# A Review on LiFi Network Research: Open Issues, Applications and Future Directions
## 8 Multi-User Access in LiFi
The study [67] also found that Space Division Multiple Access SDMA-based LiFi attocells could simultaneously serve multiple users using the full spectrum and time resources. This was achieved by replacing the single-element transmitter with an angle diversity transmitter, which conveys separate beams to users in different locations. SDMA provides considerable spectral efficiency gains compared with TDMA. Still, it has the disadvantage of increased computational complexity because of the need for the careful design of the angle diversity transmitter and user-grouping algorithms. Moreover, SDMA fails to operate in scenarios where users are located at adjacent positions due to the high ICI. In the study [205], Code Division Multiple Access CDMA-based systems and Optical orthogonal codes OOC are exploited to allow simultaneous network access for multiple users. The corresponding achievable spectral efficiency is limited by the large size and poor correlation characteristics of the OOC sequences. In the same context, Study [206], found that Orthogonal Frequency Division Multiple Access OFDMA could provide spectrum-efficient multiple access by sharing the available spectrum resources based on Orthogonal Frequency Division Multiple OFDM. In Study [207], NOMA was initially proposed as a multiuser access technique for RF cellular networks. Moreover, they discuss the use of DCO-OFDM in combination with NOMA for multiple users. The study [96] presented the idea of Index-Time Division Multiple Access I-TDMA, a novel multiple access schemes that provided significant spectral efficiency enhancement compared with TDMA, which multiplexed users in the time domain like TDMA. The proposed I-TDMA harnesses the randomness of data sequences to create a new degree of freedom. This instance allows considerable spectral-efficiency gains compared with TDMA, with minimal added circuitry that maintains its low complexity compared with other multiple-access schemes discussed earlier. An I-TDMA scheme can have minimal added complexity that involves using an XOR adder at the transmitting and receiving terminals. I-TDMA can be adapted to dynamically match the users' individual data-rate requirements by changing a single parameter [96]. Considering that no fast-fading characteristics are observed in the LiFi transmissions, high SNR statistics exist at low-frequency subcarriers. As a result, appropriate user-scheduling techniques are needed in OFDMA-based LiFi systems to ensure fairness among users. Moreover, the attained spectral gain comes at the expense of more complicated processing at the transmitter and receiver terminals. SDMA shows promising performance in terms of RF, but it cannot be directly adopted in LiFi. One of the main issues refers to the transmitter. In RF, narrow directional beams are generated by changing the amplitude and phase of the signals transmitted by an antenna array. However, this approach cannot be implemented straightforwardly in LiFi because it uses Intensity Modulation IM and Direct Detection DD, and the main reason is that in LiFi, incoherent light sources are used, and thus the phase of the emitted waves cannot be fully controlled. However, in LiFi, LEDs have the inherent feature of a confined FOV. This characteristic can be exploited for generating directional light beams. Therefore, in optical SDMA, an angle diversity transmitter, which consists of multiple directional narrow FOV LED elements, is used as the optical transmitter. By activating different transmitter elements, the angle diversity transmitter can serve multiple users at different locations simultaneously. An AP consists of a single-element transmitter that generates an omnidirectional signal. Only one user can be served within a time slot. Unlike TDMA, the SDMA AP consists of an antenna array that simultaneously generates multiple directional narrow beam signals. Hence, more than one active user can be served within a single time slot [67].