Signal-to-interference ratio
Quotient of carrier power to co-channel interference power.
The signal-to-interference ratio (SIR or S/I), also called the carrier-to-interference ratio (CIR or C/I), compares the average power of a received modulated carrier signal (S or C) to the average power of unwanted co-channel interference (I) from other transmitters. This is similar to the carrier-to-noise ratio (CNR or C/N), which measures the signal-to-noise ratio before demodulation, but the key difference is that interference (I) can be managed through radio resource allocation, whereas noise (N) typically comes from sources like additive white Gaussian noise (AWGN) that are not controllable. When interference dominates over noise—common in cellular and broadcasting systems that reuse frequency channels to maximize coverage—the CIR is the focus. In noise-limited systems, the C/N is studied instead. For cases where both interference and noise are significant, the carrier-to-noise-and-interference ratio (CNIR or C/(N+I)) may be examined.
- Also known as
- carrier-to-interference ratio (CIR or C/I)
- Compared to
- carrier-to-noise ratio (CNR or C/N)
- Studied in
- interference-limited systems
- Typical applications
- cellular radio systems, broadcasting systems
Lore & Background
The signal-to-interference ratio is a key metric in systems where interference from other transmitters dominates over noise. In such interference-limited environments, the ratio is used to assess and manage co-channel interference, which is a form of crosstalk. The CIR is distinguished from the carrier-to-noise ratio (CNR or C/N), which involves noise power from sources such as additive white Gaussian noise (AWGN). While C/N is studied in noise-limited systems, CIR is relevant where radio resource management can control interfering transmitters. When both interference and noise are significant, the carrier-to-noise-and-interference ratio (CNIR or C/(N+I)) may be studied.
Reader's Guide
The signal-to-interference ratio is significant because it directly impacts the performance of frequency-reuse systems, such as cellular networks and broadcast services, where maximizing area coverage requires balancing signal strength against interference from co-channel users. By quantifying the ratio of desired carrier power to interfering power, it enables engineers to design and optimize frequency reuse patterns and power control schemes. Its legacy lies in its role as a fundamental parameter in interference-limited system design, distinct from noise-limited scenarios. The ratio is a practical tool for radio resource management, as interfering transmitters can be controlled, unlike noise sources. Understanding CIR is essential for evaluating system capacity and quality of service in dense wireless environments.
Did You Know?
- It is the quotient of average received modulated carrier power and average received co-channel interference power.
- CIR is studied in interference-limited systems, while carrier-to-noise ratio is studied in noise-limited systems.
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