Mini-Circuits has a longstanding legacy of close collaboration with both wireless infrastructure providers and aviation equipment manufacturers. As those two industries’ interests conflicted recently with the roll-out of 5G systems in the 3.7 to 3.8 GHz frequency range (A-block spectrum), and with the 3.8-3.98 GHz portion of the band still to be auctioned off, it is not surprising to find Mini-Circuits front and center in the effort to develop a viable solution.
The aviation industry’s concern that 5G signals in the 3.7 to 3.98 GHz (C-band) frequency range will interfere with aircraft radar altimeters which operate from 4.2 to 4.4 GHz has drawn a great deal of attention in recent months. Radar altimeters incorporate inherently wideband receiver front-ends, and none of the current governing specifications reference interference mitigation. A thorough study of this interference problem prepared by Special Committee 239 (SC-239) of the Radio Technical Commission for Aeronautics (RTCA) concluded that, for Usage Category 1 (commercial air transport aircraft), aircraft operating conditions exist that may be impacted by interference between the radar altimeter receiver and a C-Band 5G signal emitted from a base station collocated near the airport.1
Mini-Circuits has responded quickly to this issue with a new surface mount design in the CBP-series of ceramic resonator bandpass filters that is compact enough to install onboard the radar altimeter. Additionally, a new component in the ZVBP-series of cavity bandpass filters is connectorized, and designed to be installed in line with the radar altimeter. Both filters perform well beyond the required safety margin and eliminate any potential interference for Usage Category 1, enabling commercial air transport aircraft radar altimeters to operate seamlessly in the presence of adjacent C-band 5G signals.
Flight Safety
Radar Altimeters are required to be certified at Safety Criticality Rating Level A for all commercial transport aircraft which fall into Usage Category 1. Safety Criticality Level A is defined by the FAA DO-178B/DO-254 as: Where a software/hardware failure would cause and/or contribute to a catastrophic failure of the aircraft flight control systems.4,5 SC-239 concluded “for Usage Category 1, the impact is limited to specific scenarios, and only for certain combinations of aircraft altitude and lateral distance between the aircraft and base station.1 Given any potential impact, it’s not surprising that this topic made headlines leading up to the most recent launch of 5G in the A-block spectrum.
The Risk to Commercial Air Transport
Figure 1 shows an illustration of how closely adjacent fundamental emissions from 5G base stations are to the radar altimeter band.1 There are two means by which interference can occur: desensitization due to 5G fundamental emissions overcoming the filtering in radar altimeter receivers, and 5G spurious emissions falling directly within the 4.2 to 4.4 GHz RA operating band.1
Figure 1: Notational emissions levels and superimposed radar altimeter filter selectivity.1
For Usage Category 1 (commercial aircraft), SC-239 determined that the safe interference limit is not exceeded by any base station configuration for the 5G spurious emissions in the 4.2 to 4.4 GHz band. For 5G fundamental emissions, the safe interference limit is exceeded by nearly 14 dB in the worst-case. The amounts by which the safe interference limits are exceeded (worst-case) for both fundamental and spurious emissions for Usage Categories 1 through 3 are shown in Table 1.
Table 1: Worst-case radar altimeter interference levels above safe limit vs. 5G emissions type and usage category.
Two Solutions to Fundamental Emissions Interference
The 3.7-3.98 GHz frequency portion of C-Band had only been occupied by low-powered satellites for decades, providing a quiet environment for the operation of radar altimeters in the 4.2-4.4 GHz frequency band.7 Radar altimeter front-end bandpass filters are therefore only specified to roll off at 24 dB per octave below 4.2 GHz and above 4.4 GHz.2,3 RA receivers consequently have very limited ability to reject fundamental emissions close to the 4.2 to 4.4 GHz band.
Fundamental 5G C-band (3.7-3.98 GHz) emissions exceed the safe limit level for every Usage Category of aircraft. Both Mini-Circuits’ CBP- and ZVBP-series of high-Q bandpass filters are capable of suppressing fundamental emissions with sharp selectivity outside the 4.2 to 4.4 GHz band from 5G C-band base stations and of providing exceptional safety margin for Usage Category 1 aircraft.
Figure 2: Notional 5G emissions levels with radar altimeter characteristics and S21 measurement plot for CBP-series filter selectivity.
The gradual radar altimeter front-end roll-off of 24 dB per octave2,3 results in approximately 7 to 9 dB of attenuation in 5G C-band, and is shown by the blue dashed line in Figure 2. By contrast, the Mini-Circuits high-Q CBP-series ceramic resonator bandpass filter exhibits dramatically steeper roll-off, achieving a typical attenuation level of greater than 45 dB across the entirety of 5G C-band as shown by the solid blue line in Figure 2. Insertion loss is typically 2.6 dB in the center of the band (4300 MHz) and 3.5 dB at the band edges (4200 and 4400 MHz).
This performance is achieved in a surface mount package size of just 28.5 x 14.0 x 4.83mm that is capable of operating from -40°C to +85°C and withstanding an input power of 30 dBm continuous at 25°C. The Mini-Circuits CBP-series filter furnishes 31 dB greater attenuation than necessary (45 dB vs. 14 dB) to completely eliminate the potential interference for radar altimeters on board commercial aircraft (Usage Category 1). The typical 45 dB of attenuation for the CBP-series filter from 3.7 to 3.98 GHz is virtually adequate to mitigate the worst-case interference levels from fundamental 5G C-band emissions for Usage Category 2 and Usage Category 3 equipment shown in Table 1.
Figure 3: Notional 5G emissions levels with radar altimeter characteristics and S21measurement plot for ZVBP-series filter selectivity.
The ZVBP-series filter is specifically designed with enough suppression to eliminate the effect of 5G fundamental emissions on radar altimeter operation, and with such low passband loss that the effect of its insertion in line with the radar altimeter is minimal. Figure 3 shows the solid blue line representing the typical ZVBP-series cavity bandpass filter roll-off along with 5G and radar altimeter emissions regions. The figure is scaled to show 90 dB of attenuation due to the excellence of the ZVBP-series filter. The lower skirt attenuation performance of the ZVBP-series cavity bandpass filter is typically 60 dB in 5G C-band, and is specified so aggressively at 4 GHz (55 dB min.) that it proves capable of adequately suppressing fundamental 5G emissions for every usage category of aircraft.
Not only that, but with a typical passband loss of just 0.8 dB, there is only a subtle increase in insertion loss/Noise Figure due to inserting the ZVBP-series bandpass filter in line with a typical radar altimeter receive antenna. Since most radar altimeters have a NF of some 6 to 12 dB, the effect of an additional 0.8 dB in NF is quite modest. The ZVBP series filter measures 97.5 x 35.1 x 25.1 mm, operates over a temperature range of -55°C to +85°C, and withstands a continuous input power of 33 dBm at 25°C.
Referencing the 5G fundamental emissions levels in excess of safe limits shown in Table 1, the minimum attenuation afforded by the Mini-Circuits ZVBP-series filter provides 41 dB greater attenuation than necessary (55 dB vs. 14 dB) to completely eliminate potential 5G interference for radar altimeters on board commercial aircraft (Usage Category 1) and 8 and 10 dB greater than necessary attenuation for 5G fundamental emissions for Usage Categories 2 and 3, respectively.
The Spurious Emissions Challege
For Usage Category 2 (commercial airplanes used for regional air transport, business and general aviation airplanes) and Usage Category 3 (transport and general aviation helicopters), worst-case 5G spurious emissions that fall within the 4.2 to 4.4 GHz radar altimeter band greatly exceed the safe limit level, as shown in Table 1. Unfortunately, these 5G spurious emissions levels incident on the radar altimeter band cannot be resolved with a filter that passes the radar altimeter band itself. The interference mitigation solution for Usage Category 2 and 3 equipment for these 5G spurious emissions must result from improved filtering of emissions from 5G C-band base stations located in close proximity to airports. An example of the effectiveness of Mini-Circuits’ CBP and ZVBP filter solutions for suppressing 5G spurious emissions can be derived from examining the performance of the upper skirt in Figures 2 and 3.
The CBP-series filter attenuates by over 40 dB in transitioning from 4400 to 4600 MHz, while the ZVBP-series filter attenuates by well over 60 dB across the same range. These two different technology filters developed by Mini-Circuits can be expected to deliver the same attenuation performance if the passband is shifted to 5G C-band and the upper skirts of their designs were transitioned to the guard band between 5G C-band and the radar altimeter band (3980-4200 MHz). A look back at Table 1 reveals that just 27 dB and 12 dB of suppression of 5G spurious emissions are required to achieve safe limit levels for Usage Category 2 and 3. Mini-Circuits has the capability to quickly develop designs that utilize these filter technologies to meet the most demanding customer requirements.
5G Interference Mitigation in a Single Component
The high rejection characteristics of the CBP-series and ZVBP-series of bandpass filters enable the fundamental 5G C-band emissions to be attenuated sufficiently for Usage Category 1 (commercial aircraft), both onboard and in line with the radar altimeter. Once a CBP-series filter is installed internal to a radar altimeter, or a ZVBP filter is installed in line with a radar altimeter, these filters eliminate the potential for 5G signals to interfere with radar altimeter operation for commercial air transport (Usage Category 1). Variants of the two models shown above may be developed to accommodate requirements for specific customer implementations.
According to the most thorough work to date on the subject, published by the RCTA, spurious emissions exist for Usage Categories 2 and 3 that greatly exceed the safe limit levels and must still be reconciled. A potential solution to the spurious emissions issue is to translate to CBP- and ZVBP-series designs’ passbands to 5G C-band and their upper skirts into the guard band between 5G C-band and the radar altimeter band. This implementation would of course require close cooperation between Mini-Circuits and wireless operators. Mini-Circuits is up to the challenge of working quickly with the avionics, aerospace, and wireless communities to solve the interference problem between 5G C-band and radar altimeters, allowing continued roll-out of 5G networks while ensuring aviation safety.
References
- RTCA Paper No. 274-20/PMC-2073
- Document 5/55, Document 5/BL/6-E, 13 December 2013, Radiocommunication Study Group 5 DRAFT NEW RECOMMENDATION ITU-R M.[RAD.ALTIM]
- Recommendation ITU-R SM.337
- Broadband Communications Usage of the 4.200 – 4.220 and 4.380 – 4.400 GHz Altimeter Band by David Vacanti Aerospace Fellow Honeywell International
- FAA DO‐178B/DO‐254
- PCMag January 20,2022 What Is C-Band, and What Does It Mean for the Future of 5G? Sascha Segan What Is C-Band, and What Does It Mean for the Future of 5G? | PCMag
- Defense News December 21, 2020 The military is scrambling to understand the aviation crash risk from a new 5G sale Valerie Insinna and Aaron Mehta The military is scrambling to understand the aviation crash risk from a new 5G sale (defensenews.com)
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