Single pilot frequency based depolarization crosstalk method and DSCM system

Abstract

The invention relates to the technical field of optical signal processing, in particular to a depolarization crosstalk method based on a single pilot frequency and a DSCM system. Polarization decrosstalk is realized through a single pilot frequency, so that a subsequent equalizer for equalization of each subcarrier is simplified into two multi-tap SISO equalizers with independent polarization, and the calculation complexity of the equalizer at a receiving end is remarkably reduced. Meanwhile, the protection frequency band between adjacent subcarriers is eliminated, and the spectrum efficiency of the signal is improved. According to the method, the influence of the XY time delay of the receiving end on the low-complexity polarization decrosstalk scheme based on the pilot frequency is considered, and the effectiveness of the polarization decrosstalk method based on the pilot frequency is further ensured by estimating and calibrating the XY time delay of the receiving end.

Description

Technical Field

[0001] The present invention relates to the technical field of optical signal processing, and more particularly to a method for depolarization crosstalk based on a single pilot and a DSCM system. Background Art

[0002] In order to meet the growing demand for data traffic, the application of coherent technology in data center optical interconnection will become a future trend to support large-capacity data transmission. However, due to the high complexity of the digital signal processing (DSP) algorithm of the traditional coherent optical communication system, it cannot be directly applied to the short-distance data center optical interconnection application scenario. Therefore, it is crucial to simplify the coherent optical communication algorithm. Compared with the traditional single-carrier coherent system, the signal based on digital subcarrier multiplexing modulation (DSCM) has better robustness to fiber dispersion and can effectively reduce the complexity of dispersion compensation. It is considered to be a very promising solution. However, in addition to dispersion compensation, the complexity of the equalizer is also the main factor leading to the high complexity of traditional coherent DSP. In the traditional coherent DSP algorithm, a multi-tap 2×2 multiple-input multiple-output (MIMO) equalizer is usually used to achieve polarization crosstalk and channel equalization, and this butterfly MIMO operation also brings the problem of high complexity.

[0003] In order to reduce the complexity of the equalizer, some solutions have been proposed. Since the impact of polarization mode dispersion (PMD) can be ignored in short-distance application scenarios, a solution has been proposed to split the multi-tap MIMO equalizer into a single-tap MIMO equalizer and two polarization-independent multi-tap single-input single-output (SISO) equalizers, and realize polarization crosstalk removal and channel equalization through single-tap MIMO and multi-tap SISO equalizers respectively. However, the polarization tracking speed that can be achieved by this adaptive equalizer is limited, and it cannot cope with scenarios with rapid polarization changes. In addition, in the DSCM system, the processing of each subcarrier needs to include a single-tap MIMO operation. In order to improve the polarization tracking speed, a polarization crosstalk removal scheme based on three pilots has been proposed and applied in the coherent DSCM system, which can realize polarization crosstalk removal before subcarrier demultiplexing, so that the subsequent equalization of each subcarrier only requires two polarization-independent SISO equalizers. However, in this scheme, when the amplitude of the received Y-polarization pilot component approaches zero, an arithmetic overflow problem will occur. In order to avoid the arithmetic overflow problem, another polarization crosstalk solution based on two pilots was proposed, which reduces the number of required pilots and reduces the computational complexity. However, the above pilot-based solutions all require a certain guard interval to be reserved between adjacent subcarriers to place pilots, thus reducing the spectrum efficiency of the signal.

[0004] In summary, in the solution based on single-tap MIMO combined with multi-tap SISO equalizer, since each subcarrier signal will be processed separately after the DSCM signal is demultiplexed, the DSP processing module of each subcarrier will include MIMO operations, which still has a certain computational complexity, and its polarization tracking speed is limited, which cannot be applied to application scenarios with rapid changes in polarization state. In the polarization crosstalk solution based on multiple pilots, the currently proposed solutions all need to reserve a certain width of protection band between adjacent subcarriers, thereby reducing the spectral efficiency of the signal. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art that the calculation complexity of the equalizer at the receiving end is complex and the signal transmission spectrum efficiency is low, and to provide a method for depolarization crosstalk based on a single pilot and a DSCM system, which significantly reduces the calculation complexity of the equalizer at the receiving end, while eliminating the guard band between adjacent subcarriers and improving the spectrum efficiency of the signal.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A method for polarization crosstalk removal based on a single pilot is provided. A pilot is inserted into a DSCM signal of X polarization to track the polarization state change experienced by the signal. The pilot components received on two polarizations are used at the receiving end to estimate the changed Jones matrix, thereby realizing polarization crosstalk removal of the signal. The method specifically comprises the following steps:

[0008] S1. Insert a pilot signal P into the X-polarized DSCM signal X (t), the signal E on the X polarization after the pilot signal is inserted is obtained X (t) and the signal E on the Y polarization Y (t);

[0009] S2. Use the polarization rotator to rotate the polarization of the transmission signal, so that the pilot signal originally located in the X polarization appears in the Y polarization. By comparing the pilot signal received at the receiving end with the pilot signal P at the transmitting end X (t), and obtain the frequency deviation Δf;

[0010] S3. Perform frequency offset compensation to obtain the compensated X-polarized signal R' X (t) and the signal R' on the Y polarization Y (t);

[0011] S4. After the pilot signal received by the receiving end is converted to a low frequency, it is filtered out with a low-pass filter to obtain the pilot components extracted from the X polarization and the Y polarization. and The parameters of the Jones matrix are obtained according to the extracted pilot components, thereby obtaining the complete Jones matrix;

[0012] S5. According to the Jones matrix obtained in step S4, the signal is depolarized and crosstalk is removed.

[0013] The present invention provides a method for polarization crosstalk removal based on a single pilot, which realizes polarization crosstalk removal through a single pilot, thereby simplifying the equalizer used for equalization of each subcarrier into two polarization-independent multi-tap SISO equalizers, significantly reducing the calculation complexity of the equalizer at the receiving end. At the same time, the guard band between adjacent subcarriers is eliminated, thereby improving the spectrum efficiency of the signal.

[0014] Furthermore, the transmitting end inserts a pilot signal P X (t) is expressed as:

[0015]

[0016] In the formula, S X (t) and S Y (t) are DSCM signals modulated on X polarization and Y polarization, respectively. X (t) and E Y (t) are the signals on X polarization and Y polarization after the pilot is inserted, P X (t) is the pilot signal inserted on the X polarization, and the pilot P X (t) is specifically expressed as:

[0017] P X (t) = A exp(j2πf1t)

[0018] Where A and f1 are the amplitude and frequency of the pilot signal, respectively, and t represents the time.

[0019] Furthermore, step S2 specifically includes:

[0020] The polarization rotator is used to rotate the polarization of the transmission signal so that the pilot originally located at the X polarization appears as a pilot component on the Y polarization, which is expressed by the following formula:

[0021]

[0022] In the formula, J XX (t), J YX (t), J YX (t) and J YY (t) are the four parameters of the Jones matrix, Δf and Represent frequency deviation and phase noise respectively, R X (t) and R Y (t) represents the received X-polarization and Y-polarization signals respectively; j is an imaginary unit; That is, after polarization rotation, the pilot signal P located at X polarizationX (t) The pilot component projected onto the Y polarization; By comparing the received pilot and the pilot signal P at the transmitting end X The frequency of (t) is obtained as the frequency deviation Δf.

[0023] Further, in step S3, the X-polarized signal R′ after frequency offset compensation is obtained by the following formula: X (t) and the Y polarization signal R′ Y (t):

[0024]

[0025] Further, in step S4, the pilot signal filtered out by the low-pass filter is expressed by the following formula:

[0026]

[0027] In the formula, and They represent the pilot components extracted from X polarization and Y polarization respectively, is the phase noise carried;

[0028] Since the Jones matrix is ​​a unitary matrix, the parameters of the Jones matrix satisfy the following relationship:

[0029]

[0030]

[0031] According to the obtained Jones matrix parameters and Get the complete Jones matrix:

[0032]

[0033] in, and They represent the four parameters of the estimated Jones matrix respectively.

[0034] Furthermore, in step S5, the polarization crosstalk of the signal is depolarized by the following formula:

[0035]

[0036] in, and They are the X-polarization and Y-polarization signals after polarization crosstalk removal.

[0037] Furthermore, when there is a receiving end XY delay, the receiving end XY delay is first estimated, then signal transmission is performed, and the estimated delay value is used for receiving end XY delay calibration, and finally steps S1 to S5 are performed. The present invention takes into account the influence of the receiving end XY delay on the pilot-based low-complexity polarization crosstalk solution, and further ensures the effectiveness of the pilot-based polarization crosstalk solution method by estimating and calibrating the receiving end XY delay.

[0038] The present invention also provides a DSCM system for depolarization crosstalk based on a single pilot, comprising:

[0039] First, the XY delay of the receiving end is tested. The digital signal processing DSP process at the transmitting and receiving end includes:

[0040] The DSP at the transmitting end: first generates a single-frequency cosine signal, and after resampling, modulates the single-frequency cosine signal onto the X-polarized optical carrier;

[0041] DSP at the receiving end: receives the pilot signals on the two polarizations, extracts the real parts of the two pilot signals, converts them to the frequency domain, then extracts the phase information of the pilots, and finally calculates the XY delay at the receiving end;

[0042] After estimating the XY delay at the receiving end, signal transmission is performed, and the estimated delay value is used for XY delay calibration at the receiving end to ensure the effectiveness of the depolarization algorithm;

[0043] Then data transmission is carried out, and the digital signal processing DSP process at the transmitting and receiving end includes:

[0044] The transmitter DSP: first maps the pseudo-random bit sequence into QAM symbols, and uses a root raised cosine filter to perform Nyquist shaping on the QAM symbols; the shaped symbols are used to generate a DSCM signal with four subcarriers; after resampling, a pilot is inserted into the X-polarized DSCM signal; finally, different polarization change speeds are simulated in the transmitter DSP;

[0045] DSP at the receiving end: first, use the estimated XY delay value of the receiving end to compensate the XY delay of the receiving signal; then use the method for connecting polarization crosstalk as described in any one of claims 1 to 6 to realize frequency offset compensation and polarization crosstalk elimination, and then demultiplex the DSCM signal by subcarriers; for each demultiplexed subcarrier signal, perform matched filtering, dispersion compensation, clock recovery, synchronization, simplified equalization and phase noise compensation in turn, and finally calculate the bit error rate.

[0046] The present invention proposes a DSCM system based on single pilot polarization crosstalk resolution. In the coherent DSCM system, a single pilot is used to implement low-complexity polarization crosstalk resolution before subcarrier demultiplexing, thereby simplifying the equalizer used in the subsequent processing of each subcarrier from the traditional multi-tap 2×2 MIMO structure to two polarization-independent multi-tap SISO structures, greatly reducing the complexity of the equalizer. At the same time, compared with the existing pilot-based polarization crosstalk resolution scheme, the proposed scheme requires fewer pilots, only one, and there is no need to reserve a certain width of protection band between adjacent subcarriers. In addition, the present invention also considers the influence of the receiving end XY delay on the effectiveness of the pilot-based polarization crosstalk resolution algorithm, and proposes a receiving end XY delay estimation algorithm based on a single pilot training sequence, and uses the estimated receiving end XY delay to calibrate the system, thereby ensuring the effectiveness of the pilot-based low-complexity polarization crosstalk resolution algorithm, and further realizing the simplification of the subsequent equalizer. Therefore, the present invention realizes a low-complexity, high-performance coherent DSCM system based on single pilot polarization crosstalk resolution.

[0047] Furthermore, when performing a delay test at the receiving end, for the transmitting end DSP, a single-frequency cosine pilot signal on the X polarization is expressed as: A is the amplitude of the pilot signal, f2 is the frequency of the pilot signal; after considering the delay and polarization rotation at the transmitting end, the pilot signal is expressed as:

[0048]

[0049] Among them, τ TXI is the I-path delay of the X polarization at the transmitting end, a and α are the parameters J XX The amplitude and phase of (t), b and β are the parameters J XY (t) is the amplitude and phase of the

[0050] Furthermore, when performing a receiving end delay test, for the receiving end DSP: after considering the receiving end delay, the received pilot signal is expressed as:

[0051]

[0052]

[0053]

[0054]

[0055] in, and are the received X-polarized I and Q signals, and Y-polarized I and Q signals, τ RXI , τ RXQ , τRYI and τ RYQ They are the X-polarization I-path delay, X-polarization Q-path delay, Y-polarization I-path delay, and Y-polarization Q-path delay at the receiving end respectively;

[0056] Delay the receiving end XY by τ RXY Defined as: RXY =τ RYI -τ RXI ;

[0057] The formula and formula Transform to the frequency domain:

[0058]

[0059]

[0060] The XY delay at the receiving end is calculated as follows:

[0061]

[0062] in, and Respectively represent the received X polarization I signal The pilot components at the positive and negative half frequencies, and Respectively represent the received Y polarization I signal Pilot components at positive and negative half frequencies, * represents conjugation operation.

[0063] Compared with the prior art, the present invention has the following beneficial effects:

[0064] Compared with the currently adopted single-tap MIMO combined with multi-tap SISO equalizer solution, the present invention can further simplify the equalizer into a structure containing only two multi-tap SISO equalizers, further reduce the complexity of the equalizer, and significantly improve the polarization tracking capability. Compared with the current multi-pilot-based polarization crosstalk solution solution, the present invention requires fewer pilots, only one, and does not need to reserve a certain width of protection band between adjacent subcarriers, thereby improving the spectral efficiency of the signal. In addition, the present invention also considers the influence of the receiving end XY delay on the effectiveness of the pilot-based polarization crosstalk solution algorithm, and proposes a receiving end XY delay estimation algorithm based on a single pilot training sequence. Based on the proposed receiving end XY delay algorithm, the receiving end XY delay can be accurately calibrated, thereby ensuring the effectiveness of the proposed pilot-based low-complexity polarization crosstalk solution algorithm, and further simplifying the subsequent equalizer, thereby realizing a low-complexity high-performance coherent DSCM system. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 The figure is a flow chart of a method for solving polarization crosstalk based on a single pilot.

[0066] Figure 2 This is a spectrum diagram of the method for depolarization crosstalk based on a single pilot in Example 1.

[0067] Figure 3 The pilot training sequence structure in Example 2; Figure 3 a represents the pilot structure at the transmitting end, Figure 3 b represents the pilot structure at the receiving end.

[0068] Figure 4 This is a schematic diagram of the simulation structure of the DSCM system in Example 3.

[0069] Figure 5 This is the bit error rate performance curve of the single pilot polarization crosstalk removal method at different polarization rotation speeds.

[0070] Figure 6 This figure shows the impact of different receiving end XY delays on the performance of the single pilot polarization crosstalk removal method.

[0071] Figure 7 is the estimation error under different preset delay values.

[0072] Figure 8 Comparison of bit error rate curves under different receiving end XY delays. DETAILED DESCRIPTION

[0073] The present invention is further described below in conjunction with specific implementation methods. The accompanying drawings are only used for exemplary descriptions and are only schematic diagrams, not actual drawings, and cannot be understood as limiting this patent; in order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the accompanying drawings may be omitted.

[0074] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the drawings, it is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limitations on this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0075] Embodiment 1:

[0076] like Figure 1 As shown, this embodiment is a method for depolarization crosstalk based on a single pilot, in which a pilot is inserted into the X-polarized DSCM signal to track the polarization state change experienced by the signal, and the pilot components received on two polarizations are used at the receiving end to estimate the changed Jones matrix, thereby realizing depolarization crosstalk of the signal; as shown Figure 2 As shown in FIG. 1 , a spectrum diagram of a single pilot polarization crosstalk solution is given. Specifically, the following steps are included:

[0077] Step 1. Insert a pilot signal P into the X-polarized DSCM signal X (t), the signal E on the X polarization after the pilot signal is inserted is obtained X (t) and the signal E on the Y polarization Y (t).

[0078] After the transmitter inserts the pilot signal, the signal can be expressed as:

[0079]

[0080] Among them, S X (t) and S Y (t) are DSCM signals modulated on X polarization and Y polarization, respectively. X (t) and E Y (t) are the signals on X polarization and Y polarization after the pilot is inserted, P X (t) is the pilot signal inserted on the X polarization, and the pilot P X (t) can be specifically expressed as:

[0081] P X (t) = A exp(j2πf1t)

[0082] Wherein, A and f1 are the amplitude and frequency of the pilot signal respectively, and t represents the time.

[0083] Step 2: Use a polarization rotator to rotate the polarization of the transmission signal so that the pilot signal originally located at the X polarization appears as a pilot signal component at the Y polarization. By comparing the pilot signal received at the receiving end with the pilot signal P at the transmitting end, X (t), and obtain the frequency deviation Δf.

[0084] like Figure 2 As shown, after polarization rotation, the pilot originally located at the X polarization will have a pilot component on the Y polarization. The process can be expressed as follows:

[0085]

[0086] Among them, J XX(t), J XY (t), J YX (t) and J YY (t) are the four parameters of the Jones matrix, Δf and Represent frequency deviation and phase noise respectively, R X (t) and R Y (t) represents the received X-polarization and Y-polarization signals respectively. j is an imaginary unit; That is, after polarization rotation, the pilot signal at the X polarization is projected onto the Y polarization. and the pilot signal P at the transmitting end X The frequency of (t) can obtain the frequency deviation Δf.

[0087] S3. Perform frequency offset compensation to obtain the X-polarized signal R' after frequency offset compensation X (t) and the signal R' on the Y polarization Y (t).

[0088] The X-polarized signal R' after compensation X (t) and the Y polarization signal R' Y (t) can be expressed as:

[0089]

[0090] Step 4. After the pilot signal received by the receiving end is converted to a low frequency, it is filtered out with a low-pass filter to obtain the pilot components extracted from the X polarization and the Y polarization. and The parameters of the Jones matrix are obtained according to the extracted pilot components, thereby obtaining a complete Jones matrix.

[0091] After the pilot frequency is transformed to a low frequency and filtered out with a low-pass filter, the extracted pilot frequency can be expressed as follows:

[0092]

[0093] in, and They represent the pilot components extracted from X polarization and Y polarization respectively, is the phase noise carried. From the above formula, we can know that the extracted pilot component is the phase noise carried The Jones matrix parameter J XX (t) and J YX (t). Since the Jones matrix is ​​a unitary matrix, its parameters satisfy the following relationship:

[0094]

[0095]

[0096] Therefore, according to the obtained Jones matrix parameters and The complete Jones matrix can be obtained:

[0097]

[0098] in, and They represent the four parameters of the estimated Jones matrix respectively.

[0099] Step 5. According to the Jones matrix obtained in step 4, the signal is depolarized and crosstalked.

[0100] The signal depolarization crosstalk process can be expressed as follows:

[0101]

[0102] in, and They are the X-polarization and Y-polarization signals after polarization crosstalk removal.

[0103] This embodiment provides a method for polarization crosstalk removal based on a single pilot, which realizes polarization crosstalk removal through a single pilot, thereby simplifying the equalizer used for equalization of each subcarrier into two polarization-independent multi-tap SISO equalizers, significantly reducing the calculation complexity of the equalizer at the receiving end. At the same time, the guard band between adjacent subcarriers is eliminated, thereby improving the spectrum efficiency of the signal.

[0104] Embodiment 2:

[0105] This embodiment provides a method for estimating the XY delay at the receiving end based on a single pilot training sequence. For the polarization crosstalk method of embodiment 1, when there is an XY delay at the receiving end, the estimated Jones matrix will be expressed as:

[0106]

[0107] Among them, τ RXY is the XY delay at the receiving end. At this time, the estimated Jones matrix will not be able to correctly reflect the change of polarization, thus causing the failure of the polarization crosstalk deconvolution algorithm.

[0108] In order to ensure the effectiveness of the low-complexity pilot-based polarization crosstalk elimination algorithm, it is necessary to calibrate the XY delay at the receiving end. Therefore, this embodiment proposes a receiving end XY delay estimation algorithm based on a single pilot training sequence. Figure 3 Given the structure of the training sequence, Figure 3 a gives the pilot structure of the transmitter, that is, a single-frequency cosine pilot signal on the X polarization, which can be expressed as:

[0109]

[0110] Wherein, A is the amplitude of the pilot signal, and f2 is the frequency of the pilot signal.

[0111] After considering the transmission delay and polarization rotation, it can be expressed as:

[0112]

[0113] Among them, τ TXI is the I-path delay of the X polarization at the transmitting end, a and α are the parameters J XX The amplitude and phase of (t), b and β are the parameters J XY (t) is the amplitude and phase of the

[0114] Considering the delay at the receiving end, the received pilot signal can be expressed as:

[0115]

[0116]

[0117]

[0118]

[0119] in, and are the received X-polarized I and Q-path signals, and Y-polarized I and Q-path signals, τ RXI , τ RXQ , τ RYI and τ RYQ They are the X-polarization I-path delay, X-polarization Q-path delay, Y-polarization I-path delay, and Y-polarization Q-path delay at the receiving end respectively.

[0120] Receiver XY delay τ RXY Defined as: RXY =τ RYI -τ RXI .

[0121] Therefore, we can use the formula and formula To get the XY delay at the receiving end. First, transform the above formula into the frequency domain:

[0122]

[0123]

[0124] The XY delay at the receiving end can be calculated as follows:

[0125]

[0126] in, and Respectively represent the received X polarization I signal The pilot components at the positive and negative half frequencies, and Respectively represent the received Y polarization I signal Pilot components at positive and negative half frequencies, * represents conjugation operation.

[0127] This embodiment takes into account the influence of the XY delay at the receiving end on the effectiveness of the pilot-based polarization crosstalk removal algorithm, and proposes a method for estimating the XY delay at the receiving end based on a single pilot training sequence. The estimated XY delay at the receiving end is used to calibrate the system, thereby ensuring the effectiveness of the low-complexity pilot-based polarization crosstalk removal algorithm and further simplifying the subsequent equalizer.

[0128] Embodiment 3:

[0129] This embodiment proposes a DSCM system based on single pilot polarization crosstalk resolution. Figure 4 As shown in the figure, a DSCM simulation system diagram is given. The optical carrier at the transmitting end is generated by a laser with a linewidth of 100kHz and an output power of 16dBm. The optical carrier is then divided into X-polarized and Y-polarized light by a polarization beam splitter (PBS), and respectively input into two IQ modulators (IQM) for signal modulation. The two 50Gbaud DSCM signals generated offline at the transmitting end are respectively loaded into two IQMs to realize electro-optical conversion. The signals output by the two modulators are coupled by a polarization beam combiner (PBC). For the XY delay estimation at the receiving end, the test is carried out in an optical back-to-back scenario, and the local oscillator (LO) is provided by the laser at the transmitting end. In addition, a polarization rotator is required to introduce polarization crosstalk. For the data transmission scenario, the signal is input into an 80-kilometer optical fiber for transmission, and an optical signal-to-noise ratio (OSNR) module is used to adjust the OSNR of the system. Another laser is used as LO at the receiving end. The rest of the settings in the XY delay estimation and data transmission stages are the same: the LO and signal are split into two orthogonal polarization states by a PBS, then input into two 90° optical mixers and detected by four pairs of photodetectors (PDs). The detected data are processed in the DSP at the receiving end. The sampling rate used in the system is 64GSa / s.

[0130] When performing data signal processing, the system receiving end XY delay is first tested and estimated, and the receiving end XY delay is obtained using the receiving end delay estimation method provided in Example 2.

[0131] The DSP algorithm flow of the transmitting and receiving end is as follows:

[0132] (1) Transmitter DSP:

[0133] In the transmitting end DSP, a single-frequency cosine signal with a frequency of 2 GHz is first generated, and after resampling, it is modulated on the X-polarized optical carrier.

[0134] (2) DSP at the receiving end:

[0135] At the receiving end, the pilot signals on the two polarizations are received, the real parts of the two pilot signals are extracted and converted to the frequency domain, the phase information of the pilots is then extracted, and finally the XY delay at the receiving end is calculated.

[0136] After estimating the XY delay at the receiving end, signal transmission is performed, and the estimated delay value is used for XY delay calibration at the receiving end to ensure the effectiveness of the depolarization algorithm.

[0137] Then the DSP performs the data transmission phase:

[0138] (1) Transmitter DSP:

[0139] In the transmitter DSP, the pseudo-random bit sequence is first mapped to 16QAM symbols, and the QAM symbols are Nyquist-shaped using a root raised cosine filter with a roll-off factor of 0.1. The shaped symbols are used to generate a 50Gbaud DSCM signal with four subcarriers. After resampling, a pilot is inserted into the X-polarized DSCM signal. Finally, different polarization change speeds are simulated in the transmitter DSP.

[0140] (2) DSP at the receiving end:

[0141] At the receiving end, the estimated receiving end XY delay value is first used to perform receiving end XY delay compensation on the received signal. Then, the polarization crosstalk method described in Example 1 is used to achieve frequency offset compensation and polarization crosstalk removal, and then the DSCM signal is subcarrier demultiplexed. For each demultiplexed subcarrier signal, matched filtering, dispersion compensation, clock recovery, synchronization, simplified equalization, and phase noise compensation are performed in turn, and finally the bit error rate is calculated.

[0142] Result analysis:

[0143] based on Figure 4 The performance of the proposed single pilot polarization crosstalk resolution scheme is first evaluated using a simulation device. Figure 5The bit error rate curves of the proposed single pilot polarization crosstalk solution at different polarization change speeds are given, and the bit error rate curve without polarization crosstalk is given for comparison. From the simulation results, it can be seen that even if the polarization change speed reaches 10Mrad / s, there is almost no OSNR loss compared with the case without polarization crosstalk, which shows that the proposed single pilot polarization crosstalk solution algorithm can effectively solve the signal crosstalk under the high-speed change of polarization state.

[0144] Figure 6 The performance impact of different receiver XY delays on the proposed single pilot depolarization crosstalk scheme is given. As can be seen from the figure, with the increase of the receiver XY delay, the performance of the pilot-based depolarization crosstalk algorithm gradually deteriorates. When there is a 2ps receiver XY delay, there is a 3.2dB OSNR loss at the hard decision forward error correction threshold. When there is a 3ps receiver XY delay, the pilot-based depolarization crosstalk algorithm will fail. Therefore, compensation for the receiver XY delay damage is a prerequisite for ensuring the effectiveness of the pilot-based depolarization algorithm, thereby further simplifying the subsequent equalizer.

[0145] Then, the accuracy of the proposed receiving-end XY delay estimation algorithm based on single pilot training sequence is evaluated. Figure 7 The estimated delay values ​​and corresponding estimation errors under different preset receiver XY delays are given. The simulation results show that the proposed receiver XY delay estimation algorithm can achieve an estimation error within ±0.15ps.

[0146] After the XY delay value of the receiving end is estimated, the XY delay of the receiving end can be calibrated. Figure 8 The bit error rate performance curves of the polarization crosstalk elimination algorithm based on a single pilot are given under different receiving end XY delays, without delay compensation and after delay compensation. At this time, the speed of polarization change is 10Mrad / s. It can be seen from the figure that without receiving end XY delay compensation, the performance deteriorates rapidly as the delay value increases. After the signal is compensated for the receiving end XY delay, the bit error rate curve is almost flat and is basically consistent with the performance without receiving end XY delay.

[0147] Compared with the currently used single-tap MIMO combined with multi-tap SISO equalizer solution, the present invention can further simplify the equalizer into a structure containing only two multi-tap SISO equalizers, further reduce the complexity of the equalizer, and significantly improve the polarization tracking capability. Compared with the current multi-pilot-based polarization crosstalk solution, the present invention requires fewer pilots, only one, and does not need to reserve a certain width of protection band between adjacent subcarriers, thereby improving the spectral efficiency of the signal. In addition, the present invention also considers the impact of the receiving end XY delay on the effectiveness of the pilot-based polarization crosstalk algorithm, and proposes a receiving end XY delay estimation algorithm based on a single pilot training sequence. Based on the proposed receiving end XY delay algorithm, accurate calibration of the receiving end XY delay can be achieved, thereby ensuring the effectiveness of the proposed pilot-based low-complexity polarization crosstalk algorithm, and further simplifying the subsequent equalizer, thereby realizing a low-complexity, high-performance coherent DSCM system.

[0148] In the specific contents of the above-mentioned specific implementation methods, the various technical features can be combined in any non-contradictory manner. In order to make the description concise, not all possible combinations of the above-mentioned technical features are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0149] Obviously, the above 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. 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 embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for depolarization crosstalk based on a single pilot, characterized in that: A pilot is inserted into the X-polarized DSCM signal to track the polarization state change experienced by the signal, and the pilot components received on two polarizations are used at the receiving end to estimate the changed Jones matrix, so as to realize the depolarization crosstalk of the signal; specifically, the following steps are included: S1. Insert a pilot signal P into the X-polarized DSCM signal X (t), the signal E on the X polarization after the pilot signal is inserted is obtained X (t) and the signal E on the Y polarization Y (t); S2. Use the polarization rotator to rotate the polarization of the transmission signal, so that the pilot signal originally located in the X polarization appears in the Y polarization. By comparing the pilot signal received at the receiving end with the pilot signal P at the transmitting end X (t), and obtain the frequency deviation Δf; S3. Perform frequency offset compensation to obtain the compensated X-polarized signal R' X (t) and the signal R' on the Y polarization Y (t); S4. After the pilot signal received by the receiving end is converted to a low frequency, it is filtered out with a low-pass filter to obtain the pilot components extracted from the X polarization and the Y polarization. and The parameters of the Jones matrix are obtained according to the extracted pilot components, thereby obtaining the complete Jones matrix; S5. According to the Jones matrix obtained in step S4, the signal is depolarized and crosstalk is removed.

2. The method for depolarization crosstalk based on a single pilot according to claim 1, characterized in that: In step S1, the transmitter inserts a pilot signal P X (t) is expressed as: In the formula, S X (t) and S Y (t) are DSCM signals modulated on X polarization and Y polarization, respectively. X (t) and E Y (t) are the signals on X polarization and Y polarization after the pilot is inserted, P X (t) is the pilot signal inserted on the X polarization, and the pilot P X (t) is specifically expressed as: P X (t)=A·exp(j2πf1t) Where A and f1 are the amplitude and frequency of the pilot signal, respectively, and t represents the time.

3. The method for depolarization crosstalk based on a single pilot according to claim 2, characterized in that: Step S2 specifically includes: The polarization rotator is used to rotate the polarization of the transmission signal so that the pilot originally located at the X polarization appears as a pilot component on the Y polarization, which is expressed by the following formula: In the formula, J XX (t), J XY (t), J YX (t) and J YY (t) are the four parameters of the Jones matrix, Δf and Represent frequency deviation and phase noise respectively, R X (t) and R Y (t) represents the received X-polarization and Y-polarization signals respectively; j is an imaginary unit; That is, after polarization rotation, the pilot signal P located at X polarization X (t) The pilot component projected onto the Y polarization; By comparing the received pilot and the pilot signal P at the transmitting end X The frequency of (t) is obtained as the frequency deviation Δf.

4. The method for depolarization crosstalk based on a single pilot according to claim 3, characterized in that: In step S3, the X-polarized signal R' after frequency offset compensation is obtained by the following formula: X (t) and the Y polarization signal R' Y (t):

5. The method for depolarization crosstalk based on a single pilot according to claim 4, characterized in that: In step S4, the pilot signal filtered out by the low-pass filter is expressed by the following formula: In the formula, and represent the pilot components extracted from X polarization and Y polarization respectively, is the phase noise carried; Since the Jones matrix is ​​a unitary matrix, the parameters of the Jones matrix satisfy the following relationship: According to the obtained Jones matrix parameters and Get the complete Jones matrix: in, and They represent the four parameters of the estimated Jones matrix, and * represents the conjugate operation.

6. The method for depolarization crosstalk based on a single pilot according to claim 5, characterized in that: In step S5, the signal polarization crosstalk is depolarized by the following formula: in, and They are the X-polarization and Y-polarization signals after polarization crosstalk removal.

7. The method for depolarization crosstalk based on a single pilot according to any one of claims 1 to 6, characterized in that: When there is a receiving end XY delay, the receiving end XY delay is first estimated, then signal transmission is performed, and the estimated delay value is used for receiving end XY delay calibration, and finally steps S1 to S5 are executed.

8. A DSCM system based on single pilot polarization crosstalk resolution, characterized in that: include: First, the XY delay of the receiving end is tested. The digital signal processing DSP process at the transmitting and receiving end includes: The DSP at the transmitting end: first generates a single-frequency cosine signal, and after resampling, modulates the single-frequency cosine signal onto the X-polarized optical carrier; DSP at the receiving end: receives the pilot signals on the two polarizations, extracts the real parts of the two pilot signals, converts them to the frequency domain, then extracts the phase information of the pilots, and finally calculates the XY delay at the receiving end; After estimating the XY delay at the receiving end, signal transmission is performed, and the estimated delay value is used for XY delay calibration at the receiving end to ensure the effectiveness of the depolarization algorithm; Then data transmission is carried out, and the digital signal processing DSP process at the transmitting and receiving end includes: The transmitter DSP: first maps the pseudo-random bit sequence into QAM symbols, and uses a root raised cosine filter to perform Nyquist shaping on the QAM symbols; the shaped symbols are used to generate a DSCM signal with four subcarriers; after resampling, a pilot is inserted into the X-polarized DSCM signal; finally, different polarization change speeds are simulated in the transmitter DSP; DSP at the receiving end: first, use the estimated XY delay value of the receiving end to compensate the XY delay of the receiving signal; then use the method for connecting polarization crosstalk as described in any one of claims 1 to 6 to realize frequency offset compensation and polarization crosstalk elimination, and then demultiplex the DSCM signal by subcarriers; for each demultiplexed subcarrier signal, perform matched filtering, dispersion compensation, clock recovery, synchronization, simplified equalization and phase noise compensation in turn, and finally calculate the bit error rate.

9. The DSCM system based on single pilot polarization crosstalk resolution according to claim 8, characterized in that: When performing a delay test at the receiving end, for the transmitting end DSP, a single-frequency cosine pilot signal on the X polarization is expressed as: A is the amplitude of the pilot signal, f2 is the frequency of the pilot signal; after considering the delay and polarization rotation at the transmitting end, the pilot signal is expressed as: Among them, τ TXI is the I-path delay of the X polarization at the transmitting end, a and α are the parameters J XX The amplitude and phase of (t), b and β are the parameters J XY (t) is the amplitude and phase of the 10. The DSCM system based on single pilot polarization crosstalk resolution according to claim 9, characterized in that: When testing the receiving end delay, for the receiving end DSP: after considering the receiving end delay, the received pilot signal is expressed as: in, and are the received X-polarized I and Q signals, and Y-polarized I and Q signals, τ RXI , τ RXQ , τ RYI and τ RYQ They are the X-polarization I-path delay, X-polarization Q-path delay, Y-polarization I-path delay, and Y-polarization Q-path delay at the receiving end respectively; Delay the receiving end XY by τ RXY Defined as: RXY =τ RYI -τ RXI ; The formula and formula Transform to the frequency domain: The XY delay at the receiving end is calculated as follows: in, and Respectively represent the received X polarization I signal The pilot components at the positive and negative half frequencies, and Respectively represent the received Y polarization I signal Pilot components at positive and negative half frequencies, * represents conjugation operation.

Images