Ray Tracing Algorithm for Reconfigurable Intelligent Surfaces

Ray tracing accelerated with graphics processing units (GPUs) is an accurate and efficient simulation technique of wireless communication channels. In this paper, we extend a GPU-accelerated ray tracer (RT) to support the effects of reconfigurable intelligent surfaces (RISs). To evaluate the electric field, we derived a RIS path loss model that can be integrated into a RT and enables further extensions for the implementation of additional features and incorporation into complex reflective scenarios. We verify the derivation and implementation of our model by comparison with empirical measurements in a lab environment. We demonstrate the capabilities of our model to support higher-order reflections from the RIS to the receiver. We find that such components have a significant effect on the received signal strength, concluding that the extensions of advanced functionality enabled by our model play an important role in the accurate modeling of radio wave propagation in an environment including RISs.

Dynamic Geometry-Based Stochastic Channel Modeling for Polarized MIMO Systems with Moving Scatterers

This paper introduces a four-dimensional (4D) geometry-based stochastic model (GBSM) for polarized multiple-input multiple-output (MIMO) systems with moving scatterers. We propose a novel motion path model with high degrees of freedom based on the Brownian Motion (BM) random process for randomly moving scatterers. This model is capable of analyzing the effect of both deterministically and randomly moving scatterers on channel properties. The mixture of Von Mises Fisher (VMF) distribution is considered for scatterers resulting in a more general and practical model. The proposed motion path model is applied to the clusters of scatterers with the mixture of VMF distribution, and a closed form formula for calculating space time correlation function (STCF) is achieved, allowing the study of the behavior of channel correlation and channel capacity in the time domain with the presence of stationary and moving scatterers. To obtain numerical results for channel capacity, we employed Monte Carlo simulation method for channel realization purpose. The impact of moving scatterers on the performance of polarized MIMO systems is evaluated using 2 by 2 MIMO configurations with various dual polarizations, i.e. V/V, V/H, and slanted 45{\deg} polarizations for different signal-to-noise (SNR) regimes. The proposed motion path model can be applied to study various dynamic systems with moving objects. The presented process and achieved formula are general and can be applied to polarized MIMO systems with any arbitrary number of antennas and polarizations.

Active Reconfigurable Intelligent Surfaces for the Millimeter-Wave Frequency Band: System Design and Measurement

Reconfigurable intelligent surfaces (RISs) will play a key role to establish millimeter wave (mmWave) ultra-reliable low-latency communication systems for sixth-generation (6G) applications. Currently, there are a few working prototypes of RISs operating in the mmWave frequency band and all of them are based on passive reflective elements. However, to fabricate an efficiently working RIS at mmWave frequencies, it is crucial to take care of the strong signal attenuation, reflective element losses and undesired radio frequency (RF) circuit effects. In this paper, we provide measurement campaign results for an active RIS in the mmWave frequency band as well as its analysis and system design. The obtained results demonstrate that an active RIS outperforms a RIS working in passive mode and provides a higher signal-to-noise-ratio (SNR). The active RIS consists of active reflective elements that amplify the impinging signal and reflect the signal to the desired beam direction. To obtain an efficient RIS in terms of power consumption and RIS state switch time, we design a hexagonal RIS with 37 elements working at 26 GHz. These elements are designed to work whether in passive state (binary phase shifting) or in active state (switch OFF or amplifying). We provide a comparison between the performance of a RIS working in passive and active mode using numerical simulations and empirical measurements. This comparison reveals that the active reflective intelligent surface (RIS) provides a received power that is at least 4 dB higher than that of the equivalent passive RIS. These results demonstrate the strong advantage of using active RISs for future ultra-reliable low-latency wireless communications.