## 1 Introduction  
Multifrequency methods of signal modulation, such as OFDM, gained a lot of attention not only in the radiocommunications, but in underwater acoustic communications (UAC) as well. Current popularity of such methods in hydro acoustic communications relates to possibilities of the effective usage of frequency resource of the underwater acoustic channel. The growing needs of autonomous underwater vehicles (AUV) capabilities for the World Ocean research and exploration demand the implementation of effective communication systems in regard to throughput and noise-sustainability, which are used for underwater navigation. AUVs and other surface/underwater objects of marine mission maintenance (vessels, buoys, autonomous surface vehicles (ASV), and stationary bottom underwater stations) demand the organization of proper underwater communication and navigation under conditions of underwater environment with highly unstable parameters [1; 2]. Usually, data transmission rate is not the main factor and its performance of a couple of tens/hundreds bits per seconds is enough for navigation and telemetry transmission purposes. To organize a connection with AUV the preferred throughput must be an order higher, in Kbit/s; and, under such conditions as alternating acceleration of underwater and surface vehicles, frequency and time selective signal fading, the presence of high power impulse noises, and Doppler frequency shifts. In that case, one of the key parameters defining the achievable throughput is the total bandwidth of receiver and transmitter paths. The defining aspect of this issue is performance of the transmitting acoustic antenna and the underwater channel itself with its strongly marked non-uniform frequency transfer response. There are several main methods of Doppler shift compensation in OFDM-based underwater acoustic communication systems. One of them is exploiting of Doppler-resilient signals (for example with linear frequency modulation) as a preamble and a postamble of OFDM signal to detect changes in its length [3]. Also, some a priori vulnerable for Doppler effects signals (like, M-sequences, OFDM test signals, etc.) can be used as a preamble for OFDM symbol. They are used in combination with a bank of matched filters in the receiver, and each of them are tuned for predetermined value of Doppler shift [4; 5]. Responses of filter bank allows for estimating the influence (vector) and value of frequency shift and to compensate it. For example, in the paper [6], simulation of UAC system utilizing pseudo-noise sequences as a preamble were conducted resulting low error rate (BER < 0.01) with peak frequency shift of 40 Hz on central frequency of 12 kHz. Other way of increasing of Doppler shift resistance is adding of null or pilot sub-carriers into OFDM packet. Such approach was used in [7], where authors were able to reach 1% BER while maximum Doppler shift (for the highest sub-carrier) was 13.36 Hz, which was more than sub-carriers frequency spacing (11.71 Hz). In addition, all of these methods can be used in together in different combinations, but adding of each new feature almost always results in a degradation of data throughput. Existing modern hydro acoustic communication equipment can be divided into three categories,according to the effectiveness of signal processing: with high spectral efficiency (from 2 to 4 bit/s/Hz) exploiting parallel frequency division of information symbols (OFDM) and M-QAM methods; with moderate spectral efficiency (from 0.1 to 1 bit/s/Hz) based on sequential frequency division (S2C, etc.) [8; 9]; and low frequency efficiency (0.1 bit/s/Hz and lower) based on direct-sequence spread spectrum (DSSS) [10]. Theoretically, the specified spectral efficiency of the system with working frequencies up to 60 kHz allows to create relatively high-speed underwater communication links (up to tens of Kbit/s), but this throughput is achievable only if the channel parameters are stable and the transmitter/receiver are tuned properly using up-to-date complex approaches.