This paper presents a transparent ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna based on a wired metal mesh (MM) structure. Transparent antennas (TAs) can be integrated on various building materials with little visual impact to facilitate flexible massive indoor wireless device deployment. The proposed antenna achieves 77% transparency by hollowing out both metal and substrate into a mesh shape, while maintaining more than 83% radiation efficiency. The MIMO antenna consists of 4 elements, each being a co-planar waveguide (CPW) fed UWB monopole and adjacent elements being placed orthogonally to produce dual polarisation. The operating band of the MIMO antenna spans over 3.2-11.2 GHz. A parasitic decoupling structure aligned with the mesh grid is designed to enhance the isolation between MIMO elements without degrading the transparency. Within the entire operating band, the isolation between any two elements is more than 20 dB. The peak gain varies from 3.3 dBi to 7.2 dBi within the bandwidth. The envelop correlation coefficient (ECC) is less than 0.004. The channel capacity and time domain response are analysed. The antenna can be easily fabricated by conventional PCB and laser cutting techniques with a low cost.

A single feed broadband circularly polarized (CP) microstrip antenna operating in millimeter wave (mmWave) band is proposed. An Archimedean spiral radiator is used to realize circular polarization. By adding a special-shaped ring slot structure with two stubs connecting the inner patch to the outer ground, the radiating direction, bandwidth and axial ratio (AR) of the proposed antenna can be significantly improved. The antenna prototype is fabricated and its performance is evaluated. Measurement results show that its impedance bandwidth (IBW) is 13 GHz from 21.1 GHz to 34.1 GHz (46.43% referring to 28 GHz). The axial ratio bandwidth (ARBW) is 8.35 GHz from 23.85 GHz to 32.2 GHz (29.82% referring to 28 GHz). The gain at 28 GHz is 6.49 dBi.

A millimeter-wave (mmWave) triple-band substrate integrated waveguide (SIW) antenna with dual-sense circular polarization (CP) etched on a single substrate is proposed. A SIW cavity operating in TM210 mode is carved with a circular slot to realize upper band radiation. A circular patch is placed in the circular slot for lower band radiation. Middle band radiation is realized by the coupling between the SIW cavity and circular patch. By introducing asymmetry on both circular slot and circular patch, CP is obtained at two bands. The polarization states of the three bands are right-hand circular polarization (RHCP), linear polarization (LP) and left-hand circular polarization (LHCP). Three pairs of vias are introduced in SIW transmission line to realize good impedance matching for the three bands. A prototype is fabricated and its performances are evaluated. Simulated and measured results agree very well. Small frequency ratios are achieved and each band can be adjusted flexibly. The proposed antenna has characteristics of low-profile and simple structure which will make it easy to be applied in future mmWave wireless communication systems.

In this work, a multifidelity stacked neural network (MFSNN) is proposed to construct surrogate model for antenna modeling and optimization. The stacked neural network consists of a low-fidelity (LF) network, a linear high-fidelity (HF) network, and a nonlinear HF network. By learning the prior from sufficient computationally cheap LF data, the MFSNN has significantly reduced the requirement of computationally expansive HF data. The correlation between LF and HF models can be learned adaptively and accurately by decomposing the correlation into linear component and nonlinear component. The feasibility of the approach is validated by two antenna structures, which shows that the MFSNN-based surrogation model can make predictions for broad ranges of input parameters with satisfactory accuracy. Then, the surrogate model is directly applied in the particle swarm optimization (PSO) framework to replace the full-wave simulation and accelerate antenna optimization procedure.

Accurate propagation characteristics are essential for future indoor millimeter-wave (mmWave) small cell network planning. This paper presents propagation measurements at 26 GHz and 38 GHz which are important candidate bands for fifth generation mmWave communication. Measurements are conducted in an indoor corridor, as well as a stairwell whose mmWave channel is seldom investigated before. In these measurements, an omnidirectional biconical antenna is used as transmitter and a steerable directional horn antenna is used as receiver. The directional and omnidirectional path loss exponents, shadow factors, cross-polarization discrimination ratios and root-mean-square delay spreads are analyzed for both line-of-sight and non-line-of-sight scenarios in both co-polarization and cross-polarization, and these characteristics are compared for different frequencies and environments. It is found obvious depolarization phenomenon in non-line-of-sight scenario for higher frequency. Compared to the corridor, the stairwell has larger path loss exponents and root-mean-square delay spreads, and the depolarization is also more evident in stairwell. The results in this paper are beneficial to building efficient and robust indoor mmWave communication systems.

The knowledge of reflection and transmission properties of building materials is crucial for understanding radio propagation and developing efficient channel models in terahertz (THz) indoor wireless communication. In this letter, such fundamental interaction mechanisms for five common building materials at 219-224 GHz are extensively investigated through experimental measurement campaign, including glass, granite, polyvinyl chloride board, ceramic, and wooden board. Measured results indicate that the reflection coefficients gradually decrease with the increasing of incident angle, whereas the transmission coefficients show weaker angle dependence. In particular, we compare the measured results with those at 40-50 GHz and investigate different characteristics of the materials at the two frequency ranges. Furthermore, the complex permittivity is extracted using least squares fitting methods based on measured reflection coefficients. The results presented in this letter can offer guidance to channel modeling at 220 GHz for indoor low-THz communication.