# A Wireless Covert Channel Based on Dirty Constellation with Phase Drift
## 6 Hardware Implementation  
The concept of the phase drift-based dirty constellation was implemented by using the Universal Software Radio Peripheral (USRP) hardware platform manufactured by National Instruments (NI, Austin, TX, USA). In this case, we used the USRP-2920 model. USRP was the essential hardware part for generating a radio signal, while the software part was provided by the LabView software with MATLAB scripts installed on a personal computer (PC). An Ethernet network adapter with a bit rate of 1 Gb/s was used to provide communication between the USRP platforms and PC via a switch. Two USRP-2920 were used to implement a test-bed for detectors in the transmitter-receiver system. The prepared test-bed was placed in an office room. The distance between the transmitter and receiver was 5 m. This configuration is illustrated in Figure 17. The parameters of the carrier signal and covert information are presented in Table 2.  
The results of the experimental research are presented in Figures 18 and 19. In this case, we determined a probability density function (PDF) of EVM and BER versus EVM graphs for two considered \(K\) values (see Figure 18). The distribution of the EVM was estimated by using a normalized histogram (see Figure 19).  
The carrier constellation is subject to distortions caused by propagation phenomena and impairments introduced by the transceiver systems. We may see that the covert signal detectability reduces if the carrier distortion (i.e., EVM) increases. Tests were performed for two average values: \(K=10\) and \(K=25\). As expected, increasing the number of averages provides the increase in the covert signal detectability. On the other hand, the transmission rate decreases.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Signal** & **Parameters** & **Value** \\ \hline \multirow{4}{*}{Cover (carrier)} & Modulation type & 4 QAM \\  & Carrier frequency & 850 MHz \\  & Bandwidth & 1 MHz \\  & Transmission rate & 4 MHz \\ \hline \multirow{4}{*}{Covert information} & \(M\) & 4 \\  & \(A_{m}\) & 0.05-\(A_{m}\) \\ \cline{1-1}  & \(\varphi_{m}\) & \(\pm\pi/4\), \(\pm 3\pi/4\) \\ \cline{1-1}  & \(K\) & 10 and 25 \\ \cline{1-1}  & \multirow{2}{*}{Transmission rate} & 400 kb/s (for \(K=10\)) \\ \cline{1-1}  & & 160 kb/s (for \(K=25\)) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters of radio signal.  
Figure 17: Test-bed based on USRP-2920 used for practical verification.  
Based on the PDF of EVM, we can conclude that for the analyzed 4 QAM and \(A_{m}=0.05\cdot A_{n}\), the average EVM oscillates between 8-12%. Comparing this value with Table 1, we can expect good transmission quality. On the other hand, increasing the distance between the transmitter and receiver will degrade the transmission quality due to the influence of the propagation conditions occurring in the radio channel. Hence, the adaptive selection of the parameters of the developed method depending on the propagation conditions will determine its future usefulness.  
Figure 19: Covert signal detection results: BER versus EVM for selected \(K\).  
Figure 18: Covert signal detection results: probability density function (PDF) of EVM.