# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 3 Hybrid RF-FSO Systems Techniques and Their Advanced Achievements
### Space Optical Satellite Communications  
The scenarios, benefits, and achievements in SOSC are described in this section. The involvement of FSO for space applications are rapidly increasing nowadays to meet near-future ultra-high-speed wireless communication requirements [24]. Only with the support of Earth surface to satellite, satellite to Earth surface, and inter-satellite FSO can near-future mandatory requirements for high bandwidth wireless communications be met [166]. In conventional satellite communications, RF waves are mainly used as data carriers. These RF waves degrade rapidly as they travel through atmospheric matters over long distances, which has prompted the widespread use of relay systems [167]. In addition, RF wave transmission requires high power and has limited bandwidth (data rate); thus, it is inadequate in space, particularly for satellite communications and long-distance Mars missions. Thus, we use FSO systems as an alternative technology to conventional RF transmission due to the advantages of focused or beam propagation with less degradation, lower cost, smaller optical antennas, and greater storage capacity [168]. In FSO communications, a finely linearly polarized laser beam transmits encrypted data into a vacuum. Moreover, the communication market's demands are rapidly increasing, and they cannot possibly be met with the current crowded RF spectrum. When using radio waves, a NASA test spaceship was able to receive a movie in 639 h, whereas an FSO system took only 8 min [169]. Only FSO systems will be able to provide future deep-space satellite communications for long-distances, as well as sophisticated operations (live HD videos, video conferencing, human expeditions beyond the surface of the moon, and so on) [24] and dependable high-volume data connections (raised by 10-100X) [25]. In order to achieve effectiveness in transmitting large amounts of information, lasers are being used to replace the ESA's existing RF space communication systems [26]. The European Space Agency (ESA) demonstrated that FSO systems could return Earth images quickly to ground stations, allowing disaster relief efforts to begin immediately. Europe recently propelled a satellite capable of collecting pictures from near-Earth satellites and transmitting them to base stations using a well-focused optical beam. With the launching of an up/downlink FSO communication satellite, Europe also started the development of a space data highway. Today's advancements and developments of FSO technology provided great hope for the deployment of more advanced space satellite optical communications equipped with lasers [27,168]. Figure 27 depicts an illustration of different installation scenarios of an FSO communication network for space point connectivity. A  
Figure 26: FSO-communication-supported WiMAX architecture. laser light was sent from Ball Aerospace & Technologies Corp. to reflect spacecraft orbiting at a 350 Km altitude in the early 1990s, which was projected down to a station on Earth [27]. The beam intensity profile was calculated using a collected optical beam, which included, primarily, air turbulence, Cn2, and spatial-temporal disruptions. In 1992, the Galileo optical mission sent a pulsed laser light from two ground stations in California and New Mexico to a deep-space spacecraft [167]. A bidirectional (earth-to-moon and back) laser connection with an air disturbance mitigation mechanism was added and demonstrated [26]. In 2008, two LEO spacecrafts were developed with an optical connection at a data throughput of 5.5 Gbps (Terra SAR-X and NFIRE) over 5500 km [167]. Various deep-space optical telecommunication technologies feasible for (i) aerial flight test systems; (ii) optical deep-space communications; (iii) laser cross-link systems; (iv) stratospheric optical payload experiments; (v) Mars laser-beam communications; (vi) aerial laser optical links, and so on, have been identified through various experiments conducted for military and aerospace requirements [167; 168].