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
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
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 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
Telecomunicacions per Fibra Òptica
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
Telecomunicacions per Fibra Òptica
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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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