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I have a question regarding the concept of carrier power, output power and power relative to carrier (dBc). Suppose I have a carrier frequency of 4Hz and signal bandwidth = 4 Hz. The signal span is from [2Hz 6Hz].
My indexing starts at 0 for simplicity.

  1. I have 7 samples with sampling frequency 7 Hz such that:

    x(n)=[1+1j 2-2j 1+3j 3+j 4 5 2+2j]; % Time domain signal

Representing the signal into frequency domain by taking the FFT with 7 points (each frequency bin is 1 Hz range 0-6 Hz) and normalizing for power,

Nfft=7;

X[n]=fft(x(n),Nfft)/Nfft;  

Therefore,
X[n]=[2.6+0.7j -0.56+0.54j -0.99-0.52j -0.5+0.07j 0.38+0.69j 0.7+0.2j -0.63-0.7j].
The output power in dBm within the Bandwidth from 2Hz to 6Hz (for simplicity we are considering up to only 4 samples excluding 6Hz sample) is:

OutPow=10*log10(sum(abs(X[:,2:5]).^2))+30; % in dBm

Therefore, the signal power is 34.34 dBm. Note: I don't think this is the average power.


I would like to generate a noise with -20 dBc. Here are two methods to generate noise power with -20dBc in frequency domain.

Method 1 (the idea is Wanted Signal Power (BW 4 Hz) -Noise at the carrier = 20 dBc):
SigPow=34.34 ; %dBm
TargetNoisePower=(34.34-20)=5.66; % Its down to 20 dBc
NoiseCarrierMagnitude=sqrt(1/1000)*10^(5.66/20)=0.0607; % Noise amplitude for carrier
Noise(n)=[0 0 0 0 0.0607 0 0];  % A single sample at frequency domain

Method 2 (The idea is Sample Power at carrier ($f_c=4$ Hz) -Noise at the carrier =20 dBc):

FcPow=27.93 ; %dBm At 4Hz the sample is 0.38+0.69j
TargetNoisePower=(27.93-20)=7.93;  % at the carrier
NoiseCarrierMagnitude=sqrt(1/1000)*10^(7.93/20)=0.079; % Noise amplitude for carrier
Noise(n)=[0 0 0 0 0.079 0 0];  % A single sample at frequency domain

My questions are the following:

  1. Which method is right for generating a noise signal in dBc?
  2. The channel might not be occupied with signal at the carrier frequency. For example within the signal BW the exact carrier frequency ($f_c=4$ Hz) sample might be 0 or have only some noise (AWGN) sample.
  3. When we say signal power within the bandwidth, do we mean the total power within the bandwidth?
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  • $\begingroup$ your signal doesn't seem to have a carrier? $\endgroup$ Aug 25 at 8:00
  • $\begingroup$ Isn't there a problem with Nyquist as well here? Sampling rate should be greater than 2*BW $\endgroup$
    – Jdip
    Aug 25 at 11:29
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    $\begingroup$ @Jdip nope, if we're doing it like this, it's basically direct synth from the DFT; we're in complex baseband! $\endgroup$ Aug 25 at 12:22
  • $\begingroup$ @MarcusMüller yup, thanks! $\endgroup$
    – Jdip
    Aug 25 at 14:55

1 Answer 1

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Your questions:

  1. You might have a carrier frequency, but you don't seem to have a carrier in your signal, so dBc is not a definable concept. You can only generate a noise with a given power in dB of carrier power if there's a carrier.
  2. Not a question!
  3. Yes

Remarks:

  • Your OutPow is not a power, the way you calculate it, but an energy, it's thus also not in dB of milliwatt.
  • I think what you're really just after is an SNR in dB. That's just the ratio of average signal power to average noise power.
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  • $\begingroup$ Muller thank you for an explanation. $\endgroup$ Aug 25 at 19:49
  • $\begingroup$ However, I think you might be wrong in number 1 because power in frequency domain sum(X(k).^2)/Ndfft^2, where X(k) is the fft sample. If possible please clarify my doubt. (2) Sorry for not articulating my question in a proper way, but "You can only generate a noise with a given power in dB of carrier power if there's a carrier." what do you mean by if there is a carrier. Suppose for some signal set "B" Fc-is the center frequency which might interfere with an existing signal. The power defference between P_Sig-P_B=X; And X is in dBc. Am I missing somethin. $\endgroup$ Aug 25 at 20:01
  • $\begingroup$ Just because you're centered around some center requency doesn't mean you transmit a carrier. But stating something in dBc requires that there is a carrier. There is really nothing to clarify here: without a carrier you can't state something as relative to the power of the carrier. $\endgroup$ Aug 25 at 22:19
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    $\begingroup$ As I already explained in my answer what you describe is not power relative to a carrier, but simply SNR. $\endgroup$ Aug 25 at 22:20

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