CWNP CWNA – Spread Spectrum Technologies Part 4

19. Direct Sequence Spread Spectrum Part4

Now, after the data has been encoded using the chipping method, the transmitter is going to need to be able to modulate the signal to create the carrier signal that carries the chips. Now, we’re going to use something like differential binary phase shifting dbpsk to be able to use phases or at least two phase shifts. One that represents a zero chip and another that represents the one. Now, I hope you remember when we talked about phase shifting. That when we saw a change of state. That change of state meant this is a one.

If the state did not change, then it was a zero. Now, to provide faster throughput the differential quadrature phase shift keying, DQ PSK utilizes phase shifts allowing you to be able to send two bits instead of one bit, basically doubling the speed that we went through. And again, we talked about that in some other parts of the course. So if you don’t remember that, you can go back and look at it. The only thing I can’t help you with is these really tough words to pronounce when they’re talking about this type of technology.

20. Orthogonal Frequency Division Multiplexing Part1

So we moved to a different type of encoding method, the orthogonal frequency division multiplexing. And it is still one of the more popular technologies that we use even in wired and wireless network communications. Technically, it’s not considered a spread spectrum even though it has a lot of properties like spread spectrum, like a low transmit power and using more bandwidth than is required. But if you technically look at, let’s say, one of these channels now remember I said that these channels are made up of smaller channels that are combined together to be one channel.

But technically what they do is they take that and they divide it into smaller sub carriers where each sub carrier can carry transmissions that can be used to be able to collect data. And in that particular case, each of these little sub carriers is kind of like its own channel, meaning it’s just enough bandwidth to submit the information that you wanted to like spread spectrum that is using more bandwidth than you really need. But by the way, by splitting it up into these smaller units, we’re actually sending more data over what everybody else would have thought was just one channel. And so that’s where it gets the kind of the name of being like a spread spectrum, but in reality really just being a little bit more like a channel or 52 channels that is just the right size for your data.

21. Orthogonal Frequency Division Multiplexing Part2

Multi Path, we’re going to find out later, is going to be actually good for the high throughput and the very high throughput. But remember, we usually have things like walls or desks where the actual signal would bounce off of them. So I might have a direct path to send my signal, but because I’m sending it, usually omnidirectional that signal is also going to bounce off a wall. Bounce off a wall and they’re going to get there at a little bit later time.

Yes, we are operating at the speed of light and if all things are consistent with the speed, this bouncing off a wall is a longer path, so it’s going to take more time to get there. Sure, you can measure that time in nanoseconds, that’s true. But one of the things that we used with Multi Path is by having multiple antennas that can hear those signals, the access point will just pick the one with the strongest signal and ignore all of the rest of them.

22. Orthogonal Frequency Division Multiplexing Part3

So with frequency hopping spread spectrum, the way it would generally work is it would pick a channel to be able to send data, transmit the data using a very small size channel, a small frequency carrier space, and then on schedule, hop to the next channel and then to the next channel. And then using that same hopping schedule, go back to the next channel to continue to transmit data. And the receiver would know what that sequence of hops were going to be so that it too, could go and continue to change to those different channels. And each one, though, like I said, is a small frequency carrier space. And then there’s also the amount of time it takes to hop from one channel to the next.

Now, one of the ways I look at that, I fly a lot. I hope that looks like an airplane. And some of the airplanes let us actually listen to the control towers. So as you’re flying across the country, I often hear them saying it’s time to go to a different airport’s operation to get more directions about their flight. So maybe they started in San Francisco and next thing I know, they’re telling them to go to Denver, and the next thing I hear, they’re saying, go to Chicago. And so that just tells me that they’re doing something very similar, not at the same speed, but something very similar with the idea of the frequency hop spread spectrum to basically get my flight from one city to the other.

23. 2.4 GHz

So the hopping sequence that I was just talking about is very well published. And so that means they had a predefined hopping sequence. I mean, really, if you think about it, if I had some antenna that was transmitting on different channels, hopping from one to the other to a receiver. And remember back in the day when I was working with this, when it first at least arrived in my life, it was on the 900 MHz range, and it was about putting what they called mobile data transmitters inside of the police cars so they could have computer access to records and everything else from the police cars. Believe it or not, that was in the 1990s, so how long ago was that? But anyway, they would use the FHSS because that was really the encoding traffic at the time.

And unlike the 1990s, today they get so much more information. I mean, those police officers that have the computers can pull up your driver’s license, they can look at your picture, make sure you’re not lying about who you are, scan it. I mean, they can do a lot more than in those days when they could just run a registration, which, by the way, if you care, it’s called a 1028 in police code. Or they could run warrants, see if you had any, which is called a 1029. Or they could run your driver’s license, which was called a 1027 just in case any of you listen to a police scanner. So you know, that kind of cool stuff. Anyway, that was mostly just text information.

So the low speed and the frequency worked out very well, but they still use the frequency hopping. Now, the problem that I saw when I first was learning about it, I thought to myself, well, if the antenna decides it’s time to change the frequency, how does the other side know what’s the next frequency that they’re going to go to? I mean, is it randomly picked? No, it’s not. They have what’s called a hopping sequence or a hopping pattern. And so both sides, when they talked to each other, would know what that pattern was and they would know how often in time cycles or time slices that they would have to go from one channel to the next. So again, it was very small carrier frequencies. We didn’t carry a lot of data and we didn’t stay on one channel for a finite amount of time. And so the FHSS radio would have to know what that hopping pattern was to be able to get that non stop flow of information.

24. 2.4 GHz (Cont.)

Now within the 2. 4 GHz range, which was something that was pretty much ratified with the eight or 211 2012 standard, we’ll divide that ism band into 14 separate channels. Now, channels are designated by their center frequency. In other words, there might be what potentially is 14 channels. But as you probably know, we only have three. Because when, when we find that center of the channel, we want to have some buffer zone between the next channel.

So we don’t have any interference. So basically, we designate the channels by their center frequency and then from there have that buffer space. So when we’re using technologies like DSS or Hrdss, the radios are going to be transmitting on their channels and each one inside is going to be 22 MHz wide. So that means that from the center frequency, we’re going to be able to still transmit on the side parts of that channel. So in an example, channel one that we use is going to be 2. 412 GHz with a plus or -11 MHz, which means that the channel actually spans from 2. 41 to 2. 4 to 3 GHz.

It should also be noted that within that distance between them, that the center frequencies between the distance is only 5. Again, that’s just because we’re trying to keep a little bit of that buffer in between. Now, when we brought OFDM into 800 and 211 A, along with the expanded use of it, and remember, A was at 5 MHz, we expanded it into the 800 and 211 G, which is 2. 4 MHz or gigahertz, excuse me. And then of course, all these other technologies we’ll talk about later, the frequency used by that channel is about 20 MHz in size.

25. 5.0 GHz

So here we’re looking at the example of having a larger frequency range and saying the center of that channel, in this case is channel one, which comes to right about here. And then we have that 11 buffer space on each side. And so, technically, as I said before, there’s many channels. But if you were going to broadcast on channel two, look at that, there’s a chance that you might overlap with channel one. So they put that 5 MHz space in between to come up with these different center channels so there’s no interference. And that’s how they came out with the three channels that we would normally see in the 2. 4 GHz range.

26. 5.0 GHz (Cont.)

Now when we look at the 5 GHz channels with things that we would use with eight or 211 A, we could use it with N, certainly with AC, they transmit in that 5 GHz uni band, uni one, uni two, unit two, and extended and uni three are all going to be able to be used by those different technologies. Now to prevent the interference with other possible bands.

Again, some extra bandwidth is going to be used to be able to basically put a guard in between the different channels in the uni one and uni two bands. The centers of the outermost channels of each band have to be 30 MHz from the band’s edge. An extra 20 bandwidth will exist in the 20 or the uni three band.

27. Adjacent, Nonadjacent, and Overlapping Channels

So some of the terms we use in coming up with these channels are things like adjacent, nonadjacent and overlapping channels. So when you’re deploying a wireless LAN, it’s important to have overlapping cell coverage for roaming, to be able to curb, but we don’t want them to step on each other. So that means it’s also important for these coverage cells not to have overlapping frequency space. So if we think about it, I might have an area of coverage from an access point, and it might be on channel one. And then, just to be safe, I might have some overlapping coverage from the next cell. In other words, you’re moving from one access point as you’re roaming to another access point.

But if they are both on channel one, then we have a problem, because in this area, we would see each of those frequencies basically stepping on each other. So we might make this channel six. And then if I have another area of overlap for the other access point, I’d make that channel eleven. Each of these coverage areas we call cells.

And where there is overlap, they do not compete with each other because they’re using different frequencies to be able to cover that. And then what I do over here, when I have the next overlapping one, guess what? That’d be channel one. Because that’s not overlapping with six and it’s not overlapping with eleven. And so that’s part of what we try to design by understanding the differences in the channels and in the design of our wireless LAN architecture.

28. Throughput vs. Bandwidth

So some of the terms we use in coming up with these channels are things like adjacent, nonadjacent and overlapping channels. So when you’re deploying a wireless LAN, it’s important to have overlapping cell coverage for roaming, to be able to curb, but we don’t want them to step on each other. So that means it’s also important for these coverage cells not to have overlapping frequency space. So if we think about it, I might have an area of coverage from an access point, and it might be on channel one. And then, just to be safe, I might have some overlapping coverage from the next cell.

In other words, you’re moving from one access point as you’re roaming to another access point. But if they are both on channel one, then we have a problem, because in this area, we would see each of those frequencies basically stepping on each other. So we might make this channel six. And then if I have another area of overlap for the other access point, I’d make that channel eleven.

Each of these coverage areas we call cells. And where there is overlap, they do not compete with each other because they’re using different frequencies to be able to cover that. And then what I do over here, when I have the next overlapping one, guess what? That’d be channel one. Because that’s not overlapping with six and it’s not overlapping with eleven. And so that’s part of what we try to design by understanding the differences in the channels and in the design of our wireless LAN architecture.

29. Section Review

So let’s talk about throughput versus bandwidth. Wireless communications are typically done within a constrained set of frequencies that we’ve talked about a lot so far. We call those the frequency band, and that band is also the bandwidth that we’re going to have. It plays a part basically in the throughput of the data.

So don’t confuse your frequency bandwidth with your data bandwidth, because I’ve already talked about the different ways in which we can encode the data. Over your frequency will actually help determine the amount of bandwidth that you have. Now, changes in speed due to the modulation and the coding is what we would call the data rates. So if you start off with FHSS and move to OFDM in the same channel, OFDM is going to send more on data rate but on the same bandwidth.