Distortion vs noise vs interference

A received signal can be written as $r(t) = f\{s(t)\} + i(t) + n(t)$: the channel distorts the signal $s(t)$, unwanted signals from other sources add interference $i(t)$, and random noise $n(t)$ is added. These three impairments differ as follows.

Distortion

Alteration of the waveform shape by the system or channel itself, so that the output is not $K\,s(t - t_d)$.

• Linear distortion: amplitude distortion (non-flat $|H(f)|$) and phase/delay distortion (non-linear phase), e.g. ISI in data channels, multipath.
• Non-linear distortion: caused by non-linear devices (amplifier saturation), producing harmonics and intermodulation.
• It exists only while the signal is present and is deterministic, so it can be corrected by equalization or by improving linearity.

Noise

Random, unpredictable unwanted electrical energy from natural sources.

• External: atmospheric (lightning), solar and cosmic, industrial (ignition, motors).
• Internal: thermal noise $P_n = kTB$, shot noise, partition and flicker noise.
• Always present, even with no signal. Cannot be removed fully; reduced by limiting bandwidth, low-noise amplifiers, higher transmit power, wideband modulation (FM) and error-correcting codes.

Interference

Unwanted signals from other communication sources that fall in the receiver band.

• Examples: co-channel and adjacent-channel interference, crosstalk between cables, image-frequency interference in superheterodyne receivers, power-line hum.
• Usually has a definite form and frequency, so it can be reduced by good filtering, shielding, frequency planning, directional antennas.