Simulink: How to set parameters of AWGN Channel Block ?

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ruozhu li
ruozhu li 2015-3-19
编辑： Greg Dionne 2015-3-23
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

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Greg Dionne
Greg Dionne 2015-3-20
编辑：Greg Dionne 2015-3-20
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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ruozhu li
ruozhu li 2015-3-23
Hi Greg,
Thanks for your answer, but unfortunately, I'm still confused. Let me declare my question again in this way.
1. I set the SNR of AWGN Channel at 20 dBw, so I think the ACPR(here is log10(power of noise in BW/power of signal and noise in BW)) should approximate -SNR when the channel power is around 30 dBm which is 1 watts.
2. When the ACPR is about -20dBc, the Channel Power is around 22.2 dBm, which is much less than 30 dBm. And the channel power before AWGN channel is 22.3 dBm.
My question is,
Why when ACPR is -20 dBc, the Channel Power is not 30 dBm referenced to 1 Ohm?
Thank you!
Best, Ruozhu
Greg Dionne
Greg Dionne 2015-3-23
编辑：Greg Dionne 2015-3-23
Hi Ruozhu,
As far as I can tell, the AWGN block doesn't actually compute the input power when in the SNR mode. It just adds a noise power that is offset from the "Input signal power". If the input signal power doesn't match your actual input power, then you'll be off by whatever difference in power is there.
Maybe an exercise would help us understand what is going on.
1. Drop in a DSP Sinewave source block. Make it have an amplitude of 1, but have the same sample rate as your original signal, additionally set its output complexity to "complex".
2. In the spectrum analyzer see if you can read back a value of 30 dBm (use the "Power" spectrum type in the Spectrum Settings dialog).
3. Now put in the AWGN block. Set it for, say, 1 Watt input power and an SNR value of 20.
4. Go to the spectrum analyzer and view the spectrum. Is it what you expect?
5. Now change the amplitude of the sinusoid to 0.1. You should see the sinusoid drop by 20 dB to 10dBm, but see the same noise power as before.
6. Look at the "Input power" control. Note that it still reads 1 not 0.1^2. Change it to 0.01. Now the SNR is "correct" in the scope.
The key point is that the SNR mode doesn't actually know anything about the actual input power of the input signal. It really is just a relative power offset from the reference input power level you typed in, not the actual input power level to the block.
Hope that helps.
-Greg

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