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Research On Some Key Technologies For Optical OFDM

Posted on:2014-06-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:J ZhangFull Text:PDF
GTID:1268330401967800Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Optical orthogonal frequency division multiplexing (OFDM) with the merits ofhigh spectral efficiency and resilience to fiber dispersion has recently attracted muchattention. It has been emerged as an advanced modulation and attractive multiplexingformat for optical communications. The suitability of optical OFDM for multi-modefiber (MMF) transmission is because it can be effectively against frequency selectivefading. In addition, the transmission performance can be improved and the spectralefficiency can be maximized by adaptively modulated optical OFDM (AMO-OFDM)modem for multi-mode fiber transmission. Optical OFDM, combined with otheradvanced techniques, e.g., high-order modulation format, polarization divisionmultiplexing (PDM) and wavelength division multiplexing (WDM) can dramaticallyincrease the transmission capacity for single fiber. It is regarded as one of the mostimportant direction of development for high-speed and long-haul backbone optical fibertransmission. The access technology based on OFDM technology can realize highspectral efficiency and large capacity optical access, and support transparenttransmission. It is able to achieve resource scheduling with different granularities andflexible resource scheduling in the time and the frequency domain, which meets thequality of service (QoS) and bandwidth requirement of different services. It has becomea very attractive way for multi-subscriber access.The application of optical OFDM technology for related fields of optical fibercommunications, e.g., multi-mode fiber local area network transmission, single modefiber (SMF) transmission and access technology, has been studied. It mainly focuses onanalyzing and solving several key issues that the OFDM technology meets in opticalfiber transmission, which provide an important reference for the actual system designand practical application. The main innovations are summarized as follows:1. An AMO-OFDM system with variable guard interval has been proposed. Accordingto the differential mode delay and the signal-to-noise ratio (SNR) threshold fordifferent modulation formats, the lower and upper limit of the cyclic prefix (CP)length can be found out. The adaptive cyclic prefix is helpful to optimize the spectral efficiency.2. Some key issues about high speed transmission have been studied and a channelestimation method based on comb-type pilot has been proposed in a single modefiber direct-detection optical OFDM (DDO-OFDM) system. The OFDM frame withcomb-type pilot subcarriers insertion uniformly can estimate the channel transferfunction of the non-pilot subcarriers by linear interpolation. After time domainaveraging to mitigate the noise, the transfer function can be used for equalization.The DDO-OFDM experiment platform based on Hermitian symmetry is setup todemonstrate the feasibility of the proposed channel estimation method atback-to-back and after50km fiber transmission. The transmission bit rate is6Gb/s.There is almost no penalty with the proposed channel estimation method comparedwith ideal channel estimation results. The required optical received power improved2dB only with linear interpolation. Meanwhile, we also conduct a simulation and aproof-of-concept experiment of a RF-Tone-assisted DDO-OFDM system todemonstrate the feasibility of the proposed channel estimation method. In simulation,a20Gb/s RF-Tone-assisted OFDM signal is transmitted over400km fibertransmission with the proposed channel estimation method. In simulation, there isalmost no OSNR penalty compared with ideal channel estimation results and2dBOSNR penalty only with linear interpolation. In experiment,6Gb/s signal istransmitted at back-to-back and over25km fiber transmission, and the results arealso consistent with the simulation results.3. More research efforts focus on multi-band coherent optical OFDM to realize Tb/soptical fiber transmissions. In simulation,a multi-band CO-OFDM system based onoptical frequency combs transmitted32×40Gb/s signal over1000km single modefiber (SMF). Because the timing and optical laser phase of individual bands inmulti-band OFDM are correlated, a method to a significant reduction of OFDMoverhead has been proposed, which is using FFT window synchronization forchannel estimation and sharing pilot subcarriers for phase estimation. Thesimulation results indicate that less than1%OFDM overhead for1Tb/s multi-bandCO-OFDM system which can also realize1000km transmission.4. The optical OFDM technology also can be considered as a multiplexing technology,and has higher spectral efficiency compared with WDM. An optical add-drop multiplexer (OADM) method among the optical subcarriers of an OOFDM signalbased on OFFT/OIFFT has been proposed, which can be applied for optical networkbased on optical OFDM technology. It is demonstrated by a multi-band PDM-QPSKsimulation platform. Simulation results show that the OFFT/OIFFT can realize theOADM function when the optical subcarriers of an OOFDM signal are orthogonalto each other and synchronized with one another.5. Optical OFDM is introduced into passive optical network (PON) for its superiorityin the physical layer. A novel architecture for upstream transmission based onorthogonal band multiplexing and collective reception has been proposed, which isdemonstrated by theoretical simulations and experiments. The synchronoustechnique among different ONUs, the tolerance to time mismatch, and the influenceof the optical laser line-width and power variation are investigated and analyzed fora10Gb/s upstream transmission. This paper also conducts a proof-of-conceptexperiment to verify the orthogonal band multiplexing and collective reception, andthe tolerance to time mismatch is twice of the CP length. Further, bandwidth-elasticOFDM-PON upstream transmission can be realized by non-uniform subcarriersdistribution and adaptive modulation for each ONU.
Keywords/Search Tags:optical fiber communication, optical OFDM, direct-detection, digital coherent demodulation, passive optical network
PDF Full Text Request
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