3 ms·

Honestly I still dont quite get it. The FCC allocated 3.7 to 3.98Ghz for cellular wireless. Then there is 20Mhz of guard band to 4.0GHz. Then there is satelite

Honestly I still dont quite get it. The FCC allocated 3.7 to 3.98Ghz for cellular wireless. Then there is 20Mhz of guard band to 4.0GHz. Then there is satelite from 4.0 to 4.2GHz. And then, finally, the altimeter fequency band from 4.2 to 4.4GHz. So the two uses appear quite far apart in the frequency domain. Do signals get sent out on the wrong frequency or something? What does it mean to interfere? And, assuming something bad is happening, why?

bbulkow 5y ago
Yeah, and more importantly, was the equipment designed to exist with that much guard band? And isn't the cell equipment specified to throw off only a certain amount of harmonics, or whatever the out of band noise would be? This seems like without either a massive engineering mistake or a tempest in a teapot?
kloch 5y ago
It does seem like an overreaction to me but there is a big difference in cell towers and satellite uplinks/downlinks Radio towers (sites) have widely varying quality control of their tenant's installations. Other transmitters can mix and amplify signals from other carriers without well designed and maintained RF filtering on each and every antenna. The more transmitters at a site the more carrier frequencies that can contribute to the mixing, and the more opportunities for poor or broken RF filters. At some busy sites there is almost nothing you can do to prevent this. Anything on the tower that acts like a diode can cause mixing, even without being amplified by another transmitter. This usually just harms other receivers at the site however. But an airplane flying close by could theoretically be affected depending on how close it is.
- NovemberWhiskey 5y ago
  >It does seem like an overreaction to me but there is a big difference in cell towers and satellite uplinks/downlinks N.B. as far as I know, the portion of the C-band that's in question was allocated for satellite downlinks; uplinks were over 6GHz. With that in mind, there is indeed a Big Difference between a satellite in orbit radiating downwards, and a 5G beam forming installation on the ground potentially radiating upwards.
NovemberWhiskey 5y ago
Radar altimeters are inherently wideband devices, designed without any historical need for front-end rejection and the specifications for them reflect that - the RF "testing mask" that's used isn't a sharp rolloff at the edge of the RA allocation. Typical implementations are, for example, only 20 dB down for frequency-dependent rejection by 3.5 GHz. With potentially high gains from beamforming, both the fundamental mode and spurious emissions from 5G radios exceed the safe interference limits for the current RA specifications by at least 10 dB.

bbulkow 5y ago

Yeah, and more importantly, was the equipment designed to exist with that much guard band? And isn't the cell equipment specified to throw off only a certain amount of harmonics, or whatever the out of band noise would be? This seems like without either a massive engineering mistake or a tempest in a teapot?

kloch 5y ago

It does seem like an overreaction to me but there is a big difference in cell towers and satellite uplinks/downlinks Radio towers (sites) have widely varying quality control of their tenant's installations. Other transmitters can mix and amplify signals from other carriers without well designed and maintained RF filtering on each and every antenna. The more transmitters at a site the more carrier frequencies that can contribute to the mixing, and the more opportunities for poor or broken RF filters. At some busy sites there is almost nothing you can do to prevent this. Anything on the tower that acts like a diode can cause mixing, even without being amplified by another transmitter. This usually just harms other receivers at the site however. But an airplane flying close by could theoretically be affected depending on how close it is.

NovemberWhiskey 5y ago
>It does seem like an overreaction to me but there is a big difference in cell towers and satellite uplinks/downlinks N.B. as far as I know, the portion of the C-band that's in question was allocated for satellite downlinks; uplinks were over 6GHz. With that in mind, there is indeed a Big Difference between a satellite in orbit radiating downwards, and a 5G beam forming installation on the ground potentially radiating upwards.

NovemberWhiskey 5y ago

Radar altimeters are inherently wideband devices, designed without any historical need for front-end rejection and the specifications for them reflect that - the RF "testing mask" that's used isn't a sharp rolloff at the edge of the RA allocation. Typical implementations are, for example, only 20 dB down for frequency-dependent rejection by 3.5 GHz. With potentially high gains from beamforming, both the fundamental mode and spurious emissions from 5G radios exceed the safe interference limits for the current RA specifications by at least 10 dB.