With the large-scale commercialization of 5th generation (5G) network globally，5G together with artificial intelligence (AI)，cloud and big data is accelerating the digitalization of the world，which push the society move towards a “digital twin”world composed of the physical world and its digital mirror.The digital twin world will enable us to predict the future and reshape the future in social governance and many industries.The ubiquitous intelligence will enable intelligent connection anytime and any place and make an intelligent world.The 6th generation (6G) mobile network is expected to be commercialized towards 2030.

New services and application scenarios such as holographic interaction，immersive social networking，sensory connection，and smart transportation will be promising usage scenarios towards 2030 and demand higher performance of 6G mobile communication network，such as extremely high data rate and capacity，ultra-low latency jitter，ultra-high security，three-dimensional coverage，ultra-high positioning accuracy，better energy efficiency and cost efficiency，etc.The requirements on the future network will certainly push the breakthroughs in new technologies，included but not limited to full-frequency access，multiple input and multiple output (MIMO)evolution，new coding and modulation，terahertz and visible light communication，new waveform and multiple access，integration of communication and sensing，integration of terrestrial network and satellite，and novel network architecture driven by information technology，communication technology，big data technology (ICDT) convergence.Depending on these potential enabling technologies，future network capabilities will be greatly enhanced to provide users with richer services and applications，adapt to the sliced and differentiated service and application scenarios.

Besides，6G network is expected to solve the issues faced by current 5G mobile network，e.g.expensive cost，high power consumption，and complicated network operation and maintenance.To achieve these objectives，6G mobile network will be featured by on demand fulfillment network，lite network，soft network，native AI，native security and digital twin network.

Next generation mobile network (NGMN)，as an alliance of global mobile operator，has started its work on 6G and published its white paper on vision and use case of 6G.The working part 5D of radio communication sector，international telecommunication union，has also started its study on 6G and targeted for the large scale commercial launch towards 2030.It is predicted that 3rd generation partner project (3GPP)，a key standardization organization of mobile industry，will initiate its 6G study around 2025.The next few years will be the key time window for the theoretical and technology breakthrough for 6G mobile communication network.

The primary goal of this feature topic is to present the state-of-the-art high-quality research advances in the theoretical and technical innovations of 6G mobile communications，including new business and application scenarios，full-frequency access，new wireless transmission technology，novel network architecture，integration of communication and sensing/computing，digital twin and network autonomy，big data and native-AI enabled network，cloud-native networks，etc.

All of the submitted papers are evaluated according to the standard reviewing process of China Communications.Following a rigorous peer-review process，9 articles are finally accepted in this special issue.

The accepted papers cover the topics about service-based radio access network (RAN)，native AI enabled network，the application of federated learning (FL) in 6G，unsourced multiple access (UMA)，sparse array based massive multiple-input multiple-output (MIMO)，joint source-channel coding(JSCC)，and orthogonal time frequency space(OTFS).We hope this special issue may inspire the existing and important future research works in 6G.Service-based architecture (SBA) as one of the promising information technology，has been preliminary introduced into the 5G core network and successfully commercialized，which verifies its feasibility and effectiveness.In the paper “Micro-service-based Radio Access Network”，Na Li，et al.proposes a service-based RAN from a protocol stack perspective.The RAN control plane (CP) services are deduced under the proposed guidelines and system operations between the CP services are defined.Fundamental problems when designing SBA RAN user plane are raised and potential solutions are given.In addition，the possible performance advantages and challenges of SBA-RAN are detailed discussed.

The 6G architecture will support native AI to enable the network agile and able to adapt itself to both predicted and unpredicted service demands，and a network brain is expected to make decisions upon inference about the network environments to maximize the service utilities and economic benefits.In the paper “Intelligent Decision Making Framework for 6G Network”，Zheng Hu et al.proposes an intelligent decision making (IDM) framework，acting as the role of network brain，based on reinforcement learning modelling philosophy to empower autonomous intelligence evolution capability to 6G network.Besides，usage scenarios and simulation demonstrate the generality and efficiency of IDM.

The next paper entitled “Cloud-assisted distributed edge brains for multi-cell joint beamforming optimization for 6G” by Juan Deng，et al.is also on the framework for model training and inference.The authors further consider the issues of centralized and decentralized architectures and propose a novel solution based on a collaborative cloud edge architecture for multi-cell joint beamforming optimization.Compared with the centralized solution，the proposed solution improves prediction accuracy by 24.66%，and reduces storage cost by 83.82%.Compared with the decentralized solution，it improves prediction accuracy by 68.26%，and improves coverage performance by 0.4dB.

Green design is an important issue of 6G.In the paper “Green Concerns in Federated Learning over 6G”，Borui Zhao，et al.focuses on green concerns in FL over 6G.It first analyzes and summarizes major energy consumption challenges caused by technical characteristics of FL and the dynamical heterogeneity of 6G networks，and models the energy consumption in FL over 6G，then classifies the basic ways to reduce energy and presents several feasible green designs for FL-based 6G network.A useful guideline is proposed based on the simulation results that different schemes should be used to achieve the minimum energy consumption at a reasonable cost of learning accuracy for different network scenarios and service requirements.

UMA is a multi-access technology for massive，low-power，uncoordinated，and unsourced Machine Type Communication (MTC) networks.It ensures transmission reliability under the premise of high energy efficiency.The critical technology of UMA is the design of its multiple-access coding scheme.In the paper “Unsourced Multiple Access for 6G Massive Machine Type Communications”，Yuanjie Li，et al.comprehensively considers the energy efficiency and computational complexity of the existing UMA coding schemes，studies the changes of the above two indexes with the increase of access scale，and considers the trade-off between the two.It is revealed by the above analysis that some guiding rules of UMA coding design.

Massive MIMO technology enables higher data rate transmission in the future mobile communications，however，exploiting a large number of antenna elements at base station makes effective implementation of massive MIMO challenging.In order to miniaturize the massive MIMO，it is crucial to reduce the number of antenna elements via effective methods such as sparse array synthesis.In the paper“Performance Analysis of Sparse Array based Massive MIMO via Joint Convex Optimization”，Mengting Lou，et al.proposes a joint convex optimization(JCO) based synthesis to construct a codebook for beamforming.The results show that with the same number of antenna elements，sparse array using the proposed JCO-based synthesis outperforms not only the uniform array，but also the sparse array with the existing CO-based synthesis method.Furthermore，with a half of the number of antenna elements that on the uniform array，the performance of the JCO-based sparse array approaches to that of the uniform array.

JSCC as the intersection of two main branches of Shannon theory，has always attracted the interest of researchers.In the paper “Joint Source-Channel Coding for 6G Communications”，Yanfei Dong et al.gives a comprehensive tutorial on the topic of low-density parity-check (LDPC) /polar-based JSCC for future 6G communications.In addition，it focuses on two new JSCC schemes called the double LDPC codes and the double polar codes，respectively，giving their detailed coding and decoding processes and corresponding performance simulations.The performance evaluation illustrates that these two JSCC schemes can achieve significant gains under long and short codes，respectively.

OTFS is another important topic of 6G which is to combat the Doppler effect.Previous works have studied the multi-antenna transmitter diversity scheme with the ideal pulse shaping in OTFS systems assuming the wireless channels of two consecutive frames duration are invariant.However，the ideal pulse shaping can not be realized in practice and the channel varies rapidly in two consecutive frames in the high-speed mobility scenarios.In the paper “Transmit Diversity Scheme Design for Rectangular Pulse Shaping Based OTFS”，Wang Dong，et al.proposes a multi-antenna transmitter diversity scheme to ensure its practicability in the rectangular-pulse based OTFS systems.Simulation results validate that the bit error rate performance of the proposed scheme has the superiority over the existing works.

Finally，the Guest Editors of this feature topic would like to thank all the authors for their valuable contributions and the anonymous reviewers for their helpful comments and suggestions.Also，we would like to acknowledge the guidance from the editorial team of China Communications.