[Starlink] starlink extensions over uk

Wed Jul 27 17:02:55 EDT 2022

I understand your points - Starlink, at most (where license allows), can use 10.7 to 12.7 GHz in downlink, split in 8 250 MHz channels, with an emissions designator that uses 240 MHz of each channel. From my initial observations, the signal is OFDMA, using the entire 240 MHz. In uplink, they can use at most 14.0 to 14.5 GHz, split in 8 62.5 MHz channels. In uplink, from my measurements, they also use OFDMA and take the entire bandwidth by a single terminal when available:

NGSO constellations are regulated at a disadvantage compared to GSO, as the latter cannot move or use alternative paths between satellite and terminal, whereas NGSO can adapt. Thus, GSO protection arc, EPFD limits, etc. - all designed to protect the GSO operators.

It is unlikely that out-of-band interference is an issue, as I’m sure both satellites and terminals have gone through a fairly rigurous design and test process. Things that I have seen in terrestrial networks, with DC carriers emanating from 3 GHz point-to-point links, which totally killed 2.4 GHz WiFi, are unlikely to take place.

Starlink will only operate in the channels it is authorized over every specific service region, so the only realistic impact is primarily from in-band interference, if, for example, GSO protection is not implemented or enforced well, where both GSO operators and Starlink are licensed to use the same frequencies (TV LNBs cover almost exactly the entire Starlink downlink band, so you can see concerns from GSO operators in downlink).

Ka band is usually more controlled, and gateway to satellite discipline is better enforced, plus the use of Cassegrain antennas reduces beamwidth and sidelobes considerably, compared to the ESAs in Ku band.

Finally, the GSO protection is calculated by the operator, based on the GIMS and Transfinite calculations, taking their system’s input parameters. Telesat, for example, filed with OFCOM with only 4.5º of GSO protection, due to their particular link characteristics.

I do think we are yet to see real world effects from multiple constellations and their additive effects, ESA side lobes, etc. - you can only simulate so much before the wonderful and mysterious world of RF takes over.

Best,

Mike
On Jul 27, 2022, 16:37 +0300, Ulrich Speidel via Starlink <starlink at lists.bufferbloat.net>, wrote:
> On 27/07/2022 1:35 am, Mike Puchol via Starlink wrote:
>
> >
> > The interference is directly in-band, as the Ku and Ka bands are used both by satellite TV, GEO data systems, and Starlink. Thus, if a Starlink satellite in-line between a satellite TV customer and the GEO satellite, the satellite TV customer would experience considerable interference. This is why Starlink satellites, gateways, or user terminals cannot transmit anywhere between 10º above and 10º below the GSO arc (18º previously).
> Hang on a second - there's a terminology issue here relating to the term "band". "Ku band" refers to frequencies between (roughly) 12-18 GHz and "Ka band" to frequencies between about 26-40 GHz. "Out-of-band interference" refers to signals emanating from a transmitter that are outside its intended signal bandwidth - and that signal bandwidth is normally a lot smaller than the bandwidth of the Ku or Ka band - unless we're talking UWB applications here, and we're not.
> A Starlink satellite or ground station will only ever transmit (wanted signal) within a small sub-band within these bands. For example, in NZ, Starlink only holds licenses for five 500 MHz sub-bands of Ka between 27.5 GHz and 30 GHz, and half a dozen 250 MHz subbands of the Ku band. So we can safely assume that any transmissions from Starlink ground stations will take place within these limits. We can also safely assume that an individual transmission between a ground station and a single satellite will occupy at most one of these sub-bands, and for Dishys it's likely to be a lot less than that.
> Any GEO sat listening to that sub-band from behind the transmission's target Starlink bird would indeed experience in-band interference. Any GEO sat listening to other sub-bands of Ka or Ku that the one used for uplink may also experience interference, but in this case it's out of band because it isn't within the sub-band that the Starlink ground station transmits in.
> Normally:
>
> • A transmitter will be designed so as to minimise emissions outside its core wanted signal spectrum in the sub-band / channel in which it's operating. Why? Because power outside that core bandwidth is wasted, and can be a source of interference to others. This is achieved with a band pass filter around the transmit frequency that is designed to let the wanted signal pass and hold anything else back.
> • Similarly, a receiver listening to a signal on a different frequency will use a band pass filter to keep signals outside that wanted frequency range out.
>
> Think of ideal signals as being vehicles on a road that stick to their own lane so they don't bump into each other. So where's the problem? The problem is twofold: Firstly, receivers are designed to handle very weak signals and amplify them to the point where they "mean something" if there's any meaning to be had in them. Secondly, those band pass filters aren't perfect. They suppress signals in the unwanted part of by a decent amount but not completely. How "decent" an amount depends on the construction of the band pass filter, and that in turn depends on size, precision, material, number of filter elements, resistive losses etc. It also depends on how far that frequency of interest that you want to have suppressed is from the band that that band pass filter will let pass. The further the better.
> For the GHz microwave spectrum in which the Ku and Ka bands lie, band pass filters are essentially just appropriately shaped pieces of metal. Size isn't so much the issue as wavelengths are very short, but precision and material / resistive losses (skin effect) are. There's only so much gold you want to put into a Dishy and gold only gets you that far.
> So in order to interfere with a GEO sat receiver, a Starlink ground terminal will have to either:
> 1) Produce a spurious signal in an unwanted part of the spectrum (not to be confused with antenna side lobes, that's a different type of unwanted signal - any part of the spectrum but wrong direction of propagation and normally at a much lower power level). In any decent transmitter with band pass filter, this is likely to be several orders of magnitude below the wanted signal. That unwanted signal has to propagate along the much longer path to the GEO sat, where it is subject to path loss. It then has to arrive at the GEO sat with sufficient signal strength in order to raise the noise + interference level at the GEO sat's receiver to a point where the signal-to-noise-plus-interference ratio at the receiver falls below the minimum required for that sat's mission.
> 2) Have its wanted signal communicate well beyond its intended target (extra path loss again) to have it suppressed to a good extent by the band pass filter at the GEO sat receiver that is there to keep out of band signals out. If the residual signal from the Starlink terminal is still strong enough to raise the noise + interference floor at the receiver enough to lower the signal-to-noise-plus-interference ratio at the GEO sat, then again we have a problem.
> Sadly, both are possible - however as I've argued, it's probably less of an issue with Starlink than with other types of NGSO services.
> Side lobes are usually orders of magnitude below the main lobe, so tend to be less of an issue than the main lobe as they project a much lower signal to start with. Plus these signals aren't correlated when coming from different transmitters, so it's a matter of "powers add" not "amplitudes add", and powers drop by one over distance squared, so... I'm not sure how seriously one should take these complaints.
> In the RF world, not everything is as it seems. When they ask you to turn your mobile (cell) off on the plane, the usual grounds proffered are some nebulous claims of interference with navigational systems - which hasn't actually stopped people from putting base stations into airliners, although their means of navigation haven't changed all that much. Plus, a lot of planes fly with active cellphones on board. The reason why airlines don't want this is because the mobile networks aren't designed for handovers at several hundred knots from 30,000 feet, where a phone can keep a large number of base stations very busy with handovers. So the mobile operators require airlines to minimise this nuisance - and that's why they play on your fear of flying, and why it's become an accepted part of flying culture that your plane might crash if you leave your phone on... ;-)
> >
> > This presentation on the subject was shared by a friend earlier, it is a really good read on the topic: https://www.itu.int/en/ITU-D/Regional-Presence/AsiaPacific/Documents/Events/2017/Aug-ISS2017/PAPER_Workshop_S3_Timur.pdf
> >
> > Phased array antennas are notoriously prone to generating considerable sidelobes, unlike e.g. a Cassegrain dish. A good article on these (and mitigations) can be found here: https://www.mwrf.com/technologies/systems/article/21143497/analog-devices-phasedarray-antenna-patterns-part-6sidelobes-and-tapering
> >
> > These sidelobes contribute to the interference, and have been a major source of complaints by the likes of Viasat and others - whereby they claim SpaceX doesn’t consider the additive effects of sidelobes from dozens or hundreds of ESAs on satellites and terminals, against a single victim earth station or satellite.
> >
> > Best,
> >
> > Mike
> > On Jul 26, 2022, 16:10 +0300, Ulrich Speidel via Starlink <starlink at lists.bufferbloat.net>, wrote:
> > > So then the difference really is just in the GSO protection settings I guess.
> > > The GSO protection is - to an extent - also patch protection. After all - who'd need satellite TV if everyone could watch the same TV via LEOs? But consider that:
> > >
> > > • Interference to GEO sats from Starlink & Co. is out of band. I'm not sure what the out-of-band emissions profiles of dishys are, but I'd imagine we'd be looking at the usual few dozen dB below peak.
> > > • Distance to GSO is around 64 times (2^6) larger than to Starlink's orbits - GEO sats see around 1/4000th ((2^6)^-2) of the power from a ground station that the Starlink satellite in front of it sees. That's 36 dB in extra separation.
> > > • Dishy is comparatively small in cross-section, and that severely limits its gain. Most serious GEO uplink applications that I'm aware of use dishes more like 8 times dishy's cross section. That's another 9 dB or so in separation between a Starlink dishy signal and signal directed specifically at a GEO sat just from the overall size. Uplinks from gateways are likely to be much more of an issue (always on and probably higher power as well as ~6 dB higher antenna gain judging from the photos I've seen), but then again these point away from the GSO when serving birds further north.
> > > • My understanding (correct me if you think I'm wrong here): It's also possible that Dishy's nature as a phased array helps here. Why? Out-of-band interference results from intermodulation and other unwanted emissions from the power amplifiers (PA) of transmitters. Where these are fed into a dish from a single PA, their amplitudes get amplified by the gain of the dish. Now say we're trying to replace that dish by an array with N antennas and N associated PAs that feed at the appropriate phase. Then each element (individual PA with associated antenna) needs to contribute P/N of the total transmit power P of the big PA & dish. Now remember that power is proportional to the square of the amplitude. As long as the wanted signal components from the PA are correlated - and they have to be for the phased array to work - their N amplitudes add up, meaning the total output power of the wanted signal across all elements is proportional to N^2. So each element only needs to contribute an amplitude proportional to 1/N in order to produce the wanted output at the correct power. However, now each PA produces its own dirt signal. But unlike the wanted signal, these unwanted signals aren't necessarily all correlated between the elements. As a result, the amplitudes of the unwanted signals from the PAs will partially cancel out as the signal combines into the beam that is being formed. So only the powers of these uncorrelated unwanted signals add up, but their amplitudes don't, meaning we now have an unwanted signal power that is proportional only to N rather than N^2, resulting in a higher ratio of wanted signal to unwanted signal. Again that'd help a lot with separation in an array with lots of elements and associated PAs. That said, published research into the unwanted emissions of phased arrays is still a bit in its infancy, and I can't profess to understand enough about Dishy's innards or the level of correlation between unwanted emissions in the array, but I'd consider it possible that this allows for some relaxation on the GSO protection parameters when it comes to Starlink.
> > >
> > > On 26/07/2022 9:06 am, Mike Puchol via Starlink wrote:
> > > > Your calculations are “Back of the Envelope Approved” :-)
> > > >
> > > > My simulator can be found at https://starlink.sx and runs on your browser (desktop only). I have just released v1.10.6, which adds a slider for setting the GSO protection. This is the difference in Sweden from 18º (as used until recently) versus 10º (as found in recent ITU filings for STEAM-1B):
> > > >
> > > > <18_vs_10_GSO.png>
> > > >
> > > > Minimum elevation in both cases is 25º, so that variable is unchanged.
> > > >
> > > > Best,
> > > >
> > > > Mike
> > > > On Jul 25, 2022, 11:35 +0300, Ulrich Speidel via Starlink <starlink at lists.bufferbloat.net>, wrote:
> > > > > I haven't got Mike's nice software but I got snail mail today, so let's try the back of one of the envelopes.
> > > > > 53 degrees north (where the existing constellation tops out) is roughly the latitude of Nottingham or Stoke-on-Trent. Note that this is also the latitude with the largest number of birds per km (or mile, if you so prefer) of parallel. They sit almost cheek to jowl there.
> > > > > Now the northern tip of the UK's main islands is at around 59 degrees north - that's six degrees more. One degree of latitude is roughly equivalent to 10,000 km / 90 degrees = 111 km, so we're talking around a devilish 666 km to the north of Nottingham here as the crow flies.
> > > > > Lets assume Dishy points itself due south at 59 degrees. The birds are at about 550 km. So doing a flat earth approximation for the moment, we're having to point at a satellite 550 km up from 660 km away - that gives an elevation of about 40 degrees (=arctan(550/660)). Ballpark. Now reduce that by the 6 degrees of difference to account for the curvature of the earth and we still have around 34 degrees, allowing for a bit of leeway either side if the closest bird isn't actually due south but a bit to the east or west. Again, ballpark, but we're nowhere near 25 degrees yet.
> > > > > So I'd say that should cover it nicely? Where does my envelope err? GEO arc protection aside for the moment, of course. And of course just because Starlink offers you service doesn't mean that it's actually continuous.
> > > > > On 25/07/2022 8:59 am, Dave Taht via Starlink wrote:
> > > > > > For those of you that don't follow mike's twitter feed...
> > > > > >
> > > > > > https://twitter.com/mikepuchol/status/1551288485713149952
> > > > > >
> > > > > > --
> > > > > > FQ World Domination pending: https://blog.cerowrt.org/post/state_of_fq_codel/
> > > > > > Dave Täht CEO, TekLibre, LLC
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