## 2 Measurements  
### Measurement System  
The wideband frequency responses of the propagation channel were measured with a measurement system based on a vector network analyzer (VNA) (MS2038C, Anritsu, Morgan Hill, CA, USA). The scattering parameter \(S_{21}\) was measured over a 0.8 GHz bandwidth and centered at 3.6 GHz and 14.6 GHz, respectively. The VNA was set to transmit 1024 continuous wave tones uniformly distributed over each frequency band, which results in a frequency step of about 0.78 MHz. The frequency resolution further yields a maximum excess delay of 1280 ns, i.e., a maximum distance range of approximately 384 m. Intermediate-frequency (IF) averaging bandwidth was set to 300 Hz. To reduce noise, six sweeps were averaged.  
The antennas used in measurements are directional horn antennas. Radiation patterns of antennas were measured in an anechoic chamber in the National Institute of Metrology, Beijing, China and are given in Figure 1 and Table 1. Although the beamwidths of the two frequency bands are not exactly the same, given the fact that the corridor is a very narrow and closed space, the angles of departure and angles of arrival for all major multipath components are expected inside the main beamwidth at both frequencies, which means that the difference of the beamwidths will not effectively influence the channel properties at the two frequencies under study. The use of the directional antennas is common at the high frequency band, e.g., [11; 12]. The antennas were mounted on wooden tripods with a small section of metal behind the directional antennas, which does not influence the pattern.  
Calibration was performed in an anechoic chamber to compensate the frequency response of antennas and cables as suggested in [13]. These two measured bands were free from interference during the measurement campaign, as confirmed by measurements with a spectrum analyzer (PSA E4445A, Agilent, Santa Clara, CA, USA). A summary of the measurement parameters is listed in Table 1.  
It is worth noting that the channel characteristics are extremely sensitive to antenna placements [14]. To extract the influence of frequency on the channel characteristics, the measurements for the two frequency bands should be carried out at the exact same antenna placement and in the exact same scenario. To this end, the measurement process is designed as follows. Firstly, the VNA is calibrated by using a \"response\" calibration [12]. The calibration is carried out for the two frequency bands, respectively. In addition, the calibration data for the two frequency bands are stored in the internal memory of the VNA. For each measurement position, calibration data for 3.6 GHz band is recalled and the measurement for 3.6 GHz band is executed. Then, immediately, without the antennas moved, calibration data for 14.6 GHz band is recalled and the measurement for 14.6 GHz band is executed. In this way, the measurements for the two frequency bands are carried out at the exact same antenna placement, in the exact same scenario, and almost at the exact same time. This design effectively reduces the uncertainty in the comparative analysis of channel characteristics between the two frequency bands.