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.

Physical layer security is a cross research hotspot of information security and wireless communications. Recently, orbital angular momentum (OAM) technology has attracted widespread attention due to its potential of improving capacity and spectrum efficiency. However, there are few researches on system security based on OAM technology. In this work, we studied the security of the proposed OAM wiretap system by applying physical layer security theory to OAM communications. Firstly, we proposed a uniform circular array-based multi-mode OAM system and derived the secrecy capacity. Secondly, the phase profiles, channel capacity and received power of OAM beams are analyzed in terms of various system parameters in oblique scenarios at 30GHz. Thirdly, we discussed the security of the proposed OAM wiretap system. The results show that the system using vortex waves is superior to conventional communication systems using planar electromagnetic waves in terms of system security due to the inherent divergence and spiral phase structure of OAM beams. Moreover, we found that the distortion of OAM beams during transmission and the center shift at receiving end will occur and that system is more stable for high-mode OAM propagation in oblique scenarios. This work can provide an effective guidance for the research and application of OAM communications.

In the millimeter-wave (mm-Wave) multiuser-multiple-input multiple-output (MU-MIMO) system, the interferences among different users generally degrade the system performance. In the meantime, orbital angular momentum (OAM) or radio vortex communication suffers significant fading due to the nature of divergence radiation. In this contribution, we utilize the divergence and propose an OAM-based independent analog beam selection (OAM-IABS) as well as a sub-optimal beam search solution to address all the above challenges. Simulation results demonstrate the superiority of the proposed scheme.

To determine propagation characteristics in building for fifth-generation (5G) wireless communication, it is necessary to obtain the complex permittivities of the construction materials at millimeter wave (mmWave) band. Cloth materials are widely used in house which have a great impact on the propagation of mmWave, however their dielectric properties in mmWave band are rarely reported. In this paper, a new method for estimation of the complex permittivities based on quantum-behaved particle swarm optimization (QPSO) algorithm and free space measurement is proposed. Compared with classical particle swarm optimization (PSO) and chaos particle swarm optimization (CPSO), QPSO is robust and efficient in searching global optima. The complex permittivities of four cloth materials (linen, leatherette, polyester fiber and latex mattress) are extracted from the measured transmission and reflection coefficients for perpendicular polarization over 40-50 GHz, and the accuracy is verified by comparing calculated and measured reflection coefficients at other incident angle. The results presented in this paper are helpful for modeling propagation of mmWave in indoor environment, and the proposed estimation method can serve as a good tool for determination of dielectric properties of construction materials in wireless network planning.

The finite-difference time-domain (FDTD) method used for the coupling analysis of the shielded cavity usually needs amount of elements, which would seriously affect the computation cost. To relieve this problem, a novel three-dimensional FDTD subgridding method combined with an automesh generator technique is presented. First, the automesh generator technique is applied to read the triangular element data from the stereolithography format file of the shielded cavity to fastly mesh the structure of the cavity by the coarse and fine grids of FDTD. Then, the subgridding interfaces are separated to temporal and spatial parts to implement temporal and spatial subgridding algorithms, respectively, to obtain the electric and magnetic fields on the interfaces. To ensure the stability and continuity of the subgridding interfaces, the missing subgrid and fine grid magnetic fields nearest temporal and spatial subgridding interfaces are obtained by the Taylor expansions and linear interpolation schemes of the coarse grid magnetic fields, respectively, at the end of each fine time step. And the coarse grid electric fields in the subgrid and fine grid nearest temporal and spatial subgridding interfaces should be updated via some interpolation schemes of the subgrid and fine grid electric fields at the end of each coarse time step, respectively. The accuracy, efficiency, and longer time stability of the presented method have been verified by comparing with the finite-integration time-domain method via the simulation of a cavity with one slot excited by the plane wave. Finally, the suitability of this presented method applied for the coupling analysis of the arbitrary shielded cavity excited by the ambient wave is confirmed.