## 4 Results of Underwater Experiment
### Performance of Single Sensor Covert Underwater Communication  
To analyze the performance of the proposed RAKE receiver, data were transmitted by varying the PN code bits \(N_{T_{c}}\). The signal is transmitted 10 times iteratively by placing a delay between data packets.  
In the RAKE receiver, the threshold value and weighted coefficient proposed in this paper were applied and the number of iterations in the decoder was set to five to represent the number of errors, and packets were displayed by time. Out of 10 packets, only packets whose data were all decoded were used to compare and analyze the performance of the RAKE receiver. The performance category was divided into three cases: one in which the threshold value and the weighted coefficient were not applied to the RAKE receiver, one in which only the threshold value was applied, and one in which both the threshold value and the weighted coefficient were applied.  
Table 2 presents a comparison and analysis of performance in the RAKE receiver using only packets data decoded out of 10 packets. Bit error rate listed in Table 2 means how many errors occurred per information bits. The bit error rate is represented based on three cases: when the threshold value and the weighted coefficient were not applied to the RAKE receiver, when only the threshold value was applied, and when both the threshold value and the weighted coefficient were applied.  
Through the experiment, it was found that when \(N_{T_{c}}\) was 8 bits, all data were decoded in three packets out of 10; when \(N_{T_{c}}\) was 16 bits, all data were decoded in four packets; and when \(N_{T_{c}}\) was 32 bits, all data were decoded in five packets. With regard to packets whose data were all decoded, the performance was analyzed when the threshold value and weighted coefficient proposed in this paper were applied. When only the existing RAKE receiver was applied, errors were generated  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**PN Code**} & \multirow{2}{*}{**Method**} & \multicolumn{5}{c}{**Packet Index**} \\ \cline{3-6}  & & **1** & **2** & **3** & **4** & **5** \\ \hline \multirow{3}{*}{\(N_{T_{c}}=8\)} & Only RAKE receiver & \(4.91\times 10^{-1}\) & \(4.78\times 10^{-1}\) & \(4.71\times 10^{-1}\) & - & - \\  & RAKE receiver with threshold & 0 & \(4.85\times 10^{-1}\) & 0 & - & - \\  & RAKE receiver with threshold & 0 & 0 & 0 & - & - \\  & and weighted coefficient & 0 & 0 & 0 & - & - \\ \hline \multirow{3}{*}{\(N_{T_{c}}=16\)} & Only RAKE receiver & \(4.94\times 10^{-1}\) & \(4.30\times 10^{-1}\) & \(4.74\times 10^{-1}\) & \(3.91\times 10^{-1}\) & - \\  & RAKE receiver with threshold & \(0.59\times 10^{-1}\) & 0 & 0 & 0 & - \\ \cline{1-1}  & RAKE receiver with threshold & \multirow{2}{*}{0} & Only RAKE receiver & \(3.21\times 10^{-1}\) & \(4.81\times 10^{-1}\) & \(4.76\times 10^{-1}\) & \(4.82\times 10^{-1}\) & \(4.69\times 10^{-1}\) \\ \cline{1-1}  & RAKE receiver with threshold & \multirow{2}{*}{0} & \multirow{2}{*}{0} & \multirow{2}{*}{0} & \multirow{2}{*}{0} & \multirow{2}{*}{0} \\ \cline{1-1}  & and weighted coefficient & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Bit error rate according to \(N_{T_{c}}=8\), 16, 32 bits by RAKE receiver.  
Figure 4: Illustration of the sea trial using the single sensor.  
with regard to all \(N_{T_{c}}\). When the threshold value was applied, data in a majority of data packets were decoded without errors. However, even if the threshold value was applied, errors occurred in the second packet in \(N_{T_{c}}=8\), in the first packet in \(N_{T_{c}}=16\), and in the third packet in \(N_{T_{c}}=32\). Accordingly, the weighted coefficient was applied and all data were decoded without errors.