## 3 Channel Characteristics Comparisons
### K-Factor  
K-factor is a measure of the severity for the small-scale fading. Knowledge of K-factor statistics is essential for the design and performance analysis of wireless systems. Furthermore, K-factor is defined as the ratio of the power of the deterministic MPC (typically LOS) and the power of all other stochastic MPCs (typically NLOS). Thus, we can obtain the K-factor according to (1), as shown in (11). However, it is difficult to distinguish which one is a LOS component and which one is an NLOS component from the measured CIRs for mmWave channels. Sometimes, one delay bin may contain both the LOS and NLOS components. Here, we applied the moment method to estimate the K-factor based upon the analysis of frequency selectivity for mmWave channels [38; 39].  
\[K=\frac{|V|^{2}}{\sum_{n=1}^{N}|a_{n}|^{2}}. \tag{11}\]  
For VLC channels, we obtain the channel characteristics by the ray tracing method. Therefore, we can distinguish the LOS and NLOS components clearly based on the source output file. The K-factor for VLC channels can be expressed according to its definition as:  
\[K=\frac{\sum_{n_{\text{{LOS}}}=1}^{N_{\text{{LOS}}}}P(n_{\text{{LOS}}})}{\sum _{n_{\text{{NLOS}}}=1}^{N_{\text{{NLOS}}}}P(n_{\text{{NLOS}}})}, \tag{12}\]  
where the \(N_{\text{{LOS}}}\) and \(N_{\text{{NLOS}}}\) are the numbers of LOS and NLOS rays, respectively.  
The CDF of the estimated K-factor and its normal fitting for mmWave channels is shown in Figure 12a. It shows that 20% of the K-factor values are less than 0 dB and these values are from the 10 th (5.43 m) and 16 th (9.0 m) measured points. It appears that NLOS components account for more power in these two points, especially in the 16 th point. This is mainly because these two points are close to the walls, which means more reflection components, resulting in the small K-factor. However, 74% of the K-factor values for VLC channels are less than 0 dB in Figure 12b, and this happens when the distance between TX and RX is larger than 5.43 m (10 th point). This indicates that NLOS components are stronger at a large distance in this conference room. Note that the K-factor in VLC bands is 9 dB lower than that in mmWave bands when the distance is 3 m. This can be explained by the definition of K-factor in (12). It is clear that whether there are one or nine LEDs placed at the TX side, the number of LOS components (\(N_{\text{{LOS}}}\)) is fixed. In our simulation, we place nine LEDs successively at TX to obtain the same coverage as mmWave antennas in azimuth (360\({}^{\circ}\)), which is required for a fair comparison. This brings more MPCs received by the detector, i.e., more NLOS rays (\(N_{\text{{NLOS}}}\)), resulting in the  
Figure 11: The CDF results of RMS DS.  
small K-factor. In addition, the \(\mu\) parameter in the normal fitting is 2.88 dB for mmWave channels, while it is \(-1.03\) dB for VLC channels. These results suggest that it is better to select mmWave technologies when large coverage is required, i.e., VLC technologies are expected for short-range communication.  
In Figure 12c,d, we plot both the RMS DS and K-factor with the distance. The distance dependence of these two parameters seems not apparent. Furthermore, we can see that these two parameters appear to present a similar trend, varying with the distance, i.e., the K-factor is proportional to the RMS DS in the conference room. Moreover, the correlation coefficients calculated by (13) are \(-0.22\) and \(0.77\) for mmWave and VLC channels, respectively. The coefficient \(-0.22\) shows a negative correlation between RMS DS and K-factor, which is demonstrated by the fact that the valley value corresponds to the peak value when the distance is 8 m in Figure 12c. It can also be verified that RMS DS and K-factor in the VLC bands have a higher correlation (\(0.77>0.22\)), as seen in Figure 12d.  
\[\rho_{X,Y}=\frac{cov(X,Y)}{\sqrt{var(X)var(Y)}}, \tag{13}\]  
where \(cov(\cdot)\) and \(var(\cdot)\) represent the covariance and variance, respectively.