Radio self-interference has been the bane of the communications industry since Marconi developed the wireless telegraph in his parents’ attic — not so long ago, in fact, in 1895.
Self-interference means that you can’t send and receive at the same time on the same frequency.
It’s because the transmitting radio creates energy that leaks into the transmitting radio’s receiver.
That leak creates meshing wave forms that manifest themselves as noise and interference — you won’t hear your correspondent, for one thing.
Consequently, the receive element in the radio must be suppressed when sending.
Unlike with a wired, landline phone call, this self-interference is the reason you can’t hold a simultaneous, two-way conversation on radios.
We’ve seen it in the movies, where the cop in the black and white presses the push-to-talk button on the microphone to report a car chase — preferably going the wrong way at a high speed in traffic. He can’t hear the dispatcher until he releases his PTT button.
Well, the PTT button isn't for dramatic effect. It’s part of the technology. And as we like to say in the IT industry: it’s a "technical limitation."
"Ah," you might say, but what about a mobile phone call, or a Wi-Fi session, surely they're simultaneously sending and receiving?
Well, they’re not, actually. They’re using blocks of adjacent frequencies, and then chopping between them quickly—they’re faking it.
Wireless full duplex: You can’t do it. But, that might be about to change, if a group of Santa Clara-based engineers' algorithms work as planned.
Kumu Networks, a startup consisting partly of Stanford University professors, reckons it’s got an algorithm that can second guess how a transmission is affected by all of the elements that create self-interference, and then cancel them. Self-interference is thus eliminated.
It’s a bit like how echo cancellation works, according to Joel Brand, Vice President of Product Management at Kumu Networks, who was interviewed by Neal Gompa for an Extreme Tech story.
Roughly how it works is that the receiver listens in, decides on the radio environment, and then predicts with algorithms how the transmission will be corrupted. It then cancels that corrupting transmission. Consequently, the self-interference is eliminated, and the receiver can hear the signal it needs to.
Noise-cancelling headphone technology is a good example of how waveforms have been used in the past to cancel each other out. The headphones work by capturing unwanted ambient sounds with microphones. They then mimic the incoming noise out-of-phase, thus canceling it.
And, indeed, a neighbor blasting his TV audio over yours, to the point that you can’t hear yours, is an example of one waveform blocking another.
Echo cancellation works by subtracting the echo from the received signal.
Kumu’s filtering is a bit more complicated, but you can read the nitty-gritty in the February 2014 issue of radiohead bible IEEE Communications.
If it works right, it’s a big deal: the advantage of being able to send and receive common radio simultaneously on the same frequency means that you’ll need fewer frequencies. You no longer need the two channels for a simultaneous duplexed conversation.
You don’t have to wait for correspondents to re-assign the channel to you after they've said what they had to say. "Over" will really be over.
It isn’t all just walkie-talkies, though. There are future tech advantages. Applications might include anywhere RF is used, including areas already optimized, like backhaul, 5G, LTE or Wi-Fi. Benefits could include denser coverage and further efficiency gains by reducing existing hardware duplexers.
Wireline RF technologies like DOCSIS could also obtain speed gains, according to Gompa's article. Gompa says faster DOCSIS connections would be possible, allowing cable to compete with fiber.
Disadvantages, though, could include reduced battery life—complex software adds calculations, which tends to use juice.
Prop warehouse of broken dreams
Sadly, though, if it all works as promised, the push-to-talk microphone will likely be consigned to Hollywood’s prop warehouse of broken dreams.
Police procedurals will never be the same if Kumu’s technology works as advertised.
Over and out.
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