In telecommunications, the CNR of a modulated signal refers to the signal-to-noise ratio of a signal after modulation. This ratio is measured in decibels (dB) and is usually expressed in terms of the ratio of the signal power to the noise power. The term is used to distinguish the CNR of the signal before modulation (e.g., the analog baseband message signal) from the CNR of the modulation-processed signal. The term SNR is sometimes used instead of CNR, with the same definition.
Digitally modulated signals (e.g. QAM or PSK) are basically made of two CW carriers (the I and Q components, which are out-of-phase carriers). In fact, the information (bits or symbols) is carried by given combinations of phase and/or amplitude of the I and Q components. It is for this reason that, in the context of digital modulations, digitally modulated signals are usually referred to as carriers. Therefore, the term carrier-to-noise-ratio (CNR), instead of signal-to-noise-ratio (SNR), is preferred to express the signal quality when the signal has been digitally modulated.
High C/N ratios provide good quality of reception, for example, low bit error rate (BER) of a digital message signal, or high SNR of an analog message signal.
The C/N ratio is measured in a manner similar to the way the signal-to-noise ratio (S/N) is measured, and both specifications give an indication of the quality of a communications channel.
In the famous Shannon–Hartley theorem, the C/N ratio is equivalent to the S/N ratio. The C/N ratio resembles the carrier-to-interference ratio (C/I, CIR), and the carrier-to-noise-and-interference ratio, C/(N+I) or CNIR.
C/N estimators are needed to optimize receiver performance. Typically, it is easier to measure the total power than the ratio of signal power to noise power (or noise power spectral density), and that is why CNR estimation techniques are timely and important.