# Lamb Wave-Based FDM-PPM Method Data Transmission Scheme in Plate Structures
## 3 Analysis of the Experimental Results for Lamb Wave FDM-PPM  
### Lamb Wave Continuous Data Transmission Experiment Based on FDM-PPM  
Based on the layout of the sensor nodes and the available bandwidth, the signal parameters were designed in this study as follows: the signal duration was set to 0.1 ms, the frame width \(T_{f}\) was 50 us, the number of time slots \(N_{ts}\) was 4, and the combination of subcarrier frequencies at 190 kHz, 230 kHz, and 270 kHz was selected. On this basis, Lamb waves were employed to transmit signals with varying numbers of symbols (ranging from  
Figure 7: (**a**) Layout of sensor node in the experiment and (**b**) experimental platform.  
two to seven), with each symbol encoding five bits of information. For example, a signal comprising seven symbols is presented (encoded information: 111 00111 00011 10001 10001 01000 100010), and the corresponding excitation signal and received Lamb wave signal are illustrated in Figure 8.  
As observed in Figure 8, the influence of the structural boundaries causes significant overlap between the direct wave and the first few reflected waves of the Lamb wave, rendering the demodulation of transmitted information using matched filtering methods impractical. Moreover, with seven distinct signal waveforms, employing cross-correlation methods would require simultaneous cross-correlation operations on multiple signals, significantly increasing the computational load for demodulation. According to the method illustrated in Figure 6, constructing a demodulation atom library allows for compensation of the Lamb wave echo signals. The compensated signals are shown in Figure 9.  
Figure 9 illustrates the Lamb wave signal after equalization compensation containing seven symbols. In the figure, the red lines mark the starting position, the 1/2 position, and the ending position of the signal; the green lines correspond to the 1/4 and 3/4 positions of the signal; and the black lines indicate the 3/8 and 5/8 positions. In benchmarking the signal wave packets, the location information carried by the signal can be precisely obtained. Additionally, the frequency and phase information of the signal is extracted through the Fourier transform, thereby enabling the complete recovery of the transmitted information.  
To verify the effectiveness of the proposed equalization algorithm in demodulating FDM-PPM signals, this study conducted multiple transmission and demodulation experiments on Lamb wave FDM-PPM signals with varying numbers of symbols (ranging from two to seven). The results demonstrate that the algorithm can effectively address the demodulation requirements for signals with different symbol counts. The detailed demodulation performances are presented in Table 1.  
Figure 8: (**a**) FDM-PPM excitation signal and (**b**) FDM-PPM received signal.  
Figure 9: (**a**–**g**) intercepted Lamb wave FDM-PPM signals corresponding to excitation numbers 1 to 7, respectively.  
As observed in Table 1, when employing the Lamb wave FDM-PPM method for data transmission, the demodulation accuracy decreases gradually with an increased number of symbols. In this Lamb wave communication system, the maximum communication rate achievable using the FDM-PPM method is 50 kbps. During the experiment, 970 bits of information were transmitted, of which 880 bits were successfully demodulated, yielding a system demodulation accuracy of 90.7%.