MATLAB: Simulink: How to set parameters of AWGN Channel Block

simulinksnr

Hi,
I'm using Simulink to simulate an OFDM communication system with AWGN Channel, and have some problems of setting parameters.
How does Spectrum Analyzer calculate the Channel Powers ? Is it based on 1 Ohm or 50 Ohm?
In my case, I use ACPR of Spectrum Analyzer to measure the SNR of signal after channel. The SNR and Input signal power, referenced to 1 ohm (watts) of AWGN Channel Block are 20 dB and 1 watts respectively. When the power of ACPR is around 20 dB , which is equal to the SNR we set in AWGN Channel Block, the Channel Powers showed in Spectrum Analyzer is around -20 dB, which I expect to be around 30 dB for referencing to 1 watts should result in -30 dB.
Where did I do wrong? What's the relationship between Input signal power, referenced to 1 ohm (watts) of AWGN Channel Block and Channel Powers showed in Spectrum Analyzer?
Best, Ruozhu

Best Answer

Hi Ruozhu,
You can adjust the reference impedance of the Spectrum Analyzer by going to the "Spectrum Settings" dialog, under the "trace options" panel, where you'll find the reference load.
The noise floor you have (as best as I can estimate from your plot) is -72 dBm/Hz. This is consistent with what you would expect from -20 dBW of total noise (or 10 dBm of noise) across your entire spectrum (~130 MHz). -72 + 10*log10(130e6) = 9.14 dBm = -20.86 dBW
My hunch is that your input signal at the input to the AWGN channel is actually at a power level of 20 dBm (-10dBW), not 30 dBm (or 0 dBW) as you were expecting. The fact that the drop in noise density between your original signal and the adjacent channel is 20 dB seems a coincidence.
If you remove the AWGN channel, do you still observe the same integrated power of 20dBm?
HTH
-Greg
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