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Spread Spectrum Communications. Originated in military communications Spread the signal over a band much wider than the signal bandwidth Advantages: Interference rejection and anti-jamming capability Multiple-access capability Multi-path diversity Mainly two types:

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Presentation Transcript
slide1

Spread Spectrum Communications

  • Originated in military communications
  • Spread the signal over a band much wider than the signal bandwidth
  • Advantages:
    • Interference rejection and anti-jamming capability
    • Multiple-access capability
    • Multi-path diversity
  • Mainly two types:
    • Frequency Hopping (FH), or Direct Sequence (DS)
slide2

Modulation and Demodulation

  • BPSK modulation with DS spreading
slide15

Narrowband Interference Rejection

f

BPSK Modulation

Spreading

BPSK Demodulation

DeSpreading

f

f

f

  • PSD Interpretation: After despreading (which is the same operation as spreading),
    • the narrowband interference is spread evenly over a bandwidth W. Hence,

Narrowband Interference

slide16

Pseudo-random Sequences

  • System requirement----direct sequence looks like random noise.
  • One kind of PN sequence----Maximal Length (Linear) Shift Register Sequence (MLSR) or m-sequence
  • Structure

Xn-1

Xn-2

Xn-k

C1

C2

Ck

+

+

slide17

Example for demonstration

Xn-1

Xn-2

Xn-3

Xn-4

+

PN sequence:

1 0 0 0 1 1 1 1 0 1 01 1 0 0

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