Handoffs

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From the course by Princeton University

Networks Illustrated: Principles without Calculus

39 ratings

Princeton University

Networks Illustrated: Principles without Calculus

39 ratings

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. Please note that per Princeton University policy, no certificates, credentials or reports are awarded in connection with this course.

From the lesson

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.

Meet the Instructors

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

So as we said, one of the initial problems with CDMA was the near-far

problem, and that's problem number one. That was the one that we focused most on

this lecture, because that concerns how your cellphone determines what it should

and shouldn't transmit. And the second problem that they had to

overcome was the idea of what's called a Handoff.

With Handoffs we're talking about what happens when a device has to switch

between two different base stations. So if this is one base station, and this

is another base station, what happens when your device is currently talking

[SOUND] to this base station? So if you're driving in your car, and

you're on your cell phone, but then you move into the coverage area of the next

space station. How does it handle that switch to the one

that is giving you the better channel quality?

Well, under FDMA or TDMA, you switch frequency channels.

So this change can happen very quickly, and it's not too much of an issue, to

have that happen. So you just, switch frequency channels

and switch base stations accordingly. But with CDMA, it's much harder to do

that because, first of all, the cells don't have dedicated frequency channels.

Everyone's transmitting in the same bands, so it's not as easy as just

switching from one channel to another. But the second problem is that when a

device enters [SOUND] another cell. The CDM, the power control algorithm are

going to have to reconverge very, very, very quickly in order for you not to see

any effect on your call quality. Because if you currently have your SIR

all figured out, when you're over in this cell, and then you suddenly transition to

this new cell tower. Then that algorithm would have to

reconverge very quickly to you depending upon the current channel conditions.

From all of the other mobile stations that are in this cell, and taking those

into account. So, this is known as a Hard Handoff, and

again Hard Handoff works fine when you have FDMA or TMA, but for CDMA, we need

another solution. We can't use the Hard Handoff.

What engineers came up with was the idea of a soft handoff instead.

And the scheme of a soft handoff is that when a device, when you're driving in

your car and you're currently talking to the cell tower.

Rather than this disconnection and reconnection happening very, very quickly

in that transition, we do it smoother, and more soft.

That's where the term soft versus hard comes from.

So, right now you're talking to this tower, and then once you move into the

intersection region, you'll actually keep connectivity [SOUND] with this tower.

While connecting to the new tower as well.

And so you won't just drop the connection really quickly from this tower and you

also won't completely rely then on this tower.

because that will give the power control algorithm enough time to converge as is

necessary for you to then move into this cell to have a smoother transition.

A more seamless transition, so you won't notice that it happened, and that's

called the Soft Handoff. And that worked very well for CDMA.

In fact it works much better withr CDMA than the Hard Handoff works for FDMA or

TDMA. because it's much smoother and gives much

better transition in call quality.

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