Tektronix OM4006D, User Manual

Page: 109 / 205

Core processing software guide
AlignTribs returns the pattern variables, PattXRe etc., as outputs in addition to
DRotM. The pattern variable outputs have additional
fi elds compared to the
pattern variable inputs, to indicate the synchronization location within the pattern.
For example, if PattXRe speci
fi es a pseudorandom bit sequence (PRBS), the
output PattXRe has a fi eld .Seed which contains the contents of the PRBS shift
register at time PattXRe.t0. Downstream functions in CoreProcessing.m are able
to generate the data sequence on the tributary from PattXRe.
Second phase estimate At this point the signal in zXSym appears ready to make a decision and count
the bit error rate. The constellation has the correct phase, comprising four tight
clusters, and the true data content on each tributary is known. However, in cases
where there is considerable phase noise (where the laser linewidths are not
narrow) the estimated phase may contain cycle slips. The phase is correct for the
initial portion of the record, and then after a cycle slip it becomes in error by a
quarter of a turn. This means the bit error rate is low or zero for the early portion,
and then suddenly rises to 0.5.
In principle the cycle slips can be avoided, given that the true data content of the
signal is known, and the purpose of the second phase estimate is to calculate the
phase without cycle slips. The starting point is the variable pSym, containing the
Jones vector of the signal resampled from the oscilloscope record. The correct
polarization rotation is applied to pSym, and the X component assigned to zXSym.
At this point zXSym can be thought of as the four-state QPSK constellation
rotating at the phase difference between signal and LO.
Next zXSym is multiplied by the converse of the (known) true data modulation on
the signal. This operation has the effect of removing the data modulation, so the
signal is equivalent to a single constellation state rotating at the phase difference
between signal and LO. EstimatePhase is called, to calculate the phase. This time
the SigType parameter passed to EstimatePhase is set to 0. With EstimatePhase,
SigType = 0 tells the function it is an unmodulated signal. The phase estimate
algorithm does not raise to the 4th power, with the inherent four-way ambiguity
in phase following the subsequent 4th root operation. The resulting ThetaSymX
does not contain cycle slips, no matter how high the level of phase noise.
With a dual polarization signal type, there is also a second polarization estimate.
The second SOP estimate is typically more accurate than the original SOP
estimate. The second SOP estimate proceeds in the same way as the second phase
estimate. The signal is multiplied by the converse of the known data modulation,
and EstimateSOP is applied with SigType = 0, as if it were an unmodulated signal.
Apply polarization & phase
estimates
The new cycle slip-free phase estimate ThetaSymX is applied to the resampled
oscilloscope record pSym via ApplyPhase, to produce a new zXSym variable.
This new zXSym inherently does not contain cycle slips.
OM4000D Series Coherent Lightwave Signal Analyzer 85