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Short note: Spread spectrum

Short answer

Spread spectrum transmits a signal over a bandwidth much larger than the data rate using a spreading code independent of the data. The receiver uses the same code to despread. Techniques: frequency hopping (FHSS) and direct sequence (DSSS). It gives processing gain, resists jamming and interference, has low probability of detection, and enables CDMA.

Write a short note on spread spectrum.

Spread spectrum is a technique in which a signal is transmitted over a bandwidth much greater than the minimum needed for the information, using a spreading code that is independent of the data. The receiver uses the same code to despread the signal and recover the data.

Principle: the spreading code makes the signal bandwidth Bss ≫ the data bandwidth B. The ratio Bss/B is the processing gain. Spread signals have low power density, so they look like noise to others.

 Data -> [Modulator] -> [Spreader] -> channel
                           ^ code
 channel -> [Despreader] -> [Demod] -> Data
                ^ same code

Main techniques

  • FHSS: carrier hops among frequencies in a pseudorandom sequence (Bluetooth).
  • DSSS: each bit is multiplied by a fast chip sequence (802.11b, GPS, CDMA).

Advantages

  • Resists jamming and narrow-band interference.
  • Low probability of detection and interception, so secure.
  • Resists multipath fading.
  • Allows many users in the same band using different codes (CDMA).

Disadvantages

  • Needs a much wider bandwidth.
  • Complex transmitter and receiver, with precise code synchronisation.

Applications

Military communication, cellular CDMA, wireless LAN, Bluetooth, GPS.

✓ Answer checked by Claude Sonnet 5.5

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