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Comment by nootropicat | Hacker News Reader

What the... is that what's possible with 5G? Amazing, and even days ago I was thinking of satellite smartphone internet as something scifi because of (clearly incorrect!) complicated antenna requirements.

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Radio's got some crazy physics.

So baseline, a modern cell phone can connect to a tower about 45 miles away. Stsrlink satellites orbir about 10 times out from that. They are solving this problem by some very clever phased array technology to be deployed on the satellites themselves, because when it comes to radio, you can often solve the problem of signal reception by boosting either the transmitter or the receiver side of the story.

This is how we've kept in touch with the Voyagers all this time. The Voyager radio technology is getting no better, but the technology we can throw at it from a ground station has been out-stripping the loss due to distance and degradation (or at least approximately keeping pace).

> So baseline, a modern cell phone can connect to a tower about 45 miles away.

Isn't that primarily because of the Earth's curvature and obstructions/hills?

Good question. I've had a bear of a time sourcing good info on this and would welcome a solid analysis.

Radio horizon is somewhat complex, but 45 miles is not entirely unreasonable from an elevated position.

https://en.wikipedia.org/wiki/Line-of-sight_propagation#Radi...

There could also be timing related constraints that limit max range: radio waves propagate at the speed of light which is fast, but not instant.

microwaves go about 10% further than the horizon, barring any tropospheric ducting - which cell towers shouldn't, in general.

The issue you have with long distances is multipath, which can throw off the complex timing required to have multiple users in realtime on a single radio.

I've heard that US spy satellites used to eavesdrop on soviet telephone calls transmitted between line-of-sight ground-based microwave relay towers. The relay towers used directional microwave antenna aimed at each other, but satellites in space could pick up those signals.

I can't find a direct reference to it on wikipedia, but I suspect this is what the Vortex satellites were doing: https://en.wikipedia.org/wiki/Vortex_(satellite)

This [1] may help but I'm wholely unfamiliar with the field. A guy at Defcon did a talk about how HAM radio in Florida allocates their bandwidth and goes into the nitty details of propegation.

[1] https://youtu.be/fH-yyTZffAk

Also 10x may not be representative since the 45 miles to a cell tower is all through atmosphere but Starlink is 400 miles up, so there's only 5-10 miles of atmosphere at any noticeable density.

The atmosphere is transparent at typical cell phone frequencies (and the part that can be a problem are clouds and the ionosphere).

Line of sight (1/r^2) often has lower attenuation than urban propagation (1/r^3 to 1/r^4), so it’s often easier to hit a LEO satellite than a terrestrial tower.

4G/LTE was commonly quoted as 3km range for rural installations (using the 2GHz bands). 5G added 700MHz and 800MHz bands that are quoted as 10-20km achievable range. For typical cell tower heights 20km is about the range where they would sink behind the horizon, so that might be the practical limit here.

Starlink satellites are about 330km up, so one order of magnitude further. But they also have basically unobstructed line of sight (if you are under blue sky). Compared to the normal conditions a cell phone has to deal with that might get you a substantial improvement. I think it's plausible that at least some existing phones can do it, with the tiny bandwidth you would expect from a poor signal.

330km is only when the satellite is directly overhead. Starlink's coverage is typically given on the assumption that the satellites may be as low as 25 degrees above the horizon, and at that elevation, the straight-line distance is more like 700km if my math is right.

None are currently at 330km, and the first shells of v2 will be at 540-570km.

GPS is a thing, so I can imagine that a phone would be able to receive satellite signals pretty easily. But for transmission, I can't imagine this working without having your phone put a ton of power into sending a powerful enough signal, perhaps more so than how much power you need to passively receive GPS signals in the absence of traditional cell towers?

It sounds like the data rate is incredibly low. That allows for a lot of signal margin. GPS receivers are sensitive to something like -165dBm.

It's probably harder for the satellite to hear from the phone. Especially at a lower frequency like 700Mhz, a phone's tiny antenna is going to be a rather inefficient transmitter. Maybe 5g technology helps by allowing the phone to do some rudimentary beam forming using multiple antennas?

(How) Does beamforming work in a moving device?

I'm not an expert, but I believe a receiving radio can make note of the relative phase of the incoming signal across all antennas, and then simply transmit its signal with the same phase offsets. This can happen incredibly quickly on the time scale of milliseconds, and so the typical device really isn't moving that quickly to matter.

I believe modern wifi has periodic "sounding packets" specifically designed for devices to robustly measure that sort of information for beam forming and multiple spatial streams.

This is all roughly right except that when you say "incredibly quickly on the time scale of milliseconds" it's often actually microseconds.

This sort of work is done either in custom hardware or FPGAs - that's the only way you can precisely control the phase offset of signals (and measure it). They can respond very quickly.

There are cell phone sized thingies that can send text messages to iridium's constellation, which is (as I understand it) way further away than starlink. I use one while backpacking.

I'd be super excited to get the same capability in a standard cell phone.

The Iridium orbit heights are not much higher (781 km vs. 550 km). You can always trade bandwidth for increased sensitivity. The most extreme example is GPS. Your phone can receive it from 20000 km away with a tiny antenna, but the data rate is only 50 bits per second.

Within the limits of a 5G receiver, there are probably limits to how much sensitivity you can give up for bandwidth.

It works because the satellites will have a HUGE antenna, 5m by 5m. And to enable the launching of this many big satellites, SpaceX will launch (most of them) on Starship.

FWIW, 5G isn’t really to “thank” for increased range. LTE was widely deployed on the 700Mhz band in the early 2010s and line of sight range could easily exceed 10 miles for useable data.

Not to forget that satellite antennas are directional and can focus all their power in a narrower beam

I think the "complicated antenna requirements" still exist - they've just put the complicated antenna on the satellite, instead of on the phone.

Yeah that's not how it works. You can't just "put the complicated antenna on the satellite, instead of on the phone" and declare this problem solved. If you believe that I have a bridge to sell you.

I'm declaring shenanigans right now on Elon's pitch but what else is new. It makes me sad that this thread is full of (probably) SWEs in a complete froth who took this at face value with little understanding of how antennas work. No, your existing phone will not be able to work on this service, full stop.

The issue is not getting signal from the satellite to your phone. That's the easy part - after all, your phone receives GPS signals. Satellites have powerful transmitters and high-gain antennas. Even GPS which is considered extremely low-power has a TX power of around 20-25 watts and EIRP is probably an order of magnitude higher after the antenna (probably enough to turn your eyeballs into Easter eggs if you were standing in front of it). The issue is going the other way. For messaging to work, your phone needs a two-way connection. Your current iPhone/Android handset is simply not designed for satellite communication.

"But muh Garmin inReach" ..... have you stared at one and wondered "why can't my iPhone do this"? First, the antenna is the size of a thumb, it looks like a helical antenna to me, which would make sense given that most satellite communications uses something called "circular polarization" to combat things like the Faraday effect. This requires an antenna specifically designed for it. Your iPhone doesn't have one, which means more signal loss. Iridium phones look similar - very Michael Douglas in Wall Street industrial design. Satellite phones and communicators are designed to direct most of the energy upwards towards the sky. Your existing phone does not so you are losing most of the energy in an omnidirectional pattern. Even things like 406 MHz emergency beacons send extremely short message bursts - just a few bytes - the link budget needed to sustain an IP connection (uplink) with an existing handset just isn't there.

So I'm expecting Elon to deliver on the magical complicated-super-antenna that works with your existing mobile handset right around the same time he gives us his FSD Cybertruck. Which is to say, never.

Phased arrays are pretty cool, if I had no idea how they worked I'd be incredulous as well.

A phased array far away is effectively a point source.

Not when it's synthesized from numerous satellites.

But it is a point source with a non-uniform radiation pattern. Hence it can send a lot more power into a specific direction.

Can you show your calculations for this? From the sound of it the Starlink antenna is going to be a 25 m^2 phased array antenna, which I believe would have impressive gain.

If this worked why is a Starlink receiver a cat-warming dish and not a cellphone antenna?

Because (existing) Starlink is for home internet where people want good bandwidth? e.g. 100Mbps per household vs the stated plans for this which is 2-4Mbps per cell region.

And because existing Starlink satellites don't have giant phased array antennas on them.

I would assume v2 only receivers/transmitters could be much smaller.

>"circular polarization" to combat things like the Faraday effect

Another factor is that the antenna on a satellite can be in a pretty arbitrary rotation compared to the receiver on the ground. Circular polarization doesn't care about orientation. Further discussion here: https://ham.stackexchange.com/questions/12414/antenna-polari...

>Satellite phones and communicators are designed to direct most of the energy upwards towards the sky. Your existing phone does not

Recent iphones run 2x2 MIMO on mmWave and 4x4 MIMO on LTE. They should be able to do a fair bit of beamforming.

This isn't a Starlink/SpaceX scheme. Non-terrestrial networks are being worked into the 3GPP specifications and highlight the same handset difficulties that need to be addressed. It's not the phone currently in your pocket that does this but might be fine on the phone you get in the next few years.

https://www.3gpp.org/news-events/partners-news/2254-ntn_rel1...

The BlueWalker 3 test satellite from AST should launch in the next couple of months. Looks like a pretty complicated phased array antenna:

https://forum.nasaspaceflight.com/index.php?topic=50349.msg2...

> It's not the phone currently in your pocket that does this but might be fine on the phone you get in the next few years.

I don't think anyone's questioning this - yet there's people here rage-posting the ludicrous assertion that the very handset in your pocket right now will work with Starlink unmodified. This is what I have an issue with. More FSD-style overpromising and underdelivering all over again.

They did go a bit hard on the existing phone's bit, tmobile guy at least used the word "aspiration" when mentioning it. I somehow doubt it, but maybe phone radio implementations are very forward looking on the standards?

In any case, late '23 never means '23. So maybe actual useful service in '25-'26 when the network, phones and sats are all ready.

Satellite phone Internet previously existed, it's just crazy expensive.

It’s $10 per month for my Garmin InReach which gives you emergency comms and let’s you upgrade to unlimited SMS and some other stuff for $60. I use it a lot and it’s very reasonable. Runs on Iridium.

I have a Garmin InReach too, but to say it’s “satellite phone internet” is a stretch. You can send little blips of data. There’s no phone service. Nor is there any internet service.

If you read the T-Mobile announcement, that’s exactly what you’ll get. Not saying it’s not great but I think people have the misconception that they’ll have usable satellite internet which is really not happening due to physics.

Satellite smartphone internet is still sci fi.

This is extremely low bandwidth and intended for sending extremely high latency text chats. Elon said in the presentation that it could be as much as 30 mins between hitting send and the message going through.

This competes with existing stuff from existing satellite providers.

It’s really really cool, and doesn’t need to be full on smartphone satellite internet to fulfill that coolness!

His 30 minute example was when there are only a few satellites in space. Once many are active, it would allegedly become near-realtime.

It's being transmitted to existing phones, so it's going over 5G, LTE, or both.

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