applied sciences Review Wireless Channel Models for Over-the-Sea Communication: A Comparative Study Arafat Habib and Sangman Moh * Department of Computer Engineering, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 61452, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-62-230-6032 Received: 18 December 2018; Accepted: 21 January 2019; Published: 28 January 2019 Abstract: Over the past few years, the modeling of wireless channels for radio wave propagation over the sea surface has drawn the attention of many researchers. Channel models are designed and implemented for different frequencies and communication scenarios. There are models that emphasize the influence of the height of the evaporation duct in the marine environment, as well as models that deal with different frequencies (2.5, 5, and 10 GHz, etc.) or the impact of various parameters, such as antenna height. Despite the increasing literature on channel modeling for the over-the-sea marine environment, there is no comprehensive study that focuses on key concepts that need to be considered when developing a new channel model, characteristics of channel models, and comparative analysis of existing works along with their possible improvements and future applications. In this paper, channel models are discussed in relation to their operational principles and key features, and they are compared with each other in terms of major characteristics and pros and cons. Some important insights on the design and implementation of a channel model, possible applications and improvements, and challenging issues and research directions are also discussed. The main goal of this paper is to present a comparative study of over-the-sea channel models for radio wave propagation, so that it can help engineers and researchers in this field to choose or design the appropriate channel models based on their applications, classification, features, advantages, and limitations. Keywords: over-the-sea communication; radio wave propagation; wireless channel; channel model; wireless channel characterization 1. Introduction Wireless channels for the radio wave propagation over the sea surface are different from the usual channels used in ground-to-ground wireless communication due to the unique nature of the marine environment. For example, neither the refraction of wave propagation, nor the effect of the evaporation duct is present in the ground-to-ground scenarios. Existing approaches to the modeling of over-the-sea channels vary with frequencies, technology used, and implementation scenario. While some approaches used millimeter waves and wide band technology, others used frequencies such as 1.9, 2, and 10 GHz, etc. Some approaches considered vessel-to-vessel mobile communication on the sea. Ship-to-ship and ship-to-shore wireless communication scenarios were also considered and implemented. So far, there are two preliminary minor approaches [1,2], that briefly describe a few models of channels for over-the-sea communication; however, with inadequate analysis and limited discussion on various models, applications, future improvements, and research directions. Readers can prompt references [1,2] for further details. Typically, the over-the-sea wave propagation environment can be divided into two types: One is “near the sea surface” and the other is “hundreds of meters above Appl. Sci. 2019, 9, 443; doi:10.3390/app9030443 www.mdpi.com/journal/applsci Appl. Sci. 2019, 8, x FOR PEER REVIEW 2 of 32 Appl. Sci. 2019, 9, 443 2 of 32 Appl. 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