## 1 Introduction  
Free-space optical (FSO) communication, which uses lasers as signal carriers, has gained significant attention as a complementary solution to radio frequency communication for numerous applications and services [1]. The key benefits of FSO communication include high data rates, broad bandwidth, strict security, and compact components [2].  
However, the development of FSO systems is constrained by atmospheric conditions. The transmitted signal is highly sensitive to atmospheric turbulence. Specifically, turbulence-induced intensity fluctuations, known as optical scintillation, are regarded as the primary limitation of the system performance [3, 4]. To counteract the effects of atmospheric turbulence, various strategies have been suggested, such as partially coherent light, adaptive optics, and spatial diversity [5; 6; 7].  
The application of spatial diversity and adaptive optics technologies significantly increases the system complexity and production costs. In comparison, the application of partially coherent light as a source in spatial optical communication systems offers a simpler and more cost-effective solution [8]. Partially coherent light, as exemplified by Gaussian Schell-model (GSM) laser beams, is less susceptible to atmospheric turbulence than fully coherent light. Consequently, numerous researchers have concentrated on utilizing partially coherent light to minimize the bit error rate (BER) of systems [9; 10].  
The emerging polarization shift keying (PolSK) technique has been proposed for partially counteracting the effects of turbulent atmosphere on space optical communication systems [11]. This method encodes information onto a carrier using the polarization state of a beam, reducing the required optical power by 3 dB compared to that of the on-off keying (OOK) modulation scheme [12].  
Most previous studies on PolSK primarily focused on the polarization characteristics of horizontal-link communications [13; 14; 15]. To fully utilize the benefits of a PolSK system using GSM laser beams, it is essential to identify the conditions necessary for preserving the polarization characteristics of the downlink [16]. Nevertheless, existing PolSK-based space laser communication systems require further enhancement, particularly in terms of communication performance and data transmission rate.  
This study innovatively integrates the advantages of partially coherent light and PolSK technology to enhance the performance of FSO-based space communication systems. A GSM-beam-based polarization transmission characteristic model for downlink was developed, and sufficient conditions for maintaining the polarization transmission characteristics were derived. To elucidate the impact of source spatial coherence on the performance of an optical communication system using a GSM-beam-based circular PolSK (CPolSK) modulation scheme, probability density distribution and scintillation index models of optical intensity were derived for an atmospheric turbulence environment. Additionally, a BER model for the CPolSK-modulated optical communication system was established. The simulation results demonstrated that, compared to an OOK system, the GSM-beam-based CPolSK-modulated system achieved a power reduction of 1.51 dB when using fully coherent light under identical conditions. These findings highlight the potential of PolSK-based optical communication for improving system reliability in turbulent environments. This work focuses on circular PolSK; an alternative of multiplexing linear OOK polarization channels is left for future study.