0:09

Well, first of all, let's just look at the connection between the end-user

device over the air to the eNobe, okay?

And let's only look at the physical layer first,

okay? So, there's no application to speak of.

Well, what is the formula of this speed here?

Roughly speaking, it goes like this. We want to look at the number of bits

transmitted, right? And then, divide by the time.

So, the number of bits transmitted over time. And there is a standard unit of

time called a sub frame of data, one unit of time, and that is one millisecond in

the standardization. Let's just count how many bits can be

sent over that one millisecond then, okay? Well, we need to know the number of

symbols per frequency block. Then, we multiply that by the number of

frequency blocks. Then, we multiply by the number of bits

per symbol. Then, we multiply by any extra gains,

such as the coding, or MIM, MIMO or multiple antenna gain,

okay? So, the formula looks something like this.

We look at the number of symbols per carrier, okay?

Different how rich of descriptive power there is

per carrier. Meaning, per small block of the frequency

that you're using, okay? Minus the control overhead that you need

for example for channel estimation, to know how good the channel is before

deciding what kind of modulation you can use,

okay? Then, you multiply that difference by the

number of carriers per frequency block. As mentioned in advanced material part of

last lecture and a quick summary here, is that the frequency block is divided into

a few blocks, okay? a number of blocks. And each block is then further divided

into different carriers, [SOUND] okay? So, signals are modulated onto different

carriers and then we got many blocks of these carriers for LTE.

2:23

So, you multiply this difference by the number of carriers you have for frequency

block, then you subtract out other overhead,

okay? Then, you multiply by the number of bits that you can transmit per symbol.

If you got a good channel, you can carry more bits per symbol through a so called

higher order modulation. Again, the detail doesn't concern us

because this is not a signal processing for communication course.

Then, we multiply the number of frequency blocks you have, all together.

And then, you multiply by the additional gains such as coding gain and MIMO

antenna gain. Now, in the ideal case, we get the

following kind of numbers, okay?

So, symbols per carrier is usually say, fourteen in the ideal case for LTE.

And the control overhead per carrier is let's say one,

[SOUND] okay? And then, we multiply by the number of carriers per frequency

block. Usually, there are twelve carriers,

okay? So, multiply by twelve, and each one has a frequency block of 180

kilohertz. And then, there's a channel estimation

overhead, okay? And this amount of overhead is used

to sense something called pilot symbols to estimate how good the channel is, and

is ideally around, ten symbols, okay? For a four by four antenna system.

And then, you look at the number of bits per symbol

that you can run. Ideally, this would be say six.

Meaning that you have two to the six which is 64, so-called 64 qam modulation,

okay? That's when the channel is very good, you can afford a higher auto

modulation as to be heard correctly at a receiver.

So, you can carry six bits for every symbol.

And then, you multiply the number of frequency blocks you have, which is

altogether a 100 frequency blocks, okay?

Together with some garband, you all together are consuming 20 megahertz then,

which is the channel with typically an LTE system.

Then, you multiply by the coding rate, [SOUND] okay? Which is basically a number

between zero and one. Between zero and one.

More efficient your error correction coding is meaning you'd only need to add

a little bit of redundancy in order to prepare for the channel distortion of

your bits then closer to one you are, for example 0.9, okay?

Now, these days, we've got very advanced error correction coding methodologies

developed in the past decade or so, that can get you very close to one but you

still can get exactly to one. Say, for the kind of a good system that

we are talking about, it get to a 0.9, okay? And then, there is the gain for

using multiple antennas. Theoretically, if you use say, four by

four MIMO system. Meaning, there are four antennas and the

transmitter for other receiver. Then, you can have four spatially

separate channels. So, you get a factor of four on the

spacing. Alright.

So, if you add these up, okay? Then, you get the following number,

315360 bits, okay? For every one millisecond.

So that implies you divide it by one millisecond, you get basically 315

megabit per second. And that's the kind of number you hear in

the physical layer speed for a 4G, okay? LTE.

Which is a huge number, if you think about it, okay?

Our home, broadband wireline access networks

usually getting something like 25 to 50 megabit per second,

okay? Wi-Fi, even if you use that 11N, a fancy

version of Wi-Fi. So, we talked about last time, you are

getting something like 100 megabit per second.

So, this is talking about a wide area wireless network.

And you can get 300, more than 300 megabit per second, okay?

This is huge, okay? In fact, if you can truly get this, you

can run ultra HD video, which requires 100 megabyte per second or so.

You can run many, many channels of HD over the air on your cell phone, LTE

cellphone, okay?

If you can truly get applications layer useful through put at over 300 megabyte

per second, okay? But in practice, what we get is the

following. Still restricting ourselves only to the

physical layer, and only to the link between the end-user device and eNobe,

okay? Not talking about any other overhand, not

talking about any other non-ideal network conditions yet.

Just this very simple case where you should get very big

number. Let's look at the practical number.

Usually, we actually get something more like twelve symbols per carrier, and the

overhead per carrier is like two symbols, okay?

You multiply it by twelve carriers per frequency.

The overhead in things like channel estimation tend to be bigger than ten.

Say, 20 symbols and practice for a mime or four by four channel.

And then, you multiply by the number bits per symbol which instead of

two to the six is often two to the four. That is sixteen plan because the channel

is not good enough or because too much interference,

non-ideal interface condition that forces you to talk slower with a lower auto

modulation. So, you multiply by four instead of six

bits per symbol, okay?

Then, you don't actually have 100 frequency blocks because you use so

called two way communication based on half duplex,

okay? Half of that goes to the one direction,

the other half goes to the other direction.

If you use time division multiplex, sometimes you get 60% of the frequency

blocks for down-link, 40% for up-link. Let's say, we're talking about down-link

from eNobe to you, okay?

That would be 60, right?

So, it's not 100. And then, you multiply by the coding

gain. And for the channel that we're dealing

with, we may only be able to get say 70% efficiency.

Then, you multiply by the MIMO multiple antenna gain.

You may have a four by four system, okay? Four transmit antenna, four receive

antenna. But they may be placed so close to each

other, and the air in between them may be such

that you don't have four independent channels.

You actually only have two independent channels.

So, you got a factor of two instead of four.

You carry out the calculation and you see a number that is [SOUND] 28,000 bits for

each millisecond. So, you divide by one millisecond, that

implies you get 28 megabit per second [SOUND] rather than 315 megabit per

second. Now, 28 megabit per second is still very

fast. If you can still actually get 28. in your application layer throughput,

useful throughput, that is great,

okay? That is faster than most Wi-Fi we are

used to. So, can you get to 28 megabit per second

for application layer useful throughput? Well, probably not.

Okay? We have not add much beyond the physical

and then lave the Mac layer. This is already 8% of ideal number.

Let's add a few more. For example, more interference among

users at peak hours. Upper layer, layers,

11:44

They all have their own overhead just in the header, okay?

So, you've got from 315 megabit per second, down to 28 or so just by physical

layer. And then after layer two, all the header,

you know, down to 25, okay?

And after further overhead in terms of header and semantics and the control

signal, and after further back on network

non-ideal situations, you can easily get another factor of two to five reduction.

That means, you know, finally are getting down to something like five to ten

megabit per second that you can experience say, in downloading a bigger

PowerPoint email attachment. Not 300 and fifteen, but five to ten.

And you see that this is like 3% of the advertised speed,

okay? Now actually, if you can get to ten megabyte per second, that's not bad at

all. You can run a pretty good experience of

video, okay?

You can do many other applications on your phone.

So, LTE compared to 3G is still great, okay?

You just have to compare apple with apple, orange with orange,

okay? You can either compare LTE's physical

layer ideal condition just between device and eNobe with the same kind of number

with 3G. Or you can compare the useful throughput

application layer for LTE with that for 3G,

okay? As long as you do a fair comparison, you

can see that 4G LTE is indeed much faster than 3G.

No doubt about that. What is tricky however, is that you can't

just take these kind of numbers and say, oh, I'm going to get 300 megabit per

second. it's going to be so fast in downloading

everything and I can watch ultra HD movie on my iPhone 5 or something,

okay? You have to say realistically by the time

you experience in the application layer end to end in a normal time of the day,

you're going to get down more like ten megabit per second.

Which is still pretty good for like a normal video.

But not HD, and way, way less than something like ultra HD.