As mobile network and device makers explore many paths to using wireless spectrum more efficiently, one possible solution is still hard to achieve: Sending and receiving data on the same frequency at the same time, in the same space.
There are multiple ongoing efforts to reuse spectrum for mobile data. Equipment vendors are working out how to have traditional "macro" base stations and smaller cells serve the same neighborhood, with automatic coordination. There are also efforts to let mobile operators and government agencies share the same spectrum while staying out of each other's way. Each of these has its own challenges.
But when it comes to a particular base station transmitting and receiving signals on the same channel at the same time, it can't be done on current 3G or 4G mobile networks, according to researchers at the University of California, Riverside. Today's networks send and then receive, or do each on a different frequency. In fact, most mobile broadband networks are built around two separate "paired" blocks of spectrum, one for "upstream" signals coming from devices to the network and one for "downstream" traffic going the other way.
The Riverside researchers think they are close to solving this limitation in a way that would only slightly increase the cost of a cellular base station. Their so-called full-duplex radio could be technically ready by the end of next year, according to Ping Liang, a member of the Riverside team. He and Professor Yingbo Hua are leading a project funded partly by a grant from the U.S. Department of Defense. Liang said a major vendor of mobile equipment, whom he can't name, is actively pursuing full-duplex and is interested the team's work. The technology might also be applied to Wi-Fi, he said.
By sending and receiving data at the same time on the same frequency, a full-duplex radio theoretically could double the efficiency of a wireless network. If a mobile operator could do that, it could offer users the same speed and capacity using just half the spectrum, saving billions on auctions for spectrum licenses. More likely, that carrier could take the same amount of spectrum it has now and make it last years longer, serving growing numbers of users and higher data demands without having to take over more frequencies.
In reality, the benefit to most applications would be less, because few uses of mobile involve subscribers sending the same amount of data they receive. But it could still make a difference, and network operators are expected to look for any advantage they can get over the next few years when it comes to efficient use of spectrum.
Full-duplex could deliver a big benefit if it let carriers take their paired upstream and downstream bands and talk both directions on each, said Ovum analyst Daryl Schoolar. "Any serious base station vendor would want to at least explore it," he said.
The problem with full-duplex operation isn't radio waves on the same frequency crashing into each other in the air, turning YouTube videos into fuzz. The problem is that signals coming from within a cellular base station drown out the ones that the base station is trying to pick up from across the wireless network. The outgoing signals are about 10 billion times more powerful than the ones coming in over the air, so the external signals get drowned out. The same thing happens inside a phone or tablet as in a base station.
What's needed is a signal cancellation circuit, according to Liang. The one his team is building generates an opposite signal that cancels out the transmission signal. It cancels the signal only within the radio, so out on the network, the transmission works fine. The key is that the radio can now "hear" the weaker incoming signals. The Riverside team developed and patented its approach in 2010 as a theory and then proceeded to test it out.
Liang and Hua aren't the first people to build a full-duplex radio. Quellan, a silicon vendor acquired by Intersil in 2009, had one on the market as early as 2005, Liang said. Intersil declined to comment on the Quellan radio. Quellan's technology has been used for GPS and other applications. However, what Quellan developed can't be used for full-duplex operations on today's mobile broadband networks, Liang said.
For one thing, the Quellan radio only works on spectrum bands that are 5MHz wide or less. Most 3G and 4G networks today use far more bandwidth than that: Both AT&T's and Verizon's LTE networks, for example, use two paired bands, each of which is 10MHz wide. The other limitation of the Quellan technology is that it can only cancel out those loud transmission signals by 30 decibels, Liang said. That leaves a signal that's still 1 million times stronger than what's coming in over the air.
Other researchers, from Microsoft, Stanford University and Rice University, more recently have advanced beyond Quellan's technology, achieving 30dB signal cancellation over a wider 10MHz channel, Liang said.
Aimed at base stations first
Liang said his team is aiming for 96dB of signal cancellation. There should be no theoretical limit to the width of the band it can work in, he said. The best place to start implementing it will be network infrastructure, partly because it takes longer to replace millions of handsets than a much smaller number of base stations, Liang said.
A base station with a full-duplex radio could receive signals from one phone and transmit to another on the same frequency at the same time. If the phones also had full-duplex radios, then the base station could send and receive data with the same handset on the same frequency, achieving even greater efficiency.
However, between development, testing, standards work and regulation, full-duplex could face a long road to commercial use in cellular networks. In fact, standards and regulation are so ingrained, there may never be full-duplex radios in cellular, Tolaga Research analyst Phil Marshall said. But the work should still pay off, he said: "You're not going to [transform] spectrum allocation with this technique, you're just going to get more efficiency."
Ovum's Schoolar thinks the concept has great potential for mobile networks but adoption might take a decade. The Wi-Fi industry might adopt the technology more quickly because it builds unlicensed networks, but it won't hit the market overnight, Schoolar said.
"Anything like that is probably going to be long term. I mean, very long term," Schoolar said.