Comment by amluto
3 hours ago
I suspect that the lack of ability to form nulls in the beam is as big or even a bigger limitation than the reduction in gain when going from a big array to a phone.
The SNR in Shannon’s Law has a log in front of it, but spectrum reuse is more or less linear. If there are five visible satellites and I can null out four of them, then I can receive from and transmit to the fifth without substantial interference. (I’m not saying this is easy! Contemplate how many WiFi generations have had MIMO and how limited it still is.)
So I believe that it’s comparatively straightforward to demonstrate a shiny new direct-to-cell system with a single phone on a stage, but achieving usefully large aggregate bandwidth in a dense area will be more challenging.
FWIW the problem with Iridium, historically anyway, was that available bandwidth was very low, so they had to charge a silly amount for usage of that bandwidth, so very few people used it. Iridium used low-ish frequencies, with narrow bandwidth, and (I think) no MIMO whatsoever, not even polarization diversity.
Yes, for more than one satellite covering the same area on the ground with a spotbeam on the same frequency at the same time to make sense, you inherently need steering/beamforming.
That's why Iridium has the constellation planned out so that you never have more satellites in the sky than strictly necessary for full coverage on the equator (where satellite density is lowest), and outer spot beams get turned off progressively as the satellites approach the poles as they'd only create interference without increasing bandwidth due to the lack of terminal-side steering.
Now I wonder if they already changed that for the second generation sats, given that there are some steered terminals available that could probably make good use of the extra satellite density near the poles, which is also an area underserved by geostationary beams?