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What Are Noise and Interference?

Preview: Learn more about noise and interference.

Noise and interference are two of the principal factors that limit the performance of communications systems. Both impair the quality of received signals and can lead to errors in the recovered information, yet they arise from different sources and often require different engineering solutions. Understanding the distinction between noise and interference is fundamental to the design of reliable communications systems.

Noise refers to unwanted random electrical energy that is generated naturally within electronic devices or by physical processes in the surrounding environment. Unlike the desired communication signal, noise carries no useful information and cannot usually be predicted or eliminated completely. Every practical communications system contains some level of noise, making it one of the unavoidable limitations of communication.

One of the most important forms of noise is thermal noise, sometimes called Johnson-Nyquist noise. It is produced by the random motion of electrons within electrical conductors and electronic components. Because this motion is caused by temperature, thermal noise is present whenever electronic equipment operates above absolute zero. It establishes a fundamental limit on the sensitivity of radio receivers, satellite Earth stations, mobile phones, and countless other communications systems.

Other forms of noise also affect communications. The Earth's atmosphere produces natural radio noise originating from lightning discharges, while the Sun and other astronomical objects generate cosmic radio noise that is particularly important in satellite and radio astronomy applications. Electronic devices such as motors, ignition systems, switching power supplies, and industrial equipment may also generate electrical noise that degrades nearby communications systems.

Interference, by contrast, originates from other transmissions rather than from random physical processes. Unlike noise, interference is often structured and may itself carry useful information intended for another receiver. Interference occurs whenever two or more communication systems attempt to use the same or nearby frequencies, causing unwanted signals to be received along with the desired transmission.

Many familiar examples of interference occur in everyday life. Two nearby radio transmitters operating on similar frequencies may interfere with one another, while neighbouring Wi-Fi networks can compete for access to the same radio channels. Mobile telephone systems, satellite communications, radar installations, and microwave links must all be carefully planned to minimise mutual interference and ensure efficient use of the available radio spectrum.

Engineers employ a variety of techniques to reduce the effects of both noise and interference. Increasing transmitter power improves the strength of the desired signal relative to background noise, while low-noise amplifiers and sensitive receivers minimise internally generated noise. Directional antennas help reject unwanted signals arriving from other directions, and carefully designed filters suppress interference from adjacent frequency channels. Modern communication systems also employ advanced modulation, channel coding, spread-spectrum techniques, adaptive equalization, and digital signal processing to improve performance under noisy or interference-prone conditions.

One of the most important measures of communication quality is the signal-to-noise ratio (SNR), which compares the strength of the desired signal with the background noise. Higher signal-to-noise ratios generally result in fewer transmission errors and improved communication quality. When interference is significant, engineers may instead consider the signal-to-interference ratio (SIR) or the combined signal-to-interference-plus-noise ratio (SINR), particularly in cellular and wireless communication systems.

The effects of noise and interference depend upon both the communication technology and the application. Analogue systems often experience a gradual reduction in quality as noise increases, producing effects such as audible hiss or visible "snow" on television pictures. Digital systems behave differently. Error-control coding and digital signal processing may maintain virtually perfect performance until conditions deteriorate beyond a critical threshold, after which communication quality can decline rapidly as the receiver is no longer able to recover the transmitted information reliably.

Today, managing noise and interference is one of the central challenges of communications engineering. Whether designing a satellite link spanning thousands of kilometres, a mobile telephone network serving millions of users, or a short-range Wi-Fi connection within a home, engineers must carefully account for both phenomena to ensure reliable operation. As demand for wireless communication continues to grow and the radio spectrum becomes increasingly crowded, controlling interference has become just as important as reducing noise.

Noise and interference therefore represent far more than unwanted disturbances. They define many of the practical limits of communications systems and have driven the development of sophisticated receivers, modulation techniques, error-control coding, and signal-processing algorithms. Much of modern communications engineering can be viewed as the continual effort to overcome the effects of noise and interference while making ever more efficient use of the world's limited communications resources.