New Full Duplex Radio Chip Transmits and Receives Wireless Signals At Once (ieee.org)

Wave723 writes: A new chip by Columbia University researchers uses a circulator made of silicon transistors to reroute signals and avoid interference from a transmitter and receiver that share the same antenna. This technology instantly doubles data capacity and could eventually be built into smartphones and tablets. The chip enables them to work around the principle of Lorentz Reciprocity, in which electromagnetic waves are thought to always travel along the same path both forward and backward. Traditionally, electronic devices required two antennas -- a transmitter and receiver -- that took turns or operated on different frequencies in order to exchange signals.

Comments

good miniaturation

[Anonymous Coward]

Circulators are used all of the place (radar, satcom), so nothing new. But one small and efficient enough to potentially work in a cellphone? Neat stuff. They come with their own set of tradeoffs, so it might not be worth it in the end for smartphone use, but will find use somewhere.

Re:good miniaturation

[MattskEE]

It's kind of new, since this is an active circulator instead of the old passive ones.

Passive ones work great except they cannot be effectively miniaturized at the low frequencies used for current cell phone communication because size is proportional to the wavelength. Active circulators, based on non-reciprocal amplifiers and appropriate phase shifted combiner/divider networks, have existed for a long time. But there's been a lot of recent attention and work to bring them to a point where they're actually useful and efficient in communication applications.

Re:good miniaturation

[Agripa]

Active circulators with an arbitrary number of ports have been known of for decades but they have to operate within the noise, power, and linearity of their active devices. When used to separate a transmitter and receiver on the same frequency, their limited isolation will cause major problems on the receive side. Their big advantage is that they can operate over wide bandwidths which makes them very useful for test instrumentation.

This design was published in 1991 and would be useless in this application. I ran across it when it was included in one of my microwave engineering books.

I suspect the refinement they came up with involves a second adaptable stage to cancel feedthrough and near end echo but in order to do this, the output of the final amplifier has to be sampled to include its high levels of noise and even if it all works perfectly, all of the noise contributed in those circuits will get added to the receiver. I looked at doing something similar by sampling an earlier stage but just the noise contributed by the final amplifier was enough to overload the receiver. You can see this effect when powering up a SSB transmitter with no modulation which promptly raises the noise level for the entire band.

Re:Whoa!

[110010001000]

That isn't a problem. I heard the iphone 7 was going to be so awesome!

The canceller is the clever bit

[HuskyDog]

The gist of what is clever here is the canceller which removes the transmitted signal from the receiver. Circulators have been around for donkey's years (not just in military systems) but they are bulky (especially at lower frequencies such as those for mobile comms). The are often used to allow a single antenna to operate at both transmit and receive either alternately (e.g. radar) or on different frequencies (e.g. satcom). Making a solid state one is clever, but this isn't the first one.

However, some of your transmit signal will always end up in the receiver for three reasons; (a) the circulator isn't perfect, (b) the antenna doesn't have a perfect match so some of the transmit energy sent to it bounces back again and (c) energy can reflect back from the immediate environment. Cancelling schemes exist, and invariably consist of some mechanism for sampling the transmitted signal and feeding just the right amount back into the receiver exactly out of phase. In theory this works, but in most practical circumstances the extremely high level of cancellation needed requires a completely unachievable precision.

For added pain, the solution tends to be very narrow band and the cancellor's settings have to be continually updated as the transmit interference changes (particularly in a mobile environment due to (c)).

If they have managed to make this work in a practical and useful way then it will be very impressive, but I would need to see some real world experiments to be convinced of its practicality.

Re:The canceller is the clever bit

[Ungrounded Lightning]

However, some of your transmit signal will always end up in the receiver for three reasons; (a) the circulator isn't perfect, (b) the antenna doesn't have a perfect match so some of the transmit energy sent to it bounces back again and (c) energy can reflect back from the immediate environment.

Combined with the many orders of magnitude strength difference between the transmitted and received signals in a typical communications application, even a miniscule imperfection in the circulator's cancellation of transmit power at the received signal port can result in the transmit signal swamping the received signal. So the circulator must be EXTREMELY GOOD to be useful in the described way.

Re:The canceller is the clever bit

[HuskyDog]

No true!

I fear that you have entirely failed to grasp the point I was making. It is true that the transmit signal is many orders of magnitude stronger than the receive signal, but one cannot fix that entirely with the circulator, no matter how good it is. Time for circulator and antenna 101!

I typical ferrite circulator has three ports (let's call them A, B and C). Energy put into port A comes out of B, energy into B and out C and in C to out A. You get the idea. Now, as with everything in life, circulators aren't perfect and they have a parameter called 'isolation'. I typical value for a modern circulator is 20dB (or a power factor of 100). This means that if I for example put 100W into port A, then 99W will come out of port B and 1W will go the wrong way and come out of port C (in practice a little bit of the power will be lost internally as heat). Supposing that I connect A to the transmitter, B to the antenna and C to the receiver. In my example I will get 1W flowing into the receiver which could be 100dB (10^10) more than the intended receive signal. Clearly something else needs to be done, but making the circulator better won't help. Why?

Because of reason (b) in my comment - the return loss of the antenna. Antennas also aren't perfect and they have a parameter called return loss. An ideal antenna will take all the power from the transmitter and convert it into electro-magnetic waves propagating away. Real antennas however have imperfections and some of the power from the transmitter goes into the antenna and bounces back out again. A typical value for a good antenna is 20dB. Really good narrow band waveguide antennas (e.g. a decent radar) might manage 30dB, but the antenna on you mobile phone or Wi-Fi base station may well only manage 10dB. So, where does that leave us?

Returning to my example. 100W comes out of the transmitter and 99W goes to the antenna. If it has a 20dB return loss (if we are lucky) then 1W (give or take) will bounce back into circulator port B and nearly all of that will emerge from port C and go into the receiver. So, the receiver is getting 1W due to circulator imperfections and about 1W due to antenna imperfections. We can improve our circulator until the cows come home and the most we will do is reduce the power into the receiver from 2W to 1W which isn't going to save the day.

As I said in my comment, the real cleverness here is not the design of the circulator (which is probably as good as it needs to be), but the amazing performance claimed for the subsequent (and not very well described) canceller.

It melted

[Mateorabi]

Geeze, what happened to those 0.1" jumpers? Looks like they melted. Did someone accidentally the soldering iron on them?

Re:Not new

[jiriw]

What you describe is not full duplex. Two radios working together, taking turns (one transmitting, then one receiving, etc.) is the very definition of half duplex. And one radio in constant operation is simplex of course. Full duplex is always a situation where two radios are used in constant operation. One sending, one transmitting.

Full duplex using two radios on different frequencies is old school. Just a matter of a good combination of filters and enough frequency separation.

Full duplex using two radios on the SAME frequency, using directional signals is difficult but not undo-able. As long as you can prevent your receiver being blown up by your own transmission signal (and hope an unexpected reflective object in your signal path doesn't undo all your careful physical transmitter-receiver antenna separation).

Full duplex using two radios (both in continuous operation, one transmitting, one receiving, as defined at the top of my post) on exactly the SAME frequency while using a SINGLE (omni-directional) antenna is a true nightmare. And apparently these guys did just that with technology that promises it to be available in hand-held appliances.

RTFA, they used a combination of a circulator on silicon (which is the most innovative part. The circulator used in the project described by the article should kill most of the transmitted signal otherwise picked up directly by the receiver) and echo cancellation (which they developed earlier and is used to subtract any transmitter signal left which should mostly be echoes from objects that reflect the transmitted signal at distance and possibly internal echo from a sub-optimal antenna) on the received signal at the receiver end, so they can then try amplifying what's left. Which should be the (weak) signals that are transmitted towards the antenna by another transceiver.

Exciting stuff :)
73, PG8W

Re: Silicon Circulator?

[bill_mcgonigle]

Amirite is a conflict mineral anyway.