01-03-2013, 04:38 PM
DIGITAL AND ANALOG SIGNAL ENCRYPTION AND DECRYPTION IN MID RF RANGE USING HYBRID ACOUSTO-OPTIC CHAOS
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ABSTRACT
Modern day communication techniques are often prone to hacking and disturbances in the communication system while in transit from one place to another. Signal encryption using chaotic waves may be a good solution to this problem. A modulation scheme uses a carrier frequency to be modulated by the signal waveform, and generally the message can be readily decoded. A chaotic signal is a non-deterministic signal and not a well defined sinusoid. Hence, a modulated chaos wave is secure and cannot be decoded without knowledge of the chaos parameters.
A chaotic signal is generated by carefully choosing the right set of parameters such as feedback gain, bias input and time delay. Encrypting a wave using a chaotic wave as a carrier also depends critically on these parameters. A signal used to encrypt a chaotic carrier can only be recovered or decoded by knowing exactly three parameter
Background of Chaos:
There is no precise definition of chaos except that chaos is a form of controlled oscillations, often appearing to be time shifted resonances. Chaos has the form of random oscillations in time with non uniformity in amplitude but essentially stable frequency characteristics. Study of chaos in the modern era dates back to late 1950s when experiments were conducted to study chaotic behavior using circuits, forced vibrations of shallow water waves in a finite container, hydro dynamical instabilities, chemical turbulence, and acousto-optic (A-O) turbulence [1].
Early experiments on chaos were conducted by Faraday using forced vibrations of shallow water waves in a finite container, the vibrations were driven into chaos when a component at a frequency f0/2 in shallow water was observed when the medium was excited with a frequency of f0. This was the likely demonstration of the first existence of chaos [2, 3].
A-O Chaos:
Acousto-optics, where acousto implies sound and optics implies light defines the interaction of sound with light. Chaos can be generated and studied using a hybrid A-O chaos system. In 1979, Ikeda predicted that chaos can be observed in an optically bistable device using a ring cavity [13]. This predicted phenomenon was first successfully observed by Gibbs in 1981 [14]. A hybrid optical system implies that a part of the optical output is electronically delayed, amplified and fed back to the driver input. Periodic optical oscillations are generated in hybrid devices by introducing delay in the feedback line.
Modulation of a Chaotic Wave:
A common motivation behind most communication systems is to transmit the data in a safe and secure manner. The basic problem has been the information security. Chaos waves may be used a carrier for transmitting data because of their random yet controlled nature. Modulation using chaos as a carrier was been first attempted in the 1990s and many schemes have been designed to encrypt chaos waves using message signals with fewer numerical algorithms as encryption keys than the classical methods [20, 21]. The chaos wave may be viewed as a carrier which is subjected to simple Amplitude
Modulation (AM) scheme. Chaos may be viewed as a random disturbance that may be used mask data and protect the data from hackers. The basic random nature of chaos and in the A-O case, its dependence on three factors viz feedback gain, bias input and time delay are the key factors that protect the data. As discussed earlier, an A-O chaos system is governed by a key set of simple nonlinear differential equations that may be used to decrypt encrypted data transmitted over a channel.
CONCLUSION
This research focusses on optical characteristics of a Hybrid A-O network. We
have proved and generated the monostable, bistable and chaos characteristics of the
hybrid A-O network. The generation of chaos has been studied for different parameters of
and time delay Td. We have generated chaos in the mid
RF range by reducing the delay time in the loop. The chaos was extended to mid RF
range in MHz and was used to encrypt and decrypt messages. The time characteristics of
the chaos have been studied and emphasis was laid on studying threshold values of
parameters in generating chaos. Chaos as a carrier was used to encrypt message which
may be used as a defense security in keeping the information safe. A receiver scheme was
developed using low pass filter and a second Bragg cell with bias input to generate local
chaos that was used to extract message using heterodyne reception.