So coding things in digital form also allowed engineers to look at a different
type of mobile access technology, and this'll be the second type that we look
at. So the idea is as follows.
Let's consider our standard Time Frequency division, and we saw before
with FDMA that every transmitter would get a different frequency band.
So person A is on this frequency, B's on this frequency, C's on this, and D's on
this one, and that's how they're distinguished over the same network
medium. That's how they share the air, if you
will. So let's try to think of a way we could
fit more people into this diagram. So what about having every frequency
carry multiple users? So something like this, first off, at the
first time, A1, B1, C1, and D1 are transmitting.
Then at the second time, A2, B2, C2, and D2 are transmitting.
Then A3, B3, C3, and D3, and so on. And so we're adding a time division on
top this frequency division so that on every frequency, we've now put 3
different people in here. It goes back to our time sharing that we
talked about in our initial analogy. Basically, these 3 users can take turns
on this one frequency. And then they can repeat again, so then
A1 would have its turn next B2 and A3 and so on for each frequency.
So we see we have three different things going on here.
Not only are people now on different frequencies, but now we can fit multiple
people on the same frequency as long as we divide them in time.
So you're probably asking, well, that's great, and now we have a threefold
improvement in capacity. Because now we can have 12 people, these
are, there's 12 blocks in here, rather than just four people.
But why didn't we do that before, with FTMA?
Well, with analog signals, it's not possible.
To compress multiple signals onto the same frequency band.
So we cant do this, we cant do this time sharing.
Analogue just doesn't allow us to do that, but digital technology allows us to
do that compression. So we can take these 3 signals, and put
them on the same frequency band and still have the receiver distinguish between
them. So, one of the advantages of digital
coding was to allow us to move to this next multiple-access technology.
So, this is called TDMA, multiple-access, as we know, again, but rather than
frequency division, we now have time division on top of that.
So this is time division multiple-access. So you're probably wondering, well, it
really still is kind of frequency division.
Because we do have different frequency bands, what we're really referring to
here with TDMA is that on top of FDMA. We're now allowing multiple people to
share the same frequency bands, so it has a TDMA in it as well.
But in the strict sense, a TDMA system would be a single frequency having
multiple users. But it doesn't make sense to do that
because we know that we can have multiple frequencies.
So when we say TDMA we're really referring to a chart like this rather
than just a chart of a single Frequency with multiple times.