TDMA

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來自 Princeton University 的課程

Networks Illustrated: Principles without Calculus

43 個評分

Princeton University

Networks Illustrated: Principles without Calculus

43 個評分

What makes WiFi faster at home than at a coffee shop? How does Google order its search results from the trillions of webpages on the Internet? Why does Verizon charge $15 for every GB of data we use? Is it really true that we are connected in six social steps or less? These are just a few of the many intriguing questions we can ask about the social and technical networks that form integral parts of our daily lives. This course is about exploring the answers, using a language that anyone can understand. We will focus on fundamental principles like “sharing is hard”, “crowds are wise”, and “network of networks” that have guided the design and sustainability of today’s networks, and summarize the theories behind everything from the social connections we make on platforms like Facebook to the technology upon which these websites run. Unlike other networking courses, the mathematics included here are no more complicated than adding and multiplying numbers. While mathematical details are necessary to fully specify the algorithms and systems we investigate, they are not required to understand the main ideas. We use illustrations, analogies, and anecdotes about networks as pedagogical tools in lieu of detailed equations. All the features of this course are available for free. It does not offer a certificate upon completion.

從本節課中

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.

與講師見面

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

So coding things in digital form also allowed engineers to look at a different

type of mobile access technology, and this'll be the second type that we look

at. So the idea is as follows.

Let's consider our standard Time Frequency division, and we saw before

with FDMA that every transmitter would get a different frequency band.

So person A is on this frequency, B's on this frequency, C's on this, and D's on

this one, and that's how they're distinguished over the same network

medium. That's how they share the air, if you

will. So let's try to think of a way we could

fit more people into this diagram. So what about having every frequency

carry multiple users? So something like this, first off, at the

first time, A1, B1, C1, and D1 are transmitting.

Then at the second time, A2, B2, C2, and D2 are transmitting.

Then A3, B3, C3, and D3, and so on. And so we're adding a time division on

top this frequency division so that on every frequency, we've now put 3

different people in here. It goes back to our time sharing that we

talked about in our initial analogy. Basically, these 3 users can take turns

on this one frequency. And then they can repeat again, so then

A1 would have its turn next B2 and A3 and so on for each frequency.

So we see we have three different things going on here.

Not only are people now on different frequencies, but now we can fit multiple

people on the same frequency as long as we divide them in time.

So you're probably asking, well, that's great, and now we have a threefold

improvement in capacity. Because now we can have 12 people, these

are, there's 12 blocks in here, rather than just four people.

But why didn't we do that before, with FTMA?

Well, with analog signals, it's not possible.

To compress multiple signals onto the same frequency band.

So we cant do this, we cant do this time sharing.

Analogue just doesn't allow us to do that, but digital technology allows us to

do that compression. So we can take these 3 signals, and put

them on the same frequency band and still have the receiver distinguish between

them. So, one of the advantages of digital

coding was to allow us to move to this next multiple-access technology.

So, this is called TDMA, multiple-access, as we know, again, but rather than

frequency division, we now have time division on top of that.

So this is time division multiple-access. So you're probably wondering, well, it

really still is kind of frequency division.

Because we do have different frequency bands, what we're really referring to

here with TDMA is that on top of FDMA. We're now allowing multiple people to

share the same frequency bands, so it has a TDMA in it as well.

But in the strict sense, a TDMA system would be a single frequency having

multiple users. But it doesn't make sense to do that

because we know that we can have multiple frequencies.

So when we say TDMA we're really referring to a chart like this rather

than just a chart of a single Frequency with multiple times.

So as we said, we could not do this with analog.

This was only made possible once digital technology was allowed.

So the first technology to employ TDMA was called GSM, or, Global System for

Mobile Communication, and that came out in 1982.

GSM was quickly shown, by 1987, to have a three times improvement over analog, the

previous analog capacity with the amps network.

And that three times improvement and capacity comes about, because now we can

have multiple users sharing the same frequency band.

And GSM was adopted in Europe rather quickly, because the European Union

favored a single standard. Because they wanted to have what's called

interoperabliltiy. Interoperability, and so they didn't want

to have multiple standards because they wanted to support things like roaming

from one area to another. So they wanted each of their cell systems

to be using these same standards. They decided that let's adopt GSM,

because it looks good at that time. And they said let's adopt this then

everyone will be using it. And that's exactly what happened.

So they had a mobile standard.

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