The increasing presence of devices for rapid wireless communication, from 5G mobile phones to sensors for autonomous vehicles, is leading to increasingly crowded radio waves. Therefore, the ability to block interfering signals, which can disrupt device operation, has become a more important and challenging problem.

With these and other new applications in mind, researchers from MIT have demonstrated a new millimeter-wave multiple input-output (MIMO) wireless reception architecture that can handle stronger spatial interference than previous designs. MIMO systems have multiple antennas, allowing them to transmit and receive signals from different directions. Their wireless receiver detects and blocks spatial interference as early as possible, before unwanted signals are amplified, improving performance.

The key to this MIMO reception architecture is a special circuit that can target and cancel unwanted signals, known as a non-reciprocal phase shift. The researchers have created a new phase shift structure that is reconfigurable, low-energy, and compact, demonstrating how it can be used to cancel interference earlier in the receiver chain.

Their receiver can block up to four times more interference than some similar devices. Additionally, the interference-blocking components can be turned on and off as needed to save energy.

In a mobile phone, such a receiver could help alleviate signal quality issues that can lead to slow and interrupted Zoom calls or video streaming.

"There is already a lot of usage in the frequency bands we are trying to use for new 5G and 6G systems. So, anything we try to add already needs to have built-in interference mitigation systems. Here we have shown that using non-reciprocal phase shifts in this new architecture provides better performance. This is quite significant, especially since we use the same integrated platform as everyone else," says Negar Reiskarimian, assistant professor of career development in the X-Window consortium in the Department of Electrical Engineering and Computer Science (EECS), a member of the Microsystems Technology Laboratory and the Research Laboratory of Electronics (RLE), and the lead author of the paper on this receiver.

Reiskarimian wrote the paper with EECS graduate students Shahabeddin Mohin, the lead author, Soroush Araei, and Mohammad Barzgari, an RLE postdoctoral fellow. The paper was recently presented at the IEEE Radio Frequency Circuits Symposium and received the award for best student paper.

Blocking Interference
Digital MIMO systems have an analog and digital part. The analog part uses antennas to receive signals, which are amplified, converted, and passed through an analog-to-digital converter before being processed in the digital part of the device. In this case, digital beamforming is needed to capture the desired signal.

However, if a strong, interfering signal from another direction hits the receiver at the same time as the desired signal, it can saturate the amplifier so that the desired signal is overwhelmed. Digital MIMO systems can filter out unwanted signals, but this filtering occurs later in the receiver chain. If the interference is amplified along with the desired signal, it is harder to filter out later.

"The output of the initial low-noise amplifier is the first place where you can do this filtering with minimal penalty, so that's exactly what we do with our approach," says Reiskarimian.

The researchers built and installed four non-reciprocal phase shifters right at the output of the first amplifier in each receiver chain, all connected to the same node. These phase shifters can pass the signal in both directions and detect the angle of the incoming interfering signal. The devices can adjust their phase until they cancel the interference.

The phase of these devices can be precisely adjusted so they can detect and cancel the unwanted signal before it passes to the rest of the receiver, blocking interference before it affects any other part of the receiver. Additionally, the phase shifters can track signals to continue blocking interference if it changes location.

"If you start losing connection or your signal quality drops, you can turn this on and mitigate that interference on the fly. Since our approach is parallel, you can turn it on and off with minimal impact on the performance of the receiver itself," adds Reiskarimian.

Compact Device
In addition to making their new phase shift architecture adjustable, the researchers designed them to occupy less space on the chip and consume less energy than typical non-reciprocal phase shifters.

After the researchers conducted an analysis that showed their idea would work, their biggest challenge was translating the theory into a circuit that achieves their performance goals. At the same time, the receiver had to meet strict size constraints and a tight energy budget, or it would not be useful in real devices.

In the end, the team demonstrated a compact MIMO architecture on a chip the size of 3.2 square millimeters that can block signals that were up to four times stronger than what other devices could handle. Simpler than typical designs, their phase shift architecture is also more energy-efficient.

Going forward, the researchers want to scale their device to larger systems and enable it to operate in new frequency bands used by 6G wireless devices. These frequency bands are prone to strong interference from satellites. Additionally, they want to adapt non-reciprocal phase shifters for other applications.

This research was partially supported by the MIT Center for Integrated Circuits and Systems.

Source: Massachusetts Institute of Technology