74 GHz Millimeter-Wave Long-Range Channel Measurements for Backhaul Communications: Sounding System and Propagation Characterization

The E-band millimeter-wave (mm-wave), i.e., at the 71-76 and 81-86 GHz frequency bands, has a potential of providing tens of Gigabits-per-second (Gbps) data rates in long-range transmissions. It is thus considered a candidate technology for building wireless backhauls, particularly to provide high-speed and low-cost connectivity in remote areas. In order to study E-band long-range propagation properties that are crucial for backhaul communications, we have developed a channel sounding system that operates at 74 GHz, supports a signal bandwidth of 1 GHz and covers a range of more than 25 km. Using the system, we performed channel measurements in across-lake, desert and sea environments, all having line-of-sight (LOS) propagation conditions and around 12-km link distances. Based on the multi-environment measurements, we characterize E-band pathloss, fading, polarization and multipath effects, which are of primary importance for system design. The results show that E-band pathloss, mainly affected by atmospheric humidity, is considerably different in the diverse environments. Vertically-polarized propagation has significantly lower attenuation than horizontally-polarized. The commonly-used ITU-R models fail to predict E-band long-range pathloss for lack of polarization modeling. Severe fading was observed in a very short time and should be considered in link budget. The cross-polarization ratios (XPR) are relatively high, indicating a possibility of using polarized multiple-input multiple-output (MIMO) to boost data rates. No significant multipath was observed, however, severe fading implies that strong and unresolvable scattering or refraction exists around the LOS path. The research reveals the feasibility of E-band backhaul communications in those remote environments, and the results contribute to future system design as well as channel modeling.