# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 3 Hybrid RF-FSO Systems Techniques and Their Advanced Achievements
### Space Optical Satellite Communications
Meanwhile, by virtue of the velocity of light speed of 3 \(\times\) 105 km/s, the connection between those planets can be estimated at about 180 s, or around 3 min. This is a tremendous achievement, if the FSO can be implemented in this case. To prepare a high-speed communication platform that can support Mars exploration as well as prepare for the landing of people on this planet, some studies have been reported to implement FSO as the unguided communication scheme to support this project. In Ref. [170], beam aperture was studied in order to find an optimal dimension of the beam that can be transmitted out through space. In space, the phenomena of atmospheric turbulence variations are unlike those in the Earth's atmosphere; thus, optical propagation is better, even for long distance data connectivity. Therefore, the primary challenge is  
Figure 28: The FSO in the configuration of deep-space optical relaying networks (DORN) as the communication platform between Earth to Mars. how light intensity can reach a destination in space which is million kilometers away. In Ref. [171], a scheme of an optical link budget (optical power allocation) was investigated for vertical optical beam propagation, which covers both the up-/down-link connections between an Earth station and deep-space terminals. An optical amplifier that is suitable for optical signal boosting for deep-space propagation is studied in [172]. A scheme of optical propagation that can reach a destination millions of kilometers away should be designed with FSO. The available solution to solve this problem is by designing an optical relaying network (ORN) in space, expanding from Earth to the Moon as a stationary space, and then forwarding a signal into the space again to reach Mars. The scheme of an ORN in the space between Earth and Mars can be called a DSORN or deep-space optical relaying network. The ORN can be implemented by configurations of many satellites or inter-satellite networks in the orbital zone of Earth, the Moon, and Mars, as illustrated in Figure 28.  
The main consideration when proposing a DSORN as the major communications platform is regarding the ability of supporting devices in FSO such as an optical amplifier. The capability of an optical amplifier to boost a signal strength right now is available and also reaching an advanced stage of development [173; 174]. An optical amplifier that has a high power output is the first requirement to transmit an optical propagation thousands of kilometers away from the geo-stationary orbit of Earth. The optical amplifiers in the ORN of a satellite is setup to amplify received signals and amplify forward relays to produce a high signal power so as to reach the next relaying node in an inter-satellite network. In free space, absorption is no longer a major threat to optical propagation, but rather the deviation of the beam profile received by each node in the receiver. Thus, scintillation and beam wandering, or beam spreading, is not a major problem anymore in free-space propagation. Thus, an optical propagation that is produced by the high power output of an optical amplifier can deliver a signal within a distance of thousands of kilometers. One of the major issues at the receiver node in an inter-satellite network is pumping-out the optimum size beam so as to reduce beam divergence to possibly extend it at the reception end. An advanced optical system/antenna has to be implemented to pump-out a well collimated beam that diverges little over a very long distance so it is effectively coupled with the subsequent electro-optics circuits/subsystems. Another improvement for signal modulation is to implement a soliton pulse into an optical propagation. A soliton pulse is an ideal pulse that has constant shape in amplitude and phase along the medium of optical fiber. The development of soliton pulses in optical fiber communications has shown good results to improve performance [175].