CN119011008B - Error Propagation Suppression Method for Decision Feedback Equalizer Based on Error Correlation - Google Patents
Error Propagation Suppression Method for Decision Feedback Equalizer Based on Error Correlation Download PDFInfo
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- H—ELECTRICITY
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- 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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Abstract
本发明提供一种基于误差相关性的判决反馈均衡器的误差传播抑制方法，接收端数字信号处理包括：采用阈值检测器检测DFE判决前的输入信号，当DFE判决前的输入信号大于预设阈值则判断当前采样点为突发误差的终点，激活纠错步骤；将DFE判决后的输出信号输入至后置滤波器，再将后置滤波器输出信号通过加法器减去DFE的输入信号得到DFE诱导的误差信息，将突发误差终点对应的DFE诱导的误差信息分别与不同长度的误差事件对应的误差相关滤波器的响应进行乘积得到不同误差相关值；输出最大的有效误差相关值；最后利用最大的有效误差相关值对应的误差事件来修正DFE带来的突发误差。本发明能够有效的抑制DFE引入的误差传播，显著提升接收机灵敏度，提高系统的传输性能。
The present invention provides an error propagation suppression method for a decision feedback equalizer based on error correlation, and the receiving end digital signal processing includes: using a threshold detector to detect the input signal before the DFE decision, when the input signal before the DFE decision is greater than a preset threshold, the current sampling point is judged to be the end point of the burst error, and the error correction step is activated; the output signal after the DFE decision is input to a post filter, and then the post filter output signal is subtracted from the DFE input signal through an adder to obtain the DFE-induced error information, and the DFE-induced error information corresponding to the burst error end point is respectively multiplied with the response of the error correlation filter corresponding to the error events of different lengths to obtain different error correlation values; the maximum effective error correlation value is output; and finally, the error event corresponding to the maximum effective error correlation value is used to correct the burst error caused by the DFE. The present invention can effectively suppress the error propagation introduced by the DFE, significantly improve the receiver sensitivity, and improve the transmission performance of the system.
Description
Technical Field
The present invention relates to an optical fiber communication technology, and in particular, to an equalizer error propagation suppression technology for a direct detection optical fiber communication system.
Background
Data center networks DCNs are continually evolving under the drive of various bandwidth-intensive services to meet the demands for larger capacity links. For cost sensitive data center networks, intensity modulated direct detection IM/DD systems are the simplest and most efficient solution. The next generation Ethernet link is expected to reach 1.6Tb/s, and the single channel exceeding 200Gb/s is expected to reduce the integration complexity. The PAM-4 signal can effectively balance bandwidth limitation and noise tolerance in different modulation formats, and is the most promising scheme for realizing single-wave 200Gb/s transmission. However, as the transmission rate increases, commercial optical transceivers impose severe bandwidth limitations, resulting in severe inter-symbol interference ISI. Therefore, in order to meet the requirements of transmission performance and power consumption of the system at the same time, a reasonable method is needed to effectively solve the problem of ISI.
Currently, many advanced digital signal processing DSP techniques have been proposed to mitigate severe ISI in bandwidth limited systems. The feedforward equalizer FFE can eliminate intersymbol interference due to bandwidth limitation and chromatic dispersion CD, but FFE enhances high-frequency noise, resulting in the colorization of white gaussian noise. The adoption of the maximum likelihood sequence estimation MLSE and the decision feedback equalizer DFE after FFE equalization can effectively inhibit noise enhancement. However, the high baud rate IM/DD channel has a long memory length, thereby increasing the computational complexity of the MLSE. On the other hand, the decision feedback operation of the DFE causes error propagation, so that the performance of the DFE is limited. Therefore, how to effectively suppress error propagation of DFE has important research significance for realizing high-speed optical transmission.
Disclosure of Invention
The invention aims to provide a method for effectively inhibiting DFE error propagation.
The invention adopts the technical proposal that the error propagation inhibition method of the decision feedback equalizer based on error correlation comprises the following steps:
A burst error detection step, namely detecting an input signal before the DFE decision by adopting a threshold detector before the DFE decision, judging that the current sampling point is the end point of the burst error when the input signal before the DFE decision is larger than a preset threshold, and activating an error correction step;
And error correction, namely inputting the output signal after the DFE judgment into a post filter with the response of G (D) =1+alpha D, wherein alpha represents a filter coefficient, D represents the delay of a single symbol interval, subtracting the input signal of the DFE from the output signal of the post filter through an adder to obtain the error information induced by the DFE, multiplying the error information induced by the DFE corresponding to the burst error end point by the responses of error correlation filters corresponding to the error events with different lengths to obtain the error correlation value of the current sampling point corresponding to the error event with different lengths respectively, obtaining the error correlation value with the maximum absolute value, judging whether the maximum error correlation value is valid or not, if so, outputting the maximum error correlation value as the maximum valid error correlation value, and correcting the burst error caused by the DFE by using the error event corresponding to the maximum valid error correlation value. If not, the output of the DFE is not modified.
Because the threshold detector can directly detect the end point of the burst error, in the error correction module, the maximum error correlation value is obtained by multiplying the error information brought by the decision feedback equalizer DFE corresponding to the position of the end point of the burst error and the response of the error correlation filter corresponding to the error event with different lengths. Meanwhile, the invention provides that the common term of the error correlation filter corresponding to the error events with different lengths is only needed to be calculated once, so that the calculation complexity is further reduced.
The method provided by the invention has the beneficial effects that error propagation introduced by the DFE can be effectively inhibited, the sensitivity of the receiver is obviously improved, and the transmission performance of the system is improved.
Drawings
FIG. 1 is a schematic block diagram of error propagation suppression for a DFE based on error correlation in accordance with the present invention;
fig. 2 is a schematic diagram of an error correlation filter bank with l.ltoreq.3, where l.ltoreq.3 is a filter bank composed of error correlation filters with three error lengths l=1l=2l=3;
Fig. 3 is a diagram of an experimental setup of a PAM-4 intensity modulated direct detection fiber optic communication system.
Detailed Description
The digital signal processing DSP at the receiving end realizes error propagation inhibition of the decision feedback equalizer EC-DFE based on error correlation by executing the following steps:
The received signal is equalized by a feedforward equalizer FFE to eliminate inter-symbol interference ISI, and then subjected to noise whitening processing using a first post filter PF having a response G (D) =1+αd. D represents the delay of a single symbol interval and α represents the filter coefficient. The DFE based on error correlation (EC-DFE) is then used to cancel the ISI introduced by the PF while suppressing the error propagation of the DFE, the specific flow is shown in fig. 1.
The EC-DFE is divided into two steps:
in the first step, before DFE decision, a threshold detector is used to detect the end point of the burst error, and assuming a PAM-4 signal, the upper threshold of the threshold detector is 3+βd, the lower threshold of the threshold detector is-3+βd, where d is the minimum euclidean distance of the PAM-4 signal, d=2, and β is a constant for adjusting the threshold, and the performance of the system depends on the value of β, so that the value of β needs to be optimized.
And secondly, when the symbol before the DFE judgment exceeds a threshold value, activating an error correction module to correct burst errors caused by the DFE.
The feedforward equalizer FFE in the receiving end DSP is connected with a first post-filter, and the output end of the first post-filter is respectively connected with the decision feedback equalizer DFE and the error correction module.
Taking a single-tap DFE as an example, the DFE includes a decision device, an adder, a single-symbol-interval delayer D and a multiplier, wherein one input end of the adder in the DFE is connected with the output end of the first post-filter, the other input end of the adder in the DFE is connected with the output end of the multiplier, one output end of the adder in the DFE is respectively connected with the input end of the decision device and the input end of the threshold detector, the output end of the decision device is connected with the input end of the single-symbol-interval delayer D, the output end of the single-symbol-interval delayer D is connected with the input end of the multiplier, and the output end of the decision device is used as the output end of the DFE.
The error correction module comprises a post filter with response G (D), an adder, error correlation filters corresponding to L max error events with different lengths, a maximum value calculation unit and an error correction unit. One input end of an adder in the error correction module is connected with the output end of the first post-filter, the other input end of the adder is connected with the output end of the post-filter in the error correction module, and the input end of the post-filter is connected with the output end of the DFE. The output ends of adders in the error correction module are respectively connected with error correlation filters corresponding to L max error events with different lengths, the output ends of the L max error correlation filters are connected with the input end of a maximum value calculation unit, the output end of the maximum value calculation unit is connected with one input end of the error correction unit, and the other input end of the error correction unit is connected with the output end of the DFE. The output end of the error correction unit outputs a signal after correcting the burst error brought by the DFE to the output end of the DFE.
Signal signalThe input decision feedback equalizer DFE, x (k) G (D) represents the transmission symbol x (k) passing through a post-filter responsive to G (D), k representing the kth sample point of the signal,Representing noise. The DFE-decided output signal can be expressed as a transmission symbol x (k) plus an error e (k), the error correction module inputs the DFE-decided output signal to a post-filter responsive to G (D) to obtain a signal x (k) G (D) +e (k) G (D), and then subtracts the DFE input signal by an adderObtaining error information E (k) induced by the DFE,When the threshold detector detects the position k e of the burst error end point, only the response of the error correlation filter corresponding to the error event with different lengths is multiplied by E (k e), so that the error correlation value of the position can be calculated. Finally, the maximum value calculating unit obtains the maximum effective error correlation value, and the error correction unit corrects the burst error caused by the DFE by utilizing the error event corresponding to the maximum effective error correlation value.
The error correlation filter is derived from the product of the error event and the response G (D -1) =α+d. Since in bandwidth limited systems DFE causes burst errors to alternate between-2 and 2. Thus, the error events that may occur for a length L are { +2, -2, +2,.} or { -2, +2, -2,.}, since the error events in the two cases differ only by one sign, the error event of length L only requires the use of an error correlation filter expressed as
Fig. 2 is a schematic diagram of the structure of an error-correlation filter bank of l.ltoreq.3, which includes a filter bank of error-correlation filters of three error lengths of l=1l=2l=3. L max denotes the maximum error length considered. Assuming we consider only error events within 3 in length, then L max is the error correlation filter bank equivalent for the maximum length error event, i.e., the error event of a different length with L max＝3.Lmax greater than 3. As can be seen from the figure, the error correlation filter expressions corresponding to the error events with different lengths contain common terms, so that the common terms are calculated only once, repeated calculation can be avoided, and the calculation complexity is reduced.
When the error event corresponding to the obtained maximum error correlation value is an unreasonable situation, that is, the DFE symbol corrected according to the error event exceeds the maximum or minimum level value, we consider that the maximum error correlation value is invalid. At this time, no modification of the DFE output is required. Otherwise, the burst error caused by the DFE is corrected by utilizing the error event corresponding to the maximum effective error correlation value. That is, the method for judging whether the maximum error correlation value is valid is that the obtained DFE symbol after error event correction corresponding to the maximum error correlation value is valid within the valid level range.
The embodiment takes a four-level pulse amplitude modulation (PAM-4) direct detection optical fiber communication system as an example to illustrate the specific working principle of the invention, and is shown in fig. 3.
In the processing of a digital signal at a transmitting end, original data (binary bit stream) is mapped into a PAM-4 signal and up-sampled, a root raised cosine filter is adopted to carry out Nyquist shaping on the signal, the shaped signal is input into a digital-to-analog converter DAC for digital-to-analog conversion after resampling, an analog electric signal is obtained, and then the electric signal is amplified by an electric amplifier EA and then is input into a Mach-Zehnder MZM modulator together with laser output by a laser for modulation, so that a transmitted optical signal is obtained.
When a sending signal is transmitted through an optical fiber with a certain distance, a variable optical attenuator VOA is adopted at a receiving end to adjust the received optical power ROP, the optical signal after attenuation realizes photoelectric conversion through a photodiode PD, and an electric signal received by the PD is acquired through an oscilloscope and then is subjected to subsequent off-line receiving end digital signal processing DSP.
In the off-line receiving end digital signal processing DSP, the received data is equalized by a feedforward equalizer FFE to eliminate ISI, and the signal after FFE equalization is subjected to noise whitening processing by a post filter PF. Then, the ISI introduced by the post-filter is removed using an error correlation based DFE, and a threshold detector is introduced before DFE decision to detect the end of the burst error. When the input symbol does not exceed the threshold, the DFE directly outputs the decided result. Once a symbol exceeding the threshold is detected, an error correction module is activated to correct the burst error caused by the DFE. The DFE-induced error information E (k) can be obtained by subtracting the input signal of the DFE from the signal obtained by convolving the DFE decision output with G (D). The end position of the burst error detected by the threshold detector is denoted by k e, and the error correlation value corresponding to the end position of the burst error can be calculated by multiplying the response of the error correlation filter corresponding to the error event with different lengths by E (k e). Then, the maximum effective error correlation value is obtained, and burst errors caused by the DFE are corrected by utilizing the corresponding error event. And finally, the corrected signal is demapped into a binary bit stream through PAM-4, and the bit error rate is calculated by comparing the binary bit stream with the transmitted data.
The invention is described by taking a PAM-4 intensity modulation direct detection optical fiber communication system as an example only to illustrate the main technical concept and characteristics of the invention, but the invention is not limited to the embodiment of the invention. Any modifications, equivalent variations, and improvements made in this application are intended to be included within the scope of the following claims.
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| CN113055321A (en) * | 2021-02-09 | 2021-06-29 | 暨南大学 | Optical communication data receiving method and system capable of suppressing burst error propagation |
| CN115314347A (en) * | 2022-07-28 | 2022-11-08 | 苏州大学 | Optical signal decision feedback equalizer based on Gaussian mixture model |
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| US12143249B2 (en) * | 2022-03-24 | 2024-11-12 | Mediatek Inc. | Error detection and correction device capable of detecting head position of suspicious error and performing forward error propagation path tracking for providing information needed by follow-up error correction and associated method |
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