Fig. 1. The symbol structures of common modulation methods for long-distance UWOC

Fig. 2. The bandwidth requirements of three modulation methods at the same communication rate

Fig. 3. PPM raw signal and received signal image at R_SN=2

Fig. 4. BER and SNR relationship of the five demodulation methods

Fig. 5. The modulation process of improved Manchester coding

Fig. 6. Data graphs before and after the sliding mean filtering module

Fig. 7. Schematic diagram of threshold free parallel demodulation for frame headers

Fig. 8. Implementation of thresholdless parallel demodulation program

Fig. 9. Image of PMT signal at the receiving end

Fig. 10. Overall block diagram of UWOC

Fig. 11. The test environment and verification methods for UWOC