**5. Conclusions**

The analysis, considering high-frequency signal propagation in the cable medium, had shown that the most useful method to provide quality parameters of BPL-PLC transmission, in hazardous areas with the use of the mining cable, is the method related with a variation of all electrical quantities of the cable with the signal frequency. The usefulness of coupling modems with the wired medium, both inductive-inductive and mixed capacitive-inductive, had been also confirmed. This fact is of grea<sup>t</sup> importance for applications, where there is no direct access to the phase, e.g., cables in a mine shaft. The positive impact of reducing the packet length, in the case of BPL-PLC transmission, was also demonstrated.

As shown, the BPL-PLC wired medium, thanks to its physical properties, can provide a reliable voice transmission system. Even in a narrowband scenario (bitrates lower than 1 Mbps), e.g., caused by bandwidth limitations and/or severe damage, etc., this technology ensures a stable and reliable connection. Whenever an emergency situation occurs, voice commands, i.e., from a supervisor or paramedic, can help during any rescue operation. Previous random events around the world have shown how important is to maintain contact and communication with cutout miners. Since voice can be transmitted effectively at 24 kbps, it may be assumed that a physical link with a speed of 1 Mbps possibly will be sufficient for this type of service. Therefore, such transmission parameters could theoretically correspond to a damaged cable. However, this particular case (a real damaged cable), has not ye<sup>t</sup> been investigated by the authors. Whereas, an investigation focused on determining the impact of noise on the quality of voice transmission in a BPL-PLC communication system, may be found in [38].

It should be noted that the results of this work are innovative and not known previously in the literature. The technical medium examination had shown that inductive-inductive coupling has a clear advantage over mixed capacitive-inductive coupling. Moreover, when examining results in the case of voice transmission, there are situations in which mode 11 proved to be superior. As shown, the BPL-PLC technology can provide stable and reliable voice transmission services at 24 kbps, regardless of the spoken language, or even type of coupling. The results of this study can aid both researchers and scientists during the design and maintenance phase of a wired BPL-PLC voice communication system. Future studies may include research on the impact of aging on the physical parameters of the cable materials, as well as real mechanical damage to its elements, on the efficiency of voice transmission via the BPL-PLC communication system.

**Author Contributions:** Conceptualization, G.D. and P.F.-G.; methodology, G.D., P.F.-G., and M.H.; software, G.D. and P.J.; validation, G.W.; formal analysis, B.M.; investigation, G.D. and P.F.-G.; resources, J.W.; data curation, B.P.; writing—original draft preparation, G.D. and P.F.-G.; writing—review and editing, G.D., P.F.-G., B.M. and M.H.; visualization, G.D., P.F.-G. and M.H.; supervision, B.M.; project administration, A.W.; funding acquisition, A.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding. The APC was funded by General Tadeusz Kosciuszko Military University of Land Forces.

**Acknowledgments:** Authors would like to thank KGHM Polska Miedz S.A. and KOMAG Institute of Mining Technology for their support during this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
