# SIC-Free Based Indoor Two-User NOMA-VLCP System
## 1 Introduction  
With the rapid development of mobile ultra-wideband, super internet of things (IoT), and artificial intelligence, 6G will provide communication services for a wide range of devices [1, 2]. Due to the multipath effect of satellite signals caused by reflections and other factors, global positioning systems (GPS) suffer from poor positioning accuracy in indoor environments. Meanwhile, radio-frequency (RF) based indoor communication and positioning technologies face electromagnetic interference and spectrum shortage problems. Compared to traditional indoor communication and positioning technologies, visible light communication and positioning (VLCP) have the advantages of wide bandwidth, resistance to RF electromagnetic interference, high biosafety and high positioning accuracy at low cost [3, 4, 5, 6]. Therefore, VLCP attracts more attention for providing information transmission and high-precision positioning services for indoor smart terminals such as smart factories and smart homes. In the visible light communication(VLC) system, the bandwidth of commercial light-emitting diode (LED) is usually several MHz, which becomes one of the difficulties hindering high-speed transmission and multi-user access. To address the issue of limited LED modulation bandwidth and to enable multiple access, researchers have proposed orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) techniques. Compared to OMA, NOMA allows multiple users to share resources such as frequency, time, and space within the non-orthogonal resource domain, improving spectral efficiency and thus system capacity [7, 8]. The NOMA system assigns higher transmit power to users with poorer channel conditions and lower transmit power to users with better channel conditions, which is fairer compared to the OMA system [9]. In order to improve the throughput of multi-user VLC systems, an adaptive multi-user association strategy based on VLC-NOMA was proposed, which obtained the optimal solution with maximum system capacity under transmission power constraints through power allocation [10]. In order to realize multi-user communication function in a single cell, a two multi-user groupingschemes was proposed, which maximizing the channel difference within the group in downlink VLC-NOMA links [11]. However, NOMA suffers from the problem of inter-user interference, which is traditionally demodulated using serial interference cancellation (SIC), resulting in the phenomenon of error propagation (EP). Adjustable bit rate superposition coding (ABRSC) and sample-and-sum successive interference cancellation (SSSIC) decoding schemes were proposed to reduce the EP effect on the bit error rate. In the case of poor VLC channel conditions, this solution introduces user bit error factors by reducing user bit rates and continuous interference elimination methods to reduce interference between users to reduce BER [12]. Moreover, in NOMA uplink, a least-reliable symbol-based ordered-successive interference cancelation (LRS-OSIC) algorithm was proposed. The algorithm comprehensively considered spectrum efficiency and transmission reliability while reducing EP [13]. In addition, an optimized optical antenna design and a signal processing method based on successive interference cancellation (SIC) and multi-layer coding (MC) are proposed to address the misalignment problem caused by transmitter/receiver mobility and the interference problem between LEDs [14]. To support multi-user communication in a cell, an improved MMSE algorithm is employed in the MIMO-OFDM VLC system, adding Cyclic Prefix and Suffix to ensure subcarrier orthogonality and reduce multipath interference [15]. To mitigate adjacent interference in dense broadband VLC networks, a joint AP selection and power allocation scheme is proposed, optimizing both to maximize data transmission rates and minimize interference [16]. In recent years, researchers have proposed many VLC systems for NOMA-based systems.