# Wireless Channel Models for Over-the-Sea Communication: A Comparative Study
## 3 Classification of Over-the-Sea Channel Models  
In this section, we will classify the existing over-the-sea wave propagation channel models reported so far in the literature. First, we will discuss the existing works briefly, and the channel models will be classified on the basis of the wireless LoS, nLoS, and bLoS wave propagations. Then, the channel models will be classified by considering the mobility of the transmitter and/or receiver. In [8], an approach was adopted to characterize a wideband channel for over-the-sea wave propagation. It presents a statistical representation and measurements of wideband mobile radio channels at 1.9 GHz. In [9], the focus was to evaluate the effects of sea surface shadowing conditions on the marine communication channel, modeled by the finite difference time domain (FDTD) method, for UAVs deployed for maritime surveillance operations. The results of measurements that characterize radio channels at 2 GHz in the open sea were presented in [10]. The results were obtained using a mobile single-antenna ship-borne transmitter and a fixed dual antenna terrestrial receiver. In [11], by means of narrowband measurements, the propagation of radio waves was analyzed at 5 GHz, taking into account the LoS near the sea surface. The work presented in [12] presents an analysis of the propagation of millimeter waves for the over-the-sea environment. In [5], a multipath channel model for radio wave propagation over the sea surface was designed for ship-to-ship and ship-to-shore wireless communication. An analytical expression for the effective width of the trapping beam of a transmitter for over-the-sea transmission was derived in [13]. Reference [14], published this year, adopts mathematical formulations to calculate the complete fading margin of the South China Sea. References [15,16] placed an effort into finding the optimal frequency and antenna height for radio wave propagation in the evaporation duct. In [17], [PERSON] et al. presented a LoS ship-to-ship wireless channel model for over-the-sea communication. [PERSON] et al. collected and analyzed the three-dimensional contour data of Diaoyu islands to generate a sea surface scenario, by using MATLAB, in order to design a channel model in that area [18]. [PERSON] et al. presented measurement results for land-to-shipoffshore wireless channel [19]. The frequency used in this channel model was 2.4 GHz. The model was investigated through the implementation of a narrowband channel measurement system between the quay and a sailing patrol boat. [PERSON] et al. presented a channel model using a Ka-band [20]. In [21], [PERSON] et al. used wideband technology to develop a channel model for UAVs to surface vessels. There are also other works on channel modeling for the over-the-sea environment with a focus on the effect of the evaporation duct height in the marine environment [22; 23], and different frequencies taking into account such parameters as signal strength or antenna height [24; 25; 26; 27]. Among all the works that have been published so far to develop wireless channel models for the ocean environment, we decided to discuss and comparatively analyze [5; 8; 9; 10; 21], because the implementation scenario and frequency used in each of these works are different. In addition, they considered a wide range of channel option parameters (fading margin, delay spread, etc.). To the best of our knowledge, no such review paper exists that has yet discussed and comparatively analyzed the existing channel models for the over-the-sea marine environment. In Section 2.1, LoS, nLoS, and bLoS wave propagations are briefly introduced with their example scenarios. One of the basic and crucial factors of wireless channel modeling is how the radio links propagate between the transmitter and receiver. A LoS between the transmitting and receiving antennas is desirable, but it is not possible to have a clear LoS always in the sea environment.