The future sixth generation (6G) networks look forward to providing revolutionary mobile connectivity and high-throughput data services through low-cost communication devices. Benefiting from the programmability and tunability of metamaterials, the reconfigurable holographic surface (RHS) inlaid with numerous metamaterial radiation elements shows its great potential to achieve such a bold vision. By leveraging the holographic principle, the RHS serves as an ultra-thin and lightweight antenna integrated with the transceiver to generate desirable directional beams with low hardware cost and power consumption. In this paper, we consider a downlink RHS-aided multi-user communication system where a base station (BS) equipped with an RHS transmits signals to users. We propose an RHS-based hybrid beamforming scheme where the digital beamforming and the holographic beamforming are performed at the BS and RHS, respectively, together with the receive combining at each user. We formulate a sum-rate maximization problem for RHS-based hybrid beamforming and decompose it into three sub-problems. A joint sum-rate maximization algorithm is then developed to solve the sub-problems in an alternating manner. Simulation results show that based on the proposed RHS-aided hybrid beamforming scheme, a moderate-sized RHS is enough to achieve a satisfactory sum-rate, which is also higher than a same-sized phased array can achieve.

Intelligent reflecting surfaces (IRSs), which are capable of adjusting radio propagation conditions by controlling the phase shifts of the waves that impinge on the surface, have been widely analyzed for enhancing the performance of wireless systems. However, the reflective properties of widely studied IRSs restrict the service coverage to only one side of the surface. In this paper, to extend the wireless coverage of communication systems, we introduce the concept of intelligent omni-surface (IOS)-assisted communication. More precisely, an IOS is an important instance of a reconfigurable intelligent surface (RIS) that can provide service coverage to mobile users (MUs) in a reflective and a refractive manner. We consider a downlink IOS-assisted communication system, where a multi-antenna small base station (SBS) and an IOS jointly perform beamforming, for improving the received power of multiple MUs on both sides of the IOS, through different reflective/refractive channels. To maximize the sum-rate, we formulate a joint IOS phase shift design and SBS beamforming optimization problem, and propose an iterative algorithm to efficiently solve the resulting non-convex program. Both theoretical analysis and simulation results show that an IOS significantly extends the service coverage of the SBS when compared to an IRS.

This paper presents a novel approximate solution for electromagnetic scattering by planar homogeneous dielectric objects with arbitrary shape. This solution is derived by introducing a semi-empirical formula of the electrical field inside objects under consideration and by exploring the volumetric integral equation along with spectral representation of dyadic Green's function. Afterward, a closed-form approximate solution has also been established by utilizing the so-called stationary phase method (SPM). It is demonstrated that our solution is uniformly valid for planar objects with moderate electrical thickness illuminated from low to high frequencies at all incidence angles. Comparisons with a full-wave computation obtained by the method of moments (MoM) have been examined, which demonstrates that the proposed formulation can provide very accurate results and can perform much better than other existing approximate techniques.

Dense media radiative transfer (DMRT) theory is used to study the multiple-scattering effects in active microwave remote sensing. Simplified DMRT phase matrices are obtained in the 1-2 frame. The simplified expressions facilitate solutions of the DMRT equations and comparisons with other phase matrices. First-order, second-order, and full multiple-scattering solutions of the DMRT equations are obtained. To solve the DMRT equation, we decompose the diffuse intensities into Fourier series in the azimuthal direction. Each harmonic is solved by the eigen-quadrature approach. The model is applied to the active microwave remote sensing of terrestrial snow. Full multiple-scattering effects are important as the optical thickness for snow at frequencies above 10 GHz often exceed unity. The results are illustrated as a function of frequency, incidence angle, and snow depth. The results show that cross polarization for the case of densely packed spheres can be significant and can be merely 6 to 8 dB below copolarization. The magnitudes of the cross polarization are consistent with the experimental observations. The results show that the active 13.5-GHz backscattering coefficients still have significant sensitivity to snow thickness even for snow thickness exceeding 1 m

A microwave model for remote sensing of snow based on dense media radiative transfer theory with random rough surface boundary conditions is developed. Both volume scattering and rough surface scattering effects are treated by numerical Maxwell model of 3D simulations (NMM3D). Detailedly, the volume scattering model is based on dense media radiative transfer (DMRT) theory by using Monte Carlo (MC) simulations of the three-dimensional solutions of Maxwell's equations for densely distributed sticky particles. The rough surface scattering is modeled by the numerical Maxwell model of 3D simulations (NMM3D), which is accelerated by sparse matrix canonical grid method (SMCG). The bistatic scattering coefficients and emissivity of rough surfaces are utilized as the boundary conditions for the DMRT. Full multiple scattering solutions are calculated by solving the DMRT numerically. The results for a layer of dry snow over a rough ground at 18.7 GHz and 38.5GHz are illustrated.

The microwave signatures of a layer of dry snow overlying ground with a rough surface are computed. We use dense medium radiative transfer (DMRT) theory with Quasicrystalline approximation (QCA) for densely distributed sticky moderate size particles to calculate the volume scattering in snow. The rough surface scattering and emission are modeled by the Numerical Maxwell Model of 3D simulations (NMM3D), which is accelerated by fast computation method known as with Sparse-Matrix Canonical Grid method (SMCG). The bistatic scattering coefficients of rough surfaces are used as the boundary conditions for the DMRT. Both active and passive cases are studied.