10 Jan
2017
10 Jan
'17
5:37 p.m.
Hi All,
I would like to realize an algorithm to build the best possible dvb-t
sdr experience. The idea is, that no matter how many sdrs and what kind
of antenna you have,
the software gets the best out of your hardware.
This is what I have thought:
The user community of a system depends on how useful it is,
how easy it is to use, build and how much money it costs.
Rtl-sdrs are quite cheap, but for now a user has no benefit of having
multiple sdrs in its system.
That is why I'm searching a way to correlate the signals of the sdrs
without hardware modification. I think everyone of you has seen the noise from
the power source. Has anyone tried to build a filter to use that noise
to calculate the delay between multiple dongles?
I mean, they get the same noise. I don't know if it is sufficient,
but it is also a good spot, to put some artificial noise in, as it can be easily
accessed and does not interfere with rf circuitry when it is switched off.
Is that an useful approach?
Do you think this could work? Did I miss something?
regards,
Steve
26 Feb
26 Feb
9:47 p.m.
Stefan Groißmeier 2017-01-10 17:37:
That is why I'm searching a way to correlate the
signals of the sdrs
without hardware modification.
Just my 2 cents:
Intriguing idea, BUT (sorry...):
For corrleation you need a common time base, that is a common clock
source. That is not the case for multiple RTL-SDR dongles. Moreover you
need a stable and known phase relation (runtimes of the clock signal)
for your dongles.
The USB bus does not provide the clock for the dongles, so this is no
way to synchronize them
The key word here would be MIMO - Multiple In Multiple Out. That is what
WLAN 801.11n does, in one device.
I do not know if any of the currently availabe dongles have a clock
input (for a common source) or a clock output (to act as clock master)
73 de Patrick OE6PSE
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Patrick Strasser-Mikhail OE6PSE <oe6pse at wirklich dot priv at>
10:30 p.m.
I've only heard of the possibility to solder out the reference
oscillator of all but one dongle and distribute the oscillator of that
to the others. That's a modification, and also, only eliminates
frequency offset - the random absolute phase (which you can remove by
correlation, assuming your antennas /mainly/ see the same signal) stays,
and what's more, the phase noise of the synthesizer inside the tuner chip.
Point is that these devices are *exactly for reasons of integration*
this cheap: there's one chip that does all the LO synthesizing and
mixing. You can't get the LO out or into that.
But, since Phase noise really is usually uncorellated to the signal
we're interested in, Patrick's right – with that modification, you can
build Multiple-input receivers. You'll get diversity gain. How much gain
(measured in reception quality improvement equivalent to an SNR
improvement, hence the name) that is depends on the method you're using
– but the fact is:
The first SDR you use gives you the most gain (namely, being able to
observe the signal at all); the second still a bit, regardless of
diversity method, the third always a bit less and so on. So whilst your
complexity tends to grow worse than linearly, the increase with every
additional receiver dwindles. What's making this worse: All these
methods depend on your ability to know the phase of every receiver path.
With RTL-SDRs you can't know that (there's no phase synchronization at
all, and even worse, no time synchronity to even start with); you have
to estimate it from your received signal, every single tune, and
continously, since the LOs *will* drift against each other in any normal
PLL'ed device. Now, estimating the phase of a reception is a pretty hard
problem and usually requires intricate knowledge of what signal you're
looking at – that's why all wireless standards have things like
preambles, pilot tones, sync words etc. With a pure cross-correlation,
your phase estimate's variance is pretty much proportional to your
sum(SNR^-1); not a good thing :(
So, in general, this is hard, and a receiver that allows you to
circumvent at least parts of these problems will be a lot more modular
and complex than an RTL-SDR dongle :(
Greetings,
Marcus
On 26.02.2017 21:47, Patrick Strasser-Mikhail wrote:
Stefan Groißmeier 2017-01-10 17:37:
That is why I'm searching a way to correlate
the signals of the sdrs
without hardware modification.
Just my 2 cents:
Intriguing idea, BUT (sorry...):
For corrleation you need a common time base, that is a common clock
source. That is not the case for multiple RTL-SDR dongles. Moreover you
need a stable and known phase relation (runtimes of the clock signal)
for your dongles.
The USB bus does not provide the clock for the dongles, so this is no
way to synchronize them
The key word here would be MIMO - Multiple In Multiple Out. That is what
WLAN 801.11n does, in one device.
I do not know if any of the currently availabe dongles have a clock
input (for a common source) or a clock output (to act as clock master)
73 de Patrick OE6PSE 
27 Feb
27 Feb
1:38 a.m.
Hi Stefan,
Rtl-sdrs are quite cheap, but for now a user has no
benefit of having
multiple sdrs in its system.
Hmmm, with several rtlsdr units one can monitor
several
different frequency bands. I am sure there are users who
find that beneficial:-)
That is why I'm searching a way to correlate the
signals of the sdrs
without hardware modification. I think everyone of you has seen the noise from
the power source. Has anyone tried to build a filter to use that noise
to calculate the delay between multiple dongles?
Surely it is possible to
synchronize two or more dongles in
software. They just have to receive the same signals to a sufficient
amount.
One would have to apply a fractional resampler to all but one
rtlsdr to compensate for different sampling clocks. Those resamplers
have to be adaptive to accomodate different frequency drifts.
This means one has to find the time shift for best correlation,
and establish how that time drifts to update the resampling ratio
for the time shift to stay constant.
There will be many interesting complications if one just uses the
normal antenna signals. It would however be fairly easy to inject
pulses at perhaps 30 Hz and use them for the synchronization.
Those pulses would be known accurately so they can be subtracted
from the data stream to not have any adverse effect on the
desired signals.
I mean, they get the same noise. I don't know if
it is sufficient,
but it is also a good spot, to put some artificial noise in, as it can be easily
accessed and does not interfere with rf circuitry when it is switched off.
Synchronization can not be switched off, stability is not good
enough for that - but the artificial noise is known so
we can subtract it:-)
Is that an useful approach?
Do you think this could work? Did I miss something?
Surely it can work. I do not
think it will be trivial to get it
running however, but you should be able to make a multi-channel
receiver that could combine phase-coherent signals from several
antennas for drastically improved S/N.
73
Leif
regards,
Steve
2622
days inactive
2670
days old
3 comments
4 participants
participants (4)
-
Leif Asbrink -
Marcus Müller -
Patrick Strasser-Mikhail -
Stefan Groißmeier