image: Schematic illustration of all-in-one radiation-scattering RIS
Credit: ©Science China Press
5G-Advanced and 6G wireless communication technologies aim to achieve the interconnection of all things. This encompasses not only traditional base stations and mobile devices but also diverse sensors, smart devices, household appliances, industrial machinery, vehicles, and more. It essentially includes the interconnection of virtually all entities capable of connecting to a network. The goal is to realize a truly interconnected world, ultimately creating a more intelligent, efficient, and connected global ecosystem. As a key candidate technology for 5G-A/6G, Reconfigurable intelligent surfaces (RIS) offer new possibilities for integrated sensing and communication (ISAC) development through their ability to dynamically manipulate electromagnetic waves. However, existing RIS architectures lack a collaborative regulation mechanism for radiation and scattering states, resulting in low hardware resource utilization and an inability to meet the future network requirements for high integration and low power consumption.
In a new paper published in National Science Review, the team of the Professor Long Li from the Xidian University, China have proposed an electromagnetic all-in-one radiation-scattering RIS. A unified and efficient theory for integrated radiation-scattering manipulation was established, along with a corresponding physical platform. This framework encompasses multi-dimensional electromagnetic properties—including phase, polarization, amplitude, waveform, frequency, and time—enabling the on-demand design of RIS. This approach provides a novel technological paradigm for the era of 6G communications and the Internet of Everything in wireless sensor networks.
The meta-atom consists of a radiating patch and a 3 dB coupler. The radiating patch determines the polarization and frequency characteristics. The diodes are loaded on the 3 dB coupler to realize the radiation scattering mode switching and the corresponding mode phase control. By loading different capacitors on the radiating patch, this design also integrates initial radiation and scattering phases into a single structure for the first time. The meta-atoms with specific polarization and frequency exhibit amplitude control, phase control, and customizable initial phase properties. Specifically, loading PIN diodes or varactor diodes on the 3 dB coupler enables 1-bit or continuous phase regulation, respectively. Under stringent hardware constraints and limited physical space, the radiation-scattering RIS provides multifunctional capabilities within a single platform. The RIS can achieve cost-effective phased arrays in radiation mode. Non-line-of-sight communication is achieved in scattering mode.
Using linearly polarized 1-bit radiation-scattering meta-atoms as an example, researchers explore their functionalities and potential applications. The meta-atom consists of a U-slot patch and a 3 dB coupler connected via metalized vias. The PIN diode is loaded on both the through and coupled branches of the 3 dB coupler. The off state of the PIN diode is defined as 0, and the on state is 1. When the PIN diodes are in states 11 and 00, 1-bit radiation phase control is achieved. When the PIN diodes are in states 10 and 01, 1-bit scattering phase control is achieved. However, both 1-bit radiation and scattering beam control result in grating lobes due to quantization errors. This severely impacts the performance of 1-bit RIS. Therefore, they load capacitors on the radiating patch, with different capacitors corresponding to different initial phases. Ultimately, they designed four types of U-slot patches loaded with different capacitors. This achieves initial scattering phases of 0°, 90°, 180°, and 270°, and initial radiation phases of 0°, 45°, 90°, and 135°. Researchers adopt the method of calculating phase delays introduced by near-field horn excitation in reflectarrays as the method for selecting the initial phase distribution. Then, the initial phase distribution is mapped onto four types of meta-atoms with different initial phases to form a 12×12 radiation-scattering RIS. It is precisely because four initial phases are introduced in both radiation and scattering modes that grating lobes in 1-bit radiation and scattering RIS are suppressed.
Based on a 12×12 electromagnetic all-in-one radiation-scattering RIS, an integrated scheme for communication, sensing, decision-making, and power supply is proposed. This scheme seamlessly integrates communication, sensing, and power supply onto a unified hardware platform, enabling multi-functional decision-making without requiring additional sensors. Within this integrated framework, the metasurface operates in transmit mode to send a priori signals to the core network of the wireless interconnected network. The core network then issues commands to the sensor nodes, which subsequently transmit communication signals back to the metasurface. Switching to receive mode, the metasurface perceives changes in the external electromagnetic environment, pinpoints the location of the sensor, and autonomously makes decisions to initiate wireless power transfer to the sensor node. Furthermore, while in receive mode, the metasurface can also harvest wireless energy. The collected energy, after rectification, can be used to charge other electronic devices or power the surface itself, demonstrating its potential application in self-powered sensing systems. Therefore, the proposed radiation-scattering RIS achieves deep integration of communication, sensing, decision-making, and power supply through a single physical platform.
A 12×12 metasurface was fabricated, and its performance as a low-cost phased array was validated in radiation mode. Experimental results demonstrate that the metasurface achieves grating-lobe-free single-beam scanning over a ±45° range, with a sidelobe level better than 10 dB and gain fluctuations maintained within a reasonable margin. Furthermore, under the '00' state in radiation mode, the structure also facilitates wireless energy harvesting, showcasing its potential for application in self-powered sensing systems. When operating in scattering mode, the metasurface was deployed to provide coverage in non-line-of-sight communication blind zones. In a practical scenario test within an L-shaped corridor, the reflected beam of the meta-surface was dynamically controlled, resulting in an average increase in signal power density of approximately 7 dB within the blind zone and a corresponding significant enhancement in communication quality.