Spread Spectrum

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Spread Spectrum

• Spread spectrum uses wide band, noise like
• ( pseudo-noise ) signals
• Spread spectrum transmitters use similar transmit
  power levels to narrow band transmitters, but
  because spread spectrum signals are so wide ,they
  can be transmitted at much lower spectral power
  density (measured in Watts per Hertz) , then
  narrow band transmitters. Since spread spectrum
  (SS) is transmitted with low power, but wide
  bandwidth, a SS and narrow band signal can occupy
  the same band with little to no interference.

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Advantages

• resistant to noise, interference, jamming,
• and unauthorized detection.
• hard to intercept or demodulate.
• multiple user can use the same band
• used for ranging or radar.

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Claude Shannon's equation

• CW log (1S/N)
• where Cchannel capacity, WBandwidth, Ssignal
  power, and Nnoise power.
• From this equation the result of increasing the
  bandwidth becomes apparent. By increasing W in
  the equation, the S/N may be decreased without
  decreased performance.

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Syarat untuk Spread Spectrum Signal

• Sinyal yang ditransmisikan jauh lebih besar
  daripada information bandwith.
• Beberapa fungsi selain mentransmisikan
  information ialah mendeterminasikan the resultant
  dari transmitted bandwidth.

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Jenis Jenis Spread spectrum

• Direct Sequence Spread Spectrum (DSSS)
• This process is achieved by by multiplying a
  radio frequency carrier with a psuedo noise. The
  pseudo noise (PN-code) is a binary signal which
  is produced at a much higher frequency then the
  data that is to be transmitted. Since this has a
  higher frequency, it has a large bandwidth, which
  spreads the signal in the frequency plain
• Frequency Hopping Spread Spectrum (FHSS)
• Time Hopping

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Frequency Hopping

• Versi awal dari frequency hopping menggunakan
  piano-roll untuk mengganti diantara 88 frekuensi,
  dan ditujukan untuk untuk membuat radio-guided
  torpedo sulit dilacak maupun dijam oleh musuh
• Bandwith dalam Frequency Hopping dibagi menjadi
  beberapa broadcast frequencies yang kemudian
  dikirimkan.
• signal ditransmisikan dengan secara cepat
• memindah carrier di banyak frekuensi channel.

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Advantages

• Frequency Hopping Spread Spectrum (FHSS)
  mempunyai data speed limitations dan lebih pendek
  dalam jangkauan
• Design FHSS lebih mudah untuk dibuat dan
  menghabiskan biaya yang lebih rendah.
• sangat bagus dalam menghindari narrow band
  interference dan user access yang tidak
  diinginkan dalam lingkungan frequency hopping.
• Mereka juga dapat dilokasikan dalam network yang
  berbeda di tempat yang sama tanpa saling memberi
  gangguan satu sama lain