Assistant professors Philip A. Levis, left, and Sachin Katti worked with advisees to create a full duplex radio device. Credit: L.A. Cicero |
“Wireless
communication is a one-way street. Over.”
Radio traffic can flow
in only one direction at a time on a specific frequency, hence the frequent use
of “over” by pilots and air traffic controllers, walkie-talkie users
and emergency personnel as they take turns speaking.
But now, Stanford
researchers have developed the first wireless radios that can send and receive
signals at the same time.
This immediately makes
them twice as fast as existing technology, and with further tweaking will
likely lead to even faster and more efficient networks in the future.
“Textbooks say you
can’t do it,” said Philip Levis, assistant professor of computer science
and of electrical engineering. “The new system completely reworks our
assumptions about how wireless networks can be designed,” he said.
Cell phone networks
allow users to talk and listen simultaneously, but they use a work-around that
is expensive and requires careful planning, making the technique less feasible
for other wireless networks, including Wi-Fi.
Sparked from a simple idea
A trio of electrical engineering graduate students, Jung Il Choi, Mayank Jain,
and Kannan Srinivasan, began working on a new approach when they came up with a
seemingly simple idea. What if radios could do the same thing our brains do
when we listen and talk simultaneously: screen out the sound of our own voice?
In most wireless
networks, each device has to take turns speaking or listening. “It’s like
two people shouting messages to each other at the same time,” said Levis. “If both
people are shouting at the same time, neither of them will hear the
other.”
It took the students
several months to figure out how to build the new radio, with help from Levis and Sachin Katti,
assistant professor of computer science and of electrical engineering.
Their main roadblock to
two-way simultaneous conversation was this: Incoming signals are overwhelmed by
the radio’s own transmissions, making it impossible to talk and listen at the
same time.
“When a radio is
transmitting, its own transmission is millions, billions of times stronger than
anything else it might hear [from another radio],” Levis said. “It’s trying to hear a
whisper while you yourself are shouting.”
But, the researchers
realized, if a radio receiver could filter out the signal from its own
transmitter, weak incoming signals could be heard. “You can make it so you
don’t hear your own shout and you can hear someone else’s whisper,” Levis said.
Jung Il Choi and Mayank Jain (with Kannan Srinivasan, not pictured) began working on a new approach listen and talk simultaneously on radio communications. Credit: L.A. Cicero |
Their setup takes
advantage of the fact that each radio knows exactly what it’s transmitting, and
hence what its receiver should filter out. The process is analogous to
noise-canceling headphones.
When the researchers
demonstrated their device last fall at MobiCom 2010, an international gathering
of more than 500 of the world’s top experts in mobile networking, they won the
prize for best demonstration. Until then, people didn’t believe sending and receiving
signals simultaneously could be done, Jain said. Levis said a researcher even told the
students their idea was “so simple and effective, it won’t work,”
because something that obvious must have already been tried unsuccessfully.
Breakthrough for communications
technology
But work it did, with major implications for future communications networks.
The most obvious effect of sending and receiving signals simultaneously is that
it instantly doubles the amount of information you can send, Levis said. That means much-improved home and
office networks that are faster and less congested.
But Levis also sees the technology having larger
impacts, such as overcoming a major problem with air traffic control
communications. With current systems, if two aircraft try to call the control
tower at the same time on the same frequency, neither will get through. Levis says these blocked
transmissions have caused aircraft collisions, which the new system would help
prevent.
The group has a
provisional patent on the technology and is working to commercialize it. They
are currently trying to increase both the strength of the transmissions and the
distances over which they work. These improvements are necessary before the
technology is practical for use in Wi-Fi networks.
But even more promising
are the system’s implications for future networks. Once hardware and software
are built to take advantage of simultaneous two-way transmission, “there’s
no predicting the scope of the results,” Levis said.
