# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 6 Contemporary Developments with UOC Technology
### FSO and VLC with Energy Harvesting  
A strong dependency and demand for the use of wireless devices and the internet of wireless things has been rapidly increasing every year. The functionalities of those wireless devices are constrained since they are powered by finite-capacity batteries. Therefore, energy harvesting (EH) from available surrounding sources becomes significant to power those devices, ensuring their availability for their intended operations. Harnessing energy from the surrounding environment and/or other possible resources and converting it into the required electrical energy is a mandatory requirement to build a reliable distributed network for indoor and/or outdoor wireless applications. As a result, EH is seen as a disruptive technology paradigm to extend the life of the sources that power wireless network devices [467]. In general, ambient EH to meet increasing energy demands and the volume reduction of devices is becoming increasingly popular. Traditional EH systems' fundamental flaw is their reliance on natural resources, such as solar and wind power, whose constancy is completely unpredictable. Harvesting energy from RF signals, which are essentially data transmission signals, is an intriguing alternative. Wireless power transmission through an RF channel has received a lot of attention in recent years, whereas optical wireless power transfer (OWPT) is a relatively new study subject with only a few works in the published literature. The different levels of throughputs existing among the devices due to their location, data acceptance angle, and channel obstruction probability suggests that there will be different EH rates [468]. This issue is being addressed by researchers with some models derived based on a few statistical/geometrical assumptions. Haas proposed VLC with EH as a new method for wireless communications. In VLC-based EH, the FoV and obstruction probability are the major performance limiting factors, and they cannot be ignored due to the directionality of visible light [469]. In the work of Fakidis et al., OWPT was accomplished using the optical wavelength of the electromagnetic (EM) spectrum of visible and IR lights. An optical signal was transmitted using a laser/LED and received using a solar cell to harvest the optical energy and convert it to electrical energy at the receiver [470]. In Ref. [471], indoor IoT devices were powered by sources energized from lighting fixtures using EH within a room. Simultaneous wireless data and power transfer were performed in a downlink VLC system with the optimization of the sum-rate maximization problem in [472]. The EH model used in this paper only supports an AC source, not DC, i.e., a solar panel. In Refs. [472; 473], a solar panel optical receiver was employed to split the AC/DC components present in the received optical signal. Employing the solar panel at the receiver to complete these tasks did not limit the performance of either data decoding or EH. The proposed model considered the linear relation between harvested energy and optical power that falls on the receiver, i.e., a solar panel. In Refs. [474; 475], a dual-hop hybrid visible light and RF communication systems was proposed. A relay was used to detect the information from a VLC link and also to harvest energy by separating the DC component present in the optical signal that falls on the optical detector. This harvested energy was applied to power another device that used an RF link to retransmit the data to a mobile terminal. Wang Hong-qiao et al. proposed a MAC protocol with an EH model as a function of distance, acceptance angle, obstruction probability, and energy adoption contention to reduce the devices' throughput unfairness problems, particularly for VLC. As a result, devices with lower EH rates may have a shorter connection window, allowing them to transmit more data [468]. The considered scenario consisted of a star topology, i.e., one access point with several devices.