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Institution of Engineering and Technology eBooks

Quantum-based satellite free space optical communication and microwave photonics

Arockia Bazil Raj, Vishal Sharma, Subhashish Banerjee

Published Jun 1, 2019• 5 citations
Publisher

Abstract

Spread-spectrum techniques are widely used in radio communication and telecommunication. Any signal like acoustic, electrical, and electromagnetic signal produced with a specific bandwidth is spread in frequency hence results in a wider bandwidth. Spread spectrum techniques are deployed in telecommunication because of many significant advantages, e.g., to achieve secure communications, to detect the eavesdropping, to resist natural interference, to bound power flux density for satellite down links and resistance to noise and jamming. In spread spectrum technique, frequency hopping (FH) is used as a basic modulation technique by which any telecommunication signal can be transmitted on a wider bandwidth (radio bandwidth) as compared to frequency value of the original signal. Spread spectrum techniques deploy FH, direct sequence (DS), or mix of both methods so that it can be used for multiple access and reduces the interference to other receivers to get the overall privacy. At the receiver side, the received signals are correlated to extract the original information being sent. The two main motivation behind spread spectrum are: to create anti-jamming for unauthenticated person and to provide low probability of interception. The classical data transmission can also be achieved by using quantum communication protocols like dense coding, quantum key distribution (QKD), and quantum teleportation. Most of these protocols use quantum states of light to transmit the information through optical fibers. The problem of channel access become apparent when number of users increases. In classical communication networks, when many users want to use the same channel at the same time, many multiple access methods are used. On the other side, quantum communication networks also deploy frequency and wavelength division multiple access techniques, hence each user make use of same channel at different frequencies and different time instance. In addition to this, many multiple access techniques employ orbital angular momentum of single photons and coherent states. Quantum spread spectrum multiple access scheme described here for optical fibers could be used for free-space data transmission. In this scheme, multiple users send their photons via same optical fiber, sharing their path, time window, and frequency band. A number of techniques have been proposed previously for data transmission for quantum optical communication which provides heavy losses while combining and separating the transmitted data from different users. The described scheme is developed to follow classical spread spectrum methods, and also follows add-drop architecture with simple extraction and combination points.

DOI
10.1049/pbte078e_ch15

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Spread-spectrum techniques are widely used in radio communication and telecommunication. Any signal like acoustic, electrical, and electromagnetic signal produced with a specific bandwidth is spread in frequency hence results in a wider bandwidth. Spread spectrum techniques are deployed in telecommunication because of many significant advantages, e.g., to achieve secure communications, to detect the eavesdropping, to resist natural interference, to bound power flux density for satellite down links and resistance to noise and jamming. In spread spectrum technique, frequency hopping (FH) is used as a basic modulation technique by which any telecommunication signal can be transmitted on a wider bandwidth (radio bandwidth) as compared to frequency value of the original signal. Spread spectrum techniques deploy FH, direct sequence (DS), or mix of both methods so that it can be used for multiple access and reduces the interference to other receivers to get the overall privacy. At the receiver side, the received signals are correlated to extract the original information being sent. The two main motivation behind spread spectrum are: to create anti-jamming for unauthenticated person and to provide low probability of interception. The classical data transmission can also be achieved by using quantum communication protocols like dense coding, quantum key distribution (QKD), and quantum teleportation. Most of these protocols use quantum states of light to transmit the information through optical fibers. The problem of channel access become apparent when number of users increases. In classical communication networks, when many users want to use the same channel at the same time, many multiple access methods are used. On the other side, quantum communication networks also deploy frequency and wavelength division multiple access techniques, hence each user make use of same channel at different frequencies and different time instance. In addition to this, many multiple access techniques employ orbital angular momentum of single photons and coherent states. Quantum spread spectrum multiple access scheme described here for optical fibers could be used for free-space data transmission. In this scheme, multiple users send their photons via same optical fiber, sharing their path, time window, and frequency band. A number of techniques have been proposed previously for data transmission for quantum optical communication which provides heavy losses while combining and separating the transmitted data from different users. The described scheme is developed to follow classical spread spectrum methods, and also follows add-drop architecture with simple extraction and combination points.
Free-space optical (FSO) communication provides a promising alternative solution to the traditional radio frequency (RF) communications. Compared to the RF technology, there are many advantages of FSO communications, such as ultra-high bandwidth, license-free spectrum, low inter-channel interference, and energy-efficient transmission. The potential applications for FSO communication include the last-one-mile access link from the fiber-based backbone network, low earth orbit (LEO) satellite communications, high-altitude base station platforms or vehicles, deep-space communications, and in-building automation connections. As the FSO technology becomes mature and increasingly involved, the security and reliability requirements of such applications also become crucial. Regarding the security issues, although the FSO communication is inherently more secure than the RF counterpart due to high directivity of its line-of-sight (LOS) link, it can still suffer from optical wire-tapping attack. There are many different application schemes that an eavesdropper can still probe the FSO link. Traditionally, to protect the communication, many works have been done on encryption layer and upper network layer, such as computationally intensive cryptographic algorithms and message exchanging protocols. In recent years, physical-layer security (PLS) has been getting more attentions as it provides an extra layer of security against eavesdropping. The PLS schemes, in general can be categorized into two main categories: classical PLS and quantum key distribution (QKD) protocols. The classical PLS is based on the information theoretic security analysis, while the QKD security is guaranteed by the quantum mechanics laws. Both techniques have been studied for FSO channels. In the first half of this dissertation, we study the PLS in a LOS FSO channel using orbital angular momentum (OAM) multiplexing and show that higher secrecy capacity can be achieved using OAM multiplexing technology in the presence of atmospheric turbulence effects. Then, we proposed an adaptation scheme for QKD system over FSO channel with different QKD protocols, i.e. BB84 and decoy state protocols. By adapting the source brightness based on the channel condition, we optimized the secret key rate (SKR) of the QKD system. Furthermore, we proposed a multiple spatial modes-based QKD system with backpropagation method to increase the total SKR through parallel channels. Regarding the reliability of an optical communication system as well as information reconciliation for both PLS and QKD schemes, the forward error correction (FEC) becomes an essential technique to enable high-speed transmission. The FEC with low-density parity-check (LDPC) codes has been studied for decades from LDPC block codes to the most recent spatially-coupled (SC) LDPC codes. For FSO communications, due to the time-varying nature of FSO channels, the capability of rate adaptation is important to achieve a consistent reliable transmission. In the second half of this dissertation, we proposed unified FPGA-based architecture to implement both quasi-cyclic (QC) LDPC codes and SC-LDPC codes with rate adaptation capability. The performance is verified through FPGA emulation to show good error floor properties. To further validate the rate adaptation capability, we have built a spatial light modulator (SLM)-based FSO channel emulator to test the adaptive coding scheme. Furthermore, we have conducted a fiber-based experiment with our rate adaptative coding scheme to show that the proposed adaptive coding scheme can be also used in dynamic fiber-based networks.
This paper presents an in-depth exploration of Quantum Spread Spectrum Code Division Multiple Access (QSSCDMA) communication systems. By manipulating the shape of quantum photon wavepackets and utilizing high-speed electro-optical modulators, we demonstrate the potential for spread spectrum encoding of quantum states of light for CDMA communication systems. This work builds on the classical spread spectrum technology’s strengths—namely, its robust interference management and low probability of intercept—by broadening the spectrum of transmitted signals through quantum mechanical principles. We rigorously characterize the mathematical framework underpinning quantum spread spectrum techniques, offering a comprehensive theoretical analysis and demonstrating the viability of QSS-CDMA systems. Our findings establish a foundation for future research and development in quantum communication systems, paving the way for advanced multi-user quantum networks.
When light is propagated through a channel that is not a waveguide, such as a fiber, it is known as a free-space optical channel. When used in communication systems, free-space optical channels provide the advantages of high communication rates, lower power, and minimal infrastructure installation. When the free-space optical channel has a random medium in it, such as the atmosphere, the distortions will be applied to the phase. The phase distortions can lead to intensity fluctuations and additional beam spreading. To compensate for the distortions adaptive optics can be used, allowing for the stability of the system to be improved. Additionally, post processing techniques on transmitted data can also be used to improve the stability of the channel. By significantly decreasing the number of photons transmitted, a classical communication channel can be considered as a quantum channel. Encoding information into the quantum states of the transmitted photons, quantum communication can be performed. When transmitting on the order of 1 photon per pulse, classical post processing techniques cannot be used to improve the stability of the channel. To stabilize the free-space optical channel, adaptive optics can be used at the transmitter or receiver. As no measurement of the quantum state is performed by the adaptive optics system, no information in the photon is lost. In this work, quantum key distribution will be implemented over the quantum channel. The focus of this work is on the design and study of how the channel degradations can be overcome, as well as the establishment of a free-space optical channel with adaptive optics in a quantum networking testbed. Quantum key distribution systems were simulated over a 30 km maritime channel as well as over a underwater channel. For the 30 km channel, the use of multiple parallel spatial modes results in the highest increases of the secure key rate at the cost of transmitting a lower average number of photons. When adaptive optics is applied, the secure key rates have a small increase, however as the strength of the turbulence increases from weak to moderate, the relative improvement is higher. For the underwater channel, operation at lower depths is optimal as fewer background photons reach the receiver and the channel attenuation is lower. In the experimental setup, it was found that the single photon detectors that are being used are the limiting the channel loss to be at most 6 dB. The free space optical link established between the Electrical and Computer Engineering building and College of Optical Sciences uses a corner cube retro-reflector to yield a distance of 1.45 km. The channel has been characterized at various times of day to determine the strength of the atmospheric turbulence, which ranges from a scintillation index of 0.07 to 2.21. A classical communication experiment was performed, resulting in error-free transmission while the input power to the EDFA before the photodetector was consistently above -25 dBm.
This paper describes the fundamental principles and mathematical foundations of Quantum Direct-Sequence Spread-Spectrum Code Division Multiple Access (QDSS-CDMA) communication systems. The evolution of quantum signals through the quantum direct-sequence spread-spectrum multiple access communication system is carefully characterized by a novel approach called the decomposition of creation operators. In this methodology, the creation operator of the transmitted quantum signal is decomposed into the chip-time interval creation operators each of which is defined over the duration of a chip. These chip-time interval creation operators are the invariant building blocks of the spread-spectrum quantum communication systems. With the aid of the proposed chip-time decomposition approach, we can find closed-form relations for quantum signals at the receiver of such a quantum communication system. Further, the paper details the principles of narrow-band filtering of quantum signals required at the receiver, a crucial step in designing and analyzing quantum communication systems. We show that by employing coherent states as the transmitted quantum signals, the inter-user interference appears as an additive term in the magnitude of the output coherent (Glauber) state, and the output of the quantum communication system is a pure quantum signal. On the other hand, if the transmitters utilize particle-like quantum signals (Fock states) such as single photon states, entanglement effect can arise at the receivers. The important techniques developed in this paper are expected to have far-reaching implications for various applications in the exciting field of quantum communications and quantum signal processing.
This paper describes the fundamental principles and mathematical foundations of quantum spread spectrum code division multiple access (QCDMA) communication systems. The evolution of quantum signals through the direct-sequence spread spectrum multiple access communication system is carefully characterized by a novel approach called the decomposition of creation operators. In this methodology, the creation operator of the transmitted quantum signal is decomposed into the chip-time interval creation operators each of which is defined over the duration of a chip. These chip-time interval creation operators are the invariant building blocks of the spread spectrum quantum communication systems. With the aid of the proposed chip-time decomposition approach, we can find closed-form relations for quantum signals at the receiver of such a quantum communication system. Further, the paper details the principles of narrow-band filtering of quantum signals required at the receiver, a crucial step in designing and analyzing quantum communication systems. We show that by employing coherent states as the transmitted quantum signals, the inter-user interference appears as an additive term in the magnitude of the output coherent (Glauber) state, and the output of the quantum communication system is a pure quantum signal. On the other hand, if the transmitters utilize particle-like quantum signals (Fock states) such as single photon states, entanglement and a spread spectrum version of the Hong-Ou-Mandel effect can arise at the receivers. The important techniques developed in this paper are expected to have far-reaching implications for various applications in the exciting field of quantum communication and signal processing.