Orthogonal Time Frequency Space Modulation in 6G Era

Weijie Yuan

Zhiqing Wei

Jiamo Jiang

Shun Zhang

Jinhong Yuan

Pingzhi Fan

Over the past decades,wireless technologies with reliable data transmissions and precise sensing ability in hostile environments are much less understood and significantly under-developed.The DD domain waveform,e.g.,orthogonal time frequency space(OTFS),has been recognized globally for its great potential to achieve high-reliable communications and robust sensing performance under various channel conditions in 6G era.

The DD domain waveform modulates the information directly in the DD domain rather than the classic TF domain for OFDM.The DD domain multiplexing changes the way of interaction between information symbols and wireless channels,which provides the benefit of high Doppler and Doppler-resilience against the highly dynamic wireless channels in hostile environments as well as a direct reflection of key sensing parameters.Besides,the two-dimensional (2D) spreading of information symbols from the DD domain to the TF domain allows each DD domain symbol to experience the full TF channel over a frame,resulting in the potential of achieving full channel diversity.Unfortunately,existing DD domain communication schemes were mainly designed based on the overlay with the classic OFDM modulation,where the essence of DD domain signal processing and how it will guild the DD domain waveform designs for communications and sensing purposes are less understood.

As a new paradigm,using DD domain for communications will introduce new critical challenges for transceiver architecture and algorithm designs.To fully unleash the potential of DD domain communications,challenging fundamental research problems and many practical design issues must be addressed,including channel estimation,detection techniques,and multiple antenna and multiple user systems design.

The main goal of this feature topic is to attract researchers in an effort to identify and discuss the major technical challenges,recent breakthroughs,and new applications related to OTFS in the 6G era.We have received a total number of 32 submissions for this feature topic.After a rigorous review process,ten papers have been accepted for publication.These works have investigated a wide spread of topics related to OTFS modulation.We summarize these articles as follows.

The first article “Secrecy Transmission of NOMA-OTFS based Multicast-Unicast Streaming,”by xx et al.,investigates the security performance of a cooperative multicast-unicast system,where the users exhibit the feature of high mobility.The authors develop the non-orthogonal multiple access (NOMA)based orthogonal time frequency space (OTFS) transmission scheme.Furthermore,the authors propose a power allocation method to strike a trade-off between the reliability of multicast streaming and the confidentiality of unicast streaming and utilize the relay selection strategy to improve the security of unicast streaming.Simulation results show that NOMA-OTFS-based cooperative transmission can significantly outperform the existing NOMA-OFDM in terms of both reliability and security in the context of multicast-unicast streaming.

The second article “Joint Multi-domain Channel Estimation Based on Sparse Bayesian Learning for OTFS System,” by XX et al.,focuses on the challenges in channel estimation caused by fractional inter-Doppler interference (IDI).The authors derive the original channel response (OCR) from the time domain channel impulse response (CIR) to reduce the dimension of OCR by using the basis expansion model (BEM) and the relationship between the time and DD domain channel.Finally,the sparse Bayesian learning (SBL) algorithm is used to estimate the basis coefficients in the BEM without any a priori information of the channel.The simulation results verify the effectiveness of the proposed algorithm.

The third article “BER Performance Analysis of OTFS Systems with Power Allocation,” by XX et al.,studies power allocation (PA) algorithms in OTFS systems.The authors propose two different PA algorithms in OTFS systems with zero forcing (ZF) and minimum mean square error (MMSE) equalization,and a suboptimal MBER power allocation method is developed to achieve better BER performance.The authors also raised a combined subsymbol allocation(SA) and PA strategy in the case of MMSE equalization and then investigate the BER performance with the proposed joint SA and PA strategy in delay-Doppler channels.

The fourth article “OTFS-based Efficient Handover Authentication Scheme with Privacy-Preserving for High Mobility Scenarios,” by XX et al.,studies Handover authentication in high mobility scenarios.The authors propose a privacy-preserving handover authentication scheme that can provide strong security guarantees with less computational cost.The scheme can achieve mutual authentication and key agreement amongst the user equipments,relay node,and authentication server.The performance evaluation results show that the proposed scheme has a small computational cost compared with other schemes and can also provide a strong guarantee of security properties.

The fifth article “Channel Estimation for OTFS System over Doubly Spread Sparse Acoustic Channels,” by XX et al.,addresses sparse channel estimation problem for the generalized linear models(GLM) in the Orthogonal time frequency space(OTFS) underwater acoustic (UWA) system. The authors develop a structured sparsity-based generalized approximated message passing (GAMP) algorithm for reliable channel estimation in quantized OTFS systems to leverage the structured sparsity of the doubly spread UWA channel.Simulation and experimental results show that the proposed algorithm can achieve accurate UWA channel state information with low computational complexity.

The sixth article “Iterative Receiver for Orthogonal Time Frequency Space with Index Modulation via Structured Prior-based Hybrid Belief and Expectation Propagation,” by XX et al.,studies the receiver for coded OTFS-IM system.The authors devise the structured prior incorporating activation pattern constraint and channel coding and develop an iterative receiver via structured prior-based hybrid belief propagation (BP) and expectation propagation (EP)algorithm.Then,they derive two variations of the proposed algorithm by using some approximations.Simulation results validate the superior performance of the proposed algorithm.

The seventh article “Low PAPR Channel Estimation for OTFS with Scattered Superimposed Pilots,”by XX et al.,investigates the problem that channel estimation schemes may result in poor peak-to-average power ratio (PAPR) performance of OTFS system or low spectrum efficiency.The authors propose a low PAPR channel estimation scheme with high spectrum efficiency.Specifically,they design a multiple scattered pilot pattern,where multiple low-power pilot symbols are superimposed with data symbols in the delay-Doppler domain.Simulation results show that the proposed multiple scattered pilot-aided channel estimation scheme can significantly reduce the PAPR while keeping the high spectrum efficiency.

The eighth article “Data-Driven Deep Learning for OTFS Detection,” by XX et al.,summarizes the existing research on OTFS detection based on data-driven deep learning (DL) and proposes three new network structures.The authors present three networks including a residual network (ResNet),a dense network(DenseNet),and a residual dense network (RDN) for OTFS detection.Specifically,the detection schemes based on data-driven paradigms do not require a model that is easy to handle mathematically.Simulation results show that RDN has the best performance among the three proposed schemes due to the combination of shallow and deep features.RDN can solve the issue of performance loss caused by the traditional network not fully utilizing all the hierarchical information.

The ninth article “OTFS modulation and PAPR reduction for IoT-railways,” by XX et al.,investigates the reliable communication in IoT for railways(IoT-R) under high mobility scenarios and strict energy constraints.The authors investigate OTFS modulation for IoT-R and use the peak windowing technique to decrease the PAPR of OTFS and promote the application of OTFS modulation in IoT-R.The simulation results show that the peak windowing technique can efficiently reduce the PAPR of OTFS for IoT-R.

The final article “Trajectory Planning for OTFS-Based UAV Communications,” by XX et al.,investigates robust air-to-ground (A2G) wireless links for high-speed UAVs.The authors investigate an orthogonal time frequency space (OTFS) based UAV communication system to tackle the challenges introduced by large path-loss and severe Doppler effect due to the high mobility of UAVs.The simulation results show that the performance of OTFS-based UAV system is superior to orthogonal frequency division multiplexing (OFDM)-based UAV systems,which can accomplish transmission tasks over shorter distances with lower energy consumption.

We sincerely hope that the papers published in this feature topic would provide valuable information reference for all researchers in the area of OTFS modulation,delay Doppler communications as well as wireless sensing.The Guest Editors would like to express their gratitude to all authors for their submissions and all reviewers for their efforts and insightful reviews that have contributed to the high quality of this Special Issue.We are also grateful to the Managing Editors and Staff Members for their great help and kind support.

Biographies

Weijie Yuan(Member,IEEE) received the B.E.degree from the Beijing Institute of Technology,China,in 2013,and the Ph.D.degree from the University of Technology Sydney,Australia,in 2019.From 2019 to 2021,he was a Research Associate with the University of New South Wales.He is currently an Assistant Professor with the Department of Electrical and Electronic Engineering,Southern University of Science and Technology,Shenzhen,China.Dr.Yuan’s research interests include orthogonal time frequency space(OTFS) and integrated sensing and communication (ISAC).He was a recipient of the Best Ph.D.Thesis Award from the Chinese Institute of Electronics and an Exemplary Reviewer from IEEE TCOM/WCL.He currently serves as an Associate Editor for the IEEE Communications Letters,an Associate Editor and an Award Committee Member for the EURASIP Journal on Advances in Signal Processing.He has led the guest editorial teams for three special issues in IEEE Communications Magazine,IEEE Transactions on Green Communications and Networking,and China Communications.He was an Organizer/the Chair of several workshops and special sessions in flagship IEEE and ACM conferences,including IEEE ICC,IEEE/CIC ICCC,IEEE SPAWC,IEEE VTC,IEEE WCNC,IEEE ICASSP,and ACM MobiCom.He is the Founding Chair of the IEEE ComSoc Special Interest Group on Orthogonal Time Frequency Space (OTFS-SIG).He has been listed in the World's Top 2% Scientists by Stanford University for citation impact.

Zhiqing Weireceived the B.E.and Ph.D.degrees from the Beijing University of Posts and Telecommunications(BUPT),Beijing,China,in 2010 and 2015,respectively.He is an Associate Professor with BUPT.He has authored one book,three book chapters,and more than 50 papers.His research interest is the performance analysis and optimization of intelligent machine networks.He was granted the Exemplary Reviewer of IEEE WIRELESS COMMUNICATIONS LETTERS in 2017,the Best Paper Award of WCSP 2018.He was the Registration Co-Chair of IEEE/CIC ICCC 2018,the publication Co-Chair of IEEE/CIC ICCC 2019 and IEEE/CIC ICCC 2020.

Shun Zhangreceived the B.S.degree in communication engineering from Shandong University,Jinan,China,in 2007,and the Ph.D.degree in communications and signal processing from Xidian University,Xi’ an,China,in 2013.He is currently with the State Key Laboratory of Integrated Services Networks,Xidian University,where he is currently a Professor.Prof.Zhang's research interests include massive MIMO,millimeter wave systems,RIS-assisted communications,deep learning for communication systems,orthogonal time frequency space (OTFS) systems,and multiple access techniques.He is an Editor for Physical Communication.He has authored or coauthored more than 80 journal and conference papers,and is the inventor of 16 granted patents(including a PCT patent authorized by US Patent and Trademark Office).He has received two Best Paper Awards in conferences,and two prize awards in natural sciences for research excellence by both China Institute of Communications and Chinese Institute of Electronics.

Jiamo Jiangreceived the B.S.degree in measurement and control technology and instrumentation and the Ph.D.degree in communication and information systems from Beijing University of Posts and Telecommunications (BUPT),Beijing,China,in 2008 and 2014,respectively.He is currently the director engineer of Mobile Communications Innovation Center (MCIC),China Academy of Information and Communications Technology (CAICT),Beijing,China.His research interests include integrated sensing and communications,machining learning and edge intelligence in wireless networks.Dr.Jiang has been engaged in research on technologies,standards,simulations and experiments of 5G and 6G for eight years.

Jinhong Yuanreceived the B.E.and Ph.D.degrees in electronics engineering from the Beijing Institute of Technology,Beijing,China,in 1991 and 1997,respectively.From 1997 to 1999,he was a Research Fellow at the School of Electrical Engineering,University of Sydney,Sydney,Australia.In 2000,he joined the School of Electrical Engineering and Telecommunications,University of New South Wales,Sydney,where he is currently a Professor and the Head of Telecommunication Group.He has published two books,five book chapters,over 300 papers in telecommunications journals and conference proceedings,and 50 industrial reports.He is a co-inventor of one patent on MIMO systems and four patents on low-density-parity-check codes.His current research interests include error control coding and information theory,communication theory,and wireless communications.He has coauthored four Best Paper Awards and one Best Poster Award,including the Best Paper Award from the IEEE International Conference on Communications,Kansas City,USA,in 2018,the Best Paper Award from IEEE Wireless Communications and Networking Conference,Cancun,Mexico,in 2011,and the Best Paper Award from the IEEE International Symposium on Wireless Communications Systems,Trondheim,Norway,in 2007.He served as the IEEE NSW Chapter Chair of Joint Communications/Signal Processions/Ocean Engineering Chapter from 2011 to 2014 and served as an Associate Editor for the IEEE Transactions on Communications from 2012 to 2017.He is currently serving as an Associate Editor for the IEEE Transactions on Wireless Communications and IEEE Transactions on Communications.

Pingzhi Fanreceived the M.Sc.degree in computer science from Southwest Jiaotong University,China,in 1987,and the Ph.D.degree in electronic engineering from Hull University,U.K.,in 1994.He has been a Visiting Professor with Leeds University,U.K.,since 1997.Since 1999,he has been a Guest Professor with Shanghai Jiaotong University.He is currently a Distinguished Professor and the Director of the Institute of Mobile Communications,Southwest Jiaotong University.He has over 300 research papers published in various international journals and eight books,including edited.He is the inventor of 26 granted patents.His research interests include vehicular communications,wireless networks for big data,and signal design and coding.He is a fellow of IET,CIE,and CIC.He also served as a Board Member for IEEE Region 10,IET (IEE) Council,and IET Asia-Pacific Region.He was a recipient of the U.K.ORS Award in 1992,the NSFC Outstanding Young Scientist Award in 1998,the IEEE VTS Jack Neubauer Memorial Award in 2018,and the IEEE SP Soc SPL Best Paper Award in 2018,the IEEE WCSP 10-Year Anniversary Excellent Paper Award from 2009 to 2019.He served as the general chair or a TPC chair of a number of international conferences.He is the Founding Chair of IEEE VTS Beijing Chapter,IEEE ComSoc Chengdu Chapter,and IEEE Chengdu Section.He is an IEEE VTS Distinguished Lecturer from 2015 to 2019 and a Distinguished Speaker from July 2019 to June 2022.