## 6 Conclusions  
Based on measurements, we presented an analysis of indoor channels for the corridor scenario at 3.6 GHz and 14.6 GHz frequency bands with the directional antennas. From the comparative analysis for the corridor, we show that the path loss is larger at 14.6 GHz frequency. The difference of the path loss exponents in the two frequency bands is less than 0.18 in the corridor. The main reason is that the path loss exponent is relevant to the relative power in the distance domain. For the RMS delay spread and coherence bandwidth, they are relevant to the relative power in the time domain. The constant reflection coefficient makes them not very sensitive to the frequency in the corridor. The differences between the two frequency bands are less than 1 ns and 6 MHz for RMS delay spread and coherence bandwidth, respectively. Based on the comparative analysis, it is hypothesized that the RMS delay spread and coherence bandwidth are independent of frequency in the typical indoor scenarios where free space propagation and/or reflection are the main mechanisms. The measurement results in the highly reflective environment (RC) and common reflective environment (corridor) verify the hypothesis.  
This analysis helps readers to understand the essence of the propagation. Both the quantitative results and qualitative analysis are useful for the needed extensions/modifications of channel modeling from the low frequency band to the high frequency band.  
**Acknowledgments:** The authors would like to thank [PERSON] for valuable guidance with regard to the antenna influence on channel characteristics. The authors also thank [PERSON] and [PERSON] for their assistance during the revision stage. This work was supported in part by the National Natural Science Foundation of China under Grant 61501020 and 61501021, by the State Key Laboratory of Rail Traffic Control and Safety under Grant RCS2016 ZJ005 and RCS2016 ZJ004, by the National Key Technology R&D Program under  
Figure 11: (color online) Coherence bandwidth in the NIM RC calculated over a 1 GHz bandwidth as a function of center frequency.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Measured Parameters**} & \multicolumn{2}{c}{**Slope**} \\ \cline{2-3}  & **3 Absorbers** & **4 Absorbers** \\ \hline RMS delay spread, 1000 MHz (ns/MHz) & \(-0.20\times 10^{-3}\) & \(-0.21\times 10^{-3}\) \\ RMS delay spread, 200 MHz (ns/MHz) & \(-0.23\times 10^{-3}\) & \(-0.32\times 10^{-3}\) \\ Coherence bandwidth, \(\rho=0.5\) & \(5.24\times 10^{-6}\) & \(9.91\times 10^{-7}\) \\ Coherence bandwidth, \(\rho=0.7\) & \(1.98\times 10^{-6}\) & \(5.25\times 10^{-7}\) \\ Coherence bandwidth, \(\rho=0.9\) & \(1.92\times 10^{-21}\) & \(4.46\times 10^{-21}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Slopes for RMS (root-mean-square) delay spread and coherence bandwidth in the NIM (National Institute of Metrology) RC (reverberation chamber).  
Grant 2014 BAK02B05, by the Fundamental Research Funds for the Central Universities under Grants 2016 JBZ006 and 2016 JBM075, by the Fundamental Funds of National Institute of Metrology under Grant 27-AKY1507 and 27-ANL604, and by the China Postdoctoral Science Foundation under Grant 2016M591355.  
[PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON], [PERSON] and [PERSON] performed the experiments; [PERSON], [PERSON], [PERSON] and [PERSON] analyzed the data; all authors wrote the paper.  
The authors declare no conflict of interest.