# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 5 Advancements in Optical Wireless Communications
### Light-Fidelity (Li-Fi)
Up/downlinks and backhaul links are all part of a full Li-Fi network. Figure 33a illustrates a simple Li-Fi indoor network with all the tasks that it can perform [234; 253]. Advanced transmission schemes of Li-Fi systems address improved resource allocation, spatial dimension, cell densification, and channel aggregation for enhanced spectral efficiency. The most effective approach among these techniques is cell densification [254]. Due to the short transmit-receive distance, and small/low-powered cells (Femto-cell/atto-cell), Li-Fi can achieve a higher SNR and significantly extend a handset's battery life. As a result, the number of active users per Hertz per unit area or SEA increases, resulting in improved coverage and capacity [243]. Therefore, the concept of a small cell, as a part of HetNets, operating with more than one concurrent access technology such as macro-cells, femtocells, microcells, and atto-cells can be formed. These HetNets offer better traffic offloading, providing sustained QoS [255]. When multiple light fixtures are used as tiny radio base stations in a room, the resultant network of small cells is known as an optical Li-Fi atto-cell network. An optical burst switching (OBS) is a light fixture that is used to serve multiple users in a room. As with an RF femtocell network, an optical atto-cell exhibits functions such as multiple access, handover, and full-duplex communication. The author of [244] investigated the downlink performance of optical atto-cell networks using optical orthogonal frequency division multiplexing (O-OFDM in terms of outage probability, SNR, and resultant cell data rates). The results were compared and confirmed that an optical atto-cell network performs better than a femtocell network. Various cell layouts in Li-Fi networks are identified and its performance is analyzed in [234]. Figure 33b illustrates the key techniques needed for creating Li-Fi optical atto-cell networks in terrestrial, underwater, and space networking applications [245]. In Li-Fi cellular networks, two methods are used to reduce the amount of feedback; they are limited frequency feedback (LFF) and limited content feedback (LCF), based on the time of the last update and minimized feedback information, respectively. These methods outperform the one-bit and full feedback schemes in terms of throughput [246]. There are a variety of protocols for multiuser access (MA) that are addressed by orthogonal and non-orthogonal multiple access (OMA and NOMA) schemes. OMA offers a better reliability of connections at the cost of reduced spectral performance. On the other hand, NOMA allows the achievable data rates to be dramatically enhanced [247; 256]. In intelligent Li-Fi connectivity, APs are developed with a new cross-layer architecture system for dynamic multiple access selections (DMAS) [247]. For multi-layered optical atto-cell networks, a top-down architecture of a multi-hop wireless backhaul system using one gateway supercell was proposed. Cell-based bandwidth scheduling (CBS) and user-based bandwidth scheduling (UBS) are  
Figure 33: (**a**) Illustration of a Li-Fi indoor network; and (**b**) Li-Fi fundamental sectional blocks. the best algorithms for analyzing Li-Fi device efficiency [248; 249]. The Li-Fi communication network can be established with color domain modulation (CDM), multi-carrier modulation (MCM), and single carrier modulation (SCM). Pulse amplitude modulation (PAM), on-off keying (OOK) modulation and pulse position modulation (PPM) are some of the most widely used SCM schemes in Li-Fi systems and have been studied or analyzed with wireless infrared (IR) region communications [242]. SCM systems are most effective when there is a need for applications with small to moderate data rates. Utilizing adaptive bit, power loading, and direct current (DC) wandering techniques, OFDM-based modulation techniques offer a practical solution for Li-Fi [250]. Nonetheless, in NLoS conditions, RF-based systems support greater mobility and better performance.