# Low-Cost Underwater Communication System: A Pilot Study
## 1 Introduction
This type of transducer is designed to work with the high coupling coefficient, so consequently, it can produce stronger output pressure, compared to traditional piezoelectric construction. Moreover, pMUT has an important property, namely, a small quality factor Q, which guarantees a wider bandwidth around the resonant frequency. Other transducers developed for underwater communication are structures based on piezocomposite materials. The main advantages of such technology are the light weight, low acoustic impedance, and broadband frequency characteristics. The last property is particularly important in the context of creating an efficient underwater communication network. Due to the very wide bandwidth of about 350 kHz with ripples of \(\pm 5\) dB [9], one might think of using broadband modulation, traditionally reserved only for terrestrial telecommunications networks. In the paper, a simplified approach is proposed. It means that the problems connected with an underwater propagation of ultrasound waves are ignored. Nevertheless, the literature on the subject indicates a few conditions for conducting experiments, where underwater propagation problems can be neglected. Very wide deliberations on ultrasound wave propagation were given in [10]. The authors pointed out that the noncoherent frequency modulation as well as phase modulation allows to avoid the difficulty of phase tracking in Doppler spread channels. In addition, they marked the possibility to utilize sophisticated modulations, such as orthogonal frequency division multiplexing (OFDM). OFDM signals, however, need to be transmitted by a broadband transducer because of their wide bandwidth. Other publications also investigate issues related to the propagation of ultrasonic waves. The theoretical limit of the underwater communication channel capacity was estimated to be 4 bits per second per Hz [11]. Consequently, for a symbol rate equal 1 kHz, it cannot be expected to achieve a baud rate greater than 4 kbps, regardless of the modulation used. Ultrasonic channel properties in the context of waveform propagation in the swimming pool can be found in [12]. Another environment of ultrasonic waveform propagation, namely, pipes, was analyzed in [13]. These research studies give important hints about the possible consequences of using underwater modems in such environments. A separate class of underwater communication systems are solutions that try to mimic sea marine mammals, which use ultrasound in a natural way. Biologically inspired experiments are conducted in various directions. In [14], researchers focused on using a time-varying carrier frequency instead of the classical chirp spread spectrum technique to achieve the highest possible degree of similarity to a dolphin's whistle. The work [15]explicitly indicates that the aim of this solution is hiding communication among naturally observed ultrasounds. A completely different approach is presented by the publication [16], where the authors focused on the development of the entire Morse alphabet composed of clicks, whistles, and songs emitted by humpback whales (Megaptera novaeangliae). All of these studies are extremely attractive; however, generating ultrasonic signals with a complex envelope and a time-varying carrier frequency requires large computational resources. The last important factor directly related to this paper is the complication of the modern device. It means that one can select some sophisticated modulation techniques, but dedicated hardware, e.g., FPGA, is required [17; 18; 19]. A similar situation addresses the choice of an ultrasound transducer--a cheap off-the-shelf device is chosen for the implementation. On the other hand, a sophisticated broadband transducer, which is less common and usually protected by some propriety rights, would bring some improvement in power or bandwidth efficiency. Therefore, at the beginning of the project, one has to decide which factors are more important.