# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Light-Fidelity (Li-Fi)
These specific RF system features can solve the few limitations of Li-Fi systems. A Wi-Fi or a femtocell AP (FAP) creates a small layer within a home. Li-Fi attocells additionally introduce a new layer, producing three-tier networks. In addition, users can connect femtocell, macro-cell, or Li-Fi atto-cell networks within a train/bus, thereby creating three-tier network structures. In such hybrid networks, mobility support, resource sharing, load balancing, packet scheduling, up/downlink sharing protocols, physical layer security, and network selection are the important concerns. In Ref. [253], the authors proposed a joint optimization problem to evaluate a selection at the network level over time for each user. The authors suggest a new fuzzy-logic-based approach [251] to lower the computing complexity necessary to tackle the optimization problem. In comparison to standard processes, the authors show that the proposed methodology boosts machine efficiency up to 68 percent, while reaching extremely low computing complexity. Wu et al. presented a mobility-aware load balancing (MALB) method for Li-Fi-Wi-Fi hybrid networks (HLWNets) in [253; 254]. In single transmission (ST) mode, a user is serviced by only one AP, which could be Li-Fi or Wi-Fi, resulting in a handover issue. In multiple transmission (MT) mode, however, each user is served simultaneously by both Li-Fi and Wi-Fi APs, and there is no transition. A combined optimization problem can occur at the same time in order to control traffic flows between Wi-Fi and Li-Fi. The authors showed that by reducing computational complexity, the proposed MALB-ST and MALB-MT methods could increase machine performance by up to 46% and 76%, respectively [255]. In Ref. [256], the authors proposed dynamic load balancing schemes based on channel state information (CSI) to enhance load balancing in Li-Fi/RF hybrid networks. The authors presented a downlink hybrid architecture with four Li-Fi APs and one Wi-Fi AP in [257]. An effective AP assignment technique that improves long-term system throughput was evaluated using a reinforcement learning (RL) algorithm. The authors compared the proposed system performance to recent benchmark methods, i.e., an iterative method of optimization, exhaustive user satisfaction search, signal strength strategy (SSS), average device throughput, power loss likelihood, and fairness are analyzed. In the case of smartphone devices, another key issue which needs to be addressed in HLWNets is handover. When a smartphone device feels a stronger Li-Fi signal under a weak Wi-Fi signal, it can turn to the Li-Fi network and vice versa. Therefore, mobile users can experience frequent handovers in HLWNets. In Ref. [245], Harald Haas suggested a fuzzy logic (FL)-based handover scheme that could greatly minimize handover numbers and achieve higher data rates. The authors evaluated the data rate efficiency of the proposed dynamic handover scheme based on FL to that of the traditional load balancing method, concluding that the suggested scheme outperforms the existing one. The industries producing commercial Li-Fi systems are given in Table 1. In short, the main pros and cons of Li-Fi technology are high security, high density, high efficiency, higher data transfer rates, low congestion, better availability, flexibility and interference, limited coverage area, and limited compatibility, respectively.