To 6G and beyond: engineers unlock the next generation of wireless communications

In the early 2010s, LightSquared, a multibillion-dollar company that promised to revolutionize mobile communications, went bankrupt. The company couldn’t figure out how to prevent its signals from interfering with those of GPS systems.

Now Penn Engineers have developed a new tool that could prevent such problems from ever happening again: an adjustable filter that can successfully prevent interference even in the higher frequency bands of the electromagnetic spectrum.

“I hope this will enable the next generation of wireless communications,” said Troy Olsson, associate professor of Electrical and Systems Engineering (ESE) at Penn Engineering and senior author of a paper in Nature communication that describes the filter.

The electromagnetic spectrum itself is one of the modern world’s most precious resources; only a small part of the spectrum, mainly radio waves, which represent less than one billionth of one percent of the total spectrum, is suitable for wireless communications.

The bands of that part of the spectrum are carefully controlled by the Federal Communications Commission (FCC), which only recently made the Frequency Range 3 (FR3) band, including frequencies from approximately 7 GHz to 24 GHz, available for commercial use. (One hertz corresponds to a single oscillation in an electromagnetic wave passing a point every second; one gigahertz, or GHz, is one billion such oscillations per second.)

Until now, wireless communication has mainly used lower frequency bands. “Right now we are working from 600 MHz to 6 GHz,” says Olsson. “That’s 5G, 4G, 3G.”

Wireless devices use different filters for different frequencies, with the result that covering all frequencies or bands requires large numbers of filters that take up significant space. (The typical smartphone contains more than 100 filters, to ensure that signals from different bands do not interfere with each other.)

“The FR3 band will most likely be rolled out for 6G or Next G,” says Olsson, referring to the next generation of mobile networks, “and right now the performance of small filter and low-loss switching technologies in those bands is very high. Having a filter that can be tuned to these bands means you don’t have to put another 100+ filters in your phone with many different switches. A filter like we created is the most feasible way to use the FR3 band.

A complication that arises when using higher frequency bands is that many frequencies are already reserved for satellites. “Elon Musk’s Starlink works in those bands,” Olsson notes. “The military has already been pushed out of much lower bands. They have no intention of giving up the radar frequencies that are right in those bands, or their satellite communications.”

As a result, Olsson’s lab – in collaboration with colleagues Mark Allen, Alfred Fitler Moore Professor in ESE, and Firooz Aflatouni, Associate Professor in ESE, and their respective groups – designed the filter to be adjustable, allowing engineers to use it selectively can use filter different frequencies, instead of having to use separate filters.

“Being tunable will be very important,” Olsson continues, “because at these higher frequencies you may not always have a dedicated block of spectrum just for commercial use.”

What makes the filter adjustable is a unique material, “yttrium iron garnet” (YIG), a mixture of yttrium, a rare earth metal, together with iron and oxygen. “The special thing about YIG is that it propagates a magnetic spin wave,” says Olsson, referring to the type of wave that is created in magnetic materials when electrons spin in a synchronized manner.

When exposed to a magnetic field, the magnetic spin wave generated by YIG changes frequency. “By adjusting the magnetic field,” says Xingyu Du, a PhD student in Olsson’s lab and the paper’s first author, “the YIG filter achieves continuous frequency tuning over an extremely wide frequency band.”

As a result, the new filter can be tuned to any frequency between 3.4 GHz and 11.1 GHz, covering much of the new territory the FCC has opened up in the FR3 band. “We hope to demonstrate that a single adaptable filter is sufficient for all frequency bands,” says Du.

The new filter is not only tunable, but also small: about the size of a quarter, unlike previous generations of YIG filters, which resembled large decks of index cards.

One reason why the new filter is so small and therefore could potentially be installed in mobile phones in the future is that it requires very little power. “We pioneered the design of a zero-static force magnetic bias circuit,” says Du, referring to a type of circuit that creates a magnetic field without requiring any energy, aside from a single pulse, to reapply the field to fit.

Although YIG was discovered in the 1950s and YIG filters have been around for decades, the combination of the new circuit with extremely thin YIG films micromachined at the Singh Center for Nanotechnology reduced the power consumption and size of the new filter dramatic. “Our filter is ten times smaller than current commercial YIG filters,” says Du.

In June, Olsson and Du will present the new filter at the 2024 Institute of Electrical and Electronics Engineers (IEEE) Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium in Washington, DC