# Lamb Wave-Based FDM-PPM Method Data Transmission Scheme in Plate Structures
## 4 Discussion
### Comparison of the FDM-PPM with ASK and PPM Method  
Based on the modulation principle of the FDM-PPM method, it significantly enhances the communication rate of the system compared with ASK, PPM, and FDM. The following section will explore the relationship between the number of signal bits, communication rate, and excitation signal energy with the time slot length and compare the performance differences between FDM-PPM and the other modulation methods. Specifically, the ASK method is independent of the number of time slots and subcarriers; the FDM method is related to the number of time slots but not to the number of subcarriers, whereas the PPM method is related to the number of subcarriers but independent of the number of time slots. In contrast, the longer the signal duration of the FDM-PPM method, the more bits can be transmitted within the same period. A schematic comparison of the symbol signal duration between the FDM-PPM method and the aforementioned three methods is shown in Figure 13.  
As illustrated in Figures 1 and 13, the signal duration for ASK and FDM is \(T_{f}\), whereas the signal duration for PPM and FDM-PPM is \(T_{s}\), where \(T_{s}\) equals Ns multiplied by \(T_{f}\) (\(T_{s}=N_{s}\times T_{f}\)). Consequently, multiple ASK and FDM signals can be accommodated within the duration of one FDM-PPM signal. Under the same signal duration condition, there are significant differences in signal bits and communication rates among ASK, FDM, PPM, and FDM-PPM. Figure 14 shows the specific differences in the number of bits between FDM-PPM and the aforementioned three methods when \(N_{s}\) equals 2. Figure 14 demonstrates that the number of bits transmitted using the FDM PPM method significantly increases compared to the ASK and PPM methods. When the number of subcarriers is low, the FDM-PPM method exhibits a higher bit count compared with FDM; however, as the number of subcarriers grows, the bit count of FDM signal transmission gradually surpasses that of FDM-PPM. According to Equation (2), the energy of the FDM  
Figure 12: The impact of FDM-PPM signal parameters on bit number: (**a**) the number of time slots, (**b**) the number of time slots, and (**c**) the number of subcarriers. Figure 13: Schematics of the duration of different types of symbol signals: (**a**) ASK, (**b**) PPM, (**c**) FDM, and (**d**) FDM-PPM. PPM signal is only \(1/N_{s}\) the energy of ASK and FDM signals (where \(N_{s}\) is the number of frames) and is comparable to the energy of PPM signals. Figure 15 illustrates the differences in the number of bits for various frame numbers when the number of subcarriers is 2. Figure 15 reveals the bit performance of FDM-PPM signals under different frame numbers. When \(N_{s}\) is 2, the number of bits in the FDM-PPM signal is significantly higher than that of the other three modulation methods. However, when \(N_{s}\) increases to 3, although the number of bits in the FDM-PPM signal is higher than that of ASK and PPM, it only exceeds that of FDM when the number of time slots is greater than 7. Furthermore, when \(N_{s}\) is 4, the number of bits in the FDM-PPM signal only surpasses that of FDM when the number of time slots exceeds 31. These results indicate that the bit advantage of FDM-PPM signals gradually emerges with increasing frame and time slot numbers, but their dependence on the number of time slots is also more pronounced. Based on the aforementioned analysis, FDM-PPM signals, when employing a small number of subcarriers, not only transmit a more significant number of bits but also exhibit the advantage of lower transmission-signal energy compared with ASK, PPM, and FDM signals. As the number of subcarriers increases, the bit count of FDM-PPM signals surpasses that of ASK and FDM signals. Although the bit count of FDM-PPM remains lower than that of FDM, its signal energy is relatively lower. Moreover, an increase in subcarriers generally leads to a decline in transmission accuracy across various communication methods.