A.L. Gelgor1, D.A. Puzko2,  Y.M. Skorodumov3, E.V. Lukoyanov4, A.E. Panarin5Pashkevich6

1,2 Peter the Great St. Petersburg Polytechnic University (St. Petersburg, Russia)

3-6 State Research Center of the Russian Federation Concern CSRI Elektropribor, JSC

1 agelgor@spbstu.ru; 2 danilapuzko@mail.ru; 3 skorum@mail.ru; 4 lukoyanov.egor@mail.ru; 5 alex32757@gmail.com, 6iv@bk.ru

Formulation of the problem. OFDM has recently taken a leading role for high data-rate underwater acoustic communications over short distances and under relatively high signal-to-noise ratios (SNR). It is due to the high efficiency of OFDM in multipath channels along with the relatively low computational complexity of its processing. At the same time, CDMA has traditionally been used for this type of communication. To develop the use of OFDM in underwater acoustic communications, an interesting task is to implement the transmission of underwater acoustic signals with OFDM imperceptibly or over long distances, i.e. in conditions where the signal level is below the noise level. This will make it possible to create universal underwater acoustic modems based on OFDM.

Objective. Разработка и апробация для гидроакустического канала методики формирования и обработки сигнально-кодовых конструкций на основе сигналов с OFDM, обеспечивающих передачу информации в условиях значений ОСШ меньше 0 дБ с возможностью адаптации к помеховой обстановке.

Results. A technique is proposed for the formation and processing of modulation and coding scheme based on OFDM for effective data transmission under SNR values below 0 dB. Specifically, firstly, it is proposed to perform coherent signal accumulation on a resource block - cells of several adjacent subcarriers and several adjacent OFDM symbols. Secondly, it is proposed to use differential modulation and thereby eliminate the need for channel estimation and equalizing. Thirdly, it is proposed to use a cascade connection of an external convolutional code and an internal repetition code. Fourthly, to ensure the possibility of detecting a signal and estimating the Doppler coefficient, it is proposed to use two successive pulses of hyperbolic frequency modulation with opposite laws of frequency change over time. Fifthly, to compensate for the Doppler effect, it is proposed to use the chirp-z transform. The proposed modulation and coding schemes were successfully tested in real cases in the waters of Lake Ladoga. Error-free data transmission was demonstrated at SNR equals –18.6 dB.

Practical significance. The presented results confirm the feasibility of using OFDM to transmit data over an underwater acoustic channel under low SNR. At the same time, the choice of parameters of the modulation and coding scheme allows you to adapt to the interference environment.

Gelgor A.L., Puzko D.A., Skorodumov Y.M., Lukoyanov E.V., Panarin A.E., Pashkevich I.V. On OFDM for Underwater Acoustic
Communications at low SNR. Radiotekhnika. 2024. V. 88. № 3. P. 48−62. DOI: https://doi.org/10.18127/j00338486-202403-06
(In Russian)

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