Asymmetric constellation transmission for a coherent free-space optical system with spatial diversity.

Spatial diversity is widely recognized as an effective technique for free-space optical (FSO) communication to combat channel fading. Most existing works on spatial diversity in FSO systems focus on intensity modulation and direct detection, while fewer mention coherent optical detection, which excels in optical receiver responsivity. Spatial diversity enables the FSO coherent system to demodulate the signal components of in-phase () and quadrature () separately and thus simplify the coherent detector.