The performances of satellite-to-ground downlink optical communications over Gamma-Gamma distributed atmospheric turbulence are studied for a coherent detection receiving system with spatial diversity. Maximum ratio combining (MRC) and selection combining (SC) techniques are considered as practical schemes to mitigate the atmospheric turbulence. Bit-error rate (BER) performances for binary phase-shift keying modulated coherent detection and outage probabilities are analyzed and compared for SC diversity using analytical results and for MRC diversity through an approximation method with different numbers of receiving aperture each with the same aperture area. To show the net diversity gain of a multiple aperture receiver system, BER performances and outage probabilities of MRC and SC multiple aperture receiver systems are compared with a single monolithic aperture with the same total aperture area (same total average incident optical power) for satellite-to-ground downlink optical communications. All the numerical results are verified by Monte-Carlo simulations.

Metal‐based thermally activated delayed fluorescence (TADF) is conceived to inherit the advantages of both phosphorescent metal complexes and purely organic TADF compounds for high‐performance electroluminescence. Herein a panel of new TADF Au(I) emitters has been designed and synthesized by using carbazole and pyrazine‐fused nitrogen‐heterocyclic carbene (NHC) as the donor and acceptor ligands, respectively. Single‐crystal X‐ray structures show linear molecular shape and coplanar arrangement of the donor and acceptor with small dihedral angles of <6.5°. The coplanar orientation and appropriate separation of the HOMO and LUMO in this type of molecules favour the formation of charge‐transfer excited state with appreciable oscillator strength. Together with a minor but essential heavy atom effect of Au ion, the complexes in doped films exhibit highly efficient (Φ∼0.9) and short‐lived (<1 μs) green emissions via TADF. Computational studies on this class of emitters have been performed to decipher the key reverse intersystem crossing (RISC) pathway. In addition to a small energy splitting between the lowest singlet and triplet excited states (ΔEST), the spin‐orbit coupling (SOC) effect is found to be larger at a specific torsion angle between the donor and acceptor planes which favours the RISC process the most. This work provides an alternative molecular design to TADF Au(I) carbene emitters for OLED application.

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