<s>
Unipolar	B-Protocol
encoding	I-Protocol
is	O
a	O
line	B-Protocol
code	I-Protocol
.	O
</s>
<s>
It	O
is	O
the	O
simplest	O
line	B-Protocol
code	I-Protocol
,	O
directly	O
encoding	O
the	O
bitstream	O
,	O
and	O
is	O
analogous	O
to	O
on-off	B-Device
keying	I-Device
in	O
modulation	O
.	O
</s>
<s>
Its	O
drawbacks	O
are	O
that	O
it	O
is	O
not	O
self-clocking	O
and	O
it	O
has	O
a	O
significant	O
DC	O
component	O
,	O
which	O
can	O
be	O
halved	O
by	O
using	O
return-to-zero	B-Protocol
,	O
where	O
the	O
signal	O
returns	O
to	O
zero	O
in	O
the	O
middle	O
of	O
the	O
bit	O
period	O
.	O
</s>
<s>
Traditionally	O
,	O
a	O
unipolar	O
scheme	O
was	O
designed	O
as	O
a	O
non-return-to-zero	B-Protocol
(	O
NRZ	O
)	O
scheme	O
,	O
in	O
which	O
the	O
positive	O
voltage	O
defines	O
bit	O
1	O
and	O
the	O
zero	O
voltage	O
defines	O
bit	O
0	O
.	O
</s>
<s>
It	O
is	O
called	O
NRZ	O
because	O
the	O
signal	O
does	O
not	O
return	B-Protocol
to	I-Protocol
zero	I-Protocol
at	O
the	O
middle	O
of	O
the	O
bit	O
,	O
as	O
instead	O
happens	O
in	O
other	O
line	B-Protocol
coding	I-Protocol
schemes	O
,	O
such	O
as	O
Manchester	B-Protocol
code	I-Protocol
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
unipolar	B-Protocol
encoding	I-Protocol
is	O
not	O
normally	O
used	O
in	O
data	O
communications	O
today	O
.	O
</s>
<s>
They	O
are	O
used	O
in	O
optical	O
communications	O
to	O
enable	O
CDMA	B-Protocol
in	O
optical	O
fiber	O
transmission	O
.	O
</s>
