Effective Transmission Method With Adaptive Nonstationary Channel Equalization for Hypersonic Reentry Communications

The reentry plasma sheath propagation environment is nonstationary and has high-dynamic characteristics. The communication signal suffers serious parasitic modulation effect since the time-varying degree of the channel is on the same order as the telemetry symbol rate. This paper proposes an effective transmission scheme to cancel the plasma sheath channel (PSC) distortion by utilizing the short spreading and adaptive nonstationary channel equalization technology. The proposed scheme spreads the telemetry signal and inserts a small number of pilot signals to determine whether the nonstationary state of the channel is changed by the channel amplitude attention value of the pilot signals. A novel adaptive nonstationary stochastic gradient recursive minimum mean square error (NSGR-MMSE) algorithm is then proposed to estimate the other channel response values in each current state. Finally, the channel equalization is performed to mitigate the damage from the parasitic modulation effect. The proposed method can accommodate the nonstationary characteristics of the channel and the small-scale channel distortion simultaneously with small bandwidth efficiency scarification and minimal algorithm complexity cost. The simulation results show that the proposed transmission scheme can ensure reliable communication under two typical reentry PSC models. Furthermore, the performance of the proposed adaptive NSGR-MMSE algorithm is improved by at least 3 dB versus the classic MMSE algorithm under the effective transmission scheme.