Reconfigurable Intelligent Surface for Industrial Automation: mmWave Propagation Measurement, Simulation, and Control Algorithm Requirements

TL;DR

This work addresses the challenge of maintaining reliable, low-latency mmWave links in indoor industrial automation when LoS is blocked. It demonstrates an enhanced active RIS with 127 elements at 23.8 GHz that uses a two-bit FET control per element and polarization transformation to coherently focus energy onto moving UEs. Through combined simulations and on-site measurements, the authors show that the RIS can achieve strong focusing in NLoS conditions, with active RIS providing about 4 dB higher received power than reflective RIS and a practical update criterion for mobile UEs. The findings highlight the viability of RIS-assisted mmWave links for industrial automation and provide concrete guidelines for RIS configuration update rates in dynamic environments.

Abstract

Reconfigurable intelligent surfaces (RISs) enable reliable low-latency millimeter wave (mmWave) communication links in cases of a blocked line-of-sight (LoS) between the base station (BS) and the user equipment (UE), i.e. a RIS mounted on a wall or the ceiling provides a bypass for the radio communication link. We present an active RIS with 127 patch antenna elements arranged in a hexagonal grid for a center frequency of 23.8 GHz. Each RIS element uses an orthogonal polarization transformation to enable amplification using a field-effect transistor (FET). The source and drain voltages of each FET is controlled using two bits. We assume that the coordinates of the UE in an industrial control scenario are known to the RIS. We measure the received power on a 2D grid of 60 cm by 100 cm with the RIS working in reflective and active mode. The results show that the RIS can successfully focus the radio signal at the desired target points. The half-power beam width is characterized in axial and radial directions with respect to the RIS position, obtaining a practical RIS configuration update criterion for a mobile UE. These results clearly show that RISs are prominent solutions for enabling reliable wireless communication in indoor industrial scenarios.

Figures (7)

• Figure 1: RIS coordinate system for a hexagonal RIS element placement in the $yz$-plane. The BS horn antenna radiates from position ${\hbox{\boldmath$a$}}$ towards the center of the RIS at ${\hbox{\boldmath$0$}}=(0,0,0)$ over a distance of $|{\hbox{\boldmath$a$}}|$. The UE monopole antenna at position ${\hbox{\boldmath$b$}}$ is within a distance of $|{\hbox{\boldmath$b$}}|$ moving on a $xy$-positioning table. The LoS is blocked between the BS and the UE. The picture is not to scale to improve clarity.
• Figure 2: Closeup of the RIS printed circuit board with 127 RIS elements.
• Figure 3: RIS testbed top view
• Figure 4: Measurement testbed with (a) and without (b) the LoS blockage object.
• Figure 5: Received power $P_\text{UE}(x, y)$ over measurement area of $xy$-positioning table. We illustrate the empirical measurement data for (a) without RIS and (b) RIS switched off