When hypersonic vehicle flies in the atmosphere at a high altitude with a high speed, plasma sheath is generated around the vehicle, and thus attenuating the electromagnetic wave signals and even interrupting the communication. Therefore the control, guidance, and navigation of hypersonic vehicle can be affected seriously by the plasma sheath. It is necessary to study this problem in reasonable ground experiment. The inductively coupled plasma (ICP) wind tunnel is an ideal equipment for studying electromagnetic transmission characteristics in plasma because it can produce uncontaminated plasma and the electrode cannot be ablated in the process of plasma production. We carry out the experiment in ICP wind tunnel. A thin slice of plasma jet is generated by a rectangular nozzle with an outlet size of m 50 mm250 mm. Plasma jets with different parameters are obtained by adjusting the operating power and inlet flow of the wind tunnel. Four kinds of states are provided with the electron densities of 7.01010, 5.01011, 3.51012 and 1.01013/cm3, and the collision frequencies of 1.5109, 1.6109, 2.0109 and 9.0109 Hz, respectively. The amplitude attenuations and phase changes of the electromagnetic waves are measured with microwave diagnostics system consisting of a vector network analyzer and high gain antennas. And electron density and collision frequency of plasma are obtained according to the transmission characteristics of electromagnetic waves in plasma. The attenuations of the electromagnetic wave in plasmas of different states are measured via microwave transmission system which is composed of a vector network analyzer and pairs of horn antennas covering a frequency range of 2.6-40 GHz. The results show that both the amplitude attenuation and attenuation band increase with the increase of electron density. The classical theory and thin layer theory are used to simulate the transmission attenuation. The results are compared with the experimental ones. The results in this paper provide basic data for further theoretical and numerical study of electromagnetic wave transmission characteristics in plasma.

In this paper, an attempt to exploit the benefits of data-driven methods in solving joint relay selection and beamforming for non-regenerative relay networks has been made. The common relay selection and beamforming optimization problem aiming to maximize the receiver's achievable rate under the constraint of relay transmit power is intrinsically hard since the mixed discrete and continuous variables. The direct map from channel state information to select a relay with optimized beamforming weights via data-driven methods often fails to yield good results. To overcome this difficulty, we propose a two-stage algorithm based on data-driven method. Firstly, we convert relay selection to a multi-class classification problem, and a Support Vector Machine(SVM) based data-driven scheme is suggested to determine the best relay. After the relay is selected, we utilize a closed-form solution to obtain the corresponding relay beamforming weights. Since the number of relays is often more than two, the sample imbalance problem exists in the classification problem, considered in our proposed data-driven scheme. The core idea of SVM based classification method is equivalent to training the optimal parameters of the SVM classifier through a large number of offline sample data. In this way, the computation of relay selection can be transferred to offline SVM training. Simulation results demonstrate that the performance of the proposed method is close to that of the global optimal relay selection scheme. Moreover, our proposed scheme has much lower complexities than the international optimal relay selection scheme, especially when the number of relays is large.

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