## 1 Introduction  
The development of telecommunication systems is moving in the direction of increasing the efficiency of using time-frequency resources. To improve the spectral efficiency, new methods of multiplexing user data are being developed, such as non-orthogonal multiple access (NOMA), and multiple-input-multiple-output (MIMO) spatial multiplexing algorithms are being improved (Massive MIMO and MU-MIMO technologies). Higher-order manipulation is used, coupled with an increase in the signal-to-noise ratio (due to a decrease in the coverage area of base stations and access points), and new group signal structures are being developed that provide greater spectral efficiency compared to cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) technology, which is the most common technology in radio data transmission systems requiring high transmission speeds. One of these technologies is the filter bank multicarrier (FBMC), which provides a lower level of out-of-band emissions by filtering signals on each subcarrier and enables an increase in spectral efficiency due to the possibility of eliminating the cyclic prefix, which makes this modulation technique a prospect for future cellular communication systems [1, 2, 3]. The idea of the FBMC system comes from attempts to change the time and frequency localization of generated discrete modulation pulses. Using perfect reconstruction filter bank theory, in [4], the authors introduced a data transmission system algorithm that allows the manipulation of the localization of data symbol pulses to achieve the higher spectral efficiency of the signal. The applicability of FBMC modulation for high-frequency (HF) channels was introduced in [5] with the addition of the spread-spectrum (SS) technique, and the robustness of the introduced modulation technology was proved for channels with low SNR values and a wide range of Doppler spread and delays. Further, the same authors in [6, 7] provide theoretical and practical comparisons of the FBMC-SS system and Walsh sequence-based direct sequence spread systems (DSSS). The same authors in [8] presented an analysis of FBMC-SS performance in doubly dispersive HF channels and showed methods of system parameter optimization to reduce the influence of doubly dispersive channel distortions. In [9], another author group provides a comparison of three types of multicarrier modulation systems--CP-OFDM, offset-QAM FBMC (FBMC-OQAM) and filtered multitone (FMT). The authors concluded that the best performance was obtained with the filtered multitone system (based on the same filter bank paradigm, except using non-overlapping subcarriers and filters instead of FBMC) thanks to the null self-interference influence, but the authors note that benefits from increased FBMC spectral efficiency are more significant and emphasize the importance of channel estimation and equalization algorithms for FBMC-OQAM systems. Another comparison of FBMC-OQAM, CP-OFDM and FMT performance was presented in [9] and shows that CP-OFDM and FMT have advantageous effects only for high SNR values, where the FBMC-OQAM self-interference noise level became comparable to channel noise. In [10], we provide an investigation of FBMC-OQAM signal properties in conditions of wideband HF channels, and the presented modeling results show the resistance abilities of FBMC-OQAM signals in the case of Watterson's channel model [11], including the influence of ionospheric dispersion distortions of signals, which greatly corrupt wideband signals. As noted in the works discussed above, the development of algorithms for qualitative channel estimation under conditions of FBMC-OQAM self-interference is an important task for such systems. Solutions to this problem are based on the generation of a training sequence that will cancel the self-interference of each symbol and will be separated from the data payload to protect the training sequence from interference generated by payload symbols.