# A Review on LiFi Network Research: Open Issues, Applications and Future Directions
## 5 Component and Architecture of a LiFi System
### Downlink Transmitter DLT  
A LiFi light bulb is used as a DLT [162]. Usually, the light bulb consists of one LED or many shaped clusters [42; 90]. Quick and massive current variations can be created to produce the optical output because only Light is used [163]. The LiFi light bulb is constantly operating and interruption-free because the LED can be turned on and off easily, and the number is quickly modulated. Thus, even though it flickers, the human eye is not able to see it [1]. This transmitter, known as the AP, can be connected to the receiver in UEs,such as PCs, laptops, and mobile phones [94]. Similar to all other wireless networking technologies, this AP spans a small region and transforms IP network knowledge into bits. Also, the coverage area created by LiFi AP is referred to as the attocell [66; 79]. Figure 5 illustrates the LiFi AP attocell. A few important factors must be considered in DLT. (i) The line of sight (LOS) component: the contributions of the received signal, wall, and surface which are reflected in the light. Therefore, the LiFi technology is not strictly oriented to the LOS [17; 120; 164]. However, LOS connections offer greater results than the Non-Line Of Sight (NLOS) link. (ii) Presenter of light: This factor depends on the irradiance angle and the environment. Most LiFi simulation studies assume that the irradiance angle in their device design is perpendicular to the floor [49; 66]. This phenomenon shows that the attocell for each AP is in an asymmetric circular form. Meanwhile, the irradiance angle is a significant feature for designing and creating methods, algorithms, and schemes, such as APS, LB, and HO management and coordination. (iii) Illumination: This factor refers to the important volumes of transmitted optical energy. This power follows the transfer of electric power to optical power [66]. Moreover, the room's height has a major effect on the optical power, which contributes to the intensity of the AP-UE communication relationship. However, more APs require more lighting, subsequently increasing the noise between the APs. Thus, the Signal-To-Interference-Plus-Noise Ratio (SINR) must be considered. Although all these factors might seem identical, they are not.  
However, they participate in AP assignment and communication resource allocations. The LiFi AP covers a specific and limited area. Moreover, attocells are cells greater in scale than the communication in the LiFi network. A strong spatial spectrum quality can be obtained by reusing the bandwidth in each attocell [94]. The overlap zone, Optical Co-Channel Interference (CCI), is recognized and treated as noise because the same bandwidth is reused by all attocells [66]. The attocell diverges scale based on the source of illumination and the form of light. (iv) The distance between the AP and the region of irradiation. The attocell boundary represents the edge of the illuminated region. CCI occurs when two or more attocell users are linked as long as they remain inside the HO circle. This phenomenon is the same as that in the LiFi service area. Thus, users will connect to the AP and receive a LiFi signal from there. Other signal forms from another AP, such as Wi-Fi, may affect the area size [66]. As users across the HO zone borders, they would experience HO where they would remain inside the HO field. Depending on the algorithm used, such as the LB algorithm, this approach requires multiple milliseconds to seconds [66; 79]. However, even within the HO zone, users will not experience HO in certain cases. This instance, which is termed as HO overhead [40; 66; 66; 165; 166], may occur even beyond the HO area because of other factors, such as: (i) the capacity of networks, (ii) the density for devices or users, (iii) signal intensity strength, and (iv) availability.  
Figure 5: LiFi attocell in DLT.