Although additive noise is independent of the useful signal, it always exists and interferes with the useful signal, thus inevitably causing harm to communications. The sources of additive noise in the channel can generally be divided into three aspects: 1. Man-made noise. Man-made noise comes from other unrelated signal sources, such as: external station signals, switch contact noise, industrial ignition radiation, etc.; 2. Natural noise. Natural noise is Refers to various electromagnetic wave sources that exist in nature, such as lightning, lightning strikes, electrical storms in the atmosphere and various universes.