Can anyone explain this from an RF/networking perspective? What frequencies does this operate at? How well matched are the antennas in most phones? How much more power are you using to be able to do this? What kind of modulation are you using for TX/RX?
This is doable only by Starlink because they're so low. They'll have very high gain antennas on the satellite - all satellite telephony needs that, these ones are presumably extra good.
I'm more interested in Doppler and timing advance. There's only limited Doppler correction in LTE - the satellites must be compensating for it. Although actually, I suppose they're mostly moving perpendicular to the direction of signal.
I'm particularly curious about TX - it's really impressive that a satellite would be able to pick out a signal coming from the tiny thing I'm holding in my hand.
It works for existing satellite messengers, and some of these are smaller and lighter than typical smartphones – and some even talk to geostationary satellites. These are almost 36000 km further away than your typical cell tower!
It’s all a question of data rates, transmit power, encoding, and (mostly in the geostationary case) large antennas on the satellite end.
> Direct to Cell works with existing LTE phones wherever you can see the sky. No changes to hardware, firmware, or special apps are required, providing seamless access to text, voice, and data.
It's also notable that this service isn't available yet, it starts (optimistically) sometime in 2024, probably very late 2024.
Everything regarding Starlink's plans would be speculation at this point, but we can look at existing comparable LEO satellite-to-mobile services, i.e. Globalstar and Iridium.
There, the answers are:
> What frequencies does this operate at?
1-2 GHz. These are also common terrestrial mobile communication frequencies.
> How well matched are the antennas in most phones?
That I don't know for Iridium (I think they're at least somewhat specialized), but for Globalstar, Apple is able to pull it off with their newer iPhones.
> How much more power are you using to be able to do this?
Iridium uses around 1-2 W – that's significantly more than LTE (I believe most devices only use about 125 mW, but I wasn't able to find precise numbers), but about the same as GSM/2G, which current phones largely still support (although I'm not sure whether they use that much transmit power practically still).
