Is this just new in Cellular?
We've been doing this in Satcomms for a few years now, I've got a link here using Comtech's Carrier-in-carrier technology which works in exactly the same way.
Full duplex! Bristol boffins demo Tx and Rx on the same frequency AT THE SAME TIME
Transmitting and receiving on the same radio frequency, at the same time, has been demonstrated by the UK's University of Bristol in a new YouTube video. The system being used is running 900MHz and 1800MHz with exactly the same equipment. There are a number of companies working on this technology, called full duplex radio, a …
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Thursday 13th August 2015 13:32 GMT Drs. Security
Re: Is this just new in Cellular?
AFAIK the Tetra system does this as well, though it isn't using the entire frequency and width of the signal for one transmission but divide it up in 4 separate channels.
As for the propagation argument: it's line-of-side anyway on these frequencies.
Meaning any reflections, ground versus sky wave and polarisation changes common in frequencies below 30mhz don't apply here.
As for tunable radio's: there are enough of them around that can cover a wide range of frequencies well enough.
The issue almost always is the resonance of the antenna system and possible the extra loss incurred on standing wave ratio's (SWR) and electrical losses in tuning components like extra capacitors and coils.
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Thursday 13th August 2015 15:51 GMT Leo_Laughlin
Re: Is this just new in Cellular?
TETRA achieves duplex operation using Frequency Division Duplexing (FDD), i.e. using separate carrier frequencies for transmitting and receiving, whereas full-duplex radios use the same carrier frequency for both directions. You are right that tunable radios exist, however the technologies used are too expensive for consumer applications; there are still some major challenges to overcome before we will see widely tunable transceivers in our smartphones, including the antenna issues you mention.
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Thursday 13th August 2015 20:35 GMT Justthefacts
Re: Is this just new in Cellular?
No - you haven't understood carrier-in-carrier.
It's (sort of) clever, but it doesn't achieve same thing. In the satcom example, downlink is the additive combination of two uplinks. That only works if it is duplex between two satellite ground stations in the same beam. What the ground station is doing, is cancelling out the re-broadcast copy of its own uplink, which is at 0dB to the wanted signal. 30dB cancellation is therefore fine. It would not work, for example, in a multibeam sat system, or earth station to Internet address.
Actually, network coding is cheaper and easier, they've been a bit silly
This invention cancels its own uplink Tx - much more general applicability, requires about 100dB overall for interest. As someone says below, this technique is 50dB, standard RF cancellation is another 30dB, and now you safely defer the remaining 20-30dB to the digital domain.
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Thursday 13th August 2015 11:01 GMT Detective Emil
While cancellation of 50dB is certainly impressive, I can see why the researchers are advocating it as a technology for future picocells. A typical mobile phone transmits at up to 27dBm, and should be usable with a received signal of -95dBm. That's a difference of 122dB. Closer to a base station, where transmit power could be dialled back to 20dBm and the received signal comes in at -60dBm, there's a difference of 80dB. Even with 50dB of cancellation, that gets you a thousand times more interference than signal.
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Thursday 13th August 2015 14:06 GMT Leo_Laughlin
You are right that 50dB is not enough isolation to enable full duplex, however our prototype can actually do much better than this. We have achieved up to 50dB isolation using the electrical balance duplexing technique only, however the system also employs a second cancellation method which actively cancels the remaining interference to achieve around 80dB isolation, which starts to become useful. Furthermore, all of this cancellation is in the RF domain, and there is the potential for digital cancellation to be applied to further suppress the interference, however we have not yet implemented this on our demonstrator.
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Thursday 13th August 2015 16:12 GMT IanDs
Does this work even with time-varying propagation conditions, including handset movement, local reflections from moving objects and so on? It's not so difficult to get good cancellation in a static demo system where you have lots of time to estimate the response accurately and cancel it out, it's a lot more difficult -- maybe impossible? -- in a time-varying mobile radio system.
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Thursday 13th August 2015 15:33 GMT ian 22
Transmit and receive on the same frequency?
I must be missing something here, as we've been doing this for donkey's years. If this is meant to be two transmitters on the same frequency being received with no mutual interference, then yes, new and exciting stuff. Although we have been extracting useful signals from vehicles (see New Horizons) in space by statistically eliminating random noise generated by the Little Green Men.
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Thursday 13th August 2015 18:06 GMT Cynic_999
I worked on Post Office (telephone) microwave links in the 1980's which received on exactly the same frequency as they transmitted - via the same (dish) aerial. I was not clever enough to understand exactly how it worked, but it appeared to be implemented by the plumbing arrangement (waveguides and cavities) rather than electronics.
The first use of full duplex on the same frequency band is probably the humble telephone - albeit at audio frequencies via copper, but the principle is essentially the same. There was a transformer (called a "hybrid") in the telephone that effectively subtracts the transmitted signal from the signal on the line before presenting it to the earpiece. Modern telephones achieve the same thing without needing a bulky transformer by the use of electronic amplifiers. The telephone has its "hybrid" circuit deliberately unbalanced so that some of the transmission gets fed back to the receiver (earpiece) where it is called "sidetone". If a telephone has no sidetone, the user tends to think the telephone is "dead" and speak too loudly. If it did not have any hybrid at all, the speaker's own voice would be far too loud (which was the case in the *very* early telephones).
Biting the hand that feeds IT
