# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 7 UOC Near-Future Network Scenarios and Research Challenges
### UOC--Today's Research Challenges
#### 7.2.4 Atmospheric Turbulence Sensing Concept for In-Situ Measurements, and VLC/Li-Fi Technology  
UOC is clearly the most promising technology to enable future wireless networks to address the rapid increase in demand for high-data-rate communications. UOC is significant from both a communications and optoelectronics point of view. Several recent technological developments are discussed in [491] to develop innovative UOC systems to establish all-optical global internet connectivity. Modulating retroreflector-based UOC duplex communications, as well as integrating OWC optics with LED/LD lights, are two potential technologies. Fundamental atmospheric physics concepts lead to reciprocity-enhanced optical communication over atmospheric turbulence. In a recent publication [496], this approach was explained, and it enables the construction of a new target-in-the-loop atmospheric sensing method for the in-situ measurements of major laser beam properties and atmospheric turbulence parameters. An overlapping integral, also known as an interference metric, is an integral connection between complex amplitudes of counter-propagating optical waves that maintains its value along the propagation route. Below is a basic explanation of the topic. The following invariant, also known as the overlapping integral or interference metric, is given by [496]  
\[J_{\text{int}}(t)=\int A(r,z,t)\psi(r,z,t)d^{2}r=\text{const} \tag{55}\]  
where \(A(\ )\), and \(\Psi(\ )\) are the complex amplitudes of the ongoing and returned waves, respectively, \(r\) is a vector in the plane of the orthogonal to the optical axis \(z\), and \(t\) is the time. The integration is carried out over the entire transverse plane. The above interference metric can be related to the coupling of the integral characteristics at the UOC transceiver \(z=0\), and target \(z=L\) planes as  
\[J_{\text{int}}(t)=\int A(r,0,t)\psi(r,0,t)d^{2}r=\int A(r,L,t)\psi(r,L,t)d^{2}r \tag{56}\]  
An interference metric is measured directly using a laser transceiver system. It consists of a transceiver telescope, single-mode fiber, and fiber-coupled photodetector. The signal measured by the transceiver is given by  
\[P_{0}(t)=\alpha\left|\int M_{0}(r)A(r,0,t)\psi(r,0,t)\ d^{2}r\right|^{2}, \tag{57}\]  
and the square of the interference metric at the target plane can be written as  
\[P_{T}(t)=\alpha\left|\int M_{T}(r)A(r,L,t)A(-r,L,t)\ d^{2}r\right|^{2}, \tag{58}\]  
where \(\alpha\) is the proportionality constant, \(M_{T}(r)\) is the stepwise function of the retro-reflector aperture, and \(M_{0}(r)\) is the stepwise aperture function. It is shown that [496]  
\[P_{0}(t)=P_{T}(t) \tag{59}\]  
The scintillation index \(\sigma_{I}^{2}(0,L)\) is derived from the following equation:  
\[\sigma_{I}^{2}(0,L)\equiv\frac{\left\langle I^{2}(0,L,t)\right\rangle}{\left \langle I(0,L,t)\right\rangle^{2}}-1=\frac{\left\langle P_{0}(t)\right\rangle }{\left\langle\sqrt{P_{0}(t)}\right\rangle^{2}}-1 \tag{60}\]The turbulence structure parameter, \(C_{n}^{2}(l)\), can also be determined from the scintillation index parameter and the path length, \(h\). Note that for UOC wireless technology to be applied between a ground and a terrestrial terminal, or space/aerial terminal, it is essential to evaluate the scintillation index to design a UOC system to effectively mitigate atmospheric turbulence in the communication channel. Transceivers can be very small optical devices to perform the operations described above, in real-time, for applications in 5G/6G or integration with IoT systems.