
Wireless Data Could Travel Twice as Fast Using a New Circuit - ghosh
http://www.technologyreview.com/news/532616/simple-circuit-could-double-cell-phone-data-speeds/
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yaantc
There's a lot of PR / excitement aroung full duplex operation on the same
channel (by opposition to what's usually called full duplex and use separate
donwlink and uplink channels), so let me be the Grinch a bit here.

The high level view of FD is that one removes the transmitted signal from the
receive path, allowing reception. This must be done at two levels: 1) a big
chunk is removed in an analog way 2) the remainder is removed in the digital
domain. The reason is that no A/D converter is good enough to work only in the
digital domain. Depending on the team, there is a claim of 85 to 100 dB
reduction in the Tx to Rx loop. That's impressive, but still a bit short when
in LTE a device can receive at ~ -100 dBm (YMMV) and transmit at +23 dBm. And
the analog removal part will have a tricky calibration: not a problem for a
controlled demo, but tricky (and potentially costly) in real life.

On top of this, all those techniques remove the internal looping of the Tx
signal to the Rx. That's already a challenge, but then there are the external
reflection: the Tx signal has a direct loop in the device, but can also bounce
on surrounding walls/stuff and get back the the Rx path this way. The issue
then is that there is a serie of time lags and distortions, so the UE doesn't
really know what needs to be subtracted. En the power level of such
reflections is big enough to kill the reception, on its own. This reflection
issue doesn't exist for some devices like line of sight micro-wave backhaul,
but is very common for all user devices (used in-door or in urban environment:
no line of sight, a lot of multi-paths).

The reflection issue makes me pessimistic for consumer devices applications.
And for other application with little / no reflections, there may be more cost
efficient ways to improve things that do not require the tricky analog
calibration.

So the teams working on FD do impressive technology, but the road to a
profitable business case is still to be proven. Good luck to them, although
I'm skeptical (but would be glad to be proven wrong ;).

~~~
DirectPlane
The device they're describing is already known in microwave engineering, it's
not a new circuit. It's a parametric-modulated circulator - the capacitance is
modulated. Not very useful in real-life: the PR-heavy letter neglects to
mention (1) the poor instantaneous bandwidth <0.5% (figure 4c), (2) the poor
linearity / poor power handling: Vm and Vdc are few volts in high-Q
environment, which translates into maximum power handling well below 0dBm (3)
the high sensitivity to analog component variation (fig 4c again) - not
something you want in mass-produced components operating at
commercial/industrial temperature ranges.

They neglected to mention the power level they used to measure the
S-parameters in the letter or the supplementary material. No self-respecting
RF engineer would forget to mention power levels - it again hints at very poor
linearity / poor power handling. Typical university "research".

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tomiko_nakamura
"Travel twice as fast" is clearly not what's going on. The article is about
full-duplex mode, i.e. doubling the throughput. The data are travelling just
as fast as before.

