Precisely.
Likewise, the famous "25 degrees over the horizon" isn't a nod to "too many trees/houses around Dishy", but there to satisfy the regulators that a transmitting beam from a satellite won't get into (more or less horizontally pointing) terrestrial communication systems (there's a softening of that requirement to 5 degrees minimum for beams to gateways at higher latitudes only).
It's also notable that this EPFD limit leads to a de-facto standardisation of the downlink as such. In a "textbook" system, you'd have some transmit power on the satellite, some antenna gain up there, you'd then take into account the path loss from spherical spreading of the signal on its way down, which gives you a PFD at your receiver site that depends on the geometry of the satellite-ground station arrangement. You'd then also look at your receive antenna gain. That's determined by the antenna aperture, which in the case of a phased array, is the collective area of all antenna elements. And since you're steering your receive beam, you'd be downrating the aperture by the cosine of your steering angle. And then you'd end up with some signal to noise ratio at the receiver which determines what modulation scheme you can use, and that along with the bandwidth of the link determines your capacity. But now you have a hard limit on your PFD imposed by the regulatory EPFD limit. So what SpaceX do according to their FCC filings is to reduce their EIRP at the satellite so the signal stays under/at the EPFD limit on the Earth's surface, and they adjust Dishy gain by using fewer of its elements the lower their steering angle is.
Put in another way: If you could "see" Starlink satellite signals, they would appear equally bright to you no matter where in the sky they come from - you wouldn't be able to tell distance from brightness. Similarly, you could think of Dishy as a pair of sunglasses that are tinted most strongly in the middle but where the tinting fades as you look sideways. Sounds complex but results in SNR's that are a perfect fit for the modulation schemes (16QAM for Gen 1, 64QAM / 256QAM for Gen 2), so saves on satellite and Dishy having to negotiate modulation.
Best,
MikeOn Jun 3, 2024 at 09:43 -0700, David Lang via Starlink <starlink@lists.bufferbloat.net>, wrote:
If the ground stations were omnidirectional antennas, you would be correct, but
since they are phased array directional antennas, they can steer the beam to
receive one satellite even while a different one is transmitting on the same
frequency to the same cell.
David Lang
On Mon, 3 Jun 2024, Ulrich Speidel via Starlink wrote:
Date: Mon, 3 Jun 2024 22:40:24 +1200_______________________________________________
From: Ulrich Speidel via Starlink <starlink@lists.bufferbloat.net>
Reply-To: Ulrich Speidel <u.speidel@auckland.ac.nz>
To: starlink@lists.bufferbloat.net
Subject: Re: [Starlink] musk: 28ms median latency on starlink
Getting the satellite density up will help, but it will only improve things
so far.
The problem on user downlink in particular is that there's a limit on the
maximum spectral power flux density that arrives from the satellite in space
on the ground. If you point all (mutually compatible) user downlink beams
from a single satellite at a single cell, you all but reach that limit there.
In fact, where SpaceX want to use two beams on the same frequency but with
opposite polarisations to the same cell, they must reduce the transmit power
on each beam by 3 dB (50%) in order to stay within the limit. More satellites
would give you more beams, but you can't point them at cells that already
have a beam on the same frequency in use from another satellite (unless you
de-rate on the power front, I guess). That seriously limits what you can
receive in terms of total capacity within a single cell to what a single
satellite's mutually compatible beams can deliver, which appears to be about
12 Gb/s on V1 and V1.5 birds, and 20 Gb/s on V2 (on Ku, if you add in Ka-band
and anticipate Dishys that can do Ka, then it's a lot more for Ka). In
practice, we know that a cell gets served by beams from different satellites,
but the overall constraint still applies - if you deploy beam X from sat A
and beam Y from sat B to the same cell, this makes the same contribution to
PFD as deploying both from the same satellite. Note that Starlink sats do
have multiple mutually incompatible beams that they can only point at
different cells, bringing Ku user downlink capacity up to 16 Gb/s on V1 and
1.5, and 48 Gb/s on V2. But that only ups your chances of getting a larger
slice of those 12 or 20 Gb/s in your cell.
Your best bet for continuing good service at the moment is literally to tell
your neighbours that Starlink is useless, so they don't sign up and you can
have your cake all to yourself ;-)
On 3/06/2024 5:13 am, Dave Taht via Starlink wrote:
Via elon musk:
Starlink just achieved a new internal median latency record of 28ms
yesterday! Great work by the engineering and operations teams.
- https://twitter.com/elonmusk/status/1797282250574184587
I of course, am very interested in y'all´s external measurements of how
well starlink is doing. For me, it is fantastic - 30Mbit uploads nowadays,
0
latency on the upload (how?)
https://www.waveform.com/tools/bufferbloat?test-id=2a1d139b-87cb-4ba4-a829-e2167801cffe
I also keep hoping that the rest of the ISP industry is now paying
attention and deploying stuff like fq_codel and cake and libreqos, but, ah
well - I will settle for starlink blowing past a lot of dsl and cable and
finding ways to get their density up.
Anyone going to the Starship launch on the 6th?
--
https://www.youtube.com/watch?v=BVFWSyMp3xg&t=1098s
<https://www.youtube.com/watch?v=BVFWSyMp3xg&t=1098s> Waves Podcast
Dave Täht CSO, LibreQos
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