Optical transmission system, control device, compensation amount determination method and program

Abstract

This optical transport system comprises an optical transmission device, an optical reception device, and a control device. The optical transmission device is provided with: a transmission signal generation unit that generates a pre-equalized transmission signal; and an optical transmission unit that converts the transmission signal from an electric signal to an optical signal and that transmits the converted optical signal. The optical reception device is provided with: an optical reception unit that converts the received optical signal to a reception signal which is an electric signal; a signal processing unit that performs a reception process including demodulation and decoding of the reception signal; and a noise loading unit that loads noise to the optical signal received by the optical reception unit or the reception signal converted by the optical reception unit. The control device is provided with: an information acquisition unit that acquires information relating to the signal quality of the reception signal when the magnitude of the noise loaded to the optical signal or the reception signal and a compensation amount in the pre-equalization have been changed; and a compensation amount determination unit that determines a compensation amount for the pre-equalization in the optical transmission device on the basis of the information acquired by the information acquisition unit.

Description

[Technical Field]

[0001] This invention relates to an optical transmission system, a control device, a method for determining compensation amounts, and a program. [Background technology]

[0002] In optical transmission systems, signal distortion caused by bandwidth limitations of analog devices is compensated for by signal processing (see, for example, Non-Patent Document 1). For example, an optical transmitter performs Digital Predistortion (DPD) to pre-equalize signal distortion caused by bandwidth limitations of the transmitter's analog devices at the transmitting end. An optical receiver performs receiver-side equalization to equalize the effects of bandwidth limitations that could not be completely removed at the transmitting end. When using DPD, it is necessary to select the optimal compensation amount after considering the noise level of the transmitter and the transmission line. If the DPD compensation amount is large, the peak-to-average power ratio (PAPR) of the signal will increase, and the effect of quantization noise will become greater. On the other hand, if the DPD compensation amount is small, the receiver-side equalization will be affected by noise enhancement when the transmission line noise level is high. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] ST Le and J. Cho, "OSNR-Aware Digital Pre-Emphasis for High Baudrate Coherent Optical Transmissions", Optical Fiber Communications Conference (OFC), March 2022, M3H.4. [Overview of the project] [Problems that the invention aims to solve]

[0004] Traditionally, the optimal DPD compensation amount could not be determined unless the transmitter noise and transmission path noise were known. Therefore, expensive equipment such as optical spectrum analyzers were required to measure the Optical Signal-to-Noise Ratio (OSNR). Furthermore, it was necessary to transmit optical signals with pre-applied DPD that fully compensated for the transmitter-side bandwidth limitation. Additionally, the post-FEC (forward error correction) BER (Bit Error Rate) characteristics—the maximum noise level the optical transmission system could withstand while remaining error-free—had to be known.

[0005] In view of the above circumstances, the present invention aims to provide an optical transmission system, a control device, a method for determining the amount of compensation on the transmitting side, and a program that can determine the amount of compensation on the transmitting side without measuring the noise of the transmitter or transmission line. [Means for solving the problem]

[0006] One aspect of the present invention is an optical transmission system comprising an optical transmitting device, an optical receiving device, and a control device, wherein the optical transmitting device comprises a transmission signal generation unit that generates a pre-equalized transmission signal, and an optical transmitting unit that converts the transmission signal from an electrical signal to an optical signal and transmits the converted optical signal; the optical receiving device comprises an optical receiving unit that receives the optical signal and converts the received optical signal to an electrical signal reception signal, a signal processing unit that performs reception processing including demodulation and decoding of the received signal, and a noise loading unit that loads noise onto the optical signal received by the optical receiving unit or the received signal converted by the optical receiving unit; and the control device comprises an information acquisition unit that acquires information on the signal quality of the received signal when the compensation amount in the pre-equalization and the magnitude of the noise loaded onto the optical signal or the received signal are changed, and a compensation amount determination unit that determines the compensation amount for the pre-equalization in the optical transmitting device based on the information acquired by the information acquisition unit.

[0007] One aspect of the present invention comprises an information acquisition unit that obtains information on the signal quality of a received signal when the compensation amount of the pre-equalization in an optical transmitting device that converts a pre-equalized transmitted signal from an electrical signal to an optical signal and transmits the converted optical signal is changed, and the magnitude of the noise loaded onto the optical signal or the received signal is changed in an optical receiving device that converts the received optical signal to an electrical signal received signal and demodulates and decodes the converted received signal, and a compensation amount determination unit that determines the compensation amount of the pre-equalization in the optical transmitting device based on the information acquired by the information acquisition unit.

[0008] One aspect of the present invention includes a transmission step in which an optical transmitting device converts a pre-equalized transmission signal from an electrical signal to an optical signal and transmits the converted optical signal; a reception step in which an optical receiving device receives the optical signal and converts the received optical signal to an electrical signal reception signal; a signal processing step in which the optical receiving device performs reception processing including demodulation and decoding of the received signal; a noise loading step in which the optical receiving device loads noise onto the optical signal received in the reception step or the received signal converted in the reception step; an information acquisition step in which a control device obtains information on the signal quality of the received signal when the compensation amount in the pre-equalization and the magnitude of the noise loaded onto the received signal are changed; and a compensation amount determination step in which the control device determines the compensation amount for the pre-equalization in the optical transmitting device based on the information acquired in the information acquisition step.

[0009] One aspect of the present invention is a program for causing a computer to function as the control device described above. [Effects of the Invention]

[0010] This invention makes it possible to determine the amount of compensation on the transmitting side without measuring the noise of the transmitter or transmission line. [Brief explanation of the drawing]

[0011] [Figure 1] This is a diagram showing the configuration of an optical transmission system according to an embodiment of the present invention. [Figure 2] It is a configuration diagram of an optical transmission system according to an embodiment of the present invention. [Figure 3] It is a configuration diagram of an optical transmission system according to the first embodiment. [Figure 4] It is a functional block diagram of a control device according to the first embodiment. [Figure 5] It is a flowchart showing the processing of an optical transmission system according to the first embodiment. [Figure 6] It is a diagram showing the relationship between the noise amount and NGMI (Normalized Generalized Mutual Information) in an optical transmission system according to the first embodiment. [Figure 7] It is a diagram showing the relationship between the filter coefficient and the noise amount in an optical transmission system according to the first embodiment. [Figure 8] It is a flowchart showing the processing of an optical transmission system according to the second embodiment. [Figure 9] It is a flowchart showing the processing of an optical transmission system according to the third embodiment. [Figure 10] It is a configuration diagram of an optical transmission system according to the fourth embodiment. [Figure 11] It is a diagram for explaining the principle of the embodiment. [Figure 12] It is a diagram for explaining the principle of the embodiment. [Figure 13] It is a diagram for explaining the principle of the embodiment. [[ID=三十六]] [Figure 14] It is a diagram for explaining the principle of the embodiment. [Figure 15] It is a diagram for explaining the principle of the embodiment. [Figure 16] It is a diagram showing the hardware configuration of the control device of the embodiment. [[Embodiments for Carrying Out the Invention]]

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0013] Figure 1 is a diagram showing the configuration of an optical transmission system 1 in an embodiment of the present invention. The optical transmission system 1 comprises an optical transmitter 2, an optical receiver 3, and a control device 4. The optical transmitter 2 and the optical receiver 3 are connected by a transmission path 5. The transmission path 5 transmits optical signals. The transmission path 5 is, for example, an optical fiber. The transmission path 5 may be equipped with an amplifier for amplifying optical signals. The optical transmission system 1 is operated with an OSNR (Responsive Operating System Routing) with a margin that is sufficiently larger than the minimum required OSNR (Required OSNR: ROSNR) for error-free decoding. Hereinafter, the difference between the ROSNR and the OSNR in operation will also be simply referred to as the "margin". The actual ROSNR of the optical transmission system 1 is unknown.

[0014] The optical transmission device 2 comprises a digital signal processing unit 21 and a transmitter 22. The digital signal processing unit 21 generates a transmission signal of an electrical signal that has undergone DPD (Digital Producer Processing) using a DSP (Digital Signal Processing). The transmitter 22 converts the transmission signal generated by the digital signal processing unit 21 from an electrical signal to an optical signal and outputs the converted optical signal to the transmission line 5. The frequency characteristics of the transmitter 22 are known.

[0015] The optical receiving device 3 comprises a receiver 31 and a digital signal processing unit 32. The receiver 31 receives the optical signal transmitted through the transmission path 5. The receiver 31 converts the received optical signal into an electrical signal and outputs it to the digital signal processing unit 32. The digital signal processing unit 32 loads the received signal output from the receiver 31 with noise simulated using random numbers by a DSP, and performs signal reception processing on the noise-loaded received signal. The digital signal processing unit 32 demodulates and decodes the received signal through signal reception processing.

[0016] The frequency characteristics of transmitter 22 can be determined as follows. For example, an optical signal without any compensation is transmitted in optical transmission device 2. Then, the frequency characteristics are obtained by examining the effect of bandwidth limitation on the optical signal after transmission through transmission path 5. Alternatively, the frequency characteristics can be confirmed from the datasheet of the analog device. Furthermore, the frequency characteristics of transmitter 22 can be obtained by any method, not limited to these.

[0017] Other configurations of the optical transmission system of this embodiment will now be described. Figure 2 is a configuration diagram of the optical transmission system 11. In the optical transmission system 11 shown in Figure 2, the same reference numerals are used for parts that are the same as those in the optical transmission system 1 shown in Figure 1, and their descriptions are omitted. The difference between the optical transmission system 11 and the optical transmission system 1 shown in Figure 1 is that the optical receiving device 6 is provided instead of the optical receiving device 3.

[0018] The optical receiving device 6 comprises a receiver 61 and a digital signal processing unit 62. The receiver 61 receives the optical signal transmitted through the transmission line 5. The receiver 61 adds noise to the received optical signal. For example, an amplifier is inserted into the receiver 61, and the amplifier amplifies the received optical signal to add noise. The receiver 61 converts the noise-added optical signal into an electrical signal and outputs it to the digital signal processing unit 62. The digital signal processing unit 62 performs DSP-based signal reception processing on the received signal from the receiver 61. The digital signal processing unit 62 demodulates and decodes the received signal using the same signal reception processing as the digital signal processing unit 32 shown in Figure 1.

[0019] With the above configuration, the optical transmitter 2 transmits a signal to which an arbitrary amount of DPD compensation is applied, and the optical receivers 3 and 6 load noise onto the received signal. The control device 4 obtains the maximum noise when the noise-loaded signal in the optical receivers 3 and 6 satisfies a predetermined signal quality. The control device 4 determines whether the predetermined signal quality is met using an index value representing the signal quality. As an index, for example, if the margin is small, NGMI (Normalized Generalized Mutual Information), Q value (quality factor), pre-FEC BER (BER before error correction), post-FEC BER (BER before error correction), FEC correction count, EVM (Error Vector Magnitude), etc. can be used. If the margin is large, post-FEC BER is used. The control device 4 determines the amount of DPD compensation applied by the optical transmitter 2 to the signal that satisfies the predetermined signal quality and has the maximum noise loaded in the optical receivers 3 and 6 as the optimal compensation amount for the system. The control device 4 instructs the optical transmitter 2 to set the optimal DPD compensation amount. Each embodiment is described in detail below.

[0020] (First Embodiment) Figure 3 is a configuration diagram of the optical transmission system 100 of the first embodiment. The optical transmission system 100 corresponds to the optical transmission system 1 shown in Figure 1. The optical transmission system 100 comprises an optical transmitter 200, an optical receiver 300, and a control device 400. The optical transmitter 200 and the optical receiver 300 are connected by a transmission path 5. The optical transmitter 200, the optical receiver 300, and the control device 400 correspond to the optical transmitter 2, the optical receiver 3, and the control device 4 shown in Figure 1, respectively.

[0021] The optical transmission device 200 comprises a transmission signal generation unit 210 and an optical transmission unit 220. The transmission signal generation unit 210 corresponds to the digital signal processing unit 21 shown in Figure 1, and the optical transmission unit 220 corresponds to the transmitter 22 shown in Figure 1.

[0022] The transmission signal generation unit 210 performs DSP. The transmission signal generation unit 210 includes an encoding unit 211, a mapping unit 212, a waveform shaping unit 213, a pre-equalization unit 214, and a digital-to-analog conversion unit 215. The encoding unit 211 outputs a transmission signal obtained by performing FEC encoding on the transmission bit sequence. The mapping unit 212 maps the transmission signal output from the encoding unit 211 to symbols and samples the mapped symbols. The waveform shaping unit 213 limits the bandwidth of the sampled transmission signal. The pre-equalization unit 214 compensates for the distortion of the waveform of the transmission signal that has been bandwidth-limited by the waveform shaping unit 213 and outputs it to the digital-to-analog conversion unit 215. The digital-to-analog conversion unit 215 is a DAC (Digital Analog Converter). Hereinafter, the digital-to-analog conversion unit 215 will be referred to as DAC215. The DAC215 converts the transmission signal input from the pre-equalization unit 214 from a digital signal to an analog signal, and outputs the converted analog signal to the optical transmission unit 220.

[0023] The optical transmission unit 220 comprises a modulator driver 221 and a modulation unit 222. The modulator driver 221 amplifies the I (in-phase) component of the X-polarization, the Q (quadrature) component of the X-polarization, the I component of the Y-polarization, and the Q component of the Y-polarization of the transmission signal output from the DAC 215, and drives the modulation unit 222 with these amplified signal components. The modulation unit 222 comprises a modulator (not shown), a light source (not shown), and a multiplexer (not shown). The modulator modulates the light from the light source using the analog signal of the I component of the X-polarization and the analog signal of the Q component of the X-polarization output from the modulator driver 221 to generate an X-polarized optical signal. Furthermore, the modulator modulates the light from the light source using the analog signal of the I component of the Y-polarization and the analog signal of the Q component of the Y-polarization output from the modulator driver 221 to generate a Y-polarized optical signal. The multiplexing unit combines the X-polarized optical signal and the Y-polarized optical signal generated by the modulator to generate a polarization-multiplexed optical signal, and outputs the generated optical signal to the transmission line 5.

[0024] The optical receiver 300 comprises a coherent receiver 310, a noise load unit 320, a demodulation / decoding unit 330, and an index value calculation unit 340. The coherent receiver 310 corresponds to the receiver 31 in Figure 1, and the noise load unit 320 and the demodulation / decoding unit 330 correspond to the digital signal processing unit 32 in Figure 1.

[0025] The coherent receiver 310 receives the optical signal transmitted through the transmission path 5 and performs coherent reception of the received optical signal. Through coherent reception, the coherent receiver 310 converts the optical signal into an electrical signal while maintaining the phase and amplitude of the polarization-multiplexed optical signal, and outputs the converted received signal. Specifically, the coherent receiver 310 separates the received optical signal into X-polarization and Y-polarization. The coherent receiver 310 converts the I and Q components extracted by interfering the X-polarization optical signal with the local oscillator light into electrical signals. Similarly, the coherent receiver 310 converts the I and Q components extracted by interfering the Y-polarization optical signal with the local oscillator light into electrical signals. The coherent receiver 310 outputs the electrical signals of the I and Q components of the X-polarization, and the electrical signals of the I and Q components of the Y-polarization, to the noise load unit 320.

[0026] The noise load unit 320 uses a DSP to load the received signal output from the coherent receiver unit 310 with noise simulated using random numbers. The noise load unit 320 outputs the noise-loaded received signal to the demodulation / decoding unit 330.

[0027] The demodulation and decoding unit 330 performs signal reception processing of the received signal using a DSP. The demodulation and decoding unit 330 comprises an analog-to-digital conversion unit 331, a wavelength dispersion compensation unit 332, a polarization fluctuation compensation unit 333, a frequency offset unit 334, a carrier phase compensation unit 335, a symbol determination unit 336, and a decoding unit 337.

[0028] The analog-to-digital conversion unit 331 is an ADC (Analog Digital Converter). Hereinafter, the analog-to-digital conversion unit 331 will be referred to as ADC331. The ADC331 converts the received signal output from the noise load unit 320 from an analog signal to a digital signal and outputs the converted digital signal to the wavelength dispersion compensation unit 332. The wavelength dispersion compensation unit 332 compensates for the wavelength dispersion experienced by the received signal in the transmission path 5 and outputs the wavelength dispersion-compensated received signal to the polarization fluctuation compensation unit 333. The polarization fluctuation compensation unit 333 compensates for the polarization fluctuations experienced by the X-polarization and Y-polarization of the received signal in the transmission path 5 and outputs the polarization fluctuation-compensated received signal to the frequency offset unit 334. The frequency offset unit 334 performs a frequency offset on the received signal whose polarization fluctuations have been compensated by the polarization fluctuation compensation unit 333. The carrier phase compensation unit 335 performs carrier phase compensation on the received signal whose frequency offset has been performed by the polarization fluctuation compensation unit 333. The symbol determination unit 336 determines the symbol of the received signal after the carrier phase compensation unit 335 has performed carrier phase compensation, and converts the determined symbol into binary data. The decoding unit 337 obtains the received bit sequence by performing error correction decoding processing such as FEC on the binary data demapped by the symbol determination unit 336.

[0029] The index value calculation unit 340 calculates an index value representing the signal quality of the received signal processed by the demodulation / decoding unit 330. The index can include NGMI, Q value, pre-FEC BER, post-FEC BER, FEC correction count, EVM, etc. Note that if the margin is large, post-FEC BER is used as the index.

[0030] Figure 4 is a functional block diagram showing the configuration of the control device 400. The control device 400 comprises a storage unit 410, a compensation amount instruction unit 420, a noise load instruction unit 430, an information acquisition unit 440, and a compensation amount determination unit 450.

[0031] The memory unit 410 stores information linking the DPD compensation amount and the noise amount. The DPD compensation amount is the amount of DPD compensation applied to the transmitted signal by the pre-equalization unit 214 of the optical transmitter 200. The noise amount represents the magnitude of the noise loaded onto the received signal by the noise load unit 320 of the optical receiver 300. The memory unit 410 may further store index value information linked to the DPD compensation amount and the noise amount. The compensation amount instruction unit 420 instructs the optical transmitter 200 on the DPD compensation amount. The noise load instruction unit 430 instructs the optical receiver 300 on the amount of noise to load onto the received signal.

[0032] The information acquisition unit 440 acquires an index value representing the signal quality of the received signal from the optical receiver 300. The information acquisition unit 440 determines whether the acquired index value satisfies predetermined conditions. The predetermined conditions are those for determining that the signal quality is better than predetermined. The information acquisition unit 440 writes information to the storage unit 410 that associates the DPD compensation amount used by the optical transmitter 200 to generate the signal for which an index value satisfying the predetermined conditions was obtained with the amount of noise that the optical receiver 300 loaded onto that signal. The information acquisition unit 440 may also write information to the storage unit 410 that associates the index value with the DPD compensation amount and the amount of noise.

[0033] The compensation amount determination unit 450 obtains a DPD compensation amount linked to a noise level greater than a predetermined value among the DPD compensation amounts whose index value satisfies predetermined conditions, based on the information stored in the storage unit 410. The noise level greater than the predetermined value is, for example, the maximum noise level. The compensation amount determination unit 450 may also obtain a DPD compensation amount corresponding to a noise level slightly lower than the maximum noise level. The compensation amount determination unit 450 determines the obtained DPD compensation amount as the DPD compensation amount to be used in the optical transmission device 200. The compensation amount determination unit 450 may also determine a value near the obtained DPD compensation amount as the DPD compensation amount to be used in the optical transmission device 200.

[0034] The control device 400 can be implemented, for example, by a computer device. In that case, the control device 400 may be implemented by recording a program for implementing its functions on a computer-readable recording medium, loading the program recorded on this recording medium into a computer system, and executing it. Here, "computer system" includes hardware such as the OS and peripheral devices. Furthermore, "computer-readable recording medium" refers to portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, and storage devices such as hard disks built into a computer system. In addition, "computer-readable recording medium" may also include those that dynamically hold programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or communication lines such as telephone lines, and those that hold programs for a certain period of time, such as volatile memory inside a computer system that acts as a server or client in such cases. Furthermore, the above program may only implement a part of the functions described above, and may also be able to implement the above functions in combination with programs already recorded in the computer system.

[0035] Furthermore, the control device 400 may be an integrated device with the optical transmitter 200 or the optical receiver 300. Also, some of the functional components of the control device 400 may be provided in the optical transmitter 200 or the optical receiver 300. Furthermore, some of the functional components of the control device 400 may be implemented by the optical transmitter 200 and the control device 400, or by the optical receiver 300 and the control device 400.

[0036] Figure 5 is a flowchart showing the processing of the optical transmission system 100. The compensation amount instruction unit 420 of the control device 400 instructs the optical transmitter 200 to change the DPD compensation amount (step S1). For example, the compensation amount instruction unit 420 selects a DPD compensation amount that has not yet been instructed to the optical transmitter 200 and sends a compensation amount change instruction including the selected DPD compensation amount to the optical transmitter 200. The pre-equalization unit 214 of the optical transmitter 200 sets the DPD compensation amount read from the compensation amount change instruction. Alternatively, the compensation amount instruction unit 420 may send a compensation amount change instruction that does not include a DPD compensation amount to the optical transmitter 200. When the pre-equalization unit 214 of the optical transmitter 200 receives the compensation amount change instruction, it selects a DPD compensation amount that has not yet been selected. The pre-equalization unit 214 sets the selected DPD compensation amount and further notifies the control device 400 of the information of the set DPD compensation amount.

[0037] The optical transmitter 200 starts transmitting an optical signal to which the DPD compensation amount set in step S1 has been applied (step S2). That is, the encoding unit 211 of the optical transmitter 200 generates a transmission signal by performing FEC encoding on the transmission bit sequence. The mapping unit 212 maps the transmission signal to symbols and samples the symbol-mapped transmission signal. The waveform shaping unit 213 limits the bandwidth of the sampled transmission signal. The pre-equalization unit 214 compensates the bandwidth-limited transmission signal with a DPD compensation amount instructed by the control device 400. The DAC 215 converts the DPD-treated transmission signal from a digital signal to an analog signal. The modulator driver 221 drives the modulation unit 222 with the transmission signal converted to an analog signal. The modulation unit 222 modulates the light from the light source with the transmission signal and generates an optical signal in which an X-polarized optical signal and a Y-polarized optical signal are polarization-multiplexed. The modulation unit 222 outputs the generated optical signal to the transmission line 5.

[0038] The noise load instruction unit 430 of the control device 400 instructs the optical receiver 300 to load noise (step S3). The noise load instruction unit 430 may also instruct the optical receiver 300 on the amount of noise. The noise load unit 320 of the optical receiver 300 starts loading noise onto the received signal. The optical receiver 300 performs signal reception processing of the optical signal transmitted from the optical transmitter 200 (step S4). That is, the coherent receiver unit 310 of the optical receiver 300 receives the optical signal transmitted from the optical transmitter 200 and transmitted through the transmission path 5. The coherent receiver unit 310 coherently receives the received optical signal and converts it into an electrical signal. The noise load unit 320 loads noise onto the electrical signal. The demodulation / decoding unit 330 performs signal reception processing of the noise-loaded received signal. Specifically, the ADC331 converts the received signal from an analog signal to a digital signal, the wavelength dispersion compensation unit 332 compensates for the wavelength dispersion of the received signal, the polarization fluctuation compensation unit 333 compensates for the polarization fluctuation of the received signal, the frequency offset unit 334 performs frequency offset on the received signal, and the carrier phase compensation unit 335 performs carrier phase compensation on the received signal. The symbol determination unit 336 determines the symbol of the received signal and converts it into binary data. The decoding unit 337 performs FEC on the binary data to obtain the received bit sequence.

[0039] The index value calculation unit 340 of the optical receiver 300 calculates an index value representing the signal quality of the received signal processed by the demodulation / decoding unit 330, and notifies the control device 400 of the calculated index value. The information acquisition unit 440 of the control device 400 receives the index value notified by the index value calculation unit 340 of the optical receiver 300. Furthermore, the information acquisition unit 440 receives information on the amount of noise loaded onto the received signal from the noise load unit 320 of the optical receiver 300 (step S5).

[0040] The information acquisition unit 440 determines whether the signal quality meets a predetermined quality by comparing the index value with a threshold (step S6). The threshold is a value that allows for the determination of whether the current OSNR can be considered to be below the ROSNR. The index value calculation unit 340 of the optical receiver 300 may output an error notification if it determines, by comparing the index value with the threshold, that the signal quality does not meet the predetermined quality. The information acquisition unit 440 of the control device 400 detects that the signal quality does not meet the predetermined quality based on the error notification output from the optical receiver 300.

[0041] If the information acquisition unit 440 determines that the signal quality meets the predetermined quality (step S6: YES), the noise load instruction unit 430 instructs the optical receiver 300 to increase the noise level. The noise load instruction unit 430 may also instruct the optical receiver 300 to increase the noise level. The noise load unit 320 of the optical receiver 300 increases the amount of noise loaded onto the received signal (step S7). The optical transmission system 100 repeats the process from step S4.

[0042] Then, if the information acquisition unit 440 determines that the signal quality does not meet the predetermined quality (step S6: NO), it performs the process in step S8. That is, the information acquisition unit 440 writes information to the storage unit 410 that links the maximum noise level when the signal quality met the predetermined quality during the period in which the current DPD compensation level is used with the current DPD compensation level instructed in step S1 (step S8). The maximum noise level is the noise level immediately before increasing to the current noise level.

[0043] The compensation amount indicator unit 420 determines whether the sweep of DPD compensation amounts has been completed (step S9). If the compensation amount indicator unit 420 determines that the sweep has not been completed and there are DPD compensation amounts that have not yet been applied to the optical transmitter 200 (step S9: NO), the process from step S1 is repeated.

[0044] When the compensation amount indicator unit 420 determines that the sweep of the DPD compensation amount is complete (step S9: YES), it notifies the compensation amount determination unit 450 of the completion of the sweep. The compensation amount determination unit 450 refers to the storage unit 410 and selects the DPD compensation amount associated with the maximum noise level (step S10). When the noise level is at its maximum, the ROSNR of the system transmission can be considered to be at its minimum. The compensation amount determination unit 450 sets the selected DPD compensation amount in the pre-equalization unit 214 of the optical transmitter 200.

[0045] An embodiment of the optical transmission system 100 is described below. In this embodiment, the transmitted signal is Uniform 64QAM (Quadrature Amplitude Modulation), the modulation rate is 100 Gbaud, and the optical receiver 300 has 3dB bandwidths of 45 GHz for the DAC 215, 66 GHz for the modulator driver 221, and 35 GHz for the modulator of the modulation unit 222.

[0046] Furthermore, a DPD (Digital Frequency Detector) was created in the pre-equalization unit 214 of the optical transmitter 200 to compensate for the transmitting-side bandwidth limitation. Assuming the frequency characteristics of the analog device used in the optical transmitter unit 220 of the optical transmitter 200 are known, let H(f) be denoted as such. f is the frequency. The inverse characteristic of this frequency characteristic, H(f), is assumed to be known in advance. -1 By performing compensation by convolving into the transmitted signal, it is possible to cancel out the effect of the bandwidth limitation of the analog device of the optical transmission unit 220. Therefore, the pre-equalization unit 214 of the optical transmission device 200 compensates for the transmission side bandwidth limitation with H(f) -α A DPD was created. α is the filter coefficient and represents the DPD compensation amount. The inverse characteristic when convolved onto the transmitted signal is H(f). -α By setting (0 ≤ α ≤ 1), the magnitude of the compensation amount can be adjusted using the filter coefficient α.

[0047] Furthermore, in the optical transmission system 100, error-free decoding is possible when the NGMI is 0.857 (NGMI threshold) in operation with an FEC of 21%. Here, the amount of noise loaded onto the received signal was obtained for each filter coefficient α when the NGMI was at the NGMI threshold. Based on these results, the control device 400 determined the filter coefficient α that would yield the maximum performance in the transmission line 5.

[0048] Figure 6 shows the relationship between noise level and NGMI for each filter coefficient α, and Figure 7 shows the relationship between filter coefficient α and noise level at the NGMI threshold. In Figures 6 and 7, the noise level is expressed as the relative noise level [dB] loaded onto the received signal. From the results in Figures 6 and 7, a filter coefficient α (compensation amount) = 0.6 was determined.

[0049] According to the embodiment described above, in an optical transmission system where the frequency characteristics of the optical receiver are known, even if the noise of the optical receiver, the noise of the transmission line, and the ROSNR are unknown, the amount of DPD compensation that maximizes noise immunity can be determined, thereby maximizing transmission performance.

[0050] (Second embodiment) In the first embodiment, noise was added to the received signal in the optical receiver. However, if the margin between the ROSNR of the optical transmission system and the OSNR in the operation of the optical transmission system is small, it is not necessary to add noise in the optical receiver. Below, the second embodiment will be described, focusing on the differences from the first embodiment.

[0051] The configuration of the optical transmission system in the second embodiment is the same as that of the optical transmission system 100 in the first embodiment shown in Figure 3. However, the optical receiver 300 does not need to have a noise load unit 320.

[0052] Figure 8 is a flowchart showing the processing of the optical transmission system 100 in the second embodiment. In Figure 8, the same reference numerals are used for the same processes as in the first embodiment shown in Figure 5, and their detailed explanations are omitted. The compensation amount instruction unit 420 of the control device 400 instructs the optical transmission device 200 to change the DPD compensation amount (step S1). The optical transmission device 200 starts transmitting an optical signal to which the changed DPD compensation amount instructed by the compensation amount instruction unit 420 of the control device 400 in step S1 is applied (step S2).

[0053] The optical receiver 300 receives the optical signal transmitted from the optical transmitter 200 and performs reception processing (step S21). That is, the coherent receiving unit 310 of the optical receiver 300 receives the optical signal transmitted by the optical transmitter 200 and transmitted through the transmission path 5. The coherent receiving unit 310 converts the received optical signal into an electrical signal. Since the noise load unit 320 is not instructed to load noise, it outputs the received signal output by the coherent receiving unit 310 to the demodulation / decoding unit 330 without loading noise onto it. Alternatively, if the optical receiver 300 does not have a noise load unit 320, the coherent receiving unit 310 outputs the received signal obtained by coherent reception to the demodulation / decoding unit 330. The demodulation / decoding unit 330 performs signal reception processing on the received signal using a DSP.

[0054] The index value calculation unit 340 of the optical receiver 300 calculates an index value representing the signal quality of the received signal processed by the demodulation / decoding unit 330, and notifies the control device 400 of the calculated index value. The index can be NGMI, Q value, pre-FEC BER, post-FEC BER, FEC correction number, EVM, etc. The information acquisition unit 440 of the control device 400 receives the index value information notified from the index value calculation unit 340 of the optical receiver 300 (step S22). The information acquisition unit 440 writes information linking the compensation amount instructed in step S1 and the received index value to the storage unit 410.

[0055] The compensation amount indicator unit 420 determines whether the sweep of the DPD compensation amount has been completed (step S9). If the compensation amount indicator unit 420 determines that the sweep has not been completed (step S9: NO), it repeats the process from step S1.

[0056] When the compensation amount indicator unit 420 determines that the sweep of the DPD compensation amount is complete (step S9: YES), it notifies the compensation amount determination unit 450 of the completion of the sweep. The compensation amount determination unit 450 refers to the storage unit 410 and obtains the DPD compensation amount that can be considered to have the minimum ROSNR (step S23). Since OSNR cannot be measured directly, for example, if NGMI or Q value is used as an indicator, the compensation amount determination unit 450 selects the DPD compensation amount with the maximum NGMI or Q value. Also, if pre-FEC BER, post-FEC BER, FEC correction count, or EVM is used as an indicator, the compensation amount determination unit 450 selects the DPD compensation amount with the minimum pre-FEC BER, post-FEC BER, FEC correction count, or EVM. The compensation amount determination unit 450 sets the selected DPD compensation amount in the pre-equalization unit 214 of the optical transmitter 200.

[0057] According to the second embodiment, in an optical transmission system where the frequency characteristics of the optical receiver are known, even if the noise of the optical receiver, the noise of the transmission line, and the ROSNR are unknown, the amount of DPD compensation that maximizes noise immunity can be determined more easily than in the first embodiment, thereby maximizing transmission performance.

[0058] (Third embodiment) In the third embodiment of the optical transmission system, when the margin is large, the optical receiver loads noise onto the received signal to search for the DPD compensation amount, and when the margin is small, the optical receiver searches for the DPD compensation amount without loading noise onto the received signal.

[0059] The configuration of the optical transmission system in the third embodiment is the same as that of the optical transmission system 100 in the first embodiment shown in Figure 3. Figure 9 is a flowchart showing the processing of the optical transmission system 100 in the third embodiment. In Figure 9, the same reference numerals are used for the same processes as in the first embodiment shown in Figure 5 and the second embodiment shown in Figure 8, and their detailed explanations are omitted.

[0060] The compensation amount instruction unit 420 of the control device 400 instructs the optical transmitter 200 to change the DPD compensation amount (step S1). The pre-equalization unit 214 of the optical transmitter 200 starts transmitting an optical signal to which the changed DPD compensation amount instructed by the compensation amount instruction unit 420 of the control device 400 in step S1 has been applied (step S2).

[0061] The noise load instruction unit 430 of the control device 400 determines whether the margin in the optical transmission system 100 is known or not (step S31). If the noise load instruction unit 430 determines that the margin is known (step S31: YES), it determines whether the margin is large enough to change the optimal DPD compensation amount between the ROSNR and the operational OSNR (step S32). If the noise load instruction unit 430 determines that the margin is not known (step S31: NO), or determines that the margin is large (step S32: YES), it instructs the optical receiver 300 to load noise (step S3). The noise load unit 320 of the optical receiver 300 starts loading noise onto the received signal. The noise load instruction unit 430 may also proceed to step S3 if the margin is small.

[0062] The optical receiver 300 performs signal reception processing of the optical signal transmitted from the optical transmitter 200 (step S4). That is, the coherent receiver 310 coherently receives the optical signal transmitted from the optical transmitter 200 and transmitted through the transmission path 5. The noise load unit 320 loads noise onto the received signal obtained by coherent reception. The demodulation / decoding unit 330 performs signal reception processing on the received signal with the noise loaded.

[0063] The index value calculation unit 340 of the optical receiver 300 calculates the index value of the received signal after the demodulation and decoding unit 330 has processed the signal reception. The post-FEC BER is used as the index. If the margin is small, NGMI, Q value, pre-FEC BER, post-FEC BER, FEC correction number, or EVM may be used as the index. The information acquisition unit 440 of the control device 400 receives index value information from the index value calculation unit 340 of the optical receiver 300 and receives information on the amount of noise loaded onto the received signal from the noise loading unit 320 (step S5).

[0064] The information acquisition unit 440 determines whether the signal quality meets a predetermined quality by comparing the index value with a threshold value (step S6). The threshold value is a value that allows for the determination of whether the current OSNR can be considered to have fallen below the ROSNR. Alternatively, the information acquisition unit 440 may detect that the current OSNR has fallen below the ROSNR by receiving an error notification from the index value calculation unit 340 of the optical receiver 300.

[0065] When the information acquisition unit 440 determines that the signal quality meets the predetermined quality (step S6: YES), the noise load instruction unit 430 instructs the optical receiver 300 to increase the noise level. The noise load unit 320 of the optical receiver 300 increases the amount of noise loaded onto the received signal (step S7). The optical transmission system 100 repeats the process from step S4.

[0066] If the information acquisition unit 440 determines that the signal quality does not meet the predetermined quality (step S6: NO), it performs the process in step S8. That is, the information acquisition unit 440 writes information to the storage unit 410 that links the maximum noise level when the signal quality met the predetermined quality during the period in which the current DPD compensation amount is used with the current DPD compensation amount instructed in step S1 (step S8). Furthermore, the information acquisition unit 440 writes information that links the current DPD compensation amount with an index value to the storage unit 410 (step S33).

[0067] The compensation amount indicator unit 420 determines whether the sweep of the DPD compensation amount has been completed (step S9). If the compensation amount indicator unit 420 determines that the sweep has not been completed (step S9: NO), it repeats the process from step S1.

[0068] On the other hand, in step S32, if the noise load instruction unit 430 of the control device 400 determines that the margin is small (step S32: NO), it instructs the optical receiver 300 to stop the noise load. The optical receiver 300 receives the optical signal transmitted from the optical transmitter 200 and performs reception processing (step S21). That is, the coherent receiver unit 310 of the optical receiver 300 converts the received optical signal into an electrical signal. The noise load unit 320 outputs the received signal output by the coherent receiver unit 310 to the demodulation / decoding unit 330 without adding noise. The demodulation / decoding unit 330 performs DSP-based signal reception processing on the received signal.

[0069] The index value calculation unit 340 of the optical receiver 300 calculates an index value representing the signal quality of the received signal processed by the demodulation / decoding unit 330, and notifies the control device 400 of the calculated index value. The index can be NGMI, Q value, pre-FEC BER, post-FEC BER, FEC correction number, EVM, etc. The information acquisition unit 440 of the control device 400 receives the index value information notified from the index value calculation unit 340 of the optical receiver 300 (step S22). The information acquisition unit 440 writes information linking the compensation amount instructed in step S1 and the received index value to the storage unit 410 (step S33). If the compensation amount instruction unit 420 determines that the sweep of the DPD compensation amount is not yet complete (step S9: NO), it repeats the process from step S1.

[0070] When the compensation amount indicator unit 420 determines that the sweep of the DPD compensation amount is complete (step S9: YES), it notifies the compensation amount determination unit 450 that the sweep is complete. The compensation amount determination unit 450 refers to the storage unit 410 and obtains the DPD compensation amount that can be considered to have the minimum ROSNR (step S34). Specifically, when noise is loaded onto the received signal in the optical receiver 300, the compensation amount determination unit 450 selects the DPD compensation amount associated with the maximum noise level. When no noise is loaded onto the received signal in the optical receiver 300, the compensation amount determination unit 450 selects the DPD compensation amount associated with the maximum NGMI or Q value, or the minimum pre-FEC BER, post-FEC BER, FEC correction number, or DPD compensation amount associated with EVM. The compensation amount determination unit 450 sets the selected DPD compensation amount in the pre-equalization unit 214 of the optical transmitter 200.

[0071] (Fourth embodiment) In the first to third embodiments, the optical receiver added noise to the received signal, which had been converted into an electrical signal, using a DSP. In the fourth embodiment, the optical receiver adds noise to the optical signal.

[0072] Figure 10 is a configuration diagram of the optical transmission system 101 of the fourth embodiment. The optical transmission system 101 corresponds to the optical transmission system 11 shown in Figure 2. In Figure 10, the same reference numerals are used for parts identical to those of the optical transmission system 100 of the first embodiment shown in Figure 3, and their descriptions are omitted. The difference between the optical transmission system 101 and the optical transmission system 100 of the first embodiment shown in Figure 3 is that the optical receiver 301 is provided instead of the optical receiver 300. The optical receiver 301 corresponds to the optical receiver 6 shown in Figure 2.

[0073] The optical receiver 301 comprises a noise load unit 350, a coherent receiver 310, a demodulation / decoding unit 330, and an index value calculation unit 340. The noise load unit 350 and the coherent receiver 310 correspond to the receiver 61 shown in Figure 2, and the demodulation / decoding unit 330 corresponds to the digital signal processing unit 62 shown in Figure 2. The noise load unit 350 is, for example, an amplifier or a variable attenuator. The noise load unit 350 receives the optical signal transmitted through the transmission path 5 and loads noise onto the received optical signal. The noise load unit 350 outputs the noise-loaded optical signal to the coherent receiver 310.

[0074] The processing of the optical transmission system 101 is the same as that of the optical transmission system 100 of the first embodiment shown in Figure 5, the optical transmission system 100 of the second embodiment shown in Figure 8, and the optical transmission system 100 of the third embodiment shown in Figure 9, except for the following point. That is, in step S3 of Figures 5 and 9, the noise load unit 350 of the optical receiver 301 starts loading noise onto the received signal. In step S4 of Figures 5 and 9, the noise load unit 350 of the optical receiver 301 receives the optical signal transmitted by the optical transmitter 200 and transmitted through the transmission path 5. The noise load unit 350 loads noise onto the received optical signal and outputs it to the coherent receiver 310. The coherent receiver 310 coherently receives the optical signal output from the noise load unit 350. The coherent receiver 310 converts the optical signal into an electrical signal received signal through coherent reception and outputs the converted received signal to the demodulation / decoding unit 330. Furthermore, in step S7 of Figures 5 and 9, the noise load instruction unit 430 instructs the noise load unit 350 of the optical receiver 301 to increase the amount of noise, and the noise load unit 350 increases the noise loaded onto the received signal. Also, in the process of step S21 of Figures 8 and 9, the noise load unit 350 of the optical receiver 301 receives the optical signal transmitted by the optical transmitter 200 and transmitted through the transmission path 5, and outputs the received optical signal to the coherent receiver 310 without loading noise onto it. The coherent receiver 310 converts the optical signal into an electrical signal by coherent reception and outputs the converted received signal to the demodulation / decoding unit 330.

[0075] (Principle of this embodiment) The principle by which the optimal DPD compensation amount can be obtained using the above-described embodiment will now be explained. Figure 11 shows the relationship between OSNR and NGMI in an optical transmission system. As shown in Figure 11, the higher the OSNR, the higher the NGMI in the optical receiver and the fewer the FEC corrections. The lower limit value indicates the lower limit of NGMI at which the demodulation and decoding result in the optical receiver becomes error-free. The ROSNR of an optical transmission system corresponds to the OSNR at which the NGMI is the lower limit value. However, since the ROSNR is unknown, optical transmission systems are usually operated with an OSNR that has an arbitrary margin from the ROSNR.

[0076] Figure 12 shows the relationship between OSNR and NGMI for different DPD compensation amounts. When noise is added to the optical receiver while the DPD compensation amount in the optical transmitter remains constant, NGMI decreases along the curve representing the relationship between OSNR and NGMI, approaching the lower limit. If even larger noise is added, NGMI falls below the lower limit. In other words, beyond a certain noise level, an error occurs in the signal reception processing of the optical receiver. The larger the amount of noise added before the error occurs, the higher the NGMI before noise was added (above the lower limit) and the lower the ROSNR.

[0077] Therefore, the optical transmission system determines the amount of noise added for each DPD with different compensation amounts until an error occurs in the optical receiver, i.e., until the NGMI falls below the lower limit. When the amount of noise added until the NGMI falls below the lower limit is maximum, the optimal compensation amount DPD with the highest noise tolerance is applied, and the ROSNR of the optical transmission system can be considered to be the lowest.

[0078] FIG. 13 is a diagram showing the relationship between OSNR and NGMI when the margin is small. When the margin is small, as shown in FIG. 13, the change in the DPD compensation amount due to the noise amount is small. In this case, since there is no problem with optimization anywhere, it is not always necessary to load noise on the received signal in the optical receiver. Therefore, the control device may obtain the index value in the optical receiver while changing the compensation amount of the DPD in the optical transmitter, and obtain the compensation amount when the index value is the best. The indexes in this case are NGMI, Q value, pre-FEC BER, post-FEC BER, number of corrections of FEC, EVM, etc.

[0079] NGMI is measured as follows. First, GMI (generalized mutual information amount) represents the information amount of one symbol of the received signal after transmission. GMI is calculated from the log-likelihood ratio (LLR) and the transmitted bits as shown in the following equation (1) (for example, see Reference 1).

[0080]

Equation

[0081] N is the number of symbols used for the calculation of GMI, n is the index representing the nth symbol, M is the number of bit levels, m is the mth digit of the bit level, b m,n and l m,n are the mth digit of the transmitted bit and the received LLR in the nth symbol. Also, H(s m ) is the entropy of s m when the probability distribution of the symbol of amplitude s m is P(s m ). GMI depends on the modulation format. Therefore, NGMI is calculated by normalizing as shown in the following equations (2) and (3). NGMI can be compared regardless of the modulation format (for example, see Reference 2).

[0082]

Equation

[0083]

number

[0084] Furthermore, BER represents the rate of erroneous data relative to all received data. Pre-FEC BER is an indicator of the quality of the communication path, using the BER before error correction. Post-FEC BER is an indicator of the quality after FEC decoding, using the BER after error correction. Typically, there are many errors before error correction, so the pre-FEC BER is a large value, and after error correction it becomes a small value such as 10-15. The Q value is an indicator of the quality of the communication path. If erfc is the complementary error function, then BER = (1 / 2)erfc(Q / √2), so the Q value is almost synonymous with pre-FEC BER. The FEC error rate, like the BER measurement, is an indicator of the error rate during error correction. EVM is an indicator that measures how much the data deviates from the original symbol point (see, for example, reference 3).

[0085] (Reference 1) Masanori Nakamura, "Research on the advancement of optical modulation and demodulation methods for long-distance, high-capacity fiber transmission," pp. 70-71, [online], 2021, Internet<https: / / ir.library.osaka-u.ac.jp / repo / ouka / all / 82241 / 31916_Dissertation.pdf>

[0086] (Reference 2) J. Cho, et al., "Normalized Generalized Mutual Information as a Forward Error Correction Threshold for Probabilistically Shaped QAM", European Conference on Optical Communication (ECOC) 2017.

[0087] (Reference 3) Osaka Institute of Technology, Faculty of Engineering, Department of Electronic and Information Communication Engineering, Intelligent Network Laboratory Homepage, Lecture Materials, 8. Digital Communication and Noise 8.5 EVM, [online], [Retrieved November 8, 2022], Internet<http: / / www.oit.ac.jp / elc / ~kumamoto / radio / 08.pdf>

[0088] Figure 14 shows the relationship between OSNR and NGMI in an optical transmission system when the margin is large. Figure 15 shows the relationship between OSNR and SNR in an optical transmission system when the margin is large. When the margin, which is the difference between ROSNR and the operational OSNR, is large, the optimal compensation amount of the DPD changes greatly depending on the amount of noise in the transmission path, as shown in Figure 14. This is because, as shown in Figure 15, with a DPD that fully compensates on the transmitting side (α=1), the SNR improves when the amount of noise in the transmission path is large (OSNR is small), but when the amount of noise in the transmission path is small (OSNR is large), the SNR deteriorates compared to when there is no compensation (α=0). Therefore, the optimal point of ROSNR is determined by loading noise in the optical receiving equipment of the optical transmission system. In this case, post-FEC BER can be used as an index representing reception quality. This is because the OSNR is unknown (cannot be measured), so even if NGMI or the like is used as an index, it does not mean that the OSNR has been optimized for the operational OSNR.

[0089] When the margin is large, if we refer to pre-error correction metrics such as NGMI, even if we optimize with ROSNR, the optimal compensation amount for DPD may change in operational OSNR. The premise of this embodiment is to "minimize ROSNR in order to maximize the performance of the optical transmission system." On the other hand, as mentioned above, in actual optical transmission systems, it is common to operate with a high OSNR, which is the ROSNR plus a margin, to avoid bit errors. If the ROSNR can be lowered, the OSNR after adding the margin to the ROSNR will also be lower, making it possible to transmit signals over longer distances (low OSNR systems).

[0090] In this case, operating at an OSNR higher than ROSNR means that indicator values ​​such as pre-FEC BER, Q value, and NGMI will show much better values ​​than those at the error-free NGMI lower limit, i.e., values ​​when operating at ROSNR, rather than values ​​near ROSNR. Even if we load noise in the OSNR region, which is much higher than ROSNR, and search for the optimal DPD compensation amount, we will not know what noise level is needed to determine the optimal DPD amount, and therefore we will not be able to achieve the original objective of minimizing ROSNR. For example, in Figure 6, the cases of α=0.6 and α=0.8 cross, which shows that the optimal DPD changes slightly between the high OSNR region (i.e., the region with a relatively small amount of loaded noise) and the region near ROSNR (i.e., the region near the NGMI lower limit).

[0091] On the other hand, post-FEC BER is error-free at OSNRs higher than ROSNR, but produces bit errors at OSNRs lower than ROSNR. This is consistent with Figure 14, where error-free occurs when NGMI exceeds the lower limit. However, when NGMI is used as an indicator, the relationship between OSNR and NGMI crosses when the margin is large, as shown in Figure 6. Therefore, when the margin is large, post-FEC BER is used as an indicator. By increasing the amount of noise loaded by the optical receiver while monitoring post-FEC BER, it is possible to determine whether the noise level has reached a level equivalent to ROSNR by whether or not bit errors occur. By comparing this noise level with signals of different DPD compensation levels, the DPD compensation level that was able to load the most noise level can be determined to be the optimal DPD compensation level near ROSNR.

[0092] The following describes an example of the hardware configuration of control devices 4 and 400. Figure 16 is a device configuration diagram showing an example of the hardware configuration of control device 4. Control devices 4 and 400 include a processor 91, a storage unit 92, a communication interface 93, and a user interface 94.

[0093] The processor 91 is a central processing unit that performs calculations and control. The processor 91 is, for example, a CPU (central processing unit) or a GPU (Graphics Processing Unit). The processor 91 reads and executes programs from the memory unit 92. The memory unit 92 further includes a work area for when the processor 91 executes various programs. The communication interface 93 connects to other devices for communication. The user interface 94 consists of input devices such as a keyboard, pointing device (mouse, tablet, etc.), buttons, and touch panel, as well as display devices such as a display. Human input is received through the user interface 94.

[0094] The functions of the control devices 4 and 400 are realized by the processor 91 reading and executing a program from the memory unit 92. Note that all or part of these functions may be implemented using hardware such as an ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or FPGA (Field Programmable Gate Array). The programs of the control devices 4 and 400 may be transmitted via a telecommunications line.

[0095] According to the embodiment described above, the optical transmission system comprises an optical transmitter, an optical receiver, and a control device. The optical transmitter comprises a transmission signal generation unit and an optical transmission unit. The transmission signal generation unit generates a pre-equalized transmission signal. The optical transmission unit converts the transmission signal from an electrical signal to an optical signal and transmits the converted optical signal. The optical receiver comprises an optical receiver, a signal processing unit, and a noise load unit. For example, the optical receiver corresponds to the coherent receiver 310 of the embodiment, and the signal processing unit corresponds to the demodulation / decoding unit 330 of the embodiment. The optical receiver receives an optical signal from the optical transmitter and converts the received optical signal into an electrical signal. The signal processing unit performs reception processing, including demodulation and decoding of the received signal. The noise load unit loads noise onto the optical signal received by the optical receiver or the received signal converted by the optical receiver. The control device comprises an information acquisition unit and a compensation amount determination unit. The information acquisition unit acquires information on the signal quality of the received signal when the compensation amount in pre-equalization and the magnitude of the noise loaded onto the optical signal or received signal are changed. The compensation amount determination unit determines the compensation amount for pre-equalization in the optical transmission device based on the information acquired by the information acquisition unit.

[0096] The transmission signal generation unit performs Digital Predistortion (DPD) processing on the transmission signal to pre-equalize signal distortion caused by bandwidth limitations of the analog devices of the optical transmission equipment.

[0097] The compensation amount determination unit obtains the compensation amount when the conditions for signal quality being better than a predetermined level are met and the noise level is at its maximum.

[0098] If the difference between the operational transmission quality of the optical signal received by the optical receiver and the minimum transmission quality required to satisfy the condition that the signal quality is better than a predetermined level is small, the information acquisition unit acquires information on the signal quality of the received signal when no noise is added and the compensation amount in pre-equalization is changed. The compensation amount determination unit determines the compensation amount for pre-equalization in the optical transmitter based on the information acquired by the information acquisition unit.

[0099] The above conditions ensure signal quality that allows for error-free decoding.

[0100] While embodiments of this invention have been described in detail above with reference to the drawings, the specific configurations are not limited to these embodiments and include designs and the like that do not depart from the spirit of this invention. [Explanation of symbols]

[0101] 1.11 Optical transmission systems 2. Optical Transmitter 3.6 Optical receiving device 4. Control device 5. Transmission lines 21 Digital signal processing unit 22 Transmitters 31 Receiver 32 Digital signal processing unit 61 Receiver 62 Digital signal processing unit 91 processors 92 Memory section 93 Communication Interface 94 User Interface 100, 101 Optical transmission systems 200 Optical Transmitter 210 Transmission signal generation unit 211 Encoding section 212 Mapping section 213 Waveform shaping section 214 Pre-equalization section 215 Digital-to-analog conversion section 220 Optical Transmitter 221 Modulator Driver 222 Modulation section 300, 301 Optical receiving device 310 Coherent Receiver 320 Noise Load Section 330 Demodulation and Decoding Unit 331 Analog-to-Digital Conversion Section 332 Chromatic dispersion compensation section 333 Polarization Fluctuation Compensation Unit 334 Frequency offset section 335 Carrier Phase Compensation Unit 336 Symbol determination unit 337 Decoding Unit 340 Indicator Value Calculation Unit 350 Noise Load Section 400 Control Unit 410 Storage section 420 Compensation amount instruction section 430 Noise Load Indicator 440 Information Acquisition Department 450 Compensation amount determination section

Claims

1. An optical transmission system comprising an optical transmitting device, an optical receiving device, and a control device, The optical transmitting device is A transmission signal generation unit that generates a pre-equalized transmission signal, The system includes an optical transmission unit that converts the transmission signal from an electrical signal to an optical signal and transmits the converted optical signal, The aforementioned optical receiving device is An optical receiving unit that receives the aforementioned optical signal, coherently receives the received optical signal and converts it into an electrical signal, A signal processing unit that performs reception processing including demodulation and decoding of the received signal, The optical receiving unit comprises a noise loader that loads noise onto the received signal coherently received by the optical receiving unit, The control device is An information acquisition unit that acquires information on the signal quality of the received signal when the compensation amount in the pre-equalization and the magnitude of the noise loaded onto the received signal are changed, The system comprises a compensation amount determination unit that determines the amount of pre-equalization compensation in the optical transmission device based on the information acquired by the information acquisition unit, Optical transmission system.

2. The transmission signal generation unit performs Digital Predistortion (DPD) processing on the transmission signal to pre-equalize signal distortion caused by bandwidth limitations of the analog device of the optical transmission apparatus. The optical transmission system according to claim 1.

3. The compensation amount determination unit determines the compensation amount for pre-equalization as the compensation amount that maximizes the amount of noise that can be added before the signal quality reaches the acceptable lower limit. The optical transmission system according to claim 1.

4. When the difference between the operational transmission quality of the optical signal received by the optical receiver and the minimum transmission quality required to satisfy the condition that the signal quality is of a predetermined or better quality is small, The information acquisition unit acquires information on the signal quality of the received signal when the noise is not applied and the compensation amount in the pre-equalization is changed. The compensation amount determination unit determines the compensation amount for pre-equalization in the optical transmission device based on the information acquired by the information acquisition unit. The optical transmission system according to claim 1.

5. The aforementioned conditions are that the signal quality allows for error-free decoding. The optical transmission system according to claim 4.

6. An information acquisition unit obtains information on the signal quality of a received signal when the compensation amount for pre-equalization in an optical transmission device that converts a pre-equalized transmission signal from an electrical signal to an optical signal and transmits the converted optical signal is changed, and the magnitude of the noise loaded onto the coherently received received signal in an optical reception device that coherently receives the received optical signal, converts it to an electrical signal received signal, and demodulates and decodes the coherently received received signal is changed. A compensation amount determination unit determines the amount of pre-equalization compensation in the optical transmission device based on the information acquired by the information acquisition unit, A control device equipped with the following features.

7. A transmission step in which an optical transmission device converts a pre-equalized transmission signal from an electrical signal to an optical signal and transmits the converted optical signal, The optical receiving device receives the optical signal, and the received optical signal is coherently received and converted into an electrical signal. The optical receiving device performs a signal processing step which includes demodulating and decoding the received signal, The optical receiving device includes a noise loading step in which it loads noise onto the received signal that was coherently received in the receiving step, An information acquisition step in which the control device obtains information on the signal quality of the received signal when the compensation amount in the pre-equalization and the magnitude of the noise loaded onto the received signal are changed, The control device performs a compensation amount determination step in which it determines the amount of pre-equalization compensation in the optical transmission device based on the information acquired in the information acquisition step, A method for determining the amount of compensation, comprising the following characteristics.

8. Computers, A program for causing the control device to function as described in claim 6.

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