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.

The Road Towards 6G: A Comprehensive Survey

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

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Rising popularity of 5G communications is making tremendous demands on the cellular network operators for providing true 5G services to the users. With limited numbers of 5G users initially, the investments for 5G services can be very high. In the early stage of 5G deployments, the 5G cells would not be lavishly spread and there would be 5G coverage holes. The operators can provide seamless services to the 5G users by inter working with the existing 4G Long-Term Evolution (LTE) network. The 5G inter working with fully deployed LTE would not only provide fast and seamless coverage but would also provide economic viability to the network operators. In this paper we survey and consolidate the 4G-5G inter working solutions that can assist in attaining the insight about various inter working possibilities and their challenges. It is important that a network operator is able to optimize its deployed infrastructure while being able to guarantee fast and seamless transition to 5G for its subscribers. To this regard, we evaluate the performance and radio resource management challenges for different 4G-5G dual connectivity options proposed by 3rd Generation Partnership Project (3GPP) standardization. We also discuss spectrum sharing possibilities between 4G and 5G wireless networks. Finally, various research challenges and discussions on path for migration to 5G standalone networks are also presented.

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A Survey on 4G-5G Dual Connectivity: Road to 5G Implementation

Sixth-generation (6G) is envisioned to be characterized by ubiquitous connectivity, extremely low latency, and enhanced edge intelligence. However, enriching 6G with these features requires addressing new, unique, and complex challenges specifically at the edge of the network. In this article, we propose a wireless digital twin edge network model by integrating digital twin with edge networks to enable new functionalities, such as hyper-connected experience and low-latency edge computing. To efficiently construct and maintain digital twins in the wireless digital twin network, we formulate the edge association problem with respect to the dynamic network states and varying network topology. Furthermore, according to the different running stages, we decompose the problem into two subproblems, including digital twin placement and digital twin migration. Moreover, we develop a deep reinforcement learning (DRL)-based algorithm to find the optimal solution to the digital twin placement problem, and then use transfer learning to solve the digital twin migration problem. Numerical results show that the proposed scheme provides reduced system cost and enhanced convergence rate for dynamic network states.

Adaptive Edge Association for Wireless Digital Twin Networks in 6G

Vehicular intelligence in 6G: Networking, communications, and computing

With the 5th Generation (5G) Mobile network being rolled out gradually in 2019, the research for the next generation mobile network has been started and targeted for 2030. To pave the way for the development of the 6th Generation (6G) mobile network, the vision and requirements should be identified first for the potential key technology identification and comprehensive system design. This article first identifies the vision of the society development towards 2030 and the new application scenarios for mobile communication, and then the key performance requirements are derived from the service and application perspective. Taken into account the convergence of information technology, communication technology and big data technology, a logical mobile network architecture is proposed to resolve the lessons from 5G network design. To compromise among the cost, capability and flexibility of the network, the features of the 6G mobile network are proposed based on the latest progress and applications of the relevant fields, namely, on-demand fulfillment, lite network, soft network, native AI and native security. Ultimately, the intent of this article is to serve as a basis for stimulating more promising research on 6G.

Vision, requirements and network architecture of 6G mobile network beyond 2030

The fifth generation (5G) mobile network is standardized and developed to explore the mobile market beyond 2020. In response to the diverse strategies of 5G deployment, five alternative network architectures have been proposed to 3GPP by different mobile operators. To fulfill the urgent deployment requirement from some operators, an early drop of 5G, termed as non-standalone (NSA) new radio (NR), was completed at the end of 2017. After that, the standardization of a new 5G system, including th standalone (SA) new radio access network, was finished in June 2018. This article analyzes and compares the SA NR and NSA NR deployment modes in terms of coverage, network capability, interworking between 4G and 5G, complexity and cost of network deployment, and the latest industry progress. NSA NR performs better in interworking performance in the initial phase, while SA NR performs better in network capabilities, device performance, simple network deployment, and cost efficiency. 5G SA NR is recommended for operators who have the ambition to explore new opportunities in the vertical and enterprise markets.

5G Deployment: Standalone vs. Non-Standalone from the Operator Perspective

The fast deployment and penetration of 4G has cultivated human behaviors on mobile data consumption, leading to explosive growth in mobile traffic and stimulating new requirements on the capabilities of mobile networks. To meet the requirements of mobile networks toward year 2020, the next generation of mobile networks (termed as IMT-2020, or 5G) is designed to support 100 Mbps-1 Gbps user-experienced data rate, 1 ms radio transmission latency, and 1 million connections per square kilometer. Recalling the vision and requirements of 5G targeting for commercial launch in 2020, this article overviews the key features of 5G and compares with those of 4G, and reports the world first field trials conducted to validate the key performance of 5G radio interface in 3.5 GHz band. The trial results show that a 1 ms transmission latency and 1 Gbps data rate are achievable.

5G features from operation perspective and fundamental performance validation by field trial

Visible light communications (VLC) is considered as an effective supplement technology for next-generation (6G) communications due to its abundant spectrum, high power efficiency and easy deployment. Optical orthogonal frequency division multiplexing (O-OFDM) is a common technology to obtain further promotion. In this paper, two typical O-OFDM schemes direct current biased O-OFDM (DCO-OFDM) and asymmetrically clipped O-OFDM (ACO-OFDM) are analyzed in terms of signal clipping at both transmitter and receiver under the constraints of maximum optical power and non-negative optical power. And effective electrical SNR models after signal clipping are proposed and verified by link simulation. Then a noise cancellation scheme is proposed based on received signal clipping and is proved to bring a significant gain for ACO-OFDM. By system simulation, we find that under a certain optical power limitation, most users can achieve above 4Gbps in indoor scenario when modulation bandwidth of the light emit diode (LED) or laser diode (LD) is 1GHz. Therefore, it can be expected that the throughput could reach tens Gbps when the LED/LD modulation bandwidth is increased and multiple LEDs/LDs are deployed.

Signal clipping at transmitter and receiver of O-OFDM for VLC under optical power constraint

Large number of antennas in transmitter and receiver side of 3D-MIMO can provide finer spatial granularity of beams and larger beamforming gain, and the number of simultaneously served UEs may also be increased compared with conventional LTE system. To support increased multi-user capacity in 3D-MIMO, DMRS needs to be further enhanced because DMRS only support up to 4 layers multi-user MIMO (MU-MIMO) transmission in current LTE specification, each layer associated with one DMRS (demodulation reference signal) port, and among the four MU-MIMO layers, two layers are orthogonal, and others are quasi-orthogonal. In this paper, link level evaluation of several enhance alternatives are analysed, considering mobility, beamforming algorithm, and orthogonal/quasi-orthogonal design. The results show that orthogonal DMRS transmission provides significant channel estimation gain than quasi-orthogonal. Therefore, in order to guarantee performance of large multiplex number of UEs, increased number of orthogonal DMRS ports is preferred. Larger orthogonal cover code (OCC) length provides good performance at lower mobility, at the same time maintaining less resource overhead, therefore slightly preferred for 3D-MIMO deployment.

Yuhong Huang

Papers

Vision, requirements and network architecture of 6G mobile network beyond 2030

5G Deployment: Standalone vs. Non-Standalone from the Operator Perspective

5G features from operation perspective and fundamental performance validation by field trial

Signal clipping at transmitter and receiver of O-OFDM for VLC under optical power constraint

Reference signal design for demodulation of higher-order MU-MIMO in 3D-MIMO systems