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2023 CO-LABS Governor’s Award

Radiofrequency Measurements to Address Electromagnetic Compatibility between Radar Altimeters and 5G Base Stations

Six smiling ITS male researchers in suits and ties posing on outdoor terrace with treetops and dramatic cloud formations in the distance behind them..

(Left to right) ITS researchers Kenneth J. Brewster, Frank H. Sanders, Geoffrey A. Sanders, Kenneth R. Tilley, Kenneth R. Calahan, and Savio Tran. Their motto: “Probare illud donec deficiat,” or “Test it until it fails.” 

Six ITS researchers accepted the CO-LABS Governor’s Award for High-Impact Research in an October 11 ceremony at the Denver Museum of Nature & Science. The research team was honored for planning, designing, and implementing first-of-their-kind quick-reaction over-the-air 5G transmitter measurements and analyses for testing compatibility between 5G signals and existing radar communication used in aircraft. ITS received this honor in the Technology Transfer category, which recognizes technological solutions with “widespread and/or significantly measurable societal utilization,” as CO-LABS puts it.  

In early 2022 C Band–auction–winning commercial cellular network operators AT&T and Verizon were preparing to commission 5G base stations throughout the U.S. using licenses in the 3.7–3.98 GHz range. These two carriers had between the two of them paid about $69B of the $80B in total proceeds generated by the Federal Communications Commission’s (FCC) C-Band auction, and they were anxious to begin realizing the benefit of their investment. Full 5G commercial deployment in the C Band was delayed to address grave concerns the Federal Aviation Administration (FAA) and Department of Defense (DOD) raised about whether 5G transmissions near airports might interfere with radio altimeters (radalts) that operate in the 4.2–4.4 GHz range, potentially leading to aircraft crashes. 

To speed resolution of the disagreement between the FAA and cellular operators on how to reach interference-free coexistence, Hill Air Force Base (AFB) organized a Joint Interagency Fifth Generation Radar Altimeter Interference (JI-FRAI) multi-stakeholder group to agree on a test design that could provide trusted ground truth data. The group included more than representatives from the NTIA, the Federal Communications Commission (FCC), the FAA, and eight other federal agencies; 5G base station equipment manufacturers; three cellular carriers; three airlines; and four aircraft and radalt manufacturers. Hill AFB put ITS in charge of the testing. 

The JI-FRAI program test design comprised four major phases of testing: (1) bench testing with hardline-injected 5G interference signals; (2) in-flight radalt performance testing in the presence of 5G radiated signals; (3) EIRP measurements of on-the-ground radalt emissions from taxiing aircraft at civilian and military airfields; and (4) controlled, calibrated measurements of emission spectra and three-dimensional radiated field strength patterns around 5G base station transmitters deployed in the 3700–3980 MHz range in the U.S. ITS led phases 2, 3, and 4, each of which involved over-the-air testing in realistic conditions.

Between January and June 2022, ITS researchers performed detailed, precision measurements of aerial radiation patterns of low C band (3300–3600 MHz) and high C band (3700–3980 MHz) multiple input multiple output (MIMO) transmitter arrays incorporated in the four radio models produced by the three known manufacturers of U.S. n77 band (3300–3980 MHz) 5G transmitter equipment being deployed in the United States. All the measurements were performed via radiated 5G base station emissions at the Table Mountain Radio Quiet Zone. ITS researchers rapidly tailored an innovative computer-controlled measurement system to measure 5G ground and skyward emissions for all types of 5G base stations being deployed in the U.S. ITS engineers operated the measurement system in the back seats of specially equipped Army helicopters as they flew above, below, around, and over the tops of every manufacturer’s 5G tower.

Results showing insignificant unwanted emissions and skyward transmissions up to 1000× less than those on the ground gave confidence that 5G can coexist with radalts equipped with suitable receive filters, and thus that 5G base stations as currently built would not cause interference in the radalts’ band. 

The Institute shared the results internationally with other spectrum regulators. This full transparency inspired trust in the results and led to consensus on a technical solution to coexistence that was accepted by all parties.

A summary of the research findings was published in NTIA Technical Report TR-22-562, “Measurements of 5G New Radio Spectral and Spatial Power Emissions for Radar Altimeter Interference Analysis,” in October of 2022, as was the data underlying the report. Detailed engineering analysis of both systems provided objective and quantifiable characteristics of out-of-band and in-band signals and yielded interference protection criteria of radalts which were submitted to the FCC on March 31, 2023, leading to full 5G commercial service deployment on July 1, 2023. The Technical Report will continue to inform regulators seeking a better understanding of 5G coexistence with different types of radars. 

A helicopter in foreground with a 5G tower in background, both against a plain light grey sky.

Photograph: A radalt risk reduction measurement in conjunction with the 5G manufacturer’s gNodeB transmitter characterization. Photograph credit: Eric Nelson.

About CO-LABS and the Governor’s Awards for High-Impact Research

CO-LABS, founded in 2007, is a nonprofit consortium of federal laboratories, research institutions, businesses and economic development organizations that provide financial and in-kind support for programs that promote the retention and expansion of Colorado scientific resources. Started in 2009, the annual CO-LABS Governor’s Awards for High-Impact Research celebrate ground-breaking discoveries and innovative research from Colorado’s ecosystem of federally-funded laboratories and institutions. A Selection Committee of professional researchers, technologists, academics and economic development experts identify remarkable research having “high impact” on society.