GUEST EDITORIAL

Wei Feng

Yunfei Chen

Jing Li

anmin Wang

Tony Q.S.Quek

With the rapid development of marine activities,there has been an increasing use of Internet-of-Things(IoT) devices for maritime applications.This leads to a growing demand for high-speed and ultra-reliable maritime communications.Current maritime communication networks (MCNs) mainly rely on satellites and on-shore base stations (BSs).The former generally provides limited transmission rate while the latter lacks wide-area coverage capability.As a result, the development of current MCN lags far behind the terrestrial fifth-generation (5G) network.

Beyond the 5G era with cutting-edge technologies such as unmanned aerial vehicles/vessels, artificial intelligence, and mobile edge computing, it is possible to build a new MCN to extend broadband coverage to the sea with higher data rate and higher reliability while maintaining a lower cost and lower delay.This new MCN is expected to be a space-air-ground-sea integrated network, which can serve randomly distributed maritime IoT devices in an on-demand manner by adaptively choosing the most efficient infrastructure and transmission resources.Nevertheless, the design of this new MCN also faces new challenges, e.g., different propagation environment compared with terrestrial systems,heterogeneous communication/computing/caching resources, and rigorous service demands for mission-critical applications.

The goal of this feature topic is to bring together the state-of-the-art original research, and the latest advances and innovations in theories, key technologies, and innovative applications for MCNs beyond 5G era.We have received 15 submissions in total for this feature topic.After a rigorous review process,seven papers have been accepted for publication.These works have investigated a wide range of topics in MCNs, including transmission signal optimization,transmission protocol design, maritime unmanned aerial vehicle (UAV) communications, and space-air-ground integrated maritime networks.We summarize these articles as follows.

The first article “Reducing cyclic prefix overhead based on symbol repetition in NB-IoT-based maritime communication,” by Xianget al., investigates Narrow-Band Internet of Things (NB-IoT) for maritime applications.The authors enhance the spectral efficiency of NB-IoT by reducing the cyclic prefix(CP) overhead in random access without causing interference.Specifically, all CPs are removed and multi-path fading effect is eliminated by using a repeated symbol to cover the disturbed symbol.Simulation results are carried out to validate the effectiveness of the proposed scheme in terms of bit error rate (BER).

The second article “Packet transport for maritime communications: a streaming coded UDP approach,”by Liet al., considers packet transport over long-distance lossy links for maritime communications.The authors propose a novel transport approach which uses user datagram protocol (UDP) along with a simple yet effective bandwidth estimator for congestion control, and with a proactive packet-level forward erasure correction (FEC) code to provide low-delay loss recovery without data retransmissions.It is shown that the approach can effectively address the key drawbacks of the transmission control protocol (TCP) in long-distance lossy links, and is promising for multimedia streaming and interactive applications.

The third article “Trajectory design for UAV-enabled maritime secure communications: a reinforcement learning approach,” by Liuet al., studies a UAV-enabled maritime secure communication network, where the UAV aims to provide the communication service to a legitimate mobile vessel in the presence of multiple eavesdroppers.The authors propose two reinforcement learning schemes based on Q-learning and deep deterministic policy gradient(DDPG) algorithms, to solve the discrete and continuous UAV trajectory design problem respectively.Simulation results are provided to validate the effectiveness and superiority of the proposed schemes.

The fourth article “Hybrid satellite-UAV-terrestrial maritime networks: network selection for users on a vessel optimized with transmit power and UAV position,” by Liet al., considers the hybrid satellite-UAV-terrestrial maritime network, where a terrestrial base station (TBS) and a satellite respectively provide wireless backhaul for the aerial base station (ABS) and the vessel-enabled base station(VBS).The network selection is jointly optimized with the transmit power of the ABS and the VBS, and the ABS’s position for improving data rate of all users.The authors solve the formulated problem by finding candidates for network selection and iteratively optimizing the transmit power and the ABS’s position for each candidate.Simulation results demonstrate that the data rate can be improved with collaborative coverage for users on a vessel.

The fifth article “Energy harvesting space-air-sea integrated networks for MEC-enabled maritime Internet of Things,” by Linet al., focuses on the complex computation tasks and low latency requirements in energy harvesting space-air-sea integrated networks(EH-SASINs).The authors first derive the optimal deployment of the tethered aerostat (TA) using the K-means method.Then, they address the issue of computation task offloading for vessels to minimize the execution delay.Finally, they propose an improved water-filling algorithm based on queuing theory.Simulation results show that the proposed method can reduce about 50% of the latency compared with local computation.

The final article “Networked TT&C for mega satellite constellations: a security perspective,” by Zhanet al., investigates the tracking, telemetry and command(TT&C) system for satellite constellations.By exploiting the synergy among multiple connected satellites, the authors propose networked TT&C,which requires much less pre-deployed infrastructures and may even perform better than traditional TT&C systems.Particularly, the authors show the unique security challenges and opportunities for networked TT&C.Then, they survey current research results on secure routing and trust mechanisms, and finally they deliver their security perspectives considering the system characteristics and security requirements of networked TT&C.

We sincerely hope that this feature topic provides valuable information to stimulate more maritime communication studies in the future.We would like to thank the authors who submitted their works to this special issue and thank all reviewers for their valuable and timely comments.We are also grateful to the Managing Editors and Staff Members for their great effort and support for promoting this feature topic.We hope all readers will enjoy the selected papers.