## 1 Introduction  
With the rapid deployment of the Internet of things (IoT) for smart cities and smart homes, the next generation of wireless communication systems are expected to have the potential to connect and support more devices, including computers, mobile phones, tablets, high-definition (HD) cameras, virtual reality (VR), etc. Communications between a large number of devices through traditional radio-frequency (RF) technology will cause serious congestion and interference problems. Visible-light communication (VLC) is a promising candidate to address the spectrum-scarcity issue, utilizing visible light as an information carrier with high security and anti-electromagnetic-interference (EMI) characteristics, simultaneously [1; 2; 3]. With the advantages of rich bandwidth resources and low cost, indoor VLC systems have great potential in solving the shortage of spectra and meeting the high data-rate requirements of systems other than 5G. However, due to the inescapable properties of lightwave transmission, line-of-sight (LOS) VLC cannot penetrate objects. VLC suffers from non-line-of-sight (NLOS) transmission constraints and a shadowing effect, which cannot guarantee stable and continuous communication, especially for indoor scenarios with random moving obstacles. Therefore, many methods are proposed to tackle this problem when the LOS link is not available. Evolving from point-to-point VLC configuration, in order to improve the inherent issues of limited transmission distance or the link barrier, the multihop framework in VLCsystems has aroused intense research interest recently. In relay systems, the introduction of relay nodes provides more possibilities for optimizing the reliability of source-to-cell links. The concepts of relaying communication that could use certain light-emitting diode (LED) lamps as relays nodes were proposed and different indoor light sources can be deployed as relay nodes, such as ceilings, desks, and floor lights [4; 5; 6; 7]. Relays between multiple VLC terminal nodes can effectively extend the transmission distance and reduce the dependence of LOS links, which have been experimentally demonstrated with improved performance [8]. The channel response of the VLC system with relay nodes based on an amplification and forwarding (AF) strategy was analyzed, and the relay auxiliary strategy in improving system reliability was simulated [9]. In addition, an experimental comparison of single-channel and relay-based VLC-link performance proved that the relayed VLC link can provide a higher data rate than the direct VLC link [10]. Until now, two types of transmission protocols have been proposed in relay communication, including decode-and-forward (DF) mode and AF mode [11]. [PERSON] et al. proposed a multihop VLC system for offshore applications to overcome the limited coverage distance [12]. [PERSON] et al. studied the performance of a full-duplex relay-auxiliary VLC system and proved that the full-duplex relay is superior to the half-duplex relay [13]. [PERSON] et al. adopted a transmission-diversity scheme and showed that it can improve the signal-to-noise ratio (SNR) and reduce outage probability [14]. [PERSON] et al. investigated the performance of dual-hop underwater optical wireless communication (UOWC) systems with simultaneous lightwave information and power transmission, and numerical results showed that using relay nodes can improve performance [15]. Overall, the relay schemes can be divided into two types: passive relay and active relay. Passive relays currently have some proposed solutions such as intelligent reflecting surfaces (IRS), different reflecting materials, and concave mirrors. These works are mainly for beam forming and beam steering of the emitting ray. Using a concave mirror cannot change the direction of the beam very well, which is usually used to increase the intensity of the beam in point-to-point channels. Another way, the active relay, we will mention below. The above-mentioned works are all based on simulations without experimental results.