## 4 Enabling Technologies for FSO Communication
### Adaptive Optics
#### 4.9.3 Pointing, Acquisition, and Tracking  
In FSO communication systems, PAT mechanisms can reduce or minimize PE through the continuous monitoring of performance metrics like the Strehl ratio and received signal power. The Strehl ratio is defined as the ratio of the intensity along the optical axis of an aberrated or corrected point spread function (PSF) to the intensity of the diffraction-limited PSF on the optical axis. These metrics help in adjusting correction elements, including mirrors, gimbals, or AO [7].  
The PAT mechanism has several key tasks: orienting the Tx towards the Rx, capturing the incoming light signal, and ensuring the FSO link remains stable by monitoring the position of the remote FSO terminal [81]. Pointing involves adjusting the Tx to align with the FoV of the Rx. Signal acquisition refers to aligning the Rx with the direction from which the beam arrives. Tracking entails continuously maintaining both pointing and signal acquisition during the optical communication between terminals [7].  
Alongside PAT mechanisms, choosing the right beam divergence can help reduce PE. Specifically, as the beam footprint at the Rx increases, the PE decreases. However, increasing the divergence angle also leads to higher geometric path loss, which is the spread of transmitted power over the communication distance. This loss is influenced by the distance between stations, the divergence angle of the beam, and the size of the Rx aperture. Various optimization models have been proposed to determine the optimal beam width to minimize the BER and OP of an optical wireless link, taking into account PE and SNR [7; 23; 406; 407].  
Moreover, due to the increased geometric path loss associated with greater beam divergence, FSO communications typically use a narrow beam with a divergence angle no larger than 1 milliradian (mrad). This narrow beam enhances received power density and boosts link margin under various weather conditions. However, it also necessitates precise alignment between the transceivers due to the beam's small footprint at the Rx. Achieving this alignment becomes more challenging when at least one of the parties involved in communication is mobile [7; 408; 409].  
PAT mechanisms are typically utilized in stationary and mobile FSO communication systems when employing a narrow beam. The primary motivations for using such beams include the high data rate associated with concentrated light intensity and the extended reach they provide. However, utilizing a wide beam can potentially reduce or even eliminate the performance demands on the PAT mechanism. Certain FSO applications have been proposed to take advantage of a wide beam to ease the requirements of PAT mechanisms. For example, utilizing the divergence angle of a wide beam can reduce the required steering speed of the FSM, a crucial component of the PAT mechanism in an FSO transceiver. It has been shown that a wide beam's divergence angle offers a greater link range and more effective coverage length compared to a narrow beam, making it advantageous for applications such as high-speed train communications [410].  
PAT mechanisms in FSO communication systems can be categorized based on their principles of operation, including mirror-based, gimbal-based, gimbal-mirror hybrid, liquid crystal, AO, and RF-FSO hybrid methods [411; 412]. Additionally, their operating principles and mechanics allow for classification based on dimensionality (i.e., 2D or 3D) as illustrated in Figure 14[7].