Cost Efficient Adaptive Technologies for High Rate Optical

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Transcript Cost Efficient Adaptive Technologies for High Rate Optical

University of Peloponnese
Adaptive Optical Technologies
for Optical Transmission Systems
Maki Nanou, George-Othon Glentis,
Kristina Georgoulakis, Chris Matrakidis,
Christina (Tanya) Politi, Alexandros Stavdas
Outline
 Basic Concepts of Optical Communications
 Fiber Impairments & Compensation Techniques
 Optical Transmission Simulations
 Results
 Conclusion
University of Peloponnese
Dept. of Inform. & Telecommunications
Traffic Growth
Until 2000: Voice Traffic dominates
(*) Cisco Forecast
60% / year
20% / year
After 2004: Data Traffic dominates
Traffic growth of 60% per year
outstrips the growth in capacity of
commercial systems.
While the entire traffic in North
American core Network could be
carried on a single fiber until 2008, in
2011 more than two fibers were
required.
Every 3 years the required number of
fibers will double.
Increased capacity ↔ Advanced Modulation
University of Peloponnese
Formats
Non Return To Zero – On – Off Keying
Output Intensity
quaternary
point bias
 Most commonly used (widely deployed)
 OOK: switching ON and OFF the
amplitude of an optical carrier signal
(Intensity Modulation Only)
Input  External Modulation: biased at the
Voltage
t
LASER
MZM optical
signal
Vπ swing
quadrature point of the MZM transfer
function, and driven by an electrical
binary NRZ-ASK signal with peak-topeak amplitude of Vπ.
 Simple Tx/Rx Configurations
University of Peloponnese
Dept. of Inform. & Telecommunications
Differential Phase Shift Keying - DPSK
Output Intensity
minimum
point bias
t
LASER
MZM
o Nearly constant envelope – higher
tolerance to non linear effects
 Higher receiver sensitivity due to the
t 3dB lower OSNR requirement to
Input
Voltage achieve a specific BER.
optical
signal
precoder
electrical
NRZ data
Phase Modulation Only
2Vπ swing
 External MZM biased at minimum
point and driven with a precoded binary
data with twice the switching voltage
required for NRZ – OOK (2Vπ)
 More complex Tx /Rx Design
University of Peloponnese
Dept. of Inform. & Telecommunications
Fiber Impairments in Single
Channel Systems
Linear
Losses
Non Linear
Dispersion
SMF
inserts ASE
noise
SPM
DCF
G
G
Tx
compensates
SMF losses
compensates
dispersion
University of Peloponnese
Dept. of Inform. & Telecommunications
compensates
DCF losses
Rx
Chromatic Dispersion Effect
Every different f travels
with different velocity
t
1
1 0
input
pulse
0
1
1
Optical Fibre
Dispersion Parameter: DSMF
Length of Transmission: LSMF
1
1
ISI
1
Some
broadening
t
Severe
broadening
 Dispersion tolerance is inversely proportional to the square of the
operating bitrate and consequently limitations due to dispersion become
more stringent as bit rate increases.
 As a linear effect, dispersion can be compensated by means of a DC
fibre, providing that the exact amount of dispersion is known in advance.
DDCF*LDCF=-DSMF*LSMF
University of Peloponnese
Dept. of Inform. & Telecommunications
Electronic Equalization
 EE attempts to reverse the distortion incurred by a signal
transmitted through a channel.
 It can be a simple linear filter or a complex algorithm.
Electric
Filter
Clock
Recovery
y(n)
y(t)
ADC
PIN
I(n)
Electronic
Equalizer
Receiver (Rx)
 EE are applied after the receiver
o no need in intervening in the already installed fibre links
 Can cope with variable amounts of dispersion
University of Peloponnese
Dept. of Inform. & Telecommunications
Electronic Equalization
 In our case we investigate the performance of the following equalizers:
• Linear Transversal Equalizer – LTE
• Decision Feedback Equalizer – DFE
• Volterra Decision Feedback Equalizer - VDFE
 All equalizers operate at supervised mode, where a training
sequence, known by the receiver is transmitted, in order to train the
equalizers about the channel characteristics.
 Fractional spacing is employed as in this case the performance of the
equalizers becomes less sensitive to the sampling phase of the receiver.
University of Peloponnese
Dept. of Inform. & Telecommunications
Linear Transversal equalizer - LTE
LTE is the simplest form of electronic equalizers. The incoming signal is
processed by a linear filter.
In order to retrieve the transmitted sequence, FS-LTE operates according to:
University of Peloponnese
Dept. of Inform. & Telecommunications
Decision Feedback equalizer - DFE
DFE consists of two parts: a Feed forward part that is driven by the
received waveform and a Feedback part that is driven by the estimations of
the previous symbols.
FS-DFE operates according to:
The performance of linear equalizers is
constrained when applied to non linear systems.
Non Linear Photodetector
 The main reason of non linearity in optical systems is induced by the
detector during the conversion of optical to electrical.
 Photodiode operates on a square law principle, in which the output
of the detector is proportional to the intensity (i.e., the square of the
input signal magnitude).
 Although it is a simple circuit, it is nonlinear and as such it is difficult
to correct linear distortions such as CD.
University of Peloponnese
Dept. of Inform. & Telecommunications
Volterra Decision Feedback equalizer - VDFE
Simplified VDFE used:
University of Peloponnese
Dept. of Inform. & Telecommunications
Complexity Calculations
University of Peloponnese
Dept. of Inform. & Telecommunications
Simulation Setup
SMF
Tx
BER Estimation
w/o EDC
DCF
G
G
Rx
equalizer
Transmission Span (x N)
BER Estimation
with EDC
10 Gb/s bitrate
10 spans x 100km (1000km)
40 Gb/s bitrate
3 spans x 100km (300km)
University of Peloponnese
Dept. of Inform. & Telecommunications
Unncompensated Results
380 km
250 km
400 km
200 km
300 km
200 km
150 km
University of Peloponnese
Dept. of Inform. & Telecommunications
NRZ-OOK Results (1)
94%
70%
98%
80 %
85 %
87.5 %
University of Peloponnese
Dept. of Inform. & Telecommunications
NRZ-DPSK Results (1)
OCR=70%-90% 10Gb/s & 40Gb/s DPSK
University of Peloponnese
Dept. of Inform. & Telecommunications
Upgrading Scenario Setup
Operating
Operatingat
at40
10 Gb/s
Gb/s
SMF
Tx
BER Estimation
w/o EDC
DCF
G
G
Rx
equalizer
Total Length of 1000 km
(10 spans x 100 km)
BER Estimation
with EDC
99 % OCR
NRZ-OOK Dispersion Tolerance
Reduces
NRZ-DPSK
NRZ-OOK
NRZ-DPSK
University of Peloponnese
Dept. of Inform. & Telecommunications
Upgrading Scenarios Results
Upgrading a system 10-40 NRZ & DPSK
University of Peloponnese
Dept. of Inform. & Telecommunications
Conclusion
 Low cost, adaptive techniques of optical transmission, consisting of
optical and electronic equalization, were studied by simulating
configurations with realistic link parameters.
 Here, the interplay between optical and electronic techniques for
physical impairment mitigation for DD optical transmission with various
performance/complexity tradeoffs, is presented.
 It has become evident that even in the absence of FEC, low
complexity equalizers can perform sufficiently well in conjunction with
optical compensation.
 Low complexity Volterra equalizers can be used to support the
migration of a system from 10 to 40 Gb/s.
University of Peloponnese
Dept. of Inform. & Telecommunications
Q&A
Thank you for your attention!
This research was funded by the Operational Program "Education and Lifelong Learning" of the Greek
National Strategic Reference Framework (NSRF) Research Funding Program: THALES PROTOMI,
grant number MIS 377322.
University of Peloponnese
Dept. of Inform. & Telecommunications