Orthogonal time frequency space (OTFS) modulation has been confirmed to provide significant performance advantages against Doppler in high-mobility scenarios. The core feature of OTFS is that the time-variant channel is converted into a non-fading 2D channel in the delay-Doppler (DD) domain so that all symbols experience the same channel gain. In available literature, the channel is assumed to be quasi-static over an OTFS frame. As for more practical scenarios, the Doppler shift is dynamic and the channel gain could be time-variant as the environment or medium changes. In this paper, we analyze the characterizations of OTFS modulation over a more general multipath channel, where the signal in each propagation path has experienced a unique rapid fading. First, we derive the explicit input-output relationship of OTFS in the DD domain for the case of ideal pulse and rectangular pulse. It is shown that the rapid fading will produce extra Doppler dispersion without impact on delay domain. We next demonstrate that OTFS can be interpreted as an efficient time diversity technology that combines space-time encoding and interleaving against the rapid fading. Simulation results reveal that OTFS is insensitive to rapid fading and still outperforms conventional multicarrier systems in these channels.

The effects of time-varying plasma to the propagation of phase modulation signals are investigated in this paper. Through theoretical analysis, the mechanism of the interaction between the time-varying plasma and the phase modulation signal is given. A time-varying plasma generator which could produce arbitrary time-varying plasma is built by adjusting the discharge power. A comparison of results from experiment and simulation prove that the time-varying plasma could cause the special rotation of QPSK (Quadrature Phase Shift Keying) constellation, and the mechanism of constellation point’s rotation is analyzed. Additionally, the experimental results of the QPSK signals’ EVM (Error Vector Magnitude) after time-varying and time-invariant plasma with different ω_p/ω are given. This research could be used to improve the TT&C (Tracking Telemeter and Command) system of re-entry vehicles.

Reentry plasma sheath diagnostic plays extremely important roles in the research of the blackout problem. In this paper, a broadband microwave reflectometer based on the invariant point of reflection data has been designed and developed for the reentry plasma sheath diagnostic. The invariant point Y(ω_p, 1) is derivable from the broadband reflection data. The electron density and collision frequency are measured simultaneously. The diagnostic algorithm was verified by the simulation analysis and diagnostic experiment. The algorithm has low computational complexity and relatively high precision. This algorithm offers a certain reference value for the plasma sheath diagnostic of reentry vehicles or hypersonic vehicles in the future. Especially, the design of broadband VVD antenna with heat-resistant wave-transparent composites has certain practical values.

Exhaust-plume-induced plasma from solid rocket engines absorbs electromagnetic (EM) waves and may cause communication interruptions. Previous papers have proposed both physical phenomena and propagation models for this problem, but few papers have analyzed random fluctuations of EM waves propagating through plumes from a communications perspective. This paper attempts to attach these fluctuations, which are caused by the plume-induced turbulent plasma spread medium, to the communications channel characteristics. A multi-parameter stochastic model is established to characterize the exhaust plume channel in which the first-order and the second-order statistical properties are presented. Accordingly, a ground firing test was conducted to validate the EM wave propagation characteristics and the channel characteristics. The experimental results are in good consistence with the theoretical results. The results indicated that the time-varying envelope of the transmitted EM waves obeys a log-normal distribution, whereas the time-varying phase shift obeys a normal distribution. Moreover, the time-frequency analysis method was applied to evaluate the EM-plume interactions in the time-frequency domain. These results are critical for designing the rocket communication systems and evaluating the systems performance.