## 6 Conclusions  
The idea of the spreading spectrum technique is to widen the signal bandwidth in order to obtain the desired transmission rate and reliability at a given noise level. However, as demonstrated by the simulation and experimental tests, in conditions of strong multipath propagation, extending the signal bandwidth deteriorates data transmission reliability. This is because of the frequency-selective fading observed in the spectrum of the signal, which undergoes numerous reflections. The simulation tests performed with the use of a measurement pool simulator have shown that in the case of a small number of signal reflections reaching the receiver (channel model with 1 virtual source), it is possible to obtain reliable data transmission at any transmission rate resulting from the bandwidth. Increasing the number of reflections of the signal has no effect on the results achieved with m-sequence in the 1 kHz and 2 kHz bandwidths, but further increasing the bandwidth increases the required SNR.  
In each of the simulation and experimental tests of the transmission of signals in a bandwidth equal to 1 kHz, the reliability described by the BER less than 0.001 was achieved without the need to use ECC encoding. However, the relatively narrow bandwidth limits the transmission rate to 3.92 bps. In cases where bandwidth is equal to 2 kHz, slightly worse BER values were obtained; however, it is possible to reach a BER value of less than 0.01 or 0.001 when using reasonably redundant ECC encoding.  
During the simulation tests using signals of bandwidth equal to or greater than 4 kHz, significantly worse reliability results were obtained than for bandwidths equal to 1 kHz and 2 kHz. In the case of BER obtained in the Watermark simulator, this value cannot be improved to the level of 0.01 while maintaining a useful data transmission rate.  
During the experimental tests in the measuring pool, the BER achieved in bandwidth equal to 8 kHz was much lower than in simulation tests. Therefore, future analysis of the adequacy of the simulation UAC channel models for real conditions in the measuring pool is necessary when the UAC system operates within s wide band. Moreover, in strong multipath propagation conditions, it is difficult to measure the transmission parameters of the underwater channel. The probe signals used are naturally wideband because the bandwidth is only an order of magnitude smaller than the value of the carrier frequency. Therefore, probe signals with a bandwidth of 1 kHz and 8 kHz make it possible to obtain significantly different estimates of the channel transmission parameters. This may also make it difficult to design the DSSS data transmission parameters, the physical layer of which should be adjusted to the channel's transmission properties.  
In all the simulation and experimental tests performed, using m-sequences allowed for better DSSS transmission reliability than the use of Gold and Kasami codes as spreading sequences.  
**Author Contributions:** Conceptualization, I.K.; methodology, R.S.; software, I.K.; validation, J.H.S. and A.M.S.; formal analysis, I.K.; investigation, J.H.S. and A.M.S.; resources, J.H.S.; data curation, A.M.S.; writing--original draft preparation, I.K. and R.S.; writing--review and editing, I.K. and J.H.S.; visualization, A.M.S.; supervision, R.S.; project administration, J.H.S.; I.K. and R.S. contributed equally to this work. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.