Two-way communication requires, of course, both send and receive capabilities. But putting them in the same device requires a filter between the send and receive circuits to provide signal isolation. Without a high performance filter, communication would be impossible.
Researchers in Cornell University's Molnar lab have devised a method for both transmitting and receiving a radio signal using a single chip without the use of a filter. This technology could potentially change the way wireless communication is done.
Separating the send and receive bands is difficult enough, but the problem is compounded by the ever-increasing number of bands in the latest devices. From GPS to Bluetooth to Wi-Fi, each band requires a filter to stop the strong transmit signals from drowning out reception of other technologies.
The researchers have come up with an ingenious way to separate the signals. Their idea lies in the transmitter - actually a series of six subtransmitters all hooked into an artificial transmission line. Each sends its signal at regular intervals, and their individually weighted outputs are programmed so that they combine to produce a radio frequency signal in the forward direction, at the antenna port, while canceling out at the receive port.
The programmability of the individual outputs allows this simultaneous summation and cancellation to be tuned across a wide range of frequencies, and to adjust to signal strength at the antenna.
You put the antenna at one end and the amplified signal goes out the antenna, and you put the receiver at the other end which it acts like a filter where the signals cancel each other out. The receiver sees the antenna through the transmission line, but it doesn’t see the transmit signal because it’s canceling itself out at that end.
This work builds on research reported six years ago by a group from Stanford University, which devised a way for the transmitter to filter its own transmission, allowing the weaker incoming signal to be heard. It’s the theory behind noise-canceling headphones. However, unlike the Stanford research, the Cornell group’s subtransmitter concept will work over a range of frequencies which is a key requirement.
This wire is a fairly broadband structure. All you need to do to make it work over a wide range of frequencies is control those different subgains of the transmitters, to make this cancellation always happen. Instead of needing a filter for every band, signal separation can be controlled digitally. Upgrading to the latest version would be like updating an app - as simple as downloading the latest software.
This work is described in “A wideband fully integrated software-defined transceiver for FDD and TDD operation,” published online Jan. 27 in the Institute of Electrical and Electronics Engineers’ Journal of Solid-State Circuits. Doctoral student Hazal Yüksel and Dong Yang, Ph.D. ’15, are co-lead authors.
