Abstracts:Under near-field conditions, spatial undersampling leads to grating lobes in the sparse multiple-input multiple-output (MIMO) array results. A fast zero migration algorithm is developed in this communication for suppressing the grating lobes. In our method, resampling in the wavenumber domain is first achieved on the received data, and then, the fast inverse Fourier transform (FT) and compensation on the image domain is leveraged for achieving zero migration of transceiver array. By applying apodization on the zero migration result, zero spreading is equivalently achieved and thus alleviate the influence of grating lobe spreading under near-field conditions and avoid the ghosts in the imaging results. Benefit from its fast FT-based processing scheme, the method achieves a reduction in computational complexity from O( ${N} ^{4}$ ) to O( ${N} ^{3}$ logN). Both simulated and measured data validate the superiority of the method in effectively suppressing grating lobes and improving the processing efficiency.

Abstracts:Nonterrestrial networks (NTNs) have attracted great interest in the development of 6G communication systems. Hence, it is very important to develop an accurate NTN clutter loss (CL) model for the deployment and optimization of NTN communications. In this work, we conduct a multiband channel measurement for the CL of NTN channels in a campus scenario, including 8.5, 10, and 13 GHz. A semideterministic model for the CL related to elevation and frequency is constructed, and a heuristic interpretation for the modeling process is provided based on: 1) the variation of the energy flux density due to the ray-tube effect of the NTN channel and 2) the channel sparsity on angular domain. The obtained results are helpful for the accurate understanding of the NTN channel, which is essential for the development of NTN communications.

Abstracts:The critical frequency of the ionospheric F2 layer (foF2), one of the essential channel parameters for high-frequency (HF) global communications via sky waves, shows significant variability in the equatorial region. This is mainly due to its nonlinear and nonstationary, which poses a considerable challenge for making efficient and accurate short-term forecasts of foF2 in low-latitude areas. Neural network algorithms have shown promising results in forecasting foF2 time series. This communication first introduces a neural network model based on the echo state network (ESN) to forecast HF propagation ionospheric parameters foF2 time series. By modeling and analyzing the foF2 data collected in 2014 and 2017 at the Sanya (18.34°N, 109.42°E) station in China, the ESN model effectively captures the characteristic trends of the foF2 time series. The results indicate that the forecast accuracy of the ESN model surpasses that of the international reference ionosphere (IRI), long short-term memory (LSTM), and bidirectional LSTM (Bi-LSTM) models, with significantly higher computational efficiency than other deep learning models. Moreover, it accurately tracks the foF2 trend during the geomagnetic storm in May 2017.

Abstracts:A circuit-informed neural network (CINN) is proposed for broadening the bandwidth of substrate-integrated waveguide (SIW)-fed slot antennas. The proposed approach optimizes the structural parameters for matching multiple stub pairs (SPs) efficiently by combining circuit knowledge and a well-trained artificial neural network (ANN) for single SP. The CINN significantly reduced the computational costs of optimization, dataset construction, and training. Experimental results illustrated the effectiveness of the proposed CINN in achieving a wide impedance fractional bandwidth of 43%. This approach features strong generalization capabilities, making it widely applicable to various SIW antennas with diverse structures and varying numbers of SPs.

Abstracts:In this communication, the completeness of surface-integral-equation (SIE)-based characteristic modes (CMs) in the modal expansion for the dielectric body is addressed. It is found that, when the excitation source is positioned inside the dielectric body, the conventional spurious-free SIE-based CMs alone are inadequate for fully characterizing the internal scattered fields and the nonradiating CMs arisen from the Chang-Harrington CM formulation will contribute to the internal fields. A numerical example where a lossless dielectric ball is excited with an internal electric dipole is provided. Theoretical analysis and numerical results demonstrate that, in the presence of internal excitations, both the radiative CMs and the nonradiative CMs should be employed to fully characterize the internal scattered fields.

Abstracts:A novel architecture of retro-reflective beamforming is proposed for wireless power transmission applications. In contrast to a conventional retro-reflective beamformer with a 2-D transmitting antenna array controlled by a 2-D receiving antenna array, the novel retro-reflective beamformer includes a 2-D transmitting antenna array controlled by multiple 1-D receiving antenna arrays. Compared with the conventional architecture of retro-reflective beamforming, the architecture proposed in this communication has better flexibility and lower complexity. Numerical results indicate that the wireless power transmission efficiency of the novel architecture has little difference from that of the conventional architecture when the location of a wireless power receiver with a clear line-of-sight path is not far off the broadside direction of the retro-reflective beamformer. Some preliminary experiments are conducted in an anechoic chamber as well as an indoor environment to verify the numerical studies. The experimental data are in agreement with the numerical results.

Abstracts:This study introduces a novel modular antenna-in-display (MAiD) concept for advanced smartphone antenna modularization. It focuses on dual-polarization integration in a compact space within the display panel, essential for millimeter-wave (mmWave) 5G smartphones operating in the n257 and n258 bands of FR2. The adaptable MAiD is compatible with various displays, including foldable and slidable types. The MAiD ingeniously utilizes the display panel’s dead space (DS), a narrow 300- $\mu $ m area, for antenna placement. This innovation is integrated into the same layer as the touch sensor (TS). We propose two $1\times 4$ antenna array configurations within the DS for dual-linear polarization, enhancing capacity through selection diversity. The antennas, named antenna-in-display parallel to DS (AiD-pDS) and antenna-in-display normal to DS (AiD-nDS), are fabricated with a 50- $\mu $ m-thick polyimide film. Their design allows embedding in a $0.03\lambda _{0}$ width of the DS. The MAiD achieves impressive 10-dB return-loss bandwidths of 26.7–28.6 GHz and 24.5–28.1 GHz, with measured boresight gains of 9.041 and 8.824 dBi for AiD-pDS and AiD-nDS, respectively. It maintains over 12-dB cross-polarization level (XPL), demonstrating its effectiveness for modern smartphone technologies.

Abstracts:This study proposes a low-profile broadband millimeter-wave magneto-electric (ME) dipole antenna with a fragmented electric dipole arm. Although the antenna has a low-profile structure, the S11 bandwidth of the proposed antenna element is widened thanks to the additional resonance points at 25 GHz frequency by fragmenting the bowtie-shaped electric dipole. By adjusting the dimensions of the fragmented bowtie, the proposed antenna has widened S11 bandwidth and 3-dB gain bandwidth toward the low-frequency band. The proposed $4\times 4$ antenna array achieves an S11 bandwidth of 41.8% covering from 25.5 to 39 GHz, a 3 dB gain bandwidth of 39.3% covering from 25.5 to 38 GHz, and a peak gain of 17.5 dBi. The cross-polarization and sidelobe levels (SLLs) are lower than −32 and −13 dB within the operating frequency, respectively.