# A Review of Variable-Beam Divergence Angle FSO Communication Systems
## 3 Existing Methods for Vbda
### Vary Beam Divergence by Adjusting the Focal Length F of the Collimating Lens
#### 3.2.3 Using Optical Wedge Pair
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multirow{2}{*}{**VBDA Range \({}^{\star}\)**} & **If Varies** & \multirow{2}{*}{**Response Time**} & \multirow{2}{*}{**Others**} \\  & & **Continuously** & & & \\ \hline \multirow{4}{*}{Using zoom group} & \multirow{4}{*}{90 \(\upmu\)rad (FWHM)–200 mrad} & & & The technology behind zoom lens groups has reached a relatively mature stage and it requires high-quality guide rails and actuators. It is not necessary to use a mechanical mechanism, which could reduce the terminal volume. However, the system may be affected by gravity and enlarging the aperture may be difficult. \\ \cline{1-1}  & & & & The principle is simple, but the communication link will experience an outage when switching angles. \\ \cline{1-1}  & & & & The parallelism of linear displacement table is very high. \\ \cline{1-1}  & & & & The principle is simple and can greatly relax the requirement for lateral positioning accuracy. \\ \cline{1-1}  & & & & The communication link will experience an outage when switching angles. \\ \cline{1-1}  & & & & The influence on the optical axis deflection is small. As the wedge angle increases, lager astigmatism is introduced. \\ \hline \hline \end{tabular} \({}^{\star}\) The data here comes from Table 1.  
\end{table} 
Table 2: Summary of the characteristics of several methods of VBDA. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**List Types** & **Year** & **Major Contribution** & **Ref** \\ \hline \hline \multirow{4}{*}{Ground terminal platform} & 2003 & Proposed a function which incorporates building many statistics, system parameters, and the required BIR to minimize transmitter power. The functions achieved this optimization by optimizing the transmitter power & [94] \\  & 2008 & Provided closed-form expressions for BIR and distributed complexity (OT) with OT/D with OOK FSO system. The OQAM was determined by [74] \\  & 2013 & Analytical the impact of beam data and convergence and the power design of FSO communication, and the OQAM was determined. [74, 75] \\  & 2016 & Deutsch the OQAM symbols expressed as divergent patterns. Found that the system availability can be significantly improved by using the OQAM. & [74, 75] \\ \hline \multirow{4}{*}{Ground terminal platform} & 2000 & Developed closed-form statistical channel model under its overall (A) and primary-spanning (GG) atmospheric turbulence models. The \\  & 2000 & Orbit, matching matching the range SNR, SNR, pump size and timing and timing. & [74, 75] \\  & 2001 & Though indoor on outdoor 3000-1000 flights.25 (2000 simultaneous experiments, it was found that increasing the beam \\  & 2002 & dropping angle or effectively the number of the communication link. & [84] \\  & 2002 & Proposed a \(CPI\) system based on quadrature detection/OFDM and N/O radio channels by moving the light source. This PMT system can achieve lower resolution and shorten the tracking of the 1/2-th of the original. & [84] \\ \hline \multirow{4}{*}{Ground terminal platform} & 2007 & This is the first work that compared the service and state beam used in RO communications for BIR, A wide beam divergence angle range of 1/300.22 (1700.22) was suggested to make the necessity of the OQAM, as well as to overcome the negative effect of Ultra Ultra W  
\begin{table}
\begin{tabular}{c c c c} \hline
**Link Types** & **Year** & **Major Contribution** & **Ref** \\ \hline Air Pattern & 2009 & Derived a closed-dem solution for the transient gain which minimized the link CP. & [81] \\ Satellite platform & & 2016 & Caused a system approximation of the circular discrete region to achieve a simple algebraic solution for the GROM and concluded that the communication capability with a dynamic-diversity control versus renewable them that birth with a fixed divergence control scheme.