Simultaneous radio frequency energy reception

[solar flex]

Regarding the concept of radio frequency interference, electromagnetic waves, as broadcast, never actually interfere with each other. The interference exists as interference to the induction measured at the receiving antenna. So my question is, has anyone ever tried to produce an intelligent antenna/array that can receive multiple simultaneous transmissions on the same frequency? This is theoretically possible but I can’t find any past research on the topic.

Thanks!

 

[berkeman]

Regarding the concept of radio frequency interference, electromagnetic waves, as broadcast, never actually interfere with each other. The interference exists as interference to the induction measured at the receiving antenna. So my question is, has anyone ever tried to produce an intelligent antenna/array that can receive multiple simultaneous transmissions on the same frequency? This is theoretically possible but I can’t find any past research on the topic.

Thanks!

Welcome to the PF.

You certainly can have multiple datastreams encoded in the same RF channel. Is that what you mean?

 

[marcusl]

Regarding the concept of radio frequency interference, electromagnetic waves, as broadcast, never actually interfere with each other. The interference exists as interference to the induction measured at the receiving antenna.

Why do you say this? The fields from two waves superpose, giving regions where the electric fields are larger and others where they are smaller, regardless of the presence of an antenna. In any case, I'm not sure what this has to do with your question:

So my question is, has anyone ever tried to produce an intelligent antenna/array that can receive multiple simultaneous transmissions on the same frequency? This is theoretically possible but I can’t find any past research on the topic.
Thanks!

You don't need an intelligent antenna, a simple antenna will do. Here is just one of many many examples: Every CDMA cell-phone base station antenna does this. CDMA handsets communicating with a single base station all broadcast at the same time on the same frequency. Tens, even hundreds of simultaneous transmissions are received by the single base station antenna, whereupon individual cell phone transmissions are differentiated on the basis of orthogonal "codes" (that is, unique and separable data waveforms).

More complex systems exist as well. Since you ask about "intelligent arrays", take a look at "smart antenna" systems of the 1990's and last decade:

http://www.arraycomm.com/docs/intellicell.pdf"
"www.jackwinters.com/cwc.ppt"[/URL]

The capability to support even more sophisticated technology is built into 4G cellular standards. It goes these days by names such as multiple antenna systems (MAS). A google search on "4G multiple antenna systems" will get you into the literature, but be warned that it gets complicated.

 

[solar flex]

Welcome to the PF.

You certainly can have multiple datastreams encoded in the same RF channel. Is that what you mean?

No. Let me give an example. Two people with 49Mhz walkie talkies are transmitting (talking) at the same time. A third person with a 49Mhz walkie talkie won't be able to receive (hear) both of them talking simultaneously. The signals cause overlapping induction at the receiving antenna of the third persons walkie talkie. HOWEVER it is theoretically possible to receive both of these signals without intereference. I am trying to find someone who has done research in this field.

Thanks!

 

[solar flex]

Why do you say this? The fields from two waves superpose, giving regions where the electric fields are larger and others where they are smaller, regardless of the presence of an antenna. In any case, I'm not sure what this has to do with your question:

I don't believe that is correct. In the same way that two beams from two separate flashlights pass right through each other unscathed, I believe the radio waves do as well.

 

[f95toli]

Your question does not really make sense. If the two walkie talkies in your example uses frequency- or amplitude modulation there is no way to avoid what you call "interference"; simply because the receivers are designed to assume that all radiation at a given frequency IS part of what it is suppose do demodulate. This has nothing to do with the antenna.

Moreover, your example with the flashlight does not work with radiowaves, the difference is that radiowaves are much more omnidirectional that visible light, so there is no such thing as a "beam" of 100MHz EM radiation. However, if you were to e.g. communicate using microwaves (or far infrared lasers) there is nothing stopping you from using "directional" communication; by simply using very directional antennas for both the transmitter and receiver.

 

[marcusl]

I don't believe that is correct. In the same way that two beams from two separate flashlights pass right through each other unscathed, I believe the radio waves do as well.

You are correct that beams of light and radio waves will pass through each other and emerge as though nothing had happened. It is also true, however, that superposition holds in the region of overlap. In that region, interference will be observed.

No. Let me give an example. Two people with 49Mhz walkie talkies are transmitting (talking) at the same time. A third person with a 49Mhz walkie talkie won't be able to receive (hear) both of them talking simultaneously. The signals cause overlapping induction at the receiving antenna of the third persons walkie talkie. HOWEVER it is theoretically possible to receive both of these signals without intereference. I am trying to find someone who has done research in this field.

Thanks!

It is possible that the third walkie talkie is in a location of wave null and receives nothing, but equally probable that it is in a region of constructive interference so that the third person hears both transmissions garbled together. This occurs all the time when you listen to a crowded ham radio band. If your walkie-talkie always blanks out in this scenario, it is because the radio receiver is overloading or is otherwise not working linearly.

In fact, the situation is more complicated than just the spatial interference you are speaking of, because the communications waves exhibit both spatial and temporal coherence. The field pattern thus also varies in time even if the transmitters and receiver are stationary (this is what allows you to hear the voices).

 

[solar flex]

You are correct that beams of light and radio waves will pass through each other and emerge as though nothing had happened. It is also true, however, that superposition holds in the region of overlap. In that region, interference will be observed.

But observed how? If one is using induction for reception then there will be interference in the superpostion region. If there is a non-inductive method of reception then wouldn't it be possible to distinguish the two in the overlap region?

P.S. Thanks very much for the replies!

 

[marcusl]

I don't understand your question. Superposition always applies in any linear system. What you call induction is just the process by which a wave produces a voltage in an antenna, and it is also linear.

 

[solar flex]

I don't understand your question. Superposition always applies in any linear system. What you call induction is just the process by which a wave produces a voltage in an antenna, and it is also linear.

You said interference will be observed in the region of overlap. I am asking if that interference will be observed with a conventional antenna. I am suggesting that a non inductive method of reception could be used that would not be susceptible to the interference that a conventional antenna would be.

 

[berkeman]

You said interference will be observed in the region of overlap. I am asking if that interference will be observed with a conventional antenna. I am suggesting that a non inductive method of reception could be used that would not be susceptible to the interference that a conventional antenna would be.

What do you mean by non-inductive antenna? Antenna RX action requires the movement of electrons, caused by the EM waves.

There is a thing called "spatial diversity" of antennas, where you use some form of an antenna array to help you get around spatial nuls in received signals. But the antennas themselves are conventional.

There are also some "fractal" antennas, which trade off gain for other properties (like you could spread out a fractal antenna to help lower the problem of spatial nuls...)

 

[solar flex]

What do you mean by non-inductive antenna? Antenna RX action requires the movement of electrons, caused by the EM waves.

That is exactly what I am talking about, a non inductive antenna for detecting EM waves at high enough resolution to distinguish multiple waves on the same frequency.

 

[berkeman]

That is exactly what I am talking about, a non inductive antenna for detecting EM waves at high enough resolution to distinguish multiple waves on the same frequency.

If you are not using spatial diversity, you can have a null in one spot that cannot be overcome. You receive no signal, because the two interfering signals result in zero EM field at that point.

Beyond that, do you have some new antenna construction technique in mind? You still haven't said what you mean by non-inductive.

 

[solar flex]

If you are not using spatial diversity, you can have a null in one spot that cannot be overcome. You receive no signal, because the two interfering signals result in zero EM field at that point.

Beyond that, do you have some new antenna construction technique in mind? You still haven't said what you mean by non-inductive.

The null is measured how, with what methodology? Using a conventional antenna perhaps?

 

[berkeman]

The null is measured how, with what methodology? Using a conventional antenna perhaps?

Yes, of course. What other "kind" of antenna would we use?

 

[pervect]

Regarding the concept of radio frequency interference, electromagnetic waves, as broadcast, never actually interfere with each other. The interference exists as interference to the induction measured at the receiving antenna. So my question is, has anyone ever tried to produce an intelligent antenna/array that can receive multiple simultaneous transmissions on the same frequency? This is theoretically possible but I can’t find any past research on the topic.

Thanks!

Can you provide a reference for where you heard the bolded section of your original? As far as I can tell, this is not standard use of the term "induction", so the basic premise of your question appears to be somewhat suspect.

As far as what you can do. As others have mentioned, you can make an antenna highly directional. This is usually called a "hi-gain" antenna. You can favor signals from a particular direction, but the antenna has to be pointed at the source.

You can do this physically, in which case you have to rotate the antenna to point at the source. Or, with phased array antennas, you can do this electronically, so it's an electronically steerable equivalent.

You could, in principle, have a phased array antenna that tracked multiple signals at the same frequency, I suppose.

 

[solar flex]

Yes, of course. What other "kind" of antenna would we use?

Thank you, that is exactly where I was going with this.

 

[solar flex]

I want to thank everyone for their time and expertise. The conversation has been extremely invaluable.

 

[solar flex]

It looks like Rearden has pulled off what I was talking about:
http://www.rearden.com/DIDO/DIDO_White_Paper_110727.pdf

Notice they skate around the physical description of what's going on at the RF level.

 

[berkeman]

It looks like Rearden has pulled off what I was talking about:
http://www.rearden.com/DIDO/DIDO_White_Paper_110727.pdf

Notice they skate around the physical description of what's going on at the RF level.

I'm not understanding the white paper on a quick initial read. I'll try to get back to it later. Looks like a lot of hype, and I can't tell yet how practical their substance is.

 

[cjameshuff]

Moreover, your example with the flashlight does not work with radiowaves, the difference is that radiowaves are much more omnidirectional that visible light, so there is no such thing as a "beam" of 100MHz EM radiation. However, if you were to e.g. communicate using microwaves (or far infrared lasers) there is nothing stopping you from using "directional" communication; by simply using very directional antennas for both the transmitter and receiver.

There is nothing fundamentally different between light and 100 MHz EM radiation. The 3-meter wavelength of the latter means that the emitter must be scaled up in size, but it can just as easily be made directional as visible light or microwaves...a big dish is all you need. That's for a beam, other directional patterns can be produced with various antenna geometries.

It's already been mentioned in a roundabout way, but you can use a complex antenna or set of antennas that can pick out multiple signals on the same frequency by direction. And as marcusl pointed out, you can modulate the signals in such a way that superposition doesn't prevent you from picking out individual transmissions...look up direct-sequence spread spectrum techniques.

 

[cjameshuff]

I'm not understanding the white paper on a quick initial read. I'll try to get back to it later. Looks like a lot of hype, and I can't tell yet how practical their substance is.

It looks like they basically operate multiple access points as nodes of a phased array to allow devices at different locations to receive different signals. Each device receives one signal from multiple access points, signals intended for other devices canceling out at its location while the signals it is intended to receive constructively interfere. It works similarly in reverse, information from multiple access points being combined to pick out transmissions from individual devices.

Should work great for wireless access points to a wired network. However, it requires all the access points to have a wired connection to a central controller for coordination of their transmissions, and for them to have very accurate and well-synchronized clocks. The waveforms sent over the wired network will also be considerably more data than the encoded information.

 

[berkeman]

It looks like they basically operate multiple access points as nodes of a phased array to allow devices at different locations to receive different signals. Each device receives one signal from multiple access points, signals intended for other devices canceling out at its location while the signals it is intended to receive constructively interfere. It works similarly in reverse, information from multiple access points being combined to pick out transmissions from individual devices.

Should work great for wireless access points to a wired network. However, it requires all the access points to have a wired connection to a central controller for coordination of their transmissions, and for them to have very accurate and well-synchronized clocks. The waveforms sent over the wired network will also be considerably more data than the encoded information.

Thanks for the info. Do they address multipath interference at all? There's no way you can do that kind of phasing in the real world with multipath reflections coming into play.

 

[cjameshuff]

Thanks for the info. Do they address multipath interference at all? There's no way you can do that kind of phasing in the real world with multipath reflections coming into play.

I didn't see any mention of it...but I'm not sure multipath interference would be a particular issue for this unless it was already bad enough to cause problems. You could take it into account if the devices echo back their received waveforms, though this isn't specific to their method. A simpler method might be to just identify access points from which there's a severe multipath issue, and not transmit/receive to that device through them.

I wonder how well the system adapts when devices move. If it's what I think, you'll need accurate distances from each access point (and measurements of multipath interference if canceling it out).