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How To Learn Sign Language

Communication over a Noisy Channel

Hi. In this problem, we’ll be
talking about communication across a noisy channel. But before we dive into the
problem itself, I wanted to first motivate the context a
little bit and talk more about what exactly a communication
channel is and what “noise” means. So in our everyday life,
we deal with a lot of communication channels, for
example, the internet, where we download data and we watch
videos online, or even just talking to a friend. And the air could be
your communication channel for our voice. But as you probably have
experienced, sometimes these channels have noise, which
just means that what the sender was trying to send isn’t
necessarily exactly what the receiver receives. And so in probability, we try
to model these communication channels and noise and
try to understand the probability behind it. And so now, let’s go into
the problem itself. In this problem, we’re dealing
with a pretty simple communication channel. It’s just a binary channel,
which means that what we’re sending is just one
bit at a time. And here, a bit just means
either 0 or 1– so essentially, the simplest
case of information that you could send. But sometimes when you send
a 0, the receiver actually receives a 1 instead,
or vice versa. And that is where
noise comes in. So here in this problem, we
actually have a probabilistic model of this channel and the
noise that hits the channel. What we’re trying to send
is either 0 or a 1. And what we know is that on
the receiving end, a 0 can either be received when a 0 is
sent, or a 1 can be received when a 0 is sent. And when a 1 is sent, we
could also have noise that corrupts it. And you get a 0 instead. Or you can have a 1 being
successfully transmitted. And the problem actually
tells us what the probabilities here are. So we know that if a 0 is sent,
then with probability 1 minus epsilon naught,
a 0 is received. And with the remaining
probability, it actually gets corrupted and turned into a 1. And similarly, if a 1 is sent,
then with probability 1 minus epsilon 1, the 1 is correctly
transmitted. And with the remaining
probability epsilon 1, it’s turned into a 0 instead. And the last bit of information
is that we know that with the probability p, any
random bit is actually is 0 that is being sent. And with probability 1 minus
p, we’re actually trying to send a 1. So that is the basic setup
for the problem. And the first part that the
problem asks us to find, what is the probability of a
successful transmission when you have just any arbitrary
bit that’s being sent. So what we can do here is, use
this tree that we’ve already drawn and identify what are the
cases, the outcomes where a bit is actually successfully
transmitted. So if a 0 is sent and a 0
is received, then that corresponds to a successful
transmission. Similarly, if a 1 is sent and
a 1 is received, that also corresponds to a successful
transmission. And then we can calculate what
these probabilities are, because we just calculate the probabilities along the branches. And so here implicitly, what
we’re doing is invoking the multiplication rule. So we can calculate the
probabilities of these two individual outcomes and their
disjoint outcomes. So we can actually just sum the
two probabilities to find the answer. So the probability here is p
times 1 minus epsilon naught– that’s the probability of
a 0 being successfully transmitted– plus 1 minus p times 1 minus
epsilon, 1, which is the probability that a 1 is
successfully transmitted. And so what we’ve done here is
actually just looked at this kind of diagram, this tree
to find the answer. What we also could have done
is been a little bit more methodical maybe and actually
apply the law of total probability, which is really
what we’re trying to do here. So you can see that this
actually corresponds to– the p corresponds to the
probability of 0 being sent. And 1 minus epsilon naught is
the probability of success, given that a 0 is sent. And this second term
is analogous. It’s the probability that a 1
was sent times the probability that you have a success, given
that a 1 was sent. And this is just an example of
applying the law of total probability, where we
partitioned into the two cases of a 0 being sent and a 1 being
sent and calculated the probabilities for each
of those two cases and added those up. So that’s kind of a review of
the multiplication rule and law of total probability. So now, let’s move on to part
B. Part B is asking, what is the probability that a
particular sequence of bits, not just a single one, but a
sequence of four bits in a row is successfully transmitted? And the sequence that we’re
looking for is, 1, 0, 1, 1. So this is how I’ll
denote this event. 1, 0, 1, 1 gets successfully
transmitted into 1, 0, 1, 1. Now, instead of dealing with
single bits in isolation, we have a sequence of four bits. But we can really just break
this out into the four individual bits and look
at those one by one. So in order to transmit
successfully 1, 0, 1, 1, that whole sequence, we first need to
transmit a 1 successfully, then a 0 successfully, then
another 1 successfully, and then finally, the last
1 successfully. So really, this is the same as
the intersection of four different smaller events, a 1
being successfully transmitted and a 0 being successfully
transmitted and two more 1’s being successfully
transmitted. So why are we able to do
this, first of all? We are using an important
assumption that we make in the problem that each transmission
of an individual bit has the same probabilistic structure
so that no matter which bit you’re talking about, they all
have the same [? error ?] probability, the same
probabilities of being either successfully transmitted or
having noise corrupt it. So because of that, it doesn’t
really matter which particular 1 or 0 we’re talking about. And now, we’ll make one more
step, and we’ll invoke independence, which is
the third topic here. And the other important
assumption here we’re making is that every single
bit is independent from any other bit. So the fact that this one was
successfully transmitted has no impact on the probability
of the 0 being successfully transmitted or not. And so because of that, we can
now break this down into a product of four probabilities. So this becomes the probability
of 1 transmitted into a 1 times the probability
of 0 transmitted into a 0, 1 to a 1, and 1 to 1. And that simplifies things,
because we know what each one of these are. The probability of 1 being
successful transmitted into a 1, we know that’s just
1 minus epsilon 1. And similarly, probability of
0 being transmitted into a 0 is 1 minus epsilon naught. So our final answer
then is just– well, we have three of these
and one of these. So the answer is going to be 1
minus epsilon naught times 1 minus epsilon 1 to
the third power. Now, let’s move on go on to
part C, which adds another wrinkle to the problem. So now, maybe we’re not
satisfied with the success rate of our current channel. And we want to improve
it somehow. And one way of doing this is
to add some redundancy. So instead of just sending a
single 0 and hoping that it gets successfully transmitted,
instead what we can do is, send three 0’s in a row to
represent a single 0 and hope that because we’ve added some
redundancy, we can somehow improve our error rates. So in particular what we’re
going to do is, for a 0, when we want to send a 0, which I’ll
put in quotes here, what we’re actually going to send
is a sequence of three 0s. And what’s going to happen is,
this sequence of three 0s, each one of these bits
is going to go through the same channel. So the 0, 0, 0 can stay and get
transmitted successfully as a 0, 0, 0. Or maybe the last 0 gets turned
into a 1, or the second 0 gets turned into a 1, or we
can have any one of these eight possible outcomes
on the receiving end. And then similarly, for a 1,
when we want to send a 1, what we’ll actually send
is a sequence of three 1’s, three bits. And just like above, this 1, 1,
1, due to the noise in the channel, it can get turned into
any one of these eight sequences on the
receiving end. So what we’re going to do now
is, instead of sending just a single 0, we’ll send three 0s,
and instead of sending a 1, we’ll send three 1s. But now, the question is, this
is what you’ll get on the receiving end. How do you interpret– 0, 0, 0, maybe intuitively
you’ll say that’s obviously a 0. But what if you get something
like 0, 1, 0 or 1, 0, 1, when there’s both 0s and 1s in
the received message? What are you going to do? So one obvious thing to do is
to take a majority rule. So because there’s three of
them, if there’s two or more 0s, we’ll say that what
was meant to be sent was actually a 0. And if there’s two or more 1s,
then we’ll interpret that as a 1 being sent. So in this case, let’s look
at the case of 0. The majority rule here would say
that, well, if 0, 0, 0 was sent, then the majority is 0s. And similarly, in these two
cases, 0, 0, 1 or 0, 1, 0, the majority is also 0s. And then finally, in this last
case, 1, 0, 0, you get a majority of 0s. So in these four received
messages, we’ll interpret that as a 0 have being set. So part C is asking, given this
majority rule and this redundancy, what is the
probability that a 0 is correctly transmitted? Well, to answer that, we’ve
already identified these are the four outcomes, where
a 0 would be correctly transmitted. So to find the answer to this
question, all we have to do is find the probability that a
sequence of 0, 0, 0 gets turned into one of these
four sequences. So let’s do that. What is the probability that
a 0, 0, 0 gets turned into a 0, 0, 0? Well, that means that
all three of these 0s had no errors. So we would have the answer
being 1 minus epsilon 0 cubed, because all three of these
bits had to have been successfully transmitted. Now, let’s consider
the other ones. For example, what’s the
probability that a 0, 0, 0 gets turned into a 0, 0, 1? Well, in this case, we need two
successful transmissions of 0s, plus one transmission
of 0 that had an error. So that is going to be 1 minus
epsilon naught squared for the two successful transmissions of
0, times epsilon naught for the single one that was wrong. And if you think about it, that
was only for this case– 0, 0, 1. But the case where 0, 1, 0 and
1, 0, 0 are the same, because for all three of these, you
have two successful transmissions of 0, plus one
that was corrupted with noise. And so it turns out that all
three of those probabilities are going to be the same. So this is our final answer
for this part. Now, let’s move on to part D.
Part D is asking now a type of inference problem. And we’ll talk more
about inference later on in this course. The purpose of this problem– what it’s asking is, suppose
you received a 1, 0, 1. That’s the sequence of
three messages, three bits that you received. Given that you received a 1, 0,
1, what’s the probability that 0 was actually the thing
that was being sent. So if you look at this, you’ll
look at it and say, this looks like something where we
can apply Bayes’ rule. So that’s the fourth
thing that we’re covering in this problem. And if you apply Bayes’ rule,
what you’ll get is, this is equal to the probability of 0
times the probability of 1, 0, 1 being received, given that 0
was what was sent, divided by the probability that 1,
0, 1 is received. So we have this basic
structure. And we also know that we can
use the law of total probability again on
this denominator. So we know that the probability
that 1, 0, 1 is received is equal to the
probability of 0 being sent times probability of 1, 0, 1
being received, given that 0 was sent, plus the probability
that 1 was sent times the probability that 1,
0, 1 is received, given that 1 is sent. And as you’ll notice in
applications of Bayes’ rule, usually what you’ll have is a
numerator is then repeated as one of the terms in the
denominator, because it’s just an application of total
probability. So if you put these pieces
together, really, what we need is just these four terms. Once we have those four terms,
we can just plug them into this equation, and we’ll
have our answer. So let’s figure out what
those four terms are. The probability of 0
being sent– well, we said that earlier. Probability of 0 being
sent is just p. And the probability of 1 being
sent is 1 minus p. That’s just from the
model that we’re given in the problem. Now, let’s figure
out this part. What is the probability of a 1,
0, 1 being received, given that 0 was sent? So if 0 was sent, then we know
that what really was sent was 0, 0, 0, that sequence
of three bits. And now, what’s the probability
that 0, 0, 0 got turned into 1, 0, 1? Wall, in this case, what we
have is one successful transmission of a 0, plus two
failed transmission of a 0. So that one successful
transmission of a 0, that probability is 1 minus
epsilon naught. And now, we have two failed
transmissions of a 0. So we have to multiply that
by epsilon naught squared. And now, for the last piece,
what’s the probability of receiving the 1, 0, 1, given
that 1 was actually sent? Well, in that case, if a 1 was
sent, what was really sent was a sequence of three 1s. And now, we want the probability
that a 1, 1, 1 got turned into a 1, 0, 1. In this case, we have two
successful transmissions of the 1 with one failed
transmission. So the two successful
transmissions will have 1 minus epsilon 1 squared. And then the one failed
transmission will give us an extra term of epsilon 1. So just for completeness, let’s
actually write out what this final answer would be. So probability of 0 is p. Probability of 1, 0, 1, given 0
is, we calculated that as 1 minus epsilon naught times
epsilon naught squared. The same term appears again
in the denominator. Plus the other term is,
probability of 1 times the probability of 1,
0, 1, given 1. So that is 1 minus epsilon
squared times epsilon 1. So that is our final answer. And it’s really just a
application of Bayes’ rule. So this was a nice problem,
because it represents a real world phenomenon that happens. And we can see that you can
apply a pretty simple probabilistic model to it and
still be able to answer some interesting questions. And there are other extensions
that you can ask also. For example, we’ve talked about
adding redundancy by tripling the number of bits,
but tripling the number of bits also reduces the
throughputs, because instead of sending one, you have
to send three bits just to send one. So if there’s a cost of that, at
what point does the benefit of having lower ever outweigh
the cost of having to send more things? And so that’s a question that
you can answer with some more tools in probability. So we hope you enjoyed
this problem. And we’ll see you
again next time.

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