Hello,
Yes, I understand the Nyquist sampling theorem and my hardware
limitations. That is why I assumed filtering would not work although I
gave it a shot anyways. I am not trying to demodulate any baseband
signal, rather I am going to be receiving a reflected signal at around
5.8 GHz and using that signal for range detection. The signal itself
will not contain any information, but the strength of the signal is the
information I need. However, I need a method for determining, or at
least estimating, SNR because that is the key variable that will change
throughout my experiment in the radar range equation and will ultimately
be the deciding factor to determine how far away the object is. That was
why I proposed doing a pre-trial calculation where my source is not
transmitting to get the average noise power, and then set that as a
constant block and subtract it in real time from my average total power
with noise and signal both included. The constant block would then be
updated prior to each run.
A necessary part of the radar range equation is the transmit power from
the source as well as the directivity. The SNR I'll be looking for is at
the receiver. So the equation takes care of any signal attenuation.
I'm building a passive RF range calculation system in conjunction with
an EO object tracking system.
Thank you,
Alex
On Fri, Jun 26, 2020 at 11:14 AM Johannes Demel <demel@ant.uni-bremen.de
<mailto:demel@ant.uni-bremen.de>> wrote:
Hi Alex,
your cut-off frequency needs to be lower than half your sampling rate.
If your sampling rate is 61.44MHz, your maximum cut-off frequency
can be
30.72MHz. And it should probably be a bit lower. You're working in
baseband here. It is really important to understand the concepts of
digital signal processing. That's also the reason I pointed out several
resources.
SNR calculation itself is not always trivial. You need a way to
distinguish samples that should only carry noise energy and those that
should carry signal energy.
Often people distinguish between SNR estimation for AWGN channels and
for fading channels. While your estimator will probably not distinguish
between the two, some estimators just fail for fading channels
especially.
For OFDM you might want to look into Schmidl&Cox preamble based SNR
estimation. There might be an M2M4 estimator for symbol based SNR
estimation.
What kind of system are you using?
Cheers
Johannes
On 26.06.20 15:49, Alex Batts wrote:
> Right, because the filter cutoff frequency needs to be at least
half the
> sampling rate, I figured I would not be able to use a filter
since the
> bladeRF I will be using has a 61.44 MHz sampling rate and I will be
> operating in the GHz range.
>
> What I will probably end up having to do is do a pre-run calibration
> where the tone is not playing, use a complex to mag^2 and average
power
> combination, set that as a constant block, and then subtract the
> calibrated constant from the total power when the tone is on to
get the
> most accurate possible signal power. While not ideal because it
is not a
> truly live SNR calculation, it is the best workaround that avoids
the
> filter I can think of.
>
> If there are any other suggestions to get a more live/real time SNR
> calculation I am open to that as well.
>
> Thank you for the help,
>
> Alex
>
>
>
> On Fri, Jun 26, 2020 at 4:17 AM Johannes Demel
<demel@ant.uni-bremen.de <mailto:demel@ant.uni-bremen.de>
> <mailto:demel@ant.uni-bremen.de
<mailto:demel@ant.uni-bremen.de>>> wrote:
>
> Hi Alex,
>
> "0 < fa <= sampling_rate/2" is correct and should always be
> enforced. If
> you try to set your filter cut-off frequency at >=
samp_rate/2, you'll
> experience aliasing.
>
> After reading your mails, I get the impression you try to set
your
> filter cut-off frequency at your carrier frequency $f_c$ +
bandwidth/2
> $B/2$. Or something in that range. That won't work in baseband.
> Effectively, your signal at $f_c$ goes through a mixer and is
shifted
> such that it would appear at $0$ in your baseband signal.
>
> There's a lot of digital signal processing fundamentals
involved. I
> like
> the explanations given in [0]. Though, of course there are
well known
> books such as the ones by Proakis or Sklar on the topic.
>
> Cheers
> Johannes
>
> [0] https://dspillustrations.com/pages/index.html
>
> On 25.06.20 22:22, Alex Batts wrote:
> > The effective noise bandwidth is part of the calculation. I'm
> using the
> > radar range equation.
> >
> > My purpose for including the bandwidth in my response was that
> any time
> > I try to use a filter with a frequency greater than my
sampling
> rate/2 I
> > get an error returned. I agree that ideally I would use a
band-pass
> > filter with very narrow cutoffs to best capture the signal
in its
> > entirety, however, I can't because the frequency I'm trying to
> set my
> > filter at is more than half my sampling rate, giving me an
error.
> Maybe
> > there is something askew with that error and it's
something else,
> but it
> > returns saying 0 < fa <= sampling_rate/2
> >
> > On Thu, Jun 25, 2020 at 3:27 PM Marcus Müller
<mueller@kit.edu <mailto:mueller@kit.edu>
> <mailto:mueller@kit.edu <mailto:mueller@kit.edu>>
> > <mailto:mueller@kit.edu <mailto:mueller@kit.edu>
<mailto:mueller@kit.edu <mailto:mueller@kit.edu>>>> wrote:
> >
> > Hi Alex,
> >
> > On 25/06/2020 21.00, Alex Batts wrote:
> > > I'm sampling an incoming signal, but only around 2
MSps.
> > >
> >
> > and that's fine! that's the *equivalent* baseband, it
has all
> the same
> > information as the RF signal.
> >
> > > I need the signal power to noise power ratio at the
> receiver as
> > part of
> > > my range calculation.
> >
> > Yes, but you'd always want to do that "signal to
noise" only
> in the
> > bandwidth that actually contains your tone; the rest will
> just contain
> > more noise, interferers... to make your measurement worse.
> >
> > > So I would need to be able to distinguish between
> > > the power of the tone vs the power of the surrounding
> noise and use
> > > those two numerical values in an equation to
calculate the
> range.
> >
> > You need to define "surrounding"! Your signal doesn't get
> worse by
> > applying a filter that only selects your tone and as
little
> else as
> > possible. So you should do that – it makes your SNR
better.
> Hence, your
> > Signal power estimate gets more reliable (which you
> definitely want).
> >
> > (that also highlights why I have a bit of doubt on your
> measurement
> > methodology – if your SNR depends on receiver
bandwidth, then
> how much
> > does it actually tell you about the range, unless you
specify the
> > bandwidth alongside with it?)
> >
> > Think about it: we typically assume noise to be white,
i.e.
> to have
> > identical power spectral density all over the
spectrum, e.g. -170
> > dBm/Hz.
> >
> > Now, if your receiver bandwidth is set to 2 MHz (because
> that's what
> > your SDR is probably configured to filter out if you
ask for
> 2 MS/s),
> > then you get twice as much noise power than if you set the
> sampling
> > rate
> > to 1 MS/s.
> >
> > It's the same thing that I always let students figure
out by
> themselves
> > the first time they use the lab spectrum analyzer:
> > Feed a 2 GHz -60 dBm tone into the spectrum analyzer.
> > Set the resolution bandwidth of the spectrum analyzer to 1
> MHz, and
> > tell
> > me what the SNR is. Now set the resolution bandwidth
to 300
> kHz and
> > tell
> > me again.
> > You get as much "N" in your SNR as you let through your
> system. In the
> > case of the spectrum analyzer, every point on the
display is
> the power
> > in 1 MHz (or 300 kHz) of filter. In the case of your
Qt plot,
> it's the
> > power in a FFT bin. There's (f_sample)/(FFT length)
bandwidth
> to each
> > bin; so your graphical analysis hinges on the setting of
> sample rate
> > and
> > FFT length (also, on window choice and labeling, and
software
> > convention). Proportionally!
> >
> > It's really hard to define "SNR" for 0-bandwidth, i.e. a
> single tone
> > (having a single tone, actually, gets tricky
physically, but
> there's a
> > lot of people who could tell you more about
oscillators than
> I could).
> >
> > If you'd be fair, the only choice for the noise filter
> bandwidth would
> > be 0 Hz, because if you chose any wider, you always
get more
> noise. But
> > in 0 Hz, there's actually 0 noise power! So, that
doesn't work.
> >
> > Instead, you need to define SNR exactly on the
bandwidth your
> detection
> > system will have to use. That's a design parameter you
haven't
> > mentioned
> > so far!
> >
> > > This
> > > is why I referenced the green and red lines on the
qt gui
> freq.
> > display,
> > > this would seem to give me signal strength in dB.
> >
> > Hopefully, above explained how much these lines depend
on your
> > configuration and aren't "SNR".
> >
> > Cheers,
> > Marcus
> >
>