# A Review on LiFi Network Research: Open Issues, Applications and Future Directions
## 8 Multi-User Access in LiFi  
LiFi is differentiated from VLC technology when it comes to multi-point connections. LiFi supports multi-point connection provides multiple access to users or devices; several users may connect to the same AP. This feature is important, especially in a dynamic environment, such as hybrid networks. However, multiple IoT devices can be supported by LiFi networks [195]. Notably, LiFi supports user mobility and multi-user access. The central controller CU in LiFi also supports multi-user access [14]. The efficient implementation of a LiFi system is critical, so the appropriate multiple access technologies are also crucial. Many techniques for establishing multiple access will be discussed in this section. In LiFi communication, several multiple access techniques were implemented, which can be divided into two categories: (i) OMA and (ii) NOMA. In OMA schemes, users allocate different orthogonal time or frequency resources [195; 196]. By contrast, NOMA allows multiple users to simultaneously utilising time and frequency resources through power domain Superposition Coded Modulation SPC and Successive Interference Cancellation SIC [197]. The performance gain of NOMA over OMA is mainly determined by the specific power allocation strategy adopted by the former [198]. Nevertheless, to apply NOMA in LiFi-enabled BID communication, the following two important issues [199; 200] shouldbe considered: 1) Energy consumption: in LiFi-enabled BID IoT communication, energy consumption originates from the LiFi AP within each optical attocell and connected IoT devices. 2) Device QoS requirements: Devices can be divided into two categories; one includes low-speed devices, such as environmental sensors and health monitors; the other includes high-speed devices, such as multimedia-capable mobile phones. NOMA is another spectrum-efficient scheme that has been recently proposed for LiFi systems [197]. NOMA is based on multiplexing users in the power domain so that each user can utilize the full spectrum and time resources. The superior spectral efficiency enhancement offered by NOMA comes with an inevitable degradation in the corresponding error performance because of the reduction of the received SINR at the users' terminals [96,194,201]. An increase in the number of simultaneously connected users reduces the benefits of NOMA. More transmitted power is allocated for users with poor channel gains, whereas less is allocated for users with satisfactory channel gains [202]. NOMA shares the entire bandwidth among all simultaneous users, thereby allowing inter-user interference to occur, while OMA orthogonally allocates resources to all users. To cope with such inter-user interference, SIC is applied on the receiving side to extract transmitted signals with superposition coding [203]. Study [203] proposed a hybrid, multiple access schemes that simultaneously uses NOMA and OMA in the same bandwidth. Moreover, the coexistence of OMA and NOMA in the entire bandwidth can reduce the actual number of simultaneous users in NOMA, thereby enhancing the effectiveness of NOMA. To improve the throughput of cell edge users, NOMA is used for RF communication systems. By utilizing the broadcasting nature of LEDs, the performance of a LiFi network can be enhanced efficiently with the application of NOMA [2]. Unlike conventional OMA technologies, NOMA can serve an increased number of users via non-orthogonal resource allocation, and it is considered a promising technology for 5G wireless communication [2]. Various multiplexing schemes of NOMA and OMA are presented in this paper. In [204], Carrier Sense Multiple Access (CSMA) was proposed. To avoid collisions, the LiFi attocell continuously senses the channel prior to transmission. Nevertheless, CSMA-based systems are highly susceptible to the hidden terminal situation, which results in significant degradation in system performance. Furthermore, CSMA involves the significant signalling overheads needed for request-to-send and clear-to-send symbols.