>> Jin Li: Hello. It's a great pleasure for us to have Dr. Sundeep Rangan from
Polytechnic Institute of NYU to come here and give a talk on femtocell,
interference coordination and next-generation wireless systems.
Dr. Rangan got his bachelor degree from University of Waterloo in Canada and a
master and PhD from UC Berkeley. Then he basically did the post-doc with
University of Michigan, Ann Arbor. And he co-founded the Flarion Technology,
which is a spinoff of Bell Labs. And they pioneered the Flash OFDM, which is
the first US use of OFDM in cellular communication technology. Flarion
Technology get acquired by Qualcomm. And Dr. Rangan then served as director
of engineering in Qualcomm, leading OFDM effort.
And recently reason he just joined Polytechnical Institute of NYU. And let's hear
what Dr. Rangan can share with us on femtocells and other topics.
>> Sundeep Rangan: Okay. Thank you, Your Honor a lot, Jin, for the
introduction. This is of course my second time here at Microsoft Research. I
was here before with Qualcomm. Now I'm here with a new Microsoft template,
Power Point template.
But the talk is still on wireless although it's actually today I'm going to talk about
femtocells. Now, Microsoft of course is not in the business of producing
femtocells itself but the motivation behind femtocells namely a massive increase
in the amount of wireless data traffic is a really of interest to everybody involved
in information and communications. So that's why I thought it would give a talk
about that here.
So let me -- I'm going to just start off the talk with the motivating trends behind
femtocells. Now, femtocells really offer a way to deploy networks much cheaper
and possibly address the issue of this massive increase in data. But there's two
basic challenges in getting femtocells to work. One is at the networking layer
and the other is in interference. I'm only going to just briefly talk about the
networking aspects.
Just to give you an idea of the femtocell concept itself, but also the way that the
network architecture revolves has some impacts on the interference. The bulk of
the talk will be on the way that we handle interference coordination.
Now, interference of course is really a central problem in any wireless system,
any time you connect more than two devices they interfere with one another.
So I wanted to start off with a bit of a historical perspective on the way that
interference is managed in cellular systems and to a lesser extent I'll talk a little
about WiFi. And in particular what we'll see here is that we really need to revisit
with femtocells how interference is managed. And in particular I want to revisit
the issue of frequency reuse which is really the central principle behind the way
that interference is managed in cellular systems.
Now, when we -- the sort of researching aspect will be in these two last subjects
here. And what I want to take a look at the interference, I want to break it into
two problems. One is just very briefly talk about femtocell and femtocell
interference and then femto and macro interference. And if I have time what I'm
going to do, this is -- this is all still somewhat of a speculative came. It really
needs the conclusion here is validation. And validation actually with applications
of actual application. That's where I think the interaction are Microsoft will be
interesting.
And I'll just talk a little briefly about how we might try to go about validating some
of these concepts in a potential test bed. Okay. So let me start off with what we
say is the wireless data crisis. So as all of you probably have experienced right
now is that we're seeing explosive demand in the amount of wireless data use,
particularly with the iPhone, but just on virtual every device is being realized right
now could potential benefit from Internet connectivity. And the result of that is
just a huge potential increase in the amount of demand that wireless data
devices are seeing.
So this graph here, which you'll see show up in lots and lots of presentations, is
and estimate. Okay, you could take these estimates for what they are. From
Cisco. Which estimates as much as like a 38 fold increase from where we are
now to where we are just by the year 2014. And you could see that the bulk of
that come from a variety of sources. And some of this is fairly recent that we've
come to learn is that the bulk of it is coming from mobile video.
The actual source of it won't be that relevant for the talk here, but this will give
some understanding. It's good to have some understanding of where it's coming
from. What was anticipated that would be a big hawk both in the wire line and
wireless is P2P. That does consume a lot, but it's actually video that's
dominating.
Now, actually I used to be at the startup. And when there was a startup these
kind of hawky six curves were always great. You would go to a venture capitalist
and that's a little point here, said okay, if you give me money I'm going to like
expand your money by this huge amount here. But this is actually not what
operators want to see because this is the amount of demand, it is not the amount
of potential revenue. So what operators are facing here is a situation where
there could be an enormous increase in potentially in data but it's not like the
revenue from that data can increase that much.
How much more revenue are operators going to get from data services, maybe
20, 30 extra dollars for an iPhone subscription a month? Maybe that's about 50
percent increase. Maybe that could even go up to say 100 percent increase.
That's nothing like what you're getting here, a 38 fold increase. So what's going
to have to happen to bottom line conclusion is you're going to have to decrease
the cost of delivering data per bit down dramatically. Usually somewhat orders of
magnitude. Now, that's why they call this crisis.
In fact I've seen this graph with various things. One graph said wireless data
apocalypse and even be more melodramatic and arrow saying you are here.
You can [inaudible] maybe that's like too much but it's definitely a situation that
all operators are actually incredibly worried about. Now, the only saving grace is
that we've actually been through this before of trying to massively increase
wireless data in the past. And if you take a look at let's say the last 20 years, you
know, going back to about the beginnings of when GSM was just being deployed
in Europe, actually the amount of wireless data capacity is actually increased
4,000 fold. You know, depending on how you count the numbers. Since that
time.
Now, how did -- it's useful when you want to come up with that last 38 of thinking
about how to -- where did that capacity could potentially come from? So capacity
you could think of that very simple formula coming from three different sources.
One source is that you could increase the number of bits per second per base
station, right? So that's really this spectral efficiency. Now, all sort of -- or most
academic researchers work on that number, all right? So it's just increasing, you
know, doing all the optimizations that's my multiple antenna technologies, you
know, better turbo codes, maybe better interference mitigation. But all these
technologies contribute to this.
So how much is all this academic and fancy techniques work and all the
mathematics? Well that's not an insignificant amount but seven. Seven out of
that 4,000, all right. And that seven is extremely hard to get. If I've been in the
standard space where, you know, an increase of what, half a dB, which is maybe
10 percent, is actually very significant. Now, can we get another -- can we get
the bulk from this increasing disk? Probably not. I mean right now we're already
running very close to capacity. I think even another seven would be extremely
doubtful, all right?
So it has to come from two other things. Now, the two other things are actually
relatively dumb. One is you can just buy more spectrum. And that actually
counts for about 10 times 10 of that factor 4,000. But that too is probably by, you
know, there's only so much we can buy there. Although new possibilities like
things with cognitive spectrum might open up more. But I will talk about that a
little at the end. But the bulk of the increase has really just come from buying
more base stations, which is, you know, what cellular operators call cell splitting
because they're basically taking a large cell and they're putting lots of smaller
cells to create smaller there. So that is the bulk.
So what's the lesson of this? If we want to get to our factor of 38 to meet Cisco's
demand then really we have to figure out a way to build base stations much,
much cheaper. And that is what's motivating femtocells.
Femtocells are essentially small personal base stations and which are -- so an
example of this femtocell is shown here on the right. This is one that is already
actually in production. There are several vendors. This is one from Huawei.
And they're basically things that look about the same form factors you get with a
wireless access point, bulk significant on the dimensions of the top of your
desktop.
Now, femtocells are actually just part of a trend. This desire to get smaller,
cheaper base stations has been well known in the wireless industry. So if you
look at a traditional macro cellular tower these things would go maybe 30 meters
high with a large number of [inaudible] at the top. I've already been in decrease
well before the introduction of femtocells. For example this is what's called a
Nokia MetroSite base station. This itself is a few several years old. That's about
maybe one, one half meters high here. So these were already going to a smaller
form factor. Femtocells are reducing that form factor much smaller and offering a
much cheaper way of deploying these base stations to try to get to you those
order of magnitude numbers that we need.
So the femtocell concept is not just different in the forum factor, the other
important part about it is that it's deployed by the consumer in the consumer's
premises. So the idea of the architecture just in this sort of cartoon here is that
you put a femtocell just like you would put a attach a wireless access point into
your home and it would connect through your personal cable modem provider or
whatever your local ISP is. And ultimately through a gateway into the macro -into the operators backbone.
But the important point about the femtocell is that it's using the operator's
spectrum. So you're using the operator's spectrum but the device that -- with a
small device that you've put in the backhaul yours. So you can see here where
the savings are potentially coming from for the operator.
First of all, the cost of the femtocell even if they have to subsidize is a much
smaller device maybe about the size you know -- it can be in the order of like one
access point. It's a few hundred dollars as opposed to a macro cellular giant
tower is maybe -- although the costs have come down, they're still probably
about 50,000 or something.
The second part is that the actual back haul eliminated. You're subsidizing that
yours. The third part is that the actual core network is completely removed. All
the traffic can potentially go through the -- through your backhaul and then into
the public Internet with only a minor amount of measurement coming in through
the operator's network. And then the fourth part is just the deployment of it. A
large part of the cost of running a big cellular network is the amount of tuning and
the installation of the cellular towers, and that is all eliminated because you're
actually deploying that yours. Yeah?
>>: So you [inaudible] I can't use it, right?
>> Sundeep Rangan: Yeah. But then you have to hand over.
But if you could make that so you'll hand over into the macro cellular network.
>>: So now [inaudible] very similar to having this WiFi box currently at home it
talks to my broad band router and sends everything out of ->> Sundeep Rangan: Okay. Yeah. So that's a good question. So the question
is what's really motivating the difference here between -- it's potentially -- it's
actually debated there's a lot of people that are saying that the femtocells just
won't pick up. Because anyway, WiFi's already a perfectly acceptable situation
in the home.
So I think that the only real -- and you know, I think that it's not clear to me which
one will actually win. But I think if one's going to make an argument for
femtocells they're really of two forms. One is that if the wireless capacity in home
was to get to such a high extent that even WiFi was getting overburdened, which
it isn't the case right now. Few people actually experience any practical data
problems with WiFi networks. Then femtocells might. Because they're operating
on a licensed background with a much more efficient cellular standard they could
potentially offer higher data rights if that's a problem. I don't think that argument
is really that valuable from a customer's perspective until you know maybe if
everyone starts streaming HDTV or something in their home over WiFi, maybe
then it will start to push the limits.
The more important aspect is this, that when it's actually outside your private
home if this is going to become a model that the operator will provide in public
spaces like in what may be called picocells or something like this where it's a -you know, at home maybe you go to WiFi but what do you start to do when you
start to roam and try to, you know, go into a suburban minimal or outside the
house in a office building in these areas? And then once it has to become
managed by the operator, then the femtocells become much more attractive.
>>: [inaudible].
>> Sundeep Rangan: Okay. So that's a good point. So what are you saving
then? Now look at from the operator's perspective. It can either deploy a whole
bunch of WiFi access points or it could deploy a whole bump of femtocell access
points. Say the cost of the femtocell and the WiFi access points are comparable
or maybe even the femtocells are just slightly more expensive. So maybe -- so
what you're buying from getting the municipal WiFi is getting maybe a slightly
cheaper device. But what the advantage of femtocells are is because it's -designed from the cellular perspective we'll have much easier coordination with
one another and so the cost then can actually dramatically drop from the
operator's perspective over WiFi.
Also, things like mobility and other stuff are much better operating in a cellular
technology. So that's really what the most basic using femtocells potentially are
over municipal WiFi. Because the dominant cost to the operators is going to be
actually on the management of these devices much more than the actual cost.
And anyway, the cost of femtocells is probably can be made comparable with
what it is for WiFi. There's other benefits also.
You know, not that these can't be solved with WiFi. When you're looking at a
mobile solution then you really want things like power saving, roaming, paging,
stuff like that. And those things are actually right now they run much less
efficiently on WiFi. For example, you know, cellular technology is much, much
more power efficient, even if -- even in active mode but then particularly for
things like sleep. And so that's -- that's all those aspects. Once you consider a
system where you want ubiquitous coverage which is what's obviously becoming
the tracking for devices like the iPhone and stuff, that's when femtocells or pee
co-cells or things that are operated in a potentially public domain become
actually an attractive solution.
Okay. Yes?
>>: So does femtocell cover larger area than [inaudible].
>> Sundeep Rangan: It will probably -- think of it as a -- in principle it could, but
they'll probably run about the same power output as a wireless access point. So
probably comparable the distance. Probably get a little bit better range because
actually the cellular technologies have -- can operate in much lower SINRs than
wireless access point. Wireless access points probably -- you know, the lowest
coding rate's around like 1, 2 dB, and because they actually don't handle fading
that well they tend to clap out actually at much higher SNRs than that.
And cellular technology been designed from the beginning to work into very, very
low SNR regime, so you can get a -- push out the range a little bit better. Okay.
Also, there's other issues you can -- you know, in a wireless access point the
minimum amount of bandwidth you have to transmit is over 20 megahertz and in
a cellular technology, you know, because it's been designed to handle things like
voice you can access a very small portion of that bandwidth if you're needing a
low rate connection. So the range would definitely be better.
So you know you could argue that because it's a cellular technology it's more
advanced, that you'll get just a general link layer improvement. But again, I'm not
so sure that's a big enough selling feature itself to -- for people to adopt wireless
to flip out WiFi access point. It's not like people complain that much about the
range of their WiFi devices.
>>: [inaudible] try to more money out of me because I'm using a spectrum. But if
I am building the [inaudible] is not there, I am not willing to pay more because
WiFi solves my purpose.
>> Sundeep Rangan: Yes, absolutely. And you know I don't really think that -- I
mean, operators are trying to do this right now with the way they're deploying
femtocells. I don't think it's going to fly, and the numbers are showing it. If you
actually tried even a nominal amount for people to buy a femtocell, people are
probably not going to buy it. The only case where people are going to buy it is
they want to get cell phone coverage for their voice service in home. For some
reason their house isn't getting a good coverage so then they put one of these
femtocells in.
And, you know, that has a limited amount of value, and it's not really the data
then is an issue. So I don't think -- you know, I think the reality is that if operators
-- this following's to be deployed, that I will have to give it away pretty much for
free. But, you know, if you believe the story that it can offer this tremendous
amount of increase in data and offload it, you know, capacity operator's networks
then they should give it away for free, which I -- you know, if femtocells take off,
that's what will probably happen.
Okay. So a little more details then about femtocells. So we've talked a little
about this. So if you compare sort of macrocells with femto or picocells you can
compare them in a number of different categories. So one aspect is just the cell
size. So cell size is, you know, in its typical suburban environment is around
right now for macro is about half a kilometer, two kilometer site difference
although the specs usually allow very large cells for, you know, rural areas, like
up to 60 kilometers.
Femtocells you're looking at their coverage that's much more like a WiFi area,
even if they were to be deployed publicly. So typically less than about 300
meters probably often need much less than that.
The deployment -- so the first part where we see the big cost savings is the
deployment. So the macro cellular model for deployment is that it's of course
planned and managed by the operator. Now, if you've actually been to an
operator you know the incredible amount of effort they -- painstaking effort they
go to deploying a macrocellular tower. When I say plan, it means the following,
that you have a bunch of RF engineers who do a very detailed similar RF
simulation to pick where the cell side is going to be placed. Then they have a
real estate team which goes out and actually negotiates a contract to actually
deploy the antenna somewhere. And then they install their services on that and
go through an elaborate process of downtilting the antennas, driving around and
testing it to get everything perfect.
I've been doing that myself and it's just incredibly painstaking process. The
femtocell comes much cheaper. It's deployed much just like you would deploy an
access point. So you open up a box, your customer installs it, plugs it in and
deploys it wherever the customer wants. So that whole cost of deployment is just
eliminated from the operator, which is in a huge part. Even if the operator were
to maintain this in principle it will have a much more plug and play kind of
deployment model at a greatly reduced cost to the operator.
The second part -- the part that is the same though is the on the air itself, that the
spectrum and actually the air interface technology for the most part are
completely the same as the cellular part. So it's deployed by the customer but
it's using the operator's spectrum.
Another difference of course is that the macrocell tends to yield multiple sectors.
So what it will have, you saw that picture actually of the tower. So what happens
is that the towers actually radiate -- they have several antenna elements, usually
three radiating like 120 degree arc into each sector. So that's to get improved
efficiency.
Here to just make it cost is just a single sector on the directional antenna. The
backhaul is the second part picture sector. The big cost to the operators is the
backhaul. And it was already a cost when they were running just these 3G
systems which run maybe a few years ago anywhere running up to only about
two megabits per second. The 4G systems and even the later releases, the 3G
systems are pushing over 100 megabits per second in principle. And those
backhaul costs would be tremendous. And it's not clear where the money's
going to come from.
If the -- in the femtocell model, that can in principle be entirely released so that it
could just go to say a subscriber's local ISP. The RF, in terms of the actual -- the
other part that's reducing cost is just the form factor, as I said, and just the overall
cost. So if you look at the raid frequency components in a macrocellular tower
you're talking about like a 20 -- for usually about a 20 watts per sector, so with
multiple sectors it could go up to a hundred watts.
And, you know, if you actually see those things, they're just giant like refrigerator
cabinets apart, boxes. And the reason is that to get that and pump that amount
of power you need whatever power amplifiers, large power amplifiers, like a PA
will be about you know maybe this by that big. And it's actually pretty heavy.
You'll want another one on the receive side of the low noise amplifier and they
stick in a rack. And then there's all sorts of other costs that come associated with
that.
You need cooling trays, you need very big cables because you don't want to
have any cable loss when you're trying to run this kind of power up to your
antenna.
On the other hand, a femtocell is usually much smaller, .1 watts to around maybe
1 watt at the absolute most. So it's kind of similar to what you get on a wireless
terminal. Which means, you know, if you look at that as a single RF chip it
comes very cheap, maybe a few bucks that can get installed. So it's just a
radically different part.
The other part of the cost savings is that the macrocellular environment is usually
servicing hundreds of users usually for voice, and that comes with all sorts of
complications, complexities. The femtocell is much smaller like a WiFi access
maybe around 4 to 32. And 32 is typically -- that number is fairly high. On the
other hand you're not sacrificing on the data rate. You still want to get those
peak data rates up there. Yeah?
>>: So from a user's point of view, would it save battery life ->> Sundeep Rangan: Over WiFi, definitely. On the terminal side it wouldn't
because -- well, at least right now, know, I dope -- there are probably fixes in the
.11 spec but the .11 spec is just -- it's actually very, very battery intensive for two
reasons. Wire actually transmitting it's just a very inefficient actually mechanism
to transmit. You're transmitting over very wide band where you have to
constantly be doing these chip level searches which actually guzzle batteries and
battery powered cell. But then also when you go into a sleep mode it becomes
even more expensive in WiFi because cellular technologies have very, very
highly optimized sleep for battery life so that you can keep your phone on and it
can pick up a call any time. So they have very efficient ways to just wake up for
a very small amount of time, check if it's being paged and then go back to sleep.
Those mechanisms, they can be added into WiFi, but they're not -- you know,
they're not -- they still don't get the same levels of efficiency that you would get.
>>: That's interesting but my question actually was [inaudible] WiFi and
femtocells but [inaudible].
>> Sundeep Rangan: Oh, sorry. Sorry, sorry. Yeah, you would save batteries
to some degree but not much. You would save the battery power in that you
would be closer to the base station. So that way would get -- but maybe not that
-- it would be significant -- the bulk of -- on a modern smart phone the bulk of the
battery drain's actually on the displays more than the -- the bulk of the battery
drains off the display more than the RF component. But even on the modem
actually at the talk time it would decrease it a little bit by dropping it. But even
that -- maybe -- maybe if you're -- I'm not sure what the number is. I'm going to
hazard a guess like 20, 30 percent or something like that if you're right beside the
access point versus away.
>>: [inaudible] coverage to the pee co-cell versus the macrocell? I mean, if let's
say picocell really take off [inaudible] coverage space?
>> Sundeep Rangan: Yes. So coverage is just a matter of function like the
number of -- picocells you can deploy. So your total data rate is, you know ->>: Because picocell may not be planned that well, right, I mean [inaudible] I
mean it may create holes ->> Sundeep Rangan: That's a very good point. So what's going to happen is -so the fear is that because of course you're making a tradeoff you put out
unplanned deployment and that's what I'm going to talk about in the rest of this
talks. Because you're doing unplanned deployment so now you can get many
more base stations. But how much is the capacity per base station go down. Is
that your question? So that's what we'll talk about in this talk. And the basic
conclusion is that there is going to be a loss but if we do the right things that loss
is not so bad. And that, you know, that loss is not so bad and we can handle
those sort of smaller corner cases where it does -- fails very badly, and in which
case then we'll still get a lot of the benefits from these radically cheaper base
stations.
>>: [inaudible] and maybe some of the data rates including home technology
dependent of operator involvement.
>> Sundeep Rangan: Yeah. But because it's -- it has to have operator
involvement because of these [inaudible]. Although many of these ideas if they
are popular from -- if there's enough benefits on the link layer side then people
could try to think about new ways to do unlicensed background. I know. If those
ideas on the air interface side are actually good, then you could think of new
ways to do it [inaudible] benefits themselves. Okay.
So those are the benefits. But here's sort of a high level understanding of where
the challenges are to get this. And so a lot of challenges are coming from
basically the fact that this is almost like an ad hoc deployment. Now, ad hoc of
course computer scientists usually mean sort of distributed routing. That's not
the sense in which I mean it here, just the sense of unplanned deployments
without the careful planning that an operator usually decides.
And there's really sort of two or really three ways in which these kind of manifest
themselves. The first is the interference environment itself. So you have, as I
said, it's an unplanned deployment. But also there's just the interaction between
the macro and femtocell to get the -- no one is going to deploy the femtocells on
dedicated spectrum, at least not initially because the demand is not there yet.
And anyway, it would be very spectrally inefficient to do that. So they're going to
be sharing the same spectrum as a macro and the femto.
And when you try to do this as a fundamental difficulty here because there's a
large power disparity between the macro and femto, and I'll talk about that. The
other difficulty though makes femtocells particularly challenging is what's called
restricted association. Our key principle in the way that cellular networks are
deployed is that the terminals can connect always to the closest base station.
And that's what makes the cellular networks work. That may not be the case in
the femtocells because you may not want somebody else to connect to your
femtocell.
And as a result, you have this restricted association, and that creates many of
the really difficult adverse interference environment.
A second problem just is really just a need for self configuration, as I said. If
we're going to get savings, these things have to be just kind of plug and play.
And that has issues both with terms of synchronization at just a physical layer
because we need for example to -- need synchronization to smooth the
handovers between the macro and femto, but then also some kind of
synchronization for interference mitigation techniques. Plus you also need other
parts which I won't talk about in this thing, just for actually the way that there are
-- devices are managed through the operator and so on.
And the third part is really just the networking aspects because no longer -- in the
previous part the networks were complete under the operator's control. Now it's
a mixture of a network partly going through a localized, partly going through a
public Internet and then finally the operator, then how do we try to do that. So I'll
only talk a little about this.
I just want to talk a little bit just about the standardization efforts. Yes?
>>: [inaudible] so when you talked about the [inaudible] of femtocell outside the
house, is handover a new challenge because now it will be more frequent as I
travel?
>> Sundeep Rangan: Absolutely. Absolutely. But on the other hand, where
you're going to likely have the density of it will be probably in areas where the
velocity will be smaller. But definitely the handover is an issue -- the handover is
an issue partly because of the -- partly because of the smaller cells so you'll be
hang off more frequently but then also the issue that you're handing over over
multiple difficult networking domains, you know, potentially handing off over one
percent femtocell connect into one ISP into an femtocell into another ISP and so
on.
I won't talk too much about that, but that's definitely an issue. Okay. So I wanted
to do this slide here just because I'm going to be throwing around some of these
acronyms. And by the way, I -- unfortunately this field is just filled with acronyms.
So if you ever see an acronym that you don't understand, just raise your hand
and I'll tell you. So femtocells are really just part of an evolution which you can -and it's just good to see it in the evolution of cellular systems more generally. So
generally femtocells -- cellular systems are characterized into what's called you
know 2G, 3G, and 4G. So 2G systems generally refer to the first digital voice
systems like back into GSM which were narrow band systems.
Now, I put a range of data rates. That's actually -- that range is really the
beginning number is sort of the original date rates into the first releases of this
pack. And you could see all these systems, you know, it's not like it ended its
evolution here. They're continuing to evolve. So you know the GSM/EDGE
systems now actually get -- actually that number is even old right now. They
have like a two megabit version of these -- or EDGE systems. In 3D systems
which are systems that are really the first generation of -- really the first data
based systems where actually the first actually offered packet services. And real
broad band data rates.
So those began in around 2000 and, you know, they were originally around about
384 kilobits but now in the later releases we'll get, you know, very high data
rates. Finally four generation systems are what's call long-term evolution and
these systems are really offering allegedly higher data rights than that and lower
latency. Now, I wanted to put this here because a lot of the -- I'm going to talk
mostly about femtocells in the context of LTE. And when LTE was actually in the
planning stage back in about 2005, a lot of these -- a lot of the benefits that
people thought were moving from -- LTEs and OFDM base system as opposed
to a CDAA system. A lot of the benefits that people really thought of that LTE
would have really not become really that valuable.
One of the benefits here was a very high data rate. Well, the very high data
rates, first of all UMTS actually was able to increase its data rates within the
system itself. And anyway, those very high peak data rates are not really that
possible in a macrocellular system, those are only possible in four antennas.
How many devices have four antennas. And you know in your situation that
you're very close to the base station which is not going to happen.
Now the low latency, sure the low latency is good. But anyway, UMTS dropped
down its delays down to 30, 40 mill second or maybe that delay isn't that much.
Where the benefit though from LTE now has become increasingly realized is
actually much more into femtocell context. So and that wasn't something that
was necessarily obvious when people started to design the system back, you
know, in 2005. So a lot of these benefits you could see are actually very
valuable in the femtocell context.
Okay. The data rights are not valuable in the macrocellular because you're not
going to get them but factual in a femtocell case it is very possible to -- it is very
possible to get these very high data rates in that environment.
The other aspect of it which is always on IP connectivity and actually more
generally just an IP, all IP back all, that actually will see here really facilitates the
easy networking between a local and private ISP and a public ISP. And the other
part about LTE which we'll talk about later is on terms of the actual interference
coordination. It's really in that domain, that LTE really excels over CDMA
systems because of a key concept which we will talk about as subband
scheduling.
So while LTE really was promoted and initially pushed as a way to try to increase
capacity in the macrocell environment, it's gains are not that significant. But
actually it's in the femtocell or just more generally in a situation what's called
heterogenous networks where LTE will really excel, and that's why I want to talk
about it in the context of LTE.
Just a little bit in terms of dates here. The actual work on femtocells began
before the standardization about the while -- while LTE was going on. It was
from pretty early on that people started talking about femtocell aspect back in
2006 and 3G in 2007 and 4G. And so as I said now there's increasing amount -a lot of the effort that worked on standardization for future releases of the
technology have really focused on femtocells. There are other groups involved
and I just put one of them here which is called the FemtoForum. So you'll see
some of the references to their work here.
All right. So let me talk just briefly about the network architecture. It's not really
my area of expertise, so that's why I'll skip over it a little bit. But also just wanted
to only talk about just so you got an idea of the femtocell concept.
Okay. So this is one of the alphabet soup kind of slides unfortunately so if you
don't remember any of these acronyms, don't worry too much. Actually if you
see pictures of network architecture for LTE, they're even much more hideous
than this. They have like 10,000 boxes of different sorts. I just picked out a few
which -- to simplify it.
Okay. So the first acronym we're going to learn is the device itself. That's called
the user equipment. I put a cell phone. That could be any PDN or it could
actually be a wireless laptop or anything like that. Anything that's getting
connectivity.
The base stations of a ridiculous acronym called E node B. The E stands for
evolved because the previous generation was called node B for a reason that
makes absolutely no sense to me. But that's the -- that's the name. So this is
the macrocell environment. And the set of them is called the EU TRAN is
another ridiculous acronym, all right, so it's called the evolved universal terrestrial
radio access network. All right.
So that's the base station. Everything behind that is the core network. So in the
core network in the user plane which is actually where the data is coming from,
the two important boxes are called the serving gateway and a packet data
network gateway. The packet data network gateway was the key box and that's
the L3 point of attachment.
So in terms of IP routing that is the first point of connectivity. If you were to do a
trace route after your connection is up, the first box you would see is the IP
address of the PDN gateway. Everything from there would be really you
consider as L2 routing. No what's specifically happening is when data arrives in
from the public Internet, it's classified into what's called an EPS bearer, all right.
A bearer is something which is specific to the UE, and the UE might have
multiple bearers to affect different quality of services for different IP traffic. And
it's mapped to one of these EPS bearers. And then all the routing between the
PDN gateway and the user equipment is based on that EPS bearer. So none of
these other boxes in between here have any concept of the UEs IP address. So
it's all L2 routing.
Now, of course it can be IP routing in parts of this like this GDB tunnel between
the PDN gateway and the base station. But that's actually transparent to this
end-to-end connection. That's just an IP connection between the PDN gateway
and the base station. All right.
>>: [inaudible] some questions?
>> Sundeep Rangan: Yes.
>>: So in the case let's say the UE has an IP address.
>> Sundeep Rangan: Yes.
>>: Is that IP address associated with a gateway? I mean basically who
[inaudible] I mean since you mentioned that most of the intermediate basically
node only have this information [inaudible].
>> Sundeep Rangan: Right.
>>: So who has the information ->> Sundeep Rangan: Only the PDN gateway.
>>: Only the gateway?
>> Sundeep Rangan: Yes, that's right. So you actually -- when you do -- so you
get your L2 connection will come up. Then you'll, you know, a Windows devices
you'll get a media up. You'll run DHCP here. And then that DHPC server is
running in the PDN gateway. So the PDN gateway will give you the IP address.
>>: Okay. [inaudible]. Is that only one or you potentially can have several?
>> Sundeep Rangan: You have several. So you have one for every type of
quality service. So typically you know your Internet traffic will run on say one
EPS bearer but then if you were to offer let's say IMS voice or maybe video
conferencing or something like that, if it was not running as a best effort service
that the operator was trying to deliver, then they would probably deliver it on
another EPS bearer to give it say a higher quality of service.
>>: And that EPS bearer would be un[inaudible] by the base station and all the
basic ->> Sundeep Rangan: That's correct.
>>: [inaudible].
>> Sundeep Rangan: So when that traffic gets routed here, it comes with -- it
says this is the -- essentially the Mac address, okay, though it's not -- it's a
different Mac address than what you would have with WiFi, and this is the bearer
IP for that Mac address. So this is the third bearer, this traffic is on the third
bearer of that.
>>: How is the quality of service of -- end up in those base stations? I mean,
let's say the base station is associated with -- can you use each of them running
->> Sundeep Rangan: Right. Because the traffic comes in here marked with a
bearer and then the bearer will have some quality of service profile so then it
schedule the one track, one bearer's traffic in with relative priority to another
bearer's traffic. So it will maintain a cue for every bearer. So actually there will
be a set of cues for every UE and then within the UE one cue for every bearer of
that UE and then when it does the scheduling it will pick from -- presumably it will
contend to favor higher priority cues over lower priority cues.
>>: [inaudible] router, Cisco routers.
>> Sundeep Rangan: Right.
>>: I mean basically I think basically [inaudible] I mean basically people talk
about basically [inaudible] and basically [inaudible] is basically [inaudible].
>> Sundeep Rangan: Yes.
>>: Say something like that.
>> Sundeep Rangan: That's right.
>>: Is those basically information available online.
>> Sundeep Rangan: Absolutely. You mean for the air link scheduling?
>>: Yes.
>> Sundeep Rangan: Yes, absolutely. So you can do -- very much you can do
those kind of things, they can do maximum weight matching or any of that. So
you have a bunch of cues, you have a priority policy on every cue and then you
can do it. The wireless scheduling actually has -- is a more complicated typically
than the switch scheduling problem because the scheduling decisions you're
making at one base station. And that's what I'll talk about in this talk. We'll of
some influences on the scheduling decisions here because they could change
the interference conditions. So it becomes a more complicated problem. Also
because the channel rates themselves change over time. So there's other things
you can get like what's called; multi-user diversity, you can try to schedule not
just based on priority but when the actual channel is better versus when the
channel is worse and you can play these kind of games. All these kinds of
games which can happen in scheduling you know wireless link that won't happen
in a wire line link.
>>: Well, another question, EPS bearer, this bearer, is there any way to
[inaudible] on the UE or on the whole side can influence the [inaudible].
>> Sundeep Rangan: Yeah. So that's the part here. So the operator will -- how
the traffic will get categorized. Now, the way that it's been done so far is that you
know everything from the public Internet whether you go to YouTube or CNN or
something like this, they are all just categorized as generally a best effort bearer.
In principle, though, and then the operator services if they were to deploy IP
based services, let's say like IMS voice or something, they could classify them on
to a separate bearer.
Now, there's nothing, though, prohibiting the operator aside from things like
network neutrality or something like this. They could say they could make a
contract with CNN and say, okay, if my client goes to CNN service, I will then -you know, based on the source IP address of the server or something, I'm going
to classify that separately and maybe give that higher priority if CNN gives me
some money. So that could happen, you know.
Or that, you know, maybe it can also from the port number recognize that this is
a VoIP application. And if they want to be nice but they probably don't but if they
wanted to be nice to say Skype or something, they could probably pick that up
from doing some packet inspection and then give that higher priority. But maybe
they don't want -- they probably don't want to be nice. Or worse. Or worse.
Yeah. That's right.
Now, that's actually a whole opening open part because another aspect of not
talking about here is a way that these networks is funding -- is probably through a
much more sophisticated -- sophisticated model, pricing model with the content
providers. Most people of looked at content -- the pricing with the subscribers,
but it's not unusually that the content providers could provide a lot of the funding
for these networks. And the operators with this system could leverage their
ability to provide different quality and services by the way it's -- you know,
marked in the PDN gateway.
>>: Basically further question is I know in the LTE there's talking about
broadcast now, right?
>>: Yes.
>> Sundeep Rangan: How is that handled? Is any basically providers deploying
broadcasting mode into the picture?
>>: I do believe -- I do believe it's definitely handled as a whole part [inaudible]
that can be -- that can absolutely be supported. And I believe that the broadcast
will also go into the PDN gateway here and then map to a Multicast ID much like
-- and then there will be some kind of Multicast bearers here.
>>: And then the user basically all the users ->> Sundeep Rangan: Can get that.
>>: [inaudible].
>> Sundeep Rangan: That's right. And then the scheduler when it comes to the
base station to do something intelligent it can try to send the data to the ones
with the -- you don't know the channel quality to all the users and then pick one
that will make it work for all of them.
>>: So are you aware of any providers using that service for -- I mean ->> Sundeep Rangan: Well, LTE itself hasn't been deployed yet. You know, but
presumably once it's deployed it will be, because they're already doing that with
the 3G systems right now. So like Verizon does some kind of time slotted basis
for some things like V cast and stuff go through some kind of time slot so I
presume that it will get -- it will definitely get used.
But you know, as we were talking about in the morning, you know, how much will
be Multicast versus how much will be On Demand. And if On Demand viewing is
the one that, you know, operators are really worried about then ->>: I think you answered my question. It was basically how the PDN gateway
will classify [inaudible].
>> Sundeep Rangan: Yeah. So it's based on something like a [inaudible]
number and, you know ->>: [inaudible].
>> Sundeep Rangan: You know, that's a good question. I think a PDN gateway
could serve in probably the neighborhood I'm thinking of like 10s of -- you know,
10s of base stations. But, you know, what will happen in these network
architecture would not -- new network architects, with hundreds of megabits per
second going potentially to each base station is, you know, it's not clear.
And with the femtocell case then the scaling issue becomes total different now.
Potentially there are hundreds of base stations for PDN gateway. So then we
need something, you know, very different on the network. Okay. So -- yeah.
>>: [inaudible] the address translation that the gateway ->> Sundeep Rangan: Oh, the NAT. So if you get -- say in the NAT would
probably -- suppose you instead of having a user equipment here you then break
this out to have -- so you probably my understanding I would get -- it will probably
much like what would happen right now with a cable modem, you would run right
now NAT at the -- I guess you would run it probably at your cable modem. So
you will get one public IP address from the -- from the operator and then if you
want to break that out you can do network address translation into your private
network. Into your private network at the -- on the customer's premises. The
same would happen here. You'll get from the PDN gateway a public IP -- a
single IP address or maybe they can offer multiple IP addresses if they want to
do that.
And then if you -- in your private -- is that your question? Yeah. So the NAT
runs -- the NAT runs into customers.
>>: [inaudible].
>> Sundeep Rangan: They will -- the specs all spec out IP V6. Now, how much
the operators networks right now are IP V6 I'm not sure. But all these all these
run -- all the protocols are compatible with IP V6. They also -- I'm not going to
talk about it here but for -- to do the -- you also want to be able to do handovers
IP mobility between from non-3GPB to 3GPB and that will -- they'll generally run
either mobile IP and that's also supported either -- they'll either run a dual stack
IP -- IP V6 or do a proxy mobile IP depending if it wants to be networked
managed, and that will also run in -- that also supports V6.
Okay. So that was the -- that was sort of a high level picture of the macro
network. Let's take a look at what happens in the femtocell network. So the
femtocell network what happens is you don't directly talk again with the serving
gateway, instead what you go through is what's called a home E node B
gateway. So the home E node B gateway is really the connection between the
operator's core network and the public Internet, all right.
So you're connected here to your home E node B. It's called the home E node B.
Remember, E node B is base station, so home E node B all right is your
femtocell. So I've kind of drawn it in a little bit different picture here. So there's a
home E node B which will be connected to your local ISP which will then go on
to the public Internet and then go to home E node B, right? The home E node B
you could think of as kind of like a concentrator. What it will do is just
concentrate the traffic, concentrate the traffic from multiple home E node Bs. But
essentially that connection it's essentially transparent. So really it looks logically
like a connection directly to this serving gateway.
Now, I put this up just to show you the picture here. The L3 point of attachment
is still the PDN gateway. Now, what this means here is that your home E node B
when it comes up, it will get an IP address actually from the local ISP. But that is
not again -- you don't see that IP address. So your home E node B gets a local
IP address from the local ISP will then get a connection to the serving gate -after he gets its IP connection there goes to the -- well, it has a -- it has a
connection here to the serving gateway. But again, if you did a trace route, still
your L3 point of attachment is here.
Now, this actually -- so if you were to ping, all right, the home E node B's IP
address, with the way I've drawn it here, you will go through this kind of ridiculous
route. Your L3 point of attachment is the PDN gateway, all right. So you will go
through your home E node B who doesn't even know that -- because he doesn't
expect your traffic that is coming to him, he will just afford it along, la, de, da,
through the ISP, through the public Internet, home E node B, then to the PDN
gateway. Through the PDN gateway then -- no, it doesn't go back this way. This
is a public Internet address so it goes out through here because he's dumb and
doesn't even recognize that it's on his network, right. So that's the way that
would work.
Now, I'll talk about that in a second to solve that problem. But I just want to point
out one big difference, though, between this. So aside from this E node B
concentrate here, one of the things here that you see between macro cells is this
X2 interface. Now, an LTE, they provide two interface. Part of that is just for
handover if you go from one base station to another. But the important other
aspect is for X is for intercell interference coordination. So there's actually
control messages that are going back between the base stations to coordinate
the interference side. You could say oh, I have a UE that I'm trying to serve,
could you try to vacate this party spectrum because this part you know I need
this is [inaudible] and so on.
So all that kind of information is there in the X too. That is no longer possible in
the home E node B because I just thought it was not scaleable or logically
difficult to try to develop and X2 interface. But you've to find the other E node Bs
and so on. Home E node Bs.
Now, because of that, that will present some challenges for intercell interference
coordination. I'll try to address that in the end of the talk.
>>: [inaudible].
>> Sundeep Rangan: That's why. Okay. So just to get around that absurd
problem of course the big issue that, you know, part of the selling point is to
offload traffic off the -- off the operators network. So that, you know, everything
has to go through the operators core network that wouldn't be good. So the way
that it's time will likely be deployed is with one of these variants. One is called
LIPA and one is called SIPTO. Okay. So let's go again. The traditional IP path
would be because this is your home UE if you're wondering what that stand for.
All right.
Normally, as I said, goes through your local ISP through the public Internet and
goes there to the cellular core network and that's your first point of attachment.
So that's if you want to get from say from YouTube you go this route here. All
right? Which is obviously not good from the customer's perspective, it's not good
from the operator's perspective either because the -- you're eating up capacity
here and that's particularly bad because these can be very high capacity lengths.
So instead what they're working on are two different solutions. One is what's
called selected IP traffic offload which is to create a connect -- direct connection.
Now, to do that though you need an L3 point of attach in the home E node B
itself. So they have this version of what's called a local PDN gateway. And
they're still being expected out to try to figure out how to do this, the different
variants of this, but there's one solution here.
That local PDN gateway could also be used for what's called local IP access. So
for example, I put it here a, you know, whatever, flat panel screen here. So you
could imagine you were trying to stream wireless data from, it may be some kind
of laptop on to that screen. So now that could run over this local IP gateway
here. The actual way it's going to be solved, there are a lot of tactical issues to
try to deal with this. You know, just basically a case of how you do handoff, how
you do security and so on. Those are still being worked out. But they will likely,
you know, be part of the femtocell situation. All right?
>>: [inaudible].
>> Sundeep Rangan: No. Really there isn't. That's the point. Really in principle
you could imagine that everybody that would get -- everything could get offloaded
or for control messaging. And like the authentication and you know, operator
management. But in principle the overwhelming majority can drop it unless it's
the operator's providing the services like IMS voice or something like that.
Okay. Another issue I'm not going to talk about it much but it's just to give a
picture of it is the security. Now, I put this up just because the traditional -- in the
traditional macrocell, you know, deployment it's been assumed that everything -the whole backhaul core network, that's everything from the cell towers and all
the network element boxes is trusted and secure. And so actually your data run
unencrypted, all right? It's encrypted of course when it goes over the air but it's
decrypted and is not encrypted anywhere else in the core network. So if you
were to tap into a macrocellular tower right now you would read everything into
clear.
Now, that's not -- not a problem in an operated -- potentially not a problem in an
operated network. And it had a lot of performance benefits by not having to do
the encryption over the backhaul. Of course that's not possible if the femtocell
situation. So generally what they do is they put what's called a security gateway
that would access, you know, between the untrusted public Internet and the
trusted IP network. And then you run actual security, the IP set over this which is
-- so it's actually encrypted at the security and decrypted here. I know, again,
there's just a huge number -- actually the have allocution of security is much like
what happened you'll see in the -- when they began to realize with WEP based
security they added some un-Godly, like 200 pages of spec into the -- you know,
if you read the .11, 2007 specs you'll see like 200 pages of specs on security.
It's actually had a similar problem in 3 GPP like 17 specs just on all sorts of
varieties of problems with security. So obviously I'm not going to address that
here.
All right. So let's go on to the main topic here which is just on interference. How
much time -- I've actually -- so I'll just talk.
>> Jin Li: I mean we can basically [inaudible].
>> Sundeep Rangan: All right. So I'll just skip through a couple of slides. So
this was the [inaudible]. So the other key problem in femtocells is the problem of
interference. So this picture was really taken from one of the specs on a bunch
of different interference scenarios that you're having. Of course interference
really the central problem is that in any wireless network it's -- you'll have
interference whenever there are more than two connections. And there was an
interference we wouldn't have a lot of the problems we have, we would just be
dealing with point to point links. But we have interference.
But femtocells really create particularly challenging interference problems.
Deployment is unplanned, the cell may be restricted and there's these issues of
the macro-femto overlay.
In addition to that, there's a desire to really make the adaptation very fast. So
every intercell interference problem could be solved by appropriate adaptation
but then the question is you want to do that quickly and you want to do that
quickly because the key to the game of using the air link officially is to be able to
dynamically allocate resources back very fast. And that's particularly needed for
low latency or bursty traffic.
I should just point out that really femtocells the issues faced with femtocells are
similar for more generally for what is called heterogenous networks, or this term
hetnets, which you'll see a lot in the cellular space. And, you know, they're
mixtures of femtos, macros but also other elements like relays and then also
mixtures of types and spectrums like between the non cellular part. So this is all
sorts of issues that operators are deploying -- facing.
Now, [inaudible] really you can think of this two ways to deal with interference, at
least in a simple way. One is what's called -- one is the traditional cellular
method which is power control. All right. And so power control is with [inaudible]
it's basically -- here are two base stations and let's say there are two mobiles and
the simplest thing is this. That both base stations will transmit on the link at the
same time and frequency -- same time and frequency. So they'll obviously when
this -- this UE here will see that entire signal as interference and this UE will see
that signal as interference.
And then the only thing you can try to do to improve the link is try to do power
control. So you can increase the power on your link which will improve your
quality at the expense of the others by creating interference there. So the only
gain is power control.
This, for reasons I'll make clear here has become the dominant method for
cellular systems and that's because of the case -- actually this is more optimal in
the case when the interference is week. And that's become the dominant
method for 3G and will continue to be the dominant method for 4G cellular
systems.
The other type of method is what you could say is orthogonalization, which is
actually a little easier to understand. All you do is at the two transmitters transmit
on different resources. All right? So well either in time or frequency. So here
that the links -- so this blue link comes on one frequency resource and doesn't
have any interference from this link.
So that has no interference problem but of course what you're doing is you're
sacrificing bandwidth that you only get half as opposed to this that you got the full
amount of bandwidth. So a lot of the questions then -- and that's really you could
[inaudible] viewed that way is really the way that things at .11 technologies work.
We'll see that -- what we're going to see is that femtocells really don't fit neatly
into one of these two categories. And what's really needed is some kind of
sophisticated hybrid scheme, and that's what I'll talk about here.
Now, to get an idea of when to use one, it's interesting to just take a look at a
couple of different interference scenarios. So let's take a look at -- let's take a
look at a few types. The first one to do is look at the interference in a
macrocellular situation. So this is obviously -- the interference is very
environmentally independent, so all you could do is run simulations and it's
accurate and, you know, as accurate as simulation model is. But let's take a look
at a few simulations.
So one part here is this model here is what's typically used in studies of
macrocellular environment. And you -- this is kind of imagine the ideal placement
of base station. So this perfect hexagon and then with a perfect sectorization
where the cell sectors are perfectly aligned and so on.
So that's you know that's one model you know what you could look at the
interference situation you could get from here. So that's kind of a model that you
would see in a traditional macro network. So let's deviate from that in two ways.
The first way as you can imagine is unplanned deployment. Let's just imagine
that I deploy cells randomly and let's look at the interference conditions here.
The second part is instead of this macro environment let's look at a femtocell
environment. So this is the femtocell environment that's used in a lot of the 3
GPP studies. You have a bunch of apartments which are 10 meter by 10 meter
and about half of them, let's say, have a femtocell in them. So that red line is one
end of that is a femtocell transmitter and the other is a receiver. So that's a link
going on in this connection here. Now, the important part about this is this
[inaudible] restricted association. So you always connect to the femtocell in your
own apartment because that's your femtocell. Even if that might -- even though
there's a wall here, it's often the case that that may not be the closest base
station. There might be, you know, might be just on the other side of the wall,
right. There may be a closer but you still won't connect to that.
So let's see what happens. So let's first look at what happens when you make
the undeployment unplanned.
So one of the common things that, you know, cellular engineers look at is these
interference or what are called geometry CDFs. So the X axis here is the SI in
our signal to interference and noise ratio. So it's a measure of the amount of
interference that the link is hearing. So this is the way you imagine all the links
transmit at the same time. You may not choose to do this, but let's say that you
did. Right. If all the links transmit at the same time and then you can look at
your signal power over the sum of all the interferences that you got. That's the
SINR, all right. And now that's a random adversarial because it depends on
randomly where you are. And this is a CDF.
So if you look here at the degredation just going from this perfect hexagon to this
random environment, you see already about a 2 dB loss. Now, 2 dB loss is not
insignificant. That's about what, maybe 70 percent. So you've already
potentially -- not 70, about 30 percent. So that's about a 30 percent loss in
capacity just from the not having a ideally deployed environment.
When you go to a femtocell situation, you get a much more radically different
interference situation. And it's sort of there's good news and bad news. Here's
the good news. The good news is that there are many links actually which have
a very, very good SNR. You know, [inaudible] as much as like 20 dB. When the
radio probably can't even benefit for more than 25 dBs. So you're about the limit
here. All right. That's because of course these apartments are relatively isolated
like maybe this link over here is isolated so it gets a very good SNR, all right? So
that's many links.
But there are some links which are really very bad, all right, down to what may be
-- you could say about eight percent or something are down about minus 10 dB.
Now, you might think okay, well what's so bad about that, it's just a small number
of links. But remember this. Imagine you were using your laptop. You're not
going to remember the 60, 70 percent of times that your laptop was really good.
You know, one out of 10 times you use your laptop and it completely fails, you're
going to remember that. So that's the actual -- that's the issue here. Even if it's a
small apparently that's the part that we really need to address. So the big
difference you see here is that there's a much large number of links in a much
worse interference condition. And that's coming primarily from this fact of the cell
selection, all right, that is restricted that you cannot connect to the strongest cell.
So this is where we see the big difference in the interference environment is
when we do this kind of graph. Now, maybe just to -- what I'll do is I'm going to
skip over all of this. Okay. Let me just take a couple of slides here. So let's take
a look at where power -- I said there were two methods to try to deal with
interference. One is power control and the other is orthogonalization. So when
you look at power control has actually been a very effective method in the cellular
environment, all right. This is a graph -- this is an algorithm that I worked on but,
you know, there's a gazillion algorithms out there and there's many deployed in
commercial systems here.
So if you look at the rate CDF actually, so when you run these algorithms and
you look at the rate CDF, you can see a couple of different aspects. First of all,
that the rates are actually quite good that you can get. So this is -- this is what
they usually measured in bits per second per hertz per user. So with 10 users,
you know, the average user is getting about maybe -- your medium user is
getting maybe around .15 bits per second, a 1.5 bits per second per hertz which
is actually a very good capacity. That means on like a 10 megahertz system you
would be getting running an average capacity around 15 megahertz which is
extremely good in a loaded cellular system.
So, you know, power control has been very effective about getting even the
minimum rate that users get can be managed quite well. So there's -- and it's a
fact there's no user in a very bad interference environment -- interference
condition. If the interference condition's bad, you can do -- you can do another
thing about these power control algorithms there's a lot of mathematical theory
about them. And then you can have systematic methods which try to improve by
changing what's called the utility function of these users and improve their
interference condition so that you can, you know, boost up the rates for the lower
mobiles at the expense of mobiles at the higher end.
>>: So that means coordination systems need to send signals to the femtocell
base.
>> Sundeep Rangan: That's right.
>>: How much power should use.
>> Sundeep Rangan: Exactly.
>>: And then basically that coordination will help to improve the interference.
>> Sundeep Rangan: That's right. I don't have time to go into it. But that's
exactly it. But all that transfer of information is already part of the specs in these
3G and 4G cellular systems and work well for power control. All right.
>>: So I mean this coordination does it need additional information for [inaudible]
do you need the location [inaudible].
>> Sundeep Rangan: You don't typically need the location what you do is you
measure -- do I have the slide on it? Oh, no. I have them -- but what you do is
you -- the mobiles make reports so they listen to the other base stations and say
this base station is 5 dB weaker than the base station to me and so you get these
kind of path ratio and you send them, you keep on reporting them to your base
station. So it measures -- it doesn't matter your geographical location is not
what's relevant anyway, what matters is your actual relative location from the
path loss. And those were always measured by the mobiles and they're
constantly reporting that to the -- so once you start talking on your cell phone it
will be constantly making those reports to the base station. That's how you can
do the power control.
And there's lots of other signals to do the power control signalling. I only have
like a little while so I'm not going to go into it. But that's all there. But the
problem is when you try to run these power control into a femtocell situation or
just any kind of local [inaudible] they kind of fail. So here's the rate CDF, all
right? So remember, so this is a CDF. So -- and this is the rate that you get. It's
under some model, all right. But imagine that this is bits per second per hertz.
So if this was a 10 megahertz then two would mean 20 megahertz, all right, so
that would be a very high rate.
So this is the situation. The blue curve, the situation all the links transmit a
maximum power, all right. There's no power controls. It's the simplest thing. All
right. And you'll see the many links, all right, again that, you know, 10, 15
percent getting virtually no rate. Right? Now, what happens when you try to do
some kind of optimal power control so you say okay, I have some link in very bad
power, I'm going to try to decrease the power on the other link to make a tradeoff
so it can get less interference. And you can play around different games on how
you do that tradeoff. But the reality is none of those curves really budge that very
much. They budge it a little bit but really it's not helping out the situation. So
there's still a lot that are suffering in a very bad interference condition.
So the reason why it's failing is again that power control fundamentally works well
when the interference is limited, all right? To illustrate that point, let's take a look
here back to what, you know, information theorists call -- what the Gaussian
interference channel. Let's take two links. All right. One transmitter and one
receiver, another transmitter and another receiver. But they interfere with one
another and this term alpha represents a kind of crossover game, all right. So
this guy here will get some power P and noise N and interference alpha P. I'm
still going to compare two things.
One is reuse one. So we'll just assume here -- we won't even bother with the
power control aspect. Let's assume that they both transmit at that time same
time and they treat the other interference as noise, all right. That's when the
dominant model for cellular systems.
The other is I could manage orthogonalization, right? I could just say that well,
I'm just going to split these two links. They'll each transmit on half the resources,
the half time half frequency and then they'll put twice the power on that. So those
are your two options. If you do this, you can look at the rates. And the rate you
get will be a function of this crossover game, this alpha. So what you get is that if
the alpha is very low, all right, that means that the weaks are a little interfering.
The blue curve is a reuse one, the rate, and the green curve is the orthogonal.
So let's look at the blue curve. When the interference is very low, obviously the
reuse one does very well. But then as the interference decreases of course it
goes down, the rate. At some point it would be better to switch over and just put
them on to orthogonal, on to two orthogonal links.
So this is the sort of high level conclusion of this very simple calculation. When
the strong interference you want to orthogonalize it, when it's weak interference
you want to do reuse one. And that makes sense of why cellular systems have
worked well with reused ones because the interference tends to be low because
of the proper cell selection.
Now, actually historically reuse one wasn't always used. It was only used
actually at the beginning of CDMA systems. 2G systems and that because they
enable a concept that you could spread this interference across the whole band
and get an averaging properties so you would see an average.
But prior to that actually the dominant way was to using reuse like what you call
reuse three, right, or something like that. And that was typically used in
conjunction with sectorization. So you would sectorize your antenna like this and
put your signal power on three different frequencies, all right? And now, for
example, the green frequency here won't interference with the blue and the red.
But of course what you're doing is you're limiting the amount of bandwidth, you're
cutting the bandwidth down to a third.
If you look at this here, does it actually improve the situation? And you could see
here why it hasn't worked well in the cellular environment. Why you get actually
an interference gain here, the amount of rate increase is fairly small. That's
because you are sacrificing that one third of the bandwidth. All right. There's a
small gain for people who have very bad interference, but the bulk of users
actually suffer quite a bit, a large number of users.
Let me just flip over. When you look at OFDMA, all right, what's happened here
is that -- it's actually what you are seeing is in OFDM when you move to 4G
systems they've actually seen a mixture of interference. So what's happened is
that you have -- maybe I should just quickly -- OFDM what you have is that you
allocate, you divide the signal into different times and frequency slots.
Within a cell you can allocate users to disjoints parts of the spectrum. So for
example these two different blue users are getting disjoint so they don't
interference with one another. So they're orthogonalized. And then you can
imagine in cell interferences tends to be high, so this is good. On the other hand
out of cell interference you randomly hop the locations of the interference of
these users within the cell, so they see kind of an averaging of the interference.
So you're going to reuse one type of interference outside the cell. Does that -okay. Let me just flip because I haven't gone to any of the new part here. Let
me just go to this.
So if you look at sort of a, you know, historical perspective here on what's
happened with reuse, early 2G systems used to be orthogonal. With the advent
of CDMA, they went to reuse 1 because fundamentally that got a much higher
efficiency because for the most part the interference was weak enough to -- was
weak enough that you would get a benefit. So both the in cell and out of cell
interference. For OFDMA systems, they got a kind of a best of both worlds. For
the out of cell interference they could use reuse 1 and but for in cell interference
they did an orthogonal. WiFi systems they didn't have a chance to talk about are
really just generally orthogonal period. Because they tend to you have no
condition of what the interference is and they just carry [inaudible] as soon as
you detect any kind of transmission they shut down. So they tend to be
orthogonal. The result, the WiFi systems tend to be simple and robust but you
get a very poor spatial reuse, all right. But you have lots of spectrums so it
doesn't matter.
The problems with femtocells, all these kind of systems are based on certain
design assumptions which, you know, which always are valid under a regularly
planned network. But what happens in a femtocell situation? In a femtocell
situation we saw here that reuse one does not work well in a small perjury but
small but significant percentage because the assumptions break down. If we
went to and all orthogonalized system then we'd have the problems that you'd
have with WiFi which is very poor spatial reuse. So you need some kind of
method, which is a hybrid. And that will motivate this last part of the talk here.
So what LTE and 4G done in general is to kind of done a -- offered a way to do
kind of a hybrid scheduling and what we call subband scheduling. In this case
you can divide the bandwidth into a bunch of subbands which I've illustrated
here. And the base agent can put different levels of power on different
subbands. And now you can offer subbands at different types. You can have
subbands which are -- they exclusively use one by one base station for
orthogonal users. You could also offer base -- subbands which are used by both
base stations.
Now, as a result what happens, you can schedule users in different parts of the
subbands. Mobiles that are not experiencing much interference can be
scheduled into one of these reuse one subbands where, you know, they could
actually benefit where mobiles at the end of the cell can then get the scheduled
on one of these other subbands.
Now, subband scheduling is not that valuable in the macrocellular environment
because the macrocellular environment because it's already orthogonal in cell
benefits almost -- gets almost everybody else you want to do in reuse 1. But this
could be very valuable in the femtocell situation.
In the femtocell situation what we want to do is some kind of -- you could think of
distributed optimization to try to properly select the subbands and try to assign
users to the cracked subbands. So you -- you could try to -- you would think of
some kind of distributed optimization, not going to have a chance to go over this,
would you use a utility based metric for this but then it's sending that metric in
some kind of distributed way. Now, because the distributed here because well of
course the selections are subbands in one base station will affect the subbands
in this situation.
When you run that algorithm, here's where you could start to see the gains. The
blue line again was when all the mobiles were transmitting at the same power
and the green line was transmitting with power people who got it a little better but
you can get a much more significant gain here by doing this kind of power -- by
doing some kind of subband scheduling. And that -- because what you can do of
course if the fewer mobiles that are suffering very badly you can then go and
assign them dedicated subband where they're not seeing any interference and
get their interference condition off a lot better.
All right. That was just a brief talk about what happens with femto to femtocells,
let me just quickly talk about what happens in the femto to macro case. Now, the
femto to macro case is -- creates a lot of problems. Logistically doing femto to
macrocell coordination is difficult. For one part it's just a scalability issue when
you want -- it's not really feasible for the femtocell large macrocell serving
hundreds of mobiles and with potentially hundreds of femtocells in its thing to try
to coordinate individual interference subbands with any of these parts. It's not
going to say, you know, do one tiny little femtocell over here, I'm going to vacate
this subband over you for, you know, which is going to affect all my subscribers
here.
There's also just, you know, a hearability issue that the signal -- there's a huge
power disparity in the signals, so it's actually just very hard to -- or at least over
the air to communicate any kind of signalling quickly between the femtocells and
a short range communication and a large -- and a macrocellular environment.
The second part is that even if the macrocell could hear you probably would
never want to yield traffic. Macro capacity as we said is very expensive, it takes
a lot of effort. The operator's never going to give away capacity of a macrocell to
try to serve just some small femtocell. So ideally you really want technologies
that operate under the radar of macro.
Now, I wanted to say this because this is sort of the perspective in a way, a kind
of dynamic version of cognitive spectrum sharing. Cognitive spectrum sharing
you could think of as a slow time scale orthogonalization. When the primary
units are occupied the spectrum all the short range communication's shut off.
When it drops, you -- when it drops then the short range communication's
detected and can come on.
What we -- what we need for femtocells is something like that, but we'd actually
like both operating simultaneously but the short range still running under the
radar of the -- under the radar of the primary user which is the macrocell. You
can do that trick doing subband scheduling. How can we do that? Let's take a
look.
Suppose we divide -- take the subband scheduling as we said. Imagine we
divide it again and what we said reuse 1 subband and reuse 3 subband, all right.
So reuse 1 subbands are typically serving mobiles that are close to the cells. So
this color picture is -- imagine on the right is a blue bay station and the left is a
green bay station. All right. So the power's kind of radiated here. The reuse one
subband's both bay stations are transmitting and they're serving mobiles who are
close to them so. The blue mobiles are being served by the blue base station,
the green by the green base station.
So obviously what can happen here is in the reuse one subband statistically
there will be very few in the middle of the cell. So if your link is at the edge of the
cell, you could kind of transmit here, you could imagine, a short range link in
between these two subbands without affecting the mobiles at the edges -mobiles close to the cell. But statistically you should be far apart. This is coming
at much lower power here. And anyway, you're not seeing much interference
from this guy because it's very far away, and you're not going to have much in
effect on the mobiles here.
On the other hand, if you end up in the unfortunate situation where close to the
base stations that's not bad either because the close base station will have reuse
three subbands, and those subbands here will be serving mobiles at the edge -the edge of the cell, so you can run on one of those subbands this -- like on this
one, the green base stations using so the blue base station's not even using it.
So if you're close to it, then you just go ahead and transmit your short-range
signal there. Does that picture make sense? Did I lose everyone on this picture.
Or was it too many colors? [laughter].
Okay. They're two base stations. There's a green base station and a blue base
station, all right. There's the blue mobiles being served by the blue base station,
the green mobiles being served by the green base station. They are the
squares. The red lines are your short range links, right, which are not getting
served by either -- by the base stations. They just have to transmit without
affecting any of this other communication.
>>: [inaudible] basically the [inaudible] links what are the basically tricks you are
using to basically make sure the basically the power for signals from base station
doesn't overwhelm.
>> Sundeep Rangan: So this part here I'm doing nothing. All I'm doing is just
saying well, as long as the signal is weak enough.
>>: Okay.
>> Sundeep Rangan: So imagine that I am very far away from both the base
stations. So then hopefully the gallon is weak enough I can transmit a little
above that signal and then I'm not going to affect it.
>>: [inaudible].
>> Sundeep Rangan: Nothing [inaudible] other than -- I just measure the receive
power -- I measure the receive power and then I say if I look at how much power
I need to transmit above that to get a decent rate. And if I'm far enough away
from every other mobile, then go ahead and [inaudible] all right? And I can be far
enough away -- okay. And I'll be far enough away because in this subband here
I'll be -- those mobiles will be close to the base station. All right? So that's the -that's the idea.
On the other hand, if you're close to the base station that's not so bad either
because what you do is if you're close to the base station there are subsubbands
which the cellular network will have content open for serving it's low edges cell
mobiles. With its edges cell mobiles it will serve because this mobile wouldn't be
scheduled here because if it was scheduled here we would see a lot of
interference between the -- it would see a weak signal from the green base
station and a -- you know, an equal power signal from the blue station. So you'll
serve that cell here.
This subband is one with a green base station only transmit when the blue base
station is off. So you can trans-- if you land up close to the blue station you just
run your short range link here, all right, because you're not even seeing any
signal from the base station, so you won't affect it. And presumably the green
mobile will be far away. Similarly if you are close to the blue base station you
transmit here. So if you're close to the blue you transmit here, if you're close to
the green, you transmit here, if you're close to neither, you transmit here.
All right. So somewhere for you to transmit, wherever you go. All right. Now, a
little part about science fiction here. You can even do better than this. Using
something called interference cancellation. Interference cancellation is this. If
it's very strong, actually, the interference you can first decode it, subtract it off,
and then transmit underneath that. That opens up a couple more points on this
color coded graph.
If you're very close to -- if you're the edge of the cell here, imagine on the uplink,
all right. In the uplink, this guy will be trend in very high powered or [inaudible]
base station. So you can very clearly hear this guy's signal. You can then
subtract it off and then transmit underneath his signal. All right. If you believe in
science fiction you can do this, all right.
On the other hand, if you are the edge of the cell, you can actually -- of course if
you're the edge of the cell, you can always transmit here because there's no
signal. But you can also transmit when you're -- when this base station's signal
is very strong. Because you will hear on the downlink you'll see a very strong
signal from the base station, so you can then decode that signal and subtract it
off, all right.
Now, of course for interference cancellation to work, you have to have -- this has
to be a decodable signal. It means that the rate has to be relatively low so you
can decode it. But any way serving edge of cell mobiles. So any way, the rate
tends to be very low. So -- which is -- which will be decodable. So you can just
make this so that these are -- so you'll tend to be able to decode the data here.
When you add all of that up -- yes?
>>: Interference cancellation is basically the hot [inaudible].
>> Sundeep Rangan: Yes.
>>: [inaudible] what are the assumptions basically in the interference
cancellation and how much of the assumption can we implement in the real
world?
>> Sundeep Rangan: Okay. So the interference cancellation just really -actually it's already -- there are Qualcomm when I was there is already producing
chips, interference cancellation. Interference avoidance is another -- but
interference cancellation is you just -- the assumptions of course is that you have
to be able to detect the assignment from the base station in this case. So that
part you just have to put the assignment at some publicly decodable part. It has
to be at a rate that's low enough that you could decode first.
So, you know, you're trying to do a very high rate to that user, you know, you'll
need a very good channel quality and it will be harder to decode, all right? So
again, you have to lower the rate. That's easy. And then you need hardware to
actually do this. The hardware is a bit of a problem, especially -- it's really -- you
know, I've been through the basic part of this part and you need really the
bottleneck is actually the memory to get that. That's because in a cellular system
you have this thing called hybrid RQ where you have to remember codes and
then try to decode them.
But the memory's actually becoming, you know, reasonable as well. You know,
at least for these cellular systems. So that's -- it's entirely feasible to do this.
>>: So [inaudible].
>> Sundeep Rangan: Yeah.
>>: [inaudible] so basically for interference cancellation the receiver will need to
try to be able to decode two signals, one is a [inaudible] signals.
>> Sundeep Rangan: Yes.
>>: Which is basically from far away [inaudible] base stations then the basically
[inaudible].
>> Sundeep Rangan: Yes.
>>: Right?
>> Sundeep Rangan: Right.
>>: So basically you try to do decoding twice.
>> Sundeep Rangan: Exactly.
>>: Right? I mean, basically first strip of -- from the one that ->> Sundeep Rangan: Yes.
>>: To the lower ones.
>> Sundeep Rangan: Exactly. Okay. So when you add this all, if you believe
this is a very preliminary simulation here. When you add this up, you get this
potentially huge amount of capacity [inaudible] 10 megahertz system. The
medium user can get what, maybe 25 megabits for free off this. There's some
minimal assumption here, some minimal affect on the macro. So like 25
megabits for free significant underneath the radar of the [inaudible] and with
interference cancellation that boosts up to around like 35 megabits. So it's
actually -- so this, I think interesting just from all sorts of people looking in
cognitive spectrum about a different way of thinking about how to share spectrum
that's maybe not so with just a simple either or kind of situation that there's a lot
of capacity.
Let me just tell you -- I can talk about this later. Maybe I'll just wrap it up right
now. We'll talk about the test bed plans that I have for poly and try to build a
wireless ->>: [inaudible].
>> Sundeep Rangan: Okay.
[applause].
>> Sundeep Rangan: Sorry to keep you here.