<s>
In	O
telecommunications	O
,	O
a	O
diversity	B-Protocol
scheme	I-Protocol
refers	O
to	O
a	O
method	O
for	O
improving	O
the	O
reliability	O
of	O
a	O
message	O
signal	O
by	O
using	O
two	O
or	O
more	O
communication	O
channels	O
with	O
different	O
characteristics	O
.	O
</s>
<s>
Alternatively	O
,	O
a	O
redundant	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
code	O
may	O
be	O
added	O
and	O
different	O
parts	O
of	O
the	O
message	O
transmitted	O
over	O
different	O
channels	O
.	O
</s>
<s>
Diversity	O
techniques	O
may	O
exploit	O
the	O
multipath	O
propagation	O
,	O
resulting	O
in	O
a	O
diversity	B-Protocol
gain	I-Protocol
,	O
often	O
measured	O
in	O
decibels	O
.	O
</s>
<s>
The	O
following	O
classes	O
of	O
diversity	B-Protocol
schemes	I-Protocol
can	O
be	O
identified	O
:	O
</s>
<s>
Time	B-Protocol
diversity	I-Protocol
:	O
Multiple	O
versions	O
of	O
the	O
same	O
signal	O
are	O
transmitted	O
at	O
different	O
time	O
instants	O
.	O
</s>
<s>
Alternatively	O
,	O
a	O
redundant	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
code	O
is	O
added	O
and	O
the	O
message	O
is	O
spread	O
in	O
time	O
by	O
means	O
of	O
bit-interleaving	O
before	O
it	O
is	O
transmitted	O
.	O
</s>
<s>
Spread	B-Architecture
spectrum	I-Architecture
,	O
for	O
example	O
frequency	B-Protocol
hopping	I-Protocol
or	O
DS-CDMA	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
wireless	O
transmission	O
,	O
it	O
can	O
be	O
achieved	O
by	O
antenna	O
diversity	O
using	O
multiple	O
transmitter	O
antennas	O
(	O
transmit	B-Protocol
diversity	I-Protocol
)	O
and/or	O
multiple	O
receiving	O
antennas	O
(	O
reception	B-Protocol
diversity	I-Protocol
)	O
.	O
</s>
<s>
In	O
the	O
latter	O
case	O
,	O
a	O
diversity	B-Protocol
combining	I-Protocol
technique	O
is	O
applied	O
before	O
further	O
signal	O
processing	O
takes	O
place	O
.	O
</s>
<s>
If	O
the	O
antennas	O
are	O
far	O
apart	O
,	O
for	O
example	O
at	O
different	O
cellular	O
base	O
station	O
sites	O
or	O
WLAN	O
access	O
points	O
,	O
this	O
is	O
called	O
macrodiversity	B-Protocol
or	O
site	O
diversity	O
.	O
</s>
<s>
A	O
special	O
case	O
is	O
phased	O
antenna	B-Algorithm
arrays	I-Algorithm
,	O
which	O
also	O
can	O
be	O
used	O
for	O
beamforming	B-Algorithm
,	O
MIMO	O
channels	O
and	O
space	B-Protocol
–	I-Protocol
time	I-Protocol
coding	I-Protocol
(	O
STC	O
)	O
.	O
</s>
<s>
A	O
diversity	B-Protocol
combining	I-Protocol
technique	O
is	O
applied	O
on	O
the	O
receiver	O
side	O
.	O
</s>
<s>
Cooperative	B-Protocol
diversity	I-Protocol
:	O
Achieves	O
antenna	O
diversity	B-Protocol
gain	I-Protocol
by	O
using	O
the	O
cooperation	O
of	O
distributed	O
antennas	O
belonging	O
to	O
each	O
node	O
.	O
</s>
<s>
An	O
important	O
element	O
in	O
communication	O
systems	O
applying	O
diversity	B-Protocol
schemes	I-Protocol
is	O
the	O
"	O
Combiner	O
"	O
,	O
which	O
processes	O
the	O
redundantly	O
received	O
signals	O
.	O
</s>
<s>
To	O
combine	O
parallel	O
redundant	O
transmitted	O
longer	O
signal	O
sequences	O
,	O
for	O
example	O
network	B-Protocol
packets	I-Protocol
,	O
the	O
principle	O
of	O
a	O
Timing	O
Combiner	O
was	O
defined	O
in	O
2012	O
.	O
</s>
<s>
Similarly	O
working	O
like	O
a	O
Selection	O
Combiner	O
,	O
the	O
first	O
fully	O
received	O
and	O
valid	O
data	B-Protocol
packet	I-Protocol
will	O
be	O
immediately	O
further	O
processed	O
,	O
whereas	O
the	O
later	O
arriving	O
redundant	O
packets	B-Protocol
will	O
be	O
immediately	O
discarded	O
after	O
reception	O
.	O
</s>
