Reception device and reception method

Abstract

This reception device includes a branch unit, a first decoding unit, a removal unit, and a second decoding unit. The branch unit branches reception waveform data of an optical signal subjected to frequency modulation and intensity modulation into a first reception signal and a second reception signal. The first decoding unit decodes an intensity modulation component of the first reception signal. The intensity modulation removal unit removes an intensity modulation component from the second reception signal using an intensity removal signal generated by inverting the intensity modulation component decoded by the first decoding unit. The second decoding unit decodes the second reception signal from which the intensity modulation component has been removed.

Description

Receiving device and receiving method

[0001] The present invention relates to a receiving device and a receiving method.

[0002] Currently, optical subscriber networks use PON (Passive Optical Network) systems to provide users with high-speed communication services economically. In a PON system, multiple subscriber devices (ONUs: Optical Network Units) share a portion of an optical fiber transmission line with an optical line terminal (OLT).

[0003] Looking further into the future, it has been proposed to use an APN (All-Photonics Network) for communication between users, including not only subscriber networks but also networks requiring low latency such as 5G, 6G, and other high-speed, low-latency wireless services, and data center networks (see, for example, Non-Patent Document 1). The APN is intended to accommodate communication in a directly optically connected network, eliminating optical-electrical conversion and electrical routing processing as much as possible on the communication path between users.

[0004] A common challenge for both of these technologies is how to increase speed and transmission distance while maintaining a simple and economical configuration of the user equipment located on the user side.

[0005] Therefore, there is a technology in PON systems that aims to increase speed and transmission distance while maintaining a simple configuration of user equipment (see, for example, Non-Patent Document 2). Figure 11 shows the configuration of a PON system using this technology. The ONU, which is the transmitter for upstream communications, uses an EA (Electro Absorption) modulator integrated direct modulation diode to generate and transmit a continuous phase frequency modulation (CPFSK) signal utilizing laser chirp. The OLT, which is the receiver for upstream communications, digitally coherently receives the signal transmitted from the ONU.

[0006] Figure 12 shows an example of the configuration of an APN (see, for example, Non-Patent Document 3). In an APN, for communication between short-distance user devices, intensity-modulated (IM) signals are transmitted and received using the same EA modulator integrated direct modulation diode as described above. These short-distance user devices communicate directly with each other using the loopback function of the PhGW (optical gateway), which is an optical node in the APN. On the other hand, for long-distance transmission, user devices use CPFSK modulation for communication between repeaters located in the local network.

[0007] Previous studies on increasing the speed of CPFSK signals have included a transmitter configuration that increases the multilevel level of the signal applied to a directly modulated laser to improve the number of information bits that can be transmitted per symbol (see, for example, Non-Patent Document 4). Assume that the multilevel modulation signal applied to the directly modulated laser of the transmitter is a four-level pulse amplitude modulation signal (PAM4; Pulse Amplitude Modulation 4). Figure 13 shows an eye pattern of the modulated signal output from the directly modulated laser, Figure 14 shows the relationship between the current applied to the directly modulated laser and the center frequency and intensity, and Figure 15 shows an example of a received constellation. As shown in Figure 15, the center frequency of the laser shifts depending on the applied current, and the signal is superimposed on the fluctuating component of this frequency. While this method also allows for increased speed, it only increases speed in the phase direction. Therefore, when a high multilevel level is used, as with multilevel PSK (Phase Shift Keying) modulation, the distance between signal points decreases, resulting in a significant increase in the required SNR to ensure a predetermined signal quality. To prevent SNR degradation due to the narrowing of the distance between signal points caused by multi-level modulation only in the phase direction, there are methods such as M-ary quadrature amplitude modulation (M-QAM). However, since the frequency-modulated signal is generated using a directly modulated laser, it is not possible to apply the CPFSK signal and the intensity-modulated signal independently. In such cases, an external modulator can be used to apply an intensity-modulated signal independently of the CPFSK signal, but the influence of the intensity-modulated signal will cause degradation of the CPFSK signal.

[0008] Kawahara et al., "Optical Full-Mesh Network Configuration Technology Supporting All-Photonics Networks," NTT Technical Journal, Vol. 32, No. 3, 2020, pp. 18-21. M. Fujiwara, R. Koma, J. Kani, K. Suzuki and A. Otaka, "Performance evaluation of CPFSK transmitters for TDM-based digital coherent PON upstream," 2017 Optical Fiber Communications Conference and Exhibition (OFC), 2017. R. Koma, K. Hara, T. Kanai, J. Kani and T. Yoshida, "Novel EA-DFB Mode-Switching Transmitter Supporting Continuous Phase Frequency Shift Keying and Intensity Modulation for All-Photonics Networks," 2021 European Conference on Optical Communication (ECOC), 2021, doi: 10.1109 / ECOC52684.2021.9605834. D. Che, F. Yuan, Q. Hu and W. Shieh, "Frequency Chirp Supported Complex Modulation of Directly Modulated Lasers," Journal of Lightwave Technology, vol. 34, no. 8, pp. 1831-1836, April 15, 2016, doi: 10.1109 / JLT.2015.2512298.

[0009] In the CPFSK signal transmission and reception methods using a directly modulated laser that have been studied so far, the focus has been on the frequency modulation component alone, without detecting the intensity modulation component that accompanies the bias modulation of the directly modulated laser. To further increase the speed, a configuration in which an intensity modulation component is applied by an external modulator could be considered, but the intensity modulation component would degrade the received signal characteristics.

[0010] In view of the above circumstances, an object of the present invention is to provide a receiving device and a receiving method that can receive an intensity-modulated and frequency-modulated signal in such a way as to improve the characteristics of the frequency-modulated component.

[0011] A receiving device according to one embodiment of the present invention comprises a branching unit that branches received waveform data of a frequency-modulated and intensity-modulated optical signal into a first received signal and a second received signal; a first decoding unit that decodes the intensity-modulated component of the first received signal; an intensity-modulation removal unit that removes the intensity-modulated component from the second received signal using an intensity removal signal generated by inverting the intensity-modulated component decoded by the first decoding unit; and a second decoding unit that decodes the second received signal from which the intensity-modulated component has been removed.

[0012] A receiving method according to one aspect of the present invention includes a branching step of branching received waveform data of a frequency-modulated and intensity-modulated optical signal into a first received signal and a second received signal; a first decoding step of decoding the intensity-modulated component of the first received signal; an intensity-modulation removal step of removing the intensity-modulated component from the second received signal using an intensity removal signal generated by inverting the intensity-modulated component decoded in the first decoding step; and a second decoding step of decoding the second received signal from which the intensity-modulated component has been removed.

[0013] The present invention makes it possible to receive intensity-modulated and frequency-modulated signals in a manner that improves the characteristics of the frequency-modulated component.

[0014] FIG. 1 is a configuration diagram of a receiving device according to a first embodiment. FIG. 2 is a diagram showing an example of the intensity and electric field of a CPFSK signal according to the first embodiment. FIG. 3 is a diagram showing an example of the configuration of a decoding processing unit according to the first embodiment. FIG. 4 is a diagram showing an example of the configuration of a decoding processing unit according to the first embodiment. FIG. 5 is a diagram showing an example of the configuration of a decoding processing unit according to the first embodiment. FIG. 6 is a diagram showing an example of the configuration of a decoding processing unit according to the second embodiment. FIG. 7 is a diagram showing an example of the configuration of an intensity removal signal generating unit according to the second embodiment. FIG. 8 is a diagram showing an example of the configuration of a decoding processing unit according to the second embodiment. FIG. 9 is a diagram showing an example of the configuration of a decoding processing unit according to the third embodiment. FIG. 10 is a diagram showing an example of the configuration of a decoding processing unit according to the fourth embodiment. FIG. 11 is a diagram showing the configuration of a PON system. FIG. 12 is a diagram showing an example of the configuration of an APN. FIG. 13 is a diagram showing the eye pattern of a multi-level modulated signal. FIG. 14 is a diagram showing the relationship between the applied current to a directly modulated laser and the center frequency and intensity. FIG. 15 is a diagram showing an example of a received constellation.

[0015] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The same parts in the drawings will be designated by the same reference numerals, and their description will be omitted. This embodiment is an optical signal receiving method that combines an intensity modulation method using an intensity modulator and a directly modulated laser with a phase-frequency modulation method. This embodiment increases the speed of a CPFSK transmission method that uses an EA-DFB (Distributed-Feedback Laser).

[0016] Conventionally, transmission speeds have been increased by improving the multilevel level of frequency modulation and polarization multiplexing. However, in a transmission / reception configuration in which intensity modulation is applied in addition to frequency modulation to increase the multilevel level, the SNR (Signal to Noise Ratio) of the frequency modulation component deteriorates due to the influence of the intensity modulation component. In this embodiment, the intensity modulation component is decoded first, and the decoded result is used to remove the intensity modulation signal component from the received signal. This is expected to improve the characteristics of the frequency modulation component.

[0017] 1 is a block diagram of a receiving device 10 according to a first embodiment of the present invention. The receiving device 10 is a receiver that simultaneously receives an intensity and CPFSK signal. That is, the receiving device 10 receives a frequency-modulated and intensity-modulated optical signal. The receiving device 10 includes a local oscillator (LO) 11, a receiving unit 12, an AD (analog-to-digital) conversion unit 13, and a decoding processing unit 14.

[0018] The local light source 11 is, for example, a laser diode (LD) and generates a local signal, which is locally oscillated light of a predetermined wavelength. The receiver 12 is a typical polarization phase diversity receiver. The receiver 12 performs digital coherent reception on the received optical signal, and performs polarization separation and phase separation on the received optical signal. That is, the receiver 12 separates the received optical signal into an X-polarized wave and a Y-polarized wave. The receiver 12 photoelectrically converts the optical signals into I and Q components of the X-polarized wave and I and Q components of the Y-polarized wave, which are obtained by interfering the X-polarized wave and the Y-polarized wave with the local signal generated by the local light source 11. The receiver 12 converts the received signal into an electrical analog signal and outputs it to the AD converter 13.

[0019] The AD conversion unit 13 is an ADC (Analog-Digital Converter). The AD conversion unit 13 receives the electrical analog signal converted by the receiving unit 12 and converts the received analog signal into an electrical digital signal. The AD conversion unit 13 outputs the received signal converted into a digital signal to the decoding processing unit 14. The decoding processing unit 14 receives the received digital signal indicating received waveform data of the optical signal from the AD conversion unit 13 and performs decoding processing to decode the intensity-modulated signal data and the CPSFK signal data.

[0020] 2 is a diagram showing an example of the electric field of the intensity-CPFSK signal received by the receiving device 10. The intensity-CPFSK signal is an optical signal obtained by applying an intensity modulation component based on IM data to a CPFSK signal obtained by CPFSK-modulating CPFSK data.

[0021] 3 is a diagram showing an example of the configuration of the decoding processing unit 200. The decoding processing unit 200 is used as the decoding processing unit 14 in FIG. 1. The decoding processing unit 200 has a chromatic dispersion compensation unit 201, a polarization compensation unit 202, an intensity signal processing unit 203, and a CPFSK signal processing unit 204. In the decoding processing unit 200 of this embodiment, the intensity signal processing unit 203 first detects and determines the intensity component of the received signal, and then the CPFSK signal processing unit 204 removes the intensity modulation component from the CPFSK signal based on the determination result.

[0022] The chromatic dispersion compensator 201 compensates for chromatic dispersion suffered by the received signal as it propagates through the fiber, and outputs the chromatic dispersion-compensated received signal to the polarization compensator 202. The polarization compensator 202 estimates polarization fluctuations of the X polarization and Y polarization suffered by the received signal as it propagates through the fiber, and compensates for the estimated polarization fluctuations. The polarization compensator 202 functions as a branching unit that branches the polarization-compensated received signal into two. The polarization compensator 202 outputs one of the branched received signals to the intensity signal processor 203, and outputs the other branched received signal to the timing adjuster 205.

[0023] The intensity signal processing unit 203 converts the received signal into an intensity information signal and decodes the converted signal. The intensity signal processing unit 203 outputs the IM data obtained by decoding to a subsequent stage. Furthermore, the intensity signal processing unit 203 detects timing information from the received signal and generates an intensity modulation signal removal pattern that is the inverse pattern of the intensity modulation signal based on the received timing information. The intensity signal processing unit 203 outputs the generated intensity modulation signal removal pattern to the intensity modulation removal unit 206 as an intensity removal signal.

[0024] The CPFSK signal processing unit 204 includes a timing adjustment unit 205, an intensity modulation removal unit 206, and a CPFSK signal decoding unit 207. The CPFSK signal processing unit 204 operates based on the output of the intensity signal processing unit 203. Therefore, the timing adjustment unit 205 matches the timing at which the intensity removal signal is input from the intensity signal processing unit 203 to the intensity modulation removal unit 206 with the timing at which the received signal is input to the intensity modulation removal unit 206. As a result, the timing adjustment unit 205 adjusts the start timing of processing so that the same symbol is input to the intensity modulation removal unit 206 from each path. Note that if the amount of delay caused by the intensity signal processing unit 203 is known and fixed, the timing adjustment unit 205 may adjust the processing timing without using timing information input from an external source.

[0025] The intensity modulation removal unit 206 removes the intensity modulation component from the received signal input from the timing adjustment unit 205, using the intensity removal signal input from the CPFSK signal processing unit 204. The intensity modulation removal unit 206 outputs the received signal from which the intensity modulation component has been removed to the CPFSK signal decoding unit 207.

[0026] The CPFSK signal decoding unit 207 performs CPFSK signal decoding processing on the received signal input from the intensity modulation removal unit 206. The CPFSK signal decoding unit 207 has a differential detection unit 301, a frequency offset compensation unit 302, an adaptive equalization filter 303, a phase compensation unit 304, and a decoding unit 305.

[0027] The differential detection unit 301 performs 1-bit differential detection of the received signal. The frequency offset compensation unit 302 compensates for the frequency offset of the received signal that has been 1-bit differentially detected by the differential detection unit 301. The adaptive equalization filter 303 applies an adaptive equalization filter to the received signal that has been frequency offset compensated for by the frequency offset compensation unit 302. The phase compensation unit 304 compensates for the phase of the received signal to which the adaptive equalization filter 303 has been applied. The decoding unit 305 decodes the phase-compensated received signal. This obtains the CPFSK data transmitted from the transmitting device. The decoding unit 305 outputs the decoded CPFSK data to a subsequent stage. Note that the configuration of the CPFSK signal decoding unit 207 is an example, and this embodiment does not depend on the configuration of this decoding unit.

[0028] A decoding processing unit 210 shown in FIG. 4 may be used instead of the decoding processing unit 14 in FIG. 1 . FIG. 4 is a diagram showing an example of the configuration of the decoding processing unit 210. In the decoding processing unit 210 shown in FIG. 4 , the same components as those in the decoding processing unit 200 shown in FIG. 3 are designated by the same reference numerals, and their description will be omitted. The decoding processing unit 210 shown in FIG. 4 differs from the decoding processing unit 200 shown in FIG. 3 in that it includes an intensity signal processing unit 211 instead of the intensity signal processing unit 203. For example, assuming a configuration in which a directly modulated laser as described in Non-Patent Document 4 is used as a transmitter, the CPFSK data obtained by the CPFSK signal decoding unit 207 decoding the received signal and the IM data obtained by the intensity signal processing unit 203 shown in FIG. 3 decoding the intensity modulated component of the received signal are the same as each other. Therefore, the intensity signal processing unit 211 shown in FIG. 4 does not output the IM data obtained by the same processing as the intensity signal processing unit 203 as an actual data signal.

[0029] A decoding processing unit 220 shown in FIG. 5 may be used as the decoding processing unit 14 of FIG. 1. FIG. 5 is a diagram showing an example of the configuration of the decoding processing unit 220. In the decoding processing unit 220 shown in FIG. 5, the same components as those in the decoding processing unit 200 shown in FIG. 3 are designated by the same reference numerals, and their description will be omitted. The decoding processing unit 220 shown in FIG. 5 differs from the decoding processing unit 200 shown in FIG. 3 in that it further includes a differential detection unit 221 and a CPFSK signal processing unit 222 instead of the CPFSK signal processing unit 204. The CPFSK signal processing unit 222 includes a timing adjustment unit 205 and a CPFSK signal decoding unit 223. The CPFSK signal decoding unit 207 differs from the CPFSK signal decoding unit 223 shown in FIG. 3 in that an intensity modulation removal unit 321 is provided between the differential detection unit 301 and the frequency offset compensation unit 302.

[0030] The differential detection unit 221 performs differential detection processing on the intensity removal signal output by the intensity signal processing unit 203, and outputs the result to the intensity modulation removal unit 321. The intensity modulation removal unit 321 removes the intensity modulation component from the received signal on which the differential detection unit 301 has performed 1-bit differential detection, using the intensity removal signal input from the differential detection unit 221. The intensity modulation removal unit 321 outputs the received signal from which the intensity modulation component has been removed to the frequency offset compensation unit 302.

[0031] In this way, when the CPFSK signal decoding unit performs differential detection processing, it may perform differential detection processing on the output from the intensity signal processing unit, and the CPFSK signal decoding unit may remove the intensity modulation component from the received signal after differential detection processing using an intensity removal signal that has been subjected to differential detection processing.

[0032] Second Embodiment In a second embodiment, the intensity signal processing unit will be described in detail. The second embodiment will be described focusing on the differences from the first embodiment.

[0033] Fig. 6 is a diagram showing an example of the configuration of a decoding processing unit 230 according to the second embodiment. The decoding processing unit 230 is used as the decoding processing unit 14 in Fig. 1. In the decoding processing unit 230 shown in Fig. 6, the same components as those in the decoding processing unit 200 shown in Fig. 3 are denoted by the same reference numerals, and their description will be omitted. The decoding processing unit 230 shown in Fig. 6 differs from the decoding processing unit 200 shown in Fig. 3 in that it has an intensity signal processing unit 231 instead of the intensity signal processing unit 203. The intensity signal processing unit 231 has a decoding unit 500 and an intensity elimination signal generation unit 600.

[0034] The decoding unit 500 includes an absolute value processing unit 501, an adaptive equalization filter 502, and a decoding processing unit 503. The absolute value processing unit 501 obtains an intensity information signal by acquiring the absolute value of a received complex signal sampled at N times the symbol rate. The adaptive equalization filter 502 performs adaptive equalization processing on the intensity information acquired by the absolute value processing unit 501 using a time filter that filters only the real axis component. The decoding processing unit 503 downsamples the signal whose waveform distortion has been compensated for by the adaptive equalization processing, and then decodes it. The decoding processing unit 503 branches the IM data obtained by decoding into two, outputs one of the branched IM data to a subsequent stage, and outputs the other branched IM data to the intensity removal signal generation unit 600.

[0035] The intensity removal signal generator 600 includes a positive / negative conversion processor 601, an upsampling processor 602, and an amplitude adjuster 603. The positive / negative conversion processor 601 converts the positive sign data of the IM data output from the decoding processor 503 into negative sign data to generate a signal. The signal output from the decoding processor 503 is a signal with one sample per symbol. Meanwhile, a signal sampled at N times the symbol rate (N is an integer equal to or greater than 1) is input to the CPFSK signal decoder 207. The upsampling processor 602 then again upsamples the signal output from the positive / negative conversion processor 601 to match the sampling rate with the signal input to the CPFSK signal decoder 207. The amplitude adjuster 603 adjusts the amplitude of the signal upsampled by the upsampling processor 602 to match the amplitude of the intensity-modulated signal to be removed by the intensity modulation remover 206. The amplitude adjusting section 603 outputs the intensity elimination signal obtained by adjusting the amplitude to the intensity modulation removing section 206. It should be noted that the intensity elimination signal generating section 600 does not necessarily have to include the amplitude adjusting section 603.

[0036] Fig. 7 is a diagram showing an example configuration of the intensity signal processing unit 232. The decoding processing unit 230 shown in Fig. 6 may have the intensity signal processing unit 232 shown in Fig. 7 instead of the intensity signal processing unit 231. In the intensity signal processing unit 232 shown in Fig. 7, the same components as those in the intensity signal processing unit 231 shown in Fig. 6 are given the same reference numerals, and their description will be omitted. The intensity signal processing unit 232 shown in Fig. 7 includes a decoding unit 500 and an intensity removal signal generation unit 610. The intensity removal signal generation unit 610 has a configuration similar to that of the intensity removal signal generation unit 600 shown in Fig. 6, and further includes a waveform adjustment unit 604 subsequent to the amplitude adjustment unit 603.

[0037] The waveform adjustment unit 604 distorts the waveform of the intensity elimination signal whose amplitude has been adjusted by the amplitude adjustment unit 603, based on the frequency characteristics of the transmitter input from outside. Alternatively, the waveform adjustment unit 604 may acquire information on the frequency characteristics of the transmitter from the adaptive equalization filter 502 of the decoding unit 500. The intensity elimination signal generation unit 610 may include both the amplitude adjustment unit 603 and the waveform adjustment unit 604 as shown in Fig. 7, or may not include the amplitude adjustment unit 603 and may have the waveform adjustment unit 604 adjust the amplitude as well.

[0038] In the configuration of this embodiment, an intensity elimination signal (intensity modulation signal elimination pattern) is generated using a signal after intensity modulation component decoding. Therefore, if there is an error in the data obtained by decoding the intensity signal, the intensity modulation component cannot be removed in the intensity modulation removal unit 206, and noise may occur. Therefore, as a modified example, an intensity elimination signal may be generated using a signal from which errors have been removed by re-encoding after error correction decoding.

[0039] Fig. 8 is a diagram showing an example of the configuration of a decoding processing unit 250. The decoding processing unit 250 is used as the decoding processing unit 14 in Fig. 1. In the decoding processing unit 250 shown in Fig. 8, the same components as those in the decoding processing unit 230 shown in Fig. 6 are denoted by the same reference numerals, and their description will be omitted. The decoding processing unit 250 shown in Fig. 8 differs from the decoding processing unit 230 shown in Fig. 6 in that an intensity signal processing unit 251 is provided instead of the intensity signal processing unit 203.

[0040] The intensity signal processing unit 251 has a decoding unit 500 and an intensity removal signal generation unit 650. The intensity removal signal generation unit 650 includes an error correction decoding unit 651, an error correction encoding unit 652, a positive / negative conversion processing unit 601, an upsampling processing unit 602, and an amplitude adjustment unit 603. The error correction decoding unit 651 performs error correction decoding on the IM data output by the decoding processing unit 503. The error correction encoding unit 652 re-encodes the IM data error-corrected by the error correction decoding unit 651 and outputs the re-encoded IM data to the positive / negative conversion processing unit 601. As a result, the intensity removal signal generation unit 650 generates an intensity removal signal from which errors have been removed.

[0041] Third Embodiment In a third embodiment, the timing adjustment circuit unit will be described in detail. The third embodiment will be described focusing on the differences from the first and second embodiments.

[0042] FIG. 9 is a diagram showing an example of the configuration of a decoding processing unit 260 according to the third embodiment. The decoding processing unit 260 is used as the decoding processing unit 14 in FIG. 1. In the decoding processing unit 260 shown in FIG. 9, the same components as those in the decoding processing unit 250 shown in FIG. 8 are denoted by the same reference numerals, and their description will be omitted. The decoding processing unit 260 shown in FIG. 9 differs from the decoding processing unit 250 shown in FIG. 8 in that a CPFSK signal processing unit 261 is provided instead of the CPFSK signal processing unit 204. The decoding processing unit 260 may further include a buffer 263. Moreover, the decoding processing unit 260 may include an intensity elimination signal generation unit 600 or 610 instead of the intensity elimination signal generation unit 650.

[0043] The CPFSK signal processing unit 261 includes a timing adjustment unit 262, an intensity modulation removal unit 206, and a CPFSK signal decoding unit 207. The timing adjustment unit 262 includes a buffer 701, an absolute value processing unit 702, and a correlation value calculation unit 703. The buffer 701 sequentially stores waveform data of the received signal output by the polarization compensation unit 202. The absolute value processing unit 702 obtains the absolute value of the partial waveform data stored in the buffer 701 by processing similar to that of the absolute value processing unit 501. The correlation value calculation unit 703 calculates the correlation value between the absolute value of the partial waveform data acquired by the absolute value processing unit 702 and the intensity removal signal output by the intensity removal signal generation unit 600. If the correlation value is equal to or greater than a predetermined value, the correlation value calculation unit 703 determines that signals with the same timing are being received, and outputs timing information to the buffer 701. When the buffer 701 receives the timing information, it inputs the stored waveform data to the intensity modulation removal unit 206 at the subsequent stage.

[0044] The buffer 263 stores the intensity removal signal output from the intensity signal processing unit 253. The buffer 263 delays the intensity removal signal by a time equivalent to the time required for the timing adjustment unit 262 to detect and determine the correlation value, and then outputs the signal to the intensity modulation removal unit 206. This allows the decoding processing unit 260 to match the timing of the symbols of the received waveform in the intensity modulation removal unit 206 with the symbols of the intensity removal signal.

[0045] (Fourth embodiment) The fourth embodiment relates to the details of the amplitude adjustment unit. In the fourth embodiment, a residual intensity component is detected at a stage subsequent to the intensity modulation removal unit and fed back to the amplitude adjustment unit.

[0046] FIG. 10 is a diagram showing an example of the configuration of a decoding processing unit 270 according to the fourth embodiment. The decoding processing unit 270 is used as the decoding processing unit 14 in FIG. 1. In the decoding processing unit 270 shown in FIG. 10, the same components as those in the decoding processing unit 260 shown in FIG. 9 are designated by the same reference numerals, and their description will be omitted. The decoding processing unit 270 shown in FIG. 10 differs from the decoding processing unit 260 shown in FIG. 9 in that it includes a CPFSK signal processing unit 271 instead of the CPFSK signal processing unit 261. In addition to the configuration of the CPFSK signal processing unit 261 shown in FIG. 9, the CPFSK signal processing unit 271 includes a residual intensity component detection unit 272 between the intensity modulation removal unit 206 and the CPFSK signal decoding unit 207. The residual intensity component detection unit 272 detects the residual intensity component of the received signal obtained by removing the intensity-modulated signal in the intensity modulation removal unit 206, and feeds the detected residual intensity component back to the amplitude adjustment unit 603. The amplitude adjusting section 603 adjusts the amplitude of the intensity removal signal based on the residual intensity component fed back from the residual intensity component detecting section 272 , and outputs the adjusted amplitude to the waveform adjusting section 604 .

[0047] According to the above-described embodiment, the receiving device includes a branching unit, a first decoding unit, an intensity modulation removal unit, and a second decoding unit. The branching unit branches received waveform data of a frequency-modulated and intensity-modulated optical signal into a first received signal and a second received signal. The branching unit corresponds, for example, to the polarization compensation unit 202 in the embodiments. The first decoding unit decodes the intensity-modulated component of the first received signal. The first decoding unit corresponds, for example, to the intensity signal processing units 203, 231, 232, 251 and the decoding unit 500 in the embodiments. The intensity modulation removal unit removes the intensity-modulated component from the second received signal using an intensity removal signal generated by inverting the intensity-modulated component decoded by the first decoding unit. The second decoding unit decodes the second received signal from which the intensity-modulated component has been removed. The second decoding unit corresponds, for example, to the CPFSK signal decoding units 207, 223 and the decoding unit 305 in the embodiments.

[0048] The receiving device may further include an intensity removal signal generator that generates an intensity removal signal by inverting the intensity modulation component decoded by the first decoder, adjusts the amplitude and / or waveform of the generated intensity removal signal, and outputs the adjusted signal to the intensity modulation removal unit.

[0049] The receiving device may further include a timing adjustment unit that adjusts the timing of inputting the second received signal and the intensity removal signal to the intensity modulation removal unit based on a correlation between the second received signal and the intensity removal signal generated by the intensity removal signal generation unit.

[0050] The intensity removal signal generation unit performs error correction decoding on the intensity modulation component decoded by the first decoding unit, and then performs error correction encoding on the intensity modulation component, and inverts the error correction encoded intensity modulation component to generate the intensity removal signal.

[0051] The intensity modulation removal unit removes the intensity modulation component from the differentially detected second received signal using an intensity removal signal that is differentially detected after inverting the intensity modulation component decoded by the first decoding unit.

[0052] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0053] REFERENCE SIGNS LIST 10 Receiver 11 Local light source 12 Receiver 13 AD converter 14 Decoding processor 200 Decoding processor 201 Wavelength dispersion compensation unit 202 Polarization compensation unit 203 Intensity signal processor 204 CPFSK signal processor 205 Timing adjustment unit 206 Intensity modulation removal unit 207 CPFSK signal decoder 210 Decoding processor 211 Intensity signal processor 220 Decoding processor 221 Delay detection unit 222 CPFSK signal processor 223 CPFSK signal decoder 230 Decoding processor 231 Intensity signal processor 232 Intensity signal processor 250 Decoding processor 251 Intensity signal processor 253 Intensity signal processor 260 Decoding processor 261 CPFSK signal processor 262 Timing adjustment unit 263 Buffer 270 Decoding processing unit 271 CPFSK signal processing unit 272 Residual intensity component detection unit 301 Differential detection unit 302 Frequency offset compensation unit 303 Adaptive equalization filter 304 Phase compensation unit 305 Decoding unit 321 Intensity modulation removal unit 500 Decoding unit 501 Absolute value processing unit 502 Adaptive equalization filter 503 Decoding processing unit 600 Intensity removal signal generation unit 601 Sign / sign conversion processing unit 602 Upsampling processing unit 603 Amplitude adjustment unit 604 Waveform adjustment unit 610 Intensity removal signal generation unit 650 Intensity removal signal generation unit 651 Correction decoding unit 652 Correction coding unit 701 Buffer 702 Absolute value processing unit 703 Correlation value calculation unit

Claims

1. A receiving device comprising: a branching unit that branches received waveform data of a frequency-modulated and intensity-modulated optical signal into a first received signal and a second received signal; a first decoding unit that decodes the intensity-modulated component of the first received signal; an intensity modulation removal unit that removes the intensity-modulated component from the second received signal using an intensity removal signal generated by inverting the intensity-modulated component decoded by the first decoding unit; and a second decoding unit that decodes the second received signal from which the intensity-modulated component has been removed.

2. The receiving device described in claim 1, further comprising an intensity removal signal generation unit that generates the intensity removal signal by inverting the intensity modulation component decoded by the first decoding unit, and performs amplitude adjustment and / or waveform adjustment on the generated intensity removal signal to output it to the intensity modulation removal unit.

3. The receiving device described in claim 2, further comprising a timing adjustment unit that adjusts the timing of inputting the second received signal and the intensity removal signal to the intensity modulation removal unit based on the correlation between the second received signal and the intensity removal signal generated by the intensity removal signal generation unit.

4. The receiving device described in claim 2, wherein the intensity elimination signal generation unit performs error correction decoding on the intensity modulation component decoded by the first decoding unit, and then performs error correction encoding on the intensity modulation component, and inverts the error correction encoded intensity modulation component to generate the intensity elimination signal.

5. The receiving device according to claim 1, wherein the intensity modulation removal unit removes the intensity modulation component from the second received signal which has been differentially detected, using an intensity removal signal which has been differentially detected after inverting the intensity modulation component decoded by the first decoding unit.

6. A receiving method comprising: a branching step of branching received waveform data of a frequency-modulated and intensity-modulated optical signal into a first received signal and a second received signal; a first decoding step of decoding an intensity-modulated component of the first received signal; an intensity-modulation removal step of removing the intensity-modulated component from the second received signal using an intensity removal signal generated by inverting the intensity-modulated component decoded in the first decoding step; and a second decoding step of decoding the second received signal from which the intensity-modulated component has been removed.

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