Breaking Lorentz Reciprocity: From Physics to New Wireless Communication Paradigms

Lorentz reciprocity is a fundamental characteristic of the vast majority of electronic and photonic structures. However, breaking reciprocity enables the realization of non-reciprocal components, such as isolators and circulators, which are critical to electronic and optical communication systems, as well as new components and functionalities based on novel wave propagation modes. Non-reciprocal components have traditionally relied on the use of magnetic materials that lose reciprocity under the application of an external magnetic field through the Faraday effect, rendering them bulky, expensive and incompatible with IC fabrication. In this talk, I will present a novel approach to break Lorentz reciprocity based on linear periodically-time-varying (LPTV) circuits. We have demonstrated the world's first CMOS passive magnetic-free non-reciprocal circulator through spatio-temporal conductivity modulation Since conductivity in semiconductors can be modulated over a much wider range than the more traditionally exploited permittivity, our structure is able to break reciprocity within a compact form factor with very low loss and high linearity.

One of the emerging applications of non-reciprocity is within the next generation of wireless communication networks. Various emergent wireless technologies are under investigation to enable the "5G" revolution, including massive MIMO and full-duplex wireless. Enabling these technologies requires a re-evaluation and redesign of various layers of the communication system, from the PHY layer all the way up to the application layer. In the case of the PHY layer, conventional reciprocal antenna interfaces impose fundamental limitations on the radio front-end design. I will present our recent efforts on designing full-duplex receivers, which take advantage of our integrated circulator to co-design and co-optimize the system performance as a whole.

Looking to the future, I am broadly interested in exploring novel fundamental physical concepts that have strong engineering applications. Specifically, I am interested in new physical phenomena and functionalities enabled by space-time-modulated metamaterials. I am investigating achieving non-reciprocity using only the electric field, enabling the realization of "inductor-less" circulators and non-reciprocal components. I am also investigating topological metamaterials, which would enable reconfigurable and robust "beyond-circulator" non-reciprocal antenna interfaces for large-scale arrays that will combine massive MIMO wireless with full-duplex.