Re: [Discuss-gnuradio] USRP output power

[Marcus D. Leech]

On 25/04/2011 11:42 AM, Nick Foster wrote:
> The USRP does not have a calibrated output or input and you will have to
> calibrate it using a measuring receiver, calibrated signal generator, or
> power meter in order to obtain meaningful results.
>
> --n
I don't think precise values are really required here.  The typical 
Eb/N0 estimates are just that, *estimates*.

Let's assume that those bits leave the transmitter at "full-power" 
(+20dBm), and radiate isotropically through free-space,
  until they get to the receiver.  At the receiver, the signal is 
received against a locally-generated noise-background of some
  magnitude.  I think the noise figure of the RFX2400 is in the 
neighbourhood of 5-6dB at maximum gain.  A noise figure of 3dB
  is equivalent to a noise power of -174dBm/Hz of bandwidth, so let's 
call the noise power at the receiver -170dBm/Hz to
  make the math easier.

Now, what is the equivalent bandwidth of each bit?  Let's say it's 10KHz 
for argument's sake.  That means that the receiver
  noise power over the bandwidth of a bit is -170dBm/Hz + 40dBHz = 
-130dBm of noise power at the receiver over the bandwidth
  occupied by our theoretical 10KHz bit.

The signal itself leaves the transmitter, and suffers free-space path 
loss in a roughly inverse-cube law.  Over a 1M path, that
  signal suffers (assuming a perfectly-isotropic transmit antenna at 
2.4GHz) a path loss of about 40dB, which brings the received signal
  down to about -20dBm.  So that -20dBm is set "against" a receiver 
noise over the same bandwidth of -130dBm.  The calculation of
  the linear-units EB/N0 is left as an exercise for the reader.