# Multi-Wavelength Path Loss Model for Indoor VLC with Mobile Human Blockage
## 1 Introduction  
With the commercialization of the fifth-generation (5G) networks, the exploration of the sixth-generation (6G) mobile communication system has been carried out [1, 2]. Given that the dearth of spectrum in the conventional radio frequency (RF) bands coupled with increasing wireless traffic, industry and academia are inclined to consider employing visible light communication (VLC) as one of the candidate technologies for the next generation of wireless access networks. VLC is a high-speed communication technology that uses an unauthorized spectrum ranging from 400 to 800 THz, which can provide a huge communication capacity. Moreover, VLC can use the light emitting diode (LED) to realize lighting and communication functions simultaneously [3], which can be used to fulfill the high capacity demand in indoor scenarios where the maximum data usage occurs [4]. At the same time, the dual-use of LED makes the VLC close to the \"zero energy consumption\" communication of light source which can help to achieve the goals of \"carbon peaking\" and \"carbon neutralization\". Therefore, VLC can be used as a supplement to the existing RF system and can become one of the key technologies supporting 6G. In the designing and optimizing of VLC systems, since channel properties determine the performance limit of wireless communication systems, VLC channel modeling is regarded as a foundational step. Also, the visible light channel model can be used to evaluate the performance of VLC technologies [5]. Notably, in the VLC channel, becausethe wavelength of visible light signals is extremely short, and the size of the receiver is usually millions of square wavelengths, the small-scale fading of signals over several wavelengths will not occur [6]. And, it is the path loss that dominates VLC performance that has critical importance in accurately estimating the link budget and the capacity of the VLC channel [7; 8]. Therefore, an accurate path loss model plays a decisive role in VLC system design, performance evaluation, and testing. Due to the propagation nature of light, the path loss of the VLC channel is susceptible to wavelength dependence and blockage effect [9; 10]. Firstly, the path loss shows a dependence on the visible light signal wavelength due to the inherently wideband (380-780 nm) of the light source in VLC. The variation of signal wavelength will lead to a significant change in the reflection coefficient of the reflecting material, thus changing the propagation characteristics of the VLC channel. According to Friis free space path loss formula, the higher the signal frequency, that is, the lower wavelength, the greater the path loss is. And, it has been verified by measurements that there is an increase of 2.26 dB between the path loss under 405 nm and that under 625 nm at the transceiver distance of 10 m [11]. What's more, in wireless transmission, path loss is susceptible to human blockage due to the human body obstructing the line-of-sight (LoS) component and it is particularly severe for VLC systems [12]. Taking an example of the millimeter wave signal, there will appear four states when encountering blockage: falling, blocking, rising, and returning to the unblocked state. In the process of falling and rising, the signal intensity remains high for a certain period, which can maintain communication between transceivers. However, it should be highlighted that for VLC, the wavelength of light is only a few hundred nanometers, and it is almost impossible for light to bypass the macro blockages by diffraction [13]. Once encountering blockage during propagation, the signal intensity will rapidly fluctuate on a large scale and fall to an extremely small level without the process of rising and falling. Consequently, the receiving position can hardly receive any signal once blockage occurs, reducing the stability of the VLC system [12]. Therefore, it is eminent to propose a path loss model which can characterize the wavelength dependence properties and the human blockage effect for VLC.