Tektronix OM4006D, User Manual

Page: 108 / 205

Core processing software guide
This worked example is for a single polarization QPSK signal. If a dual
polarization signal were used, then before the polarization rotation each
polarization of pSym would contain a mixture of the two polarization tributaries.
After polarization rotation the top row of pSym.Values would contain one
polarization tributary, and the bottom row the other.
EstimateSOP has pSym as an input, and produces a Jones matrix RotM as an
output. RotM is a type of matrix called an orthogonal matrix, which represents
a rotation. Then pSym is rotated by the inverse of RotM to separate the X and
Y tributaries. zXSym is assigned to the X polarization part, and zYSym the Y
polarization part. zXSym and zYSym are time series of complex numbers, not
Jones vectors like pSym.
Initial phase estimate
zXSym contains the signal, but it does not appear as four separate constellation
points yet, because the phase of the signal is continually changing. Instead zXSym
appears as a ring of points. The phase is calculated by EstimatePhase and assigned
to variable ThetaSymX. The phase has two components. ThetaSymX.CentFreq
is the heterodyne frequency, the difference frequency between the center of the
signal spectrum and the local oscillator. The constellation effectively spins at
this difference frequency before phase correction. ThetaSymX.Values contains
the random phase component, in radians, which arises from phase noise in the
signal and local oscillator lasers.
zXSym is corrected for ThetaSymX by the function ApplyPhase. At this point
zXSym appears as a conventional QPSK signal, four clusters of constellation
points on the corners of a square.
Align signal tributaries
with data content
Although the signal appears to be a good QPSK constellation, a further adjustment
in phase is typically required. The phase estimation algorithm in EstimatePhase
randomly produces a phase offset which is a multiple of 90° away from the
true phase. The constellation may be 0, 1, 2 or 3 quarter turns from the true
constellation. The function AlignTribs compares the actual data content of the
two tributaries with the known data content speci
fi ed in PattXRe and PattXIm.
These two variables are loaded by the OUI with the data patterns speci fi ed in
the Analysis Parameters window. (See page 32, Analysis Parameters window .)
If the inphase component of zXSym contains the data pattern of the quadrature
component, and vice versa, then AlignTribs produces an output DRotM which is
equivalent to a quarter turn to correct the phase of zXSym. AlignTribs decides the
phase correction. (See page 91,
AlignTribs .) The function uses several criteria,
including tie-breaker criteria if both tributaries have the same data pattern.
When a dual polarization signal is used AlignTribs may exchange the SOPs as
well as adjusting the phase. If AlignTribs were not applied then the tributaries
would be randomly mapped to different components in the OUI display each
acquisition. If one tributary had different features from the others, for example a
bias offset, then that feature would appear to randomly jump between the different
eye diagrams each acquisition, instead of staying in the same place.
84 OM4000D Series Coherent Lightwave Signal Analyzer