# A Review of Variable-Beam Divergence Angle FSO Communication Systems
## 5 Conclusions  
This paper organizes a comprehensive overview of various VBDA techniques used in FSO communication. It mainly contains the following content:  
1. Explanation of the application significance of VBDA technology in FSO communication, emphasizing its ability to enhance system performance in terms of higher acquisition probability, greater tracking ability, larger communication link range, and lower power consumption;
2. Introduction to the transmission theory of Gaussian beams and the basic principle of the VBDA method, laying the foundation for the following discussion;
3. In-depth research on different types of VBDA methods, including: 1. Using zoom groups or axial translation of fiber or lens: This method has the advantage of mature zoom lens groups technology and simple principles. However, it requires high-quality guide rails and actuators; 2. Using zoom lenses: This method has the advantages of having no mechanical mechanism and being small in size, but the system may be affected by gravity, and enlarging the aperture may be difficult; 3. Using lenses with different \(F\) or inserting parallel plates: These methods, respectively, have the advantage of simplicity and can greatly relax the require ment for lateral positioning accuracy. However, the communication link will experience an outage when switching angles;
* Using an optical wedge-pair: The advantage is that the influence on the optical axis deflection is small. However, a trade-off must be made between the VBDA range and larger astigmatism;
* The research on using VBDA technologies for different types of links and how they can enhance performance.  
These characteristics require the appropriate choose of VBDA method for the given FSO link situation and communication terminal conditions. Factors such as power consumption, beam quality, cost, and environmental robustness must be carefully considered;
* Emphasis on the challenges faced by VBDA technology and potential future research directions.  
The comprehensive analysis and discussion provided in the paper aim to contribute to the advancement of VBDA technology and its successful integration into practical FSO systems.  
Writing--original draft preparation, G.Z.; writing--review and editing, G.Z., X.W., J.W., L.M., Y.L., X.Y. and S.G.; and supervision, S.G. All authors have read and agreed to the published version of the manuscript.  
This work was supported by National Natural Science Foundation of China (No. 62101527), Funding Program of Innovation Labs by CIOMP (Y9J132E).  
Not applicable.  
We would like to express our gratitude to [[https://pixabay.com](https://pixabay.com)]([https://pixabay.com](https://pixabay.com)) (accessed on 1 May 2023), [[https://www.pexels.com](https://www.pexels.com)]([https://www.pexels.com](https://www.pexels.com)) (accessed on 1 May 2023), https://ill illustrations.co (accessed on 1 May 2023), [[https://lukaszadam.com/illustrations](https://lukaszadam.com/illustrations)]([https://lukaszadam.com/illustrations](https://lukaszadam.com/illustrations)) (accessed on 3 May 2023), [[https://undraw.co/illustrations](https://undraw.co/illustrations)]([https://undraw.co/illustrations](https://undraw.co/illustrations)) (accessed on 3 May 2023), [[https://www.flaticon.com/free-icon/analysis_10711078](https://www.flaticon.com/free-icon/analysis_10711078)]([https://www.flaticon.com/free-icon/analysis_10711078](https://www.flaticon.com/free-icon/analysis_10711078)) (accessed on 7 May 2023), and [[https://www.manypixels.co](https://www.manypixels.co)]([https://www.manypixels.co](https://www.manypixels.co)) (accessed on 5 May 2023) for providing the images used in this article. Their contributions greatly enhanced the visual presentation and overall quality of our work.  
The authors declare no conflict of interest.