Signal processing circuits, optical receivers, and optical transmission systems

Abstract

The ability to perform initial pull-in of adaptive equalization processing even with a small number of TS symbols in the frame. [Solution] The signal processing circuit 103 includes a tap coefficient adaptive control processing unit 141 that processes a received signal including a training sequence TS and a pilot symbol PS, and performs tap coefficient update processing using information of multiple symbols including PS; an FIR filter 132 that performs adaptive equalization processing using the updated tap coefficients obtained by the tap coefficient adaptive control processing unit 141; a TS synchronization detection unit 122 that performs TS synchronization detection from the TS information of the received signal based on the output data of the FIR filter 132; and pilot extraction processing units 152 and 153 that extract multiple symbols including PS included in the received signal based on the TS synchronization information obtained from the TS synchronization detection unit 122. By performing initial pull-in of adaptive equalization in the FIR filter 132, the tap coefficients can be obtained even in frames with a small number of TS symbols.

Description

Technical Field

[0001] The present invention relates to a signal processing circuit, an optical receiver, and an optical transmission system.

Background Art

[0002] In optical communication transmission, transmission path distortions such as polarization fluctuations and polarization mode dispersion accompanied by time variations are adaptively compensated by an adaptive equalizer (AEQ: Adaptive EQualizer) provided in an optical transmission device. When the transmission path characteristics of the transmission path are unknown, at the initial startup of the optical transmission device (AEQ), it is necessary to perform frame synchronization and generate the initial tap coefficients of the AEQ. For example, by using a method using a specific known signal, frame synchronization is performed from the information of a training sequence (TS), and then a periodically embedded pilot symbol (PS) is extracted. Utilizing that the PS is a known signal, adaptive equalization processing such as polarization mode dispersion (PMD: Polarization Mode Dispersion) compensation and polarization rotation compensation is performed.

[0003] As a conventional technique, there is one that estimates the frequency offset and the amount of wavelength dispersion in an optical receiver and estimates the timing of a signal based on the spectrum shift of a known BPSK signal included in a transmission signal (see, for example, Patent Documents 1 and 2 below).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] In optical digital coherent transmission, the baud rate used is increasing to achieve higher capacity, requiring either an increase in the number of data symbols within a frame or a decrease in the number of TS symbols. For example, while the existing DP-16QAM has 192 TS symbols, the new OIF (Optical Internetworking Forum) 800ZR communication standard has only 11 TS symbols. When using TS as a specific known signal, as in conventional technology, 11 TS symbols is too short, making accurate frame synchronization difficult and PS acquisition challenging.

[0006] In one aspect, the present invention aims to enable the initial pull-in of adaptive equalization processing even when the number of TS symbols in a frame is small. [Means for solving the problem]

[0007] According to one aspect of the present invention, the signal processing circuit is a signal processing circuit that processes a received signal including a training sequence and pilot symbols, and comprises: a tap coefficient adaptive control processing unit that performs tap coefficient update processing using information of a plurality of symbols including pilot symbols; an FIR (Finite Impulse Response) filter that performs adaptive equalization processing using the updated tap coefficients obtained by the tap coefficient adaptive control processing unit; a TS (Training Sequence) synchronization detection unit that performs TS synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter; and a pilot extraction processing unit that extracts a plurality of symbols including pilot symbols included in the received signal based on the TS synchronization information obtained from the TS synchronization detection unit, wherein the initial pull-in of adaptive equalization in the FIR filter is required. [Effects of the Invention]

[0008] According to one aspect of the present invention, the initial pull-in of adaptive equalization processing can be performed even if the number of TS symbols in the frame is small. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a diagram showing a signal processing circuit according to Embodiment 1. [Figure 2] Figure 2 shows a signal processing circuit as an example. [Figure 3] Figure 3 is a flowchart showing an example of signal processing using a reference signal processing circuit. [Figure 4A] Figure 4A shows an example of an existing transmission frame. [Figure 4B] Figure 4B shows an example of a new transmission frame. [Figure 5] Figure 5 is a flowchart showing an example of signal processing by the signal processing circuit of Embodiment 1. [Figure 6] Figure 6 shows a signal processing circuit according to Embodiment 2. [Figure 7A] Figure 7A is an explanatory diagram for the regeneration of tap coefficients due to equivalence convergence and deequilibrium. (Part 1) [Figure 7B] Figure 7B is an explanatory diagram for the regeneration of tap coefficients due to equivalence convergence and deequilibrium. (Part 2) [Figure 8] Figure 8 is a flowchart showing an example of signal processing by the signal processing circuit of Embodiment 2. [Figure 9] Figure 9 shows the steady-state function of the signal processing circuit according to Embodiment 2. [Figure 10] Figure 10 is a flowchart showing an example of steady-state signal processing in the signal processing circuit according to Embodiment 2. [Figure 11] Figure 11 shows an example of the configuration of an optical receiver. [Figure 12] Figure 12 shows an example of the configuration of an optical transmission system. [Modes for carrying out the invention]

[0010] Referring to the drawings below, embodiments of the disclosed signal processing circuit, optical receiver, and optical transmission system will be described in detail. The signal processing circuit of the embodiment is applied to optical digital coherent optical transmission and has an AEQ to adaptively compensate for transmission path distortions such as polarization fluctuations and polarization mode dispersion that change over time. In the embodiment, at the time of startup of the AEQ when the transmission path characteristics of the transmission path are unknown, the initial pull-in of the adaptive equalization process by the AEQ can be performed even if the number of TS symbols in the frame is small.

[0011] In the embodiment, for example, quadrature amplitude modulation (QAM) is used for optical communication transmission, and probabilistic constellation shaping (PCS) is used for the transmission signal. In PCS, the values of the bit sequence are converted to increase the usage frequency of the inner symbol points closer to the center of the constellation, forming the probability distribution of the mapping. By increasing the usage frequency of the inner symbol points with lower power, when the same average power is achieved, the Euclidean distance can be ensured compared to the prior art, the noise tolerance is improved, and efficient data transmission can be performed.

[0012] (Embodiment 1) FIG. 1 is a diagram showing a signal processing circuit according to Embodiment 1. The signal processing circuit of Embodiment 1 corresponds to the receiving DSP 103 (Digital Signal Processor) shown in FIG. 1. In the optical transmission system, the optical transmission device on the transmission side transmits an optical signal via the optical transmission path L, and the optical transmission device (optical receiver) R on the receiving side receives it. The optical transmission device R on the receiving side includes an O / E conversion unit 101, an AD converter (ADC) 102, and a receiving DSP 103. The O / E conversion unit 101 photoelectrically converts the received optical signal and outputs it to the ADC 102. The ADC 102 performs analog-digital conversion on the electrical signal (received signal) after photoelectric conversion and outputs it to the receiving DSP 103.

[0013] The receiving DSP 103 performs data processing on the received signal and includes a FEQ (Fixed EQualizer) 111, an adaptive equalization processing unit 112, a CPR / FOC 113, and a control unit 114. CPR stands for Carrier Phase Recovery, and FOC stands for Frequency Offset Compensation.

[0014] The FEQ 111 performs dispersion compensation, linear compensation, non-linear compensation, etc. The adaptive equalization processing unit 112 performs compensation for the DGD (Differential Group Delay) of two orthogonal polarization states, residual dispersion compensation, etc. The CPR / FOC 113 compensates for the deviation between the carrier frequency of the received optical signal and the frequency of the locally generated light, and restores the phase of the carrier. The control unit 114 controls the FEQ 111, the adaptive equalization processing unit 112, and the CPR / FOC 113.

[0015] The adaptive equalization processing unit 112 includes an AEQ (Adaptive EQualizer) 121 and a TS synchronization detection unit 122. The AEQ 121 includes a tap coefficient update unit 131 and a FIR (Finite Impulse Response) filter 132.

[0016] Based on the output data of the FIR filter 132, the TS synchronization detection unit 122 performs frame synchronization based on the information of the training sequence (TS) of the received signal and outputs the TS synchronization information to the tap coefficient update unit 131 of the AEQ 121.

[0017] The tap coefficient update unit 131 of the AEQ 121 includes a tap coefficient adaptive control processing unit 141. The tap coefficient adaptive control processing unit 141 performs tap coefficient adaptive control by the blind CMA (Constant Modulus Algorithm). In the blind CMA, as an adaptive algorithm, adaptive equalization processing is performed by the CMA of blind equalization from unknown signal symbols.

[0018] In Embodiment 1, the adaptive equalization processing unit 112 includes an initial tap coefficient setting unit 151, and the tap coefficient adaptive control processing unit 141 of AEQ 121 performs tap coefficient adaptive control processing using blind CMA with only pilot symbols (PS). For this reason, the tap coefficient adaptive control processing unit 141 of AEQ 121 has a pilot extraction processing unit 1 (152) and a pilot extraction processing unit 2 (153) before and after it.

[0019] The pilot extraction processing unit 1 (152) extracts peripheral data including pilot symbols (PS) from the input data to the FIR filter 132 and outputs it to the tap coefficient adaptive control processing unit 141. The pilot extraction processing unit 2 (153) extracts pilot symbols (PS) from the output data of the FIR filter 132 and outputs it to the tap coefficient adaptive control processing unit 141.

[0020] The AEQ121 receives input data and output data from the FIR filter 132 in the tap coefficient adaptive control processing unit 141 of the tap coefficient update unit 131, calculates updated tap coefficients that follow the fluctuations in transmission line characteristics, and sets them in the FIR filter 132. The updated tap coefficients have separate tap coefficients for orthogonal H and V polarizations.

[0021] The tap coefficient adaptive control processing unit 141 of Embodiment 1 performs tap coefficient adaptive control processing using blind CMA with only pilot symbols (PS) as the basis. The tap coefficient adaptive control processing unit 141 performs pilot extraction on the input and output data of the FIR filter 132 and passes the extracted pilot symbols (the input includes surrounding data of the pilot) to the tap coefficient adaptive control processing.

[0022] The signal processing circuit of Embodiment 1 performs the following signal processing during the initial pull-in of the adaptive equalization process. The control unit 114 controls the following signal processing.

[0023] 1. (First frame synchronization processing) During the initial pull-in, such as the startup of the signal processing circuit (AEQ121), TS synchronization detection is performed by the TS synchronization detection unit 122 before the adaptive equalization processing is performed by AEQ121. In this state, the TS synchronization accuracy is rough.

[0024] In Embodiment 1, the pilot extraction processing units 1 and 2 (152 and 153) extract multiple symbols in the vicinity of the PS symbol included in the frame of the received signal as the pilot extraction range W (see Figure 4B) during TS detection. The pilot extraction range W is set to correspond to the number of symbols equal to the tap coefficient of the FIR filter 132. For example, the number of symbols in the pilot extraction range W is an odd number of symbols including the tap center plus several symbols before and after it, resulting in 31 symbols (1 symbol) + 15 symbols before and after it.

[0025] 2. Next, the AEQ121 sets the initial tap coefficient set in the initial tap coefficient setting unit 151 to the tap coefficient adaptive control processing unit 141.

[0026] 3. Next, the tap coefficient adaptive control processing unit 141 determines updated tap coefficients using a blind CMA with only PS based on the set initial tap coefficients, sets the updated tap coefficients in the FIR filter 132, and performs adaptive equalization processing such as PMD compensation and polarization rotation compensation.

[0027] 4. (Second frame synchronization process) Next, the TS synchronization detection unit 122 performs TS synchronization detection and determines the TS synchronization information. 5. Next, the AEQ121 performs tap coefficient correction based on the TS synchronization information, obtains the corrected updated tap coefficients, sets the updated tap coefficients in the FIR filter 132, and performs adaptive equalization processing such as PMD compensation and polarization rotation compensation. For example, if the AEQ121 determines that synchronization is achieved in the judgment process based on the TS synchronization information, it performs the tap coefficient update process.

[0028] As described above, the initial pull-in process, such as the startup of the signal processing circuit (AEQ121), is completed even though the transmission characteristics of the optical transmission path L are unknown.

[0029] (Example assignment) Next, I will explain the assignment using an example.

[0030] Figure 2 shows a signal processing circuit based on an example. Figure 2 shows an example configuration of an entire optical transmission system that performs frame synchronization and adaptive equalization processing using known signals TS and PS. The transmitting optical transmission device (optical transmitter) T transmits an optical signal via the optical transmission path L, and the receiving optical transmission device (optical receiver) R receives the optical signal via the optical transmission path L.

[0031] The optical transmitter T includes a transmitting DSP 201, a digital-to-analog converter (DAC) 202, and an E / O conversion unit 203. The transmitting DSP 201 includes a PCS unit 211 that converts input data into PCS, a bit / symbol conversion unit 212 that maps the bits of the PCS-converted input data to symbols, and a transmitting frame generation unit 213 that generates a transmission frame from the input data after symbol conversion. The DAC 202 converts the input data from digital to analog and outputs it to the E / O conversion unit 203. The E / O conversion unit 203 converts the input data into an optical signal and sends it to the optical transmission path L.

[0032] The optical receiver R includes an O / E conversion unit 221, an ADC 222, and a receiving DSP 223. The O / E conversion unit 221 converts the received optical signal photoelectrically and outputs it to the ADC 222. The ADC 222 outputs the electrical signal (received signal) after photoelectric conversion to the receiving DSP 223.

[0033] The receiving DSP223 processes the received signal data and includes an FEQ231, an adaptive equalization processing unit232, and a CPR / FOC233. The adaptive equalization processing unit232 in the reference example includes an AEQ241, a frame synchronization and initial tap coefficient generation unit242.

[0034] During the initial startup of the AEQ241, it is necessary to perform frame synchronization and generate the initial tap coefficients for the AEQ241. When using a specific known signal as shown in the example, frame synchronization is performed from the TS information, and then the periodically embedded PS is extracted. By utilizing the fact that the PS is a known signal, adaptive equalization processing such as PMD compensation and polarization rotation compensation is performed.

[0035] Figure 3 is a flowchart showing an example of signal processing by a reference signal processing circuit. The initial pull-in process of adaptive equalization performed by the adaptive equalization processing unit 232 of the signal processing circuit (optical receiver R) shown in Figure 2 during the startup of the signal processing circuit (AEQ241) will be explained below.

[0036] First, the TS synchronization process roughly estimates the starting position of the TS (Step S301). Next, the TS signal contained in the received frame is used to perform MMSE (Minimum Mean Square Error) processing to generate the tap coefficients of the AEQ241 (Step S302).

[0037] Next, a centroid correction process is performed on the generated tap coefficients (step S303). Then, a compensation process is performed using the tap coefficients after the centroid correction process, and the TS starting position is estimated with high accuracy in the TS synchronization process, and the PS position is detected (step S304). With these steps completed, the frame synchronization and initial tap coefficient generation processes are finished, and the initial pull-in process of the signal processing circuit (AEQ241) is completed.

[0038] Figure 4A shows an example of an existing transmission frame. The transmitting optical transmission device in the optical transmission system generates the transmission frame shown in Figure 4A. Figure 4A shows a transmission frame used in DP-16QAM. The transmitting optical transmission device inserts known TS and PS at regular intervals into the input data symbols. Here, the number of TS symbols is 192.

[0039] Figure 4B shows an example of a new transmission frame. Figure 4B shows an example of a transmission frame using the new OIF 800ZR communication standard, and the number of TS symbols has been increased to 11 in order to accommodate the increased bitrate and thus the number of data symbols in the frame. The number of TS symbols in Figure 4B, 11, is a significant decrease compared to the number of TS symbols in Figure 4A, which is 192.

[0040] In the example shown in Figure 4A, if the number of symbols in the TS is large, frame synchronization using known signals can be performed. However, as shown in Figure 4B, if the number of symbols in the TS decreases, frame synchronization cannot be performed accurately, and the PS cannot be acquired.

[0041] Here, it is conceivable to use a blind CMA that does not utilize known signals.

[0042] However, when using blind CMA, in the transmission frame shown in Figure 4B, the application of PCS to the signal symbols leads to a concentration of use of closely spaced symbol points near the center of the constellation. In this case, the difference in intensity between the symbols used becomes so small that CMA calculations may not be possible.

[0043] Furthermore, when simply using blind CMA during startup of the signal processing circuit (AEQ241), in the transmission frame shown in Figure 4B, there is an error in TS synchronization detection due to the shortened number of TS symbols (interval), and PS symbols cannot be accurately detected. In the example, if the extracted symbols do not include PS symbols and only data symbols are extracted, the CMA calculation cannot be performed because the symbols are shaped by the PCS.

[0044] In response to the above-mentioned problems in the reference example, Embodiment 1 performs the processes described in steps 1 to 5 above. In Embodiment 1, during TS detection, the pilot extraction processing units 1 and 2 (152 and 153) extract multiple symbols, including PS symbols, that are included in the frame of the received signal. The pilot extraction range W to be extracted is shown in Figure 4B. The tap coefficient adaptive control processing unit 141 of the AEQ241 then performs tap coefficient adaptive control processing using a blind CMA that uses only PS. The CMA in the reference example differs from Embodiment 1 in that it performs tap coefficient adaptive control processing by referring to all data, including TS and PS. The pilot extraction processing units 1 and 2 (152 and 153) extract PS on the input and output sides of the received signal to the FIR filter 132, thereby updating the tap coefficients in the tap coefficient adaptive control processing unit 141.

[0045] The pilot extraction range W has a predetermined range, and since PS is included in the number of extracted symbols, CMA calculation becomes possible based on PS. Thus, according to Embodiment 1, even if the number of TS symbols included in the received frame decreases, the initial pull-in of the adaptive equalization process can be performed.

[0046] (Signal processing example of Embodiment 1) Figure 5 is a flowchart showing an example of signal processing by the signal processing circuit of Embodiment 1. The initial pull-in process for adaptive equalization at startup of the signal processing circuit (optical receiver R) shown in Figure 1 will be explained below. The following processes are controlled by the control unit 114. In Embodiment 1 as well, the tap coefficient update process is performed for TS and PS, which are known signals.

[0047] During the initial pull-in, TS detection is performed by the TS synchronization detection unit 122 before the adaptive equalization processing by AEQ 121 (step S501). At this time, multiple symbols including PS symbols are extracted within the pilot extraction range W. The accuracy of TS synchronization detection in step S501 is coarse. Next, the initial tap coefficient is set in the tap coefficient adaptive control processing unit 141 (step S502).

[0048] After setting the initial tap coefficients, the tap coefficient adaptive control processing unit 141 performs tap coefficient adaptive control processing using blind CMA with only PS (step S503). In this case, the tap coefficient adaptive control processing unit 141 uses PS included in the pilot extraction range W. As a result, the updated tap coefficients are set in the FIR filter 132.

[0049] Next, the TS synchronization detection unit 122 performs TS synchronization detection and determines the TS synchronization information (step S504). After this, the tap coefficient adaptive control processing unit 141 corrects the tap coefficients based on the TS synchronization information (step S505). With this TS synchronization detection, TS synchronization is performed with high accuracy, and the tap coefficients are corrected to suit the transmission line characteristics. As a result, the initial pull-in at the start-up of the signal processing circuit (AEQ121) is completed, and the signal processing circuit moves to steady-state processing during operation.

[0050] During steady-state processing, the control unit 114 stops TS synchronization detection in the TS synchronization detection unit 122. Furthermore, the control unit 114 switches the tap coefficient update unit 131 to tap coefficient adaptive control processing using LMS (Least Mean Square) with only PS.

[0051] According to Embodiment 1, TS detection is performed before AEQ equalization to perform a rough TS detection. At this time, multiple symbols including PS symbols are extracted within the pilot extraction range W. Then, after setting the initial tap coefficients, a tap coefficient update process is performed using blind CMA with only PS symbols, enabling accurate TS synchronization detection and allowing the determination of tap coefficients corresponding to the transmission line characteristics. As a result, even if the number of TS symbols in a frame decreases, the initial pull-in of the applied equalization process at the startup of the signal processing circuit can be performed.

[0052] (Embodiment 2) Figure 6 shows a signal processing circuit according to Embodiment 2. It shows another configuration example of the signal processing circuit of Embodiment 1 (adaptive equalization processing unit 112 in the receiving DSP 103) described in Figure 1, and the same reference numerals are used for the same components as in Figure 1. Embodiment 2 resolves the problem that occurs when equivalence convergence occurs during TS synchronization detection (equivalent to step S504 in Figure 5). In addition, a function is added to correct the center shift relative to the tap center for the centroid of each orthogonal polarization (H,V) of the updated tap coefficient.

[0053] As seen in the configuration in Figure 6, the difference from Figure 1 is that the tap coefficient update unit 131 is equipped with a centroid correction processing unit 601. The updated tap coefficients obtained by the tap coefficient adaptive control processing unit 141 are input to the centroid correction processing unit 601. The centroid correction processing unit 601 corrects the center shift relative to the tap center for the centroid of each orthogonal polarization (H,V) of the updated tap coefficients. The centroid correction processing unit 601 then outputs the corrected tap coefficients to the tap coefficient adaptive control processing unit 141.

[0054] If equivalence convergence occurs during TS synchronization detection, a blind CMA using only PS will not yield the correct output from the FIR filter 132, and the TS synchronization detection unit 122 will not function correctly. During equivalence convergence, unless polarization rotation or other changes occur in the input data to the AEQ 121, the equivalence convergence will not be released, and the initial pull-in cannot be completed.

[0055] In Embodiment 2, the control unit 114 determines whether or not equivalence convergence has occurred based on the TS synchronization detection performed by the TS synchronization detection unit 122. When equivalence convergence is determined, the control unit 114 stops the tap coefficient update performed by the tap coefficient adaptive control processing unit 141. The control unit 114 also completes the TS synchronization detection process by feeding back the TS synchronization information (synchronous / asynchronous information, symbol shift information during asynchronous operation, etc.) as information for the next TS synchronization detection.

[0056] Figures 7A and 7B illustrate the regeneration of tap coefficients by equivalence convergence and deequilibrium. In Figures 7A and 7B (a) to (d), the horizontal axis represents the tap number of the FIR filter 132, and the vertical axis represents the amplitude of each polarization (X-axis polarization tap coefficients: HH, VH, Y-axis polarization tap coefficients: HV, VV).

[0057] Equivalence convergence is a state in a polarization-multiplexed coherent optical receiver where multiple polarizations converge to the same source. In the example shown in Figure 7A(a), HH and HV have the same tap number 7, and VH and VV have the same tap number 11 (before equivalence convergence is released). Figure 7A(b) shows only the H-side coefficients (HH, VH) from Figure 7A(a), and Figure 7B(c) shows only the V-side coefficients (HV, VV) from Figure 7A(a).

[0058] To prevent equivalence convergence, the tap coefficients of one polarization are regenerated based on the tap coefficient output of the other polarization (equivalence convergence is disabled). For example, when generating the V-side coefficient from the H-side coefficient shown in Figure 7A(b), the H-side coefficient is folded back at the tap center (tap number 9, the center of tap numbers 7 and 11), and the folded coefficient is obtained by complex conjugate substitution as the V-side coefficient (Figure 7B(d), after equivalence convergence is disabled). In this case, Figure 7A(b) is used for the H-side coefficient and Figure 7B(d) is used for the V-side coefficient (Figure 7B(c) is not used as the V-side coefficient).

[0059] Furthermore, when the control unit 114 determines that equivalence convergence has occurred, it stops the tap coefficient update by the tap coefficient adaptive control processing unit 141. The reason for stopping the tap coefficient update is that when equivalence convergence is determined by the TS synchronization information determination, it is necessary to stop the tap coefficient update and perform TS synchronization detection for the following reasons.

[0060] For example, suppose equivalence convergence occurs during blind CMA using only PS, and both the H-side and V-side output data are H-side. In this case, the H-side receiving pilot position can be detected. To release equivalence convergence, the V-side coefficient is calculated from the H-side tap coefficient. We would like to update the coefficient using this regenerated V-side tap coefficient, but since the expected pilot position on the V-side is unknown, the coefficient update fails. Therefore, in other embodiments, the control unit 114 performs TS synchronization detection without updating the tap coefficient after releasing equivalence convergence, calculates the accurate pilot positions on the H-side and V-side, and then resumes updating the tap coefficient.

[0061] (Signal processing example of Embodiment 2) Figure 8 is a flowchart showing an example of signal processing by the signal processing circuit of Embodiment 2. The initial pull-in process for adaptive equalization during startup of the signal processing circuit (optical receiver R) shown in Figure 6 will be explained below. The following processes are controlled by the control unit 114.

[0062] During the initial pull-in, TS detection is performed by the TS synchronization detection unit 122 before the adaptive equalization processing by AEQ 121 (step S801). At this time, multiple symbols including PS symbols are extracted within the pilot extraction range W. The accuracy of TS synchronization detection in step S801 is coarse. Next, the initial tap coefficient is set in the tap coefficient adaptive control processing unit 141 (step S802).

[0063] After setting the initial tap coefficients, the tap coefficient adaptive control processing unit 141 performs tap coefficient adaptive control processing using blind CMA with only PS (step S803). In this case, the tap coefficient adaptive control processing unit 141 uses PS included in the pilot extraction range W. As a result, the updated tap coefficients are set in the FIR filter 132.

[0064] Furthermore, the center of gravity correction processing unit 601 performs a center of gravity correction process (step S804). The center of gravity correction processing unit 601 corrects the tap center misalignment for the updated tap coefficients obtained by the tap coefficient adaptive control processing unit 141, and outputs the corrected tap coefficients to the tap coefficient adaptive control processing unit 141.

[0065] Next, the TS synchronization detection unit 122 performs TS synchronization detection (step S805) and determines the TS synchronization information (step S806). If the determination result in step S806 is "equivalent convergence", the process proceeds to step S807; if the determination result is "TS synchronization", the process proceeds to step S809.

[0066] In step S807, the control unit 114 performs the process of disabling equivalence convergence (see Figures 7A and 7B) (step S807). Then, the control unit 114 stops updating the tap coefficients (step S808) and returns to the process of step S805. By returning to the process of step S805, equivalence convergence disabling and tap coefficient updating can be resumed based on the time-changing subsequent received input data.

[0067] In step S809, the tap coefficient adaptive control processing unit 141 receives symbol shift information indicated by the TS synchronization information from the TS synchronization detection unit 122, and performs tap coefficient correction to suppress the symbol difference between polarizations based on this shift information (step S809). The tap coefficients are corrected to be accurately TS synchronized and adapted to the transmission line characteristics by this TS synchronization detection. Based on the corrected tap coefficients, the tap coefficient adaptive control processing unit 141 determines the updated tap coefficients to be set in the FIR filter 132. If the tap coefficient update had been stopped due to equivalence convergence, the tap coefficient update is restarted (step S810). With the above steps completed, the initial pull-in at the start-up of the signal processing circuit (AEQ 121) is completed, and the signal processing circuit moves to steady-state processing during operation.

[0068] (Example of steady-state functionality) Figure 9 shows the steady-state function of the signal processing circuit according to Embodiment 2. It shows the function after the completion of the initial pull-in process during startup, as explained using Figure 6. As shown in Figure 9, during steady-state operation, the control unit 114 stops the functions of the TS synchronization detection unit 122 and the centroid correction processing unit 601. The control unit 114 also switches the tap coefficient adaptive control processing unit 141 to tap coefficient adaptive control processing using LMS with only PS.

[0069] Figure 10 is a flowchart showing an example of steady-state signal processing in the signal processing circuit according to Embodiment 2. After the processing in Figure 8 is completed (after initial startup is complete), the control unit 114 performs the steady-state processing shown in Figure 10.

[0070] First, the control unit 114 stops the center of gravity correction processing by the center of gravity correction processing unit 601 and the TS synchronization detection by the TS synchronization detection unit 122 (step S1001). Next, the control unit 114 switches the tap coefficient adaptive control processing unit 141 (step S1002). At this time, the control unit 114 causes the tap coefficient adaptive control processing unit 141 to perform tap coefficient adaptive control processing using LMS with only PS (step S1003). As a result, during operation, the steady-state processing shown in Figure 10 is performed.

[0071] According to Embodiment 2, TS detection is performed before AEQ equalization to perform a rough TS detection. At this time, multiple symbols including PS symbols are extracted within the pilot extraction range W. Then, after setting the initial tap coefficients, a tap coefficient update process is performed using blind CMA with only PS, enabling accurate TS synchronization detection and allowing the determination of tap coefficients corresponding to the transmission line characteristics. In addition, in Embodiment 2, the centroid shift of the tap coefficients is corrected, and the stopping of tap coefficient updates based on the determination of whether or not equivalence convergence has occurred is controlled, enabling the initial engagement of the applied equalization process. As a result, even if the number of TS symbols in the frame decreases, the initial engagement of the applied equalization process can be performed when the signal processing circuit is started up.

[0072] (Example configuration of optical transmission equipment: optical receiver) Figure 11 shows an example of the configuration of an optical receiver. The signal processing circuit described above can be applied to the optical receiver R located on the receiving side of the optical transmission device shown in Figure 11. In Figure 11, the same reference numerals are used for functions that are the same as those in Figure 1. The signal processing circuit shown in Figure 1 corresponds to the function of the adaptive equalization processing unit (AEQ) 112 shown in Figure 11.

[0073] As shown in Figure 11, the optical receiver R receives the coherent detection result (analog electrical signal) of the received signal as input to the ADC 102, which then converts it to digital and outputs it. The dispersion compensation unit 1101 compensates for waveform distortion due to dispersion such as polarization mode dispersion (PMD). The sampling phase detection and adjustment unit 1102 adjusts the phase position when sampling digital data and outputs it to the adaptive equalization processing unit 112. The adaptive equalization processing unit 112 performs the adaptive equalization processing by CMA described above.

[0074] The synchronization detection / frequency offset monitor / compensation unit 1103 includes the functions of the TS synchronization detection unit 122 and FOC described above, and detects and compensates for the difference (frequency offset) between the carrier frequency of the received signal and the frequency of the station emission. The carrier phase restoration unit 1104 includes the functions of CPR described above, and restores the phase of the carrier wave. For example, the amount of frequency offset can be detected by a well-known method, and the frequency offset is compensated by reversing the rotation of the constellation at a phase rotation speed corresponding to the detected frequency error.

[0075] The IQ distortion compensation unit 1105 compensates for IQ distortion (such as IQ imbalance or IQ incompleteness) that occurs within the optical receiver R. The received frame synchronization unit 1106 synchronizes the received signal frame. The error correction decoding unit 1107 corrects bit errors using an error correction code from an FEC (Forward Error Correction code) decoder, decodes the received signal, and outputs it.

[0076] (Example of an optical transmission system configuration) Figure 12 shows an example of the configuration of an optical transmission system. The signal processing circuits described in Embodiments 1 and 2 above were explained using an optical receiver located on the receiving side of the optical transmission device as an example. As shown in Figure 12, optical transceivers 1 and 2 (1201 and 1202) are located at both ends of the optical transmission path L as optical transmission devices.

[0077] Optical transceiver 1 (1201) transmits an optical signal via the downstream optical transmission path L1, and optical transceiver 2 (1202) receives the optical signal via the receiving side R. On the other hand, optical transceiver 2 (1202) transmits an optical signal via the upstream optical transmission path L2, and optical transceiver 1 (1201) receives the optical signal via the receiving side R.

[0078] To describe the configuration of the transmitting side T of the optical transceiver 1 (1201), the framer 1211 frames the input signal from the client on the optical transceiver 1 (1201) side, and the transmitting DSP of the digital signal processing unit 1212, which consists of a DSP, processes the data of the transmitted signal. In the optical transceiver unit 1213, the DAC 1221 converts the transmitted signal from digital to analog, the E / O conversion unit 1222 converts the electrical signal to an optical signal, and the optical signal is sent to the optical transmission path L1. The light source 1223 is a local light emitter for generating the optical signal to be transmitted, and the optical signal is transmitted after being subjected to a predetermined optical modulation.

[0079] The configuration of the receiving side R of the optical transceiver 2 (1202) is as follows: The O / E conversion unit 1241 of the optical transceiver unit 1231 converts the optical signal into an electrical signal, and the ADC 1242 converts the received signal from analog to digital. The light source 1243 is a local light emitter for demodulating the received optical signal. The receiving DSP of the digital signal processing unit 1232, which consists of a DSP, performs reception processing. This receiving DSP corresponds to the receiving DSP 103 in Figure 1 described above and includes the functions of the adaptive equalization processing unit 112. The output of the receiving DSP is framed via the framer 1233 and output as an output signal to the client on the optical transceiver 2 (1202) side.

[0080] The configurations of the receiving side R of optical transceiver 1 (1201) are the same as those of the receiving side R of optical transceiver 2 (1202). Similarly, the configurations of the transmitting side T of optical transceiver 2 (1202) are the same as those of the transmitting side T of optical transceiver 1 (1201). In Figure 12, identical components are denoted by the same reference numerals.

[0081] As shown in Figure 12, each optical transmission device (optical transceiver) located at both ends of the optical transmission path L has the functions of an optical transmitter T and an optical receiver R. The adaptive equalization processing unit (AEQ) 112 described in the above embodiment can be applied to the optical receiver R.

[0082] Incidentally, the adaptive equalization processing unit 112 mentioned above requires high-speed signal processing, and currently uses a dedicated DSP. However, it is not limited to this; the adaptive equalization processing unit 112 can also be configured using an ASIC or FPGA that supports high-speed processing. Furthermore, a high-speed CPU could be used as the control unit 114 of the adaptive equalization processing unit 112 in the future. ASIC stands for Application Specific Integrated Circuit, and FPGA stands for Field Programmable Gate Array.

[0083] The signal processing circuit of the embodiment described above is a signal processing circuit that processes a received signal including a training sequence and a pilot symbol, and comprises: a tap coefficient adaptive control processing unit that performs tap coefficient update processing using information of multiple symbols including a pilot symbol; an FIR filter that performs adaptive equalization processing using the updated tap coefficient obtained by the tap coefficient adaptive control processing unit; a TS synchronization detection unit that performs TS synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter; and a pilot extraction processing unit that extracts multiple symbols including the pilot symbol included in the received signal based on the TS synchronization information obtained from the TS synchronization detection unit, thereby performing initial pull-in of adaptive equalization in the FIR filter. In this way, rough TS synchronization is performed before the equalization control processing, and multiple symbols including the PS symbol are extracted in the pilot extraction range W. Then, by updating the first updated tap coefficient using only the PS, it becomes possible to obtain the updated tap coefficient even when receiving frames with a small number of TS symbols due to high bitrates, at the initial startup of the equalization control processing when the transmission path characteristics are unknown.

[0084] Furthermore, in the signal processing circuit of this embodiment, after the tap coefficient update process, the TS synchronization detection unit performs TS synchronization detection based on the training sequence of the received signal, and the tap coefficient adaptive control processing unit corrects the updated tap coefficients when the TS synchronization detection unit detects TS synchronization. As a result, even when the number of TS symbols in the frame is small, accurate TS synchronization can be achieved, tap coefficients corresponding to the transmission line characteristics can be determined, and the initial startup of the equalization control process can be completed.

[0085] Furthermore, the signal processing circuit of the embodiment further includes a centroid correction processing unit that corrects the center deviation of the updated tap coefficient with respect to the tap center, which is updated by the tap coefficient adaptive control processing unit. This allows the center deviation of the first updated tap coefficient to be corrected, enabling the initial startup of the equalization control process to be completed with greater accuracy.

[0086] Furthermore, in the signal processing circuit of this embodiment, the tap coefficient adaptive control processing unit stops updating the tap coefficients when the TS synchronization detection unit detects equivalence convergence. This enables tap coefficient adaptive control using only the PS to respond to equivalence convergence, and allows for the determination of accurate tap coefficients.

[0087] Furthermore, in the signal processing circuit of this embodiment, during steady-state operation after the initial pull-in, the TS synchronization detection unit stops processing, and the tap coefficient adaptive control processing unit switches to tap coefficient adaptive control processing using LMS with only pilot symbols. This allows for a smooth transition to steady-state processing after the initial startup of the equalization control processing is completed.

[0088] Furthermore, in the signal processing circuit of this embodiment, the tap coefficient adaptive control processing unit updates the tap coefficients based on a blind CMA using only the pilot symbols extracted by the pilot extraction processing unit. This enables adaptive equalization processing from unknown signal symbols, and allows initial pull-in of adaptive equalization processing even when the number of TS symbols in the received signal is small due to high rates.

[0089] Furthermore, the optical receiver of this embodiment includes an O / E conversion unit that converts the received optical signal into an electrical signal, an ADC that performs analog-to-digital conversion of the received signal after conversion by the O / E conversion unit, and the above-mentioned signal processing circuit that receives and processes the received signal output by the ADC, which includes a training sequence and pilot symbols. Thus, the signal processing circuit can be applied to an optical receiver that receives a received optical signal.

[0090] Furthermore, the optical transmission system of this embodiment includes an optical transmitter that transmits an optical signal including a training sequence and pilot symbols, and an optical receiver that receives an optical signal including a training sequence and pilot symbols, wherein the optical receiver has the above-described signal processing circuit that receives and processes the received signal including the training sequence and pilot symbols. Thus, the signal processing circuit can be applied to an optical transmission system that transmits and receives optical received signals.

[0091] With regard to the embodiments described above, the following additional information is disclosed.

[0092] (Note 1) In a signal processing circuit that processes a received signal including a training sequence and pilot symbols, A tap coefficient adaptive control processing unit performs tap coefficient update processing using information from multiple symbols, including pilot symbols. An FIR (Finite Impulse Response) filter performs adaptive equalization processing using the updated tap coefficients obtained by the aforementioned tap coefficient adaptive control processing unit, A TS synchronization detection unit performs TS (Training Sequence) synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter, The system includes a pilot extraction processing unit that extracts a plurality of symbols, including pilot symbols, included in the received signal, based on TS synchronization information obtained from the TS synchronization detection unit, The initial draw-in of adaptive equalization in the FIR filter is performed. A signal processing circuit characterized by the following features.

[0093] (Note 2) After the update process of the tap coefficient, The TS synchronization detection unit performs TS synchronization detection based on the training sequence of the received signal. The tap coefficient adaptive control processing unit corrects the updated tap coefficient when the TS synchronization detection unit detects TS synchronization. The signal processing circuit described in Appendix 1, characterized by the above.

[0094] (Note 3) The tap coefficient adaptive control processing unit further comprises a centroid correction processing unit that corrects the center deviation of the updated tap coefficient with respect to the tap center, which is updated by the tap coefficient adaptive control processing unit. The signal processing circuit described in Appendix 1 or 2, characterized by the above.

[0095] (Note 4) The tap coefficient adaptive control processing unit stops updating the tap coefficients when the TS synchronization detection unit detects equal convergence. The signal processing circuit described in Appendix 2, characterized by the above.

[0096] (Appendix 5) During steady-state operation after the initial pull-in, The TS synchronization detection unit stops processing, The signal processing circuit described in Appendix 2, characterized in that the tap coefficient adaptive control processing unit switches to tap coefficient adaptive control processing using LMS (Least Mean Square) with only pilot symbols.

[0097] (Note 6) The signal processing circuit according to Note 1, characterized in that the tap coefficient adaptive control processing unit updates the tap coefficients based on a blind CMA (Constant Modulus Algorithm) using only the pilot symbols extracted by the pilot extraction processing unit.

[0098] (Note 7) An O / E conversion unit that converts the received optical signal into an electrical signal, The O / E conversion unit includes an ADC that converts the received signal after conversion from analog to digital, The ADC includes a signal processing circuit that receives and processes the received signal, which includes a training sequence and pilot symbols, output by the ADC. The aforementioned signal processing circuit is A tap coefficient adaptive control processing unit performs tap coefficient update processing using information from multiple symbols, including pilot symbols. An FIR (Finite Impulse Response) filter performs adaptive equalization processing using the updated tap coefficients obtained by the aforementioned tap coefficient adaptive control processing unit, A TS synchronization detection unit performs TS (Training Sequence) synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter, The system includes a pilot extraction processing unit that extracts a plurality of symbols, including pilot symbols, included in the received signal, based on TS synchronization information obtained from the TS synchronization detection unit, The initial draw-in of adaptive equalization in the FIR filter is performed. An optical receiver characterized by the following features.

[0099] (Note 8) An optical transmitter that transmits an optical signal including a training sequence and pilot symbols, Includes an optical receiver that receives an optical signal containing a training sequence and pilot symbols, The optical receiver has a signal processing circuit that receives and processes a received signal including a training sequence and pilot symbols. The aforementioned signal processing circuit is A tap coefficient adaptive control processing unit performs tap coefficient update processing using information from multiple symbols, including pilot symbols. An FIR (Finite Impulse Response) filter performs adaptive equalization processing using the updated tap coefficients obtained by the aforementioned tap coefficient adaptive control processing unit, A TS synchronization detection unit performs TS (Training Sequence) synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter, The system includes a pilot extraction processing unit that extracts a plurality of symbols, including pilot symbols, included in the received signal, based on TS synchronization information obtained from the TS synchronization detection unit, The initial draw-in of adaptive equalization in the FIR filter is performed. An optical transmission system characterized by the following:

[0100] (Note 9) In Note 8, The optical transmission system is further characterized in that the optical transmitter transmits an optical signal that has been processed by PCS. [Explanation of Symbols]

[0101] 101 O / E conversion unit 102 ADC 103 Receiving DSP (Signal Processing Circuit) 111 FEQ 112 Adaptive Equalization Processing Unit 113 CPR / FOC 114 Control Unit 121 AEQ 122 TS Synchronization Detection Unit 131 Tap coefficient update section 132 FIR filter 141 Tap coefficient adaptive control processing unit 151 Initial tap coefficient setting section 152,153 Pilot extraction processing unit 601 Center of gravity correction processing unit 1101 Dispersion compensation section 1102 Sampling Phase Detection and Adjustment Unit 1103 Synchronization Detection / Frequency Offset Monitor / Compensation Unit 1104 Carrier phase restoration section 1105 IQ distortion compensation section 1106 Received frame synchronization section 1107 Error Correction and Decoding Unit 1211,1233 Framer 1212,1232 Digital signal processing unit 1213,1231 Optical Transceiver Unit 1223,1243 light source L(L1,L2) Optical transmission line PS Pilot Symbol R Optical Receiver T optical transmitter TS training sequence (training pattern) W Pilot extraction range

Claims

1. In a signal processing circuit that processes a received signal including a training sequence and pilot symbols, A tap coefficient adaptive control processing unit performs tap coefficient update processing using information from multiple symbols, including pilot symbols. An FIR (Finite Impulse Response) filter performs adaptive equalization processing using the updated tap coefficients obtained by the aforementioned tap coefficient adaptive control processing unit, A TS synchronization detection unit performs TS (Training Sequence) synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter, The system includes a pilot extraction processing unit that extracts a plurality of symbols, including pilot symbols, included in the received signal, based on TS synchronization information obtained from the TS synchronization detection unit, The initial draw-in of adaptive equalization in the FIR filter is performed. A signal processing circuit characterized by the following features.

2. After the update process of the tap coefficient, The TS synchronization detection unit performs TS synchronization detection based on the training sequence of the received signal. The tap coefficient adaptive control processing unit corrects the updated tap coefficient when the TS synchronization detection unit detects TS synchronization. The signal processing circuit according to feature 1.

3. The aforementioned tap coefficient adaptive control processing unit further comprises a centroid correction processing unit that corrects the center misalignment of the updated tap coefficient with respect to the tap center. The signal processing circuit according to claim 1 or 2.

4. The tap coefficient adaptive control processing unit stops updating the tap coefficients when the TS synchronization detection unit detects equal convergence. The signal processing circuit according to feature 2.

5. During the steady state after the initial pull-in, The TS synchronization detection unit stops processing. The signal processing circuit according to claim 2, characterized in that the tap coefficient adaptive control processing unit switches to tap coefficient adaptive control processing using LMS (Least Mean Square) with only pilot symbols.

6. The signal processing circuit according to claim 1, characterized in that the tap coefficient adaptive control processing unit updates the tap coefficients based on a blind CMA (Constant Modulus Algorithm) using only the pilot symbols extracted by the pilot extraction processing unit.

7. An O / E conversion unit that converts an optical received signal into an electrical signal, The O / E conversion unit performs analog-to-digital conversion of the received signal after conversion using an ADC, The ADC includes a signal processing circuit that receives and processes the received signal, which includes a training sequence and pilot symbols, output by the ADC. The aforementioned signal processing circuit is A tap coefficient adaptive control processing unit performs tap coefficient update processing using information from multiple symbols, including pilot symbols. An FIR (Finite Impulse Response) filter performs adaptive equalization processing using the updated tap coefficients obtained by the aforementioned tap coefficient adaptive control processing unit, A TS synchronization detection unit performs TS (Training Sequence) synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter, The system includes a pilot extraction processing unit that extracts a plurality of symbols, including pilot symbols, included in the received signal, based on TS synchronization information obtained from the TS synchronization detection unit, The initial draw-in of adaptive equalization in the FIR filter is performed. An optical receiver characterized by the following features.

8. The system includes an optical transmitter that transmits an optical signal containing a training sequence and pilot symbols, and an optical receiver that receives an optical signal containing a training sequence and pilot symbols, The optical receiver has a signal processing circuit that receives and processes a received signal including a training sequence and pilot symbols. The aforementioned signal processing circuit is A tap coefficient adaptive control processing unit performs tap coefficient update processing using information from multiple symbols, including pilot symbols. An FIR (Finite Impulse Response) filter performs adaptive equalization processing using the updated tap coefficients obtained by the aforementioned tap coefficient adaptive control processing unit, A TS synchronization detection unit performs TS (Training Sequence) synchronization detection from the training sequence information of the received signal based on the output data of the FIR filter, The system includes a pilot extraction processing unit that extracts a plurality of symbols, including pilot symbols, included in the received signal, based on TS synchronization information obtained from the TS synchronization detection unit, The initial draw-in of adaptive equalization in the FIR filter is performed. An optical transmission system characterized by the following:

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