## 1 Introduction  
Underwater acoustics communication systems are used in both civil and military applications, including control of Autonomous Underwater Vehicles (AUVs) and military objects on the seabed, instrument monitoring, pollution control, climate recording, prediction of natural disturbances, search and survey missions, and the study of marine life [1]. If the system operates in shallow water, the transmitted signal suffers from strong time dispersion due to multipath propagation. The received signal consists of reflections from the sea-bottom and the water's surface and other objects present in the water [2]. Multipath propagation additionally includes strong refraction caused by a significant change in sound velocity as a function of depth. Both multipath propagation and refraction produce time dispersion of the transmitted signal, which 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, correct information detection possibilities are significantly limited due to the Doppler effect causing signal spectrum distortions. Spread-Spectrum (SS) techniques are used in modern wireless communication systems due to their interference suppression benefits, energy density reduction, fine time resolution, and enabling of multiple access. The many features of pseudo-random signal processing techniques that are important for spread spectrum communication include the ability to cope with multipath propagation, resistance to interference, and the potential of sharingspectrum resources with other users. Spread-spectrum systems have a low probability of intercept (LPI). Transmitter-receiver pairs using independent random carriers can operate in the same bandwidth, with minimal co-channel interference. Moreover, SS systems have cryptographic capabilities, where the data modulation cannot be distinguished from the carrier modulation, and the carrier modulation is effectively random to an unwanted observer [3]. One of the SS techniques is the Direct-Sequence Spread Spectrum (DSSS) method. The DSSS signal occupies a much wider bandwidth than the minimum bandwidth needed to send the information. The spreading is implemented as a multiplication of the sequence of information bits with a Pseudo-Noise (PN) sequence independent of the data. At the receiver, recovery of original data is performed by correlation of the received spread signal with a replica of the spreading sequence [4]. Spread spectrum techniques are attractive for designers of UAC systems operating in shallow-water channels [5; 6]. The DSSS technique is used in systems with one-to-one communication due to its resistance to interference caused by multi-path propagation. In addition, it is useful in underwater acoustic networks, making the implementation of Direct-Sequence (DS) Code-Division Multiple Access (CDMA) possible. A PN-sequence used in a DSSS system as a spreading sequence should have good autocorrelation properties to achieve matched filtration benefits at the receiver. Hence, one-to-one communication systems often use m-sequences. However, in CDMA systems, the spreading sequences should have both good autocorrelation and cross-correlation properties. In such a system, each communication participant uses a unique PN-sequence when sending and receiving messages. The transmitted signals should not interfere with each other; therefore the PN-sequences should be mutually orthogonal, and thus its cross-correlation should be equal to zero. There are very few m-sequences of any given length with good cross-correlation properties. As a consequence, when numerous PN-sequences are needed, one needs to use Gold sequence sets or Kasami sequence sets, which have good cross-correlation properties and good autocorrelation properties [7]. There are numerous reported UAC systems using the DSSS technique.