1. Adaptive Optical Technologies for Optical Transmission Systems
Maki Nanou, George-Othon Glentis,
Kristina Georgoulakis, Chris Matrakidis,
Christina (Tanya) Politi, Alexandros Stavdas

2. Outline Basic Concepts of Optical Communications
Fiber Impairments & Compensation Techniques
Optical Transmission Simulations
Results
Conclusion
University of Peloponnese
Dept. of Inform. & Telecommunications

3. Increased capacity ↔ Advanced Modulation Formats
Traffic Growth
Until 2000: Voice Traffic dominates
(*) Cisco Forecast
60% / year
After 2004: Data Traffic dominates
Traffic growth of 60% per year outstrips the growth in capacity of commercial systems.
20% / year
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 Formats
University of Peloponnese

4. Non Return To Zero – On – Off Keying
Output Intensity
Most commonly used (widely deployed)
quaternary point bias
OOK: switching ON and OFF the amplitude of an optical carrier signal (Intensity Modulation Only) t
Input Voltage
External Modulation: biased at the quadrature point of the MZM transfer function, and driven by an electrical binary NRZ-ASK signal with peak-to-peak amplitude of Vπ.
optical
signal
MZM
LASER
Vπ swing
Simple Tx/Rx Configurations
University of Peloponnese
Dept. of Inform. & Telecommunications

5. Differential Phase Shift Keying - DPSK
Phase Modulation Only
Nearly constant envelope – higher tolerance to non linear effects
Higher receiver sensitivity due to the 3dB lower OSNR requirement to achieve a specific BER.
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
Output Intensity
minimum
point bias t t
Input Voltage
optical
signal
LASER
MZM
electrical
NRZ data
precoder
2Vπ swing
Differential-phase-shift-keying (DPSK) MF is a promising candidate to improve the performance of optical communication systems.
In the DPSK modulation format, the information is carried by the phase of the optical carrier signal.
The most interesting benefit of DPSK signals when compared to the conventional ASK format is the high receiver sensitivity due to the ~3dB lower OSNR required to achieve a given BER. DPSK can be used to extend the transmission distance, reduce optical power requirements, and relax component specifications.
A commonly used NRZ-DPSK transmitter setup consists of an external MZM biased at minimum transmission and driven with a precoded binary data with twice the switching voltage required for OOK modulation 2Vπ.
The advantage of DPSK over OOK is obtained at the expense of increased complexity and cost in the transmitter and receiver structure, such as a need of a differential encoder at the transmitter, a delay- interferometer at the receiver side and two photodiodes to construct a balanced receiver.
University of Peloponnese
Dept. of Inform. & Telecommunications

6. Fiber Impairments in Single Channel Systems
Linear
Non Linear
Losses
Dispersion
inserts ASE noise
SPM
SMF
DCF Rx Tx G G
compensates dispersion
compensates DCF losses
compensates SMF losses
University of Peloponnese
Dept. of Inform. & Telecommunications

7. Chromatic Dispersion Effect
Every different f travels with different velocity t 1
Optical Fibre 1
Some broadening
input pulse t
Dispersion Parameter: DSMF
Length of Transmission: LSMF 1
Severe
broadening
ISI
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

8. 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.
Receiver (Rx)
Electric
Filter
Clock
Recovery
PIN
y(t)
y(n)
I(n)
Electronic Equalizer
ADC
EE are applied after the receiver
no need in intervening in the already installed fibre links
Can cope with variable amounts of dispersion
University of Peloponnese
Dept. of Inform. & Telecommunications

9. 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

10. 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

11. 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.

12. 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

13. Volterra Decision Feedback equalizer - VDFE
Simplified VDFE used:
University of Peloponnese
Dept. of Inform. & Telecommunications

14. Complexity Calculations
University of Peloponnese
Dept. of Inform. & Telecommunications

15. Transmission Span (x N)
Simulation Setup G
Transmission Span (x N) Tx Rx
BER Estimation
w/o EDC
equalizer
with EDC
SMF
DCF
10 spans x 100km (1000km)
3 spans x 100km (300km)
10 Gb/s bitrate
40 Gb/s bitrate
Here is a block diagram of the performed simulation setup. It consists of N number of identical spans. In case of 10 Gb/s the total tranmission length is 1000 km, while in case of 40 Gb/s it is 300 km. Every span has 2 amplifiers, one for compensating the SMF losses and another to compensate DCF Losses. Every amplifier has a noise figure of 5 dB.
In the case of the scenario of uncompensated dispersion no DCF is used, therefore all the amount of dispersion is compensated only by the means of EE. In the scenario with variable amounts of ODCR, variable amounts of residual dispersion are accumulated at the end of the link. In all cases after the receiver, the 3 different equalizers are used in order to assess their performance.
University of Peloponnese
Dept. of Inform. & Telecommunications

16. Unncompensated Results
380 km
250 km
400 km
200 km
300 km
200 km
150 km
University of Peloponnese
Dept. of Inform. & Telecommunications

17. NRZ-OOK Results (1) University of Peloponnese 94% 70% 98% 80 % 85 %
87.5 %
University of Peloponnese
Dept. of Inform. & Telecommunications

18. NRZ-DPSK Results (1) OCR=70%-90% 10Gb/s & 40Gb/s DPSK
University of Peloponnese
Dept. of Inform. & Telecommunications

19. Upgrading Scenario Setup
Operating at 40 Gb/s
Operating at 10 Gb/s
BER Estimation
w/o EDC
SMF
DCF Tx Rx G G
equalizer
Total Length of 1000 km
(10 spans x 100 km)
BER Estimation
with EDC
99 % OCR
NRZ-OOK
NRZ-OOK
In our project we investigate a scenario of upgrading an existed link of 10 Gb/s to 40 Gb/s.
99% of its dispersion is compensated by the means of DCF as 100% DC is in practice difficult to be achieved.
However even a residual dispersion of 5% becomes of major importance, since the dispersion tolerance reduces dramatically as the bitrate increases.
We then deploy EE in order to investigate if the performance of the system is improved.
This scenario is performed for both modulation formats.
Dispersion Tolerance
Reduces
NRZ-DPSK
NRZ-DPSK
University of Peloponnese
Dept. of Inform. & Telecommunications

20. Upgrading Scenarios Results
Upgrading a system NRZ & DPSK
University of Peloponnese
Dept. of Inform. & Telecommunications

21. 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

22. Thank you for your attention!
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
University of Peloponnese
Dept. of Inform. & Telecommunications