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Overview of Wireless Communications

20. Communication Fundamentals Part4

Now, as we talk about some of the other fundamentals of communications, one of the things we'll see is what's called a keying method. And a keying method is what changes a signal into a carrier signal. Basically, it's a way in which we can provide that signal with the ability to encode data so that it can be communicated or transported to the receiver.

21. Communication Fundamentals Part5

Now keying methods, and there are many of them, are ways in which the receiver can see a change in that transmission and realise how the change is communicating the information of ones and zeros. One of them that we might see in some of the current types anyway is what we call amplitude shift keying. Or, as K, we also have the frequency shift keying, or FSK, or, as you might say, the phase shift keying. PSK. We're going to talk about each of those here as we continue on.

22. Communication Fundamentals Part6

Now, when we're talking about some of these methods, there is something that's important for us to look at, and that is the idea of current state and state transition. So current state is a method of determining if ones or zeros are being transmitted, and that's done by noticing the state of the signal at any expected point in time and comparing it with the same signal at the next point in time. Now, we might also say, "Well, hey, the state of that signal has transitioned." And that method, again, helps us determine if ones and zeros are being generated based on how that transmitted signal changed its state, such as going out of phase. And I think it is easier when we diagram these for you to get a better understanding of what we're trying to say about current states and state transitions.

23. Communication Fundamentals Part7

So let's take a look at this signal. If we were to measure this signal, I would hope that you'd see that it's the same frequency. In other words, the distance between the peaks will be consistent throughout this section. But what's different is the strength of the signal or the amplitude. And so what we're seeing here is that when we see a smaller amplitude, that would be an indication that, in this case, transmitting is zero. And then when the transmitter increases the amplitude, that would be considered a one. And so as we're looking at that frequency and we see this pattern of low and high amplitude, that means it continues to change. We looked at the current state here, and we saw the current state for this lower amplitude. And then, when we looked again over a period of time, we saw the other change in that state. And that is one method of being able to integrate, whether you're sending a zero or a one.

24. Communication Fundamentals Part8

Frequency shift keying is a little bit different. So now we're looking at transmissions here that have different frequencies over a certain amount of time. Again, as we're looking and watching at this or listening to this transmission over time, what we're seeing here is that when there's a lower frequency, we'd call that a lower frequency, and we have a zero. And then when we change to a higher frequency, those are the ones. So that's a noticeable change for the receiver. And that's how we could easily then say, "Hey, here's the zero, here's the 1001," and be able to again send this information. Now, I realise that this looks like it's a little bit slow in that you're thinking, "Wow, look at how much time did it take?" If this is a measure of time, how much time does it take to send all these ones and zeros? Well, remember that this doesn't have to be for each frequency we measure. By frequency, we meant how many times we see this topcycle of the wave over 1 second of time; that doesn't mean it has to last for a full second. This change is going to happen depending on the frequency, but it's going to happen very quickly, very fast.So we are still able to send a lot of data.

25. Communication Fundamentals Part9

Now, the phase shift keying is a little bit more difficult to try to explain. So let's take a look here. When we see that we're communicating on our frequency, the notes change; in other words, it looks like the same frequency as we're measuring it. That would be a zero. But then when the transmitter says, Hey, I want you to know that I'm sending one, Now, at that point, we'll change the phase of that signal. In other words, you notice that it has started coming up, and we would have expected it to continue to go up, but it didn't. Instead, we've changed the phase of that. How much we change the phase can help give an indication of whether we're sending a one or a zero, as you'll see with some other methods. But just the fact that the phase changed means that we are sending one. Then, when the phase no longer changes, we're back to zero. We're back at zero. Then I changed the phase right here to get back into saying, Hey, once you see that change, then I'm transmitting a one. If you don't see any changes in the phase, then I'm transmitting zeros.

26. Communication Fundamentals Part10

Now, the idea of multiphase shift keying, an advanced version of what we just talked about, is trying to be able to send multiple bits per symbol instead of just using two phases. Now, remember, when we talk about phases, we're talking about from zero to 360 degrees. And some of these changes can be broken down into four components: basically zero, 90 degrees, 180 degrees, a change from 270 degrees, and the rest of that. And if I get back to 360, I'm basically back to zero. So now what we see is that we are in code and we say, "Look, if we have a 90-degree change in phase, then we want to let you know we're sending you a zero and a one." If we do not change the phase at all, which was what happened last time, when there's no change, then we're sending you two zeros. If we have a 270-degree phase change, then we're sending you a one and a zero, or a 180-degree phase change, then we're sending you two ones. Those are binary numbers, by the way. I'm not talking about it looking like the number ten, but it's actually two digits, one and a zero. And we can see that on the chart that, as we saw this frequency, we then had that 90-degree change. So it came up to here, and at this point we started it down here. There's the 90-degree change that becomes the ones and zeros. And then here, of course, it's going along with no change. As a result, we see no change in the next time you read this frequency. So it becomes two zeros, 270 deg. And we were down, all the way down, at the bottom of this arc. And then we saw the 270-degree change, and then that was the ones and zeros. Then you can see this other 180-degree change. That indicates that we're sending the two ones.

27. Section Review

So in this module, we talked a bit about the history of wireless, the different standards, the different organisations like the FCC, the ITUC, the IETF, and the rest of those. A brief overview of the wired infrastructure, with some idea of what the OSI model was attempting to assist us with. We looked at the hierarchical model, the access, and the distribution of the core. We talked about different types of ways we can communicate over the carrier signal. Remember, a carrier signal is basically a transmission that is indicating the actual data, the ones and zeros. And we looked at some of the fundamentals of the different types of methods that can be used to depict the ones and zeros as they are sent from the transmitter and seen or heard by the receiver.

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