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Lecture: 8 Physical Layer Impairments in Optical Networks. Ajmal Muhammad, Robert Forchheimer Information Coding Group ISY Department. Outline. Introduction to Physical Layer Impairments (PLIs) PLIs Classification Linear and non-linear Signal Quality Estimation - PowerPoint PPT Presentation
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Lecture: 8 Physical Layer Impairments in Optical Networks
Ajmal Muhammad, Robert ForchheimerInformation Coding Group
ISY Department
Outline
Introduction to Physical Layer Impairments (PLIs) PLIs Classification
Linear and non-linear Signal Quality Estimation PLIs Aware Routing and Wavelength Assignment
Physical Layer Impairments
Optical signals traverse the optical fibre links, passive and/or active optical components
Signals encounter many impairments that affect their intensity level, as well as their temporal, spectral and polarization properties
If the received signal quality is not within the receiver sensitivity threshold, the receiver may not be able to correctly detect the optical signal
Physical Layer Impairment Awareness
Important for network designers and operators to know:
Various important Physical Layer Impairments (PLIs)Their effects on lightpath (connection) feasibilityPLI analytical modeling, monitoring and mitigation techniquesTechniques to communicate PLI information to network layer and control plane protocolsHow to use all these techniques to dynamically set-up and manage optically feasible lightpaths
PLIs Dependence
PLIs and their significance depend on: network type, reach, type of network applications
Network type: opaque (signal undergoes OEO conversion at all intermediate nodes along its path), translucent (undergoes OEO at some intermediate nodes), transparent (lightpaths are switched completely in the optical domain)
Reach: Access, metro, or core/long-haul network
Type of applications: Real-time, non-real time, mission-critical, etc
Maximum Transparency Length
The maximum distance an optical signal can travel and be detected by a receiver without requiring OEO conversion
The maximum transparency length of an optical path depends on:
The optical signal powerThe fibre distanceType of fibre and design of links (e.g., dispersion compensation)The number of wavelengths on a single fibreThe bit-rate per wavelengthThe amplification mechanism and the number of amplifiersThe number and type of switching elements through which the signals pass before reaching the egress node or before regeneration
PLIs Classification
PLIs are broadly classified into two categories: linear and non-linear
Optical systems that operate below a certain input power threshold exhibit a linear relationship between the input and output signal power
The loss and refractive index (n) of the fibre are independent of the signal power, i.e., static in nature
Important linear impairment are: fibre attenuation, component insertion loss, Amplifier Spontaneous Emission (ASE) noise, Chromatic Dispersion (CD) (or Group Velocity Dispersion (GVD)), Polarization Mode Dispersion (PMD), crosstalk and Filter Concatenation (FC)
PLIs Classification: Non-linear
Non-linear impairments refer to phenomena that only occur when the signal energy propagating in a medium attains sufficiently high intensities
This can be due to high launch power and/or the confinement of energy in extremely small areas, i.e., fibre core
Non-linear impairments induce phase variation and introduce noise into the optical signal
Important non-linear impairments are: Self Phase Modulation (SPM), Cross Phase Modulation (XPM), Four Wave Mixing (FWM)
Outline
Introduction to Physical Layer Impairments (PLIs)
PLIs Classification Linear and non-linear
Signal Quality Estimation
PLIs Aware Routing and Wavelength Assignment
Signal Attenuation & Insertion Loss
Signal attenuation: refers to the loss of power of a signal propagating through optical fibre as distance increases
Causes: material absorption, Rayleigh scattering
Material absorption: impurities within fibre absorb propagating signal power, often convert the energy into heat
Rayleigh scattering: photons can interact with the atoms in the fibre causing energy to be scattering in all directions
If a scattered photon does not propagate in the same direction as the original signal, then signal attenuation or loss occur
Insertion loss: loss of signal power resulting from the insertion of a device in an optical fibre and is usually expressed in decibels (dB)
Amplified Spontaneous Emission (ASE)
Amplifiers are used to overcome fibre losses
Optical noise is added by each amplifier -spontaneously emitted photons have random characteristics and manifest in the amplified signal as noise
ASE noise within the signal bandwidth cannot be removed and is subject to gain from any other amplifier downstream in the optical link
Optical Signal to Noise Ratio (OSNR)
Power in optical signal divided by the power in 0.1 nm of the noise spectrum
Expressed in dBFor amplifiers and a line system, delivering a high ONSR is goodFor a receiver, tolerating a low OSNR is good
Km
dB
Chromatic DispersionMaterial Dispersion: Since refractive index (n) is a function of wavelength, different wavelengths travel at slightly different velocities.
Waveguide Dispersion: Signal in the cladding travels with a different velocity than the signal in the core. This phenomenon is significant in single mode conditions.
Group Velocity (Chromatic) Dispersion = Material Disp. + Waveguide Disp.
Group Velocity Dispersion
Effects of Dispersion and Attenuation
Polarizations of Fundamental Mode
Two polarization states exist in the fundamental mode ina single mode fiber
Polarization Mode Dispersion (PMD)
Each polarization state has a different velocity PMD
CrosstalkOptical switches are prone to signal leakage, giving rise to crosstalk
Inter-channel crosstalk: occurs between signals on adjacent channels. Can be eliminated by using narrow pass-band receivers.
Intra-channel crosstalk: occurs among signals on the same wavelengths, or signals whose wavelengths fall within each other’s receiver pass-band.
Outline
Introduction to Physical Layer Impairments (PLIs)
PLIs Classification Linear and non-linear
Signal Quality Estimation
PLIs Aware Routing and Wavelength Assignment
Kerr Effect
The refractive index (n=c/v) of optical fibre dependent on the optical signal intensity, I: n=n0 + n2I = n0 + n2 P/Aeff
Where P is optical signal power, Aeff is the effective area of the fibre core cross section, n0 is the linear refractive index, n2 is the “nonlinear index coefficient”
When I is large, the nonlinear component of the refractive index becomes significant, resulting in the kerr effect (change in the refractive index of a material in response to an applied electric field)
Kerr effect
Self and Cross Phase Modulation (SPM & XPM )
The refractive index changes induced by the kerr effect cause phase changes in different parts of the optical pulse to travel at different speeds, resulting in new frequencies being introduced into the pulse
The kerr effect inducing phase changes of a signal due to its own intensity variation is known as self phase modulation
The kerr effect induces phase modulation in a signal due to intensity variations in other channels, this effect is known as cross phase modulation
Four Wave Mixing (FWM)
Multiple channels at different wavelengths (frequencies) propagate down a single fibre. The signals of these channels interact to produce new signals
In general, for N signal channels, the number of generated mixing product M will be:
M= N2.(N-1)/2
And the generated FWM frequencies are given by: fijk=fi+fj-fk , i!=k, j!=k
FWM generated by two signals f1 & f2
FWM generated by three signals
Digital Processing for Impairments Compensation
Tx Processing
CustomerTraffic comingInto the chip
Encoding forError correction
Compensationfor nonlinearity
DSP for compensatingdispersion & shaping the spectrum
Compensation forImperfection in the modulator
22 M GatesDSP= 20 Mgates
Receive Processing
Undoing the polarization effects
Inverting the differenceb/w the transmitter laser &the receiver laser
70 T ops/s32 nm CMOS150 M gates3.7 km wire (copper)
Outline
Introduction to Physical Layer Impairments (PLIs)
PLIs Classification Linear and non-linear
Signal Quality Estimation
PLIs Aware Routing and Wavelength Assignment
Eye Diagram
Overlay the received bit stream in the time domain over a three-bit sliding window
Eight 3-bit sequence
Superimposition of multipleinstances of the eight 3-bitbinary sequences
Eye Diagram in the Presence of Signal Degradation
When a received signal is degraded by optical impairments, the eye diagram becomes partially closed and distorted
For ASE, this corresponds to anincrease in thestandard deviationof the levels
For PMD and CD, thiscorresponds to distortions inthe slope of the bit transitionsand an increase in the timingjitter
Bit Error Rate (BER) and Q-factor
BER: number of bits received in error as a ratio of the total number of transmitted bits
idec is the signal level at the decision instant
For Gaussian distributions with mean and standarddeviations given by
Q-factor
and
Optimal decision threshold value
Perror minimized when
Q-factor and BER
Typical BER levels range from 10-9 to 10-12, correspond to Q-factor of 6 to 8, respectively
Using forward error correction (FEC), a system may tolerate up to levels of 10-3 corresponding to a Q-factor of 3
Outline
Introduction to Physical Layer Impairments (PLIs)
PLIs Classification Linear and non-linear
Signal Quality Estimation
PLIs Aware Routing and Wavelength Assignment
PLI-RWA Proposals
When selecting a lightpath (route and wavelength), a PLI-RWA algorithm for a transparent or translucent network has to take into account the physical layer impairments as well as wavelength availability
The PLIs are either considered as constraints for the RWA decisions (i.e., physical layer impairment constrained or PLIC-RWA) or the RWA decisions are made considering these impairments (i.e., physical layer impairment aware or PLIA-RWA)
In PLIA-RWA, it is possible to find alternate routes considering the impairments, while in PLIC-RWA the routing decisions are constrained by PLIs
Approach 1
Compute the route and the wavelength in the traditional way and finally verify the selected lightpath considering the physical layer impairments
Approach 2
Considering the physical layer impairments values in the routing and/or wavelength assignment decisions
Approach 3
Considering the physical layer impairments values in the routing and/or wavelength assignment decisions and finally also verify the quality of the candidate lightpath
PLIverification