Link Performance Analysis of X-Ray Communication System
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Establishing the theoretical relationship between the unique properties of X-ray photon (large energy, non-dispersive, strong penetrability, etc.) and the link performance is the primary problem needed to be resolved in X-ray communication system design. This paper firstly outlines the research histories of X-ray communication at home and abroad, the effects of link parameters such as the detector sensitivity, the divergence angle and the propagation distance on the propagation loss are studied by numerical simulation. Then, the theoretical link equation of X-ray communication is derived, and the solutions to the key technologies which restricting X-ray communication performance are proposed in the end to provide theoretical support for X-ray application in large capacity and high reliable information transmission in space.
KeywordsX-ray communication Link model Performance analysis
X-ray has a extremely short wavelength ranging from 0.01 to 10 nm which was discovered by German physicist W.K. Roentgen in 1895. Recent studies show that when the X-ray photon energy is greater than 10 keV, with the wavelength less than 1 nm and the atmospheric pressure lower than 10−1 Pa, the transmittance of X-ray is approximate to 100%, which means the propagation of X-ray in free space is almost non-attenuated. The extremely short wavelength of X-ray has the potential advantages to lower requirements on SWAP; the exceedingly high carrier frequency which is greater than 1018 Hz means significantly larger bandwidths for a capability that would allow the transmission of gigabits per second throughout the solar system; and its strong penetrability means the safe and reliable communication in some special environments as the electromagnetic shielding area or blackout zone [1, 2]; which makes X-ray a new choice in future space communication of deep space detection, inter-satellite high-speed data link and space networks.
2 Overview of X-Ray Communication
Dr. Gendreau at Goddard first put forward the concept of X-ray communication in 2007; then, a point-to-point communication system used X-ray photons was demonstrated, which verified the feasibility of X-ray communication in vacuum environment [3, 4]. Dr. Daniel in the Johns Hopkins Applied Physics Laboratory studied the information-theoretic limits for the performance of X-ray Communication . Then, the X-ray communication technology was considered as the revolutionary technology in NASA Communication and Navigation System Roadmap in 2012 . Based on the above work, NASA launched the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) project in 2013, which combines the Neutron-star Interior Composition ExploreR(NICER) project with the X-ray Communication(XCOM) project, so SEXTANT = XNAV + NICER + XCOM. The payload was launched and fixed on the International Space Station (ISS) in June 2017, and a space demonstration of X-ray communication using NASA’s NavCube at a distance about 50 m is planned in spring 2019 [6, 7]. In 2014, researchers at NASA ASTER Laboratory proposed the idea of integrating X-ray directly into radio and optical communication systems to achieve a radio, optical and X-ray integrated communication system (iROX: integrated radio, optical and X-ray communications system) .
Meanwhile, domestic researchers are mainly focusing on the X-ray communication application modes in special environment, such as low-rate and high-reliable X-ray communication in space [9, 10, 11]; integrating X-ray pulsar navigation (XPNAV) and X-ray communication (XCOM) to improve the XPNAV signal strength ; X-ray propagation in plasma sheath when the spacecraft reenters into the atmosphere [13, 14]; a new interplanetary communication method based on modulated X-ray array ; and with a few theoretical studies on Bit Error Rate (BER) and modulation performance of X-ray communication [9, 16].
The above studies show that using X-ray as a carrier in information transmission is feasible. However, the experiments used in demonstration were mainly based on the existing technology in the field of X-ray astronomy, which limited the performance improvement of X-ray communication due to different needs in their respective fields. To take full advantages of X-ray properties, the basic theory of X-ray space communication needs to be further improved. Based on the traditional optical communication theory, this paper tries to derive the link equation of X-ray communication, makes clear the constraints between link elements with X-ray photon properties, and provides technical supports for X-ray communication system design.
3 X-Ray Communication Link Model
3.1 Receiving Sensitivity
Receiving sensitivity at different transmission rates
3.2 Transmission Loss
3.3 Link Equation of X-Ray Communication
4 Key Technologies and Solutions
Because of the unique properties of X-ray photon energy, the requirement on detector receiving sensitivity is lower than the traditional visible light communication. The main factors affect X-ray communication performance are the X-ray source and X-ray optical system; the key technologies needed to breakthrough are discussed as follows in a practical point of view.
4.1 High-Speed X-Ray Modulation Technology
The existing X-ray source modulation schemes for X-ray communication are based on the principle of X-ray tube and its improvement method [19, 20, 21, 22]. However, due to the limitation of the X-ray bremsstrahlung radiation, whether using photocathode or hot cathode, the electron generation, interior Grid modulation and electron acceleration were coupled together, which limits the emitted power in several nW scales and the modulation bandwidth in several KHz scales. The future researches need focus on the new cathode materials such as carbon nanotubes, and the high-speed external modulation methods.
4.2 High-Efficiency X-Ray Optics Technology
The traditional optical focusing principle of visible light is no longer suitable for X-ray as its refractive index is close to 1; only the grazing incidence with the incidence angle less than 3 degrees can change the incident direction. At present, the focusing efficiency of multilayer nested Wolter-I optical system can reach about 50% theoretically , and the divergence angle of X-ray optical system is generally in several mrad scales by grazing incidence. It is hopeful to develop some new X-ray optical technologies to improve the divergence angle to μrad level.
The theoretical link model of X-ray communication was established with the constraints analyzed among the link parameters such as the detection sensitivity, transmitted power, beam divergence angle, propagation distance, etc. The results show that due to the large energy of X-ray photon, the constraints on detector sensitivity were reduced. The X-ray transmission rate and distance are mainly affected by its transmitted power and divergence angle. In order to achieve long distance and high transmission rate, high speed X-ray modulation method and X-ray microporous optical technology need to be further studied.
Supported by the National Natural Science Foundation of China (Grant No. 61601463/11405265).
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