Communication Systems: Components, Noise, and Modulation

Posted on Mar 9, 2026 in Electronics

1. Communication System Components

  • Information Source: Generates the message (voice, image, video, text, or data). Example: Human voice in mobile communication.
  • Transmitter: Converts the message into a suitable electrical signal. Functions include amplification, modulation, and encoding to prepare for long-distance transmission.
  • Channel: The medium for signal travel (e.g., twisted pair, coaxial cable, optical fiber, or free space). Signals may suffer from attenuation and distortion here.
  • Noise Source: Unwanted disturbances (thermal, atmospheric, or man-made) that reduce signal quality.
  • Receiver: Reconstructs the original message via amplification, demodulation, and decoding.
  • Destination: The final user (e.g., a listener or television viewer).

2. Types of Noise

External Noise

  • Atmospheric Noise: Caused by lightning and natural disturbances; also called static noise.
  • Man-Made Noise: Generated by motors, power lines, and vehicles; also called industrial noise.
  • Extraterrestrial Noise: Includes Solar Noise (from the sun) and Cosmic Noise (from stars and galaxies).

Internal Noise

  • Thermal Noise: Caused by random electron motion in resistors; increases with temperature.
  • Shot Noise: Occurs due to sudden charge carrier flow in diodes and transistors.
  • Transit Time Noise: Significant at high frequencies when electron travel time is notable.
  • Flicker Noise: Also called 1/f noise; occurs at low frequencies in semiconductors.
  • Partition Noise: Occurs when current divides into multiple paths.

3. Signal Analysis and Repeaters

Signal Power and PSD

  • Signal Power: The average power of a signal over time, measured in Watts.
  • Power Spectral Density (PSD): Describes power distribution over frequency (Watt/Hz). It is the Fourier Transform of the autocorrelation function.

Regenerative Repeater

Used in digital systems to amplify, reshape, and retime signals, removing noise before retransmission.

Energy Spectral Density (ESD)

Shows energy distribution in the frequency domain (Joule/Hz) for finite energy signals.

Concept of Negative Frequency

A mathematical tool in Fourier analysis representing clockwise rotation in the complex plane; essential for symmetric spectrum analysis.

4. Modulation Techniques

Modulation Index

The ratio of the modulating signal amplitude to the carrier signal amplitude. Defines the modulation state: under, 100%, or over-modulation.

DSB-SC (Double Sideband Suppressed Carrier)

Transmits both sidebands while suppressing the carrier to save power.

SSB-SC (Single Sideband Suppressed Carrier)

Transmits only one sideband and suppresses the carrier, optimizing both bandwidth and power for long-distance communication.