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Link Performance Analysis of X-Ray Communication System

  • Jing MengEmail author
  • Qing-qing Lin
  • Xiao-min Bei
  • Qiang Chen
  • Yao-jun Wu
Review
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Abstract

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.

Keywords

X-ray communication Link model Performance analysis 

1 Introduction

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 [5]. Then, the X-ray communication technology was considered as the revolutionary technology in NASA Communication and Navigation System Roadmap in 2012 [1]. 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) [8].

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 [12]; 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 [15]; 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

The receiving sensitivity refers to the minimum input power required by the receiver at the required BER. The main factor affecting receiving sensitivity is the receiver noise. Generally, the noise of photodetector mainly includes shot noise, electron multiplier noise, thermal noise, etc. [17, 18]. The shot noise is caused by the interaction between signal photon, back ground photon and dark current in detector, and the dark current is usually considered as part of background radiation in practical analysis. Without considering the noise caused by the electron multiplier, the total noise of the photodetector can be approximately expressed as
$$ \sigma^{2} = 2e\left( {I_{\text{p}} + I_{\text{d}} } \right)\Delta f + \left( {\frac{4KT}{{R_{\text{L}} }}} \right)\Delta f, $$
(1)
where \( e \) is the electronic charge, \( I_{\text{p}} \) is the output current of photodetector, \( I_{\text{d}} \) is the dark current of photodetector, \( \Delta f \) is the equivalent bandwidth, \( K \) is the Boltzmann constant, \( T \) is the absolute temperature, \( R_{\text{L}} \) is the equivalent resistance. When the received optical power is low, undering the thermal noise limit, the minimum average received power can be expressed as
$$ \overline{{P_{\hbox{min} } }} = \frac{{Q\sigma_{\text{T}} }}{R} = \frac{Q}{R}\left[ {\left( {\frac{4KT}{{R_{\text{L}} }}} \right)\Delta f} \right]^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} , $$
(2)
where \( Q = (I_{1} - I_{0} )/(\sigma_{1} + \sigma_{0} ) \), \( I_{1} \) and \( \sigma_{1} \) are the output current and noise when transmitting bit “1”, respectively, \( I_{0} \) and \( \sigma_{0} \) is the output current and noise when transmitting bit “0”, respectively, \( \sigma_{\text{T}} \) is the thermal noise, \( R = \eta e\lambda /hc \) is the responsivity of detector, \( \eta \) is the quantum efficiency, \( e \) is the electronic charge, \( \lambda \) is the wavelength, \( h \) is the Planck constant, \( c \) is the speed of light in vacuum.
When using the optimal decision, the BER of OOK modulation can be expressed as
$$ {\text{BER = }}\frac{1}{2}\text{erfc}\left( {Q/\sqrt 2 } \right) $$
(3)
For X-ray communication link, the minimum received optical power of the detector can be calculated according to Eqs. (2) and (3). Assumed BER requirement is 10−7, the X-ray photon energy is 1 keV, the equivalent resistance is 50 \( \varOmega \), the receiving sensitivity at different transmission rates is shown in Table 1. It can be seen that lowering the same BER requirement, the receiving sensitivity of X-ray detector decreases with the transmission rate increases, which means the requirement on minimum receiving X-ray power increases. To achieve 1-Gbps transmission rate, the minimum receiving optical power of the X-ray detector should be in mW level, which is a great challenge for long-distance communication. Nevertheless, considering the X-ray photon energy is greater than 1 keV, it is easy to satisfy the power requirement in receiver. At the same time, another advantage is that undering the same requirement on optical power, the number of X-ray photons needed to be detected is significantly reduced. Consequently, the shot noise of detector is reduced; the SNR can be enhanced.
Table 1

Receiving sensitivity at different transmission rates

Transmission rate

1 Kbps

1 Mbps

1 Gbps

Sensitivity (dBm)

− 35.41

− 20.41

− 5.41

3.2 Transmission Loss

The X-ray transmission loss in free space is shown in Fig. 1; here, the radius of the emitted beam is \( d/2 \), the divergence angle is \( \theta \), the transmitted distance \( R_{0} \), the radius of the emitted beam at the detector is extended to \( r \). Assume that the X-ray power distribution is uniform and the receiving and transmitting optical antennas are the same, the transmission loss can be expressed as [17]
$$ L = \frac{{\pi (d/2)^{2} }}{{\pi r^{2} }}, $$
(4)
where the radius \( r \) can be described as:
$$ r = d/2 + R_{0} \text{tg}\theta $$
(5)
Fig. 1

Illustration of transmission loss of X-ray in free space

The variation of transmission loss with the divergence angle at different transmission distances (100 km/1000 km/10,000 km) is shown in Fig. 2. The transmission loss increases with the increase of transmission distance and divergence angle obviously. With the fixed effective area of detector and the transmission distance, compressing the divergence angle is an efficient way to reduce transmission loss, especially in long-distance communication. Based on the diffraction-limited theory, the X-ray wavelength is three orders of magnitude smaller than the visible light, which means that the X-ray can use more concentrated energy beam for long-distance propagation.
Fig. 2

Transmission loss vs divergence angle at different distances

3.3 Link Equation of X-Ray Communication

Based on the traditional optical communication [18], the link equation of X-ray communication can be modified as
$$ P_{\text{r}} = P_{\text{t}} \cdot \eta_{\text{t}} \cdot L \cdot \eta_{\text{r}} \cdot L_{{\text{APT}}} \cdot M, $$
(6)
where \( P_{\text{r}} \) is the received optical power at the detector, \( P_{\text{t}} \) is the transmitted optical power, \( L \) is the transmission loss in free space, \( \eta_{\text{t}} \) and \( \eta_{\text{r}} \) are the efficiency of transmitted and received optical system, respectively, \( L_{{\text{APT}}} \) is the pointing errors, \( M \) is the link margin. In X-ray communication link, supposing the X-ray tube is used at the transmitter, the transmitted power can be described as \( P_{\text{t}} { = }P_{\text{E}} \cdot \eta_{\text{e}} \), \( P_{\text{E}} \) is the electrical power and \( \eta_{\text{e}} \) is the efficiency of X-ray tube. \( \eta_{\text{t}} \) and \( \eta_{\text{r}} \) are mainly determined by the transmittance of the transmitted/received optical system, if only considering the incident window materials, when using beryllium (Be) as window material, its transmittance in soft X-ray band is better than 0.8. So, the received X-ray power can be expressed as
$$ P_{\text{r}} = P_{\text{E}} \cdot \eta_{\text{e}} \cdot \eta_{\text{t}} \cdot \eta_{\text{r}} \cdot L_{{\text{APT}}} \cdot M \cdot \left( {\frac{d/2}{{d/2 + R_{0} \text{tg}\theta }}} \right)^{2} $$
(7)
To study the constraints between the X-ray link parameters, set the X-ray communication link parameters as follows: the X-ray tube power is 0.1 W, the tube efficiency is 0.01, the optical antenna aperture is 0.2 m, the efficiency of transmitted and received optical system is 0.5. Figure 3 shows that the received power of X-ray detector varies with the divergence angle at different transmission distances; the received power decreases rapidly with the increase of transmission distance and divergence angle. The condition to establish basic communication links is that the receiving power of the detector must be greater than its receiving sensitivity. Considering the link margin should be greater than 3 dB, if the transmission distance of X-ray is 1000 km and the transmission rate is 1 Mbps, the divergence angle of the X-ray beam at the transmitter should be better than 20 μrad.
Fig. 3

Received power vs divergence angle at different distances

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 [6], 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.

5 Conclusion

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.

Notes

Acknowledgements

Supported by the National Natural Science Foundation of China (Grant No. 61601463/11405265).

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Copyright information

© Chinese Society of Astronautics 2019

Authors and Affiliations

  • Jing Meng
    • 1
    Email author
  • Qing-qing Lin
    • 1
  • Xiao-min Bei
    • 1
  • Qiang Chen
    • 1
  • Yao-jun Wu
    • 1
  1. 1.Qian Xuesen Laboratory of Space TechnologyBeijingChina

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