Modulation Technology And Link Characteristics Of X-ray Communication In The Communication Blackout Region

In recent years,with the rapid evolution of the aerospace technology,the development and utilization of the near-space has become a focus point.When high speed spacecraft re-entries the atmosphere of earth,the friction between the spacecraft and the atmosphere is very intense because of the shock wave.The heat-resistant material near the spacecraft’s surface would be dissociated and ionized to form plasma sheath.The plasma sheath would weaken and distort the communication signal,and interrupt the radio communication signal between the re-entry spacecraft and ground-based station.Previous studies demonstrated that X-ray could penetrate the plasma sheath without attenuation,which means that using X-rays as the communication carrier may be feasible to eliminate reentry blackout.Based on the transmission theory of space X-ray communication（XCOM）,we proposed XCOM link model of communication satellite relay and stratospheric airship relay.And the communication performance of XCOM system in blackout occasion was studied.The main research contents are as follows:（1）According to the distribution of the atmospheric density and components,the near-earth atmosphere was divided into several layers.For two relay schemes of communication satellite and stratospheric airship,the propagation process of X-ray beams in different scenarios were simulated based on MCNP5 code.The results shown that the energy of X-rays needs to be increased to improve the transmission performance when the blackout phenomenon occur at low altitude region.In addition,the effective signal transmission can be realized by using soft X-ray energy band in high altitude environment.Furthermore,with the increase of the zenith angle,the transmission coefficient of X-ray decreases.（2）The signal modulation and demodulation technologies of XCOM system were studied in this chapter.The intensity modulation/direct detection（IM/DD）method was used to modulate the X-ray signal.The bandwidth requirements,power utilization,and bit error rate of modulation methods in IM/DD system were analysis.It was found that PPM modulation is the first choice of XCOM system in the blackout region,due to its low transmission power and high communication performance.（3）The communication link equation can systematically reflect the channel characteristics of XCOM in blackout region and the power conversion process in XCOM system.Combining with the noise source of XCOM system in blackout region,a noise model was established to obtain the relationship between SNR and BER.At the same time,the influence of factors on the communication performance was analyzed.The results shown that X-ray beams with 30-50 ke V and a CZT detector could effectively improve communication performance when the altitude of blackout region is 40 km and the satellite altitude is 330 km.Moreover,low-energy X-rays and the SDD detector system are highly suitable at altitudes above 60 km.Thus,the X-ray energy and type of detector should be chosen according to the required communication altitude.In order to realize the communication rate of 1 Mbps and the BER of 10^-6,the XCOM system with the energy of 30 ke V needs the transmitting power of 14.5 W when the altitude of stratospheric airship is 40 km and the reentry altitude is 100 km.This research work of the thesis will enrich the transmission theory of XCOM in blackout region and point out the direction for the development of core components.At the same time,it will provide a theoretical basis for the engineering of XCOM in blackout region.