Upgrading your cell phone to get the best networking speed available is expensive.
Upgrading your cell-phone towers to get a faster connection is a lot more expensive.
You could double the amount of data one cell phone can send and receive if you could convince it to listen to a radio signal at the same time it was broadcasting. Because cell phone radio links work just like walkie-talkies, though, they can only talk and listen simultaneously if they're doing it on different frequencies – cutting in half the number of devices a cell tower can support.
Full duplex connections – which allow each node on a network to both send data and receive it at the same time, are no small trick, especially via radio.
Even wired Ethernet networks make full duplex work only by sending data along one of a pair of twisted wires while receiving it on the other.
Researchers at Rice University think they've solved the problem of full-duplex cell phone networks by using selective hearing – a trick teenage humans have used to advantage for centuries.
In 2010 Ashutosh Sabharwal, a Rice University professor of electrical and computer engineering published a paper showing it was theoretically possible to create a full duplex connection using a single frequency by making each end of one connection deaf to its own voice, allowing it to hear the other.
Rice Univ. on full duplexing cell nets
Each device on a radio-frequency connection has one node that broadcasts its signal to the other device, and a second that receives responses. When a device broadcasts, its receiver is overwhelmed by the signal, so it can't hear anything else even when it should be able to do so.
Sabharwal and other members of the team – Melissa Duarte and Chris Dick, Achaleshwar Sahai and Gaurav Patel, also from Rice – solved that by creating a second outgoing signal that was the mirror image of the first – cancelling it out, but only in a very local area: right around the device's receiver.
Since two opposite waves cancel each other out, the receiving node ends up sitting in silence while its opposite number broadcasts its data. That allows it to hear a signal from the device on the other end of the connection and process it just as if its own broadcast node were not talking at the same time, according to Sabharwal.
It was an additional step to make the exchange asynchronous – that is, to make it possible for the receiver on one end to begin taking in a new data stream while its broadcaster is already sending.
Otherwise creating a full duplex connection would mean forcing each pair of linked devices negotiate the timing every time they tried to make a connection, which would put a lot of small kinks in the hose, reducing the amount of water that can flow through it at any given moment.
The approach is an adapted version of multiple-input/multiple-output (MIMO) WiFi networking that is common in home routers.
In July the Rice team published a paper describing how the wave-cancellation, asynchronous and duplexing connections should work, both theoretically and in their own implementation.
The advantage, from a carrier's point of view, is that Sabharwal's full-duplex approach requires very little refitting or additional equipment to work on existing cell networks.
"Our solution requires minimal new hardware, both for mobile devices and for networks, which is why we've attracted the attention of just about every wireless company in the world," a statement from Rice quoted Sabharwal as saying. "The bigger change will be developing new wireless standards for full-duplex. I expect people may start seeing this when carriers upgrade to 4.5G or 5G networks in just a few years."
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