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SPREAD SPECTRUM
 DEVELOPED INITIALLY FOR MILITARY AND
INTELLIGENCE REQUIREMENTS
 IDEA: TP SPREAD THE INFORMATION SIGNAL
OVER A WIDER BANDWIDTH TO MAKE JAMMING
AND INTERCEPTION MORE DIFFICULT
 TWO BASIC METHODS: FREQUENCY HOPPING
AND DIRECT SEQUENCE
 A MODEL OF A SPREAD SPECTRUM SYSTEM, SEE
FIGURE
 INPUT IS FED INTO A CHANNEL ENCODER THAT
PRODUCES AN ANALOG SIGNAL WITH A NARROW
BANDWIDTH
 THIS SIGNAL IS FURTHER MODULATED BY USING
A SEQUENCE OF DIGITS = SPREADING CODE
 THE SPREADING CODE IS GENERATED BY A
PSEUDORANDOM NUMBER GENERATOR
 THE EFFECT OF THE MODULATION IS TO
INCREASE THE BANDWIDTH
 THE SIGNAL IS FED INTO CHANNEL
 ON THE RECEIVING END THE OPPOSITE
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WHAT ARE THE BENEFITS OF THIS TECHNIQUE?
 IMMUNITY FROM JAMMING, NOISE AND
MULTIPATH DISTORSION
 FOR HIDING AND ENCRYPTING SIGNALS
 SEVERAL USERS CAN INDEPENDENTLY USE THE
SAME BANDWIDTH WITH VERY LITTLE
INTERFERENCE
FREQUENCY HOPPING SPREAD SPECTRUM (FHSS)
 THE SIGNAL IS BROADCAST OVER A SEEMINGLY
RANDOM SERIES OF RADIO FREQUENCIES
 HOPPING FROM FREQUENCY TO FREQUENCY AT
FIXED INTERVALS
 THE RECIEVER IS SYNCHRONIZED WITH THE
TRANSMITTER
 THE TRANSMITTER OPERATES IN ONE CHANNEL
FOR A FIXED INTERVAL, THE IEEE 802.11
WIRELESS LAN STANDARD USES A 300-MS
INTERVAL
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 THE SEQUENCE OF CHANNELS USED IS DICTATED
BY A SPREADING CODE
 BOTH TRANSMITTER AND RECEIVER USE THE
SAME CODE
 BINARY DATA ARE FED INTO A MODULATOR
USING SOME DIGITAL-ANALOG ENCODING
SCHEME SUCH AS FSK OR BPSK
 THE RESULTING SIGNAL IS CENTERED AROUND
SOME CARRIER FREQUENCY
 A PSEUDORANDOM NUMBER SOURCE SERVES AS
AN INDEX INTO A TABLE OF FREQUENCIES
 EACH k BITS OF THE PN SOURCE SPECIFIES 2k
CARRIER FREQUENCIES
 ON RECEPTION: THE SAME SEQUENCE IS USED
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DIRECT SEQUENCY SPREAD SPECTRUM (DSSS)
 EACH BIT IN THE ORIGINAL SIGNAL IS
REPRESENTED BY MULTIPLE BITS IN THE
TRANSMITTED SIGNAL
 THE SPREADING CODE SPREAD THE SIGNAL
ACROSS A WIDERN FREQUENCY BAND IN DIRECT
PROPORTION TO NUMBER OF BITS USED
 ONE TECHNIQUE: XOR (EXCLUSIVE-OR)
 SEE BELOW
 IT CAN BE SHOWN THAT THE POWER OF AN
JAMMING SIGNAL IS BEEN REDUCED BY A
FACTOR Wd/Ws WHERE Ws IS THE SPREAD
SPECTRUM SIGNAL BANDWIDTH AND Wd IS THE
SIGNAL BANDWIDTH
 THE INVERSE OF THE FACTOR IS THE GAIN IN
SIGNAL-TO-NOISE RATIO
 A TYPICAL SPECTRUM IS GIVEN BELOW
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 SIGNAL ENERGY AS A FUNCTION OF
FREQUENCY