Carrier-to-Noise Versus Signal-to-Noise
- Carrier-to-noise ratio — C/N ratio or CNR
- Signal-to-noise ratio — S/N ratio or SNR
- In the world of telecommunications, "SNR" and "CNR" are often used interchangeably
- For this discussion, the 'noise' in CNR and SNR is assumed to be additive white Gaussian noise (AWGN).
- In the world of cable, we generally use CNR and SNR to represent quite different measurement parameters: one in the RF domain and the other in the baseband domain.
- Technically speaking, when measuring CNR or SNR against real thermal noise, one actually is measuring (C+N)/N or (S+N)/N. Not normally an issue unless carrier-to-noise or signal-to-noise ratios are very low—say, single digit values.
Carrier-to-Noise Ratio
- CNR is generally accepted to be a pre-detection measurement, that is, one made at RF.
- From the perspective of analog TV channels, CNR is the difference, in decibels, between the amplitude of a TV channel's visual carrier and the rms amplitude of system noise in a specified bandwidth.
- According to the FCC's cable rules in §76.609(e), system noise is the "total noise power present over a 4 MHz band centered within the cable television channel."
- Good engineering practice targets end-of-line analog TV channel CNR in the 45 to 49 dB range.
Signal-to-Noise Ratio
- In cable industry vernacular, SNR is generally accepted to be a pre-modulation or post-detection measurement, that is, one made on a baseband signal such as video or audio.
- Baseband video SNR is the ratio of the peak-to-peak video signal, excluding sync, to the noise within that video signal.
CNR vs. SNR Recap
- CNR is a pre-detection measurement performed on RF signals. Raw carrier power to raw noise power in the RF transport path only. Ideal for characterizing network impairments.
- SNR is a pre-modulation or post-detection measurement performed on baseband signals. Includes noise in original signal, transmitter or modulator, transport path, and receiver & demodulator. Ideal for characterizing end-to-end performance.
Digitally Modulated Carrier CNR
- DOCSIS specifies a minimum 35 dB CNR for downstream digitally modulated carriers, and 25 dB for upstream digitally modulated carriers.
- Carrier amplitude is the digitally modulated carrier's average power level.
- The correct noise power bandwidth is one that is equivalent to the digitally modulated carrier's symbol rate.
CNR Noise Power Bandwidth — Downstream
| Channel Bandwidth | Symbol Rate | Modulation Format | Noise Power Bandwidth |
|---|---|---|---|
| 6 MHz | 5.056941 Msym/sec | 64-QAM | 5.06 MHz |
| 6 MHz | 5.360537 Msym/sec | 256-QAM | 5.36 MHz |
| 8 MHz | 6.952 Msym/sec | 64 or 256 QAM | 6.95 MHz |
Digitally Modulated Carrier CNR vs. BER
| Modulation Format | 1.0E-04 BER | 1.0E-06 BER | 1.0E-08 BER | 1.0E-10 BER | 1.0E-12 BER |
|---|---|---|---|---|---|
| ASK & FSK | 7 dB | 9 dB | 10 dB | 11 dB | 12 dB |
| BPSK | 9 dB | 11 dB | 12 dB | 13 dB | 14 dB |
| QPSK | 12 dB | 14 dB | 15 dB | 16 dB | 17 dB |
| 16-QAM | 19 dB | 21 dB | 22 dB | 23 dB | 24 dB |
| 32-QAM | 21 dB | 23 dB | 24 dB | 25 dB | 26 dB |
| 64-QAM | 25 dB | 27 dB | 28 dB | 29 dB | 30 dB |
| 256-QAM | 32 dB | 34 dB | 35 dB | 36 dB | 37 dB |
Modulation Error Ratio (MER)
Why Measure MER?
- Somewhat analogous to in-channel signal-to-noise ratio ("MER" and "SNR" are often used interchangeably)
- Direct measure of modulation quality
- Direct linkage to BER
- Can be used in conjunction with adjacent channel power to estimate linear distortions
- Good metric for end-to-end health of a network, but provides little insight about the type of impairment
MER = 10log(average symbol power / average error power)
- Minimum recommended downstream MER (includes 3 to 4 dB headroom for reliable operation): 64-QAM: 27 dB | 256-QAM: 31 dB
- Typical headend and node downstream MER should be 34 to 36 dB or greater
Error Vector Magnitude (EVM)
EVM: The magnitude of the vector drawn between the ideal (reference) state position and the measured state position.
EVM = (RMS error magnitude / maximum symbol magnitude) × 100%
By convention, EVM is reported as a percentage of peak signal level, usually defined by the constellation's corner states.
Why Measure EVM?
- Many engineers are familiar with EVM
- More comfort with a linear measurement
- Links directly with the constellation display
- Linear relationship between EVM and constellation "cloud" size
- No direct link between EVM and BER
Summary
- CNR is a measure of pre-detection signal quality; ideal for characterizing performance of the cable network
- SNR, MER and EVM are measures of post-detection signal quality
- SNR is a useful metric for quantifying a baseband signal — e.g., the video quality seen on a TV set
- SNR, MER and EVM — along with bit error rate — are ideal for characterizing the health of a digital signal