# A Review of Variable-Beam Divergence Angle FSO Communication Systems
If the beam is not detected, the system enters the next phase and continues to scan. At this time, \(\phi\) = 1, and the value of \(\theta\) becomes \(\theta_{0}/(2\times 1+1)\). This process is repeated until the receiver detects the beam and starts tracking. Simulation results demonstrate that this new scanning strategy reduces acquisition time by approximately 80%, leading to a significant improvement in the probability of acquisition. Greater tracking ability. Upon entering the tracking phase, the beam divergence angle can be further compressed by utilizing information from the coarse and fine tracking residuals. This results in improved optical power received by the coarse and fine tracking units, typically cameras and four quadrant detectors, respectively. The increase in optical power enhances the tracking ability. When coarse and fine tracking residuals increase due to turbulence or jitter in platform itself, appropriately increasing the beam divergence angle can improve the robustness of the coarse and fine tracking units to these disturbances, leading to more reliable communication due to an enlarged allowable pointing error. Larger communication link range. In certain 6G use cases, the distance between FSO nodes would dynamically change [9], such as for communication between ground stations and UAVs. When the communication distance decreases, the beam radius has not yet  
Figure 4: The acquisition strategy of using VBDA scheme [8]. Reprinted/adapted with permission from Ref. [8]. 2017, SPIE. Figure 3: Advantages of FSO communication systems with VBDA scheme. significantly expanded, resulting in a smaller allowable pointing error. Consequently, the pointing error occurring during the beam steering interval easily exceeds the allowable limit. This indicates that the FSO link is more likely to be disconnected as the moving node approaches its paired node. To address this, appropriately increasing the beam divergence angle can effectively cover the uncertain area, thereby potentially reducing the minimum communication distance. When the communication distance increases, the optical power loss increases by 6 dB every time the communication distance doubles [10]. Although it could be compensated for by a larger receiving aperture or higher transmission power, these compensation methods are very limited because a larger aperture or higher power light source will increase the SWaP (size, weight, and power) requirements of the optical system [11]. At this time, appropriately reducing the beam divergence angle can increase the received optical power, thereby further increasing the maximum communication distance. In summary, the use of a VBDA scheme could effectively increase the FSO communication link range. Less power consumption. Fixed-beam divergence angle systems are often designed based on the worst-case scenario. In addition, the beam divergence angle will be slightly larger than the optimal beam divergence angle (OBDA) during communication, which results in a certain amount of energy waste. In applications where energy consumption is a critical factor, such as in satellites, UAVs, and personal handheld terminals, it is desirable to reduce unnecessary energy waste to extend the equipment's endurance time. By selecting the appropriate beam divergence angle and optical power based on real-time link conditions, it is possible to reduce energy consumption while maintaining communication rates. Better communication performance. During FSO communication, the performance of the link can be influenced by various factors. These include system parameters such as beam divergence angle, transmitted optical power, receiver's field of view (FOV) angle, attenuation loss, atmospheric turbulence, pointing error, and angle-of-arrival (AOA) fluctuations. By adjusting the beam divergence angle and transmitted optical power, improved communication performance can be attained, resulting in lower outage probability (OP) and bit error rate (BER).