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PMD mitigation Techniques
PMD mitigation techniques are divided into 3-
approaches:
Passive mitigation or increasing PMD tolerance
Electronic PMD compensation &
Optical PMD compensation
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Passive Techniques
Passive techniques includes:
PMD robust linecoding format
Soliton transmission
Allocating more margin to PMD distortion
Forward error correction (FEC) coding
Polarization scrambling
Selectively replace those segments in the link known to be dominant contributors tooverall link DGD
Regenerating the optical signal by placing a regenerator at a point where the DGDeffects is intolerable.
For DWDM systems, moving traffic off from the PMD impaired channel on to sparechannel. Channel switching.
Using specially manufactured very low PMD fiber.
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Advantage of passive techniques
They do not require any dynamically adjustedcomponents.
Bit rate independent
Easy to implement
Less costly, only requires encoder at transmitterand decoders at receiver while using PMD robustlinecoding or modulation formats.
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Line Coding
The manner in which data bits, or blocks of data
bits, are represented on a line. Examples
include 4B3T, 4B/5B, alternate mark inversion
(AMI), and Manchester.
In telecommunication, a line code (also calleddigital baseband modulation) is a code
chosen for use within a communications system
for transmission purposes.
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Line Coding
For digital data transport line coding is often used. Line coding
consists of representing the digital signal to be transported, by an
amplitude- and time-discrete signal, that is optimally tuned for the
specific properties of the physical channel (and of the receiving
equipment).
The waveform pattern of voltage or current used to represent the 1s
and 0s of a digital signal on a transmission link is called lineencoding. The common types of line encoding are uni-polar, polar,
bipolar and Manchester encoding.
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NRZ coding
Manchester linecoding
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PMD: Electronic compensation
Electronic PMD compensation involves
equalizing the electrical signal before the
receiver & after the photodetector.
Electronic compensator minimizes the ISI at
the receiving end regardless of the
phenomenon that causing it.
Electronic compensators require high speed
electronics for better performance.
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Adv. of electronic equalization
Electronic PMD compensators are
compact and low cost compared to optical
one.
Electronic equalizers can be integrated
into the receiver, thus saving the
installation costs
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Electronic PMD Comp. Techniques
Different electronic equalizers are:
(i) Linear traversal filter (LTF)
(ii) Feed forward equalizer (FFE)
(iii) Decision feedback equalizers (DFE) &
(iv) Maximum likelihood sequence estimation(MLSE)
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LinearTraversal Filter
The LTF divides the signal into two copies,
delays the copies by constant delay
stages and superimposes the differentially
delayed signals at output port.
The tap weights are adjusted to minimize
ISI in the received signal.
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LTF & DFE
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DFE
The DFE is a non-linear filter.
Non-linear filters are advantageous in thesense that they can improve signal quality
even the received eye-diagram is poor.
However, the DFE needs high speed
processors.
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MLSE scheme
The MLSE scheme is based on the correlation
between an undistorted signal sequence and an
estimate of the received signal sequence, over
many bits.
The selection of sequence and the maximization
of the correlation are the factors based on whichthe decision for each individual bit is made.
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Disadvantages of electronic equalization
As the data rate rise beyond 10 Gb/s, it will bechallenging task to perform electronicequalization.
Because of the difficulty in finding electronicdelay stages and filters that can operate at suchhigh speed ( >10Gb/s).
The square law detection drops all polarizationand phase information, they are not as effectiveas optical compensators.
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Optical PMD compensation
A typical optical PMD compensator usuallyconsists of 4-parts:
(i) A compensating element/ optical delay
element
(ii) A feedback signal generator
(iii) a feedback loop.
(iv) A control system
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Optical Compensator
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PMD monitoring: feedback signal
Observation of the power level in the received RFspectrum of the baseband signal.
Received eye diagram monitoring
Monitoring the degree of polarization (DOP) of thereceived optical signal. PMD reduces the DOP.
- DOP modulation format & bit rate independent
- DOP is also affected by ASE noise, non-linear effects.
dUnpolarizepolarized
polarized
PP
PDOP
!
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Optical comp. : Order of Compensation
Halforder: Comprises a polarization controller (PC) & afixed delay element. Feedback control mechanism toprovide control signals to the PC.
First order: it has variable delay element. A feedbackmechanism provides control signals for adjusting bothPC & delay element
Second order: It has two PC & two pieces of Hi-Bi fiber.The compensators PSPs are made to vary linearly withfrequency so as to compensate for PMD over a largerbandwidth.
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Disadvantage ofoptical compensation ofPMD
Optical scheme require expensive and relativelybulky optical components.
Also, because of the dynamic nature of PMD,compensators must be adaptive.
Adaptation is not easily achieved in the optical
domain because of the relative lack of flexibilityin optical components and because of thedifficulty in extracting an appropriate error signalto control the adaptation