Theoretical and Experimental Investigation of Differential Modulation and Detection in FSO Systems

In free-space optical (FSO) communication systems, on–off keying (OOK) modulation is widely used due to its simplicity. However, systems applying OOK suffer from the BER floor in atmospheric turbulence channels, leading to persistently high BER even at high SNR. To mitigate this limitation in atmospheric turbulence channels, differential modulation and detection (DMD) can be adopted. An in-depth theoretical and experimental investigation of DMD in FSO systems is conducted in this paper, considering the effects of turbulence. A comprehensive derivation of the system performance for DMD under atmospheric turbulence channels is also provided, with the results of research revealing that DMD outperforms OOK in high-SNR regions. To validate the theoretical analysis, an experimental platform is set up to sample the fluctuation of light intensity. Furthermore, the system performance of DMD is analyzed under varying scintillation indices, modulation depths, and transmission rates in this paper. Based on the data acquired from experiments, the results corroborate the analytical findings, confirming the great advantages of DMD in turbulent environments. The insights provided in this study establish a foundation for practical FSO system design, enabling the development of simpler and more reliable communication systems.