Testing looks at “high C-Band” transmissions in the upper portion of the band
Concerns over potential interference between legacy radar altimeters in aircraft and new terrestrial 5G deployments in the C-Band spectrum sparked several federal testing and evaluation efforts. Amid the open question of whether operators will be asked to continue— or possibly, be mandated to continue—restricted C-Band operations around airports (as the Federal Aviation Administration and an airline industry group have asked the Federal Communications Commission), a new report from NTIA details some of the recent testing and results.
In some ways, this report looks both at and beyond current deployments, because it looks at operations across all of 3.7-3.98 GHz even though the entire swath of auctioned C-band will not be available to operators until December 2023.
In response to the worries over the potential clash between 5G operating at 3.7-3.98 GHz and altimeter receivers operating at 4.2-4.2 GHz interference concerns, the Joint Interagency Fifth Generation (5G) Radar Altimeter Interference (JI-FRAI) Quick Reaction Test (QRT) was formed to gain a better understanding of potential 5G interference issues, develop methods for testing 5G interference, and to provide answers to questions from government and industry communities. This is the first of several reports.
Essentially, the report said, the testing seeks to answer two engineering-related questions: When or if 5G interferes with radalt receivers, what is the physical mechanism of that interference?; and two, at what distances and in what directions between radalts and 5G transmitters does that interference occur?
Key takeaways from the report include:
-The testing focused primarily on radar altimeters (radalts) used by the military, but some of the models are also used in civilian aircraft.
-There were four major phases of testing: Bench testing in a lab setting of the radalts, with 5G signals added and power increased until harmful interference was caused; in-flight testing under controlled conditions at a military base in Utah and a civilian airport in Texas; collection of radio frequency power level measurements at two airfields (one military and one civilian); and the characterization of 3D radiation patterns of the 5G C-Band base station transmitters by the three manufacturers whose equipment is being deployed in the U.S. The new report focuses primarily on the results of the last aspects of the testing, with additional reports on the others to come.
-Radiated testing of 5G gNodeBs in three dimensions was conducted between January and June of this year, with ITS engineers working with AT&T and Verizon, with support from the U.S. Army, at the U.S. Department of Commerce’s Table Mountain Radio Quiet Zone (TMRQZ) north of Boulder, Colorado. The report notes that the measurements were performed using helicopter-borne measurement and data-recording systems and calibrated antenna arrays (made by ITS) on the bellies and sides of the aircraft. Other measurements were taken from the ground.
-Emission spectrum measurements of the 5G transmitters under those circumstances showed “radiated 5G base station emission spectra across a wide frequency range (3500–4400 MHz) and with wide dynamic range (i.e., with 80 to 95 decibels of total spectrum dynamic range, depending on the 5G radio model),” the report concluded.
-In general, the testing found that the manufacturers’ engineering spec sheets and simulation data for the 5G MIMO arrays’ EIRP patterns was similar to what played out in the actual testing—so future studies on radiation patterns can use that data rather than additional airborne measurements.
-In addition to details on exactly how the testing was conducted and data collected, the report presented two “substantial stand-alone results.” The testing found that the emission spectra of the three C-Band radio models show that “all three of these 5G base station transmitters incorporate effective bandpass filtering in their output stages,” which means that the base stations filter power such that they transmit very little above 4 GHz as opposed to where they are supposed to be operating, at 3.7-3.98 GHz. “Above 4 GHz (i.e., within the radalt band of 4200–4400 MHz) these radios’ spectrum emissions are as much as 106 decibels lower than their on-tuned intentional-radiation power in the 5G frequency band 3700–3980 MHz,” the report says, adding, “This low level of unwanted 5G emissions within the radalt spectrum band reduces the potential for a 5G-to-radalt harmful interference scenario which would be due to 5G unwanted emissions on radalt receiver frequencies. … This measurement-based observation increases the likelihood that, to the extent that any EMC problem exists between 5G transmitters and adjacent-band radalt receivers, the technical solution to such a problem might be the installation or retrofitting of more-effective RF power-rejection filters on radalt receivers for frequencies below 4200 MHz.”
-The testing also concluded that the radiated patterns direct much more energy at the ground—where cellular users are typically located—than what airborne measurement equipment picked up. “Airborne radiation patterns show measurably, significantly less power than is found in 5G base station main antenna beams directed toward [user equipment] at ground level,” the reported said, though it added, “The amount of power reduction in the sky is variable and needs to be examined by researchers in detail, using the collected data that we have made available.” The report also noted the presence of essential “nulls”—not quite zero power, but close—directly above the MIMO arrays, and a “distinct near-far effect in our airborne measurements on pairs of transportable 5G base stations” or C-Band Cells on Wheels (CoWs). The nearer transmitter is dominant in the receiver and the emissions from further-off transmitters “rapidly [fade] to insignificance,” the report says, which has implications for aggregated interference effects.
-The report is available here and is one of three that will be produced. The other three reports will focus on the bench testing, flight testing and measurements from aircraft sitting on the ground or taxiing. “The combined materials in all of these reports will provide a thorough description of the extent, if any, to which EMC problems exist between high 3700–3980 MHz 5G emissions and radalt receivers, and will point the way toward practical and effective technical solutions of any such potential problems.”
