Today's fast wireless communication, including 5G mobile phones and sensors for autonomous vehicles, increasingly clogs radio frequencies. This makes blocking interference that can disrupt device operation more important and challenging.

With these and other applications in mind, researchers at MIT have demonstrated a new architecture for a millimeter-wave multiple input and output (MIMO) wireless receiver that can handle stronger spatial interference than previous designs. MIMO systems have multiple antennas that enable sending and receiving signals from different directions. Their wireless receiver detects and blocks spatial interference as early as possible, before unwanted signals are amplified, which improves performance.

The key to this MIMO receiver architecture is a special circuit that can target and cancel unwanted signals, known as a non-reciprocal phase shifter. By designing a new phase shifter structure that is reconfigurable, low-power, and compact, the researchers demonstrate 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 reduce signal quality issues that can lead to slow and choppy Zoom calls or video streaming.

Blocking Interference
Digital MIMO systems have both an analog and a 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 device's digital domain. In this case, digital beamforming is required to capture the desired signal.

But 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 MIMOs can filter unwanted signals, but this filtering happens later in the receiver chain. If 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 you can perform this filtering with minimal penalty, so that's exactly what we're doing with our approach,” says Reiskarimian.

The researchers built and installed four non-reciprocal phase shifters immediately 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 sense the angle of the incoming interfering signal. The devices can adjust their phase until they cancel out the interference.

The phase of these devices can be precisely tuned so they can sense 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 they change location.

“If you start losing connection or your signal quality drops, you can turn this on and mitigate those interferences 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 shifter architecture adjustable, the researchers designed them to take up less space on the chip and consume less power 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 achieved their performance goals. At the same time, the receiver had to meet strict size constraints and a tight energy budget, otherwise, it would not be useful in real devices.

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

In the future, the researchers want to scale their device to larger systems as well as enable it to operate in new frequency ranges used by 6G wireless devices. These frequency ranges are prone to strong interference from satellites. Additionally, they would like to adapt non-reciprocal phase shifters for other applications.

This research was supported, in part, by the MIT Center for Integrated Circuits and Systems.

Source: Massachusetts Institute of Technology