# Lamb Wave-Based FDM-PPM Method Data Transmission Scheme in Plate Structures
## 1 Introduction  
Traditional sensor networks primarily rely on wired data transmission. However, in certain specific scenarios, the structural complexity of the monitored components or harsh operating environments can limit the feasibility of wired transmission [1, 2, 3]. With their low cost and long-distance propagation advantages, Lamb waves have become an important technical means for structural condition monitoring [4, 5, 6]. In specific scenarios, complex working conditions limit the use of cables and electromagnetic waves. For instance, in the online monitoring systems of the aerospace field, the installation of cables compromises structural integrity and increases the difficulty of installation and maintenance. Although the use of electromagnetic wave wireless communication can achieve lightweight sensor networks, the Faraday effect causes metal panels to attenuate electromagnetic waves, significantly affecting the data transmission's reliability. Additionally, the electromagnetic waves radiated outward by the sensor networks may interfere with onboard aircraft equipment, posing potential risks. Integrating Lamb waves from non-destructive testing as a carrier to achieve a sensing network that combines detection and communication is important for promoting the development of structural health monitoring networks [7]. Currently, Lamb wave data transmission's modulation and demodulation methods include on-key control, phase-shift keying, frequency-division multiplexing, code-divisionmultiple access, and other methods [8; 9; 10]. [PERSON] et al. took the sinusoidal pulse signal processed by the Hanning window as the carrier signal and modulated it using on-off keying, achieving a data transmission rate of 100 kbps on a glass fiber composite plate [11]. In addition, to address the Lamb wave communication among multiple sensor nodes, [PERSON] et al. proposed a Lamb wave communication method based on time-reversal pulse position [12]. This method leverages the differences in transfer functions along different propagation paths to enable data transmission among multiple nodes on an aluminum plate. However, when the differences in transfer functions between sensor nodes are minor, these nodes cannot be effectively distinguished using the aforementioned method. The research team drew inspiration from code-division multiple access (CDMA) to address this. It employed Kasami sequences as spreading codes, which were modulated by single-cycle sinusoidal signals to generate Lamb wave CDMA signals [13]. Despite this, the CDMA method suffers from a low transmission rate and requires dispersion compensation for Lamb wave dispersion. To resolve these issues, [PERSON] et al. proposed a Lamb wave communication method based on FDM, which allocates different subcarrier frequencies in the frequency domain to enable data transmission among multiple nodes [14; 15]. Building on this, [PERSON] et al. divided the waveguide bandwidth in FDM sensor networks into communication and defect bands [16]. Subsequently, [PERSON] et al. used finite element models and experiments to study the selection of Lamb wave excitation frequencies for FDM on stiffened plates, verifying that the proposed method achieves a high signal-to-noise ratio [17]. [PERSON] et al. experimentally compared Lamb wave communication methods using on-off keying (OOK), ASK, and binary phase-shift keying (BPSK) and found that the BPSK method exhibited superior interference resistance [18; 19; 20]. [PERSON] et al., starting from the perspective of sparse compression of Lamb waves, used a phase-sparse reconstruction method to recover information from Lamb wave data and proposed a pulse code frequency-division multiple access (PCF-DMA) method to achieve the multi-access transmission of Lamb wave data [21; 22; 23]. Due to the impact of Lamb wave multimodal and dispersive characteristics on communication signal encoding methods, many efficient communication techniques are challenging to apply to directly transmit Lamb wave acoustic data.