Re: [Discuss-gnuradio] usrp_fft.py axis values and units

[Martin Braun]

On Thu, Jul 15, 2010 at 10:51:58AM +0200, address@hidden wrote:
> Thanks a lot for the reply! But I have some follow-up questions:
> About the statement above: "There are other methods for estimating
> SNR if you indeed want to get that calculation.":
> 
>           What I need to measure is the following: I need the
> Transmit Power of the USRP (Pt) and the Noise Power at the Input of
> the USRP (Pn), and if possible also the Received-Signal Power (Pr).
> These values would then enable me to calculate SNR, distance between
> Sender and Receiver (of course with an appropriate Path-Loss Model),
> etc. Those values will then be used for some other calculations and
> Measurements.
> 
> Of course it would be great if the measuring of Pn, Pt, Pr would be
> possible using the USRP itself (e.g. using the usrp_oscilloscope or
> usrp_fft), but if this is really that bad I'd be happy to get some
> other suggestions. Any Ideas?

Hi Björn,

this gets asked so often on this mailing list--you should have another
browse through the archives. And like everyone else, I will tell you
it's not that easy; measuring real powers means tedious calibration, and
even then, it will only work reliably once, and for one specific setup.

Even if you had Pt and Pr, calculating SNR still depends on many things.
E.g., the typically given equation for a BER for BPSK modulation in AWGN
depending on SNR assumes a very specific SNR value (i.e., the noise
distributed over the bandwidth related to the pulse shaping filter).

For digital signal processing purposes, you very rarely actually need
true powers (although of course I don't know what you're up to), but
there's usually a way to _estimate_ SNR (for a given definition of SNR)
for a specific application.

Cheers
MB

-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

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Building 05.01
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Phone: +49 721 608-3790
Fax: +49 721 608-6071
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KIT -- University of the State of Baden-Württemberg and
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