Distributed Antennas Push The Limits Of High-Frequency Signals

Princeton University researchers have found a way to extend the near-field range for sub-Terahertz wireless systems using off-the-shelf hardware. By using multiple synched arrays, the team realized advanced near-field capabilities that were previously thought to require much larger and more expensive systems. Technologies using this high-frequency radio spectrum promise to unlock extraordinary data rates and entirely new sensing and security capabilities for telecom and defense applications.

“Near-field operation gives us much greater control over how electromagnetic energy is distributed in space,” said Yasaman Ghasempour, assistant professor of electrical and computer engineering, who led the study. “And that opens up exciting possibilities for communication, tracking or secure wireless transmission.”

Previous work in this area suggested that extending the near-field region for the sub-Terahertz band would require building much larger and more expensive hardware.

In a paper published April 22 in Nature Communications, the Princeton team showed how to extend the near field with standard hardware in a distributed, coordinated system.

“You can obtain essentially the same near-field beam quality, but in a much more practical way,” said Ghasempour, who is also co-director of the Princeton NextG initiative.

Decades of work had shown engineers how to link such distributed systems for the far field, where nearly all of today’s wireless systems operate. Translating those older methods to the near field meant reinventing the mathematical models from scratch, starting at the foundational equations for electromagnetism.

Ghasempour’s team, including collaborators at Northeastern University, established the new mathematical models and demonstrated the utility of those models in real systems.

“By coordinating multiple smaller arrays to work as a team, we can project highly innovative beams, like those that focus with surgical precision or heal themselves around obstacles, across much larger, practical distances that match the scale of real-world applications,” said Josep Miquel Jornet, a distinguished professor of electrical and computer engineering and the associate dean of research at Northeastern. “Just as the wireless industry previously evolved from single-antenna beamforming to distributed MIMO systems, we are now taking the next leap from single-aperture near-field beam shaping to distributed near-field wavefront engineering.”

While sub-Terahertz signals carry far more data than current radio signals, they are more vulnerable to adverse real-world conditions: blocked by walls, ripped apart by electromagnetic interference and absorbed into the atmosphere. To mitigate those vulnerabilities, engineers developed advanced beamforming techniques such as focusing to pinpoint accuracy, curving beams around obstructions, or repairing degraded signals in midair.

“Near-field beams objectively give us better performance,” said the paper’s first author Atsutse Kludze, a graduate student in Ghasempour’s lab. “But only at a certain distance.”

He said the first question to answer was how far from the transmitter these beams can be used effectively.

Scientists have long used the Fraunhofer distance, a theoretical measure of how far a signal can travel and still be considered in the near field range. The Princeton team took a closer look at the math and determined that the Fraunhofer distance overestimates the range over which exotic techniques can shape sub-Terahertz signals.

Kludze determined that the effective near-field range is much smaller. While the actual calculations depend on the details of the system, including signal frequency and antenna size, in their experimental setups the effective near-field range was around one-sixth or one-seventh the Fraunhofer distance.

Using the new, more conservative calculations, the researchers were then able to extend the effective near-field without building enormous antenna arrays. Instead, they showed how a distributed network of smaller antenna arrays made advanced beam forming useful at distances that are realistic, with important implications for larger systems relevant to communication, sensing and security systems.

“Our solution is let’s use distributed arrays,” Kludze said. “So, multiple arrays working together. Doing that, we were able to push the effective near field range farther out. We can push past the limits of a single array.”