The increasing prevalence of high-speed wireless devices, from 5G smartphones to autonomous vehicle sensors, is leading to increasingly congested airwaves. This congestion creates a significant challenge: effectively blocking interfering signals that can hinder device performance. To address this, MIT researchers have demonstrated a new millimeter-wave multiple-input-multiple-output (MIMO) wireless receiver architecture capable of handling stronger spatial interference than previous designs. MIMO systems utilize multiple antennas to transmit and receive signals from various directions. This new receiver identifies and blocks spatial interference at the earliest stage before unwanted signals are amplified, resulting in improved performance.
At the heart of this MIMO receiver architecture lies a unique circuit called a nonreciprocal phase shifter, designed to target and eliminate unwanted signals. The researchers developed a novel phase shifter structure that is reconfigurable, energy-efficient, and compact, demonstrating its ability to cancel interference earlier in the receiver chain.
“There is already a lot of utilization happening in the frequency ranges we are trying to use for new 5G and 6G systems. So, anything new we are trying to add should already have these interference-mitigation systems installed. Here, we’ve shown that using a nonreciprocal phase shifter in this new architecture gives us better performance. This is quite significant, especially since we are using the same integrated platform as everyone else,” says Negar Reiskarimian, the X-Window Consortium Career Development Assistant Professor in the Department of Electrical Engineering and Computer Science (EECS), a member of the Microsystems Technology Laboratories and Research Laboratory of Electronics (RLE), and the senior author of a paper on this receiver.
This receiver can block up to four times more interference than comparable devices. Additionally, the interference-blocking components can be activated or deactivated as needed to conserve energy. In a mobile phone, this type of receiver could help alleviate signal quality issues that often lead to choppy video calls or lagging video streams.
Digital MIMO systems consist of analog and digital components. The analog portion utilizes antennas to receive signals, which are then amplified, down-converted, and processed through an analog-to-digital converter before undergoing digital processing. This process requires digital beam forming to extract the desired signal. However, if a strong interfering signal from a different direction reaches the receiver simultaneously with the desired signal, it can overwhelm the amplifier, effectively drowning out the desired signal. While digital MIMOs can filter out unwanted signals, this filtering occurs later in the receiver chain. Amplifying interference alongside the desired signal makes it more difficult to filter out later.
“The output of the initial low-noise amplifier is the first place you can do this filtering with minimal penalty, so that is exactly what we are doing with our approach,” Reiskarimian explains.
The researchers integrated four nonreciprocal phase shifters directly at the output of the first amplifier in each receiver chain, all connected to the same node. These phase shifters, capable of bidirectional signal passing, can detect the angle of an incoming interfering signal and adjust their phase to cancel it out.
The phase of these devices can be finely tuned to detect and neutralize an unwanted signal before it reaches the rest of the receiver, effectively blocking interference before it impacts other receiver components. Moreover, the phase shifters can track signals to maintain interference blocking even if the interference source changes location.
If you start getting disconnected or your signal quality goes down, you can turn this on and mitigate that interference on the fly. Because ours is a parallel approach, you can turn it on and off with minimal effect on the performance of the receiver itself,
Reiskarimian adds.
Beyond making their novel phase shifter architecture tunable, the researchers prioritized minimizing its chip footprint and power consumption compared to conventional nonreciprocal phase shifters. After analysis confirmed the viability of their concept, the team’s primary challenge was translating theory into a circuit that met their performance objectives. Simultaneously, the receiver had to adhere to strict size and power constraints to ensure practicality in real-world devices.
Ultimately, the team successfully demonstrated a compact MIMO architecture on a 3.2-square-millimeter chip capable of blocking signals up to four times stronger than what other devices could handle. Their phase shifter architecture, simpler than typical designs, also boasts greater energy efficiency.
Looking ahead, the researchers aim to scale up their devices for larger systems and enable operation within the new frequency ranges utilized by 6G wireless devices. These frequency ranges are susceptible to strong interference from satellites. Additionally, they plan to explore the adaptation of nonreciprocal phase shifters for other applications.
This research was supported, in part, by the MIT Center for Integrated Circuits and Systems.
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