Characteristics of Reentry Plasma Sheath Channels in the Delay-Doppler Domain for OTFS-Based Communication Systems

The absorption characteristics of the plasma sheath have a significant impact on the propagation of electromagnetic (EM) waves. The high-speed motion of hypersonic vehicles, combined with multipath transmission, further intensifies the selective fading of reentry plasma sheath channels in the time-frequency (TF) domain. These effects can severely degrade communication quality and, in extreme cases, lead to complete radio blackout. Recently, a novel modulation technique—orthogonal time-frequency space (OTFS)—has been proposed to enable reliable communication in high-mobility environments by operating in the delay-Doppler (DD) domain. However, the nonstationary flow field around reentry vehicles causes fluctuations in plasma sheath parameters, resulting in rapid time-varying (RTV) fading. To date, no studies have explored the behavior of such channels in the DD domain, which is essential for the effective deployment of OTFS in this context. This article presents a theoretical analysis demonstrating that RTV fading in reentry plasma sheath channels induces Doppler-axis energy leakage in the DD domain channel impulse response (CIR), with a leakage pattern that aligns with the spectral characteristics of the time-varying fading coefficients. In addition, we analyze the impact of two dominant dynamic factors in such channels—time-varying electron density and time-varying Doppler shift—on the CIR. Furthermore, we show that the parameter configuration of the OTFS system also plays a critical role in shaping the CIR. Finally, we verify the performance of the OTFS system through simulations, confirming its potential for robust communication under extreme reentry conditions.