## 1 Introduction  
Hypersonic vehicles have the ability to fly at hypersonic speeds in near space, with speeds reaching over Mach five. They can reach any corner of the Earth within two hours, greatly reducing flight time and possessing the ability to break through all current ground military defense equipment [1, 2, 3]. Therefore, hypersonic vehicles have broad applicational prospects in both civilian and military fields, and research on their related technologies is receiving increasing attention both domestically and internationally [4, 5, 6, 7]. When a vehicle flies at hypersonic speed in the atmosphere, it violently impacts the air. At the front of the vehicle, the air is compressed violently, producing a very strong bow shock wave. Shock waves convert some of the kinetic energy of a hypersonic vehicle into thermal energy, which is then transmitted into the air to form high-temperature gases. Due to friction between high-temperature air and the surface of the vehicle, the gas near the surface of the vehicle further heats up, and a large number of air molecules are thermally ionized into ions, electrons, and charged particles. In addition, the surface of the vehicle will also be eroded by high-temperature gases, and the ablation products will participate in complex chemical processes. Subsequently, a layer of high-temperature charged gas is formed, covering the flight surface, known as a plasma sheath [8; 9; 10; 11]. When high-density free electrons interact with incident electromagnetic waves, charged particles in the plasma, especially free electrons, oscillate. The plasma sheath strongly absorbs and reflects electromagnetic waves, which attenuate the energy of communication signals. At the same time, due to the instability of hypersonic flow fields, the parameters of the plasma sheath medium vary randomly in space and time, adding severe multiplicative interference to communication signals and causing waveform distortion and spectral dispersion of signals. Conventional TT&C (Tracking, Telemetering, and Command) communication signals are difficult to receive and correctly demodulate, and in severe cases, communication interruptions and \"blackouts\" occur [12; 13; 14]. At present, the mainstream means to alleviate a \"blackout\" mainly include the following: (1) designing a more reasonable vehicle shape to reduce gas ionization on the surface of the vehicle [15; 16]; (2) releasing electrophilic liquids, neutralizing free electrons, and reducing the electron density of the plasma sheath [17; 18]; (3) with an additional magnetic field, utilizing Lorentz force to displace electrons and form low-density regions [19; 20; 21]; and (4) changing the carrier frequency of communication signals, utilizing the characteristics of plasma media to find communication windows, and improving the energy transmittance of electromagnetic waves [22; 23]. The first three physical methods can alleviate the influence of plasma sheaths on electromagnetic signals to a certain extent, but the shape design is constrained by various factors, such as the vehicle's volume and function, and cannot unilaterally align with the rationality of an aerodynamic shape. Electronic liquids and high-grade magnets are difficult to achieve a lightweight design and require a large amount of vehicle load. Therefore, changing the carrier frequency of communication signals has become the most easily breakthrough communication method for hypersonic vehicles. There are two main methods for changing communication frequency: the high-frequency method and the low-frequency method: (1) Plasma is a dispersive medium. When the frequency of electromagnetic waves (terahertz band) is greater than the frequency of plasma, the attenuation effect of plasma decreases, and electromagnetic waves can effectively penetrate the plasma sheath [24; 25; 26]. However, high-frequency electromagnetic waves will suffer from more severe path loss and atmospheric loss, requiring the use of satellites to achieve signal coverage and limiting the communication transmission distance.