# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 1 Introduction to Unguided Optical Communication (UOC)
### Working and Installation Principles of Unguided Optical Communications
The probability density function (PDF) of \(\alpha_{A}\) can be obtained from the long-term observation of the receiver signal. \(V\) data are used by meteorological atmosphere monitoring departments and airport authorities to quantify the attenuation coefficient as [27]  
\[\alpha_{A}\bigg{(}\frac{\text{dB}}{\text{km}}\bigg{)}=4.43\frac{3.91}{V} \bigg{(}\frac{\lambda}{550}\bigg{)}^{-q}, \tag{5}\]The value of \(q\) must be correctly selected depending on the value of \(V\) as in [14; 26]. The communication link accessibility estimated in Equation (4) is for a given time. Tracking mechanisms are employed in many long-range FSO links so that beam centroid wandering errors are minimized and, hence, they are not required to be considered when determining connection consistency [38]. For example, if the atmospheric challenges follow a log-normal distribution, then the PDF of the irradiance (\(I\)) can be characterized by  
\[p(I)=\frac{1}{2\sqrt{2\pi}l\sigma_{X}}\text{exp}\left(\frac{\left(\ln\ I-\ln\ I_{0} \right)^{2}}{8\sigma_{X}^{2}}\right), \tag{6}\]  
where the average intensity denoted by \(I_{0}\) is for zero turbulence and \(\sigma_{X}\) is the variance. We can see that the irradiance cumulative probability \(p(I)\) and a threshold intensity (\(l_{th}\)) can be used to determine the data link reliability \(\tau(I)\) as given by [39]  
\[\tau(I)=\int_{l_{th}}^{\infty}p(I)dI=\frac{1}{2}-\frac{1}{2}\text{erf}\left( \frac{ln(I_{th}/I_{0})}{2\sigma_{X}\sqrt{2}}\right), \tag{7}\]  
where erf denotes the error function. When the reliability of the FSO link exceeds a certain \(I_{th}\) level, then it can be detected. The reliability value ranges between 0 to 1 and is affected by the communication range, i.e., lower the reliability, the longer the connection. In [40], an analysis related to link dependability, BER, SNR, probability function, link margin, and average outage capacity is given. In air-to-water/within-water OWC systems, an optical beam is used as the data carrier, as shown in Figure 3c. UOWC architecture is almost the same as OWC except for the waterproof packaging. The system parameters are decided based on the specific applications, such as underwater sensing, underwater imaging, air-to-water/water-to-air data connectivity, and air-to-water control/guidance. As in Figure 3c, underwater sensors (S1, S2 and S3) are connected using laser beams directly by using data hubs. The data hubs act as coordinating nodes to bridge the data flow control from/to the sensors, ships, and surface buoy. The surface buoy is made up of necessary electrooptic systems to function as an air-water interface to establish data connectivity with the aerial platform (i.e., UAV), which uses FSO links with a low Earth orbit (LEO) satellite, sea shore stations, and neighboring UAVs. A separate laser beam, from the surface buoy, can also be used for the control/guidance of submarine vehicles. Another emerging underwater communication/imaging technology is opto-acoustic communications, where acoustic waves are produced by a high-power laser beam at the air-water interface point (or surface), as shown in Figure 3c. The opto-acoustic-generated sound waves then travel within water, reaching the objects and sensors. The object-scattered acoustic waves travel upwards and are collected by the acoustic sensor located in the UAV. Since the depth of sensor is known, the received echo from the underwater object is processed to extract information. In both OWC techniques, the altitude of the UAV from the water surface must be chosen based on the applications and the parameters of underwater systems (sensor and submarine vehicles).