Design and implementation of PPM modulation and demodulation algorithm in atmospheric turbulence channel

A pulse position modulation (PPM) algorithm is designed and implemented to counter atmospheric turbulence interference in free-space optical (FSO) communication. A homemade turbulence simulation device based on thermal wind has been constructed to simulated a atmospheric turbulence channel, and it has been utilized to check the performance of the FSO communication system in atmospheric turbulence conditions. Both the signal modulation schemes of 4-ary pulse position modulation (4-PPM) and non-return-to-zero (NRZ) on-off keying (OOK) are implemented and compared in the system, and their corresponding modulation and demodulation algorithms have been realized using the field-programmable gate arrays (FPGA). To verify the effectiveness and practical performance of the PPM algorithm, extensive experiments have been carried out on the FSO communication system under laboratory- simulated atmospheric turbulence conditions, and the bit error rates (BER) of the PPM and OOK modulation schemes have been obtained and compared. The experimental results in the simulating atmospheric turbulence channel show that the PPM modulation system designed in this study yields a lower BER than the OOK modulation system under different turbulence intensities. Furthermore, as the turbulence intensity increases, the BER’s improvement of the PPM modulation system becomes more remarkable. The research results indicate the FSO communication system with PPM modulation possesses superior performance in an atmospheric turbulence channel.