Designing devices capable of effectively blocking unwanted signals is no easy task, especially as 5G networks become more widespread and future generations of wireless communication systems are developed. Conventional techniques use many filters to block a range of signals, but these filters are bulky, expensive, and drive up production costs.
MIT researchers have developed a circuit architecture that blocks unwanted signals at a receiver's input without affecting its performance. They borrowed a technique from digital signal processing and used some tricks that allow it to work effectively in a radio frequency system across a wide frequency range.
Its receiver blocked even high-power unwanted signals without introducing additional noise or inaccuracies into the signal processing operations. The chip, which performs about 40 times better than other broadband receivers at blocking a specific type of interference, requires no additional hardware or circuitry. This would facilitate its mass production.
"We are interested in developing electronic circuits and systems that meet the demands of 5G and future generations of wireless communication systems. When designing our circuits, we draw inspiration from other fields, such as digital signal processing and applied electromagnetism. We believe in the elegance and simplicity of circuits and strive to create multifunctional hardware that doesn't require more power or chip surface area," says lead author Negar Reiskarimian, Associate Professor of Professional Development for the X-Window Consortium in the Department of Electrical and Computer Engineering (EECS) and a core faculty member of the Microsystems Technology Laboratories.
Reiskarimian co-authored the paper with EECS graduate students Soroush Araei, the lead author, and Shahabeddin Mohin. The work is being presented at the International Conference on Solid State Circuits.
Harmonic interference
The researchers developed the receiver chip using what is known as a mixer-first architecture. This means that when the device receives a radio frequency signal, it immediately converts it to a lower frequency signal before passing it to the analog-to-digital converter to extract the digital bits it carries. This approach allows the radio to cover a wide range of frequencies while filtering out interference near the operating frequency.
Although effective, receivers with mixers are susceptible to a particular type of interference known as harmonic interference. Harmonic interference comes from signals whose frequencies are multiples of a device's operating frequency. For example, if a device operates at 1 gigahertz, 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.
"Many other broadband receivers don't do anything about harmonics until it comes 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 eliminate harmonics as early as possible so as not to lose information," Araei explains.
To achieve this, the researchers drew inspiration from a digital signal processing concept known as digital block filtering. They adapted this technique to the analog domain using capacitors, which hold electrical charges. The capacitors are charged at different times as the signal is received, and then turned off so that the charge is retained and can be used later to process the data.
These capacitors can be connected in various ways, including parallel connection, which allows the capacitors to exchange stored charges. While this technique can combat harmonic interference, it results in significant signal loss. Stacking capacitors is another possibility, but this method alone is insufficient to resist harmonics.
Most radio receivers already use switched capacitor circuits for frequency conversion. These frequency conversion circuits can be combined with block filtering to combat harmonic interference.
A precise arrangement
The researchers discovered that placing the capacitors in a specific arrangement, connecting some of them in series and sharing the charge, allowed them to block harmonic interference without losing information.
"These techniques had been used separately until now, but never together. We've discovered that both techniques must be applied simultaneously to achieve this benefit. Furthermore, we've discovered how to do this passively within the mixer without using any additional hardware, maintaining signal integrity and reducing costs," he says.
They tested the device by simultaneously sending a desired signal and harmonic interference. The chip effectively blocked the harmonic signals with only a slight reduction in signal strength. It was capable of handling signals 40 times stronger than the most advanced broadband receivers.
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Written by Adam Zewe, MIT News Office
