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![OOppttiiccaall FFiilltteerr
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).
Telecomunicacions per Fibra Òptica
12/10/14 15](https://image.slidesharecdn.com/ofdminopticals-141012092630-conversion-gate01/85/Ofdm-in-opticals-15-320.jpg)
Uploaded byYatish Bathla
Ofdm in opticals
This document provides an overview of optical OFDM (orthogonal frequency-division multiplexing) systems. It discusses why OFDM is used, including its ability to mitigate chromatic dispersion and intersymbol interference. It then describes the basic principles of OFDM, including how data is divided into parallel streams and transmitted on separate subcarriers. It also outlines the key components of an optical OFDM system, such as lasers, modulators, filters, photodiodes, and how these components are used in an optical OFDM transmitter and receiver. It shows examples of output constellations and error vector magnitude with and without equalization components.
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Ofdm in opticals
- 1. OOFFDDMM AACCCCEESSSS IINN OOPPTTIICCAALL CCOOMMMMUUNNIICCAATTIIOONN YYaattiisshh BBaatthhllaa ((yyaattiisshhbbaatthhllaa@@hhoottmmaaiill..ccoomm)) CCuurrss 22001100 –– 22001111 Telecomunicacions 12/10/14 per Fibra Òptica 1
- 2. WWhhyy OOFFDDMM ???????????? 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 Telecomunicacions per Fibra Òptica 12/10/14 2
- 3. IInnttrroodduuccttiioonn Electrical Domain OFDM Basic Principle FFT/ IFFT Cyclic Prefix Modulation Type Block Diagram Telecomunicacions per Fibra Òptica 12/10/14 3
- 4. IInnttrroodduuccttiioonn 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 Telecomunicacions per Fibra Òptica 12/10/14 4
- 5. DDDD--OOFFDDMM SSYYSSTTEEMM DDEESSCCRRIIPPTTIIOONN Telecomunicacions per Fibra Òptica 12/10/14 5
- 6. OFDM TRANSMITTER Telecomunicacionsper Fibra Òptica
- 7. OOFFDDMM--BBaassiicc Availablespectrum divided into many 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 Telecomunicacions per Fibra Òptica narrow bands sin(x)/x spectrum . 12/10/14 7
- 8. OOFFDDMM--BBaassiicc Eachsubcarrier has a different FDM OFDM SPECTRAL EFFICIENCY GAIN Telecomunicacions per Fibra Òptica 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 12/10/14 8
- 9. BBlloocckk DDiiaaggrraamm OOFFDDMM Telecomunicacions per Fibra Òptica 12/10/14 9
- 10. PPuullssee SShhaappiinngg 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 Telecomunicacions per Fibra Òptica 12/10/14 10
- 11. RF-UPCONVERSION SineGenerator 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 Telecomunicacions per Fibra Òptica
- 12. OOppttiiccaall LLaasseerr TheLaserDriver 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. Telecomunicacions per Fibra Òptica 12/10/14 12
- 13. OOppttiiccaall LLaasseerr 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 Telecomunicacions per Fibra Òptica 12/10/14 13
- 14. MMaacchh ZZeehhnnddeerr MMoodduullaattoorr 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 Telecomunicacions per Fibra Òptica 12/10/14 14
- 15. OOppttiiccaall FFiilltteerr 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). Telecomunicacions per Fibra Òptica 12/10/14 15
- 16. PPhhoottooddiiooddee Amodel 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 Telecomunicacions per Fibra Òptica 12/10/14 16
- 17. OOFFDDMM RREECCEEIIVVEERR Telecomunicacionsper Fibra Òptica 12/10/14 17
- 18. OOuuttppuutt DDiissppllaayy((SSEEEE,,SSAA)) 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. Telecomunicacions per Fibra Òptica 12/10/14 18
- 19. PPoowweerr SSppeeccttrruumm wwiitthhoouuttEEqquuaalliisseerr Telecomunicacions per Fibra Òptica 12/10/14 19
- 20. CCoonnsstteellllaattiioonn aanndd EEVVMMwwiitthhoouutt EEqquuaalliisseerr EVM=1.007 Telecomunicacions per Fibra Òptica 12/10/14 20
- 21. PPoowweerr SSppeeccttrruumm wwiitthhEEqquuaalliisseerr Telecomunicacions per Fibra Òptica 12/10/14 21
- 22. CCoonnsstteellllaattiioonn aanndd EEVVMMwwiitthh EEqquuaalliisseerr EVM=0.177 Telecomunicacions per Fibra Òptica 12/10/14 22
- 23. PPoowweerr SSppeeccttrruumm wwiitthhoouuttooppttiiccaall AAmmpplliiffiieerr Telecomunicacions per Fibra Òptica 12/10/14 23
- 24. CCoonnsstteellllaattiioonn aanndd EEVVMMwwiitthhoouutt ooppttiiccaall aammpplliiffiieerr EVM=0.703 Telecomunicacions per Fibra Òptica 12/10/14 24
- 25. PPoowweerr SSppeeccttrruumm wwiitthhoouuttooppttiiccaall AAmmpplliiffiieerr aanndd EEqquuaalliisseerr Telecomunicacions per Fibra Òptica 12/10/14 25
- 26. CCoonnsstteellllaattiioonn aanndd EEVVMMwwiitthhoouutt ooppttiiccaall aammpplliiffiieerr aanndd EEqquuaalliisseerr EVM=1.048 Telecomunicacions per Fibra Òptica 12/10/14 26
- 27. OOFFDDMM ddiissaaddvvaannttaaggee Large PAPR(peak to avarage power ratio) Overcome by different coding scems or clipping Sensitivity for frequency and phase noise Telecomunicacions per Fibra Òptica 12/10/14 27
- 28. RReessoouurrcceess 1. JeanArmstrong; 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 Telecomunicacions per Fibra Òptica 1. VPI Software 4. Notes of Professor Maria C.Santos 12/10/14 28
Editor's Notes
- #4 I will start with OFDM brief discription and then explain how OFDM Symbols are generated.
- #5 Here I will explain how OFDM symbols are transmitted in optical fibre. Basically I focus on Direct detection and Using VPI software plot some graphs of optical spectrum and received constellation in different conditions( Without equalizer, optical filter and cyclic prefix). Then overview of Coherent detection technique and finally drawbacks of OFDM and its application in another areas.
- #6 Diagram of Direct detection OFDM using VPI software. It uses 64 sub carriers(in my case) in one OFDM symbol. Cyclic prefix is 0.2 it means 13 sub carriers( 0.2*64) is replicated from bottom to front in order to avoid Inter symbol interference and inter-carrier interference. In this case, it transmit 10 Gbps over distance of 100 k.m. v.i.a. optical cable. OFDM transmitter Block: output of this block is logical channel which contains information of output modulated OFDM symbols and PRBS generator input. Laser Drive circuit: it 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. Optical laser: The Laser is a Distributed FeedBack laser producing a continuous wave (CW) optical signal MZM 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 Optical 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). Optical filter can convert double side band signal into sigle side band signal in order to proper reception of signal otherwise opposite phase cancel each other ( if both band signal are out of phase then at receiver we cannot receive RF output) Loop and optical Amplifier: One loop = 10 k.m. so we take 100 k.m.( long haul for optical communication). EDFA amplifier and optical filteris used in order to increase the gain of optical signal and filter the signal into single side band. With the help of Universal fibre it can also make attenuation and dispersion equal to zero( Hypothetical case) then there is no need of amplifier and filter. Photodiode: 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 . A model of PIN and APD photodiodes. These can be simulated on base of predefined responsivity, avalanche multiplication, dark current and noise. OFDM receiver: it can demodulate, remove cyclic prefix and apply FFT in order to receive original data from OFDM symbols. Output Display: it uses Signal Error Estimator and Signal Analyzer to calculate Signal Error Ratio, Error Vector Magnitude and output optical spectrum
- #7 It is divided into three parts: OFDM Coding: Bits are input from PRBS( Pseudo random Bit Sequence) generator. Bits are modulated( in my case it uses 4-QAM), IFFT(inverse fourier transform), add cyclic prefix( to avoid ISI and ICI) and DAC( Digital to analog converted). At output of this block it get real and imaginary part of OFDM symbol. Pulse 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. The ideal filter is not realizable and a practical odd-symmetric extension is a raised cosine characteristic fitted to the ideal low-pass filter. RF up-conversion: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
- #28 CO-OFDM holds the promise of delivering superior performance in spectral efficiency, receiver sensitivity, and polarization-dispersion resilience,67,68 but implementation is much more complex than for DDO-OFDM.14,15 The transmission experiments of CO-OFDM produced(page 29)