## 1 Introduction
For example, in [8], DSSS signaling was used to increase the Signal-to-Noise Ratio (SNR) per data symbol and resolve multipath components for a single-user, short-range application. The system utilized 10 kHz of bandwidth to transmit the information with a rate equal to 625 bps at a distance of 1 km in shallow water. In [9] an improvement in the reliability of data transmission from BER of less than \(10^{-1}\) to BER less than \(10^{-2}\) in a system with QPSK modulation was achieved after implementing the DSSS technique. The Mediterranean Sea experimental results presented in [10] include data transmission tests at a distance of 45 nautical miles, using Gold sequences of rank 6 and achieving a data rate equal to 35.7 bps. An experiment at a distance of 20 km is also described, where using Kasami sequences of rank 6 made it possible to transmit data with a rate equal to 24 bps. In [11], the authors proposed a UAC modem, in which two Gold codes of length 2047 were used to spread differentially coded data bits. The bit rate of the system was 100 bps, and the carrier frequency was set to 12 kHz. The proposed modem was tested in the Baltic sea at an approximate range of about 3000 m. The authors obtained errorless transmission of blocks of 200 data bits. A DS CDMA underwater acoustic communication network is described in [12]. Communication between four users was demonstrated. Simulation tests showed that it is possible to achieve reliable data transmission at a rate of up to 2.5 kbps per user at a distance of 2 km, using the Quadrature Phase-Shift Keying (QPSK) modulation scheme and Kasami spreading codes of lengths 15, 63, and 255. The carrier frequency was equal to 33 kHz. In [13] the results of the stationary communication experiment are described. The data has been transmitted with a rate equal to about 0.5 bps at a distance of up to 8 km, using the DSSS signal of bandwidth equal to 2 kHz over carrier frequency equal to 12 kHz. M-sequence of rank 8 has been used as a spreading sequence. The article [14] describes a UAC modem performing data transmission using several physical layer configurations. One of them is the DSSS transmission mode, in which every QPSK symbol is spread by a portion of an m-sequence of rank 13. The band of the signal is equal to 8-16 kHz. During the experimental tests at a distance of 1.5 km, a data transmission rate of 100 bps was achieved. The Autonomous Underwater Vehicle, HUGIN, by Kongsberg Maritime, uses a commercial DSSS-based UAC modem, which operates at a horizontal range of up to 4 km in shallow water with a data transmission rate equal to 50 bps, but the manufacturer reserves that this value depends on the sound profile and acoustic noise [15]. The DSSS-based UAC systems described in the literature use specific PN sequences and fixed transmission bandwidth. To the best of the authors' knowledge, there are no publications presenting an analysis of an underwater DSSS system's performance depending on its bandwidth or the PN sequence used. This paper presents the results of simulation and experimental communication tests of a DSSS-based UAC system using three types of spreading sequence, namely m-sequences, Kasami codes, and Gold codes, and occupying different bandwidths from 1 kHz to 8 kHz around a center frequency equal to 30 kHz. The simulation tests of such a system using an Additive White Gaussian Noise (AWGN) channel model, Rician fading channel model, and a replay channel modeled by impulse responses measured during the inland water experiment were previously described in [16]. In this paper, the UAC channel was modeled by impulse responses calculated by the virtual sources method and by the Watermark simulator. The experimental tests were conducted in the model pool. During the tests, the bit error rate (BER) of data transmission was estimated. The goal of this simulation and experimental research was to estimate the transmission rate which is possible to achieve in strong multipath propagation conditions, assuming a reliability determined by the BER of lower than 0.001.