Improving the performance of Gamma–Gamma FSO links with adaptive optics, dynamic beam steering, and strong modulation formats

Abstract Although FSO communication provides high data-rate, license-free links, atmospheric turbulence causes deep fades and irradiance scintillation that significantly reduce reliability. This study compares four modulation formats (OOK, 4-PPM, DPSK, and DCO-OFDM) and two turbulence-mitigation strategies: adaptive optics (AO) and dynamic beam steering (DBS). It also presents a simulation-based performance evaluation of an IM/DD FSO link over a Gamma–Gamma turbulence channel. The results of Monte-Carlo evaluation for a 1550-nm, 1-km link under very strong turbulence C n 2 = 5 × 10 − 14 m − 2 / 3 $\left({C}_{n}^{2}=5\times {10}^{-14}\hspace{0.17em}{\mathrm{m}}^{-2/3}\right)$ show that DPSK outperforms OOK (≈4.1 × 10 −3 at 20 dB) by achieving BER = 4.2 × 10 −5 at 20 dB and <10 −6 at 30 dB. With DBS using four spatial paths, realistic operation under estimation error achieves BER ≈ 8 × 10 −5 at 20 dB and 9 × 10 −6 at 30 dB, indicating substantial diversity gain. AO modeled with a residual factor η AO = 0.3 yields moderate gains (BER ≈ 9.5 × 10 −4 at 20 dB). In general, the most dependable design for high-speed FSO links in turbulent atmospheric conditions combines spatial diversity (DBS) and robust modulation (DPSK).