## 1 Introduction  
The designers of shallow-water communication systems try to implement the techniques of modern radiocommunications, but both the BER and data transmission rates achieved are much lower in the case of UAC systems. This is due to the disadvantageous properties of the UAC channels, namely the sea and inland waters, but also due to the technical capabilities of the generation and reception of acoustic waves. The range of a UAC system is determined mainly by the absolute value of absorption attenuation and varies in proportion to the square of the frequency of the system [1]. Differences in attenuation of frequency components of the transmitted signal due to growth in the range have degrading influence on the shape of the signal spectrum, and thus the time-domain waveform is distorted. To avoid these distortions, the bandwidth should be reduced as the system's range increases. Therefore, the differences in attenuation have a limiting effect on the bandwidth of the system and reduces its throughput. Another phenomenon that strongly impacts the transmission properties of the UAC channel consists of reflections from the sea-bottom and the water's surface, as well as other objects present in the water. This causes multipath propagation, which goes hand-in-hand with strong refraction, caused by a significant change in sound velocity as a function of depth. Both multipath propagation, as well as refraction, produce time dispersion of the transmitted signal. The time dispersion causes Inter-Symbol Interference (ISI), the consequence of which is frequency-selective fading observed in the received signal spectrum. Moreover, if the UAC transmitter or receiver,or objects reflecting the signal remain in motion, the possibilities of correct information detection are significantly limited due to the Doppler effect causing the signal spectrum distortions, which reveal as Inter-Carrier Interference (ICI) in case of multi-carrier systems. The Orthogonal Frequency-Division Multiplexing (OFDM) technique is a digital modulation scheme used by many wireless communication standards, such as WiFi (IEEE 802.11 a/g/n), WiMAX (IEEE 802.16), and the fourth generation (4G) cellular systems. The popularity of OFDM stems from its capability to convert a long multipath channel in the time domain into multiple parallel single-tap channels in the frequency domain, thus considerably simplifying receiver design. Such a feature makes OFDM an attractive choice for UAC systems [2]. UAC systems with OFDM technique are characterized by a high flexibility of modulation parameters, which allows the signal to be adapted to the specificity of a particular communication channel. At very short ranges of the order of several hundred meters, OFDM systems work in the frequency bandwidth from several to several dozen kHz, allowing transmission rates of tens of kbps, but with a Bit Error Rate not less than \(10^{-1}\). The use of Error-Correction Coding allows achieving a BER less than \(10^{-3}\), while reducing the transmission rate to single kbps [3; 4; 5]. At a long distance over 50 km the OFDM technique allows to achieve a transmission rate of several dozen bps, but such a system characterizes with low reliability (BER not less than \(10^{-1}\)), despite the use of ECC [6]. The paper presents the results of underwater acoustic OFDM communication test performed at a distance of 1 km in a very shallow inland-water channel. The aim of the experiment was to adapt the OFDM parameters to the propagation conditions of a shallow-water channel in such a way that it is possible to achieve high reliability of data transmission with a BER less than \(10^{-4}\). Such reliability was achieved using OFDM technique together with the channel equalization using pilot tones and ECC. A similar OFDM experiment carried out over a distance of 340 m using two pilot tone schemes, but without Error Correction Coding, is described in [7]. The OFDM data transmission was tested with different configurations of modulation parameters.