ofdm in opticals

Telecomunicacions per Fibra Òptica12/10/14 1

OFDM ACCESS IN OFDM ACCESS IN OPTICAL COMMUNICATIONOPTICAL COMMUNICATION

Yatish Bathla ([email protected])Yatish Bathla ([email protected])

Curs 2010 – 2011 Curs 2010 – 2011

Telecomunicacions per Fibra Òptica

Why OFDM ??????Why OFDM ??????

Vast demand on Bandwidth Robustness against chromatic despersion, ISI and

ICI Simple equalizer(popular in Broadband system) Increased efficiency because carrier spacing is

reduced (orthogonal carriers overlap) Ease of Dynamic Channel Estimation and mitigation More resistant to fading Capability of Dynamic Bit and Power Loading

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IntroductionIntroduction

Electrical Domain OFDM Basic Principle

FFT/ IFFT Cyclic Prefix Modulation Type Block Diagram

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IntroductionIntroduction

Optical Domain Direct Detection• System Diagram• Optical Laser• Optical Modulator

Type• Optical Filter• Optical Amplifier• Photodiode

• Ouput Spectrum • Output Constellation

and EVM Overview Coherent

Detection Drawbacks OFDM OFDM Applications

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DD-OFDM SYSTEM DESCRIPTIONDD-OFDM SYSTEM DESCRIPTION

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OFDM TRANSMITTER


OFDM-BasicOFDM-Basic

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Available spectrum divided into many narrow bands

Data is divided into parallel data streams each transmitted on a separate band

Symbol period is much longer than for a serial system with the same total data rate

ISI affects at most one symbol and equalization is simplified

Each OFDM subcarrier has

sin(x)/x spectrum

.


OFDM-BasicOFDM-Basic

Each subcarrier has a different frequency

The subcarrier frequencies are chosen so that the signals are mathematically orthogonal over one ODFM symbol period

Modulation/Demodulation and Multiplexing/Demultiplexing is performed digitally by IFFT/FFT

Data is carried by varying the phase or amplitude of each subcarrier

QPSK, 4-QAM, 16-QAM, 64-QAM

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FDM OFDM

SPECTRAL EFFICIENCY GAIN


Block Diagram OFDMBlock Diagram OFDM

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Pulse ShapingPulse Shaping

generates a Nyquist response from an incoming electrical impulse

An ideal linear low-pass filter with a cut-off frequency or Nyquist frequency fN = SymbolRate/2.

An ideal low-pass filter has a sinc function impulse-response with equidistant zero-crossings at the sampling instants and hence no intersymbol interference (ISI).

The ideal filter is not realizable and a practical odd-symmetric extension is a raised cosine characteristic fitted to the ideal low-pass filter

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RF-UPCONVERSION

Sine Generator can be used to generate a sine signal of arbitrary amplitude and initial phase, as adjusted with Amplitude and Phase

The frequency may be chosen from zero Hz (DC) up to a value lower than half the sample rate.

Real part of OFDM signal is multiply by sine function and imaginary part is multiplied with Cosine function.

Logical Add Channel provides the ability to “assign” logical channels stored in a global list to the signal


Optical LaserOptical Laser

The LaserDriver module is used for driving TLM lasers and modulators that require an arbitrary current or voltage swing. It is also useful for attenuating, multiplying and offsetting electrical signals.The LaserCW module models a DFB laser producing a continuous wave (CW) optical signal The module produces a time dependent field E(t) describing the radiation of a CW laser with the specified power, frequency, linewidth, and polarization.

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Optical LaserOptical Laser

Light contains photons measured by power (square of Electric field) always positive

current contains electrons so positive or negative Converting photon to electron (vice versa) is a

nonlinear process Electric power is twice of Optical power Laser is Distributed Feedback Laser because it use

Single mode fibre

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Mach Zehnder ModulatorMach Zehnder Modulator

The optical power Pout at the output of MZM, depends on the phase difference ΔΦ between the two modulator branches

d(t) is the power transfer function and ΔΦ1(t) and ΔΦ2(t) are the phase changes in each branch caused by the applied modulation signal data(t). The phase changes take place due to the electro-optical effect.

the modulator will have a large extinction ratio, and a low chirp

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Optical FilterOptical Filter

Optical filters are key components of optical communication systems. They are widely used for WDM signal demultiplexing, noise and distortion suppression, fiber dispersion compensating

Filter characteristics can be defined completely by the transfer function. Module and argument of the complex-valued transfer function H(ω) describe the magnitude and phase frequency responses of the filter on the input harmonic signals E(t) = exp[j(ωt+φ0)]. If the filter transfer function is known, then the filtered signal in the frequency domain can be found simply as a product of the input signal spectrum and the filter transfer function: Eout(ω) = H(ω)Ein(ω).

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PhotodiodePhotodiode

A model of PIN and APD photodiodes. These can be simulated on base of predefined responsivity, avalanche multiplication, dark current and noise

Photodiode is the one of key components of optical receivers that converts light into electricity due to photoelectric effect. The output current is described by sum of photocurrent, dark current, shot and thermal noise

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OFDM RECEIVEROFDM RECEIVER

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Output Display(SEE,SA)Output Display(SEE,SA)

Signal Error Estmator estimates the Symbol Error Rate (SER) and Error Vector Magnitude (EVM) of an electrical mQAM signal, taking I and Q electrical signals as inputs. The module automatically performs clock recovery, amplitude and phase correction of the received constellation. It uses probability density function fitting, assuming a combination of Exponential and Gaussian statistics.

Signal Analyser is used used to display and analyze electrical and optical signals.

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Power Spectrum without EqualiserPower Spectrum without Equaliser

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Constellation and EVM without Constellation and EVM without EqualiserEqualiser

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EVM=1.007


Power Spectrum with EqualiserPower Spectrum with Equaliser

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Constellation and EVM with Constellation and EVM with EqualiserEqualiser

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EVM=0.177


Power Spectrum without optical Power Spectrum without optical AmplifierAmplifier

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Constellation and EVM without Constellation and EVM without optical amplifieroptical amplifier

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EVM=0.703


Power Spectrum without optical Power Spectrum without optical Amplifier and EqualiserAmplifier and Equaliser

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Constellation and EVM without Constellation and EVM without optical amplifier and Equaliseroptical amplifier and Equaliser

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EVM=1.048


OFDM disadvantageOFDM disadvantage

Large PAPR(peak to avarage power ratio) Overcome by different coding scems or clipping

Sensitivity for frequency and phase noise

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ResourcesResources1. Jean Armstrong; OFDM for Optical Communications; Journal of

Lightwave Technology, Vol.27, no.3, February 1, 2009

1. Wiliam Shieh, Ivan Djordjevic; Orthogonal Frequency Devision Multiplexing for Optical Communications, ISBN 978-0-12-374879-9, 2010

1. VPI Software

4. Notes of Professor Maria C.Santos

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ofdm in opticals

Education

fibra ptica121014

wwhhyy ooffddmm

capability of dynamic

chromatic despersion

isreduced orthogonal

bandwidth robustness

power loadingtelecomunicacions

vast demand