During hypersonic flight, the communication link is affected by the multipath effect and plasma sheath. As a result, envelopes and phases of the wireless signals fluctuate with time. In this paper, an integrated channel model is proposed based on geometric propagation theory. The statistical characteristics of the plasma sheath channel are carefully studied. The probability density functions (PDF), the correlation coefficient between envelope and phase, the autocorrelation function (ACF), the level-crossing rate (LCR) and the average fade duration (AFD) are derived and detailed based on electromagnetic propagation theory. Two numerical examples are also used to demonstrate the effectiveness of the model. All statistical curves agree with the theoretical curves. The proposed model comprehensively considers the multipath effect, the Doppler effect and the plasma sheath effect. It is crucial to plausibly understand the blackout problem to facilitate performance evaluations of hypersonic vehicle communication systems.

Synchronization of large acquisition bandwidth brings great challenges to the traditional second-order phase-locked loop (PLL). To address the contradiction between acquisition bandwidth and noise suppression capability of the traditional PLL, a new second-order PLL coupled with a nonlinear element is proposed. The proposed nonlinear second-order PLL regulates the loop noise bandwidth adaptively by the nonlinear module. When a large input–output phase error occurs, this PLL reduces the frequency offset quickly by taking advantage of the large bandwidth. When the phase error is reduced by the loop control, the proposed PLL suppresses noises by using the small bandwidth to increase the tracking accuracy. Simulation results demonstrate that the tracking speed of the proposed PLL is increased considerably, and its acquisition bandwidth is increased to 18.8 kHz compared with that of the traditional second-order PLL (4 kHz).

Large and rapidly changing Doppler shift poses a huge challenge on fast carrier acquisition for hypersonic vehicle communications. In this study, a novel Doppler shift acquisition method is proposed to increase the acquisition probability and reduce the acquisition time simultaneously. Firstly, the Doppler rate is estimated with a proposed fast blind estimation algorithm. The order of Doppler shift is reduced though twice instantaneous delay autocorrelation operation on the received signal and the modulated information is cancelled, and the Doppler rate can be obtained by the corresponding spectrum analysis. Secondly, the received signal is compensated by the estimated Doppler rate. Finally, the maximum value of the average power spectrum of the compensated result over multiple symbols is detected, and the Doppler shift and Doppler rate can be jointly determined. Furthermore, to improve the acquisition speed, a corrected narrower range, which is determined by the flight conditions of the hypersonic vehicles, is proposed to constrain the search range of the power spectrum peak. Simulation results indicate that, under typical hypersonic flight conditions, the acquisition probability of the Doppler rate increases from 0.52 to 0.86 under − 20 dB. The processing speed can be improved by 75.47% when the length of the combined symbols is 1000.

Time-varying plasma sheath likely causes parasitic amplitude and phase modulation effects on communication signals. It can increase the carrier tracking error of tracking telemetry and command systems for hypersonic vehicles flying in near space, and it may lead to loss of lock under serious conditions. This paper comprehensively analyzes the influence mechanism of the amplitude modulation effects on the loss threshold of Type3-phase-locked loop (PLL) in phase-locked receivers. Analysis result shows that amplitude fading in time-varying plasma sheaths is similar to the Gaussian distribution unrelated to the fluctuant densities. The standard deviation of amplitude fading increases with the fluctuant intensity. The 8% fluctuant intensity of electron density begins to have a significant impact on the lock probability of Type3-PLL. On the basis of the analysis results and the amplitude modulation effects, Type3-PLL with a prefiltering (PF-Type3-PLL) method is proposed to mitigate the amplitude modulation effects. PF is placed before the discriminators of Type3-PLL to restrict the amplitude variation within a reasonable range. The tracking loop PF-Type3-PLL is tested with spatial–temporal fluctuant plasma sheath model and compared with the traditional Type3-PLL. Tracking results show that PF-Type3-PLL is more robust than the traditional Type3-PLL. The phase error jitter is significantly reduced when the fluctuant intensity is 8%, and the loss threshold is reduced by 4 dB. The robustness is also enhanced under different loop noise bandwidths (5–15 Hz).