Latest Breakthroughs in 6G RF/Microwave Front-End Filtering and Reconfigurable Devices

Dear Colleagues,

The shift to 6G wireless communication demands a revolution in the RF/microwave front-end to handle higher frequencies up to 100 GHz and beyond, massive bandwidths, and new capabilities like communication and integrated sensing. The primary advancement lies in designing highly reconfigurable and tunable devices to achieve the required flexibility and performance.

6G will use a much wider, broken-up spectrum, including the upper mid-band (7–24 GHz) and extended millimeter-wave bands. This necessitates filters that can dynamically adapt their characteristics. To satisfy these requirements, tunable and multimode filters should be used to dynamically adjust their center frequency, bandwidth, and transfer function. Furthermore, using high Q-factor filters helps preserve performance and reduce power loss at high frequencies. The design of filtering reconfigurable intelligent computational surfaces is a novel concept that integrates filtering capability into reconfigurable intelligent surfaces. To minimize circuit size and insertion loss, there is a strong push for integrating filters directly with other components such as filter–antenna co-design.

Reconfigurability is the main requirement for the 6G RF/microwave front-end design, enabling one hardware platform to support multiple frequency bands, standards, and functionalities like communication and sensing. Traditional semiconductor switches often suffer from high insertion loss and slow switching speeds. Therefore, new nonvolatile RF switches—memristors—are being explored to create highly agile hardware. These devices enhance energy efficiency and reconfigurability by eliminating the use of static energy in both the ON and OFF states.

Over the past ten years, numerous nonvolatile memory concepts—including those based on resistive RAM (RRAM), conductive-bridge RAM (CBRAM), phase-change memory (PCM), and 2D materials—have been proposed. These nonvolatile approaches offer high switching speed, low energy consumption, high cutoff frequency, and nanoscale dimensions, making them suitable for CMOS integration.

The development of nonvolatile RF switches began with devices like the conductive-bridge RAM (CBRAM) in 2015, which introduced the concept of a nanoscale memristive switch. Today, the focus has shifted to atomristors, realized using 2D resistive memory materials such as monolayer MoS₂ and h-BN. These 2D switches could be a promising solution for next-generation systems; for instance, a monolayer MoS₂ switch meets 6G requirements with data rates of approximately 100 Gbit/s and operating frequencies reaching 0.5 THz (a frequency range where h-BN switches perform significantly slower). This makes 2D RF switches suitable for applications that require reliability, low power, and MIMO support.

Nonvolatile RF switches are expected to be used for the design of numerous microwave circuits. Some potential microwave applications with memristors are quasi-lumped components, filters, phase shifters, patch antennas and phased array antenna, attenuators, oscillators, and amplifiers.

In order to inspire a discussion and offer an overview of technology trends, we invite researchers from both industry and academia to contribute to this Special Issue with their ongoing research and visions of the future design of RF/microwave filters and memristive devices, circuits, and systems. The contributions should consider, but are not limited to, the following topics:

• RF/microwave filters;
• Multimode resonators;
• Multiband filters;
• Planar filter;
• Filtering reconfigurable intelligent computational surfaces;
• Waveguide filters;
• Substrate-integrated waveguides filters;
• Dielectric resonator filter;
• Tunable/reconfigurable filters and multiplexers;
• Programmable multifunctional RF/microwave circuit;
• Multifunctional filtering power dividers/baluns/couplers/antennas;
• Optimization techniques for filter design;
• 3D printing of microwave and millimeter-wave filtering devices;
• Wireless sensors;
• Memristor modeling and applications;
• Memristive devices;
• Memristive tunable/reconfigurable microwave and millimeter waves circuits;
• Memristors for intelligent RF applications;
• Memristive CMOS circuits;
• Memristive sensors.

Prof. Dr. Milka Potrebić Ivaniš
Prof. Dr. Dejan Tošić
Guest Editors

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• RF/microwave filters
• multimode resonators
• multiband filters
• planar filter
• waveguide filters
• substrate-integrated waveguides filters
• dielectric resonator filter
• tunable/reconfigurable filters and multiplexers
• programmable multifunctional RF/microwave circuit
• multifunctional filtering power dividers/baluns/couplers/antennas
• 3D printing of microwave and millimeter-wave filtering devices
• memristive devices
• memristors for intelligent RF applications