# A Method for Underwater Wireless Data Transmission in a Hydroacoustic Channel under NLOS Conditions
## 1 Introduction  
The development of underwater robotics and environmental monitoring systems has created the need to provide wireless communication in the water. In addition, there are the needs of the military and state services, e.g., in the field of minefield control or surveillance systems in strategic areas, such as ports [1]. Despite significant progress in the development of telecommunications systems, wireless communication in the aquatic environment still causes many problems. In such an environment, acoustic waves propagate the best, so it is primarily used as a transmission medium. Unfortunately, the propagation of a sound wave in water is accompanied by many unfavorable phenomena that affect the overall quality of transmission expressed by the range of data transmission, bit error rate or bit rate. Among these phenomena, the most important are the multipath propagation leading to intersymbol interference and the Doppler effect which, due to the relatively low speed of sound propagation in the water environment, is hundreds of thousands of times stronger than that it is in the case of radio communication [2]. Besides, serious difficulties exist in obtaining relatively fast (about kilobits) and reliable (BER about \(10^{-3}\)) transmission are caused by waters with intensive hydrotechnical infrastructure, such as ports [3]. Communication under NLOS conditions is a special case. Ensuring such communication is necessary, for example, when exploring wrecks. Often, such exploration is performed by a diver or with the use of an underwater vehicle does not allow the useof cables due to the need to enter into the wreckage and the high risk of entanglement or deadlock of the transmitting cables. In this case, the complex layout of the corridors in the wreckage requires wireless communication. Such communication must be ensured with a relatively low bit error rate of \(10^{-2}\)-\(10^{-3}\) and a relatively high bit rate, in the range of kilobits, which would allow, for example, with the use of appropriate vocoders, for voice communication with the diver. Unfortunately, currently proposed commercial solutions do not allow communication under NLOS conditions with such parameters. Therefore, this work is aimed at developing a method of wireless underwater communication under NLOS conditions. Generally, there are three methods of wireless transmission in water, described in the literature. Radio frequency (RF) transmission is possible at very short distances in sensor networks [4]. Optical communication, of great interest in recent years [5; 6], is also feasible as a technique that promises high throughputs over long distances. However, at present most research is still carried out under line-of-sight (LOS) laboratory conditions. The authors of this publication found one article discussing wireless optical communication in water under NLOS conditions [7]. However, the tests in [7] were carried out only in laboratory conditions, in which the distance between the receiver and the transmitter was only 90 cm. The third method is acoustic underwater communication, which has a long history [8]. The physical phenomena associated with it are well known, what allows for the creation of simulators of hydroacoustic channels based on measurements carried out in various reservoirs and with various measurement methods [9]. Despite the extensive knowledge about the phenomena occurring during the propagation of acoustic waves in water [2; 10; 11; 12], this environment is still a challenge in the field of designing underwater wireless communication systems. This is mainly due to the high variability of the propagation environment over time, especially in shallow waters [13], hence the great interest of reliable and efficient methods of wireless acoustic communication underwater. In the literature, the wireless underwater acoustic communication systems using linear frequency modulation [14], BPSK, QPSK and 4-QAM modulations [15; 16; 17], as well as proposals for the use of the OFDM technique [18; 19] can be found.