Working in his research lab, Andrea Alù is focused on a single objective: doubling wireless bandwidth. "The final goal is to realize a compact device that can enable full-duplex for wireless communications, transmitting and receiving at the same time on the same frequency channel," says Alù, an associate professor of electrical and computer engineering at University of Texas at Austin.
While the world goes increasingly wireless, data rates continue to play tag-along, always a step or two slower than the speeds emerging apps—and their users—require. As compression and other existing bandwidth enhancement techniques reach their limits, academic researchers worldwide are investigating a variety of technologies and methods that promise to bring wireless broadband up to speed—and beyond—by allowing transmit and receive channels to occupy the same spectrum space.
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Andrea Alù
"[Full duplex] would allow us to better use the scarce spectrum available for wireless communications,” Alù says. "As our world increasingly depends on wireless communications and data transfer for everything from phones to the Internet of things, the demand on limited spectrum is growing."
Non-Magnetic Circulator
The key to Alù's research is a magnetic-less radio wave circulator. Engineers have known for over half a century that magnetic-based circulators are capable of enabling two-way communication on the same frequency. Yet the technique has never been widely adopted due to the size, weight and cost drawbacks associated with using magnets and magnetic materials.
Released from a dependency on magnetic effects, the circulator developed by Alù and his research team offers a much smaller footprint and uses less expensive materials than current circulators. The new device's cost and size efficiencies promise to make the circulator a standard technology in phones and other mobile devices, enabling faster and more spectrum efficient service, Alù says.
The team's prototype circulator has a two-centimeter diameter. The device could eventually be scaled down to just a few microns, Alù says. The prototype is based on materials widely used in integrated circuits, including small amounts of gold, copper and silicon, which makes it easy to integrate the circulator into circuit boards.
The circulator mimics the way magnetic materials break the symmetry in transmissions between two points, an attribute that allows magnetic circulators to selectively route radio waves. The new circulator achieves the same effect, but replaces the magnetic bias with a traveling wave spinning around the device.
"Having such a piece of hardware opens up something very important for radio-wave communications," Alù says. The technology enables a radio to have a pair of isolated paths leading to the same antenna. "This allows transmitting and receiving on the same frequency channel, effectively increasing the available spectrum bandwidth," Alù notes.
Alù is looking forward to making the technology commercially available. He is currently CTO of Austin-based Silicon Audio RF Circulator, a company that holds an exclusive license to the invention. "We hope to have a full-duplex system based on this technology two years from now," Alù says.
New transceiver architecture
A University of Bristol research group has created a new full duplex transceiver architecture that can estimate and cancel out interference from a user's own transmission, enabling a mobile device to transmit and receive on the same channel simultaneously. Since the technology requires only one channel for two-way communication, it uses only half as much spectrum space as conventional technology.
Doctoral student Leo Laughlin developed the transceiver architecture based on research begun by supervisor Mark Beach, a Bristol professor of radio systems engineering. The system's key enabling technologies are analog and digital cancellation technologies. "The transceiver combines electrical balance isolation and active radio frequency cancellation to suppress interference by a factor of over 100 million," Laughlin says. A current prototype uses low-cost, small form factor technologies designed for use in smartphones and tablets.
Researchgate
Leo Laughlin
Division-free duplexing can theoretically provide a two-fold increase in spectrum efficiency, yet a real world system will likely offer something less than a two-fold improvement. "However, any increase in spectral efficiency would translate into a range of benefits, including increased data rates and reduced cost and power consumption in the network infrastructure," Laughlin says.
There are already a handful of full duplex technologies on the market, Laughlin notes, targeted at backhaul infrastructure and LTE relay applications. "However, these systems use different circuitry and signal cancellation techniques to the technology we are developing," he says.
Laughlin expects that it will take several years for the technology to become available in commercial products.
A chip-oriented approach
A team of Columbia University researchers believes that enabling reliable and efficient full duplex wireless communication is a task best addressed at the chip level. The researchers, led by Harish Krishnaswamy, an associate professor of electrical engineering, have developed full-duplex radio integrated circuits that can be implemented in nanoscale CMOS to enable simultaneous transmission and reception on the same frequency.
"Having a transmitter and receiver use the same frequency offers the potential to immediately double network data capacity," Krishnaswamy says. "Our work is the first to demonstrate an IC that can receive and transmit simultaneously," he says. CMOS is the dominant technology used for radio ICs inside phones and other radio-equipped mobile devices.
The biggest challenge the team faced during its research was canceling transmitter echo, a phenomenon that makes usable full duplex impossible. "What you really need to do is to cancel-out that echo to the point where it’s eliminated almost perfectly and the residual echo is extremely small—smaller than the received signal, the desired signal—that you’re trying to receive from the distant cell tower," he says.
Columbia
Harish Krishnaswamy
Since the echo is over a billion times more powerful than the received signal, echo cancellation circuits must operate highly precisely. "We need echo cancellation circuits that are something like one-part-per-billion-level accurate," Krishnaswamy explains.
Such precision is difficult to achieve in software alone without killing overall device performance. "This really is something that needs to be done in hardware," Krishnaswamy says. "That level of precision in the echo cancellation, and the need to handle such a loud echo, cannot be done purely in signal processing."
To achieve optimal quality, the researchers applied multiple layers of echo cancellation to their software. "The echo is at least a billion to 10 billion times more powerful than the signal that we’re trying to receive, so basically you want to cancel that factor, and that’s hard to do with one signal echo canceller," Krishnaswamy says. "So the way these full duplex systems are likely to be successful is to have multiple layers of echo cancellation, just hitting that echo canceling again, and again, and again.
Krishnaswamy and doctoral student Jin Zhou are now testing the full-duplex technology on various nodes to understand exactly what gains might be possible at the network level. "We are looking forward to being able to deliver the promised performance improvements," Krishnaswamy says.
Extra antenna
A Rice University researcher team believes that full duplex wireless communication can be achieved relatively simply and inexpensively by adding a tiny extra antenna and new software to future phones and other mobile communication devices.
The approach relies on MIMO (Multiple-Input Multiple-Output), a wireless technology that uses several transmitter and receiver antennas for increasing a radio's data transfer capacity. "We utilized a multiple antenna approach for our full-duplex system because it requires only a minimal amount of new hardware, both on mobile devices and the network hardware," says team leader Ashutosh Sabharwal, a Rice professor of electrical and computer engineering. "So far, we've attracted the interest of nearly every wireless company in the world."
Ashutosh Sabharwal, a Rice professor of electrical and computer engineering
Sabharwal says that the approach is designed to address the needs of both device users and manufacturers. "On the device side, we've proven that we can supply full duplex easily and cost effectively as an additional mode on existing hardware," he says. "As mobile devices become smaller, inside space becomes increasingly scarce and valuable, so manufacturers really appreciate an approach that doesn't require them to add new hardware simply to provide a full duplex capability."
The Rice technology uses a pair of signals that are designed to cancel each other out at the receiving antenna. "The canceling effect is entirely local, so the other node can still hear what we're sending," Sabharwal says. He notes that although the cancellation concept is not particularly new, and is relatively simple in theory, no one else had determined a way to implement the idea in a way without requiring complex and expensive new radio hardware.
The researchers have also developed an asynchronous full-duplex mode that allows a wireless node to start receiving a signal even when it is currently transmitting. "Asynchronous transmission is critical for carriers who want to maximize network traffic," Sabharwal says.
Rice University
Ashutosh Sabharwal
Sabharwal notes that perhaps the biggest challenge still facing full duplex researchers and the entire wireless industry is the development of new wireless standards supporting the technology. Nevertheless, he believes that widespread adoption of full duplex technology is simply a matter of time. "It appears possible that full duplex will debut when carriers upgrade to 4.5G or 5G networks a few years from now," Sabharwal says.
Edwards is a freelance writer. He can be reached at jedwards@johnedwardsmedia.com.
Photo Caption. Prototype circuit incorporating a magnetic-less radio wave circulator that promises to boost wireless network bandwidth while conserving spectrum space. Photo credit: University of Texas at Austin.