# A Review of Variable-Beam Divergence Angle FSO Communication Systems
); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (X.X.W. ); [EMAIL_ADDRESS] (X.X.X)  
[PERSON]  
1 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,  
Changchun 130033, China; [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (X.X.W. ); [EMAIL_ADDRESS] (X.X.X)  
[PERSON]  
1 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,  
Changchun 130033, China; [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (X.X.X)  
[PERSON]  
1 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,  
Changchun 130033, China; [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (X.W.) [PERSON]  
1 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,  
Changchun 130033, China; [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (X.X.W. ); [EMAIL_ADDRESS] (X.X.) [PERSON]  
1 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,  
Changchun 130033, China; [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (X.X.W. ); [EMAIL_ADDRESS] (X.X.X)  
[PERSON]  
1 Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,  
Changchun 130033, China; [EMAIL_ADDRESS] (C.Z. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (J.W. ); [EMAIL_ADDRESS] (Taking the intensity modulation/direct detection (IM/DD) with on-off keying (OOK) FSO communication system as an example, Figure 2 illustrates its typical optical path. A laser diode (LD) is commonly used as the light source, although some manufacturers may use high-power LEDs [5]. At the transmitting end, the data is encoded and modulated [6]. The modulated light beam is then amplified and shaped by an optical amplifier, collimator, and optical antenna before entering free space. At the receiving end, the optical antenna focuses the received beam onto an optoelectronics diode (PD), which converts the received optical signal into electrical current. Finally, the demodulation process is performed to obtain the original transmission data. At present, FSO communications mostly use fixed-beam divergent angle scheme. It has several benefits, such as a wide selection of commercial components, compact structure, and ease of installation. However, there is still room for improvement in areas such as link reliability, communication link range, and power consumption [7]. Compared to fixed-beam divergent angle scheme, the variable-beam divergent angle (VBDA) scheme offers four significant advantages, which are illustrated in Figure 3. Each of these advantages will be discussed in detail below:  
Figure 1: The extensive communication scenario contained in 6G. Figure 2: Optical path diagram of FSO communication terminal. Higher acquisition probability. When targeting objects with a large uncertain area (such as targets moving at high speeds or drifting due to Global Positioning System (GPS) unavailability), traditional solutions involve using a large divergence angle beam to cover the uncertain area or scanning with a small divergence angle beam. However, these methods have drawbacks, such as reduced detection range or increased acquisition time. To address this problem, a solution using VBDA can be employed. As in [8], the beam divergence angle \(\theta\) is set to \(1/(2\phi+1)\) times the size of the uncertain area, whose angle value is \(\theta_{0}\), as shown in Figure 4. Here, \(\phi\) = 0, 1, 2,... represents the scanning phase, and scanning starts from the \(\phi\) = 0 phase.