# Multi-Wavelength Path Loss Model for Indoor VLC with Mobile Human Blockage
## 1 Introduction
### Literature Review  
Looking from the existing literature, there are many studies that pay attention to path loss in indoor scenarios for VLC. For instance, the channel path losses for various receiving positions in indoor environments was presented in [14]. An optical ray-tracing approach for minimizing path loss in a variable link was discussed in [15]. The distance (CI) model of VLC was derived in [16]. The impact of transceiver characteristics on path loss was studied in [17]. However, in these studies, wavelength dependence and human blockage were not considered.  
Furthermore, some works investigate the impact of wavelength dependence on path loss without considering the human blockage effect. In [11], a measurement campaign was conducted and a wavelength-dependent path loss model was proposed based on the measurement data. In [9; 18], the path loss was investigated with consideration of the radiant power spectral density (PSD) of white LED and the wavelength-dependent spectral reflection coefficient in obtaining the channel impulses response (CIR). Based on the derived CIR, in [9], a path loss model was proposed in the form of the close-in (CI) model which is a distance-dependent function. In [18], the path loss was modeled as a logarithmic function of two parameters by curve fitting. The influence of material reflection characteristics on path loss was explored and the model parameters were given under two kinds of specific reflection materials. However, in these works, the wavelength dependence is integrated within the proposed model and the relationship between the wavelength and the path loss has not been reflected.  
Moreover, the path loss is investigated with the presence of human blockage without considering the wavelength dependence. In [19], it was conducted a measurement campaign with realistic human blockage during office hours in an office scenario. In [20], a solution framework was proposed to enhance the communication performance in a realistic indoor environment where blockage from both the static objects and mobile human persons exists. In [21], a measurement campaign was conducted to study path loss with a human body staying in a fixed position. However, in a realistic application scenario, the human is usually moving and the path loss with the static human blockage can not reflect the link instability induced by human mobility. Also, in [21], there is no proposed model to describe the impact of human blockage on path loss. To involve human mobility in the investigation, path loss with human blockage was studied based on a human moving trajectory. In [22], the human moving trajectory was generated by a random trajectory generator algorithm in which step direction angle and step length are chosen uniformly. In [18], the human moving trajectory was a fixed route next to the wall and furniture. And, in [18; 22], the path loss with mobile human blockage was both modeled as a random variable with the log-normal distribution. However, in these works, the human moving trajectory which dominates the variation in the channel states does not exhibit the characteristics of realistic human walking behavior, thereby reducing the accuracy of the path loss model.  
In a word, the research on path loss that considers the impact of wavelength dependence and the human blockage has been done in some literature. However, due to the lack of a realistic human moving trajectory, the existing path loss model can not accurately reflect the effect of mobile human blockage. Furthermore, to the best of the authors' knowledge, there is no standardized channel model or a comprehensive model which can characterize the impact of both the wavelength dependence and the mobile human blockage on path loss for indoor VLC channels.