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Cells & 1G
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En provenance du cours de Princeton University
Networks Illustrated: Principles without Calculus
43 notes
À partir de la leçon
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.
Rencontrer les enseignants
Christopher BrintonLecturer
Electrical Engineering
Mung ChiangProfessor
Electrical Engineering
So the concept of atenuation led engineers to start dividing reasons
geographically into what we call cell. Now we're not speaking of a biological
cell clearly so don't get confused there speaking of the cell in terms of a
cellular network or a cellular system. And the way that we represent a cell is
geometrically, as a hexagon. So we divide our region into a bunch of
hexagons, and the reason that we divide them into hexagons is be, perhaps beyond
our scope here. We can point out one important idea.
And one of the reasons is that at the intersection each of these cells we have
what we will be called cell towers or base stations.
And rather than having a base station at the middle of each cell serving to the
ends of each cells, we have a base station serving a portion of 3 different
cells. So, if there a bit serve this portion,
[UNKNOWN] serve this portion, they're going to serve this portion that's on.
So that's one of the reasons, and the other ones are perhaps beyond our scope
here. So now, within each of these cells, we
have our phones and each of our phones, or our mobile subscriptions, lie within
one of these cells. And we call cell phones, or any device
that can transmit and receive according to a cellular standard, a Mobile Station,
or an MS. And the MS's are going to connect to base
stations depending upon where they are. And another term you may have heard for
base station could be node B or evolvable node B meaning E node B that's a 4G term.
And we're not going to use that term here, even though you could call it node
B or enode B, depending upon what you're speaking in.
For our purposes here they're really synonymous, so we'll just stick with the
term, Base Station. So now, with FDMA, we have to determine
which frequency bands allocate to which cell.
And with the cellular network there's certain rules that this imposes upon that
from having a detrimental impact on conversations.
So what we say with the cellular network is that each cell is going to get a band
of frequencies. So naturally within each cell they are
all going to use one band of frequencies. But each mobile station has to be on a
different channel. So if this is the channel allocation for
this cell each mobile station on this channel is going to be on different one
of these channels. Each of the adjacent cells to this one
,so this cell right here is adjacent because it is touching, this cell is
adjacent this one also is adjacent. Each of them has to use a block of
frequencies that's different than the one that this cell is using.
So this cell up here would have to use a different set of frequencies, than this
cell over here. But, as long as the cells are not
adjacent, we can reuse the existing frequencies.
So up here for instance, in this cell, we can use the same frequency bands that
this cell down here is using. Because they're far enough away, that the
attenuation would cut down the signal level so far when we travel from here up
to here. That you won't even be able to hear the
conversations anymore, even if these people are using the same frequency bands
as the ones in this cell. So we have to determine how to allocate
the bands of frequencies in order to use the minimum number of channels.
Because that will clearly be the most specially efficient as we would say.
Or its making best use of our available capacity so that's a pretty difficult
problem mathematically if you have a really large saw region.
To determine what each cell would get, what frequency banny cell would get.
So that we conform to these ideas of only reusing if the cell blocks are not
adjacent, as we said. And using the minimum number, because
that's the most special efficient is to use the minimum, number of frequency
bands. So, we can look at this simple example
of, is it small enough, and we can just eyeball it would be in order to use the
minimum number. The way that we can go about doing that
is by assigning each one a different color, and a color here will be a band of
frequencies. So we can start by coloring this first
cell, blue over here. And now, as we said, these are each
adjacent to this blue cell. So, we have to color them a different
color than this blue. So, we'll choose red, green and orange
maybe. Now, let's try to color this cell over
here. Now, this cell, is adjacent to green and
red. But it is not adjacent to either blue or
orange. So we can color this either blue or
orange, and so rather than adding a fifth color, or a fifth set of frequencies.
We can reuse, as we choose, this orange over here, to make it more spectrally
efficient. Which means we're making better use of
our available frequency, capacity. We could have also taken this orange over
here and made this one green. Because these are not adjacent, but we
just chose three different colors off the bat over here.
Because on this one, without it being able to be green it would have had to
have been orange. So it's still using four, and so we just
want to make sure we're using the minimum number of colors possible.
Now, we can continue to allocate colors to each of these cell blocks.
And we might get something like this, and there's not only one answer.
There's different allocations clearly that would work.
But the idea is that we just want to make sure we're using them minimum number of
colors possible. Because that's going to give us the
maximum use of our available capacity. And so, you can verify on this diagram
that no two adjacent cells have the same color, and that's the idea.
That's what we want to get across here. And so, if we look at a frequency chart
down here, just to draw this out. We may say that the blue is on channels
one, two, and three if there were only three channels within each frequency
band. That's clearly not anywhere near enough.
But just for illustration purposes, we would say that these three would be one,
two, and three. So now anyone in the red bands would use
4, 5, and 6 to allocate to their cell blocks, which is separate from 1, 2, and
3. Separate color, separate bands, and 7, 8,
and 9, for green, and ten, 11, and 12, for Orange.
So now, as we have more and more channels within each frequency block, we would
need to add more spectrums. So we would have to increase the size of
this accordingly. But right here we're just showing three
for each for a total of 12 channels, divided accordingly.
So rather than needing, so now, we're using 12 channels to allocate to this
entire spectrum in here. So we need 12 channels to do this
allocation if we want each, if we want each cell to have three channels.
So now, if we count up the number of cells, we have 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13. So if we wanted to give Each cell a
different band of frequencies you would need 13 times 3, 3 per cell which is 39
channels. And we're doing this in only 12, so we
have that factor of improvement. Instead of 39 we've now reduced it now to
only the number 12 to do this same allocation.
So clearly, using cells give us a lot of capacity advantage over not using cells.
Now, one other term we should point out is that of the Frequency Reuse Factor,
which is exactly what we were just discussing here.
The Frequency Reuse Factor is the number of colors that you need in this diagram.
It's really the number of bands that you need to use.
So, in here, we have a frequency reuse factor of four, because we're using four
different colors or four different bands of frequencies.
So cells were first used in the first generation, and that's when we started to
use the name cell, as we said. So we evolved from zero G then so what we
call 1G, which is when we started to use this style concept.
And it was first used in the AMPS system, which was pushed out by Bell in 1986.
AMPS is called the Advanced Mobile Phone System, which operated around in the 800
MHz band. The cellular concept marked a transitions
from 0G to 1G technology and the first such 1G or First Generation Technology
was called AMPS. Or the Advanced Mobile Phone System which
was introduced in the United States in 1978 and in other countries a little
later. Now the first field trial of AMPS was in
Chicago, and it consisted of 10 cells and 90 people.
And AMPS operated in the 800 MHz band. And this really demonstrated the
feasibility of the cell concept and the cell concept is employed to the present
day. Under AMPS, the number of mobile phone
users skyrocketed. In the United States, by the mid 1990s,
there were already around 25 million mobile subscribers.
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