Researchers at Stanford University have come up with a surprisingly simple way to allow a single frequency to be used for simultaneous transmission and reception--full duplex. To understand the significance of this, consider that today's duplex transmission and reception either requires double the spectrum, with separate frequencies for transmit and receive or a time-sharing of a single frequency between two transmitters with send/receive switching going on fast enough so that the connectivity appears to be full duplex. However, the sum of the data rates of the transmissions from the transmitters at each end of the link can't exceed the total data rate available for a one-way transmission on the link, so this is not really an ideal solution.
"Textbooks say you can't do it," said Philip Livis, assistant professor of computer science and electrical engineering at Stanford. "The new system completely reworks our assumptions about how wireless networks can be designed."
The problem is how do you receive a weak signal from a distant transmitter while transmitting to a distant receiver on the same frequency?
The solution that Livis and graduate students Jung Il Choi, Mayank Jain and Kannan Srinivasan came up with is surprisingly simple--it uses two techniques to cancel out the local transmitted signal at the receiver.
The first of these involves the use of two transmitting antennas and one receiving antenna positioned so that the signals from the two transmit antennas reach the receive antenna exactly 180 degrees out of phase, canceling out each other. Antenna cancellation, combined with a noise cancellation circuit, reduces self-interference by about 50 dB before the RF is demodulated. Digital cancellation, similar to what we've seen used in on-channel digital TV repeaters, removes the residual interference. The current prototype has 84 percent higher throughput in 802.15.4-like physical layer experiments using software radios with minimal loss of link reliability.
As you can imagine, the current technology has limitations. First, the antenna cancellation only works at one precise frequency. Deviate from that frequency and the signals from the two transmitting antennas are no longer exactly out of phase. Second, any reflections of the transmitted signal from nearby objects will not be canceled at the receive antenna. Active noise cancellation effectiveness is also degraded by variations in frequency or amplitude. The researchers are considering the possibility of generalizing the full-duplex design and extending it to multiple input/multiple output (MIMO) systems. The critical antenna spacing would also make it difficult to implement in portable devices.
One other problem, which is not described on the Stanford Full-Duplex Wireless Design Web page is that communication is rarely symmetrical over a link. When browsing the Internet, or listening to or viewing streaming content, one side of the link is doing most of the transmitting while the other side is sending very little data.
The good news is this shows that there are still some techniques being discovered that allow more efficient spectrum use. This is something to consider before crippling existing services such as television broadcasting to recover spectrum that ultimately may not be needed.
