Special issue on 6G and satellite communications

The fifth-generation (5G) technology has recently been consolidating its function as an infrastructure for new services, such as ultrareliable low-latency communications (URLLC) and massive machine-type communications, by further expanding the high-speed data service of the mobile communication system. Due to the successful deployment of the 5G technology, the sixth generation (6G) is expected to develop in the direction of further dramatically improving 5G services, such as the ultramobile broadband, ultra-low-latency reliability and security, and ultrahigh-sensing low-latency communications. The 6G technology will make a decisive contribution to the realization of new service concepts, such as metaverse services combined with new technologies like artificial intelligence and block chain. These new 6G services may be implemented by more intelligent infrastructure and ultra-wideband media that go beyond 5G. Beyond the conventional terrestrial communication coverage, 6G is expected to support non-terrestrial coverage using unmanned aerial vehicles and low Earth orbit (LEO) satellites to serve the three-dimensional spatial coverage, such as high-rise buildings, aerial places, and seas. This will further accelerate newly emerging services by unmanned vehicles that will cross the ground and air in the future. In this sense, for this special issue, we have selected eight key papers in the three aspects of the 6G technology: (i) intelligent networks and protocols for enhancing reliability and user privacy; (ii) improved topology and transceiving for boosting performances; and (iii) extension to non-terrestrial networks. The first two papers discussed intelligent networks and protocols for enhancing reliability and user privacy. Their proposed algorithms enabled networks to provide more reliable paths and stronger security, respectively. In their paper titled [1] “Reliability-guaranteed Multi-path Allocation Algorithm in Mobile Network,” Lee and Ko investigated the method of selecting higher reliable network paths for further enhancing URLLC services. To achieve a more cost-efficient reliability, this study proposed a constrained Markov decision process (CMDP)-based algorithm for selecting paths over wireless and wired networks, which can consider both the path setup time and the dynamicity of path reliability. This work demonstrated that the proposed algorithm reduces the network cost by 30% through event-driven simulations. Furthermore, the CMDP-based algorithm well reveals design guidelines to adapt the numbers of allocated gNBs and CN paths to the required reliability. The paper titled [2] “Dynamic ID Randomization for User Privacy in Mobile Network” by Sarker and others proposed ID-RZ, a new dynamic randomization scheme managing the temporary secure IDs for 5G and 6G networks. ID-RZ is designed to proactively change the IDs of mobile devices, providing better security at an arbitrary moment. This work demonstrates that ID-RZ is lightweight compared to conventional ID reallocation schemes retrieving a randomized ID from the network owing to its property of local generation of unpredictable temporary IDs using a hash-chain-based construction starting from the seed ID originally provided by the network. The four papers below discussed the improved topology and transceiving for improving performances. They investigated mmWave cell-free massive MIMO, optical wireless networks, deep learning (DL)-based channel estimation, and beamforming for the mmWave URLLC, respectively. The paper [3] “Energy-Efficient mmWave Cell-Free Massive MIMO Downlink Transmission with Low-Resolution DACs and Phase Shifters” proposes a novel framework for energy-efficient mmWave CFmMIMO systems using low-resolution digital–analog converters (DACs) and phase shifters (PSs) to introduce low-complexity hybrid precoding. The simulation results show that the proposed hybrid precoding and pilot allocation scheme outperform the existing schemes. Further, they show that low-resolution DACs and PSs can effectively increase the energy efficiency by compromising the spectral efficiency and network power consumption. Nath and others provided the second paper [4] “Interference and Noise Analysis for Hybrid FSO/RF based 6G Mobile Backhaul,” which investigates hybrid free-space optics (FSO) and radio frequency (RF) systems as possible configurations for a mobile backhaul in the 6G network. Cognitive radio technology was used to combine RF with FSO to enhance the link availability, and the paper presented performance analysis in terms of outage probability and average bit error rate by considering the impact of optical channel turbulence, RF interference, and other losses. It was shown that the presented configuration could enhance the utilization of existing RF resources, reducing the waste of RF spectrum. Authors also suggested interference management techniques as a future study item. The paper [5] “Deep Learning-based Scalable and Robust Channel Estimator for Wireless Cellular Networks” presents a two-stage scalable channel estimator (TSCE) that uses a DL-based scalable and robust channel estimator comprising two DL networks to efficiently support different resource allocation sizes and reference signal configurations. The results show that the proposed TSCE system can learn the wireless propagation channels correctly and outperform both traditional estimators and baseline DL-based estimators. The paper titled [6] “Frequency divided group beamforming with sparse space–frequency code for above 6GHz URLLC systems” proposes a limited-feedback-based frequency divided group beamforming with sparse space–frequency transmit diversity coded orthogonal frequency division multiplexing (OFDM) system for URLLC systems. In this system, a novel power allocation method is proposed based on cooperative game theory to manage multipoint transmission structure realized by the distributed panels. The last two papers investigated the resource allocation for non-terrestrial networks using unmanned aerial vehicles and LEO satellites. The paper titled [7] “BandBlock: Bandwidth Allocation in Blockchain-Empowered UAV-based Heterogeneous Networks” by Kuna and others proposed a blockchain-enabled bandwidth allocation framework for secure bandwidth trading between terrestrial cellular base stations (CBSs) and UAV-assisted flying BSs (UBSs). Non-terrestrial UBSs can play a key role in complementarily serving UEs with unexpected dynamic traffic demands (TDs) or those experiencing poor coverage. The proposed blockchain-empowered bandwidth trading contributed to the enhancement of the transaction security and privacy without any centralized third party. The Cournot oligopoly game model was applied to the bandwidth allocation framework. This game model balanced the tradeoff between the bandwidth demands of the CBSs and the cost charged by the UBSs. The simulation results demonstrated that the proposed method enhances the network's transaction security and privacy protection for bandwidth trading and maximizes the UBS and CBS utilities. The paper [8] “Dynamic Power and Bandwidth Allocation for DVB-based LEO Satellite Systems” proposes an efficient power and bandwidth allocation method employing two linear machine learning algorithms with inputs of channel conditions and TD. The simulation results conducted on multibeam frequency-reuse in LEO satellite systems show that the proposed method outperforms the existing methods. The guest editors would like to thank all authors, reviewers, and editorial staff of ETRI Journal for making this special issue a success. We are most pleased to have been part of the effort of getting these high-quality technical papers timely. These leading studies for 6G will contribute to the design and realization of future 6G systems. We would like to thank all the authors for their contributions. We, unfortunately, had to reject some interesting contributions owing to space and time limitations associated with a special issue and its natural deadlines. We are highly grateful to the reviewers for their effort and to the ETRI Journal editorial board and the editorial staff. The authors declare that there are no conflicts of interest. Taesoo Kwon received the BS, MS, and PhD degrees from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea, in 2001, 2003, and 2007, respectively, all in electrical engineering and computer science. From 2007 to 2011, he was a Senior Researcher with Samsung Advanced Institute of Technology, Yongin, Republic of Korea, where he was involved in research on LTE-Advanced systems and beyond 4G communication systems. In 2008 and 2011, he was a Visiting Scholar with the Department of Electrical Engineering, Stanford University, Stanford, CA, USA. From 2011 to 2012, he was a Postdoctoral Fellow with the Department of Electrical and Computer Engineering, the University of British Columbia, Vancouver, Canada. From 2013 to 2015, he was a Senior Researcher with the Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea, where he was engaged in research on 5G communication systems. Since 2015, he has been with the Department of Computer Science and Engineering, Seoul National University of Science and Technology (SeoulTech), Seoul, Republic of Korea. His research interests include wireless networks, stochastic geometry, optimization, and machine learning. Sooyoung Kim received the BS degree in electrical and electronics engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea, in 1990. After having worked Satellite Communication Technology Division, Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea from February 1990 to September 1991, she received the MSc and the PhD degrees in electrical and electronics engineering from University of Surrey, UK in 1992 and 1995, respectively. From November 1994 to June 1996, she was employed as a research fellow at the Centre for Satellite Engineering Research, University of Surrey, UK. In 1996, she re-joined the Satellite Communication Technology Division, ETRI and worked as a team leader until February 2004 to develop efficient transmission techniques for digital satellite communication systems. She is now a professor in Jeonbuk National University, Jeonju, Republic of Korea. Her research interests include coded MIMO schemes, forward error correction coding, physical layer security schemes, and satellite communications. She is actively working on Working Party 4B of Radiocommuncation Sector of International Telecommunication Union (ITU-R) for standardization activities for satellite communications, and she has been appointed as an international standardization expert in Korea. She has published more than 100 technical papers in the field of wireless/satellite communications. She is an editorial board member of the International Journal of Satellite Communication Systems and Networking. Kyunghan Lee is currently an associate professor in the Department of Electrical and Computer Engineering at Seoul National University. He received his BS, MS, and PhD degrees in Electrical Engineering from KAIST (Korea Advanced Institute of Science and Technology), Daejeon, Republic of Korea, in 2002, 2004, and 2009, respectively. Prior to joining Seoul National University, Seoul, Republic of Korea, he had been with the school of ECE at UNIST from 2012 to 2020. He is serving as an Editor for IEEE/ACM Transactions on Networking, IEEE Transactions on Vehicular Technology, Computer Networks (Elsevier), and ICT Express. He had served as a General Co-Chair of ACM MobiHoc 2022. He is a Next-generation Member of KAST (The Korean Academy of Science and Technology). He received IEEE ComSoc William R. Bennett Prize in 2013 and 2016, respectively, and ACM MobiSys Best Paper Award in 2021. His research interests include performance-guaranteed networking and computing systems for 6G. Jong-Moon Chung received the BS and MS degrees in electronic engineering from Yonsei University, Seoul, Republic of Korea, and the PhD in electrical engineering from the Pennsylvania State University, PA, USA. Since 2005, he has been a professor in the School of Electrical and Electronic Engineering at Yonsei University, where he is also the associate dean of the College of Engineering, and (joint appointment) professor in the Department of Emergency Medicine of the College of Medicine at Yonsei University. From 1997 to 1999, he was an assistant professor and instructor at the Pennsylvania State University in the Department of Electrical Engineering. From 2000 to 2005, he was with the Oklahoma State University (OSU), OK, USA, as a tenured associate professor in the School of Electrical & Computer Engineering. Currently, he serves as a Vice President of the IEEE Consumer Technology Society and the IEEE Product Safety Engineering Society, Senior Editor of the IEEE Transactions on Consumer Electronics, Section Editor of the Wiley ETRI Journal, Co-Editor-in-Chief of the KSII Transactions on Internet and Information Systems, and a member of the IEEE Eta Kappa Nu (HKN) Honor Society. Dr. Chung has served as the General Co-Chair of IEEE ICCE 2022 and was the General Chair of IEEE ICCE-Asia 2020 and IEEE MWSCAS 2011.

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