Decrypting X-ray Space Communication
X-ray communication, as the name suggests, uses X-rays as carrier waves to load information onto one or several physical parameters of X-ray photons and send them outward.
In fact, it is still a communication method that uses electromagnetic waves, and it is not essentially different from traditional communication methods such as microwave communication and laser communication.
X-rays will encounter serious attenuation when propagating in the atmosphere, but when the X-ray photon energy is greater than 10keV and the atmospheric pressure is less than 10-1Pa, the X-ray transmittance can reach 100%, that is It is said that there is no physical attenuation of X-ray propagation in a vacuum environment.
However, the wavelength of X-rays is shorter, and the bandwidth of the communication system is theoretically higher. PorterGeorge believes that the maximum theoretical rate of X-ray communication can reach 40,000Tbps; the energy of a single photon is higher, and is affected by the interaction between high-energy particles and space electricity and magnetic fields. The impact is smaller and more Meets the requirements for communication in complex aerospace environments; the X-ray beam divergence angle is very small, and the free space loss is very small, so it is expected to achieve long-distance space transmission with smaller volume, weight, and power consumption; in addition, X-ray penetration capabilities Strong, the use of X-rays for communication is highly directional and confidential.
Unlike traditional microwave, laser and other communication methods, which are greatly reduced in reliability or even unable to communicate when affected by shielding interference and space weather changes, X-ray communication can work normally under the influence of electromagnetic shielding and complex space environments.
It is foreseeable that space X-ray communication is not only a supplement to microwave and laser communication, but also a disruptive replacement for traditional communication methods in complex space environments and special applications.
In 2007, Dr. Keith Gendreau of the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center first proposed
Researcher Zhao Baosheng’s team at the Xi’an Institute of Optics and Mechanics of the Chinese Academy of Sciences proposed the concept of X-ray communication for the first time in China in 2011, and applied for an international patent with a completely different technical solution from the United States.
On January 19, 2012, " The headline on the fourth page of the China Science Journal was titled "Proposed new method for space X-ray communication" and made a relevant report on the research progress of Zhao Baosheng's team, which attracted great attention from domestic and foreign experts.
In the plan. A method based on grid modulated X-ray transmitter (GMXT, patent authorization number: 2011102600121520) as the X-ray pulse radiation source is proposed, and the MCP single photon detector with weak light signal detection capability is used as the receiving transducer.
The developed principle prototype has been initially verified in the laboratory
X-ray space communication test Demo system
For any wireless communication system, it mainly consists of three parts: the transmitter, the receiver and the communication channel.
The same is true for space X-rays. First, a device that can load information into the X-ray is required. Modulated emission source on the physical parameters of ray photons, as well as being sensitive to the X-ray band and able to Detection device for information parameter restoration.
Due to the magical properties of X-rays, when applied to the field of space communication, it will achieve better performance than traditional communication methods, but it will also Core components such as transmission, reception, and modulation have put forward more stringent requirements.
1. Research on X-ray communication theory
Based on the space environment at different orbital heights, theory and experiment are combined to study the interaction mechanism between X-ray photons, high-energy particles, and plasma; study the random interference caused by the space environment and detection mechanism, and provide interference Model calculation process; establish a space channel model for X-ray communication, giving the communication distance, communication rate, and transmission power and error characteristics and other main index calculation equations.
Aims to use theoretical analysis and software simulation to give a clear analysis of the transmission process of X-ray communication, and to establish a channel model for X-ray space communication based on this. and noise models, to discover the advantages of X-rays as carriers early and implement them in engineering applications.
2. Generation of X-rays
How to generate X-ray photons suitable for communication and select appropriate X-ray parameters as information carriers.
And in space communications, the operating distance is very long (for example, the transmission distance of space links between satellites can reach thousands of roads, or even tens of thousands of roads), and the background light interference is strong under such conditions. Under the circumstances, a high-power, stable light source is an important prerequisite to ensure fast and reliable data transmission at the receiving end.
In addition, the higher the power of the X-ray transmitter, the more transmission is required to close a given link. The smaller the aperture, this can reduce the size of the entire device.
Space X-ray communication systems are mostly used on satellites and some portable devices. Since the power resources on these devices are very precious, high power output is required. At the same time, low energy consumption must be ensured, which requires the X-ray transmitter to have high efficiency.
In addition, its lifespan is also an important performance indicator, especially for applications on satellites. It should match the service life of the entire system and satellite.
The research goal is to optimize the X-ray emission source. It is planned to study the use of carbon nanotube technology X-ray emission tubes to replace the traditional hot cathode emission tubes. To obtain high-efficiency, low-power, wide-bandwidth X-ray pulse modulation emission source
3. X-ray encoding modulation
, and even the entire optical communication field, first of all due toThe carrier frequency is very high and its particle nature begins to appear.
Information is transmitted through discrete X-ray photons. For the time resolution capability of existing X-ray detectors, its fluctuation is not very obvious, which is reflected on the receiving end detector. , X-ray photons are characterized as pulse trains that are discrete in time.
So for current X-ray communication systems, IM/DD (intensity modulation/direct detection), that is, direct envelope detection of intensity modulated optical carrier signals, is still a common practice.
The next research goal is to find coding and modulation methods with higher efficiency and better error characteristics, in order to obtain better communication effects.
4. Detector research
Depending on the application, appropriate detection needs to be selected based on the X-ray parameters of the so-called information carrier device.
For example, when the X-ray signal reaching the detector is relatively weak, it is more appropriate to choose an MCP (MicroChannelPlate) detector with single-photon testing capabilities.
For another example, when using analog amplitude modulation, it is necessary to perform energy resolution on the received X-rays. Then a detector with strong time resolution and high detection efficiency should be selected, such as SDD (Silicon Drift Detector, Silicon Drift detector.
) It should be noted that different detectors correspond to different signal demodulation methods, and their corresponding noise interference models are also different.
In general, different detector noise models are considered based on detector type and background noise level.
When a photon counter detector is used, the noise mainly comes from the photon detector dark current and low-energy background light interference. At this time, the noise distribution conforms to the Poisson noise model. In other words, the noise is expressed as Wrong photon pulse.
When considering the Gaussian noise model, most of the noise is thermal noise.
In addition, excessive interference can result when background light is not effectively filtered out, and these noise situations also apply to Gaussian models.
