time hopping

8/8/2019 Time Hopping

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TIME-HOPPING

SPREAD SPECTRUM


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the period andduty cycleofa pulsed RF

carrier are variedin a pseudorandommanner

underthe control ofa coded sequence.


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HYBRID SYSTEM

Frequency Hopping DirectSequence


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Such schemes will typicallyemployfrequency

hopping ofthe carrier wave, while

concurrentlyusing a DS modulation technique

tomodulatethedata upon the carrier.


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To successfullyintercept such a signal you

mustfirst crack the FH code, andthen crack

the DS code. Ifyou wantto befurther secure,

youencryptyourdata stream with a very

secure crypto code beforeyoufeeditinto

your DS modulator, andemploy

cryptographically secure PN codes forthe DSand FH operations.


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Chirp Spread Spectrum


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Chirp Spread Spectrum (CSS) is a spread

spectrumtechniquethatuses wideband linear

frequencymodulated chirp pulses toencode

information.

a chirp is a sinusoidal signal whosefrequency

increases ordecreases over a certain amount

oftime.


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unlike DSSS ofFHSS in thatitdoes not add any

pseudo-randomelements tothe signal tohelpdistinguishitfrom noiseon the channel,

insteadrelying on the linear natureofthe

chirp pulse.

CSS is ideal for applications requiring low

powerusage and needing relatively low

amounts ofdata rate (1Mbits or less)


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PSEUDO-RANDOM NOISE CODE


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Maximum length sequence

1 1 -1 -1 -1 1 -1 -1 1 1 -1 1 -1 1 1


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Gold Sequences

m-sequences gave only one sequence oflength . By combining two of these

sequences, we can obtain up to 31 plus

the two m-sequences themselves,

generate 33 sequences that can be used

to spread different input messages


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Barker Code:

Barker codes,originallydevelopedforradar, are short

(13 bits or less) sequences that are normallyusedinone-shot schemes, as comparedtomostotherspreading codes whichrun continually. Barkersequences are notthe natural productoflinearfeedback shiftregisters, butrather arehard-coded.

R2: 10 (or 11)R3: 110R4: 1011 or (1001)R5: 11101R7: 1110010

R11: 11100010010R13: 1111100110101


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Hadamard-Walsh Codes

time hopping

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