With the rapid development of marine activities, there has been an increasing number of Internet-of-Things (IoT) devices on the ocean. This leads to a growing demand for high-speed and ultrareliable maritime communications. It has been reported that a large performance loss is often inevitable if the existing fourth-generation (4G), fifth-generation (5G), or satellite communication technologies are used directly on the ocean. Hence, conventional theories and methods need to be tailored to this maritime scenario to match its unique characteristics, such as dynamic electromagnetic propagation environments, geometrically limited available base station (BS) sites and rigorous service demands from mission-critical applications. Toward this end, we provide a survey on the demand for maritime communications enabled by state-of-the-art hybrid satellite-terrestrial maritime communication networks (MCNs). We categorize the enabling technologies into three types based on their aims: 1) enhancing transmission efficiency; 2) extending network coverage; and 3) provisioning maritime-specific services. Future developments and open issues are also discussed. Based on this discussion, we envision the use of external auxiliary information, such as sea state and atmosphere conditions, to build up an environment-aware, service-driven, and integrated satellite-air-ground MCN.

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Hybrid Satellite-Terrestrial Communication Networks for the Maritime Internet of Things: Key Technologies, Opportunities, and Challenges

With the rapid development of marine activities, there has been an increasing number of maritime mobile terminals, as well as a growing demand for high-speed and ultra-reliable maritime communications to keep them connected. Traditionally, the maritime Internet of Things (IoT) is enabled by maritime satellites. However, satellites are seriously restricted by their high latency and relatively low data rate. As an alternative, shore & island-based base stations (BSs) can be built to extend the coverage of terrestrial networks using fourth-generation (4G), fifth-generation (5G), and beyond 5G services. Unmanned aerial vehicles can also be exploited to serve as aerial maritime BSs. Despite of all these approaches, there are still open issues for an efficient maritime communication network (MCN). For example, due to the complicated electromagnetic propagation environment, the limited geometrically available BS sites, and rigorous service demands from mission-critical applications, conventional communication and networking theories and methods should be tailored for maritime scenarios. Towards this end, we provide a survey on the demand for maritime communications, the state-of-the-art MCNs, and key technologies for enhancing transmission efficiency, extending network coverage, and provisioning maritime-specific services. Future challenges in developing an environment-aware, service-driven, and integrated satellite-air-ground MCN to be smart enough to utilize external auxiliary information, e.g., sea state and atmosphere conditions, are also discussed.

The recent advances in wireless communication technologies allow mobile users to access various data services anytime and anywhere on land, while it is one of challenging issues to provide reliable data communications for maritime users due to the geographic features on the sea. Considering the increasing demands of maritime digital data services, we need to develop maritime communications supporting high-speed data rates and extended communication coverage. In this paper, we present the state-of-the-art works related to the data requirements of maritime services and the technical characteristics of existing maritime networks. Then, we introduce a long-term evolution for maritime (LTE-Maritime) that is an ongoing research project in South Korea. The objective of LTE-Maritime is to develop a maritime communication infrastructure supporting the data rates in the order of megabits per second within the communication coverage of 100 km. In order to confirm the feasibility of LTE-Maritime, we implemented a testbed for the LTE-Maritime which consisted of ships equipped with LTE-Maritime routers, base stations (BSs) along the coast, and an operation center. The experimental results show that the LTE-Maritime could be a practical solution for ship-to-shore data communication. Furthermore, we discuss a set of open issues related to the development of LTE-Maritime network.

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LTE-Maritime: High-Speed Maritime Wireless Communication Based on LTE Technology

Different from conventional cellular networks, a maritime communication base station (BS) has to cover a much wider area due to the limitation of available BS sites Accordingly the performance of users far away from the BS is poor in general This renders the fairness among users a challenging issue for maritime communications In this paper, we consider a practical massive MIMO maritime BS with hybrid digital and analog precoding Only the large-scale channel state information at the transmitter (CSIT) is considered so as to reduce the implementation complexity and overhead of the system On this basis, we address the problem of fairness-oriented pre-coding design A max-min optimization problem is formulated and solved in an iterative way Simulation results demonstrate that the proposed scheme performs much better than conventional hybrid precoding algorithms in terms of minimum achievable rate of all the users, for the typical three-ray maritime channel model

Fairness-oriented hybrid precoding for massive MIMO maritime downlink systems with large-scale CSIT

The freshness of status information is of great importance for time-critical Internet-of-Things (IoT) applications. A metric measuring status freshness is the Age of Information (AoI), which captures the time elapsed from the status being generated at the source node (e.g., a sensor) to the latest status update. However, in intelligent IoT applications such as video surveillance, the status information is revealed after some computation-intensive and time-consuming data processing operations, which would affect the status freshness. In this article, we propose a novel metric, Age of Processing (AoP), to quantify such status freshness, which captures the time elapsed of the newest received processed status data since it is generated. Compared with AoI, AoP further takes the data processing time into account. Since an IoT device has limited computation and energy resources, the IoT device can choose to offload the data processing to the nearby edge server under constrained status sampling frequency. We aim to minimize the average AoP in a long-term process by jointly optimizing the status sampling frequency and processing offloading policy. We first formulate this online problem as an infinite-horizon constrained Markov decision process (CMDP) with an average reward criterion. We then transform the CMDP problem into an unconstrained Markov decision process (MDP) by leveraging a Lagrangian method, and accordingly propose a Lagrangian transformation framework for the original CMDP problem. Furthermore, we integrate the framework with a perturbation-based refinement mechanism for achieving the optimal policy of the CMDP problem. Our investigation shows that to minimize the average AoP: 1) for processing offloading: the policy exploits good channel state to offload processing to the edge server and 2) for status sampling: the waiting time presents a threshold structure. Extensive numerical evaluations show that the proposed algorithm outperforms the benchmarks, with an average AoP reduction up to 30%.

Age of Processing: Age-Driven Status Sampling and Processing Offloading for Edge-Computing-Enabled Real-Time IoT Applications

This paper describes a nonorthogonal multiple access (NOMA)-based device-to-device (D2D) communication system underlaying in a cellular network sharing time and frequency resource. To optimize the total transmit power for all users in the network, we investigate a comprehensive resource allocation solution consisting of joint subchannel assignments, user pairings, and power control. The optimization problem is a mixed integer linear program, which is solved in three low-complexity steps. We first group D2D users to minimize mutual interference from cellular users (CU). Second, we demonstrate a heuristic solution for coupling D2D users in each group. Finally, we minimize the total transmit power of all devices in the network while maintaining the SINR requirements for both CU and D2D users. The optimal solution is derived off-line using the Rayleigh fading channels for any given QoS level of CU and NOMA-D2D users. We recognize that both subchannel assignment and user pairings play the important roles in reducing the total transmit power of the network, in which both cellular and D2D users coexist. However, optimization of the subchannel assignment contributes more toward minimizing the total network transmit power than the proposed model of optimizing user pairings.

Resource Allocation for NOMA-Based D2D Systems Coexisting With Cellular Networks

In this work, the performance evaluation and the optimization of dual-hop LoRa network are investigated. In particular, the coverage probability (Pcov) of edge end-devices (EDs) is computed in closed-form expressions under various fading channels, i.e., Nakagami- $m$ and Rayleigh fading. The Pcov under Nakagami- $m$ fading is computed in the approximated closed-form expressions; the Pcov under Rayleigh fading, on the other hand, is calculated in the exact closed-form expressions. In addition, we also investigate the impact of different kinds of interference on the performance of the Pcov, i.e., intra-SF interference, inter-SF interference (or capture effect) and both intra- and inter-SF interference. Our findings show that the impact of imperfect orthogonality is not non-negligible, along with the intra-SF interference. Moreover, based on the proposed mathematical framework, we formulate an optimization problem, which finds the optimal location of the relay to maximize the coverage probability. Since it is a mixed integer program with a non-convex objective function, we decompose the original problem with discrete optimization variables into sub-problems with a convex feasible set. After that, each sub-problem is effectively solved by utilizing the gradient descent approach. Monte Carlo simulations are supplied to verify the correctness of our mathematical framework. In addition, the results manifest that our proposed optimization algorithm converges rapidly, and the coverage probability is significantly improved when the location of relay is optimized.

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Performance Analysis and Optimization of the Coverage Probability in Dual Hop LoRa Networks With Different Fading Channels

A single-hop LoRa ( Lo ng Ra nge) network suffers from a limitation in coverage caused by high signal attenuation when deployed in industrial areas, which often include wireless unfriendly zones (WUZs). Thus, some end nodes in WUZs may fail to transmit data to the gateway (GW). This paper proposes a reliable two-hop real-time LoRa protocol in which the nodes in WUZs transmit data to another node that relays the received data to the gateway for reliable transmission. The proposed protocol allocates transmission slots to nodes with different data transmission periods, so the nodes not only achieve high reliability but also meet time constraints or periods for delivering data to a gateway. In this case, 1-hop nodes (one hop away from GW) may consume more energy than 2-hop nodes (two hops away from GW). For energy balancing, the protocol uses a data aggregation approach to minimize the number of transmissions. We show by analysis that the proposed protocol can support up to 200 nodes using a single-channel GW when every node sends one packet with a payload of 30 bytes within 17 seconds, and 30% of the nodes are 2-hop nodes. In addition, we show by experiment that the proposed protocol can achieve high reliability in data transmission.

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A Two-Hop Real-Time LoRa Protocol for Industrial Monitoring and Control Systems

LoRa technology draws attention for its use in industrial monitoring and control systems in which each end device or task is required to send data periodically to a (cloud) server. Despite its provision of a stable link, it suffers from data loss by signal suppression and interference. A real-time LoRa protocol is proposed that uses a slot scheduling to remove collision and device or node grouping based on signal attenuation to deal with signal suppression. Based on the definition of a frame-slot structure, a logical slot indexing algorithm is devised to tag a logical index to each slot. The logical indices enable the easy allocation of slots to nodes such that if each node sends data in the allocated slots, it can satisfy time constraint. To handle external interference caused by other networks, the protocol uses a multiple listen-before-talk (mLBT) mechanism that allows channel detection multiple times within one slot. Our protocol is compared analytically and experimentally with other ones to show its superior throughput and reliability against signal suppression and interference.

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A Real-Time LoRa Protocol for Industrial Monitoring and Control Systems

In this study, we shortly introduce MariComm bridge of MariComm (Maritime Broadband Communication) project to provide broadband internet/multimedia services available at a rate of 1 Mbps or more on the sea. MariComm bridge enables MariStations to form the maritime heterogeneous relay networks. Thanks to this bridge function, MariStation can act as a gateway node or a relay node according to the connection possibility of a terrestiral base station. And finally, marine performance tests of this noble maritime communication system on 4 vessels in the Korean southern sea are introduced.

Dae-Seung Yoo

Papers

Resource Allocation for NOMA-Based D2D Systems Coexisting With Cellular Networks

Performance Analysis and Optimization of the Coverage Probability in Dual Hop LoRa Networks With Different Fading Channels

A Two-Hop Real-Time LoRa Protocol for Industrial Monitoring and Control Systems

A Real-Time LoRa Protocol for Industrial Monitoring and Control Systems

Implementation of MariComm bridge for LTE-WLAN maritime heterogeneous relay network