Noise, thermal noise, delay distortion

Noise

Noise is any unwanted signal that gets added to the useful signal between transmitter and receiver. It is the main factor limiting the performance of a channel and the reason for errors.

Types of noise

• Thermal noise: caused by random motion of electrons in conductors. It is present in all electronic devices, uniform over all frequencies (white noise), and N = kTB.
• Intermodulation noise: produced when signals of different frequencies share a medium with non-linear behaviour; new frequencies f₁ ± f₂ appear.
• Crosstalk: unwanted coupling between nearby wires, for example hearing another call.
• Impulse noise: short bursts of high amplitude from lightning, power-line surges, switching. It produces burst errors in digital data.
• Shot noise and atmospheric noise also occur in practice.

Thermal noise power density

N₀ = kT watts/Hz, with k = 1.38 × 10⁻²³ J/K.

T = 17 + 273 = 290 K

N₀ = 1.38 × 10⁻²³ × 290 = 4.0 × 10⁻²¹ W/Hz

Answer: N₀ ≈ 4.0 × 10⁻²¹ W/Hz (about 4.004 × 10⁻²¹ if 273.15 is used).

Delay distortion

In a guided medium, the velocity of propagation varies with frequency, so the frequency components of a signal arrive at different times and the pulse shape is distorted (spreading and overlapping of neighbouring pulses). It is critical for digital data. It is corrected by equalisation, using equalisers (delay-equalising filters) that give the faster components more delay, so all components arrive together.

Why digital transmission is preferred

• Repeaters regenerate the signal, so noise and distortion do not accumulate.
• Error detection/correction codes can be used.
• Easy multiplexing, encryption, compression and processing.
• Voice, data and video can share one integrated network, with cheap digital hardware.