# Resource Allocation in Downlink VLC-NOMA Systems for Factory Automation Scenario
###### 1 Introduction  
Advancements in light emitting diode (LED) technology have caused LEDs to have long life cycles and to overheat less during operation, making them more cost- and energy-efficient. This has resulted in their large-scale use. LEDs are not only used as a light source but can also be used as transmitters for visible light communication (VLC) [1]. As VLC can provide high speeds and security, it can be an appropriate solution for indoor wireless communications [2]. VLC can be a solution to overcome the shortage of radio frequency resources, as it adopts an unlicensed spectrum. As such, the potential of VLC can be advantageous for industrial wireless applications. There have also been considerations to apply VLC to industrial wireless communications [3]. As non-orthogonal multiple access (NOMA) can achieve high spectral efficiency by adopting superposition coding (SC), it can simultaneously transmit signals to both the near-user (NU) and the far-user (FU) using the same frequency resource [4]. Therefore, NOMA was proposed for VLC [5, 6, 7, 8]. The signal for FU can be canceled by successive interference cancellation (SIC); then, the signal for NU can be decoded [9]. Instead, as the signal for FU is with the highest power allocation, the signal can be decoded without SIC. VLC-NOMA systems were evaluated in [7, 10] in terms of their coverage probability and ergodic sum rate. [PERSON] et al. evaluated the performance of VLC-NOMA compared with orthogonal frequency-division multiple access in terms of data rate [11]. [PERSON] et al. evaluated the bit error rate (BER) of a NOMA system made up of a user pair including three or more users [12]. In this study, we describe the VLC-NOMA system applied to factory automation, proposing a deep neural network (DNN)-based power allocation scheme and a priority-based user pairing (PBUP) scheme. As mentioned above, VLC adopts unlicensed bandwidth, is free from radio frequency interference, and has secure transmission by signals not penetrating walls [8]. Therefore, the advantages of VLC can be considered solutions for factory automation. For more additions, factory automation should handle periodic transmission for each node [13]. Therefore, handling periodic transmission should be scheduled, and the usage of resources for scheduling should be reduced to avoid missed deadlines of real-time transmission. Adopting NOMA can reduce the usage of resources, as NOMA can transmit multiple signals within the same resource. We derived the BER performance of both users in a user pair, in terms of both perfect and imperfect SICs, and showed a balanced power ratio that considers both NU and FU in terms of reliability. Reliability is a key performance indicator for defining the stability and accuracy of a system [14]. To improve reliability, DNN-based power allocation (DBPA) was proposed with the derived BER performance. Previous studies have applied DNNs to VLC. [PERSON] et al. proposed a deep learning framework for the design of an on-off keying (OOK)-based binary signaling transceiver in dimmable VLC systems [15]. [PERSON] et al. suggested a VLC module that uses a DNN to reduce the complexity caused by handling both the error rate and level of illumination simultaneously [16]. However, these studies did not consider NOMA. Although [PERSON] et al. suggested a signal DNN-based demodulator in a VLC-NOMA system [17], their study applied a DNN to a demodulator rather than a power allocation algorithm. In contrast, the proposed DBPA in this study focuses on finding the minimum point of average BER to improve reliability for both users. In addition, there have been studies for resource allocation in NOMA systems with radio frequency. [PERSON] et al. suggested a power allocation algorithm using DNN in a device-to-device NOMA system to maximize sum capacity [18]. [PERSON] et al. studied user-clustering and power allocation in downlink NOMA systems to maximize throughput [19]. [PERSON] et al.