The LICRIS Research Project: Building the 6G Radio Channel

What you’ll learn:

The wireless ecosystem and going beyond the Shannon model.
How reconfigurable intelligent surfaces can address smart system applications.
About the 6G Liquid Crystal Reconfigurable Intelligent Surfaces (LICRIS) project.

Even though the next mobile communications standard is still a few years away from introduction, we already know two things: 6G will enable new use cases and it will increase wireless communications between devices.

To open up more bandwidth, 6G uses parts of the higher and largely unused frequency spectrum, but higher attenuation and increased shadowing from buildings will also be factors during signal transmission. Both reduce network coverage. More transmitting antennas can compensate for the attenuation and shadowing, though only with greater power consumption.

Another option is to modify how radio waves are propagated. Reconfigurable intelligent surfaces (RIS) hold a lot of promise. Suspending them from building facades, they can control the reflection of 6G radio waves to provide sufficient network coverage with far fewer transmitting antennas and far greater energy efficiency.

Beyond the Shannon Model

The Shannon model is the classic transmitter-receiver model used in communications engineering and assumes that a radio channel is a fixed quantity. This model focuses on encoding and decoding processes to maximize data throughput.

RIS can be used to actively adapt the radio channel during operation for the first time so that the channel is no longer passive. This opens the door to completely new optimization methods for radio communications far beyond the familiar territory of the Shannon model.

Radio channels and wave propagation are generally regarded as constant. Radio channels were fixed, for example, when passing through the walls of a building indoors. RIS now makes it possible to actively reflect and target radio channels and wave propagation for better coverage in the future.

Reconfigurable Intelligent Surfaces

RIS are antennas and integrated circuits that use special diodes or liquid crystals. They form a flat structure and can be configured to reflect incoming radio waves and forward them to receivers. Compared to classic RF amplifiers (repeaters) that contain a transmitter and receiver, RIS are cheaper to purchase and more efficient to operate.

Current development projects use RIS as passive components that simply reflect signals, with no amplification. We observe the frequency range below 6 GHz (FR1) or the millimeter-wave range (FR2). Using the surfaces as active signal amplifiers might be interesting, but higher-energy demands and construction costs mean that this use isn’t a top priority.

In general, RIS are best in densely populated areas and in industrial environments. They improve spectral efficiency and make it easier to generate and manage directional radio beams. When radio transmission requirements change, the RIS adjusts radio channels during operation. When installed on a building facade, an RIS will direct mobile communications into streets and interior spaces—areas where cellular coverage has been difficult.

Figure 1 shows how RIS can increase network coverage when the line of sight between the transmitter and receiver is blocked. This will apply to both indoor and outdoor scenarios in the future. Also important to the mobile communications industry: RIS can improve indoor base-station signal transmissions by integrating them into window glass as meta-lenses.¹ RIS are best installed close to the transmitter or close to the receiver to effectively minimize the loss of very high-frequency signals (FR2) over the transmission path.

1. Mobile communications in the FR2 band are sensitive to obstacles in the transmission path. RIS can eliminate such gaps in network coverage.

The LICRIS Project

The 6G Liquid Crystal Reconfigurable Intelligent Surfaces (LICRIS) project is developing RIS using liquid crystals.² These are better in the high-frequency ranges than semiconductors (also an option).

The project is funded by the German Federal Ministry of Education and Research (BMBF) and covers phases for developing new liquid-crystal materials, constructing RIS, and testing them in a mobile radio test network. The final step will place 6G LICRIS technology in a real network environment to demonstrate data transmission from one device to another (end-to-end data transmission).

Metamaterials in the RF Range

To keep power consumption low, RIS uses metamaterials to reflect electromagnetic waves. Metamaterials have properties that go beyond those of natural materials. In optics, metamaterials can have a negative refractive index (natural materials always have a positive refractive index). They’re used in lenses with very high imaging quality or, more experimentally, to guide light around objects to act as a cloaking device. RIS metamaterials have excellent electromagnetic properties in the RF range.

Metamaterials have certain basic structures called meta-atoms. The geometry and arrangement of meta-atoms determine the properties of a metamaterial—a real advantage since both can be controlled in the manufacturing process. The metamaterial properties can be tailored to a specific application.

Meta-atoms are dimensioned to the application and always have wavelengths shorter than those of the signals reflected by the metamaterial. Light has a short wavelength and requires much smaller meta-atoms than radio signals, which have longer wavelengths.

Metamaterials can influence the propagation of electromagnetic waves in a way that’s not possible with any other material. As a result, metamaterials give researchers an enormous amount of freedom when designing RIS and can improve properties such as the reflection, absorption, and transmission of electromagnetic waves.³

Liquid Crystals for RF Applications

Liquid crystals are a mature technology, which is a major advantage. Liquid crystals have been used in PC monitors and smartphone screens for decades. Their electrical control methods are well known, and the industry has mastered production of large-area liquid-crystal layers.

The technology is now being transferred to the RF sector. For example, liquid crystals are used in smart antennas.² Not only can they change the polarization of light, they’re also capable of changing effective capacitor permittivity or capacitance. They’re perfect for setting up phase shifters for directional RF antennas. Similar to screen pixels, phase-shifter pixels can be constructed and combined to form large-area directional antenna arrays or large RIS. Figure 2 shows one such setup.

2. Flat phased-array directional antenna (left) consisting of many identical antenna elements in a square arrangement. Phase shifters are the central components, controlling the phase of each antenna element and generating the directional effect (right).

Liquid crystals were discovered in 1888 by Friedrich Reinitzer, who recognized them as a previously unknown phase of liquids. He was surprised to discover that their physical properties are also anisotropic (direction-dependent) in the liquid state, which until then had been considered a classic characteristic of solid crystal structures.

Today, many different classes of liquid crystals are known, and the underlying mechanisms are well understood.6 This research was so fruitful and fundamental that Pierre-Gilles de Gennes was awarded the Nobel Prize in Physics in 1991.

Their anisotropy is due to rod-shaped molecules (Fig. 3), which are uniformly oriented in the liquid-crystalline phase and form an axis. Figure 4 illustrates the nematic phase that occurs in the class of thermotropic liquid crystals. Liquid-crystal displays (LCDs) use this type of liquid crystal. In this case, anisotropy refers to the fact that the extent to which the nematic liquid crystals change the polarization of an electromagnetic wave (in this case, light from the LCD backlight) depends on the angle at which the wave strikes the axis.

3. Example of a rod-shaped molecule in a liquid crystal. The anisotropic molecular shape leads to anisotropic physical properties such as the different electrical permittivity ε in parallel and perpendicular directions of incidence.

4. Thermotropic liquid crystals change their phase depending on the temperature. At the melting point Tm, the crystalline material passes into the nematic phase and turbidity sets in. At the transparency point Tc, the uniform orientation of the molecules is lost and the liquid becomes transparent. One current research objective is to create liquid-crystal mixtures that express their “tunability.” — 4. Thermotropic liquid crystals change their phase depending on the temperature. At the melting point T_m, the crystalline material passes into the nematic phase and turbidity sets in. At the transparency point T_c, the uniform orientation of the molecules is lost and the liquid becomes transparent. One current research objective is to create liquid-crystal mixtures that express their “tunability.”

In an LCD, an electric field changes the orientation of this axis to change the polarization direction of the light in a controlled manner. The liquid crystals are located between two polarization filters arranged perpendicular to one another and act like a light valve that controls the brightness of the individual display pixels (Fig. 5). The Merck Group in Darmstadt, Germany, the global market leader in the production of liquid crystals, is one of the partners in the 6G LICRIS project.

5. Variable plate capacitor with liquid crystals as a dielectric between two glass substrates for RF applications. When no voltage is applied, the top layer (called the orientation layer or alignment layer) stimulates the liquid crystals to align the molecules parallel to the substrate. As the voltage increases, the molecules gradually align themselves vertically and the capacitance C of the capacitor increases (left). How this component can be integrated into a (simplified) antenna structure is shown on the right.

Two RIS Variants

The 6G LICRIS project is developing two RIS versions that work on the principle of phased-array directional antennas. The first type is for high frequencies from 26 to 27 GHz (FR2) and uses liquid crystals. The second type is for signals between 6 and 7 GHz and has conventional, reconfigurable RF components that can be manufactured using PCB technology. In principle, liquid-crystal-based RIS also work with significantly higher frequency signals.

So far, usability up to 100 GHz has been demonstrated. The limiting factor in such a metamaterial concept is the switching time when the electric field is turned off. Liquid crystals have a certain rotational viscosity based on liquid-crystal layer thickness, with a relaxation period before all of the liquid crystals can be arranged parallel to the alignment layer again. Layer thickness of 5 m can take tens of milliseconds, but this switching time is good enough for many practical applications.

Before the project officially ends in 2025, it will also look at specific use cases for RIS, create concepts for network integration, and develop measurement concepts to characterize them. One possibility for the latter is based on a conventional setup with a feed antenna in an anechoic chamber (Fig. 6).

6. The image on the top shows a test setup for characterizing phased array antennas in an anechoic chamber, which can also be used in a similar form for RIS. You can see both the device under test (marked with a crosshair) and the feed antenna. The image on the bottom shows the 3D reflection pattern of a real RIS provided by Greenerwave.

Future Prospects for Reconfigurable Intelligent Surfaces

RIS technology is a promising approach to mobile network coverage in the FR2 range and beyond. Further technological development will focus on reducing costs to compete with existing alternatives such as access points and repeaters. After all, use cases for network operators also depend on economic factors.

RIS will be used where the benefits of installing them exceed the costs (development and acquisition costs, possibly also rental payments for installation on building facades). The potential is certainly there.

Another interesting use case for Rohde & Schwarz is reconfigurable intelligent surfaces that can change electromagnetic conditions in measuring chambers. This would allow testing teams to create quiet zones in any required shape and size or change channel conditions to suit specific applications. Together with project partners, Rohde & Schwarz continues to explore the potential of liquid crystal in RF applications and drives their commercial use in 6G mobile communications, which will be introduced around 2030.

References

1. Kitayama, D. et al., “Research of Transparent RIS Technology toward 5G evolution & 6G.” NTT Technical Review, November 2021, vol. 19, issue 11, pp. 26-34.

2. “6G LICRIS Reconfigurable surfaces extend 6G network coverage.”

3. Bette, F.; Mellein, H., “Reconfigurable Intelligent Surfaces.” Rohde & Schwarz white paper, 2023.

4. Wittek, M.; Fritzsch, C; Schroth, D., “Employing Liquid Crystal-Based Smart Antennas for Satellite and Terrestrial Communication. Information Display,” January/February 2021, vol. 37, issue 1, pp. 17-22.

5. Rohde & Schwarz, “Rohde & Schwarz and Greenerwave collaborate to verify RIS modules and drive 6G research.” May 9, 2023.

6. De Gennes, P.G.; Probst, J., “The Physics of Liquid Crystals. 2nd edition, International Series of Monographs on Physics,” 1993, Oxford University Press.