To meet the requirements of the engineering of high-speed Underwater Wireless Optical Communication (UWOC) technology, and to address the issue of high Bit Error Rate (BER) in underwater channels when using modulation methods such as On/Off Keying (OOK) that rely on threshold demodulation for high-speed communication, this paper conducts comparative simulation analysis of commonly used underwater optical communication modulation methods and proposes an improved modulation and demodulation algorithm that combines Pulse Position Modulation (PPM) with Manchester encoding and does not require threshold decision-making.

The paper uses sliding mean filtering to improve the Signal to Noise Ratio (SNR). The frame synchronization is then achieved by parallel multi-path threshold-free decision-making on the PPM frame header, providing a time calibration basis for Manchester encoding and demodulation. Finally, high-speed communication system with low BER is realized by using Manchester encoding sub-frame headers and Reed-Solomon (RS) error-correcting codes in the data part. The algorithm is designed and implemented on a Field Programmable Gate Array (FPGA), and a UWOC system using this method is built to conduct BER tests under different environments and distances.

The test results show that the UWOC system based on this demodulation algorithm has an BER of 10^-5 in indoor clear water pools and 10^-3 in nearshore lake water at a communication speed of 50 Mbit/s and a communication distance of 10 m.

The proposed algorithm in this paper is applied in nearshore lake water with a distance of 10 meters, compared to modulation methods such as OOK and PPM, using modified Manchester encoding modulation can achieve more reliable 50 Mbit/s communication.