Delay-Augmented Stacked Intelligent Surfaces: Potential, Challenges, and Opportunities

Stacked intelligent surfaces (SIS)s have been proposed recently as an enabling technology for Holographic Multiple Input Multiple Output (HMIMO) and Ultra-massive MIMO (umMIMO) technologies. Their utility can extend beyond spatial wave-domain processing of signals if they are enhanced with strategically-tuned symbol-duration level delays to enable temporal processing as well. In this work, we introduce the idea of a delay-augmented SIS (DA-SIS). We shed light on the feasibility of realizing delay units in an SIS. Then, we discuss the relevance of the proposed DA-SIS and present a use case that illustrates its potential, wherein the DA-SIS serves as an analog equalizer that aids in eliminating multi-path-induced inter-symbol-interference (ISI). We show how the number of elements affect the equalization process using the bit error rate (BER) as a metric, and demonstrate the potential of the DA-SIS in equalization via comparing with digital equalizers as a benchmark. Finally, we present opportunities and future research directions that can be undertaken to bring this idea to fruition.

A Tunable Reflection Surface with Independently Variable Phase and Slope

A reconfigurable intelligent surface (RIS) is an essential component in the architecture of the next generation of wireless communication systems. An RIS is deployed to provide a controllability to the multi-path environment between the transmitter and the receiver, which becomes critical when the line-of-sight signal between them is blocked. In this work, we design an electrically tunable linearly polarized RIS at 2.5 GHz that yields a controllable reflection phase and phase-frequency slope; in other words, we add tunability of the phase-frequency slope to the tunability of the resonance center frequency. The proposed design consists of two layers of unit cells placed over a ground plane, with dog-bone-shaped elements in the top layer and patch elements in the bottom layer. Each patch and dog-bone element is loaded with a varactor, whose reverse bias voltage is controlled to provide a phase-frequency profile with a slope value of 9 degrees/MHz or 0.95 degrees/MHz, and a phase shift range of 320 degrees.