## 2 System Model of RSST-SR-DCSK
### Transmitter Structure of RSST-SR-DCSK  
Figure 5 illustrates the transmitter of the RSST-SR-DCSK system. In the front end of the reference slot, the data selector is utilized to transmit \(N\) information bits \(b_{0}\cdots b_{N-1}\), and the chaotic signal from the chaotic signal generator is formed by selecting one of the \(2^{N}\) scramblers to constitute the reference signal. In the information slot, after replication P times and delaying by \(\beta\), the delayed chaotic signal is multiplied by the pre-set M information bits to obtain the information signal. Taking the \(k\)th frame transmission signal as an example, the chaotic signal generated by the chaotic signal generator is scrambled by a scrambler to serve as the reference signal \(\hat{x}_{i,k}\) for that symbol period. The reference signal is then replicated P times and delayed by R to obtain \(x_{i-R,k}\). Finally, the two modulated information signals are added together, and the overall result serves as the system's transmission signal. Here, \(x_{0,k}\), \(x_{1,k}\cdots x_{R-2,k}\), \(x_{R-1,k}\) represents the chaotic sequence of length \(R\) for the \(k\)th frame, and \(\hat{x}_{0,k}\), \(\hat{x}_{1,k}\cdots\hat{x}_{R-2,k}\), \(\hat{x}_{R-1,k}\) represents the reference signal of length \(R\) after being scrambled. The scrambler unit consists of \(2^{N}\) scramblers, each corresponding to the pre-set \(N\) information bits. Each scrambler receives a chaotic signal and generates a scrambled chaotic signal, which is then transmitted to the selection switch. The selection switch chooses the corresponding scrambler based on the pre-set \(N\) information bits and obtains the scrambled chaotic signal. This scrambled chaotic signal is used to transmit the \(N\) information bits, generating the reference signal. The main purpose of the scrambler is to reduce the correlation between the reference signal and the information signal, thereby improving the security performance of the system. The scrambler configuration can adopt stream cipher construction methods, such as altering the order of the chaotic signal with a finite length, to disrupt the correlation between systems and enhance security. Alternatively, it can use matrix transformations to change signal rows and columns, among other methods. Assuming that \(N\) is set to 2, which corresponds to 4 scramblers, the selection of a particular scrambler is determined based on the two transmitted information bits. For example, if the transmitted information bit is 00, scrambler 1 is selected, and the working principle of scrambler 1 is to swap the signal values corresponding to adjacent positions in a chaotic signal; if the transmitted information bit is 01, scrambler 2 is selected, and the working principle of scrambler 2 is to evenly divide a chaotic signal into two segments,  
Figure 4: RSST-SR-DCSK demodulation frame. Figure 5: RSST-SR-DCSK transmitter. and swap the signal values corresponding to the even positions of the first half of the segment and the odd positions of the second half; if the transmitted information bit is 10, scrambler 3 is selected, and the signal values corresponding to the odd positions of the second half are swapped; if the transmitted information bit is 10, scrambler 3 is selected, and the signal values corresponding to the even positions of the first half are swapped.