# Worldwide Coverage Mobile Systems for
## 6 Discussion and Future Trends
### Satellites Optical Communications  
The emerging alternative to satellite radio communications is the satellite optical communications, which are attracting the attention of scientists and engineers. Nowadays, the scientific community is starting to experience a new revolution in satellite communications: optical communications, which are promising unexpected data rates transmission, data security, and resilience to meet commercial requirements in the next ten years. An example of serious initiatives in such direction is the ScyLight (Secure and Laser communication technology) programme promoted by the ESA which is aimed at supporting the research, development, innovation, and evolution of optical communication technologies mainly based on laser and offers researchers flight opportunities for in-orbit verification of prototypes ([[https://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/ScyLight](https://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/ScyLight)]([https://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/ScyLight](https://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/ScyLight))).  
The frequency bands used in satellite communications suffer from some congestion and the bottleneck effect due to the growing demand for satellite communications services. Beaconless acquisition using a laser link means that lower orbits, which are congested, are avoided, and that mobile terminals can be lighter, with lower power consumption, providing greater accuracy and safety than a conventional radio link.  
Among the benefits of optical links, it is worth highlighting the benefits of avoiding interferences and detection, due to the difficulty of exploring the space by searching for laser beacons to intercept. It results in being more difficult than in radio frequency communications. Optical links are also capable of exchanging large amounts of data, more than the existing alternatives. Accordingly, in the case of new technologies and applications, such as Big Data, it becomes a technology of capital importance for the development of future services.  
Under the ScyLight program, the ESA is conducting a system and market study to analyse how optical communications technologies can be developed and applied more efficiently to meet the demand. The industry can also participate in this ESA initiative, for example, to apply for ScyLight programs by submitting a proposal to request assistance for any research and development project, even being able to perform real tests in orbit.  
\begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline
**Antennas** & **Type** & **Freq. (GHz)** & **Num.** & **Size (cm\({}^{2}\))** & **AR BW (G dB) (\%)** & **Effc. (\%)** & **Peak Gain (dB)** & **Polariz.** \\ \hline
[21] & multilayer planar slot array & Ku & \(16\times 16\) & - & 10 & 30 & 30 & orthog. linear \\
[22] & T-shaped microstrip patch antenna & Ku & \(10\times 10\) & - & 2 & 28 & 30 & orthog. linear \\
[26] & wide-angle scanning antenna array & Ku & \(2\times 15\) & \(50\times 45\) & 40 & 55 & 16.75 & single \\
[27] & Ovvaldi Antenna & X,Ku & \(4\times 4\) & \(4\times 4\) & 20 & 70 & 19.4 & dual \\
[28] & hat-feed based reflector antenna & Ku & 1 & 80 & 30 & 70 & 30 & single \\
[20] & array antenna with high-gain dual-band and dual-polarized & Ku & - & - & 4.2/6.9 & 38/46 & 32.2/32.3 & dual \\
[31] & rectangular grating waveguide slot array & Ku & 8 & \(9\times 9\) & 1.2 & 68.3/88.8 & 15.82 & single \\
[33] & CT MED & Ku & - & - & 29/43 & 29.3 & 11.45 & dual \\
[34] & Dielectric resonator antenna & Ku & 4 & - & 20 & 15.1 & CP \\
[39] & radiant split ring slots-SW cavity & 10.40 & - & - & 1.54 & 10.8 & 44 & CP \\
[41] & HHSBW & 5 & \(1\times 2\) & \(5\times 7.7\) & 40 & 93 & 5.8 & CP \\ Authors & SIW & Ku & 4 & \(10\times 10\) & 20 & 70 & 6 & dual \\ proposal & \multicolumn{1}{c}{} & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 24: Features comparison: proposed SIW antenna vs. existing antennas.