## 1 Introduction
### The Challenging Nature of Communications in Maritime Environments  
Communication is the most critical process in underwater technology. It can be established either by wired or wireless connections. Both methods have their own advantages and disadvantages,depending on the application. The current trend favors wireless communications as the most suitable way to transfer information, especially when it comes to deal with environments like the one described in this manuscript, where wired connections are either not practical or just impossible [9]. Unlike terrestrial-based applications, establishing underwater wireless communications is not a straightforward process. When considering the underwater communications, the main concerns that researchers consider involve the channel model (in this case, underwater), attenuation, transmission distance, power consumption, Signal-to-Noise ratio, bit error, symbol interference, error coding, modulation strategies, instrumentation and underwater interferences. Dealing with interferences for underwater research is a complex task due to the dynamic nature of water, which makes it difficult to plan communication links ahead. As far as AUVs are concerned, issues of navigation and communications are often the most difficult problems to address, as there are few options for transmitting messages underwater [2]. Unlike radio links in terrestrial applications, challenges are quite different for underwater wave propagation. Water itself becomes the main source for the signal interference, due to unpredictable changes in some of the parameters that define most prominent features of seawater, like permittivity [10]. Indeed, characteristics like water turbidity, temperature and water noise have an impact in acoustic wave propagation [11]. In addition to that, common terrestrial phenomena like scattering, reflection or refraction also happens in underwater application domains [12]. There are various ways to get reflection in underwater environment, for example the signal can bounce off the sea floor and other underwater geographic structures, including softer mediums such as the ocean surface and layers of water separated by differences in temperature or density. In the underwater area of knowledge there are three types of carrier waves that are most commonly used in wireless communications: (a) electromagnetic waves, (b) optical waves and (c) acoustic waves [13; 14]. Using electromagnetic waves, the communication can be established at a higher frequency and bandwidth, but there are limitations due to high absorption/attenuation, which has a significant effect on the transmitted signal. A large antenna is also needed for this type of communication, thus affecting the design complexity and cost. Optical waves also offer high data rate transmission. Unfortunately, the signal is rapidly absorbed in water and suffers from scattering effect [3; 4; 5; 6], which affects data transmission accuracy. Acoustic is the most preferred signal used as carrier by many applications because of its low absorption characteristics for underwater communication. Even though the data transmission is slower compared to other carrier signal, low absorption features enable the carrier to travel at longer range as less absorption is faced by the carrier. Depending on the distance between endpoints of a data transmission, bandwidth can range from 2 kbits [15] to 87 kbits per second [16]. When ensuring effective underwater communication, the communication system design plays a vital role. Factors such as transducer parameters (sensitivity, power consumption, noise immunity, transduction mechanism, directivity, resolution) and properly matched impedance must be taken into account during the process. Also, from the instrumentation system point of view, the size of devices is one of the main concerns. Manufacturers from the electronics industry are competing to produce a device with better performance and smaller size for the overall efficient system.