As of today, the fifth generation (5G) mobile communication system has been rolled out in many countries and the number of 5G subscribers already reaches a very large scale. It is time for academia and industry to shift their attention towards the next generation. At this crossroad, an overview of the current state of the art and a vision of future communications are definitely of interest. This article thus aims to provide a comprehensive survey to draw a picture of the sixth generation (6G) system in terms of drivers, use cases, usage scenarios, requirements, key performance indicators (KPIs), architecture, and enabling technologies. First, we attempt to answer the question of “Is there any need for 6G?” by shedding light on its key driving factors, in which we predict the explosive growth of mobile traffic until 2030, and envision potential use cases and usage scenarios. Second, the technical requirements of 6G are discussed and compared with those of 5G with respect to a set of KPIs in a quantitative manner. Third, the state-of-the-art 6G research efforts and activities from representative institutions and countries are summarized, and a tentative roadmap of definition, specification, standardization, and regulation is projected. Then, we identify a dozen of potential technologies and introduce their principles, advantages, challenges, and open research issues. Finally, the conclusions are drawn to paint a picture of “What 6G may look like?.” This survey is intended to serve as an enlightening guideline to spur interests and further investigations for subsequent research and development of 6G communications systems.

Next-generation cellular technologies, commonly referred to as the sixth generation (6G), are envisioned to support a higher system capacity, better performance, and network sensing capabilities. The terahertz (THz) band is one potential enabler to this end due to the large unused frequency bands and the high spatial resolution enabled by the short signal wavelength and large bandwidth. Different from earlier surveys, this paper presents a comprehensive treatment and technology survey on THz communications and sensing in terms of advantages, applications, propagation characterization, channel modeling, measurement campaigns, antennas, transceiver devices, beamforming, networking, the integration of communications and sensing, and experimental testbeds. Starting from the motivation and use cases, we survey the development and historical perspective of THz communications and sensing with the anticipated 6G requirements. We explore the radio propagation, channel modeling, and measurement for the THz band. The transceiver requirements, architectures, technological challenges, and state-of-the-art approaches to compensate for the high propagation losses, including appropriate antenna design and beamforming solutions. We overview several related technologies that either are required by or are beneficial for THz systems and networks. The synergistic design of sensing and communications is explored in depth. Practical trials, demonstrations, and experiments are also summarized. The paper gives a holistic view of the current state of the art and highlights the open research challenges towards 6G and beyond.

We argue for network slicing as an efficient solution that addresses the diverse requirements of 5G mobile networks, thus providing the necessary flexibility and scalability associated with future network implementations. We elaborate on the challenges that emerge when designing 5G networks based on network slicing. We focus on the architectural aspects associated with the coexistence of dedicated as well as shared slices in the network. In particular, we analyze the realization options of a flexible radio access network with focus on network slicing and their impact on the design of 5G mobile networks. In addition to the technical study, this article provides an investigation of the revenue potential of network slicing, where the applications that originate from this concept and the profit capabilities from the network operator s perspective are put forward.

The Fifth Generation (5G) of mobile communication system aims to deliver a ubiquitous mobile service with enhanced Quality of Service (QoS). It is also expected to enable new use-cases for various vertical industrial applications – such as automobiles, public transportation, medical care, energy, public safety, agriculture, entertainment, manufacturing, etc. Rapid increases are predicted to occur in user density, traffic volume, and data rate. This calls for novel solutions to the requirements of both mobile users and vertical industries in the next decade. Among various available options, one that appears attractive is to redesign the network architecture – more specifically, to reconstruct the Radio Access Network (RAN). In this article, we present an inclusive and comprehensive survey on various RAN architectures towards 5G, namely Cloud-RAN, Heterogeneous Cloud-RAN, Virtualized Cloud-RAN and Fog-RAN. We compare them from various perspectives such as energy consumption, operations expenditure, resource allocation, spectrum efficiency, system architecture, and network performance. Moreover, we review the key enabling technologies for 5G systems, such as Multi-access Edge Computing, Network Function Virtualization, Software Defined Networking and Network Slicing; and some crucial Radio Access Technologies (RATs), such as Millimeter Wave, Massive Multi-Input Multi-Output, Device-to-Device Communication and Massive Machine-Type-Communication. Last but not least, we discuss the major research challenges in 5G RAN and 5G RATs, and identify several possible directions of future research.

The global roll-out phase of the fifth-generation of mobile communication systems is currently underway. The industry and academia have already begun research on potential sixth-generation (6G) communication systems. The 6G communication system is anticipated to provide network connectivity for an extensive range of use cases in a variety of emerging vertical industries. Consequently, a new set of challenging requirements and more stringent key performance indicators have to be considered, a novel architecture has to be designed, and unique enabling technologies shall be developed in order to fulfill the technical, regulatory, and business demands of the communication service customers. These requirements place enormous pressure on the players in the telecommunications industry, including network operators, service providers, hardware suppliers, standards development organizations (SDOs), and regulatory authorities aimed at developing, standardizing, and regulating an energy-efficient, cost-effective, performing, and sustainable 6G communication ecosystem. One area of focus for 6G communication systems is the digital twin (DT) technology, which is a well-defined set of tools designed to create virtual representations of physical objects that serve as their digital counterparts. This article explores the applicability of the DT technology in the context of 6G communication systems by viewing it as a promising tool to make research, development, operation, and optimization of the next-generation communication systems highly efficient. The major contribution of this article is fivefold. Firstly, we provide critical analysis of the state-of-the-art literature in the field of DT technology in order to capture its essence in several application areas since its inception. Secondly, we conduct a comprehensive survey of the research concerning the deployment of DT technology in 6G communication systems. Thirdly, we discuss potential use cases and key areas of applications (along with detailed examples) of 6G communication systems that can benefit from DT technology. Fourthly, we present an overview of the activities of several SDOs that are active in the field of DT technology. Finally, we identify several open research challenges and future directions that need to be addressed before the end-to-end deployment of DT technology in 6G communication systems.

In the context of fifth-generation mobile networks, the concept of “Slice as a Service”promotes mobile network operators to flexibly share infrastructures with mobile service providers and stakeholders. However, it also challenges with an emerging demand for efficient online algorithms to optimize the request-and-decision-based inter-slice resource management strategy. Based on genetic algorithms, this paper presents a novel online optimizer that efficiently approaches toward the ideal slicing strategy with maximized long-term network utility. The proposed method encodes slicing strategies into binary sequences to cope with the request-and-decision mechanism. It requires no a priori knowledge about the traffic/utility models and therefore supports heterogeneous slices while providing solid effectiveness, good robustness against non-stationary service scenarios, and high scalability.