|This article does not cite any references or sources. (June 2007)|
The signal-to-interference ratio (S/I or SIR), also known as the carrier-to-interference ratio (C/I, CIR), is the quotient between the average received modulated carrier power S or C and the average received co-channel interference power I, i.e. cross-talk, from other transmitters than the useful signal.
The CIR resembles the carrier-to-noise ratio (CNR or C/N), which is the signal-to-noise ratio (SNR or S/N) 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 (AWGN).
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, C/(N+I) or CNIR may be studied.
See also 
- Crosstalk (electronics)
- Co-channel interference (CCI)
- signal-to-noise ratio (SNR or S/N)
- carrier-to-noise ratio (CNR or C/N)
- SINAD (ratio of signal-plus-noise-plus-distortion to noise-plus-distortion)
- Carrier-to-receiver noise density C/N0