IEEE TRANSACTIONS ON NETWORK AND SERVICE MANAGEMENT, VOL. XX, NO. XX, XX 2021 1 A Comprehensive Survey on 6G Networks: Applications, Core Services, Enabling Technologies, and Future Challenges Amin Shahraki*, Member, IEEE, Mahmoud Abbasi, Member, IEEE, Md. Jalil Piran*, Senior Member, IEEE and Amir Taherkordi [Note: This work has been submitted to the IEEE Trans- generations of cellular networks due to the limitation of the actions on Network and Service Management journal for previous generations. possible publication. Copyright may be transferred without Since 2019, the fifth-generation (5G) of cellular networks notice, after which this version may no longer be accessible ] Abstract—Cellular Internet of Things (IoT) is considered as de is commercially available in several countries [3]. 5G uses facto paradigm to improve the communication and computation revolutionary technologies, e.g., higher frequencies, network systems. Cellular IoT connects massive number of physical function virtualization (NFV), software defined networking and virtual objects to the Internet using cellular networks. (SDN), and network slicing and evolutionary technologies, The latest generation of cellular networks, e.g. fifth-generation e.g., massive multiple-input and multiple-output (MIMO) to (5G), use evolutionary and revolutionary technologies to notably improve the performance of wireless networks. However, given make a significant improvement in data rates, energy effi- the envisioned new use-cases, e.g., holographic communication, ciency, reliability, and connection density. Meanwhile, 5G and the ever-increasing deployment of massive smart-physical network is used in a wide range of applications such as IoT, end-devices in IoT, the volume of network traffic has considerably smart city, Industry 4.0 [4], e-health, wearables, smart utilities raised, and therefore, the current generation of mobile networks [5], etc. Generally, the main 5G service classes include: 1) cannot wholly meet the ever-increasing demands. Hence, it is envisioned that the next generation, sixth generation (6G) enhanced mobile broadband (eMBB), 2) ultra-reliable low- networks, need to play a critical role to alleviate such challenges latency communications (URLLC), and 3) massive machine- in IoT by providing new communication services, network type communication (MTC) (MTC). eMBB is capable of capacity, and ultra-low latency communications (uRLLC). In providing high data rates of 1G bits/s for mobile users [6] this paper, first, the need for 6G networks is discussed. Then, while URLLC focuses on the reliability (99.999%) and latency the potential 6G requirements and trends, as well as the latest research activities related to 6G are introduced e.g., Tactile (milliseconds) of the communication, especially for the appli- Internet and Terahertz (THz). Furthermore, the key performance cations such as industrial IoT (IoT) and vehicle to everything indicators, applications, new services, and the potential key (V2X) [6]. mMTC emphasizes on the number of connected enabling technologies for 6G networks are presented. Finally, devices in IoT employment (up to 1 million connections per several potential unresolved challenges for future 6G networks km2) [7]. are presented. However, By taking the present-day and emerging advance- Index Terms—Internet of Things (IoT), 6G, 5G, uRLLC, THz, ments of wireless communications into account, 5G may not Tactile Internet, cellular IoT. meet the future demands for the following reasons: • Given the ever-increasing growth of the deployment of I. INTRODUCTION IoT devices, there is a particular need to improve further the connection density (10 million connections per km2) arXiv:2101.12475v2 [cs.NI] 13 Jun 2021 Internet of things (IoT) is the most prevalent framework and coverage of 5G-enabled IoT networks [8], [9]. in the realm of Information and Communication Technology • New emerging services of IoT such as extended real- (ICT). Cisco predicted that by 2023 there will be 14.7 billion ity (XR), telemedicine systems, mind-machine interface devices connect to IoT [1] with various applications that are (MMI), and flying cars will challenge the original 5G generating a huge volume of data [2]. Moreover, the popularity service classes. To effectively provide IoT services such of multimedia services over wireless networks is exploding as XR and telemedicine systems for mobile devices, [1]. Therefore, supporting such a volume of data demands new future mobile networks must simultaneously provide high transmission rates, high reliability, and low latency, which Amin Shahraki (Corresponding Author) is with the Department of Infor- matics, University of Oslo, Oslo, Norway (e-mail: [email protected]) significantly exceeds the original goals of the 5G net- Mahmoud Abbasi is with Islamic Azad University, Mashhad, Iran (email: works [10]–[12]. [email protected]) • It is expected that future mobile networks will be an Md. Jalil Piran (Corresponding Author) is with the Department of Computer Science and Engineering, Sejong University, Seoul, 05006, South Korea ultra-large-scale, highly dynamic, and incredibly complex (email: [email protected]) system, e.g. the massive and heterogeneous devices in Amir Taherkordi is with University of Oslo, Oslo, Norway (email: the IoT. However, the architecture of the current wireless amirhost@ifi.uio.no) Manuscript received xxx xx, 2021; revised xxx xx, 2021. networks (e.g., 4G and 5G) are often fixed, and the *Equal contribution in case of corresponding author optimization tasks are defined to cope with specific and IEEE TRANSACTIONS ON NETWORK AND SERVICE MANAGEMENT, VOL. XX, NO. XX, XX 2021 2 Figure 1: The organizational structure of the survey. 3CLS Control, Localization, and Sensing 3D 3-Dimensional I. Introduction 3GPP 3rd generation partnership project II. Related Work AES Advanced Encryption Standard AI artificial intelligence III. Requirements AID Autonomous intelligent driving and Trends AMP approximate message passing The key KPIs APs access points IV. Research and Use-cases AR augmented reality Activities and AU aerial user Motivation New Service Classes CAD cooperative automated driving 6G Survey CRAN cloud radio access network V.Evolutionary CSI channel state information Technologies D2D Device-to-device VI.Revolutionary DL deep learning Technologies E2E end-to-end VII. Challenges ECC Elliptic Curve Encryption and Future EI Edge Intelligence Directions EM emit electromagnetic eMBB enhanced mobile broadband VIII. Conclusion ETSI European Telecommunications Stan- dards Institute GPS global positioning system identified challenges and services. Hence, the prevailing HMIMOS holographic MIMO surface manual-based optimization and configuration tasks are no ICT Information and Communication longer appropriate for future networks [6], [13]–[15]. Technology To deal with the challenges mentioned above, 6G networks are IDS intrusion detection system expected to provide new service classes, use new spectrum for IIoT industrial IoT wireless communications, enormous network capacity, ultra- IoT Internet of things low latency communications, and adopt novel energy-efficient IR infrared transmission methods [16]. ITU International Telecommunication We aim to present a comprehensive survey on 6G cellular Union networks by considering a wide ranges of 6G aspects. Our KPI key performance indicator contributions can be summarized, but not limited, as follows. LIS Large Intelligent Surfaces • Discussing the need of a new generation of cellular LoRaWAN Long Range Wide Area Network networks beyond 5G. LoS line-of-sight • Providing a detailed review on the existing works on 6G. MEC mobile edge computing • Introducing the 6G key performance indicators (KPIs) MIMO multiple-input and multiple-output and new use-cases e.g., holographic communication and ML machine learning Industrial automation. MMI mind-machine interface • Studying various potential enabling technologies that will mMTC massive MTC have important contributions in 6G. mmWave millimetre wave • highlighting the potential 6G requirements, challenges, MTC machine-type communication and trends e.g., Green 6G and 3D coverage. NB-IoT Narrow band IoT • Outlining several 6G future research directions. NFV network function virtualization The rest of this paper is organized as follows. Section II NMO Network Management and Orchestra- reviews the related survey and magazine articles on 6G. In tion Section III, we present the requirements and trends of 6G. NOMA Non-orthogonal multiple access The research activities and motivation are discussed in Section NTC network traffic classification IV. Moreover, in this section, we provide a comprehensive list NTMA network traffic monitoring and anal- of 6G KPIs and use-cases, as well as new service classes. In ysis Sections V and VI, we introduce the important evolutionary NTP network traffic prediction and revolutionary technologies respectively that contribute to OFDM Orthogonal frequency-division multi- the future 6G networks. We discuss several 6G challenges in plexing Section VII. Finally, Section VIII draws the conclusion. OMA orthogonal multiple access OWC optical wireless communication LIST OF ABBREVIATIONS IEEE TRANSACTIONS ON NETWORK AND SERVICE MANAGEMENT, VOL. XX, NO. XX, XX 2021 3 PLMN public land mobile network highlight that these changes are essential for cellular networks QC quantum computing to satisfy future use cases’ demand. In [21], the authors QKD quantum key distribution paint a comprehensive picture of 6G, foreseen applications,