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CARRIER CENTERS ARE PUT ON ORTHOGONAL
FREQUENCIES
ORTHOGONALITY - THE PEAK OF EACH SIGNAL
COINCIDES WITH TROUGH OF OTHER SIGNALS
SUBCARRIERS ARE SPACED BY 1/TS
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OFDM HAS RECENTLY BEEN APPLIED TO OPTICAL COMMUNICATIONS.
THIS PARTLY BECAUSE OF RECENT DEMAND FOR INCREASED DATA
RATES ACROSS DISPERSIVE OPTICAL MEDIA AND PARTLY BECAUSE
DEVELOPENTS IN DSP TECHNOLOGY MAKES PROCESSING AT OPTICAL
DATA RATE WHICH HAS BECOME FEASIBLE.
WITH THE HELP OF O-OFDM TECHNIQUE IT IS POSSIBLE TO ACHIEVE
SPAN DISTANCES UP TO 100 KM RESULTING IN CONVERGENCE OF
ACCESS AND METRO NETWORKS WHICH MEANS REDUCTION OF COST
IN CLOUD COMPUTING ENVIRONMENT.
WITH OFDM IT IS POSSIBLE TO ACHIEVE HIGH SPECTRAL
EFFICIENCY. WITH A GREATER PERCENTAGE OF POWER IN THE
CENTER OF THE WAVEFORM THAN CONVENTIONAL TECHNIQUES,THISATTRIBUTE ENABLE O-OFDM SIGNALS TO PASS THROUGH CASCADED
FILTERS SUCH AS THOSE FOUND IN A STRING OF RECONFUGRABLE
OPTICAL ADD / DROP MULTIPLEXERS (ROADMS) MORE CLEANLY THAN
CONVENTIONAL SIGNALS.
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DATADATAFIBER CHANNEL
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O-OFDM IS A MULTICARRIER TECHNIQUE WHERE A HIGH SPEED
DATA STREAM IS DIVIDED INTO MULTIPLE PARALLEL LOWER
SPEED STREAMS AND MODULATED ONTO SUB CARRIER OF
DIFFERENT FREQUENCIES FOR TRANSMISSION.
OFDM IMPROVES THE ROBUSTNESS OF HIGH BIT RATE SIGNALS
AGAINST ADVERSE EFFECTS SUCH AS CHROMATIC DISPERSION
(CD),WHICH CAN CAUSE INTER SYMBOL INTERFERENCE
RESULTING IN HIGH BIT ERROR RATE. HENCE, SPLITTING THE
SIGNAL IN TO LOWER BIT RATE SIGNALS PER SUBCARRIER AND
USING A CYCLIX PREFIX COUNTERACTS THE PENALTIESINDUCED BY THE OPTICAL FIBER.
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IT MAY BE NOTED THAT FOR ORDINARY OFDM WITH A RECTANGULAR PULSE
DISPERSIVE CHANNEL CAUSES ISI. IT CAN BE MITIGATED BY A TIME DOMAIN GUARD
INTERVAL BETWEEN OFDM SYMBOLS(BLOCKS), OR BY A CYCLIC PREFIX TO THE OFDM
BLOCKS.THE PROPOTIONAL OVERHEAD COST IS MINIMIZED BY THE USE OF LONG
OFDM SYMBOLS.
A GUARD INTERVAL EQUAL TO THE MEMORY (DISPERSION TIME ) OF THE CHANNEL,
DURING WHICH NO ENERGY IS TRANSMITTED,IS THE SIMPLEST SOLUTION.
HOWEVER,THE CHANNEL IS USED MORE EFFICIENTLY IF SOMETHING IS TRANSMITTED
DURING THE GUARD SPACE THAT CONTRIBUTES TO THE SIGNAL ENERGY WITHOUT
INTRODUCING ISI.
THIS IS KNOWN AS CYCLIC PREFIX (CP ),THE REPITITION OF LAST PART OF THE
TRANSMITTED SIGNAL DURING PREFIX INTERVAL. NEXT FIGURES SHOWS THE TIMEDOMAIN SEQUENCE OF OFDM SYMBOLS SHOWING THE CYCLIC PREFIX AND TIME
DOMAIN COMPONENTS OF SUB CARRIER FOR TWO SYMBOLS SHOWING CYCLIC
PREFIX.
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(i) O-OFDM is a multicarrier technique where a high speed data stream is divided
into multiple parallel lower speed streams and modulated onto sub carrier of differentfrequencies . OFDM improves the robustness of high bit rate signals against adverse
effects such as chromatic dispersion (CD),which can cause inter symbol interference
resulting in hight bit error rate. Hence, splitting the signal in to lower bit rate signals
per subcarrier and using a cyclix prefix counteracts the penalties induced by the
optical fiber.
(ii) O-OFDM is a good candidate for implementation in new network systems to meet
the growing demand of cloud computing in future optical access networks in terms of
extended reach and bit rate. This format is appropriate to meet the constraints
brought about by the convergence of metro/Access networks targeted by long reach
Passive Optical networks (LR PONs).
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(iii) Application of O-OFDM to dynamically allocate sub carriers or services to
the users is another advantage of O-OFDM, which is one of the primary
requirement of cloud computing.
(iv) OFDM format does not require dispersion compensation, which is another
improvement . Also, the co-existence of different modulation formats Such as
NRZ and OFDM in the same architecture is feasible.
(v) By considering the application of O-OFDM in WDM-PON or TDM-PON
architecture, it is possible to overlay next generation equipment facilities on top
of a previously installed optical access network such as GPON, BPON or
EPON by just changing the terminal equipment.
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(vi) Due to O-OFDM's high spectral efficiency it is possible to have low-cost
components such as Vertical Surface Emitting Lasers (VCSELs) and Distributed
Feedback Lasers (DFB's).
(vii) OFDM is also more robust to Rayleigh back scattering in the fiber.
(viii) OFDM can precisely tailor the transmitted signal to the frequency
characteristics of the channel,e.g., by avoiding frequencies with low SNR, and
increasing the constellation size on good subcarriers.
France Telecomes orange Lab has already demonstrated 100 km transmission on a
PON architecture using-OFDM technique.
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