SPREAD SPECTRUM
In spread spectrum (SS), we combine signals
from different sources to fit into a larger
bandwidth, but our goals are to prevent
eavesdropping and jamming. To achieve these
goals, spread spectrum techniques add
redundancy.
docsity.com
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Guidelines and tips
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Spread Spectrum-Data Communication Systems-Lecture Slides, Slides of Digital Systems Design
This lecture is part of lecture series on Data Communication Systems. It was delivered by Prof. Prajin Ahuja at Birla Institute of Technology and Science. Its main points are: Spectrum, Jamming, Redundacy, Modulated, Demolated, Hopping, Frequency, Synthersizer, Transmitter, Pseudonoise
Typology: Slides
2011/2012
1 / 24
This page cannot be seen from the preview
Don't miss anything!
Related documents
Partial preview of the text
Download Spread Spectrum-Data Communication Systems-Lecture Slides and more Slides Digital Systems Design in PDF only on Docsity!
SPREAD SPECTRUM^ In spread spectrum (SS), we combine signalsfrom^ different^
sources^ to^ fit^ into
a^ larger bandwidth,^ but^ our
goals^ are^ to^ prevent eavesdropping and jamming. To achieve thesegoals,^ spread^ spectrum
techniques^ add redundancy.
Spread Spectrum^ ^ Analog or digital data^ ^ Frequency hoping^ ◦^ Signal broadcast over seemingly random series of frequencies^ ^ Direct Sequence^ ◦^ Each bit is represented by multiple bits in transmitted signal^ ◦^ Chipping code
Spread Spectrum Concept^ ^ Input fed into channel encoder^ ◦^ Produces narrow bandwidth analog signal around centralfrequency^ ^ Signal modulated using sequence of digits^ ◦^ Spreading code/sequence^ ◦^ Typically generated by pseudonoise/pseudorandom numbergenerator^ ^ Receiver uses same sequence to demodulate signal^ ^ Demodulated signal fed into channel decoder
General Model of Spread SpectrumSystem
Figure^ Frequency selection in FHSS
Gains^ ^ Immunity from various noise andmultipath distortion^ ◦^ Including jamming^ ^ Can hide/encrypt signals^ ◦^ Only receiver who knows spreading code canretrieve signal
Basic Operationk^ ^ Typically 2^ carriers frequencies formingk^2 channels^ ^ Channel spacing corresponds withbandwidth of input^ ^ Each channel used for fixed interval^ ◦^ 300 ms in IEEE 802.11^ ◦^ Sequence dictated by spreading code
Frequency Hopping Example
Figure^ Bandwidth sharing
Slow and Fast FHSS^ ^ Frequency shifted every T
secondsc^
^ Duration of signal element is T
secondss^
^ Slow FHSS has T^ c
^ T^ s
^ Fast FHSS has Tc^
< Ts
^ Generally fast FHSS gives improvedperformance in noise (or jamming)
Fast Frequency Hop Spread Spectrum UsingMFSK (M=4, k=2)
FHSS Performance Considerations^ ^ Typically large number of frequenciesused^ ◦^ Improved resistance to jamming
Figure^ DSSS
Figure^ DSSS example