[Ground-station] Baseband => decimation - questions
Zach Leffke
zleffke at vt.edu
Sat Jan 26 23:51:36 PST 2019
I'll attempt to clarify without adding to much noise......I think I'm
seeing both sides of this here......Though I'm no DSP expert, so fair
warning I might misspeak a bit.
A lot of what has been discussed so far is about down conversion, not
decimation.........
The trick is the 'guts' of the SDR, specifically the RFIC and the ADC
used. Say we have an IF of 700 MHz coming out of an LNB. The upper
edge of our signal would be at 705 MHz. According to Nyquist, you would
have to sample that IF signal at 1410 Megasamples per second (at
least). Any lower than that, and images of the sampled signal will
overlap and corrupt the data (Aliasing). Say your ADC had 14
bits.......that's a lot of data, probably more than most FPGAs can handle.
The trick in the SDR (pick your vendor, I'm staying generic here) is
that it has a tunable front end that performs another level of down
conversion in the analog domain before sampling (for example, the AD9361
in Ettus B210s, E310s, etc). This takes the 700 MHz signal down to say
20 MHz (made that number up), with an ADC running at a fixed rate.....
say 80 MHz (also made that number up).
At this point, my DSP-fu is weak, but where I'm going with this is that
the 80 MHz sampling is real samples, represents 40 MHz of RF bandwidth
(Nyquist). This can be represented as a stream of complex samples (IQ)
running at half the rate, though each sample is twice the size, one for
I and one for Q .......(Nyquist isn't violated, because even at half the
rate, you have two samples, one In-phase, and one Quadrature)......
So now we have a stream at 40 MSPS (complex), spanning from 0 to +40
MHz, with our signal camped out at +20 MHz. So then we tune digitally
to center our signal at 0 MHz...aka complex baseband. Now we have a
stream still at 40 MSPS (complex), but spanning from -20 MHz to +20
MHz. Negative frequencies are OK in the complex domain. Our signal of
interest is now centered at 0 MHz, but is spanning from -5 MHz to +5 MHz.
I think what Michelle is getting at is that we don't want to have an
FPGA processing a 40 MHz wide stream of data, when we only need to worry
about 10 MHz. Since our signal is centered at 0 MHz, we can start
throwing away samples........Decimation. We can toss 3 out of every 4
samples out (decimate by 4, and filter), which leaves us with a 10 MSPS
stream of complex samples. This also has some benefit in that we are
also rejecting the noise contributions of those unnecessary samples that
we tossed out (linked to the B in kTB).
Everything that was (horribly) described above is handled 'under the
hood' by the UHD drivers in Ettus products for example. When you tell a
UHD source block the sample rate you want (usually the 'samp_rate'
variable that shows up in the flowgraph) what you are actually telling
it is the 'requested' sample rate on the output of the above process.
The onboard ADC still runs at the fixed higher rate (80 Mhz in my
example), but based on your input into the UHD source block, it will
automatically select the right parameters to ensure that after sampling,
conversion to complex baseband (including tuning your requested center
frequency to 0 MHz), decimation and filtering, that the rate you
requested is fed out to the host computer. This is why you have to be
careful about selecting sample rates with UHD.........some of you have
probably seen the debug output where it warns you that the division of
the ADC clock rate by the requested sample rate is not an even integer
and to expect 'CIC rolloff'........basically it couldn't cleanly do the
decimation you requested (again....something here about the value of
half band filters in the FPGA that are part of the conversion to complex
baseband and decimation.....sorry for my weak DSP-fu). You know you got
it wrong when you see a 'hump' of spectrum when the spectrum should be
'flat'.
Now we can run that 10 MSPS complex stream of samples through the
demodulation process and extract the 'frames of interest' for any
particular user........that would be demultiplexing (throw out the
frames for everyone else, I only want the frames for me).
I would offer that on the ground side, there is no channelizer in the
mix.....you must receive the entire 10 MHz signal to recover the full
downlink data stream, since there is only one time multiplexed, 10 MHz
wide signal.
On the satellite payload on the other hand....you WILL want a
channelizer. Lets say the uplink is 10 MHz wide, and supports 1000
channels, so each 10 kHz wide. In order to demodulate 1000 channels in
parallel, somewhere in there you need to tune 1000 times to center each
uplink signal to complex baseband and decimate to 10 ksps (complex).
executing 1000 'tunes' in parallel at the full 10 MSPS rate is very very
wasteful........Think of the channelizer as a really efficient way of
performing the tuning and decimation so that each output channel of the
channelizer is only 10 ksps and it is properly 'centered' on the desired
uplink channel. it will require more horsepower than a single tune and
single decimate 'string' but is less processing intensive than 1000 of
those strings running in parallel.
I didn't mention anything about other sampling tricks (Nyquist zones,
and potential spectral inversion issues), or the benefits of
oversampling (dynamic range), integer vs float
representation.........maybe on another thread one day.
So if you are going to roll your own hardware..........first a lot of
downconversion from 10 GHz (maybe an LNB to get to say L-Band or maybe
something to get to a 'ham band IF' at 432 or 144). Thats not enough.
You'll have to then downconvert again to something that can be handled
by the selected ADC and whatever clock rate it is running at (Nyquist
rules apply here). Filtering and careful consideration of mixing
products will matter! phase noise of the LOs will matter in all that
downconversion.......Also, gain gain gain... will matter to make sure
that you are fully exercising the full range of the ADC and not just
toggling the the lowest couple of bits (but not too much
gain.....clipping). Then the complex baseband conversion and decimation
and on to demodulation, demultiplexing, etc. etc.......A lot of the
above is what is handled 'under the hood' by most of the commercial SDRs
out there (i.e. UHD) so that the end user can easily get up and running
with the 'more interesting' stuff downstream.........
Hopefully this helped clarify the issue.....sorry if it added more noise
(my DSP-fu is weak). Not sure if I actually answered any questions.......
-Zach, KJ4QLP
Research Associate
Aerospace Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305
On 1/25/2019 7:27 PM, Ron Economos via Ground-Station wrote:
>
> Okay. De-multiplexing is a much better and less confusing terminology.
> As you stated, decimation is a DSP thing and channelizing the downlink
> payload has nothing to do with DSP (all the DSP has already been down
> in order to deliver payload packets).
>
> Ron W6RZ
>
> On 1/25/19 16:17, Michelle Thompson wrote:
>> To me, decimation is what we do in order to channelize in the payload.
>>
>> I don't think that's exactly what I'm being asked about in the ground
>> station receiver, though.
>>
>> -Michelle W5NYV
>>
>>
>>
>>
>> On Fri, Jan 25, 2019 at 4:14 PM Ron Economos <w6rz at comcast.net
>> <mailto:w6rz at comcast.net>> wrote:
>>
>> I'm not sure we are talking about the same thing yet. So what
>> exactly do you expect to decimate and why?
>>
>> Ron W6RZ
>>
>> On 1/25/19 16:07, Michelle Thompson wrote:
>>> The beginning of wisdom being the definition of terms and all,
>>> it would be good to make sure we're all talking about the same
>>> thing.
>>>
>>> So far, I've used LNBs and USRPs for receive, with the LNB doing
>>> an IF at 618MHz (LNB-on-a-Stick) and giving reasonable performance.
>>>
>>> Decimation to me is a DSP thing, or used to reduce power
>>> consumption when you don't need to sample as high as you can.
>>>
>>> -Michelle W5NYV
>>>
>>>
>>>
>>>
>>> On Fri, Jan 25, 2019 at 3:52 PM Ron Economos via Ground-Station
>>> <ground-station at lists.openresearch.institute>
>>> <mailto:ground-station at lists.openresearch.institute> wrote:
>>>
>>> The standard IF for DVB-S2 receivers is 950 to 2150 MHz.
>>>
>>> DB6NT was selling a down-converter from 10489-10500 MHz to
>>> 1129-1140 MHz for P4A.
>>>
>>> https://shop.kuhne-electronic.com/kuhne/en/shop/new/MKU+LNC+10+OSCAR+P4A/?card=1832
>>>
>>> I'm not sure what decimation has to do with receiving
>>> DVB-S2. The entire 10 MHz signal needs to be demodulated.
>>> Individual baseband frames will be selected for processing,
>>> but I call that de-multiplexing.
>>>
>>> Ron W6RZ
>>>
>>> On 1/25/19 15:32, David Vieira via Ground-Station wrote:
>>>> Michelle - Thanks for posting. I'll frame some of the
>>>> questions.
>>>>
>>>> Typical 10 GHz terrestrial contesting rigs are Heterodyne;
>>>> that is a Mixer works with a Local Oscillator (LO) to take
>>>> the RF down to an IF (Intermediate Frequency).
>>>> For an SDR, that IF can be digitized by an Analog-Digital
>>>> Converter.
>>>>
>>>> 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.
>>>>
>>>> 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.
>>>>
>>>> There are a lot of proof of existence designs for a 10 GHz
>>>> Mixed down to an IF; and lots of off the shelf ADC
>>>> dev-boards. (catch me off thread for details).
>>>>
>>>> Some questions I have are:
>>>> ---from an FPGA side of the SDR, what data rate(s) can the
>>>> FPGA absorb in to a decimator?
>>>>
>>>> Must we decide upfront on a single frequency; or
>>>> preferably allow flexibility in the RF front end design
>>>> (ie, Mixer, PLL and Local Oscl hardware choices) by
>>>> allowing a wide and programmable variety of ADC and
>>>> decimation rates?
>>>>
>>>> {This is where RF and Digital folks must communicate across
>>>> walls.} ;-)
>>>>
>>>> Comments welcome.
>>>>
>>>> regards,
>>>> David
>>>> KI6CLA
>>>>
>>>>
>>>> On Friday, January 25, 2019, 2:41:54 PM PST, Michelle
>>>> Thompson via Ground-Station
>>>> <ground-station at lists.openresearch.institute>
>>>> <mailto:ground-station at lists.openresearch.institute> wrote:
>>>>
>>>>
>>>> While we are striving to enable all sorts of wonderful
>>>> designs by putting prototypes into GNU Radio, a central
>>>> goal is to design our own hardware.
>>>>
>>>> We've had a lot of progress on the protocol and algorithm
>>>> front (GSE, LDPC, some of the polyphase).
>>>>
>>>> Some fundamental decisions about our own hardware need to
>>>> be made.
>>>>
>>>> When we receive, we expect to have to decimate. This is
>>>> because we are receiving at a relatively high frequency
>>>> (10GHz).
>>>>
>>>> Our bandwidth is (up to) 10MHz. For DVB-S2/X, we fix our
>>>> sampling rate, depending on what bandwidth we want to
>>>> support. We have a lot of freedom here.
>>>>
>>>> Picking the right frequencies for the receive chain is
>>>> therefore important.
>>>>
>>>> What are our options?
>>>>
>>>> What options make the best sense?
>>>>
>>>> I'd like to build and test as soon as possible, so let's
>>>> get some discussion going.
>>>>
>>>> -Michelle W5NYV
>>>>
>>>>
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