Abstract:
An optical transmission device for transmitting and receiving a multilevel-modulated optical signal includes a plurality of transmission frame processors for generating transmission frame signals accommodating a plurality of client signals that are each subjected to error correction processing and scrambling/descrambling processing, and a digital modulator/demodulator for mapping the transmission frame signals that are input to and output from the plurality of transmission frame processors to a multilevel signal. The digital modulator/demodulator performs digital modulation/demodulation, in which the plurality of transmission frame processors each have a function of shifting a phase of a pattern between a plurality of transmission frames to be mapped to a multilevel signal and to be digitally modulated/demodulated.

Description:
TECHNICAL FIELD 
     The present invention relates to an optical transmission device for transmitting and receiving a multilevel-modulated optical signal. 
     BACKGROUND ART 
     Standards for an optical signal for realizing large-capacity and long-haul communication are defined in the Optical Transport Network (OTN) as specified in ITU-T G.709. Among those, an OTN maintenance signal is formed of, for example, a repeating pattern such as ODU AIS (all “1”s), OCI (repeating “0110”), or LCK (repeating “0101”). In addition, in order to prevent signal degradation on a transmission path, such an OTN maintenance signal is specified to be subjected to scrambling operation before being output (see, for example, Non Patent Literature 1). 
     Further, there is disclosed an example of a 100 G long-haul transceiver for performing error correction processing for long-haul transmission and transmitting and receiving a polarization multiplexed QPSK signal (see, for example, Non Patent Literature 2). 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] Recommendation ITU-T G.709/Y.1331 (December 2009) “Interface for the Optical Transport Network (OTN)” 
         [NPL 2] OIF-MSA-100 GLH-EM-01.1, Optical Interface Forum, Sep. 20, 2011 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the related art has the following problem. 
     In a transmission system, in which independent optical channel transport unit—k (OTUk) frames of a plurality of systems are subjected to time-division multiplexing, the following problem arises when the related art is applied to perform mapping to a multilevel signal. 
     Every OTN maintenance signal has a fixed pattern even after scrambling. Accordingly, for example, in a case where 100 G OTU4 signals of two systems are mapped to a 200 G polarization multiplexed 16 QAM signal, when the 100 G signals are the same maintenance signals in both of the systems, a pattern having the same value for two consecutive bits is mapped to a 200 G polarization multiplexed 16 QAM pattern. 
     With this, even when the signal is randomized by the scrambling, the fixed pattern is output as a result, which may cause degradation of transmission characteristics. 
     The present invention has been made in order to solve the above-mentioned problem, and it is an object of the present invention to provide an optical transmission device capable of preventing performance degradation in an optical communication system using a multilevel (e.g., polarization multiplexed 16 QAM scheme) signal of several hundred-Gbps or, more, even at the time of transmission of data having a repeating or periodic pattern, such as an OTN maintenance signal. 
     Solution to Problem 
     According to one embodiment of the present invention, there is provided an optical transmission device for transmitting and receiving a multilevel-modulated optical signal, including: a plurality of transmission frame processors for generating transmission frame signals accommodating a plurality of client signals that are each subjected to error correction processing and scrambling/descrambling processing; and a digital modulator/demodulator for mapping the transmission frame signals that are input to and output from the plurality of transmission frame processors to a multilevel signal and performing digital modulation/demodulation, in which the plurality of transmission frame processors each have a function of shifting a phase of a pattern between a plurality of transmission frames to be mapped to a multilevel signal and be digitally modulated/demodulated. 
     Advantageous Effects of Invention 
     The optical transmission device according to the one embodiment of the present invention has the function of shifting the phase of the pattern between the plurality of transmission frames to be mapped to the multilevel signal and be digitally modulated/demodulated. Thus, it is possible to obtain the optical transmission device capable of preventing performance degradation in an optical communication system using a multilevel (e.g., polarization multiplexed 16 QAM scheme) signal of several hundred-Gbps or more, even at the time of transmission of data having a repeating or periodic pattern, such as an OTN maintenance signal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an optical transmission device according to a first embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a format of an OTUk frame to be processed by OTU4 framers of the optical transmission device according to the first embodiment of the present invention. 
         FIG. 3  is a diagram illustrating a format of an OTUkV frame to be processed by FEC processing circuits of the optical transmission device according to the first embodiment of the present invention. 
         FIG. 4  is a diagram illustrating a symbol transition in polarization multiplexed 16 QAM at the time of transmission of an OTN maintenance signal (ODU-AIS) in the optical transmission device according to the first embodiment of the present invention. 
         FIG. 5  is a diagram illustrating a symbol transition in polarization multiplexed 16 QAM at the time of transmission of an OTN maintenance signal (ODU-LCK) in the optical transmission device according to the first embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a configuration of an optical transmission device according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, a description is given of an optical transmission device according to preferred embodiments of the present invention with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a diagram illustrating a configuration of an optical transmission device according to a first embodiment of the present invention. For example, as a scheme for transmitting an optical signal, in order to realize an optical communication system realizing high-speed and long-haul transmission, such a scheme is adopted that information having a large number of bits is transmitted in a 1-symbol time period by combining quadrature amplitude modulation such as 16 quadrature amplitude modulation (16 QAM) with polarization multiplexing using polarization channels (X polarization and Y polarization). In addition, with in-phase and quadrature phase components for multilevel modulation and the X polarization and Y polarization for polarization multiplexing, eight kinds of signal components are transmitted. 
       FIG. 1  illustrates an exemplary case where 100 G signals of two systems are transmitted and received by 200 G polarization multiplexed 16 QAM. The optical transmission device illustrated in  FIG. 1  includes OTU4 framers  11  and  12  for processing a 100 G OTU signal, a 200 G transceiver  21  for processing a 200 G signal, an E/O  31  for converting an electrical signal into an optical signal, and an O/E  32  for converting an optical signal into an electrical signal. 
     The OTU4 framers  11  and  12  respectively include the following components.
         OTU4 frame generation/termination units  111  and  121  for 100 G signals #1 and #2   Maintenance generation signal processors  112  and  122  for the 100 G signals #1 and #2   OTU4 MLD units  113  and  123  for interfacing with the 200 G transceiver  21  by multiple lane distribution (MLD)       

     Further, the 200 G transceiver  21  includes the following components.
         OTU4 MLD units  211  and  221  for 100 G systems #1 and #2 for interfacing with the OTU4 framers  11  and  12  by the MLD   Memories (FIFOs)  212  and  222  capable of temporarily storing an OTU4 frame to shift a frame phase   TEC processing circuits  213  and  223  for the 100 G systems #1 and #2   Scramblers/descramblers  214  and  224  for the 100 G systems #1 and #2   A digital modulator/demodulator  215  for mapping a signal output from the scramblers  214  and  224  to symbols of an optical signal and performing digital modulation/demodulation processing   A DA converter (DAC)  216  for generating a modulated optical signal (analog)   An AD converter (ADC)  226  for converting a modulated optical signal (analog) into a digital signal       

     Herein, the memory (FIFO)  212 , the FEC processing circuit  213 , and the scrambler/descrambler  214 , and the memory (FIFO)  222 , the FEC processing circuit  223 , and the scrambler/descrambler  224  correspond to a plurality of transmission frame processors arranged respectively to a plurality of client signals. 
       FIG. 2  is a diagram illustrating a format of an OTUk frame to be processed by the OTU4 framers  11  and  12  of the optical transmission device according to the first embodiment of the present invention. More specifically, the OTUk frame illustrated in  FIG. 2  is formed of the following components.
         A payload for storing actual information data such as a client signal   A frame alignment overhead (FA OH) for frame synchronization   An OTUk OH and optical channel data unit-k overhead (ODUk OH) for maintenance and monitoring information   An optical channel payload unit-k overhead (OPUk OH) for payload mapping   An FEC redundant area for storing information of an error correction code for correcting bit errors caused due to deterioration in an optical quality after transmission       

     A Reed-Solomon code (hereinafter referred to as “RS code”) is usually used as an error correction code. Note that, in general, a part formed of FA OH, OTUk OH, ODUk OH, and OPUk OH is called “overhead”. 
     On the other hand,  FIG. 3  is a diagram illustrating a format of an OTUkV frame to be processed by the FEC processing circuits  213  and  223  of the optical transmission device according to the first embodiment of the present invention. As illustrated in  FIG. 3 , for example, an FEC redundant area is extended as compared with the FEC redundant area of  FIG. 2  to enhance error correction performance, to thereby realize long-haul transmission. 
       FIG. 4  is a diagram illustrating a symbol transition in polarization multiplexed 16 QAM at the time of transmission of an OTN maintenance signal (ODU-AIS) in the optical transmission device according to the first embodiment of the present invention. Only symbols having pairs of two consecutive bits having the same value make transitions. It is assumed that as illustrated in the left part of  FIG. 4 , after the ODU-AIS signal (all “1”s) is subjected to scrambling, a fixed pattern of “10 10 11 01 00” is obtained. In other words, as a result of calculating “EXOR” of “ODU-AIS signal (fixed value)×PRBS (fixed value)”, as indicated by “(A)” of  FIG. 4 , the same patterns are obtained in both of the two systems. 
     When the 100 G signals of the two systems (200 G) having a uniform frame phase are transmitted by the polarization multiplexed 16 QAM, symbol map data is as illustrated in the lower left part of  FIG. 4 . When this data is represented on a constellation map, symbols are mapped at positions surrounded by the broken line in the diagram of the right part of  FIG. 4 . In other words, only four symbol points from a top-left symbol point to a right-bottom symbol point make transitions. 
     Similarly,  FIG. 5  is a diagram illustrating a symbol transition in polarization multiplexed 16 QAM at the time of transmission of an OTN maintenance signal (ODU-LCK) in the optical transmission device according to the first embodiment of the present invention. Only symbols having pairs of two consecutive bits having the same value make transitions. It is assumed that as illustrated in the left part of  FIG. 5 , after the ODU-LCK signal (repeating “0101”) is subjected to scrambling, a fixed pattern of “00 11 01 00 10” is obtained. 
     When the 100 G signals of the two systems (200 G) having a uniform frame phase are transmitted by the polarization multiplexed 16 QAM, symbol map data is as illustrated in the lower left part of  FIG. 5 . When this data is represented on a constellation map, symbols are mapped at positions surrounded by the broken line in the diagram of the right part of  FIG. 5 . In other words, only four symbol points from a top-left symbol point to a right-bottom symbol point make transitions. 
     Accordingly, in the first embodiment, the FIFOs  212  and  222  are arranged individually for the respective 100 G systems so that the phase of the OTU4V frame to be output in the form of the optical signal is shifted for each 100 G system. With this configuration, the pattern at the time of transmission of the OTN maintenance signal is shifted for each 100 G system, and hence it is possible to prevent the occurrence of the fixed patterns at the optical-symbol level, which occur in  FIG. 4  and  FIG. 5 . In other words, it is possible to provide the optical transmission device capable of preventing the performance degradation even at the time of transmission of data having a repeating or periodic pattern, such as an OTN maintenance signal. 
     As described above, according to the first embodiment, the memory (FIFO) capable of temporarily storing the OTU4 frame to shift the phase of the frame is arranged in the 200 G transceiver for each 100 G system. As a result, the pattern at the time of transmission of the OTN maintenance signal can be shifted for each 100 G system, and hence it is possible to provide the optical transmission device capable of preventing the performance degradation. 
     Second Embodiment 
       FIG. 6  is a diagram illustrating a configuration of an optical transmission device according to a second embodiment of the present invention. As compared with the configuration of  FIG. 1  according to the first embodiment, the configuration of  FIG. 6  according to the second embodiment differs in that the FIFOs  212  and  222  are removed, and that a mechanism for changing a seed value for generating a pseudo-random pattern is added into each of the scramblers/descramblers  214  and  224 . Other components and functions are the same as those of the first embodiment described above. 
     For example, in ITU-T G.709, at the end of the FA OH of the OTUkV frame illustrated in  FIG. 3 , each frame is initialized with an all-“1”s seed value. Thus, when the seed value for initialization is changed for each 100 G system, the value of the pseudo-random pattern is shifted as a result. 
     In this manner, in the second embodiment, the mechanism for changing the seed value for generating the pseudo-random pattern for each 100 G system is arranged in each of the scramblers/descramblers  214  and  224 . Accordingly, the value of a random signal can be varied for each 100 G system, and hence it is possible to prevent the occurrence of the fixed patterns at the optical-symbol level, which occur in  FIG. 4  and  FIG. 5 . In other words, it is possible to provide the optical transmission device capable of preventing the performance degradation even at the time of transmission of data having a repeating or periodic pattern, such as an OTN maintenance signal. 
     As described above, according to the second embodiment, the mechanism for changing the seed value for generating the pseudo-random pattern for each 100 G system is arranged in each of the scramblers/descramblers of the 200 G transceiver for each 100 G system. As a result, the pattern at the time of transmission of the OTN maintenance signal can be shifted for each 100 G system, and hence it is possible to provide the optical transmission device capable of preventing the performance degradation. 
     Note that, the configuration having the two systems is described in the first and second embodiments described above, but it is apparent that similar effects can be acquired even in a configuration having three or more systems by configuring the optical transmission device in a similar manner. Further, the example of the mapping to the polarization multiplexed 16 QAM signal is described in the first and second embodiments described above, but it is apparent that similar effects can be acquired even when the optical transmission device is configured to perform mapping to another type of multilevel signal such as a 64 QAM signal.