Noise is any undesired signal in a communication circuit. Another definition calls noise unwanted disturbances superimposed on a useful signal, which tends to obscure its information content. There are many varieties of noise; however, the four most important to the telecommunication/data communication technologist are thermal noise, intermodulation noise, crosstalk and impulse noise.
Thermal noise occurs in all transmission media and communication equipment, including passive devices. It arises from random electron motion and is characterized by a uniform distribution of energy over the frequency spectrum with a Gaussian distribution of levels. Every equipment element and the transmission medium itself contribute thermal noise to a communication system if the temperature of that element or medium is above absolute zero. Whenever molecules heat above absolute zero, thermal noise will be present. The more heat generated or applied, the greater the level of thermal noise.
Intermodulation (IM) noise is the result of the presence of intermodulation products. If two signals of frequencies F1 and F2 are passed through a nonlinear device or medium, the result will contain IM products that are spurious frequency energy components. These components may be inside or outside the frequency band of interest for a particular device. IM products may be produced from harmonics of the desired signals in question, either as products between the harmonics or between a harmonic of one of the signals and the other basic signal or between both signals themselves. The products result when two (or more) signals beat together or "mix."
Crosstalk refers to unwanted coupling between signal paths. There are essentially three causes of crosstalk: (1) electrical coupling between transmission media, such as between wire pairs on a voice-frequency (VF) cable, (2) poor control of frequency response (i.e., defective filters or poor filter design) and (3) nonlinear performance in analog (FDM) multiplex systems. Excessive level may exacerbate crosstalk. Analog transmission is distorted by crosstalk and it will deteriorate the BER performance of a digital path.
Impulse noise is a noncontinuous series of irregular pulses or noise "spikes" of short duration, broad spectral density and of relatively high amplitude. In the language of the trade, these spikes are often called "hits." Impulse noise degrades telephony only marginally, if at all. However, it may seriously corrupt error performance of a data circuit.
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