[awelkie]

I feel like limits on EIRP are overly conservative and restrict the usefulness of phased arrays. If the limit were on total radiated power, then your 1 watt WiFi router could have the range of a kilowatt transceiver with a reasonable number of antenna elements, while emitting the same total power as interference. But since the limit is on EIRP, the phased array is limited to the same range, and so there's no point in using a phased array over a single antenna.

Does anyone know if there's a good reason to use EIRP that I'm missing? I figure satellite communication terminals can have huge EIRPs because they're all pointed at the sky, but the FCC can't guarantee that the beams won't cross for other bands, so they limit the EIRP, but I still think we would all be better off of our systems were spatially selective.

[adrian_b]

If you use a directive antenna to concentrate the radiated power into a small solid angle, to reach a distant receiver, you also increase in the same proportion the interference for another receiver that is located in the same direction as yours, but which does not want to receive your signal.

So limiting EIRP provides a limit for the interference suffered by a receiver that happens to be in the direction towards which you transmit, for which it does not matter at all which is the total power that you transmit in all directions.

[tyzoid]

True, but it dissuades folks from using directional signals, broadcasting RF energy in more directions and increasing the noise floor for everyone. I feel like there should be some sort of middle point here.

[adrian_b]

Directional signals are easy to use only when both the access points and the wireless stations are in fixed locations.

When this is true, it is trivial to use classic directive antennas to achieve very long range communications with standard WiFi. There is no need for expensive phased array antennas.

For mobile stations and/or access points, phased array antennas are not enough. See my other reply.

WiFi, Bluetooth and the other kinds of communication protocols standardized for use in the unlicensed bands are intended mainly for cheap mobile devices, and mobility at a modest price restricts the antennas to be omnidirectional.

[BenjiWiebe]

Directional antennas also are a benefit on the receiving side.

[pc486]

EIRP is good at reducing uintentional interference. After all, you'd probably wouldn't like me pointing a 20 element yagi antenna through your house, denying your ability to use the spectrum in a reasonable manner, just so I could do a point-to-point fixed link.

EIRP minimizes regulations. It's a good trade-off over operator and installation licencing.

[MPSimmons]

It sounds like this is very definitely targeted at high speed personal-area-networks.

[ballooney]

Yes, for the same reason that I can look into a 5mW LED but 5mW of laser can blind me. Your neighbour's WiFi routewr might be entirely DoS'd by the Maser of RF coming through the wall at it from your phased array, even thoughh it's only 100mW.

[awelkie]

Sure, but the probability would be low-ish of that happening, and the other system could either switch frequencies or beamform a null in the direction of the interferer if they were also a phased array.

Maybe the EIRP shouldn't be unlimited, but I still think it would be beneficial to encourage spatially selective systems.

[adrian_b]

In an unlicensed band, everyone may have receivers and transmitters, so the probability of other receivers in the same direction as yours is not low at all, but it is very high, unless you live in the middle of nowhere, so you do not have neighbors.

There is no justification for imposing additional costs for others in order to accommodate your desires that do not matter for them.

Nobody stops you to use a phased array antenna only to obtain a higher gain for reception, in order to increase the communication range.

Even without phased array antennas, using just classic directive antennas that are placed on high masts, it is possible to communicate through WiFi at tens of km (but only at low bit rates and not in all countries, as some have more severe EIRP limits).

The problem of directive antennas is that they are usable only for fixed positions of access points and wireless stations.

Phased array antennas are not enough to enable mobility, because initially a mobile wireless station must discover the direction of the AP and the AP must discover the direction of the station, by using omnidirectional transmission, which limits the range to what can be achieved without phased array antennas.

To use a mobile wireless network that works at distances greater than possible with omnidirectional antennas requires much more sophisticated equipment than just the phased array antennas. You also need means to determine the coordinates of each station (and of the access points, if they are also mobile) and maps with the locations of the access points so that a station that wants to associate with them will know in what direction to transmit. You also need a protocol different from standard WiFi, e.g. the access point may need to scan periodically all directions in order to allow new associations from distant stations.

[Dylan16807]

> so the probability of other receivers in the same direction as yours is not low at all, but it is very high

But on average, even with unlimited EIRP, I'll see 1/n as many interfering signals that are each n times as strong. That's not a bad tradeoff.

But having a moderate EIRP increase for focused signals would make things better for everyone. Let's say a signal that's 10x as focused can have 3x the EIRP, and everyone switches their equipment over. That drops the total power output by 3.3x, and interference drops significantly for almost everyone.

> initially a mobile wireless station must discover the direction of the AP and the AP must discover the direction of the station, by using omnidirectional transmission, which limits the range to what can be achieved without phased array antennas

You can do discovery at a lower bit rate to get a big range boost.

[adrian_b]

For a radio receiver it is irrelevant how many interfering signals exist.

The only things that matter are the radiant intensity (i.e. power per solid angle) of the interfering transmitter and the percentage of the time when that transmitter is active.

A single interfering transmitter with high radiant intensity (a.k.a. EIRP) will blind the radio receiver for all the time when it is active.

Doing discovery at a low bit rate is fine, but that means that your fancy phased array antenna cannot achieve any higher distance for communication than an omnidirectional antenna, but it can only increase the achieved bit rate at a given distance.

That would be OK, except that it is achieved by interfering with your neighbors, exactly like when using a transmitter with a higher total power than allowed.

Limiting EIRP is the right thing to do in order to limit the interference that you can cause to your neighbors.

The law does not stop you to use a phased array antenna or any other kind of directive antenna, with the purpose of lowering the power consumption of your transmitter, while maintaining the same quality for your communication and the same interference for your neighbors.

What you want to do, i.e. increase the interference for your neighbors, is the wrong thing to desire. If that were allowed, your neighbors would also increase the radiant intensity of their transmitters and then everybody would have worse reception conditions and you would gain nothing.

The hope that only you will increase your radiant intensity and your neighbors will not, is of course illusory.

[Dylan16807]

> For a radio receiver it is irrelevant how many interfering signals exist.

> The only things that matter are the radiant intensity (i.e. power per solid angle) of the interfering transmitter and the percentage of the time when that transmitter is active.

And if a transmitter isn't pointed at you, then it isn't an interfering transmitter. This is a crucial factor in the math.

Or for a more realistic analysis of directionality, the radiant intensity is only high for a small fraction of observers, and is very low for the rest of them.

In the first scenario I talked about, total interference is probably the same.

In the second, total interference is almost always much less.

> A single interfering transmitter with high radiant intensity (a.k.a. EIRP) will blind the radio receiver for all the time when it is active.

If a moderate boost blinds the receiver, then the alternative is being almost blind for much longer (because there are more interfering transmitters), so I'm not convinced that's a problem.

> Doing discovery at a low bit rate is fine, but that means that your fancy phased array antenna cannot achieve any higher distance for communication than an omnidirectional antenna, but it can only increase the achieved bit rate at a given distance.

I don't understand what you mean.

If you don't care about speed, the maximum distance is the same for both antennas, and is defined by obstacles alone.

You can always slow down to compensate for a lack of gain. And it's a proportional slowdown, not very expensive. Especially when you only need to send a beacon that's a few bytes long to initiate contact.

Just some example numbers: Your functional requirements are 1Mbps of bandwidth with pretty tight focus. You send the few bytes of initial omnidirectional contact at 1Kbps. Your slow omnidirectional signal actually reaches further than your fast focused signal.

> Limiting EIRP is the right thing to do in order to limit the interference that you can cause to your neighbors.

If your only concern is the worst case of everyone being pointed at the same spot, yes. In normal situations the average level of interference matters more.

> The law does not stop you to use a phased array antenna or any other kind of directive antenna, with the purpose of lowering the power consumption of your transmitter, while maintaining the same quality for your communication and the same interference for your neighbors.

The law says that I can maintain quality and decrease interference, but I don't gain any real benefit because I'm only saving half a watt. So I'm not very motivated to do so. I'd prefer if it was legal to split the difference between increased signal quality and somewhat decreased interference.

> What you want to do, i.e. increase the interference for your neighbors, is the wrong thing to desire.

Where do you think I said that?

Edit:

> your neighbors would also

Here, I'll elaborate on a scenario.

Originally, me and my 6 neighbors are all transmitting omnidirectionally and causing 1 unit of interference to each other person. Everyone gets 6 units of interference.

I want to lower my transmit power but double my EIRP. I am entirely selfishly motivated, and just want a better signal to my devices. As a consequence I will now cause 2 units of interference to a single neighbor, and 0.1 units of interference to all other neighbors.

What happens when everyone thinks this way and does the same thing? Well now the neighborhood receives 2.5 units of interference on average instead of 6. Even with a bit of variance as devices move around, everyone is better off now. I love that my neighbors did the same thing I did.

[palata]

This (and the parent) really sound super interesting to me, but I don't understand. Before I spend hours on Wikipedia reading about EIRP and phased array (and probably give up), is there a chance one of you could explain this briefly in words I may understand? :)

[GrantMoyer]

A phased array is capable of "beamforming", that is, sending an electromagnetic signal most strongly in a specific, programmable direction, as opposed to broadcasting the signal in many or all directions.

EIRP is a measure of the maximum power in any direction, so a phased array that transmits 1W only forward has the same EIRP as an omnidirectional antenna that transmits 1W forward, but also 1W backward, and up, and down, etc. Overall the omnidirectional antenna may transmit much more power total, but still only 1W in any particular direction.

[ac29]

> an omnidirectional antenna that transmits 1W forward, but also 1W backward, and up, and down, etc

By definition, any antenna with positive gain must concentrate power in one or more directions. So in practice, omnidirectional antennas are only omnidirectional in the horizontal plain, they have greatly reduced power in the up and down directions (which makes sense as generally there is no need to transmit into the sky or ground).

> Overall the omnidirectional antenna may transmit much more power total

This doesnt make sense - an antenna is a passive device, not an amplifier. Regardless of gain, you cant get more energy out of an antenna than you put into it.

[KK7NIL]

EIRP adjusts the total radiated power for the antenna gain.

So if you're transmitting 1 W and your antenna has a gain of 30 dBi (1000x), that's equivalent (from the perspective of whatever it's pointed at) to an isotropic antenna (no gain) emitting 1000 W at the same distance.

It should be obvious that EIRP is what matters for interference and human safety, hence why the FCC regulates EIRP instead of power output.

[20after4]

edit: GrantMoyer's answer is better.

A phased array is a very high-tech method of selective transmission so that you can send a radio signal that is stronger in one specific direction. The way I think of it is that it creates a virtual directional antenna where the direction is adjustable without actually physically moving the antenna elements. The actual tech involved is pretty heavy math and physics (and I don't fully understand it myself) so if you want to really understand it then you may still need to spend hours reading about it.

[Junk_Collector]

In the simplest possible terms, EIRP is the equivalent of power density. It takes into account how narrow the beam from your antenna is.

[Aurornis]

Modern MIMO is about utilizing the combined channel efficiently, not necessarily beam forming. In most cases you can still extract more capacity from a channel with two or more antennas within the same EIRP envelope as a single antenna.