4 PMD Compensation

8/6/2019 4 PMD Compensation

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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

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4 PMD Compensation