SNR vs CNR-C/N ratio(CNR) versus S/N ratio(SNR)

This page describes on SNR vs CNR describes signal to Noise Ratio(S/N ratio) and Carrier to Noise Ratio(C/N ratio).

C/N Ratio (CNR) stands for Carrier to Noise Ratio. It is measured after modulation.

S/N Ratio (SNR) stands for Signal to Noise Ratio. It is measured before modulation.

Carrier to Noise Ratio versus Signal to Noise Ratio

S/N = Psignal/Pnoise = [Asig(rms)/Anoise(rms)]2

(S/N)dB= 10 log(Psignal/Pnoise) Both CNR and SNR are expressed in Decibel (dB).

Relation between C/N Ratio (CNR)and Eb/N0 ratio

Eb/N0 = ((C/Fb )/ (N/BW))
Where Fb is bit rate, C is the total carrier power,
N is the total noise power, BW is the bandwidth of the modulated signal.

Eb/N0 and S/N ratio required for complex digitally modulated signal for WiMAX system is as follows.

Modulation/Code rate Eb/No(dB) S/N ratio(dB)
QPSK-1/2 10.5 9.4
QPSK-3/4 10.5 11.2
16QAM-1/2 14.5 16.4
16QAM-3/4 14.5 18.2
64QAM-2/3 19 22.7
64QAM-3/4 19 24.4

SNR vs CNR is very useful for RF field engineers to analyze complex power spectrum using spectrum analyzer and readings obtained in the satellite modem used for VSAT.

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