# Using Fast Frequency Hopping Technique to Improve Reliability of Underwater Communication System
## 1 Introduction  
In the water environment, due to the fact that electromagnetic waves are strongly suppressed, acoustic waves are used to implement the underwater communication systems. Systems whose most important task is reliable operation in shallow coastal waters must overcome numerous limitations. One of the most important factors is multipath propagation. Its occurrence is the result of signal reflections from the water surface and the bottom (characteristic of horizontal channels) and any obstacles present in water reservoir. The acoustic wave may also be refracted and dispersed over the inhomogeneities of the medium. As a result, inter-symbol interference (ISI) is observed, which is undesirable in communication systems. Another important factor is the Doppler effect caused by the movement of the transmitter and/or receiver relative to each other. Although these adverse factors also occur in other communication systems, which are implemented using a different transmission medium, it is in the water environment and using acoustic waves where they become particularly emphasized. This is due to the low speed of propagation of the acoustic waves and the narrow available bandwidth, which directly results in limiting the throughput of the hydroacoustic channel [1]. Their seasonalvariability is also observed, which is an additional difficulty in clearly defining the acoustic parameters of the underwater communication channels [2; 3]. The already developed and commercially used hydroacoustic modems usually use a carrier frequency below 100 kHz, while for technological reasons, hydroacoustic transducers offer a bandwidth on the order of several kilohertz. All of these restrictions taken together are the reason for the complexity of the built receivers, whose task is to decode a symbol on the basis of the received signal. It is generally known that the underwater acoustic channel is one of the most demanding channels for the implementation of modern digital communication. This applies in particular to transmission through a horizontal channel occurring in shallow coastal waters. In wireless underwater communication, various modulation techniques are used, mainly frequency and phase modulation of the carrier wave [4; 5; 6; 7; 8]. They are used in both single-carrier and multi-carrier systems. The use of non-coherent modulations allows the undesirable effects of multipath propagation and the Doppler effect to be effectively overcome, especially in a horizontal underwater channel, where they provide transmission rates from several dozen to several hundred bits per second (bps) [9; 10]. These modulations are most often used in devices requiring long-term operation from battery power sources, due to the simple design of the receiver. Coherent modulations are used for systems operating in the environment of deep waters as well as vertical channels. Coherent modulations use the available frequency band more efficiently, offering higher transmission rates, but require a more complex receiver that performs adaptive correction of the transmission channel characteristics [11; 12]. Compared to non-coherent modulations, they require a signal with a higher signal-to-noise ratio for proper operation. In general, coherent modulation techniques remain less reliable than is desirable. Obtaining improving transmission reliability or higher transmission rates for this modulation method is possible as a result of transmission over many independent propagation paths, which is obtained by using multiple hydroacoustic antennas [13]. However, for compact applications, this means inconvenient hardware system complications. Another of the modulation techniques, the spread spectrum technique, is widely used in military ground communication systems due to its ability to work with a low signal-to-noise ratio and its good tolerance to multipath propagation and system jamming. In addition, the use of channel coding together with interleaving reduces the bit error rate.