<s>
Link	B-Protocol
adaptation	I-Protocol
,	O
comprising	O
adaptive	B-Protocol
coding	I-Protocol
and	I-Protocol
modulation	I-Protocol
(	O
ACM	O
)	O
and	O
others	O
(	O
such	O
as	O
Power	O
Control	O
)	O
,	O
is	O
a	O
term	O
used	O
in	O
wireless	O
communications	O
to	O
denote	O
the	O
matching	O
of	O
the	O
modulation	B-Application
,	O
coding	O
and	O
other	O
signal	O
and	O
protocol	O
parameters	O
to	O
the	O
conditions	O
on	O
the	O
radio	O
link	O
(	O
e.g.	O
</s>
<s>
For	O
example	O
,	O
WiMAX	O
uses	O
a	O
rate	O
adaptation	O
algorithm	O
that	O
adapts	O
the	O
modulation	B-Protocol
and	I-Protocol
coding	I-Protocol
scheme	I-Protocol
(	O
MCS	O
)	O
according	O
to	O
the	O
quality	O
of	O
the	O
radio	O
channel	O
,	O
and	O
thus	O
the	O
bit	O
rate	O
and	O
robustness	O
of	O
data	O
transmission	O
.	O
</s>
<s>
The	O
process	O
of	O
link	B-Protocol
adaptation	I-Protocol
is	O
a	O
dynamic	O
one	O
and	O
the	O
signal	O
and	O
protocol	O
parameters	O
change	O
as	O
the	O
radio	O
link	O
conditions	O
change	O
—	O
for	O
example	O
in	O
High-Speed	O
Downlink	O
Packet	O
Access	O
(	O
HSDPA	O
)	O
in	O
Universal	B-Protocol
Mobile	I-Protocol
Telecommunications	I-Protocol
System	I-Protocol
(	O
UMTS	B-Protocol
)	O
this	O
can	O
take	O
place	O
every	O
2	O
ms.	O
</s>
<s>
Adaptive	B-Protocol
modulation	I-Protocol
systems	O
invariably	O
require	O
some	O
channel	O
state	O
information	O
at	O
the	O
transmitter	O
.	O
</s>
<s>
Adaptive	B-Protocol
modulation	I-Protocol
systems	O
improve	O
rate	O
of	O
transmission	O
,	O
and/or	O
bit	O
error	O
rates	O
,	O
by	O
exploiting	O
the	O
channel	O
state	O
information	O
that	O
is	O
present	O
at	O
the	O
transmitter	O
.	O
</s>
<s>
Especially	O
over	O
fading	O
channels	O
which	O
model	O
wireless	O
propagation	O
environments	O
,	O
adaptive	B-Protocol
modulation	I-Protocol
systems	O
exhibit	O
great	O
performance	O
enhancements	O
compared	O
to	O
systems	O
that	O
do	O
not	O
exploit	O
channel	O
knowledge	O
at	O
the	O
transmitter	O
.	O
</s>
<s>
In	O
HSDPA	O
link	B-Protocol
adaptation	I-Protocol
is	O
performed	O
by	O
:	O
</s>
<s>
Choice	O
of	O
modulation	B-Application
type	O
—	O
the	O
link	O
can	O
employ	O
QPSK	O
for	O
noisy	O
channels	O
and	O
16QAM	B-Device
for	O
clearer	O
channels	O
.	O
</s>
<s>
The	O
former	O
is	O
more	O
robust	O
and	O
can	O
tolerate	O
higher	O
levels	O
of	O
interference	O
but	O
has	O
lower	O
spectral	B-Protocol
efficiency	I-Protocol
,	O
which	O
means	O
it	O
may	O
transmit	O
at	O
a	O
lower	O
bit	O
rate	O
for	O
a	O
given	O
bandwidth	O
.	O
</s>
<s>
The	O
latter	O
has	O
double	O
the	O
spectral	B-Protocol
efficiency	I-Protocol
,	O
but	O
is	O
more	O
prone	O
to	O
errors	O
due	O
to	O
interference	O
and	O
noise	O
hence	O
it	O
requires	O
stronger	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
coding	O
which	O
in	O
turn	O
means	O
more	O
redundant	O
bits	O
and	O
lower	O
information	O
bit	O
rate	O
;	O
</s>
<s>
Choice	O
of	O
FEC	O
code	O
rate	O
—	O
the	O
FEC	O
code	O
used	O
has	O
a	O
rate	O
of	O
1/3	O
,	O
but	O
it	O
can	O
be	O
varied	O
effectively	O
by	O
bit	O
puncturing	O
and	O
hybrid	B-Error_Name
automatic	I-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
HARQ	B-Error_Name
)	O
with	O
incremental	O
redundancy	O
.	O
</s>
<s>
In	O
very	O
bad	O
link	O
conditions	O
retransmissions	B-Protocol
occur	O
due	O
to	O
HARQ	B-Error_Name
which	O
ensures	O
correct	O
reception	O
of	O
the	O
sent	O
information	O
but	O
further	O
decreases	O
the	O
bit	O
rate	O
.	O
</s>
<s>
Thus	O
HSDPA	O
adapts	O
to	O
achieve	O
very	O
high	O
bit	O
rates	O
,	O
of	O
the	O
order	O
of	O
14	O
megabit/sec	O
,	O
on	O
clear	O
channels	O
using	O
16-QAM	B-Device
and	O
close	O
to	O
1/1	O
coding	O
rate	O
.	O
</s>
