This paper studies the secrecy performance of an intelligent reflecting surface (IRS)-aided indoor wireless communication where the IRS is capable of adjusting the direction and phase shift of reflected signal on its surface and assists a source to communicate with an authenticated user in the presence of several unauthenticated users, which can be potential eavesdroppers. The goal of this paper is to design a tile-allocation-and-phase-shift-adjustment (TAaPSA) strategy for the IRS to optimize the average secrecy rate (ASR); moreover, the respective secrecy outage probability (SOP) for this TAaPSA is evaluated. To achieve this goal, the ray model and the Rice distribution are adopted to describe the propagation of the IRS's reflected signals and the fading process, respectively. Closed-form analytical expressions for the ASR and the SOP are derived. Using these analytical results, a genetic algorithm (GA) is utilized to find an optimal TAaPSA strategy for the IRS. The accuracy of the analytical results and the improvement in ASR using GA-based TAaPSA strategy are verified by simulation results.

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A three-dimensional frequency selective surface (3D-FSS) that provides stability for an angle of incidence and miniaturised unit cell size is presented. The proposed 3D-FSS is easy to fabricate and is implemented using via holes in a multilayer printed circuit board structure. Frequency transmission characteristics for different angles of both TE and TM polarisations are presented through simulations. The proposed structure was fabricated so as to verify the simulation results. The comparisons between the simulation and the measured results show good agreement. The results also show that the proposed 3D-FSS can provide better frequency stability for different incidence angles and polarisations as well as miniaturised unit cell size.

A radar absorbing structure was designed on the basis of a circuit analogue absorber for the implementation of broadband characteristics by using a metal-mesh transparent electrode with an improved optical transmittance. To realise a high optical transmittance and broadband absorbing performance in the X-band, a resistive frequency selective surface, in which soda-lime glass, a transparent dielectric, was patterned with a metal-mesh, was used. The simulation for the reflection characteristics of the proposed absorber is performed and showed its validity through experiment. The measurement results were in good agreement with the simulation results, confirming the broadband reflection loss characteristics. Since the proposed structure has a high optical transmittance and stable broadband absorbing performance, it can be used in various fields including military applications, where facilities requiring secure wireless signals or transparent radar absorbers are necessary.

Reconfigurable intelligent surface (RIS)-aided wireless communications systems are one the promising wireless communication system where the wave can be guided by the RIS. It is envisioned that beyond-5G/6G communication will have a low-cost, high spectral efficiency, high energy efficiency, and smart wireless environment. In this paper, initially, different measurement techniques of the RIS have been discussed, which are available in the literature. Then, a new type of RIS has been proposed. Finally, a different parameter measurement technique for our proposed RIS has been presented. A low-cost FR4 substrate with a height of 1.6 mm was considered to design the RIS in the sub-6 GHz frequency band. Another important thing is that our proposed IRS is a single-layer substrate backed by a copper plate. The area of each unit cell was 42 mm × 42 mm. The RIS was designed to operate at the central frequency of the 3.5 GHz frequency band. The novelty of the proposed RIS is that it is a polarization-independent structure. Thus, polarization-related losses can be overcome using this structure. A 10×10-unit cell array was designed to check the radiation performance. The magnitude of the reflection coefficients was measured in our laboratory for the proposed configuration.

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To reduce the electromagnetic wave interference caused by cavity resonance or electromagnetic wave leakage, herein, an optical transparent radar absorbing structure (RAS) was designed using transparent conductive oxides (TCOs) with a high optical transmittance and electrical conductivity, and a procedure was proposed for detecting possible defects in the fabrication and operation and for assessing the influence of the defects on the electromagnetic performance. To detect locally occurring defects in planar and three-dimensional absorbing structures, a measurement system based on an open-ended near-field antenna capable of producing small beam spots at a close distance was constructed. Moreover, the reflection characteristics of the transparent RAS were derived from a simplified multiple reflection equation, and the derived results were compared with the analysis results of an equivalent circuit model to predict the surface resistance of the TCO coating layer, based on which the presence of defects could be confirmed. By using the experimental results, the predicted surface resistance results of the coating layer and the results measured using a non-contact sheet resistance meter were compared and were found to correspond, thereby confirming the effectiveness of the proposed defect detection method.

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