Combined equalization method based on generalized noise whitening filter

Abstract

The invention relates to a joint equalization method based on a generalized noise whitening filter, which comprises the following steps of: after an MIMO (Multiple Input Multiple Output) linear equalizer is used for processing sideband residual crosstalk and inter-symbol crosstalk at a receiving end, whitening and equalizing amplified low-frequency and high-frequency noise through a cascaded three-tap generalized noise whitening filter; and a maximum likelihood sequence estimation (MLSE) algorithm is used to eliminate the known inter-symbol crosstalk introduced by the whitening filter so as to realize joint equalization. According to the invention, the influence of sideband residual crosstalk and intersymbol crosstalk on the double-single sideband signal can be suppressed at the same time, the problem of amplifying low-frequency and high-frequency noise at the same time in linear equalization can be solved, the transmission performance of the system is improved, meanwhile, a protection frequency band can be removed, and the spectrum utilization rate of the system is improved.

Description

Technical Field

[0001] The present invention relates to a technology in the field of optical communication, specifically a joint equalization method based on a generalized noise whitening filter for a Dual-SSB (Dual-Single Sideband) signal transmission system. Background Art

[0002] Existing methods for suppressing the influence of sideband residual crosstalk and inter-symbol interference on Dual-SSB signals include adding a guard band between the signal and the carrier and using a multiple-input multiple-output (MIMO) linear equalizer. However, the method of adding a guard band will reduce the spectral efficiency of the system and at the same time make the signal suffer from more severe bandwidth limitations; while the MIMO linear equalization method has the disadvantage of amplifying in-band noise, so the equalization performance is limited. Summary of the Invention

[0003] In view of the deficiencies of the existing adaptive equalization technology using finite steep filtering, such as residual interference and the low-frequency and high-frequency noises simultaneously enhanced by MIMO linear equalization, the present invention proposes a joint equalization method based on a generalized noise whitening filter, which can simultaneously suppress the influence of sideband residual crosstalk and inter-symbol interference on Dual-SSB signals, and can solve the problem of the low-frequency and high-frequency noises simultaneously amplified by linear equalization, improve the transmission performance of the system, and at the same time can remove the guard band to improve the spectral utilization rate of the system.

[0004] The present invention is realized through the following technical solutions:

[0005] The present invention relates to a joint equalization method based on a generalized noise whitening filter. After using a MIMO linear equalizer at the receiving end to process sideband residual crosstalk and inter-symbol interference, the equalized amplified low-frequency and high-frequency noises are whitened by cascading a three-tap generalized noise whitening filter, and then the known inter-symbol interference introduced by the whitening filter is eliminated by using the maximum likelihood sequence estimation (MLSE) algorithm to achieve joint equalization.

[0006] The maximum likelihood sequence estimation selects a maximum likelihood path as the surviving path from all possible paths, specifically including:

[0007] Step 1: Use a Viterbi decoder to compare all states at each stage in the convolutional code trellis diagram: the distances of the two paths entering this state, that is, the Hamming distance between the coding sequence represented by this path and the received sequence.

[0008] Step 2: For the two paths entering the same state, retain the path with the smaller distance, where the path with the smallest distance is the surviving path, and the corresponding input information bits are the final result.

[0009] The present invention relates to a system for implementing the above method, including: a MIMO linear equalizer, a three-tap generalized noise whitening filter bank, and a maximum likelihood sequence estimation unit, where: the MIMO linear equalizer jointly processes the information of the received left and right sidebands, realizes adaptive linear equalization based on the training sequence, and obtains a data sequence after compensating the channel response; the three-tap generalized noise whitening filter optimizes the tap coefficients based on the equalized data sequence according to the lowest bit error rate, suppresses the low-frequency and high-frequency noises enhanced by the equalization, and obtains a data sequence after noise whitening; the maximum likelihood sequence estimation unit is used to compensate for the known inter-symbol interference introduced by the noise whitening filter, and obtains the finally recovered data sequence.

[0010] The MIMO linear equalizer described above includes: four parallel finite impulse response (FIR) filters, where: the first and third FIR filters receive the left sideband signal, the second and fourth FIR filters receive the right sideband signal, the first and second FIR filters are superimposed as the left sideband output, and the second and third FIR filters are superimposed as the right sideband output.

[0011] The three-tap generalized noise whitening filter bank described above includes: two three-tap FIR filters, each of which receives the left and right sideband outputs from the MIMO linear equalizer, and the time-domain impulse response is expressed as h(t) = 1 + αδ(t - T s ) + βδ(t - 2T s ), where: T s is the reciprocal of the baud rate of the signal, α and β are tap coefficients, and α, β ∈ [-1, 0). Technical effects

[0012] The present invention whitens the equalization-enhanced noise through a three-tap generalized noise whitening filter, and at the same time realizes the suppression of the low-frequency and high-frequency noises enhanced by the linear equalization, breaking through the performance limitations of the traditional two-tap noise whitening filter. Description of the drawings

[0013] Figure 1 It is a schematic diagram of the system of the present invention;

[0014] Figure 2 It is a schematic diagram of the MIMO linear equalizer of the present invention;

[0015] Figure 3 It is a schematic diagram of the system of the embodiment;

[0016] Figure 4 It is a comparison diagram of the bit error performance of the present method and the conventional method under different received signal powers in Embodiment 1. Specific implementation manners

[0017] Such as Figure 1As shown in the figure, a joint equalization system based on a generalized noise whitening filter according to this embodiment includes: a MIMO linear equalizer, a three-tap generalized noise whitening filter bank, and a maximum likelihood sequence estimation unit.

[0018] As Figure 2 shown, the MIMO linear equalizer includes: four FIR filters with taps ωl, hl, ωr, and hr respectively; the input left-sideband signal passes through FIR1 and FIR3, and the input right-sideband signal passes through FIR2 and FIR4; the outputs of FIR1 and FIR2 are the left-sideband signals after MIMO equalization, and the outputs of FIR3 and FIR4 are the right-sideband signals after MIMO equalization; subsequently, the left and right sideband signals are respectively input into a three-tap FIR filter with a time-domain impulse response of h(t) = 1 + αδ(t - T s ) + βδ(t - 2T s ) to suppress low-frequency and high-frequency noises, and then the maximum likelihood sequence estimation is used to recover the signal.

[0019] As Figure 3 shown, a joint equalization system based on a generalized noise whitening filter according to this embodiment includes: an arbitrary waveform generator, an electrical amplifier, a polarization controller, a laser, a dual-drive Mach-Zehnder modulator, an adjustable optical attenuator, a single-mode fiber, an erbium-doped fiber amplifier, an optical bandpass filter, a photodetector, and a digital-to-analog converter, where: a 4th-order pulse amplitude modulation (PAM) signal with a 44 GHz baud rate generated by the arbitrary waveform generator is amplified by the electrical amplifier and then drives the dual-drive Mach-Zehnder modulator; the laser-generated laser is modulated by the dual-drive Mach-Zehnder modulator to obtain a double-sideband signal, which is amplified by the erbium-doped fiber amplifier and then transmitted into an 80-kilometer single-mode fiber; the received signal is divided into two paths for reception after passing through the adjustable optical attenuator and the erbium-doped fiber amplifier; the two paths of signals respectively pass through the optical bandpass filter and then enter the photodetector for optoelectronic conversion; finally, the digital-to-analog converter samples the electrical signal and outputs it to the joint equalization system to obtain a recovered signal.

[0020] The arbitrary waveform generator converts the digital signal into an analog electrical signal.

[0021] The bias point of the dual-drive Mach-Zehnder modulator is located at the quadrature point.

[0022] The photodetector converts the optical signal into an electrical signal.

[0023] The digital-to-analog converter converts the analog electrical signal into a digital signal.

[0024] After specific experiments, a carrier wave of 1550.4 nm is output by a laser, and then it enters a dual-drive Mach-Zehnder modulator for electro-optic modulation. An electrical signal with a baud rate of 44 G is generated by an arbitrary waveform generator with a sampling rate of 65 GSa / s and a 3-dB bandwidth of 25 GHz. After 80-kilometer transmission, at the receiving end, a filter with a 3-dB bandwidth of approximately 70 GHz is used to filter out the left and right sidebands respectively, and the steepness of the upper and lower edges of its passband is 320 dB / nm and 200 dB / nm respectively. The filtered signal is input into an analog-to-digital converter with a sampling rate of 80 GSa / s and a 3-dB bandwidth of 36 GHz through a photodetector with a bandwidth of 40 GHz for offline processing. In the optical back-to-back case without transmitting optical fiber, the bit error rates corresponding to the traditional linear equalization method and the MIMO linear equalization method are 5.8×10 -2 and 1.8×10 -2 respectively. In contrast, the bit error rate of the present invention is 7.1×10 -3 ; in the scenario of 80-kilometer optical fiber transmission, the bit error rates corresponding to the traditional linear equalization method and the MIMO linear equalization method are 7.8×10 -2 and 3.2×10 -2 respectively. In contrast, the bit error rate of the method proposed by the present invention is 2.3×10 -2 .

[0025] In summary, compared with the prior art, the present method can suppress the influence of sideband residual crosstalk and inter-symbol interference on the dual-SSB signal, and can simultaneously solve the problems of enhanced low-frequency and high-frequency noise introduced by linear equalization. Therefore, it can avoid inserting a guard band between the signal and the carrier wave and improve the spectrum utilization rate.

[0026] The above specific implementation can be locally adjusted in different ways by those skilled in the art without departing from the principles and purposes of the present invention. The protection scope of the present invention is subject to the claims and is not limited by the above specific implementation, and all implementation solutions within its scope are subject to the constraints of the present invention.

Claims

1. A joint equalization method based on a generalized noise whitening filter, characterized in that After processing the sideband residual crosstalk and inter-symbol interference using a MIMO linear equalizer at the receiving end, the low-frequency and high-frequency noises amplified by the equalization are whitened by cascading a three-tap generalized noise whitening filter, and then the known inter-symbol interference introduced by the whitening filter is eliminated using the maximum likelihood sequence estimation (MLSE) algorithm to achieve joint equalization.

2. The joint equalization method based on a generalized noise whitening filter according to claim 1, characterized in that, The maximum likelihood sequence estimation selects a maximum likelihood path from all possible paths as the surviving path, specifically including: Step 1: Use a Viterbi decoder to compare all states at each stage in the convolutional code trellis diagram: the distances of the two paths entering this state, that is, the Hamming distance between the coding sequence represented by this path and the received sequence. Step 2: For the two paths entering the same state, retain the path with the smaller distance, where the path with the smallest distance is the surviving path, and the corresponding input information bits are the final result.

3. A joint equalization system based on a generalized noise whitening filter for implementing the method according to claim 1 or 2, characterized in that, including: A MIMO linear equalizer, a three-tap generalized noise whitening filter bank, and a maximum likelihood sequence estimation unit, where: the MIMO linear equalizer jointly processes the information of the left and right sidebands received, and realizes adaptive linear equalization based on the training sequence to obtain a data sequence after compensating the channel response; the three-tap generalized noise whitening filter optimizes the tap coefficients based on the equalized data sequence according to the lowest bit error rate to suppress the low-frequency and high-frequency noises enhanced by the equalization, and obtains a data sequence after noise whitening; the maximum likelihood sequence estimation unit is used to compensate for the known inter-symbol interference introduced by the noise whitening filter to obtain the finally recovered data sequence.

4. The joint equalization system based on a generalized noise whitening filter according to claim 3, wherein, The MIMO linear equalizer described above includes: four parallel finite impulse response (FIR) filters, where: the first and third FIR filters receive the left sideband signal, the second and fourth FIR filters receive the right sideband signal, the first and second FIR filters are superimposed as the left sideband output, and the second and third FIR filters are superimposed as the right sideband output.

5. The joint equalization system based on a generalized noise whitening filter according to claim 3, characterized in that, The described three-tap generalized noise whitening filter bank includes: two three-tap FIR filters, each of which receives the left and right sideband outputs from the MIMO linear equalizer, and the time-domain impulse response is expressed as h(t) = 1 + αδ(t - T s ) + βδ(t - 2T s ), where: T s is the reciprocal of the baud rate of the signal, and α and β are tap coefficients, with α, β ∈ [-1, 0).

6. A joint equalization system based on a generalized noise whitening filter, characterized in that, including: An arbitrary waveform generator, an electrical amplifier, a polarization controller, a laser, a dual-drive Mach-Zehnder modulator, an adjustable optical attenuator, a single-mode fiber, an erbium-doped fiber amplifier, an optical bandpass filter, a photodetector, and a digital-to-analog converter, where: the 44 GHz baud rate 4-order pulse amplitude modulation (PAM) signal generated by the arbitrary waveform generator is amplified by the electrical amplifier and then drives the dual-drive Mach-Zehnder modulator; the laser-generated laser is modulated by the dual-drive Mach-Zehnder modulator to obtain a double-sideband signal, which is amplified by the erbium-doped fiber amplifier and then transmitted into an 80-kilometer single-mode fiber; the received signal is divided into two paths for reception after passing through the adjustable optical attenuator and the erbium-doped fiber amplifier; the two paths of signals respectively enter the photodetector for photoelectric conversion after passing through the optical bandpass filter; finally, the digital-to-analog converter samples the electrical signal and outputs it to the joint equalization system described in any one of claims 3-5 to obtain the recovered signal.

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