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Spread Spectrum CommunicationsPowerPoint Presentation

Spread Spectrum Communications

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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:
- Frequency Hopping (FH), or Direct Sequence (DS)

Modulation and Demodulation

- BPSK modulation with DS spreading

PSD of DS Spread Spectrum

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

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

+

+

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