The fifth generation (5G) wireless communication networks are being deployed worldwide from 2020 and more capabilities are in the process of being standardized, such as mass connectivity, ultra-reliability, and guaranteed low latency. However, 5G will not meet all requirements of the future in 2030 and beyond, and sixth generation (6G) wireless communication networks are expected to provide global coverage, enhanced spectral/energy/cost efficiency, better intelligence level and security, etc. To meet these requirements, 6G networks will rely on new enabling technologies, i.e., air interface and transmission technologies and novel network architecture, such as waveform design, multiple access, channel coding schemes, multi-antenna technologies, network slicing, cell-free architecture, and cloud/fog/edge computing. Our vision on 6G is that it will have four new paradigm shifts. First, to satisfy the requirement of global coverage, 6G will not be limited to terrestrial communication networks, which will need to be complemented with non-terrestrial networks such as satellite and unmanned aerial vehicle (UAV) communication networks, thus achieving a space-air-ground-sea integrated communication network. Second, all spectra will be fully explored to further increase data rates and connection density, including the sub-6 GHz, millimeter wave (mmWave), terahertz (THz), and optical frequency bands. Third, facing the big datasets generated by the use of extremely heterogeneous networks, diverse communication scenarios, large numbers of antennas, wide bandwidths, and new service requirements, 6G networks will enable a new range of smart applications with the aid of artificial intelligence (AI) and big data technologies. Fourth, network security will have to be strengthened when developing 6G networks. This article provides a comprehensive survey of recent advances and future trends in these four aspects. Clearly, 6G with additional technical requirements beyond those of 5G will enable faster and further communications to the extent that the boundary between physical and cyber worlds disappears.
This work was supported by National Key RD Program of China (Grant No. 2018YFB1801101), National Natural Science Foundation of China (Grant Nos. 61960206006, 61901109), Frontiers Science Center for Mobile Information Communication and Security, High Level Innovation and Entrepreneurial Research Team Program in Jiangsu, High Level Innovation and Entrepreneurial Talent Introduction Program in Jiangsu, National Postdoctoral Program for Innovative Talents (Grant No. BX20180062), Research Fund of National Mobile Communications Research Laboratory, Southeast University (Grant No. 2020B01), and Fundamental Research Funds for the Central Universities (Grant No. 2242020R30001).
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Figure 1
(Color online) A vision of 6G wireless communication networks.
Figure 2
(Color online) An overview of 6G wireless communication networks.
Figure 3
(Color online) Organization of this paper.
Figure 4
(Color online) Performance metrics of 6G communication networks.
Figure 5
(Color online) Application scenarios of 6G communication networks.
Figure 6
(Color online) Comparison of 5G and 6G requirements of key performance metrics and application scenarios.
Figure 7
(Color online) Cloud VR reference architecture
Figure 8
ICDT-based C-V2X architecture.
Figure 9
(Color online) The schematic of the digital twin body area network to help manage the epidemic.
Figure 10
(Color online) The connection between traditional mobile communications and micro-mobile communications in body, body surface, and body domain
Figure 11
OTFS Tx and Rx.
Figure 12
Parallel concatenated encoder and decoder
Figure 13
Serially concatenated encoder and decoder.
Figure 14
Three-stage serially concatenated encoder and decoder.
Figure 15
(Color online) An illustration of blockchain-based networks.
Figure 16
(Color online) Concepts of direct O/T and T/O conversion.
Figure 17
(Color online) A high-level overview of SDN architecture
Figure 18
(Color online) A high-level overview of NFV architecture
Figure 19
(Color online) Network slicing conceptual outline
Figure 20
SBA for 5G core network
Figure 21
(Color online) An illustration of DEN$^2$.
Figure 22
(Color online) CF massive MIMO system model.
Figure 23
(Color online) SDN-based CF architecture
Figure 24
(Color online) Multi-tier computing network architecture, which integrates cloud, edge, and fog computing technologies to enable intelligent services for anything at anytime and anywhere
Figure 25
(Color online) Integration of terrestrial and satellite communication networks.
Figure 26
(Color online) Maritime MTC network functional view and topology for service-centric and software-defined networking
Figure 27
(Color online) International frequency allocations for maritime MTC
Figure 28
(Color online) Space-air-ground-sea integrated network control architecture.
Figure 29
(Color online) Microphotograph of the multiplier with on-chip antenna.
Figure 30
(Color online) (a) SNR of a COTS red LED at a distance of 1.6 m and bit loading pattern. Binary phase shift keying (BPSK) is still achieved between 900 MHz–1 GHz which is well above the 3-dB bandwidth of the device. (b) Channel gains of the different colors (red, green, blue, and yellow) of the wavelength division multiplexing (WDM) system which achieved 15.7 Gbps with COTS LEDs
Figure 31
(Color online) Underwater communication system using micro LEDs
Figure 32
(Color online) DL-aided Bayesian optimal estimators for physical layer communications.
Figure 33
The framework of 5G network optimization based on DRL with the help of physical model.
Figure 34
(Color online) An illustration of virtual vehicular networks.
Figure 35
(Color online) The self-evolution closed-loop structure of the intelligence-endogenous network.
| Abbreviation | Definition | Abbreviation | Definition |
| 2D | two-dimensional | MBD | mobile big data |
| 3D | three-dimensional | MCS | modulation coding scheme |
| 3GPP | 3rd generation partnership project | MEC | mobile edge computing |
| 4G | fourth generation | MIMO | multiple-input multiple-output |
| 5G | fifth generation | ML | machine learning |
| 5G-ACIA | 5G alliance for connected industries and automation | MLSE | maximum likelihood sequence estimation |
| 5G PPP | 5G infrastructure public private partnership | MMSE | minimum mean squared error |
| 6G | sixth generation | mMTC | massive machine type communications |
| ACK | acknowledgment | mmWave | millimeter wave |
| A-CPI | application-controller plane interface | MPC | multipath component |
| A/D | analog-to-digital | MR | maximum ratio |
| AI | artificial intelligence | MRC | maximum ratio combining |
| AMF | access and mobility management function | MRT | maximum ratio transmission |
| ANN | artificial neural network | MS | mobile station |
| AP | access point | MTC | machine type communication |
| AR | augmented reality | MUST | multi-user superposition transmission |
| ASK | amplitude shift keying | MZM | Mach-Zehnder modulator |
| AUSF | authentication server function | NAS | non-access stratum |
| BB | baseband | NEF | network exposure function |
| BCH | Bose-Chaudhuri-Hocquenghem | NFV | network functions virtualization |
| BCJR | Bahl, Cocke, Jelinek, and Raviv | NFVI | NFV infrastructure |
| BER | bit error rate | NFV M&O | NFV management and orchestration |
| BPF | bandpass filter | NGMN | next-generation mobile network |
| B-RAN | blockchain radio access network | NG-RAN | next-generation radio access network |
| BS | base stations | NLOS | non-line-of-sight |
| BSS | business support system | NOMA | non-orthogonal multiple access |
| CC | central controller | NP | non-deterministic polynomial |
| CF | cell-free | NR | new radio |
| CGM | continuous glucose monitoring | NRF | network function repository function |
| CIR | channel impulse response | NR-V | new radio vehicle |
| CNN | convolutional neural network | NWDA | network data analytics |
| CoMP | coordinated multi-point | OAM | orbital angular momentum |
| COTS | commercial off-the-shelf | OFDM | orthogonal frequency-division multiplexing |
| CPM | continuous phase modulation | OMA | orthogonal multiple access |
| CPU | central processing units | ONF | open networking foundation |
| CR | cognitive radio | O-RAN | open radio access network |
| CRC | cyclic redundancy check | OSS | operations support system |
| CSA | cognitive service architecture | OTFS | orthogonal time frequency space |
| CSI | channel state information | OWC | optical wireless communication |
| CT | computed tomography | P2P | peer-to-peer |
| C-V2X | cellular vehicle to everything | PAPR | peak to average power ratio |
| D/A | digital-to-analog | PCC | parallel concatenated codes |
| DAS | distributed antenna system | PCF | policy control function |
| D-CPI | data-controller plane interface | PDA | placement delivery array |
| DEN$^2$ | deep edge node and network | probability density function | |
| DetNet | deterministic networking | PDP | power delay profile |
| DL | deep learning | PLS | physical layer security |
| DN | data network | PSD | power spectral density |
| DNN | deep neural network | QAM | quadrature amplitude modulation |
| DOF | degree of freedom | Q-D | quasi-deterministic |
| DRL | deep reinforcement learning | QKD | quantum key distribution |
| (To be continued on the next page | |||
| Use case (high level) | Availability | Cycle time | Typical payload | # of | Typical service area | ||
| (%) | (ms) | size | devices | ||||
| Motion control | Printing machine | textgreater99.9999 | textless 2 | 20 bytes | textgreater100 | 100 m$\times$100 m$\times$30 m | |
| Machine tool | textgreater99.9999 | textless 0.5 | 50 bytes | $\sim$20 | 15 m$\times$15 m$\times$3 m | ||
| Packaging machine | textgreater99.9999 | textless 1 | 40 bytes | $\sim$50 | 10 m$\times$5 m$\times$3 m | ||
| Mobile robots |
| textgreater99.9999 | 1 | 40–250 bytes | 100 | $<$ 1 km$^2$ | |
| textgreater99.9999 | 10–100 | 15–150000 bytes | 100 | $<$ 1 km$^2$ | ||
| Mobile control | Assembly robots or | textgreater99.9999 | 4–8 | 40–250 bytes | 4 | 10 m$\times$10 m | |
| panels with | milling machines | ||||||
| safety functions | Mobile cranes | textgreater99.9999 | 12 | 40–250 bytes | 2 | 40 m$\times$60 m | |
| Process automation (process monitoring) | textgreater99.99 | textgreater 50 | Varies | 10000 devices per km$^2$ | |||
| (Continued | |||
| Abbreviation | Definition | Abbreviation | Definition |
| EAP | extensible authentication protocol | QoE | quality of experience |
| ECG | electrocardiogram | QoS | quality of service |
| EEG | electroencephalogram | QSDC | quantum secure direct communication |
| EIRP | effective isotropic radiated power | RAN | radio access network |
| eMBB | enhanced mobile broadband | RAU | remote antenna unit |
| EMG | electromyogram | RF | radio frequency |
| EMS | element management system | RMS | root mean square |
| ETSI | European telecommunication standards institute | RSC | radio side controller |
| euRLLC | enhanced-uRLLC | RSS | received signal strength |
| EXIT | extrinsic information transfer | RTT | round trip time |
| FBMC | filter bank multi-carrier | Rx | receiver |
| FCC | federal communications commission | RZF | regularized zero-forcing |
| FDD | frequency division duplex | SBA | service-based architecture |
| FDMA | frequency division multiple access | SC | successive cancellation |
| FEC | forward error correction | SCC | serially concatenated code |
| FSO | free space optical | SCM | spatial channel model |
| FTTA | fiber-to-the-antenna | SCMA | sparse code multiple access |
| GBSM | geometry-based stochastic model | SDN | software defined network |
| GFDM | generalized frequency division multiplexing | SFFT | symplectic finite Fourier transform |
| GPS | global positioning system | SGS | satellite ground station |
| HA | horn antenna | SIMO | single-input multiple-output |
| HAP | high altitude platform | SINR | signal-to-interference-plus-noise ratio |
| HARQ | hybrid automatic repeat request | SISO | single-input single-output |
| HCC | hybrid concatenated codes | SLC | satellite local controller |
| HD | high definition | SMF | session management function |
| HST | high-speed train | SNR | signal-to-noise ratio |
| IC | integrated circuits | SR | symbiotic radio |
| ICDT | information, communication, and data technology | S-V | Saleh-Valenzuela |
| ICI | inter-carrier interference | TCP | transmission control protocol |
| ICT | information and communication technology | TDD | time division duplex |
| IEN | intelligence endogenous network | TLC | terrestrial local controller |
| IETF | Internet engineering task force | THz | terahertz |
| ILDP | interactive learning design paradigm | ToF | THz-over-fiber |
| IoE | Internet of everything | TSAP | terrestrial-satellite access point |
| IoT | Internet of things | Tx | transmitter |
| IP | Internet protocol | UAV | unmanned aerial vehicle |
| IRS | intelligent reflection surface | UDM | unfied data management |
| ISFFT | inverse symplectic finite Fourier transform | UDN | ultra-dense network |
| ISM | industrial, scientific, and medical | UE | user equipment |
| IT | information technology | uHDD | ultra-high data density |
| ITS | intelligent transportation system | uHSLLC | ultra-high-speed with low-latency |
| ITU | international telecommunication union | communications | |
| KPI | key performance indicator | uMUB | ubiquitous mobile ultra-broadband |
| LAP | low altitude platform | UPF | user plane function |
| LDPC | low-density parity-check code | uRLLC | ultra-reliable and low latency communications |
| LED | light emitting diode | UTC-PD | uni-traveling-carrier photodiode |
| LEO | low Earth orbit | UV | ultraviolet |
| LO | local oscillator | V2V | vehicle-to-vehicle |
| LOS | line of sight | V2X | vehicle to everything |
| LTE | long term evolution | VHF | very high frequency |
| LTE-LAA | LTE license assisted access | VLC | visible light communication |
| LTE-U | LTE unlicensed | VNF | virtualized network function |
| LTE-V | long time evolution vehicle | VR | virtual reality |
| MAC | media access control | XR | extended reality |
| MAEC | multi-access edge computing | ZF | zero forcing |
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