## 2 Concept, System Setups, and Methods  
In Figure 1, a relaying VLC system based on multiple micro-LEDs is investigated, which consists of the source node, the relay node, and the end-user. The corresponding experimental setup of dual-hop system is shown in Figure 2. Experimental parameters based on Hop 1 and Hop 2 are summarized in Table 1. The proposed framework can be applied in indoor VLC scenarios, which means many high-speed applications including virtual reality, real-time HD communication, second-level download, and real-time cloud storage can be realized. High-speed data transmission not only improves the ultimate sensory experience for users but also creates greater value for operators. However, the transmission system may be limited by NLOS due to the characteristics of VLC. Therefore, we consider the downlink transmission of a dual-hop system, consisting of two micro-LEDs and a single end-user. The system concept is shown in Figure 1a. As illustrated in Figure 1a, the link between the first transmitter (Tx.1) and the second receiver (Rx.2) suffers NLOS transmission constraints due to the obstacle. The use of a relay node composed of Rx.1 and Tx.2 is an alternative method to solve this NLOS problem. We used a micro-LED as the light source for the dual-hop VLC system; therefore, the transmitter can be considered as a monochromatic-point light source with Lambertian radiation pattern. The VLC system is based on the AF relay method. Among various relay strategies based on VLC, we chose AF relay as our relay node, which is very simple but will inevitably synchronously cause amplification of noise. In our dual-hop VLC system, channel-intensity modulation with direct detection (IM/DD) is the preferred choice, which can be modeled as a base-band linear system. At the end-user receiver, the noise is composed of larger shot noise and thermal noise due to the introduction of additional relay node. In a multi-relay VLC system that implements communication, there is a positive correlation between the communication-performance deterioration of the end-user receiver and the number of introduced nodes. The setup for the dual-hop VLC system is depicted in Figure 1b. A long pseudo-random binary sequence with a length of \(2^{31}-1\) is modulated by OOK format and generated from bit-error-rate tester (BERT, MP2100B, Anritsu), the system also used to measure BER performance. Before implementing the intensity modulation of micro-LED, the signal is amplified first and superimposed with direct current (DC) by a bias-tee. After 2 m free-space transmission, the optical signal is received by Rx.1. In this experiment, Rx.1 and Rx.2 are two avalanche photodiode (APD, APD210, Thorlabs) modules. Following transmission over the first optical link, the AF relay scheme is used. At the relay node, Rx.1 is used to convert the received optical signals into electrical signals and provide high gain. In order to ensure the signal quality and  
Figure 1: **(a)** Schematic diagram of dual-hop VLC-link setup. **(b)** Diagram of dual-hop VLC-link setup. **(c)** The cascade structure of Amp.2 with detailed order and type. (Tx. : Transmitter; Rx. : Receiver; Amp. : Amplifier; BERT: Bit-error-rate tester; DC: Direct current). Figure 2: Experimental setup of dual-hop VLC link. (Inset Cascade Amp.2). adjust the voltage dynamically, Amp.2 is composed of two amplifiers (SHF, Mini-circuits, and DR-AN-20-MO, iXblue), as shown in Figure 2. To obtain the optimum voltage and modulation depth for relay signals, we can traverse different values to select the appropriate voltage corresponding to the lowest BER, specific voltage values can be seen in Table 1. Similarly, Rx.2 can detect the optical signal transmitted from the Tx2 after a 2 m distance in another direction. Finally, to evaluate communication performance, a real-time oscilloscope (RTO, DPO75902 SX, Tektronix) is used to record the signal after dual-hop structure, and then analyze the signal quality and capture the eye diagrams.