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The weather/5G frequency use conflict reminds me of the FAA 5G filter fiasco 8 monthes ago: FAA Shows ‘Sample NOTAMs’ for Possible 5G Restrictions https://news

by guerby 4y ago

The weather/5G frequency use conflict reminds me of the FAA 5G filter fiasco 8 monthes ago: FAA Shows ‘Sample NOTAMs’ for Possible 5G Restrictions https://news.ycombinator.com/item?id=29694085 https://news.ycombinator.com/item?id=29694085 My comment at the time https://news.ycombinator.com/item?id=29696273 https://news.ycombinator.com/item?id=29696273 This is all ridiculous, there's still a 200 MHz band guard between the FAA band and the 5G proposed band. Here is what a $1 ESP wifi dongle has to follow: https://en.wikipedia.org/wiki/IEEE_802.11 "The mask requires the signal to be attenuated a minimum of 20 dB from its peak amplitude at ±11 MHz from the center frequency" So 2 dB/MHz filter. I let you do the math. FAA is just ridiculous here if they let old junk radio hardware handle safety landings for airplanes, but well after 737 max what do you expect... And obviously this was in line with reality, FAA finally admitted it didn't do its job of preventing crap filters to be kept in planes for decades: https://www.faa.gov/newsroom/faa-statements-5g https://www.faa.gov/newsroom/faa-statements-5g Airlines and other operators of aircraft equipped with the affected radio altimeters must install filters or other enhancements as soon as possible." Now I haven't looked in details yet on this new frequency use conflict and Sabine mentionned a scientific study that seemed legit. One thing is different: around 20 GHz there's lots of frequencies available (vs 5GHz) so we could have larger guard band without significant impact.

mlyle 4y ago
The technical analysis you're putting out here is a bit of a canard. Yes, better filters help, but radars are really, really picky. After all, there's an inverse fourth power relationship between return strength and distance. So we have our doppler weather radar transmitting at 450kW, traveling out a big distance to a storm (inverse square), and reflecting at very low efficiency, and traveling back (inverse square). Compare to a base station putting out 40W that's closer and just subject to inverse square law. It can easily be 10 orders of magnitude stronger. You need pretty good filtering for this. > One thing is different: around 20 GHz there's lots of frequencies available (vs 5GHz) so we could have larger guard band without significant impact. You need a much larger guard band. It's easy to make a 1MHz wide filter at 10MHz, and really hard at 100000MHz. Another important thing: parasitics start to matter a whole lot, too. You can have a filter that sharply rolls off around your fundamental, but then above the resonant frequencies of your passives/filter elements become transmissive again. It's pretty hard to keep 20GHz out of a receiver that was designed for a lower frequency before 20GHz was a major concern.
- beecafe 4y ago
  nitpick: it's not inverse fourth power, it's just inverse square with twice the distance, which is 1/((2 * distance) ^ 2) = 1/(4 * distance ^ 2) (ignoring reflection efficiency, but that isn't determined by distance)
  - mlyle 4y ago
    > nitpick: it's not inverse fourth power, it's just inverse square with twice the distance, which is 1/((2 * distance) ^ 2) = 1/(4 * distance ^ 2) No. For a diffuse reflection, the amount of light hitting your target is inverse square. And then it is scattered and inverse square back on the return journey. 1/x^2 * 1/x^2 = 1/x^4. https://en.wikipedia.org/wiki/Radar#Radar_range_equation https://en.wikipedia.org/wiki/Radar#Radar_range_equation "In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term Rt² Rr² can be replaced by R^4, where R is the range." (This all assumes that your target is smaller than the beam size, of course-- which is not as true for the two cases of a radar altimeter or a doppler radar as it is for e.g. tracking aircraft... but it's still close enough in practice).

by guerby 4y ago

mlyle 4y ago

The technical analysis you're putting out here is a bit of a canard. Yes, better filters help, but radars are really, really picky. After all, there's an inverse fourth power relationship between return strength and distance. So we have our doppler weather radar transmitting at 450kW, traveling out a big distance to a storm (inverse square), and reflecting at very low efficiency, and traveling back (inverse square). Compare to a base station putting out 40W that's closer and just subject to inverse square law. It can easily be 10 orders of magnitude stronger. You need pretty good filtering for this. > One thing is different: around 20 GHz there's lots of frequencies available (vs 5GHz) so we could have larger guard band without significant impact. You need a much larger guard band. It's easy to make a 1MHz wide filter at 10MHz, and really hard at 100000MHz. Another important thing: parasitics start to matter a whole lot, too. You can have a filter that sharply rolls off around your fundamental, but then above the resonant frequencies of your passives/filter elements become transmissive again. It's pretty hard to keep 20GHz out of a receiver that was designed for a lower frequency before 20GHz was a major concern.

beecafe 4y ago
nitpick: it's not inverse fourth power, it's just inverse square with twice the distance, which is 1/((2 * distance) ^ 2) = 1/(4 * distance ^ 2) (ignoring reflection efficiency, but that isn't determined by distance)
- mlyle 4y ago
  > nitpick: it's not inverse fourth power, it's just inverse square with twice the distance, which is 1/((2 * distance) ^ 2) = 1/(4 * distance ^ 2) No. For a diffuse reflection, the amount of light hitting your target is inverse square. And then it is scattered and inverse square back on the return journey. 1/x^2 * 1/x^2 = 1/x^4. https://en.wikipedia.org/wiki/Radar#Radar_range_equation https://en.wikipedia.org/wiki/Radar#Radar_range_equation "In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term Rt² Rr² can be replaced by R^4, where R is the range." (This all assumes that your target is smaller than the beam size, of course-- which is not as true for the two cases of a radar altimeter or a doppler radar as it is for e.g. tracking aircraft... but it's still close enough in practice).