WO2014194940A1 - Récepteur optique cohérent - Google Patents

Récepteur optique cohérent Download PDF

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Publication number
WO2014194940A1
WO2014194940A1 PCT/EP2013/061533 EP2013061533W WO2014194940A1 WO 2014194940 A1 WO2014194940 A1 WO 2014194940A1 EP 2013061533 W EP2013061533 W EP 2013061533W WO 2014194940 A1 WO2014194940 A1 WO 2014194940A1
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WIPO (PCT)
Prior art keywords
coherent optical
signal
optical signal
tedc
digital
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PCT/EP2013/061533
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English (en)
Inventor
Nebojsa Stojanovic
Original Assignee
Huawei Technologies Co., Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Application filed by Huawei Technologies Co., Ltd. filed Critical Huawei Technologies Co., Ltd.
Priority to CN201380077046.7A priority Critical patent/CN105393487B/zh
Priority to PCT/EP2013/061533 priority patent/WO2014194940A1/fr
Publication of WO2014194940A1 publication Critical patent/WO2014194940A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/0016—Arrangements for synchronising receiver with transmitter correction of synchronization errors
    • H04L7/002—Arrangements for synchronising receiver with transmitter correction of synchronization errors correction by interpolation
    • H04L7/0029—Arrangements for synchronising receiver with transmitter correction of synchronization errors correction by interpolation interpolation of received data signal
    • 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
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/616—Details of the electronic signal processing in coherent optical receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/02—Speed or phase control by the received code signals, the signals containing no special synchronisation information
    • H04L7/033—Speed or phase control by the received code signals, the signals containing no special synchronisation information using the transitions of the received signal to control the phase of the synchronising-signal-generating means, e.g. using a phase-locked loop
    • H04L7/0334—Processing of samples having at least three levels, e.g. soft decisions
    • H04L7/0335—Gardner detector
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/0054—Detection of the synchronisation error by features other than the received signal transition
    • H04L7/007—Detection of the synchronisation error by features other than the received signal transition detection of error based on maximum signal power, e.g. peak value, maximizing autocorrelation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/02—Speed or phase control by the received code signals, the signals containing no special synchronisation information
    • H04L7/027—Speed or phase control by the received code signals, the signals containing no special synchronisation information extracting the synchronising or clock signal from the received signal spectrum, e.g. by using a resonant or bandpass circuit

Definitions

  • the present invention relates to a coherent optical receiver and a method for clock recovery in a coherent optical receiver.
  • optical fiber systems An important goal of long-haul optical fiber systems is to transmit the highest data throughput over the longest distance without signal regeneration in optical-electrical- optical regenerators. Given constraints on the bandwidth imposed by optical amplifiers and ultimately by the fiber itself, it is important to maximize spectral efficiency. Most current systems use binary modulation formats, such as on-off keying encoding one bit per symbol.
  • Advanced modulation formats in combination with coherent receivers enable high capacity and spectral efficiency.
  • Polarization multiplexing, quadrature amplitude modulation and coherent detection are seen as a winning combination for the next generation of high- capacity optical transmission systems since they allow information encoding in all the available degrees of freedom.
  • FIG. 1 A Block diagram of a coherent optical receiver 100 is shown in Fig. 1. Since the digital signal is mapped into both polarization a 90°hybrid 101 is used to mix the input signal 102 with the local oscillator (LO) signal 104 that results in four output signals 106 (two signals per polarization). The optical signal 102 is converted to an electrical signal via an optical front end (OFE) 103 consisting of photo diodes (single PIN or balanced) and a transimpedance amplifier (TIA).
  • OFE optical front end
  • TIA transimpedance amplifier
  • AGC automatic gain control
  • DSP digital signal processing
  • a fast DSP hardware part 109a a fast DSP hardware part 109a
  • a slow DSP software part 109b a slow DSP block 109b.
  • DSP block 109 one compensates for chromatic dispersion (CD), polarization mode dispersion (PMD), polarization rotation, nonlinear effects, LO noise, LO frequency offset, etc.
  • Estimation of slow processes can be done in the software part 109b of the DSP circuit 109.
  • Basic DSP blocks 200 are presented in Fig. 2. After offset and gain correction 201 the four signals 202 are equalized for chromatic dispersion in frequency domain using two fast Fourier transformation (FFT) blocks 203. Frequency offset is removed in a frequency recovery block 205.
  • FFT fast Fourier transformation
  • FIR finite impulse response
  • ⁇ 0 is the signal wavelength
  • f s is the sampling frequency
  • N is the FFT size
  • c is the speed of light
  • n is the tap number
  • L is fiber length
  • D dispersion coefficient
  • IFFT inverse FFT
  • each receiver In digital communication systems, the heart of each receiver is a clock recovery circuit that extracts frequency and phase from incoming data and forces a local clock source to control the sampling rate and the sampling phase of the ADC.
  • the second feature is not too important in over-sampled systems, as data processing blocks are less sensitive to the sampling phase.
  • phase detectors Several phase detectors (PD) have been proposed for digital systems. Some of them are frequently used in practical systems: The Mueller and Mijller phase detector is described in [K. H. Mueller and M. Muller, IEEE Transaction on Comm. 24, 516-531 (1976)].
  • the Alexander phase detector is described in [J. D. H. Alexander, Electron. Lett. 111 , 541 -542 (1975)].
  • the Gardner phase detector is described in [F.
  • the Mueller and Muller PD works with one sample per symbol. Other PDs are used with two-fold oversampling.
  • the Gardner PD TEDC can be described for complex signals as:
  • TEDC( ) E[real(x(kT - T 12 + ⁇ )(x * (kT + ⁇ ) - x * (kT - T + ⁇ )))] (1 ) where 7 is symbol interval, x is input signal, ⁇ is the sampling instant (between 0 and 7), £ is expectation operator, and * denotes complex conjugate operation.
  • the Godard PD can be easily translated in FFT domain as
  • the received signal is oversampled (two samples per symbol).
  • the Nyquist transmission based on Nyquist pulses is used to frequency limit the channel bandwidth. This enables better channel packaging and automatically higher spectral efficiency.
  • the raised-cosine filter is an implementation of a low-pass Nyquist filter, i.e., one that has the property of vestigial symmetry. This means that its spectrum exhibits odd symmetry about 1/27, where 7 is the symbol-period of the communications system. Its frequency-domain description is given by: otherwise
  • the roll-off factor, ⁇ is a measure of the excess bandwidth of the filter, i.e. the bandwidth occupied beyond the Nyquist bandwidth of 1/27.
  • Frequency 400a and impulse response 400b of the Nyquist filter are shown in Fig. 4.
  • the minimum signal bandwidth is achieved for a roll-off factor equal to 0.
  • the results shown in Fig. 6 illustrate the clock recovery problem. TEDCs are very small and desynchronized.
  • This approach uses specific pre-filter 701 and narrowband filter 703 to filter out the clock tone. It enables the clock extraction for small ROF value but fails for higher ROF values.
  • the complete system 700 is realized in the analog domain where there is no any limitation in terms of sampling frequency and signal digitalizing before the clock extraction.
  • a technique for improved clock recovery can be realized by using feed-forward and feed- backward clock recovery coupled by a phase detector providing a timing error detection characteristic (TDEC) signal for clock offset and phase offset compensation.
  • TDEC timing error detection characteristic
  • OFE optical front end
  • PIN positive intrinsic negative
  • AGC automatic gain control
  • ADC analog-to-digital converter
  • DSP digital signal processing
  • CD chromatic dispersion
  • PMD polarization mode dispersion
  • FFT Fast Fourier Transform
  • TEDC timing error detection characteristic
  • ROF roll-off factor
  • VCO voltage controlled oscillator
  • CDU clock distribution unit
  • OFDM orthogonal frequency division multiplex
  • the invention relates to a coherent optical receiver, comprising: analog-to-digital conversion means configured for sampling an analog coherent optical signal into a digital coherent optical signal; channel equalization means configured for equalizing the digital coherent optical signal; channel transfer function calculation means configured for calculating a channel transfer function based on the digital coherent optical signal and the equalized digital coherent optical signal interpolated by interpolation means and configured for adjusting the channel equalization means based on the calculated channel transfer function; phase detection means configured for providing a timing error detection characteristic, TEDC, signal, based on the equalized digital coherent optical signal; feed-backward timing recovery means configured for adjusting the sampling of the analog-to-digital conversion means based on the TEDC signal with respect to a frequency offset compensation criterion; and feed-forward timing recovery means configured for adjusting the interpolation means with respect to a phase offset compensation criterion.
  • analog-to-digital conversion means configured for sampling an analog coherent optical signal into a digital coherent optical signal
  • channel equalization means configured for equalizing the digital coherent optical signal
  • the coherent optical receiver provides clock extraction in Nyquist systems. By using the feed-forward and feed-backward timing recovery means, clock extraction can be enabled independently on the roll-off factor, ROF, ⁇ .
  • the coherent optical receiver provides a timing signal for feed-back timing recovery by using the feed-backward timing recovery means.
  • the coherent optical receiver enables feed-forward timing recovery tolerating large and fast jitter by using the feed-forward timing recovery means.
  • the coherent optical receiver By using the interpolation means, the coherent optical receiver generates the quadrature component using the simplest linear interpolation that does not require four samples per each complex signal component which is easy to implement
  • the coherent optical receiver can be operated independently on modulation formats.
  • the TEDC signal indicates a phase offset and a frequency offset of the analog coherent optical signal with respect to a local oscillator, the local oscillator controlling the sampling of the analog-to-digital conversion means.
  • phase offset and a frequency offset of the analog coherent optical signal can be compensated.
  • the interpolation means comprises first interpolation means configured for interpolating the digital coherent optical signal and second interpolation means configured for interpolating the equalized digital coherent optical signal.
  • the first interpolation means provides two output samples for two input samples; and the second interpolation means provides one output sample for two input samples.
  • interpolation can be correctly adjusted for precisely calculating the channel transfer function.
  • the feed-forward timing recovery means comprises position calculation means configured for calculating clock positions based on the TEDC signal.
  • clock positions can be precisely adjusted with respect to phase offset compensation.
  • the interpolation means is configured to provide samples of the digital coherent optical signal and the equalized digital coherent optical signal at the calculated clock positions.
  • the channel equalization means can accurately equalize the digital coherent optical signal, thereby reducing the bit error rate.
  • the TEDC signal is filtered by an infinite impulse response, MR, low-pass filter before being provided to the position calculation means.
  • the coherent optical receiver comprises: carrier recovery means configured to recover a carrier signal of the analog coherent optical signal based on the equalized digital coherent optical signal.
  • the coherent optical receiver is flexible for using the carrier signal or the digital coherent optical signal for detecting the channel transfer function.
  • the channel transfer function calculation means is configured for calculating the channel transfer function based on the carrier signal.
  • the digital coherent optical signal comprises two samples per symbol.
  • the phase detection means is configured for providing the TEDC signal based on samples of two contiguous symbol intervals.
  • a TEDC signal based on samples of two contiguous symbol intervals can be easy calculated.
  • a simple filter with one delay element can be applied for that calculation.
  • the coherent optical receiver is configured for receiving optical signals using Nyquist pulses, configured for receiving optical signals of any QAM or PSK modulation format and/or configured for receiving optical signals with signal bandwidth smaller than a minimum defined Nyquist bandwidth.
  • the coherent optical receiver can be flexibly used for receiving any QAM or PSK modulation format.
  • Such a phase detector provides a linear TEDC signal having a strong clock tone and low jitter.
  • the phase detector is configured for providing a linear TEDC signal, in particular by applying the relations according to the second aspect to the digital coherent optical signal and a shifted version thereof.
  • phase detector can be easily implemented by using standard FIR filtering or MR filtering.
  • the invention relates to a method for clock recovery in a coherent optical receiver, the method comprising: sampling an analog coherent optical signal into a digital coherent optical signal; equalizing the digital coherent optical signal; calculating a channel transfer function based on interpolations of the digital coherent optical signal and the equalized digital coherent optical signal and adjusting the channel equalization means based on the calculated channel transfer function; providing a timing error detection characteristic, TEDC, signal, based on the equalized digital coherent optical signal; adjusting the sampling of the analog-to-digital converter based on the TEDC signal with respect to a frequency offset compensation criterion; and adjusting the interpolations of the digital coherent optical signal and the equalized digital coherent optical signal with respect to a phase offset compensation criterion.
  • TEDC timing error detection characteristic
  • Such a method can be advantageously applied for clock extraction in Nyquist systems.
  • the clock extraction can be enabled independently on the roll-off factor.
  • the method thus provides a timing signal for feedback timing recovery and enables feed-forward timing recovery tolerating large and fast jitter.
  • the method is able to generate the quadrature component using the simplest linear interpolation that does not require four samples per each complex signal component which is easy to implement. The method therefore allows operating a coherent optical receiver independently on modulation formats.
  • the methods, systems and devices described herein may be implemented as software in a Digital Signal Processor (DSP), in a micro-controller or in any other side-processor or as hardware circuit within an application specific integrated circuit (ASIC).
  • DSP Digital Signal Processor
  • ASIC application specific integrated circuit
  • the invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations thereof, e.g. in available hardware of conventional mobile devices or in new hardware dedicated for processing the methods described herein.
  • Fig. 1 shows a block diagram illustrating a conventional coherent optical receiver 100
  • Fig. 2 shows a block diagram illustrating basic DSP blocks 200 of the coherent optical receiver 100 depicted in Fig. 1 ;
  • Fig. 3 shows a block diagram illustrating a CD compensation block 300 of the basic DSP blocks 200 depicted in Fig. 2;
  • Fig. 4 shows diagrams of frequency 400a and impulse response 400b of a conventional raised-cosine filter with various roll-off factors
  • Fig. 5 shows a diagram illustrating QPSK Gardner timing error detection characteristics (TEDC) 500 for roll-off factors from 0 to 1 in steps of 0.1 for the raised-cosine filter depicted in Fig. 4;
  • TDC QPSK Gardner timing error detection characteristics
  • Fig. 6 shows a diagram illustrating QPSK Gardner TEDCs 600 for a roll-off factor of 1 for the raised-cosine filter depicted in Fig. 4;
  • Fig. 7 shows a block diagram illustrating signal preprocessing of a 4 th power low analog system 700 for clock tone extraction
  • Fig. 8 shows a block diagram illustrating a coherent optical receiver 800 according to an implementation form
  • Fig. 9 shows a diagram illustrating a TED characteristic 900 of a phase detector according to an implementation form
  • Fig. 10 shows eye diagrams of real 1000a and imaginary 1000b parts of QPSK modulated signal
  • Fig. 1 1 a shows a block diagram of a phase detector 1 100 with linear TEDC according to an implementation form
  • Fig. 1 1 b shows a block diagram of a low pass filter 1 150 used in the phase detector 1 100 depicted in Fig. 1 1 a according to an implementation form;
  • Fig. 13 shows a block diagram of a phase detector 1300 with linear TEDC according to an implementation form
  • Fig. 14 shows a block diagram of a phase detector 1400 with linear TEDC comprising a circuit for changing the sampling phase according to an implementation form
  • Fig. 22 shows a block diagram of a transmitter circuit 2200 and a receiver circuit 2250 illustrating Nyquist super-channel clocking according to an implementation form
  • Fig. 23 shows a block diagram illustrating a circuit 2300 comprising parallel implemented phase detectors according to an implementation form
  • Fig. 24 shows a schematic diagram illustrating a method 2400 for clock recovery in a coherent optical receiver according to an implementation form.
  • FIG. 1 shows a block diagram illustrating a conventional coherent optical receiver 100 as described above.
  • Fig. 2 shows a block diagram illustrating basic DSP blocks 200 of the coherent optical receiver 100 depicted in Fig. 1 as described above.
  • Fig. 3 shows a block diagram illustrating a CD compensation block 300 of the basic DSP blocks 200 depicted in Fig. 2 as described above.
  • Fig. 4 shows diagrams of frequency 400a and impulse response 400b of a conventional raised-cosine filter with various roll-off factors as described above.
  • Fig. 5 shows a diagram illustrating QPSK Gardner timing error detection characteristics (TEDC) 500 for roll-off factors from 0 to 1 in steps of 0.1 for the raised-cosine filter depicted in Fig. 4 as described above.
  • TDC QPSK Gardner timing error detection characteristics
  • Fig. 6 shows a diagram illustrating QPSK Gardner TEDCs 600 for a roll-off factor of 1 for the raised-cosine filter depicted in Fig. 4 as described above.
  • Fig. 7 shows a block diagram illustrating signal preprocessing of a 4 power low analog system 700 for clock tone extraction as described above.
  • Fig. 8 shows a block diagram illustrating a coherent optical receiver 800 according to an implementation form.
  • the coherent optical receiver 800 comprises analog-to-digital conversion means 801 , e.g. an analog-to-digital converter (ADC) configured for sampling an analog coherent optical signal 802 into a digital coherent optical signal 804.
  • the coherent optical receiver 800 comprises channel equalization means 803 configured for equalizing the digital coherent optical signal 804.
  • the coherent optical receiver 800 comprises channel transfer function calculation means, e.g.
  • a channel transfer function calculator 805 configured for calculating a channel transfer function 808 based on the digital coherent optical signal 804 and the equalized digital coherent optical signal 806 interpolated by interpolation means 807a, 807b and configured for adjusting the channel equalization means 803 based on the calculated channel transfer function 808.
  • the digital coherent optical signal 804 passes a delay element 831 and first interpolation means, e.g. a first interpolator 807a before being provided to the channel transfer function calculation means 805.
  • a switch S2 is used for switching the digital coherent optical signal 804 either directly or after having passed the delay element 831 and the first interpolator 807a to the channel transfer function calculation means 805.
  • the analog coherent optical signal 802 may correspond to the signal 108 described above with respect to Fig. 1 .
  • the analog-to-digital converter (ADC) 801 may correspond to the ADC 107 described above with respect to Fig. 1 .
  • the digital coherent optical signal 804 may correspond to the signal 1 10 described above with respect to
  • the coherent optical receiver 800 comprises phase detection means 809, e.g. a phase detector as described below with respect to Figures 1 1 , 13 and 14, configured for providing a timing error detection characteristic, TEDC, signal 810, based on the equalized digital coherent optical signal 806.
  • phase detection means 809 e.g. a phase detector as described below with respect to Figures 1 1 , 13 and 14, configured for providing a timing error detection characteristic, TEDC, signal 810, based on the equalized digital coherent optical signal 806.
  • the coherent optical receiver 800 comprises feed-backward timing recovery means 81 1 configured for adjusting the sampling of the analog-to-digital conversion means 801 based on the TEDC signal 810 with respect to a frequency offset compensation criterion.
  • TEDC signal 810 passes a digital-to-analog converter 833 and a low pass filter 835 before being provided to a local oscillator of a voltage controlled oscillator 815.
  • the voltage controlled oscillator (VCO) 815 controls the adjusting of the sampling rate and sampling time of the analog-to-digital converter 801 such that a frequency offset between the digital coherent optical signal and the analog coherent optical signal is compensated.
  • the coherent optical receiver 800 comprises feed-forward timing recovery means 813 configured for adjusting the interpolation means 807a, 807b with respect to a phase offset compensation criterion.
  • the TEDC signal 810 passes a low pass filter 819 before being provided to a position calculator 817 which uses the filtered TEDC signal to calculate adequate clock positions for the first interpolator 807a and for a second interpolator 807b.
  • the second interpolator 807b interpolates the equalized digital coherent optical signal 806 and provides the interpolated equalized digital coherent optical signal to the channel transfer function calculator 805.
  • the first interpolator 807a interpolates the digital coherent optical signal 804 delayed by the delay element 831 and provides the interpolated delayed digital coherent optical signal to the channel transfer function calculator 805.
  • the channel transfer function calculator 805 calculates the channel transfer function based on the interpolated equalized digital coherent optical signal and based on the interpolated delayed digital coherent optical signal.
  • a switch S1 can be used to selectively provide a recovered carrier signal 812, recovered by a carrier recovery unit 821 or the interpolated equalized digital coherent optical signal, to the channel transfer function calculator 805.
  • the feed-forward (FF-) 813 and feed-backward (FB-) 81 1 timing recovery (TR) in coherent optical systems are presented in Fig. 8.
  • the feed-backward timing recovery (FB-TR) 81 1 comprises a phase detector (PD) 809, a digital-analog converter (DAC) 833, a low-pass filter (LPF2) 835 and a voltage controlled oscillator (VCO) 815 as a local oscillator (LO).
  • the feed-forward timing recovery (FF-TR) 813 comprises interpolation means 807a, 807b, the phase detector (PD) 809, a position calculation means 817, a low-pass filter (LPF1 ) 819 and a delay element 831 .
  • the ADC 801 normally delivers complex signals from both polarizations (x' and y'; four data lines).
  • the signal denoted as the digital coherent optical signal 804 is twice oversampled, although it is possible to work with less than 2 samples per symbol using polyphase filters.
  • the channel equalizer 803 compensates for CD, PMD, nonlinear effects, etc.
  • Channel transfer function consisting of linear and nonlinear cascaded functions is estimated in the block "Channel transfer function calculation" 805. This block can use a training sequence to estimate the channel transfer function (switch S2 is in the position 4 and switch S1 in the neutral position).
  • the signal x and y denoted as the equalized digital coherent optical signal 806 is used in the phase detector (PD) 809 that is shared with FF-TR 813 and FB-TR 81 1 .
  • the phase detector 809 outputs the signal (denoted as TEDC signal 810) that contains information about the clock parameters.
  • the right clock signal has to be:
  • the current clock signal is equal to
  • the parameter A is practically irrelevant for clock extraction (only plays role in the design of timing recovery circuits; mostly influencing the reaction time of the recovery blocks.
  • the clock offset ⁇ and phase offset ⁇ are the negative effects that have to be compensated.
  • the FB-TR 81 1 is responsible for clock offset compensation via the feedback loop (small bandwidth loop). Since there is a large delay between the phase detector and ADC blocks 801 the FF-TR 813 is required to follow/compensate the fast sampling phase variations (not related to self-phase jitter).
  • the PD block 809 outputs the signal 810 that is proportional to the residual phase difference between the received data and Rx VCO clocks. This signal 810 is filtered
  • the FF-TR 813 filters the PD output 810 using MR low-pass filter (LPF1 , 819) in the digital domain.
  • the filtered signal contains information about the correct clock position. This is used in a block for position calculation 817. Based on this position and samples after the channel equalizer 803 the interpolator 807b (lnterpolation2; 2 samples in; one sample out) delivers samples at the correct sampling instances.
  • the channel estimator 805 can work in either blind or decision directed mode and the data before the channel equalizer 803 are delayed 831 (shift register) and interpolated 807a (Interpolationl ; 2 samples in; 2 samples out) as the data after channel equalization 803. This way, the data at the correct sampling phases are used for channel estimation 805.
  • Data after carrier recovery 821 also can be used for channel estimation 805 (switch S1 in position 2) to improve channel estimator 805 accuracy.
  • the enhanced PD 809 works with complex (can also work with real signal) modulation formats.
  • the received signal 806 is twice over-sampled (two samples per symbol). Samples within one symbol interval n are denoted by A(n) and B(n). Then, the TEDC signal 810 is calculated by using the equation
  • Using imaginary part in equation (7) results in TEDC having the constant value over one unit interval (Ul; symbol interval).
  • Ul unit interval
  • Such a TEDC cannot be used directly for clock extraction.
  • the conjugate operation in Eq. 7 can be avoided to improve performance in some specific transmission scenarios.
  • the parameter a is used to improve clock performance for different modulation formats and pulse shapes and to additionally adjust the sampling phase.
  • the TEDC signal 810 indicates a phase offset and a frequency offset of the analog coherent optical signal 801 with respect to a local oscillator 815, wherein the local oscillator 815 is controlling the sampling of the analog-to-digital conversion means 801.
  • the interpolation means 807a, 807b comprises first interpolation means 807a configured for interpolating the digital coherent optical signal 804 and second interpolation means 807b configured for interpolating the equalized digital coherent optical signal 806.
  • the first interpolation means 807a provides two output samples for two input samples; and wherein the second interpolation means 807b provides one output sample for two input samples.
  • the feed-forward timing recovery means 813 comprises position calculation means 817 configured for calculating clock positions based on the TEDC signal 810.
  • the interpolation means 807a, 807b is configured to provide samples of the digital coherent optical signal 804 and the equalized digital coherent optical signal 806 at the calculated clock positions.
  • the TEDC signal 810 is filtered by an infinite impulse response, MR, low-pass filter 819 before being provided to the position calculation means 817.
  • the coherent optical receiver 800 comprises carrier recovery means 821 configured to recover a carrier signal 812 of the analog coherent optical signal 802 based on the equalized digital coherent optical signal 806.
  • the channel transfer function calculation means 805 is configured for calculating the channel transfer function 808 based on the carrier signal 812.
  • the digital coherent optical signal 804 comprises two samples per symbol.
  • the phase detection means 809 is configured for providing the TEDC signal 810 based on samples of two contiguous symbol intervals.
  • the coherent optical receiver 800 is configured for receiving optical signals using Nyquist pulses, configured for receiving optical signals of any QAM or PSK modulation format and/or configured for receiving optical signals with signal bandwidth smaller than a minimum defined Nyquist bandwidth.
  • the phase detection means 809 is configured for providing a TEDC signal 810 based on a digital coherent optical signal 804 according to equations (7) and (8).
  • the phase detection means 809 is configured for providing a linear TEDC signal 810, in particular by applying the equations (7) and (8) to the digital coherent optical signal 804 and a shifted version thereof.
  • Fig. 9 shows a diagram illustrating a TED characteristic 900 of a phase detector according to an implementation form. The phase detector can be applied in a coherent optical receiver 800 as described above with respect to Fig. 8. By analyzing the TED
  • phase detector generates the clock at 0.25UI.
  • the VCO will be locked at this phase; rising zero TEDC crossing.
  • the clock should be at 0.5UI which is in the center of the diagram.
  • Fig. 10 shows eye diagrams of real 1000a and imaginary 1000b signals at the input of the phase detector depicted in Fig. 9.
  • the eye diagrams show real and imaginary parts of QPSK signals. Comparing Fig. 9 and 10 one can conclude that the phase detector generates clock at 0.25UI, the VCO will be locked at this phase; rising zero TEDC crossing, but the clock should be at 0.5UI, i.e. in the center of the eye.
  • Fig. 1 1 a shows a block diagram of a phase detector 1 100 with linear TEDC according to an implementation form.
  • the phase detector 1 100 can be applied in a coherent optical receiver 800 as described above with respect to Fig. 8.
  • the PD block is modified with respect to the phase detector shown in Figures 9 and 10 to provide the correct sampling phase.
  • the input signal 1102 is processed by two PDs 1101 , 1103 (eq. 7 and 8).
  • One part of the signal is interpolated at sampling instances shifted by UI/4 1105.
  • the signal W 2 is generated that has sin shape and the W-i signal that has cos shape.
  • the block calculating angle function (atan) values 1107 can be lookup table; LUT) from - ⁇ to + ⁇ and normalizing these values by 2 ⁇
  • the output signal 1110 from this block 1107 takes values between -0.5 and +0.5.
  • This value ⁇ is directly used for interpolation.
  • the T value can be processed by an unwrapping function.
  • Fig. 1 1 b shows a block diagram of a low pass filter 1 150 used in the phase detector 1 100 depicted in Fig. 1 1 a according to an implementation form.
  • the LPF1 1150 is realized in digital domain using MR structure having transfer function: ⁇ ⁇ , /2 ⁇
  • the interpolator for the second phase detector 1103 generating the sin function can be realized as the interpolators shown in Fig. 8. For example, a cubic interpolator using four samples may be used to interpolate the signal.
  • the linear TEDC is presented in Fig. 12. This function is periodical with period equal to one Ul.
  • such a phase detector is applied in a coherent optical receiver 800 as described above with respect to
  • Fig. 13 shows a block diagram of a phase detector 1300 with linear TEDC according to an implementation form.
  • the phase detector 1300 can be applied in a coherent optical receiver 800 as described above with respect to Fig. 8.
  • Two adjacent samples 1302, 1304 are added 1301 and fed to the basic phase detectors 1307, 1309 of the main PD 1300.
  • the basic phase detectors 1307 and 1309 change the output signal powers and the signal W 2 has different maxima than the signal W-i . It results in non-atan function. Using appropriate equations this power change can be accurately calculated.
  • the signal W 2 is multiplied 1303 by a parameter g before entering to the lookup-table (LUT) 1305. This parameter is equal to:
  • Fig. 14 shows a block diagram of a phase detector 1400 with linear TEDC comprising a circuit for changing the sampling phase according to an implementation form.
  • the phase detector 1400 can be applied in a coherent optical receiver 800 as described above with respect to Fig. 8.
  • W m and W n are W-i or gW 2 and W m is not equal to W n .
  • the signals W1 and gW2 are almost identical except of a shift by 90 degree that is wanted. It proves that the parameter g was appropriately calculated. The small difference between these two functions did not degrade the linearity of the TEDC function. As can be seen from Fig. 15c, the TEDC signal 1500c is linear and convenient for the use in the interpolators.
  • the range of the atan function can be extended using an unwrapping function to cover a large jitter that is not limited to one Ul.
  • the phase shift can be defined as:
  • the unwrapping function range depends on the available registers length. To avoid large phase fluctuations especially in the acquisition phase (clock frequency offset acquiring) this function can be limited to several symbols.
  • Fig. 22 shows a block diagram of a transmitter circuit 2200 and a receiver circuit 2250 illustrating Nyquist super-channel clocking according to an implementation form.
  • N transmitters Tx1 , Tx2, TxN are integrated to save power, size and price.
  • the super-channel transmitter circuit 2200 shares one VCO 2201 to all transmitters Tx1 , Tx2, TxN. It is realized as shown in Fig. 22.
  • One VCO 2201 supplies the clock distribution unit (CDU, 2203) that clocks all N transmitters Tx1 , Tx2, TxN.
  • Fig. 23 shows a block diagram illustrating a phase detector circuit 2300 comprising parallel implemented phase detectors according to an implementation form.
  • PD outputs denoted by v are summed up in a summing unit 2301 providing a PD output signal 2302 which is filtered by LPF1 as described above with respect to Fig. 8 to be used for interpolation.
  • the summed up (averaged signal) 2302 is DAC converted, e.g. by using the DAC 833 and used in the FB-TR 811 controlling VCO 815 frequency and phase.
  • the VCO output clocks the ADC circuit 801 as described above with respect to Fig. 8.
  • the phase detector circuit 2300 is applied as phase detector 809 in the coherent optical receiver 800 as described above with respect to Fig. 8.
  • Fig. 24 shows a schematic diagram illustrating a method 2400 for clock recovery in a coherent optical receiver according to an implementation form.
  • the method 2400 comprises sampling 2401 an analog coherent optical signal into a digital coherent optical signal.
  • the method 2400 comprises equalizing 2403 the digital coherent optical signal.
  • the method 2400 comprises calculating 2405 a channel transfer function based on interpolations of the digital coherent optical signal and the equalized digital coherent optical signal and adjusting the channel equalization means based on the calculated channel transfer function.
  • the method 2400 comprises providing 2407 a timing error detection characteristic, TEDC, signal, based on the equalized digital coherent optical signal.
  • the method 2400 comprises adjusting 2409 the sampling of the analog-to- digital converter based on the TEDC signal with respect to a frequency offset
  • the method 2400 comprises adjusting 241 1 the interpolations of the digital coherent optical signal and the equalized digital coherent optical signal with respect to a phase offset compensation criterion.
  • the present disclosure also supports a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the performing and computing steps described herein.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Optical Communication System (AREA)

Abstract

L'invention concerne un récepteur optique cohérent (800) qui comporte : un moyen de conversion analogique-numérique (801) conçu pour échantillonner un signal optique analogique cohérent (802) en signal optique numérique cohérent (804) ; un moyen d'égalisation de canal (803) conçu pour égaliser le signal optique numérique cohérent (804) ; un moyen de calcul de fonction de transfert de canal (801) conçu pour calculer une fonction de transfert de canal (808) en fonction du signal optique numérique cohérent (804) et du signal optique numérique cohérent égalisé (801), interpolé par des moyens d'interpolation (807a, 807b) et conçu pour régler le moyen d'égalisation de canal (803) suivant la fonction de transfert de canal calculée (808) ; un moyen de détection de phase (809) conçu pour fournir un signal TEDC, de caractéristique de détection d'erreur de calendrier (810), en fonction du signal optique numérique cohérent égalisé (806) ; un moyen de récupération temporelle en arrière (811) conçu pour le réglage de l'échantillonnage du moyen de conversion analogique-numérique (801), en fonction du signal TEDC (810) par rapport à un critère de compensation de décalage de fréquence ; un moyen de récupération temporelle en avant (813) conçu pour le réglage du moyen d'interpolation (807a, 807b), par rapport à un critère de compensation de décalage de phase.
PCT/EP2013/061533 2013-06-05 2013-06-05 Récepteur optique cohérent WO2014194940A1 (fr)

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CN107710669A (zh) * 2015-09-21 2018-02-16 华为技术有限公司 一种数据传输方法、收发设备及系统
CN107710669B (zh) * 2015-09-21 2020-06-02 华为技术有限公司 一种数据传输方法、收发设备及系统
US10097276B2 (en) 2016-03-14 2018-10-09 Beijing University Of Posts And Telecommunications Method and device for sending and receiving an optical signal
WO2017174103A1 (fr) * 2016-04-04 2017-10-12 Huawei Technologies Co., Ltd. Appareil et procédé de récupération du rythme dans des récepteurs de détection directe
WO2018035174A1 (fr) * 2016-08-15 2018-02-22 Greensand Networks Inc. Récepteur cohérent analogique de liaisons de communication à fibre optique
US10771162B2 (en) 2016-08-15 2020-09-08 Greensand Networks Inc. Coherent optical receiver with improved noise performance for optical fiber communication links
US9998274B2 (en) 2016-09-24 2018-06-12 Huawei Technologies Co., Ltd. Method and apparatus for robust clock recovery in coherent optical systems
WO2020242518A1 (fr) * 2019-05-24 2020-12-03 Google Llc Récepteur cohérent basse puissance pour communication optique à courte portée
US11476947B2 (en) 2019-05-24 2022-10-18 Google Llc Low power coherent receiver for short-reach optical communication
US10767974B1 (en) 2019-12-10 2020-09-08 Rockwell Collins, Inc. Wide range optical wavelength fast detection for narrowband signal

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