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1、1ContentsList of Figures.3List of Tables.41.Introduction to Terahertz Communication.51.1THz Channel Characteristics and Influences.51.2Research on THz Communication in China and Overseas.51.3Progress in Standard Development.71.4THz Spectrum Utilization.81.5Government-Funded Research Programs on THz

2、Communication.92.Typical THz Applications.132.1Indoor/Outdoor Wireless Transmission.132.2THz Communication at the Micro-nano Scale.162.3Inter-Satellite Communication in Air-and-Space Integration.172.4Joint Communication and Sensing based on the THz Band.173.Core Devices of THz Communication.203.1THz

3、 Signal Source.203.2THz Related Key Devices.223.2.1THz Power devices.223.2.2THz Frequency Converter.233.2.3General-purpose THz Components and Subsystems.243.3THz Antenna.253.4THz RF System Integration.264.Thoughts on THz Channel Measurement and Modeling.284.1New Thoughts on THz Channel Measurement T

4、echnology.284.1.1Ultra-broadband Channel Measurement.284.1.2THz/Sub-THz Band Channel Measurement.294.1.3Overcoming High Path Loss.294.1.4Angle Domain Channel Parameters Measurement.304.2THz Channel Estimation and Channel Modeling.314.2.1Channel Parameters Estimation.314.2.2Channel Modeling.325.Key T

5、echnologies and Challenges of THz Communication.345.1New OTATechnologies.3425.1.1Modulation and Coding Technology.345.1.2Waveform Design.355.1.2.1Waveform Design Strategy.365.1.2.2THz(New)Waveform Design.385.1.2.3Physical Layer Numerology Design.415.1.3Synchronization.455.2Massive MIMO Technology.45

6、5.2.1PhasedArray Antenna Architecture.455.2.2Beam Tracking.485.2.3Spatial Multiplexing.495.3THz-Related Network Layer Technology.505.4THz and Intelligent Reflecting Surface(IRS).515.5Joint Communication and Radio/Radar Sensing.526.Challenges to Design and Testing of THz Communication Prototype Syste

7、ms.576.1Challenges to the Design of THz Hardware Systems.576.2Development and Testing of THz Prototype Systems.617.6G Development Planning and Suggestions based on the THz Band.688.References.703List of FiguresFigure 1:Channel Attenuation Caused by Different Environmental Factors in DifferentFrequen

8、cy Bands 1.5Figure 2:THz Publications in Recent Years 1.7Figure 3:Target Application Scenarios of IEEE 802.15.3d.8Figure 4:High-speedAccessApplication(Source:Internet).13Figure 5:FWAOperating Mode.14Figure 6:Application of Backhaul Link.14Figure 7:Application of Secure Communication(Source:Internet)

9、.15Figure 8:High-speed In-Vehicle Communication.16Figure 9:THz-based Micro-nano Communication.16Figure 10:THz-based Inter-Satellite Communication(Source:Internet).17Figure 11:(a)Specular reflection of objects in the microwave frequency band;(b)Diffusereflection of the same objects in the visible and

10、 infrared bands;(c)Specular reflectionand diffuse reflection in the THz band 15.19Figure 12:Technology Routes of THz Signal Source 16.21Figure 13:The 50 to 250 GHz Broadband InP HBT Power Amplifier MMIC Series Releasedby Teledyne.23Figure 14:ADiode Design.24Figure 15:PAs Saturated Output Power vs Ca

11、rrier Frequency 31.37Figure 16:Symbol Error Rate(SER)Comparison Between OFDM and SC-FDE.39Figure 17:Sub-Carrier(1024),Data Block Size(256),Oversampling Factor(4).PAPRComparison Between DFT-s-OFDM and OFDM.40Figure 18:PAPR Comparison Between OFDM,DFT-s-OFDM,UW-DFT-s-OFDM,SC-FDE,OTFS and DFT-s-OTFS.41

12、Figure 19:BER Comparison Between OFDM,DFT-s-OFDM,UW-DFT-s-OFDM,SC-FDE,OTFS and DFT-s-OTFS in Case of Doppler Shift.41Figure 20:Numerology Defined in Communication System 1.42Figure 21:Link-Level Performance When Phase Noise Exists:a)OFDM and b)SC-FDMAuse Rel-15 compliant PTRS;c)OFDM and d)SC-FDMA us

13、e enhanced PTRS structures,90 GHz carrier,rank-2.43Figure 22:Major Design Principles/Methods of Numerology in Traditional Communication4System 34.45Figure 23:Fully-Connected Architecture of Hybrid Beamforming.47Figure 24:Sub-array Architecture of Hybrid Beamforming.47Figure 25:Hierarchical Generaliz

14、ed Architecture of Hybrid Beamforming.48Figure 26:JCAS Application on WiFi and 6G.53Figure 27:CSI Measurement Error Sources with Typical WiFi 2.4 GHz Receiver with DirectDown Conversion Architecture 44.55Figure 28:Phase Noise of Sub-THz Up-Converter Design.58Figure 29:Phase Noise Simulation Result.5

15、9Figure 30:Band-Pass Filter and Power Amplifier Added to the Sub-THz Up-ConverterDesign.60Figure 31:Simulation Result with the Band-Pass Filter and Power Amplifier Added.60List of TablesTable 1:Overview of Global THz Studies.6Table 2:List of THz Communication Spectrums Identified by WRC-19 11.9Table

16、 3:List of Government-Funded THz Programs.10Table 4:Characteristics of Typical Semiconductor Materials.2351.Introduction to Terahertz CommunicationThis chapter briefly introduces the characteristics and the status quo of terahertz(THz)communication,including THz channel characteristics,research stat

17、us at home and abroad,IEEEs progress in developing THz standards,frequency spectrum involved,and somegovernment-funded research programs on THz communication.1.1THz Channel Characteristics and InfluencesTo enable efficient wireless communication in the THz band,investigations of channelcharacteristi

18、cs in this frequency range are needed.The THz band features the uniquereflection and scattering attenuation of high frequency bands,as well as the spatialdistributions of specular reflection and non-specular reflection along the propagation paths.Highly directional beams are usually used to combat t

19、he high path loss in the THzband,however they result in beam misalignment in the frequency domain even for servingmobile users in a small area only.The main propagation characteristics of THz waves areshown in Figure 1,which must be considered in THz channel modeling.Existing channelmodels cannot be

20、 applicable to the THz band,because they cannot capture and emulatedifferent phenomena,including attenuation and noise due to molecular absorption,scatteringcaused by particles with an equivalent wavelength of THz,and scintillation resulted from THzradiation.Such propagation characteristics ask for

21、a new channel model to effectivelycharacterize channels in the THz band.In particular,in the lower THz spectrum,such as thefrequency band below 500 GHz,channel characteristics need to be further explored andanalyzed.Figure 1:Channel Attenuation Caused by Different Environmental Factors in Different

22、Frequency Bands 11.2Research on THz Communication in China and Overseas6The term Terahertz was first proposed by the International Microwave Symposium in the1970s to describe interferometers frequency spectrum,diode detectors coverage,and water-laserresonance among others 1234.In around 2000,THz was

23、 referred to as sub-millimeterwaves in the frequency range from 100 GHz to 10 THz.However,there was not a clear definitionon the boundary between sub-millimeter wave and far-infrared.The concept of using THz forultra-broadband communication based on non-line-of-sight(NLOS)signal components was first

24、proposed in article 5,which was considered as a viable solution for extremely high data ratetransmission.Since then,the application of THz technology in communication has been a focus inacademic studies.The relevant studies have been captured in the statistics of IEEE and Web ofScience publications

25、in recent years(as shown in Figure 2).Through joint efforts,research teamsaround the world are developing new designs,new materials and new processes,highlighting thehuge prospect of the THz technology.Table 1 is an overview of global studies in the field of THz(partially sourced from 1).Table 1:Ove

26、rview of Global THz StudiesTeam/LabRegionResearch InterestsMittleman Lab,Brown UniversityUnitedStatesTHz physical layer,THz spectroscopy,THzdetectionBroadband Wireless Networking Lab,Georgia Institute of TechnologyUnitedStatesTHzphysicallayeraccesslayer,THzmicro-nano communication,THz devicesNaNoNet

27、working CenterSpainTHz micro-nano communicationUltra-broadbandNano-Communication Laboratory,University at BuffaloUnitedStatesTHzphysicallayeraccesslayer,THzmicro-nano communication,THz devicesTerahertz Electronics Laboratory(UCLA)UnitedStatesTHzsources,detectors,spectrometers,reconfigurablemeta-film

28、,imagingandspectroscopyMIT Terahertz Integrated ElectronicsGroupUnitedStatesSensing,metrology,securityandcommunication in the THz bandFraunhofer Institute forApplied SolidState Physics IAFGermanyTHz physical layer access layer and RFelectronicsTerahertz CommunicationsLabGermanyChannel measurement an

29、d modeling,THzreflectorsNTT Core technology laboratoryGroupJapanTHzintegrateddevicesandmodulartechnologyTexas Instrument KilbyLabUnitedStatesSub-THz ultra-low power CMOS systemsTonouchi Lab,Osaka UniversityJapanTHz nanoscience,THz bioscience,THzbiosensing,and industrial applicationsTHz Electronics S

30、ystems Lab,KoreaUniversitySouthKoreaTHz physical layer access layer and RFelectronicsNanocommunications Center,Tampere University of TechnologyFinlandTHzphysicallayerandmicro-nanocommunication7University of Electronic Science andTechnology of ChinaChinaHigh-power THz sources,new THz sources,THzquasi

31、-opticaldevices,THzcommunication,THz active metasurface,key technologies in the front part of THzimaging systems,THz high-speed directcontrol devices and applicationsCETC No.13 Research InstituteChinaTHzmixers,frequencymultipliers,amplifiersFigure 2:THz Publications in Recent Years 11.3Progress in S

32、tandard DevelopmentStandard development for future wireless communication systems in the THz band wasinitiated by the IEEE 802.15 THz Interest Group in early 2008.In 2013,the IEEE 802.15WPAN Task Group 3-D 100 Gbit/s wireless(TG 3d 100 G)6 was officially established todevelop 100 Gbps wireless commu

33、nication standards for the frequency range from 275 GHzto 325 GHz.Thanks to the TGs efforts,IEEE 802.15.3d-2017,the worlds first wirelesscommunication standard,was approved on September 28,2017 and released on October 12,2018 7.This standard targets at a series of THz communication application scena

34、rios,including Kiosk download,intra-device communication,wireless backhaul and fronthaul,andradio links in data centers(as shown in Figure 3).Relevant application cases,performanceand system function requirements are defined in the application requirements document ofIEEE 802.15 3d 8.8Figure 3:Targe

35、t Application Scenarios of IEEE 802.15.3dThe standard covers the new physical layer technology and the MAC layer technologythat support 8 channels with a bandwidth from 2.16 GHz to 69.12 GHz.In addition,itsupports seven modulation formats(BPSK,Quadrature Phase Shift Keying(QPSK),8-PSK,16-QAM,64-QAM

36、and OOK)and three encoding formats(RS(240,224),14/15-LDPC and11/14-LDPC).The standard also supports single-carrier mode and OOK mode at the physicallayer.The channel modeling document 9 summarizes channel propagation characteristics oftarget scenarios,and specifically proposes channel models based o

37、n different applicationscenarios.As the next step,the standard will look into the interference of THz communicationon the frequency bands identified by the International Telecommunication Union(ITU)andmake it available for radio astronomy,satellite earth exploration,and space research servicesamong

38、other applications 10.As is known to all,the power loss in THz propagation ismainly attributed to the absorption of water vapor.Therefore,an important research area inthe next step is detailed analysis of regional propagation characteristics.The wireless THz connectivity specified in IEEE 802.15.3d

39、is only applicable to fixedpoint-to-point links.THz-based applications in WLAN are still to be explored further.Currently speaking,IEEE 802.15 TAG THz is also exploring new solutions for the physicallayer and the link layer in the sub-THz and THz bands within the 10 GHz to 100 GHz range.The ideas an

40、d solutions proposed by IEEE(802.15.3d)will contribute to the implementationof THz communication and set a foundation for 6G wireless systems in the future.1.4THz Spectrum UtilizationIn terms of THz communication spectrum,while ensuring some passive services,such asradio astronomy and Earth Explorat

41、ion Satellite Service(EESS),can be protected from harmfulinterference,radio regulations allow the use of spectrum above 275 GHz.The 2015 WorldRadiocommunication Conference(WRC)gradually identified the spectrums for terrestrial mobileservices and fixed services in the range from 275 GHz to 450 GHz wh

42、ile ensuring no interferenceto passive services,and these spectrums were discussed under the WRC 2019 agenda item 1.15.The frequency bands identified by the WRC 2019(275-296 GHz,306-313 GHz,318-333 GHz9and 356-450 GHz)will be used for implementing land mobile services and fixed services,but inthe ra

43、nge of 296-306 GHz,313-318 GHz,and 333-356 GHz,certain restrictions need to beintroduced.Adding upon the spectrum previously allocated from 252 to 275 GHz,a total of 160GHz can be used for THz communication in the range from 275 GHz to 450 GHz with no specificEESS protection requirements.Table 2:Lis

44、t of THz Communication Spectrums Identified by WRC-19 11Frequency Band(GHz)Rules for Spectrum Use252-275Prioritize land mobile services and fixed services275-296For land mobile services and fixed servicesNo specific EESS protection requirements306-313318-333356-450296-306Limited use for land mobile

45、services and fixed services,withEESS protection requirementsThe Confederation of European Posts and Telecommunications(CEPT)completed usagespecifications for the 92 to 114.25 GHz(W band)and the 130 to 174.8 GHz(D band)in 2018 12.Other than that,there are so far limited regulations on using 90 GHz an

46、d above frequencies inEurope.Fixed wireless services operating in the spectrums defined above feature the following:1)Very large available bandwidth that allows low-cost data traffic in the areas of diversified serviceproviders;2)The deployment of radio links is more feasible than that of wired conn

47、ections;3)Low signal interference and low interception probability,which ensuring high security of signals.In 2019,in order to address the ETSI requirements for radio measurement applications in thefrequency range from 120 GHz to 260 GHz,CEPT developed standards for the correspondingspectrums.Also i

48、n 2019,the Federal Communications Commission(FCC)decided to apply newregulations to frequency bands above 95 GHz,so as to accelerate the application of sub-THz andTHz bands 13.This means that experimental licenses can be obtained for the frequency rangefrom 95 GHz to 3 THz,as well as the unlicensed

49、spectrum at 21.2 GHz.1.5Government-Funded Research Programs on THz CommunicationThe THz band guarantees a large throughput,and can theoretically expand the availablefrequency spectrum to several THz and achieve a capacity of Tbps 10.Because of the hugepotential of THz technology,extensive research i

50、scarrying out,along withnew designs,newmaterials and new manufacturing technologies in the THz band.Governments and scientificinstitutes around the world wish to open a new horizon for 6G in terms of communication anddevices,and are therefore providing robust support for the various THz programs.Tab

51、le 3 lists thelatest THz programs projects at home and abroad(partially sourced from 1).10Table 3:List of Government-Funded THz ProgramsProgramFunded byCommencementObjectivesR&DProgramonExpandingRadioSpectrum ResourcesTheMinistryofInformationandCommunicationsand the Ministry ofEducation,Culture,Spor

52、ts,Science andTechnology,Japan2008Explore new technologies toincrease spectrum utilization,promote shared spectrum useandadoptionofhigherfrequency bandsWirelessLANCommunicationTechnologyinTHzbandSouthKoreangovernment/IITA2008ExploreTHzWLAN/PANcommunicationsystemsbased on electronic devicesRoomtemper

53、atureTHzemissionanddetectionsemiconductornano-devices(ROOTHz Program)The EU FrameworkProgrammeforResearch2010Manufacturesolid-statetransmittersanddetectorsoperating in the THz bandTERAPAN:ControllableAntenna-basedUltra-High Data RateTransmissionSystemfor the THz BandFederal Ministry ofEducationandRe

54、search,Germany2013DevelopanindoorTHzadaptivewirelesscommunicationprototypewith a data rate of 100 GbpsiBROW:Omni-presentUltra-BroadbandWirelessCommunication basedonTHzTransceiverTechnologyHorizon 2020 of EU2015Exploreinnovativeultra-broadbandshort-rangewirelesstransceivertechnology with a low costan

55、d high efficiencyTERAPOD:THz-basedUltra-BroadbandWirelessAccessNetworkHorizon 2020 of EU2017Demonstrate THz radio linkcommunicationandproof-of-conceptThoR:THzEnd-to-endWirelessCommunicationSystemDeliveringHigh Date RateHorizon 2020 of EUandtheNationalInstituteofInformationandCommunicationsTechnology

56、(NICT),Japan2018Propose solutions to supportthedatabackhaulandfronthaul of the 300 GHzfrequency band11ULTRAWAVE:mmWaveTravelingWaveTube-basedLarge-capacityWirelessCommunication in the100 GHz and aboveFrequency BandsHorizon 2020 of EU2017Achieve 5G cellular intensityby employing the 100 GHzand above

57、frequency bandsto develop large-capacitybackhaul linkTERRANOVA:THzWireless TransmissionTechnology to Deliverthe Quality Experienceof Optical NetworksHorizon 2020 of EU2017Provide a reliable connectionwith high rate and almostzero latency to realize theevolution from optical fiberto wirelessEPIC:Next

58、-generationof Channel Encodingfor Tbps TransmissionHorizon 2020 of EU2017Explore new FEC encodingtechnology to enable Tbpswireless transmissionDREAM:D-bandWirelessSolutiontoAchieveaReconfigurableMeshNetwork beyond 100GbpsHorizon 2020 of EU2017Explore a wireless backhaulsolutionwitha datarateexcellin

59、g the current V-bandand E-band to achieve thedatarateof opticalfibersystemsWORTECS:WirelessOptical and Radio THzCommunicationsHorizon 2020 of EU2017Developawirelesstransmissiontechnologycapable of delivering Tbpsrate in the frequency bandabove 90 GHz through theintegration of photonic andelectronic

60、technologiesTeraNova:IntegratedTest Platform for THzCommunicationNationalScienceFoundation(NSF)2017Develop the first R&D andtestplatformfortheultra-broadbandTHzcommunication networkEAGER:Research andDevelopment of THzComponents based onHigh-performanceOptical PhononsNationalScienceFoundation(NSF)201

61、7Systematically explore howtoimplementinnovativeTHzsourcesindifferentworking modesInnovativeTHzGeneratorsbasedonMagnetic MaterialsNationalScienceFoundation(NSF)2017ExploreinnovativeTHzgeneratorsbasedonthetheory of transforming THzwavesthroughmagneticoscillation12mmWaveandTHzWirelessCommunicationTech

62、nologyDevelopmentTheNational863Program of China2010Research on three terahertzcommunication architecturescovering the frequency rangefrom 0.1 THz to 7 THzTHzWirelessCommunicationTechnologiesandSystemsAmajorspecialprogramoftheMinistry of Scienceand Technology ofChina2018Inresponsetotheapplication req

63、uirements ofhigh-speedspacetransmissionandthenext-generationofmobilecommunication,explore anoverall technical solution forTHzhigh-speedcommunicationsystems;explore channel model ofTHz spatial and terrestrialcommunication;explorehigh-speedandhigh-precisionTHzsignalcapturingandtrackingtechnology;explo

64、rehigh-speed baseband signalprocessingtechnologyfeaturinglowcomplexityand low power consumptionas well as the IC designmethod;exploreTHzhigh-speedcommunicationbaseband platform;exploreTHz high-speed modulationtechnologies,including THzdirectmodulation,THzmixed modulation,and THzphotoelectricmodulati

65、on;explore RF units for THzhigh-speedcommunication;integrateTHzcommunicationbaseband,RF and antenna to developanexperimentalTHzhigh-speedcommunicationsystem,and complete THzhigh-speedcommunicationtesting132.Typical THz ApplicationsThe THz band can provide a large amount of available bandwidth to mee

66、t the requirements ofultra-high-speed wireless connection and the Tbps rate in the future.Because of the large path lossand environmental impacts in the THz band,it has a limited coverage and is therefore moresuitable for high-speed communication within a smaller range.How to effectively utilize the

67、 largebandwidth of the THz band while ensuring reasonable coverage is one of the key problems to beresolved in the study of THz application scenarios.In consideration of the large bandwidth and theunique channel characteristics,regardless of the challenges ahead,THz communication can stillfoster a w

68、ide range of unique applications to address the potential needs of humankind in the 6Gera,such as ground-based indoor and outdoor wireless access,minimum-range communication,air-and-space integration,and joint communication and sensing services.2.1 Indoor/Outdoor Wireless TransmissionTaking overall

69、consideration of the THz channel characteristics and the current ITUallocation of the THz spectrum,the application prospects of the low-frequency part of THzspectrum in the 6G era are discussed in the below.Therefore,the THz band referred to in thebelow is mainly from 100 GHz to 500 GHz,that is,the

70、sub-THz spectrum.High-speed wireless access is the most prominent application of wireless communication.THz communication can be used in 6G cellular cells to establish a hierarchical cellular network orheterogeneous network in combination with the low frequency band systems,providingultra-high-speed

71、 data communication within a coverage of 10 m to 50 m,and offering largecapacity transmission service to hotspots.Apart from hotspot coverage,the THz band also needsto address the requirements and challenges of continuous high-speed outdoor coverage.Continuous coverage is a key requirement of wirele

72、ss communication applications.Theapplication outlook of the THz band in this area is an important indicator when considering it as akey 6G candidate technology.Figure 4:High-speedAccessApplication(Source:Internet)FWA(fix wireless access)is a wireless access operating mode that provides fixed and mob

73、ile14users with seamless connectivity between ultra-high-speed wired network and wireless devices.Itmeets the transmission requirements of HD multimedia streaming and UHD video conferencing,and addresses the last bit of challenge when extending network coverage from outdoor to indoor.Figure 5:FWAOpe

74、rating ModeTHz can support short-range communication and effectively minimize the negative effectsdue to high path attenuation and molecular attenuation in the THz band.The typical applicationscenarios include short-range high-speed download(such as KIOSK).Such applications requireterminals to be ca

75、pable of transmitting at high data rates.The distance between the user and theterminal is usually less than 1 m,so as to fit in the short transmission range and the point-to-point(P2P)network topology.THz communication enables seamless interconnection between the ultra-high-speed wirednetwork and pe

76、rsonal wireless devices.By coordinating the wireless and wired link transmissionrates,it can support bandwidth-intensive applications such as HD holographic video conferencing.It can be used to establish a Tbps link among adjacent devices,such as the ultra-high-speed datatransmission between persona

77、l devices,greatly improving the data transmission rate.The THz band lays a solid foundation for the development of large-capacity wirelessfronthaul/backhaul links,and can be expanded further for the deployment of ultra-dense networksand coordinated multipoint transmission.Taking the large bandwidth

78、of THz into consideration,inthe application scenarios of backhaul and fronthaul links,large-scale high-gain directional antennaarrays can be employed at the transmitting and receiving ends to increase the beamforming gain,hence to enable long-range transmission.Figure 6:Application of Backhaul Link1

79、5Apart from the above,THz can also be used for secure communication.The mainapplications involve the following:using ultra-broadband secure communication links in militaryapplications to detect dangerous objects and investigate targets;using the multi-antennatechnology to generate extremely narrow b

80、eams that eliminate interception;using the spreadspectrum technology on ultra-broadband channels to defend interference attacks.Figure 7:Application of Secure Communication(Source:Internet)The vehicle-to-vehicle,vehicle-to-infrastructure,and in-vehicle communications on thehorizon require high bandw

81、idth connections.Autonomous vehicles need to be capable ofprocessing information and batch-downloading data in real time,so as to allow ultra-high-speeddata transmission and massive data upload,and optimize the large-scale cloud traffic.The THztechnology is a reliable solution for such applications(

82、as shown in Figure 8).But we need toresolve many related challenges,such as vehicle scheduling,autonomous link establishment,inter-regional vehicle control and switching,map planning,and THz spectrum utilizationefficiency,etc.16Figure 8:High-speed In-Vehicle Communication2.2 THz Communication at the

83、 Micro-nano ScaleA high-speed THz wireless link can connect two or more PCBs,or connect different chips onthe same PCB of a device,to enable wireless high-speed data exchange within a small section ofa device.Through planar nano-antennas,THz communication allows the wireless on-chipnetwork to scale

84、effectively,shaping an ultra-high-speed link that meets the strict requirements offootprint-limited and communication-intensive on-chip scenarios,usually with a target data rate ofup to Tbps.In addition,short-range communication based on the THz band can be used forwireless connections between large

85、-scale data centers,allowing high-speed communication amongthe servers and eliminating the complexity of wired network design and cabling.Figure 9:THz-based nano Communication17THz features a wavelength of the molecular scale,which makes it possible to monitor blood,cholesterol,tumor biomarkers,etc.

86、through nano sensors,build nano sensor networks to collecthealth-related user data,and report health data via the wireless interface between nano sensors andmicro devices,thereby enabling health monitoring.Nano-sensors can be used for quick detectionof chemical compounds,which require a network with

87、 a large number of nano-sensor nodes to bedeployed.Different nano-scale devices can be interconnected through the IoNT,which use nanotransceivers and nano antennas for different tracking purposes.The nano-scale Internet allowsmicro devices at nano-scale to be interconnected,and supports super-scale

88、connection betweenobjects with its ultra-large bandwidth,enabling the mMTC to expand further.2.3 Inter-Satellite Communication inAir-and-Space IntegrationAs one of the key 6G enablers,air-and-space integration ensures three-dimensional coverageof the future 6G system around the globe.Space communica

89、tion no longer suffers attenuationcaused by molecular absorption or other environmental impacts.Despite the free space loss,it canefficiently utilize the large bandwidth of THz to achieve highly reliable inter-satellite datatransmission at ultra-high data rate and low latency with low energy consump

90、tion,meeting therequirements of air-and-space integrated communication in future.Figure 10:THz-based Inter-Satellite Communication(Source:Internet)2.4 Joint Communication and Sensing based on the THz BandBecause of its unique characteristics,the THz band can be used in scenarios includingwireless co

91、mmunication,recognition,sensing,imaging,positioning,and navigation.Among them,the sensing-communication integration as a major trend in 6G development has great applicationprospects,which allows highly accurate perception of the environment based on the THz band.This is mainly because the frequency

92、band can achieve very high perception accuracy at almost allphysical scales(including distance,angle,and Doppler information).18The application of the THz band greatly facilitates the joint implementation of thesensing-positioning-communication integration.THz allows environmental perception in real

93、 time.Its sub-wavelength characteristics,frequency selective resonance,and material absorptionproperties make it possible to perceive the environment according to the signal characteristicsobserved.The ultra-wide frequency spectrum of THz provides an effective link for wireless datasensing and real-

94、time calculation.The high-frequency characteristics of THz enable THz imagingand centimeter-level positioning,effectively improve positioning accuracy and realize theintegration of sensing,imaging and positioning,laying a solid foundation for holographiccommunication.The THz system can reconstruct a

95、 physical space within a wide angle throughbeam scanning.Electronic beam scanning can be done in real time;therefore,THz systems can beused to measure complex environments such as offices in real time.Unlike optical imaging ofcameras,it is not affected by the light,and can penetrate some obstacles,s

96、uch as curtains,therebyallowing us to perceive in a wireless manner.In addition,wireless signals in the THz bandproduce specific vibrations to or absorption to specific objects and gases,therefore THz signalscan be used to detect specific chemicals in beverages or in the air based on the frequencych

97、aracteristics,and compact THz systems can be integrated in 6G terminals for general inspection.THz signals can also be integrated in the terminals to enable gesture recognition,thereby realizingcontactless human-computer interaction or safe scanning of the human body.By perceiving the highly accurat

98、e reconstruction of the surrounding environment,THzsystems can predict the channel characteristics of a mobile device during communication,assistthe calibration of directional antennas,enable the adaptability of real-time positioning andwireless functions,thereby allowing the sensing function to ass

99、ist and support informationtransmission.Such information fusion can also be applied to intelligent transportation,shoppingand other retail activities.Specifically,unlike cameras and other optical devices,THz systems can be used to observeNLOS objects,and are therefore suitable for rescue detection,u

100、nmanned driving and positioning.Because the light wavelength is smaller than the surface roughness of the object,observing NLOSobjects using an optical system requires more sophisticated equipment and computation.Moreover,low-frequency(10 GHz)radar systems have poor imaging capabilities and can be e

101、asily affectedby multipath reflections in a complex environment.THz systems have combined a number ofadvantages of microwave and visible light systems.The short wavelength and large bandwidthallow THz systems to achieve a sensing accuracy comparable to that of optical devices.Other thanthat,THz sign

102、als can be used for detection as microwave systems according to the spectruminformation and signal reflection.19Figure 11:(a)Specular reflection of objects in the microwave frequency band;(b)Diffuse reflection of the sameobjects in the visible and infrared bands;(c)Specular reflection and diffuse re

103、flection in the THz band 15In comprehensive consideration of the THz application in communication and sensing,theTHz-based communication and sensing integration can be used for active or passive sensingimaging.The passive imaging technology employs array imaging sensors to capture object imagesin an

104、 incoherent manner by leveraging the inherent reflection characteristics of the objects surface.The active imaging technology extracts distance,Doppler and angle information by emittingpurpose-built detection waveforms as well as analyzing and processing coherent reflection signals.These two technic

105、al solutions fit in a wide range of application scenarios:Passive THz communication and sensing integration:At present,although IC-basedthermal radiometers are highly sensitive and technically proven,the way that sensors andCMOS circuits are designed in results in the high cost of the solution.THz i

106、maging systemsbuilt on integrated CMOS have very high cost advantageActive THz communication and sensing integration:The active THz imaging technologyhas two distinct applications:active radar and object sensing.The sensing method similar tothat of active radar allows 3D imaging to evolve into 4D im

107、aging,adding Dopplerinformation to the traditional RGB channel.Active THz communication and sensing systemscan be used for security inspection scanning,smart shopping,gaming and entertainmentapplications.In summary,because of the super-large bandwidth available,THz is a promising technologyfor terre

108、strial,air,and space wireless communication.It supports communication at micro-nanoscale,and has a great potential in the study of communication and sensing integration.Analyzing,troubleshooting,and solving THz coverage problems to achieve high-speed coverage within acertain range is of great practi

109、cal significance to accelerate the evolution and maturity of the THztechnology.203.Core Devices of THz CommunicationAs the development of mobile communication systems progresses,the communication chipand devices industry chain has matured.But for THz band,there are still great challenges in thedesig

110、n,integration,and manufacturing of many core devices.This is especially true for the RFpart above 300 GHz.For example,1)how to generate an efficient signal source,2)how to amplify,transmit and receive signals,3)how to effectively realize antenna array and beamforming,are stillin the early R&D stage.

111、It is still a long way to the maturity of large-scale commercial use.Therefore,there remain many uncertainties in the constraints and impact of the development ofTHz RF devices on the final system design.The following describes the current situation,opportunities and challenges related to THz core d

112、evices from the perspectives of signal source,antenna,and RF system integration.3.1 THz Signal SourceTHz RF signal sources for communications need to be able to generate pure THz continuouswave stably at room temperature for a long time.High quality signal source is very important forwirelesscommuni

113、cation.TherearesomecommercialTHzsignalsourceproductsinnon-communication fields such as security inspection,remote sensing,imaging,and otherapplications.But most of the sources are pulse signal sources,or the source efficiency,size orstability cannot meet the requirements needed for wireless communic

114、ation systems.21Figure 12:Technology Routes of THz Signal Source 16The frequency range that a signal source can output is essentially limited by the maximumoperatingfrequencyofitsphysicalmaterial.Differentmaterialshavedifferentdevicemanufacturing processes,which leads to more dimensional considerati

115、ons such as cost,integration,and functional flexibility(whether it is convenient to integrate programmable logic circuits).Inprinciple,THz signal sources suitable for THz communications can be roughly divided into thefollowing technical routes:1617THz oscillation source is generated based on III-V c

116、ompound semiconductordevices such as Schottky diodes.Such devices have been used in 5G millimeter wave RFsystem,belong to one of the research hotspots of monolithic microwave integrated circuit(MMIC).Because III-V materials(such as GaAs and InP)are used as substrate devices,theyhave high electron mo

117、bility,can produce high-power RF signals,and have a maximumoperating frequency of more than 1THz.They can not only be used as signal source,but alsocommonly used to realize amplification and mixing functions.They are powerful candidatesfor many key parts of THz RF system.However,most of the current

118、research is still focusedon the millimeter wave band below 300GHz.Even in the frequency band above 100 GHz,mature products are rarely seen,especially in China.THz signals can also be generated by mixing two lasers in a photodiode.Thismethod usually uses single carrier photodiodes(UTC-PD),which have

119、high bandwidth.Because the laser frequency is very high,it is very easy to get a frequency difference ofhundreds of GHz to several THz in UTC-PD.However,due to the problem of energy22conversion efficiency,the THz signal power produced by UTC-PD is low,affecting thesystem coverage.Silicon based integ

120、rated circuit THz source.Although the maximum frequencysupported by silicon based semiconductors is below 300GHz,and the efficiency is far lowerthan that of III-V semiconductors,todays the CMOS,bipolar cmos process and directintegration with logic control circuits andthe potential ofD-band and E-ban

121、d in 6Gcommunication is very mature,which is an attractive advantage for them to become terahertzsignal sources.Some studies have proposed using silicon-based CMOS designs to realizedirect chip surface current regulation and radiate THz signal,which is a promising newimplementation method of RF syst

122、em.The development of silicon-based photonicsintegrated chip may also provide some new design ideas of THz system from the perspectiveof photoelectric combination.In summary,at present,the development of THz communication technology is still subject tohigh-power signal generation technology and high

123、-sensitivity detection technology,especiallyabove 300GHz.No matter the III-V transistor,UTC-PD or silicon-based integrated circuit,it is aninefficient method to produce continuous wave THz signals,and it is difficult to meet the practicalapplication requirements of wireless communications.In additio

124、n,it should be noted that thefrequency and power of signal generation are only the most basic considerations in system design,and factors such as integration difficulty,programmable control and process cost will have animportant impact on the final technical route selection.3.2 THz Related Key Devic

125、esBesides THz signal sources,THz communication also faces a variety of challenges in thedesign and manufacturing of core devices,chips,and antenna arrays.3.2.1 THz Power DevicesMillimeter-wave and THz power devices mainly include power amplifiers(PA)and Lownoise amplifiers(LNA).PA and LNA are core p

126、ower components in millimeter wave and THztransceiver system.They are widely used in THz security check imaging,THz communication,THz radar,Radio astronomy and other fields.GaN,GaAs,InP III-V materials with higher electronmobility and better radiation resistance are the most widely used materials in

127、 THz integratedcircuits and other fields(Refer to Table 3-1).Chip level suppliers of MMW and THz power devices mainly include Teledyne,HRL,ADI,Macom,Gotmic,Ommic,Fraunhofer,Nanjing Chip Valley,CETC13,CETC55,Xiongan Taixin,Milliway,MKR,etc.MMW and THz PA and LNA module packaging and testing mainly in

128、clude:Nanjing Nuozhijie,Shanghai AT microwave,Suzhou TeraHub and so on.Under the background of international embargo,domestic power devices are stronglysupported by the government.At present,the commercial LNA covers 270 GHz and the PAfrequency covers 230 GHz.In some frequency bands,the performance

129、indicators are equal to or23better than those of foreign manufacturers,and have the ability of commercial supply.As the development of Millimeter-wave and THz application technology progresses,ordershave surged in the past two years.However,domestic chips have been subject to a series ofproblems,suc

130、h as low modeling accuracy,poor batch consistency and lack of on-chip test system.Fortunately,it has It solved the problem of the embargo.However,in general,key chips such asMillimeter-wave and THz PA and LNA,especially in China,can not reach the state of large-scalecommercial,and further research a

131、nd development and investment are needed.Table 4:Characteristics of Typical Semiconductor MaterialsMaterialBand Gap(Ev)ThermalConductivity(W/(cm.K)ElectronMobility(cm/V.s)Electrons SaturationDrift Velocity(*107cm/s)FrequencyRange(GHz)GaAs1.420.455500-70002.110-300GaN3.491.3400-16002.510-150InP1.350.

132、6810000-120002.350-1100Figure 13:The 50 to 250 GHz Broadband InP HBT Power Amplifier MMIC Series Released by Teledyne3.2.2 THz Frequency ConverterMillimeter-wave and THz frequency converting devices mainly include frequency multipliers,mixers,detectors,nonlinear transmission lines,and Gunn oscillato

133、rs and so on.Frequency converting devices in the Millimeter-wave and THz solid-state field are mainlydeveloped based on the planar Schottky diode.Schottky diodes are high-speed devices based on ametal semiconductor contact.Due to the high mobility of the majority carriers,they are able torealize mix

134、ing and frequency multiplying at terahertz frequencies for spectrum conversion.Yuenie Lau(CEO of OML,Since 1991,creatively designing of frequency multiplier,mixer,frequency converter,etc)、Dr.Thomas W.Crowe(CEO of VDI,designing diodes and etc.,now)and David Porterfield(CEO of MicroHarmonics,designing

135、 varactor diode,high-efficiencyhigh-power frequency multiplier,Gunn oscillator,transceiver system,etc.)are advanced mastersof THz field design.At present,based on planar Schottky diode,it can transmit,receive andmeasure frequencies above 3 THz.Many commercial companies in Europe and USA,such as VDI,

136、24OML,Teratech,RPG,ACST,and ABmilimeter have mature chip or module products and havebeen commercialized,but the price is high and maintenance is difficult.Anti-parallel diodes for frequency mixingAnti-series diodes for frequency multiplyingFigure 14:ADiode DesignThe manufacturing of frequency conver

137、ting devices mainly includes diode manufacturing,circuit design,device packaging,testing,etc.In China,Schottky diode manufacturers mainlyinclude CETC13,CETC55,Sinano and the CAS Institute of Microelectronics.Circuit design,device packaging and testing manufacturers mainly include the CAS National Sp

138、ace ScienceCenter,UESTC,CETC41,China Academy of Engineering Physics and so on.Of course,there arealso companies with strong strength and independent design ability.At present,domesticfrequency converters have just broken through 1 THz,and the performance and reliability of thisfrequency band are sti

139、ll far from those of mainstream manufacturers in Europe and the USA.However,domestic producers can still compete below 300 GHz.There is a long way to go beforelarge-scale commercialization.3.2.3 General-purpose THz Components and SubsystemsGeneral components and subsystems in Millimeter-wave and THz

140、 solid-state field includepowerdividers,couplers,filters,duplexers,isolators,circulators,voltagecontrolledattenuators,transceiver systems,up and down converters,etc.At present,this field is in thetrend of all flowers blooming together.Many commercial companies in Europe and USA,such as Micro Harmoni

141、cs(26-500 GHz isolator,hybrid circulator,voltage controlledattenuator,etc.),Millitech(18-325Ghz),MiWAVE(18-220GHz),Elva-1,etc.,have matureproducts.At present,there are mainly CETC13，CETC55，CETC41 in China,and the commercialcompanies are not mature.However,the current general-purpose components are m

142、ainlyconcentrated in W-band and below with a few offerings available the high-frequency bands ofMillimeter-wave and THz.In terms of passive components,At present,ferrite devices(subject to magnetic core materials)do not have domesticmanufacturers of isolators/circulators with broadband above 40GHz.A

143、t MMW andTHz frequencies.MicroHarmonics is the leader in the industry.Directional couplers are subject to high-frequency absorber materials,and there arefew mature domestic manufacturers above 110GHz.Filters,duplexers,power dividers,etc.above 110GHz are mostly customized.Customized services are main

144、ly providedby Millitech,Miwv,Elmika,Elva-1,Flann,Eravant,Nanjing Nuozhijie,Shanghai ATmicrowave,Suzhou TeraHub,etc.Fundamental mixers and sub-harmonic mixers and high order harmonic mixers aremainly supplied by OML,VDI,Nanjing Nuozhijie,Suzhou Terahub,Shanghai AT25Microwave,etc.In terms of active co

145、mponents,Solid-state noise sources from 50 to 330 GHz are basically dependent on imports.Voltage controlled attenuator/programmable attenuator,etc.-at present,there are nodomestic manufacturers that realize truly independent broadband above 40GHz.Thereare Millitech,Micro Harmonics,Miwv,Elmika，Flann

146、and so on.Transceiver systems,up and down converters-It needs to be customized.It is worthmentioning that Nanjing Nuozhijie has developed band Q/V/E/W/D/220 GHz/300 GHz/400 GHz/500GHz transceiver systems,up and down converters(the SSBis 8dB typ).They have been applied to Millimeter-wave and THz rada

147、r,imagingsystems,channel simulations,6G communication prototypes,etc.,and supplied tomajor domestic scientific research institutes,universities,and enterprises.Frequency multipliers are mainly supplied by VDI,OML,RPG,ACST,Teratech,CETC-13,CETC-41,Nanjing Nuozhijie,Suzhou Terahub,Shanghai AT Microwav

148、e,etc.3.3 THzAntennaTransmission line losses are high in the THz band.Therefore,THz communication systemsrequire unprecedented high gain antennas to compensate for the huge path loss.In both academicresearch and practical application,the most commonly used THz antenna is the horn antenna.Inaddition

149、to horn antennas,patch and slot antennas,reflector antennas,and lens antennas arecommonly used.The horn antenna has the advantages of simple structure,good radiation direction,low crosspolarization,broadband operation,and can achieve tens of dB antenna gain.Reflector antennasand lens antennas compri

150、se a feed antenna and an aperture surface with a focusing function.Theycan form directional high gain beams and are mostly used in the field of radio astronomy.However,these antennas belong to mechanical antennas,which are only suitable for fixeddirection(or with extremely limited adjustable angle)p

151、oint-to-point communication scenarios.They have large volumes and are difficult to integrate with signal processing chips.In a mobile communication scenario,the most ideal THz communication should adopt alarge-scale antenna array similar to millimeter wave.Through the complex feed network,theantenna

152、 array can provide more flexible beam forming and signal gain and is closely combinedwith the logic control chip to realize dynamic beam forming.Theoretically,the antenna size ofTHz band is smaller than millimeter wave,which is more suitable for large-scale array,but it hasnot been realized in pract

153、ice.This is because,at THz frequencies,the loss of feed networks andsubstrate materials will lead to a significant reduction in the radiation efficiency of the antenna.Abasic fact is that at present,only the prototype chip of D-band(110 170 GHz)is reported in theworld for 5G millimeter wave antenna

154、array,which has not yet reached the scale of commercialuse,and there are no research achievements made in this area in China.However,there are a lot oftechnical gaps in how to realize the large-scale electronically controlled THz antenna array above300GHz,which can be broken through only by combinin

155、g a large amount of innovative work onnew materials and integration technology.These are great challenges for THz frequency band in266G communication application.For example,in terms of new materials,carbon nanotube and graphene are hot schemes inthe research of THz array antenna.New materials have

156、advantages in device miniaturization andlarge-scale antenna array design,but like THz source,performance is not the only consideration.In terms of practicability,how to reduce cost and integrate with digital processing chip is a moreimportant direction of THz antenna design.It is also a simple and f

157、easible implementation schemeto form an array based on UTC-PD integrated micro antenna.UTC-PD can mixer a laser with asignal with another CW laser to directly generate a THz RF signal and radiate it through anintegrated antenna.The signal light can propagate through a low loss integrated optical wav

158、eguide.At the same time,it can adjust the gain and delay with a mature optical signal processing method,so as to avoid the problem of directly operating the feed network in THz band.However,asmentioned above,the radiated power and sensitivity of UTC-PD as a receiver need to be furtheroptimized.In fa

159、ct,THz antenna array is facing not only the challenge of antenna design,but also thechallenge of the whole RF system integration and signal processing architecture.There are stillmany challenges to be solved on the road of large-scale business.3.4 THz RF System IntegrationIn the last ten years,THz R

160、F system integration technology has made great progress.Thisincludes the study of different substrate technologies,such as III V semiconductors and silicon,field effect transistor devices and heterojunction bipolar devices,as well as all electron and hybridelectron photon systems.Although the method

161、s of electronics and photonics often look different,there is a trend of technology integration in the terahertz frequency range.Many emerging systemscan be classified as photonically excited or optoelectronic hybrid.This trend is essentially in linewith the More than Moore development route promoted

162、 by the semiconductor industry in theface of the slowdown of Moores law 17.However,there are many other unique systemic challenges in the design of THz RF systemscompared with millimeter wave and lower frequency bands.For example,the traditional RFantenna can often be designed independently and then

163、 connected to other parts of the RF system,while in the THz band,the antenna and other modules of the RF system cannot even be connectedby cable.If the waveguide is used to build the whole RF link,the size and flexibility of the systemwill be unacceptable.In order to reduce the connection loss and r

164、educe the size and cost of RFsystem,it is necessary to consider the highly integrated design of antenna array and RF circuitfrom the beginning.The key progress of THz RF system integration may not come from the improvement of asingle device,but from the new multifunctional and reconfigurable archite

165、cture.Many traditionalelectronic system design methods will be broken.A notable example is that the size of the THzband antenna becomes smaller,and the signal wavelength can even be smaller than the size of thechip itself,which makes the on-chip antenna perhaps the most direct solution.The on-chipan

166、tenna can avoid the large loss of external antenna and reduce the challenge of broadbandswitching design.Further,each module in the traditional RF link,such as low noise amplifier27(LNA),power amplifier,mixer,up converter,detector,modulator,voltage controlled oscillator(VCO),phase shifter,switch,etc

167、.,to be implemented in the integrated chip of THz band.Whethersome traditional RF processing steps in the electrical domain can be replaced by opticaltechnology is another problem that needs to be evaluated and considered.In addition,although the high-speed ADC/DAC chip does not belong to the RF cir

168、cuit,itwill also affect the design of the whole THz system.If the goal of THz communication system isto achieve hundreds of Gbps to 1Tbps capacity,it is actually close to the capacity of the currentbackbone optical fiber transmission system.At present,the high-speed ADC/DAC chip used inthe mainstrea

169、m high-capacity optical transmission system can support up to 200Gsample/ssampling rate.However,when the power density is certain,the high quantization accuracy andhigh sampling rate of ADC/DAC can be considered as a pair of contradictions.One of thereasons why commercial high-capacity optical fiber

170、 transmission systems always use singlecarrier QAM instead of OFDM is that high-speed ADC/DAC chip is difficult to supportquantization accuracy of more than 8bit.For THz systems,the power consumption andcomplexity caused by the number of ADC/DAC chips should also be considered whensupporting multi a

171、ntenna MIMO.Therefore,the comprehensive trade-off of ADC/DAC quantity(power consumption),quantization accuracy and sampling rate will also become one of the keydesign constraints in THz communication systems.It can be predicted that for THz communication RF systems above 100 GHz and even above300GHz

172、,hardware technology routes with significant differences may be differentiated fordifferent application scenarios in the future.For example,according to the different geographicalenvironment,communication distance,number of users and mobility,the system can make atrade-off among many factors such as

173、 cost,power consumption and size,and choose to use acertain antenna form and an electric or optical or hybrid RF chip integration scheme.Compared with the upper system design,the integration of THz bottom core devices andchips may becomethe mainobstacle tothe reallarge-scaleimplementationof THzcommu

174、nication technology.The maturity of related technologies still needs the continuouspromotion of the semiconductor industry from materials,devices,processes,and other aspects,soas to be expected to reach the maturity of todays millimeter and lower band device market withinfive to ten years.This also

175、puts forward higher requirements for communication system designers,including the need to work closely with the upstream semiconductor industry to accelerate theiterative innovation and real application of THz related technologies.284.Thoughts on THz Channel Measurement and ModelingStudies on wirele

176、ss channel characteristics mainly cover channel measurement,channelestimation and modeling.The THz wireless frequency band means higher frequency,largerpropagation loss and Doppler effects.For channel measurement,the following specialrequirements need to be considered:Bandwidth:The main advantage of

177、 the THz band is to provide a large bandwidth that meetsthe large-capacity transmission requirements in future.The bandwidth required by typicalTHz applications varies from several GHz to tens of GHz,so THz channel measurement alsoneeds to support a measurement bandwidth of tens of GHz.Measuring dis

178、tance:In THz indoor communication applications,the maximum lineardistance between the receiving and transmitting terminals is around 10 meters.However,consideringthesingle/multiplesignalreflections,thechannelmeasurementrangecorresponding to the maximum path delay is about 100 meters.For outdoor appl

179、icationscenarios,the maximum path delay for measurement needs to be extended to several hundredmeters.Doppler frequency:In a static/quasi-static scenario,the transmitter and receiver arestationary.Pedestrians are the typical mobile targets in the channel,who move at a speedabout 5 km/h,corresponding

180、 to a Doppler shift of about 1.4 kHz in the 300GHz frequencyband.In certain outdoor application scenarios(such as wireless fronthaul/backhaul),thetarget(such as a vehicle)moves faster and will produce a higher Doppler shift.Based on the new channel characteristics of the above-mentioned THz band and

181、 the newrequirements for channel measurement,new thoughts have been put forth on the correspondingchannel measurement technology and channel modeling.4.1 New Thoughts on THz Channel Measurement TechnologyIn comparison with the frequency range covered by the 5G channel measurement system,themeasureme

182、nt of THz/sub-THz wireless channels are facing more technical challenges,including:higher bandwidth,higher frequency range,larger path loss and support for angle-domainmeasurement.4.1.1 Ultra-broadband Channel MeasurementIn a frequency domain channel measurement system,VNAs frequency sweep range may

183、exceed tens of GHz,satisfying the THz channel measurement requirements.In a time domainchannel measurement system,either the sliding correlation method or the broadband correlationmethod needs to use the broadbands baseband signal generator to generate stimulus signals at thetransmitting terminal.Hi

184、gh-performance arbitrary waveform generators(AWG),such asKeysights M8196Abroadband arbitrary-waveform generator,support an analog bandwidth of upto 32 GHz and a sampling frequency of 96 GSa/s,which resolve the problem of generatingbroadband stimulus signals.29At the receiving terminal,a sliding corr

185、elation system only needs to capture CIR data with asampling frequency being only 1/of the rate to transmit stimulus signals,with usually rangingfrom hundreds to thousands.It means that a low sampling frequency digitizer will be sufficient.Ina broadband correlation system,a broadband digitizer with

186、a sampling frequency that matches thebandwidth of the stimulus signal is required.In such a system,a broadband oscilloscope(such asthe Keysight UXR high-performance oscilloscope that provides up to 110 GHz samplingbandwidth and 256 GSa/s sampling frequency)or a broadband digitizer(such as the Keysig

187、htM8131A that provides up to 12.5 GHz sampling bandwidth and 32 GSa/s sampling frequency)canbe used.Another issue to be considered is how to store the channel measurement data.In theVNA-based method and the sliding correlation method,the receiving data rate is not high,amemory system that requires a

188、 data stream transmission rate of several hundred MBps can realizethe gapless storage of data.In the broadband correlation method,the higher receiving data raterequires a RAID system with ultra-broadband data transmission interfaces.For example,theKeysight M8131A digitizer has 4 optical digital inte

189、rfaces(ODI)and can transmit data at amaximum rate of 640 Gbps,capable of storing the channel measurement data received in a largedisk array system.4.1.2 THz/Sub-THz Band Channel MeasurementSignal transmitting and receiving in the THz/sub-THz band can be realized by using externalfrequency expansion

190、modules,such as an external up-converter connected to the transmitter,anexternal down-converter connected to the receiver,or an RF instrument(signal generator,signalanalyzer,oscilloscope,or VNA).As a consequence,a variety of commercial up/down convertermodules from 60 GHz to 1.1 THz can be used to e

191、xtend the frequency range of a channelmeasurement system.In a frequency domain channel measurement system,a low-frequency cable connects theVNA port and the up/down converter module to transmit IF signals.An additional LO source isrequired to provide the LO signal to the LO signal input port of the

192、up/down converter for LOsignal transmission.In a time domain channel measurement system,AWG generates IF signals at the transmittingterminal,sends them to the IF input of the up-converter,and modulates them from IF signals toTHz/sub-THz RF signal.At the receiving terminal,down-converter may be used

193、to convertTHz/sub-THz RF signals back to IF.In a sliding correlation system,IF signals are transmitted tothe sliding correlator to generate time-expanded CIR signals.In a broadband correlation system,IF signals are directly sampled by a broadband digitizer,and followed by digital down-conversionand

194、cross-correlation,in order to get the CIR data.Both the transmitting and the receivingterminal need to be connected to the LO signal,and they need to be frequency synchronized.4.1.3 Overcoming High Path Loss30According to the Free Space Path Loss(FSPL)equation,the THz/sub-THz wireless channelproduce

195、s bigger path loss than the 5G channel.For example,in a distance of 1 m,the FSPL isabout 42 dB at 3 GHz frequency,62 dB at 30 GHz frequency,or 82dB at 300 GHz frequency.Achannel measurement system needs to consider and overcome the big path loss in order to getsufficient reception power and dynamic

196、range.However,in the THz/sub-THz frequency range,the maximum output power of poweramplifiers(PA)and other RF components is much lower than the corresponding RF output powerof the 5G band.Therefore,increasing the transmitters transmission power cannot solve theproblem of path loss increase.A common a

197、nd effective approach is to use highly directional antennas at the transmittingand receiving terminals,which provide additional antenna gain to partially compensate the pathloss.Such an approach has certain application value for the time domain and frequency domainchannel detection systems.The time

198、domain channel measurement system takes another approach to increase thedynamic range,that is,to use a waveform with a longer sequence length to achieve biggercorrelation gain.For example,a PN sequence with a sequence length of 1023 gets a correlationgain of about 30 dB from the cross-correlation pr

199、ocessing,while a PN sequence with a sequencelength of 16383 gets a correlation gain of more than 42 dB,that is,an additional dynamiccoverage of 12 dB.Obviously,bigger waveform length results in longer sequence measurementtime.However,considering the large bandwidth in the THz/sub-THz channel measure

200、ment(which shortens the symbol time of the stimulus signal),the total stimulus signal time is stillacceptable for channel measurement.4.1.4Angle Domain Channel Parameters MeasurementTo cope with the larger path loss,communication systems in the THz/Sub-THz band will usehigh-gain horn antennas,large

201、antenna arrays,or other highly directional antennas to control thebeam width of transmitters and receivers.Spatial information such as the angle of arrival(AoA)and the angle of departure(AoD)of channels marks the basic features of the THz/sub-THzchannel,therefore,the angle domain channel measurement

202、 is necessary.Two methods apply in measuring the angle domain information,the Rotating Directional Antenna(RDA)method and the Uniform Array(UA)method.In the RDA method,a directional antenna(such as a horn antenna)is fixed on amechanical turntable,and circular scanning in made with a fixed step in th

203、eazimuth/elevation direction.High-gain directional antennas contribute to better dynamicrange measurement with the RDA method;however,because it takes a relative longtime to cover the required scanning range,the RDA method applies to static channelsonly.Besides,the angular resolution of the RDA meth

204、od depends on the beam width ofthe directional antenna,which is usually about 10 degrees,hence the angularmeasurement resolution is low.31In the UA method,the antenna array can be a uniform physical array or a uniformvirtual array(UVA).Array elements in the UA method are usually omnidirectional orlo

205、w-directional antennas,therefore this method cannot benefit from the high antennagain as the RDA method.On the other hand,by combining the measurement signalsfrom multiple array elements,additional measurement gain can be provided according tothe antenna arrays size.To use a physical array,multiple

206、transmitting/receiving RF channels are required.Alternatively,a single RF channel can be connected to each array antenna unit via RF switches,forming a MIMO channel measurement system that has faster measurement speed and higherangular resolution.However,for a channel measurement system in the THz/s

207、ub-THz band,someimplementation issues need to be considered before using a physical array:First,how to connectthe antenna array.A half-wave spacing antenna array in THz/sub-THz is only a few millimeters insize.For example,in the 300 GHz frequency band,a uniform linear array(ULA)of 4 elementshas a si

208、ze of 2 mm only if the element spacing is half a wavelength(i.e.,0.5 mm),while antennaswith a waveguide connector are much bigger.Integrating THz RF and antenna circuits makes apossible solution.However,at present,RF switch modules with a frequency of above 100 GHz arenot commercially available and

209、there are no commercialization cases reported,especially inChina.The UVA method does not use a physical array.Instead,it uses a single antenna for translationalscanning in the 2D/3D space domain,and forms a virtual antenna array by integrating detectionsignals of all scanning positions.The UVA metho

210、d eliminates the problems of connecting smallerdimension antenna arrays and not having RF switches available,and leverages larger virtual arraysto obtain higher angular resolution.On the other hand,the UVA method requires more time toform a virtual array,which means that it can be used only for meas

211、uring static channels as theRDA method.Therefore,it also faces challenges in terms of accuracy and time.4.2 THz Channel Estimation and Channel ModelingWith the channel measurement data available,further post-processing is needed to extract thechannel characteristics,including:1)extracting channel pa

212、rameters from the CIR data,that is,channel parameters estimation;2)analyzing the statistical characteristics of channel parametersand establishing mathematical/physical models,that is,channel modeling.4.2.1 Channel Parameters EstimationA number of transient characteristic parameters of channels can

213、be extracted from the CIR datameasured,including:Time domain parameters,such as the power delay profile(PDP)that covers multipathpath delay and path lossSpace domain parameters,such as theAoA/AoD of each pathFrequency domain parameters,such as the Doppler shiftThere are three commonly seen channel p

214、arameters estimation algorithms:estimation basedon beamforming,estimation based on sub-space,and estimation based on maximum likelihood.32Estimation based on beamforming:The basic idea is to control the beam direction of thetransmitting/receiving arrays and measure the signal power,search for the be

215、am directioncorresponding to the maximum signal power,and use it forAoA/AoD estimation.Theoutput of the beamforming algorithm can be expressed as a linear combination of thesignals received by array sensors.Different beamforming algorithms use differentweighting vectors.Typical beamforming algorithm

216、s include the Bartlett algorithm and theCapon algorithm.Algorithms based on beamforming are simple to implement,but theirperformance are relatively low.Estimation based on sub-space:This algorithm uses the characteristic structure of thecovariance matrix to estimate the covariance matrix of the rece

217、ived signal.Thesubspace-based algorithms commonly seen nowadays include the MUSIC(MultipleSignal Classification)algorithm,the WSF(Weighted Subspace Fitting)algorithm,theESPRIT(Estimation Signal Parameters via Rotation Invariance)algorithm and theirenhancements.Subspace-based algorithms contribute go

218、od angular resolution toAoA/AoD estimation,and some of the enhancements are very useful for coherent signalestimation.On the downside,in subspace-based algorithms,the number of estimablepaths cannot exceed that of receivers array elements.Maximum likelihood-based algorithms basically define the like

219、lihood of changes in thenumerology to be estimated(path delay/path loss/AoA/AoD/Doppler),and then searchfor the numerology to maximize the likelihood.The search result is the estimation of theparameters.ML-based joint parameter estimation is fairly accurate,and the number ofrecognizable paths is not

220、 limited by the number of antennas.Because multi-dimensionalparameter search is required for the maximum likelihood estimation,ML-basedalgorithms require more time for computation.Among the various ML-based algorithms,the Spatial Alternate Generalized Expectation(SAGE)Maximization algorithmeffective

221、ly reduces the computational complexity and is one of the most popular channelestimation algorithms at present.4.2.2 Channel ModelingWireless channels have large-scale characteristics(such as path loss and shadow fading)andsmall-scale characteristics(such as multi-path).In general,there are three ty

222、pes of channel models:deterministic channel model,stochastic channel model,and semi-deterministic channel model.Deterministic channel models determine channel characteristics according to the physicalbehavior of electromagnetic wave propagation,which include the measurement-based model,the ray traci

223、ng model,and the finite difference time domain(FDTD)-based model.Deterministic channel models can accurately describe channel characteristics,but theyrequire a large amount of computation and are hence fairly complicated.Stochastic channel models extract a mathematical model based on the channel par

224、ametersestimation,can provide relatively simplified and flexible channel characterization with lowercomplexity and accuracy.Stochastic channel models include the geometry-based channelmodel(GBCM),the correlation-based channel model(CBCM)and the beam domain channelmodel(BDCM),each having a different

225、modeling method.33Semi-deterministic models,also referred to as hybrid channel models,can be considered as acombination of the two model types in the above,which not only simplify the physicalbehavior of electromagnetic waves,but also take the statistical channel characteristics in themodel into con

226、sideration.Semi-deterministic models have well balanced complexity andmodeling accuracy,and established some standardized channel models,such as the WINNERmodel and the SCM/SCME model.Amongst the various THz channel modeling methods,deterministic channel models may bemore suitable for link level sim

227、ulation because of the high accuracy;while stochastic channelmodels and semi-deterministic channel models are more suitable for system level simulationbecause of the low computation cost.The high frequency and large transmission bandwidth ofTHz communication will cause non-stationarity in the freque

228、ncy domain of channels.Due to thelarge path loss,large/ultra-large antenna arrays are usually used at the transmitting and receivingterminals,resulting in non-stationarity in the space domain of channels.The mobility oftransmitting and receiving devices will cause non-stationarity in the time domain

229、 of channels.Atpresent,relevant organizations are making helpful explorations on the measurement and modelingof THz frequency channels.For example,the THz Wireless Communication Laboratory ofShanghai Jiao Tong University reviewed the studies on THz wireless channels in collaborationwith the Technica

230、l University of Braunschweig and the University of Southern California 25.First of all,the article introduced and compared three commonly used THz channel measurementmethods at present,the vector network analyzer-based frequency domain channel measurement,the sliding-correlation-based time domain ch

231、annel measurement,and the THz time domainspectrometer-based time domain channel measurement,and discussed the measurement activitiescorresponding to the three methods.Secondly,present channel modeling methods are divided intodeterministic,stochastic,and hybrid categories.The article captured the mos

232、t advanced THzchannel models at present,and introduced the channel simulators developed based on suchchannel models.It also elaborated and analyzed the channel characteristics of the THz band.Atthe end,the article outlined the issues existing in the research of 6G THz wireless channels as wellas fut

233、ure research interests.Specific THz channel modeling theories and practical methods are yetto be further explored.345.Key Technologies and Challenges of THz CommunicationCore components of THz systems,such as power amplifiers,suffer chip non-linearity,I/Qimbalance,and phase noise.These unwanted char

234、acteristics have a great impact on the design andoptimization of the system solution and pose a challenge to the realization of target performances.The current 3GPP standards mainly talk about design issues of systems below 71 GHz.Therefore,new thoughts,designs and studies are required to develop ke

235、y technologies for the THz band.THz communication needs to deal with ultra-large bandwidth and ultra-high-speed datatransmission,and deal with problems such as extremely high path loss,serious time broadeningeffects,frequency spectrum window changes,large delay propagation,frequent synchronizationpr

236、ocesses,carrier frequency offset and serious phase noise,etc.,should be addressedCharacteristics,need to redesign the THz waveform,modulation and coding,synchronization andother OTA technologies,adopt Ultra Massive MIMO(UM-MIMO)technology to achieve highdirectional beams to overcome attenuation,and

237、achieve THz through technologies such as beamcontrol and mobility management The network layer control of the system expands thecombination of THz technology and IRS,and supports the integrated network of sensorycommunication.In terms of THz system design and key technology research,5G system design

238、and the physical layer technology should be comprehensively considered,so that the applicationand corresponding enhancement of the future 6G THz system design are implemented.5.1 New OTATechnologiesBeing a critical assurance for the performance of THz communication systems,new OTAtechnologies mainly

239、 include modulation and coding,waveform design,multi-antenna,keyparameter selection for the physical layer,and so on.5.1.1 Modulation and Coding TechnologyModulation mode plays a significant role in THz communication,and helps improvefrequency spectrum efficiency and data rate.When rethinking the TH

240、z modulation and codingtechnology,a number of factors including system performance,complexity,power consumption,and non-linear effects of devices should be comprehensively considered based on the 5Gmodulation and coding technology and the channel characteristics of the THz band.The THz modulation te

241、chnology should focus on the design of low PAPR modulation codes,in order to avoid the THz PA efficiency reduction and the high-level distortion in the saturationregion.The unwanted noises of local oscillators and active devices as well as the phase noise dueto device damage shall be effectively dea

242、lt with.Although traditional modulation schemes can beused in THz systems,they cannot take full advantage of the THz band characteristics.Channeldistance and frequency-dependent characteristics have accelerated the development of newmodulation schemes in the THz band.In traditional wireless communic

243、ation systems,the35modulation scheme is determined according to the worst case(SNR of receiver at the far end).Similarly,the bandwidth is also defined by the available bandwidth of the receiver at the far end.Because the available bandwidth in the THz band changes with the distance,such a modulation

244、scheme is inefficient in the aspect of the higher bandwidth available for receivers in a closer range.In addition,molecular absorption defines multiple transmission windows at a certain distance.Asthe distance increases,the absorption peak becomes stronger and wider,causing the transmissionwindow to

245、 shrink correspondingly.Therefore,how to define and effectively utilize the availablebandwidth of each user is a key concern.A potential thought is to use distance information foradaptive modulation,and maximize the user data rate by optimizing the bandwidth and power ofsub-band and the modulation o

246、rder.For multiple users,the interaction between the user-basestation distance and the bandwidth mapping information,such as the distance-bandwidth mappingtable,should be considered.Hierarchical bandwidth and spatial modulation should be employed todeal with THz channels with distance characteristics

247、.For example,within a short distance wherethe available bandwidth is large and the path loss is low,the symbol duration can be shorter thanthat over a longer distance.Besides,the THz modulation technology also considers low-order modulation methods withlow envelope changes such as/4 QPSK,and use lin

248、ear amplitude phase shift keying(APSK)andother modulation methods that are less sensitive to nonlinear phase distortion to resist PAsnonlinear phase distortion.Denser phase noise mapping should be leveraged to deal with thephase noise and reduce inter-carrier interference(ICI),and the resistance of

249、silent transmission tomolecular absorption should be analyzed.Low-order modulation methods impair systemperformance,which need to be employed in combination with space multiplexing and otherrelevant technologies to reach the target Tbps transmission rate for 6G.With regard to the codingtechnology,th

250、e THz multi-molecule absorption noise and multi-path fading need to be thoroughlyanalyzed,and the coding/decoding capabilities as well as time resource requirements need to berationally designed,so that the optimal coding weight is determined and dynamically setaccording to the transmission conditio

251、ns to minimize the overall transmission and decoding powerconsumption.Of course,for the sub-THz spectrum below 500 GHz,the impact of molecular absorption onthe transmission window should be analyzed based on the thorough exploration of its channelcharacteristics.The impact of transmission range on t

252、he available bandwidth should also becarefully evaluated,and an optimal modulation and link optimization strategy should be selectedin consideration of the support from relevant key chips(such as ADC),so as to increase systemcapacity while ensuring a reasonable coverage.5.1.2 Waveform DesignIn the r

253、esearch of the THz physical layer,waveform design is one of the most criticalcontents.Due to the THz band characteristics and the non-ideality of key devices,efficiency,power consumption and complexity should be considered in waveform design,and key devicesshould be maintained in proper working cond

254、ition,so that system coverage is ensured and36optimized.5.1.2.1 Waveform Design StrategyAlthough THz boasts a wealth of frequency spectrum resources,the uniqueness of THzwave propagation as well as equipment,devices and chips pose severe challenges in systemdesign,especially waveform design.On the o

255、ne hand,an optimized adaptive THz waveformstrategy should be developed with respect to the specific THz communication channelcharacteristics.The modulation scheme should be optimized and integrated to the base bandwaveform design,in order to identify the most suitable spectrum.On the other hand,beca

256、use thesystem operates at a higher frequency,the unwanted characteristics of the transceiver hardwareand the consequences will increase drastically.When it comes to the THz band,the waveformdesign strategy is more susceptible to transceivers characteristics,which needs to take thefollowing channel c

257、haracteristics,device influences and so on into consideration:26272829First,in comparison with those in a lower frequency band,FSPL of electromagnetic waves inthe THz band is much higher(increasing by quartic times with the frequency).In this case,directional antenna systems can provide higher anten

258、na array gain and compensate for thegreat path loss.Therefore,as the width of beam directed at the communication receiverdecreases,the delay spread decreases while the coherence bandwidth increases.Second,the THz band is subject to severe reflection and scattering,which causes asignificant reduction

259、 in the quantity of main paths.Due to high penetration loss,THzelectromagnetic waves are almost incapable of penetrating obstacles,which puts a limit to therange of THz communication,especially in indoor scenarios and building-intensive outdoorenvironment.Therefore,the energy of THz signals received

260、 may be concentrated in the LoSpath and several specular reflection paths,reducing the root mean square(RMS)delay spread.For example,at a transmission distance of 3 m and a center frequency of 0.7 THz,thecoherent bandwidth of multipath propagation may reach 3.87 GHz.This may pose a majorimpact on th

261、e selection of THz waveforms.Third,atmospheric effects may impair the THz wave propagation and result in molecularabsorption.Such an impact is more significant at certain THz frequencies and less obvious atlower frequency bands.Because of molecular absorption peaks,the THz band is divided intosevera

262、l THz windows of different widths according to the transmission range.An adaptivemodulation scheme is hence required to extend the transmission range or maximize the datarate.Current waveform technologies do not support such a range-adaptive modulationscheme.Fourth,the THz system has a large bandwid

263、th,with the ultra-broadband width of a singlespectrum window going up to several GHz or even tens of GHz.If the current narrow-bandsystem design strategy is considered,a huge number of sub-bands will be required,leading toa highly complicated system design.Dividing each frequency spectrum window int

264、o a groupof sub-windows makes multi-broadband transmission possible.In this circumstance,inter-symbol interference,inter-band interference,and so on in multi-band systems need to bestudied.Finally,the highly cohesive connectivity and diversified applications require the THzwaveform design to be more

265、 flexible,so that the various requirements of application services37are addressed,including but not limited to communication and sensing.As a result,thehybrid design of several different waveforms will become a possible trend of future systems.In conclusion,the THz channel characteristics mentioned

266、above should be leveraged in theTHz waveform design process to rationally design and select a waveform technology for thephysical layer.The design objective is to improve the various key performance indicators of thecommunication system,including effectiveness and reliability,data rate,and bit error

267、 rate(BER).Adding to that,spectrum efficiency,latency,and connectivity are also critical indicators of thenext-generation wireless systems,which need to be considered in the THz waveform design.The complexity of wireless communication transceivers continues to grow,carrierfrequency is getting increa

268、singly higher,the overall system performance becomes more sensitiveto the unwanted characteristics of the RF analog front end.In particular,the waveforms supportedby transceivers in the THz band,PA efficiency,phase noise(PN)robustness,power leakage andother performance indicators are to be considere

269、d.PAs saturation characteristics may result innon-linear distortion at the output when the input is too big(much bigger than the nominal value).When the maximum output power is constrained by the saturated power,the input powerwill fall back,so that operation is maintained in the linear region.PAs a

270、verage output power is dBm=dBm dB,where represents the PA fallback power.The biggerfallback generated from PAs peak power will result in low transmitting power and low powerefficiency,which can be calculated with the ratio of transmitting power to PA power consumption30.In general,such a fallback is

271、 proportional to the peak-to-average power ratio(PAPR)of thesignal transmitted.In order to maximize the transmitting power and power efficiency of thetransmitter,power fallback must be lowered by reducing the PAPR.As shown in Figure 15,withthe increase of the carrier frequency,the saturated output p

272、ower decreases rapidly.For example,saturated power in the 100 GHz frequency band is at least 10 dB lower than that in the 10 GHzfrequency band 31.Transmitters PA efficiency is more sensitive to PAPR.As a consequence,future THz communication systems should consider waveform technologies with a lower

273、PAPR toprovide higher coverage and improve the effectiveness of THz communication systems.Figure 15:PAs Saturated Output Power vs Carrier Frequency 31When complex base band signals are up-converted to RF transmitters carrier frequencyand pass-band signals are down-converted to receivers base band si

274、gnal,there is phase noise(PN)in the local oscillator(LO).PN leads to random phase rotation in time domain signals,henceinter-carrier interference(ICI)in the frequency domain.For every single increase in carrier38frequency,PN increases by 6 dB.Therefore,it is essential to evaluate the influence of PN

275、distortion when designing THz waveforms.As mentioned above,out-of-band power leakage is another critical consideration in THzwaveform design.Larger out-of-band power will result in adjacent channel interference(ACI).Inthis circumstance,a guard band is needed to minimize the ACI impact.But this will

276、lead to alower spectrum efficiency and needs to be comprehensively thought about.5.1.2.2 THz Waveform DesignFor carrier-based modulation in digital communication,two concepts dominate thewaveform design and application:single-carrier waveform and multi-carrier waveform.With lowPAPR and high power ef

277、ficiency,single-carrier waveforms are suitable for scenarios with limitedcoverage,so as to compensate for the high transmission loss.Multi-carrier waveforms can providehigh spectrum efficiency,support flexible resource allocation in the frequency domain,and can beeasily integrated into the multi-ant

278、enna technology.In a single-carrier communication system,thetransmission symbol is sent based on the pulse by the transmission filter at the transmitting end.However,the equalizers high complexity is a notable issue,because ICI increases as the data rateincreases.For multi-carrier waveforms,the most

279、 popular technology at present is the orthogonalfrequency division multiplexing(OFDM)modulation scheme used in the 5G NR downlink(DL).Such a technology can achieve higher spectral efficiency for time-invariant frequency selectivechannels,but it will produce high PAPR.For single-carrier waveforms,the

280、 discrete Fouriertransform spread spectrum OFDM(DFT-s-OFDM)used in 5G NR up-link(UL),the single-carrierfrequency domain equalization(SC-FDE)and other techniques have certain or potential prospectsin the THz down-link.In comparison with the traditional down-link multi-carrier OFDMtechnique,the single

281、-carrier scheme has certain advantages in the aspect of PAPR and etc.Therefore,the advantages of single-carrier and multi-carrier techniques in all aspects should becomprehensively considered,and the THz channel characteristics and requirements should becombined to ultimately design and determine th

282、e key technology suitable for the THz band.OFDM features high spectral efficiency and can effectively resist frequency selectivefading.Such key advantages are vital for meeting the high-speed data transmission requirements.This waveform technology can be widely used in 5G NR down-link systems,and ha

283、s beenverified by practical applications.Therefore,when analyzing and designing the THz waveformtechnology,OFDMs application prospects should be prioritized.The major advantages of OFDMwaveform technology in wireless communication systems can be summarized as follows:For a given channel delay spread

284、,OFDM introduces the cyclic prefix(CP)to avoidintersymbol interference(ISI)and uses a frequency domain equalizer to transmit largebandwidth signals on wireless channels.The design of an OFDM receiver is far simpler thana single carrier system with a time domain equalizer.The data rate of each sub-ca

285、rrier can be adjusted according to the signal-to-noise ratio(SNR)of a sub-carrier to significantly improve channel capacity,which is also known as the waterfilling algorithm of frequency domain link adaptation.In addition to these advantages,the application of OFDM in the THz band is encounteredwith

286、 many challenges,including high PAPR,robustness against phase noise(PN),out-of-bandleakage and complexity of implementation.Given that the antenna/beam directionality increases39and the number of LOS paths decreases in the THz band,the coherence bandwidth of the THzchannel is large.In most cases,the

287、 THz channel is flat,so there is little significance to selectchannel robustness for the OFDM frequency.Another possible way to overcome multi-path effectsis to employ the SC-FDE waveform with CP.SC-FDE has the same performance trend as OFDMin nature and can solve the problems of ISI and frequency s

288、electivity through FDE and DFT/IDFT.Figure 16 shows the results of performance(symbol error rate(SER)analysis for SC-FDE andOFDM in different channel scenarios in the 300 GHz frequency band.It is clear that SC-FDE andOFDM have similar SERs in the AWGN channel,while in a sparse multipath environment(

289、oneLOS plus three reflection paths),the SER of SC-FDE is lower than OFDM when the SNR isgreater than 8 dB.Therefore,OFDM has little advantage over SC-FDE in terms of robustness offrequency domain selectivity in the THz band.Moreover,OFDM is subject to the limitations andconstraints of hardware,which

290、 further affects its application in high-frequency bands.Figure 16:Symbol Error Rate(SER)Comparison Between OFDM and SC-FDEIn addition,as the important single carrier waveform technology,DFT-s-OFDM can betterserve multi-user scenarios based on further optimization of SC-FDE.It is currently applied i

291、n theup-link transmission of 5G NR and can be considered one of the alternative THz waveformtechnologies.This technology boasts a lower PAPR,ensuring relevant devices work in a betterstate.Figure 17 compares the performance of OFDM and DFT-s-OFDM in terms of PAPR.It isobvious that the single-carrier

292、 technology excels the conventional multi-carrier technology inPAPR.40Figure 17:Sub-Carrier(1024),Data Block Size(256),Oversampling Factor(4).PAPR Comparison BetweenDFT-s-OFDM and OFDM.Besides traditional OFDM,DFT-s-OFDM,SC-FDE,some other alternatives have also beendiscussed in the 4G and 5G standar

293、dization,including OTFS,UW-DFT-s-OFDM,andDFT-s-OTFS jointly proposed by Nokia Shanghai Bell and Shanghai Jiao Tong University 32.Of the aforementioned technologies,OTFS shows remarkable advantages over OFDM in channelenvironments with large Doppler shift,such as higher spectral efficiency and lower

294、block errorrate(BLER).UW-DFT-s-OFDM is a flexible guard interval(FGI)solution derived fromDFT-s-OFDM in response to the considerable return loss and short delay spread of the THzchannel.It helps reduce cyclic prefix overhead and improve resource utilization to increase thedata transmission rate.As r

295、egards DFT-s-OTFS,it further reduces its PAPR while maintaining theadvantages of OTFS.It can be applied to address the impact of Doppler shift due to frequencyincrease and keep vital high-frequency devices working in optimal condition.Hence,it is alsoseen as an alternative in the studies of THz wave

296、forms.Nokia Shanghai Bell and Shanghai JiaoTongUniversityhaveevaluatedvariouspotentialwaveformtechnologiesfromdistinctperspectives such as PAPR,out-of-band leakage and BER.The preliminary assessment results areshown in Figures 16-19.Further discussion,determination and optimization of waveformtechno

297、logies applicable to the THz band will derive from these results.41Figure 18:PAPR Comparison Between OFDM,DFT-s-OFDM,UW-DFT-s-OFDM,SC-FDE,OTFS andDFT-s-OTFSFigure 19:BER Comparison Between OFDM,DFT-s-OFDM,UW-DFT-s-OFDM,SC-FDE,OTFS andDFT-s-OTFS in Case of Doppler ShiftTo sum up,with respect to effic

298、iency,PAPR,channel robustness,complexity,and hardwarelimitations and constraints,the application of OFDM in the THz band still necessitates furtherresearch and analysis,whereas single-carrier-based waveform technologies are promising.Therefore,when considering the waveform technology for the THz ban

299、d,the existingsingle-carrier waveform technologies can be analyzed and optimized or new technology can bedesigned to maximize the advantages of the THz band and satisfy the large-capacity requirementsin the 6G era with reasonable coverage.In addition,more detailed and comprehensive evaluationof link

300、-level or even system-level simulation is required.Certainly,multi-carrier technologiesneed to be included at the present stage,and further analysis and evaluation are necessary.5.1.2.3 Physical Layer Numerology DesignBasic definitionThe design of physical layer numerology is an important system des

301、ign and standardizationwork of 6G.The main purpose is to determine and optimize the set of a series of basic physical42layer parameters of the communication system designed based on waveforms,includingsub-carrier spacing(SCS),cyclic prefix,TTI duration,system bandwidth,etc.Figure 20:Numerology Defin

302、ed in Communication System 1Although the THz physical layer waveform technology has not yet been finalized,relevantstrategic principles and constraints can be studied and considered during the research of 6Gnumerology to better guide 6G-related standardization in the future.In the first place,theinf

303、luence of numerology in the 5G NR era over the 6G system and the value thereof that is worthreferencing need to be studied and analyzed to facilitate the efficient transmission in the THz band.To this end,Nokia Bell Labs has carried out preliminary research and analysis,and proposed themeans of opti

304、mizing the key parameter design for the physical layer of the THz communicationsystem based on the settings of physical layer numerology of 5G NR 35.The studies haveanalyzed potential waveform technologies and numerology design from the perspectives of PNrobustness,system performance and more.Releva

305、nt performance evaluation results are also givenunder the condition of 90 GHz carrier frequency.Considering the large coherent bandwidth ofTHz,it is important to adopt higher SCS to improve and support high system throughput,but thisrequires lots of modifications to the physical layer design in orde

306、r to address the challenges ofshorter scheduling cycles and possible poorer coverage of control channel.In addition towaveform design,the design of phase tracking reference signal(PTRS)can also be optimized tocope with the influence of significant phase noise(PN)of the THz communication system,inclu

307、ding optimization pertaining to the time domain and/or frequency domain to enable moreadvanced transmission and 6GTbps transmission capacity.Figure 21 shows the systemperformance under different SGS circumstances.43Figure 21:Link-Level Performance When Phase Noise Exists:a)OFDM and b)SC-FDMA use Rel

308、-15 compliantPTRS;c)OFDM and d)SC-FDMA use enhanced PTRS structures,90 GHz carrier,rank-2SCS(Sub-carrying space)designing is one of the key parameters of physical layernumerology and exerts a great influence on the THz communication system design.ImprovedSCS design is crucial for solving the problem

309、 of Doppler shift influence due to frequency increase,reducing system signaling load,and optimizing system design.It requires further research andanalysis according to the physical layer waveform technology and characteristics of the THzchannel.If the waveform technology based on OFDM and its varian

310、ts is to be adopted for thefuture THz system,the following principles should be considered for the design and optimizationof SCS:6G services require the THz band of a wide range and a large coherence bandwidth withthe way of deployment varies.Thereby,it is necessary to design a flexibly extensible S

311、CS.For example,the sub-carrier spacing of 5G NR is 15 2.On the basis of thestandard 15 KHz sub-carrier spacing,larger sub-carrier spacing should be designed giventhe greater coherence bandwidth of THz to provide users with services containing strictrequirements for time delay,such as URLLC services.

312、The design of scheduling and reference signals should be simplified.Sub-carrier spacingis inversely proportional to OFDM symbol duration.In all the numerologies of 5G,theOFDM symbol quantity/time slot parameter in all the numerologies should be set to 14,simplifying the design of scheduling and refe

313、rence signals.The time slot shortens as thesub-carrier spacing increases.44Channel bandwidth should be diverse.The maximum number of OFDM FFT samples andsub-carrier spacing determine channel bandwidth.Numerologies of large sub-carrierspacing require a greater signal bandwidth,and vice versa for thos

314、e of small sub-carrierspacing.For example,the frequency band used by the 5G NR can be roughly divided intotwo segments:1)frequency band less than 6 GHz:450 MHz-6 GHz;2)millimeter wave(mmWave:approximately 24 GHz-52 GHz).R17 has already begun the discussion of 52GHz-72 GHz).Numerologies with the sub-

315、carrier spacing being15 kHz and 30 kHz areused in the sub 6 GHz band and the maximum bandwidth is 100 MHz.Numerologieswith the sub-carrier spacing exceeding 120 kHz are used in the mmWave band and themaximum bandwidth is 400 MHz.Both of the frequency bands are applicable tonumerologies with 60 kHz s

316、ub-carrier spacing.Moreover,3GPP is able to supportsub-carrier spacing up to 120 kHz to enable larger bandwidth and frequency bands.In thefuture THz system,the bandwidth available will be considerably more and SCS willincrease further in accordance with the condition of coherence bandwidth.Hence,the

317、application of 480 K,960 K,1.92 M or greater SCS can be considered and analyzed inthe THz system.The upper and lower limits of SCS parameters should be studied and analyzed based onthe characteristics of the THz channel.It is expected that the smaller the sub-carrierspacing,the more efficient the tr

318、ansmission performance.However,if the spacing is toosmall,phase noise tends to be triggered,to get rid of which will impose higherrequirements on the local crystal oscillator.As a result,the performance is susceptible tothe interference of Doppler frequency offset and a large signaling load is cause

319、d.Nevertheless,if the spacing is set too large,CP duration will be shorter and delay spreadcannot be eliminated.CP duration(or channel delay spread)determines the maximum ofsub-carrier spacing.Therefore,it is necessary to design the appropriate SCS range basedon THz channel characteristics,such as s

320、parse channel characteristics,delay spread,coherence bandwidth and other factors.With this as the foundation,SCS can be designedand optimized in various application scenarios.Certainly,if OFDM or relevant waveform technologies are not adopted for the future THzcommunication system,the aforementioned

321、 principles need to be subject to the specific waveformtechnology.In general,the numerology design in the THz communication system needs to enableflexibility and scalability to support the differential requirements of various application scenariosand the requirements of a large number of continuous

322、or discrete operating frequency bands.Itshould be determined based on the specific waveform technology in standard discussion anddevelopment.The basic principles though are similar to the numerology design principles of thetraditional communication system,as shown in the following figure.45Figure 22

323、:Major Design Principles/Methods of Numerology in Traditional Communication System 345.1.3 SynchronizationSynchronization is a formidable challenge in the ultra-wideband(sub-)THz communicationnetwork,because it is difficult to sample signals at the Nyquist rate while executing complexsignal processi

324、ng tasks at the Tbps data rate.In addition,users have independent local oscillatorsto generate carrier frequencies,resulting in a large frequency offset of the device as a whole.In synchronization design,pulse-based modulation and ON-OFF switch control modulationfor channel access can be considered

325、regarding nano-networks.Since a robust and accuratesynchronization mechanism is required for longer communication distances,time and frequencydomain synchronization can be considered.Special attention should be paid to reducing thecollection time of receivers for fast receipt and synchronization.Mul

326、ti-path effects and channelsparsity need to be analyzed to design new synchronization algorithms,and jointly design andoptimize waveform,modulation and synchronization to enable more effective use of bandwidthresources in the THz band.5.2Advanced multiple-antenna TechnologyThe power limitation and h

327、igh attenuation of the THz transceiver lead to a significantreduction in the communication distance.The introduction of Ultra Massive MIMO(UM-MIMO)plays a key role in ensuring reasonable coverage while improving the capacity of the THzcommunication system.5.2.1 PhasedArray Antenna ArchitectureThe hi

328、gh frequency of THz makes it possible to develop UM-MIMO in a very small areawhile also bringing higher requirements for hardware design.At present,a feasible solution for46phased array antenna architecture is to increase the number of antenna components anddynamically control arrays using new plasm

329、a materials.Plasma materials support surface plasmonpolaritons(SPP)transmission and make adjustable the resonant frequency of graphene-basedplasma antennas by dynamic adjustment of limiting factors.Apart from that,conventionalmeta-materials and communication networks can be combined to dynamically c

330、ontrol theperformance of meta-materials,effectively change the dielectric constant of meta-materials andmodify the limiting factor in real time 36.By dynamically adapting to and adjusting the amplitude and time/delay/phase of the signal ofeach antenna,different UM-MIMO operating modes such as beamfo

331、rming,spatial multiplexingand multi-band UM-MIMO can be realized.The communication distance is greatly reduced due tothe THz transceivers power limitation and considerable path loss.UM-MIMO can thereby bebrought in to sustain flexible highly directional beam,improve the signal-to-noise ratio gain of

332、signals and increase the communication distance.By doing so,the rich bandwidth resources in theTHz band are effectively utilized to achieve a communication network capacity of more thanTerabits per second.With regarding to UM-MIMO,a feasible solution is to transmit data using an analog/digitalhybrid

333、 structure.It combines the beamforming of the analog domain and the digital domain andcan flexibly adjust the number of RF channels to bring about a balance between the hardware costand the number of data streams available.In the analog domain,it uses the phase shifter networkto obtain sufficient antenna array gain on each RF channel to solve the formidable problem ofsmall THz band coverage.Multip