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Boffins concoct interference-busting radios

What’s the frequency, Kenneth?

Radio interference can be a pain to deal with, regardless of whether it's a rogue baby monitor interrupting your Wi-Fi or a stadium full of smartphone signals drowning each other out.

However, brainiacs at MIT say they've developed a radio chip that can see through the noisiest RF hellscape by actively blocking unwanted frequencies before they can scramble messages.

The chip was developed to address the growing challenges associated with 5G and other wireless communications standards. It takes inspiration from several adjacent domains – including digital signal processing and applied electronics – explained Negar Reiskarimian, assistant professor of electrical engineering and computer science at MIT.

The work, presented at the International Solid-State Circuits Conference (ISSCC) last week and detailed in a recent blog post, combines a number of existing technologies into a novel radio chip, which researchers say can contend with RF interference 40 times higher than existing wideband receivers. What's more, they say the method doesn't require large, bulky filtering equipment.

Even in its current development stage, the chip is small enough – just 0.65mm square – for use in mobile devices, according to Reiskarimian and Soroush Araei, an MIT grad student working on the project. And while 5G is highlighted as a potential application for radios based on the design, they note there's no reason it can't be used for other wireless signals like Wi-Fi.

"5G has a wide range of frequencies that it covers," they explained to The Reg. "Many of those frequency bands are on top of or very close to other technologies such as Wi-Fi and Bluetooth. Claiming this work to be useful for 5G is an umbrella statement that covers all signal frequencies in the sub-6GHz band."

The basic design of the chip is based on a mixer-first architecture, so called because it strips out unwanted frequencies before decoding the signal. However, this approach alone can't prevent harmonic interference.

If you're not familiar, harmonic interference occurs at multiples of the target frequency. For example, 1GHz, 2GHz, and 5GHz would all be harmonic frequencies, the researchers explained. They add that this kind of interference is particularly tricky to filter out because electronic equipment often finds it difficult to distinguish between them.

"A lot of other wideband receivers don't do anything about the harmonics until it is time to see what the bits mean," Araei explained. "We want to remove harmonics as soon as possible to avoid losing information."

To combat these frequencies, researchers applied a technique used in digital signal processing, called block-digital filtering, to an analog environment using capacitors. Different arrangements of capacitors connected in parallel or in sequence can effectively block these harmonics – but often have the side effect of degrading the signal.

To avoid this signal loss, Reiskarimian's team used a carefully calibrated combination of stacked capacitors arranged in series. The result was the majority of interference could be filtered out without compromising the integrity of the desired frequencies.

"People have used these techniques – charge sharing and capacitor stacking – separately before, but never together," Araei said. "We found that both techniques must be done simultaneously to get this benefit."

The researchers believe this technology has practical applications in noisy RF environments. And as 5G networks – which are composed of large swaths of high and low frequency spectrum – become more prevalent, they expect this tech to become increasingly important.

There are some challenges yet to be addressed. Two of them, we're told, are extending the frequency range and optimizing the performance of the chip to address specific use cases.

However, Reiskarimian and Araei don't expect it will take long for their work to find its way into smartphones and other wireless tech. 

"The world of integrated circuit development moves very fast, especially when it comes to wireless technology," they said. "Our technology and the techniques we use to make it possible can easily find their way to commercial radios within a couple of years." ®

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