# Covert Channel Based on Quasi-Orthogonal Coding
## 7 Summary  
Creating a covert channel in the physical layer of wireless communications is an issue that is difficult to implement in practice. The low probability of detecting such a channel and the need to affect the cover's signal as little as possible entails the low power that can be allocated to covert transmission. The natural solution in such a situation is to increase energy per bit by increasing its duration while accepting a lower transmission speed. Such a solution encounters a serious problem, arising from the estimation of channel parameters, which becomes apparent in the inability to extract the covert signal. The purpose of the article was to identify solutions to this type of problem. First, it was noted that higher transmission speed can be achieved by using FSK modulation, which does not require an increase in signal power if covert modulation order is increased, since the energy per bit remains constant, and this is done at the expense of signal bandwidth. However, as noted, FSK modulation is more difficult to receive in non-perfect SIC compared to low-value PSK modulation. The solution in such a case may be the use of sorting, which aims to more easily extract the signal through greater separability of signals in the frequency domain. Although, at the transmitter, the primary FSK signal is converted to a pseudo-noise sequence, the final reception is performed by a traditional FSK demodulator. The proposed solution for creating quasi-orthogonal signals can also be applied to other modulations, which was also simulated in this paper. Importantly, the sorting method is determined by the cover signal, hence there is no need to send additional information between the transmitter and receiver. Although a correct decoding of a block of cover data is required to receive a single or several covert symbols, this is not a major limitation, since a cover signal is a strong signal by its very definition.  
Conceptualization, K.G.; Methodology, Z.P. and J.M.K.; Validation, K.G., Z.P. and J.M.K.; Formal analysis, K.G.; Investigation, Z.P.; Writing-original draft, K.G. and J.M.K.; Writing-review & editing, Z.P. and J.M.K.; Visualization, K.G.; Funding acquisition, J.M.K. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Carrier Frequency** & **850 MHz** & \\ \hline \multirow{4}{*}{Cover (carrier)} & Modulation & 16 QAM \\  & Bandwidth & 8 MHz \\  & Transmission rate & 32 Mb/s \\  & Block length & 16 \\  & Power & \(P_{0}\) \\ \hline \multirow{4}{*}{Cover information} & Modulation & 8 FSK \\  & Number samples per symbol & 16 \\ \cline{1-1}  & Number symbols in block & 1 \\ \cline{1-1}  & Transmission rate & 1.5 Mb/s \\ \cline{1-1}  & Power & \(0.01\cdot P_{0}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters of radio signal.  
Figure 16: BER versus SNR for covert signal transmission with and without sorting.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Acknowledgments:** The authors would like to express their great appreciation to the sensors journal editors and anonymous reviewers for their valuable suggestions, which have improved the manuscript quality.  
**Conflicts of Interest:** The authors declare no conflict of interest.