1 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 2 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 3 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 4 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 5 SIGNAL ENERGY AS A FUNCTION OF FREQUENCY