Following on from developing a circuit to resolve interference, Dr. Daiki Hatanaka, Senior Research Scientist at NTT Basic Research Laboratories shares more
5G has ushered in a new era for the IoT landscape, with 5G-enabled connected devices accelerating enterprises’ digital transformation while expanding horizons for the further adoption of robotics, artificial intelligence, virtual/augmented reality and more.
The IoT Analytics State of IoT—Spring 2023 report “shows that the number of global IoT connections grew by 18% in 2022 to 14.3 billion active IoT endpoints” and “the number of 2G/3G IoT devices has a predicted negative annual growth rate of 20 percent up to 2029” according to Ericsson.
However, with an increase in the number of connected devices comes a commensurate problem: a rise in radio waves traveling through space, resulting in signal reflections and backscattering, ultimately, interference between devices.
According to Onomondo, a global cellular operator for the IoT, signal reflections occur when a mismatch is present between the impedance of the transmission line and the load. This impedance difference causes part of the signal to bounce back to the source, leading to signal distortion at the receiver.
According to DevX, backscatter occurs when waves, particles, or signals are reflected back toward their source. This phenomenon can lead to interference, degrade signal quality and cause inaccurate readings in some applications. Signal reflections and backscattering are one of the most common issues in reliable connection integrity. If overcome, the path towards an even more expansive landscape of connected devices becomes much clearer.
Ultrasonic filters, “a filter is an electronic circuit or software algorithm that lets certain frequencies pass through it and restricts others” according to Partstack, are vital to overcoming signal reflections, but modern ultrasonic filters cannot keep up with the exponentiating increase in signals caused by the continued adoption of connected devices as they reach their current limits of efficiency, and as the physical space to add more filters to devices runs out.
Recently, scientists with NTT Corporation and Okayama University published research towards a possible solution: the world’s first gigahertz ultrasonic circuit that utilitises topology to enable smaller, more efficient radio frequency filters. The research raises important questions: What is topology, what are the unique beneficial properties of ultrasonic technologies—and, how have the two been combined to advance interference-filtering capabilities?
What is topology?
Topology is the branch of mathematics that studies the shape of objects and the properties of space, focusing on properties that do not change when the object is subjected to “continuous deformation” (such as bending and stretching). In other words, topology focuses on how objects are connected. Writing for AFP, Mariëtte Le Roux and Marlowe Hood describe a popular metaphor:
A topologist is a person who cannot tell the difference between a coffee mug and a doughnut… In the metaphor, the mug and the doughnut are one and the same. If they were made out of rubber, one could be twisted and stretched into the shape of the other without changing its essence … The two are considered topologically equivalent as each has a hole—the ear of the mug and the centre of the doughnut.
Ultrasonic waves are sound waves at frequencies higher than the range from low frequencies to the top of the audible frequency range (0 Hz to 20 kHz) and are inaudible to humans. Ultrasonic communication technology is becoming more valuable for various location-based services; for example, an app detects beacons and recommends location-specific services or content when your device is within range of certain ultrasonic signals. This technology is utilised for providing information for many industries, including entertainment events such as guided tours, streamlining ticketing processes at stadiums and other venues, and creating unique experiences like synchronised light shows.
A cell phone utilises ultrasonic waves. All cell phones must precisely extract and receive only the desired signal and ultrasonic filters play an important role in this process. Today, it is possible to realise filters that are much smaller and more energy-efficient than filters made from electronic components due to the low energy leakage property of ultrasonic technologies.
Cell phones are equipped with nearly 100 ultrasonic filters, which allow them to efficiently send and receive signals in different bands. However, with further 5G and even 6G adoption, will further increase the amount of signals that IoT devices, such as cell phones, rely on, with the potential for interference thus rising comensurately. So, more and more filters will be required—how do you fit them all into a relatively small, finite space?
Fitting more signals into a small space
NTT and Okayama University researchers have developed a special material for future filters called “ultrasonic topological phononic crystal,” which was developed using an artificial elastic structure consisting of periodic arrays of microscopic holes. This new structure takes advantage of the “valley pseudospin-dependent conduction phenomenon”.
A valley pseudospin is a quantum number that typically indicates that an electron or ultrasonic wave is in a particular valley (a valley is a specific region in phase space where the energy bands of electrons and ultrasonic waves dip, like a valley). This phenomenon resulted in a robust and stable traveling wave protected by topological order, a state in which matter is not determined by symmetry disturbances, local defects, or fluctuations in physical variables, but by how the matter is connected.
For example, when two materials with different topologies are connected, such as the topological ultrasonic waveguide in this experiment, the conduction state of the ultrasonic wave appears at the junction between different topology. The local order of the junction does not guarantee the existence of this state but by the nonlocal order of how the two topological structures that form the junctions are connected.
The result? Stable and robust ultrasonic wave propagation against structural fluctuations in the waveguide, meaning the wave does not reflect backward like normal ultrasound waves, but instead travels smoothly (Figure 1).
Using the ultrasonic topological phononic crystal structure, researchers successfully propagated ultrasonic waves stably without backscattering, even on complicated and microscopic channels. Just as importantly, the researchers simultaneously succeeded in miniaturising the size of an ultrasonic filter to hundreds of square micrometers, less than 1/100 of the size of a conventional ultrasonic filter (which is tens of thousands of square micrometers).
While still in the research phase, these results are expected to enable the necessary miniaturisation, integration and multifunctionality of ultrasonic filters widely used in wireless communication terminals. This innovation promises to propel the IoT industry towards a more advanced and efficient future, underscoring the transformative potential of integrating cutting-edge mathematics and technology in addressing real-world challenges.
Author: Dr. Daiki Hatanaka, Senior Research Scientist of the Nanomechanics Research Group in the Advanced Applied Physical Science Laboratory at NTT Basic Research Laboratories
There’s plenty of other editorial on our sister site, Electronic Specifier! Or you can always join in the conversation by commenting below or visiting our LinkedIn page.
