A chaotic direct sequence spread-spectrum communication system
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Transcript of A chaotic direct sequence spread-spectrum communication system
112/4/2016
Guided byDr. Saugata Sinha
Department of Electronics EngineeringVNIT, Nagpur
Visvesvaraya National Institute Of Technology, Nagpur
Presentation onA Chaotic Direct-Sequence Spread-Spectrum Communication System
Presented By:LOKESH GAHANE (MT15CMN007)
MOHIT CHIMANKAR (MT15CMN008) ARUNJITH SAHADEVAN (MT15CMN004)
MOHIT GARADE (MT15CMN009)
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• Introduction• Chaotic System• Spread Spectrum• Spread Spectrum Techniques• Chaotic Sequences• CSK Modulation• Advantages• Carrier Regeneration Detectors• Code Clock Extraction Detectors• Conclusion• References
Contents
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• The introduction of chaos into communication systems offers several opportunities for improvement.
• What is spread spectrum?• What is DSSS? • What are Discrete Time Dynamical System?
Introduction
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• is a deterministic system• random-like behaviour• Hence can disguise the modulation as noise.• Can produce large number of such signals
which can be reconstructed.
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Chaotic Systems
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• It is a technique in which the frequency of the transmitted signal is deliberately varied, this results in a much greater bandwidth than the signal would have had if its frequencies weren’t varied.
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What is spread spectrum?
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1. Constant frequency signal is subject to catastrophic interference
2. Easy to intercept.• These vulnerabilities are overcome by
spread spectrum technique.
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Problems with conventional wireless communication.
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• Hence the frequency of the transmitted signal is deliberately varied.
• Varied according to a specific but complicated mathematical function.
• Only source and receiver know the frequency variation. Hence can’t be intercepted.
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Spread Spectrum
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• Direct Sequence Spread Spectrum.• Message signal is used to modulate a bit
sequence known as the Pseudo Noise(PN) code.
• Smaller the chip duration, the larger the bandwidth and more immune the resultant signal is to interference.
What is DSSS?
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• This paper is only concerned with discrete-time dynamical systems.
• It is defined by the state equation
• Starting with an initial condition xo, repeated applications of the map f give rise to the sequence of points {x; k =0, 1, 2, ...} called an orbit of the discrete-time system.
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Discrete-time Dynamical system
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• Example
where, 1 < r <4, and r is called the bifurcation parameter
Depending on ‘r’ it can exhibit periodicity or chaos.• For 3.57.. . < r 5 4, the sequence is, for all practical
purposes, non-periodic and non-converging.
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Continued..
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SPREAD SPECTRUM TECHNIQUES
• Frequency Hopping Spread Spectrum(FH-SS) -The message is transmitted in seemingly random part of the pre-
assigned band width. -This can be visualized as the transmitted signal hopping in
frequency domain.
• Direct Sequence Spread Spectrum(DS-SS) -The message is directly spread in to larger band-width. -It will be difficult for a narrow band jammer to interfere
destructively with the signal because the signal power is spread across the larger band-width and jamming a wide band is not practical in terms of power.
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Cont’d
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Characteristic of ideal code that can be used for DS-SS
• Zero or near zero cross-correlation and impulse like Auto-correlation• The codes need to be wide band to spread the signal energy over a
wide section of the spectrum• The codes need to be easy to generateThe examples of spreading codes are• Pseudo Noise codes• Walsh Sequence• Gold codes• M-sequencesThese codes belongs to the binary category so that it can take only two Values +1&-1.ie the degree of freedom is very less.
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chaotic sequences• High degree of freedom.• The future state of the process have to be deterministic if the initial
condition is exactly known.• The future state of the process have to be un-predictable if the initial
condition is not known.Example : Lorenz Attractor• The initial condition of the equation change by small number ,the
oscillation trajectory will be completely different.
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Cont’d
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Existing method of Generating chaotic sequence for BPSK DS-SS System
• Assign different initial condition to the users• Start the chaotic map and repeatedly generate points on the orbit.• Let N be the length of the spreading sequence needed for each bit
of information . So every consecutive N points generated by the chaotic map can be taken to be a signature sequence for a bit of information.
• Each bit gets a different sequence of chips.Disadvantage • Occurrence of periodicities in the generated sequence. -cross-correlation never be zero over there , so it will deviate from
the characteristics of ideal code.
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CSK Modulation• A message symbol must have a code sequence nearly
orthogonal to those of the other symbols.• This can be accomplished by assigning different Bifurcation
parameters to each symbol.• Once the symbol is decided, one code sequence will be
associated with it. The last chip of that sequence will be in the initial point of the next.
• So error in detection of one symbol leads to incorrect generation of all the code sequence that follows.
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Scheme proposed for the generator
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Cont’d
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AdvantagesIts advantages are :I. Chaotic Sequences are easy to generate and
store.II. Provide Secure Communication as;a) The transmitted signal look like noise.b) The sequences are no longer binary.c) A Receiver who may know its parameters
and functions involved, it is still difficult for him to estimate it.
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The generation of chaotic sequences can easily be made as complicated as desired.
For example, multi-dimensional chaotic maps may be used instead of the one-dimensional. Or doing cascading of several chaotic systems.
Incorporation of the chaotic sequences into the DS/SS system has dramatically enhanced the LPI (low probability of intercept) performance of these systems.
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• It reduces the PSD of the Transmitted Signal.• Difficult to intercept because of Spectral
Spreading.• However Binary DS/SS does not provide much
protection against two particular interception methods :
I. Carrier Regeneration Detectors.II. Code Clock Regeneration Detectors.
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Carrier Regeneration Detectors :• It is used for the detection of Signals which
suppress the carriers like DS/SS systems with BPSK, QPSK, SQPSK etc.
• It consists of squaring the received signal in order to wipe out the modulation.
• Using the resultant double frequency term to detect the BPSK signal and Quadrupled frequency for QPSK and SQPSK.
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Block diagram of carrier regenerating detector for Binary BPSK DS/SS
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Block diagram of carrier regenerating detector for Binary QPSK DS/SS
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Code Clock Extraction Detector
• Rc = 1/Tc = Chip rate• Tc = Chip duration
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It is technique used for extraction of chip rate called “Delay and Mix”
Delay the spreading sequence and multiply with undelayed sequence
This method is effective on Binary DS/SSChaotic DS/SS is resistant to this method of
detection
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CONCLUSION
Chaotic sequence for spectral spreading in DS/SS system has several advantage over conventional method
Same error performanceEnormous number of different sequencesGeneration and regeneration of chaotic
sequences is simpleCode sequences are easily made independent
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Chaotic DS/SS is inherently difficult to intercept conventional dispreading procedure
Offers increased LPIProvides more security
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Reference G. Heidari-Bateni, C.D. McGillem, “A Novel Multiple-Address Digital
Communication System Using Chaotic Signals,” Proceedings of International Conference on Communications, Chicago, IL, June 15-18,1992.
T.S. Parker and L.O. Chua, “Chaos: A Tutorial for Engineers,” Proc. IEEE, Special h u e on Chaotic System, August 1987.
G. Heidari-Bateni, “Chaotic Signals for Digital Communication,” Ph.D. Dissertation, School of Electrical Engineering, Purdue University, W. Lafayette, IN, Dec. 1992.
D.J. Torrieri, Principles of Secure Communication Systems, Artech House, Inc., 1985.
D.E. Reed, “Comparison of S ymbol-Rate Detector and Radiometer Intercept Receiver Performances in a Nonstationary Environment,” Proceedings of 1989 IEEEMilitary Communications Conference, vol. 1, Boston, Massachusetts, October 1989, pp. 19.5.1-19.5.5.12/4/2016