Researchers at MIT, Massachusetts have developed a circuit architecture that targets and blocks unwanted signals at a receiver’s input without hurting its performance. They borrowed a technique from digital signal processing and used a few tricks that enable it to work effectively in a radio frequency system across a wide frequency range.
This innovation is especially relevant for crowded settings. Imagine sitting in a packed stadium for a pivotal football game — tens of thousands of people are using mobile phones at the same time, perhaps video chatting with friends or posting photos on social media. The radio frequency signals being sent and received by all these devices could cause interference, which slows device performance and drains batteries.
MIT’s receiver blocked even high-power unwanted signals without introducing more noise, or inaccuracies, into the signal processing operations. The chip, which performed about 40 times better than other wideband receivers at blocking a special type of interference, does not require any additional hardware or circuitry. This would make the chip easier to manufacture at scale.
“We are interested in developing electronic circuits and systems that meet the demands of 5G and future generations of wireless communication systems. In designing our circuits, we look for inspirations from other domains, such as digital signal processing and applied electromagnetics. We believe in circuit elegance and simplicity and try to come up with multifunctional hardware that doesn’t require additional power and chip area,” says senior author Negar Reiskarimian, the X-Window Consortium Career Development Assistant Professor in the Department of Electrical Engineering and Computer Science (EECS) and a core faculty member of the Microsystems Technology Laboratories.
The team advances the work using something called a “mixer-first architecture” to identify and block unwanted interferences without harming a phone’s performance. In this setup, a radio frequency signal is converted into a lower frequency as soon as it is received by a device. From there, the signal’s digital bits are extracted via an analog-to-digital converter.
While effective, mixer-first receivers are susceptible to a particular kind of interference known as harmonic interference. Harmonic interference comes from signals that have frequencies that are multiples of a device’s operating frequency. For instance, if a device operates at 1 gigahertz, then signals at 2 gigahertz, 3 gigahertz, 5 gigahertz, etc., will cause harmonic interference. These harmonics can be indistinguishable from the original signal during the frequency conversion process.
“A lot of other wideband receivers don’t do anything about the harmonics until it is time to see what the bits mean. They do it later in the chain, but this doesn’t work well if you have high-power signals at the harmonic frequencies. Instead, we want to remove harmonics as soon as possible to avoid losing information,” said Soroush Araei, a graduate student at the department of Electrical Engineering and Computer Science (EECS).
The researchers also found that arranging capacitors in a specific layout, by connecting some of them in series and then performing charge sharing, enabled the device to block harmonic interference without losing any information.
They tested the device by simultaneously sending a desired signal and harmonic interference. Their chip was able to block harmonic signals effectively with only a slight reduction in signal strength. It was able to handle signals that were 40 times more powerful than previous, state-of-the-art wideband receivers.
“In short, our research can make your devices work better with fewer dropped calls or poor connections caused by interference from other devices,” says Araei.
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