Spread Spectrum - Data Communications - Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Chapter 9

Spread Spectrum

Spread Spectrum

• Analog or digital data
• Analog signal
• Spread data over wide bandwidth
• Makes jamming and interception harder
• Frequency hoping — Signal broadcast over seemingly random series of frequencies
• Direct Sequence — Each bit is represented by multiple bits in transmitted signal — Chipping code

General Model of Spread

Spectrum System

Gains

• Immunity from various noise and multipath

distortion

—Including jamming

• Can hide/encrypt signals

—Only receiver who knows spreading code can retrieve signal

• Several users can share same higher bandwidth

with little interference

—Cellular telephones —Code division multiplexing (CDM) —Code division multiple access (CDMA)

Frequency Hopping Spread

Spectrum (FHSS)

• Signal broadcast over seemingly random series

of frequencies

• Receiver hops between frequencies in sync with

transmitter

• Eavesdroppers hear unintelligible blips

• Jamming on one frequency affects only a few

bits

Basic Operation

• Typically 2k^ carriers frequencies forming 2 k

channels

• Channel spacing corresponds with bandwidth of

input

• Each channel used for fixed interval

—300 ms in IEEE 802. —Some number of bits transmitted using some encoding scheme

• May be fractions of bit (see later) —Sequence dictated by spreading code

Frequency Hopping Spread

Spectrum System (Transmitter)

Frequency Hopping Spread

Spectrum System (Receiver)

Slow Frequency Hop Spread

Spectrum Using MFSK (M=4, k=2)

Fast Frequency Hop Spread

Spectrum Using MFSK (M=4, k=2)

Direct Sequence Spread

Spectrum (DSSS)

• Each bit represented by multiple bits using spreading code
• Spreading code spreads signal across wider frequency band — In proportion to number of bits used — 10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code
• One method: — Combine input with spreading code using XOR — Input bit 1 inverts spreading code bit — Input zero bit doesn’t alter spreading code bit — Data rate equal to original spreading code
• Performance similar to FHSS

Direct Sequence Spread

Spectrum Example

Direct Sequence Spread

Spectrum Transmitter

Direct Sequence Spread

Spectrum Using BPSK Example