Fwd: How to get out the maximum from cheap sdrs

Marcus Müller marcus.mueller at ettus.com
Sun Feb 26 21:30:51 UTC 2017

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 :(



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

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