High Capacity Turbulence-Resilient Free-Space Chaotic Optical Communication Based on Vector Optical Field Manipulation

Researchers from China have proposed a hollow top-hat beam (HTB) diffractive optical element (DOE) shaping method, for use in space uplink chaotic communications systems (SUCCS). Simulation results show that by shaping the beam into a HTB, the bit error-rate (BER) improvement is substantial when compared to the traditional hollow Gaussian beam (HGB). The results could be significant for the optimisation of designs relating to SUCCS. Laser communication is a type of communication technique that, unsurprisingly, uses laser beams as a signal carrier, allowing the transmission of images, sound and other data in free space. It has been conceptualised since the development of the first lasers in the 1960s. When compared to more traditional microwave communication, laser communication has the advantages of allowing faster transmission due to higher data-rate (in principle anywhere from 1,000 to 100,000 times faster), a larger communication capacity, smaller terminal size and lower power consumption. It is anticipated that, due to its higher data-rate, laser communication will be the preferred candidate for the problem of bottlenecking when it comes to space uplink communications. Prof Mi Li (left of centre) and his research group. (a) original Gaussian distribution of laser beam (b) hollow Gaussian beam (HGB) shaped by DOE (c) hollow top-hat beam (HTB) shaped by DOE. Space chaotic laser communication system. Reducing the complexity of designing laser communications systems is a hot topic in the research field of free-space communication. Optical antennas play an important role, where they function as beam collators, expanders and convergers – all of which can improve the performance of free-space communication. One of the most commonly used antennas is the Cassegrain reflective optical antenna, featuring a short mirror body, high magnification and elimination of spherical aberrations (which result in blurry images due to numerous undesired focal points). Cassegrain antennas do come with a drawback, however, in that the secondary mirror can block part of the transmission beam, affecting the transmission efficiency of the optical system. The work presented in this issue of Electronics Letters is applicable to space chaotic laser communications systems, which are more complex than ground optical fiber chaotic communication systems. The authors use a DOE to shape a carrier beam into a hollow, top-hat shape (see inset images). DOEs have been widely used in the field of beam-shaping due to the advantages of high diffraction efficiency, flexible design and simple structure. Intelligent shaping of the beam can eliminate the problem of secondary mirrors in a Cassegrain antenna blocking part of the transmission beam. It can also be used to improve the transmission efficiency of the optical sub-system, meaning that the design pressure on other sub-system parameters is eased. DOEs can be used to shape and split laser beams in an energy-efficient manner and are often used in production facilities for laser material processing, or in medical laser treatments and diagnostics. They also see use in lighting, printing and lithography. In short, DOEs are frequently used in areas where shaping and splitting with minimal light loss is necessary. In order to effectively demonstrate how their suggested HTB improves the quality of the communication system, the authors use BER to evaluate their work. They also compare the HTB to a HGB, which they used in previous research works, to see if there is any difference in quality when considering the two shapes for SUCCS. The research considers strong and weak atmospheric conditions, as atmospheric turbulence also plays a role in the BER of signals moving between Earth and orbit. The work shows that, at higher beam power, BER is reduced by around 10−0.9 when using the HTB shaping method at a low chaotic mismatch and 10−1.5 at a high chaotic mismatch. In both scenarios the HTB shaping outperforms the HGB shaping, showing that the HTB is more suitable for SUCCS. In recent years there have been a number of exciting achievements in the free-space laser communication field. Laser communication has moved from concept to practice, Mbps rates to Gbps rates and we are now seeing the next iteration of communication technology. Research scientists are carrying out research work on small-size, high-speed laser communication stations. The work presented here not only improves the efficiency of the beam, but also makes steps towards reducing the design difficulty of other sub-systems within the communication terminal. The authors hope that this work will also allow a meshing of 5G ground wireless communications and space laser communications networks, and even pave the way for easier realisation of 6G in the future.

In recent years, a large amount of work on chaos-based cryptosystems has been published (Kocarev (2001); Millerioux et al. (2008)). A general methodology for designing chaotic and hyperchaotic cryptosystems has been developed using the control systems theory (Grassi et al. (1999); Liao et al. (1999); Yang et al. (1997a;b)). The chaotic communication system is closely related to the concept of chaos synchronization. An overview of chaotic secure communication systems can be found in (Yang (2004)). He classified the continuous-time chaotic secure communication systems into four generations. In the third generation, the combination of the classical cryptographic technique and chaotic synchronization is used to enhance the degree of security. Specifically, Yang et al. proposed a new chaos-based secure communication scheme in an attempt to thwart the attacks (Yang et al. (1997a;b)). They have combined both conventional cryptographic method and synchronization of chaotic systems. Their cryptographic method consists of an encryption function (the multi-shift cipher), a decryption function (the inverse of the encryption function), a chaotic encrypter that generates the key signal for the encryption function, and a decrypter that estimates the key signal. The approach has a limitation since the cryptosystem design may fail if different chaotic circuits are utilized. So far, this generation has the highest security in all the chaotic communication systems had been proposed and has not yet broken. From the control theoretic perspective, the transmitter and the receiver in the chaotic communication system can be considered as the nonlinear plant and its observer, respectively. Grassi et al. proposed a nonlinear-observer-based decrypter to reconstruct the state of the encrypter (Grassi et al. (1999); Liao et al. (1999)). They extended the Chua’s oscillator to the observer-based decrypter. The cryptosystem does not require initial conditions of the encrypter and the decrypter belonging to the same basin of attraction. If we can design a decrypter without the knowledge of the parameters of the encrypter, the chaos-based secure communication systems are not secure, because the parameters of the encrypter is selected as static secret keys in the cryptosystem. Parameter identification and adaptive synchronization methods may be effective for intruders in building reconstruction mechanisms, even when a synchronizing system is not available. Therefore, it is important for secure issues to investigate whether adaptive identifiers without the system information of encrypter can be constructed or not. We have recently designed an observer-based chaotic communication system combining the cryptosystems proposed by Grassi et al. (Grassi et al. (1999)) and by Liao et al. (Liao et al. (1999)) that allows us to assign the relative degree and the zeros of its encrypter system (Matsuo et al. (2004)). Specifically, we constructed three cryptosystems based on a Chua’s circuit by assigning its relative degree and zeros. The cryptosystem consists of 20

Chaotic optical communication at high bit rates has attracted great attention because of its potential application in secure communications. In a chaotic optical communication system, a message can participate in the dynamics of the nonlinear system and further affect the characteristics of the chaotic carrier waveform. Message encoding can also change the synchronization quality if an encoding scheme breaks the symmetry between a transmitter and a receiver. In this presentation, we experimentally investigate the influence of message encoding on the characteristics of chaotic dynamics, chaos synchronization, and chaotic communication, using three encoding schemes, namely chaos shift keying, chaos masking, and chaos modulation, with the setup of a chaotic optical communication system with optoelectronic feedback. The chaos shift keying and chaos modulation schemes are found to be able to increase the complexity of the chaotic carrier waveform due to the random nature of the encoded message. Meanwhile, the chaos shift keying and chaos masking schemes are found to deteriorate the synchronization quality when the amplitude of the message is increased. Consequently, only the chaos modulation scheme has the desired feature in increasing the complexity of the chaotic carrier waveform while maintaining the quality of chaos synchronization, which is an important feature in secure communication.

In this paper, we propose chaotic wireless communication system using digital retrodirective array antenna (RDA) for improving security and receive performance at receiver. Chaotic modulation schemes have studied for reducing the probability of interception and interrupt. As a result, chaotic communication systems have enhanced security. But the receive performance at receiver is degraded. We propose digital RDA system based on chaotic modulation for improving the reception performance while maintaining the security. Simulation results show that to overcome degradation of reception performance in CDSK, digital RDA with array elements up to five is required compared to BPSK modulation scheme.KeywordsDCSKCDSKSecuritytent mapretrodirective array antenna

This paper presents a new approach to transmit and retrieve an information-bearing signal via chaotic communication system. The transmitter in the proposed communication system is using the chaotic mask circuit. This circuit uses a chaotic signal to mask the information signals. The information signals are detected at the receiver by a synchronous chaotic subsystem augmented with a nonlinear filter. SPICE simulation results and experiment are presented to verify our approach.

Implementation of chaotic secure communication systems based on OPA circuits

: We have made impressive research progress on several related aspects of our research grant during the period; namely, (1) robustness of chaotic synchronization schemes, (2) efficiency of chaotic synchronization systems, (3) design of practical chaotic spread-spectrum communication systems, (4) channel capacity of chaotic spread-spectrum communication systems, and (5) potential commercial applications of chaotic spread spectrum communication systems.

A novel universal approach for design and implementation of chaotic digital communication system based on IEEE-754 standard and field programmable gate array technology (FPGA) is proposed, combing chaos encryption with traditional cipher realization. By using the Euler algorithm and appropriate discrete processing, the continuous chaotic system is converted to a discrete chaotic system. Using FPGA hardware design system, digital chaotic sequence is generated as the key. Scrambling and expansion encryption algorithm is realized and analyzed. Driven and response of secure communication system is designed by constructing a loop including the signal and achieving chaotic synchronization between sender and receiver. Taking grid Chua chaotic system as an example, the secure communication system using FPGA hardware platform is implemented. The technical development process, algorithm flow chart, hardware design and realization result are also given.

In wireless communications, chaotic communications have been a field of interest due to its low complexity in hardware implementation and low power consumption in chaotic signal generation. Among the modulation schemes using the chaotic signal, Differential Chaos Shift Keying (DCSK) is a robust noncoherent technique. Multiple-input multiple-output (MIMO) system is a technology that uses multiple transmit and/or multiple receive antennas in order to improve the system performance in wireless systems. In our paper, we have proposed a new scheme of MIMO-DCSK which utilizes the benefits of MIMO system into the chaotic communication system by transmitting and receiving DCSK modulated signals through multiple antennas. Our analysis and simulation results show how the chaotic communications in the new MIMO-DCSK benefits over the single input single output (SISO) system and the BER performance of DCSK in additive white Gaussian noise (AWGN) channels using Alamouti space-time code and the maximum likelihood decoding is analyzed.

Alternant-channel chaotic laser secure communication system and its chaos alternant coding (CAC) are presented and deeply studied via two coupled lasers synchronizing or desynchronizing with another single laser. The coupled lasers are used as two chaotic transmitters and transmit two chaotic carriers to mask two information signals, respectively. One conventional encoding scheme is that the single laser is used as a receiver and obtain synchronization with one of two chaotic transmitters for chaos decoding. And another CAC scheme as a non-conventional coding is presented and its coding principle is defined for secure communication. CAC coding and scheme is that one laser of chaotic transmitters transmits a chaotic carrier to mask an information signal, where the carrier is modulated by an information signal, while the decoding of CAC is performed via synchronization or de-synchronization between another laser of two chaotic transmitters and a receiver. A physical model of the synchronous system is presented using a drive-feedback technique while a chaotic synchronization is obtained between a chaotic transmitter and a receiver. And alternant-channel chaotic secure communication and CAC are successfully implemented while a novel CAC technique is validated. The system has the characteristics of high dimension, many degrees of freedom in phase-space, a high degree of nonlinearity, a nonconventional CAC, and many secret keys. So it has high security and can transmit securely two chaotic carries masking two information signals, respectively.

In this paper, we numerically study chaotic synchronization communication system based on incoherent optical feedback and incoherent optical injection. The characteristics of the system, such as synchronization time, synchronization error, chaos shift keying encoding/decoding and modulation rate are analyzed. The results of simulation show that the system has good synchronization quality and good abilities of robust secure. The synchronization time is 1ns and the chaos shift keying encoding can reach a high rate of 1 Gbit/s. The system has a good ability of robust secure. It proves the feasibility of the optical secure communication.

Chaotic signals can be applied to confidential communications because of their noise-like, broad-spectrum and unpredictable properties. However, using the internal parameters of the laser as the key for communication is not only not easy to change, but also the key parameter space is limited. Therefore, increasing the key space is crucial for the security of chaotic confidential communication systems. In this paper, we propose to increase the key space of chaotic optical communication system by using multi-section semiconductor laser (MSSL), which increases the number of key parameters and increases the key space of the system. Finally, the validity of the ideas proposed in this paper is verified through experiments.

A numerical investigation of the performance of an open-loop optical chaotic communication system for the isochronous synchronization solution has been carried out, under strong optical injection conditions achieved using antireflective coating at the input facet of the receiver laser in combination with an optical erbium-doped fiber amplifier (EDFA). Different message encoding techniques have been considered and tested at multigigabit rates and for different levels of optical injection to the receiver. The effects induced by the amplified spontaneous emission (ASE) noise of the EDFA to the performance of the chaotic communication system have also been studied. The performance of all the examined encryption methods for the 1 Gb/s bit-rate messages was quite satisfactory and was characterized by Q-factor values that exceeded 10, after synchronizing in the strong injection regime. For higher message bit rates, the Q-factor values for all methods decrease considerably due to the residual frequency components of the chaotic carrier that are now more significant in the message spectral region. The effect of the amplifier's ASE noise to the system's performance was deteriorated as long as the EDFA chaotic input was kept in relatively high power levels.

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In this paper we study the problem of blind channel identification in chaotic communications. An adaptive algorithm is proposed, which exploits the boundness property of chaotic signals. Compared with the EKF-based approach, the proposed algorithm achieves a great complexity gain but at the expense of a slight accuracy degradation. However, our approach enjoys the important advantage that it does not require the a priori information such as nonlinearity of chaotic dynamics and the variances of measurement noise and the coefficient model noise. In addition, our approach is applicable to the ARMA system.