RF Propagation

Summary

RF Propagation

Video Transcript

- Hi, welcome to this talk on RF Propogation. My name's Bob Baxley, and I'm the Chief Engineer at Bastille, where I work on the Radio and Data Science teams. So the objective of this talk is to give you a feeling and an intuition for how radio waves propagate, which in turn, gives you a feeling for how wireless devices communicate, and specifically, how well they communicate in various channel conditions, so walls, and those sorts of things.

So I've got a piece of clip art here that illustrates the various propagation mechanisms. So when my signal propagates from here to there, it, various things happen. One thing that happens is I send a line of sight signal. A line of sight signal goes from here to there, directly to there.

Another mechanism that happens is, things bounce off the walls, so these are reflections. And those signals end up at the destination. There's also this mechanism of shadowing, so if there's a wall between us, some signal would permeate through the wall, but it would go through the wall with reduced signal strength.

And so, as we say, it's shadowed a bit. And there's scattering, which is like diffused reflection. There's a bunch of reflectors gathered together, and you have this scattering event. There's all these propagation mechanisms. In the next few slides, I want to give you a feeling for how they affect the signal.

Because what we're ultimately interested in is, you know, how much signal gets from the transmitter to the receiver, because that dictates the signal quality, how likely is the link to be maintained. So let's first start with shadowing. Here, I've got a simulation where I've drawn two walls, kind of like this, and I've got a transmitter in two different spots.

So I've got a star transmitter, and a triangle transmitter. And you can see that the, what the heat map shows is the signal intensity in space. So as the signal emanates from that star, and it hits the wall, it's shadowed by the wall. And that's why, behind the wall, directly on the other side of the wall, we have this blue region, where the signal is attenuated.

As you can see, some signal ends up wrapping around the wall. And depending on the wall material, the wall material dictates how much permeates through the wall, and how much gets absorbed or reflected, so that's shadowing. And it's pretty intuitive, if there's a wall in the way, especially a lead wall, it's going to adversely affect your wireless system.

The next most, if we go down the level of intuition, the next one is this path loss. Just as I travel out from my signal source, surely, I lose some power, right? And so if I want to transmit a long way away, I need more power. So one way to visualize that, and think about that is, if this is my transmitter, and the signal leaves this transmitter, and it propagates in an isotropic pattern, it's kind of like a sphere blowing up from this point.

And so at this point, I have a certain amount of power, and as I make the sphere bigger, the surface area of the sphere contains, the sum of the surface area contains all the power. So the further away I get, the more surface area there is, which means there's more like, there's less energy density, essentially.

So if I get a long way away, my receiver's only seeing a tiny portion of the sphere. If the receiver is close, then it sees a much bigger portion of the sphere. If you work out the math behind all of that, you end up showing that under that propagation mechanism, the power falls off with the square of distance.

So the further away, if I take the distance from here to there and I square it, that's how much less power I have. That squaring, that two, is called the path loss exponent. So in free space, we say the path loss exponent is two. In other propagation environments, the path loss exponent can be higher, like, three or four.

So if it was three, that means the power falls off with the cube of distance, which is worse. So depending on the propagation setup, you might have more or less power loss. The last kind of interesting mechanism is called multipath. So just like when I'm speaking now, my signal, my voice, is going directly to the microphone, but it's also bounding off the walls and then coming back to the microphone.

And depending on the geometry of the walls, that will dictate how these multiple signals cohere, or decohere, at the receiver. So you may be familiar with, like, audiophiles who try to optimize their living rooms so that the speakers all cohere at the center of the couch, so they can hear the audio really well.

And if you move off from that a little bit, you might be in an audio dead spot. Well, the exact same mechanisms apply in RF. The only difference is, we're talking about the speed of light instead of the speed of sound, so that's much faster, on much smaller distance scales.

So here, I've got a, an animation where I show a signal emanating from a black dot in the middle of the space, and you can see these wave patterns. And then on the right, there's a pink dot, and I'm plotting the black and pink signal up above. And so this is constructive multipath.

As the signal bounces off those walls, and then arrives at the pink dot, you can see the pink sinusoid, pink sine wave is actually larger, I'll mark it out here. It's 50% larger than the black signal is transmitted. So this is coherent interference. I'm getting more than I sent.

There's also destructive interference, so if I move that pink receiver just a little bit spatially, it might be such, the geometry is, that the signal is attenuated a lot. So that's what we're seeing here. As soon as all these multipaths cohere, you can see the signal almost dies, and is, you know, 1/2 of what it was when I transmitted it.

You can see a similar phenomenon when you're in a car in an urban canyon, so there's buildings all around you, and you're at a stop light listening to the radio station, and the reception is not great. If you inch up your car just a little bit, the reception might change drastically.

And it's the same mechanism, the signals are bounding off of the buildings. And whether you're in a dead spot or not depends on the geometry of all this. So now I'm going to show you a demo, where I'm going to plug in this radio and kind of move my hand around, and you'll be able to see in the signal, as I move my hand around, how the multipath changes.

So here, we're showing a demo of multipath. What I've done is I've taken my software defined radio, and what you can see on the screen over here is a spectrograph. And what I've tuned the radio to is the 500 to 600 megahertz band, and in this band, what we're seeing are TV stations.

So TV stations seem to be fairly power constant. And you can see, this frequency pattern is kind of wavy, okay. So what I'm going to demonstrate is, when I start moving, you'll see the effect of multipath. Remember how I said, slight changes in the physics of the environment can change the characteristics of the signal.

So here are the characteristics changing with the multipath changing, it's going to be manifested by this frequency selective pattern changing. So you cal already kind of see, as I move my hands, the pattern changes. So I'm going to move my hands closer to the antenna. When I move them erratically, you'll see the spectrum changing as my hands change.

So I do that, that's very obvious that you can see my hands change. So this is a very similar phenomenon to the phenomenon that radar uses. So radar's looking for planes, and the effect that it's really looking for is the multipath change of the plane, as it flies by, it's creating with the signal.

So again, you can see, as I flatten my arm like this, it's very clear in the spectrogram that somebody's moving nearby. So it's kind of a super cool phenomenon. And again, to the higher level point, it's just getting your own intuition for how the physics of the environment can affect your signal.

Again, I'm Bob Baxley with Bastille, thanks for listening.