With Handoffs we're talking about what happens when a device has to switch
between two different base stations. So if this is one base station, and this
is another base station, what happens when your device is currently talking
[SOUND] to this base station? So if you're driving in your car, and
you're on your cell phone, but then you move into the coverage area of the next
space station. How does it handle that switch to the one
that is giving you the better channel quality?
Well, under FDMA or TDMA, you switch frequency channels.
So this change can happen very quickly, and it's not too much of an issue, to
have that happen. So you just, switch frequency channels
and switch base stations accordingly. But with CDMA, it's much harder to do
that because, first of all, the cells don't have dedicated frequency channels.
Everyone's transmitting in the same bands, so it's not as easy as just
switching from one channel to another. But the second problem is that when a
device enters [SOUND] another cell. The CDM, the power control algorithm are
going to have to reconverge very, very, very quickly in order for you not to see
any effect on your call quality. Because if you currently have your SIR
all figured out, when you're over in this cell, and then you suddenly transition to
this new cell tower. Then that algorithm would have to
reconverge very quickly to you depending upon the current channel conditions.
From all of the other mobile stations that are in this cell, and taking those
into account. So, this is known as a Hard Handoff, and
again Hard Handoff works fine when you have FDMA or TMA, but for CDMA, we need
another solution. We can't use the Hard Handoff.
What engineers came up with was the idea of a soft handoff instead.
And the scheme of a soft handoff is that when a device, when you're driving in
your car and you're currently talking to the cell tower.
Rather than this disconnection and reconnection happening very, very quickly
in that transition, we do it smoother, and more soft.
That's where the term soft versus hard comes from.
So, right now you're talking to this tower, and then once you move into the
intersection region, you'll actually keep connectivity [SOUND] with this tower.
While connecting to the new tower as well.
And so you won't just drop the connection really quickly from this tower and you
also won't completely rely then on this tower.
because that will give the power control algorithm enough time to converge as is
necessary for you to then move into this cell to have a smoother transition.
A more seamless transition, so you won't notice that it happened, and that's
called the Soft Handoff. And that worked very well for CDMA.
In fact it works much better withr CDMA than the Hard Handoff works for FDMA or
TDMA. because it's much smoother and gives much
better transition in call quality.