WO2012139165A2 - Codage conjoint pour mrof optique - Google Patents

Codage conjoint pour mrof optique Download PDF

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WO2012139165A2
WO2012139165A2 PCT/AU2012/000377 AU2012000377W WO2012139165A2 WO 2012139165 A2 WO2012139165 A2 WO 2012139165A2 AU 2012000377 W AU2012000377 W AU 2012000377W WO 2012139165 A2 WO2012139165 A2 WO 2012139165A2
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subcarriers
optical
pair
subcarrier
ofdm
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WO2012139165A3 (fr
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Arthur James Lowery
Emanuele Viterbo
Yi Hong
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Monash University
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Monash University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2697—Multicarrier modulation systems in combination with other modulation techniques
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H04B10/5055—Laser transmitters using external modulation using a pre-coder
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B2210/00—Indexing scheme relating to optical transmission systems
    • H04B2210/25—Distortion or dispersion compensation
    • H04B2210/254—Distortion or dispersion compensation before the transmission line, i.e. pre-compensation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/36—Modulator circuits; Transmitter circuits
    • H04L27/362—Modulation using more than one carrier, e.g. with quadrature carriers, separately amplitude modulated
    • H04L27/364—Arrangements for overcoming imperfections in the modulator, e.g. quadrature error or unbalanced I and Q levels

Definitions

  • the present invention relates to optical communications, and more particularly to methods and apparatus for improving the sensitivity of receivers, and hence the overall available power budget, in optical Orthogonal Frequency Division Multiplexed (OFDM) transmission systems.
  • OFDM optical Orthogonal Frequency Division Multiplexed
  • Optical Orthogonal Frequency Division Multiplexing is a term referring to a range of proposals and technologies enabling highly spectrally-efficient and potentially robust transmission of high-bandwidth information channels over optical links and networks.
  • O-OFDM Coherent Optical Orthogonal Frequency Division Multiplexing
  • CO-OFDM Coherent Optical Orthogonal Frequency Division Multiplexing
  • an optical signal comprising numerous orthogonal subcarrier channels is transmitted via an optical link or network without the inclusion of a corresponding optical carrier tone.
  • the recovery of a CO-OFDM signal at the receiver requires the use of a separate local oscillator laser, along with the necessary optical and optoelectronic components (e.g. an optical hybrid and balanced detectors) to perform coherent detection of the received signal.
  • the independence of the local oscillator laser from the transmitter laser results in phase noise due to the randomly-evolving relative phase between the two lasers, which must be compensated using suitable techniques, such as the inclusion of a pilot tone within the CO-OFDM signal band.
  • CO-OFDM theoretically provides the highest possible receiver sensitivity amongst all O-OFDM techniques, it also involves additional complexity in the system overall, and particularly at the receiver.
  • DDO-OFDM Frequency Division Multiplexing
  • the transmitted carrier optimally comprises approximately half of the received optical power, but contributes nothing to the information content.
  • intermixing noise and interference can be substantially eliminated within a CO-OFDM receiver, using balanced detectors.
  • electrical noise generated by the mixing of optical noise components can also be substantially eliminated through the use of balanced detectors. It is possible to improve the performance of the DDO-OFDM receiver in respect of the second and third contributing factors, by extracting the transmitted carrier and using it as a local oscillator' (so-called self-coherent optical OFDM), enabling the benefits of balanced detection to be obtained.
  • this approach reintroduces much of the complexity of the CO-OFDM system.
  • the post-detection noise varies with frequency, i.e. the signal quality is a function of the subcarrier number within the OFDM signal band.
  • the error rate i.e. overall performance
  • subcarriers having the highest noise and/or interference levels relative to the signal power have the lowest quality, and contribute the largest number of errors to the overall error rate.
  • electrical components may exhibit frequency-dependent noise, gain and/or attenuation
  • optical amplifiers may exhibit gain slope
  • both electrical components and long-haul transmission links may exhibit nonlinearities that result in frequency-dependent distortions and interference. It is, accordingly, an object of the present invention to exploit frequency dependence of signal quality in O-OFDM systems to provide improvements in system performance.
  • the invention is directed to mitigating the impact of the performance of the subcarriers having lower received signal quality, in order to improve the overall performance of the transmission system.
  • Embodiments of the invention seek to reduce overall error rate for a given received optical signal power, or equivalently enable a given error rate to be achieved with a lower received optical signal power or OSNR, i.e. to improve the receiver sensitivity. In turn, this results in an increase in the available power budget in an O-OFDM transmission system.
  • the invention provides a method for generating an Optical Orthogonal Frequency Division Multiplexed (O-OFDM) signal providing improved receiver sensitivity in an optical communications system which comprises an optical channel, wherein a received signal quality is variable with frequency, the method comprising:
  • an overall improvement in receiver sensitivity can be obtained.
  • the effects of subcarriers having low signal quality on the overall error rate are mitigated by coding information on these subcarriers jointly with subcarriers having a higher signal quality. This can be shown to result in an overall net improvement, which translates directly into improved receiver sensitivity.
  • the improvements provided by joint coding may allow a reduction in transmitted carrier power within a DDO-OFDM system, thereby assigning a larger proportion of available power to the subcarriers for a given OSNR, resulting in direct improvements to the sensitivity of the receiver, and to the power budget of the transmission system overall.
  • O-OFDM systems may transmit OFDM signals on either one or two orthogonal polarisation states of the optical carrier.
  • subsets of subcarriers may be selected within each polarisation state and/or across polarisation states.
  • Embodiments of the invention preferably employ a small number of selected subcarriers within each subset, for example at least two subcarriers, and no more than four subcarriers.
  • a preferred embodiment employs 'pairwise' coding, wherein each subset comprises a selected pair of subcarriers, wherein at least one pair is selected according to an imbalance of received signal quality.
  • Pairwise coding is advantageous because it enables improvements in receiver sensitivity to be achieved without requiring significant processing resources for encoding or decoding. Pairwise coding is therefore practical in O-OFDM systems, which may be required to operate at very high bit rates, e.g. tens of gigabits per second or higher.
  • a measure of the imbalance of received signal quality is based upon a ratio of the received signal quality of subcarriers in a selected pair. This may be, for example, a ratio between the Signal-to-lnterference and Noise Ratio (SINR) of the subcarriers, or of some convenient function of this ratio, such as the square root.
  • SINR Signal-to-lnterference and Noise Ratio
  • a preferred form of the method comprises the following steps for selection of pairs:
  • this pair selection approach results in a pairing of the 'worst' subcarrier with the 'best' subcarrier, the 'second best' with the 'second worst', and so forth.
  • the joint-coding algorithm does not introduce redundancy, i.e. a rate 1 code is employed.
  • a rate 1 code is employed.
  • the use of a rate 1 code avoids additional complexity in the decoder of a corresponding receiver.
  • the joint coding algorithm comprises a phase rotation applied to each selected pair of subcarriers, wherein the method includes coding steps of:
  • the method includes a further coding step of applying an in-phase/quadrature (IQ) component interleaving over each pair of phase-rotated subcarriers.
  • IQ in-phase/quadrature
  • the method includes performing a theoretical calculation of predicted received signal quality of each subcarrier as a basis for partitioning of the subcarriers, wherein the theoretical calculation is based upon estimated and/or measured properties of the optical channel.
  • the O-OFDM transmission system is a DDO-OFDM system, and the theoretical calculation is based upon a computation of the interference and noise components generated by square-law detection.
  • the method includes performing a numerical simulation of predicted received signal quality of each subcarrier as a basis for partitioning of the subcarriers, wherein the simulation is based upon estimated and/or measured properties of the optical channel.
  • the method includes performing a measurement of received signal quality of each subcarrier as a basis for partitioning of the subcarriers.
  • the method may include measurement steps of transmitting a predetermined training data set over the optical channel, and determining an error rate of each subcarrier of the received signal.
  • the error rates may be determined at the receiver, and corresponding signal quality information sent back to the transmitter via a separate (e.g. out-of-band) channel.
  • the invention provides a method for recovering digital information in an O-OFDM receiver of an optical communication system, wherein a received optical signal comprises a plurality of orthogonal subcarriers to which the digital information has been mapped and which have been partitioned to form a plurality of subcarrier pairs, each pair having been coded by applying a phase rotation and an in-phase/quadrature (IQ) component interleaving, the method comprising:
  • the invention provides an O-OFDM transmitter comprising:
  • a digital processor configured and/or programmed at least to
  • subcarriers partition the subcarriers to form a plurality of subsets of subcarriers, and apply a joint coding algorithm to encode information for transmission via each subset of subcarriers, wherein the subcarriers are selected such that an improvement in sensitivity of an associated optical receiver is obtained, when compared with a corresponding independent coding of each subcarrier, and
  • DAC digital-to-analog converter
  • the digital processor is configured and/or programmed to partition the subcarriers such that each subset comprises a selected pair of subcarriers, wherein at least one pair is selected according to an imbalance of received signal quality, according to a method comprising the following steps for selection of pairs:
  • the digital processor is further configured and/or programmed to apply a joint coding algorithm which comprises a phase rotation applied to each selected pair of subcarriers, according to a method comprising coding steps of:
  • the digital processor is further configured and/or programmed to execute a further coding step of applying an IQ component interleaving over each pair of phase-rotated subcarriers.
  • the invention provides an O-OFDM receiver comprising: an optical-to-electrical (O/E) conversion unit having an optical input and at least one electrical output, which is configured to detect an O-OFDM signal received at the input and to generate a corresponding electrical OFDM signal at the electrical output;
  • O/E optical-to-electrical
  • ADC analog-to-digital converter
  • a digital processor configured and/or programmed at least to receive the signal samples from the ADC
  • FIG. 1 is a schematic diagram of a DDO-OFDM transmission system embodying the invention
  • Figure 2 shows schematic spectra illustrating frequency dependence of received noise and interference in the DDO-OFDM system of Figure 1 ;
  • Figure 3 is a graph of simulated and calculated received signal, interference and noise levels as a function of frequency within the DDO-OFDM system of Figure 1 ;
  • Figure 4 is a graph illustrating variation of signal-to-interference noise ratio across the subcarrier band according to embodiments of the invention.
  • Figure 5 is a graph illustrating optimal angle of pairing of subcarriers according to embodiments of the invention.
  • Figure 6 is graph showing required optical signal-to-noise ratio for a bit error ratio of 10 "3 as a function of the ratio of carrier power to sideband power, with and without pairing in accordance with an embodiment of the invention.
  • Embodiments of the present invention are applicable to a range of Optical Orthogonal Frequency Division Multiplexing (O-OFDM) transmission systems and networks, in which the signal quality varies across the OFDM subcarriers.
  • OFDM Optical Orthogonal Frequency Division Multiplexing
  • a central principle exploited by the invention is the recognition that by performing joint coding over subcarriers having different signal quality, it is possible to 'share' the benefits of higher-quality subcarriers amongst those of lower quality or, conversely, to 'distribute' the adverse impact of poor quality subcarriers across those of higher quality.
  • DDO-OFDM Direct Detection Optical Orthogonal Frequency Division Multiplexing
  • the DDO-OFDM system 100 has a data input 102 at which digital information bits are received.
  • the input digital data is then processed in the transmitter by a digital processor which implements, conceptually, a number of distinct processing functions represented as blocks within the schematic diagram of the system 100.
  • the digital processor may be implemented in software executing on a suitable central processing unit (e.g. a DSP device), or as custom, or semi-custom, hardware, such as an Application-Specific Integrated Circuit (ASIC), or programmable hardware, such as a Field-Programmable Gate Array (FPGA).
  • ASIC Application-Specific Integrated Circuit
  • FPGA Field-Programmable Gate Array
  • An input mapping 104 is applied, in which groups of input data bits are buffered and transferred to modulation blocks 106, 108.
  • modulation blocks 106, 108 In the embodiment 100 shown in Figure 1 , 4-QAM modulation is employed, whereby two input data bits are provided to each modulation block 106, 108, each of which outputs a corresponding complex number, which takes on one of four values.
  • These complex values are conveniently represented in Cartesian form, as a real, or in-phase (I) component, and an imaginary, or quadrature (Q) component.
  • the joint coding algorithm employed is a 'pairwise' algorithm, i.e. one which performs coding across subsets comprising two subcarriers each.
  • the two complex-valued outputs of the modulation blocks 106, 108 represent the signal information that is to be coded across a corresponding pair of subcarriers.
  • the pairs of 4-QAM values output from the modulators 106, 108 are jointly coded by firstly applying a phase rotation, in the rotation blocks 1 10, 1 12, which is dependent upon an imbalance in received signal quality of a corresponding pair of subcarriers of the DDO-OFDM signal.
  • the pair of rotated complex values are then in-phase/quadrature (IQ) interleaved, as indicated by the crossed signal paths 1 14. That is, the in-phase, or real, components of both input values are mapped to one output value, while the quadrature, or imaginary, components are mapped to the other output.
  • the resulting pair of jointly coded values are then mapped to a corresponding selected pair of subcarriers of the transmitted signal via the operation of the commutator 1 16 and serial-to-parallel conversion 1 18 processing blocks.
  • FFT Transform
  • a cyclic prefix is added 122, and then parallel-to-serial conversion performed 124, in order to produce two serial sequences of real-valued digital samples, corresponding with in-phase and quadrature components of a transmitted signal.
  • These sequences of digital samples are converted to corresponding electrical signal waveforms via a pair of digital-to-analog converters (DACs) 126.
  • DACs digital-to-analog converters
  • a carrier tone 130 is then added to the electrical waveforms, which will act as the optical carrier during direct detection of the transmitted optical signal.
  • the RF frequency of the carrier tone 130 is selected so as to provide a frequency spacing between the resulting carrier and the edge of the subcarrier signal bandwidth of B gap .
  • the addition of a carrier tone within the electrical domain enables the transmitted electrical signal waveforms to remain at baseband, so as to take maximum advantage of the bandwidth of the relevant electrical components.
  • An optical source such as a semiconductor laser 132
  • a complex optical modulator 134 is then modulated with the in-phase and quadrature components of the signal using a complex optical modulator 134.
  • the resulting optical spectrum 136 comprises the carrier tone 138, and an OFDM subcarrier band 140, having a bandwidth of B sc .
  • the O-OFDM signal 136 is then transmitted via amplified fibre link 142, which comprises a number of spans of single-mode optical fibre, along with optical amplifiers, such as Erbium-Doped Fibre Amplifiers (EDFAs), arranged to compensate for attenuation within the fibre spans.
  • EDFAs Erbium-Doped Fibre Amplifiers
  • an optical bandpass filter 144 is used to reject Amplified Spontaneous Emission (ASE) noise generated by the EDFAs outside the desired signal bandwidth.
  • the resulting signal is directly detected, for example by the photodiode 146.
  • Front-end analog components 148 amplify, filter and down-convert the received signal, resulting in detected in-phase and quadrature signal waveforms, which are converted to corresponding sequences of digital signal samples using a pair of analog-to-digital converters (ADCs) 150.
  • ADCs analog-to-digital converters
  • the receiver includes a digital processor configured to demodulate and recover the transmitted digital information.
  • the receiver digital processor may be implemented via software, custom ICs, ASICs, FPGAs, other suitable digital processing components, and/or any combination of these.
  • the required processing is described with reference to specific conceptual functional blocks.
  • a serial-to-parallel converter 152 receives fixed-length blocks (i.e. received OFDM symbols) and converts them into a corresponding parallel array of complex values. The cyclic prefix is then stripped in block 154. A time/frequency transformation, in the form of FFT 156, is applied to recover a corresponding block of frequency domain, i.e. subcarrier, values. Equalisation 158 is applied, to compensate for transmission impairments such as chromatic dispersion, polarisation-mode dispersion, and linear distortions due to non-ideal characteristics of optical and electrical components. The jointly coded pairs of subcarriers are then selected by the demapping block 160. A parallel-to-serial converter 162 then outputs the values corresponding with these selected pairs in sequence.
  • IQ de-interleaving is performed, as indicated by the crossed signal paths 164, which reverses the interleaving process performed in the transmitter 1 14.
  • the resulting pair of complex received subcarrier values which have been degraded by processes such as additive noise and nonlinear distortion, are then processed in order to recover (i.e. estimate) the corresponding values of transmitted information bits.
  • an output mapping 170 is applied, to produce a digital data output
  • a transmission system such as the exemplary system 100 will not be designed to achieve a zero raw Bit Error Ratio (BER), which is neither possible nor optimal in practice.
  • BER Bit Error Ratio
  • FEC Forward Error Correction
  • the improvement in overall system power budget achieved by utilising FEC is more than sufficient to compensate for the additional overhead required for the error detection/correction coding.
  • any error coding is applied prior to the data input 102, and subsequent to the data output 172, and will therefore not be discussed in any greater detail.
  • embodiments of the invention are applicable to O-OFDM systems in which the signal quality varies across the subcarrier band.
  • subcarrier signal quality is affected by additive noise, distortion, and interference, such as crosstalk from other subcarriers and other signals within the system, which may be generated, for example, through nonlinear processes, including the square-law detection process occurring at the photodiode 146 of the system 100.
  • SINR Signal-to-lnterference and Noise Ratio
  • the SINR is frequency-dependent, even for flat ASE spectrum, and even if all of the characteristics of the various electrical and optical components are ideal. This arises because of the mixing that occurs at the photodetector 146, which outputs a photo current proportional to the square of the input optical field.
  • Figure 2 shows schematically a series of spectra illustrating the frequency dependence of received noise and interference in the DDO-OFDM system 100.
  • the optical spectrum 200 of the received signal comprises the carrier tone 202, the band of information-bearing subcarriers 204, and an approximately flat 'floor' of ASE noise 206. (Note that all of the schematic spectre in Figure 2 are shown in two-sided form.)
  • the spectrum 208 of the desired OFDM signal which results from mixing between the subcarrier band 204 and the carrier tone 202, comprises the received subcarriers 210.
  • the spectrum 212 shows the interference 214 which results from mixing of the subcarriers 204 with themselves.
  • the spectrum 216 comprises the spontaneous-spontaneous beat noise 218, which results from mixing of ASE noise 206 with itself.
  • the spectrum 220 represents the carrier-spontaneous beat noise, resulting from mixing of the ASE noise 206 with the carrier tone 202.
  • This noise component has a 'step' form, having a higher noise power level 222 around DC, and a lower noise power level 224 across the subcarrier signal band.
  • the spectrum 226 resulting from mixing of the ASE noise 206 with the subcarriers 204 has the relatively complex form 228.
  • ⁇ fo is the carrier frequency
  • ⁇ A is the amplitude of the optical carrier field
  • ⁇ So is the mean power spectral density of the optical OFDM signal
  • ⁇ NASE is the ASE noise power spectral density
  • ⁇ B m is the bandwidth of the optical bandpass filter 144
  • ⁇ Bg a p is the width of the frequency gap between the carrier 202 and the subcarrier band 204;
  • ⁇ Bsc is the total bandwidth of the OFDM subcarrier band 204;
  • ⁇ B L is the bandwidth of unrejected AS E noise at frequencies lower than the carrier frequency
  • ⁇ BH is the bandwidth of unrejected AS E noise above the optical OFDM subcarrier band.
  • G y (f) 2G f)* ⁇ -[S(f -f 0 ) + S(f + f 0 )] + G s (f)* G s (f) + G n (f)* G n (f)
  • G s (/) (equal to S 0 over the subcarrier band) and G n ⁇ f) (equal to N A SE over the optical filter bandwidth) are the PSD of the OFDM signal and of the ASE noise in both polarizations, respectively.
  • the above expression is valid under the assumption that both noise and signal are zero-mean Gaussian random processes. This assumption is valid when the number of subcarriers is sufficiently large.
  • Equation (1 ) represents the power spectrum of the useful OFDM signal which has been down-converted by the subcarriers mixing with the carrier upon photodetection ; (2) is the autocorrelation of G s (/) accounting for the unwanted mixing tones; (3) is the autocorrelation of G n ⁇ f); (4) is noise that has been down converted by mixing with the carrier; and (5) arises from mixing of noise with the OFDM subcarriers.
  • the sum of the terms (2), (3), (4) and (5) represents the noise and interference impairments.
  • the DC component arises from the optical carrier and the unwanted tones, while the (2/ ) arises from the optical carrier.
  • FIG. 3 provides a further illustration of the inherent frequency dependence of SINR in a DDO-OFDM receiver.
  • the figure comprises a graph 300 of simulated and calculated received signal, interference and noise levels in the receiver as a function of frequency.
  • the resolution bandwidth used in generating the spectrum is 14.648 MHz.
  • This bandwidth is one quarter of the subcarrier spacing, and accordingly the SINR that may be read from the graph 300 (i.e. the difference between the signal power density and the associated noise power density at a particular frequency) is 6 dB more than the actual SINR experienced by an individual subcarrier after demodulation.
  • the graph 300 shows RF power in dBm (within the resolution bandwidth) on the vertical axis 302, as a function of RF frequency, in gigahertz, on the horizontal axis 304.
  • Three sets of points are shown in the graph 300, being the RF signal level across the subcarrier band 306, the inter-modulation components 308 (i.e. resulting from subcarrier mixing), and RF noise 310, resulting from the combined ASE mixing terms.
  • the scattered points result from a system simulation performed using VPItransmissionMakerTM Version 8.5, from VPIphotonicsTM, while the smooth curves are computed using the above analytical expression of the electrical power spectral density.
  • the SINR of the subcarriers spans the range from 6 dB (at 30 GHz) to about 9dB (at 60 GHz).
  • Example calculations are shown in the graph 400 of Figure 4.
  • the graph 400 shows SINR, in dB, on the vertical axis 402, as a function of the RF frequency, in gigahertz, on the horizontal axis 404.
  • the curves 406, 408, 410, 412, respectively, represent the SINR for an OSNR of 19 dB, 16 dB, 13 dB and 10 dB.
  • MATLABTM simulations have been conducted comparing the OSNRs at which the DDO-OFDM system can achieve a raw BER of 10 "3 as a function of the carrier-to-signal power ratio.
  • the OSNR was swept to obtain values of the BER as a function of OSNR, from which the OSNR corresponding with a BER of 10 "3 was derived.
  • the results are summarised in the graph 600 of Figure 6, which shows results with and without 'pairing' (i.e. joint coding).
  • the graph 600 shows the required OSNR for a BER of 10 "3 , in dB, on the vertical axis 602, as a function of the carrier-to-sideband power ratio on the horizontal axis 604.
  • the upper set of points 606 represents the results without pairwise coding, i.e. in accordance with the prior art.
  • Three additional points, e.g. 608, have been obtained using a full system simulation in
  • VPItransmissionMakerTM software Excellent agreement is obtained between the
  • the lower set of points 610 corresponds to MATLABTM simulations including optimum pairwise coding (see appendix), in combination with ML estimation in the receiver. In all cases simulated, a reduction in the required OSNR to achieve a BER of 10 "3 is achieved. This reduction in required OSNR corresponds with an improvement in receiver sensitivity or, equivalently, an increase in overall system power budget.
  • the corresponding SINR may be calculated, for example based upon a Gaussian approximation, or more-detailed model of the system and receiver.
  • the measured SINR may then be communicated back to the transmitter, for example using an out-of-band channel, such as a signalling channel.
  • the quantity of information involved in this feedback step i.e. a single SINR value representing each subcarrier
  • the measured data is then used in the transmitter in order to compute optimised pairwise partitions, and corresponding coding parameters.
  • predetermined OFDM training symbols may be periodically re-transmitted, in order to update the measured SINR values, and track the time variations.
  • embodiments of the invention employ joint coding, exemplified by pairwise coding in the present examples, which is shown to be beneficial in systems for which the received SINR varies as a function of frequency, i.e. subcarrier number.
  • this is an inherent characteristic of DDO-OFDM systems, for which a performance improvement of 0.7 dB has been demonstrated for particular exemplary embodiments.
  • a further advantage of the joint-coding approach in the exemplary embodiments is that an improvement may be achieved in conjunction with a reduction in the relative power of an optical carrier tone. This reduction in power is potentially useful for reducing the effects of fibre nonlinearity.
  • the sub-channels may first be reordered into a list with monotonically increasing (or decreasing) SINR, before applying the same pairing method, and mapping back to the original subcarrier numbers.
  • the actual coding is performed across a pair (indexed by k) of M-QAM information symbols a k and b k by multiplying by rotation factor yielding two rotated complex symbols a k el ek and b k el ek , where ⁇ 3 ⁇ 4 is the rotation angle for the Ar-th pair.
  • the optimal rotation angle denoted by 0 k opt , is derived analytically in S. K. Mohammed, E. Viterbo, Y. Hong, and A. Chockalingam, "MIMO precoding with X- and Y-codes," IEEE Trans, on Information Theory (201 0) for 4-QAM, to minimize the total error probability and is given by:
  • condition number of the pair of subcarriers ⁇ p k , q k ), and The condition number describes the SINR unbalance between the two subcarriers.
  • the graph 500 shows the optimum rotation angle on the vertical axis 502 as a function of the subcarrier pairing index / on the horizontal axis 504.
  • the curves 506, 508, 510, 512, 514 correspond with OSNR of 7 dB, 10 dB, 13 dB and 19 dB respectively.
  • OSNR 7 dB and 19 dB
  • IQ Component interleaving is used over the two precoded symbols and b k ek .
  • the real part of b k ek and the imaginary part of a ⁇ ek are interchanged (IQ interleaving) to obtain two coded complex symbols for transmission:
  • X Re(3 ⁇ 4e 73 ⁇ 4 ) + j Re(3 ⁇ 4e 73 ⁇ 4 )
  • X Im(3 ⁇ 4e 73 ⁇ 4 ) + j Im(3 ⁇ 4e 73 ⁇ 4 )
  • Re(.) and lm(.) denote the real and imaginary parts of a complex symbol.

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  • Optical Communication System (AREA)

Abstract

Un émetteur à multiplexage par répartition orthogonale de la fréquence optique (MROF-O) selon l'invention comprend un processeur numérique qui est configuré pour recevoir des bits d'informations numériques d'entrée sur une entrée de données (102) et mapper les bits reçus vers une pluralité de sous-porteuses orthogonales. Les sous-porteuses sont partitionnées pour former une pluralité de sous-ensembles de sous-porteuses et un algorithme de codage conjoint (110, 112, 114) est appliqué pour coder les informations pour transmission par le biais de chaque sous-ensemble de sous-porteuses. Les sous-porteuses sont choisies (116, 118) de telle sorte qu'une amélioration de sensibilité d'un récepteur optique associé soit obtenue, par rapport à un codage indépendant correspondant de chaque sous-porteuse. Le processeur numérique réalise en outre une transformation (120) des sous-porteuses codées pour produire une séquence temporelle correspondante d'échantillons de signaux MROF. L'émetteur comprend également au moins un convertisseur numérique-analogique (126) couplé fonctionnellement au processeur numérique et configuré pour convertir les échantillons de signaux MROF en au moins un signal MROF électrique correspondant. Une source optique (132, 134) comprenant au moins une entrée électrique et une sortie de signal optique est conçue pour recevoir le signal MROF électrique et pour moduler une porteuse optique au moyen du signal MROF électrique pour générer un signal MROF optique correspondant. L'invention concerne également un récepteur correspondant et un système de transmission MROF-O correspondant.
PCT/AU2012/000377 2011-04-14 2012-04-13 Codage conjoint pour mrof optique Ceased WO2012139165A2 (fr)

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AU2011901404A AU2011901404A0 (en) 2011-04-14 Joint Coding for Optical OFDM
AU2011901404 2011-04-14

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CN103546417A (zh) * 2013-11-08 2014-01-29 武汉邮电科学研究院 偏振正交信号的光ofdm直接检测方法及装置
CN104486025A (zh) * 2014-11-26 2015-04-01 梁宇 一种光码分复用系统编解码方法
CN109314579A (zh) * 2016-06-13 2019-02-05 三菱电机株式会社 光传输方法和光传输系统
CN116866139A (zh) * 2023-08-14 2023-10-10 清华大学深圳国际研究生院 一种无线光通信的ofdm增强调制方法

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US8204377B2 (en) * 2008-10-23 2012-06-19 Alcatel Lucent System, method and apparatus for joint self phase modulation compensation for coherent optical polarization-division-multiplexed orthogonal-frequency division-multiplexing systems

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Publication number Priority date Publication date Assignee Title
CN103546417A (zh) * 2013-11-08 2014-01-29 武汉邮电科学研究院 偏振正交信号的光ofdm直接检测方法及装置
CN103546417B (zh) * 2013-11-08 2017-01-11 武汉邮电科学研究院 偏振正交的光ofdm信号直接检测方法及装置
CN104486025A (zh) * 2014-11-26 2015-04-01 梁宇 一种光码分复用系统编解码方法
CN109314579A (zh) * 2016-06-13 2019-02-05 三菱电机株式会社 光传输方法和光传输系统
CN109314579B (zh) * 2016-06-13 2021-06-18 三菱电机株式会社 光传输方法和光传输系统
CN116866139A (zh) * 2023-08-14 2023-10-10 清华大学深圳国际研究生院 一种无线光通信的ofdm增强调制方法

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