# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 2 FSO Communication Channel Effects and Their Mitigation Techniques
### FSO Communication Channels  
FSO communication offers an attractive solution for large bandwidth information transportation in general and where optical fiber cable installation becomes unrealistic. Incredible electro-optic component progressions in the areas of light sources, modulators, highly sensitive light detectors, and light processing optics with reduced unit size and weight, facilitate the establishment of reliable large bandwidth FSO data transmission systems. However, in FSO communication, environmental constraints to the transmission path, such as sand-wind, haze, storm, fog, aerosol, dust, Tx/Rx platform instability, etc., have a negative impact on information rate consistency and so, total connectivity efficiency deteriorates [41]. The emitted light signal power is greatly reduced by environmental particles (reflection and absorption) and the wavefront of the light beam is heavily disrupted by FSO channel turbulence instabilities. The environmental properties for the transmission of an optical signal brings power fading, multiple scattering, and an instantaneous power reduction at the receiver surface. Optical parameters such as polarization, refraction, reflection, signal amplification, dispersion, and cohesiveness are altered during the transmission through the environment. The key cause of this is that the FSO channel is made up of a variety of compounds of ground atmosphere that constantly change their energy levels in response to changes in ambient parameters. It is possible to calculate the ambient parameters' inhomogeneity as a function of atmospheric parameters. When the refractive index structure parameter (\(C_{n}\)2) has large dissimilarities, it results in random light signal wandering and/or power variation at a frequency range of approximately 0.01 Hz through to 200 kHz, and even above [26]. In a long-range terrestrial FSO communication system, the optical beam interacts with the environment along the way; hence, optical power is more severely affected than the vertical path. The scintillation index measures random temporal/spatial power variations introduced by light power fluctuations at the detection plan. Even for relatively low scintillation effects, the bit error rate (BER) grows rapidly, particularly in long-range Gbps FSO data links. Thus, to assess the maximum possible link range and data rate, it is essential to calculate the scintillation index fluctuation profile prior to system installation [10]. Overall, the various deleterious atmospheric features lead to severe signal fading and complete optical link disconnection [41]. This must be taken into consideration when measuring the LM in terms of atmospheric condition, since it is crucial for the reliability of the FSO system's efficiency. We can find various LM models that provide constraints of atmospheric channels as a function of weather parameters in [42; 43]. These models support the optimization of the both the transmitter and the receiver base station parameters to install an FSO communication link with the highest possible data rate in a variety of atmospheric turbulence conditions. The optical data link classifications based on the channel (beam path) are (i) terrestrial channel (buildings, towers, and between vehicles); (ii) earth-to-space channel (near-earth spacecraft, drones, UAVs, and space-shuttles); (iii) space channel (between satellites); (iv) enclosed (interior) channel (office electronic appliances); and (v) optic connectivity channel (IC inter connections). The transmitter and receiver installations for these classifications are as follows. (i) Horizontal link: preferred for urban zone data connectivity between the roofs of high constructions/buildings, windows and rooftops, and windows and tall towers, with a maximum link distance of a few kilometers. The effect of the atmosphere is assumed to be uniform throughout the channel and the link parameters such as the scintillation and turbulence strength index are calculated accordingly.