Wei Jiang, Ph.D., is a Senior Researcher with the German Research Center for Artificial Intelligence (DFKI). He has served as the leader of many EU or German research projects on 5G, 6G, AI, and Industry 4.0, and has extensive experiences in developing new technology and international standards related to wireless communications and networking by co-authoring 3 books, 90 articles and holding 30 patents. Fa-Long Luo, Ph.D., is a Full Professor (Affiliate) of the ECE Department at the University of Washington, USA. He has over 39 years of research and leadership experience with holding 86 patents and having published 7 well-received books and 110 articles on wireless and signal processing.

• Preface xvList of Abbreviations xxiPart I The Vision of 6G and Technical Evolution 11 Standards History of Cellular Systems Toward 6G 31.1 0G: Pre-Cellular Systems 41.2 1G: The Birth of Cellular Network 61.2.1 Nordic Mobile Telephone (NMT) 71.2.2 Advanced Mobile Phone System (AMPS) 81.3 2G: From Analog to Digital 91.3.1 Global System for Mobile communications (GSM) 101.3.2 Digital Advanced Mobile Phone System (D-AMPS) 111.3.3 Interim Standard 95 (IS-95) 111.3.4 Personal Digital Cellular (PDC) 121.3.5 General Packet Radio Service (GPRS) 121.3.6 Enhanced Data Rates for GSM Evolution (EDGE) 141.4 3G: From Voice to Data-Centric 151.4.1 Wideband Code-Division Multiple Access (WCDMA) 161.4.2 Code-Division Multiple Access 2000 (CDMA2000) 181.4.3 Time Division-Synchronous Code-Division Multiple Access(TD-SCDMA) 211.4.4 Worldwide Interoperability for Microwave Access (WiMAX) 221.5 4G: Mobile Internet 231.5.1 Long-Term Evolution-Advanced (LTE-Advanced) 251.5.2 WirelessMAN-Advanced 281.6 5G: From Human to Machine 301.7 Beyond 5G 371.8 Conclusions 39References 392 Pre-6G Technology and System Evolution 432.1 1G –AMPS 442.1.1 System Architecture 442.1.2 Key Technologies 462.1.2.1 Frequency Reuse 462.1.2.2 Cell Splitting 472.1.2.3 Sectorization 482.1.2.4 Handover 482.1.2.5 Frequency-Division Multiple Access 492.2 2G –GSM 492.2.1 System Architecture 502.2.1.1 Mobile Station Subsystem 502.2.1.2 Bases Station Subsystem 502.2.1.3 Network and Switching Subsystem 512.2.1.4 Operation and Support Subsystem 512.2.1.5 General Packet Radio Service 522.2.1.6 Gateway GPRS Support Node 532.2.2 Key Technologies 532.2.2.1 Time-Division Multiple Access 532.2.2.2 Frequency Hopping 542.2.2.3 Speech Compression 552.2.2.4 Channel Coding 552.2.2.5 Digital Modulation 562.2.2.6 Discontinuous Transmission (DXT) 562.3 3G –WCDMA 562.3.1 System Architecture 572.3.1.1 User Equipment 572.3.1.2 UMTS Terrestrial Radio Access Network 582.3.1.3 Core Network 592.3.2 Key Technologies 602.3.2.1 Code-Division Multiple Access 602.3.2.2 Rake Receiver 632.3.2.3 Turbo Codes 632.4 4G – LTE 642.4.1 System Architecture 652.4.1.1 Evolved Universal Terrestrial Radio Access Network 652.4.1.2 Evolved Packet Core 652.4.2 Key Technologies 682.4.2.1 Orthogonal Frequency-Division Multiplexing 702.4.2.2 Carrier Aggregation 712.4.2.3 Relaying 712.4.2.4 Heterogeneous Network 722.4.2.5 Coordinated Multi-Point Transmission and Reception 732.4.2.6 Device-to-Device Communications 732.4.2.7 License-Assisted Access 742.5 5G –New Radio 752.5.1 System Architecture 762.5.1.1 5G Core Network 772.5.1.2 Next Generation Radio Access Network 792.5.2 Key Technologies 812.5.2.1 Massive MIMO 812.5.2.2 MillimeterWave 822.5.2.3 Non-Orthogonal Multiple Access 832.5.2.4 SDN/NFV 842.5.2.5 Network Slicing 852.5.2.6 Polar Codes 862.6 Conclusions 87References 873 The Vision of 6G: Drivers, Enablers, Uses, and Roadmap 893.1 Background 903.2 Explosive Mobile Traffic 923.3 Use Cases 943.4 Usage Scenarios 983.5 Performance Requirements 1023.6 Research Initiatives and Roadmap 1073.6.1 ITU 1083.6.2 Third Generation Partnership Project 1103.6.3 Industry 1103.6.4 Europe 1103.6.5 The United States 1133.6.6 China 1163.6.7 Japan 1163.6.8 South Korea 1173.7 Key Technologies 1173.7.1 MillimeterWave 1183.7.2 Terahertz Communications 1183.7.3 Optical Wireless Communications 1193.7.4 Massive MIMO 1203.7.5 Intelligent Reflecting Surfaces 1213.7.6 Next-Generation Multiple Access 1223.7.7 Open Radio Access Network 1233.7.8 Non-Terrestrial Networks 1243.7.9 Artificial Intelligence 1253.7.10 Communication-Computing-Sensing Convergence 1273.8 Conclusions 128References 128Part II Full-Spectra Wireless Communications in 6G 1314 Enhanced Millimeter-Wave Wireless Communications in6G 1334.1 Spectrum Shortage 1344.2 mmWave Propagation Characteristics 1364.2.1 Large-Scale Propagation Effects 1374.2.1.1 Free-Space Propagation Loss 1374.2.1.2 NLOS Propagation and Shadowing 1394.2.1.3 Atmospheric Attenuation 1414.2.2 Small-Scale Propagation Effects 1434.2.3 Delay Spread and Coherence Bandwidth 1454.2.4 Doppler Spread and Coherence Bandwidth 1464.2.5 Angular Spread 1494.3 Millimeter-Wave Channel Models 1524.3.1 Large-Scale Fading 1524.3.2 3GPP Channel Models 1554.3.2.1 Urban Micro Scenario 1554.3.2.2 Urban Macro Scenario 1564.3.2.3 Indoor Scenario 1574.3.3 Small-Scale Fading 1594.4 mmWave Transmission Technologies 1634.4.1 Beamforming 1634.4.1.1 Digital Beamforming 1644.4.1.2 Analog Beamforming 1684.4.1.3 Hybrid Beamforming 1694.4.1.4 3D Beamforming 1734.4.2 Initial Access 1754.4.2.1 Multi-Beam Synchronization and Broadcasting 1764.4.2.2 Conventional Initial Access in LTE 1784.4.2.3 Beam-Sweeping Initial Access in NR 1814.4.3 Omnidirectional Beamforming 1834.4.3.1 Random Beamforming 1854.4.3.2 Enhanced Random Beamforming 1874.4.3.3 Complementary Random Beamforming 1904.5 Summary 192References 1935 Terahertz Technologies and Systems for 6G 1955.1 Potential of Terahertz Band 1965.1.1 Spectrum Limit 1965.1.2 The Need of Exploiting Terahertz Band 1985.1.3 Spectrum Regulation on Terahertz Band 2035.2 Terahertz Applications 2055.2.1 Terahertz Wireless Communications 2055.2.1.1 Terabit Cellular Hotspot 2055.2.1.2 Terabit Wireless Local-Area Network 2065.2.1.3 Terabit Device-To-Device Link 2065.2.1.4 Secure Wireless Communication 2075.2.1.5 Terabit Wireless Backhaul 2075.2.1.6 Terahertz Nano-Communications 2085.2.2 Non-Communication Terahertz Applications 2095.2.2.1 Terahertz Sensing 2095.2.2.2 Terahertz Imaging 2105.2.2.3 Terahertz Positioning 2125.3 Challenges of Terahertz Communications 2125.3.1 High Free-Space Path Loss 2135.3.2 Atmospheric Attenuation 2155.3.3 Weather Effects 2225.3.4 Blockage 2245.3.5 High Channel Fluctuation 2265.4 Array-of-Subarrays Beamforming 2285.5 Lens Antenna 2315.5.1 Refraction of RadioWaves 2325.5.2 Lens Antenna Array 2335.6 Case Study – IEEE 802.15.3d 2365.6.1 IEEE 802.15.3d Usage Scenarios 2375.6.2 Physical Layer 2405.6.2.1 Channelization 2405.6.2.2 Modulation 2425.6.2.3 Forward Error Correction 2425.6.3 Medium Access Control 2445.6.4 Frame Structure 2465.6.4.1 Preamble 2475.6.4.2 PHY Header 2475.6.4.3 MAC Header 2485.6.4.4 Construction Process of Frame Header 2485.7 Summary 250References 2516 Optical and Visible Light Wireless Communicationsin 6G 2536.1 The Optical Spectrum 2546.1.1 Infrared 2546.1.2 Visible Light 2566.1.3 Ultraviolet 2576.2 Advantages and Challenges 2586.3 OWC Applications 2626.4 Evolution of Optical Wireless Communications 2646.4.1 Wireless Infrared Communications 2656.4.2 Visible Light Communications 2666.4.3 Wireless Ultraviolet Communications 2676.4.4 Free-Space Optical Communications 2686.5 Optical Transceiver 2686.6 Optical Sources and Detectors 2716.6.1 Light-Emitting Diode 2736.6.2 Laser Diode 2766.6.3 Photodiode 2806.7 Optical Link Configuration 2836.8 Optical MIMO 2866.8.1 Spatial Multiplexing 2866.8.2 Spatial Modulation 2896.9 Summary 292References 292Part III Smart Radio Networks and Air InterfaceTechnologies for 6G 2957 Intelligent Reflecting Surface-Aided Communications for6G 2977.1 Basic Concept 2987.2 IRS-Aided Single-Antenna Transmission 3027.2.1 Signal Model 3037.2.2 Passive Beamforming 3067.2.3 Product-Distance Path Loss 3097.3 IRS-Aided Multi-Antenna Transmission 3107.3.1 Joint Active and Passive Beamforming 3107.3.1.1 SDR Solution 3127.3.1.2 Alternating Optimization 3147.3.2 Joint Precoding and Reflecting 3157.4 Dual-Beam Intelligent Reflecting Surface 3187.4.1 Dual Beams Over Hybrid Beamforming 3187.4.2 Dual-Beam IRS 3217.4.3 Optimization Design 3227.5 IRS-Aided Wideband Communications 3257.5.1 Cascaded Frequency-Selective Channel 3257.5.2 IRS-Aided OFDM System 3277.5.3 Rate Maximization 3307.6 Multi-User IRS Communications 3317.6.1 Multiple Access Model 3327.6.2 Orthogonal Multiple Access 3337.6.2.1 Time-Division Multiple Access 3347.6.2.2 Frequency-Division Multiple Access 3367.6.3 Non-Orthogonal Multiple Access 3377.7 Channel Aging and Prediction 3397.7.1 Outdated Channel State Information 3417.7.1.1 Doppler Shift 3417.7.1.2 Phase Noise 3437.7.2 Impact of Channel Aging on IRS 3437.7.3 Classical Channel Prediction 3457.7.3.1 Autoregressive Model 3457.7.3.2 Parametric Model 3477.7.4 Recurrent Neural Network 3487.7.5 RNN-Based Channel Prediction 3517.7.5.1 Flat-Fading Channel Prediction 3527.7.5.2 Frequency-Selective Fading Channel Prediction 3537.7.6 Long-Short Term Memory 3557.7.7 Deep Learning-Based Channel Prediction 3587.8 Summary 359References 3598 Multiple Dimensional and Antenna Techniques for 6G 3638.1 Spatial Diversity 3648.2 Receive Combining 3668.2.1 Selection Combining 3688.2.2 Maximal Ratio Combining 3708.2.3 Equal-Gain Combining 3738.3 Space-Time Coding 3748.3.1 Repetition Coding 3758.3.2 Space-Time Trellis Codes 3778.3.3 Alamouti Coding 3798.3.4 Space-Time Block Codes 3818.4 Transmit Antenna Selection 3838.5 Beamforming 3868.5.1 Classical Beamforming 3868.5.2 Single-Stream Precoding 3908.6 Spatial Multiplexing 3938.6.1 Single-User MIMO 3948.6.2 MIMO Precoding 4008.6.2.1 Full CSI at the Transmitter 4008.6.2.2 Limited CSI at the Transmitter 4038.6.3 MIMO Detection 4068.6.3.1 Maximum-Likelihood Detection 4068.6.3.2 Linear Detection 4078.6.3.3 Successive Interference Cancelation 4108.7 Summary 413References 4139 Cellular and Cell-Free Massive MIMO Techniques in 6G 4179.1 Multi-User MIMO 4189.1.1 Broadcast and Multiple-Access Channels 4199.1.2 Multi-User Sum Capacity 4229.1.3 Dirty Paper Coding 4259.1.4 Zero-Forcing Precoding 4289.1.5 Block Diagonalization 4299.2 Massive MIMO 4329.2.1 CSI Acquisition 4339.2.2 Linear Detection in Uplink 4359.2.2.1 Matched Filtering 4369.2.2.2 ZF Detection 4369.2.2.3 MMSE Detection 4379.2.3 Linear Precoding in Downlink 4379.2.3.1 Conjugate Beamforming 4389.2.3.2 ZF Precoding 4389.2.3.3 Regularized ZF Precoding 4399.3 Multi-Cell Massive MIMO 4399.3.1 Pilot Contamination 4419.3.2 Uplink Data Transmission 4449.3.3 Downlink Data Transmission 4469.4 Cell-Free Massive MIMO 4479.4.1 Cell-Free Network Layout 4489.4.2 Uplink Training 4499.4.3 Uplink Signal Detection 4519.4.3.1 Matched Filtering 4529.4.3.2 ZF Detection 4529.4.3.3 MMSE Detection 4529.4.4 Conjugate Beamforming 4539.4.5 Zero-Forcing Precoding 4559.4.6 Impact of Channel Aging 4579.4.6.1 Channel Aging 4579.4.6.2 Performance Degradation 4609.5 Opportunistic Cell-Free Communications 4649.5.1 Cell-free Massive Wideband Systems 4649.5.2 Opportunistic AP Selection 4669.5.3 Spectral Efficiency Analysis 4689.6 Summary 472References 47210 Adaptive and Non-Orthogonal Multiple Access Systems in6G 47510.1 Frequency-Selective Fading Channel 47610.2 Multi-Carrier Modulation 48010.2.1 The Synthesis and Analysis Filters 48010.2.2 Polyphase Implementation 48310.2.3 Filter Bank Multi-Carrier 48610.3 Orthogonal Frequency-Division Multiplexing 48710.3.1 DFT Implementation 49110.3.2 Cyclic Prefix 49310.3.3 Frequency-Domain Signal Processing 49610.3.4 Out-of-Band Emission 49910.4 Orthogonal Frequency-Division Multiple Access 50310.4.1 Orthogonal Frequency-Division Multiple Access 50310.4.2 Single-Carrier Frequency-Division Multiple Access 50510.4.3 Cyclic Delay Diversity 50710.4.4 Multi-Cell OFDMA 51010.5 Cell-Free Massive MIMO-OFDMA 51210.5.1 The System Model 51310.5.2 The Communication Process 51610.5.2.1 Uplink Training 51610.5.2.2 Uplink Payload Data Transmission 51810.5.2.3 Downlink Payload Data Transmission 51810.5.3 User-Specific Resource Allocation 51910.6 Non-Orthogonal Multiple Access 52010.6.1 Fundamentals of NOMA 52110.6.1.1 Downlink Non-Orthogonal Multiplexing 52210.6.1.2 Uplink Non-Orthogonal Multiple Access 52510.6.2 Multi-User Superposition Coding 52810.6.3 Uplink Grant-Free Transmission 53110.6.4 Code-Domain NOMA 53310.6.4.1 Low-Density Signature-CDMA/OFDM 53310.6.4.2 Sparse Code Multiple Access 53610.7 Summary 538References 538Index 541

Wei Jiang, Ph.D., is a Senior Researcher with the German Research Center for Artificial Intelligence (DFKI). He has served as the leader of many EU or German research projects on 5G, 6G, AI, and Industry 4.0, and has extensive experiences in developing new technology and international standards related to wireless communications and networking by co-authoring 3 books, 90 articles and holding 30 patents. Fa-Long Luo, Ph.D., is a Full Professor (Affiliate) of the ECE Department at the University of Washington, USA. He has over 39 years of research and leadership experience with holding 86 patents and having published 7 well-received books and 110 articles on wireless and signal processing.

• Preface xvList of Abbreviations xxiPart I The Vision of 6G and Technical Evolution 11 Standards History of Cellular Systems Toward 6G 31.1 0G: Pre-Cellular Systems 41.2 1G: The Birth of Cellular Network 61.2.1 Nordic Mobile Telephone (NMT) 71.2.2 Advanced Mobile Phone System (AMPS) 81.3 2G: From Analog to Digital 91.3.1 Global System for Mobile communications (GSM) 101.3.2 Digital Advanced Mobile Phone System (D-AMPS) 111.3.3 Interim Standard 95 (IS-95) 111.3.4 Personal Digital Cellular (PDC) 121.3.5 General Packet Radio Service (GPRS) 121.3.6 Enhanced Data Rates for GSM Evolution (EDGE) 141.4 3G: From Voice to Data-Centric 151.4.1 Wideband Code-Division Multiple Access (WCDMA) 161.4.2 Code-Division Multiple Access 2000 (CDMA2000) 181.4.3 Time Division-Synchronous Code-Division Multiple Access(TD-SCDMA) 211.4.4 Worldwide Interoperability for Microwave Access (WiMAX) 221.5 4G: Mobile Internet 231.5.1 Long-Term Evolution-Advanced (LTE-Advanced) 251.5.2 WirelessMAN-Advanced 281.6 5G: From Human to Machine 301.7 Beyond 5G 371.8 Conclusions 39References 392 Pre-6G Technology and System Evolution 432.1 1G –AMPS 442.1.1 System Architecture 442.1.2 Key Technologies 462.1.2.1 Frequency Reuse 462.1.2.2 Cell Splitting 472.1.2.3 Sectorization 482.1.2.4 Handover 482.1.2.5 Frequency-Division Multiple Access 492.2 2G –GSM 492.2.1 System Architecture 502.2.1.1 Mobile Station Subsystem 502.2.1.2 Bases Station Subsystem 502.2.1.3 Network and Switching Subsystem 512.2.1.4 Operation and Support Subsystem 512.2.1.5 General Packet Radio Service 522.2.1.6 Gateway GPRS Support Node 532.2.2 Key Technologies 532.2.2.1 Time-Division Multiple Access 532.2.2.2 Frequency Hopping 542.2.2.3 Speech Compression 552.2.2.4 Channel Coding 552.2.2.5 Digital Modulation 562.2.2.6 Discontinuous Transmission (DXT) 562.3 3G –WCDMA 562.3.1 System Architecture 572.3.1.1 User Equipment 572.3.1.2 UMTS Terrestrial Radio Access Network 582.3.1.3 Core Network 592.3.2 Key Technologies 602.3.2.1 Code-Division Multiple Access 602.3.2.2 Rake Receiver 632.3.2.3 Turbo Codes 632.4 4G – LTE 642.4.1 System Architecture 652.4.1.1 Evolved Universal Terrestrial Radio Access Network 652.4.1.2 Evolved Packet Core 652.4.2 Key Technologies 682.4.2.1 Orthogonal Frequency-Division Multiplexing 702.4.2.2 Carrier Aggregation 712.4.2.3 Relaying 712.4.2.4 Heterogeneous Network 722.4.2.5 Coordinated Multi-Point Transmission and Reception 732.4.2.6 Device-to-Device Communications 732.4.2.7 License-Assisted Access 742.5 5G –New Radio 752.5.1 System Architecture 762.5.1.1 5G Core Network 772.5.1.2 Next Generation Radio Access Network 792.5.2 Key Technologies 812.5.2.1 Massive MIMO 812.5.2.2 MillimeterWave 822.5.2.3 Non-Orthogonal Multiple Access 832.5.2.4 SDN/NFV 842.5.2.5 Network Slicing 852.5.2.6 Polar Codes 862.6 Conclusions 87References 873 The Vision of 6G: Drivers, Enablers, Uses, and Roadmap 893.1 Background 903.2 Explosive Mobile Traffic 923.3 Use Cases 943.4 Usage Scenarios 983.5 Performance Requirements 1023.6 Research Initiatives and Roadmap 1073.6.1 ITU 1083.6.2 Third Generation Partnership Project 1103.6.3 Industry 1103.6.4 Europe 1103.6.5 The United States 1133.6.6 China 1163.6.7 Japan 1163.6.8 South Korea 1173.7 Key Technologies 1173.7.1 MillimeterWave 1183.7.2 Terahertz Communications 1183.7.3 Optical Wireless Communications 1193.7.4 Massive MIMO 1203.7.5 Intelligent Reflecting Surfaces 1213.7.6 Next-Generation Multiple Access 1223.7.7 Open Radio Access Network 1233.7.8 Non-Terrestrial Networks 1243.7.9 Artificial Intelligence 1253.7.10 Communication-Computing-Sensing Convergence 1273.8 Conclusions 128References 128Part II Full-Spectra Wireless Communications in 6G 1314 Enhanced Millimeter-Wave Wireless Communications in6G 1334.1 Spectrum Shortage 1344.2 mmWave Propagation Characteristics 1364.2.1 Large-Scale Propagation Effects 1374.2.1.1 Free-Space Propagation Loss 1374.2.1.2 NLOS Propagation and Shadowing 1394.2.1.3 Atmospheric Attenuation 1414.2.2 Small-Scale Propagation Effects 1434.2.3 Delay Spread and Coherence Bandwidth 1454.2.4 Doppler Spread and Coherence Bandwidth 1464.2.5 Angular Spread 1494.3 Millimeter-Wave Channel Models 1524.3.1 Large-Scale Fading 1524.3.2 3GPP Channel Models 1554.3.2.1 Urban Micro Scenario 1554.3.2.2 Urban Macro Scenario 1564.3.2.3 Indoor Scenario 1574.3.3 Small-Scale Fading 1594.4 mmWave Transmission Technologies 1634.4.1 Beamforming 1634.4.1.1 Digital Beamforming 1644.4.1.2 Analog Beamforming 1684.4.1.3 Hybrid Beamforming 1694.4.1.4 3D Beamforming 1734.4.2 Initial Access 1754.4.2.1 Multi-Beam Synchronization and Broadcasting 1764.4.2.2 Conventional Initial Access in LTE 1784.4.2.3 Beam-Sweeping Initial Access in NR 1814.4.3 Omnidirectional Beamforming 1834.4.3.1 Random Beamforming 1854.4.3.2 Enhanced Random Beamforming 1874.4.3.3 Complementary Random Beamforming 1904.5 Summary 192References 1935 Terahertz Technologies and Systems for 6G 1955.1 Potential of Terahertz Band 1965.1.1 Spectrum Limit 1965.1.2 The Need of Exploiting Terahertz Band 1985.1.3 Spectrum Regulation on Terahertz Band 2035.2 Terahertz Applications 2055.2.1 Terahertz Wireless Communications 2055.2.1.1 Terabit Cellular Hotspot 2055.2.1.2 Terabit Wireless Local-Area Network 2065.2.1.3 Terabit Device-To-Device Link 2065.2.1.4 Secure Wireless Communication 2075.2.1.5 Terabit Wireless Backhaul 2075.2.1.6 Terahertz Nano-Communications 2085.2.2 Non-Communication Terahertz Applications 2095.2.2.1 Terahertz Sensing 2095.2.2.2 Terahertz Imaging 2105.2.2.3 Terahertz Positioning 2125.3 Challenges of Terahertz Communications 2125.3.1 High Free-Space Path Loss 2135.3.2 Atmospheric Attenuation 2155.3.3 Weather Effects 2225.3.4 Blockage 2245.3.5 High Channel Fluctuation 2265.4 Array-of-Subarrays Beamforming 2285.5 Lens Antenna 2315.5.1 Refraction of RadioWaves 2325.5.2 Lens Antenna Array 2335.6 Case Study – IEEE 802.15.3d 2365.6.1 IEEE 802.15.3d Usage Scenarios 2375.6.2 Physical Layer 2405.6.2.1 Channelization 2405.6.2.2 Modulation 2425.6.2.3 Forward Error Correction 2425.6.3 Medium Access Control 2445.6.4 Frame Structure 2465.6.4.1 Preamble 2475.6.4.2 PHY Header 2475.6.4.3 MAC Header 2485.6.4.4 Construction Process of Frame Header 2485.7 Summary 250References 2516 Optical and Visible Light Wireless Communicationsin 6G 2536.1 The Optical Spectrum 2546.1.1 Infrared 2546.1.2 Visible Light 2566.1.3 Ultraviolet 2576.2 Advantages and Challenges 2586.3 OWC Applications 2626.4 Evolution of Optical Wireless Communications 2646.4.1 Wireless Infrared Communications 2656.4.2 Visible Light Communications 2666.4.3 Wireless Ultraviolet Communications 2676.4.4 Free-Space Optical Communications 2686.5 Optical Transceiver 2686.6 Optical Sources and Detectors 2716.6.1 Light-Emitting Diode 2736.6.2 Laser Diode 2766.6.3 Photodiode 2806.7 Optical Link Configuration 2836.8 Optical MIMO 2866.8.1 Spatial Multiplexing 2866.8.2 Spatial Modulation 2896.9 Summary 292References 292Part III Smart Radio Networks and Air InterfaceTechnologies for 6G 2957 Intelligent Reflecting Surface-Aided Communications for6G 2977.1 Basic Concept 2987.2 IRS-Aided Single-Antenna Transmission 3027.2.1 Signal Model 3037.2.2 Passive Beamforming 3067.2.3 Product-Distance Path Loss 3097.3 IRS-Aided Multi-Antenna Transmission 3107.3.1 Joint Active and Passive Beamforming 3107.3.1.1 SDR Solution 3127.3.1.2 Alternating Optimization 3147.3.2 Joint Precoding and Reflecting 3157.4 Dual-Beam Intelligent Reflecting Surface 3187.4.1 Dual Beams Over Hybrid Beamforming 3187.4.2 Dual-Beam IRS 3217.4.3 Optimization Design 3227.5 IRS-Aided Wideband Communications 3257.5.1 Cascaded Frequency-Selective Channel 3257.5.2 IRS-Aided OFDM System 3277.5.3 Rate Maximization 3307.6 Multi-User IRS Communications 3317.6.1 Multiple Access Model 3327.6.2 Orthogonal Multiple Access 3337.6.2.1 Time-Division Multiple Access 3347.6.2.2 Frequency-Division Multiple Access 3367.6.3 Non-Orthogonal Multiple Access 3377.7 Channel Aging and Prediction 3397.7.1 Outdated Channel State Information 3417.7.1.1 Doppler Shift 3417.7.1.2 Phase Noise 3437.7.2 Impact of Channel Aging on IRS 3437.7.3 Classical Channel Prediction 3457.7.3.1 Autoregressive Model 3457.7.3.2 Parametric Model 3477.7.4 Recurrent Neural Network 3487.7.5 RNN-Based Channel Prediction 3517.7.5.1 Flat-Fading Channel Prediction 3527.7.5.2 Frequency-Selective Fading Channel Prediction 3537.7.6 Long-Short Term Memory 3557.7.7 Deep Learning-Based Channel Prediction 3587.8 Summary 359References 3598 Multiple Dimensional and Antenna Techniques for 6G 3638.1 Spatial Diversity 3648.2 Receive Combining 3668.2.1 Selection Combining 3688.2.2 Maximal Ratio Combining 3708.2.3 Equal-Gain Combining 3738.3 Space-Time Coding 3748.3.1 Repetition Coding 3758.3.2 Space-Time Trellis Codes 3778.3.3 Alamouti Coding 3798.3.4 Space-Time Block Codes 3818.4 Transmit Antenna Selection 3838.5 Beamforming 3868.5.1 Classical Beamforming 3868.5.2 Single-Stream Precoding 3908.6 Spatial Multiplexing 3938.6.1 Single-User MIMO 3948.6.2 MIMO Precoding 4008.6.2.1 Full CSI at the Transmitter 4008.6.2.2 Limited CSI at the Transmitter 4038.6.3 MIMO Detection 4068.6.3.1 Maximum-Likelihood Detection 4068.6.3.2 Linear Detection 4078.6.3.3 Successive Interference Cancelation 4108.7 Summary 413References 4139 Cellular and Cell-Free Massive MIMO Techniques in 6G 4179.1 Multi-User MIMO 4189.1.1 Broadcast and Multiple-Access Channels 4199.1.2 Multi-User Sum Capacity 4229.1.3 Dirty Paper Coding 4259.1.4 Zero-Forcing Precoding 4289.1.5 Block Diagonalization 4299.2 Massive MIMO 4329.2.1 CSI Acquisition 4339.2.2 Linear Detection in Uplink 4359.2.2.1 Matched Filtering 4369.2.2.2 ZF Detection 4369.2.2.3 MMSE Detection 4379.2.3 Linear Precoding in Downlink 4379.2.3.1 Conjugate Beamforming 4389.2.3.2 ZF Precoding 4389.2.3.3 Regularized ZF Precoding 4399.3 Multi-Cell Massive MIMO 4399.3.1 Pilot Contamination 4419.3.2 Uplink Data Transmission 4449.3.3 Downlink Data Transmission 4469.4 Cell-Free Massive MIMO 4479.4.1 Cell-Free Network Layout 4489.4.2 Uplink Training 4499.4.3 Uplink Signal Detection 4519.4.3.1 Matched Filtering 4529.4.3.2 ZF Detection 4529.4.3.3 MMSE Detection 4529.4.4 Conjugate Beamforming 4539.4.5 Zero-Forcing Precoding 4559.4.6 Impact of Channel Aging 4579.4.6.1 Channel Aging 4579.4.6.2 Performance Degradation 4609.5 Opportunistic Cell-Free Communications 4649.5.1 Cell-free Massive Wideband Systems 4649.5.2 Opportunistic AP Selection 4669.5.3 Spectral Efficiency Analysis 4689.6 Summary 472References 47210 Adaptive and Non-Orthogonal Multiple Access Systems in6G 47510.1 Frequency-Selective Fading Channel 47610.2 Multi-Carrier Modulation 48010.2.1 The Synthesis and Analysis Filters 48010.2.2 Polyphase Implementation 48310.2.3 Filter Bank Multi-Carrier 48610.3 Orthogonal Frequency-Division Multiplexing 48710.3.1 DFT Implementation 49110.3.2 Cyclic Prefix 49310.3.3 Frequency-Domain Signal Processing 49610.3.4 Out-of-Band Emission 49910.4 Orthogonal Frequency-Division Multiple Access 50310.4.1 Orthogonal Frequency-Division Multiple Access 50310.4.2 Single-Carrier Frequency-Division Multiple Access 50510.4.3 Cyclic Delay Diversity 50710.4.4 Multi-Cell OFDMA 51010.5 Cell-Free Massive MIMO-OFDMA 51210.5.1 The System Model 51310.5.2 The Communication Process 51610.5.2.1 Uplink Training 51610.5.2.2 Uplink Payload Data Transmission 51810.5.2.3 Downlink Payload Data Transmission 51810.5.3 User-Specific Resource Allocation 51910.6 Non-Orthogonal Multiple Access 52010.6.1 Fundamentals of NOMA 52110.6.1.1 Downlink Non-Orthogonal Multiplexing 52210.6.1.2 Uplink Non-Orthogonal Multiple Access 52510.6.2 Multi-User Superposition Coding 52810.6.3 Uplink Grant-Free Transmission 53110.6.4 Code-Domain NOMA 53310.6.4.1 Low-Density Signature-CDMA/OFDM 53310.6.4.2 Sparse Code Multiple Access 53610.7 Summary 538References 538Index 541

Wei Jiang, Ph.D., is a Senior Researcher with the German Research Center for Artificial Intelligence (DFKI). He has served as the leader of many EU or German research projects on 5G, 6G, AI, and Industry 4.0, and has extensive experiences in developing new technology and international standards related to wireless communications and networking by co-authoring 3 books, 90 articles and holding 30 patents. Fa-Long Luo, Ph.D., is a Full Professor (Affiliate) of the ECE Department at the University of Washington, USA. He has over 39 years of research and leadership experience with holding 86 patents and having published 7 well-received books and 110 articles on wireless and signal processing.

• Preface xvList of Abbreviations xxiPart I The Vision of 6G and Technical Evolution 11 Standards History of Cellular Systems Toward 6G 31.1 0G: Pre-Cellular Systems 41.2 1G: The Birth of Cellular Network 61.2.1 Nordic Mobile Telephone (NMT) 71.2.2 Advanced Mobile Phone System (AMPS) 81.3 2G: From Analog to Digital 91.3.1 Global System for Mobile communications (GSM) 101.3.2 Digital Advanced Mobile Phone System (D-AMPS) 111.3.3 Interim Standard 95 (IS-95) 111.3.4 Personal Digital Cellular (PDC) 121.3.5 General Packet Radio Service (GPRS) 121.3.6 Enhanced Data Rates for GSM Evolution (EDGE) 141.4 3G: From Voice to Data-Centric 151.4.1 Wideband Code-Division Multiple Access (WCDMA) 161.4.2 Code-Division Multiple Access 2000 (CDMA2000) 181.4.3 Time Division-Synchronous Code-Division Multiple Access(TD-SCDMA) 211.4.4 Worldwide Interoperability for Microwave Access (WiMAX) 221.5 4G: Mobile Internet 231.5.1 Long-Term Evolution-Advanced (LTE-Advanced) 251.5.2 WirelessMAN-Advanced 281.6 5G: From Human to Machine 301.7 Beyond 5G 371.8 Conclusions 39References 392 Pre-6G Technology and System Evolution 432.1 1G –AMPS 442.1.1 System Architecture 442.1.2 Key Technologies 462.1.2.1 Frequency Reuse 462.1.2.2 Cell Splitting 472.1.2.3 Sectorization 482.1.2.4 Handover 482.1.2.5 Frequency-Division Multiple Access 492.2 2G –GSM 492.2.1 System Architecture 502.2.1.1 Mobile Station Subsystem 502.2.1.2 Bases Station Subsystem 502.2.1.3 Network and Switching Subsystem 512.2.1.4 Operation and Support Subsystem 512.2.1.5 General Packet Radio Service 522.2.1.6 Gateway GPRS Support Node 532.2.2 Key Technologies 532.2.2.1 Time-Division Multiple Access 532.2.2.2 Frequency Hopping 542.2.2.3 Speech Compression 552.2.2.4 Channel Coding 552.2.2.5 Digital Modulation 562.2.2.6 Discontinuous Transmission (DXT) 562.3 3G –WCDMA 562.3.1 System Architecture 572.3.1.1 User Equipment 572.3.1.2 UMTS Terrestrial Radio Access Network 582.3.1.3 Core Network 592.3.2 Key Technologies 602.3.2.1 Code-Division Multiple Access 602.3.2.2 Rake Receiver 632.3.2.3 Turbo Codes 632.4 4G – LTE 642.4.1 System Architecture 652.4.1.1 Evolved Universal Terrestrial Radio Access Network 652.4.1.2 Evolved Packet Core 652.4.2 Key Technologies 682.4.2.1 Orthogonal Frequency-Division Multiplexing 702.4.2.2 Carrier Aggregation 712.4.2.3 Relaying 712.4.2.4 Heterogeneous Network 722.4.2.5 Coordinated Multi-Point Transmission and Reception 732.4.2.6 Device-to-Device Communications 732.4.2.7 License-Assisted Access 742.5 5G –New Radio 752.5.1 System Architecture 762.5.1.1 5G Core Network 772.5.1.2 Next Generation Radio Access Network 792.5.2 Key Technologies 812.5.2.1 Massive MIMO 812.5.2.2 MillimeterWave 822.5.2.3 Non-Orthogonal Multiple Access 832.5.2.4 SDN/NFV 842.5.2.5 Network Slicing 852.5.2.6 Polar Codes 862.6 Conclusions 87References 873 The Vision of 6G: Drivers, Enablers, Uses, and Roadmap 893.1 Background 903.2 Explosive Mobile Traffic 923.3 Use Cases 943.4 Usage Scenarios 983.5 Performance Requirements 1023.6 Research Initiatives and Roadmap 1073.6.1 ITU 1083.6.2 Third Generation Partnership Project 1103.6.3 Industry 1103.6.4 Europe 1103.6.5 The United States 1133.6.6 China 1163.6.7 Japan 1163.6.8 South Korea 1173.7 Key Technologies 1173.7.1 MillimeterWave 1183.7.2 Terahertz Communications 1183.7.3 Optical Wireless Communications 1193.7.4 Massive MIMO 1203.7.5 Intelligent Reflecting Surfaces 1213.7.6 Next-Generation Multiple Access 1223.7.7 Open Radio Access Network 1233.7.8 Non-Terrestrial Networks 1243.7.9 Artificial Intelligence 1253.7.10 Communication-Computing-Sensing Convergence 1273.8 Conclusions 128References 128Part II Full-Spectra Wireless Communications in 6G 1314 Enhanced Millimeter-Wave Wireless Communications in6G 1334.1 Spectrum Shortage 1344.2 mmWave Propagation Characteristics 1364.2.1 Large-Scale Propagation Effects 1374.2.1.1 Free-Space Propagation Loss 1374.2.1.2 NLOS Propagation and Shadowing 1394.2.1.3 Atmospheric Attenuation 1414.2.2 Small-Scale Propagation Effects 1434.2.3 Delay Spread and Coherence Bandwidth 1454.2.4 Doppler Spread and Coherence Bandwidth 1464.2.5 Angular Spread 1494.3 Millimeter-Wave Channel Models 1524.3.1 Large-Scale Fading 1524.3.2 3GPP Channel Models 1554.3.2.1 Urban Micro Scenario 1554.3.2.2 Urban Macro Scenario 1564.3.2.3 Indoor Scenario 1574.3.3 Small-Scale Fading 1594.4 mmWave Transmission Technologies 1634.4.1 Beamforming 1634.4.1.1 Digital Beamforming 1644.4.1.2 Analog Beamforming 1684.4.1.3 Hybrid Beamforming 1694.4.1.4 3D Beamforming 1734.4.2 Initial Access 1754.4.2.1 Multi-Beam Synchronization and Broadcasting 1764.4.2.2 Conventional Initial Access in LTE 1784.4.2.3 Beam-Sweeping Initial Access in NR 1814.4.3 Omnidirectional Beamforming 1834.4.3.1 Random Beamforming 1854.4.3.2 Enhanced Random Beamforming 1874.4.3.3 Complementary Random Beamforming 1904.5 Summary 192References 1935 Terahertz Technologies and Systems for 6G 1955.1 Potential of Terahertz Band 1965.1.1 Spectrum Limit 1965.1.2 The Need of Exploiting Terahertz Band 1985.1.3 Spectrum Regulation on Terahertz Band 2035.2 Terahertz Applications 2055.2.1 Terahertz Wireless Communications 2055.2.1.1 Terabit Cellular Hotspot 2055.2.1.2 Terabit Wireless Local-Area Network 2065.2.1.3 Terabit Device-To-Device Link 2065.2.1.4 Secure Wireless Communication 2075.2.1.5 Terabit Wireless Backhaul 2075.2.1.6 Terahertz Nano-Communications 2085.2.2 Non-Communication Terahertz Applications 2095.2.2.1 Terahertz Sensing 2095.2.2.2 Terahertz Imaging 2105.2.2.3 Terahertz Positioning 2125.3 Challenges of Terahertz Communications 2125.3.1 High Free-Space Path Loss 2135.3.2 Atmospheric Attenuation 2155.3.3 Weather Effects 2225.3.4 Blockage 2245.3.5 High Channel Fluctuation 2265.4 Array-of-Subarrays Beamforming 2285.5 Lens Antenna 2315.5.1 Refraction of RadioWaves 2325.5.2 Lens Antenna Array 2335.6 Case Study – IEEE 802.15.3d 2365.6.1 IEEE 802.15.3d Usage Scenarios 2375.6.2 Physical Layer 2405.6.2.1 Channelization 2405.6.2.2 Modulation 2425.6.2.3 Forward Error Correction 2425.6.3 Medium Access Control 2445.6.4 Frame Structure 2465.6.4.1 Preamble 2475.6.4.2 PHY Header 2475.6.4.3 MAC Header 2485.6.4.4 Construction Process of Frame Header 2485.7 Summary 250References 2516 Optical and Visible Light Wireless Communicationsin 6G 2536.1 The Optical Spectrum 2546.1.1 Infrared 2546.1.2 Visible Light 2566.1.3 Ultraviolet 2576.2 Advantages and Challenges 2586.3 OWC Applications 2626.4 Evolution of Optical Wireless Communications 2646.4.1 Wireless Infrared Communications 2656.4.2 Visible Light Communications 2666.4.3 Wireless Ultraviolet Communications 2676.4.4 Free-Space Optical Communications 2686.5 Optical Transceiver 2686.6 Optical Sources and Detectors 2716.6.1 Light-Emitting Diode 2736.6.2 Laser Diode 2766.6.3 Photodiode 2806.7 Optical Link Configuration 2836.8 Optical MIMO 2866.8.1 Spatial Multiplexing 2866.8.2 Spatial Modulation 2896.9 Summary 292References 292Part III Smart Radio Networks and Air InterfaceTechnologies for 6G 2957 Intelligent Reflecting Surface-Aided Communications for6G 2977.1 Basic Concept 2987.2 IRS-Aided Single-Antenna Transmission 3027.2.1 Signal Model 3037.2.2 Passive Beamforming 3067.2.3 Product-Distance Path Loss 3097.3 IRS-Aided Multi-Antenna Transmission 3107.3.1 Joint Active and Passive Beamforming 3107.3.1.1 SDR Solution 3127.3.1.2 Alternating Optimization 3147.3.2 Joint Precoding and Reflecting 3157.4 Dual-Beam Intelligent Reflecting Surface 3187.4.1 Dual Beams Over Hybrid Beamforming 3187.4.2 Dual-Beam IRS 3217.4.3 Optimization Design 3227.5 IRS-Aided Wideband Communications 3257.5.1 Cascaded Frequency-Selective Channel 3257.5.2 IRS-Aided OFDM System 3277.5.3 Rate Maximization 3307.6 Multi-User IRS Communications 3317.6.1 Multiple Access Model 3327.6.2 Orthogonal Multiple Access 3337.6.2.1 Time-Division Multiple Access 3347.6.2.2 Frequency-Division Multiple Access 3367.6.3 Non-Orthogonal Multiple Access 3377.7 Channel Aging and Prediction 3397.7.1 Outdated Channel State Information 3417.7.1.1 Doppler Shift 3417.7.1.2 Phase Noise 3437.7.2 Impact of Channel Aging on IRS 3437.7.3 Classical Channel Prediction 3457.7.3.1 Autoregressive Model 3457.7.3.2 Parametric Model 3477.7.4 Recurrent Neural Network 3487.7.5 RNN-Based Channel Prediction 3517.7.5.1 Flat-Fading Channel Prediction 3527.7.5.2 Frequency-Selective Fading Channel Prediction 3537.7.6 Long-Short Term Memory 3557.7.7 Deep Learning-Based Channel Prediction 3587.8 Summary 359References 3598 Multiple Dimensional and Antenna Techniques for 6G 3638.1 Spatial Diversity 3648.2 Receive Combining 3668.2.1 Selection Combining 3688.2.2 Maximal Ratio Combining 3708.2.3 Equal-Gain Combining 3738.3 Space-Time Coding 3748.3.1 Repetition Coding 3758.3.2 Space-Time Trellis Codes 3778.3.3 Alamouti Coding 3798.3.4 Space-Time Block Codes 3818.4 Transmit Antenna Selection 3838.5 Beamforming 3868.5.1 Classical Beamforming 3868.5.2 Single-Stream Precoding 3908.6 Spatial Multiplexing 3938.6.1 Single-User MIMO 3948.6.2 MIMO Precoding 4008.6.2.1 Full CSI at the Transmitter 4008.6.2.2 Limited CSI at the Transmitter 4038.6.3 MIMO Detection 4068.6.3.1 Maximum-Likelihood Detection 4068.6.3.2 Linear Detection 4078.6.3.3 Successive Interference Cancelation 4108.7 Summary 413References 4139 Cellular and Cell-Free Massive MIMO Techniques in 6G 4179.1 Multi-User MIMO 4189.1.1 Broadcast and Multiple-Access Channels 4199.1.2 Multi-User Sum Capacity 4229.1.3 Dirty Paper Coding 4259.1.4 Zero-Forcing Precoding 4289.1.5 Block Diagonalization 4299.2 Massive MIMO 4329.2.1 CSI Acquisition 4339.2.2 Linear Detection in Uplink 4359.2.2.1 Matched Filtering 4369.2.2.2 ZF Detection 4369.2.2.3 MMSE Detection 4379.2.3 Linear Precoding in Downlink 4379.2.3.1 Conjugate Beamforming 4389.2.3.2 ZF Precoding 4389.2.3.3 Regularized ZF Precoding 4399.3 Multi-Cell Massive MIMO 4399.3.1 Pilot Contamination 4419.3.2 Uplink Data Transmission 4449.3.3 Downlink Data Transmission 4469.4 Cell-Free Massive MIMO 4479.4.1 Cell-Free Network Layout 4489.4.2 Uplink Training 4499.4.3 Uplink Signal Detection 4519.4.3.1 Matched Filtering 4529.4.3.2 ZF Detection 4529.4.3.3 MMSE Detection 4529.4.4 Conjugate Beamforming 4539.4.5 Zero-Forcing Precoding 4559.4.6 Impact of Channel Aging 4579.4.6.1 Channel Aging 4579.4.6.2 Performance Degradation 4609.5 Opportunistic Cell-Free Communications 4649.5.1 Cell-free Massive Wideband Systems 4649.5.2 Opportunistic AP Selection 4669.5.3 Spectral Efficiency Analysis 4689.6 Summary 472References 47210 Adaptive and Non-Orthogonal Multiple Access Systems in6G 47510.1 Frequency-Selective Fading Channel 47610.2 Multi-Carrier Modulation 48010.2.1 The Synthesis and Analysis Filters 48010.2.2 Polyphase Implementation 48310.2.3 Filter Bank Multi-Carrier 48610.3 Orthogonal Frequency-Division Multiplexing 48710.3.1 DFT Implementation 49110.3.2 Cyclic Prefix 49310.3.3 Frequency-Domain Signal Processing 49610.3.4 Out-of-Band Emission 49910.4 Orthogonal Frequency-Division Multiple Access 50310.4.1 Orthogonal Frequency-Division Multiple Access 50310.4.2 Single-Carrier Frequency-Division Multiple Access 50510.4.3 Cyclic Delay Diversity 50710.4.4 Multi-Cell OFDMA 51010.5 Cell-Free Massive MIMO-OFDMA 51210.5.1 The System Model 51310.5.2 The Communication Process 51610.5.2.1 Uplink Training 51610.5.2.2 Uplink Payload Data Transmission 51810.5.2.3 Downlink Payload Data Transmission 51810.5.3 User-Specific Resource Allocation 51910.6 Non-Orthogonal Multiple Access 52010.6.1 Fundamentals of NOMA 52110.6.1.1 Downlink Non-Orthogonal Multiplexing 52210.6.1.2 Uplink Non-Orthogonal Multiple Access 52510.6.2 Multi-User Superposition Coding 52810.6.3 Uplink Grant-Free Transmission 53110.6.4 Code-Domain NOMA 53310.6.4.1 Low-Density Signature-CDMA/OFDM 53310.6.4.2 Sparse Code Multiple Access 53610.7 Summary 538References 538Index 541