<s>
Carrier-Suppressed	B-Protocol
Return-to-Zero	I-Protocol
(	O
CSRZ	B-Protocol
)	O
is	O
an	O
optical	O
line	B-Protocol
code	I-Protocol
.	O
</s>
<s>
In	O
CSRZ	B-Protocol
the	O
field	O
intensity	O
drops	O
to	O
zero	O
between	O
consecutive	O
bits	O
(	O
RZ	B-Protocol
)	O
,	O
and	O
the	O
field	O
phase	O
alternates	O
by	O
π	O
radians	O
between	O
neighbouring	O
bits	O
,	O
so	O
that	O
if	O
the	O
phase	O
of	O
the	O
signal	O
is	O
e.g.	O
</s>
<s>
In	O
its	O
standard	O
form	O
CSRZ	B-Protocol
is	O
generated	O
by	O
a	O
single	O
Mach	O
–	O
Zehnder	O
modulator	O
(	O
MZM	O
)	O
,	O
driven	O
by	O
two	O
sinusoidal	O
waves	O
at	O
half	O
the	O
bit	O
rate	O
BR	O
,	O
and	O
in	O
phase	O
opposition	O
.	O
</s>
<s>
The	O
signal	O
format	O
Alternate-Phase	B-Protocol
Return-to-Zero	I-Protocol
(	O
APRZ	B-Protocol
)	O
can	O
be	O
viewed	O
as	O
a	O
generalisation	O
of	O
CSRZ	B-Protocol
in	O
which	O
the	O
phase	O
alternation	O
can	O
take	O
up	O
any	O
value	O
ΔΦ	O
(	O
and	O
not	O
necessarily	O
only	O
π	O
)	O
and	O
the	O
duty	O
cycle	O
is	O
also	O
a	O
free	O
parameter	O
.	O
</s>
<s>
CSRZ	B-Protocol
can	O
be	O
used	O
to	O
generate	O
specific	O
optical	O
modulation	O
formats	O
,	O
e.g.	O
</s>
<s>
CSRZ-OOK	O
,	O
in	O
which	O
data	O
is	O
coded	O
on	O
the	O
intensity	O
of	O
the	O
signal	O
using	O
a	O
binary	O
scheme	O
(	O
light	O
on	O
=	O
1	O
,	O
light	O
off	O
=	O
0	O
)	O
,	O
or	O
CSRZ-DPSK	O
,	O
in	O
which	O
data	O
is	O
coded	O
on	O
the	O
differential	O
phase	O
of	O
the	O
signal	O
,	O
etc	O
.	O
</s>
<s>
CSRZ	B-Protocol
is	O
often	O
used	O
to	O
designate	O
APRZ-OOK	O
.	O
</s>
<s>
The	O
characteristic	O
properties	O
of	O
an	O
CSRZ	B-Protocol
signal	O
are	O
those	O
to	O
have	O
a	O
spectrum	O
similar	O
to	O
that	O
of	O
an	O
RZ	B-Protocol
signal	O
,	O
except	O
that	O
frequency	O
peaks	O
(	O
still	O
at	O
a	O
spacing	O
of	O
BR	O
)	O
are	O
shifted	O
by	O
BR/2	O
with	O
respect	O
to	O
RZ	B-Protocol
,	O
so	O
that	O
no	O
peak	O
is	O
present	O
at	O
the	O
carrier	O
and	O
power	O
is	O
ideally	O
zero	O
at	O
the	O
carrier	O
frequency	O
(	O
hence	O
the	O
name	O
)	O
.	O
</s>
<s>
Compared	O
to	O
standard	O
RZ-OOK	O
,	O
the	O
CSRZ-OOK	O
is	O
considered	O
to	O
be	O
more	O
tolerant	O
to	O
filtering	O
and	O
chromatic	O
dispersion	O
,	O
thanks	O
to	O
its	O
narrower	O
spectrum	O
.	O
</s>
