CN113726434A - Low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC - Google Patents

Low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC Download PDF

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CN113726434A
CN113726434A CN202110977738.3A CN202110977738A CN113726434A CN 113726434 A CN113726434 A CN 113726434A CN 202110977738 A CN202110977738 A CN 202110977738A CN 113726434 A CN113726434 A CN 113726434A
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CN113726434B (en
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李凡
殷明珠
邹冬冬
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Sun Yat Sen University
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    • 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
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
    • H04B10/25133—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion including a lumped electrical or optical dispersion compensator
    • 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
    • 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
    • H04B10/516—Details of coding or modulation
    • 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
    • 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
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    • H04B10/66—Non-coherent receivers, e.g. using direct detection

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Abstract

The invention relates to a low-cost IM/DD system long-distance transmission method based on a low-quantization bit width DAC. And a transmitting end DSP: mapping a pseudorandom bit sequence with the length of l into a PAM-4 symbol, performing up-sampling on the mapped symbol, and realizing a pulse-shaped signal A (t) through a raised cosine wave with a roll-off factor of d; the shaped signal realizes dispersion and compensation through a dispersion compensation scheme of an improved GS algorithm; the pre-compensated signals pass through an FIR filter to realize digital pre-equalization; noise shaping is carried out on the pre-equalized signal, quantization is carried out simultaneously, quantization noise is shaped, and the obtained quantized signal is input into a DAC; DSP at the receiving end: and carrying out error code calculation, PAM-4 inverse mapping, linear equalization, synchronization, clock recovery, matched filtering and down-sampling treatment on the signals obtained by PD detection. The invention eliminates the dispersion problem in the optical fiber by using the dispersion compensation scheme of the improved GS algorithm, the convergence speed of the system is higher, and the required calculation complexity is lower; the cost is effectively reduced.

Description

Low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC
Technical Field
The invention belongs to the technical field of optical communication systems and high-speed optical signal processing, and particularly relates to a low-cost IM/DD system long-distance transmission method based on a low-quantization bit width DAC.
Background
With the continuous emergence of new applications such as social media, cloud computing, ultra-high definition television and the like, the rapid increase of optical communication traffic of a data center and a metropolitan area is promoted. The widespread use of data centers has resulted in data centers being interconnected over distances in excess of 40 kilometers and even up to 80 kilometers. For a transmission network below 40 km, 8 × 50Gb/s and 4 × 100Gb/s wavelength division multiplexing schemes using Pulse Amplitude Modulation (PAM) signals in the O-band to transmit 40 km of Single Mode Fiber (SMF) are effective solutions. However, the high attenuation of the O-band and the immature amplifier technology result in a transmission of over 40 km or even 80 km in the C-band being preferred. Therefore, the following schemes are continuously proposed for a system transmitting a communication distance of 40 km to 80 km in the C band: (1) IQ modulator & coherent receiver: extensive long-distance research is mostly based on the use of an IQ modulator or a coherent receiver, which mainly utilizes the characteristic that the IQ modulator or the coherent receiver can directly perform phase compensation, but the use of such devices causes the device cost of the system to be greatly increased, so that the device is not suitable for a low-cost communication system; (2) digital nonlinear equalization techniques: complex digital signal processing algorithms can be used for cancellation of inter-symbol interference (ISI) of signals after long distance transmission in intensity modulated direct detection systems. Such as maximum likelihood equalizers (MLSE), Artificial Neural Networks (ANN), and the like. But the complex calculation is not suitable for the IM/DD system with low cost and low time delay; (3) iterative dispersion compensation of a transmitting end based on a GS algorithm: the GS algorithm realizes the dispersion pre-compensation of a transmitting end in a mode of continuously iterating through Fourier transform (FFT) and inverse Fourier transform (IFFT) or a dispersion transmission function and a dispersion inverse transmission function. The iterative dispersion compensation of the transmitting end based on the GS algorithm can effectively eliminate the interference of optical fiber dispersion, but the PAPR of the signal after iteration is large, and when the system cost is reduced by further reducing the DAC digit, the quantization noise is too large, so that the quantization signal-to-noise ratio is reduced.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides a low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC, and the cost of system devices is effectively reduced.
In order to solve the technical problems, the invention adopts the technical scheme that: a low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC comprises the following steps:
s1, in a DSP (digital signal processor) at a transmitting end, firstly mapping a pseudorandom bit sequence with the length of l into a PAM-4 symbol, performing up-sampling on the mapped symbol, and realizing a pulse-shaped signal A (t) through a rising cosine wave with a roll-off factor of d;
s2, the shaped signal realizes dispersion and compensation through a dispersion compensation scheme of an improved GS algorithm; the dispersion compensation scheme of the improved GS algorithm comprises the following steps:
s21, initializing phase
Figure BDA0003228015180000021
And simulated receiver amplitude E:
Figure BDA0003228015180000022
E(t)＝A(t)；
s22, starting GS iteration: forI ═ 1,2, …, IteNum;
s23, reverse dispersion transmission:
Figure BDA0003228015180000023
s24, limiting and carrying out dispersion transmission by a sending end:
Figure BDA0003228015180000024
s25, calculating an iteration error: errt (t) ═ Eodd(t)-Aodd(t)；
S26, simulating receiving end limitation: eodd(t)＝Aodd(t)-errt(t)；
S27, judging whether to finish circulation according to conditions;
s28, finishing iteration, and outputting a signal B (t) after iterative dispersion compensation;
s3, the precompensated signals are subjected to digital precompensation through an FIR filter;
s4, performing noise shaping and quantization on the pre-equalized signal, performing quantization on the pre-equalized signal, shaping quantization noise, and inputting the obtained quantization signal into a DAC;
and S5, in the DSP of the receiving end, performing down-sampling processing, matched filtering, clock recovery, synchronization, linear equalization, PAM-4 inverse mapping and error code calculation on the signal obtained by PD detection.
Compared with the traditional GS algorithm dispersion compensation scheme, the improved GS dispersion compensation scheme reserves the amplitude of up-sampling of the symbol in the iterative process, thereby better utilizing the optimizable space of the signal and improving the iterative convergence speed and the final dispersion compensation effect. Compared with the traditional delta-sigma modulation (DSM) and Digital Resolution Enhancer (DRE), the Noise Shaping (NS) technology does not need high oversampling rate, and the algorithm implementation has low calculation complexity, thereby being more beneficial to the implementation of the low-cost IM/DD system.
Further, the length l of the pseudo-random bit sequence in step S1 is 211。
Further, the roll-off factor d is 1.
Further, in the step S4, the pre-equalized signal is quantized 3/4/5/6-bit.
Further, the step of noise shaping in step S4 includes:
s41, determining quantization noise at the previous moment;
s42, carrying out low-frequency filtering on the quantization noise through a constructed filter;
and S43, adding the noise after filtering into the signal to enable the noise to be close to the standard DAC level.
Further, in the step S42, the number of taps of the constructed filter is calculated by:
s421. output Y (e) of noise shaping structurejω) Expressed as:
Y(ejω)＝X(ejω)+(1+H(ejω))N(ejω)
in the formula, H (e)jω) And X (e)jω) Respectively representing the channel response of the feedback filter and the signal before quantization; quantization noise N (e)jω) Is the difference between the data before and after entering the DAC;
s422. to minimize the quantization noise in the signal band, the quantization noise after the noise shaping technique structure is output should be smaller than the quantization noise before this operation, so we can:
∫|(1+H(ejω))N(ejω)|2dω<∫|N(ejω)|2dω
in the formula, omega belongs to (0-omega)s),ωsIs the angular frequency of the signal;
s423, the problem reflected by the formula in the step S422 is converted into the following optimized problem:
Figure BDA0003228015180000031
s424, assume that the feedback filter is an FIR filter, and the expression is:
H(ejω)＝h1e-jω+h2e-j2ω+…hne-jnω
the discrete form of the formula in step S423 is:
Figure BDA0003228015180000032
the definition vector F is:
Figure BDA0003228015180000033
suppose that:
Figure BDA0003228015180000041
the discrete form of the formula in step S423 is written as:
Figure BDA0003228015180000042
s425, the number of taps of the feedback filter is as follows:
h＝E-1×1
in the formula, E-1Representing the pseudo-inverse of matrix E.
The invention also provides a low-cost IM/DD system long-distance transmission system based on the low-quantization bit width DAC, which comprises the following components:
and a transmitting end DSP: a signal A (t) used for mapping a pseudorandom bit sequence with the length of l into a PAM-4 symbol, up-sampling the mapped symbol, and realizing pulse forming through a raised cosine wave with a roll-off factor of d; the shaped signal realizes dispersion and compensation through a dispersion compensation scheme of an improved GS algorithm; the pre-compensated signals pass through an FIR filter to realize digital pre-equalization; noise shaping is carried out on the pre-equalized signal, quantization is carried out simultaneously, quantization noise is shaped, and the obtained quantized signal is input into a DAC;
DSP at the receiving end: the method is used for carrying out error code calculation, PAM-4 inverse mapping, linear equalization, synchronization, clock recovery, matched filtering and down-sampling processing on the signals obtained by PD detection.
Further, the improved GS algorithm comprises:
an initialization unit: for initialising the phase
Figure BDA0003228015180000043
And simulated receiver amplitude E:
Figure BDA0003228015180000044
GS iteration unit: for starting GS iteration: forI ═ 1,2, …, IteNum;
reverse dispersion transmission unit: for reverse dispersion transmission:
Figure BDA0003228015180000045
a transmission limiting unit: and (3) limiting and carrying out dispersion transmission by a sending end:
Figure BDA0003228015180000046
Figure BDA0003228015180000047
an error unit: for calculating the iteration error: errt (t) ═ Eodd(t)-Aodd(t)；
A reception limiting unit: simulation of receiving end limitation: eodd(t)＝Aodd(t)-errt(t)；
A judging unit: used for judging whether to finish the circulation according to the condition;
an output unit: for outputting the signal b (t) after the iterative compensation of chromatic dispersion after the iteration is finished.
Further, at the transmitting end, the generated off-line data is loaded to a DAC for digital-to-analog conversion, then an electric amplifier is used for amplifying the signal, and then the MZM modulator is used for realizing photoelectric conversion; the optical carrier of the modulator comes from a tunable external cavity laser.
Furthermore, at a receiving end, an erbium-doped fiber amplifier is used for realizing small signal amplification, and a band-pass filter is used for realizing out-of-band noise filtering; before the receiving end directly detects the signal, an optical attenuator is used for adjusting the receiving power of the signal; the optical signal after attenuation is detected and received by the PD; and the analog electric signal output by the PD is collected by an oscilloscope and is subjected to offline DSP.
Compared with the prior art, the beneficial effects are: the invention provides a low-cost IM/DD system long-distance transmission method based on a low-quantization bit width DAC, which solves the problem of chromatic dispersion in an optical fiber by using a chromatic dispersion compensation scheme for improving a GS algorithm, and has higher convergence speed and lower required calculation complexity compared with the traditional noise shaping scheme. In addition, aiming at the problem that the PAPR of signals after algorithm iteration is large, the invention provides a low-quantization bit DAC combined with a low-computation-complexity noise shaping technology to further reduce the cost of system devices.
Drawings
FIG. 1 is a schematic structural diagram of an apparatus for carrying out the method of the present invention.
Fig. 2 is the IM/DD transmitting end iterative dispersion compensation principle of the present invention based on the improved GS algorithm.
Fig. 3 is a schematic diagram of the structure of the noise shaping technique employed by the present invention.
Fig. 4 is a schematic diagram of the working principle of the noise shaping technique adopted by the present invention in the time domain.
Fig. 5 is a schematic diagram of the working principle of the noise shaping technique adopted by the present invention in the frequency domain.
Fig. 6 is a graphical illustration of the SNR enhancement of the present invention versus the number of feedback taps.
FIG. 7 is a graph of bit error rate versus PAPR for 28Gbaud PAM-4 signals generated by different bit DACs in an embodiment of the present invention.
FIG. 8 is a graph of bit error rate versus number of quantization bits for a 28Gbaud PAM-4 signal with and without noise shaping techniques in an embodiment of the present invention.
Fig. 9 to 11 are graphs showing the relationship between the bit error rate and the received optical power of the 28Gbaud PAM-4 signal under different equalization schemes in the embodiments of the present invention, and 5, 10, and 20 GS iterations are respectively used.
Detailed Description
The drawings are for illustration purposes only and are not to be construed as limiting the invention; for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted. The positional relationships depicted in the drawings are for illustrative purposes only and are not to be construed as limiting the invention.
As shown in fig. 1, a low-cost IM/DD system long-distance transmission method based on a low-quantization bit width DAC includes the following steps:
s1, in the DSP of the transmitting end, firstly, the length is 211The pseudo-random bit sequence is mapped into a PAM-4 symbol, the mapped symbol is subjected to up-sampling, and a signal A (t) of pulse forming is realized through a raised cosine wave with a roll-off factor of 1;
s2, as shown in FIG. 2, the shaped signal realizes dispersion and compensation through a dispersion compensation scheme of an improved GS algorithm; the dispersion compensation scheme of the improved GS algorithm comprises the following steps:
s21, initializing phase
Figure BDA0003228015180000061
And simulated receiver amplitude E:
Figure BDA0003228015180000062
E(t)＝A(t)；
s22, starting GS iteration: forI ═ 1,2, …, IteNum;
s23, reverse dispersion transmission:
Figure BDA0003228015180000063
s24, limiting and carrying out dispersion transmission by a sending end:
Figure BDA0003228015180000064
s25, calculating an iteration error: errt (t) ═ Eodd(t)-Aodd(t)；
S26, simulating receiving end limitation: eodd(t)＝Aodd(t)-errt(t)；
S27, judging whether to finish circulation according to conditions;
s28, finishing iteration, and outputting a signal B (t) after iterative dispersion compensation;
s3, the precompensated signals are subjected to digital precompensation through an FIR filter;
s4, performing noise shaping on the pre-equalized signal, simultaneously performing 3/4/5/6-bit quantization, shaping quantization noise, and inputting the obtained quantized signal into a DAC;
and S5, in the DSP of the receiving end, performing down-sampling processing, matched filtering, clock recovery, synchronization, linear equalization, PAM-4 inverse mapping and error code calculation on the signal obtained by PD detection.
Wherein the step of noise shaping in step S4 includes:
s41, determining quantization noise at the previous moment;
s42, carrying out low-frequency filtering on the quantization noise through a constructed filter;
and S43, adding the noise after filtering into the signal to enable the noise to be close to the standard DAC level.
As shown, FIG. 3 illustrates the structure of NS technology, where Q represents a simulated DAC, primarily for quantization noise acquisition, where quantization noise N (e)jω) Is the difference between the data before and after entering the DAC. From FIG. 3 we can obtain the NS structure output Y (e)jω) Can be expressed as:
Y(ejω)＝X(ejω)+(1+H(ejω))N(ejω) (1)
in the formula, H (e)jω) And X (e)jω) Respectively representing the channel response of the feedback filter and the signal before quantization; quantization noise N (e)jω) Is the difference between the data before and after entering the DAC;
in order to minimize the quantization noise in the signal band, the quantization noise after the noise shaping technique structure output should be smaller than the quantization noise before this operation, so it can be:
∫|(1+H(ejω))N(ejω)|2dω<∫|N(ejω)|2dω (3)
in the formula, omega belongs to (0-omega)s),ωsIs the angular frequency of the signal;
the problem reflected by equation (2) translates to optimizing the following problem:
Figure BDA0003228015180000071
assume that the feedback filter is a FIR filter and the expression is:
H(ejω)＝h1e-jω+h2e-j2ω+…hne-jnω (5)
the discrete form of equation (3) is:
Figure BDA0003228015180000072
the definition vector F is:
Figure BDA0003228015180000073
suppose that:
Figure BDA0003228015180000074
the discrete form of equation (5) can be written as:
Figure BDA0003228015180000075
the number of taps of the feedback filter is:
h＝E-1×1
in the formula, E-1Representing the pseudo-inverse of matrix E.
The operating principle of the NS technique is to reduce the in-band quantization noise by adding noise to the unused frequency band of the signal, bringing the signal close to the DAC output level. The out-of-band noise is obtained from the output of the feedback FIR filter, where the input is the quantization noise at the last instant as shown in the structure in fig. 3. Fig. 4 shows a time domain signal diagram of the signal after addition and non-addition of out-of-band noise, respectively, of the time domain signal generated using a 3-bit DAC. It has been found that proper addition of out-of-band noise can bring the signal closer to the standard DAC output level and effectively reduce the in-band quantization noise. The working principle of NS (noise shaping) techniques can also be described in the frequency domain. Under the condition that the signal is generated by using the low-quantization DAC and combined with the NS technology, the quantization noise is mostly distributed out of the signal band, and only a small amount of quantization noise remains in the signal band, as shown in FIG. 5. Fig. 5 shows electrical spectrum diagrams of an input signal, a non-NS technique quantized output, and an NS technique quantized output, respectively.
The principle of the NS technique is to bring the signal close to the DAC standard output level by adding noise in the unused frequency band of the signal, where the high frequency quantization noise is obtained by low frequency filtering the quantization noise at the previous moment through a feedback FIR filter, as shown in fig. 3. It is therefore desirable to find the optimal number of taps for the FIR filter to balance the in-band quantization noise removal capability and computational complexity. In order to obtain the optimal number of taps for the FIR filter, the simulation calculated the curve relationship between the quantized signal-to-noise ratio (SQNR) enhancement of the transmitted signal at 3/4/5/6-bit quantization bits and the number of taps of the filter, as shown in fig. 6. As shown in fig. 6, the degree of increase of SQNR increases with the number of taps when the number of taps of the feedback filter is small, and there is no significant performance improvement when the number of taps reaches 5. Thus, for the experimental signal, the NS technique uses a feedback filter with a tap length of 5. The implementation of noise shaping depends mainly on the feedback filter and the quantization operation, and the multiplication and addition complexity of the filter is L × N and (L-1) × N, where L is the tap length of the feedback filter and N is the transmission signal length. The quantization process requires M × N comparisons and (M +1) × N subtractions, where M denotes the number of quantization levels of the DAC.
Experimental apparatus:
as shown in FIG. 1, the system adopted by the present invention is an IM/DD system, first, at the transmitting end, offline data generated by MATLAB is loaded to DAC with 80-GSa/s sampling rate and 3-dB bandwidth of 16.7GHz for digital-to-analog conversion, and then the signal is amplified by an Electrical Amplifier (EA) with a gain of 23 dB. And then the MZM modulator is used for realizing photoelectric conversion. The optical carrier of the modulator comes from a tunable external cavity laser, the wavelength of the optical carrier used in the experiment is 1552.524-nm, and the output power is 16-dBm. The optical power output by the modulator is about 10dBm, and the optical power of the optical fiber is adjusted to be about 6dBm through the attenuator because the larger optical power of the optical fiber can cause larger nonlinearity. After the signal led into the optical fiber is transmitted through the 80-km single-mode optical fiber, the small signal amplification is realized at the receiving end by utilizing an erbium-doped optical fiber amplifier, and the out-of-band noise filtering is realized by utilizing a band-pass filter. An optical attenuator is used to adjust the received power of the signal before the signal is directly detected at the receiving end. The optical signal after attenuation is detected and received by a PD (PIN-TIA) (Finisar MPRV 1331A). The analog electric signal output by the PD is collected by an oscilloscope with the cutoff bandwidth of 36-GHz and the sampling rate of 80-GSa/s and is subjected to off-line DSP.
And (4) analyzing results:
the PAPR of the signal is high after the signal is subjected to iterative dispersion compensation, and if a low resolution DA is used to generate the signal, quantization noise can seriously degrade the signal performance. In terms of solving the problem of the PAPR, clipping is a commonly used technology, so a curve relation between the 28Gbaud PAM-4 signal bit error rate and the clipping PAPR is tested experimentally to find the best signal PAPR when various quantization bit numbers of DACs are used for signal generation, and the experimental result is shown in FIG. 7. It can be observed that the PAPR after optimization for the 28Gbaud PAM-4 signal produced by the 3/4/5 bit resolution DAC is 9/11/12-dB. In the following experiment, the PAPR of a signal will be set according to the PAPR obtained from the measurement experiment in fig. 7. FIG. 8 shows the BER performance of a 28Gbaud PAM-4 signal versus the physical number of bits (PNoB) of the DAC. Noise shaping techniques can bring significant improvements when the number of quantization bits is 3-5 bits. The performance of the 28Gbaud PAM-4 signal produced by the 4/5/6 bit resolution DAC and the NS technique may approach that produced by an 8 bit resolution DAC. Therefore, the 28Gbaud PAM-4 signal generated by the 4-bit resolution DAC combined with the NS technology can well meet the requirement of a low-cost IM/DD system.
Figures 9 to 11 show the BER performance of a signal after transmission through a single mode fibre for up to 80 km, under conditions where the signal is generated by a 4-bit DAC and modulated in a C-band optical signal. The experimental scheme employs the GS algorithm for 5, 10 and 20 iterations in combination with a linear equalizer, and also explores the performance without post-equalization, as shown in fig. 9-11. It can be observed that the 28Gbaud PAM-4 signal using the GS algorithm, over 5 iterations, combined with the 11 tap linear equalizer, can reach the HD-FEC threshold with an ROP of-8.5 dBm after 80 km single mode fiber transmission. Furthermore, using the GS algorithm over 5 iterations in combination with an 11 tap FFE, the error performance of the signal produced by the 4-bit DAC in combination with noise shaping can approach that of the signal produced by the 8-bit DAC. For the case where post-equalization is not performed at the receiver, the error rate of the 28Gbaud PAM-4 signal after transmission in the C-band beyond 80 kilometers of SMF may reach the HD-FEC threshold at ROP below-2 dBm when the signal is generated by a 4-bit resolution DAC in combination with a 5-tap noise technique.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.
It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

Claims (10)

1. A low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC is characterized by comprising the following steps:
s1, in a DSP (digital signal processor) at a transmitting end, firstly mapping a pseudorandom bit sequence with the length of l into a PAM-4 symbol, performing up-sampling on the mapped symbol, and realizing a pulse-shaped signal A (t) through a rising cosine wave with a roll-off factor of d;
s2, the shaped signal realizes dispersion and compensation through a dispersion compensation scheme of an improved GS algorithm; the dispersion compensation scheme of the improved GS algorithm comprises the following steps:
s21, initializing phase
Figure FDA0003228015170000011
And simulated receiver amplitude
Figure FDA0003228015170000012
S22, starting GS iteration: forI ═ 1,2, …, IteNum;
s23, reverse dispersion transmission:
Figure FDA0003228015170000013
s24, limiting and carrying out dispersion transmission by a sending end:
Figure FDA0003228015170000014
s25, calculating an iteration error: errt (t) ═ Eodd(t)-Aodd(t)；
S26, simulating receiving end limitation: eodd(t)＝Aodd(t)-errt(t)；
S27, judging whether to finish circulation according to conditions;
s28, finishing iteration, and outputting a signal B (t) after iterative dispersion compensation;
s3, the precompensated signals are subjected to digital precompensation through an FIR filter;
s4, performing noise shaping and quantization on the pre-equalized signal, performing quantization on the pre-equalized signal, shaping quantization noise, and inputting the obtained quantization signal into a DAC;
and S5, in the DSP of the receiving end, performing down-sampling processing, matched filtering, clock recovery, synchronization, linear equalization, PAM-4 inverse mapping and error code calculation on the signal obtained by PD detection.
2. The low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC, as claimed in claim 1, wherein the length l of the pseudo-random bit sequence in step S1 is 211。
3. The low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC, as claimed in claim 1, wherein the roll-off factor d is 1.
4. The low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC of claim 1, wherein in step S4, 3/4/5/6-bit quantization is performed on the pre-equalized signal.
5. The low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC, according to any one of claims 1 to 4, wherein the noise shaping step in step S4 comprises:
s41, determining quantization noise at the previous moment;
s42, carrying out low-frequency filtering on the quantization noise through a constructed filter;
and S43, adding the noise after filtering into the signal to enable the noise to be close to the standard DAC level.
6. The low-cost IM/DD system long-distance transmission method based on low-quantization bit width DAC according to claim 5, wherein in step S42, the number of taps of the constructed filter is calculated by:
s421. output Y (e) of noise shaping structurejω) Expressed as:
Y(ejω)＝X(ejω)+(1+H(ejω))N(ejω)
in the formula, H (e)jω) And X (e)jω) Respectively representing the channel response of the feedback filter and the signal before quantization; quantization noise N (e)j ω) Is the difference between the data before and after entering the DAC;
s422. to minimize the quantization noise in the signal band, the quantization noise after the noise shaping technique structure is output should be smaller than the quantization noise before this operation, so we can:
∫|(1+H(ejω))N(ejω)|2dω<∫|N(ejω)|2dω
in the formula, omega belongs to (0-omega)s),ωsIs the angular frequency of the signal;
s423, the problem reflected by the formula in the step S422 is converted into the following optimized problem:
Figure FDA0003228015170000021
s424, assume that the feedback filter is an FIR filter, and the expression is:
H(ejω)＝h1e-jω+h2e-j2ω+…hne-jnω
the discrete form of the formula in step S423 is:
Figure FDA0003228015170000022
the definition vector F is:
Figure FDA0003228015170000023
suppose that:
Figure FDA0003228015170000031
the discrete form of the formula in step S423 is written as:
Figure FDA0003228015170000032
s425, the number of taps of the feedback filter is as follows:
h＝E-1×1
in the formula, E-1Representing the pseudo-inverse of matrix E.
7. A low-cost IM/DD system long-distance transmission system based on low-quantization bit width DAC is characterized by comprising:
and a transmitting end DSP: a signal A (t) used for mapping a pseudorandom bit sequence with the length of l into a PAM-4 symbol, up-sampling the mapped symbol, and realizing pulse forming through a raised cosine wave with a roll-off factor of d; the shaped signal realizes dispersion and compensation through a dispersion compensation scheme of an improved GS algorithm; the pre-compensated signals pass through an FIR filter to realize digital pre-equalization; noise shaping is carried out on the pre-equalized signal, quantization is carried out simultaneously, quantization noise is shaped, and the obtained quantized signal is input into a DAC;
DSP at the receiving end: the method is used for carrying out error code calculation, PAM-4 inverse mapping, linear equalization, synchronization, clock recovery, matched filtering and down-sampling processing on the signals obtained by PD detection.
8. The low-cost IM/DD system long-distance transmission system based on low-quantization bit width DAC of claim 7, wherein the improved GS algorithm comprises:
an initialization unit: for initialising the phase
Figure FDA0003228015170000033
And simulated receiver amplitude
Figure FDA0003228015170000034
Figure FDA0003228015170000035
GS iteration unit: for starting GS iteration: forI ═ 1,2, …, IteNum;
reverse dispersion transmission unit: for reverse dispersion transmission:
Figure FDA0003228015170000036
a transmission limiting unit: and (3) limiting and carrying out dispersion transmission by a sending end:
Figure FDA0003228015170000037
Figure FDA0003228015170000038
an error unit: for calculating the iteration error: errt (t) ═ Eodd(t)-Aodd(t)；
A reception limiting unit: simulation of receiving end limitation: eodd(t)＝Aodd(t)-errt(t)；
A judging unit: used for judging whether to finish the circulation according to the condition;
an output unit: for outputting the signal b (t) after the iterative compensation of chromatic dispersion after the iteration is finished.
9. The low-cost IM/DD system long-distance transmission system based on the low-quantization bit width DAC, as claimed in claim 7, wherein at the transmitting end, the generated off-line data is loaded to DAC for digital-to-analog conversion, then the signal is amplified by the amplifier, and then the MZM modulator is used to realize photoelectric conversion; the optical carrier of the modulator comes from a tunable external cavity laser.
10. The low-cost IM/DD system long-distance transmission system based on low-quantization bit width DAC, wherein at the receiving end, an erbium-doped fiber amplifier is used for amplifying small signals, and a band-pass filter is used for filtering out-of-band noise; before the receiving end directly detects the signal, an optical attenuator is used for adjusting the receiving power of the signal; the optical signal after attenuation is detected and received by the PD; and the analog electric signal output by the PD is collected by an oscilloscope and is subjected to offline DSP.
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