## 4 Enabling Technologies for FSO Communication
### Mode Diversity Reception  
Using lasers as information carriers, FSO communication with fiber-based optical receivers presents an excellent solution for achieving high data rates, ultra-wide bandwidth, and low power consumption [134]. However, atmospheric turbulence during free-space propagation induces refractive index disturbances. This results in laser signals experiencing beam spread, beam drift, intensity scintillation, and angle-of-arrival fluctuations. These effects reduce the coupling efficiency of the spatial optical beam to the fiber, leading to significant power fading and scintillation, which consequently degrades the performance of practical laser transmission systems [135; 136]. Several methods have been investigated to mitigate the effects of atmospheric turbulence and enhance the performance of high-speed FSO communication systems. One of the most commonly used and effective technologies is the AO system, which improves the coupling efficiency of free-space light into a single-mode fiber (SMF) by correcting wavefront aberrations within the aperture [137; 138]. However, AO FSO systems still face challenges in terms of reliability under strong turbulence. Due to the limited spatial resolution and response time of the optics, AO does not achieve perfect wavefront correction in these conditions [139]. Additionally, high-speed FSO communication with bandwidths on the order of GHz demands higher accuracy and faster response times from AO systems,which can result in slight deficiencies in communication system reliability under strong turbulence [140]. Modes diversity offers a method to counteract wavefront distortion within the aperture, thereby enhancing the performance of FSO communication systems under atmospheric turbulence [139]. To this end, mode diversity reception (MDR) has been proposed and examined as an effective strategy for improving the reception performance of high-speed FSO communication systems. This approach involves receiving optical signals using few-mode fibers (FMFs) or multimode fibers (MMFs) that support multiple orthogonal modes [140; 141; 142]. Furthermore, the distorted wavefront caused by atmospheric turbulence can be considered a combination of a fundamental mode and multiple high-order modes. MDR captures as many high-order mode optical signals as possible. Using mode demultiplexers, the turbulent light is effectively split into an array of SMFs. Different methods are then used to combine the outputs from the single-mode ends of the demultiplexers to enhance signal quality. The probability of all copies, which carry the same information in the SMF array, simultaneous experiencing deep fading is low. Thus, mode diversity technology effectively improves the quality of the received signal and can be applied to satellite-to-ground optical communication systems [143]. The atmospheric turbulence compensation scheme using MDR involves two key technologies: mode reception and signal combination. In fiber-based MDR systems, mode demultiplexers are essential for decomposing spatial light affected by turbulence into SMFs. The two common types of mode demultiplexers used in these systems are multi-plane light converters (MPLCs) and photonic lanterns (PLs). Additionally, there are three primary methods for combining multichannel signals: equal gain combining (EGC), selection combining (SC), and maximum ratio combining (MRC) [140; 142; 144; 14]. For different FSO communication scenarios, numerous studies have explored MDR using various mode demultiplexers and combining technologies to enhance system performance. For instance, ref. [145] proposes and verifies two simplified MDM schemes for FSO communication systems under moderate-to-strong turbulence conditions. These MDM schemes are designed and implemented for coherent detection in FSO communication systems using FMF coupling. The first scheme replaces the coherent receiver with a KK receiver (field reconstruction), significantly reducing the number of photodetectors and high-speed analog-to-digital converters by 75%.