[Ground-station] Baseband => decimation - questions

[Michelle Thompson]

We have block diagrams from David Viera and Scott Cowling, a standard we
are well on our way to implementing, our own additional amateur-centric
requirements, a list of questions, and an understanding of the current
state of the art for SDR.

We have a widely used RFIC in the AD9361, a solid choice in the LTC2208, a
related part already in the diagram from TAPR Space Weather Station
(LTC2145), and some idea of gear that can and will be kit-bashed for many
stations (PLL equipped LNBs).

Questions with some Answers and More Questions:

-=-=-=-=-=-=-=-=-=-=-=-=-=-

Must we decide on a single frequency?

No*.

*But, yes, we do, for our manufactured solution. We need to pick sweet
spots so we don't go broke while also ensuring enough performance to do the
things we want (ACM, flexible use, all the queueing and quality of service
possibilities available through GSE).

The choice of intermediate frequencies (and symbol rate) is not limited by
DVB-S2/X. The standard doesn't specify implementation, so the management of
frequencies for this build is up to us. As mentioned, if you have too many
layers of conversion, and you end up with too much phase noise, then you
will have a compromised system.

Fortunately there's a lot in DVB-S2 that helps. We will use short frames
and pilots and we expect to be using lower order constellations. This
reduces (but does not eliminate) phase noise requirements.

The most common IFs are the ones we've discussed. 950-2150MHz for
receivers, and 600-700MHz (like when you hook up your commercial LNB to an
SDR for ham 10GHz).

As far as symbol rate - anything from the low hundreds of samples per
second up to 500Msps+ is done with DVB-S2/X. We expect to max out at 10MHz
but that's due to the band plan, not a technical limitation. It's well
worth taking a hard look at this limitation and deciding whether or not
it's a false one. Yes, 120MHz might be overkill, but the Space Weather
Station wants to cover 60MHz. If it's no difference to include the part,
then we get design reuse.

-=-=-=-=-=-=-=-=-=-=-=-=-=-

LNBs
(Skip to bottom of this section for TL;DR)

Something Kent said is a big deal. There's a lot of us that build cheap
receivers for 10GHz out of LNBs and DSS dishes and things like RTL-SDRs.

However, we're full duplex for P4G.

We have a dual-band feed design. We put the time and effort into it for a
reason. It's hard enough to point a receive dish at a GEO and get it right.
If you haven't done this, it can either go really well and seem pretty
easy, or be a very grumpy process where you end up pointing at the wrong
thing or getting nothing and then you find something else to do for a while.

How are you going to point your transmitter reliably, if it's separate?

Our system will not pass your traffic if you're not heard on the downlink.

Once you set up your receiver and are receiving, and you have a separate
antenna system for the transmitter, how easy is it to align? How will
someone be able to tell the difference between not aligned and something
else not working? A broad beamwidth at 5GHz helps here. Can we get enough
gain that way? I believe the answer is yes, but I don't think that P4G 5GHz
RF volunteers have prototyped anything that shows this (yet).

We know what we need to close the link. We have link budgets and lots of
people that have used 5GHz in the field.

One dish, one feed, and you're good. This is the best experience. Two
antennas, two feeds, and we have potentially a lot of customer support -
especially if the uplink has a narrow beamwidth or the operator just
doesn't have the best sense of direction.

Where does this fit into the block diagram? In selecting the parts for the
transmitter and receiver. We need to start seriously thinking about what a
dual band feed RF hardware chain looks like, and not pick something that
can ONLY be built with two separate antennas.

BUT If you want two separate antennas, then you should be able to do that.
Can we achieve this, on this dev board? I think we can, since the inputs to
the two feed points were prototyped with SMAs. If you don't want the dual
bander, then you should be able to simply connect your RX and TX to
whatever antenna you want.

Can we surgically alter an LNB and graft it into our dual band feed? That
would give us a lot of cheap hardware alternatives.

If you want the dual bander, cut up your LNB and mangle it in there. Want
to use an LNB stand-alone? Just use it, and have your transmitter separate.

Is there something other than a mangled LNB that would drop in? There's an
awful lot of silver boxes out there for sale. Which one works for us>

TL;DR:
We need to allow for the use of a commercial LNB as a receiver without
requiring it.

-=-=-=-=-=-=-=-=-=-=-=-=-=-

Doppler. There's not a lot of it at GEO. We make the most sense at GEO and
HEO. We could work at LEO, but we're leaving a lot of the fun and potential
community building on the table.

-=-=-=-=-=-=-=-=-=-=-=-=-=-

IQ gain-phase corrections are well-studied with widely documented
techniques to mitigate.

-=-=-=-=-=-=-=-=-=-=-=-=-=-

Do we have to worry about the other signals in the vicinity of the
frequency of interest?

This is an interesting question. I'm going to say "no".

For terrestrial deployments, there will be very few other signals in the
experimental 10GHz ham band. The signals seen will be local. If we do it
right, the Groundsat will be the only signal, and it will "sound" like a
bit more noise.

For space deployments, there appear to be very few other signals in the
10GHz downlink.

I agree with Ahmet. For the payload/Groundsat, "it is more important to
have a fixed and low jitter ADC clock for the channelization as the fine
tuning and equalization for each user needs to be done separately after
channelization in the digital domain anyway."

This is the only place we can really make up for what may be a very wide
range of quality signals. I am very hopeful that the sort of
experimentation and DIY going on with Es'Hail 2 (Phase 4A) will happen
here. That means that there could be some very sketchy 5GHz uplink.
Pointing errors, timing errors, cheap hardware, bad choices, too much
bourbon, whatever. The best possible receiver and channelizer will capture
them and allow us to regenerate them as beautiful downlink packets. Yes,
we'll need a higher dynamic range here. Since there are much fewer payloads
than ground stations, we can and should budget the performance

-=-=-=-=-=-=-=-=-=-=-=-=-=-

David Viera writes:

"The most popular IF for contesting/SSB rigs is 144 MHz.  For a data BW of
10 MHz that may or may not be a fast enough IF carrier.  If we can digitize
and recover the data, it would allow a lot of re-use of existing equipment."

He continues with:

"I've heard suggestions/proposals up to the 1.2 GHz Ham band.
In some sense, the IF carrier could be 144/220/440/915/1200 MHz, or even
any Non-Ham frequency in between."

I'm edgy about 144MHz as an IF. I know DB6NT has gear that does this for
10GHz, but it's a lot of converting. I have two sets of DB6NT 10GHz
transceivers. One of them failed, and I got a front-row seat to to the
conversion process.

How about 10GHz to 700MHz to <something that might work for both TAPR Space
Weather System and Phase 4 Ground>?

10GHz => 700MHz => 30MHz?

Would this give us enough traction with a wider market? Does it allow for
someone to graft in a commercial LNB with relatively little pain?

-Michelle W5NYV

"Vincit qui se vincit."



On Mon, Jan 28, 2019 at 10:04 AM Phil Karn via Ground-Station
<ground-station at lists.openresearch.institute> wrote:
>
> I see one important  design decision related to decimation, sample
> rates, IF selections and so forth.
>
> If you need, say, 10 MHz of bandwidth you can get it by complex sampling
> a direct conversion receiver at 10 MHz. That gives you a complex base
> band stream extending from -5 MHz to +5 MHz.
>
> This does have some drawbacks; the question here is whether they are
> sufficiently serious for this application to switch to another
> technique. The main drawback is the spike at 0 Hz (DC) in the base band
> due to mixer imbalance, usually from the LO cross feeding into the mixer
> input.
>
> This DC component is pretty stable unless you retune, so I've found it
> fairly easy to remove with simple exponential high-pass filters (one
> each in I and Q). If the signal has a residual carrier component, it'd
> be wise to not tune it down exactly to DC, though this is actually hard
> to do except in simulation with perfect oscillators. The carrier
> tracking that puts the carrier at exactly 0 Hz will be done later in DSP
> where these imperfections don't exist.
>
> Although the DC spike is easy to remove, there is still a small mound of
> 1/f noise surrounding the now-suppressed DC spike. This may or may not
> pose a problem to the specific modulation; it depends on how much signal
> energy is in this region vs the whole thing. For a broadband 10 MHz PSK-
> or QAM-like signal, it probably won't cause much problem.  Again the
> signal can be shifted slightly in frequency if necessary, with this
> shift compensated for in the subsequent carrier tracking.
>
> There are two other imperfections that have to be corrected. The I and Q
> channels won't have exactly the same gain, and they won't be in exact
> quadrature. This shows up as imperfect image suppression between the
> negative and positive frequencies. You fix the first problem with AGCs
> to make the two channels have the same average power. You fix the second
> by rotating the Q axis slightly to drive the average dot product between
> the I and Q channels to zero.
>
> This doesn't handle frequency-dependent imperfections, but it may be
> good enough for our modulation. In a recent local IEEE talk, Fred Harris
> talked about how images can be a serious problem but his examples were
> of extremely complex OFDM signals with thousands of 2048-QAM carriers
> (or similarly absurd numbers). It's probably not significant with a
> single signal carrier with low order modulation designed for a satellite
> channel with a relatively low SNR. (Has one of the DVB-S2 formats been
> chosen yet?)
>
> But if these imperfections *do* turn out to be a problem, there's
> another approach involving faster sampling and more DSP: you shift the
> center of your signal to +/-Fs/4. That is, you center it halfway between
> zero and either the negative or positive Nyquist frequency and filter
> out (throw away) the signal on the other side of zero. This is sometimes
> called a "low IF" system since the last IF is less than the sample rate
> (it's 1/4 of it) and DSP is still used to get rid of the mixer image.
>
> The DC spike and 1/f noise are now gone (they're actually moved to the
> Nyquist frequency on one side, which you're not using anyway). Any
> images are now just thermal noise instead of signal (unless we have some
> nearby interferers which seems unlikely). This obviously requires A/D
> converters that are twice as fast, followed by DSP that can handle twice
> the samples. The Fs/4 frequency shift is easy, you just rotate each
> complex sample by 90 degrees: 1, j, -1, -j, 1...
>
> It provides a cleaner signal but again it may not be necessary for our
> specific modulation. And anything we do to keep the sample rate down
> will help keep costs down.
>
> 73, Phil
>
>
>
> _______________________________________________
> Ground-Station mailing list
> Ground-Station at lists.openresearch.institute
> http://lists.openresearch.institute/mailman/listinfo/ground-station
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openresearch.institute/pipermail/ground-station-openresearch.institute/attachments/20190201/8f0a9f08/attachment.html>