# 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
FSO connectivity can be installed successfully by maintaining a direct _LoS_ path (or drone/HPA-based pend-pipe-supported path) between the transmitter and the receiver antennas. FSO systems have several advantages over RF wireless technologies, including no spectrum licensing, no frequency coordination, no RF-induced interference with other channels, and inherent security (i.e., hard to intercept). Figure 3: (**a**) The architecture of a WOC network installation; (**b**) a ‘k’ baseband channel FSO communication system; and (**c**) OWC systems. The available data rates of FSO technology are comparable to existing fiber-optic communications. Particularly, the coherence and low divergence of light beams enable the implementation of huge parallel (MIMO) FSO links at certain locations. In addition to ground-based FSO systems, there are other applications for the use of FSO systems such as inter-satellite up/downlinks, communication between a UAV and ground platforms, connectivity between mobile space stations, etc. [20; 21; 22]. The unique characteristics of a light beam and its coherence make high-data-rate modulation and long-range free-space data transmission possible. An FSO system covering long-distances through the atmosphere uses a highly shaped optical beam [21]. The most popular modulation methods for FSO links are digital pulse-width/pulse-position, digital pulse interval, on-off keying (OOK), and differential phase-shift keying (DPSK) [23]. A digital pulse position modulation (PPM) scheme is very suitable in FSO systems, in multiple scattering environments [24; 25]. In communication systems, the channel capacity developed by Claude Shannon in 1949 [23] is given as  
\[C=B\,\log_{2}\,\left(1+r\frac{E_{b}}{N_{0}}\right)\,b/s, \tag{1}\]  
where \(B\) represents channel bandwidth, \(N_{0}\) represents noise PSD, \(E_{b}\) is the energy of the bit, and \(r\) denotes the spectral efficiency. An signal-to-noise ratio (SNR) of \(-1.6\) dB is required to maintain the \(0.7\) ppb Shannon sensitivity [23] to achieve the maximum data rate with proper channel/source coding and modulation methods. FSO technology is becoming more and more popular and being adapted to conventional network models to attain fast terrestrial or global data connectivity [26]. However, FSO links are impacted by atmospheric conditions [27]. Variations in atmospheric weather conditions decide the phenomena relating the optical propagation through it. For example, dispersion (scattering) and attenuation are more noticeable in adverse weather conditions. Following the analysis of weather conditions, FSO system parameters can be specified in order to ensure quality of service [28]. The link margin can be expressed as [29]  
\[LM=P_{t}-L_{p}-N_{b}, \tag{2}\]  
where \(P_{t}\) is transmitter power, \(N_{b}\) is receiver sensitivity, and \(L_{p}\) is propagation loss. As in Equation (2), the \(LM\) is used to describe the dependability of an optical data link. The \(LM\) can also be defined as [30]  
\[LM=\frac{\text{Receivedpower}}{\text{powernecessary}\text{forspecifiedBER}}, \tag{3}\]  
The \(LM\) express the amount of power margin required for the FSO link to compensate for the scattering/absorption of the atmosphere [31]. FSO link visibility (\(V\)) and attenuation at \(785\) nm under different weather conditions is investigated in [32; 33]. FSO links must meet the accessibility [34] and availability standard of \(99.999\%\) for data transport [35], which means the FSO link being down for \(5\) min over a year, i.e., \([(525,\!600-5)/525,\!600]\times 100\approx 99.999\%\). Note that where the information speed exceeds the requisite value, the link performance is measured in terms of the time average, \(T_{av}\)[36], as a function of the probability loss (\(p(\alpha_{A})\)) as given by [37]  
\[T_{av}=100\%\int_{0}^{\alpha_{A}\lim}p(\alpha_{A})d\alpha_{A}, \tag{4}\]  
where \(\alpha_{A}\lim\) is the integral limit of the attenuation coefficient.