[Starlink] Starlink "beam spread"

Wed Aug 31 10:51:08 EDT 2022

"DNS on Starlink satellites: Good idea, lightweight, and I'd suspect
maybe already in operation?"

Are the satellites processing IP packets? Are the ISLs even in
operation? I have been told Starlink satellites are transparent.

> Date: Thu, 1 Sep 2022 01:41:07 +1200
> From: Ulrich Speidel <u.speidel at auckland.ac.nz>
> To: David Lang <david at lang.hm>
> Cc: Sebastian Moeller <moeller0 at gmx.de>, Ulrich Speidel via Starlink
> 	<starlink at lists.bufferbloat.net>
> Subject: Re: [Starlink] Starlink "beam spread"
> Message-ID: <56e56b0f-07bd-fe0c-9434-2663ae9d4404 at auckland.ac.nz>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> Um, yes, but I think we're mixing a few things up here (trying to bundle
> responses here, so that's not just to you, David).
>
> In lieu of a reliable Starlink link budget, I'm going by this one:
>
> https://www.linkedin.com/pulse/quick-analysis-starlink-link-budget-potential-emf-david-witkowski/
>
> Parameters here are a little outdated but the critical one is the EIRP
> at the transmitter of up to ~97 dBm. Say we're looking at a 30 GHz Ka
> band signal over a 600 km path, which is more reflective of the current
> constellation. Then Friis propagation gives us a path loss of about 178
> dB, and if we pretend for a moment that Dishy is actually a 60 cm
> diameter parabolic dish, we're looking at around 45 dBi receive antenna
> gain. Probably a little less as Dishy isn't actually a dish.
>
> Then that gives us 97 dBm - 178 dB + 45 dB = -36 dBm at the ground
> receiver. Now I'm assuming here that this is for ALL user downlink beams
> from the satellite combined. What we don't really know is how many
> parallel signals a satellite multiplexes into these, but assuming at the
> moment a receive frontend bandwidth of about 100 MHz, noise power at the
> receiver should be around 38 pW or -74 dBm. That leaves Starlink around
> 38 dB of SNR to play with. Shannon lets us send up to just over 1.25
> Gb/s in that kind of channel, but then again that's just the Shannon
> limit, and in practice, we'll be looking a a wee bit less.
>
> That SNR also gives us an indication as to the signal separation Dishy
> needs to achieve from the beams from another satellite in order for that
> other satellite to re-use the same frequency. Note that this is
> significantly more than just the 3 dB that the 3 dB width of a beam
> gives us. The 3 dB width is what is commonly quoted as "beam width", and
> that's where you get those nice narrow angles. But that's just the width
> at which the beam drops to half its EIRP, not the width at which it can
> no longer interfere. For that, you need the 38 dB width - or thereabouts
> - if you can get it, and this will be significantly more than the 1.2
> degrees or so of 3dB beam width.
>
> But even if you worked with 1.2 degrees at a distance of 600 km and you
> assumed that sort of beam width at the satellite, it still gives you an
>  >12 km radius on the ground within which you cannot reuse the downlink
> frequency from the same satellite. That's orders of magnitude more than
> the re-use spatial separation you can achieve in ground-based cellular
> networks. Note that the 0.1 deg beam "precision" is irrelevant here -
> that just tells me the increments in which they can point the beam, but
> not how wide it is and how intensity falls off with angle, or how bad
> the side lobes are.
>
> Whether you can re-use the same frequency from another satellite to the
> same ground area is a good question. We really don't know the beam
> patterns that we get from the birds and from the Dishys, and without
> these it's difficult to say how much angular separation a ground station
> needs between two satellites using the same frequency in order to
> receive one but not be interfered with by the other. Basically, there
> are just too many variables in this for me to be overly optimistic that
> re-use by two different sources within a Starlink cell is possible. And
> I haven't even looked at the numbers for Ku band here.
>
> CDNs & Co - are NOT just dumb economic optimisations to lower bit miles.
> They actually improve performance, and significantly so. A lower RTT
> between you and a server that you grab data from via TCP allows a much
> faster opening of the congestion window. With initial TCP cwnd's being
> typically 10 packets or around 15 kB of data, having a server within 10
> ms of your client means that you've transferred 15 kB after 5 ms, 45 kB
> after 10 ms, 105 kB after 15 ms, 225 kB after 20 ms, and 465 kB after 25
> ms. Make your RTT 100 ms, and it takes half a second to get to your 465
> kB. Having a CDN server in close topological proximity also generally
> reduces the number of queues between you and the server at which packets
> can die an untimely early death, and generally, by taking load off such
> links, reduces the probability of this happening at a lot of queues.
> Bottom line: Having a CDN keeps your users happier. Also, live streaming
> and video conferencing aside, most video is not multicast or broadcast,
> but unicast.
>
> DNS on Starlink satellites: Good idea, lightweight, and I'd suspect
> maybe already in operation? It's low hanging fruit. CDNs on satellites:
> In the day and age of SSDs, having capacity on the satellite shouldn't
> really be an issue, although robustness may be. But heat in this sort of
> storage gets generated mostly when data is written, so it's a function
> of what percentage of your data that reaches the bird is going to end up
> in cache. Generally, on a LEO satellite that'll have to cache baseball
> videos while over the US, videos in a dozen different languages while
> over Europe, Bollywood clips while over India, cooking shows while over
> Australia and always the same old ads while over New Zealand, all the
> while not getting a lot of cache hits for stuff it put into cache 15
> minutes ago, would probably have to write a lot. Moreover, as you'd be
> reliant on the content you want being on the satellite that you are
> currently talking to, pretty much all satellites in the constellation
> would need to cache all content. In other words: If I watch a cat video
> now and thereby put it into the cache of the bird overhead, and then
> send you an e-mail and you're in my neighbourhood and you watch it half
> an hour later, my satellite would be on the other side of the world, and
> you'd have to have it re-uploaded to the CDN on the bird that's flying
> overhead our neighbourhood then. Not as efficient as a ground-based CDN
> on our ground-based network that's fed via a satellite link.
>
> As long as Starlink is going to have in the order of hundreds of
> thousands of direct users, that problem won't go away.
>
> On 31/08/2022 7:33 pm, David Lang wrote:
>
>> On Wed, 31 Aug 2022, Ulrich Speidel via Starlink wrote:
>>
>>> This combines with the uncomfortable truth that an RF "beam" from a
>>> satellite isn't as selective as a laser beam, so the options for
>>> frequency re-use from orbit aren't anywhere near as good as from a
>>> mobile base station across the road: Any beam pointed at you can be
>>> heard for many miles around and therefore no other user can re-use
>>> that frequency (with the same burst slot etc.).
>>
>> not quite, you are forgetting that the antennas on the ground are also
>> steerable arrays and so they can focus their 'receiving beam' at
>> different satellites. This is less efficient than a transmitting beam
>> as the satellites you aren't 'pointed' at will increase your noise
>> floor, but it does allow the same frequency to be used for multiple
>> satellites into the same area at the same time.
>>
>> David Lang
>>
> --
> ****************************************************************
> Dr. Ulrich Speidel
>
> School of Computer Science
>
> Room 303S.594 (City Campus)
>
> The University of Auckland
> u.speidel at auckland.ac.nz
> http://www.cs.auckland.ac.nz/~ulrich/
> ****************************************************************
>