Transmission impairments

Transmission impairments are the imperfections in a channel that make the received signal differ from the transmitted one. They cause errors in digital systems and loss of quality in analog systems. The three main types are attenuation, distortion and noise.

1. Attenuation

Loss of signal strength with distance because energy is absorbed or dissipated in the medium. It increases with distance and frequency. It is measured in decibels: dB = 10 log₁₀(P₂/P₁), a negative value for loss.

• Example: a signal falls from 10 mW to 5 mW, so loss = 10 log₁₀(0.5) = -3 dB.
• Remedy: amplifiers (analog) or repeaters (digital).

2. Distortion

Change in the shape of a composite signal because its frequency components travel at different speeds in the medium (delay distortion) or are attenuated unequally (attenuation distortion).

• Example: the harmonics of a square pulse arrive at different times, so the pulse spreads and overlaps with the next one (inter-symbol interference).
• Remedy: equalisers.

3. Noise

Unwanted signals added between transmitter and receiver.

• Thermal noise: random motion of electrons, N = kTB.
• Intermodulation noise: sum or difference of frequencies when several signals share a non-linear medium.
• Crosstalk: coupling from a neighbouring wire, for example overhearing another call on a telephone line.
• Impulse noise: short, high-amplitude spikes from lightning or switching. It destroys a burst of bits.

Effect

The quality of a link is measured by SNR = signal power / noise power. A low SNR increases the bit error rate. By Shannon, the channel capacity C = B log₂(1 + SNR) drops when noise rises.