Method for suppressing low-frequency noise

Abstract

The invention relates to the technical field of optical communication, in particular to a method for suppressing low-frequency noise, which comprises the following steps: at a transmitting end, converting an input original service signal into a digital signal to obtain a digital signal sequence; using a root raised cosine filter to perform shaping filtering operation on the digital signal sequence after up-sampling, and then performing frequency spectrum inversion operation on the filtered digital signal sequence to obtain an inverted digital signal sequence; in a channel, transmitting the inverted digital signal sequence, and meanwhile, introducing a low-frequency noise signal; and at a receiving end, performing frequency spectrum inversion operation on the digital signal sequence into which the low-frequency noise signal is introduced, and then performing root raised cosine matched filtering and down-sampling operation to obtain and output a final digital signal. No extra device needs to be introduced, so that the cost of a communication system is reduced; the method is low in algorithm complexity, is high in low-frequency noise suppression flexibility, and still has a good suppression effect even for randomly changing low-frequency noise.

Description

Technical Field

[0001] The present invention relates to the technical field of optical communication, and in particular to a method for suppressing low-frequency noise. Background Art

[0002] With the introduction of diversified multimedia and data services such as the Internet of Things, cloud computing, and remote surgery, as well as the rapid development of 5G communication networks, global network data traffic has exploded. The explosive growth of data traffic naturally poses a major challenge to data center optical interconnection. Intensity Modulation Direct Detection (IMDD) is widely used in high-speed short-distance optical interconnection scenarios such as data center optical interconnection due to its advantages such as low cost, low power consumption, simple configuration, and high reliability. In addition, the pulse amplitude modulation (PAM) format has been studied and applied to improve the transmission capacity of the intensity modulation direct detection (IMDD) system. However, the damage caused by multipath interference to the IMDD transmission system based on pulse amplitude modulation cannot be ignored. Among them, multipath interference is caused by multiple reflections of the optical signal when passing through the contaminated optical fiber connector, and the noise generated by multipath interference is low-frequency noise (MHz level).

[0003] In recent years, various solutions have been proposed at both the transmitter and receiver to alleviate the low-frequency noise caused by multipath interference in IMDD transmission systems, such as:

[0004] Since the noise of multipath interference is low-frequency noise, and the range of low-frequency noise is at the megahertz level, the conventional digital signal processing (DSP) end digital filter requires thousands of taps to implement, resulting in excessive DSP power consumption. For this reason, YJ Wen, Y. Cui and Y. Bai proposed in the document “Mitigation of optical multipath interference impactfor directly detected PAMn system” to add an analog high-pass filter (a first-order RC analog high-pass filter, i.e., composed of a resistor and a capacitor) after the photodetector (PD), eliminating the complex DSP filtering algorithm and reducing DSP power consumption; however, since the multipath interference noise overlaps with the low-frequency part of the signal, when the high-pass filter is used to directly filter out the low-frequency part, the low-frequency of the original service signal will also be filtered out; N. Cheng in the document “Optical Multipath Interference Mitigation for PAM4 Transmission Using Line Coding and High-pass In order to alleviate the signal damage caused by the high-pass filtering scheme, it is proposed to use DC balanced coding at the transmitting end in "Filtering". DC balanced coding divides the signal sequence into blocks and makes the DC of each block zero through coding. In terms of spectrum, after DC balanced coding, the zero-frequency component of the signal disappears, and the energy of the low-frequency part of the signal is transferred to other spectrum ranges. In this way, the overlapping part of the low-frequency part of the original service signal and the low-frequency noise generated by multipath interference becomes smaller, reducing the damage caused by high-pass filtering.

[0005] C. Huang et al. proposed two filtering algorithms in the paper “Optical Multipath Interference Mitigation for High-Speed ​​PAM4 IMDD Transmission System”: Algorithm 1 uses sliding window filtering to remove the amplitude offset caused by low-frequency noise; Algorithm 2 uses a sliding window to reconstruct and subtract the multipath interference noise using the decision value, and uses the algorithm to alleviate the low-frequency noise caused by multipath interference in the IMDD transmission system.

[0006] In summary, the defects in the prior art are as follows:

[0007] (1) The solution of adding an analog filter after the photodetector introduces additional devices, increases device cost, and changes the existing architecture. In addition, the solution of using a high-pass filter alone will cause significant damage to the original service signal while filtering out the low-frequency noise caused by multipath interference.

[0008] (2) DC balanced coding is used at the transmitter, which increases the complexity of the DSP at both the transmitter and receiver.

[0009] (3) The size of the sliding window in the filtering algorithm is often selected to be hundreds or thousands. The algorithm is highly complex and requires decision values ​​and optimized weighting factors. It has poor flexibility for randomly changing noise. Summary of the invention

[0010] To this end, the technical problem to be solved by the present invention is to overcome the problems in the prior art of introducing high-pass filter devices, increasing device costs and changing the existing architecture; at the same time, using a high-pass filter alone damages the original service signal; using DC balanced coding at the transmitting end increases the complexity of the DSP at the transmitting and receiving ends; using a filtering algorithm, the algorithm has high complexity and poor flexibility to randomly changing noise.

[0011] In order to solve the above technical problems, the present invention provides a method for suppressing low-frequency noise, comprising:

[0012] At the transmitting end, an original service signal is input, and the original service signal is converted into a digital signal to obtain a digital signal sequence; based on a preset upsampling factor, an upsampling operation is performed on the digital signal sequence to obtain an upsampled digital signal sequence; a root raised cosine filter is used to perform a shaping filtering operation on the upsampled digital signal sequence to obtain a filtered digital signal sequence; a spectrum inversion operation is performed on the filtered digital signal sequence, wherein the spectrum inversion operation is to invert all odd bits or even bits in the filtered digital signal sequence to obtain an inverted digital signal sequence;

[0013] In the channel, the reverse digital signal sequence is transmitted and at the same time, a low-frequency noise signal is introduced;

[0014] At the receiving end, after the spectrum inversion operation is performed on the digital signal sequence that introduces the low-frequency noise signal, a matched filtering operation is performed using the same root raised cosine filter as the transmitting end to obtain a new digital signal sequence; after the new digital signal sequence is downsampled, the final digital signal is obtained and output.

[0015] Preferably, the performing of a spectrum inversion operation on the filtered digital signal sequence, wherein the spectrum inversion operation is to invert all odd bits or even bits in the filtered digital signal sequence to obtain an inverted digital signal sequence, comprising: Select the filtered digital signal sequence The odd bits of the filtered digital signal sequence are inverted; The odd bits of the reverse sequence are reversed. , whose expression is: ; in, express is an odd number of digits; Represents the sequence number in the filtered digital signal sequence.

[0016] Preferably, the step of performing a shaping filtering operation on the upsampled digital signal sequence by using a root raised cosine filter to obtain a filtered digital signal sequence comprises: By means of convolution, each symbol in the upsampled digital signal sequence is filtered using a root raised cosine filter to obtain a filtered digital signal sequence. , whose expression is: ; in, represents the digital signal sequence after upsampling; represents the impulse response of the root raised cosine filter;

[0017] The spectrum bandwidth corresponding to the filtered digital signal sequence is limited to the bandwidth parameter value of the root raised cosine filter, that is: According to the symbol rate of the digital signal, its Nyquist bandwidth is obtained, and its expression is: ; in, Represents the Nyquist bandwidth of a digital signal; Indicates the symbol rate of the digital signal;

[0018] According to the Nyquist bandwidth of the digital signal and the roll-off factor of the root raised cosine filter, the bandwidth parameter value of the root raised cosine filter is calculated, and its expression is: ; in, Indicates the limit value of the spectrum bandwidth of the digital signal; Represents the Nyquist bandwidth of a digital signal; Represents the roll-off factor of the root raised cosine filter.

[0019] Preferably, the roll-off factor of the root raised cosine filter is adjusted based on the frequency domain response of the root raised cosine filter. value to ensure the required filtering bandwidth for low-frequency noise.

[0020] The above technical solution of the present invention has the following beneficial effects compared with the prior art:

[0021] (1) A method for suppressing low-frequency noise described in the present invention can adopt a joint equalization scheme of the transmitting end and the receiving end when low-frequency noise interference exists in the channel, that is, spectrum inversion and RRC filtering operation, without the need to introduce additional devices, thereby reducing the cost of the communication system and not changing the existing system architecture;

[0022] (2) The method for suppressing low-frequency noise described in the present invention is that at the transmitting end, the original service signal is converted into a digital signal, and the up-sampled digital signal is filtered and shaped by using an RRC filter, so that the high-frequency component in the filtered digital signal is suppressed, and then the spectrum inversion operation is performed, and the low-frequency component and the high-frequency component are swapped. The low-frequency component in the spectrum characteristics of the final signal at the transmitting end is very small; because the spectrum characteristics of noise are more prominent in the low-frequency component, after passing through the channel with low-frequency noise interference, the spectrum overlap part of the final signal at the transmitting end and the noise is small, and it can be known that the spectrum overlap part of the original service signal and the noise is small ; At the receiving end, the spectrum of the inverted digital signal and the noise is inverted at the same time. As the low-frequency noise becomes high-frequency noise, RRC matched filtering is performed again, which is equivalent to performing a low-pass filtering operation on the entire receiving sequence. Based on the fact that the low-frequency noise has become high-frequency noise after the spectrum inversion, the low-frequency noise interference in the channel can be successfully filtered out; since the spectrum overlap between the original service signal and the noise is small, the damage to the original service signal in the process of filtering out the low-frequency noise interference in the channel is relatively small; this method has low algorithm complexity and high flexibility in suppressing low-frequency noise. Even for randomly changing low-frequency noise, this method can still have a good suppression effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0024] Figure 1 It is a schematic diagram of multipath interference generation;

[0025] Figure 2 is a flow chart of a method for suppressing low-frequency noise provided by the present invention;

[0026] Figure 3 is a schematic diagram of the spectrum inversion operation;

[0027] Figure 4 It is a schematic diagram of the low-frequency component and the high-frequency component of the digital signal being swapped after the spectrum inversion operation;

[0028] Figure 5 It is a schematic diagram of a method for suppressing low-frequency noise provided by the present invention in suppressing multipath interference. DETAILED DESCRIPTION

[0029] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0030] like Figure 1 As shown in the figure, multipath interference is caused by multiple reflections of the optical signal when passing through the contaminated optical fiber connector; the reflected signal has random optical delay and random optical attenuation relative to the original signal. After being detected by the square law of the photodetector (PD), the phase noise is converted into intensity noise, resulting in a larger bit error rate (BER). JL Gimlett and NK Cheung have proved in the literature "Effects of phase-to-intensity noise conversion by multiple reflections on gigabit-per-second DFB laser transmission systems" that the noise generated by multipath interference is low-frequency noise (MHz level). Pulse amplitude modulation formats with different number of levels have different tolerances to multipath interference. The more levels there are, the lower the tolerance to multipath interference.

[0031] In order to solve the low-frequency noise caused by multipath interference, the present invention provides a method for suppressing low-frequency noise. Figure 2 As shown, including:

[0032] S1: At the transmitting end, an original service signal is input, and the original service signal is converted into a digital signal to obtain a digital signal sequence; in a specific embodiment of the present invention, the digital signal sequence may be a PAM4 sequence;

[0033] Based on a preset upsampling factor, an upsampling operation is performed on the digital signal sequence to obtain an upsampled digital signal sequence; wherein the sampling rate of the digital signal sequence is increased to a multiple of the sampling rate by upsampling; in a specific embodiment of the present invention, the preset upsampling factor may be 2, that is, the sampling rate of the digital signal sequence is increased to 2 times the sampling rate;

[0034] The root raised cosine filter is used to perform shaping filtering operation on the upsampled digital signal sequence to obtain a filtered digital signal sequence, including: By means of convolution, each symbol in the upsampled digital signal sequence is filtered using a root raised cosine filter to obtain a filtered digital signal sequence. , whose expression is: ; in, represents the digital signal sequence after upsampling; represents the impulse response of the root raised cosine filter;

[0035] The spectrum bandwidth corresponding to the filtered digital signal sequence is limited to the bandwidth parameter value of the root raised cosine filter, that is: According to the symbol rate of the digital signal, its Nyquist bandwidth is obtained, and its expression is: ; in, Represents the Nyquist bandwidth of a digital signal; Indicates the symbol rate of the digital signal;

[0036] According to the Nyquist bandwidth of the digital signal and the roll-off factor of the root raised cosine filter, the bandwidth parameter value of the root raised cosine filter is calculated, and its expression is: ; in, Indicates the limit value of the spectrum bandwidth of the digital signal; Represents the Nyquist bandwidth of a digital signal; represents the roll-off factor of the root raised cosine filter; The roll-off coefficient of the root raised cosine filter is 0.5; the appropriate optimization, Should be as large as possible; in practical applications, the roll-off factor of the root raised cosine filter is adjusted based on the frequency domain response of the root raised cosine filter. The value of , ensures the required low-frequency noise filtering bandwidth, and the relative intensity noise of the low-frequency noise power density spectrum within the required bandwidth is less than -145dB / Hz, typically for a signal with a bandwidth of 56Ghz, , at this time, the filtering bandwidth of low-frequency noise is about 4Ghz;

[0037] Performing a spectrum inversion operation on the filtered digital signal sequence, wherein the spectrum inversion operation is to invert all odd bits or even bits in the filtered digital signal sequence to obtain an inverted digital signal sequence, including:

[0038] Preset selection of filtered digital signal sequence The odd bits of the filtered digital signal sequence are inverted; The odd bits of the reverse sequence are reversed. , whose expression is: ; in, express is an odd number of digits; Represents the sequence number in the filtered digital signal sequence; for details, please refer to Figure 3 As shown, Figure 3 All odd-numbered signal sequences in are their corresponding negative values;

[0039] S2: In the channel, the reverse digital signal sequence is transmitted and at the same time, a low-frequency noise signal is introduced;

[0040] S3: At the receiving end, after performing a spectrum inversion operation on the digital signal sequence into which the low-frequency noise signal is introduced, a matched filtering operation is performed using the same root raised cosine filter as that at the transmitting end to obtain a new digital signal sequence; after performing a downsampling operation on the new digital signal sequence, a final digital signal is obtained and output.

[0041] In a specific embodiment of the present invention, the upsampling factor is selected to be 2, the roll-off coefficient of the root raised cosine filter is 0.5, the symbol rate of the digital signal is 56 Gbaud, and the Nyquist bandwidth is 28 GHz, then the spectrum bandwidth of the filtered digital signal will be limited to 28*(1+0.5)=42 GHz.

[0042] Figure 4 The effect of the spectrum inversion operation is intuitively demonstrated. That is, after the spectrum inversion operation, the overall spectrum is inverted, such as Figure 4 As shown by the dark line in Figure 4 It can be clearly seen that the spectrum results are also limited to 42GHz.

[0043] based on Figure 4 It can be seen that the spectrum inversion operation will swap the low-frequency and high-frequency components of the digital signal. After the root-raised cosine filter (RRC) performs shaping filtering on the digital signal sequence, the spectrum bandwidth of the digital signal will be limited to a certain range. Therefore, the root-raised cosine filter is equivalent to a low-pass filter.

[0044] In summary, from the overall analysis we can see that:

[0045] At the transmitter, after performing the RRC filtering and shaping operation, the high-frequency component of the digital signal is suppressed. After performing a spectrum inversion operation, the low-frequency and high-frequency components are swapped, that is, the final signal spectrum characteristic at the transmitter is that the low-frequency component is very small. The effect is as follows: Figure 4 The dark line of the signal; after passing through a channel with low-frequency noise interference, the low-frequency part of the signal overlaps with the noise; and due to the previous RRC filtering and spectrum inversion operations at the transmitting end, the spectrum overlap between the original service signal and the noise is small.

[0046] At the receiving end, a spectrum inversion is first performed, and the signal and noise spectra are inverted together, and the low-frequency noise becomes high-frequency noise; then an RRC matched filter is performed, which is equivalent to performing a low-pass filtering operation on the entire received sequence. In addition, the low-frequency noise has become high-frequency noise after the spectrum inversion, and the low-frequency noise interference in the channel can be successfully filtered out.

[0047] Taking multipath interference suppression as an example, the specific process is as follows: Figure 5 As shown, Figure 5 In the present invention, an additional section of optical fiber 2 is added to the optical fiber link, which is only used to simulate the transmission of the reflected signal when the multipath effect is applied to verify the effectiveness of the proposed scheme. A delay device is added to simulate the random optical delay of the reflected signal, and an adjustable optical attenuator 1 is added to simulate the random optical attenuation of the reflected signal.

[0048] based on Figure 5 At the transmitting end, a four-level pulse amplitude modulation (PAM4) sequence is generated, upsampled to multiple sampling rates, shaped and filtered by a root-raised cosine filter (RRC), and then all odd / even bits of the signal sequence are inverted to perform a spectrum inversion operation; through the simulated multipath interference link, the low-frequency part of the signal overlaps with the multipath interference noise spectrum.

[0049] based on Figure 5 At the receiving end, the odd / even bits of the signal sequence are first inverted again and the spectrum inversion is repeated to invert the overall spectrum; the root raised cosine matched filter is used for shaping filtering to suppress the high-frequency noise spectrum with its low-pass filtering characteristics; finally, downsampling is performed and the bit error rate is calculated.

[0050] Obviously, the above embodiments are merely examples for the purpose of clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for suppressing low-frequency noise, characterized in that: include: At the transmitting end, an original service signal is input, and the original service signal is converted into a digital signal to obtain a digital signal sequence; based on a preset upsampling factor, an upsampling operation is performed on the digital signal sequence to obtain an upsampled digital signal sequence; a root raised cosine filter is used to perform a shaping filtering operation on the upsampled digital signal sequence to obtain a filtered digital signal sequence; a spectrum inversion operation is performed on the filtered digital signal sequence, wherein the spectrum inversion operation is to invert all odd bits or even bits in the filtered digital signal sequence to obtain an inverted digital signal sequence; In the channel, the reverse digital signal sequence is transmitted and at the same time, a low-frequency noise signal is introduced; At the receiving end, after the spectrum inversion operation is performed on the digital signal sequence that introduces the low-frequency noise signal, a matched filtering operation is performed using the same root raised cosine filter as the transmitting end to obtain a new digital signal sequence; after the new digital signal sequence is downsampled, the final digital signal is obtained and output.

2. A method for suppressing low-frequency noise according to claim 1, characterized in that: The performing of a spectrum inversion operation on the filtered digital signal sequence, wherein the spectrum inversion operation is to invert all odd bits or even bits in the filtered digital signal sequence to obtain an inverted digital signal sequence, comprising: Select the filtered digital signal sequence The odd bits of the filtered digital signal sequence are inverted; The odd bits of the reverse sequence are reversed. , whose expression is: ； in, express is an odd number of digits; Represents the sequence number in the filtered digital signal sequence.

3. The method for suppressing low-frequency noise according to claim 1, characterized in that: The method of performing shaping filtering operation on the upsampled digital signal sequence by using a root raised cosine filter to obtain a filtered digital signal sequence includes: By means of convolution, each symbol in the upsampled digital signal sequence is filtered using a root raised cosine filter to obtain a filtered digital signal sequence. , whose expression is: ； in, represents the digital signal sequence after upsampling; represents the impulse response of the root raised cosine filter; The spectrum bandwidth corresponding to the filtered digital signal sequence is limited to the bandwidth parameter value of the root raised cosine filter, that is: According to the symbol rate of the digital signal, its Nyquist bandwidth is obtained, and its expression is: ； in, Represents the Nyquist bandwidth of a digital signal; Indicates the symbol rate of the digital signal; According to the Nyquist bandwidth of the digital signal and the roll-off factor of the root raised cosine filter, the bandwidth parameter value of the root raised cosine filter is calculated, and its expression is: ； in, Indicates the limit value of the spectrum bandwidth of the digital signal; Represents the Nyquist bandwidth of a digital signal; Represents the roll-off factor of the root raised cosine filter.

4. A method for suppressing low-frequency noise according to claim 3, characterized in that: Adjust the roll-off factor of the root-raised cosine filter based on its frequency domain response value to ensure the required filtering bandwidth for low-frequency noise.

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