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Full Duplex Wireless Tech Could Double Bandwidth

Soulskill posted more than 2 years ago | from the now-telcos-can-throttle-us-even-more dept.

Wireless Networking 60

CWmike writes "Rice University researchers announced on Tuesday that they have successfully demonstrated full-duplex wireless tech that would allow a doubling of network traffic without the need for more cell towers. Professor Ahutosh Sabharwal said the innovative technology requires a minimal amount of new hardware for both mobile devices and networks. However, it does require new standards, meaning it might not be available for several years as carriers move to 5G networks, he added. By allowing a cell phone or other wireless device to transmit data and receive data on the same frequency, unlike with today's tech, the new standard could double a network's capacity. Rice has created a Wireless Open-Access Research Platform (WARP) with open source software that provides a space for researches from other organizations to innovate freely and examine full-duplex innovations."

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60 comments

Great! (1, Funny)

Cornwallis (1188489) | more than 2 years ago | (#37325430)

I'll be able to drop calls that much faster.

Re:Great! (2)

elrous0 (869638) | more than 2 years ago | (#37325694)

And I'll be able to reach my download caps in half the time!

Re:Great! (1)

Anonymous Coward | more than 2 years ago | (#37325782)

And I'll be able to reach my download caps in half the time!

Yay! We'll quadruple our profits! //The Phone company

Quick! Someone patent "Full Duplex" (0)

Anonymous Coward | more than 2 years ago | (#37325432)

This mysterious "Full Duplex" thingy doodad is completely new technology. I hope someone patents it right away because whomever thought of having a two way conversation is a galldarn genius!

Re:Quick! Someone patent "Full Duplex" (1)

ceoyoyo (59147) | more than 2 years ago | (#37325506)

Full duplex on one channel using a radio is a LITTLE bit innovative.

And this is why we don't put Slashdot in charge of the patent system.

Re:Quick! Someone patent "Full Duplex" (1)

Chrisq (894406) | more than 2 years ago | (#37325650)

And this is why we don't put Slashdot in charge of the patent system.

Seriously .... could it be any worse than it is now?

Re:Quick! Someone patent "Full Duplex" (2)

SlashV (1069110) | more than 2 years ago | (#37325850)

Seriously .... could it be any worse than it is now?

Very dangerous question. "just when you're thinking things couldn't possibly get any worse, they suddenly do."

Re:Quick! Someone patent "Full Duplex" (2)

Ksevio (865461) | more than 2 years ago | (#37326104)

However there's quite a difference between patenting the vague "A method of using phones Full Duplex" and patenting the actual implementation (which the article didn't actually explain).

Patent Agent: "Dupe" (2)

bill_mcgonigle (4333) | more than 2 years ago | (#37326404)

And this is why we don't put Slashdot in charge of the patent system.

At least we have a way to identify prior art. [slashdot.org]

Re:Quick! Someone patent "Full Duplex" (1)

BitZtream (692029) | more than 2 years ago | (#37326670)

No its not, I first heard of it 10 years ago, and it wasn't actually new then either. What happens is someone says 'OMFG NEW WAY TO DO FULLDUPLEX ON A SINGLE FREQ!!!!@$%!#%!@\!'

And then 10 years later we are still hearing 'well, theres just a few minor problems, but once we work those out, it'll also cure cancer!'

FIRST POST (-1)

Anonymous Coward | more than 2 years ago | (#37325434)

FTW!

will it work (1)

calin2k (763711) | more than 2 years ago | (#37325436)

will it work on my wrt54g?

Re:will it work (1)

datapharmer (1099455) | more than 2 years ago | (#37325468)

Not unless you want terrible performance due to interference or are talking about the wired connection (which is already full duplex). There are plenty of full duplex wireless routers out there though.

Cool! (2)

scubamage (727538) | more than 2 years ago | (#37325460)

I wish more than the simple abstract was available, I'm greatly interested to find out more in how they've done it. All I can seem to glean is they came up with some sort of cancellation technology. I'm going to assume its CSMA/CA evolved. Should be some neat tech if its not hyped media stuff.

Re:Cool! (2)

_merlin (160982) | more than 2 years ago | (#37325536)

Nah, CSMA/CA is half-duplex on one frequency (listen for some quiet before you talk). If they've come up with some form of code domain duplex that actually works, congratulations to them. I'd just like to see it implemented in something affordable before I get excited.

PDF of the actual paper (0)

Anonymous Coward | more than 2 years ago | (#37325580)

Re:Cool! (2)

Whiternoise (1408981) | more than 2 years ago | (#37328146)

All the links from TFA lead to full papers (caveat, I'm on a University network, but arxiv should be accessible anywhere):

http://arxiv.org/pdf/1107.0607v1 [arxiv.org]
http://arxiv.org/pdf/1107.1276v1 [arxiv.org]
http://warp.rice.edu/trac/attachment/wiki/Asilomar2010_FullDuplex/MDAsilomar2010.pdf [rice.edu]
http://warp.rice.edu/trac/attachment/wiki/Asilomar2011_FullDuplex/Everett11FullDuplexDirectionalDiveristy.pdf [rice.edu]

Re:Cool! (1)

scubamage (727538) | more than 2 years ago | (#37328932)

Awesome links, thank you!!

We have this already (2, Interesting)

Anonymous Coward | more than 2 years ago | (#37325478)

It's called 802.11n (which has been working for quite some time now), this is just doing it with cell phones.

Rice's team overcame the full-duplex hurdle by employing an extra antenna and some computing tricks.

We repurposed antenna technology called MIMO, which are common in today's devices

Yup. [ieee.org]

Re:We have this already (0)

Anonymous Coward | more than 2 years ago | (#37325718)

makes u wonder why the big carriers that spend millions/billions deploying their networks didn't think of this (or if they did, why they didn't attempt it)

Not the same. (1)

Namarrgon (105036) | more than 2 years ago | (#37326048)

MIMO (and related techniques like beamforming) have been standard [wikipedia.org] in WCDMA/UMTS (3G) all along; it's not something that's unique to 802.11n.

This also uses multiple antennae, but it goes beyond standard MIMO by using different modulation on each antenna that cancels out the other direction, effectively giving you two separate channels on the same frequency.

Not really a breakthrough (0)

Anonymous Coward | more than 2 years ago | (#37325482)

I still dont understand why someone would use this technology over beamforming or Multi-User MIMO to reuse the same spectrum for multiple users (instead of uplink/downlink). Depending on the channel conditions MU-MIMO a system with N-antennas can offer N-times the spectral efficiency.

Full paper available at:
http://arxiv.org/pdf/1107.0607v1

Re:Not really a breakthrough (1)

gmack (197796) | more than 2 years ago | (#37326262)

It's a doubling of raw bandwidth but in practice, it will be a lot more than that. TCP runs more efficiently at full duplex.

Doubling is not enough. (2, Interesting)

Anonymous Coward | more than 2 years ago | (#37325496)

In a world where bandwidth demands are increasing exponentially, a simple doubling of capacity ought to get us by for, oh, I don't, know, 6 months? This needs to be paired with a more fundamental upgrade that will get us by for several years to be worthy of incrementing the 'G' number to 5G, for any reasons other than marketing blather that is.

Re:Doubling is not enough. (1)

vlm (69642) | more than 2 years ago | (#37325710)

In a world where bandwidth demands are increasing exponentially, a simple doubling of capacity ought to get us by for, oh, I don't, know, 6 months?

Demand might be increasing, but the supply is being choked by throttling and overage charges. Great, now I can reach my cap in 10 minutes instead of an hour. Whoo hoo.

Also there's a pretty hard limit to BW demands... What would I do with a tiny little hand held device with a two hour battery life that would use more than HDTV bandwidth? I reached this limit at home around 2003, and both my monthly bill and comparative service levels have gone from "exotic" to "average" in that interval. I could certainly afford to upgrade to 100 meg service, but I have no idea what I'd do with it that I'm not already doing...

More than double? (3, Insightful)

Geoff-with-a-G (762688) | more than 2 years ago | (#37325508)

In wired Ethernet topologies, going full duplex yields significantly more than double the throughput, since you no longer have collisions, back-offs, and re-sends. The article doesn't elaborate whether their full-duplex wireless would still be multi-access (think WiFi, with many clients on the same AP and same channels) or if each frequency would be carved out for one client and the base-station (in which case you'd see the same gains you did on wired Ethernet).

M point is that while they cite "allow a doubling of network traffic", the reality is even better than that. Full duplex gets you more than double throughput, as well as improved jitter/latency since you no longer have to randomly re-transmit frames (or randomly wait to transmit, as with WiFi collision avoidance).

Re:More than double? (1)

ultramutalisk (881933) | more than 2 years ago | (#37325626)

In wired Ethernet topologies, going full duplex yields significantly more than double the throughput, since you no longer have collisions, back-offs, and re-sends. The article doesn't elaborate whether their full-duplex wireless would still be multi-access (think WiFi, with many clients on the same AP and same channels) or if each frequency would be carved out for one client and the base-station (in which case you'd see the same gains you did on wired Ethernet).

M point is that while they cite "allow a doubling of network traffic", the reality is even better than that. Full duplex gets you more than double throughput, as well as improved jitter/latency since you no longer have to randomly re-transmit frames (or randomly wait to transmit, as with WiFi collision avoidance).

I may be wrong, but I believe we already have "Full Duplex" in a sense as explained by you by using different frequencies for transmit and receive. The only difference here is now we can use the same frequency for transmit and receive.

Re:More than double? (1)

c0lo (1497653) | more than 2 years ago | (#37326450)

In wired Ethernet topologies, ...

M point is that while they cite "allow a doubling of network traffic", the reality is even better than that. Full duplex gets you more than double throughput, as well as improved jitter/latency since you no longer have to randomly re-transmit frames (or randomly wait to transmit, as with WiFi collision avoidance).

Mmmm... yes and no. Wired full-duplex is still easier - it's still point-to-point. When switching to wireless and have more than two transmission nodes, one still need to establish a medium access control. See here [microsoft.com] some proposals (I know, I know - came as a surprise to me as well to see MS is involved in some actual scientific research and not only in taking 5 years+ to release a crappy OS, "getting the facts" or writing Halloween memos).

Re:More than double? (1)

Geoff-with-a-G (762688) | more than 2 years ago | (#37327270)

Ahhh, indeed that's what I was afraid of, you still need CSMA with back-off times since the medium (the air) is still open to other transmitters. So essentially the "full duplex" here is just using one frequency for both directions at once, just the "double traffic" summarized by TFA. Thanks for the clarification.

And for what it's worth, I've always heard good things about Microsoft Research. It's when the researched ideas make their way (or fail to make their way) into finished products with price tags on them that the friction arises...

COWBOY NEAL BUSTED RUNNIN NAKED IN NYPD STATION !! (-1)

Anonymous Coward | more than 2 years ago | (#37325510)

When asked, "Why'd you do it, Cowbody?" He replied, "Because I'm one drunk-son-bitch!", as one of New York's finest led him off to booking.

more on why this is difficult (5, Informative)

Trepidity (597) | more than 2 years ago | (#37325518)

The idea, as they mention, has been around for a while, in fact since at least the early 1970s, with some information-theoretic work putting bounds on ideal full-duplex operation. The main idea is that you can cancel your own transmitted signal locally because you know what you're transmitting. The difficulty is that the transmitted signal is much stronger locally than the received signal, so there is little margin for error for imperfect cancellation; even if you cancel out 99.9% of the signal, there might still be too much noise left to decode the incoming signal. Errors can come from nearly anything; slightly imperfect knowledge of the characteristics of your device, changes due to weather or motion, interference from surrounding objects, etc.

Also note that terminology here is a bit confusing. In some uses (esp. radio), "full-duplex" just means any system that is capable of having people speak in both directions simultaneously, even if it's done by using separate frequencies for each direction, or by using a multiplexing scheme. In contrast, this usage of full-duplex means that both directions are transmitting simultaneously on the same channel, without segmenting or multiplexing it.

I don't actually know how they solved the problem, though, and the article is light on details.

Re:more on why this is difficult (0)

Anonymous Coward | more than 2 years ago | (#37325628)

Errors can come from nearly anything; slightly imperfect knowledge of the characteristics of your device, changes due to weather or motion, interference from surrounding objects, etc.

Sounds like it should work in a lab environment ...
I hope there is more to this.

Re:more on why this is difficult (5, Interesting)

vlm (69642) | more than 2 years ago | (#37325900)

Errors can come from nearly anything; slightly imperfect knowledge of the characteristics of your device

Non-linear effect anywhere in the RX or TX chain, or intermod from surrounding objects is a big problem.

I've done quite a bit of RF design work, microwave ham radio stuff, etc. The big problem is historically low noise stuff which makes a great receiver tends to blow up when subjected to power, and high power gear tends to have horrific weak signal noise characteristics.

A great low noise fractional dB noise figure preamp is off the shelf and cheap, and it'll be vaporized by say 20 dBmW.
A great 30 dBmW MMIC 1 GHz amp is off the shelf and cheap, I have used the watkins johnson devices (yes I know they have a new marketing name which I've temporarily forgotten), and its weak signal noise performance ... is not good.

In contrast, this usage of full-duplex means that both directions are transmitting simultaneously on the same channel, without segmenting or multiplexing it.

I don't actually know how they solved the problem, though, and the article is light on details.

If I had to do it, I'd do traditional 70s era spread spectrum code division multiple access CDMA. Imagine a psuedorandom voltage generator feeding the RX VCO attached to the RX mixer. Then imagine a different psuedorandom voltage gen, or at least the same generating polynomial at a different offset, feeding the TX VCO attached to the TX mixer. Two completely separate RF paths, maybe up to the antenna. Synchronizing two separate psuedorandom voltage gens is merely twice as fun as just one, kinda, I guess.

The other way was to use an old fashioned yet highly effective RF circulator. They are large, and heavy, and frankly kinda hard to make. Think like a hockey puck of ferrite with a big ole magnet. RF only flows clockwise. This is old, old stuff. Larger and heavier than a "brick" cellphone from the 90s, although they worked perfectly fine at the base station.

There's another way to do it using PLLs and the two transmitters in quadrature, but that's getting bizarre (like, have I been drinking this morning already?) and synchronization is gonna be an absolute bear. The hard part isn't static stability, but dynamic as it switches in and out of sync, or multipath interferes with it.

Re:more on why this is difficult (1)

AB3A (192265) | more than 2 years ago | (#37326498)

Mod parent up. People are treating this as if it is voodoo technology that never existed before. In reality, these methods have existed for many decades. The problem is mass production and getting it down to a cost, size, and weight that people can want and afford.

Some of these problems can be overcome with newer technologies and improved linearity of amplifier and mixer technologies. My guess is that with a combination of that, frequency diversity, orthogonal antenna polarization, quadrature phase (TX PLL methods aren't the only way to achieve this, quadrature hybrids will do this too), and perhaps orthogonal code spread spectrum, there may be just enough separation to make this work.

But all this is a guess. The article doesn't reveal very much about what these researchers actually did to achieve this performance. In truth, it is easy to build a receiver and transmitter with better performance than what you have in your phone, if you don't care what the size, cost, and weight is. The phone of today is a marvelous series of compromises that yields a mass market device that is "good enough."

That, dear Slashdot readers, is what engineering is REALLY all about. It will be interesting to see how small, inexpensive, and light weight the Rice University researchers managed to make their prototype...

Re:more on why this is difficult (1)

anethema (99553) | more than 2 years ago | (#37326524)

From what I gleaned, they are just using a form of MIMO. Basically taking another antenna and relying on multipath and computer tricks. 802.11n makes free use of this. Not sure if they are also relying on polarity differences which is common in the point to point microwave area, but doubt it would work well on a mobile device. As soon as RF bounces off of something the polarity is usually all over the place. The article kind of sucks on detail.

Re:more on why this is difficult (0)

Anonymous Coward | more than 2 years ago | (#37331958)

99.9% is nothing close. Consider that a typical GSM receive level is something around the -100dBm mark, whereas it may transmit at something like 20dBm. So that's 120dBm difference, or 12 orders of magnitude. So more like 99.9999999999%.

Re:more on why this is difficult (1)

Trepidity (597) | more than 2 years ago | (#37334636)

Yeah, that's true (and important); I was a bit sloppy with the figures there, in a way that actually is relevant to the point. Theoretically, you can cancel your outgoing signal because it's known to you, but in practice, there is virtually zero margin for error in doing so.

Re:more on why this is difficult (0)

Anonymous Coward | more than 2 years ago | (#37332176)

99.9% isn't even close.

Take GSM as an example. A typical GSM handset might transmit at 20dBm. A typical receive level is in the region of -100dBm. That's a power difference of 120dB, i.e. 12 orders of magnitude.

Small-scale DIDO? (3, Interesting)

Namarrgon (105036) | more than 2 years ago | (#37325530)

MIMO uses multiple antennas and the Rice team was able to send two signals in a way that they cancel each other out, allowing a clear signal to go through over the single frequency.

Doesn't this sound an awful lot like the DIDO approach (pdf [rearden.com] ) that Steve Perlman was talking up [wired.com] recently?

Yeuch... (1)

Chrisq (894406) | more than 2 years ago | (#37325664)

Doesn't this sound an awful lot like the DIDO approach (pdf [rearden.com] ) that Steve Perlman was talking up [wired.com] recently?

I misread that as like the DIDO approach that Steve Perlman was taking up recently!

I'd better steer clear of Freudian anal ists

That should have been DILDO (nt) (0)

Chrisq (894406) | more than 2 years ago | (#37325676)

(nt)

Re:That should have been DILDO (nt) (1)

ArsenneLupin (766289) | more than 2 years ago | (#37326444)

Don't worry, everybody understood... after all, the l of talking had to go somewhere...

Wasn't Stanford first? (1)

c0lo (1497653) | more than 2 years ago | (#37325564)

Well, about 1 year ago??? Demonstrated [sigmobile.org] at Mobicon 2010?

Re:Wasn't Stanford first? (1)

c0lo (1497653) | more than 2 years ago | (#37325576)

Seems like it was. [stanford.edu]

Re:Wasn't Stanford first? (0)

Anonymous Coward | more than 2 years ago | (#37325812)

Real-time Full-duplex MAC

We’ve implemented a real-time full-duplex MAC for an access point based network. The MAC design is based on the Contraflow [microsoft.com] generalized full-duplex MAC from Microsoft Research. The design is implemented and tested using the WARP [rice.edu] software radio platform from Rice University. Using this MAC, we’ve shown in experiments with real nodes that full-duplex can reduce hidden terminal losses by up to 88% and significantly improve fairness in WLAN based networks.

and thus, the circle is complete...

Re:Wasn't Stanford first? (1)

c0lo (1497653) | more than 2 years ago | (#37326298)

and thus, the circle is complete...

So:
1. Rice University had (and has) WARP [rice.edu] - a sophisticated research platform of software controlled radios, build from the ground up using open source software
2. Stanford has the idea of using self-interference and demoes [stanford.edu] single-channel full-duplex wireless in 2010 at mobicon
3. Microsoft Research UK [microsoft.com] had some other ideas (May 2011) on self-interference and meshing for medium access control (check the citations: Stanford is mentioned)
4. Rice University takes a step further and establishes [arxiv.org] a math model and (this is the actual novelty) a way of doing it using an unexpensive setup.

Can't stop but wonder: in all the above there's no patent? Is it the promotion of "the progress of science and useful arts" possible outside "securing for limited times to authors and inventors the exclusive right"? And this is not communism?

Re:Wasn't Stanford first? (1)

bill_mcgonigle (4333) | more than 2 years ago | (#37326454)

And this is not communism?

Any form of historically implementable communism would now require a command control agency to dictate to industry how they will implement the commercial hardware and what it will cost.

This isn't very helpful (4, Insightful)

The O Rly Factor (1977536) | more than 2 years ago | (#37325604)

When AT&T, et. al. are in a position where they are the DeBeers of wireless bandwidth. I think instead of actually spending money to upgrade infrastructure, they would rather just continue to artificially limit the amount of available bandwidth so they can keep it grossly overvalued. Gotta keep those profits rolling in for the shareholders somehow.

Re:This isn't very helpful (1)

grumling (94709) | more than 2 years ago | (#37325906)

Yes and no. Yes, they can divide up available spectrum into finer and finer pieces by adding towers and directional antennas. But overall they have much less spectrum than you might think. One major problem is getting rid of old technology. running parallel networks (2G, 3G and 4G) is incredibly wasteful. Back in the old days, when Cellular One was moving over to digital, they had "trade in" parties, where customers could trade in their car phones for the new digital handsets (without having to sign new contracts... and we got hot dogs and hamburgers too). This was so they could re-allocate bandwidth and increase capacity. Could you see that happening today? Yet, it's ultimately in their best interest to get everyone on the same technology.

However, I do agree they value bits at a higher rate than they should. If they really wanted to, they could do a much better job of managing their bandwidth. There's a lot of roof space not being utilized that could go to more towers too.

I stopped reading when (0)

Anonymous Coward | more than 2 years ago | (#37326232)

I saw "could" in the title.

Full duplex is good but it's not awesome at all ! (2)

macpacheco (1764378) | more than 2 years ago | (#37326470)

This is NOT a big deal. Every new wireless telephony increases bandwidth by at least 10x. Basic GSM to basic 3G (UMTS) did that. From EDGE to HSxPA same thing. From HSxPA to LTE same.
While doubling performance is good, 5G networks needs to increase bandwidth 5x fold at an absolute minimum in order to be of any interest to wireless carriers.
And notice that today's wireless is full duplex a far as the user is concerned, but it happens using separate upstream/downstream channels. The biggest bottleneck is that both channels are shared among hundreds of users and that's where the congestion lies.
The 5G idea that was discussed is having all phones act as intelligent repeaters, allowing for very low power transmissions where user density is highest, the lowest power transmissions are, the more often spectrum can be reused dynamically by those repeaters. Also this could help with the shadow spots, since all it would take is one user in contact with the tower and with a shadow spot to illuminate that spot with signal. Also it would be expected to involve 10x more basestations in the form of femtocells / nanocells, using the same low power principle.
Wireless communications will never ever be not even close as efficient as wired communications for bandwidth intensive applications. Unlimited plans were always a fish and bait scheme, designed to create interest and dependency, until they limited bandwidth to increase revenue (i.e. jack up the prices).

Everything old is new again. (1)

Nethead (1563) | more than 2 years ago | (#37326718)

My first mobile telephone [wikipedia.org] was full duplex, and had a vacuum tube final amplifier.

Re:Everything old is new again. (0)

Anonymous Coward | more than 2 years ago | (#37332816)

It wasn't full duplex on a single frequency through was it? The base station and the mobile used different frequencies to transmit.

we've heard this before... (0)

Anonymous Coward | more than 2 years ago | (#37326744)

http://tech.slashdot.org/story/11/02/15/1745204/Two-way-Radio-Breakthrough-To-Double-Wi-Fi-Speeds

yu0 ]fail it (-1)

Anonymous Coward | more than 2 years ago | (#37327016)

CDMA doesn't have this problem (1)

Bengie (1121981) | more than 2 years ago | (#37328382)

Why don't they just use CDMA? TDMA/FDMA is quite inferior, although cheaper.

Oh, Great! (1)

ThatsNotPudding (1045640) | more than 2 years ago | (#37329302)

Twice the cancer!

couldn't care less (0)

Anonymous Coward | more than 2 years ago | (#37329940)

cellular wireless technologies seem to be mostly limited by bufferbloat anyhow, much more important if they finally fixed that

Don't confuse us.This's just typical antenna array (2)

wifiengineer (1997052) | more than 2 years ago | (#37330464)

The idea sounds great. But when you think realistically, you will know there is always a tradeoff. If you look at the paper and ideas by ignoring all the marketing messages, you can see there are flaws and it cannot apply to MIMO solutions.

Basically, the idea is to place the 1 Rx antenna between two Tx antennas so that the signals from two Tx antennas are out of phases at the Rx antenna position. Yes, it works for the purpose that you want the Rx antenna receives no signal from Tx antennas. However, this also changes the antenna beam pattern which will reduce the coverage for the reception of the signal by half. The placement of 2 Tx antennas is the same as have a Uniform Linear Array (ULA) with 2 antennas with some inter-element distances. For example, if you place the antennas with lambda/2 away (where lambda is the wavelength), you will receive very weak signal at the broadside angle. In the other word, you trade the reception quality at broadside angle for the null-out effect at your Rx antenna.

Just look at the image here [microwaves101.com] and you can choose the null wherever you like

Actually, you have two degrees of freedom here to choose any position/angles you want to have the weakest signals (null-out effect). 1) by changing the phase difference between two Tx antennas. For example, making them 180 degree out of phase, the null will happen at the middle between two antennas. In this case, you trade your reception along the direction perpendicular to your antenna array with the so-call single duplex. 2) by changing the inter-distance between two antennas. For example, you can make the distance with lambda*3/4 away to create the 180 degree out of phase. The drawback is the same. You trade the reception at certain area with this null-out at certain position.

People can argue, you can increase the inter-distance more, so that more grating lobes can be observed. Because there are multiple paths, eventually, the whole space can be covered. Yes, you are right. But how far away two antennas should be separated? It definitely depends on the environment. You can make it adaptive. So a dedicated engineer needs stand at the access point to change the antenna separations per request. And if you place the antennas so far away, is that really practical to have such systems?

So, they are just using one mode of the ULA to achieve the signal cancellation at the receiver point. But since they are twisting the antenna patterns, if you add more antennas trying to achieve 3x3 or 4x4, you are doomed. Because the degree of freedom has been used to cancel the signal at receivers, you have no more degree of freedoms for more spatial streams. So, yes, they can increase the capacity by two fold (in some area of coverage), but not beyond as in MIMO solution!

I believe there is an use case. But we should know the tradeoff and whether it is suitable for real deployment.

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