TECH

A low-tech solution to the 6G problem—metacrystal panels offer cheap way to guide wireless signals around corners

The advent of sixth-generation (6G) and future wireless technologies will transform communications by offering higher data rates, improved energy efficiency, and lower latency1. However, the realization of high data rates necessitates the exploration of new frequency bands, such as millimeter (mm) waves and sub-THz bands. While these frequencies offer vast amounts of bandwidth, they also present considerable challenges due to their high atmospheric attenuation, free-space path loss, and harsher scattering effects when encountering obstacles. Therefore, reliance on traditional multipath propagation is no longer feasible, and directional beams must be used for communication. Moreover, higher-frequency signals are often blocked by obstacles, such as walls, requiring a denser network of base stations and relays. Recently, metasurfaces, also referred to as intelligent surfaces, have been proposed to mitigate these challenges by efficiently redirecting communication signals in free space to bypass obstacles. These artificial surfaces, strategically positioned on walls, ceilings, and even windows, can substantially enhance both indoor and outdoor signal coverage through anomalous reflection or refraction, requiring minimal to no energy for their operation.

Most of the existing studies on intelligent surfaces focused on achieving reconfigurable responses. Programmable metasurfaces are capable of dynamically manipulating several wave characteristics, including wave vector, polarization, frequency, and wavefront, within a unified structure. However, they have proven to be too expensive for widespread adoption in the communication industry. This is primarily due to their requirement to operate at high frequencies (above 30–50 GHz), their large physical footprint (approximately one square meter) even for incorporating a single communication channel, and the need for highly tunable constituent elements. Consequently, their non-reconfigurable (completely passive) counterparts have recently gained great attention due to their significantly lower manufacturing and maintenance costs. In fact, in many real-world scenarios, reconfigurability is not necessary because the positions of the receivers and transmitters are fixed or weakly varying. For instance, in industrial settings, machinery and sensors are usually installed in fixed locations; the infrastructure and major pathways in large public hubs remain constant; and in office environments, the locations of access points are typically fixed.

While various pathways for the analytic design of passive intelligent surfaces were proposed (e.g., anomalous reflectors, smart skins, metagratings, and aperiodic gratings), all of them lack the sufficient versatility for realistic applications. Indeed, in most practical scenarios, it is necessary for the intelligent surface to operate effectively across both signal polarizations, multiple frequency bands, various angles of arrival, and even all at once. Realizing such versatile surfaces with current analytical or semi-analytical design techniques remains very challenging, as these techniques rely on specific homogenization models (e.g., based on polarizability, susceptibility, or surface impedance tensors). Factors, such as frequency dispersion, nonlocality, and anisotropy make the implementation of the unit cells with required material parameters hardly possible. Recent work on multifunctional metasurfaces at microwave and sub-THz frequencies falls into two main classes: multi-incidence and multi-dimensional. 

Multi-incidence designs operate under multiple incidence angles or wave vectors; examples include angle-dependent/independent metasurfaces, directional Janus metasurfaces, and schemes multiplexing guided and space waves. Multi-dimensional designs simultaneously control several wave properties (polarization p, propagation direction/wavefront angle θ, phase ϕ, and amplitude A) typically for a single incident wave. Demonstrations include concurrent control of polarization and direction, wave-vector modulation across frequencies using multi-band metasurfaces, and co-modulation of polarization and wavefront.

Basements, tunnels, large buildings—a weak Wi-Fi or mobile signal in these hard-to-reach places is frustrating. The usual solution is to add more electronics like routers, repeaters and base stations. Yet, as we move towards a 6G mobile network, this kind of complex infrastructure can be unsustainable and prohibitively expensive. Higher-frequency channels of 6G communications aim to provide vastly more data bandwidth than the current 5G, but those channels are more easily blocked by walls, people and other obstacles.

A passive 3D-printed metacrystal panel redirects radio waves around obstacles and toward users, offering a low-cost way to improve indoor/outdoor wireless coverage without adding base stations, wiring or powered electronics(image above). Credit: Aalto University / Mahdi Asgari

To tackle this, researchers at Aalto University have developed a new solution in the form of metacrystals: passive, 3D-printed smart panels that can shape wireless signals without electronics, a power supply or active tuning. The paper, "Metacrystals: Inversely-designed 3D-printed intelligent panels for 6G communications" is published in Nature Communications.

"When a room is too dark, you can bring in more lamps—or use simple mirrors to guide the already available light. This is what these metacrystals do, but with radio waves," explains doctoral researcher Mahdi Asgari. "Unlike previously proposed single-layer intelligent surfaces, these volumetric metacrystals can be designed to control multiple incoming signals or frequency bands independently—a key requirement for realistic wireless communication."

The panels could be installed on walls, ceilings, furniture, or other surfaces to redirect signals around corners, into shadowed areas or toward specific users or devices.

Unlike many existing intelligent surfaces, which often perform only one task for one signal direction, the panels can handle several incoming waves at the same time, operate over different frequency bands simultaneously, work in reflection or transmission mode, and even fully absorb unwanted signals.

3D printed, custom elements...Conventional reconfigurable intelligent surfaces require many tunable elements and complicated control circuits, which makes them expensive and difficult to deploy widely. However, the metacrystal panels can be fabricated using 3D printing, leaving the estimated price of consumable material at a few tens of euros per piece. This also allows for creating custom panels for specific environments, rather than having one universal device.

"For industry, the most attractive use cases are static or slowly changing environments like factories, indoor 5G/6G networks, warehouses, and long corridors," says Asgari. "In such places, a passive panel designed for a known layout could be much cheaper and simpler than an actively controlled surface that requires continuous maintenance."

Asgari says that complex electromagnetic functionality can now be realized as a low-cost, single-piece plastic structure ready to be put on a wall. These panels can quietly improve wireless connections in the background. Once installed, geometry does all the work.

Metacrystals could become part of everyday architecture...The researchers are now looking into pathways to commercialize the discovery and are seeking engagement from industrial collaborators interested in programmable metasurfaces, intelligent wireless infrastructure and low-cost passive signal-control technologies.

"The hope would be that in the future we can see these scalable, smart wireless environments put to practical application in indoor spaces and outdoor urban settings," says Asgari.

The next step is to move from static towards reconfigurable panels that can adapt when the wireless environment changes, he says. Today's reconfigurable intelligent surfaces are often too costly and complex for broad industrial use, so the team is exploring simpler ways to fabricate tunable panels while keeping them affordable and practical.

Provided by Aalto University