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SPREAD SPECTRUM

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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


				
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