Full duplex may be the next breakthrough in mobile networking

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Stanford startup Kumu Networks didn’t receive much notice at Mobile World Congress this week as the giants of the mobile industry revealed their plans for 2015, but it did get the attention of two rather important mobile carriers. At their separate booths, Telefónica and SK Telecom were showing off a Kumu-built radio transmission system called full duplex, which both carriers said could eventually become one of the key technologies of any future 5G standard.

When the mobile companies pull out the 5G card, they’re usually trying to signal that something is a really big deal, and in the case of Kumu, they could very well be right. What full duplex does is solve a fundamental problem in wireless communications that limits a network’s full capacity potential: the inability to transmit and receive signals on a radio channel at the same time. The problem is known as self-interference, but the concept is not quite as complex as it sounds.

Shouting

Imagine two people are having a conversation, which itself is one of the simplest two-way — or duplex — communication channels. If both people are talking at the same time, neither one can understand what the other is saying. The words one person speaks get drowned out by the other’s voice before it ever reaches his ears. The same principle holds for wireless transmissions. When a radio is transmitting its signals bleed over into its own receiver interfering with the signals it’s trying to listen for.

For that reason wireless networks have always been built in something called half-duplex mode, which basically prevents them from ever transmitting and receiving in the same channel at the same time. It’s why most mobile networks in the world today use different sets of frequencies for downlink and uplink transmissions (For instance in many U.S. LTE systems, our devices receive data from the tower in a 2100 MHz channel, but they send information back at 1700 MHz). And it’s why a Wi-Fi router flip-flops between transmitting and receiving when it talks to your laptop or smartphone. Half-duplex has served the wireless industry well, but using it means you’re only using half of the total capacity of your airwaves at any given time.

Kumu Networks is based in Santa Clara but its roots are in Stanford where its founders started their full duplex research.

Kumu Networks is based in Santa Clara but its roots are in Stanford where its founders started their full duplex research.

 

As my colleague Signe Brewster wrote in Gigaom’s first look at the Stanford startup in 2013, Kumu claims to have developed the mathematical breakthrough necessary to solve the problem of self-interference at a practical level. And now it’s claiming to have produced a commercially viable full-duplex radio system that can transmit and receive simultaneously without turning its connection to mush. According to Kumu VP of product development Joel Brand, the company accomplished this by becoming a very smart listener.

Essentially Kumu is constantly scanning the radio environment, gauging the exact state of the airwaves at any given time, Brand said. Using internally developed algorithms, Kumu can “hear” how the transmission the radio is pumping out is changing the signal environment a the receiver. It can then compensate for those changes as signals heading the opposite direction arrive. It’s like echo cancellation applied to radio waves instead of sound.

Full Duplex demo

Kumu supplied some photos of the full duplex rig it demoed at Mobile World Congress, and I’ll be the first to admit it doesn’t look very impressive. But at MWC I asked Vish Nandlall, CTO of Australian multinational mobile carrier [company]Telstra[/company], about the technology, and he said it was the real deal. Full duplex isn’t some crazy new concept Kumu just made up one day, he said. Full duplex is used today in regular phone lines, and its application to wireless has been kicking around scientific papers and academic research labs for some time. But what Kumu did was come up with a viable technology that could be applied to real world networks, Nandlall said.

The impact could be quite significant. If you remove the self-interference barrier, carriers could use all of their spectrum for both uplink and downlink at the same time, which would double the capacity or double the number of connections any network could support. Wi-Fi networks would no longer have to alternate between sending data and receiving it, thus dramatically improving their download and upload speeds. It might not solve the so-called spectrum crunch, but it would go a long way to making wireless networks a lot more efficient.

Right now Kumu is pitching the technology to carriers as a backhaul system, so they could use their 4G spectrum to concurrently communicate with phones and the core network. But Brand says in the future full duplex can easily be applied to the access network connecting our devices. In fact, Kumu’s MWC demos were using off-the-shelf radio smartphone chips from [company]Qualcomm[/company], just with the duplexer ripped out. That kind of change would require a redesign of both our networks and our devices, which isn’t going to happen overnight. That’s why Kumu and its carrier partners [company]Telefónica[/company] and [company]SK Telecom[/company] are looking ahead to 5G.

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