## 3 CubeSats and Inter-Satellite Links (ISLs)  
While ISLs play a crucial role in satellite communication, enabling direct and efficient data exchange in space, their deployment in LEO constellations is less common compared to LEO-GND or GEO-LEO links, as shown in Figure 2 and Table 2. This scarcity is primarily due to the technical and operational challenges associated with implementing ISLs in LEO satellites, which are often smaller and have limited resources compared to satellites in other orbital regimes [59]. The advent of the miniature satellites with a standardized dimension, known as Cube-sat's, have paved the way for the space-based laser communication system (lasercom) to be deployed for ISLs [60]. Compared to their larger, traditionally designed counterparts, Cubesat's offer significant advantages in cost, development time, and launch opportunities. Their compact size and modular design enable faster development cycles and lower pro  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ref. ** & **Year** & **Mission** & **Link Type** & **Wavelength** & **Modulation** & **Data Rate** \\ \hline \multirow{4}{*}{[9]} & 1994 & ETS-VI (NICT, Japan) & GEO-GND & 0.8/0.5 \(\upmu\)m & IMDD & 1 Mbps \\  & 1995 & GOLD (NASA [FL, USA] & GEO-GND & 0.8/0.5 \(\upmu\)m & IMDD & 1 Mbps \\  & 2000 & STRV-2 (BMDO, USA)-fail & LEO-GND & 0.8 & IMDD & 1.2 Gbps \\  & 2001 & GOLITE (NRO, USA) & GEO-GND & - & - & - \\ \hline \multirow{4}{*}{[14]} & \multirow{4}{*}{2001} & \multirow{4}{*}{SILEX (ESA)} & GEO-LEO, & \multirow{4}{*}{0.8 \(\upmu\)m} & \multirow{4}{*}{IMDD} & \multirow{4}{*}{50 Mbps} \\  & & & GEO-Air & & & \\ \hline \multirow{4}{*}{[13]} & \multirow{4}{*}{2005} & \multirow{4}{*}{OICETS (JAXA/NICT)} & GEO-LO, & \multirow{4}{*}{0.8 \(\upmu\)m} & \multirow{4}{*}{IMDD} & \multirow{4}{*}{50 Mbps} \\  & & & LEO-GND & & & \\ \hline \multirow{4}{*}{[36]} & \multirow{4}{*}{2003} & \multirow{4}{*}{NFIRE (MDA, USA)} & LEO-LEO & \multirow{4}{*}{1.06 \(\upmu\)m} & BPSK & 5.6 Gbps \\ \cline{1-1} \cline{5-5}  & & & LEO-LEO, & & & \\ \cline{1-1} \cline{5-5}  & & & LEO-GND & & & \\ \hline \multirow{4}{*}{[37, 38]} & \multirow{4}{*}{2011} & \multirow{4}{*}{HY-2 (China)} & LEO-GND & \multirow{4}{*}{1.5 \(\upmu\)m} & IMDD & \multirow{4}{*}{504 Mbps} \\  & & & BILS (Russia) & LEO-GND & \multirow{4}{*}{1.55/0.85 \(\upmu\)m} & IMDD & \multirow{4}{*}{125 Mbps} \\ \cline{1-1} \cline{5-5}  & & & LLD (NASA GSFC) & Lunar-GND & \multirow{4}{*}{1.5 \(\upmu\)m} & PPM & \multirow{4}{*}{622 Mbps} \\ \cline{1-1} \cline{5-5}  & & & GEO-LEO, & & & \\ \cline{1-1} \cline{5-5}  & & & GEO-GND & & & \\ \hline \multirow{4}{*}{[43, 44]} & \multirow{4}{*}{2014} & \multirow{4}{*}{SOCRATES/SOTA (NICT)} & LEO-GND & \multirow{4}{*}{0.98/1.5 \(\upmu\)m} & IMDD & \multirow{4}{*}{10 Mbps} \\ \cline{1-1} \cline{5-5}  & & & LEO-GND & & & \\ \hline \multirow{4}{*}{[45]} & \multirow{4}{*}{2014} & \multirow{4}{*}{OPALS (NASA JPL)} & LEO-GND & \multirow{4}{*}{1.5 \(\upmu\)m} & IMDD & \multirow{4}{*}{30–50 Mbps} \\ \cline{1-1} \cline{5-5}  & & & LEO-GND & & & \\ \hline \multirow{4}{*}{[46]} & \multirow{4}{*}{2015} & \multirow{4}{*}{OSCD-A (Aero. Corp., USA)-fail} & LEO-GND & \multirow{4}{*}{1.5 \(\upmu\)m} & IMDD & \multirow{4}{*}{5–50 Mbps} \\ \cline{1-1} \cline{5-5}  & & & LEO-GND & & & \\ \hline \multirow{4}{*}{[47]} & \multirow{4}{*}{2016} & \multirow{4}{*}{OSRISV1-2 (DLR)} & LEO-GND & \multirow{4}{*}{1.5 \(\upmu\)m} & IMDD & \multirow{4}{*}{20–100 Mbps} \\ \cline{1-1} \cline{5-5}  & & & LEO-GND & & & \\ \hline \multirow{4}{*}{[48]} & \multirow{4}{*}{2017} & \multirow{4}{*}{Micius (China)} & LEO-GND & \multirow{4}{*}{0.85/0.532/0.671 \(\upmu\)m} & - & - \\ \cline{1-1} \cline{5-5}  & & & LEO-GND & & & \\ \hline \multirow{4}{*}{[49]} & \multirow{4}{*}{2018} & \multirow{4}{*}{GCND-B/ Aerobe-7B (Aero.