Single-Frequency, Full-Duplex Could Relieve Spectrum Crunch

In the early days of radio communications, you pushed a button on your microphone to say want you wanted, and then –– usually after saying “over”–– released the button to listen for the reply on the same frequency.

Today, radios need to transmit and receive information at the same time and this takes more spectrum. Researchers at the University of California Bourns College in Riverside, California have developed a way to transmit and receive on the same frequency at the same time. The method is outlined in the recently published paper A method for broadband full-duplex MIMO radio in the journal IEEE Signal Processing Letters. It’s co-authored by Yingbo Hua and Ping Liang, who are both electrical engineering professors, and three of their graduate students: Yiming Ma, Ali Cagatay Cirik and Qian Gao.

One of the problems with implementing a full-duplex radio is the interference from the transmitter into the receiver. The UC Riverside researchers solve the problem using “time-domain transmit beamforming.” It digitally creates a time-domain cancellation signal, which is coupled to the receiver's front-end, canceling the transmitted signal and allowing reception of weak signals on the same frequency.

“We believe the future applications of full duplex radios are huge, ranging from cell towers, backhaul networks and wireless regional area networks to billions handheld devices for data intensive application such as FaceTime,” said Ping Liang.”

He added the researchers have had discussions with several major telecommunication equipment companies. The technology should also have applications in cognitive radios.

The paper includes drawings and test results, which show a 47 to 50 dB cancellation of self-interference. The abstract states, “We present a time-domain transmit beamforming (TDTB) method for self-interference cancellation (SIC) at the radio frequency (RF) frontend of the receivers on broadband full-duplex MIMO radios. It is shown that the conventional frequency-domain transmit beamforming (FDTB) method along with the orthogonal frequency division multiplexing (OFDM) framework does not generally perform SIC in the prefix region of a transmitted frame. A hardware based test of the TDTB method shows a 50 dB SIC over a bandwidth of 30 MHz.”

I wonder if this technology could be used to allow devices receiving COFDM TV signals to transmit a return signal in the same band? One obvious problem that would have to be solved would be how to deal with interference to receivers near the transmitting device that would need some way to cancel the signal from the nearby device to avoid interference.

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.