# A Review on LiFi Network Research: Open Issues, Applications and Future Directions
## 3 Taxonomy
### Simulation  
The articles in this class (n = 105) were used to produce a comprehensive overview of studies on LiFi that use simulation in implementing their approach. Network simulation is a technique whereby a software program models the behavior of a network by calculating the interaction among different network entities (e.g., routers, switches, nodes, APs, and links). This section comprises four subcategories: (i) Category 1 (C1) consists of common problems in LiFi, which contains 94 articles and is mostly focused on considering issues, such as Handover (HO), mobility, and interference problems. (ii) Category 2 (C2) consists of common methods used in LiFi simulation studies, and it contains 45 articles. Most of these works used Load Balancing (LB), AP selection/assignment, and user/Atto-cell selections (ACS). (iii) Category 3 (C3) consists of common environments in which LiFi systems could be implemented, containing (n = 71) articles. The majority focused on conducting their study in indoor scenarios while the rest considered outdoor scenarios. (iv) Category 4 (C4) consists of the transmission direction of data, and it consists of (n = 48) articles. Most of the aforementioned categories focus on the downlink (DL) and bidirectional (BID) transmission, and only a few have considered the uplink (UL) transmission. In this section, the simulation studies are classified based on these four categories, as illustrated in Table 1. Many studies have proposed novel methods and schemes for access point assignment (APA) and access point selection (APS) within different environments by assessing the literature. Few of these studies considered working on UL, whereas most studies focused on the DL side of the transmission. This section focuses on simulation studies. A total of 32 out of the 105 articles involved simulation studies that used the Monte-Carlo simulation method [28; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60]. For other studies, different simulation software, including MATLAB and OptSim [61], or MATLAB only [62; 63], MATLAB and OPTI-WAVE [64], NS2 [65], TracePro [55], and Mininet emulation environment [66], were used. These four classes have been used to summarize all the previous literature on LiFi studies as shown in Table 1. All the classes and categories in Table 1 include 93 out of 105 articles which are oriented on simulation studies. Aside from the simulation studies that fall under the previously discussed problems (i.e., C1, C2, C3, and C4), other simulation studies that do not fall in the previously discussed categories still exist. To begin with, [63] proposed a novel modulation technique called Hybrid DC-Biased Asymmetrically Clipped Pulse-Amplitude-Modulated (HDAP-OFDM) power-efficient communication, as well as a comprehensive dimming control. In [67], researchers presented a network congestion characterization  
Figure 4: Taxonomy of literature on LiFi. related to LiFi and a High throughput satellite (HTS) under the Ka-band modulation schemes during adverse weather conditions in tropical regions. The study in [68] proposed a fascinating system for massive data transmission that could be performed by free-space communication and was proven to be helpful for links within high and large buildings. The respective data transmission capability can be reached via the big data concept [69] and developed a smart vehicular communication system that protected vehicular collisions on the roads using LiFi technology. The study in [15] simulated a Direct Current Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) modulation-based MIMO system used for LiFi technology. In [70], the authors designed a smart traffic system to communicate between automobiles and their traffic environment. This system is usually called V2X technology. In the work of [37], they proposed a new modulation format called HDC-OFDM for LiFi by combining the aspects of the existing Diversity-combined asymmetrically clipped optical OFDM DACO-OFDM and DCO-OFDM.