KIRAN:  Hi.  I'm Kiran and I'll be talking about how Wi-Fi has 
gotten faster over the last 20 years.  When the first -- when 
the IAAA put out the first Wi-Fi standard in 1987, it had a max 
upper speed of 20 megabits per second.  The newest amendment is 
a new transmission of 3.39 gigabits.  Per second.  That's a 17 
fold increase in the last 20 years.  We talk a lot about Moore's 
Law, and its two times transistory density.  So it's a grab bag 
of magic tricks that we've thrown together to make this happen.  
So this is how Wi-Fi works.  We get data in, and we get 
estimated data out.  So the first thing we do is compression.  
This is how JZITS works and then you run this through channeling 
algorithms and then this throws in redundancy and error 
correcting.  It's kind of thinking about uncompression where you 
take your data and think about it three times over.  But it's 
not quite.  It's more like complicated checksums that sends the 
in the end.  So once you have the bits you're sending, you take 
your bits and you, like, mess with the sine wave to turn it into 
information you can send.  And then you just mess it up 
completely.  You send it over the air which is a totally noisy 
channel.  There are people talking all the time.  There's 
microwaves which are the worst.  And then you repeat the whole 
process in reverse.  You demoed late, you decode, you 
decompress.  So I'm going to concentrate on demoedlation and 
compression.  Which is the least programy part of this but this 
is where we've seen most increases happen.  So let's talk about 
why this is hard.  So there's a theorem that says, for a given 
noise contamination there's a maximum rate of data transmission 
that you can send error free.  So this, the bandwidth, the 
channel capacity in bits depends on your analogue bandwidth.  
The number of frequencies that you can talk over and your signal 
to noise ratio which is the signal that you're transmitting 
compared to the background noise and so on.  So we've been 
squeezing out both variables.  So getting more on with 
bandwidth.  If you have more space, you can encode more 
information in.  And so let's do a quick overview of modulation. 
 You may have heard of AM, or FM radio, amplitude modulation, or 
frequency modulation.  The method Wi-Fi uses, you adjust your 
phase in amplitude in order to encode a different number of bits 
in.  So RF engineers represent this as a constellation diagram.  
You can see that as polar coordinates, as a phase-off in this 
direction and then the amplitude that you're changing.  So the 
way that we've been doing is to make that information denser.  
You can have different values for each of these things.  So we 
saw a 8-piece constellation here.  This is 16 and we've gone up 
to 256.  That's super density.  That's 16 bits per symbol that 
you're sending out in a four nanosecond per second space and 
it's crazy that we can actually detect those changes at such a 
small level.  So this again, noise.  Noise is hard.  And noise 
knocks out the symbols that you see into different places and so 
the separation between these symbols gives you your noise 
tolerance and if you can't tell what the symbol originally was, 
you can't really decode it.  So what happens -- we use something 
called link adaptation to do this.  If you see that your 
channel's especially noise youy, you can back up and make your 
modulation less dense which is one of the reasons that as you 
back away from your router, things get slower and slower and 
slower, you go down to a lower modulation.  So these conditions 
are highly dependent on your signal to noise ratio.  So another 
way that you can optimize more bandwidth is by having more 
bandwidth.  So you take a 20 megahertz channel and you can split 
that into 52 streams that are talking independently at once.  
The newest draft protocol uses 186 megahertz channels so you can 
send things eight times as quickly.  So your transmission speeds 
increase almost linearly with your bandwidth because you can 
just split it up into more channels.  The issue here is 
interference.  You can think about slicing off a piece of 
bandwidth for yourself as getting a table at a restaurant.  
There's a lot of stuff going on at 2.4 gigahertz.  So move up 
into the spectrum to five gigahertz where not many people use 
it, or up to 60 gigahertz, which is the wild west, there's no 
one up there, partially because there's no one with it.  So we 
might see seeing faster Internet but you can't use it, like, 
across your five-story house which I wish I had.  So we'll talk 
about getting more out of your signal to noise ratio, as we were 
talking before, your signal to noise ratio depends on how 
tightly you can modulate things.  So basically if you're 
speaking louder, you can speak faster and we can't fix 
background noise.  There is noise in the air and our signal 
power is limited by the FCC at 1 watt maximum.  
So you can't really, like, keep sending more information out and 
turning up the power partially because you might mess up with 
people in your area, and partially because of FCC.  And so we'll 
talk about tricky tricks about increasing our power in localized 
points instead of the average power.  One of these is 
beamforming.  It's kind of like throwing stones in a pond and 
seeing this warped checker board pattern go out with different 
power levels.  And so I'm going to use quantum mechanics to 
explain this because you may have seen the double slit bit.  
It's silly relying on that as a tool that people understand... 
but, you can use this interference to double the intensity at 
points.  So you can see here that there are -- this is the same 
intensity at both points.  On both diagrams but there are some 
points where you have double the power and some points where you 
have no power.  And so you end up with a watt on average.  And 
this is directional.  By changing the phase difference between 
the two antennas, you can send it out to particular points.  
This is kind of like how your ears hear direction where one of 
the transmitters, the receiver sends out a synchronization 
pulse, and the transmitter listens on multiple antennas and 
shoots a signal back with a phase offset.  They were kind of 
like yeah, people will figure out how to do feedback for 
beamforming.  But now it's something that's baked in.  And so we 
might see this soon.  And that's why you have antennas on your 
routers, it's not because they look cool, but that's one thing.  
And I promised you some cheating of Shannon's Law.  And as I 
said before, it's not really sheeting.  You just account for 
more variables being thrown in.  So this is another form that 
you can use multiple antennas to deal with things.  This is 
called spatial multiplexing, and you hope that the channel's 
different enough for each transmission because of things that 
are bouncing off of places that you can't resolve where the 
stream is coming from at the original point.  So one way to kind 
of think about this is, like, your -- is like the lens in your 
eye being able to distinguish between things at multiple places 
and seeing streams coming from multiple places but this is 
really wild and the math is crazy.  And this only really works 
in some very small cases but this is one way where you can send 
multiple streams out at the same frequency, using the same 
channel and still be able to resolve it at the end.  So this is 
what exists now in the protocols we have now.  In the future, we 
might see more crazy things.  One of the most exciting things to 
me is multiuser MIMO, which is multiuser using many antennas.  
This is kind of like the beamforming that we saw before where 
instead of -- but here instead of talking to one station at a 
time, you can talk to multiple stations by relying on 
beamforming to increase intensity at some points and null at 
others.  So I can have four conversations with four people at 
once by directing who exactly I'm talking to, and expecting it 
to null out elsewhere, which is insane.  We're going from being 
able to use one antenna at one time to one person, so utilizing 
the channel to talk to many, many people.  And the other thing 
that I was alluding to earlier was having more bandwidth.  If 
you go up to 60 gigabits which was what was proposed, you can 
still carve out to two gigabits and still be okay, and that's 
still a huge bandwidth increase both in the analogue and digital 
sense.  So, thanks.
[ Applause ]