# Propagation Characteristics of Modulated EHF Signal in the Wake Region of Plasma Sheath
###### Abstract  
A large number of studies have confirmed that the wake region may be a more ideal antenna installation area for the reentry vehicle communication problem, but many practical issues such as how to choose the modulation mode, carrier frequency, and antenna orientation are still pending. Based on numerical simulations, the characteristics of tail channels and the bit error rate (BER) of extremely high-frequency (EHF) communication in the wake region of the plasma sheath of hypersonic vehicle are studied. It is found that, with an increase in the angle between the tail channel and the tail of the vehicle, the attenuation of the EHF signals decreases and the phase shift fluctuates more severely. In order to obtain better communication performance, 2 PSK with a carrier frequency of 140 GHz or 225 GHz, or 4 QAM (QPSK) modulation with a carrier frequency of 140 GHz, and the tail channel with an angle between 50\({}^{\circ}\) and 60\({}^{\circ}\) to the tail of the vehicle can be selected. This study reveals the propagation characteristics and BER performance of EHF signals in the wake region of plasma sheath, which can provide a valuable reference for the design of the hypersonic vehicle communication system. plasma sheath; wake; modulated EHF signa; SMM; antenna location +
Footnote †: journal: Journal: Journal: Aerospace  
Footnote †: journal: Journal: Journal: Journal
Some groups have studied the propagation characteristics of an EHF wave in inhomogeneous colliding magnetic plasma [14], non-magnetic plasma [12; 15; 16; 17], high-temperature, non-magnetic plasma sheath [18], and realistic plasma sheath [11; 12; 13] from many aspects. [PERSON] et al. found that the total attenuation of an EHF signal is affected by the plasma distribution structure on the signal transmission path [19]. Moreover, the spatiotemporal characteristics of plasma sheath have also been widely studied. The results showed that the spatial distribution of plasma sheath is inhomogeneous and time-varying [20; 21; 22; 23]. In addition, the installation positions of the communication antenna has been a matter of concern in the recent decade. Among them, the wall and the nose of the vehicle were studied earlier and more comprehensively, because the direct communication link between ground station and vehicle is usually considered. The antennas for RAM C-I and RAM C-II flight tests are installed near the nose and on the wall of the vehicles, respectively [1]. [PERSON] et al. showed that installing a remote antenna module in a dedicated small container on top of the vehicle can provide uninterrupted radio communication with the reentry vehicle, although there are many problems related to the heating and thermal protection of vessels and towers [6]. On the other hand, some research studies have revealed that installing the antenna on the wall close to the tail is helpful for mitigating the \"blackout\" for a blunt-coned reentry vehicle [11; 24]. In those works, the communication signals are usually supposed to penetrate the plasma sheath via the aft region, which is near the wall and close to the tail of the vehicle. In other words, the channel is supposed to be in the aft region in those works. On the other hand, the wake region has also been suggested to possess potential channels by some researchers. The wake region was investigated by scientists and engineers soon after the blackout was discovered in 1950s [25]. In 1960s, the US launched the FIRE project, which partly aimed at investigating the flow field of the wake region of reentry plasma sheaths [26]. Since then, researchers have studied the evolution of the structure of wake regions and its mechanics. According to previous work, although the energy loss of propagating signals could occur due to the scattering caused by turbulences in wake regions [27], the electron densities in the wake region of blunt-coned plasma sheaths are lower than that in the aft region [28; 29].