Experimental Research on Performance Comparison of Multi-modulation Formats for Ocean Turbulent Optical Communication Based on QC-LDPC Coded

The Underwater Wireless Optical Communication （UWOC） system has advantages such as high transmission speed and high reliability. The presence of ocean turbulence can induce fluctuations in light intensity， thereby degrading the communication performance of the system. Quasi-Cyclic Low-Density Parity-Check （QC-LDPC） codes can mitigate the impact of ocean turbulence on Bit Error Rate （BER） and enhance the communication performance of UWOC systems. However， research on QC-LDPC codes in UWOC has primarily focused on theoretical analysis and numerical simulation. The gain effect of QC-LDPC coding may vary under different modulation formats， and specific experimental verification is lacking. In response to the absence of experimental research on the coding gain of different modulation formats in turbulent environments， this paper investigates the influence of QC-LDPC codes on the coding gain of different modulation formats in weak ocean turbulent channels under mixed coding modulation schemes.A mathematical simulation model for weak turbulent channels in UWOC is established based on the Log-normal distribution. The check matrix H is constructed using the Dayan sequence， and QC-LDPC encoding is carried out based on the minimum row-column product LU decomposition algorithm to obtain the encoded sequence， which exhibits lower computational complexity compared to direct encoding algorithms. At the receiving end， the Minimum Sum Algorithm （MSA） with low computational complexity is selected for decoding， which is convenient for hardware implementation. The coding gain and communication characteristics of QC-LDPC encoding under different modulation formats， scintillation indices， transmission distance， encoding lengths， and other parameters are simulated and analyzed. Subsequently， a blue-green laser communication experimental platform is designed and built based on a simulated ocean turbulence pool， and comparative experimental research is conducted on the communication performance of QC-LDPC-encoded ocean turbulence optical communication with multiple modulation formats at a transmission distance of 1.5 m and a transmission rate of 10 Mbps. Field Programmable logic Gate Array （FPGA） is employed for QC-LDPC encoding， signal modulation and demodulation. Temperature-induced turbulence is simulated by injecting hot water of different temperatures into a transparent water tank， and salt-induced turbulence is simulated by injecting salt water of different concentrations， and normalized scintillation index δI2is introduced to describe the fluctuation of light intensity at the receiving end， in order to measure turbulence intensity. Aiming to research the encoding gain of QC-LDPC for different modulation formats in weak turbulence environments.The simulation results indicate that the performance improvement brought by adding QC-LDPC encoding in low Signal-to-Noise Ratio （SNR） On-Off Keying （OOK） modulation and Binary Phase-Shift Keying （BPSK） control modulation may be insufficient to offset the increase in decoding complexity， leading to an increase in the BER. Incorporating techniques such as low-pass filtering or probability shaping at the receiving end to improve the waveform can appropriately reduce the BER and increase the encoding gain. The BER of the QC-LDPC coded Differential Phase-Shift Keying （DPSK） modulation system decreases as the code length increases， and the best anti-turbulence effect is achieved when the code length is 2 400. In contrast， the BER of the QC-LDPC coded DPSK modulation system increases with the increase in the scintillation indices， communication distance， and data rate. When the communication distance is 10 m and the scintillation indices is 0.14， the BER of DPSK modulation with QC-LDPC coding is reduced to 4×10⁻⁵. The experimental results show that QC-LDPC encoding can effectively mitigate the impact of weak turbulence on the BER. Compared with the uncoded modulation format， when the BER is 10⁻⁵， the encoding gains of QC-LDPC encoding for OOK modulation in tap water， temperature-induced turbulence （δI2=0.056）， and salt-induced turbulence （δI2=0.034） environments are 0.46 dB， 0.49 dB， and 1.31 dB， respectively； while the encoding gains for DPSK modulation are 0.83 dB， 0.79 dB， and 1.63 dB， respectively. The QC-LDPC encoding gains for OOK modulation and DPSK modulation in salt-induced turbulence are higher than those in temperature-induced turbulence.The coding gain of QC-LDPC codes for OOK modulation and DPSK modulation in salt-induced turbulence is higher than that in temperature-induced turbulence. Adding the mixed modulation of QC-LDPC coding in the UWOC system is conducive to reducing the underwater turbulence effect and has a better turbulence suppression effect compared with DPSK modulation alone. The combination of the QC-LDPC+DPSK coding modulation system has higher coding gain and better anti-interference ability compared with the QC-LDPC+OOK coding modulation system.