|Subject:||Re: [Discuss-gnuradio] Measure the power using USRP|
|Date:||Tue, 8 Dec 2015 12:17:03 +0100|
|User-agent:||Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0|
To emphasize on what the other Marcus said:|
GNU Radio only knows numbers, not physical units. The USRP delivers numbers, relative to full ADC range, to the computer. What these numbers represent depends on a lot of factors, most of which are in the analog domain and different for every device, every application etc.
Now, the USRPs and their daughterboards are pretty linear for a very large range of powers , so you can measure once with a signal source of known power (a standard), and derive a factor between "digital number power" and "physical receiver power" for a given frequency/bandwidth/gain/sample rate/antenna/... configuration, as long as the powers you observe don't leave the linear range (again, ).
Problem is: OFDM is notorious for having a large dynamic range due to its potentially high PAPR. Usually, you configure your receiver gain so that it gets a high signal power out of OFDM symbols that have a modest PAPR, because it's not as bad to lose a bit of precision with a symbol that has an extreme PAPR as to constantly have bad SNR. Now, for exactly these high-PAPR symbols, your receiver gets pushed into nonlinearity, and then your power estimate will be a little too low (if you're not really tricky about correcting that, but that would be very close to equalization based on data knowledge, and has little to do with receive strength).
So: get yourself a signal source of known power, connect it to your USRP, state very clearly what you use as a reference (makes a difference whether you incorporate antenna efficiency or not, for example), and run a quick calibration, then you'll have a factor between digital power and physical power. Be aware that for high digital powers, your physical power estimate is less exact.
 compare your daughterboard's data from http://files.ettus.com/performance_data/
On 08.12.2015 04:38, Marcus D. Leech wrote:
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