EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 255)

Optical Modulation Analyzer 247
D Modulation Schemes
This appendix describes modulation schemes supported by the PSO-200.
All formats can be RZ (pulse carved) or NRZ. The pulse carver is usually
placed after the data modulator, and synchronized with the symbol rate.
Hence, when a pulse carver is used, it does not affect the constellation
points, but only the transitions, since the power goes down to zero
between each symbol.
As an example, the table in this section shows QPSK with (RZ) and without
(NRZ) pulse carving. Your pulse carver can be operated in different modes
depending on your transmitter and signal, for example, 33 % RZ, 50 % RZ,
or 66 % RZ (carrier suppressed), which affects the optical spectral content.
A general transmitter may be considered as a “black box” that only
describes the mapping, as shown below.
The mapping concept must be understood in order to specify a mapping
for a specific transmitter. As an example, consider the 16-QAM default
implementation below.
The X
0
-X
1
arm has an I/Q modulator that generates a QPSK constellation,
whereas the X
2
-X
3
arm generates a smaller QPSK constellation (since it
has an attenuator included). Hence, the X
0
-X
1
arm determines the
quadrant in the 16-QAM constellation, whereas the X
2
-X
3
arm determines
the symbol in each quadrant, as illustrated.
Q
I
Tx
B
3
, B
2
, B
1
, B
0
, ...
MZM π/2
MZM
MZM π/2
MZM
I
Q
I
Q
X
0
X
1
X 2
X
3
Attn