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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:

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Spread Spectrum Communications

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  1. 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)

  2. Modulation and Demodulation • BPSK modulation with DS spreading

  3. PSD of DS Spread Spectrum

  4. 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

  5. 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 + +

  6. 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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