So, we just looked at what was initially proposed solution to the near-far problem
and we said it was called the TPC or transmit power control algorithm.
The TPC algorithm tries to do is to equalize the received signal power, but,
it turns out that signal power isn't really what we need the calls to be
equalizing. We need to equalize something else, which
is called the signal quality. Now, quality is different than power,
because quality takes interference into account.
So when we looked at the TPC algorithm, we were only concerned with what issue
the transmitters were sending, so this one was sending may be 2 milliwatts and
this guy closer to the station was send 1 milliwatt if the station was over here or
something. And, while we, we were looking at this
person's link to the transmitter and this person's link to the transmitter.
And we said, while this person needs to then increase his power to 4 milliwatts
and this person cuts it down to 0.5 milliwatts I'm just making these numbers
up, then everything might be okay. The problem is that when this person
increases up to 4 millawatts that he's causing extra interference to this
transmitter right over here. So we kind of have a vicious cycle, where
as this person increases his power and this person would have higher
interference, then he might have to increase his power, which would cause
this person more interference. Then he might have to increase his power
and so on and it'd be a vicious repetitive cycle or an arms race if you
will to the maximum transfer of power. So we need a way to do that, and we'll
look at now, exactly what signal quality really is and we'll define this.
So suppose we have three transmitters, A, B, and C, and they're all looking to
transmit to a base station. Now we can consider the receivers on the
single base station as co-located. So this is receiver A, receiver B, and
receiver C. Even though they're all going to a point
on the base station, just look at them as three separate places on the base
station. So if we look at receiver, if we look at
transmitter B for a second, let's focus on him first.
We have a few parameters here. The first one is the direct channel.
So B has a direct channel right to his receiver B.
This is transmitter B, this is receiver B.
And that's where he wants to get his signal, so he wants to get it to receiver
B. But now, if you look at these receiver,
what we also have to take into consideration is the fact that A and C
are going to cause interference. So, this is the direct channel, and this
is the indirect or interference channel. And direct here really means desire to
indirect this undesired, and this is also an indirect as well.
And, we have the same exact thing for each of the other transmitters and
receiver pairs. So for A, he has a direct channel and his
interference comes from interfering channels with channels B and C.
And then in addition, each of these receivers themselves are going to have
some amount of internal noise. It's just basically random fluctuations
of electrical signal. We won't go into it in much detail, but
each receiver has a certain amount of a noise that is within it as well.