Doug Lung /
Single-Frequency, Full-Duplex Could Relieve Spectrum Crunch
UC Riverside researchers solve problem using “time-domain transmit beamforming"
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
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.
Sound of Music: On Stage, On the Small Screen
“By and large, the few mistakes made were human and, if anyone noticed them, just reminded the viewer that the show was live.” ~ Robert Dickinson, lighting designer "The Sound of Music Live"