Data and Computer Data and Computer Communications Communications

Data and Computer Data and Computer

Communications Communications

Chapter 9 – Spread Spectrum

Chapter 9 – Spread Spectrum

Spread Spectrum Spread Spectrum

All creative people want to do the unexpected.

—Ecstasy and Me: My Life as a Woman,

Hedy Lamarr

Spread Spectrum Spread Spectrum

 important encoding method for wireless important encoding method for wireless communications

communications

 analog & digital data with analog signal analog & digital data with analog signal

 spreads data over wide bandwidth spreads data over wide bandwidth

 makes jamming and interception harder makes jamming and interception harder

 two approaches, both in use: two approaches, both in use:



Frequency Hopping Frequency Hopping



Direct Sequence Direct Sequence

General Model of Spread General Model of Spread

Spectrum System

Spectrum System

Spread Spectrum Advantages Spread Spectrum Advantages

 immunity from noise and multipath immunity from noise and multipath distortion

distortion

 can hide / encrypt signals can hide / encrypt signals

 several users can share same higher several users can share same higher bandwidth with little interference

bandwidth with little interference



CDM/CDMA Mobile telephones CDM/CDMA Mobile telephones

Pseudorandom Numbers Pseudorandom Numbers

 generated by a deterministic algorithm generated by a deterministic algorithm



not actually random not actually random



but if algorithm good, results pass reasonable but if algorithm good, results pass reasonable tests of randomness

tests of randomness

 starting from an initial seed starting from an initial seed

 need to know algorithm and seed to need to know algorithm and seed to predict sequence

predict sequence

 hence only receiver can decode signal hence only receiver can decode signal

Frequency Hopping Spread Frequency Hopping Spread

Spectrum (FHSS) Spectrum (FHSS)

 signal is broadcast over seemingly random signal is broadcast over seemingly random series of frequencies

series of frequencies

 receiver hops between frequencies in sync receiver hops between frequencies in sync with transmitter

with transmitter

 eavesdroppers hear unintelligible blips eavesdroppers hear unintelligible blips

 jamming on one frequency affects only a jamming on one frequency affects only a few bits

few bits

Frequency Hopping Example

Frequency Hopping Example

FHSS (Transmitter)

FHSS (Transmitter)

Frequency Hopping Spread

Frequency Hopping Spread

Spectrum System (Receiver)

Spectrum System (Receiver)

Slow and Fast FHSS Slow and Fast FHSS

 commonly use multiple FSK (MFSK) commonly use multiple FSK (MFSK)

 have frequency shifted every T have frequency shifted every T _{c c} seconds seconds

 duration of signal element is T duration of signal element is T _{s s} seconds seconds

 Slow FHSS has T Slow FHSS has T _{c c}   T T _{s s}

 Fast FHSS has T Fast FHSS has T _{c c} < T < T _{s s}

 FHSS quite resistant to noise or jamming FHSS quite resistant to noise or jamming



with fast FHSS giving better performance with fast FHSS giving better performance

Slow MFSK FHSS

Slow MFSK FHSS

Fast MFSK FHSS

Fast MFSK FHSS

Direct Sequence Spread Direct Sequence Spread

Spectrum (DSSS) Spectrum (DSSS)

 each bit is represented by multiple bits each bit is represented by multiple bits using a spreading code

using a spreading code

 this spreads signal across a wider this spreads signal across a wider frequency band

frequency band

 has performance similar to FHSS has performance similar to FHSS

Direct Sequence Spread Direct Sequence Spread

Spectrum Example

Spectrum Example

Direct Sequence Spread Direct Sequence Spread

Spectrum System

Spectrum System

DSSS Example Using BPSK

DSSS Example Using BPSK

Approximate Approximate

Spectrum of Spectrum of

DSSS Signal

DSSS Signal

Code Division Multiple Code Division Multiple

Access (CDMA) Access (CDMA)

 a multiplexing technique used with spread a multiplexing technique used with spread spectrum

spectrum

 given a data signal rate D given a data signal rate D

 break each bit into break each bit into k k chips according to a chips according to a fixed chipping code specific to each user fixed chipping code specific to each user

 resulting new channel has chip data rate resulting new channel has chip data rate kD kD chips per second chips per second

 can have multiple channels superimposed can have multiple channels superimposed

CDMA Example

CDMA Example

CDMA for DSSS

CDMA for DSSS

Summary Summary

 looked at use of spread spectrum looked at use of spread spectrum techniques:

techniques:

 FHSS FHSS

 DSSS DSSS

 CDMA CDMA