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In this paper, a synthesis and implementation method for generating Bessel beams based on time-reversal theory incorporating electromagnetic (EM) meta-lens is proposed. As is known, time-reversal EM waves has the unique characteristic of adaptive backtracking. Therefore, with this characteristic, the EM characteristics of the radiation aperture can be obtained and further utilized to generate Bessel waves with any departure angle. Based on this concept, two meta-lenses for generating Bessel beams tilted in different directions were designed. Both meta-lenses were designed at the center frequency of 15 GHz, and the simulation results were consistent with the target expectation. A representative meta-lens was fabricated and measured. The final size of the meta-lens was 350 mm × 350 mm, and a Bessel beam with a 30° emergence angle was generated by this structure. The experimental results were in good agreement with the simulation results and the theoretical derivation. This synthetic method of Bessel beam generation using the time-reversal operation may be of great use for the application of Bessel beams in microwave communications, and it can broaden the application scope of non-diffracting beams.

Structurally integrated antenna is a kind of highly integrated microwave device with a load-bearing function, and it is usually installed on the structural surface of the air, water and ground vehicles. This paper presents the design, fabrication and testing of a novel structurally integrated Ka-band active antenna for airborne 5G wireless communications. The proposed antenna is mainly composed of three parts: a package layer, a control and signal process layer and a RF layer. In the RF layer, the microstrip antenna array, tile transmitting (Tx) modules, micro-channel heat sinks and a stripline feeding network are highly integrated into a functional block with a thickness of 2.8 mm. Electromechanical co-design methods are developed to design the active antenna array with the superstrates, and two schemes for designing micro-channel heat sinks are evaluated to obtain a uniform temperature distribution. The RF layer is fabricated by using the low-temperature cofired ceramic process, and the three layers are assembled to form the full-size antenna prototype. The mechanical and electromagnetic experiments are carried out, and the results demonstrate the feasibility of the structurally integrated active antenna for airborne wireless communications.

The resolution limitation is a main problem in the time-reversed field shaping system. In this paper, a subwavelength field shaping approach is proposed by combining time reversal (TR) technique and defective resonant metasurfaces. The subwavelength 2D and 3D field patterns can be effectively realized in both single-target and multi-target field-shaping systems. The resolution in both single-target and multi-target field-shaping systems aided with defective metasurfaces is approximately 0.1 λ, which is corresponding to the dimension of the applied metasurface cell. The proposed technique can be used for wireless power transfer, industrial heating, and localized wireless communications.

A transmitting coil with a tumbler structure is presented to improve lateral misalignment tolerance of a wireless power transfer (WPT) system with a planar receiver. The tumbler is made of an acrylic ball and soil. A planar spiral receiving coil wound by Litz wire is centered by Nd-Fe-B magnets. A spherical cap transmitting coil centered also by Nd-Fe-B magnets sticks on the inner surface of the acrylic ball. When lateral misalignment occurs, the transmitting and receiving coils can achieve self-align with the action of magnetic force, and thus the transfer efficiency is improved. A WPT system consisting of two coils with 6 cm outer radii resonated at 107 kHz is studied. A concise theoretical analysis based on circuit model is given first, and then experimental data are reported. The positive correlation between magnetic force and tracking range is given by comparison. Especially, when the transfer distance is 1.5 cm, the WPT system has a 75.5% peak efficiency and a 3.52 W load power at zero lateral misalignment and the tracking range is from 0 to 8.5 cm; In the tracking range the WPT system has a 70.0% average efficiency and the load power maintains over 94.7% of the peak load power. With the increase of transfer distance, the improvements of transfer efficiency and the tracking range both decrease gradually, and the efficiency loss within the tracking range also increases gradually.

A time-reversal (TR) multi-channel transmission approach with only one receiving antenna is theoretically proposed, numerically analyzed, and experimentally demonstrated in this paper. By using a single receiving antenna with parallel or serial constructed post-channel circuits, information can be received independently from each receiving channel in the proposed TR system. The simulated and experimented results show that the proposed TR system with a single receiving antenna has similar parallel transmission performance to the traditional TR system with multiple receiving antennas. The proposed approach can be used in the TR high-capacity communication systems.

The emerging fifth-generation (5G) wireless and mobile communications demand high-performance phase shifters to enable the multi-channel transmit/receive systems with phase control ability. In this work, a size-miniaturized wideband phase shifter based on slow-wave half-mode substrate integrated waveguide (SW-HMSIW) is presented. The proposed SW-HMSIW is consisting of conventional HMSIW section and periodically-patterned non-uniform microtrip polyline unit cells. Patterning polyline unit cells in HMSIW can effectively enhance the product of the equivalent permittivity and permeability of SW-HMSIW, so that the slow-wave effect can be exhibited and its associated size reduction will be further achieved. Moreover, lengths and widths of segments of the proposed polyline unit cell can influence the equivalent permittivity and permeability of the SW-HMSIW, and consequently change its phase constant and cutoff frequency. Hence, for a SW-HMSIW with a fixed physical length, an electrical length variation, i.e, phase shift, can be achieved as segments of the polyline employ various lengths or widths. Based on this principle, a wideband equal-width equal-length SW-HMSIW phase shifter is implemented. Measured results are in good agreement with simulated ones, with a phase shift of 90.5 ± 3 degree and a fractional bandwidth of 43.0% achieved. Compared with some similar reported works, the proposed one shows a size reduction more than 80% as well as good magnitude and phase deviations performance. Moreover, its miniaturized size and equal-width equal-length structure make it suitable for the complex multi-channel modules and networks of the 5G applications.

As a novel financing method, peer-to-peer (P2P) lending has drawn extensive attention as it provides those financers who cannot participate in the traditional financial market with funds. In P2P lending marketplaces, one of the crucial challenges that P2P online lending platforms are facing is to accurately predict the default risk of each loan by tapping into default prediction models, thus effectively helping P2P lending companies avoid credit risks. That traditional credit risk prediction models fail to meet the demand of P2P lending companies for default risk prediction, which is because of the uneven distribution of credit data samples in the P2P lending marketplaces (i.e., the default sampled data are scarce). In this paper, we designed a multi-round ensemble learning model based on heterogeneous ensemble frameworks to predict default risk. In this model, an extreme gradient boosting (XGBoost) is initially used for ensemble learning, and the XGBoost, deep neural network, and logistic regression are then regarded as heterogeneous individual learners to undergo a linear weighted fusion. To verify the designed default risk prediction model, real credit data from a famous P2P online lending marketplace in China were used in a test. The results of the experiment indicate that this model can effectively increase the predictive accuracy compared with traditional machine learning models and ensemble learning models.

In radar signal processing, the detection and parameter estimation of high-speed maneuvering targets, which often utilize a coherent pulse train signal with linear frequency modulation, have been receiving increasing attention. Fundamentally and significantly, the Cramer-Rao lower bound (CRLB), as a cornerstone for evaluating the estimation performance of high-order kinematic parameters, has been derived and investigated here. In this paper, a 2-D echo signal model expressed in the fast-frequency and slow-time domains is adopted. The scaled orthogonal Legendre polynomials are deliberately introduced to solve the inverse problem of the Fisher information matrix, and then, a linear mapping relationship between different polynomial parameters can be used to obtain the analytical CRLB expressions. The main contributions included are: 1) the CRLBs, which are exact and closed form, have been extended to arbitrary motion model orders and reference time instants; 2) the influences of the motion model order, the reference time instant, as well as the radar parameters on the CRLBs are exploited comprehensively; and 3) some specific cases, including four low-order motion models and two preferred reference times, are also presented to better demonstrate the CRLB performance relationships. It highlights the fact that the reference time instant corresponding to the middle of the pulse train is a reasonable and compromised choice for parameter estimation, although it is not necessarily optimal for the kinematic parameters of all models and orders. The above research results are illustrated with numerical simulations and further verified using the maximum likelihood estimation method combined with Monte Carlo experiments.