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

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 <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mrow> <m:mfenced close=")" open="(" separators="|"> <m:mrow> <m:msubsup> <m:mi>C</m:mi> <m:mi>n</m:mi> <m:mn>2</m:mn> </m:msubsup> <m:mo>=</m:mo> <m:mn>5</m:mn> <m:mo>×</m:mo> <m:msup> <m:mn>10</m:mn> <m:mrow> <m:mo>−</m:mo> <m:mn>14</m:mn> </m:mrow> </m:msup> <m:mtext> </m:mtext> <m:msup> <m:mi mathvariant="normal">m</m:mi> <m:mrow> <m:mo>−</m:mo> <m:mn>2</m:mn> <m:mo>/</m:mo> <m:mn>3</m:mn> </m:mrow> </m:msup> </m:mrow> </m:mfenced> </m:mrow> </m:math> (Cₙ²=5× 10⁻¹⁴0.17emm-2/3) 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).