Imagine you’re sitting in a crowded stadium for a crucial football game – tens of thousands of people are using cell phones at the same time, perhaps video chatting with friends or posting photos on social media. The radio frequency signals sent and received by all of these devices can cause interference that slows device performance and drains batteries.
Designing devices that can efficiently block unwanted signals is not an easy task, especially as 5G networks become more universal and future generations of wireless communication systems are developed. Traditional techniques use many filters to block a range of signals, but filters are bulky, expensive, and increase production costs.
MIT researchers have developed a circuit architecture that targets and blocks unwanted signals at the input of a receiver without degrading its performance. They borrowed a technique from digital signal processing and used some tricks that allow them to work effectively in a high frequency system over a wide range of frequencies.
Their receiver even blocked unwanted high-power signals without introducing more noise or inaccuracies into the signal processing operations. The chip, which was about 40 times better than other broadband receivers at blocking a specific type of interference, requires no additional hardware or circuitry. This would make it easier to manufacture the chip on a large scale.
“We are interested in developing electronic circuits and systems that meet the requirements of 5G and future generations of wireless communication systems. When designing our circuits, we look for inspiration from other fields, such as digital signal processing and applied electromagnetics. We believe in the elegance and simplicity of circuits and try to design multifunctional hardware that does not require additional power and chip area,” says senior author Negar Reiskarimian, assistant professor of career development of the X Window Consortium in the Department of Electrical Engineering and Computer Science (EECS ) and a core faculty member of Microsystems Technology Laboratories.
Reiskarimian co-authored the paper with EECS graduates Soroush Araei, the lead author, and Shahabeddin Mohin. The work will be presented at the International Solid-States Circuits Conference.
The researchers developed the receiver chip using a so-called mixer-first architecture. This means that a high-frequency signal received by the device is immediately converted to a lower-frequency signal before being passed to the analog-to-digital converter to extract the digital bits it contains. This approach allows the radio to cover a wide frequency range while still filtering out interference that is close to the operating frequency.
Mixer-first receivers, while effective, are susceptible to a specific type of interference known as harmonic interference. Harmonic interference is caused by signals with frequencies that are many times the operating frequency of a device. For example, if a device operates at 1 gigahertz, then signals at 2 gigahertz, 3 gigahertz, 5 gigahertz, etc. will cause harmonic interference. These harmonics can become indistinguishable from the original signal during the frequency conversion process.
“Many other broadband receivers don’t do anything about the harmonics until it’s time to see what the bits mean. You do this later in the chain, but that doesn’t work well when you have high power signals at the harmonic frequencies. Instead, we want to remove harmonics as quickly as possible to avoid information loss,” says Araei.
The researchers were inspired by a concept from digital signal processing, known as block digital filtering. They adapted this technique to the analog domain using capacitors that hold electrical charges. The capacitors are charged at different times when the signal is received, then they are switched off to allow the charge to be held and used later to process the data.
These capacitors can be connected together in a variety of ways, including a parallel connection, which allows the capacitors to exchange stored charges. While this technique can target harmonic interference, the process results in significant signal loss. Stacking capacitors is another possibility, but this method alone is not enough to ensure harmonic immunity.
Most radio receivers already use switched capacitor circuits to perform frequency conversion. This frequency conversion circuit can be combined with block filtering to combat harmonic interference.
A precise arrangement
The researchers found that arranging capacitors in a specific layout, by connecting some of them in series and then performing charge sharing, allowed the device to block harmonic interference without losing information.
“People have used these techniques, charge sharing and capacitor stacking, separately before, but never together. We have found that both techniques must be performed simultaneously to achieve this benefit. Additionally, we figured out how to do this in a passive way within the mixer without using additional hardware, while preserving signal integrity and keeping costs down,” he says.
They tested the device by simultaneously sending a desired signal and harmonic interference. Their chip was able to effectively block harmonic signals with only a minor reduction in signal strength. It was able to handle signals 40 times stronger than previous state-of-the-art broadband receivers.