Researchers at Columbia Engineering have developed a technology for full-duplex radio integrated circuits (ICs) that can be implemented in nanoscale CMOS to enable simultaneous transmission and reception at the same frequency in a wireless radio. Last year, the researcher had developed a nanoscale CMOS radio that required two antennas, one for the transmitter and one for the receiver. Now, the team, led by Electrical Engineering Associate Professor Harish Krishnaswamy, has developed a breakthrough technology that needs only one antenna, thus enabling an even smaller overall system. This is the first time researchers have integrated a non-reciprocal circulator and a full-duplex radio on a nanoscale silicon chip.
This technology could revolutionize the field of telecommunications. Systems with seperate Tx and Rx antennas can now operate using a single antenna and systems based on WiFi will see capacity double using a nanoscale silicon chip with a single antenna.
This group has been working on silicon radio chips for full duplex communications for several years and became particularly interested in the role of the circulator, a component that enables full-duplex communications where the transmitter and the receiver share the same antenna. In order to do this, the circulator has to “break” Lorentz Reciprocity, a fundamental physical characteristic of most electronic structures that requires electromagnetic waves travel in the same manner in forward and reverse directions.
Reciprocal circuits and systems are quite restrictive as you can’t control the signal freely. The traditional way of breaking Lorentz Reciprocity and building radio-frequency circulators has been to use magnetic materials such as ferrites, which lose reciprocity when an external magnetic field is applied. But these materials are not compatible with silicon chip technology, and ferrite circulators are bulky and expensive.They were able to design a highly miniaturized circulator that uses switches to rotate the signal across a set of capacitors to emulate the non-reciprocal “twist” of the signal that is seen in ferrite materials. Aside from the circulator, they also built a prototype of their full-duplex system—a silicon IC that included both their circulator and an echo-cancelling receiver—and demonstrated its capability at the 2016 IEEE International Solid- State Circuits Conference in February.
Non-reciprocal circuits and components have applications in many different scenarios, from radio-frequency full-duplex communications and radar to building isolators that prevent high-power transmitters from being damaged by back-reflections from the antenna. The ability to break reciprocity also opens up new possibilities in radio-frequency signal processing that are yet to be discovered. Full-duplex communications is of particular interest to researchers because of its potential to double network capacity, compared to half-duplex communications that current cell phones and WiFi radios use. To further improve the performance of their circulator, and exploring "beyond-circulator" applications of non-reciprocity, work is already going on.
This group was able to demonstrate a practical RF circulator integrated with a full-duplex receiver that exhibited a factor of nearly a billion in echo cancellation, making it the first practical full-duplex receiver chip and which led to the publication in the 2016 IEEE ISSCC.
The work has been funded by DARPA MTO (Microsystems Technology Office) under the ACT (Arrays at Commercial Timescales) and the RF-FPGA programs, and by the National Science Foundation (ECCS-1547406).
