Noise Immunity

the ability of an apparatus or system to perform its functions when interference (noise) is present. Noise immunity is rated according to the noise intensity at which the disruption of the equipment’s functions is still within permissible limits. The higher the level of noise at which the equipment maintains its operating ability, the higher its noise immunity.

The diversity of equipment and of the tasks it handles, on the one hand, and the variety of types of noise, on the other, require a specialized approach to each particular case. The noise-immunity requirements of various types of equipment differ greatly: it is sometimes considered acceptable for individual stations in a radar system to miss up to half the objects under observation in one scan of the area being monitored, but in the data-transmission systems used for electronic digital computers, the loss of even one character out of an extremely large number (for example, of the order of 10⁹) is intolerable.

The concept of noise immunity is most frequently used in characterizing data-transmission equipment, such as communications lines, or for observation equipment, such as radar stations. In most such cases it is possible to establish the concept of a “signal” and thus to evaluate noise immunity by finding the signal-to-noise ratio that provides a specified quality of operation. For radar, this would be the signal-to-noise ratio that ensures a given level of reliability of detection (the probability of correct detection with a given probability of a false alarm).

If the statistical characteristics of signals and noises are known, the maximum attainable noise immunity, called the potential noise immunity, may be determined theoretically. The production of “optimum” equipment, which would provide such noise immunity, is usually too complicated, and the unavoidable technical imperfections of the equipment would prevent full achievement of the potential noise immunity. Therefore, the simplest equipment that closely approximates the optimum is usually considered satisfactory. When additive noises are affecting data-transmission systems, the noise immunity can be improved by increasing the power of the transmitted signals. In the case of multiplicative (nonadditive) noises in communications lines or passive noises in radar, it is impossible to improve the noise immunity substantially by increasing the signal power, and a radical change in methods is required, such as the use of noise-proof coding or adaptive reception.