## 1 Introduction  
Visible light communication (VLC) is deemed to be an efficacious candidate for high data rate communication and efficient ubiquitous connectivity, and a remarkable complement to traditional radio frequency (RF) communication. It has the capability to overcome the spectrum scarcity and capacity limitation of RF communication. VLC offers several promising features such as an unlicensed spectrum, low implementation cost, low power consumption, high transmission rate, high confidentiality, relatively low health hazards, and immunity to interference from electromagnetic radiation. With the increasing inherent drawbacks of RF spectrum congestion and the unprecedented demand for high-speed communication, VLC can provide a high-speed and large-capacity solution for indoor wireless communication networks and is considered to a wireless communication technology with inevitable application prospects. It offers several unique transmission capabilities making it feasible in various scenarios, such as visible light positioning (VLP) [1], optical camera communication [2; 3], underwater communication [4; 5; 6; 7], free-space optical communication (FSO) [8; 9], and integrated lighting and communication [10]. Efficient transmission is the key point to communication system design. Traditional RF communication is based on fixed resource allocation by orthogonal allocation of the time, frequency, and code domains of the system [11; 12], which makes users communicate independently of the resources and do not interfere with each other. The latest wireless optical code division multiple access technology has also been studied and implemented [13; 14]. However, the number of nodes that can be served by an orthogonal multiple access system at the same moment cannot exceed the number of orthogonal resources; therefore, the system throughput performance is limited. In order to meet the diverse and stringent transmission needs in the envisioned indoor intelligent network transmission, it is necessary to study the access resource allocation for VLC access systems. Indoor optical access is significantly different from the multiple access technology of RF communication due to the limited radiation distance and angle range. Therefore, it is important to use its directional radiation to form a beam for indoor multiple optical access. In [15], the new non-orthogonal multiple access (NOMA) was reported for indoor environments for multi-user downlink communication by taking advantage of light intensity distribution or uneven transmission distance without frequency and time conflicts. Thus, this key distinguishing feature makes it a promising approach for single-receiver multiple-input and multiple-output (MIMO) VLC in an uplink system [16]. The authors proposed a NOMA scheme incorporated with orthogonal frequency division multiplexing access (OFDMA) for VLC, which provided a high system capacity, flexible bandwidth, and high throughput for a large number of users [16]. The results indicated that inter-user interference could be effectively eliminated through accurate channel estimation. In order to experimentally realize a MISO system, one key constraint is etendue conservation. Increasing the active area of the photodetector receiver can significantly enhance the signal-to-noise-ratio (SNR), but it also raises the concern of lower data rate due to limited bandwidth. A traditional approach to enhance the SNR is to focus light through optical lenses. However, these optical concentrators are usually constrained by the conservation of etendue, resulting in a tradeoff between field-of-view (FoV) and optical gain [17]. Hence, the fluorophore, which initially absorbs impingent light and then re-emits it at a longer wavelength, is designed to overcome the above shortcoming. Both single and double fluorescent layers have been considered as concentrators to attain high gain and wide FoV in VLC systems [18; 19].