# Feasibility Test and Analysis for Heavy Haul Freight Using WDPS Based on MIMO-OFDM
## 3 Measurement and Test
### WDPS Performance Measurement and Analysis  
Before applying WiFi-based WDPS, the propagation environment for interference was measured in the test section. Since WiFi operates in the 2.4/5 GHz ISM band, it must be to check the effect  
Figure 4: Heavy haul freight configuration. of the same band device used in houses and factories around the track. As a result of propagation environment, 2.4/5 GHz noise was less than \(-90\) dBm, and a 2.4 GHz band AP with low signal strength was detected intermittently in the downtown area. It is determined that the detected AP will not significantly affect WDPS performance. Detailed WiFi specifications are given in Appendix D.  
Measured WDPS SNR Analysis  
The heavy haul freight was consisted as shown in Figure 4, and operated 21.365 km from Jillye station to Busansinhang station. There are five tunnels and 20 bridges in the test section, of which there are four tunnels over 1 km and one bridge over 1 km. The SNR results measured at the master locomotive are shown in the Figure 5. From Figure 5, we can see that WiFi-based WDPS is applicable to distributed control of heavy haul freight. The fluctuation of signal strength exists depending on the track environment within the test section, but no functional error occurred because the 2.4/5 GHz band signals did not drop below 10 dB simultaneously, except for one short point. The section where the 1 and 2 occurred is an uphill gradient open area with three short bridges continuously, and the curve radius is R = 1400 and cant = 150. SNR fluctuation due to NLOS occurred for a short time in the corresponding section. This section 3 is a tunnel with a length of 2.5 km. It is a curved section while changing from an upper gradient to a down gradient at the entrance of the tunnel. At that time, the heavy haul freight train had a serious meandering behavior and the NLOS was happened, so the communication state was unstable. But no signal breakdown occurred because the 2.4/5 GHz band signals did not drop below 10 dB simultaneously, excluding an approximately 480-m section. At that point, the 2.4/5 GHz band signal dropped below 10 dB at the same time, but with very short moments, no functional error occurred in the WDPS operation. We can find an important fact that there was no frequency diversity effect in specific sections. Overall, the 5 GHz signal was more stable than 2.4 GHz in the test section, but it was confirmed that the attenuation of the signal was greater than 2.4 GHz in the specific section where tunnels, curves, and gradients existed simultaneously. These results are evidence that the higher the frequency, the more vulnerable to performance in NLOS condition under the influence of the characteristics more sensitive to fading. Therefore, under the premise that frequency diversity is applied, the following methods can be considered for stable operation of WDPS. The first method should be arranged so that the wireless device satisfies the LOS condition when applying the high-frequency band. The second method when choosing a frequency, we can allocate two or more other channels in the 2.4 GHz band or consider the latest communication methods in the band below the 2.4 GHz band (e.g., LTE in 850, 950, 900 MHz, 1.8 GHz, etc.). The section where 4 occurred is the open area, low slope and radius of curvature R = 1000, and cant = 30. There are five short bridges in a row. NLOS caused by the influence of this terrain is considered to be the cause of signal attenuation. 5 is open area, the upper gradient and R = 2000 with cant = 40. This is a section that passes over a bridge over 1 km in length and leads to a tunnel. This was a section in which LOS was not guaranteed. 6 is a section of three consecutive tunnels with 5.5 km length. Unlike the same tunnel section 3, 6 was a straight-line section, so the LOS was guaranteed. Therefore, the SNR was constant.