This paper proposes a coherent multilevel polarization shift keying (MPOLSK) modulation scheme using the spatial diversity detection in a free-space optical (FSO) turbulence channel. The symbol error probability (SEP) analysis for the proposed MPOLSK system under various turbulence regimes has been carried out. The power penalty caused by the phase tracking error has been investigated. For instance, the error floors of 1.3× 10<sup>-5</sup> and 3.4× 10<sup>-3</sup> are observed for 8POLSK with the phase tracking variances of σ<sub>Δ</sub> = 0.3 and 0.5, respectively. The maximum ratio combining technique has been applied in the MPOLSK receiver in order to improve the SEP performance. To achieve a SEP of 10<sup>-9</sup>, the spatial diversity gains required for a 16POLSK-FSO employing four detectors are ~5.8, ~15.8, and ~13.7 dB in weak, moderate, and strong turbulence regimes, respectively.
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