Millimeter-wave frequencies get a lot of media coverage these days for two reasons. First, they’re baked into 5G standards to offer immense amounts of spectrum that will eventually be needed for applications that require very wide channel bandwidths. Second, thanks to significant advances in silicon device fabrication, it’s finally possible to overcome the enormous propagation challenges at these frequencies. Multiple vendors have created small, packaged RF subsystems that combine an entire RF signal chain (minus baseband) and active phased-array antennas with 64 elements (or more). They are beginning to be deployed in some places.
However, C-band frequencies offer the greatest benefits for wireless carriers, as they are available now rather than when millimeter-wave infrastructure is likely to be widely deployed, allow wireless carriers to demonstrate that they have “true” 5G, and provide a good combination of range, penetration, and transmission speeds. In addition, as signals travel far further than at higher frequencies, less infrastructure is required to cover a specific area. All of which make C-band frequencies a must-have for wireless carriers.
C-band for Cellular
C-band starts at 4 GHz and ends at 8 GHz but for cellular communications purposes it is between 3.7 and 4.2 GHz. If these frequencies sound familiar it’s because they have been used since the 1970s by satellite providers to downlink signals from space to receive terminals on the ground. In fact, C-band was where satellite TV originated, and to receive the signals you needed a 10 ft. parabolic dish antenna, some of which can still be found rotting away in American backyards.
Over the years, the satellite TV industry moved upwards in frequency to Ku-band where antennas are much smaller. And later, satellite TV itself began its descent into history as broadband delivered by cable and fiber offers better picture quality, higher broadband data rates, and no need for an antenna.
With the emergence of 5G, more spectrum was needed, ideally in the “mid-band” frequencies between 3.7 and 3.98 GHz where propagation conditions are well suited to wireless communications and where antenna size is reasonable (Figure 1). This so-called propagation “sweet spot” was already densely packed with allocations for cellular and other services, with only small snippets of spectrum available to auction.
As a band-aid until better solutions could be found, carriers have relied on aggregating spectrum at different frequencies that collectively form channels wide enough to support high-bandwidth services. Another approach was to reallocate TV channels—the so-called TV white spaces—to produce additional spectrum.
As satellite TV had moved to Ku-band, the FCC considered the C-band spectrum used by satellite providers to be “underserved,” and thus fodder for wireless applications, as long as the satellite providers could be encouraged to compromise. There was only one problem: under-utilized does not mean unused, and satellite providers legally owned licenses for the 3.7 to 4.2 GHz spectrum and still use them because C-band satellites are still in orbit and are valuable assets. Prying spectrum away from providers would take considerable finesse.
But as a testament to how important these frequencies were to wireless carriers, they ultimately agreed not only to pay satellite providers to use only the lower part of their C-band allocations (leaving the rest for them), offer financial incentives for this to happen at an accelerated rate, and even to pay to launch new satellites to replace them, as well as covering some of the cost of replacing or modifying earth stations with new RF hardware.
Although this was extremely expensive, the satellite providers were ultimately able to make use of less spectrum, primarily because digital signals require less bandwidth. So, AT&T and Verizon have been able to start using the 3.7 to 3.8 GHz region of the C-band spectrum in cities throughout the U.S., although there are some exceptions such as Washington, D.C. and Baltimore, Atlanta, and Denver because the transition is not fully completed. Once this so-called “clearing” process is complete, satellite providers will be restricted to 4 to 4.2 GHz, which appears to be adequate not just for digital TV broadcasting but data services as well.
All seemed to be going as planned for the FCC to release the C-band frequencies in January 2022, but once again there was a problem, one that continues to be widely debated and is likely to be recorded as one of the most politically fraught, mishandled situations in FCC history.
The Great C-band Fiasco
In December 2021 just before “launch time,” Boeing and Airbus called on the U.S. government to delay the rollout of C-band because of potential interference issues with sensitive aircraft instruments, primarily radio altimeters operating at 4.2 to 4.4 GHz. Radio altimeters track an aircraft’s altitude during takeoff and landing. They function by reflecting a signal from the ground and sending the results immediately to the cockpit (Figure 2). While this is most important when visibility is low, some feed the data to automated navigation and crash-avoidance systems that control engines and braking systems.
Boeing and Airbus were joined by almost the entire aviation industry that collectively determined that wireless carriers with base stations operating at C-band frequencies near airports could present significant safety issues because of out-of-band emissions. Specifically, a report based on a study conducted by the Radio Tactical Commission for Aeronautics (RTCA) stated they posed:
“a major risk that 5G telecommunication systems in the 3.7 to 3.98 GHz band will cause harmful interference to radar altimeters on all types of civil aircraft,” and “clearly indicates that this risk is widespread and has the potential for broad impacts to aviation operations in the United States, including the possibility of catastrophic failures leading to multiple fatalities in the absence of appropriate mitigations. Radio altimeter anomalies that are undetected by the aircraft automation or pilot, particularly close to the ground… could lead to loss of continued safe flight and landing.”
After continuous back-and-forth deliberations and heated exchanges between the airline industry, the FCC, Congress, the CTIA, and wireless carriers, Verizon and AT&T announced that until July 5, 2022, they would not turn on their C-band equipment at 600 transmission towers near the runways at 87 airports and would reduce radiated power at others in response to those concerns.
Once the delay was announced, the FAA defined the specific airports that would need to have buffer zones between wireless towers and airfields, but even then the agency warned that there might still be some problems in some cases. And last February, the FAA released an airworthiness directive warning airlines operating Boeing 737s, the world’s most widely used commercial aircraft, that some of their radio altimeters were problematic.
As this is written, the FAA has decided that “sensitive” equipment in these and other aircraft should be replaced, which would cost the industry enormous amounts of money and, according to the airline industry, would require at least two years. For the moment, the truce between the FCC and wireless carriers on one side and the FAA and the aviation industry on the other seems to be stable. More than 1000 aircraft from the four major carriers are currently banned from landing in low visibility conditions at many airports.
However, more challenges are on the way because T-Mobile and other wireless carriers have been approved to roll out their own C-band deployments by the end of next year that are even closer in frequency to those used by radar altimeters.
It Comes Down to Filters
Band-pass filters are key components in virtually every RF system, so from a technical perspective the solution would have been to specify filters for altimeter receivers with greater out-of-band rejection in the first place. Cost could be a consideration but these components contribute a small amount to the bill of materials for an altimeter. There is also the position that radar altimeters, especially older ones, are not particularly selective because they didn’t need to be: there were no signals near their operating frequencies, so inexpensive filters were more than adequate for their intended function at the time of manufacture. With the introduction of C-band this is no longer the case.
Spurious emissions such as harmonics from 5G transmissions can leak into an altimeter’s receiver, potentially preventing it from receiving signals reflected from the ground. The lack of a sharp cutoff in the band-pass filters in an altimeter’s receiver is the main point of concern. The fundamental frequency could possibly create blocking interference in the altimeter when a strong signal outside of its receive band is not sufficiently filtered to prevent front-end overload or other effects.
Spurious emissions, in contrast, can fall within the normal receive bandwidth of the altimeter and can desensitize the receiver by reducing the signal-to-interference-plus-noise ratio (SINR), creating false altitudes if it determines the signal is from a radar return or some other source.
The altimeter fiasco has raised serious questions about how the process was handled and in particular, why it only appeared at the 11th hour, years after it became known that the C-band spectrum could be used for wireless services. Another issue is that there is a 200 MHz guard band between where the wireless signals are located and the aeronautical band used by altimeters (Figure 3).
Compared to the tiny slivers of spectrum between wireless bands at various frequencies, 200 MHz is at least ten times wider, which is no doubt why countries throughout the world have already begun rolling out networks at C-band frequencies and no problems have thus far been reported.
Further, the positions taken by FCC commissioners favored the wireless industry while the FAA acted on behalf of the airline industry, although it can argued that this industry has more at stake than the wireless industry.
The investigative journalistic organization Pro Publica1 delved into this issue and found that failures by the FCC as well as the FAA significantly contributed to the problem. Specifically, the FCC has consistently advocated for the interests of the telecommunications industry while the FAA did the same for the aviation industry, even though both agencies are supposed to serve only the public.
The C-band disaster doesn’t reflect well on any of the participants, from the White House to Congress and private industry. One way or another the altimeter issues will be solved, and consumers will rejoice in their ability to achieve faster downloads than before. Wireless carriers, especially Verizon, that until recently staked its 5G reputation on millimeter wavelengths, can promote its “5G Ultra Wideband” offering. Hopefully, when T-Mobile and other carriers introduce their C-band offerings, this problem won’t arise, or at least not with the same level of torment.
1. ProPublica, “Inside the Government Fiasco That Nearly Closed the U.S. Air System,” May 5, 2022.