Outline

Romain Rousseau

Tutor: Zaki Ahmed

Master of Science (MSc)Communication Engineering and Signal ProcessingDepartment of Communication and Electronic Engineering

University of Plymouth, Faculty of TechnologySeptember 2007Research in PRML for High Speed Optical Communications1147

2.b) Issues3

Extracted from: Introduction to Fibre Optics and Optical Networks (2007) Available http://www.engineeringlab.com/fiberoptics.html#demoBackgroundISIPRML technologyProjectRealisationResultsConclusionFuture works

Outline441.a) ISI from Fibre optics551.a) InterSymbol Interference

signal pulse of a bitRespectively amplitude of electric and magnetic fieldsAngular frequency

Propagation constant

Normalised FrequencyRefractive Index Difference

61.a) InterSymbol Interference7

From a polarisation point of view :Fast axis: Mode propagation along the x-axis has the smaller effective mode indexSlow axis: Mode propagation along the y-axis has the smaller effective mode index

Birefringence-Polarization Mode Dispersion effects

Modal birefringenceBeat length

Phase birefringence :1.a) InterSymbol Interference8Total Dispersion or Chromatic Dispersion

velocity of the light becomes wavelength dependent

1.a) InterSymbol Interference91.a) InterSymbol Interference10Fibre will have material dispersion if

Material dispersion

Is the core refractive indexMaterial Dispersion

1.a) InterSymbol Interference11A waveguide is a device such as coaxial cable or glass fibre, designed to confine and direct the propagation of electromagnetic waves. In optical fibers the confinement is achieved by having a region with a larger refractive index sterberg Ulf L Waveguide Dispersionfibre will have waveguide dispersion if

is constant propagation

waveguide dispersion

1.a) InterSymbol Interference12

rms pulse broadening data rate limitation total dispersion parameter Total Dispersion

1.a) InterSymbol Interference13Line CodingLine coding consists of modulating binary data in order to map symbols into specific waveform, before sending them to the receiver.3 points that need to be opposed when you want to choose your line coding:

The coded spectrum goes to zero at the frequency approaches 0 (DC energy can not be transmitted)A clock can be recovered from the coded data stream (necessary for synchronous systems and detection)They can detect errors or even correct errors 1.a) InterSymbol Interference14

Different modulations for data sequence 010111001Line Coding1.a) InterSymbol Interference15Non Return to Zero (NRZ) format also known as NRZ-OOK (On Off Keying). This modulation is composed of two sub-modulation Unipolar NRZ: values interpreted by the photodiode is +v for a logical 1 and 0v for a logical 0. Polar NRZ: values interpreted by the photodiode is +v for a logical 1 and v for a logical 0.

Line Coding

NRZ format needs all the bandwidth to transmit information Can not be used for synchronous systems because 1.a) InterSymbol Interference16Return to Zero (RZ) format is composed of two sub-modulationUnipolar RZ: values interpreted by the photodiode for a logical 1 is +v for and 0v for next In the case of a logical 0, current delivered by photodiode is a 0v for a full period of Polar RZ: values interpreted by the photodiode for a logical 1 is the same than Unipolar RZ logical 1. For a logical 0, voltage is v for a period of and then 0v for next

Line Coding

Both RZ format require 2 times bandwidth than a NRZ format.RZ format increases lifetime of laser. RZ Polar format enables to extract timing for synchronous-systems.1.a) InterSymbol Interference17Line CodingSummary

1.b) PRML Technology18PRML is a combination of a PR target (created with an equalizer) associated to a Maximum Likelihood Sequence Decoder (MLSD). 181.b) PRML Technology19In 1962 Adam Lender presents PR targets in his paper The Duobinary Technique for High-Speed Data Transmission [5]It was first introduced in magnetic recording system by Kobayashi and Tang, two researchers of IBM, in 1970Recently, many scientists have proved that duobinary signalling allows high bandwidth frequency and is a very efficient way to counter dispersion effects of fibre opticsPR targets require less power and complexity than other Multi level schemes.The aim of duobinary signalling is to accept and to transform ISI into a specific pattern. Then the receiver is built depending on this pattern and it makes use of the nature of the ISI instead of cancelling it.1.b) PRML Technology20In 1966 Kretzmer shorted Partial response channels in different class specifying the partial response polynomial H(D), where D is a delay

For a minimum bandwidth system must have in H(D), but for k>>0 number of levels is increase so error performance is degraded

factor reduces low-frequency component in spectrum needed

1.b) PRML Technology21The characteristic polynomial of widely used channel is a composition of

This partial response is called Modified Duobinary or PR Class IV (PR4) For channel models are usually referred to as extended Class-4 models

and denoted by

22

1.b) PRML Technology23

Basically an equalizer analyses channel distortions by receiving a specific impulse response. Differences between original pulse and received pulse by equaliser correspond to the channel deformationsBy adjusting parameters {Cn} of equalizer we can correct attenuationsPreset Equalizer1.b) PRML Technology24

Ch(t) is channels pulse response Preset EqualizerOutput of equalizer

Example for a EPR4 target (sequence number 1,2,1) and =T

1.b) PRML Technology25Convolutional codes are codes with memory which means that the output depends on previous inputs.

The code rate of convolutional codes is a ratio between numbers of input symbols k over numbers of output symbols n. PR code rate considered as 1-rate.

The length of input block K is often called constraint length. Maximum Likelihood Sequence DecoderPR4 system is non-recursive convolutional code.

Recursive convolutional codes which have both feed forward and feedback nodes.Non-recursive convolutional codes which have only feed forward nodes1.b) PRML Technology26Maximum Likelihood Sequence Decoder

Trellis Creation:

1.b) PRML Technology27Most famous MLSD: Viterbi AlgorithmDecoding Process:

Estimate the new path metric, by adding the path metric with the survivor path from state k-1 respectively for each states available on the trellis.For each path, select the survivor path with the best metric.Store the survivor path and metric for each vertex at time k, increment k and repeat the three steps above until end of data.Choose the minimum value from all possible vertexes and trace back from end to first state and output the bit information using the path matrix saved.

1.b) PRML Technology2.a) Investigations28282.a) My Realisation29

Single mode fibre link1st block: Attenuation generated with a 7th order elliptic Low Pass filter using a normalised cutoff frequency of 0.21. The ripple in the passband is 0.3 dB and 50dB for stopband.2nd block:Preset equalizer with EPR4 and E3PR4 targets.3rd block: AWGN noise. It gives details about strength of system especially the decoder part. 4th block: Detector which consists of a Soft Input Hard Output (SIHO) Viterbi Algorithm

2.a) My Realisation301st Block: Low Pass Filter

On the left, this is the pulse used to estimate the coefficient for equalizer On the right is a typical attenuation for single mode fibre2.a) My Realisation31

2nd Block: EqualizerEPR4E3PR4

2.a) My Realisation32 2nd Block: Equalizer

2.a) My Realisation333rd Block: AWGN noiseAWGN noise is to observe the strength of our system especially the decoder part. this noise can be assumed as a noise created by photodiode when converting optical impulse into electric voltage. We know that the best photodiode systems to convert data from optical to electric are avalanche photodiode, and they produce a significant noise 2.a) My Realisation344th Block: Viterbi Algorithm

EPR4E3PR42.a) My Realisation354th Block: Viterbi Algorithm

EPR4E3PR42.a) My Realisation364th Block: Viterbi Algorithm

E3PR4

2.b) Results37372.b) Results38

Number of taps for equalizerEPR4 = 11 tapsE3PR4=17 taps2.b) Results39Number of taps for equalizer

EPR4 = 11 tapsE3PR4=17 taps2.b) Results40Result of equalization

NRZ-OOK EPR4=11taps

E3PR4=17 taps2.b) Results41Strength of PR target

2.b) Results42CPU time/delay created by Viterbi Algorithm

The processor used was an Intel Centrino Duo T2500@2 GHz2.b) Results43Global view

44

2.b) ResultsIssuesIdeal Coefficients {Cn}3.a) Future works45MLSD is not optimised and deeper searches must be done .Bonek, et al. developed a method to simplify 16-state trellis to 8 states by considering a butterfly architecture. It reduces 47% wiring and has generalised the algorithm for n-states trellis. Farkas-Weiss and Kalet (1989) Simulation of a Trellis Code with Duobinary Signaling IEEE The Sixteenth Conference of Electrical & Electronics Engineers in Israel Pages:1 4

Ozarow and Calderbank have also proposed appropriate trellis to duobinary channel resulting in a rate code. The code rate is also a part of MLSD that can be investigated for next projectsWeiss S.F. and Russell A. (????), Simulation of a trellis code with Duobinary Signaling, Tel-Aviv UniversityPR targetGPRML provides 4.5dB gain compare to EPR4ML according to Suns paperSun D., Xotta A., and Abidi. A.A., (2005) A 1 GHz CMOS Analog Front-End for a Generalized PRMLRead Channel. IEEE Journal of Solid-State Circuits, vol. 40, no. 11, pp. 2275-2285.Code is essentialPR associated to serial turbo scheme (A turbo coding much interesting for data storage) and LDPC codes obtained 5dB gain compare to an uncoded PRML channel. (Song et.al.)Song H.,Liu J.,Kumar V. and Kurtas E. (2001) Iterative soft decoded partial response channels for hybridmagneto-optical recording, Magnetics, IEEE Transactions on, Volume 37, Issue 2 Page(s):676 681Questions ???Thanks you for your attention46Elijah Wu (2006), Measuring Chromatic Dispersion of Single-Mode Optical Fibres using White Light Interferometry , University of Auckland, MsC ThesisSenior J. (1992), Optical Fiber Communication: Principles and Practice Prentice Hall; 2nd editionReeves, 2007, Optical Fibres, University of Plymouth LectureLender A. (1963) The Duobinary Technique for High-Speed Data Transmission IEEE Transactions on Communication and Electronics, Vol. 82, pp. 214-218Kobayashi H. and Tang D. (1970) "Application of Partial-Response Channel Coding to Magnetic Recording Systems", IBM Journal. of Res. & Dev., vol. 14, no. 4, p. 368Bosco G.,Carena A., Curri V.and Poggiolini P. (2006) Best Optical Filtering for Duobinary Transmission from Optical Communication Theory and Techniques Springer US Lyubomirsky I. (2006) Coherent detection for optical duobinary communication systems, Photonics Technology Letters, IEEE, Vol 18, Issue 7 pp. 868 870Kretzmer E. R. (1966) "Generalization of a Technique for Binary Data Communication, IEEE Transactions on Communication Technology, February 1966, pp. 67-68Kabal, P. and Pasupathy S. (1975) "Partial-Response Signaling", IEEE Transactions On Communications, vol. Com-23, No. 9, pp. 921-934.4648

2.b) Issues492.b) Issues

conclusion