Table Of Content

Ultra-Dense Heterogeneous Networks Driven by the ever-increasing amount of mobile data, cellular networks evolve from small cell network to ultra-dense heterogeneous networks, to provide high system capacity and spectrum efficiency. By bringing base stations (BSs) to similar number magnitude of served users, ultra-dense heterogeneous networks would definitely bring unprecedented paradigm changes to the network design. Firstly, along with densification of small cells, inter-cell interference becomes severe and may deteriorate performance of mobile users. Assigning network resources including bandwidth and time slots, while avoiding interference, desires serious consideration. Secondly, the coverage area of BSs becomes small and irregular, resulting in much frequent and complicated handovers when mobile users move around. How to ensure continuous communication and implement effective mobility management, inter-cell resource allocation and cooperation, remains a challenging issue. Thirdly, such dynamic change in spatial dimension moti- vates us to re-investigate available and ongoing communications and networking techniques, such as massive MIMO, CoMP, millimeter waves (mmWaves), carrier aggregation, full duplex radio, and D2D communications. To address the aforementioned challenging research issues, this book will investigate the service and QoE provisioning in ultra-dense heterogeneous networks. In particular, firstly we introduce ultra-dense heterogeneous networks by care- ful definition regarding spatial deployment, generic characteristics, and require- ments of ultra-dense heterogeneous networks in order to ensure QoE of mobile users. Secondly, we depict the resource management among small cells in close proximity, mobility management for mobile users (addressing the super-frequent handovers), and interference management (dealing with the interference due to frequency-reuse in the vicinity). Thirdly, we study the enabling factors, and the integration of ultra-dense heterogeneous networks with enabling technologies, such as massive-MIMO, cloud-RAN, mmWaves, D2D, IoT. Finally, we conclude the book and indicate future directions and challenges. Ultra-Dense Heterogeneous Networks Wen Sun Qubeijian Wang Nan Zhao Haibin Zhang Chao Shen First edition published 2023 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2023 Wen Sun, Qubeijian Wang, Nan Zhao, Haibin Zhang and Chao Shen Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. 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Contents Chapter1 Introduction.....................................................................................1 1.1 Prospectsandreviewsofcommunicationnetworks...............1 1.1.1 FrontEndNetwork....................................................3 1.1.2 CoreNetwork.............................................................7 1.1.3 Backhauling...............................................................8 1.2 Ultra-densenetwork..............................................................10 1.2.1 DefinitionsofUDN..................................................10 1.2.2 CharacteristicsofUDNs..........................................12 1.2.3 PerformanceMetrics................................................14 1.3 Outline..................................................................................15 Chapter2 ResourceandInterferenceManagement.......................................17 2.1 Modelingtechniques.............................................................17 2.1.1 StochasticGeometry................................................17 2.1.2 GameTheory...........................................................23 2.2 Resourcemanagementinultra-denseheterogeneous network.................................................................................28 2.2.1 QoEProvisioningTechniqueswithOFDMAor NOMA.....................................................................28 2.2.2 LoadBalancing........................................................32 2.2.3 Energy-SavingTechniques......................................36 2.3 Interferencemanagement......................................................40 2.3.1 InterferenceCoordination........................................40 2.3.2 SpectrumSharing.....................................................44 2.3.3 Beamforming...........................................................48 2.4 Summary...............................................................................52 Chapter3 MobilityManagement...................................................................55 3.1 UEassociation......................................................................55 3.1.1 UEAssociationinConventionalCellular Networks..................................................................55 3.1.2 CellRangeExpansion(CRE)inUDNs...................59 3.1.3 CoordinatedMultiplePointsinUDNs.....................61 3.2 Mobilitymanagement...........................................................63 3.3 Summary...............................................................................67 i ii Contents Chapter4 EnablingFactorsandEmergingTechniques.................................69 4.1 Massive-MIMO....................................................................69 4.2 Machinelearning..................................................................74 4.3 Softwaredefinednetwork(SDN).........................................82 4.4 Cloud-radioaccessnetwork(C-RAN)..................................87 4.5 mmWaves..............................................................................91 4.6 Proactivecaching..................................................................96 4.7 Summary...............................................................................98 Chapter5 PromisingApplications...............................................................101 5.1 Internetofthings.................................................................101 5.2 Mobileedgecomputing......................................................103 5.3 Wirelessenergyharvesting.................................................104 5.4 Self-drivingorautonomousvehicles..................................106 5.5 Summary.............................................................................108 Chapter6 SummaryandFutureWork..........................................................111 References.............................................................................................................113 Index......................................................................................................................129 1 Introduction 1.1 PROSPECTSANDREVIEWSOFCOMMUNICATIONNETWORKS According to the forecast of Cisco Global Cloud Index, there will be more than 847 ZB data generated each year, of which more than 50% of the data needs to be analyzed, processed, and stored at the edge of the network [1]. Meanwhile, world- wideonlinedeviceswouldexceed28billionandgenerateupto4.8ZBoftrafficper year [2]. The transmission and analysis of such massive data poses unprecedented challengesonthecurrenttelecommunicationsystem. Nowadays, to meet the rapidly rising data demand, researchers have started to conceptualize 6G with the vision of connecting everything, transmission over millimeter wave (mmWave) and THz, and integrating sensing, communication, com- putation, and control functionalities. Several companies (Nokia, Ericsson, Huawei, Samsung, LG, Apple, Xiaomi), as well as several countries (Finland, British, Ger- many, and China), have shown interest in 6G networks. In 2018, Finland took the leadinannouncingresearchplanstodevelopacomplete6Gecosystem,whichwill lastfor8yearswithatotalamountof$290million[3].Afterthat,BritainandGer- manybeganinvestinginprojectsincludingquantumtechnology.OnNovember20, 2019, China announced the launch of 6G research and began to develop spectrum technologies such as terahertz and mmWave [4]. Although the development of 6G isstillinitsinfancy,6Gnetworksareexpectedtoprovideextremelyhighdatarate andsupportapplicationsbeyondcurrentmobileusescenarios,suchasvirtualreality (VR),augmentedreality(AR),andMetaverse. Spectrum extension and spectrum efficiency improvement (e.g., cell splitting and densification) have been considered the most effective means to deliver ever- increasingdatacapacityandimprovingqualityofuserexperience.Spectrumexten- sionaimstoachievehighdataratethroughtheextensionofunusedelectromagnetic spectrum,e.g.,mmWave,Terahertz.Whereas,theefficiencyofspectrumcanbeef- fectively improved using emerging techniques, such as Massive-MIMO and ultra- densenetworks. SpectrumExtensionTechnology As shown in Fig. 1.1, electromagnetic spectrum can be categorized into radio, microwave,infrared,visiblelight,ultraviolet,X-Ray,andGamma-Ray.Wirelesscom- munications typically exploit the radio bands and the microwave bands within 6 GHz. 5G extended the usage of spectrum resources to the range of 24 GHz and 100 GHz to increase communication capacity. However, the expanded frequency band are still insufficient to meet the growing bandwidth demand. To further sup- port bandwidth-hungry and latency-intolerant applications, terahertz transmission becomesacomplementarywirelesstechnologyforBeyond5Gand6G. DOI:10.1201/9781003148654-1 1 2 Ultra-DenseHeterogeneousNetworks Figure1.1 SpectrummapincludingmmWaveandterahertzfrequencybands • Terahertzcommunication Terahertzband,alsoknownassubmillimeterband,consistsofthespectral bandwithinthefrequenciesfrom0.3to3THz.Terahertzcommunication canprovidetensofGHzofavailablebandsforhigh-speeddatatransmission (exceedingseveralhundredsofgigabitsfersecond).Furthermore,terahertz communicationhashighdirectionalityandshortpulsetime,whichcanre- duceinterferencewhileensuringtransmissionsecurity.Terahertztransmis- sionhasbecomeawirelessbackhaulextensionoftheopticalfibersforsce- nariosincludinghotspotsandlast-meterindoorwirelessaccess.Thelatter iseitherinfeasibleorverycostlyfortransmissioninruralorremoteregion. However,theterahertzwavecanbeeasilyabsorbedintheairduetoitspoor diffractive ability. In the future, Terahertz communication will inevitably becomethefeasiblebackhaulingtechnologyforultra-densenetworks[5]. • Visiblelightcommunication(VLC) Visible light communication, as a subset of optical wireless communications, refers to a data communication using visible light between 400 and 800THz.Thetechnologyenablestransmissionat10Kbit/susingordinary lamps,orbeyond500Mbit/susingmicro-LED.Itisbelievedthatwiththe development of VLC and LED technology, the transmission rate of VLC willreachTbit-levelpersecond[6].Comparedwithtraditionalradiocom- munications, VLC is feasible for ubiquitous communication medium, as lightingdevicesareusedeverywhere,e.g.,indoor/outdoorlamps,TVs,car headlights, and traffic signs. VLC can provide a large amount of poten- tially available spectrum resources without the authorization of the spec- trumregulator.Moreover,VLCisinsusceptibletoexternalelectromagnetic Introduction 3 interferencewhilegeneratingnoelectromagneticradiation.Sincethetrans- missionrangeofvisiblelightislimitedbythestructureofbuildings,VLC canreduceinter-cellinterferenceandensurereliabletransmission. SpectralEfficiencyImprovementTechnology • Massive-MIMO Massive-MIMO is an extension of MIMO that groups antennas together atthetransmitterandreceiverforimprovedthroughputandspectrumeffi- ciency. In 5G, the 8-antennas MIMO system evolves toward the massive- MIMOsystemwith256−1024antennas,whichrapidlyincreasethecapac- ityandspectralefficiencyofthesystem.Itisexpectedthatthenumberof antennasdeployedbytheMIMOsystemwillexceed10,000in6Gsystems. Alargenumberofantennaswillbringthefollowingadvantages:(1)byap- plyingspatialmultiplexingtechnologytotransmithundredsofparalleldata streamsonthesamechannel,thesystemcansignificantlyincreaseenergy efficiencyandreducelatency;(2)byutilizinghundredsofavailablebeams, thesystemcansimultaneouslyservemultipleUEstosignificantlyincrease network throughput; (3) by forming ultra-narrow beams to overcome the propagationlossofmmWaveandtheterahertzwave,thesystemcangreatly reduce inter-cell interference. Despite the promising advantages, complex algorithmsaredesiredtofindtheexactlocationofUEssoastoapplyac- curatebeamformingtechnology.Beammanagementandbeamsteeringare key technologies to enable massive-MIMO to be applied in practical sce- narios[7]. • UDN Bybringingbasestations(BSs)totheapproximatespatialscaleandnumber magnitude of users, ultra-dense networks (UDNs) would definitely bring unprecedentedparadigmchangestothenetworkdesign.Firstly,alongwith densificationofsmallcells,inter-cellinterferencebecomessevereandmay deteriorate performance of mobile users. Assigning network resources in- cludingbandwidthandtimeslots,whileavoidinginterference,desiresseri- ousconsideration.Secondly,thecoverageareaofBSsbecomessmalland irregular,resultinginmuchfrequentandcomplicatedhandoverswhenmo- bileusersmovearound.Howtoensurecontinuouscommunicationandim- plement effective mobility management, inter-cell resource allocation and cooperation,remainsachallengingissue.Thirdly,suchdynamicchangein spatial dimension needs us to re-investigate available and ongoing communications and networking techniques, such as massive MIMO, CoMP, millimeter waves (mmWaves), carrier aggregation, full duplex radio, and D2Dcommunications[8]. 1.1.1 FRONTENDNETWORK ThefrontendnetworkgenerallyreferstothenetworkthatprovidesaccessforUEs. Therapidlyincreasingdatademandurgesthefrontendnetworkoraccessnetwork