Signal-to-interference ratio

The signal-to-interference ratio, also known as the carrier-to-interference ratio, is the quotient between the average received modulated carrier power S or C and the average received co-channel interference power I, i.e. crosstalk, from other transmitters than the useful signal.
The CIR resembles the carrier-to-noise ratio, which is the signal-to-noise ratio of a modulated signal before demodulation. A distinction is that interfering radio transmitters contributing to I may be controlled by radio resource management, while N involves noise power from other sources, typically additive white Gaussian noise.

Carrier-to-noise-and-interference ratio (CNIR)

The CIR ratio is studied in interference limited systems, i.e. where I dominates over N, typically in cellular radio systems and broadcasting systems where frequency channels are reused in view to achieve high level of area coverage. The C/N is studied in noise limited systems. If both situations can occur, the carrier-to-noise-and-interference ratio may be studied.