# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 6 Contemporary Developments with UOC Technology
### FSO and VLC with Energy Harvesting
D and C are used here to block the AC and DC components, respectively. In accordance with the switching time, the receiver switches, in time, between EH and data decoding, as shown in Figure 40b. This means that for a given fraction of time, the received signal is processed either for EH or data decoding. In the second architecture, a power splitter is used to equally divide the incoming power into two paths: one for EH, and another for data decoding, as shown in Figure 40c. In this scheme, an inductor (L) is used at the upper path to remove the AC component and allow the DC component. In the bottom path, a C is used to perform the reverse function. In the third architecture, simultaneous k-channel EH and data decoding can be performed, as shown in Figure 40d. In this scheme, higher amounts of EH are achieved [481]. In all these cases, a photoelectric converter (PEC) consisting of a photodiode and a solar panel is used to capture the optical signals. Behrooz Makki et al. proposed and analyzed an FSO link scheme, transferring both power and data, considering two different channel conditions: quasi-static and fast-fading. Expressions, for optimal power allocation statistics, outage probability, and throughput, were derived in a closed form and simulated. Finally, an RF data link was combined with the FSO link to yield a hybrid data link (RF-FSO) aiming to reduce the data-carrying beam outage probability. The experimental results evidenced the significance of such a hybrid implementation compared to a single FSO data link case [481]. Two stages, namely power splitting and time splitting, were proposed to increase the utilization of a carrier signal while simultaneously transferring data and power. In the first scheme, the received signal is split into two paths for EH and data decoding, whereas in the second method, the received signal is alternately switched between two points from where the signal is taped for EH and data decoding [482]. Mohamed Ridha Zenaidi et al. highlighted the design of a more efficient and reliable communication system with an EH technique. The authors presented a two-LED, two-receiver dual-hop VLC/RF wireless communication system. The proposed technique had EH functionality in a decode-and-forward relaying system to power intermediate Tx-Rx devices. A series of intermediate Tx-Rx devices were used here to increase the coverage of VLC systems. A model was designed to estimate the achievable data rate and rate region using the successive interference cancellation technique. Finally, the authors devised a rate-region maximization issue and discovered a solution for the best EH time-switching protocol design [482]. Ahmed H. Abd El-Malek et al. established a multiple-input single-output (MISO) RF-combined FSO network and studied the possibility of EH and data transmission via them. In their study, due to the constraints of the power source, every secondary user communicated to its destination following the route of a hybrid MISO RF-FSO relay. The hybrid MISO RF-FSO relay and backbone network provided power to the secondary users. Every relay consisted of two antennas: one for power transmission, while the other was for data decoding. The secondary user and relay units were linked by the RF channel and the relay and destination were linked by the FSO channel. Expressions for the ergodic capacity, average BER and the outage probability in closed form were derived based on the Nakagami-m and Malaga-M fading models. The derived expressions were validated  
Figure 40: (**a**) VLC-based energy harvesting model; (**b**) receiver based on time switching; (**c**) receiver based on signal component separation; and (**d**) receiver based on PEC architectures. with simulation results [483]. The combined formation of heterogeneous cellular networks with good energy efficiency and EH nowadays follows the concept of small cells. John Fakidis et al. investigated the construction of an FSO data link system with the provision to transfer the power that is harvested by the small cell inside the laboratory.