<s>
Hybrid	B-Error_Name
automatic	I-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
hybrid	B-Error_Name
ARQ	I-Error_Name
or	O
HARQ	B-Error_Name
)	O
is	O
a	O
combination	O
of	O
high-rate	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
and	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
error-control	O
.	O
</s>
<s>
In	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
,	O
the	O
original	O
data	O
is	O
encoded	O
with	O
an	O
FEC	O
code	O
,	O
and	O
the	O
parity	O
bits	O
are	O
either	O
immediately	O
sent	O
along	O
with	O
the	O
message	O
or	O
only	O
transmitted	O
upon	O
request	O
when	O
a	O
receiver	O
detects	O
an	O
erroneous	O
message	O
.	O
</s>
<s>
The	O
ED	O
code	O
may	O
be	O
omitted	O
when	O
a	O
code	O
is	O
used	O
that	O
can	O
perform	O
both	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
in	O
addition	O
to	O
error	B-Error_Name
detection	I-Error_Name
,	O
such	O
as	O
a	O
Reed	B-Error_Name
–	I-Error_Name
Solomon	I-Error_Name
code	I-Error_Name
.	O
</s>
<s>
As	O
a	O
result	O
,	O
hybrid	B-Error_Name
ARQ	I-Error_Name
performs	O
better	O
than	O
ordinary	O
ARQ	O
in	O
poor	O
signal	O
conditions	O
,	O
but	O
in	O
its	O
simplest	O
form	O
this	O
comes	O
at	O
the	O
expense	O
of	O
significantly	O
lower	O
throughput	O
in	O
good	O
signal	O
conditions	O
.	O
</s>
<s>
There	O
is	O
typically	O
a	O
signal	O
quality	O
cross-over	O
point	O
below	O
which	O
simple	O
hybrid	B-Error_Name
ARQ	I-Error_Name
is	O
better	O
,	O
and	O
above	O
which	O
basic	O
ARQ	O
is	O
better	O
.	O
</s>
<s>
The	O
simplest	O
version	O
of	O
HARQ	B-Error_Name
,	O
Type	O
I	O
HARQ	B-Error_Name
,	O
adds	O
both	O
ED	O
and	O
FEC	O
information	O
to	O
each	O
message	O
prior	O
to	O
transmission	O
.	O
</s>
<s>
When	O
the	O
coded	O
data	O
block	O
is	O
received	O
,	O
the	O
receiver	O
first	O
decodes	O
the	O
error-correction	B-Error_Name
code	O
.	O
</s>
<s>
In	O
a	O
more	O
sophisticated	O
form	O
,	O
Type	O
II	O
HARQ	B-Error_Name
,	O
the	O
message	O
originator	O
alternates	O
between	O
message	O
bits	O
along	O
with	O
error-detecting	O
parity	O
bits	O
and	O
only	O
FEC	O
parity	O
bits	O
.	O
</s>
<s>
Also	O
,	O
two	O
consecutive	O
transmissions	O
can	O
be	O
combined	O
for	O
error	B-Error_Name
correction	I-Error_Name
if	O
neither	O
is	O
error	O
free	O
.	O
</s>
<s>
To	O
understand	O
the	O
difference	O
between	O
Type	O
I	O
and	O
Type	O
II	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
,	O
consider	O
the	O
size	O
of	O
ED	O
and	O
FEC	O
added	O
information	O
:	O
error	B-Error_Name
detection	I-Error_Name
typically	O
only	O
adds	O
a	O
couple	O
of	O
bytes	O
to	O
a	O
message	O
,	O
which	O
is	O
only	O
an	O
incremental	O
increase	O
in	O
length	O
.	O
</s>
<s>
FEC	O
,	O
on	O
the	O
other	O
hand	O
,	O
can	O
often	O
double	O
or	O
triple	O
the	O
message	O
length	O
with	O
error	B-Error_Name
correction	I-Error_Name
parities	O
.	O
</s>
<s>
In	O
standard	O
ARQ	O
a	O
transmission	O
must	O
be	O
received	O
error	O
free	O
on	O
any	O
given	O
transmission	O
for	O
the	O
error	B-Error_Name
detection	I-Error_Name
to	O
pass	O
.	O
</s>
<s>
In	O
Type	O
II	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
,	O
the	O
first	O
transmission	O
contains	O
only	O
data	O
and	O
error	B-Error_Name
detection	I-Error_Name
(	O
no	O
different	O
from	O
standard	O
ARQ	O
)	O
.	O
</s>
<s>
If	O
data	O
is	O
received	O
in	O
error	O
,	O
the	O
second	O
transmission	O
will	O
contain	O
FEC	O
parities	O
and	O
error	B-Error_Name
detection	I-Error_Name
.	O
</s>
<s>
If	O
received	O
in	O
error	O
,	O
error	B-Error_Name
correction	I-Error_Name
can	O
be	O
attempted	O
by	O
combining	O
the	O
information	O
received	O
from	O
both	O
transmissions	O
.	O
</s>
<s>
Only	O
Type	O
I	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
suffers	O
capacity	O
loss	O
in	O
strong	O
signal	O
conditions	O
.	O
</s>
<s>
Type	O
II	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
does	O
not	O
because	O
FEC	O
bits	O
are	O
only	O
transmitted	O
on	O
subsequent	O
re-transmissions	O
as	O
needed	O
.	O
</s>
<s>
In	O
strong	O
signal	O
conditions	O
,	O
Type	O
II	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
performs	O
with	O
as	O
good	O
capacity	O
as	O
standard	O
ARQ	O
.	O
</s>
<s>
In	O
poor	O
signal	O
conditions	O
,	O
Type	O
II	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
performs	O
with	O
as	O
good	O
sensitivity	O
as	O
standard	O
FEC	O
.	O
</s>
<s>
This	O
is	O
called	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
with	O
soft	O
combining	O
(	O
Dahlman	O
et	O
al.	O
,	O
p.120	O
)	O
.	O
</s>
<s>
There	O
are	O
two	O
main	O
soft	O
combining	O
methods	O
in	O
HARQ	B-Error_Name
:	O
</s>
<s>
Chase	B-Error_Name
combining	I-Error_Name
:	O
every	O
re-transmission	O
contains	O
the	O
same	O
information	O
(	O
data	O
and	O
parity	O
bits	O
)	O
.	O
</s>
<s>
Because	O
all	O
transmissions	O
are	O
identical	O
,	O
Chase	B-Error_Name
combining	I-Error_Name
can	O
be	O
seen	O
as	O
additional	O
repetition	B-Error_Name
coding	I-Error_Name
.	O
</s>
<s>
For	O
example	O
,	O
in	O
partial	O
Chase	B-Error_Name
combining	I-Error_Name
only	O
a	O
subset	O
of	O
the	O
bits	O
in	O
the	O
original	O
transmission	O
are	O
re-transmitted	O
.	O
</s>
<s>
An	O
example	O
of	O
incremental	O
redundancy	O
HARQ	B-Error_Name
is	O
HSDPA	O
:	O
the	O
data	O
block	O
is	O
first	O
coded	O
with	O
a	O
punctured	O
1/3	O
Turbo	B-Error_Name
code	I-Error_Name
,	O
then	O
during	O
each	O
(	O
re	O
)	O
transmission	O
the	O
coded	O
block	O
is	O
usually	O
punctured	O
further	O
(	O
i.e.	O
</s>
<s>
Although	O
the	O
HSDPA	O
standard	O
supports	O
both	O
Chase	B-Error_Name
combining	I-Error_Name
and	O
incremental	O
redundancy	O
,	O
it	O
has	O
been	O
shown	O
that	O
incremental	O
redundancy	O
almost	O
always	O
performs	O
better	O
than	O
Chase	B-Error_Name
combining	I-Error_Name
,	O
at	O
the	O
cost	O
of	O
increased	O
complexity	O
.	O
</s>
<s>
HARQ	B-Error_Name
can	O
be	O
used	O
in	O
stop-and-wait	B-Error_Name
mode	O
or	O
in	O
selective	B-Error_Name
repeat	I-Error_Name
mode	O
.	O
</s>
<s>
Stop-and-wait	B-Error_Name
is	O
simpler	O
,	O
but	O
waiting	O
for	O
the	O
receiver	O
's	O
acknowledgment	O
reduces	O
efficiency	O
.	O
</s>
<s>
Thus	O
multiple	O
stop-and-wait	B-Error_Name
HARQ	B-Error_Name
processes	O
are	O
often	O
done	O
in	O
parallel	O
in	O
practice	O
:	O
when	O
one	O
HARQ	B-Error_Name
process	O
is	O
waiting	O
for	O
an	O
acknowledgment	O
,	O
another	O
process	O
can	O
use	O
the	O
channel	O
to	O
send	O
some	O
more	O
data	O
.	O
</s>
<s>
There	O
are	O
other	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
codes	O
that	O
can	O
be	O
used	O
in	O
an	O
HARQ	B-Error_Name
scheme	O
besides	O
Turbo	B-Error_Name
codes	I-Error_Name
,	O
e.g.	O
</s>
<s>
extended	O
irregular	O
repeat-accumulate	O
(	O
eIRA	O
)	O
code	O
and	O
Efficiently-Encodable	O
Rate-Compatible	O
(	O
E2RC	O
)	O
code	O
,	O
both	O
of	O
which	O
are	O
low-density	B-Error_Name
parity-check	I-Error_Name
codes	I-Error_Name
.	O
</s>
<s>
HARQ	B-Error_Name
is	O
used	O
in	O
HSDPA	O
and	O
HSUPA	O
which	O
provide	O
high	O
speed	O
data	O
transmission	O
(	O
on	O
downlink	O
and	O
uplink	B-Application
,	O
respectively	O
)	O
for	O
mobile	O
phone	O
networks	O
such	O
as	O
UMTS	B-Protocol
,	O
and	O
in	O
the	O
IEEE	O
802.16-2005	O
standard	O
for	O
mobile	O
broadband	O
wireless	O
access	O
,	O
also	O
known	O
as	O
"	O
mobile	O
WiMAX	O
"	O
.	O
</s>
<s>
Type	O
I	O
Hybrid	B-Error_Name
ARQ	I-Error_Name
is	O
used	O
in	O
ITU-T	O
G.hn,	O
a	O
high-speed	O
Local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
standard	O
that	O
can	O
operate	O
at	O
data	O
rates	O
up	O
to	O
1	O
Gbit/s	O
over	O
existing	O
home	O
wiring	O
(	O
power	B-Device
lines	I-Device
,	O
phone	O
lines	O
and	O
coaxial	B-Protocol
cables	I-Protocol
)	O
.	O
</s>
<s>
G.hn	B-General_Concept
uses	O
CRC-32C	O
for	O
Error	B-Error_Name
Detection	I-Error_Name
,	O
LDPC	B-Error_Name
for	O
Forward	B-Error_Name
Error	I-Error_Name
Correction	I-Error_Name
and	O
Selective	B-Error_Name
Repeat	I-Error_Name
for	O
ARQ	O
.	O
</s>
