# Low-Cost Underwater Communication System: A Pilot Study
## 1 Introduction  
Underwater communication systems are constantly improved and developed. The vast majority of these systems are specially designed for large-budget projects. For this reason, the solutions used in these projects are not widely used, mainly due to the high price, patents, and other proprietary technology protection. It is beyond the scope of this article to discuss the meaningfulness of using these safeguards. However, the visible effect of the applied protection mechanisms is the limited access to knowledge and devices that allow for the widespread use of underwater communication. This document presents attempts to develop a device that can be constructed with a limited budget. In the literature, one can find scientific articles that describe experimental underwater communication systems using commonly available elements [1,2]. The first paper describes an experimental multiagent system which utilizes autonomous underwater vehicles (AUV). The whole system is composed of AUVs, transponders, and sea buoys. All of these components have to communicate with each other. To achieve this aim, the ultrasound modems were specially designed in this project. Scientists developed an experimental communication technology called 2 PSK. As it is stated by its name, double frequency shift keying modulation is utilized in each agent/modem. Two carrier frequencies, i.e., 35 and 38 kHz, are modulated separately, so a stream of two-bit symbols can be transmitted. The modem software works on PC104 computers with 64 MB of RAM, thus, the computing resources are quite large. Nonetheless, the system is designed to communicate at a rate of a few hundred bits per second. Therefore, a special communication protocol utilizing short 40-bit code words was designed. The main goal of the other project [2] was to construct a compact ultrasound modem capable of communicating in an underwater environment. The modem was designed with the use of the ATmega128 microcontroller, so it can be powered by a 9 V battery. Waterproof plastic-shielded ultrasound transducers were chosen mainly because of their low price. The transducer resonant frequency is 30 kHz. The communication system is designed asfollows: bit '1' is transmitted using a carrier frequency of 30 kHz, whereas for bit '0', there is no signal at all, so it is a kind of amplitude modulation. Despite the simplicity of such a modulation scheme, the authors reported a quite high 1 kbps transmission rate, which is a good result. Unfortunately, this kind of modulation is not resistive to the multipath propagation of ultrasound waves. Similar work was reported in [3] but only short distance communication was reached. A low power transceiver working with 200 kHz carrier frequency was deliberated in [4]. The authors reported the use of on-off keying (OOK) modulation with a 250 bps data rate, although the paper was focused on the design of an optimal amplifier, driving a piezoelectric transducer. Similar results were reported in [5]. The paper deliberated the possibility of implementing OOK modulation on the Cortex M3 microcontroller. The main advantage of this work was the use of a spherical piezoelectric transducer with a 70 kHz resonant frequency. The use of a spherical transducer allows for omni-directional transmission, which is an essential factor in underwater robotics. The paper reported a 1700 bps data rate for a short distance and 200 bps for a distance equal to 40 m.  
In most underwater modern constructions, piezoelectric transducers are used. They are relatively easy to fabricate and tune their frequency parameters, so any resonant frequency can be chosen depending on the application. Additionally, a piezoelectric transducer can be simply modeled by its capacitance only [6]. Therefore, an amplifier circuit drives a voltage to charge and discharge this capacitance, so a class-D amplifier is a standard for this kind of construction [4]. Nevertheless, other transducers are taken into account in the research, such as a piezoelectric micro-machined ultrasound transducer (pMUT) [7; 8].