90 GHz Channel Measurement Results for Mixed LOS and NLOS Aircraft Maintenance Hangar Environments

Commercial aviation is experiencing rapid growth. Although terrestrial millimeter wave (mmWave) communication systems are advancing steadily, mmWave communication system applications in aviation and airport settings are still in their infancy. Millimeter wave communication systems employ directional antennas to ensure adequate link range and must often "search" in the angular domain for a signal of significant strength in non-line-of-sight (NLOS) regions. Research on wireless channel characterization for the aviation environments is an active area of study for reliable communication link design. In this paper, we quantify some 90 GHz band channel characteristics for an unusual aviation setting: an aircraft maintenance hangar. This setting is a crowded environment with multiple aircraft, metallic objects, and other obstacles, whose positions move throughout the day. Hence our study focuses on mixed environments, i.e., both LOS and NLOS. Our results are empirical, based upon measurements using a 500-MHz bandwidth chirp signal. We first estimate propagation path loss, and model this using a single-frequency close-in (CI) free space reference distance model. As expected, pathloss exponents exceed that for free space, with a maximum of 3.14 for the mixed environments. We also quantify the root mean-square (RMS) delay spread, finding a maximum value of 16.23 ns for the NLOS case. We also determined that the RMS delay spread decreased by approximately 4 ns (25%) when our receiver aimed to the maximum received power azimuth angle, representing a strong obstacle reflection. These results will be of use to mmWave network designers in this unique aviation application.