## 2 FSO Communication System
### Receiver Side
#### 2.3.2 Coherent FSO Systems  
Commercial FSO communication systems traditionally use IM/DD techniques due to their lower implementation complexity and cost effectiveness [24]. However, there is a growing shift towards coherent FSO systems. This trend is driven by recent advancements in the fabrication of integrated coherent Rxs and high-speed digital signal processing (DSP) integrated circuits, which have significantly enhanced the practicality of coherent detection for wired and wireless communications. Fortunately, the suitable bands for FSO transmission coincide with the wavelengths used in fiber optics, specifically the C band, which lowers the cost of optical and optoelectronic components due to mass production [25, 26, 27]. In coherent FSO systems, information is encoded using modulation techniques such as amplitude and phase modulation. When the signal reaches the Rx, it is optically combined with a locally generated reference signal (LO, optical oscillator, usually a laser with the same wavelength of the received optical signal). This mixing process is crucial for detecting and decoding the information carried by the signal [25, 26, 27]. Coherent detection in FSO systems retrieves the complete electric field of a signal by measuring both of its field quadratures (i.e., its magnitude and phase) in two polarizations. Figure 6: Block diagram of (**a**) coherent and (**b**) IM/DD FSO Rxs. TLNA: Trans-Impedance Low-Noise Amplifier, AGC: Automatic Gain Control, LPF: Low-Pass Filter. This approach maximizes the degrees of freedom (DoF) available in the optical channel for carrying information. At the Rx, the incoming signal beam is combined with a LO beam, which provides a phase reference for downconversion as illustrated in Figure 6a. This mixing process with the LO amplifies the received signal, making the detection process primarily limited by shot noise. As a result, coherent detection is often viewed as a method for enhancing Rx sensitivity in FSO systems. Furthermore, coherent detection is effective in rejecting background noise, thermal noise of the TIA and intentional interferences [28]. An intriguing aspect of coherent systems is the ability to send information via the phase, amplitude, or polarization of the optical field, significantly increasing the system's spectral efficiency (SE). Typical modulation schemes in coherent systems include multilevel Phase-Shift Keying (PSK), Quadrature Amplitude Modulation (QAM), and multilevel Polarization Shift Keying (POLSK). These advanced modulation techniques contribute to the robust and efficient performance of coherent FSO communication systems [1; 27]. In the realm of coherent Rxs, there exist two primary methodologies for signal detection: homodyne detection and heterodyne detection. Homodyne detection is known for its superior detection sensitivity, making it highly effective in accurately capturing weak signals. However, this approach necessitates the use of an exceptionally precise optical phase-locked loop (PLL). The implementation of such a highly accurate PLL is technologically demanding and incurs significant costs, rendering it economically challenging to realize on a large scale [27]. As a result of these constraints, heterodyne detection has garnered more widespread attention and consideration in the academic and research literature. While heterodyne detection may not match the sensitivity levels of homodyne detection, it offers a more practical and cost-effective solution. This is because heterodyne systems do not require the same degree of precision in their PLLs, thereby reducing both complexity and expense. Consequently, researchers and engineers have favored heterodyne detection in many applications where the balance of performance, feasibility, and cost is a critical consideration [1; 27]. Moreover, the application of DSP techniques to coherent receivers has facilitated the transmission of spectrally efficient advanced modulation formats and effective compensation for impairments.