRC Radio Resource Control
RRH Remote Radio Head
RRU Remote Radio Unit
RSRP Reference Signal Received Power RTL Register-Transfer Level
UE User Equipment
UL Uplink
UUT Unit Under Test
SA Signal Analyzer
SCO Sampling Clock Offset
SDAP Service Data Adaptation Protocol SMS Short Message Service
SRS Sounding Reference Signal
SS/PBCH Synchronization Signals/PBCH block SSS Secondary Synchronization Signal
TDD Time Division Duplex
TR-RS Tracking Reference Signal
URLLC Ultra-reliable and low-latency communication VHDL VHSIC Hardware Description Language VSG Vector Signal Generator
CHAPTER
1
Introduction
1.1 Scope
Since the beginning of mobile communications, around 1980, many techniques and technologies were developed in order to allow new services and improve the network characteristics, leading to wider transmissions bandwidths, higher data rates, and lower latencies. Furthermore, the introduction of new mobile services and the quality of the communications improved the user experience. Cellular communications from the first generation (1G) up to the fourth generation (4G) have evolved notably, but the greatest advance comes with the development of the 5G. As shown in Figure 1.1, the increase in data rate due to the upcoming of new generations, led to other advances in the technology. For example, from the 1G to 2G, the analog transmission fell in disuse as the digital transmission was introduced alongside the Short Message Service (SMS).
3G introduced the mobile broadband and, at the moment, 4G provides reasonable data rates. However, a true networked society is expected with the 5G, due to the improved data rates, scalability, connectivity and energy efficiency [2].
The 5G mobile communications provides a new 5G radio access network called NR technology presenting new features that can benefit society by contributing to the development of a wide range of new services, like autonomous driving, smarter agriculture, Industry 4.0, smart cities, among others. In this context, it is important to highlight three use cases [3]:
• Enhanced mobile broadband (eMBB);
• Massive machine-type communication (mMTC);
• Ultra-reliable and low-latency communication (URLLC).
Larger data volumes and higher data rates (10 to 20 Gbps peak user data rate, with 10000 times more traffic), support for high mobility (500 km/h), massive number of devices (200000 to 1000000 devices per km2), very low device cost and device energy consumption, low latency (1 ms maximum latency) and high reliability (99.999 %) are requirements that will be present
in 5G NR to meet the services needs [4], illustrated in figure 1.2.
Figure 1.2: 5G use cases classification (retrieved from [5]).
5G NR extends its view across the frequency spectrum by exploring new frequency ranges - FR1, from 450 MHz to 6 GHz, and FR2 from 24.25 GHz to 52.6 GHz - providing more flexibility with the increase of numerologies, wider channel bandwidth, more slot configurations and the introduction of Millimeter Waves (mmWave) [6]. Another important aspect of 5G NR is the reuse of the features and structures of Long Term Evolution (LTE), although the requirements on NR are more extensive and, for this reason, it is necessary to think in a different set of technical solutions.
The 3rd Generation Partnership Project (3GPP), the entity responsible for the study and specification of mobile communications, is developing the full set of Fifth Generation Core Network (5G CN) specification that include the 5G Core Network (CN) and NR radio-access technology.
1.2 Motivation
The typical radio-access architecture is composed by a CN, a Base Station (BS) and a radio head. LTE is based on this architecture and, in terms of 3GPP nomenclature, the CN is called Evolve Packet Core (EPC), the BS and the radio head is called Evolved Node B (eNB). Due to the LTE architecture evolution towards a Cloud Radio Access Network (C-RAN) architecture, the eNB is splitted in Baseband Unit (BBU) and Remote Radio Unit (RRU), as depicted in Figure 1.3.
The 5G architecture follows the same scheme, but the RAN has some differences. The Base Station is split in three parts: the Central Unit (CU), the Distributed Unit (DU) and the Remote Unit (RU) (sometimes also called RRU) [7], as depicted in Figure 1.3. That structure can provide more flexibility in terms of implementation of the layer functions with the purpose of the data rates decrease in the fronthaul. Furthermore, the constant evolution of this RAN architectures, from the moment when all processing functions were together in BS up until the C-RAN, led to the necessity of function virtualization and this new split architecture facilitates this virtualization operation. 3GPP only considers the CU and DU in terms of split architecture [8], but a split architecture consisting in three elements, CU, DU and RU [7] was adopted.
Figure 1.3: Evolution from 4G/LTE to 5G New Radio transport architecture (based on [7]). The traditional interface between DU and RU using Option 8 (Common Public Radio Interface (CPRI) or Open Base Station Architecture Initiative (OBSAI) protocol) is inde-pendent of user traffic; in other words, it requires a continuous bitrate transport [7]. This
split point is very demanding in terms of latency and bandwidth requirements due to the centralization of all layer processing functions and, for this reason, the fronthaul requires high transport capacity and transport latency up to a few µsec. So, it is necessary to take into account another split options; some options in the protocol stack are presented in Figure 1.4.
Figure 1.4: Optional split points (retrieved from [9]).
With the increase of data rates in 5G, the conventional fronthaul implementation needs to change. A new functional split architecture that takes into account cost-effective and technical tradeoffs between functional centralization, latency and throughput, will be discussed and presented in this dissertation.
The option 7.1, or split 7.1, has a considerable reduction of data rate comparing with the traditional split 8, keeping a simple DU in terms of processing functions and a high level of flexibility. Implementing the processing functions in Field Programmable Gate Array (FPGA), the DU can support the processing requirements and results in a simple integrated board with the Radio Frequency (RF) front-end. This implementation can be re-programmable in future implementations and eliminates the necessity to use a powerful computer to support the processing functions.
1.3 Objectives
In this context, the DU 5G NR Receiver Uplink chain implementation on FPGA is the main goal of this dissertation. Furthermore, with this implementation, the goal is to decrease the requirements in fronthaul while relieving the processing requirements in the BS. The focus of this work will be the implementation of some Physical Layer (PHY) functions; more specifically the Cyclic Prefix (CP) removal and Fast Fourier Transform (FFT). An alternative method will be used to design the UL DU, allying a high level of abstraction with Simulink models to the VHDL synthesis tool HDL Coder.
To achieve this goal, the following steps are outlined: • Study and Matlab modeling of Uplink reception chain; • Simulink modeling of the Uplink reception chain; • VHDL synthesis with HDL Coder and simulation; • FPGA validation with ILA.
1.4 Document Structure
After this introduction, this dissertation is organized as follows:
• Chapter 2: 5G New Radio Overview - This chapter presents a brief description of the 5G NR, in particularly the new frequency bands, the key technologies, the CN, the next generation NodeB (gNB), and an overview of NR protocol stack;
• Chapter 3: Physical Layer - The study about the PHY layer, where the physical channels and physical signals are explained and contextualized, is presented. The frame structure is explained and the OFDM is studied, including its synchronization issues. The split justification and study is showcased with the RAN deployment scenarios;
• Chapter 4: Matlab Modeling and Simulation - The entire reception is explained and analyzed, from the signal generation to the proposed receiver architecture;
• Chapter 5: Simulink Modeling and Simulation - Simulink models of the DU are obtained, and the compatibility with the HDL Coder is implemented;
• Chapter 6: Synthesis and Functional Simulation -The VHDL synthesis with HDL Coder is performed and the behavioral simulation is realized.
• Chapter 7: Implementation and Results - The IP generation and successive tests and validations are presented;
• Chapter 8: Conclusions and Future Work - In this chapter, conclusions are presented and future lines are proposed.
CHAPTER
2
5G New Radio Overview
2.1 Introduction
The 5G NR development started in 2016 as a study item in the 3GPP Release 14, where different technical solutions were studied after the work item phase, resulting in the first version of the NR specifications. At the same time, a new 5G core network has been developed, providing a complete network [10]. However, before the 5G core network was concluded, a non-standalone 5G NR approach was implemented without affecting the basic radio technology, once the radio access was the same. The non-standalone approach tests the use of gNB separately and reuse the EPC for new mobile deployments, as depicted in Figure 2.1. In fact, the main difference between non-standalone and standalone approach is the core network.
Figure 2.1: Standalone and non-standalone approach (based on [11]).
In terms of spectrum, throughout the generations, in order to meet the necessary require-ments, the used frequency bands start occupying the higher end of the spectum. Nevertheless, 5G NR keeps the compatibility with the frequency bands used for previous generations. To follow the new requirements, it is necessary to take into account the different bands character-istics due to the propagation properties. Signals at higher frequencies suffer more attenuation,
which leads to a weaker received signal. This makes high frequencies bands ideal to dense deployments and low frequencies bands to wide-area coverage [5].
In order to meet the requirements of the proposed use cases, the 5G NR supports eMBB scenario that will require higher data rates and capacities in dense deployments, leading to the use of higher frequencies that can surpass the 24 GHz (often called mmWave).
Similar to the LTE, two duplex modes are supported, Time Division Duplex (TDD) and Frequency Division Duplex (FDD), and new bands were defined.
2.2 Frequency Bands
A fundamental aspect of global mobile services is the possibility of a RAN operating in different frequency bands, which will ease the operation in different regions of the world [12]. In terms of the radio access functionality perspective, NR does not operate in a specific frequency band, but spans within a large range of frequencies. However, some frequency ranges only operate at a certain NR numerology [6].
In the new mmWave frequency bands for devices and base stations, Figure 2.2, the RF requirements are different and new technology is implemented, from massive Multiple Input Multiple Output (MIMO), beamforming and an highly integrated advanced antenna systems. For this reason, frequency bands were divided into two frequency ranges [6]
• FR1 includes new bands, from 450 MHz to 6 GHz; • FR2 includes new bands, from 24.25 GHz to 52.6 GHz.
Figure 2.2: New frequency bands (retrieved from [13]).
This frequency bands operate in paired and unpaired spectra and require flexibility in the duplex arrangement, and for this reason, NR supports both FDD and TDD operations.
To group a certain set of RF requirements in a frequency range for Uplink (UL) and/or Downlink (DL), 3GPP defines operating bands. The release 15 [6], includes 26 operating bands in FR1 and 3 in FR2. The scheme is compatible with LTE and the band number hasn’t change over time.
For NR, bands have a scheme of assigned numbers ranging n1 to n512. This assignment obeys the following rules:
• The LTE band numbers that are reused for NR, by adding an ’n’;
• The range n65 to n256 is reserved for new NR frequency bands in FR1 and the range n257 to n512 is reserved for new NR frequency bands in FR2.
Some NR frequency bands are targeted to specific regions due to the way the frequency bands are implemented and, in some cases, the frequency bands are partly or fully overlapping.
The frequency bands used by 5G NR are listed with more detail in [6].
2.3 Key Technology in NR
These new features in terms of spectrum are accompanied by other new characteristics the of radio access technology. The leap from LTE to NR, is based on [5]:
• Higher-frequency bands to support large transmission bandwidths and the associated data rates;
• Ultra-lean design to reduce interference and improve energy performance; • Forward compatibility;
• Low latency to boost performance;
• Extensive usage of beamforming and a massive number of antenna elements.
2.3.1 Higher-frequency operation
NR has available a large range of spectrum, below 1 GHz up until to 52.6 GHz, however at mmWave frequencies a higher radio-channel attenuation narrows the network coverage. To contradict this paradigm, advanced multi antenna usage paired with the use of both lower frequencies, to guarantee a large network coverage, and high frequencies to support high traffic capacity and extreme data rates, is used.
2.3.2 Duplex schemes
As mentioned above, NR operate in both paired and unpaired spectra and allocations in lower frequency bands are often paired (use FDD) while in high frequency bands are unpaired (use TDD). The NR frame structure is designed to support both paired and unpaired spectra, and half-duplex and full-duplex operation.
In dense deployments working at higher frequencies with small cells (with a small number of users per cell), the cell traffic varies extremely fast. So, dynamic TDD was introduced to dynamically assign and reassign time-domain resources in both directions of transmission. In this sense, if a small group of users is connected to a cell and need to download a large amount of information, DL has more resources than UL [5].
2.3.3 Scheduling and Data Transmission
Another characteristic where resources are dynamically shared between users is found in
channel-dependent scheduling. Devices send channel-quality reports to the base station, where
the scheduler runs. Based on these reports the scheduler takes scheduling decisions which leads to different traffic priorities and quality-of-service requirements. These scheduling decisions also try to mitigate the fast variations of the instantaneous channel conditions stemming from frequency-selective fading, distance-dependent path loss and random interference variations. The information about channel-quality is contained in Physical Downlink Control Channel
(PDCCH) and Physical Uplink Control Channel (PUCCH). NR grants the option of disabling
the dynamic scheduling, and in this case the device is configured in advance with the respective resources for UL data transmission or/and DL data reception [5].
2.3.4 Control Channels
More concretely, the control channels provide feedback information and scheduling decisions, in UL and DL respectively. To improve the poor latency in LTE, flexibility in the control channels was introduced, as well as support for beamforming. PDCCH can be configured to occupy a part of the carrier bandwidth with one or more Control Resource Sets (CORESET), allowing devices with different bandwidth capabilities (forward compatibility principle) share the channel. PUCCH demonstrates the effort on low latency in the NR design. There are a lot of PUCCH formats, and PUCCH is transmitted in the last one or two symbols of a slot, where the delay after the chain is just a few tens of microseconds depending on the numerology used (comparing to LTE this is a big difference, 3 ms). PUCCH transmits uplink control information, from hybrid-ARQ acknowledgments, channel state feedback and scheduling request to the uplink data [14].
2.3.5 Ultra-lean design
One problem of the current mobile communications technologies is the existence of signals that are transmitted regardless if they have data to send. For example, the reference signal for channel estimation is used for detection and broadcast in dense networks constituting a substantial part of the network transmission, which causes a decrease of the average traffic load per network node. This decreases the achievable network energy performance and causes interference between cells, reducing the data rates.
The ultra-lean design principle aims to mitigate these problem by reducing this always signals, or in this context, aggregating data within them. It is the case of 5G NR demodulation reference signal that is used for channel estimation, and only is transmitted when have data available to transmit. Similarly, another procedures have been modified to take account for the ultra-lean design principle [15].
2.3.6 Forward compatibility and LTE coexistence
One concern described in NR specifications is the insurance of forward compatibility in the radio-interface design, that is intrinsically difficult to guarantee. However 3GPP identify some basic principles to help NR achieving forward compatibility [16].
• Flexible use of time and frequency resources; • Ultra-lean design principle;
• Very flexible signals and channels for the physical layer.
This flexibility allows for the future introduction of new types of transmissions, without the need to change the physical layer.
NR is compatible with LTE as well, i.e., NR needs to be compatible with past and future architectures. This interworking with systems operating at lower frequencies is beneficial to complement the network, for example in the coverage imbalance between UL and DL, intra-NR carrier aggregation, dual connectivity and handover. One of the fundamental tools for this coexistence is the NR numerology based on 15 kHz subcarrier spacing (used in LTE).
Adjacent to this, the independence and flexibility between slots (division of the subframes) permits the introduction of new types of transmission in the future [5].
2.3.7 Transmission scheme, bandwidth parts and frame structure
Another thing that 5G NR inherited from LTE is the use of OFDM. Once again, OFDM is used due to its sturdiness to time dispersion and ease of management. It uses both the time and frequency domain to define channels and signals with different structures. Nevertheless, NR uses conventional non-DFT-precoded OFDM, but this theme will be further explored in the next chapter [17].
2.3.8 Low-latency Support
The new RAN architecture demands new requirements in terms of latency with this being an important characteristic of NR. To minimize the decoding delay, the reference signals and DL control signals carrying scheduling information are placed at the beginning of the 5G frame and do not use time domain interleaving across OFDM symbols, allowing the device to start processing the received data immediately [18]. It is possible to divide the slot in mini-slot and transmit individually, that minimizes the decoding delay. With all the new features in NR, the time requirements in the devices and on the network are more tightened.
2.4 Overall System Architecture
The 5G NR system, which connects the User Equipment (UE) and the network, has the challenge of integrate the new radio access technologies with the previous technologies. Handling the CN functions separately, before of integration them into the RAN, is beneficial because allows the reuse of this CN for others radio access technologies. This happens when NR operates in non-standalone mode.
The 5G NR system is composed by the RAN and the CN, where the RAN has the responsibility to ensure all the way since the scheduling until the multi-antenna schemes whereas the CN has the responsibility to complete other necessary functions to provide a complete network.
2.4.1 Core Network
Support for network slicing, service-based architecture and control-plane/user-plane split are new characteristics of 5G CN, while keeping some core functions similar to those used in EPC and introducing new ones, like as NR Repository Function.
The 5G CN is a service-based architecture, as shown in 2.3, that consists in a group of functional elements that provide flexibility in terms of services and functionalities [19].
Network slicing is introduced to support the most demanding requirements, creating end-to-end logical networks serving isolated needs. Slices run in the same physical core and radio networks, but they appear as independent networks from the user perspective [3].
The core network functions can be implemented in a single physical node, distributed across multiple nodes or executed on a cloud platform. These functions were divided into
Figure 2.3: High-level core network architecture (retrieved from [19]).
user-plane and control-plane functions, where user-plane is a gateway between RAN and external networks, and consist of User Plane Functions. The control-plane functions consist of: Session Management Function, Access and Mobility Management among others.
2.4.2 Radio Access Network
To enable LTE compatibility in the radio access network, the 5G CN supports two types of nodes:
• A gNB, for NR devices;
• A next generation eNodeB (ng-eNB), for LTE devices.
The term RAN is a simplification of New Generation Radio Access Network (NG-RAN) and corresponds to the ng-eNB for LTE radio access and to the gNB for NR radio access. It is important to refer that gNB and ng-eNB are not physical implementations but logical nodes [20].
The functions like, radio resource management, admission control, connection establish-ment, among others, are the responsibility of gNB as well as ng-eNB, in one or several cells. One very common implementation is comprised of one baseband processing unit (CU) with various DU connected, as depicted in Figure 2.4.
The interface between gNB and 5G CN is called NG interface, more concretely NG-u to the user-plane part (UPF) and NG-c to the control-plane part (AMF) [20]. Between various gNB the Xn Interface is used and supports dual connectivity and active-mode mobility.
The option to split the gNB in several parts is possible, as depicted in Figure 2.4. The gNB-Centralized Unit (gNB-CU or just CU) and the gNB-Distributed Unit (gNB-DU or just DU) are separated using the F1 interface. Between the user device, also called UE, and the gNB the interface used is the Uu Interface [20]. This division is very flexible, at the point that the functions of each block were not assigned statically, making possible their
Figure 2.4: RAN Interfaces (based on [5]).
change depending on the deployment scenarios and according to the necessary functions and requirements. This topic will be studied in subchapter 3.9.
So, for a UE to establish connection with the BS, it is necessary one cell to supports uplink and downlink transmissions, which means that all data and control flows are handled by the cell. This more simple configuration is used nowadays. But, allowing the connection with several cells can be beneficial as increases the data rates (user-plane aggregation) or, in the case where control-plane and user-plane are separated, a device can be connected to two cells (dual connectivity) [5].
The NG-RAN have the responsibility to support and satisfy the new requirements, as well as, being flexible.
2.5 Protocol Stack Architecture
The network node for 5G NR is termed the gNB by 3GPP, and the base station based on gNB protocol can be implemented in various ways. As stated above, the radio protocol architecture can be separated in control-plane functions and user-plane functions, where the user-plane is responsible to transport user data, while the control-plane controls the process (connection setup, mobility and security).
From the point of view of the user-plane, the information arrives through Internet Protocol (IP) packets then goes to the Service Data Adaptation Protocol (SDAP), follows to the Packet Data Convergence Protocol (PDCP), then to the Radio Link Control (RLC), and Medium Access Control (MAC) and, finally, reaches the PHY before arriving to the RF chains
(waveform in time domain). The next Figure 2.5 illustrates the user-plane protocol stack of NR, which functionalities are summarized in table 2.1 [21].
Figure 2.5: NR user-plane protocol stack (based on [5]).
Table 2.1: Main functionalities of layers.
Layer Description
SDAP Mapping between quality of service and radio bearers
IP packets are mapped to radio bearers according to QoS requirements PDCP IP header compression/decompression
IP header reordering and duplicate detection IP header ciphering/deciphering and integrity protection RLC Error correction (Automatic Repeat Request (ARQ) mechanism)
Segmentation/resegmentation of IP packets Delivery of data units to higher layers
MAC Error correction (hybrid Automatic Repeat Request (HARQ) mechanism) UL and DL scheduling
Multiplexing data across multiple component carriers
PHY Coding/decoding
Modulation/demodulation Multiantenna processing
Mapping signals and channels to physical time-frequency resources
The responsibilities of connection setup, mobility and security are the function of the control-plane. The control signalling is provided by the Core Network or by the Radio Resource Control (RRC) layer in gNB. Beyond this control function, the RRC layer performs various control services [21], such as:
• Broadcasting of system information; • Transmission of paging messages; • Security management;
• Handovers;
• Cell selection/reselection; • QoS management;
• Detection and recovery from radio link failures.
Regarding the control-plane protocol stack, the unique difference is that the RRC layer is added and stays above of SDAP layer. This architecture layer is practical because the RRC messages are transmitted by the remaining of layers.
The PHY layer is the basis of radio protocol architecture so, it is necessary to understand very well the Physical Layer because it is the backbone of any RAN [21].
According to [22], the model of the physical layer of the UE for Uplink and Downlink is described based on the corresponding channels, this happens because the data channels include all necessary functions. In the case of UL, is the Physical Uplink Shared Channel (PUSCH) and in the case of DL is the Physical Downlink Shared Channel (PDSCH).
2.6 Physical Layer in gNB
The PHY interfaces the MAC and the RRC, that provides a transport channel to MAC (transfer information over the radio interface), and MAC offers different logical channels to RLC, as depicted in Figure 2.6 [17]. The difference between logical and transport channel is the information that is transferred over the layers.
Figure 2.6: PHY Interfaces (based on [17]).
Furthermore, it is important to mention that signals like the Control Channel are provided by higher layers, in other words, higher layers configure the physical layer. The different cell specific signals that are used in Cell Search procedures for Downlink reception are generated in the PHY, before the layer mapping operation. The different channels or signals, as Sounding Reference Signal (SRS) whose the UE uses to send information about the channel or the UE random access (PRACH), are detected in PHY.
The Physical Layer is responsible for data transport services to higher layers and the access to these services is through the use of transport channels via the MAC sub-layer. To
provide the data transport service, the PHY performs the following functions, according to [22]:
• Error detection;
• FEC encoding/decoding; • Hybrid ARQ soft-combining; • Rate matching;
• Mapping in physical channels; • Power weighting;
• Modulation and demodulation; • Frequency and time synchronization; • Radio characteristics measurements; • MIMO antenna processing;
• RF processing.
The PHY functions are the main aspect in DU design, so the focus of this dissertation is the PHY layer and, for this reason, Figure 2.7, besides the relation with other layers.
Figure 2.7: Description of NR PHY processing chain (retrieved from [23]).
The 5G NR radio interface, at the level of the of physical layer, should approach different features to satisfy the requirements, being them basic frame structure, numerology, initial access procedures and scheduling for operation [24].
CHAPTER
3
Physical Layer
As previously stated in the Chapter 2, the channels and signals needs processing functions at level of PHY layer, therefore, a study and understanding about each physical layer processing chain is necessary, and consequently the function of channels and signals.
3.1 Physical Time-Frequency Resources
A resource element is an element of the resource grid for the antenna port p and subcarrier spacing configuration µ, is uniquely identified by (k, l)p,µ, where k is the index in the frequency domain and l is the relative symbol position, in the time domain, to the reference point [14]. The resource element (k, l)p,µ corresponds to the complex value ap,µk,l of the subcarriers, where
the indices p and µ may be omitted, resulting in ak,l. A resource block (RB) is a group of 12 consecutive subcarriers in the frequency domain, that correspond to 12 consecutive resource elements in the resource grid.
The resource grid is a group of resource elements, which number depends on the numerology and carrier, that is defined by a number of subcarriers (Ngridsize,µNscRB), that corresponds to the RB resource grid length multiplied by the subcarriers number per RB, and OFDM symbols. In the Figure 3.1 was considered an OFDM symbols number correspondent to one subframe (Nsymbsubf rame,µ). A single resource grid is given for an antenna port, and a specific transmission direction (DL or UL).
The resource block grids has a common reference point, called Point A. This reference point normalizes the resource blocks position with himself, originating the reference point for the common resource blocks.
The common resource blocks, nµCRB, depends on the used numerology and it is given by nµCRB =jNkRB
sc
k
, where k is defined relative to point A. A group of contiguous common resource blocks is called Bandwidth Part (BWP), for a given numerology (µi) in bandwidth part i. Inside the BWP, the resource blocks are called Physical Resource Block and numbered from 0 to NBW P,isize − 1, where i is a BWP and Nsize
Figure 3.1: Resource grid (based on [4]).
within the BWP. The UE expects to receive the channels inside an active bandwidth part [14].
An antenna port, p, is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed [14]. In other words, each antenna ports is a logical concept and it allocates a specific set of reference signals in the resource grid, so the channel properties for a single resource element can be assumed for the entire reference signal. This fact indicates that a data demodulation can use the channel information obtained by a single analysis to a resource element, applied to entire reference signal [25].
This means that physical channels and physical signals are transmitted using the resource elements that will be mapped in resource grid.
3.2 Frame Structure
The resource grid is dependent on the subcarrier spacing configuration, which corresponds to the multiple OFDM numerologies where the number of OFDM symbols per subframe will change according to the numerology.
The higher layers configure the parameter subcarrierSpacing and cyclicPrefix that change the spacing between the subcarriers and consequently the frame structure, according to the next table [14] (Table 3.1). The numerology 5 (480 kHz and cyclic prefix normal) is referenced in some literature, but not specified in Release 15. For transmissions below the 6 GHz, numerologies up to two (60 kHz) are more indicated, and above 6 GHz other numerologies are more appropriate.
The subcarrier spacing for a given numerology, ∆fscµ, can be express simply by ∆fsc,
because in NR is implicit that the subcarrier spacing is specific for a given numerology.
Table 3.1: Supported numerologies (Release 15).
µ ∆f = 2µ· 15[kHz] Cyclic prefix 0 15 Normal 1 30 Normal 2 60 Normal, Extended 3 120 Normal 4 240 Normal
3GPP [14] defines two variables in time domain referring to the basic time unit for NR (Tc) and for LTE (Ts), with a constant relationship between them, κ, presented on Equation 3.1 and 3.2, respectively.
Tc= 1/(∆fmax· Nf), ∆fmax = 480 · 103, Nf = 4096; (3.1)
Ts= 1/(∆fref · Nf,ref), ∆fref = 15 · 103, Nf,ref = 2048; (3.2)
The basic time unit reference for NR is the inverse of the multiplication between the subcarrier spacing and the FFT length (explained in OFDM subchapter 3.7.1). The basic time unit for LTE is used as reference for all NR numerologies, and is calculated as the inverse of the multiplication between the LTE reference subcarrier spacing and the LTE FFT length.
The constant κ is defined by the division of the basic time unit for LTE by the basic time unit for NR, Equation 3.3. This constant is used to calculate different parameters in frame structure, for example, the CP samples [14].
κ = Ts Tc = 1 ∆fref·Nf,ref 1 ∆fmax·Nf = ∆fmax· Nf ∆fref· Nf,ref = ∆fmax ∆fref · Nf Nf,ref = 64 (3.3)
This constant κ is the normalization between the subcarrier spacing and the normalization of the FFT length, and corresponds to the proportion between LTE and NR. In this case, the maximum values are used for NR, but the values are changed accordingly to the case, more concretely, depending of the bandwidth channel and the numerology.
DL and UL transmissions are organized into frames, each one with 10 ms duration and consisting of ten subframes, each with 1 ms. Another division is effectuated in half-frames, where each frame is divided in two equally-sized subframes. This structure in frames, subframes and half-frames is always constant (Figure 3.2).
The structure that varies depending the numerology is the number of slots per subframe, which is given by 2µ. That is, the number of slots increases with the numerology. The slots
are numbered per subframe, nµs, with a total slots per subframe, Nslotsubf rame, or the slots are numbered per frame, nµs,f, with a total slots per frame, Nslotf rame.
Figure 3.2: Frame and Subframe coexistence.
Each slot always has the same number of OFDM symbols, 14 for cyclic prefix normal and 12 for cyclic prefix extended, independently the numerology. OFDM symbols can be classified as “downlink”, “flexible” or “uplink”, in a slot. It happens that, the number of OFDM symbols in a subframe is Nsymsubf rame,µ = Nsymbslot · Nslotsubf rame,µ, where the Nsymbslot = 10 and the last parameter is variable depending on the numerology, as depicted in Figure 3.3 [14].
Figure 3.3: Slots in different numerologies.
The start of first OFDM symbol is aligned with the start of the slot, and in a DL transmission the UE can assume that symbols are only “downlink” or “flexible”. In the UL transmissions, the same can be assumed, but in this case, the symbols are only “uplink” or “flexible”.
The OFDM symbols per frame, the number of OFDM symbols per slot, and the number of slots per subframe, are summarized in Table 3.2.
Table 3.2: Number of slots per subcarrier and consequences.
µ Nslotsubf rame,µ Cyclic prefix Nsymbslot Nslotf rame,µ Nsymbf rame,µ
0 1 Normal 14 10 140 1 2 Normal 14 20 280 2 4 Normal 14 40 560 2 4 Extended 12 40 480 3 8 Normal 14 80 1120 4 16 Normal 14 160 2240
Because this structure is divided in slots, this permits that transmissions are self-contained, so that the data in a slot is decodable without depending on other slots but implies that the reference signals to be used are included in the slot. This frame structure provide timing flexibility between slots, avoiding static timing relations and allowing different transmission directions across slots.
Allying the flexibility of the NR frame structure to the fact that the transmissions can be TDD or FDD, new types of transmissions are enabled, such as, transmissions of variable length, dynamic TDD, fast HARQ acknowledgments and very low latency. To improve the latency, mini-slots (2, 4 or 7 OFDM symbols) are very useful because they can start at any time and they are more shorter than a slot, making the transmission more faster. It is the case of low-latency scenarios, Ultra-reliable and low-latency communication, where the transmission needs to begin immediately [21].
In NR, the concept of mini-slots has introduced by 3GPP to support transmissions shorter than regular slot duration. Mini-slots can start at any OFDM symbol and are useful in various scenarios.
• Low-latency transmissions;
• Transmissions in an unlicensed spectrum; • Transmissions in the millimeter-wave band.
The 5G NR frame structure is designed to obtain low latency, and for this reason the control signals and reference signals are sent in the first symbols.
Each frame carriers the time-frequency resources, where different channels and signals are transmitted. The relation between them differs in DL and UL transmissions.
3.3 DL Overall View
A general view of the DL in PHY is showed in the Figure 3.4, where the DL PHY is decomposed in its different physical channels and signals.
Next, the various DL physical channels and signals are described, as well as, the types of information they carry and the global behavior.
3.3.1 DL Physical Channels
Physical channels carries information from higher layers. Table 3.3 describes the physical channels functions for DL.
Table 3.3: A brief description about the DL physical channels.
Physical channels Description
Physical Downlink Shared Channel (PDSCH) Downlink data transmission
Physical Downlink Control Channel (PDCCH)
Downlink control information (Includes scheduling decisions
of PDSCH reception and permissions for PUSCH transmission
by the UE)
Physical Broadcast Channel (PBCH) Broadcasting system information (Required by the UE to access)
In the DL, the UE monitors the PDCCH as often as required to enable ultra low latency transmissions. After that, when the UE detects a valid PDCCH, it receives one transport block on the PDSCH. Then, the respective answer with HARQ acknowledgment is sent to gNB, for the respective treatment [21].
3.3.2 DL Physical Signals
While the physical channels carry information from higher layers, physical signals do not establish contact with higher layers. The physical signals are generated in physical layer and are used as reference signals for demodulation, channel estimation, channel state information and synchronization.
The table 3.6, shows the different physical signals that are used in UL.
Table 3.4: Physical signals used in the Downlink.
Downlink
Demodulation Reference Signal DM-RS Phase Tracking Reference Signal PT-RS Channel State Information Reference Signal CSI-RS
Primary Synchronization Signal (PSS) Secondary Synchronization Signal (SSS)
NR only transmits the reference signals when necessary, thus avoiding the so-called
always-on transmissialways-ons, this is in calways-ontrast to LTE. The main improvements are network energy
efficiency and interference reduction. The reference signals enhance the synchronization and removal of errors introduced over the link.
Nevertheless, each reference signal have a specific function in the PHY. The two syn-chronization signals play the main role in the initial connection. Regarding the DL, the PSS is used to know the timing space of frame and, with this timing reference space, the SSS specifies the cell-ID. Those signals have a particular sequence that have the same frequency response, the m-Sequence. These signals, together with Physical Broadcast Channel (PBCH) and the respective Demodulation Reference Signal (DM-RS), forms the Synchronization Signals/PBCH block (SS/PBCH). In LTE, the PSS is a little different because it uses the Zadoff-Chu sequence.
SS/PBCH Block
The SS/PBCH block, in time domain, is a group of four OFDM symbols that are mapped with a specific order [14]:
1. PSS in the first position;
2. PBCH with DM-RS in second, third and fourth position; 3. SSS in the third position.
The structure described above is depicted in the Figure 3.5.
Figure 3.5: SS/PBCH block (retrieved from [4]).
An SS/PBCH block is also configured by higher layers, for example in terms of subcarrier offset, and consists in 240 continuous subcarriers, that correspond to 20 physical resource blocks (will be explained below).
The DM-RS that belongs to SS/PBCH is a signal that is always with the physical channels (as well as the Phase Tracking Reference Signal (PT-RS)) and this signal is used for demodulation and estimation of the radio channel. The DM-RS and PT-RS design supports low-latency applications, both in low-speed or in high-speed scenarios. This is possible because of its location at the beginning of a slot and because the time density of DM-RS in a slot is dynamically assigned.
In comparison with LTE, the NR introduces a new reference signal, the Phase Tracking Reference Signal, to compensate the oscillator phase noise, which is directly proportional to
the oscillator carrier frequency. In other words, this signal is introduced because in mmWave frequencies, the oscillator phase noise causes mainly Common Phase Error (CPE), which leading to a phase rotation in all subcarriers. Thanks to CPE, the phase rotation is identical for all subcarriers in OFDM symbols, however, across OFDM signals there is low correlation of phase noise.
Therefore, the frequency density for the PT-RS is one subcarrier in a specific Physical Resource Block (PRB), and the timing density of the DM-RS is a determined OFDM symbol [21]. So, the DM-RS and PT-RS in a time-frequency structure has a relative position between them, as shown in the figure 3.6.
Figure 3.6: An example of DM-RS and PT-RS in a time-frequency structure (based on [21]). These two reference signals are UE-specific, that is, the UE confines the signals in a resource scheduler to be allocated. In this case, the PT-RS configurations depend on the quality of the oscillator, because being the objective to mitigate the oscillator phase noise, the scheduler needs to take them into account.
The PSS and SSS are DL reference signals, while the DM-RS and PT-RS helps in OFDM symbols demodulation in both directions. The latter signals are used in the demodulation of each slot, instead of providing information about the transmission (Cell-ID, per example). Another DL reference signal is the Channel State Information Reference Signal (CSI-RS). It is used for Channel State Information (CSI) acquisition, beam management, time/frequency tracking and uplink power control. Similar to the other signals, it is also flexible to support diverse use cases. At the level of CSI acquisition, the signal is used to determine different parameters for link adaptation and for determining precoders. The interference measurements in the UE can be made with CSI interference measurement (CSI-IM) resource, even as the CSI-RS evaluates candidate transmissions beams by measuring the Reference Signal Received Power (RSRP) for each beam. For channel estimation and demodulation, the SRS is also used and have a specific name, a Tracking Reference Signal (TR-RS) that can be used for fine time and frequency synchronization, delay spread and Doppler estimation [21].
3.4 UL Overall View
A general view about UL physical channels and signals in PHY is depicted in the Figure 3.7.
Figure 3.7: Channels and signals for uplink.
3.4.1 UL Physical Channels
Similar to the DL, the UL physical channels carries information from higher layers, and Table 3.5 describes the physical channels functions for UL.
Table 3.5: A brief description about the UL physical channels.
Physical channels Description
Physical Uplink Shared Channel (PUSCH) Uplink data transmission Uplink control information Physical Uplink Control Channel PUCCH (HARQ feedback acknowledgments,
scheduling request and channel-state information for link adaptation)
Physical Random Access Channel (PRACH) UE request connection setup (For the random access)
In the case of the UL, the UE needs physical time-frequency resources to transmit data. For this, a scheduling request is sent over the PUCCH to the gNB. After the reception of this request, the gNB sends a scheduling grant over the PDCCH which grants time-frequency resources to the UE for transmission. Respecting the scheduling order, the UE sends the data over the PUSCH and the gNB receives uplink data and respond with HARQ acknowledgement in order to indicate if the Uplink data transmission was adequately decoded or not, and in the last case a retransmission is scheduled [21].
To activate ultra low-latency communication data, the network can configure data trans-mission resources for a UE (grant-free transtrans-mission). One problem of this scheme, is that the UE does not need these time-frequency resources every time.
3.4.2 UL Physical Signals
The UL physical signals includes some physical signals used also in DL, which is the case of the DM-RS and the PT-RS. This happens because these signals are intended for channel estimation and the BS uses them to demodulate the PUSCH and PUCCH.
Table 3.6: Physical signals used in the Uplink.
Uplink
Demodulation Reference Signal DM-RS Phase Tracking Reference Signal PT-RS
Sounding Reference Signal SRS
The SRS is a "CSI-RS" of the uplink, because it is used for CSI measurements, for link adaptation and scheduling, this signal is modular and flexible to support UE and gNB capabilities, including the reciprocity-based precoder. For NR, uplink beam management and massive MIMO is supported by SRS.
The transmission of data between the gNB and UE is ruled by the exchange of information contained in each physical channel, depending of their function.
The main feature about all of these reference signals is flexibility, the communication between UE and gNB, making available updates about radio channel and the status of each of the stakeholders in communication.
3.5 Modulation and Channel Coding
To meet the requirements imposed by the use cases, in the case of Massive machine-type
communication (mMTC) new modulation schemes are introduced to reduced peak-to-average
power ratio and enhanced power efficiency at lower data rates, which is the case of π/2-BPSK for the uplink. To cover a greater range of use cases, new modulation schemes may be introduced, such as 1024-QAM, for different UE categories [17].
NR also supports Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), 64 QAM and 256 QAM modulation formats for uplink and downlink (as in LTE).
Low Density Parity Check (LDPC) codes helps in guarantee the Enhanced mobile broadband scenario, because LDPC codes in a multigigabits-per-second data rates are very attractive from an implementation perspective. Moreover they use a rate-compatible structure that allows transmissions at different code rates, besides for HARQ operation they use an incremental redundancy. For the physical layer control signaling, NR uses polar codes. Concatenating with another code, it is acquired a good performance for short block length, by performing successive cancellation list decoding (for example, Reed-Muller code) [21].
