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Telecomunicacions per Fibra Òptica 24/04/22 1 OFDM ACCESS IN OFDM ACCESS IN OPTICAL COMMUNICATION OPTICAL COMMUNICATION Yatish Bathla ([email protected]) Yatish Bathla ([email protected]) Curs 2010 – 2011 Curs 2010 – 2011

Ofdm Access in OFT

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Page 1: Ofdm Access in OFT

Telecomunicacions per Fibra Òptica09/04/23 1

OFDM ACCESS IN OFDM ACCESS IN OPTICAL COMMUNICATION OPTICAL COMMUNICATION

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

Curs 2010 – 2011 Curs 2010 – 2011

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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 Software

Consideration

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IntroductionIntroduction

Optical Domain Optical Fiber Laser Optical Filter Optical Amplifier Optical Modulator Direct Detection System Diagram

Output Spectrum Output Constellation

and EVM Drawback Direct

Dtection Coherent Detection

overview Drawbacks OFDM OFDM Applications

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ELECTRICAL DOMAIN

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OFDM-BASICOFDM-BASIC

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frequency-division multiplexing(FDM) scheme used as a digital multi-carrier modulation method

Available spectrum divided into many narrow subcarriers

Each OFDM subcarrier has

sin(x)/x or sinc spectrum OFDM subcarriers are closely

spaced, orthogonal and used to carry

data Data is divided into parallel data

streams each transmitted on a separate subcarrier

Each sub-carrier is modulated with a conventional modulation scheme (QAM or PSK) at a low symbol rate

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PRINCIPLE

• In single carrier modulation, data is sent serially over the channel by modulating one single carrier

• In a multipath fading channel, the time dispersion can be significant compared to the symbol period, which results in inter symbol interference (ISI).

• A complex equaliser is then needed to compensate for the channel distortion.

• The basic idea of multicarrier modulation was introduced and patented in the mid 60's by Chang [1]: the available bandwidth W is divided into a N number of subbands, commonly called subcarriers, each of width= W/N

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PRINCIPLE

•Instead of transmitting thedata symbols in a serial way, a multicarrier transmitterpartitions the data stream into blocks of Nc data symbols that are transmitted in parallel by modulating the Nc carriers.• The symbol duration for a multicarrier scheme is Tc=Nc/R•Condition for flat fading within the sub bands a reasonable range for Nc can be derived as

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FFT/IFFT

IFFT block is the main component in the transmitter and the FFT in the receiver

input to the IFFT is the complex vector 1 2 3, , ,...,

T

NX X X X X

Where N is the size of IFFT. Each of the elements of represents the data to be carried on the corresponding subcarrier. For IFFT

1

0

1 2exp( )

N

m kk

j kmx X

NN

For 0 m N-1

For FFT 1

0

1 2exp( )

N

k mm

j kmX x

NN

For 0 k N-1

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CYCLIC PREFIX

With cyclic prefix the OFDM symbol is extended by copy-pasting the beginning of the symbol to the end (or vice versa)

Chromatic dispersion and PMD cause the subcarriers to drift relatively to eachother. As a result a cyclic prefix is required to prevent power leakage from neighboring OFDM symbols

The cyclic prefix allocated must be larger than the walk off of the transmission link

When the cyclic prefix is not sufficient. Inter-symbol-interference (ISI) occurs and with that the subcarriers affected lose their orthogonality

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MODULATION TYPE

Basically 6 types: QPSK, 4-QAM, 8-QAM, 16-QAM,32-QAM and 64-QAM

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BLOCK DIAGRAMBLOCK DIAGRAM

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Description

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OFDM TRANSMITTER

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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

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OPTICAL DOMAIN

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Telecomunicacions per Fibra Òptica09/04/23 18

OPTICAL FIBRE(SMF)OPTICAL FIBRE(SMF)

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LASERLASER

The LaserCW module models a Distributed Feedback 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 FILTEROPTICAL FILTER

key components of optical communication systems.

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)].

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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OPTICAL AMPLIFIER

Types of Optical Amplifiers

Semiconductor Optical Amplifier (SOA)

Fiber Optical Amplifier Doped-Fiber Amplifiers

(EDFA) Non-Linear Optical

Amplifier• Raman Amplifier• Parametric Amplifier

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OPTICAL AMPLIFIER(EDFA)

EDFA consist four parts

• Erbium-doped fiber• An optical pump (to

invert the population).• A coupler• An isolator to cut off

back propagating noise

Pump can be configured in different ways on Erbium- doped fiber

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OPTICAL MODULATION/DETECTION TECHNIQUE

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OPTICAL MODULATION (MACH OPTICAL MODULATION (MACH ZEHNDER MODULATOR)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 MODULATION (MACH OPTICAL MODULATION (MACH ZEHNDER MODULATOR)ZEHNDER MODULATOR)

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Non Coherent Detection (PHOTODIODE)Non Coherent Detection (PHOTODIODE)

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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Non Coherent Detection(PHOTODIODE)Non Coherent Detection(PHOTODIODE)

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DD-OFDM SYSTEM DESCRIPTIONDD-OFDM SYSTEM DESCRIPTION

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OFDM RECEIVEROFDM RECEIVER

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Description

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Output DISPLAY (SEE,SA)Output DISPLAY (SEE,SA)

Signal Error Estimator(SEE) estimates the Symbol Error Rate (SER) and Error Vector Magnitude (EVM) of an electrical m-QAM 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 Analyzer(SA) is used to display and analyze electrical and optical signals(received modulated optical signal).

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OFDM Output(OFDM Output(without optical Amplifier )without optical Amplifier )

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OFDM Output(OFDM Output(with optical Amplifier )with optical Amplifier )

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OFDM Output(OFDM Output(without Equalizer)without Equalizer)

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EVM=1.007

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OFDM Output(OFDM Output(with Equalizer)with Equalizer)

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EVM=0.177

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OFDM Output(OFDM Output(without Cyclic Prefix)without Cyclic Prefix)

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EVM=0.199

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Drawback(Non Coherent Detection)Drawback(Non Coherent Detection)

detection based on energy measurement allows signals to encode only one degree of freedom (DOF) per polarization per carrier

reducing spectral efficiency and power efficiency loss of phase information prevents full equalization

of linear channel impairments like CD and PMD by linear filters

achievable performance is suboptimal compared with optical or electrical equalization making use of the full electric field

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Coherent DetectionCoherent Detection

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OFDM DRAWBACKOFDM DRAWBACK

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

clipping Sensitivity for frequency and phase noise Greater complexity More expensive transmitters and receivers Efficiency gains reduced by requirement for

guard interval

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APPLICATION

Optical fibre to home DAB HDTV-DVB Wireless LAN Networks HIPERLAN/2 IEEE 802.11ª IEEE 802.11g IEEE 802.16 Broadband Wireless Access

System

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ResourcesResources

1. VPI Software

2. Optical Device Notes

3. Optical fibre telecommunication Notes

4. 2. Jean Armstrong; OFDM for Optical Communications; Journal of Lightwave Technology, Vol.27, no.3, February 1, 2009

5. Wikipedia.org

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ResourcesResources5. Xia Li, R. Mardling and J. Armstrong; Channel Capacity of IM/DD

Optical Communication Systems and of ACO-OFDM; Monarsh University

6. Jean Armstrong, Brendon J.C. Schmidt, Dhruv Kalra, Himal A. Suraweera and Arthur J. Lowery; Performance of Asymmetrically Clipped Optical OFDM in AWGN for an Intensity Modulated Direct Detection System; Monarsh University

7. Jean Armstrong, Brendon J.C. Schmid: Comparison of Asymmetrically Clipped Optical OFDM and DC-Biased Optical OFDM in AWGN, 2008

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