## 1 Introduction
In [33], a joint detection method using phase pre-distortion for multi-user detection in a NOMA-VLC system was proposed, which had acceptable pre-distortion effects at low, medium, and high SNR, better than the interference cancellation detection algorithm. In [34], symmetric superposition coding and symmetric successive interference cancellation decoding techniques were implemented, and the effectiveness was experimentally verified, which eliminated 90% of the demodulation error compared to the conventional orthogonal multiple access system. The aforementioned methods investigated NOMA techniques based on power or code domain resources and did not consider NOMA techniques under multidimensional resource multiplexing. Additionally, these methods cannot solve the problem of multi-user detection under multidimensional resources. One crucial challenge is how to allocate power to NOMA users when there are limited resources. The power allocation to each user is performed in a sophisticated manner in order to prevent a specific user from accumulating the total power. It has a substantial impact on the performance of the NOMA technique, such as interference management and rate distribution among users. The performance of NOMA schemes strongly depends on the power allocation, where inaccurate power allocation can cause interference among superimposed users [35]. Consequently, it will degrade the BER performance, rate distribution, and outage of NOMA users. Interference between the NOMA users can be avoided by choosing power allocation accurately. An appropriate power allocation with low complexity is highly desirable to exploit the power domain optimally in practical NOMA-based VLC systems. Power allocation is important in NOMA systems to improve the system throughout, spectral efficiency, energy efficiency, capacity, quality of service (QoS), and user fairness. Moreover, accurate power allocation and an appropriate user pairing technique [36] must be compatible to achieve better performance of NOMA systems. Important parameters that are required to design power allocation schemes are total power constraint,QoS requirements, maximizing objective function, channel conditions, CSI availability, and many more. Some of the performance metrics used for power allocation schemes are the number of admissible users, energy efficiency (EE), SNR, channel gain, fairness index, and sum rate [37]. An energy efficient NOMA scheme for bidirectional VLC systems was proposed in [38] to maximize energy efficiency under certain QoS targets through an optimal power allocation approach. At present, although many studies have applied NOMA technology in optical communication systems, the power allocation and the SIC signal detection technology of NOMA have not been thoroughly studied, and the error propagation problem is more prominent. In [39], the challenges faced by introducing a NOMA-VLC system were summarized, including power allocation, how to combine it with optical orthogonal frequency-division multiplexing (OFDM) modulation, and how to combine it with multiple-input multiple-output (MIMO). In [40], a gain ratio power allocation (GRPA) strategy was proposed to allocate power to individual users according to their channel status, which improved the effectiveness and fairness of the system. In [41], a power allocation scheme based on GRPA was introduced with a look-up table method. The system complexity was reduced by designing a channel gain table for power allocation. In [42], the NOMA technique was introduced into the MIMO multi-user VLC system. For the characteristics of MIMO technology, the paper proposed a normalized gain difference power allocation (NGDPA) method to improve the system capacity and transmission efficiency. A summary of recent works on NOMA systems is provided in Table 1.