《物联网技术及应用》课程教学资料（参考资料）A Survey on Green 6G Network - Architecture and Technologies

IEEEAcceSSSPECIAL SECTION ON GREENINTERNETOF THINGSaaidshlany:BReceived November 15,2019, accepted November 29,2019, date of publication December 4, 2019dateofcurrentversionDecember18,2019Digital Objecr Identifier 10. 1109/ACCESS.2019.2957648ASurveyonGreen6GNetwork:Architecture and TechnologiesTONGYI HUANG1,WUYANG2,JUN WU1,(Member,IEEE)JINMA',XIAOFEI ZHANG3,ANDDAOYINZHANG3'School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China2Information Security Research Center, Harbin Engincering University, Harbin 150001, China3State Grid Electric Power Research Institute,Nanjing 211000, ChinaCorresponding authors: Wu Yang (yangwu@hrbeu.edu.cn)and Jun Wu (junwuhn@sjtu.edu.cn)This work was supported in part by the National Natural Science Foundation of China under Grant 61831007 and Grant 61972255, in partby the State Key Laboratory of Smart Grid Protection and Control under Grant SGNROOOOGZJS1808084, and in part by the Research onKey Technologies of Security Vulnerabilities and Risk Experiment Capabilities of State Grid Headquarters Science and TechnologyProjects.:ABSTRACT While 5G is being commercialized worldwide, research institutions around the world havestarted to look beyond 5G and 6G is expected to evolve into green networks, which deliver high Qualityof Service and energy efficiency.To meet the demands of future applications,significant improvementsneed to be made in mobile network architecture.We envision 6G undergoing unprecedented breakthroughand integrating traditional terrestrialmobile networks with emerging space,aerial and underwater networksto provide anytime anywhere network access.This paperpresents a detailed survey on wireless evolutiontowards 6G networks.In this survey,the primefocus is on the newarchitectural changes associated with 6Gnetworks,characterized by ubiquitous 3D coverage,introduction of pervasive AI and enhanced networkprotocol stack.Along with this, we discuss related potential technologies that are helpful in formingsustainableand socially seamless networks,encompassing terahertz and visiblelight communication,newcommunication paradigm,blockchain and symbioticradio.Our work aims to provide enlighteningguidancefor subsequentresearch of green 6G.:INDEX TERMS 6G,architecture,green networks, VLC, blockchain.LINTRODUCTIONacomplete6Gecosystem[3].TheU.K.andGermangov-With the completion of the first full set of 5G standards,ernments have invested in some potential technologiesforthe initial commercial deploymentof 5Gwireless networks6G such as quantum technology,and theUnited Statesbeganhas begun in 2019.5G wireless network marks the begin-research on terahertz-based 6Gmobile networks.The Min-ning of a true digital society and achieves significant break-ister of Industry and Information Technologyin China hasmadetheofficial pronouncementthatthecountryhas focusedthroughsintermsoflatency,datarates.mobilityandnumberof connected devices in contrast to previous generations [1]onthedevelopmentof6GLooking back at the evolution of mobile communication,Novel service requirements and scale increases are theit takes about onedecade from the initial concept researchdrivingforcebehindtheevolutionofwirelessnetwork.Theto the commercial deployment, while its subsequent usagerapid development of emerging applications results in alasts for at least another 10 years. That is, when the previ-never-ending growth in mobile data traffic.According to theforecast byInternational Telecommunication Union (ITU)ousgenerationmobilenetworkentersthecommercialphase.the next generation begins concept research.As 5G is in theglobal mobiledatatrafficwill reach5zettabytesby2030initial stages of commercialization,nowis the righttime to[4], as shown in Fig.1. Upcoming applications (e.g.e-healthlaunch research on 5G's successor.andautonomousdriving)havemorestringentrequirementsIn the past few years, some countries have issued stratefor latencyand throughput,which will eventuallyexceed thegic plans for the development of 6G [2]. In 2018, Finlandlimits of 5Gnetworks.It is expected that5Gwill reach itsannounced the 6Genesis Flagship program, an eight-yearlimitsinadecadeorsoandtomeetthesedemands.themainprogram with the overall volume of s290 million to developtechnical objectivesfor 6Gnetworks will be. Ultra-high data rate (up to ITbps) and ultra-low latency.The associate editor coordinating the review of this manuscript andapproving it for publication was Zhenyu Zhou.High energy efficiency for resource-constraineddevices.This work is licensed under a Creative Commons Attribetion 4.0 License.For more information,seehttp://creativecommons.org/licenses/by/4.0/175758VOLUME7,2019

SPECIAL SECTION ON GREEN INTERNET OF THINGS Received November 15, 2019, accepted November 29, 2019, date of publication December 4, 2019, date of current version December 18, 2019. Digital Object Identifier 10.1109/ACCESS.2019.2957648 A Survey on Green 6G Network: Architecture and Technologies TONGYI HUANG 1 , WU YANG2 , JUN WU 1 , (Member, IEEE), JIN MA1 , XIAOFEI ZHANG3 , AND DAOYIN ZHANG3 1School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2 Information Security Research Center, Harbin Engineering University, Harbin 150001, China 3State Grid Electric Power Research Institute, Nanjing 211000, China Corresponding authors: Wu Yang (yangwu@hrbeu.edu.cn) and Jun Wu (junwuhn@sjtu.edu.cn) This work was supported in part by the National Natural Science Foundation of China under Grant 61831007 and Grant 61972255, in part by the State Key Laboratory of Smart Grid Protection and Control under Grant SGNROOOOGZJS1808084, and in part by the Research on Key Technologies of Security Vulnerabilities and Risk Experiment Capabilities of State Grid Headquarters Science and Technology Projects. ABSTRACT While 5G is being commercialized worldwide, research institutions around the world have started to look beyond 5G and 6G is expected to evolve into green networks, which deliver high Quality of Service and energy efficiency. To meet the demands of future applications, significant improvements need to be made in mobile network architecture. We envision 6G undergoing unprecedented breakthrough and integrating traditional terrestrial mobile networks with emerging space, aerial and underwater networks to provide anytime anywhere network access. This paper presents a detailed survey on wireless evolution towards 6G networks. In this survey, the prime focus is on the new architectural changes associated with 6G networks, characterized by ubiquitous 3D coverage, introduction of pervasive AI and enhanced network protocol stack. Along with this, we discuss related potential technologies that are helpful in forming sustainable and socially seamless networks, encompassing terahertz and visible light communication, new communication paradigm, blockchain and symbiotic radio. Our work aims to provide enlightening guidance for subsequent research of green 6G. INDEX TERMS 6G, architecture, green networks, VLC, blockchain. I. INTRODUCTION With the completion of the first full set of 5G standards, the initial commercial deployment of 5G wireless networks has begun in 2019. 5G wireless network marks the begin￾ning of a true digital society and achieves significant break￾throughs in terms of latency, data rates, mobility and number of connected devices in contrast to previous generations [1]. Looking back at the evolution of mobile communication, it takes about one decade from the initial concept research to the commercial deployment, while its subsequent usage lasts for at least another 10 years. That is, when the previ￾ous generation mobile network enters the commercial phase, the next generation begins concept research. As 5G is in the initial stages of commercialization, now is the right time to launch research on 5G’s successor. In the past few years, some countries have issued strate￾gic plans for the development of 6G [2]. In 2018, Finland announced the 6Genesis Flagship program, an eight-year program with the overall volume of $290 million to develop The associate editor coordinating the review of this manuscript and approving it for publication was Zhenyu Zhou . a complete 6G ecosystem [3]. The U.K. and German gov￾ernments have invested in some potential technologies for 6G such as quantum technology, and the United States began research on terahertz-based 6G mobile networks. The Min￾ister of Industry and Information Technology in China has made the official pronouncement that the country has focused on the development of 6G. Novel service requirements and scale increases are the driving force behind the evolution of wireless network. The rapid development of emerging applications results in a never-ending growth in mobile data traffic. According to the forecast by International Telecommunication Union (ITU), global mobile data traffic will reach 5 zettabytes by 2030 [4], as shown in Fig. 1. Upcoming applications (e.g. e-health and autonomous driving) have more stringent requirements for latency and throughput, which will eventually exceed the limits of 5G networks. It is expected that 5G will reach its limits in a decade or so and to meet these demands, the main technical objectives for 6G networks will be • Ultra-high data rate (up to 1Tbps) and ultra-low latency. • High energy efficiency for resource-constrained devices. 175758 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see http://creativecommons.org/licenses/by/4.0/ VOLUME 7, 2019

IEEEAcceSST. Huang et al: Survey on Green 6G Network: Architecture and TechnologiesTraffic/monthA.FROM1GTO3G6000The first generation mobile network was introduced in the50150001980s,which was designedfor voiceservices,with adata rateup to2.4kbps.It used analog signal totransmit information3327and there was no universal wireless standard, leading to many194Adrawbacks such asproblematichand-off,lowtransmission2000efficiency and no security [6]. Compared to first-generation1000systems, 2G was based on digital modulation technologies5791158249such as Time Division Multiple Access(TDMA) and Code20202021202220232024202520262027202820292030Division Multiple Access(CDMA). It has a data rate up tonon-M2MtrafficM2Mtrafic64kbps,supporting notonlybettervoiceservices,butalsoFIGURE 1.Global mobile data traffic in 2020-2030 forecast by ITU.services like Short Message Service (SMS).The all-dominantmobile communication standard in the 2G era was theGSM(Global System for MobileCommunication)[7].The third.Ubiquitous global network coverage.generation was proposed in 2000 with the goal of offering.Trusted and intelligent connectivity across the wholehigh-speed datatransmission.3G network provides a datanetwork.transfer rate of at least 2 Mbps as well as high speed accessto Internet [8].It enables advanced services not supported byIn this paper, we discuss some emerging ideas about1Gand2G networks,includingWebbrowsing,TVstreaming.potential architecture and technologies of 6G. The rest ofnavigational maps and video services. In order to achievethe paper is organized as follows. Section II presents theglobal roaming, an organization called 3rd Generation Part-evolution of mobile communication networks. Section IIInership Project (3GPP) was established to define technicalgives the detailed description of architectural changes of 6G.specifications and continue the work by defining mobileSectionIV provides a brief overview of some visionarytech-standardsand systems[9]nologies that may be key parts of 6G. Finally, this paper isconcludedin Section V.B.4GII.EVOLUTIONOFMOBILECOMMUNICATIONNETWORK4G is an all IP based network introduced in the lateThere has been a phenomenal advancement inmobile com-2000s,which is capable of providing high-speed datamunication network since the first emergence of analogrates up to1Gbits/s inthe downlink and500Mbits/scommunications networkinthe198Os.Thisadvancementin the uplink. It apparently improves spectral efficiencyis not a one-step process, but consists of several generaand reduces latency,accommodating the requirements settions which have different standards, capacities and tech-by advanced applications like Digital Video Broadcasting(DVB), High Definition TV content and video chat. More-niques.New generation have been introduced nearly everyten years [5]. The evolution of mobile network is shownover, 4G enables terminal mobility to provide wireless ser-in Fig. 2.vices at anytime and anywhere, through automatic roaming4BlockchainTerahertzonnona~TbpsBDMASDN6GMm-WaveWiMAX~20GbpsUMTSLTE-ATD-SCDMA5GTDMA<2GbpsCDMA2000GSMWCDMA4GEDGEFDMA2-100MbpsGRPSAnalog3G<64Kbps草<2.4KbpsP2G好1GED国YHNCMobile InternetFullyintelligentandWeblloTSMSVoice serviceofapplicationscoveredconnection198019902000201020202030YearsFIGURE 2. Evolution of mobile wireless systems.175759VOLUME 7, 2019

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies FIGURE 1. Global mobile data traffic in 2020-2030 forecast by ITU. • Ubiquitous global network coverage. • Trusted and intelligent connectivity across the whole network. In this paper, we discuss some emerging ideas about potential architecture and technologies of 6G. The rest of the paper is organized as follows. Section II presents the evolution of mobile communication networks. Section III gives the detailed description of architectural changes of 6G. Section IV provides a brief overview of some visionary tech￾nologies that may be key parts of 6G. Finally, this paper is concluded in Section V. II. EVOLUTION OF MOBILE COMMUNICATION NETWORK There has been a phenomenal advancement in mobile com￾munication network since the first emergence of analog communications network in the 1980s. This advancement is not a one-step process, but consists of several genera￾tions which have different standards, capacities and tech￾niques. New generation have been introduced nearly every ten years [5]. The evolution of mobile network is shown in Fig. 2. A. FROM 1G TO 3G The first generation mobile network was introduced in the 1980s, which was designed for voice services, with a data rate up to 2.4 kbps. It used analog signal to transmit information and there was no universal wireless standard, leading to many drawbacks such as problematic hand-off, low transmission efficiency and no security [6]. Compared to first-generation systems, 2G was based on digital modulation technologies such as Time Division Multiple Access(TDMA) and Code Division Multiple Access(CDMA). It has a data rate up to 64kbps, supporting not only better voice services, but also services like Short Message Service (SMS). The all-dominant mobile communication standard in the 2G era was the GSM (Global System for Mobile Communication) [7]. The third generation was proposed in 2000 with the goal of offering high-speed data transmission. 3G network provides a data transfer rate of at least 2 Mbps as well as high speed access to Internet [8]. It enables advanced services not supported by 1G and 2G networks, including Web browsing, TV streaming, navigational maps and video services. In order to achieve global roaming, an organization called 3rd Generation Part￾nership Project (3GPP) was established to define technical specifications and continue the work by defining mobile standards and systems [9]. B. 4G 4G is an all IP based network introduced in the late 2000s, which is capable of providing high-speed data rates up to 1Gbits/s in the downlink and 500Mbits/s in the uplink. It apparently improves spectral efficiency and reduces latency, accommodating the requirements set by advanced applications like Digital Video Broadcasting (DVB), High Definition TV content and video chat. More￾over, 4G enables terminal mobility to provide wireless ser￾vices at anytime and anywhere, through automatic roaming FIGURE 2. Evolution of mobile wireless systems. VOLUME 7, 2019 175759

IEEEAcceSST. Huang et al.: Survey on Green 6G Network: Architecture and Technologiesacross geographic boundaries of wireless networks. Longthroughput, ultra-low latency and reliability. ThereforeTerm Evolution-Advanced (LTE-A) and Wireless Interoper-forward-lookingresearchon futurenetwork frameworks isnecessary. FG NET-2030 has established Sub-Group 3 toabilityforMicrowaveAccess(WiMAX)areconsideredas4G standards [10].LTE integrates existing and newtech-formulate architectureof Network2030.However,it is unre-nologies suchas coordinatedmultipletransmission/receptionalistic to accurately illustrate what the future network archi-(CoMP), multiple-input multiple-output (MIMO), orthogo-tecturewill be.Sub-Group3has reached a compromisenal frequency division multiplexing (OFDM).interpreting the architecture from different dimensions ratherthan defining a unified framework. In this section, we intro-C.5Gducethe architectural changes associated with 6G from threeThe fifth generation mobile communication network hasdimensions, as shown in Fig. 3.almost completed the initial basictests,hardwarefacilitiesconstruction and standardization process,and will soon be putA.FROMTERRESTRIALTOUBIQUITOUS3DCOVERAGEintocommercial use.Thegoal of 5G is tomakerevolutionaryOne target of the next generation network architecture is toadvances in data rates, latency, network reliability, energyexpandthebreadth and depthof communication coverageefficiency,and massive connectivity[11].It not onlyusesThe current network architecture based on legacy terres-the new spectrum of the microwave band (3.3-4.2 GHz),trialcellularinfrastructurehasthefollowingtwodrawbacks:but also innovatively uses themillimeter-wave band for theinability to meet the high-altitude and deep-sea commu-first time, greatly increasing data rates (up to 10 Gbps).nication scenarios.which is an inevitable requirementfor5G applies advanced accesstechnologies,includingBeamfuture services; prohibitively expensive provisioning cost forDivisionMultiple Access (BDMA)and Filter Bank multidense cellular networks to provide connectivity in theglobalcarrier (FBMC). Many emerging technologies are integratedscale.In order to cover the above drawbacks, 6G will inte-into5Gtoimprovenetworkperformance:MassiveMIMOgrate non-terrestrial networks to provide full wireless cover-forcapacityincreaseSoftwareDefinedNetworks(SDN)forage [16].Preliminary envision about Space-Air-Ground-Seaflexibility in network,device-to-device(D2D)for spectralintegrated communication has been discussed in [17]efficiency.InformationCentricNetworking(ICN)forreduction in network traffic and network slicing for quickdeploy-1)SPACENETWORK:LEOSATELLITESYSTEMment of various services [12]-[14]. IMT 2020 proposedHigh throughput satellite (HTS) systems are capable ofthreemajor 5G usage scenarios:Enhanced mobile broad-broadband Internet access service comparableto terrestrialband(eMBB),Ultra-reliableandlowlatencycommunica-services interms of pricing and bandwidth.Most commu-tions (URLLC)and Massivemachinetype communicationsnications satellites arein geostationaryorbit(GEO)atan(mMTC).altitudeof35.786km,naturallyleadingtoexcessivedelayand infeasibility of integration with terrestrial mobile net-D.VISIONOFGREEN6Gwork.Non-geostationary orbit(NGSO)satellitesystemisAs5Gisenteringthecommercialdeploymentphase,researchproposed to provide low-latency,high-bitrateglobal Internetinstitutions around the world have begun to pay attention toconnectivity and several satellite constellations areabout to6G, which is considered to be deployed in about 2030. Greenbegin commercialization:6G is expectedto enhancetheperformanceof information: Starlink: American company SpaceX plans to launchtransmission - peak data rates up to 1 Tbps and ultra-lowStarlink,a constellationof 4.425lowEarthorbitLEO)latency inmicroseconds.Itfeaturesterahertzfrequency com-satellites and 7518 VLEO satellites in approximatelymunication and spatial multiplexing,providing as muchas340km orbits.The plan was authorized by theFederal1000timeshighercapacitythan5Gnetworks.Onegoal of6GCommunicationsCommission(FCC)[18]andwillbeis to achieve ubiquitous connectivity by integrating satellitefullydeployed in2027.communication networksandunderwater communicationsto.OneWeb:OnFebruary27,2019,OneWeb successfullyprovideglobal coverage[19].Energy harvestingtechnologieslauncheditsfirst six satelliteintoorbits.Theconstel-and the use of new materials will greatly improve the systemlation consists of 720LEO satellite[20] and hasgotenergy efficiency and realize sustainable green networks.authorization fromUK andFCCThreenew6Gserviceclassesweredescribedin[15]:ubiqui.Hongyan: China Aerospace Science and Technologytousmobileultrabroadband(uMUB),ultrahigh-speed-with-Corporation(CASC)willlaunchnineLEOsatellitesaslow-latencycommunications(uHSLLC)andultrahighdataa pilot demonstration for the Hongyan system, whichdensity (uHDD)ultimatelywillcomprise320satellitesandbecompletedby 2025.III.ARCHITECTURESOFGREEN6GNETWORKGreen 6G networks are expected to achieve energy-efficientAlthough there is still a long time before overall deploy-and socially seamless wireless connections in a global scope,mentof NGSOsatellite systems and convergenceof satellitewhile the existing network architecture is nuable to guar-communications and mobile wireless networks,advantagesof LEO satellite networks have been confirmed in theoryantee future application delivery constraints ultra-high175760VOLUME7,2019

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies across geographic boundaries of wireless networks. Long Term Evolution-Advanced (LTE-A) and Wireless Interoper￾ability for Microwave Access (WiMAX) are considered as 4G standards [10]. LTE integrates existing and new tech￾nologies such as coordinated multiple transmission/reception (CoMP), multiple-input multiple-output (MIMO), orthogo￾nal frequency division multiplexing (OFDM). C. 5G The fifth generation mobile communication network has almost completed the initial basic tests, hardware facilities construction and standardization process, and will soon be put into commercial use. The goal of 5G is to make revolutionary advances in data rates, latency, network reliability, energy efficiency, and massive connectivity [11]. It not only uses the new spectrum of the microwave band (3.3-4.2 GHz), but also innovatively uses the millimeter-wave band for the first time, greatly increasing data rates (up to 10 Gbps). 5G applies advanced access technologies, including Beam Division Multiple Access (BDMA) and Filter Bank multi carrier (FBMC). Many emerging technologies are integrated into 5G to improve network performance: Massive MIMO for capacity increase, Software Defined Networks (SDN) for flexibility in network, device-to-device (D2D) for spectral efficiency, Information Centric Networking (ICN) for reduc￾tion in network traffic and network slicing for quick deploy￾ment of various services [12]–[14]. IMT 2020 proposed three major 5G usage scenarios: Enhanced mobile broad￾band (eMBB), Ultra-reliable and low latency communica￾tions (URLLC) and Massive machine type communications (mMTC). D. VISION OF GREEN 6G As 5G is entering the commercial deployment phase, research institutions around the world have begun to pay attention to 6G, which is considered to be deployed in about 2030. Green 6G is expected to enhance the performance of information transmission - peak data rates up to 1 Tbps and ultra-low latency in microseconds. It features terahertz frequency com￾munication and spatial multiplexing, providing as much as 1000 times higher capacity than 5G networks. One goal of 6G is to achieve ubiquitous connectivity by integrating satellite communication networks and underwater communications to provide global coverage [19]. Energy harvesting technologies and the use of new materials will greatly improve the system energy efficiency and realize sustainable green networks. Three new 6G service classes were described in [15]: ubiqui￾tous mobile ultrabroadband (uMUB), ultrahigh-speed-with￾low-latency communications (uHSLLC) and ultrahigh data density (uHDD). III. ARCHITECTURES OF GREEN 6G NETWORK Green 6G networks are expected to achieve energy-efficient and socially seamless wireless connections in a global scope, while the existing network architecture is nuable to guar￾antee future application delivery constraints — ultra-high throughput, ultra-low latency and reliability. Therefore, forward-looking research on future network frameworks is necessary. FG NET-2030 has established Sub-Group 3 to formulate architecture of Network 2030. However, it is unre￾alistic to accurately illustrate what the future network archi￾tecture will be. Sub-Group 3 has reached a compromise — interpreting the architecture from different dimensions rather than defining a unified framework. In this section, we intro￾duce the architectural changes associated with 6G from three dimensions, as shown in Fig. 3. A. FROM TERRESTRIAL TO UBIQUITOUS 3D COVERAGE One target of the next generation network architecture is to expand the breadth and depth of communication coverage. The current network architecture based on legacy terres￾trial cellular infrastructure has the following two drawbacks: inability to meet the high-altitude and deep-sea commu￾nication scenarios, which is an inevitable requirement for future services; prohibitively expensive provisioning cost for dense cellular networks to provide connectivity in the global scale. In order to cover the above drawbacks, 6G will inte￾grate non-terrestrial networks to provide full wireless cover￾age [16]. Preliminary envision about Space-Air-Ground-Sea integrated communication has been discussed in [17]. 1) SPACE NETWORK: LEO SATELLITE SYSTEM High throughput satellite (HTS) systems are capable of broadband Internet access service comparable to terrestrial services in terms of pricing and bandwidth. Most commu￾nications satellites are in geostationary orbit (GEO) at an altitude of 35,786 km, naturally leading to excessive delay and infeasibility of integration with terrestrial mobile net￾work. Non-geostationary orbit (NGSO) satellite system is proposed to provide low-latency, high-bitrate global Internet connectivity and several satellite constellations are about to begin commercialization: • Starlink: American company SpaceX plans to launch Starlink, a constellation of 4,425 low Earth orbit LEO) satellites and 7518 VLEO satellites in approximately 340 km orbits. The plan was authorized by the Federal Communications Commission (FCC) [18] and will be fully deployed in 2027. • OneWeb: On February 27, 2019, OneWeb successfully launched its first six satellite into orbits. The constel￾lation consists of 720 LEO satellite [20] and has got authorization from UK and FCC. • Hongyan: China Aerospace Science and Technology Corporation (CASC) will launch nine LEO satellites as a pilot demonstration for the Hongyan system, which ultimately will comprise 320 satellites and be completed by 2025. Although there is still a long time before overall deploy￾ment of NGSO satellite systems and convergence of satellite communications and mobile wireless networks, advantages of LEO satellite networks have been confirmed in theory 175760 VOLUME 7, 2019

IEEEAcceSST. Huang et al: Survey on Green 6G Network: Architecture and TechnologiesControl view:IntelligentconnectionDistributedArtificial IntelligenceReal-timeIntelligentEdge6G applicationsIntelligent RadioContent-driven routingGEOXQuantunLEOateNetworkrepeaterSatellitFSORrlintSpaceview:networkEnhancedStratificationQuantumHAPBlockchannelntchainAirInfrastructureAerial4BackhoneXMannetworkview:planeUAVLUbiquitousArtuida3D coverageVLCSateliDParTerrestrialStetonnetwork611THZ4commkAetrumacsPUnderseanetworkUaderseaRF/optical/aconensotFIGURE 3.Different dimensions of the architecture of green 6Gand simulation environment.LEO network with laser andan altitude of no more than several kilometers.Comparedradio frequency(RF)co-routing mechanism can provideto LAP,HAP networks arecapable of widercoverageandlowerlatency communicationsthanterrestrial optical fiberlonger endurance, but the advantages of HAP overlapwithnetworks when communication distances are greater thanLEO satellitenetworkto someextent.On theotherhand,about3000km[21].LAP networks based on unmanned aerial vehicle (UAV)Apotential architecture of the space-terrestrial integratedcan be swifter to deploy, more flexibly reconfigured tonetwork (STIN)has shown in [22].comprised of thebest suit the communication environment,and present betterspace-basedbackbonenetwork (SBN)ofGEO satellites andperformance in short-range communication[25].Besides,the inter-satellite links (ISLs)connecting them,terrestrial net-flying base stations like UAV can work as relay nodes inworks (TN)and space-based accessnetworks (SAN)of LEOlong-distance communication to promote the integration ofand medium earth orbit(MEO)satellites.SBNis capableofterrestrial and non-terrestrial networks.Thesefeatures makeUAV-based wireless network a potential integral componentextending coverage and ensuring reliable space-ground con-nectivity while SAN is essential for integration with terres-of next-generation mobile communication system.In[26]a fully integrated, multi-layer vertical architecture for 6Gtrial and HTS networks to support ubiquitous global wirelessaccess. Several emerging technologies are embraced to facil-network was presented, including heterogeneous terrestrialitate the integration of terrestrial and satellite networks.SDNnetworks,UAV-basedLAPs,HAPs,LEO and GEO satelliteand ICNhavebeen introduced into STIN with the advantagesnetworks.The most attracting characteristic of UAV wireless net-of flexiblenetwork control,efficientnetwork configurationand small requestdelay[23].Theperformance of Multipathwork is that it enables mobile communication in situationsTCP (MPTCP) was evaluated in [24] and the result indicateswhere there are heavily compromised infrastructures or eventhat MPTCP strategy improves throughput and provides unin-no infrastructures,especially in catastrophic and emergencyterrupted connections duringhandover.situation. UAV network has been applied to temporary emer-gency communication services, however, there are issues to2)AERIALNETWORK:FLEXIBLERELAYSERVICESbetackled before stable and reliable UAVnetwork can beAerial network can be broadly classified into two categories,introduced into common application scenarios [27].First ofall,energyefficiency is critical tolong-term network servicehigh altitude platforms (HAP)which generally operate in thestratosphere and low altitude platforms (LAP)typically atPropulsion and directional adjustment consume most of theVOLUME7,2019175761

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies FIGURE 3. Different dimensions of the architecture of green 6G. and simulation environment. LEO network with laser and radio frequency (RF) co-routing mechanism can provide lower latency communications than terrestrial optical fiber networks when communication distances are greater than about 3000 km [21]. A potential architecture of the space-terrestrial integrated network (STIN) has shown in [22], comprised of the space-based backbone network (SBN) of GEO satellites and the inter-satellite links (ISLs) connecting them, terrestrial net￾works (TN) and space-based access networks (SAN) of LEO and medium earth orbit (MEO) satellites. SBN is capable of extending coverage and ensuring reliable space-ground con￾nectivity while SAN is essential for integration with terres￾trial and HTS networks to support ubiquitous global wireless access. Several emerging technologies are embraced to facil￾itate the integration of terrestrial and satellite networks. SDN and ICN have been introduced into STIN with the advantages of flexible network control, efficient network configuration and small request delay [23]. The performance of Multipath TCP (MPTCP) was evaluated in [24] and the result indicates that MPTCP strategy improves throughput and provides unin￾terrupted connections during handover. 2) AERIAL NETWORK: FLEXIBLE RELAY SERVICES Aerial network can be broadly classified into two categories, high altitude platforms (HAP) which generally operate in the stratosphere and low altitude platforms (LAP) typically at an altitude of no more than several kilometers. Compared to LAP, HAP networks are capable of wider coverage and longer endurance, but the advantages of HAP overlap with LEO satellite network to some extent. On the other hand, LAP networks based on unmanned aerial vehicle (UAV) can be swifter to deploy, more flexibly reconfigured to best suit the communication environment, and present better performance in short-range communication [25]. Besides, flying base stations like UAV can work as relay nodes in long-distance communication to promote the integration of terrestrial and non-terrestrial networks. These features make UAV-based wireless network a potential integral component of next-generation mobile communication system. In [26], a fully integrated, multi-layer vertical architecture for 6G network was presented, including heterogeneous terrestrial networks, UAV-based LAPs, HAPs, LEO and GEO satellite networks. The most attracting characteristic of UAV wireless net￾work is that it enables mobile communication in situations where there are heavily compromised infrastructures or even no infrastructures, especially in catastrophic and emergency situation. UAV network has been applied to temporary emer￾gency communication services, however, there are issues to be tackled before stable and reliable UAV network can be introduced into common application scenarios [27]. First of all, energy efficiency is critical to long-term network service. Propulsion and directional adjustment consume most of the VOLUME 7, 2019 175761

IEEEAcceSST. Huang et al: Survey on Green 6G Network: Architecture and TechnologiesTABLE 1.Comparison of diffirent undersea wireless communication technologies.RFAcousticOpticalHighAttenuationLowestxturbidityData rates [34]~Mbps~Kbps~GbpsModerateHighLowLatency [34]Transmission distance<10m<100km<100mHighPowerconsumptionModerateLowenergyin UAV communications,thereforenew trajectoryAl-driven approach where intelligence will be an endogenousoptimizationandrouteplanning schemes areproposed andcharacteristic of 6G architecture.Initial intelligence is that a relatively isolated networksignificantly improve energy efficiency [28], [29]. In [30]a learning-based computing offloading approach was pre-entity can intelligently adjust the configuration based onsentedtoreduceenergyconsumption andtotalcost.Secondly,multiple predefined options in a different yet determinis-extreme weather condition should be considered in UAVtic manner [35], which is actually an implementation ofcommunication.Free spaceoptics(FSO)is introduced intoperceptual AI without the capability to respond to unin-backhaulframework where UAVstransmit informationviatended scenarios. As the network is evolving into an extremepoint-point FSO links, enabling UAV network to offer highcomplex and heterogeneous system because of diversifieddata rates in different weather condition [31].Thirdly, dueservice requirements and explosively growing number ofto frequent topology change caused by high-speed mobility,connected devices, a novel AI paradigm of self-aware, self-more advanced mobile ad hoc protocols are demanded. Theadaptive,self-interpretive and prescriptive networking ismuch needed [38]. It requires not only embedding intelli-authors in [32] proposed adaptivehybrid communication pro-tocols which outperform existingprotocols.gence across whole network,but also embedding the logicof AI into the network structure,in which perception andinference interact in a systematic way,eventually enabling3)UNDERSEANETWORKallnetworkcomponentstoautonomouslyconnectandcontrolThere is a lot of controversy about whether undersea networkwith the ability to recognize unexpected situations and adaptis able to become a part of the future 6G network [17]. Under-to them.The ultimate expectation of intelligent networks issea wireless communication mainly involves RE, acousticthe autonomous evolution of networks.Wehighlight threeand optical communication.The comparison between thekey enablers for intelligent network, as described below.above three communicationtechnologies is shown in Table1.Unpredictableand complexunderwater environmentleadsto1)REAL-TIMEINTELLIGENTEDGE（RTIE)intricate network deployment, severe signal attenuation andNext generation network will require the support of interac-physical damage to equipment [33], leaving plenty of issuesto beresolved.tive AI-powered services and some services like autonomousvehicles are sensitive to response latency,which needs tointeract intelligently with their environments in real timeB.TOWARDSINTELLIGENTNETWORKCentralized cloud AI dealing with static data is incapable ofArtificial intelligence (AI), more specifically machine learn-achieving such services and there is an urgent need for theing (ML), has attracted a lot of attention from industry andRTIE,where intelligentprediction,inference anddecision areacademia in recentyearsand initial intelligence has beenmade onlive data.Major academiaand industryhavebegunapplied to many aspects of 5G cellular networks [35],fromtodeveloptechnologiesand softwarecomponentsthatmeetphysical layer applications such as channel coding and estithe real-time requirements in collaborative research labs suchmation,toMAClayerapplicationssuchasmultipleaccess,as theBerkeleyRISELab[39].High-performancehardwaretonetworklayerapplications suchas resourceallocation andis another driving factor for RTE and a specialized real-timeerror correction.and etc.In addition,the combination ofAI processor has been designed in [40].artificial intelligence and edge computing proves to improveQuality of Experience andreduce costs[36].Edgelearn2)INTELLIGENTRADIO(IR)ing also provides new possibilities for the implementationIn contrast to deployed physical (PHY) layer with initial intel-of many applications, such as healthcare[37].However,the application of AI in 5G networks is limited to the opti-ligence,IR is a broader and deeper conception that separatesmizationoftraditional networkarchitectureanditisdifficulthardware and transceiver algorithms. It operates as a unifiedto fully realize the potential of AI in the 5G era since theframework where hardware capabilities are estimated and5G networkdid not take AI into account at the beginningtransceiver algorithms can dynamically configure themselvesof architecture design. To fulfill the vision of intelligentaccording to the hardware information.From the perspectiveof PHY layer,IR is able to access theavailable spectrum,network,the design of 6G architecture should considerpos-sibilities of Al in network comprehensively and follow ancontrol transmission powerand adjusttransmission protocols175762VOLUME7,2019

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies TABLE 1. Comparison of diffirent undersea wireless communication technologies. energy in UAV communications, therefore new trajectory optimization and route planning schemes are proposed and significantly improve energy efficiency [28], [29]. In [30], a learning-based computing offloading approach was pre￾sented to reduce energy consumption and total cost. Secondly, extreme weather condition should be considered in UAV communication. Free space optics (FSO) is introduced into backhaul framework where UAVs transmit information via point-point FSO links, enabling UAV network to offer high data rates in different weather condition [31]. Thirdly, due to frequent topology change caused by high-speed mobility, more advanced mobile ad hoc protocols are demanded. The authors in [32] proposed adaptive hybrid communication pro￾tocols which outperform existing protocols. 3) UNDERSEA NETWORK There is a lot of controversy about whether undersea network is able to become a part of the future 6G network [17]. Under￾sea wireless communication mainly involves RF, acoustic and optical communication. The comparison between the above three communication technologies is shown in Table 1. Unpredictable and complex underwater environment leads to intricate network deployment, severe signal attenuation and physical damage to equipment [33], leaving plenty of issues to be resolved. B. TOWARDS INTELLIGENT NETWORK Artificial intelligence (AI), more specifically machine learn￾ing (ML), has attracted a lot of attention from industry and academia in recent years and initial intelligence has been applied to many aspects of 5G cellular networks [35], from physical layer applications such as channel coding and esti￾mation, to MAC layer applications such as multiple access, to network layer applications such as resource allocation and error correction, and etc. In addition, the combination of artificial intelligence and edge computing proves to improve Quality of Experience and reduce costs [36]. Edge learn￾ing also provides new possibilities for the implementation of many applications, such as healthcare [37]. However, the application of AI in 5G networks is limited to the opti￾mization of traditional network architecture and it is difficult to fully realize the potential of AI in the 5G era since the 5G network did not take AI into account at the beginning of architecture design. To fulfill the vision of intelligent network, the design of 6G architecture should consider pos￾sibilities of AI in network comprehensively and follow an AI-driven approach where intelligence will be an endogenous characteristic of 6G architecture. Initial intelligence is that a relatively isolated network entity can intelligently adjust the configuration based on multiple predefined options in a different yet determinis￾tic manner [35], which is actually an implementation of perceptual AI without the capability to respond to unin￾tended scenarios. As the network is evolving into an extreme complex and heterogeneous system because of diversified service requirements and explosively growing number of connected devices, a novel AI paradigm of self-aware, self￾adaptive, self-interpretive and prescriptive networking is much needed [38]. It requires not only embedding intelli￾gence across whole network, but also embedding the logic of AI into the network structure, in which perception and inference interact in a systematic way, eventually enabling all network components to autonomously connect and control with the ability to recognize unexpected situations and adapt to them. The ultimate expectation of intelligent networks is the autonomous evolution of networks. We highlight three key enablers for intelligent network, as described below. 1) REAL-TIME INTELLIGENT EDGE (RTIE) Next generation network will require the support of interac￾tive AI-powered services and some services like autonomous vehicles are sensitive to response latency, which needs to interact intelligently with their environments in real time. Centralized cloud AI dealing with static data is incapable of achieving such services and there is an urgent need for the RTIE, where intelligent prediction, inference and decision are made on live data. Major academia and industry have begun to develop technologies and software components that meet the real-time requirements in collaborative research labs such as the Berkeley RISELab [39]. High-performance hardware is another driving factor for RTE and a specialized real-time AI processor has been designed in [40]. 2) INTELLIGENT RADIO (IR) In contrast to deployed physical (PHY) layer with initial intel￾ligence, IR is a broader and deeper conception that separates hardware and transceiver algorithms. It operates as a unified framework where hardware capabilities are estimated and transceiver algorithms can dynamically configure themselves according to the hardware information. From the perspective of PHY layer, IR is able to access the available spectrum, control transmission power and adjust transmission protocols 175762 VOLUME 7, 2019

IEEEAcceSST. Huang et al: Survey on Green 6G Network: Architecture and TechnologiesTABLE 2. Inherent shortcomings of current Internet.Internet characteristicInherent shortcomingBest-effort deliveryNo guarantee of throughput and latencyradio retransmission are not synchronized with TCP flow controlTCPwastefullyretransmits packetsNetwork is not aware of the needs of application layersUnable to provide deterministic servicesFixed protocol fieldsEncapsulation redundancyBlind and independent congestion controlUnnecessary retransmission and dropIP address doesn't represent the real communication entitiesHard to satisfy different requestsTunnels over tunnels and duplicate header fieldsHigh header taxes and low protocol efficiencywith the aid of AI [41].By decoupling transceiver algorithmsupper layer, the routing andforwarding strategies would befrom hardware, new design paradigm allows agile adaptationefficient and direct.Similarly,the transport layer necessitates further enhance-to upgradable and diversified hardware [15]ment (e.g. differential priorities for data and high-precisionsynchronizationformultipathtransmission)toaccommodate3）DISTRIBUTEDAIfuture communication needs.In[45],a cross-layer transportThe future network will be a large decentralized system,layer was advocated. It merges traditional network and trans-where intelligent decisions are made at different granularport layer and carries out functions of both layers at thelevels. To accelerate the learning and improve the inferentialsame time. The benefit of the combined layer is that it takesreliability,distributedAI leverages distributed C4 resourcesinto account the requirements of applications and the overall(computation.communication.cachingandcontrol)[42]innetwork state,thus reducing congestion more effectivelythe network through parallel training process that requiresFurthermore, the cross-layer design breaks the end-to-endsplitting the data and model in an appropriate manner.principleandprovides advancednetworkfunction suchasA recently developed distributed AI paradigm is federatedflow multiplexing.learning[43]modelsaretrainedatedgebasedonlocalIV.PROMISINGTECHNOLOGIESOFGREEN6Gsamplepatterns and sent to centralized cloud for modelA.SPECTRUMCOMMUNICATIONTECHNIQUESaveraging,thereby obtaining a shared global model.It alsoSpectrum is thefoundation ofmobile communications andstrengthens security and privacy by keeping data at edge.since the rise of mobile networks in the 1980s, we havewitnessed tremendous expansion of spectrum resources inC.NEWNETWORKPROTOCOLSTACKARCHITECTUREevery newgeneration dueto the endless pursuitfordata rates.The existing Internet protocol stack architecture,typicallyOne of the main targets of 6G is to provide Tbps aggregatedTCP/IP, was originally designed for data delivery and hasbit rate and it is inevitable to operate at higher frequenciesachieved huge success over the past 40 years. Nonethe-for available spectrum and bandwidth.Terahertz (THz)andless, current Internet has encountered many unprecedentedvisible light are two attractive candidate spectrums [46]challenges and cannot guarantee futuristic application deliv-1)THZCOMMUNICATIONery constraints (e.g., deterministic throughput and latency)In recentyears,some protocolsbuilt on TCP/IP such asThe THz band is the spectral band between microwaveQUIC (Quick UDP Internet Connections) have eased theseand optical bands, with frequency ranging from 0.1 THzchallenges to some extent.Unfortunately.these patch-liketo 10 THz.Except for abundant undeveloped spectrumprotocolsmaketheInternetmorecomplicatedandarenotresources,there are many uniquecharacteristics thatmotivateable to completely remedy the inherent Internet shortcomingstheuseofTHzbandforfuturecommunication networks[47](shown in Table 2) [44].This brings the need to rethink(1)THzcommunication systemispromisedto supportdatatheTCP/IP protocols and newdevelopments areexpected toratesontheorderof 1o0GbpsormorewithtensofGHzprovide services beyond end-to-end transportation.availablebands inTHz spectrum,whilethere is only9GHzCurrent network layer packets follow a fixed “"header +bandwidth within mm-Wave band [48]. (2) THz wave couldpayload" mode, which is isolated from the requirements ofrealize secure communication dueto narrowbeamand shortupper applications.To support abundant future applicationspulse duration that drastically limit theprobability of eaves-and providedifferentiated network services,meta-data anddropping.(3)THz waves are able to penetrate some materialscommands defined by theapplication designer may becomewithasmallattenuation.whichissuitableforsomespecialimportantcomponentofnewIPprotocol.Suchprotocolfieldsscenes.With such characteristics,THz wavehasbroadappli-could include identifiers of real communication entities andcation prospect in ultra-high speed wireless communicationinformation aboutflow states,applications'requirements,and space communication,thusglobal regulatorybodiesandnetwork measurement metrics and security details.Ideally,standard agencies are already trying to expedite the develop-the network functions couldperceivethe expected packetmentofnewcommunicationstechnologiesinTHzspectrumformat and implementflexible policies based on meta-data:InMarch2019,theFCCvotedtopermitenhancedexper-and commands. With the aid of additional information fromimental licensing and unlicensed applications within the175763VOLUME7,2019

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies TABLE 2. Inherent shortcomings of current Internet. with the aid of AI [41]. By decoupling transceiver algorithms from hardware, new design paradigm allows agile adaptation to upgradable and diversified hardware [15]. 3) DISTRIBUTED AI The future network will be a large decentralized system, where intelligent decisions are made at different granular levels. To accelerate the learning and improve the inferential reliability, distributed AI leverages distributed C4 resources (computation, communication, caching and control) [42] in the network through parallel training process that requires splitting the data and model in an appropriate manner. A recently developed distributed AI paradigm is federated learning [43] — models are trained at edge based on local sample patterns and sent to centralized cloud for model averaging, thereby obtaining a shared global model. It also strengthens security and privacy by keeping data at edge. C. NEW NETWORK PROTOCOL STACK ARCHITECTURE The existing Internet protocol stack architecture, typically TCP/IP, was originally designed for data delivery and has achieved huge success over the past 40 years. Nonethe￾less, current Internet has encountered many unprecedented challenges and cannot guarantee futuristic application deliv￾ery constraints (e.g., deterministic throughput and latency). In recent years, some protocols built on TCP/IP such as QUIC (Quick UDP Internet Connections) have eased these challenges to some extent. Unfortunately, these patch-like protocols make the Internet more complicated and are not able to completely remedy the inherent Internet shortcomings (shown in Table 2) [44]. This brings the need to rethink the TCP/IP protocols and new developments are expected to provide services beyond end-to-end transportation. Current network layer packets follow a fixed ‘‘header + payload’’ mode, which is isolated from the requirements of upper applications. To support abundant future applications and provide differentiated network services, meta-data and commands defined by the application designer may become important component of new IP protocol. Such protocol fields could include identifiers of real communication entities and information about flow states, applications’ requirements, network measurement metrics and security details. Ideally, the network functions could perceive the expected packet format and implement flexible policies based on meta-data and commands. With the aid of additional information from upper layer, the routing and forwarding strategies would be efficient and direct. Similarly, the transport layer necessitates further enhance￾ment (e.g. differential priorities for data and high-precision synchronization for multipath transmission) to accommodate future communication needs. In [45], a cross-layer transport layer was advocated. It merges traditional network and trans￾port layer and carries out functions of both layers at the same time. The benefit of the combined layer is that it takes into account the requirements of applications and the overall network state, thus reducing congestion more effectively. Furthermore, the cross-layer design breaks the end-to-end principle and provides advanced network function such as flow multiplexing. IV. PROMISING TECHNOLOGIES OF GREEN 6G A. SPECTRUM COMMUNICATION TECHNIQUES Spectrum is the foundation of mobile communications and since the rise of mobile networks in the 1980s, we have witnessed tremendous expansion of spectrum resources in every new generation due to the endless pursuit for data rates. One of the main targets of 6G is to provide Tbps aggregated bit rate and it is inevitable to operate at higher frequencies for available spectrum and bandwidth. Terahertz (THz) and visible light are two attractive candidate spectrums [46]. 1) THZ COMMUNICATION The THz band is the spectral band between microwave and optical bands, with frequency ranging from 0.1 THz to 10 THz. Except for abundant undeveloped spectrum resources, there are many unique characteristics that motivate the use of THz band for future communication networks [47]: (1) THz communication system is promised to support data rates on the order of 100 Gbps or more with tens of GHz available bands in THz spectrum, while there is only 9 GHz bandwidth within mm-Wave band [48]. (2) THz wave could realize secure communication due to narrow beam and short pulse duration that drastically limit the probability of eaves￾dropping. (3) THz waves are able to penetrate some materials with a small attenuation, which is suitable for some special scenes. With such characteristics, THz wave has broad appli￾cation prospect in ultra-high speed wireless communication and space communication, thus global regulatory bodies and standard agencies are already trying to expedite the develop￾ment of new communications technologies in THz spectrum: • In March 2019, the FCC voted to permit enhanced exper￾imental licensing and unlicensed applications within the VOLUME 7, 2019 175763

IEEEAcceSST. Huang et al: Survey on Green 6G Network: Architecture and TechnologiesIn order to support THz communication, the followingaspects need further research: (1)Transistor and hardwareFECmaterials with excellent high frequency characteristics.TheADXDLLProcesRFfrontendBascband Procmost potential material employed forhardware design isgraphene which has high thermal and electrical conductiv-FIGURE 4. THz wireless radio transceiver hardware.ities and plasmonic effects[54].(2)Robust beam form-ing and scanning algorithms such as hybrid beamformingspectrumbetween95GHzand3THzandlaunchedaapproaches.(3)Low-complexity,low-powerhardwarecir-new license type-Spectrum Horizons License withcuits.(4)Channel and noise modeling.It is hard to establishlowbarriersandminimalcostinordertoacceleratethea common THz channel model andmixturemodelsmaybedevelopment ofTHz technologies [49]the solutionto this problem.(5)Energy efficient modulation.In Europe,Horizon 2020, thebiggest EU Research andschemesandlow-density channelcodes.(6)Ultra-massiveInnovation program, has funded several projects in theMIMO system.Nano antennas willplay an importantcallNetworking research beyond 5G",most of whichrole in MIMO systems. (7) Powerful synchronizationare aiming at THz technologies.schemes.IEEE has begun to take the first steps for THzcommunication standardization by forming the IEEE2)VISIBLELIGHTCOMMUNICATIONS802.15 wirelesspersonal area network(WPAN)TaskOptical Wireless Communications (OWC) are considered asGroup 3-D 100Gb/s Wireless (TG3d 100 G)[50]a complementary technology for RF-based mobile commu-and createdthefirst worldwidewireless communicationnications and the frequency range includes infrared, visiblestandard in the 250-350 GHz frequency rangewhichlight and ultraviolet spectrum. The visible light spectrumchannelizes theband into32channelsof 2.16GHz.(430-790THz) is the most promising spectrum of OWC·TheInternationalTelecommunicationUnion(ITU)hasdue to the technological advances and widespread adop-decided to classify 0.12THz and 0.2THz into wirelesstion of LED.Oneof themost striking aspects of LEDcommunication.differentfromolder illuminationtechnologyisthat it can.TheMinistryof Internal Affairs and Communicationsswitch to different light intensity levels very quickly,whichofJapan hasestablished anR&DprogramonKeyenables data to be encoded in emitted light in a variety ofTechnology in Terahertz Frequency Bands and supways [55].ported a lot of related work, such as CMOS technologyVisible light communication (VLC) takes full advantage ofin the THz band.LED to achieve the dual purposes of lightning and high-speedThere are tremendous challenges ahead for mature THzdata communication.VLC for short range links (up to fewwireless communication system, but global research hasmeters)hasmanyattractiveadvantagesoverclassicalradiomade some breakthroughs. In [51], the authors designed acommunication[56].Firstly,visible light spectrum pro-300-GHz band wireless transceiver chipset and a module thatvides ultra-high bandwidth (THz), and the spectrum is freeconnects CMOS chipset to external waveguide antenna toandunlicensed.Secondly,visiblelight,VLC'stransmissionovercome the defectof low antenna gain of on-chip antenna.medium, cannot penetrate opaque obstructions.This meansRecently,a full-featured single-chip transceiver capable ofthat thetransmission of network information is confined todata rates of 80Gb/s was introduced in[52].Owing to suchone building and receivers outside the building are incapablehighfrequencies,thereflections,cross-talks,andattenuationof receiving the signals, which obviously guarantees theonthebonding wires lead tomanyproblems in thedesign ofinformation transmission security and reduces the inter-cellRF-frontend and baseband such as gain reduction. [53] advo-interference that is very serious in high-frequency RF com-catesshiftingtheseproblemstodatalink layer(DLL)andmunication.Thirdly,VLC utilizes illumination sources asfabricatesaDLLprocessor withlow-chiparea.Fig.4depictsbasestations.whichdoesnotneedexpensivebasestationthe hardware structure of THz wireless radio transceiver.construction and maintenance costs that are required inTABLE 3. Comparison of THz communication and VLC.THZVLCAvailable bandwidthTenstohundredsofGHzHundreds of THzLOSTransmission distancenon-line-of-sight (NLOS)noelectromagnetic radiationves100Gbps10GbpsData rate achievedSpectrum regulatorylicensedunlicensedPenetration ability Special opaque materialsTransparent materialsInter-cell interferenceseriousnoneCostexpensivecheapTransmission powerHighlowlicensedunlicenseddiffusereflectionlosses175764VOLUME7,2019

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies FIGURE 4. THz wireless radio transceiver hardware. spectrum between 95GHz and 3THz and launched a new license type— Spectrum Horizons License with low barriers and minimal cost in order to accelerate the development of THz technologies [49]. • In Europe, Horizon 2020, the biggest EU Research and Innovation program, has funded several projects in the call ‘‘Networking research beyond 5G’’, most of which are aiming at THz technologies. • IEEE has begun to take the first steps for THz communication standardization by forming the IEEE 802.15 wireless personal area network (WPAN) Task Group 3-D 100 Gb/s Wireless (TG 3d 100 G) [50] and created the first worldwide wireless communication standard in the 250-350 GHz frequency range which channelizes the band into 32 channels of 2.16 GHz. • The International Telecommunication Union (ITU) has decided to classify 0.12THz and 0.2THz into wireless communication. • The Ministry of Internal Affairs and Communications of Japan has established an R&D program on Key Technology in Terahertz Frequency Bands and sup￾ported a lot of related work, such as CMOS technology in the THz band. There are tremendous challenges ahead for mature THz wireless communication system, but global research has made some breakthroughs. In [51], the authors designed a 300-GHz band wireless transceiver chipset and a module that connects CMOS chipset to external waveguide antenna to overcome the defect of low antenna gain of on-chip antenna. Recently, a full-featured single-chip transceiver capable of data rates of 80Gb/s was introduced in [52]. Owing to such high frequencies, the reflections, cross-talks, and attenuation on the bonding wires lead to many problems in the design of RF-frontend and baseband such as gain reduction. [53] advo￾cates shifting these problems to data link layer (DLL) and fabricates a DLL processor with low-chip area. Fig. 4 depicts the hardware structure of THz wireless radio transceiver. In order to support THz communication, the following aspects need further research: (1) Transistor and hardware materials with excellent high frequency characteristics. The most potential material employed for hardware design is graphene which has high thermal and electrical conductiv￾ities and plasmonic effects [54]. (2) Robust beam form￾ing and scanning algorithms such as hybrid beamforming approaches. (3) Low-complexity, low-power hardware cir￾cuits. (4) Channel and noise modeling. It is hard to establish a common THz channel model and mixture models may be the solution to this problem. (5) Energy efficient modulation schemes and low-density channel codes. (6) Ultra-massive MIMO system. Nano antennas will play an important role in MIMO systems. (7) Powerful synchronization schemes. 2) VISIBLE LIGHT COMMUNICATIONS Optical Wireless Communications (OWC) are considered as a complementary technology for RF-based mobile commu￾nications and the frequency range includes infrared, visible light and ultraviolet spectrum. The visible light spectrum (430-790THz) is the most promising spectrum of OWC due to the technological advances and widespread adop￾tion of LED. One of the most striking aspects of LED different from older illumination technology is that it can switch to different light intensity levels very quickly, which enables data to be encoded in emitted light in a variety of ways [55]. Visible light communication (VLC) takes full advantage of LED to achieve the dual purposes of lightning and high-speed data communication. VLC for short range links (up to few meters) has many attractive advantages over classical radio communication [56]. Firstly, visible light spectrum pro￾vides ultra-high bandwidth (THz), and the spectrum is free and unlicensed. Secondly, visible light, VLC’s transmission medium, cannot penetrate opaque obstructions. This means that the transmission of network information is confined to one building and receivers outside the building are incapable of receiving the signals, which obviously guarantees the information transmission security and reduces the inter-cell interference that is very serious in high-frequency RF com￾munication. Thirdly, VLC utilizes illumination sources as base stations, which does not need expensive base station construction and maintenance costs that are required in TABLE 3. Comparison of THz communication and VLC. 175764 VOLUME 7, 2019

IEEEAcCeSST. Huang et al: Survey on Green 6G Network: Architecture and TechnologiesRF communication.Finally,VLCdoes notgenerate electro-and inalienablelaw.Furthermore,QCcanimprovedataratesmagnetic radiation andis immuneto external electromagneticdue to the superposition nature of qubits. After decades ofinterference.Therefore, it is suitable for special situationsexploration,there are manybranches of QC:quantumkeysensitivetoelectromagnetic radiation[57] such asaircraftanddistribution, quantum teleportation, quantum secret sharinghospital.andquantum securedirectcommunication[66]The maximum achievable data rate depends largely onA number of works have practically implemented ini-lightingtechnology[58].Thedatarateof VLCbased ontial quantum network and associated protocols of variousphosphor coated blueLED is upto1 Gb/s,while thedatabranches.The properties and capacities of QC channels wereratebased onRGBLEDcan reachmulti-Gb/s.Thebestper-summarized in [67].Another attractivefeature of QC is itsforming LED technology is micro-LED, which has achievedhuge potential in long-distance communication.Quantumdata rates of more than 10 Gb/s in the lab [59].With therepeaters are critical devices for long-distance global quan-continuousimprovementofthelifeandluminousefficiencytumnetworkwhichiscapableofdividingthedistanceofof LEDlamps and the advancementof related technologiesQCinto shorter intermediate segments and correcting both(e.g.digital modulation technology),VLC is expected tophoton loss and operation errors [68].A recent work has real-ized intercontinentalQCwith amaximaldistanceof7600kmreachthedatarateofhundredsof Gb/s,orevenTb/s,inthe6Gera.relying on LEO satellites as relay.B.NEWCOMMUNICATIONPARADIGMC.FUNDAMENTALTECHNIQUES1)BLOCKCHAINFORDECENTRALIZEDSECURITY1)MOLECULARCOMMUNICATIONThere are manyenvironments (i.e.nanonetwork insidebody)Blockchains are distributed ledger-based databases wherewhereconventionalwirelesscommunicationbasedonelectransactions can be securely registered and updated with-tromagnetic (EM)waves may not be feasible or efficientout central intermediaries [69].The inherent features ofA new communication paradigm inspired by nature is con-blockchain such as decentralized tamper-resistanceandsideredapossible solutionwhich usesbiochemical signalsanonymity make it ideal for various applications [70]In 2018 Mobile World Congress Americas (MWCA)to transfer information,referred to as molecular communi-the FCC commissioner, Jessica Rosenworcel,indicated thatcations (MC) [60]. In MC, biochemical signals are typicallysmall particles of a few nanometers to a few micrometersblockchain will play an important role in next-generationin size such as lipid vesicles and particles, which usuallywirelessnetwork [71].Blockchain is considered as the next revolution for futurepropagate in aqueous or gaseous medium.Compared with radio communication,MChas certainmobile communication technologies.Itguarantees strongeradvantages in both micro and macro scale. On the one hand,security features throughout the communication sinceitat nanoscale dimension,electromagnetic communication isenables various network entitiesto securely access the criticalconstrained bythe ratio of antenna sizeto EM wavelengthdata and an untamable distributed ledger containing all data iswhileMC signals arebiocompatible,and consume very littleshared among all relevant entities [72].Apart from securityenergyforgenerationandpropagation[61].whichmakesMCblockchain provides severalbenefitsin resource orchestra-ideal for intrabody nanonetworks [62].On the other hand,tion and network access.Decentralized control mechanismEM communication is notreliable in someharshenviron-basedonblockchainenablesdirectcommunicationlinkstoments such as underground tunnels and gas pipelines due tobeestablishedbetweennetworkentities.whichreducesthehigh propagation path loss. An emerging view is that MCadministrative costs[73].Instead of centralized database,is highly dynamic, leading to the initial research of mobilethe integration ofblockchain in spectrum sharing system canMC[63].Mobilecarriersareusefulfornetworkimplemen-increase spectral efficiency. In addition,blockchain facilitatestation and increase data transfer rates.Ultimately,MC sys-the integration of individual systems developed by differenttems are expected to interface with the Internet and mobileoperatorsbyprovidinga unified authentication and authoriza-networks and the two major challenges are the interfacestion mechanism and billing system [74], and allows roamingbetween electrical and chemical domain, and security assur-across operators and networks.ance methods [64].2)FLEXIBLEANDINTELLIGENTMATERIALS2）QUANTUMCOMMUNICATIONDespite the tremendous success in wireless communica-Quantumcommunication(QC)isanotherpromisingcom-tion system over the past decades, the performance ofmunication paradigm with unconditional security.The fun-traditional semiconductor materials like silicon seems todamental difference between quantum communication andreach its limits and materials with better high-frequencyclassical binarybased communication is whether eavesdrop-and high-temperature characteristics are in urgent need forping can be detected on-site [65]. The information is encodedultrahigh-speed communication. Novel materials such asin quantum state using photons or quantum particles andGallium Nitride, Indium Phosphide, Silicon Germanium andcannot be accessed or cloned without tampering it due toGraphene have been used todesign next-generation commuquantum principles such as correlation ofentangled particlesnications devices. Moreover, fluid materials are introducedVOLUME7, 2019175765

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies RF communication. Finally, VLC does not generate electro￾magnetic radiation and is immune to external electromagnetic interference. Therefore, it is suitable for special situations sensitive to electromagnetic radiation [57] such as aircraft and hospital. The maximum achievable data rate depends largely on lighting technology [58]. The data rate of VLC based on phosphor coated blue LED is up to 1 Gb/s, while the data rate based on RGB LED can reach multi-Gb/s. The best per￾forming LED technology is micro-LED, which has achieved data rates of more than 10 Gb/s in the lab [59]. With the continuous improvement of the life and luminous efficiency of LED lamps and the advancement of related technologies (e.g. digital modulation technology), VLC is expected to reach the data rate of hundreds of Gb/s, or even Tb/s, in the 6G era. B. NEW COMMUNICATION PARADIGM 1) MOLECULAR COMMUNICATION There are many environments (i.e. nanonetwork inside body) where conventional wireless communication based on elec￾tromagnetic (EM) waves may not be feasible or efficient. A new communication paradigm inspired by nature is con￾sidered a possible solution which uses biochemical signals to transfer information, referred to as molecular communi￾cations (MC) [60]. In MC, biochemical signals are typically small particles of a few nanometers to a few micrometers in size such as lipid vesicles and particles, which usually propagate in aqueous or gaseous medium. Compared with radio communication, MC has certain advantages in both micro and macro scale. On the one hand, at nanoscale dimension, electromagnetic communication is constrained by the ratio of antenna size to EM wavelength, while MC signals are biocompatible, and consume very little energy for generation and propagation [61], which makes MC ideal for intrabody nanonetworks [62]. On the other hand, EM communication is not reliable in some harsh environ￾ments such as underground tunnels and gas pipelines due to high propagation path loss. An emerging view is that MC is highly dynamic, leading to the initial research of mobile MC [63]. Mobile carriers are useful for network implemen￾tation and increase data transfer rates. Ultimately, MC sys￾tems are expected to interface with the Internet and mobile networks and the two major challenges are the interfaces between electrical and chemical domain, and security assur￾ance methods [64]. 2) QUANTUM COMMUNICATION Quantum communication (QC) is another promising com￾munication paradigm with unconditional security. The fun￾damental difference between quantum communication and classical binary based communication is whether eavesdrop￾ping can be detected on-site [65]. The information is encoded in quantum state using photons or quantum particles and cannot be accessed or cloned without tampering it due to quantum principles such as correlation of entangled particles and inalienable law. Furthermore, QC can improve data rates due to the superposition nature of qubits. After decades of exploration, there are many branches of QC: quantum key distribution, quantum teleportation, quantum secret sharing and quantum secure direct communication [66]. A number of works have practically implemented ini￾tial quantum network and associated protocols of various branches. The properties and capacities of QC channels were summarized in [67]. Another attractive feature of QC is its huge potential in long-distance communication. Quantum repeaters are critical devices for long-distance global quan￾tum network, which is capable of dividing the distance of QC into shorter intermediate segments and correcting both photon loss and operation errors [68]. A recent work has real￾ized intercontinental QC with a maximal distance of 7600km relying on LEO satellites as relay. C. FUNDAMENTAL TECHNIQUES 1) BLOCKCHAIN FOR DECENTRALIZED SECURITY Blockchains are distributed ledger-based databases where transactions can be securely registered and updated with￾out central intermediaries [69]. The inherent features of blockchain such as decentralized tamper-resistance and anonymity make it ideal for various applications [70]. In 2018 Mobile World Congress Americas (MWCA), the FCC commissioner, Jessica Rosenworcel, indicated that blockchain will play an important role in next-generation wireless network [71]. Blockchain is considered as the next revolution for future mobile communication technologies. It guarantees stronger security features throughout the communication since it enables various network entities to securely access the critical data and an untamable distributed ledger containing all data is shared among all relevant entities [72]. Apart from security, blockchain provides several benefits in resource orchestra￾tion and network access. Decentralized control mechanism based on blockchain enables direct communication links to be established between network entities, which reduces the administrative costs [73]. Instead of centralized database, the integration of blockchain in spectrum sharing system can increase spectral efficiency. In addition, blockchain facilitates the integration of individual systems developed by different operators by providing a unified authentication and authoriza￾tion mechanism and billing system [74], and allows roaming across operators and networks. 2) FLEXIBLE AND INTELLIGENT MATERIALS Despite the tremendous success in wireless communica￾tion system over the past decades, the performance of traditional semiconductor materials like silicon seems to reach its limits and materials with better high-frequency and high-temperature characteristics are in urgent need for ultrahigh-speed communication. Novel materials such as Gallium Nitride, Indium Phosphide, Silicon Germanium and Graphene have been used to design next-generation commu￾nications devices. Moreover, fluid materials are introduced VOLUME 7, 2019 175765

IEEEAcceSST. Huang et al.: Survey on Green 6G Network: Architecture and Technologiesinto the design of frequency-reconfigurable antennas to pro-REFERENCESvide more flexibility [75][1] D. Soldani and A. Manzalini, "Horizon 2020 and beyond: On the 5GOperating system for a true digital society," IEEE Veh. Technol. Mag.Metamaterial and metasurface can be deployed in avol. 10, no. 1, pp. 32-42, Mar. 2015.radio-controllablewirelessenvironment[76].Metamaterials[2] P. Yang,Y. Xiao, M.Xiao, and S.Li,"6G Wireless communications:are artificial structures composed by periodic or quasi-peri-Vision and potential techniques"IEEE Netw., vol. 33, no. 4, pp. 70-75,Jul./Aug.2019.odicmeta-atoms with EM properties acrossanyfrequency[3] M. Katz, M. Matinmiko-Blue, and M. Latva-Aho, "6Genesis flagdomain.Software-controlledplanar metamaterials arepossi-ship program: Building the bridges towards 6G-enabled wirelessble to reduce the interference by deterministic control oversmart society and ecosystem," in Proc.IEEE IOth Latin-Amer Conf.Commun. (LATINCOM),Nov. 2018, Pp.1-9.the properties of the environment [77].[4] IMT Traffic Estimates for the Years 2020 to 2030, document ITU-R SG05Jul. 2015.[5] K. David and H. Bernd, "6G vision and requirements: Is there any need3)ENERGYHARVESTINGANDMANAGEMENTfor beyond 5G?" IEEE Veh. Technol. Mag-, vol. 13, no. 3, Pp. 72-80,The consistent computation demands for AI processing andSep. 2018.[6] A. Gupta and E. R K. Jha, "A survey of 5G network: Architecture andincreasing proliferation of IoT devices are posing signif-emerging technologies"IEEE Access, vol.3,pp.1206-1232, Jul. 2015.icant challenges to the energy efficiency of communica-[7]K.David and H.Berndt, "6G vision and requirements: Is there any need fortion equipment.Therefore,energy-efficient communicationbeyond 5g?"IEEE Veh.Technol. Mag., vol.13, no.3.pp.7280,Sep.2018.[8] P. Sharma, "Evolution of mobile wireless communication networks-1Gtechnologies will shine in 6G where communication dis-to 5G as well as future prospective of next generation communicationtance is much shorter.There have been numerous effortsnetwork,"Int.J.Compu.Sci.Mobile Compu.,vol.2,no.8,pp.47-53,2013.spent on energy harvesting and management researches over[9] A. U. Gawas, "An overview on evolution of mobile wireless communica-the past decade.A technology called symbiotic radio (SR)tionnetworks:1G-6G."Int.J.Recen InnovTrends Comput.Commun..offers a possible solution to the energy problem, whichvol. 3, no. 5, pp. 31303133, 2015.integrates passive backscatter devices with active trans-[10] J. Parikh and A. Basu, "LTE advanced: The 4G mobile broadband technology,"Int.J.Compu.Appl.,vol.13, no.5, pp.17-21,2011.mission system [78]. A typical example of SR is ambi-[Il] M. Shafi, A. F. Molisch, P. J. Smithein, P. Zhu, P. De Silva.Hausteentbackscattercommunication,inwhichnetworkdevicesF. Tufvesson, A. Benjebbour, and G. Wunder, "5G: A tutorial overviewutilize ambient RF signals to transmit information with-of standards, trials, challenges, deployment, and practice," IEEE J. Sel.Areas Commun.,vol.35,no. 6, pp.1201-1221,Jun.2017.out requiring active RF transmission, making battery-free[12] Z. Zhou, M. Dong, K. Ota, G. Wang, and L. I. Yang, "Energycommunication possible [79]. Smart energy managementefficient resource allocation for D2D communications underlaying cloud-is another promising mechanism with the goal of dynam-RAN-based LTE-A networks, IEEE Intemet Things J., vol. 3, no. 3,pp. 428-438, Jun. 2016.ically optimizing the balance between energy demand and[13] J.Wu,M.Dong.K.Ota,J.Li,W.Yang.andM.Wang."Fog-computing-supply [80].enabled cognitivenetwork function virtualization for an information-centric future Intermet."IEEE Commun.Mag-, vol.57, no.7,pp.48-54,Jul.2019.V.CONCLUSION[14] J. Wu, M. Dong, K. Ota, J. Li, and Z. Guan, "Big data analysis-basedsecure cluster managementfor optimized control plane in software-definedAlmost exponential increase in wireless data,especially mul-networks," IEEE Trans.Netw. Service Manag-, vol. 15, no. 1., pp.27-38.timedia data, and rapid proliferation of all kinds of smartMar. 2018.devices are setting the stage for next wireless evolutionC.Fan,X.Wang,X.Duan, B.Wang, and J.Wang.[15] B.Zong-"6G technologies: Key drivers, core requirements, system architectures.towards 6G.6G wireless networks are promising a signifi-and enabling technologies,"IEEE Veh.Technol.Mag.,vol.14, no.3.cant increase in QoS and sustainable future. In this paper,pp. 18-27, Sep. 2019.we present a detailer survey on wireless evolution towards[16] Z. Zhang, Y. Xiao, Z. Ma, M. Xiao, Z. Ding, X. Lei, G. K. Karagiannidisand P. Fan, "6G wireless networks: Vision, requirements, architecture, andgreen 6Gnetworks.Wecommence withtheevolutionofkey technologies," IEEE Veh. Technol. Mag., vol. 14, no. 3, Pp. 28-41,mobilewirelessnetworkfrom1Gto5G,whichindicatesSep. 2019.the development trend of 6G to some extent. Subsequently[17] Y. Zhao, G. Yu, and H. Xu, "6G mobile communication network: Vision.challenges and key technologies," 2019, arXiv:1905.04983. [Online].the new architectural paradigm shift is explained with threeAvailable: https://arxiv.org/abs/1905.04983brand new characteristics, including the integration of ter-[18]MemorandumOpinion,OrderandAuthorizationintheMatlerofApplica-restrial and non-terrestrial networks,truly intelligent connec-tionforApproval for Orbital Deploymentand Operating AuthoriryfortheSPACEX NGSO Satellite System, document FCC-18-38, Mar. 2018.tionsenabled bypervasiveAI and enhancednetworkprotocol[19] A. Yastrebova, R. Kirichek, Y. Koucheryavy, A. Borodin, andstack framework.Finally,wefocus our attention on emergingA. Koucheryavy,"Future networks 2030: Architecture & requirements,technologies.New spectrum technologies, like THz com-inProc.10thInt.Ultra Mod. Telecommun. Control Syst.CongrWorkshops (ICUMT),Nov.2018,pp.1-8.munication and VLC,and new communication paradigms.[20] J. Radtke, C. Kebschull, and E. Stoll, "Interactions of the space debrissuch as molecular and quantum communication, have beenenvironmentwith mega constellationsUsing the example oftheOneWebdiscussed, which are potential to dramatically improve dataconstellation," Acta Astronautica, vol.131, pp. 5568, Feb. 2017.[21] M. Handley,"Delay is not an option: Low latency routing in space," inrates and becomekey parts ofa seamless society.InnovationsProc.17thACMWorkshopHotTopicsNehw.,2018,pp.85-91in fundamental technologies are also explained, including[22] H. Yao, L. Wang, X. Wang, Z. Lu, and Y. Liu, "The space-terrestrialintroduction of blockchain,flexible and intelligent materialinteeratednetwork:AnoverviewIEEECommun.Mag..vol.56,no.9pp.178-185, Sep.2018and ambient backscatter communication.This survey may[23] Z. Liu, J. Zhu, C. Pan, and G. Song. "Satellite network architecture designserve as an enlightening guideline for future research worksbased on SDN and ICN technology." in Proc. 8th In. Conf. Electron. Inf.ingreen 6G communications.Emergency Commun. (ICEIEC), Jun.2018,pp.124-131.175766VOLUME7,2019

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies into the design of frequency-reconfigurable antennas to pro￾vide more flexibility [75]. Metamaterial and metasurface can be deployed in a radio-controllable wireless environment [76]. Metamaterials are artificial structures composed by periodic or quasi- peri￾odic meta-atoms with EM properties across any frequency domain. Software-controlled planar metamaterials are possi￾ble to reduce the interference by deterministic control over the properties of the environment [77]. 3) ENERGY HARVESTING AND MANAGEMENT The consistent computation demands for AI processing and increasing proliferation of IoT devices are posing signif￾icant challenges to the energy efficiency of communica￾tion equipment. Therefore, energy-efficient communication technologies will shine in 6G where communication dis￾tance is much shorter. There have been numerous efforts spent on energy harvesting and management researches over the past decade. A technology called symbiotic radio (SR) offers a possible solution to the energy problem, which integrates passive backscatter devices with active trans￾mission system [78]. A typical example of SR is ambi￾ent backscatter communication, in which network devices utilize ambient RF signals to transmit information with￾out requiring active RF transmission, making battery-free communication possible [79]. Smart energy management is another promising mechanism with the goal of dynam￾ically optimizing the balance between energy demand and supply [80]. V. CONCLUSION Almost exponential increase in wireless data, especially mul￾timedia data, and rapid proliferation of all kinds of smart devices are setting the stage for next wireless evolution towards 6G. 6G wireless networks are promising a signifi- cant increase in QoS and sustainable future. In this paper, we present a detailer survey on wireless evolution towards green 6G networks. We commence with the evolution of mobile wireless network from 1G to 5G, which indicates the development trend of 6G to some extent. Subsequently, the new architectural paradigm shift is explained with three brand new characteristics, including the integration of ter￾restrial and non-terrestrial networks, truly intelligent connec￾tions enabled by pervasive AI and enhanced network protocol stack framework. Finally, we focus our attention on emerging technologies. New spectrum technologies, like THz com￾munication and VLC, and new communication paradigms, such as molecular and quantum communication, have been discussed, which are potential to dramatically improve data rates and become key parts of a seamless society. Innovations in fundamental technologies are also explained, including introduction of blockchain, flexible and intelligent material and ambient backscatter communication. This survey may serve as an enlightening guideline for future research works in green 6G communications. REFERENCES [1] D. Soldani and A. Manzalini, ‘‘Horizon 2020 and beyond: On the 5G operating system for a true digital society,’’ IEEE Veh. Technol. Mag., vol. 10, no. 1, pp. 32–42, Mar. 2015. [2] P. Yang, Y. Xiao, M. Xiao, and S. Li, ‘‘6G Wireless communications: Vision and potential techniques,’’ IEEE Netw., vol. 33, no. 4, pp. 70–75, Jul./Aug. 2019. [3] M. Katz, M. Matinmikko-Blue, and M. Latva-Aho, ‘‘6Genesis flag￾ship program: Building the bridges towards 6G-enabled wireless smart society and ecosystem,’’ in Proc. IEEE 10th Latin-Amer. Conf. Commun. (LATINCOM), Nov. 2018, pp. 1–9. [4] IMT Traffic Estimates for the Years 2020 to 2030, document ITU-R SG05, Jul. 2015. [5] K. David and H. Berndt, ‘‘6G vision and requirements: Is there any need for beyond 5G?’’ IEEE Veh. Technol. Mag., vol. 13, no. 3, pp. 72–80, Sep. 2018. [6] A. Gupta and E. R. K. Jha, ‘‘A survey of 5G network: Architecture and emerging technologies,’’ IEEE Access, vol. 3, pp. 1206–1232, Jul. 2015. [7] K. David and H. Berndt, ‘‘6G vision and requirements: Is there any need for beyond 5g?’’ IEEE Veh. Technol. Mag., vol. 13, no. 3, pp. 72–80, Sep. 2018. [8] P. Sharma, ‘‘Evolution of mobile wireless communication networks-1G to 5G as well as future prospective of next generation communication network,’’ Int. J. Comput. Sci. Mobile Comput., vol. 2, no. 8, pp. 47–53, 2013. [9] A. U. Gawas, ‘‘An overview on evolution of mobile wireless communica￾tion networks: 1G-6G,’’ Int. J. Recent Innov. Trends Comput. Commun., vol. 3, no. 5, pp. 3130–3133, 2015. [10] J. Parikh and A. Basu, ‘‘LTE advanced: The 4G mobile broadband tech￾nology,’’ Int. J. Comput. Appl., vol. 13, no. 5, pp. 17–21, 2011. [11] M. Shafi, A. F. Molisch, P. J. Smith, T. Haustein, P. Zhu, P. De Silva, F. Tufvesson, A. Benjebbour, and G. Wunder, ‘‘5G: A tutorial overview of standards, trials, challenges, deployment, and practice,’’ IEEE J. Sel. Areas Commun., vol. 35, no. 6, pp. 1201–1221, Jun. 2017. [12] Z. Zhou, M. Dong, K. Ota, G. Wang, and L. T. Yang, ‘‘Energy￾efficient resource allocation for D2D communications underlaying cloud￾RAN-based LTE-A networks,’’ IEEE Internet Things J., vol. 3, no. 3, pp. 428–438, Jun. 2016. [13] J. Wu, M. Dong, K. Ota, J. Li, W. Yang, and M. Wang, ‘‘Fog-computing￾enabled cognitive network function virtualization for an information￾centric future Internet,’’ IEEE Commun. Mag., vol. 57, no. 7, pp. 48–54, Jul. 2019. [14] J. Wu, M. Dong, K. Ota, J. Li, and Z. Guan, ‘‘Big data analysis-based secure cluster management for optimized control plane in software-defined networks,’’ IEEE Trans. Netw. Service Manag., vol. 15, no. 1, pp. 27–38, Mar. 2018. [15] B. Zong, C. Fan, X. Wang, X. Duan, B. Wang, and J. Wang, ‘‘6G technologies: Key drivers, core requirements, system architectures, and enabling technologies,’’ IEEE Veh. Technol. Mag., vol. 14, no. 3, pp. 18–27, Sep. 2019. [16] Z. Zhang, Y. Xiao, Z. Ma, M. Xiao, Z. Ding, X. Lei, G. K. Karagiannidis, and P. Fan, ‘‘6G wireless networks: Vision, requirements, architecture, and key technologies,’’ IEEE Veh. Technol. Mag., vol. 14, no. 3, pp. 28–41, Sep. 2019. [17] Y. Zhao, G. Yu, and H. Xu, ‘‘6G mobile communication network: Vision, challenges and key technologies,’’ 2019, arXiv:1905.04983. [Online]. Available: https://arxiv.org/abs/1905.04983 [18] Memorandum Opinion, Order and Authorization in the Matter of Applica￾tion for Approval for Orbital Deployment and Operating Authority for the SPACEX NGSO Satellite System, document FCC-18-38, Mar. 2018. [19] A. Yastrebova, R. Kirichek, Y. Koucheryavy, A. Borodin, and A. Koucheryavy, ‘‘Future networks 2030: Architecture & requirements,’’ in Proc. 10th Int. Congr. Ultra Mod. Telecommun. Control Syst. Workshops (ICUMT), Nov. 2018, pp. 1–8. [20] J. Radtke, C. Kebschull, and E. Stoll, ‘‘Interactions of the space debris environment with mega constellations—Using the example of the OneWeb constellation,’’ Acta Astronautica, vol. 131, pp. 55–68, Feb. 2017. [21] M. Handley, ‘‘Delay is not an option: Low latency routing in space,’’ in Proc. 17th ACM Workshop Hot Topics Netw., 2018, pp. 85–91. [22] H. Yao, L. Wang, X. Wang, Z. Lu, and Y. Liu, ‘‘The space-terrestrial integrated network: An overview,’’ IEEE Commun. Mag., vol. 56, no. 9, pp. 178–185, Sep. 2018. [23] Z. Liu, J. Zhu, C. Pan, and G. Song, ‘‘Satellite network architecture design based on SDN and ICN technology,’’ in Proc. 8th Int. Conf. Electron. Inf. Emergency Commun. (ICEIEC), Jun. 2018, pp. 124–131. 175766 VOLUME 7, 2019

IEEEAcceSST. Huang et al: Survey on Green 6G Network: Architecture and Technologies[24] P. Du, S. Nazari, J. Mena, R. Fan, M. Gerla, and R. Gupta, "Multipath[47] L.E. Akyildiz, J. M. Jomet, and C. Han, "Terahertz band: Next frontier forTCP in SDN-enabled leo satellite networks," in Proc.IEEE Mil. Commun.wireless communications,"Phys.Commun.,vol.12,pp.16-32,Sep.2014.Conf. (MILCOM),Nov.2016,pp.354-359.[48] N. Khalid, N. A. Abbasi, and O. B. Akan, "300 GHz broadband transceiver[25] Y. Zeng, R. Zhang, and T. J. Lim, "Wireless communications withdesign for low-THz band wireless communications in indoor Intermetunmanned aerial vehicles: Opportunitiesand challenges”IEEEof Things,"in Proc.IEEE Int. Conf. Intermet Things (iThings)IEEECommun.Magvol.54,no.5,pp.3642.May2016.GreenComput.Commun.(GreenCom)IEEECyber,Phys.Social Comput.[26] H. Yanikomeroglu, "Integrated terrestrial/non-terrestrial 6G networks for(CPSCom) IEEE Smart Data (SmarData), Jun.2017.Pp.770-775.ubiquitous 3D super-connectivity" in Proc. 2Ist ACM Int. Conf. Model.,[49] First Report and Order in the Matter of Spectrum Horizons,Anal.Simulation Wireless Mobile Syst.,2018,pp.3-4.documentFCC-19-19.Mar.2019[27] L. Gupta, R. Jain, and G. Vaszkun, "Survey of important issues in UAV[50] IEEE 802.15 WPAN Srudy Group 100 Gbit/s Wireless (sg100g).communication networks,"IEEE Commun. Surveys Tuts.,vol.18, no.2,Accessed: Aug. 13, 2018. [Online]. Available: http:/www.ieee802.org/15]pp.1123-1152,2nd Quart.,2016pub/sg100g.html[28] Y. Zeng and R. Zhang, "Energy-efficient UAV communication with tra-[51] M.Fujishima, "300GHz-band CMOS wireless transceiver," in Proc. IEEEjectoryoptimizationEEETrans.Wireless Commun,vol.16,no.6In. Symp. Radio-Freq. Integr. Technol. (RFIT), Aug. 2018, pp. 1-3.pp. 3747-3760, Jun. 2017.[52] S. Lee, R. Dong, T. Yoshida, S. Amakawa, S. Hara, A. Kasamatsu, J. Sato,[29] Z. Zhou, J. Feng, B. Gu, B. Ai, S. Mumtaz, J. Rodriguez, andand M. Fujishima,"9.5 an 80Gb/s 300GHz-band single-chip CMOSM. Guizani, "When mobile crowd sensing meets UAV: Energy-efficienttransceiver," in IEEE ISSCC Dig. Tech. Papers,Feb.2019,pp.170-172.[53] L. Lopacinski, M. Marinkovic, G. Panic, M. H. Eissa, A. Hasanitask assignment and route planning."IEEE Trans.Commun.,vol. 66,K. Krishnegowda, and R. Kraemer, "Data link layer processor for 100no. 11. pp. 5526-5538, Nov. 2018.[30] N. Cheng. F. Lyu, W. Quan, C. Zhou, H. He, W. Shi, andGbps terahertz wireless communications in 28 nm CMOS technology,X.Shen,"Space/aerial-assistedcomputing offloadingforloTapplications:IEEE Access, vol.7, pp.44489-44502, 2019.[54] K. Tekblyik, A. R. Ekti, G. K. Kurt, and A. Gorcin, "Terahertz band com-A learning-based approach,"IEEE J.Sel. Areas Commun., vol. 37,no.5munication systems: Challenges, novelties and standardization efforts,pp. 1117-1129, May 2019.[3] M. Alzenad, M Z. Shakir, H. Yanikomeroglu, and M-S. Alouini.Phys. Commun., vol. 35, Aug.2019, Art. no. 100700.[55] P. H. Pathak,X.Feng, P. Hu, and P. Mohapatra, "Visible light communica-"FSO-based vertical backhaul/fronthaul framework for 5G+ wireless net-tion, networking, and sensing: A survey, potential and challenges,"IEEEworks,"IEEECommun.Mag.,vol.56,no.1,pp.218-224,Jan.2018Commun,Surveys Tuts.,vol.17,no.4,pp.20472077,4th Quart.,2015.[32] Z. Zheng, A. K. Sangaiah, and T. Wang. "Adaptive communication pro-[56] S.Cho, G.Chen, and J.P.Coon,"Enhancement of physical layer securitytocols in flying ad hoc network,"IEEE Comumun. Mag., vol. 56, no.1,with simultaneous beamforming and jamming for visible light commu-pp. 136142, Jan. 2018.[33] E.Felemban,FK Shaikh, U.M.Qureshi, A.A. Sheikh, and S.B. Qaisar.nication systems,"IEEE Trans. Inf. Forensics Security, vol. 14, no.10,"Underwater sensor network applications: A comprehensive survey," Int.pp.2633-2648, Oct.2019.[57] L. U. Khan, "Visible light communication: Applications, architecture,J. Distrib. Sensor Nerw., vol. 2015, no. 11, 2015, Art. no. 896832.[34] H. Kaushal and G. Kaddoum, "Underwater optical wireless communica-standardization and research challenges,"Digit. Commun. Netw., vol. 3,no. 2. pp. 7888, May 2016.tion,"IEEEAccess, vol.4,pp.1518-1547,2016.[35] R. Li, Z. Zhao, Z. Xuan, G. Ding. C. Yan, Z. Wang. and H. Zhang.[58] H. Haas, "LiFi is a paradigm-shifting 5G technology," Rev Phys., vol. 3,"Intelligent 5G: When cellular networks meet artificial intelligence,"pp. 26-31, Nov. 2017.[59] M. S. Islim, R. X. Ferreira, X. He, E. Xie, S. Videv, S. Viola, S. Watson,IEEEWireless Commun.,vol.24,no.5,pp.175-183,Oct.2017.[36] Z. Zhou, H. Liao, G. Gu, K. M. S. Huq, s. Mumtaz, and J. Rodriguez,N. Bamiedakis, R. V. Penty, I. H White, A. E. Kelly, E. Gu, H Haas,"Robust mobile crowd sensing: When deep learming meets edge comput-and M. D, Dawson, "Towards 10 Gb/s orthogonal frequency divisioning.IEEENetw.,vol.32,no.4.pp.5460,Jul.2018multiplexing-based visible light communication using a GaN violet micro-[37] G. Li, G. Xu, A. K. Sangaiah, J. Wu, and J. Li, "EdgeLaaS: Edge learningLEDPhoton.Res,vol.5,no.2. pp.A35-A43,207Maharaj,as a service for knowledge-centric connected healthcare," IEEE Netw, to[60] U.RChude-Okonkwo, R.Malekian, B. T.andAV.Vasilakos,"Molecular communicationbe published.A.and nanonetwork for[38] M.G.Kibria, K. Nguyen, G.P Villardi, O.Zhao, K. Ishizu, andF.Kojima,targeted drug delivery: A survey,"IEEE Commun. Surveys Tuts., vol.19,"Big data analytics, machine learning, and artificial intelligence in next-no. 4, pp. 30463096,4th Quart., 2017.[6]] N. Farsad, H. B. Yilmaz, A. Eckford, C.-B. Chae, and W. Guo, "A com-generation wireless networks"IEEE Access, vol. 6, pp. 32328-32338,2018.prehensive survey of recent advancements in molecular communica-[39] RISELAB. Real-Time Intelligent Secure Explainable Systems. [Online].tion,"IEEE Commun. Surveys Tuts., vol. 18, no.3, pp.1887-1919, 3rdAvailable: https://rise.cs.berkeley.eduQuart.,2016.[40] J. Fowers, K. Ovtcharov, M. Papamichael, T. Massengil, M. Liu, D. Lo.[62] O. B. Akan, H. Ramezani, T. Khan, N. A. Abbasi, and M. Kuscu, "Fun-S.Alkalay, M. Haselman, L.Adams,M. Ghandi, S..Heil, P.Patel,damentals of molecular information and communication science," Proc.A. Sapek, G. Weisz, L. Woods, S. Lanka, S. K. Reinhardt, A. M. Caulfield,IEEE,vol.105,no.2,pp.306318,Feb. 2017E. S. Chung. and D. Burger, "A configurable cloud-scale DNN proces-[63] T.Nakano, Y.Okaie, S. Kobayashi, T. Hara, Y. Hiraoka, and T. Haraguchi,sor for real-time AI," in Proc. ACM/IEEE In. Symp. Comput. Archit.,"Methods and applications of mobile molecular communication," Proc.Jun. 2018, pp. 114.IEEE, vol. 107, no. 7, Ppp. 14421456, Jul. 2019.[4]] C. Jiang, H. Zhang, Y. Ren, Z. Han, K.-C. Chen, and L. Hanzo, "Machine[64] F.Akyildiz, M, Pierobon, S.Balasubramaniam, and Y. Koucheryavy.learning paradigmsfor next-generation wireless networks,"IEEE Wireless"The Intemet of bio-nano things," IEEE Commun. Mag., vol. 53, no.3,Commun,vol.24,no.2.pp.98-105,Apr.2017pp. 32-40, Mar. 2015.[42] A.Ndikumana, N.H.Tran,.M.Ho, Z.Han, w.Sad,D.Niyato, and[65] J.-Y. Hu, B. Yu, M.-Y. Jing, L.-T. Xiao, S.-T. Jia, G.-Q. Qin, andC.S.Hong,"Joint communication, computation, caching, and control inG.-L.Long, "Experimental quantum secure direct communication withsinglephoonsLigh,Sci.pplo5,no.9,26tno44big data multi-access edge computing."IEEE Trans.Mobile Comput.,to[66] W. Zhang, D.-S. Ding. Y-B. Sheng, L. Zhou, B.-S. Shi, and G.-C. Guo,be published[43] S. Samarakoon, M. Bennis, W. Saad, and M. Debbah, "Federated learn-"Quantum secure direct communication with quantum memory," Phys.Rev:Let.,vol.118no.22,p.22001,2017ing for ultra-reliable low-latency V2V communications," in Proc. IEEE[67] L. Gyongyosi, S. Imre, and H. V. Nguyen, "A survey on quantum channelGlobal Commun.Conf. (GLOBECOM),Dec.2018,pp.1-7.[44] H. Zhang. W. Quan, H.-C. Chao, and C. Qiao, "Smart identifier network:capacities," IEEE Commun. Surveys Turs., vol. 20, no. 2. pp. 1149-1205,2nd Quart., 2018.A collaborative architecture for the future Internet," IEEE Netw., vol. 30,[68] S. Muralidharan, L. Li, J. Kim, N. Lutkenhaus, M. D. Lukin, and L, Jiang,no.3, pp.4651, May/Jun. 2016.[45] M. F. Zhani and H. EIBakoury, "FlexNGIA: A flexible Intermet archi-"Optimal architectures for long distance quantum communication,"Sci.tecture for the next-generation tactile Intemet,"2019, arXiv:1905.07137.Rep., vol. 6, 2016, Art. no. 20463.[69] X. Lin, J. Li, J. Wu, H. Liang, and w. Yang."Making knowledge trad-[Online].Available: https:Jlarxiv.org/abs/1905.07137[46] E. C. Strinati, S. Barbarossa, J.L. Gonzalez-Jimenez, D. Ktenas,able in edge-ai enabled IoT: A consortium blockchain-based efficient andincentiveapproachEEETransInd.InfomattobepublishedN. Cassiau, L. Mare, and C. Dehos, "6G: The next frontier: From holo-[70] H. Liang, J. Wu, S. Mumtaz, J. Li, X. Lin, and M. Wen, "MBID:graphic messaging to artificial intelligence using subterahertz and visibleMicro-blockchain-based geographical dynamic intrusion detectionlight communication,"IEEE Veh.Technol.Mag.,vol. 14, no.3, pp.4250,forV2x."IEEE Commun.57,no. 10, pp-7783,Sep. 2019.Mag-,vol.175767VOLUME 7, 2019

T. Huang et al.: Survey on Green 6G Network: Architecture and Technologies [24] P. Du, S. Nazari, J. Mena, R. Fan, M. Gerla, and R. Gupta, ‘‘Multipath TCP in SDN-enabled leo satellite networks,’’ in Proc. IEEE Mil. Commun. Conf. (MILCOM), Nov. 2016, pp. 354–359. [25] Y. Zeng, R. Zhang, and T. J. Lim, ‘‘Wireless communications with unmanned aerial vehicles: Opportunities and challenges,’’ IEEE Commun. Mag., vol. 54, no. 5, pp. 36–42, May 2016. [26] H. Yanikomeroglu, ‘‘Integrated terrestrial/non-terrestrial 6G networks for ubiquitous 3D super-connectivity,’’ in Proc. 21st ACM Int. Conf. Model., Anal. Simulation Wireless Mobile Syst., 2018, pp. 3–4. [27] L. Gupta, R. Jain, and G. Vaszkun, ‘‘Survey of important issues in UAV communication networks,’’ IEEE Commun. Surveys Tuts., vol. 18, no. 2, pp. 1123–1152, 2nd Quart., 2016. [28] Y. Zeng and R. Zhang, ‘‘Energy-efficient UAV communication with tra￾jectory optimization,’’ IEEE Trans. Wireless Commun., vol. 16, no. 6, pp. 3747–3760, Jun. 2017. [29] Z. Zhou, J. Feng, B. Gu, B. Ai, S. Mumtaz, J. Rodriguez, and M. Guizani, ‘‘When mobile crowd sensing meets UAV: Energy-efficient task assignment and route planning,’’ IEEE Trans. Commun., vol. 66, no. 11, pp. 5526–5538, Nov. 2018. [30] N. Cheng, F. Lyu, W. Quan, C. Zhou, H. He, W. Shi, and X. Shen, ‘‘Space/aerial-assisted computing offloading for IoT applications: A learning-based approach,’’ IEEE J. Sel. Areas Commun., vol. 37, no. 5, pp. 1117–1129, May 2019. [31] M. Alzenad, M. Z. Shakir, H. Yanikomeroglu, and M.-S. Alouini, ‘‘FSO-based vertical backhaul/fronthaul framework for 5G+ wireless net￾works,’’ IEEE Commun. Mag., vol. 56, no. 1, pp. 218–224, Jan. 2018. [32] Z. Zheng, A. K. Sangaiah, and T. Wang, ‘‘Adaptive communication pro￾tocols in flying ad hoc network,’’ IEEE Commun. Mag., vol. 56, no. 1, pp. 136–142, Jan. 2018. [33] E. Felemban, F. K. Shaikh, U. M. Qureshi, A. A. Sheikh, and S. B. Qaisar, ‘‘Underwater sensor network applications: A comprehensive survey,’’ Int. J. Distrib. Sensor Netw., vol. 2015, no. 11, 2015, Art. no. 896832. [34] H. Kaushal and G. Kaddoum, ‘‘Underwater optical wireless communica￾tion,’’ IEEE Access, vol. 4, pp. 1518–1547, 2016. [35] R. Li, Z. Zhao, Z. Xuan, G. Ding, C. Yan, Z. Wang, and H. Zhang, ‘‘Intelligent 5G: When cellular networks meet artificial intelligence,’’ IEEE Wireless Commun., vol. 24, no. 5, pp. 175–183, Oct. 2017. [36] Z. Zhou, H. Liao, G. Gu, K. M. S. Huq, S. Mumtaz, and J. Rodriguez, ‘‘Robust mobile crowd sensing: When deep learning meets edge comput￾ing,’’ IEEE Netw., vol. 32, no. 4, pp. 54–60, Jul. 2018. [37] G. Li, G. Xu, A. K. Sangaiah, J. Wu, and J. Li, ‘‘EdgeLaaS: Edge learning as a service for knowledge-centric connected healthcare,’’ IEEE Netw., to be published. [38] M. G. Kibria, K. Nguyen, G. P. Villardi, O. Zhao, K. Ishizu, and F. Kojima, ‘‘Big data analytics, machine learning, and artificial intelligence in next￾generation wireless networks,’’ IEEE Access, vol. 6, pp. 32328–32338, 2018. [39] RISELAB. Real-Time Intelligent Secure Explainable Systems. [Online]. Available: https://rise.cs.berkeley.edu [40] J. Fowers, K. Ovtcharov, M. Papamichael, T. Massengill, M. Liu, D. Lo, S. Alkalay, M. Haselman, L. Adams, M. Ghandi, S. Heil, P. Patel, A. Sapek, G. Weisz, L. Woods, S. Lanka, S. K. Reinhardt, A. M. Caulfield, E. S. Chung, and D. Burger, ‘‘A configurable cloud-scale DNN proces￾sor for real-time AI,’’ in Proc. ACM/IEEE Int. Symp. Comput. Archit., Jun. 2018, pp. 1–14. [41] C. Jiang, H. Zhang, Y. Ren, Z. Han, K.-C. Chen, and L. Hanzo, ‘‘Machine learning paradigms for next-generation wireless networks,’’ IEEE Wireless Commun., vol. 24, no. 2, pp. 98–105, Apr. 2017. [42] A. Ndikumana, N. H. Tran, T. M. Ho, Z. Han, W. Saad, D. Niyato, and C. S. Hong, ‘‘Joint communication, computation, caching, and control in big data multi-access edge computing,’’ IEEE Trans. Mobile Comput., to be published. [43] S. Samarakoon, M. Bennis, W. Saad, and M. Debbah, ‘‘Federated learn￾ing for ultra-reliable low-latency V2V communications,’’ in Proc. IEEE Global Commun. Conf. (GLOBECOM), Dec. 2018, pp. 1–7. [44] H. Zhang, W. Quan, H.-C. Chao, and C. Qiao, ‘‘Smart identifier network: A collaborative architecture for the future Internet,’’ IEEE Netw., vol. 30, no. 3, pp. 46–51, May/Jun. 2016. [45] M. F. Zhani and H. ElBakoury, ‘‘FlexNGIA: A flexible Internet archi￾tecture for the next-generation tactile Internet,’’ 2019, arXiv:1905.07137. [Online]. Available: https://arxiv.org/abs/1905.07137 [46] E. C. Strinati, S. Barbarossa, J. L. Gonzalez-Jimenez, D. Ktenas, N. Cassiau, L. Maret, and C. Dehos, ‘‘6G: The next frontier: From holo￾graphic messaging to artificial intelligence using subterahertz and visible light communication,’’ IEEE Veh. Technol. Mag., vol. 14, no. 3, pp. 42–50, Sep. 2019. [47] I. F. Akyildiz, J. M. Jornet, and C. Han, ‘‘Terahertz band: Next frontier for wireless communications,’’ Phys. Commun., vol. 12, pp. 16–32, Sep. 2014. [48] N. Khalid, N. A. Abbasi, and O. B. Akan, ‘‘300 GHz broadband transceiver design for low-THz band wireless communications in indoor Internet of Things,’’ in Proc. IEEE Int. Conf. Internet Things (iThings) IEEE Green Comput. Commun. (GreenCom) IEEE Cyber, Phys. Social Comput. (CPSCom) IEEE Smart Data (SmartData), Jun. 2017, pp. 770–775. [49] First Report and Order in the Matter of Spectrum Horizons, document FCC-19-19, Mar. 2019. [50] IEEE 802.15 WPAN Study Group 100 Gbit/s Wireless (sg100g). Accessed: Aug. 13, 2018. [Online]. Available: http://www.ieee802.org/15/ pub/sg100g.html [51] M. Fujishima, ‘‘300GHz-band CMOS wireless transceiver,’’ in Proc. IEEE Int. Symp. Radio-Freq. Integr. Technol. (RFIT), Aug. 2018, pp. 1–3. [52] S. Lee, R. Dong, T. Yoshida, S. Amakawa, S. Hara, A. Kasamatsu, J. Sato, and M. Fujishima, ‘‘9.5 an 80Gb/s 300GHz-band single-chip CMOS transceiver,’’ in IEEE ISSCC Dig. Tech. Papers, Feb. 2019, pp. 170–172. [53] L. Lopacinski, M. Marinkovic, G. Panic, M. H. Eissa, A. Hasani, K. Krishnegowda, and R. Kraemer, ‘‘Data link layer processor for 100 Gbps terahertz wireless communications in 28 nm CMOS technology,’’ IEEE Access, vol. 7, pp. 44489–44502, 2019. [54] K. Tekbıyık, A. R. Ekti, G. K. Kurt, and A. Görçin, ‘‘Terahertz band com￾munication systems: Challenges, novelties and standardization efforts,’’ Phys. Commun., vol. 35, Aug. 2019, Art. no. 100700. [55] P. H. Pathak, X. Feng, P. Hu, and P. Mohapatra, ‘‘Visible light communica￾tion, networking, and sensing: A survey, potential and challenges,’’ IEEE Commun. Surveys Tuts., vol. 17, no. 4, pp. 2047–2077, 4th Quart., 2015. [56] S. Cho, G. Chen, and J. P. Coon, ‘‘Enhancement of physical layer security with simultaneous beamforming and jamming for visible light commu￾nication systems,’’ IEEE Trans. Inf. Forensics Security, vol. 14, no. 10, pp. 2633–2648, Oct. 2019. [57] L. U. Khan, ‘‘Visible light communication: Applications, architecture, standardization and research challenges,’’ Digit. Commun. Netw., vol. 3, no. 2, pp. 78–88, May 2016. [58] H. Haas, ‘‘LiFi is a paradigm-shifting 5G technology,’’ Rev. Phys., vol. 3, pp. 26–31, Nov. 2017. [59] M. S. Islim, R. X. Ferreira, X. He, E. Xie, S. Videv, S. Viola, S. Watson, N. Bamiedakis, R. V. Penty, I. H. White, A. E. Kelly, E. Gu, H. Haas, and M. D. Dawson, ‘‘Towards 10 Gb/s orthogonal frequency division multiplexing-based visible light communication using a GaN violet micro￾LED,’’ Photon. Res., vol. 5, no. 2, pp. A35–A43, 2017. [60] U. A. K. Chude-Okonkwo, R. Malekian, B. T. Maharaj, and A. V. Vasilakos, ‘‘Molecular communication and nanonetwork for targeted drug delivery: A survey,’’ IEEE Commun. Surveys Tuts., vol. 19, no. 4, pp. 3046–3096, 4th Quart., 2017. [61] N. Farsad, H. B. Yilmaz, A. Eckford, C.-B. Chae, and W. Guo, ‘‘A com￾prehensive survey of recent advancements in molecular communica￾tion,’’ IEEE Commun. Surveys Tuts., vol. 18, no. 3, pp. 1887–1919, 3rd Quart., 2016. [62] O. B. Akan, H. Ramezani, T. Khan, N. A. Abbasi, and M. Kuscu, ‘‘Fun￾damentals of molecular information and communication science,’’ Proc. IEEE, vol. 105, no. 2, pp. 306–318, Feb. 2017. [63] T. Nakano, Y. Okaie, S. Kobayashi, T. Hara, Y. Hiraoka, and T. Haraguchi, ‘‘Methods and applications of mobile molecular communication,’’ Proc. IEEE, vol. 107, no. 7, pp. 1442–1456, Jul. 2019. [64] I. F. Akyildiz, M. Pierobon, S. Balasubramaniam, and Y. Koucheryavy, ‘‘The Internet of bio-nano things,’’ IEEE Commun. Mag., vol. 53, no. 3, pp. 32–40, Mar. 2015. [65] J.-Y. Hu, B. Yu, M.-Y. Jing, L.-T. Xiao, S.-T. Jia, G.-Q. Qin, and G.-L. Long, ‘‘Experimental quantum secure direct communication with single photons,’’ Light, Sci. Appl., vol. 5, no. 9, 2016, Art. no. e16144. [66] W. Zhang, D.-S. Ding, Y.-B. Sheng, L. Zhou, B.-S. Shi, and G.-C. Guo, ‘‘Quantum secure direct communication with quantum memory,’’ Phys. Rev. Lett., vol. 118, no. 22, p. 220501, 2017. [67] L. Gyongyosi, S. Imre, and H. V. Nguyen, ‘‘A survey on quantum channel capacities,’’ IEEE Commun. Surveys Tuts., vol. 20, no. 2, pp. 1149–1205, 2nd Quart., 2018. [68] S. Muralidharan, L. Li, J. Kim, N. Lütkenhaus, M. D. Lukin, and L. Jiang, ‘‘Optimal architectures for long distance quantum communication,’’ Sci. Rep., vol. 6, 2016, Art. no. 20463. [69] X. Lin, J. Li, J. Wu, H. Liang, and W. Yang, ‘‘Making knowledge trad￾able in edge-ai enabled IoT: A consortium blockchain-based efficient and incentive approach,’’ IEEE Trans. Ind. Informat., to be published. [70] H. Liang, J. Wu, S. Mumtaz, J. Li, X. Lin, and M. Wen, ‘‘MBID: Micro-blockchain-based geographical dynamic intrusion detection for V2X,’’ IEEE Commun. Mag., vol. 57, no. 10, pp. 77–83, VOLUME 7, 2019 175767