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FDMA
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Из курса от партнера Princeton University
Networks Illustrated: Principles without Calculus
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Power Control in Cellular Networks
How is it possible that we can all communicate effectively without disrupting each other's calls, messages, or Internet usage? In this lesson, we will take a look at some of the methods that have been developed for letting us "share" the air over which our phones communicate.
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Christopher BrintonLecturer
Electrical Engineering
Mung ChiangProfessor
Electrical Engineering
So now that we've brainstormed a little bit about different ways that we could
have people share the same network meeting without causing too much
interference with one another. Which is called multiple access, we can
look at our first multiple access technology and that's called Frequency
Division Multiple Acts, or FDMA for short.
We just said what multiple access means. And the way that we go about
accomplishing that is by dividing the different lengths by assigning them
different frequencies. So each link is going to get a different
frequency or really a band of frequencies as we will see, in order to divide them
accordingly. So, really the idea is that, if we're
having some process occurring over time, so let's have this be time, everything
that's happening over time. And have this be frequency, where
assigning each person or each link gets a different frequency in which they're
going to communicate. So Anna, or whoever is receiving Anna's
message, knows what Anna was saying. Because they'll know and they'll see the
message come in at this frequency. And Ben, whoever's receiving Ben's
message will see that come in on Ben's message.
And Charlie, and Dana, accordingly. So the first time that FDMA was used was
in the multiple telegraph. Where we had a wire, which we would see
right here, and then we had different transmitters and receivers.
Let's draw the transmitters in this side, the transmitters could also be over here.
Every person could be a transmitter or receiver depending on what's currently
happening and the receivers. And so, if this is one link, this would
get one frequency band. Say this is Anna, and this is whoever is
receiving Anna's message. And this is Ben, and this is whoever is
receiving Ben's message. This would be on one frequency band, say
it's A. And this is on another one, we'll say
that's B, so now what really is a frequency how do we define that how we
define what it does. Well time you can't really feel and you
can't hear time you can't really perceive time other then the fact that it happens,
and frequency you can actually hear. If you sustain a frequency you get a
pitch so different musical instruments as we said before have different pitches.
So FDMA is really analogous to separating amongst pitches as we said before.
But it's separating them enough so that if you're listening in for this frequency
over here. That you will never be able to hear this
frequency over here because you're only listening at a select band.
And so, the way that we perceive and define what a frequency is, is by this
quantity called Hertz. And, what a Hertz means is it's looking
at a wave, or just some power, some electromagnetic signal.
As we can see right here and this would be one signal and we always draw things
as sinusoids. Because that's the definition that's
really how we define and come about the idea for frequencies in terms of the
sinusoid. And so this is one sinusoid and over
time, because we have time on this axis down here.
We see the level of the wave, over time and then it comes back and eventually it
hits zero again, so it's done one full cycle once it gets back over to here.
So, we define a Hertz, as being one cycle in one second.
So this wave we say is, has a frequency of 1 Hertz, and that's how we would
define it. And then so a frequency of 2 Hertz or 3
Hertz is higher. And that 2 Hertz would mean that we do 2
cycles in 1 second, 3 Hertz would mean that we do 3 cycles in one second.
Like this wave right here we can see it's done, it's come up and down, 1 time, and
then up and down 2 in 1 second. So we say that this wave would be a
frequency of 2 Hertz and so on and so forth.
Now the frequency bands that we look at in terms of communications are very much
higher than 1 Hertz or 2 Hertz. And very much higher then anything you
could ever hear. If your in the kHz range the human, the,
the human ear has a range up to about, somewhere between 12 and 14 kHz.
And kilo means times 10 to the 3, or 1,000.
So, 12 kHz is 12,000 hertz, which means that this cycle would have to repeat up
and down, up and down, back again, and so on.
12,000 times before we get to one second, very, very fast.
Now, on communications, we're speaking of frequencies that are way, way higher than
even what we can perceive. And instead of being in the kHz range,
we're actually in the MHz range, which is a million Hertz.
And so a frequency of 5 MHz means that the wave repeats 5 million times, in one
second. That it goes up and down, so, you can
think even faster than this will have to be doing 5 million times before it gets
to one second. You can barely ever even think of drawing
that out. So, as we said though, if we look at this
frequency access, it's not. Simply just one frequency that each of
these channels are on. They're actually on a band of
frequencies, so in between here and here, that's one channel.
In between here and here, that's another channel.
In between here and here, that's another channel, and so on.
So if we take this axis and rotate it down, remember, now we're not talking
about time anymore. Whereas up here we're looking at time now
this is frequency on this axis. So just make sure you remember which axis
we're speaking of. Suppose we have between 20 and 120 MHz or
as we've said, it's millions of Hertz. So this would be 120 million Hertz and
this is 20 million Hertz. So if we wanted to put, say, five
channels between 20 and 120 MHz, kind of, here we have four channels.
But suppose we had a fifth one, and we wanted to put five of them within this
range. It's really a simple math problem to
figure out how to divide that up. So at the end over here, you have 120
MHz, and at the beginning you're at a 20. So in between that we have 120 minus 20,
right? And then if we want to put five of those
channels in there, we would divide this by 5 to figure out how quick.
How much each needs to be spaced, and so if we do that division out, we get 20
MHz. So each of the channels would have to be
spaced by 20 MHz in order to put five of them in between.
So this first one's between 20 and 40, second one would be between 40 and 60,
third one between 60 and 80, 80 and 100, 100 and 120.
It fits just like a puzzle. So, just keep in mind that when we speak
of frequency, we're really meaning a frequency band.
That's really the proper term is that one of these conversations would be between
20 and 40 MHz. The other one would be between 40 and 60.
Between 16, 80, 80, and 100 and so on. And actually, technically we need some
space in between them so we can't have them intersecting or else we will have
some interference. So the easiest way to understand FDMA is
to think about you on your car radio. And if you're turning the dial to switch
stations really what you're doing is you're switching, you're changing your
receiver on your car to listen in for a different frequency.
And that's how the radio stations are distinguished is by what frequency
channel they're on. And so if this, if you're on, say, 95.5,
and you've probably heard on a radio station before, they'll say, you're now
listening to 95.5 WPLJ. What they really are saying is that your
receiver is tuned in to listen to the station of 95.5, and we're up in the MHz
range there. So, that's really saying that your car
radio is listening in for 95.5 MHz, and it's tuned in to listen to that station.
And there's another station out here, which if you turned your receiver you
would move up, and then listen in to this next one.
And that dial will switch between the channels for you, and listen in to what
that radio station is currently broadcasting.
And so right here we see one radio station second radio station and a third
radio station.
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