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 Multipath 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 LowResolution 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 lowcomplexity 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 DOI: 10.4218/etr2.12551