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External vs internal noise; interference sources; need for modulation

Short answer

External noise originates outside the receiver (e.g., atmospheric, industrial), while internal noise is generated within components (thermal, shot). Interference sources include co-channel, adjacent-channel, image frequency, crostalk, intermodulation products, power-line hum, multipath, and spurious emissions. Modulation is needed for practical antenna size, multiplexing, noise immunity, long-range propagation, and narrowbanding.

Distinguish between external and internal noise. List out the sources of interferences. Write the needs of modulation in communication system.

External vs internal noise

BasisExternal noiseInternal noise
SourceGenerated outside the receiverGenerated within receiver components
TypesAtmospheric, solar, cosmic, industrialThermal, shot, partition, flicker, transit-time
CauseLightning, sun, stars, motors, ignitionRandom electron motion, carrier arrival
Frequency rangeMostly significant at LF–VHFPresent at all frequencies (thermal is white)
MeasurementDifficult to predictCan be calculated, e.g. kTBkTB
ReductionShielding, site selection, directional antennasLow-noise devices, cooling, narrow bandwidth

Sources of interference

  1. Co-channel interference: another transmitter on the same frequency (e.g. distant cells reusing the frequency).
  2. Adjacent-channel interference: strong signals in neighbouring channels leaking through imperfect filters.
  3. Image-frequency interference in superheterodyne receivers.
  4. Crosstalk between neighbouring wire pairs or cables.
  5. Intermodulation products from non-linear amplifiers mixing several signals.
  6. Power-line hum, switching supplies and electrical machines.
  7. Multipath echoes and reflections.
  8. Harmonics and spurious emissions from other transmitters.

Needs of modulation

  1. Practical antenna size: antenna length should be about λ/4\lambda/4. A 10 kHz signal has λ=30\lambda = 30 km (7.5 km antenna), but on a 10 MHz carrier the antenna is only 7.5 m.
  2. Multiplexing: different messages on different carriers can share one channel (FDM), e.g. many radio stations.
  3. Noise and interference reduction: wideband modulation (FM, spread spectrum) improves SNR.
  4. Narrowbanding: reduces the high-to-low frequency ratio, easing antenna and amplifier design.
  5. Long-range propagation and fitting the signal into the allotted frequency band of the channel.

✓ Answer checked by Claude Opus 5.5 (high effort)

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