Spread Spectrum Communications
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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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Spread Spectrum Communications

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Spread spectrum communications

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)


Spread spectrum communications

Modulation and Demodulation

  • BPSK modulation with DS spreading


Spread spectrum communications

PSD of DS Spread Spectrum


Spread spectrum communications

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


Spread spectrum communications

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

+

+


Spread spectrum communications

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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