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)

2

• Introduction• Chaotic System• Spread Spectrum• Spread Spectrum Techniques• Chaotic Sequences• CSK Modulation• Advantages• Carrier Regeneration Detectors• Code Clock Extraction Detectors• Conclusion• References

Contents

12/4/2016

3

• 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

12/4/2016

4

• 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.

12/4/2016

Chaotic Systems

5

• 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.

12/4/2016

What is spread spectrum?

6

1. Constant frequency signal is subject to catastrophic interference

2. Easy to intercept.• These vulnerabilities are overcome by

spread spectrum technique.

12/4/2016

Problems with conventional wireless communication.

7

• 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.

12/4/2016

Spread Spectrum

8

• 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?

12/4/2016

9

• 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.

12/4/2016

Discrete-time Dynamical system

10

• 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.

12/4/2016

Continued..

11

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.

12/4/2016

12

Cont’d

12/4/2016

13

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.

12/4/2016

14

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.

12/4/2016

15

Cont’d

12/4/2016

16

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.

12/4/2016

17

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.

12/4/2016

18

Scheme proposed for the generator

12/4/2016

19

Cont’d

12/4/2016

20

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.

12/4/2016

21

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.

12/4/2016

22

• 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.

12/4/2016

23

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.

12/4/2016

24

Block diagram of carrier regenerating detector for Binary BPSK DS/SS

12/4/2016

25

Block diagram of carrier regenerating detector for Binary QPSK DS/SS

12/4/2016

26

Code Clock Extraction Detector

• Rc = 1/Tc = Chip rate• Tc = Chip duration

12/4/2016

27

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

12/4/2016

28

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

12/4/2016

29

Chaotic DS/SS is inherently difficult to intercept conventional dispreading procedure

Offers increased LPIProvides more security

12/4/2016

30

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