A University of Nebraska–Lincoln researcher is using National Science Foundation funding to tackle spectrum scarcity, an increasingly urgent problem as wireless traffic soars to new heights.
Husker engineer Shubhendu Bhardwaj received a $550,000 grant from NSF’s Faculty Early Career Development program to develop a new, energy efficient approach to long-range wireless communication links that would represent a paradigm shift in the field. Rather than manipulating signals to compress data — a traditional approach called signal processing — Bhardwaj is designing special antennas capable of transmitting and receiving corkscrew-shaped electromagnetic waves with spinning wavefronts. The system would enable the simultaneous manipulation of the waves’ spin and orbital angular momentum.
These spiraling waves — called “vortex waves” — will allow the multiplexing of many different data streams into a single frequency, which overcomes a fundamental limit of current wireless communications systems: Because of interference issues, only one channel can travel across any given frequency of the electromagnetic spectrum. That data signal can only be compressed to a certain point before it begins to lose special features.
Transforming the waves’ angular momentum solves that problem, enabling multiple data streams to travel across the same frequency. This is key, as frequencies are a finite natural resource regulated by the government. Bhardwaj’s approach will allow efficient reuse of the spectrum.
“We need fundamental innovations that go above and beyond traditional compression methods,” said Bhardwaj, associate professor of electrical and computer engineering. “This project moves the needle in redesigning the physical layer side of the communication link, which means that antenna and microwave circuit designs will guide the proposed wireless communication innovations. This is an alternate, energy-efficient way of reusing the same frequency to allow higher data rates.”
Bhardwaj will build antennas capable of creating, sending and receiving waves with a high degree of polarization purity, meaning their orientation is precise and stable. Bhardwaj will achieve this by using a specially designed microwave circuit, called a Rotman lens, which controls the antennas’ ability to produce and receive differently oriented waves. While circuits will generate the required signal, the antennas will serve as novel waveguide structures, generating the helical wavefronts and sending them to a receiver.
Though the use of waveguides for antennas is well established, Bhardwaj’s approach is distinct for engineering them to generate vortex modes. His waveguide antennas will produce helical waves that can travel long distances without distortion.
“The trick is to use novel structures as radiators with a very high mode-purity of the transmission signal, meaning we create perfect modes as compared to prior methods, which generate these modes with mixing and distortion,” Bhardwaj said. “This ultimately shortens the link distance in prior methods.”
His research is novel for integrating both properties of waves — spin and angular momentum — to maximize reuse of a frequency. Other researchers have explored changing waves’ orbital angular momentum to transmit more data in a technique called OAM multiplexing. But Bhardwaj’s vortex wave multiplexing goes further by producing waves with altered angular and spin momentum, which will further enhance transmission rates.
“Adding spin gives you another degree of freedom to add more data channels,” Bhardwaj said.
This differentiation is what allows a single frequency to accommodate multiple data streams. If each stream is carried by a wave with a unique angular and spin momentum, they don’t encroach on each other, and the specialized antenna on the receiving end can easily sort the incoming information.
A major feature of the proposed system is its energy efficiency. Compared to math-heavy data compression — which requires significant power — Bhardwaj’s hardware solutions are a passive approach that reduce complexity and do not require any specific circuits or modules to perform signal processing.
The educational component of the project will focus on aligning the University of Nebraska–Lincoln’s engineering courses with industry needs. Bhardwaj will conduct workshops where members of industry will review course syllabi and provide input on how to equip students for work at companies.
“I’m really excited about integrating industry-facing ideas into our educational plans,” he said. “These partners will tell us how we can evolve our courses to align with what they need.”
Bhardwaj will also experiment with inverted classroom methods, which flip the script of conventional teaching. Rather than spending class time lecturing, Bhardwaj will deliver instructional content as homework. Then, he will guide students in engaging more deeply with the material during class hours through active learning mechanisms like collaborative discussion, projects and problem-solving.
Bhardwaj will also invite Husker students to experiment with the antenna testbed that he is developing as part of the CAREER project. The testbed will enable specialized measurements of the waves and experimentation with the linkages between sending and receiving antennas.
The National Science Foundation’s CAREER award supports pre-tenure faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research.