# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 2 FSO Communication Channel Effects and Their Mitigation Techniques
### FSO Communication System: Temporal Challenges  
The unique features and benefits of FSO systems come at the cost of the uncontrollable atmospheric channel effects. In fiber-optic communications, the amount of power obtained at the detector end is well-known, while, owing to agitation variations in the open atmosphere and spatially scattered route impacts, an FSO system has a number of dynamic inhomogeneous and random power fluctuation issues. Thus, the data-carrying light signal transmitted through the atmosphere decides the output of FSO systems. As depicted in Figure 4, fog, ice, polluted air, haze, desert, turbulence speed variations, building motion, etc., have a significant impact on optical beam transmission [15; 16; 17]. Water dewdrops and aerosol units with a diameter of a few hundred microns will change the properties of a light wave as it propagates, and as a result, we obtain a random wavefront value, which results in signal amplitude and phase fluctuations. By the mutual action of the absorption, scattering, and reflection of the atmospheric particles [9], link availability is greatly limited and the system BER is, therefore, increased [44]. Similarly, fog absorbs a lot of visible and infrared light. An FSO system's environmental loss is similar to an RF system's rainfall loss [14]. These massive ambient channel molecules expand or decay in response to moisture levels, and their impacts can be calculated using the Rayleigh or Mie scattering principle because of the spectrum of light and molecule dimensions [27]. Photonic attenuation for atmospheric absorption spectrum and extinction varies with the optical range, with higher wavelengths having more attenuation, i.e., the air molecules feebly interfere with the optical field above one micrometer [45]; as a result, optical scattering becomes negligible. A prime criterion for the effective connection of an FSO system is the specification of the optical wavelength in compliance with local climatic factors. The discrepancy of ambient intensity disturbance in the light system, due to the solar non-uniform heat of channel air pockets, is the primary cause of the light beam reference that we set at the detector. At the output port [46], scintillation causes intensity/phase variations over time, which must be dealt with via statistical approaches. In short-distance communication (<1 km) light signal transmission, the scintillation properties are negligible, whereas, in medium-(\(\leq\)3 km)/long (>3 km)-range communication, they become one of the most important factors affecting an FSO system's overall efficiency and reliability. Air temperature, moisture, pressure, height, the spatial geography of the implementation location, wind direction, and other environmental conditions, all influence the power level of FSO channels [46]. Bird species, insects/bees, helicopters, mini-UAVs, tree branches, building motion, pollution from power plants, and other transient physical obstacles in the _LoS_ can cause total or fractional clogging of an optical field. MIMO FSOC transmitter and receiver devices can successfully prevent these forms of clogging/disruptions [47]. To avoid object obstruction, an FSO system uses redundancy MIMO transceiver connections. An obstacle affecting a large portion of one channel's optical field would give little impact on the other channel; thus, the total required power will not fall below the minimum. Additionally, FSO systems are mounted on buildings and towers; hence, building/tower sway causes beams to wander on the detector plane [48]. Strong winds, frequent heating and cooling, and tiny movements over time are the major factors in this kind of sway. Today's modern professional FSO systems have built-in protocols to manage the tilt/sway of buildings where they are deployed and preserve beam centroid stabilization on the detection plane, such as beam-tracking-alignment systems, beam divergence mix, well-matched FoV, and clock recapture PLL.