# Editorial for Special Issue: Underwater Acoustics, Communications, and Information Processing
## 2 Underwater Channel Characterization  
To design a conventional communication system, the standard approach is to understand the transmission medium and characterize the channel with a proper model. The communication system design is to optimize an objective function for an efficient transmission over the channel model. Historically, the channel model for basic terrestrial communications was rather simple, such as a randomly homogeneous one (e.g., the additive white Gaussian) or the fading amplitude one represented as a multiplicative model, either in the signal domain or in the log-scale power domain [2]. However, the basic underwater medium is expressed rather as a complex time-varying, temperature- and depth-dependent one and more properly expressed as a kind of multi-layer heterogeneous model [3], which makes underwater system optimization difficult. In other words, underwater channel characterization is more important for the underwater communication system design and implementation, especially considering various unique environmental effects of the specific sites for the system deployment. It is noteworthy that a coastal area becomes more complex because of a boundary problem, whereas open ocean problems neglect geological boundary conditions. Subsequently, the channel characterization of underwater acoustics is still interesting, both experimentally and analytically. To obtain a realistic perspective on a coastal sea channel near Vladivostok, [PERSON] et al. performed an experimental study in [4] in the shallow sea area, to understand the special aspects of scalar-vector sound field spatial structures as the first step of underwater channel characterization. They towed a low-frequency acoustic source on the continental shelf of the Sea of Japan, with the source emitting a 134 Hz acoustic signal, at a depth of 20 m at distances of up to 10 km from the combined receiver. The main results include the possibility of a practical application of the quantitative characteristics and features of the formation of signal interference at several depths along the tracks. It is interesting to note that, from the results of comparing horizontal and vertical field components, it follows that it is possible to identify the presence of vortex structures in the acoustic source field on several tracks. Further in [5], \"Variability of Hydroacoustic Noise Probability Density Function at the Output of Automatic Gain Control System\", [PERSON] et al. characterized the underwater channel noise through experiments. These resulted in the estimation of temporal variability of the underwater acoustic noise probability density function (PDF) in shallow waters within the frequency band of 0.03-3.3 kHz, along the Primorsky Aquarium on Russky Island, Vladivostok, Russia. The comparative results show a white Gaussian noise with respect to the sea noise PDF estimates in the same frequency range. [PERSON] et al. numerically investigated the effects of viscous flux vectors on hydrofoil cavitation flow, dynamically to characterize and model the radiated flow noise in [6], to resolve computational complexity and numerical instability problems. Their work justified the conclusion that the thin-layermodel could provide predictions as accurate as the full viscous model, but required less computational time. They examined the effects of the viscous flux vectors on the predicted flow fields and its radiated noise. Additionally, the hydro-dynamic forces, velocity distribution, volume fraction, far-field sound directivities, and sound spectrum of the three approaches were compared. To develop a practical channel model through analysis, applicable to realistic ocean applications, [PERSON] et al. estimated doubly spread acoustic channels based on wavelet transform (WT) in [7]. Typically, a simple model represents underwater acoustic channels, usually with severe delay spread and significant Doppler effects.