Optical Orthogonal Frequency Division Multiplexing for Future Optical Fiber Networks

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Transcript of Optical Orthogonal Frequency Division Multiplexing for Future Optical Fiber Networks

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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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