<s>
Space	O
–	O
time	O
block	O
coding	O
based	O
transmit	B-Protocol
diversity	I-Protocol
(	O
STTD	O
)	O
is	O
a	O
method	O
of	O
transmit	B-Protocol
diversity	I-Protocol
used	O
in	O
UMTS	B-Protocol
third-generation	B-Architecture
cellular	O
systems	O
.	O
</s>
<s>
STTD	O
is	O
one	O
of	O
numerous	O
open	O
loop	O
transmit	B-Protocol
diversity	I-Protocol
schemes	O
which	O
also	O
include	O
Phase	O
Switched	O
Transmit	B-Protocol
Diversity	I-Protocol
(	O
PSTD	O
)	O
,	O
Time	O
Switched	O
Diversity	O
(	O
TSTD	O
)	O
,	O
Orthogonal	O
Transmit	B-Protocol
Diversity	I-Protocol
(	O
OTD	O
)	O
and	O
Space	O
Time	O
Spreading	O
(	O
STS	O
)	O
 [ 1 ] 	O
.	O
</s>
<s>
Diversity	O
improves	O
link	O
reliability	O
for	O
each	O
user	O
over	O
time	O
,	O
especially	O
near	O
cell	O
edges	O
(	O
in	O
the	O
absence	O
of	O
soft	B-Protocol
handoff	I-Protocol
)	O
,	O
and	O
also	O
the	O
average	O
performance	O
of	O
an	O
ensemble	O
of	O
users	O
at	O
any	O
particular	O
instant	O
.	O
</s>
<s>
However	O
,	O
an	O
open	O
loop	O
transmit	B-Protocol
diversity	I-Protocol
scheme	O
must	O
not	O
degrade	O
performance	O
for	O
a	O
user	O
close	O
to	O
the	O
base	O
station	O
where	O
the	O
channels	O
may	O
be	O
line	O
of	O
sight	O
and	O
nearly	O
ideal	O
.	O
</s>
<s>
STTD	O
can	O
be	O
applied	O
to	O
single	O
symbols	O
in	O
QAM	O
,	O
CDMA	O
code	O
words	O
,	O
or	O
subcarrier	O
symbols	O
in	O
OFDM	B-Protocol
and	O
the	O
transmit	O
method	O
has	O
become	O
standardised	O
,	O
especially	O
in	O
3G	B-Architecture
cellular	I-Architecture
wireless	O
 [ 2 ] 	O
as	O
described	O
below	O
.	O
</s>
<s>
Note	O
that	O
in	O
the	O
inscrutable	O
3G	B-Architecture
specifications	O
,	O
for	O
example	O
TS125.211	O
,	O
a	O
consecutive	O
pair	O
of	O
transmitted	O
QPSK	O
symbols	O
,	O
after	O
coding	O
,	O
interleaving	O
etc.	O
,	O
is	O
defined	O
by	O
a	O
logical	O
binary	O
string	O
of	O
four	O
bits	O
:	O
</s>
<s>
In	O
OFDM	B-Protocol
applications	O
such	O
as	O
Long	O
Term	O
Evolution	O
(	O
LTE	O
)	O
two	O
transmit	O
element	O
STTD	O
is	O
optionally	O
applied	O
just	O
as	O
above	O
while	O
there	O
is	O
also	O
a	O
4-element	O
option	O
.	O
</s>
