6G New Multiple Access Technology

Yifei Yuan

As a basic technology at physical layer of mobile communications,non-orthogonal multiple access has been attracting wide attention across the academia and the industry.During the standardization of the fifth-generation (5G) of mobile communications,3GPP conducted preliminary study on non-orthogonal multiple access without reaching the consensus to standardize the technology.Since 2017,there have been a number of breakthroughs in the area of massive access by academic researchers.With the consideration of finite codeword length,several information-theoretic capacity bounds were derived for non-orthogonal unsourced transmission,showing the enormous performance advantage over orthogonal multiple access.Meanwhile,channel coding for multi-user systems has become a hot topic in academia,and is being combined with the compressed sensing technology for active user detection and the estimation of wireless channels in massive access.Extensive papers on scheduling-based superposition transmissions are also seen in academia,for instance,rate-splitting multiple access (RSMA),etc.In early 2022,an emerging technology initiative(ETI) was formed in IEEE–Next-Generation Multiple Access (NGMA) with the purpose of encouraging research on multiple access in academia and industry,and their collaboration.In May 2022,new multiple access (NMA) task force group was created in IMT-2030 Promotion Group of China,with the hope to accelerate the study on transmission schemes,channel coding and modulation,receiver algorithms,system performance analysis,etc.The NMA group would facilitate the early harmonization of technical solutions,prepare for the input to ITU-R Technical Trends of IMT-2030 and provide design guidance of multiple access in 6G air interface standards to be started in 2025.

This feature topic is aimed at encouraging academic researchers and industry engineers to explore and discuss use scenarios,fundamental theories,technical challenges and breakthrough of 6G new multiple access.Most of the guest editors are affiliated with the leader institutes or companies that see the importance of engineering feasibility and would play important roles in pushing forward new multiple access technology.

The accepted papers cover topics about unsourced multiple access for massive Internet-of-Things (IoT)including both terrestrial networks and low-earth orbit (LEO) satellites,multiple access technologies for high mobility scenarios and device-to-device(D2D) scenarios,the multi-antenna-based spatial-dimension multiple access schemes.A brief account of each of these papers is given below.We hope this special issue may inspire future research works in related fields.

The connection density requirement for 6G massive IoT can be more than a magnitude higher than that of 5G,e.g.,from 106 devices/km2 to 107～108 devices/km2.Massive IoT traffic also features small payload,making traditional multi-step random access highly inefficient.Unsourced multiple access is a promising research direction to support a large number of IoT devices with very low signaling overhead and access delay.In the article entitled “Design Framework of Unsourced Multiple Access for 6G Massive IoT”,the authors proposed a rather complete design of unsourced multiple access in which two key parts: a compressed sensing module for active user detection,and a sparse interleaver-division multiple access (SIDMA) module are simulated side by side on a same platform.With a proper combination of compressed sensing matrix,a convolutional encoder,receiver algorithms,the simulated performance results appear superior to the state-of-the-art benchmark,yet with relatively less complicated processing.

Unsourced multiple/random access with compressed sensing can be also be used for LEO systems when each satellite is equipped with massive uniform planar array equipped.In the article entitled “OFDMA-Based Unsourced Random Access in LEO Satellite Internet of Things”,the authors expanded the inner codebook with predefined timing and frequency offsets and formulate the inner decoding as a tractable problem to cope with severe time and frequency offsets.The authors also leveraged the inherent sparsity in angular domain of massive antennas,thereby enabling the outer decoder to support more active devices.Furthermore,the outputs of the outer decoder are used to reduce the search space of the inner decoder,therefore cutting down the computational complexity and accelerates the convergence of the inner decoding.The merits of the proposed scheme were verified by simulations.

Another example of combining massive antennas and massive connections can be seen in the article entitled “Cluster-Based Massive Access for Massive MIMO Systems” a novel cluster-based massive access method is proposed.By exploiting angular domain characteristics,devices are separated into multiple clusters with a learned cluster-specific dictionary that can enhance active device identification.For detected active devices whose data recovery fails,power domain non-orthogonal multiple access(PD-NOMA) with successive interference cancellation (SIC) is employed to recover their data via re-transmission.Simulation results show that the proposed scheme and algorithm achieve improved performance on active user detection and data recovery.

For grant-free transmissions to support massive IoT,devices can initiate the access without multi-step process to synchronize the timing at the receiver.Rateless coding can be used to achieve lower access delay and high reliability.In the article entitled “A Joint Activity and Data Detection Scheme for Asynchronous Grant-Free Rateless Multiple Access”,the authors proposed a joint activity and data detection(JADD) scheme to support asynchronous grant-free rateless multiple access in more practical scenarios.Approximate message passing (AMP) is used for active user set detection and transmitted signals estimation.To determine the starting time for transmission,a maximum posterior probability (MAP)estimation problem is solved,leveraging the intrinsic power mode in the codeword.The effectiveness of the proposed solution is verified by simulations

For high mobility scenarios,hybrid carrier (HC)scheme has been used to spread the signal energy in time and frequency domain via weighted fractional Fourier transform (WFRFT) to form a fused signal containing the single carrier components and multi-carrier components.In the article entitled “The Extended Hybrid Carrier-based Multiple Access Technology for High Mobility Scenarios”,the authors studied the extended hybrid carrier (EHC)multiple access scheme to realize a power layered multiplexing framework that exhibits enhanced interference suppression capability owing to the more uniform energy distribution design.The implementation method is applicable for both uplink and downlink,and the performance analysis under varying channel conditions is provided.In addition,the non-orthogonal multiple access (NOMA) method of the EHC system was explored and the EHC sparse code multiple access scheme was developed.Simulation results have verified the feasibility and advantages of the proposed scheme compared with existing HC multiple access schemes.

Generally speaking,MIMO is also a form of multiple access which is often called space division multiple access (SDMA) that has played key roles in improving the system capacity of wireless networks.In the article entitled “Design and Implementation of Nonlinear Precoding for MIMO-SDMA Toward 6G Wireless”,the authors presented a novel and robust nonlinear precoding (NLP) design and detection structure specifically tailored for SDMA.The scheme aims to effectively mitigate the impact of imperfect channel estimation by leveraging the channel fluctuation mean square error (MSE) for reconstructing a highly accurate precoding matrix.Furthermore,a simplified receiver structure was introduced to eliminate the need for equalization,resulting in reduced interference and notable enhanced system performance.The efficacy of the proposed approach was verified by both computer simulations and experimental tests.

In conclusion,the Guest Editors of this feature topic would like to thank all the authors for their valuable contributions and the anonymous reviewers for their constructive comments and suggestions.We also would like to acknowledge the professional guidance and timely assistance from the Editor-in-Chief as well as the editorial team of China Communications.

Biographies

Yifei Yuan(Fellow,IEEE),received his Bachelor & Master degrees from Tsinghua University in China,and a Ph.D degree from Carnegie Mellon University,USA.He was with Alcatel-Lucent from 2000 to 2008,working on 3G/4G key technologies.From 2008 to 2020,he was with ZTE as technical director and Chief Engineer,responsible for standards research on LTE-Advanced and 5G.Since January 2020,he has been with China Mobile Research Institute as a Chief Expert,responsible for advanced research of 6G air interface.His research interests include MIMO,channel coding,non-orthogonal multiple access (NOMA),internet-of-things(IoT),and their standardizations in 3GPP.He has more than 80 journal/conference papers and 150 granted patents.He is an Editor ofIEEE Communications Lettersand an Editor ofChina Communications.He was the rapporteur of 3GPP 5G study on non-orthogonal multiple access (NOMA).He is the recipient of the Best Paper Award by IEEE Communications Society Asia-Pacific Board for a paper on NOMA in IEEE Communications Magazine.He is the Chair of Task force group of New Multiple Access in IMT-2030 (6G) Promotion Group of China.He has authored/co-authored 10 books on LTE-Advanced relay (in English) published by Springer,LTE-Advanced key technologies & system performance,narrow-band (NB) IoT and its standards evolution,5G NR channel coding(also in English by CRC Press),5G ultra-dense networks(UDN),5G non-orthogonal multiple access(also in English by CRC Press),5G random access enhancements,5G massive MIMO enhancements,5G unlicensed spectrum,respectively.yuanyifei@chinamobile.com

Yongpeng Wureceived the B.S.degree in telecommunication engineering from Wuhan University,Wuhan,China,in 2007,the Ph.D.degree from Southeast University,Nanjing,China,in 2013.He is currently a Professor with the Department of Electronic Engineering,Shanghai Jiaotong University,China.He was senior research fellow with Technical University of Munich,Germany and Humboldt research fellow and senior research fellow with Institute for Digital Communications,University,Germany.During his doctoral studies,he conducted cooperative research at the Department of Electrical Engineering,Missouri University of Science and Technology,USA.His research interests include massive MIMO/MIMO systems,massive machine type communication,physical layer security,and signal processing for wireless communications.Dr.Wu was awarded the IEEE Student Travel Grants for IEEE International Conference on Communications (ICC) 2010,the Alexander von Humboldt Fellowship in 2014,the Travel Grants for IEEE Communication Theory Workshop 2016,the Excellent Doctoral Thesis Awards of China Communications Society 2016，the Exemplary Editor Award ofIEEE Communication Letters2017,Young Elite Scientist Sponsorship Program by CAST 2017,and Excellent Youth Science Fund Project of National Natural Science Foundation of China 2021.He has been the leading guest editor ofIEEE Journal of Selected Topics in Signal Processing,IEEE Journal on Selected Areas in CommunicationsandIEEE Wireless Communications.He is currently an editor ofIEEE Wireless Communications,and was an Editor ofIEEE Transactions on CommunicationsandIEEE Communications Letters.He has been symposium chairs of various conferences,including Globecom,ICC,VTC,and PIMRC,etc.yongpeng.wu@sjtu.edu.cn

Lei Wangreceived the Ph.D.in electrical and computer engineering from Carleton University,Ottawa,ON,Canada,in 2017,where he was awarded the Senate Medal.He is currently a Full Professor with the School of Communication and Information Engineering,Chongqing University of Posts and Telecommunications,Chongqing,China.Prior to this,he was a cross-appointed Postdoctoral Fellow with the Department of Systems and Computer Engineering,Carleton University as well as Huawei Ottawa Research & Development Centre from 2017 to 2019.He is with the Editorial Boards ofEURASIP Journal on Wireless Commun.&Net.and Trans.Emerging Telecomm.Tech.He was the reviewer and the TPC Member of many IEEE journals and conferences.His research interests include wireless communications,satellite networks,Internet protocols and optimization theory.wanglei888@huawei.com

Zhifeng Yuanreceived MS degree in signal and information processing from Nanjing University of Post and Telecommunications in 2005.From 2004 to 2006,he was mainly engaged in FPGA/ SOC ASIC design.He has been as a member of the wireless technology advance research department at ZTE since 2006 and has been responsible for the research of the new multiple access group since 2012.His research interests include wireless communication,MIMO systems,information theory,multiple access,error control coding,adaptive algorithm,and high-speed VLSI design.yuan.zhifeng@zte.com.cn

Wei Baireceived his Ph.D.degree from Shanghai Jiaotong University,Shanghai,China,in 2003.He joined China Information Communication Technology (CICT) Group,Beijing,China,in 2016,where he was involved in the standardization of the Third-Generation Partnership Project New Radio(3GPP NR).He is currently the senior researcher of CICT Mobile Communications Technology Co.Ltd.He has contributed to the research of 6G key technologies.His current research interests include B5G/6G mobile communication systems,random access and multiple access technologies.baiwei1@cictmobile.com.

Yuanwei Liuis a Senior Lecturer (Associate Professor) with the School of Electronic Engineering and Computer Science,Queen Mary University of London. His research interests include non-orthogonal multiple access,reconfigurable intelligent surface,integrated sensing and communications,and machine learning.He is a Fellow of the IEEE,a Web of Science Highly Cited Researcher,an IEEE Communication Society Distinguished Lecturer,an IEEE Vehicular Technology Society Distinguished Lecturer,the academic Chair for the Next Generation Multiple Access Emerging Technology Initiative,the rapporteur of ETSI Industry Specification Group on Reconfigurable Intelligent Surfaces on work item of “Multi-Functional Reconfigurable Intelligent Surfaces(RIS): Modelling,Optimisation,and Operation”,and the UK representative for the URSI Commission C on “Radio Communication Systems and Signal Processing”.He was listed as one of 35 Innovators Under 35 China in 2022 by MIT Technology Review.He received IEEE ComSoc Outstanding Young Researcher Award for EMEA in 2020.He received the 2020 IEEE Signal Processing and Computing for Communications (SPCC) Technical Committee Early Achievement Award,IEEE Communication Theory Technical Committee (CTTC) 2021 Early Achievement Award.He received IEEE ComSoc Outstanding Nominee for Best Young Professionals Award in 2021.He is the co-recipient of the Best Student Paper Award in IEEE VTC2022-Fall,the Best Paper Award in ISWCS 2022,the 2022 IEEE SPCC-TC Best Paper Award,and the 2024 IEEE ICCT Best Paper Award.He serves as the Co-Editor-in-Chief of IEEE ComSoc TC Newsletter,an Area Editor of IEEE CL,and an Editor of IEEE COMST/TWC/TVT/TNSE.yuanwei.liu@qmul.ac.uk.