Offset frequency expansion optical signal demodulation device and optical signal demodulation method

Abstract

The present invention provides an offset frequency expanded optical signal demodulator and optical signal demodulation method that not only transmits and receives codes in accordance with time-domain index modulation rules, but also enables coherent detection of the arriving optical symbol. [Solution] In a communication receiving device, the demodulation unit receives complex digital signals CSx and CSy obtained by converting the transmitted optical signal, and the polarization index demodulation unit demodulates an index decoded code based on the position of the symbol in the time slot within one frame, and the complex digital signal is input and the symbol decoded code DEx AP , Dey AP The system has a polarization symbol demodulation unit that demodulates the symbols. When the polarization index demodulation unit receives a time slot containing a symbol, it sends a notification signal to the polarization symbol demodulation unit indicating the presence of the symbol, and the polarization symbol demodulation unit decodes the symbol based on the notification signal.

Description

[Technical Field]

[0001] The present invention relates to an optical signal demodulator applicable to optical fiber transmission networks, particularly optical access networks. Compared to the prior art, the present invention realizes an optical signal demodulator capable of receiving optical signals based on a time-domain index optical modulation scheme that can maintain communication line connectivity even with low average optical signal power. [Background technology]

[0002] Optical access networks are now used in over 20 million households in Japan, making them an indispensable social infrastructure system for our advanced information society.

[0003] In particular, in recent years, optical access network services that achieve a maximum transmission rate of 10 Gbit / s in each household are being deployed in accordance with the IEEE 802.3av (10GE-PON) standard (Non-Patent Literature 1). In most cases, this system employs a configuration in which optical fiber lines are branched in a tree-like fashion from optical line terminals (OLTs) installed at telecommunications bureaus in each local city, and connected to multiple (up to 32) optical network units (ONUs) installed in each household (Figure 24).

[0004] In this system, the optical loss budget for the OLT and ONU is 29 dB, and the transmission distance is 20 km. If these conditions are no longer met, it will be impossible to maintain the communication line, and the 10 Gbit / s transmission rate will be completely interrupted.

[0005] By employing a time-domain index optical modulation scheme (Non-Patent Literature 2), it becomes possible to maintain a constant average optical power while increasing the peak optical signal power and thus maintaining the optical signal-to-noise ratio by downsampling optical symbols in accordance with changes in transmission path conditions between the OLT and the ONU (Figure 25). In this case, although the absolute value of the mutual information of the communication path decreases, it becomes possible to maintain a normalized mutual information value close to 1 bit / symbol, and with the support of error correction functions, it becomes possible to maintain the connectivity of the communication line. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2024-100734 [Patent Document 2] International Publication No. 2014 / 115840 (Patent No. 5886984) [Non-patent literature]

[0007] [Non-Patent Document 1] IEEE Std 802.3av-2009, IEEE Standard for Ethernet-Physical Layer Specifications and Management Parameters for 10Gb / s Passive Optical Networks. [Non-Patent Document 2] M. Nakao, T. Ishihara, and S. Sugiura, “Dual-Mode Time-Domain Index Modulation for Nyquiest-Criterion and Faster-Than-Nyquist Single-Carrier Transmissions,” IEEE Access, vol. 5, no. 11, pp. 27659-27667, Nov. 2017. [Non-Patent Document 3] W. Imajuku, D. Aoki, Y. Yamaga, K. Yamamoto, S. Takahashi, “Time-Domain Single Carrier Index Modulation for Elastic Optical Access Links” Journal of Lightwave Technology, vol. 42, No.6, pp. 1853―1860, March, 2024. [Non-Patent Document 4] K. Kikuchi, “Fundamentals of coherent optical fiber communications,” J.Lightw. Technol., vol.34, no.1, pp.157-179, Aug. 2016. [Non-Patent Document 5] H. Imai and S. HIRAKAWA, “A New Multilevel Coding Method Using Error-Correcting Codes,” IEEE Trans.On Information Theory, vol.IT-23, no.3, pp.371-377, Aug. 1977. [Non-Patent Document 6] Suzuki, S., Miyamoto, Y., Tomizawa, M., Sakano, T., Murata, K., Mino, S., Shibayama, M., Shibuya, M., Fukuchi, K., Onaka, H., Hoshida, T., Komaki, K., Mizuochi, T., Kubo, K., Miyata, Y., and Kamio, T., "Research and Development of Digital Coherent Signal Processing Technology for Increasing the Capacity of Optical Communication Networks," Journal of the Institute of Electronics, Information and Communication Engineers, vol. 95, no. 12, pp. 1100-1116, December 2012. [Non-Patent Document 7] K.Itoh, “Analysis of the phase unwrapping algorithm,” Applied Optics, vol. 21, no.14, pp.2470, July 1982. [Non-Patent Document 8] A. Alvarado, T. Fehenberger, B. Chen, and FJ Willems, “Achievable Information Rates for Fiber Optics Applications Aand Computations,” J.Lightw. Technol., vol.36, no.2, pp.424-439, Jan. 2018. [Non-Patent Document 9] J. Li, S. Lin, K. Abdel-Ghaffar, WERyan, and D. L. Costello, Jr., “LDPC Code Designs, Constructions, and Unification, Chapter 11” Cambridge University Press, 2017, ISBN978-1-107-17568-6. [Overview of the project] [Problems that the invention aims to solve]

[0008] As exemplified by the IEEE 802.3av (10GE-PON) standard (Non-Patent Literature 1), phase diversity coherent optical communication is considered fundamental for high-capacity optical communication. However, due to the increasing communication capacity per transmission resulting from the proliferation of video content, the anticipated social demand for extended transmission distances, and the expected increase in access during disasters, the optical loss budget may exceed 29 dB in the future.

[0009] Even in such cases, it is considered necessary to provide best-effort optical access services in accordance with communication traffic conditions and transmission path conditions. In other words, the objective of the present invention is to provide a demodulation device for optical communication that can maintain the communication state even if the communication rate is reduced, even when the optical loss budget exceeds 29 dB.

[0010] Furthermore, in order to achieve the above objective, the present invention combines a coherent optical modulation method and a time-domain index optical modulation method (Patent Document 1). In this case, it is necessary to be able to perform frequency offset compensation, phase compensation, etc., for intermittently incoming optical symbols, similar to conventional coherent optical signal receiving devices. However, the time-domain index optical modulation method has the problem that the maximum allowable offset frequency at which frequency offset compensation is possible is reduced to about 1 / 10 (Non-Patent Document 3). [Means for solving the problem]

[0011] In view of the above problems, the present invention expands the maximum allowable offset frequency which is a constraint of the time-domain index optical modulation scheme, and provides a demodulation device and method that can stably provide best-effort optical access services in accordance with communication traffic conditions and transmission path conditions, even when the optical loss budget exceeds 29 dB.

[0012] More specifically, the optical signal demodulation device according to the present invention is as shown in Figure 10, An optical signal demodulator that receives an optical signal on which the original signal (INFx, INFy) is transmitted for each of the X-polarization and Y-polarization components, using symbols that are placed as complex digital signals (CSx, CSy) in at least one of the time slots, and an index code formed at the time position of the time slot on which the symbols are placed, and demodulates the original signal (INFx, INFy). A receiver (RX) performs phase diversity detection on the optical signal and converts it into the complex digital signals (CSx, CSy) for each of the X-polarization and Y-polarization components. The system includes a demodulation unit (6) that receives the complex digital signals (CSx, CSy) for each of the X-polarization and Y-polarization components and demodulates the original signals of the X-polarization and Y-polarization components. The demodulation unit (6) consists of an X-polarization demodulation unit (6X), a Y-polarization demodulation unit (6Y), and a common frequency offset compensation unit (22com). The X-polarization demodulation unit (6X) is, a complex digital signal of an X-polarization component (CSx) output from said receiving unit (RX), and a frequency offset-compensated complex digital signal of an X-polarization component (CSxf) input from said common frequency offset compensation unit (22com), an X-polarization activation pattern signal (APx CS ) that notifies the position of said time slot in which said symbols are carried, and an X-polarization index decoded code (DEx Id ) obtained by demodulating said index code, and in accordance with said X-polarization activation pattern signal (APx CS ), generating an X-polarization symbol decoded code (DEx AP ) obtained by decoding symbols of said frequency offset-compensated complex digital signal of an X-polarization component (CSxf), from said X-polarization index decoded code (DEx Id ) and said X-polarization symbol decoded code (DEx AP ), generating and outputting an X-polarization decoded original signal (DE(INFx)), outputting said X-polarization activation pattern signal (APx CS ) to said common frequency offset compensation unit (22com), and said Y-polarization demodulation unit (6Y): a complex digital signal of a Y-polarization component (CSy) output from said receiving unit (RX), and a frequency offset-compensated complex digital signal of a Y-polarization component (CSyf) input from said common frequency offset compensation unit (22com), a Y-polarization activation pattern signal (APy CS ) that notifies the position of said time slot in which said symbols are carried, and a Y-polarization index decoded code (DEy Id ) obtained by demodulating said index code, and in accordance with said Y-polarization activation pattern signal (APy CS ), a Y-polarization symbol decoded code (DEy) obtained by decoding symbols of said frequency offset-compensated complex digital signal of a Y-polarization component (CSyf)AP ) generates, The aforementioned Y-polarization index decoding code (DEy Id ) and the Y-polarization symbol decoding code (DEy AP ) generates and outputs the Y-polarized decoded original signal (DE(INFy)), The aforementioned Y-polarization activation pattern signal (APy CS ) is output to the common frequency offset compensation unit (22com), The aforementioned common frequency offset compensation unit (22com) is, The complex digital signal (CSx) of the X-polarization component, The complex digital signal (CSy) of the Y-polarization component, The X-polarization activation pattern signal (PAx) output from the X-polarization demodulation unit (6X) CS )and, The Y polarization activation pattern signal (PAy) output from the Y polarization demodulation unit (6Y) CS ) was entered, The complex digital signal (CSxf) of the X-polarization component with the frequency offset compensated, The complex digital signal (CSyf) of the Y-polarization component, with the frequency offset compensated, is output to the X-polarization demodulation unit (6X) and the Y-polarization demodulation unit (6Y), respectively.

[0013] Furthermore, the optical signal demodulation method according to the present invention is An optical signal demodulation method that receives an optical signal on which the original signal (INFx, INFy) is transmitted for each of the X-polarization and Y-polarization components, using symbols that are placed as complex digital signals (CSx, CSy) in at least one of the time slots, and an index code formed at the time position of the time slot on which the symbols are placed, and demodulates the original signal (INFx, INFy), wherein each frame is composed of multiple time slots for each X-polarization component and Y-polarization component, and the original signal (INFx, INFy) is demodulated. A receiving process (RX) that performs phase diversity detection on the optical signal and converts it into the complex digital signals (CSx, CSy) for each of the X-polarization component and the Y-polarization component, The system includes a demodulation step (6) for demodulating the original signals of the X-polarization component and the Y-polarization component from the complex digital signals (CSx, CSy) for each of the X-polarization component and the Y-polarization component. The demodulation process (6) consists of an X-polarization demodulation process (6X), a Y-polarization demodulation process (6Y), and a common frequency offset compensation process (22com). The X-polarization demodulation process (6X) is performed as follows: The complex digital signal (CSx) of the X-polarization component obtained in the above receiving process (RX), From the frequency-offset-compensated X-polarization complex digital signal (CSxf) obtained from the common frequency offset compensation process (22com), X-polarized activation pattern signal (APx) that indicates the position of the time slot on which the symbol is placed. CS )and, The X-polarized index decode (DEx) obtained by demodulating the aforementioned index code. Id The process of generating ) The aforementioned X-polarization activation pattern signal (APx CS The X-polarization symbol decoding code (DEx) is obtained by decoding the complex digital signal (CSxf) of the X-polarization component that has been frequency offset compensated in accordance with the above. AP The process of generating ) The aforementioned X-polarization index decoding code (DEx Id ) and the X-polarization symbol decoding code (DEx AP The process includes generating the X-polarized original signal DE(INFx) from ). The aforementioned Y polarization demodulation process (6Y) is performed as follows: The complex digital signal (CSy) of the Y polarization component obtained by the receiver (RX), From the frequency-offset-compensated Y-polarization complex digital signal (CSyf) obtained from the common frequency offset compensation unit (22com), A Y-polarized activation pattern signal (APy) indicates the position of the time slot on which the aforementioned symbol is placed. CS )and The Y-polarized index decode (DEy) obtained by demodulating the aforementioned index code. Id The process of generating ) The aforementioned Y-polarization activation pattern signal (APy CS The Y-polarization symbol decoding code (DEy) is obtained by decoding the Y-polarization complex digital signal (CSyf) of the Y-polarization component that has been frequency offset compensated in accordance with the above. AP The process of generating ) The aforementioned Y-polarization index decoding code (DEy Id ) and the Y-polarization symbol decoding code (DEy AP The process includes generating the Y-polarized decoded original signal DE(INFy) from ). The aforementioned common frequency offset compensation process (22com) is: The complex digital signal (CSx) of the X-polarization component, The complex digital signal (CSy) of the Y-polarization component, The X-polarization activation pattern signal (PAx) generated in the X-polarization demodulation step (6X) CS )and, The Y polarization activation pattern signal (PAy) generated in the Y polarization demodulation step (6Y) CS ) and from, The complex digital signal (CSxf) of the X-polarization component with the frequency offset compensated, The method is characterized by generating a complex digital signal (CSyf) of the Y-polarization component with the frequency offset compensated, and outputting it to the X-polarization demodulation process (6X) and the Y-polarization demodulation process (6Y), respectively. [Effects of the Invention]

[0014] According to the optical signal demodulation apparatus and method of the present invention, communication can be maintained by transmitting using a coherent optical modulation scheme as long as the optical loss does not exceed 29 dB, and by also using a time-domain index modulation scheme when it exceeds 29 dB. Furthermore, this method enables the same operation as the frequency offset compensation circuit, phase compensation circuit, and phase slip reduction circuit built into conventional digital coherent optical signal receivers, even for optical symbols that arrive at irregular timings in accordance with the time-domain index modulation rules of a time-domain index optical modulation signal.

[0015] This enables not only the transmission and reception of codes according to time-domain index modulation rules, but also coherent detection of the arriving optical symbol. In other words, it becomes possible to transmit and receive codes according to M-phase orthogonal phase amplitude optical modulation rules, just like conventional digital coherent optical signal receivers.

[0016] As a result, the present invention can realize multiple operating modes as described later. Specifically, a system employing the present invention achieves data transmission using the M-phase quadrature-amplitude (M-QAM) optical modulation method in the normal operating mode, ensuring communication capacity (normal mode). On the other hand, when customer data traffic is low, the system employing the present invention switches to the index optical modulation operating mode, reducing the average optical signal power while ensuring the peak power of the optical signal (ECO mode).

[0017] On the other hand, if the transmission path distance is long or the transmission path conditions are poor, the system switches to index optical modulation mode and increases the peak power of the optical signal to ensure connectivity of the communication line (Long Reach / Fault Tolerant mode). In this way, the present invention makes it possible to provide a "best-effort" optical access service in accordance with the communication traffic state and transmission path conditions.

[0018] Furthermore, the frequency offset compensation unit provided in the optical signal demodulation device of the present invention can improve the frequency offset compensation performance of the receiver, thereby relaxing the requirements regarding the accuracy of the oscillation optical frequency of the local oscillator light source provided in the receiver, and contributing to a reduction in the cost of the receiver.

[0019] The optical signal demodulation device of the present invention enables operation similar to that of the frequency offset compensation circuit, phase compensation circuit, and phase slip reduction circuit built into conventional digital coherent optical signal receivers, even for optical symbols that arrive at irregular timings in accordance with the time-domain index modulation rules of time-domain index optical modulation signals. This makes it possible not only to transmit and receive codes in accordance with the time-domain index modulation rules, but also to perform coherent detection of the arriving optical symbols. In other words, it becomes possible to transmit and receive codes in accordance with the M-phase orthogonal phase amplitude optical modulation rules, just like conventional digital coherent optical signal receivers. [Brief explanation of the drawing]

[0020] [Figure 1] An example of a modulation rule in time-domain indexed optical modulation is shown. [Figure 2] An example of a modulation rule in an M-phase orthogonal phase-amplitude optical modulation scheme is shown. [Figure 3] This figure shows the relationship between transmission path loss (which can be converted to transmission distance) and communication capacity for the time-domain indexed optical modulation scheme to be realized in the present invention. [Figure 4] This diagram shows the logical model of the time-domain indexed optical modulation scheme to be realized in this invention. [Figure 5] This diagram illustrates the benefits realized by the system to which the present invention is applied. [Figure 6] This diagram shows the difference in optical frequency between the optical signal and the local emission. [Figure 7] This is a diagram illustrating offset frequency. [Figure 8] This figure shows the transmission modes in which the present invention can be implemented. [Figure 9] This is a diagram showing the configuration of the communication receiving device described in Patent Document 1. [Figure 10] This figure shows the configuration of the communication receiving device of the present invention. [Figure 11] This figure shows the configuration and connection of the X-polarization demodulation unit and the common frequency offset compensation unit of the optical communication receiving device of the present invention. [Figure 12]This diagram shows the frame and symbol configuration of the indexed optical modulation scheme. [Figure 13] This diagram shows the configuration of the common frequency offset compensation unit. [Figure 14] This figure shows the effect of frequency offset compensation. [Figure 15] This figure shows the expansion of the offset frequency, which is the effect of frequency offset compensation. [Figure 16] This diagram shows the configuration of the phase compensation section of the demodulation unit. [Figure 17] This diagram shows the effect of the phase compensation unit. [Figure 18] This diagram shows the configuration of the demodulation unit, including the provisional judgment calculation unit. [Figure 19] Figure 18 shows the configuration and connection of the X-polarization demodulation unit and the common frequency offset compensation unit. [Figure 20] This diagram shows the configuration when a provisional determination calculation unit is used in a common frequency offset compensation unit that has a provisional determination calculation unit. [Figure 21] This diagram shows the configuration of the provisional judgment calculation unit. [Figure 22] This diagram shows the configuration of the phase compensation unit using the provisional determination calculation unit. [Figure 23] This diagram shows the configuration of the provisional judgment calculation unit used in the phase compensation section. [Figure 24] This diagram shows the relationship between the OLT and the ONU. [Figure 25] This diagram shows the signal configuration when using the index optical modulation method. [Modes for carrying out the invention]

[0021] The configuration and operation of the optical signal demodulation device 3 according to the present invention will be explained below with reference to the drawings. The following description illustrates one embodiment and one example, and the present invention is not limited to the following description. The following description may be modified without departing from the spirit of the present invention. In addition, the parts referred to as "~ section" in the following embodiments may include either hardware or software components. Also, the parts referred to as "~ section" may be referred to as "~ process" which has the function to be performed. Furthermore, the addition section, multiplication section, and division section are elements that perform complex calculations even if they are not explicitly described as "complex".

[0022] Furthermore, in the explanation of the demodulation unit 6 and beyond, the components of the processing related to the X-polarization component will be referred to as the "X-polarization system," and the components of the processing related to the Y-polarization component will be referred to as the "Y-polarization system." Also, "signal" and "information" may be used synonymously. In addition, this invention deals with both X-polarization and Y-polarization components, and in order to simplify the explanation, the explanation will focus on the elements related to the X-polarization component. However, similar components exist for the Y-polarization component to those related to the X-polarization component.

[0023] Furthermore, when referring to the components of the X-polarization system and the Y-polarization system collectively, the phrase "each polarization" is added to the beginning. Note that, for more detailed aspects, distinctions such as "each polarization," "X-polarization," and "Y-polarization" may not be made; instead, the signs "x" and "y" may be omitted, or only "x" or "y" may be added to distinguish them. Also, when processing an input signal to obtain an output signal, each component can be understood as generating and outputting the output signal.

[0024] The principle of the time-domain index optical modulation scheme, which is the premise of the present invention, is as follows. First, Figure 1 shows the frame configuration used in the time-domain index optical modulation scheme. The time-domain index optical modulation scheme constructs an index optical modulation signal frame (hereinafter simply referred to as "frame") with multiple time slots. Optical signals are transmitted and received using this frame as a single unit. Note that "1 frame" means "one frame" (the same applies hereafter in this specification). A time slot is a predetermined time width. There is a time gap between one time slot and the next time slot. However, if the sender and receiver are perfectly synchronized, there may be no time gap. The time width of a time slot, including the time gap, is represented by "Ts".

[0025] Under these conditions, consider the case where the average optical signal power of each frame is operated at a constant value near the upper limit of the transmitter. In this case, by decimating a fixed number of optical symbols within each frame, the amplitude of the optical symbols, and consequently the signal-to-noise ratio of the optical symbols, increases. In addition, by changing the transmission and reception time slots of the decimated optical symbols in accordance with the time-domain index modulation rules, it becomes possible to transmit and receive the code.

[0026] Here, a symbol refers to the light (optical symbol carrier wave; hereafter also simply called "optical signal LS") on which the sender carries information during a time slot. Symbol decimation refers to a state in which there is no light or the intensity is low (dark) during a time slot. Time slot changes include changing the time of the time slot or changing the intensity of the symbols within the time slot.

[0027] Figure 1 shows eight frames. Each frame consists of eight time slots. In each time slot, the black time slots contain optical symbols. The optical symbols contain information using the M-QAM method, such as QPSK. In other words, complex digital signals are carried in the time slots. For example, in the third frame, the third time slot contains an optical symbol. White time slots are time slots that do not contain optical symbols.

[0028] Figure 1 shows the case where a light symbol is placed in one of the eight time slots. Eight activation patterns are possible depending on the position of the light time slot on which the light symbol is placed. By assigning an Index bit to each activation pattern, three bits of information can be specified.

[0029] Furthermore, by simultaneously performing coherent detection of the optical symbols, it becomes possible to transmit and receive codes using M-QAM optical modulation symbols, similar to conventional digital coherent optical signal receivers. Figure 2 shows an example of signal points of optical symbols transmitted in time slots. Figure 2(a) is a QPSK (Quadrature Phase Shift Keying) signal, and (b) is a 16QAM (16 Quadrature Amplitude Modulation) signal.

[0030] Figure 3 shows an example of the relationship between the normalized mutual information of the QPSK method (thin line) and the (8,1,QPSK) time-domain index optical modulation method of the present invention (thick line), which will be described in detail later. "(8,1,QPSK)" means that one frame is composed of eight time slots, and index modulation communication is performed by loading a symbol into one of these time slots, and that symbol is modulated with QPSK to load information.

[0031] Referring to Figure 3, the horizontal axis represents optical loss (dB), and the vertical axis represents the normalized mutual information R (bits / symbol). In existing QPSK optical modulation schemes, when the optical loss exceeds the budget of 29 dB, bit errors are introduced, and the communication line is disconnected. Here, a complete disconnection of the communication line is defined as the normalized mutual information R being 0.81 bits / symbol or less.

[0032] However, by implementing a (forward) error correction function (labeled "FEC: Forward Error Correction" in Figure 3) in the receiver and adopting a time-domain index optical modulation scheme (labeled "Index modulation"), the margin of the optical loss budget increases by approximately 10 dB in total. This value corresponds to the optical loss over a 20 km section, assuming an optical fiber loss coefficient of 0.5 dB / km for the optical access network. In other words, it suggests that the transmission distance of the access optical network can be doubled to 40 km.

[0033] The communication channel realized by the time-domain index optical modulation scheme is logically a communication channel in which an index bit (hereinafter "INDBit") channel, which transmits and receives codes by changing the symbol position within one frame of the transmission and reception time slot of optical symbols in accordance with the time-domain index modulation rules, and an amplitude-phase modulation bit (hereinafter "APMBit") channel, which transmits and receives codes by M-QAM optical modulation, are in parallel.

[0034] Figure 4 is a conceptual diagram of the overall configuration of a communication system using INDBit and APMBit. On the transmitting side (optical signal transmitter 1), the original signal INF is split into INDBit and APMBit by the serial / parallel conversion unit (S / P). Each signal is then subjected to error correction codes (FEC Enc.), mapped (MAP), and transmitted from the transmitter (TX).

[0035] It can also be seen that INDBit and APMBit transmit simultaneously in two sequences, one with X polarization and the other with Y polarization. Therefore, it can be said that the original signal INF is composed of an X-polarized original signal INFx, which is modulated with X polarization, and a Y-polarized original signal INFy, which is modulated with Y polarization.

[0036] In transmission path 2, logically, the INDBit channel (Index ch) and the APMBit channel (APM ch) are transmitted. Physically, the optical signal LS, with a phase-modulated signal superimposed during the time slot shown in Figure 1, is transmitted.

[0037] At the receiving end (optical signal demodulator 3), the signal received by the receiving unit (RX) is decoded as an index modulated signal (DMAP) and error correction is performed (FEC Dec.) in the index demodulation unit 10. Furthermore, in the symbol demodulation unit 20, the amplitude-phase modulated signal is frequency offset compensated (FC), phase shift compensated (PC), decoded (DMAP), and error correction is performed (FEC Dec.). In addition, in the optical signal demodulator 3 of the present invention, the activation pattern signal AP, described later, is basically sent from the index demodulation unit 10 to the symbol demodulation unit 20. CS It will be sent.

[0038] In this way, INDBit and APMBit are demodulated. These signals are converted back to serial signals by the parallel / serial conversion unit 29 (hereinafter also referred to as "P / S conversion unit 29") to obtain the decoded original signal DE(INF) corresponding to the original signal INF. The decoded original signals decoded from the X-polarization and Y-polarization signals are denoted as the X-polarization decoded original signal DE(INFx) and the Y-polarization decoded original signal DE(INFy), respectively. The total communication capacity C (bit / sec) is given by equation (1).

[0039]

number

[0040] Here, N is the number of symbols per frame of the time-domain index optical modulation signal, K is the number of symbols to which optical symbols are inserted, [] is the Gaussian symbol, and M is the number of signal points defined for the M-QAM modulated optical symbols. The first term on the right-hand side corresponds to the communication capacity of the INDBit channel, and the second term corresponds to the communication capacity of the APMBit channel.

[0041] Figure 5 shows the relationship between the number of optical symbols per frame (K / N) and the data transfer rate B (bits / symbol). The open white marks indicate the case where only amplitude-phase modulation bits (APMbits) are used, and the filled black marks indicate the case where INDBit is also used. In both cases, APMbit is QPSK. The square marks indicate the case where one frame is 16 time slots, the circle marks the case where one frame is 8 time slots, and the triangle marks the case where one frame is 4 time slots.

[0042] Here, as methods for transmitting codes in a low average optical signal power state with the average number of optical symbols reduced to 1 / 16 to 1 / 4, there are two methods: one that assigns optical symbols to fixed time slots (labeled FIM in Figure 5) and another that uses time-domain (N, K, QPSK) index optical modulation (labeled IM in Figure 5).

[0043] For example, as a method of transmitting codes in a low average optical signal power state with the average number of optical symbols reduced to 1 / 8, the time-domain (8,1,QPSK) index optical modulation transmission method (in Figure 5, the circle indicates K is 1, so K / N = 0.125) can expand the communication capacity to about 2.5 times compared to the 1 / 8 rate QPSK modulation transmission method (0.25 bits / symbol) which also reduces the number of optical symbols to 1 / 8.

[0044] As shown in Figure 1, the present invention is an optical signal demodulation apparatus and method for demodulating an optical signal in which one frame is composed of multiple time slots and symbols are placed on at least one of the time slots, and aims to increase the allowable frequency difference between the signal light and the local light emission.

[0045] Figure 6 illustrates the frequency difference between the optical signal LS and the local light emission Lo, which is a challenge for the optical signal demodulator 3, and shows the relationship between the frequencies of the optical signal LS and the local light emission Lo. Here, the optical signal LS is a signal in which one frame is composed of N time slots, and the M-QAM symbol is superimposed in K of these slots. In other words, it is a "time-domain (N,K,M-QAM) index optical modulation signal".

[0046] In Figure 6, the horizontal axis represents optical frequency, and the vertical axis conceptually represents light intensity. In the optical signal demodulator 3, it is desirable to use light of the same frequency as the optical signal LS as the local emission Lo, but it is not easy to finely adjust the frequency of the local emission Lo, and the two lights have a frequency difference of Δf (hereinafter also called "offset frequency Δf").

[0047] Figure 7 shows the relationship between the offset frequency Δf and the code error rate of the received signal (Non-Patent Literature 3). Referring to Figure 7, the horizontal axis represents the offset frequency Δf normalized by the symbol rate B. The vertical axis shows the relationship with the code error rate BER (logarithmic representation) of the received signal.

[0048] As shown in the graph of Figure 7 in Non-Patent Document 3, when the optical frequency difference Δf between the two beams of light, divided by the symbol rate, exceeds 0.1, even a slight difference in the offset frequency Δf causes the code error rate BER to change by an order of magnitude. Therefore, compensating for this offset frequency Δf becomes difficult. Increasing the maximum allowable offset frequency Δfmax, which is the maximum value of the offset frequency Δf, is essential for stable system operation and lowering the cost of laser light sources.

[0049] Figure 8 shows one frame transmitted in the section from the OLT (see Figure 24) to the ONU. When transmission path conditions are good, conventional M-QAM optical modulation symbols are transmitted (Figure 8(a): normal mode). On the other hand, when transmission path conditions are poor and the optical loss budget exceeds 29 dB, the system operates based on the time-domain index optical modulation scheme and thins out the optical symbols (Figure 8(b): ECO mode). In an optical receiver that realizes such operation, it is necessary to be able to demodulate both M-QAM optical modulation symbols and time-domain index optical modulation symbols in the same optical receiver, and to efficiently realize the expected operation by increasing the common parts of the receiver.

[0050] Furthermore, if the transmission path distance is long or the transmission path conditions are poor, the system switches to index optical modulation operation mode and increases the peak power of the optical signal to ensure connectivity of the communication line (Figure 8(c): L / F mode [Long Reach / Fault Tolerant mode]). The optical modulation operation itself is the same as in ECO mode, but the peak power of the optical signal is increased. In this case, the INDbit demodulation unit determines the presence or absence of optical symbols, generates an activation pattern, and inputs it to the APMbit demodulation unit. The APMbit demodulation unit demodulates M-QAM optical modulation symbols only for time slots where optical symbols exist, according to this activation pattern.

[0051] In the ECO mode shown in Figure 8(b), under conditions where optical symbols arrive irregularly according to the index optical modulation rule, it is necessary to decode the transmission code of the APMbit channel while simultaneously performing frequency offset compensation and phase compensation, in order to decode the data according to the index optical modulation rule.

[0052] The present invention provides a means for realizing an intradyne digital coherent detection optical signal decoding unit that can handle time-domain index optical modulation schemes that can solve these problems. The present invention not only provides a means for demodulating time-domain (N,K,M-QAM) index optical modulation signals (Patent Document 1), but also provides a means for efficiently realizing the decoding of conventional M-QAM optical modulation symbols and time-domain (8,1,M-QAM) index optical modulation symbols while increasing the maximum allowable offset frequency Δfmax.

[0053] (First Embodiment) To clarify the differences between the present invention and Patent Document 1, first, using Figure 9, the optical signal demodulation device 3 (see Figure 4) of Patent Document 1 will be explained with a block diagram of its main parts.

[0054] The optical signal LS transmitted via optical fiber is received by the phase diversity detector RXa and converted into a digital signal by the analog-to-digital converter RXb. Note that the optical signal LS transmits separate information in X-polarization and Y-polarization. Subsequently, polarization compensation is performed by the optical polarization compensation unit RXc. The optical polarization compensation unit RXc includes a dispersion compensation unit, a timing phase synchronization unit, and a polarization rotation compensation unit. These are the receiving unit RX, which performs phase diversity detection on the optical signal LS and converts it into a complex digital signal CS. The receiving unit RX outputs two complex digital signals CSx and CSy, each containing two polarization components. These complex digital signals CS still include the offset frequency Δf (see Figure 6) between the station light emission Lo and the optical signal LS, and the phase error of the carrier component of the M-QAM signal.

[0055] The two complex digital signals CSx (complex digital signal with X polarization component; hereafter also simply referred to as "X-polarization component") and CSy (complex digital signal with Y polarization component; hereafter also simply referred to as "Y-polarization component") output from the receiver RX are input to the respective polarization demodulation units 6 in the subsequent stage, one for each signal component. Note that the polarization demodulation units 6 in Figure 9 represent the X-polarization demodulation unit 6X and the Y-polarization demodulation unit 6Y. The two X-polarization complex digital signals CSx and the Y-polarization complex digital signal CSy are then decoded by the respective polarization index demodulation units 10 contained within each polarization demodulation unit 6 into the respective polarization index decoding code DE for the INDbit channel. Id It is decrypted as follows.

[0056] In each polarization symbol demodulation unit 20, after frequency offset compensation and phase compensation are performed, the signal of the APMbit channel is converted into each polarization symbol decoding code DE AP It is decoded as follows. In this invention, a common frequency offset compensation unit 22com, which performs this frequency offset compensation for both the X-polarization component and the Y-polarization component, is provided outside each polarization symbol demodulation unit. Error correction may be performed in each polarization index demodulation unit 10 and each polarization symbol demodulation unit 20. Furthermore, error correction may also be performed in the final stage of decoding.

[0057] Each polarization index decoding code DE Id and each polarization symbol decoding code DE AP The original signal INF is decoded into serial data by each polarization parallel / serial conversion unit 29 (referred to as "each polarization P / S conversion unit 29"), and each polarization decoded original signal DE(INF) is obtained. Each polarization decoded original signal DE(INF) includes the X polarization decoded original signal DE(INFx) and the Y polarization decoded original signal DE(INFy) from the X polarization. Each polarization index decoded code DE Id and each polarization symbol decoding code DE AP There are codes for the X-polarization component and the Y-polarization component, and the X-polarization index decoding code DEx is for each. Id Y-polarization index decoding code DEy Id and X-polarized symbol decoding code DEx APY-polarization symbol decoding code DEy AP It is called that.

[0058] An embodiment of the present invention is shown in Figure 10. Figure 10 shows in more detail the demodulation unit 6 of the optical signal demodulation device 3 according to the present invention, starting from the optical polarization compensation unit RXc in Figure 9. This embodiment is basically constructed with a similar concept to that of Patent Document 1. The main difference is that the polarization frequency offset compensation units (not shown) that were contained within each polarization symbol demodulation unit 20 of each polarization component in Figure 9 are extracted and driven as a common frequency offset compensation unit 22com common to both the X-polarization component and the Y-polarization component.

[0059] The demodulation unit 6 disclosed in Figure 9 generates a polarization activation pattern signal AP for each time slot in which a symbol exists within a frame. CS This information is sent to each polarization symbol demodulation unit 20, and the offset frequency is compensated. However, if there are few symbols in one frame and the positions of the symbols are far apart in consecutive frames, the opportunities for offset frequency compensation decrease. As a result, if the offset frequency widens, compensation becomes impossible (the maximum allowable offset frequency Δfmax becomes smaller).

[0060] However, if the phases of the X-polarization and Y-polarization pulses are detected by a common frequency offset compensation unit 22com, the probability of detecting a pulse in the other polarization component increases even if the pulse interval widens in one polarization component. Therefore, the pulse interval to be detected is effectively narrowed, and phase changes can be tracked and compensated for at short time intervals. As a result, the maximum allowable offset frequency Δfmax can be increased compared to when the offset frequency Δf is compensated for each polarization.

[0061] Referring to Figure 10, the demodulation unit 6 according to the present invention consists of an X-polarization demodulation unit 6X, a Y-polarization demodulation unit 6Y, and a common frequency offset compensation unit 22com. The X-polarization demodulation unit 6X includes an X-polarization index demodulation unit 10x, an X-polarization symbol demodulation unit 20x, and an X-polarization P / S conversion unit 29x. Similarly, the Y-polarization demodulation unit 6Y includes a Y-polarization index demodulation unit 10y, a Y-polarization symbol demodulation unit 20y, and a Y-polarization P / S conversion unit 29y.

[0062] Furthermore, the common frequency offset compensation unit 22com receives both the X-polarized complex digital signal CSx and the Y-polarized complex digital signal CSy, which are output from the optical polarization compensation unit RXc, which performs residual dispersion, polarization dispersion, and polarization rotation compensation for the received optical signal LS.

[0063] In addition to these, the common frequency offset compensation unit 22com receives the X-polarization activation pattern signal APx output from the X-polarization demodulation unit 6X. CS and the Y-polarization activation pattern signal APy output from the Y-polarization demodulation unit 6Y. CS The input is also a frequency offset compensated X-polarized complex digital signal CSx f The X-polarization symbol demodulation unit 20x of the X-polarization demodulation unit 6X outputs this to the X-polarization symbol demodulation unit 20x. In addition, the common frequency offset compensation unit 22com outputs the Y-polarization complex digital signal CSy with the frequency offset Δf compensated. f This is output to the Y-polarization symbol demodulation unit 20y of the Y-polarization demodulation unit 6Y.

[0064] The optical signal demodulation that realizes the time-domain (N,K,M-QAM) index optical modulation scheme, performed by the X-polarization demodulator 6X and the Y-polarization demodulator 6Y, will be explained using Figure 11. Figure 11 shows the X-polarization demodulator 6X and the common frequency offset compensation unit 22com. The explanation will focus on the X-polarization demodulator 6X, but the Y-polarization demodulator 6Y has similar components and operates in a similar manner.

[0065] The X-polarization demodulation unit 6X and the Y-polarization demodulation unit 6Y receive an X-polarization complex digital signal CSx and a Y-polarization complex digital signal CSy output from the optical polarization compensation unit RXc of the receiving unit RX illustrated in FIG. 10. The X-polarization complex digital signal CSx is input to an X-polarization amplitude calculation unit 11x of an X-polarization index demodulation unit 10x and a common frequency offset compensation unit 22com.

[0066] The X-polarization amplitude calculation unit 11x of the X-polarization index demodulation unit 10x calculates the absolute value (or the square of the absolute value) of the X-polarization complex digital signal CSx, calculates the intensity of the X-polarization complex digital signal CSx corresponding to the received optical symbol, and uses this as an output value. This output is defined as the X-polarization symbol intensity Px CS .

[0067] An X-polarization frame timing extraction unit 12x extracts the start timing of an X-polarization index optical modulation frame based on the output value of the X-polarization amplitude calculation unit 11x (the X-polarization symbol intensity Px CS ) and a code pattern decoded by the X-polarization index demodulation unit 10x (an X-polarization index decoded code DEx Id ). Then, the X-polarization start notification signal Sx indicating the start timing is output to an X-polarization IndexBit demapping unit 14x. TOP

[0068] <X-polarization frame timing extraction unit 12x> In the case of a time-domain (N,K,M-QAM) index optical modulation scheme, the X-polarization frame timing extraction unit 12x identifies the start timing of a frame composed of N symbols. FIG. 12 shows a frame composed of N symbols. Referring to FIG. 12, the horizontal axis represents time, with the direction toward the right indicating the past. In FIG. 12, one frame is composed of 8 time slots (white rectangles), and a symbol (black rectangle) is placed in one time slot within one frame. FIG. 12 shows a frame composed of 1 symbol.

[0069] ​Refer to Figure 11 again. Two main methods are used to realize the X-polarized frame timing extraction unit 12x: (1) a method of determining the starting timing from the training symbols included in the main signal optical symbols, and (2) a method of using timing extraction from the control optical channel signals.

[0070] In this implementation, as shown in Figure 12, we illustrate a method of extracting from training frames included in the main signal optical symbol (1). Of course, timing can also be extracted from the control optical channel signal (2). In this case, a specific frame pattern consisting of N symbols is repeated for J frames, and this is used as the training frame. Here, a symbol is always placed in the first time slot of the training frame. Such training frames are also known to be inserted in existing digital coherent transmission systems at a rate of about 1 / 100 to 1 / 10 of the transmitted signal, and a method of outputting a frame timing signal in accordance with the symbol placed at the beginning of the frame is known (Non-Patent Literature 5).

[0071] Furthermore, the method of determining the frame's starting timing from a specific bit pattern is a traditional technique applied in many existing transmission systems. For example, the starting position of an Ethernet frame is extracted from the 1010101 pattern while timing synchronization is performed. Also, Synchronous Digital Hierarchy (SDH) transmission systems, which have been applied in 10-gigabit class transmission systems, are designed to identify the frame's starting position by inserting a specific code pattern. The X-polarized frame timing extraction unit 12x of the present invention also recognizes the frame's starting symbol in such a known way.

[0072] In this embodiment, the training symbol is confirmed to be positioned as the first symbol of the frame from the bit pattern decoded by the X-polarized IndexBit demapping unit 14x described later, while the X-polarized leading notification signal Sx TOP The X-polarized leading notification signal Sx, which represents the beginning time slot of the frame, is output at the appropriate time. TOPand outputs the same. For example, if the system is designed according to the time-domain index modulation rule as shown in FIG. 1, the X-polarization frame timing extraction unit 12x obtains, from the X-polarization IndexBit demapping unit 14x, an X-polarization index decoded code DEx corresponding to the activation pattern of "000" ("10000000") Id ), and controls the X-polarization start notification signal Sx to be output at appropriate timing while confirming that the above output is generated TOP .

[0073] <X-polarization index demodulation unit 10x> Next, the operation of the X-polarization index demodulation unit 10x will be described. In the delay unit 13x, the X-polarization frame timing extraction unit 12x is driven correctly, and the X-polarization start notification signal Sx is aligned with the frame start symbol TOP is transmitted, and the delay is adjusted such that the timing at which the signal is input to the X-polarization IndexBit demapping unit 14x is synchronized with the timing at which the start symbol of the frame is input to the X-polarization IndexBit demapping unit 14x.

[0074] The X-polarization IndexBit demapping unit 14x receives the X-polarization start notification signal Sx from the X-polarization frame timing extraction unit 12x TOP and the output signal of the delay unit 13x (the absolute value of the X-polarization complex digital signal CSx: symbol strength) Px CS as inputs. The X-polarization IndexBit demapping unit 14x identifies the frame timing of the Index symbol based on the X-polarization start notification signal Sx from the X-polarization frame timing extraction unit 12x TOP , demaps IndexBits from the X-polarization symbol strength Px, which is an input signal from the delay unit 13x CS (the absolute value (or the square of the absolute value) of the complex digital signal CSx calculated by the X-polarization amplitude calculation unit 11x), and outputs the X-polarization index decoded code DEx, which is a decoded code Id .

[0075] The demapping rule may be based on, for example, the time-domain index modulation rule shown in Figure 1. In this case, the input signal from the delay unit 13x (X-polarization symbol intensity Px CS ) is the activation pattern signal AP shown in Figure 1. CS The activation pattern is then demapped to the index bit and decoded. Note that the X-polarization activation pattern signal APx CS From this, you can find out the time slot in which a symbol is located within a single frame.

[0076] <Register section 16x> Refer again to Figure 11, and see the decoded X-polarization index decoded code DEx Id This is input to register section 16x. Register section 16x receives the X-polarization leading notification signal Sx via delay section 15x. TOP It is driven based on the X-polarization leading notification signal Sx TOP Only when ON, the X-polarization index decode DEx is output from the X-polarization IndexBit demapping unit 14x. Id The data is captured and held. Note that error code decoding is performed before register section 16x.

[0077] On the other hand, the X-polarization IndexBit demapping section 14x processes the input signal (X-polarization symbol intensity Px) within one frame. CS Since it is determining the X-polarization activation pattern signal APx CS This can be created. This X-polarization activation pattern signal APx CS This is transmitted to the X-polarization APMBit demodulation unit 25x of the X-polarization symbol demodulation unit 20x and the common frequency offset compensation unit 22com. This completes the operation of the index demodulation unit 10.

[0078] <Common frequency offset compensation unit 22com> The common frequency offset compensation unit 22com aims to estimate the offset frequency Δf in each polarization complex digital signal CS and remove that component. This offset frequency Δf of each polarization complex digital signal CS is generated by the phase diversity detector RXa (see Figure 9) installed in the receiver RX of the present invention (see Figure 6).

[0079] The phase diversity detector RXa attempts to extract phase information from the received optical symbol by interfering it with the local oscillator laser beam to detect the received optical symbol. However, the optical frequencies of the received optical symbol and the local oscillator laser beam do not necessarily match, and a difference in optical frequency exists (Figure 6). This difference in optical frequency becomes the oscillation frequency of the electrical signal, and is the offset frequency Δf of each polarization complex digital signal CS.

[0080] Next, with reference to Figure 13, the common frequency offset compensation unit 22com will be described in detail. The common frequency offset compensation unit 22com of the present invention removes the modulation effect from each polarization complex digital signal CS using a method called the power method and extracts the phase rotation angle of the received optical symbol carrier.

[0081] The common frequency offset compensation unit 22com receives the X-polarization activation pattern signal APx CS and Y-polarization activation pattern signal APy CS The X-polarized complex digital signal CSx and the Y-polarized complex digital signal CSy are input. These are then input to the multiplexing unit 101 located at the input stage of the common frequency offset compensation unit 22com.

[0082] The multiplexing unit 101 outputs the X-polarization activation pattern signal APx from the X-polarization IndexBit demapping unit 14x and the Y-polarization IndexBit demapping unit 14y. CS and Y-polarization activation pattern signal APy CS The timing of this signal and the timing of the branched complex digital signals CSx and CSa are synchronized by an internal delay adjustment.

[0083] Furthermore, the multiplexing unit 101 receives the X-polarization activation pattern signal APx CS and Y-polarization activation pattern signal APy CS The two signals are multiplexed and output. Here, the X-polarization activation pattern signal APx is used at the timing. CS and Y-polarization activation pattern signal APy CS If the signals are input to the multiplexing unit 101 at the same time, they are either combined into a single signal or one of the signals is selected and output. Similarly, either the complex digital signals CSx or CSy is selected and output. The selection of both signals is made to be equal.

[0084] Furthermore, within the common frequency offset compensation unit 22com, the X-polarization activation pattern signal APx is used from the multiplexing unit 101 onwards. CS and Y-polarization activation pattern signal APy CS Furthermore, the distinction between the X-polarized complex digital signal CSx and the Y-polarized complex digital signal CSy becomes unnecessary, so it simply becomes the complex digital signal CS and the activation pattern signal AP. CS The explanation continues.

[0085] The complex digital signal CS output from the multiplexing unit 101 is input to the power calculation unit 102. Here, the data modulation component contained in the complex digital signal CS is removed.

[0086] For example, in the case of a complex digital signal CS generated from QPSK optically modulated optical symbols, the phase angles of the data modulation components are π / 4, 3π / 4, -π / 4, and -3π / 4 (rad), so the complex representation of the complex digital signal CS is expressed as shown in equation (2).

[0087]

number

[0088] As a means of removing the effect of data modulation components, we perform a fourth-power operation. That is, raising both sides of equation (2) to the fourth power yields equation (3).

[0089]

number

[0090] The phase angles of the data modulation component, π / 4, 3π / 4, -π / 4, and -3π / 4 (rad), are canceled out by raising them to the fourth power. Here, Δωt is the angular frequency representation of the frequency offset Δf. The reason for raising it to the fourth power here is that the phase modulation symbol was 4-phase; in general, if the phase modulation symbol is M-phase, raising it to the M power will cancel out the data modulation component. That is, the influence of the data modulation component is removed, and the carrier component of the optical symbol and the optical angular frequency offset Δω and optical phase difference Δθ (phase shift) of the local light emission Lo are eliminated (carrier offset × phase component CC). ωθ The carrier offset / phase component CC (called CC) is generated. The power calculation unit 102 then processes this carrier offset / phase component CC. ωθ The output is sent to sample and hold units 103a and 103b. Carrier wave offset and phase component CC ωθ When written out explicitly, it becomes equation (4).

[0091]

number

[0092] The sample and hold units 103a, 103b, and 103c generate a timing signal (or activation pattern signal AP) at the timing when it is determined that an optical symbol exists in the X-polarized IndexBit demapping unit 14x or the Y-polarized IndexBit demapping unit 14y. CS At the time of arrival, the carrier wave offset and phase components CC input to the sample and hold units 103a and 103c are detected. ωθ It holds the activation pattern signal AP. CS Carrier offset / phase component CC corresponding to the light symbol at the ON timing ωθ It can hold.

[0093] The output of the sample-and-hold unit 103a (carrier offset phase component CC ωθ ) is input to the delay unit 104. The delay unit 104 applies a delay corresponding to the time slot time of an optical symbol, and outputs the delayed signal to the sample-and-hold unit 103c. Accordingly, if the timing signal (or activation pattern signal AP CS ) arrives in two consecutive cycles at equal intervals with the optical symbol time slot, the carrier offset phase component CC held in the sample-and-hold unit 103c ωθ is a value one optical symbol time slot older than the carrier offset phase components CC held in the sample-and-hold units 103a and 103b ωθ .

[0094] Further, if the interval between arrivals of the timing signal (or activation pattern signal AP CS is N optical symbol time slots, the carrier offset phase component CC held in the sample-and-hold unit 103c ωθ is a value N optical symbol time slots older than the carrier offset phase components CC held in the sample-and-hold units 103a and 103b ωθ .

[0095] Note that the carrier offset phase component CC held in the sample-and-hold unit 103c ωθ is defined as "CC ωθ1 ", and the carrier offset phase components CC held in the sample-and-hold units 103a and 103b ωθ are defined as "CC ωθ2 ". The carrier offset phase component CC ωθ1 is the carrier offset phase component CC ωθ2 of a symbol that temporally precedes the carrier offset phase component CC ωθ .

[0096] In any case, the activation pattern signal AP CSCarrier offset / phase component CC corresponding to the light symbol at the ON timing ωθ The carrier wave offset and phase components CC are held continuously. ωθ The time interval can also be determined from the number of optical symbol time slots from when data is held in the sample-and-hold unit 103c until data is held in the sample-and-hold unit 103b. This makes it possible to determine the phase difference per unit time even when the arrival of optical symbols is intermittent due to the introduction of index optical modulation, and thus it becomes possible to reconstruct the optical angular frequency offset Δω (corresponding to the offset frequency Δf) between the optical symbol and the local light emission Lo.

[0097] The complex conjugate calculation unit 105 processes the carrier offset and phase component CC, which is the input signal from the sample and hold unit 103c. ωθ1 The complex conjugate of is calculated. Then, the carrier offset and phase component CC, which is the input signal from the sample-and-hold unit 103b, is calculated. ωθ2 Multiplying by the multiplier 106 yields the carrier component of the optical symbol and the optical phase angle difference of the local emission (hereinafter, the signal with Δθ removed (this is the carrier offset component CC)). ω It is called ). )) is obtained. That is, it can be found as in equation (5).

[0098]

number

[0099] This carrier offset component CC ω (t) is the carrier offset and phase component CC, which is the input signal from the sample and hold unit 103b. ωθ2 And the carrier wave offset and phase component CC, which is the input signal from the sample and hold section 103c. ωθ1 It has phase difference information. Here, CC ωθ1 * (t) is CC ωθ1The complex conjugate of (t) is t2-t1, where t2-t1 is the time interval between each symbol. Alternatively, if the hold values ​​of sample-and-hold units 103c and 103b are complex digital signals sampled and held at time intervals equal to the N optical symbol time slot, then the carrier offset component CC in equation (5) ω It can be seen that (t) contains information about the amount of phase change of the complex digital signal (corresponding to the carrier wave component of the reconstructed optical symbol) that changed during the time elapsed for N optical symbol time slots.

[0100] The sample and hold section 103a, delay section 104, sample and hold sections 103c and 103b, and multiplier section 106 control the carrier offset and phase components CC. ωθ and activation pattern signal AP CS The input is the carrier offset component CC. ω This can be described as the carrier offset component extraction unit 22B1 that extracts the carrier offset component. In Figure 11, the carrier offset component extraction unit 22B1 is written as "22B1:103a, 104, 103c, 103b, 106" to mean that it includes the sample and hold unit 103a, delay unit 104, sample and hold units 103c, 103b, and multiplier unit 106.

[0101] The phase angle calculation unit 107 outputs Δω(t2-t1), which is the exponent part of equation (5). That is, it outputs the value of the phase angle. Here, the phase angle value is limited to the range -π≦Δω(t2-t1)≦π. Therefore, if the phase angle value jumps abruptly from a value near π to near -π, an error will occur. Therefore, the unwrap unit 108 performs signal processing to minimize the jump error in δ1=Δω(t2-t1). For example, the algorithm for this operation may be adopted from Non-Patent Literature 6. The phase rotation value φΔf, which is the output of the unwrap unit 108, is input to the sample and hold unit 103e. Note that the phase rotation value φΔf is the phase rotation value due to the offset frequency Δf, so it is called the phase rotation value φΔf due to the offset.

[0102] The sample and hold section 103e receives the activation pattern signal AP. CSWhen the ON state is activated, the phase rotation value φΔf due to the offset input from the unwrapping unit 108 can be output appropriately.

[0103] The counter unit 109 counts the number of time slots according to the clock and outputs the value. Then, it outputs the activation pattern signal AP. CS When the signal from the power calculation unit 102, which is output in accordance with the timing, is output, the counter value is reset. The sample and hold unit 103d is driven by the rising timing of the activation pattern signal APCS to save the value of the output of the counter unit 109 just before it is reset, and outputs that value to the division unit 110. This allows the division unit 110 to calculate the phase rotation value φΔf per time slot. The averaging unit 111 calculates the offset cumulative value ΣφΔf while averaging the output of the division unit 110. That is, after n symbol time slots t n The cumulative offset value in ΣφΔf(t n If the phase rotation amount at time t1 is θ1, the cumulative offset value ΣφΔf can be expressed as shown in equation (6).

[0104]

number

[0105] This cumulative offset value ΣφΔf(t) is nothing more than the sum of the phase rotation amounts (phase rotation value φΔf due to offset) caused by frequency offsets that occurred at the same time. Therefore, this cumulative offset value ΣφΔf(t) is input to the averaging unit 111.

[0106] The averaging unit 111 calculates the output value of the division unit 110, which is ΣφΔf(t n The ΣφΔf(t) is taken as input and processed by an averaging unit 111 to remove the effects of noise, and then the resulting signal is output. n )av is output, and the exponential calculation unit 112 calculates exp{-jΣφΔf(t nThe complex value of )av is output and input to the multiplication units 22A6x and 22A6y. This complex value is used to obtain the complex frequency offset compensation amount C. ΣφΔf It is called that.

[0107] The angle calculation unit 107, unwrap unit 108, counter unit 109, division unit 110, averaging unit 111, and exponential calculation unit 112 are responsible for the carrier wave offset component CC. ω From the complex frequency offset compensation amount C ΣφΔf This can be described as the complex frequency offset compensation amount calculation unit 22B2. Note that in Figure 13, the complex frequency offset compensation amount calculation unit 22B2 is written as "22B2:107, 108, 109, 110, 111, 112x, 112y" to mean that it includes the angle calculation unit 107, the unwrap unit 108, the counter unit 109, the division unit 110, the averaging unit 111, and the exponential calculation units 112x and 112y.

[0108] The multiplication units 22A6x and 22A6y calculate the complex frequency offset compensation amount C output from the exponential calculation unit 112 to the X-polarized complex digital signal CSx and the Y-polarized complex digital signal CSy input to the common frequency offset compensation unit 22com. ΣφΔf This is multiplied by the following: This compensates the offset frequency Δf of the complex digital signal CS described in equation (2) input to the common frequency offset compensation unit 22com, and yields the output of the complex digital signal shown in equation (7). This complex digital signal is a signal with the offset frequency Δf compensated, and the X-polarization component and Y-polarization component are respectively compensated by the X-polarization offset compensation signal CSx f and Y polarization offset compensation signal CSy f It is called that.

[0109]

number

[0110] The outputs of the multipliers 22A6x and 22A6y are passed through a suitably adjusted delay unit (not shown) to generate an activation pattern signal AP. CSWhen the ON timing, the X-polarized complex digital signal CSx has its offset frequency Δf compensated. f and Y-polarized complex digital signal CSy f Therefore, the common frequency offset compensation unit 22com receives the X-polarized complex digital signal CSx and the Y-polarized complex digital signal CSy as inputs, and the X-polarized offset compensation signal CSx is produced by compensating the offset frequency Δf generated at the receiver RX. f and Y polarization offset compensation signal CSy f Outputs.

[0111] Figure 14 shows the effect of the overall operation of the common frequency offset compensation unit 22com. In all the photographs in Figure 14, the horizontal axis corresponds to the real number axis and the vertical axis corresponds to the imaginary number axis. The experiment was conducted with the X-polarization component for explanatory purposes. The results show the effect of applying the common frequency offset compensation unit 22com to a pseudo-random data pattern of PN7 as the data modulation component. As shown in Figure 14(a), the value of the complex digital signal CSx before it is input to the common frequency offset compensation unit 22com shows a behavior of rotation on the complex plane over time.

[0112] On the other hand, when compensation processing is performed by the common frequency offset compensation unit 22com, as shown in Figure 14(b), the value of the complex digital signal CSx at the output of the common frequency offset compensation unit 22com becomes static. Similar results are obtained for the Y polarization component. This output value is the X polarization offset compensation signal CSx f and Y polarization offset compensation signal CSy f This is input to the X-polarization phase compensation unit 23x and the Y-polarization phase compensation unit 23y of the X-polarization demodulation unit 6X and the Y-polarization demodulation unit 6Y.

[0113] Furthermore, due to the effect of the multiplexing unit 101, the interval between complex digital signals CS is narrowed. Accordingly, the amount of phase change between complex digital signals before and after comparing arguments is reduced. Therefore, even when the offset frequency Δf increases, accurate comparison of arguments can be achieved. FIG. 15 shows this effect. The horizontal axis represents the offset frequency normalized by the optical symbol rate, and the vertical axis represents the bit error rate.

[0114] The broken line indicates the result obtained by the conventional compensation method, and the solid line indicates the result obtained when the common frequency offset compensation unit 22com according to the present invention is applied. Circle marks represent (8,1,BPSK) signals, triangle marks represent (8,2,BPSK) signals, and square marks represent (8,3,BPSK) signals. For identical marks between the broken line and the solid line, when compared at the value of the normalized offset frequency (indicated by the arrow in the figure) at which the pre-forward error correction bit error rate (Pre-Fec BER) is 10 -3 , it can be seen that the value of the maximum allowable offset frequency Δfmax, which is the maximum value of the offset frequency Δf that can keep the bit error rate low, is expanded from 1.4 times to 1.8 times.

[0115] <X偏波位相補償部23x> Next, the X-polarization phase compensation unit 23x will be described. As shown in FIG. 11, the X-polarization phase compensation unit 23x, which receives the output of the common frequency offset compensation unit 22com at the X-polarization demodulation unit 6X, is configured as shown in FIG. 16. First, the input complex digital signal (X-polarization offset compensation signal CSx f ) is split into two branches. One of the branches is input to the power operation unit 231x. Here, the data modulation component included in the complex digital signal (X-polarization offset compensation signal CSx f ) is removed, and the phase difference information between the carrier phase of the received optical symbol and the local oscillation light phase is reproduced. The operation of this power operation unit 231x may be the same as that of the power operation unit 102 of the common frequency offset compensation unit 22com. The output of the power operation unit 231x is defined as the carrier phase component CCx θ . When the carrier phase component CCx θ is explicitly written, it is expressed by Equation (8) below.

[0116]

number

[0117] The averaging unit 232x processes the complex digital signal (carrier phase component CCx) output from the power calculation unit 231x. θ The average value of L symbols for each of the real and imaginary parts of ) Av(CCx θ The phase calculation unit 233x calculates the average value Av(CCx) of the carrier phase component of the received optical symbol output from the averaging unit 232x. θ The optical phase difference xΔθ is calculated from the given values. Here, the optical phase difference xΔθ is the optical phase difference between the carrier component of the optical symbol and the local emission.

[0118] The optical phase difference xΔθ is limited to the range -π ≤ xΔθ(t2-t1) ≤ π. Therefore, if the optical phase difference xΔθ suddenly jumps from a value near π to near -π, an error occurs. To address this, the unwrapping section 234x performs signal processing to minimize the jump error δ1 = xΔθ(t2-t1). The operation of this unwrapping section 234x is the same as that of the unwrapping section 108 of the common frequency offset compensation section 22com. The optical phase difference xΔθ, which is the output of this unwrapping section 234x, is multiplied by 1 / M in the division section 235x to obtain the phase shift rotation value xΦθ. Here, M is a value that depends on the modulation scheme of the optical symbol; for example, in the case of QPSK, M = 4.

[0119] The exponential calculation unit 236x takes the phase shift rotation value xΦθ, which is the output value of the division unit 235x, as input, outputs the complex value of exp(-jxΦθ), and inputs it to the multiplication unit 238x. This complex value is the complex phase shift compensation amount Cx Φθ This is called [the function]. The unwrapping unit 234x, the division unit 235x, and the exponential calculation unit 236x receive the optical phase difference xΔθ as input and calculate the complex phase shift compensation amount Cx Φθ This can be called the complex phase shift compensation amount calculation unit 23B1x, which calculates the following:

[0120] The delay unit 237x receives the X-polarized complex digital signal CSx input to the X-polarized phase compensation unit 23x.f (Offset compensation signal CSx f The input is CSx, an X-polarized complex digital signal with frequency offset compensated, which is branched off to the power calculation unit 231x. f (X polarization offset compensation signal CSx) f The multiplication unit 238x is output with a delay equal to the time it takes for the result to be input to the multiplication unit 238x via the exponential calculation unit 236x.

[0121] The multiplier 238x receives the complex digital signal CSx, which has its frequency offset compensated, input to the X-polarization phase compensation unit 23x. f (X polarization offset compensation signal CSx) f ) and the complex phase shift compensation amount Cx output from the exponential calculation unit 236x Φθ Multiply by . The complex digital signal output from the multiplier 238x is phase-compensated by this multiplication process so that the phase angle of the data modulation component matches the phase angle of the defined signal point, and the optical phase difference xΔθ between the carrier component of the optical symbol and the local emission is compensated. Since this signal has offset frequency and phase difference compensated, the offset & phase compensated signal CSx fθ This is called the offset and phase compensation signal CSx, which is the output from this multiplier unit 238x. fθ This is taken as the output of the entire X-polarization phase compensation section 23x and input to the X-polarization APMBit demapping section 25x. Offset & phase compensation signal CSx fθ This is expressed as shown in equation (9). Note that equation (9) is for each polarization offset & phase compensation signal CS fθ This is shown as follows.

[0122]

number

[0123] The above describes the overall configuration of the X-polarization phase compensation unit 23x. As shown in Figure 16, the complex digital signal CSx before it is input to the X-polarization phase compensation unit 23x is also shown. f (X polarization offset compensation signal CSx) f), the value deviates from the position of the signal point defined on the complex plane. On the other hand, as a result of undergoing compensation processing by the X-polarization phase compensation unit 23x, the output of the X-polarization phase compensation unit 23x is a complex digital signal (offset & phase compensation signal CSx fθ ), the value is near the defined signal point as shown in FIG. 17. This output value is input to the X-polarization APM Bit demapping unit 25x (see FIG. 11).

[0124] <X偏波APMBitデマップ部25x> Referring again to FIG. 11, the X-polarization ApmBit demapping unit 25x will be described. The X-polarization APM Bit demapping unit 25x receives the X-polarization activation pattern signal APx CS operates in synchronization with. Here, the X-polarization activation pattern signal APx CS is the complex digital signal detected when an optical symbol is ON (offset & phase compensation signal CSx fθ ) is input to the X-polarization APM Bit demapping unit 25x via the delay unit 24x so as to achieve timing synchronization with. The X-polarization APM Bit demapping unit 25x receives the X-polarization activation pattern signal APx CS provides a function of decoding the complex digital signal (offset & phase compensation signal CSx fθ ) input at the ON timing into a digital code.

[0125] As a method for decoding the complex digital signal (offset & phase compensation signal CSx fθ ) into a digital code, there is a method of assigning Gray codes to signal points on the complex plane. For example, in the case of QPSK modulation, codes "11", "01", "10", "00" are assigned to signal points 1+j, -1+j, 1-j, -1-j on the complex plane.

[0126] The X-polarization APM Bit demapping unit 25x processes the input complex digital signal (offset & phase compensation signal CSx fθThe likelihood is calculated for each signal point (Non-Patent Document 7), and the code of the signal point with the highest likelihood is selected and decoded. The output of the X-polarized APMBit demapping unit 25x is converted to the X-polarized symbol decoded code DEx. AP This is the case. Furthermore, error correction processing may be performed between the X-polarized APMBit demapping section 25x and the register section 27x.

[0127] Furthermore, the signal point arrangement exhibits rotational symmetry of 90° in phase angle. Therefore, when decoding optical symbols, there is a possibility that the interpretation of the I-axis and Q-axis of the complex plane may be reversed. To avoid this, the X-polarized APMBit demapping unit 25x has an IQ channel discrimination function.

[0128] To drive this IQ channel discrimination function, the transmitter periodically inserts training optical symbols (complex value 1+j) to provide a phase reference for the received optical symbols. The IQ channel discrimination function identifies the training optical symbols, then determines the phase reference (complex value 1+j) of the optical symbols, and correctly determines the phase rotation of the optical symbols, i.e., the IQ channel.

[0129] Furthermore, in an optical transmission system having the functions of the present invention, the IQ channel discrimination function of the X-polarized APMBit demapping unit 25x can be omitted by employing a differential coding scheme. In the X-polarized APMBit demapping unit 25x corresponding to the differential coding scheme, the differentially coded complex digital signal S input at the timing of the N-1th activation pattern input N-1 And the differentially coded complex digital signal S input at the timing of the Nth activation pattern input. N Decoding is performed using the phase difference. For example, when the phase difference is 0, it is decoded as 00; when it is π / 2, it is decoded as 01; when it is -π / 2, it is decoded as 10; when it is π, it is decoded as 11, and so on.

[0130] <Register section 27x> The register section 27x receives the X-polarized activation pattern signal APx via the delay section 26x. CSwhich receives, as an input, an X-polarization activation pattern signal APx CS acquires and holds a decoded code string of APMBit when it is on, i.e., the X-polarization symbol decoding code DEx AP ). Herein, the delay unit 26x is set with a delay time such that arrival timings match between the X-polarization activation pattern signal APx CS and the corresponding X-polarization symbol decoding code DEx decoded by the X-polarization APMBit demapping unit 25x AP .

[0131] The AMPBit data (the X-polarization symbol decoding code DEx AP ) stored in this register unit 27x is read out while being synchronized in timing with the code string of IndexBit decoded by the X-polarization index demodulation unit 10x and stored in the register unit 16x (the X-polarization index decoding code DEx Id ). The above is the operation of the X-polarization symbol demodulation unit 20x.

[0132] <X-polarization P / S conversion unit 29x> The X-polarization P / S conversion unit 29x reads the X-polarization index decoding code DEx of IndexBit decoded by the X-polarization index demodulation unit 10x and stored in the register unit 16x Id and the X-polarization symbol decoding code DEx of APMBit decoded by the X-polarization symbol demodulation unit 20x and stored in the register unit 27x AP , converts these into a serial code string SDATx, decodes the transmission code string (original signal INFx) to be transmitted by the optical signal transmission device 1, and obtains a decoded original signal (X-polarization decoded original signal DE(INFx)). Here, the X-polarization P / S conversion unit 29x receives a trigger signal whose timing is adjusted by the delay unit 28x, and performs reading processing of the codes accumulated in the register unit 16x and the register unit 27x based on this signal.

[0133] The above describes the first embodiment of the present invention, which is characterized in that it has the function of receiving optical symbols generated based on a time-domain (N,K,M-QAM) index optical modulation scheme, while also being able to receive optical symbols generated based on an M-QAM optical modulation scheme.

[0134] When receiving only M-QAM optical symbols, demapping (conversion from complex digital signal to code) in the X-polarization IndexBit demapping section 14x (and Y-polarization IndexBit demapping section 14y) is stopped, but the activation pattern signal AP is not. CS This is continuously output to the common frequency offset compensation unit 22com. As a result, the common frequency offset compensation unit 22com operates continuously in accordance with the arrival period of the optical symbol.

[0135] As a result, frequency offset compensation can be achieved even for optical symbols generated based on the M-QAM optical modulation scheme. Furthermore, the phase compensation unit 23 and APMBit demapping unit 25 within each polarization demodulation unit 6 of the present invention are originally designed to operate continuously at the period of the optical symbol time slot, regardless of whether it is a time-domain (N,K,M-QAM) index optical modulation scheme or an M-QAM optical modulation scheme, and therefore operate in common regardless of the scheme. However, for the identification of training optical symbols, the circuit is switched according to the operating mode of the time-domain (N,K,M-QAM) index optical modulation scheme and the M-QAM optical modulation scheme. Note that this mode switching and the cessation of demapping in the IndexBit demapping unit 14 within each polarization demodulation unit 6 may be received as separate signals.

[0136] Each polarization parallel / serial conversion unit 29 within each polarization demodulation unit 6 changes the read patterns from each polarization register unit 16 and each polarization register unit 27 located in the preceding stage, according to the operating modes of the time-domain (N,K,M-QAM) index optical modulation method and the M-QAM optical modulation method. For example, in the case of the time-domain (8,1,QPSK) index optical modulation method, one frame is composed of time slots for 8 optical symbols, and only one optical symbol is turned ON in one frame. In this case, there are 8 types of ON / OFF patterns for the symbol, and 3 bits of transmission is possible with the Index Bit.

[0137] On the other hand, APMBit can transmit 2 bits because only one QPSK optically modulated optical symbol arrives. When each polarization parallel / serial conversion unit 29 operates with a time-domain (N,K,M-QAM) index optical modulation scheme, each polarization activation pattern signal AP CS When the input is received, 3 bits of the code are read from each polarization register section 16 and 2 bits of the code are read from each polarization register section 27.

[0138] On the other hand, when receiving a QPSK optically modulated optical symbol, each polarization parallel / serial converter 29 within each polarization demodulation unit 6 operates in a time-domain (N,K,M-QAM) index optical modulation scheme, and each polarization activation pattern signal AP for each polarization component is generated. CS When the input is received, two bits of the code are read from each polarization register section 27 within each polarization demodulation section 6. This operation enables switching between the operating modes of the time-domain (N,K,M-QAM) index optical modulation method and the M-QAM optical modulation method.

[0139] Thus, the optical signal transmitting device 1 of the present invention is characterized by its ability to handle both time-domain (N,K,M-QAM) index optical modulation and M-QAM optical modulation with substantially the same circuit configuration. Furthermore, it also has the advantage of expanding the maximum allowable offset frequency Δfmax for time-domain (N,K,M-QAM) index optical modulation signals.

[0140] (Second embodiment) In the second embodiment, instead of the common frequency offset compensation unit 22com, the power calculation unit 102 (see Figure 13) and each polarization power calculation unit 231 (see cumulative calculation unit 231x in Figure 16 for the X polarization system) located in each polarization phase compensation unit 23 (see Figure 11 for the X polarization system) located in each polarization symbol demodulation unit 20 within each polarization demodulation unit 6, the provisional determination calculation unit 102a (see Figure 20) and each polarization provisional determination calculation unit 231a (see Figure 22), which will be described later, are applied. The provisional determination calculation unit 102a and each polarization provisional determination calculation unit 231a provide the same function as the power calculation unit 102 and each polarization cumulative calculation unit 231 in the first embodiment, in that they are applied as means to remove the influence of data modulation components from the complex digital signal CS corresponding to the received optical symbol.

[0141] The common frequency offset compensation unit 22com, which has a provisional determination calculation unit 102a and each polarization provisional determination calculation unit 231a, and each polarization phase compensation unit 23 in each polarization demodulation unit 6 are referred to as the common frequency offset compensation unit 22coma and each polarization phase compensation unit 23a.

[0142] Specifically, each polarization phase compensation unit 23a refers to the X-polarization phase compensation unit 23xa and the Y-polarization phase compensation unit 23ya. Furthermore, each polarization symbol demodulation unit 20 having the X-polarization phase compensation unit 23xa and the Y-polarization phase compensation unit 23ya is called the X-polarization symbol demodulation unit 20xa and the Y-polarization symbol demodulation unit 20ya. Similarly, each polarization demodulation unit 6 having the X-polarization symbol demodulation unit 20xa and the Y-polarization symbol demodulation unit 20ya is called the X-polarization demodulation unit 6Xa and the Y-polarization demodulation unit 6Ya. Finally, the demodulation unit having the X-polarization demodulation unit 6Xa and the Y-polarization demodulation unit 6Ya is called the demodulation unit 6a.

[0143] Figures 18, 19, and 20 show the configuration of the demodulation unit 6a and each polarization demodulation unit 6a (specifically, the X-polarization demodulation unit 6Xa) in this embodiment, as well as the common frequency offset compensation unit 22coma and the X-polarization demodulation unit 6Xa. The Y-polarization demodulation unit 6Ya is not shown, but it is composed of the same corresponding elements as the X-polarization demodulation unit 6Xa.

[0144] The common frequency offset compensation unit 22coma and each polarization phase compensation unit 23a within each polarization demodulation unit 6 perform the same operation as the common frequency offset compensation unit 22coma and each polarization phase compensation unit 23 within each polarization demodulation unit 6. Therefore, the operation of each polarization symbol demodulation unit 20a within each polarization demodulation unit 6a, including the common frequency offset compensation unit 22coma and each polarization phase compensation unit 23a within each polarization demodulation unit 6 (X polarization demodulation unit 6Xa and Y polarization demodulation unit 6Ya), is the same as in the first embodiment.

[0145] Figure 20 shows the configuration of the common frequency offset compensation unit 22coma. The differences from the common frequency offset compensation unit 22coma in Figure 13 are that the power calculation unit 102 is replaced by the provisional determination calculation unit 102a, and the complex frequency offset compensation amount C is the output of the exponential calculation unit 112. ΣφΔf The point is that this is input to the provisional determination calculation unit 102a.

[0146] Figure 21 shows the configuration of the provisional determination calculation unit 102a incorporated into the common frequency offset compensation unit 22coma. First, the provisional determination calculation unit 102a receives each polarization complex digital signal CS of equation (7) below, corresponding to the optical symbol (output CS of the multiplexing unit 101 in Figure 20). Here, it is assumed that the optical symbol is QPSK optically modulated, and the optical angular frequency offset between the carrier component of the optical symbol and the local emission is Δω, and the optical phase difference is Δθ. When the complex digital signal CS represented by equation (10) is input to this provisional determination calculation unit 102a, the complex digital signal CS is split into two, and one is input to the multiplication unit 2211 and the other to the multiplication unit 2213.

[0147]

number

[0148] On the other hand, the exponential calculation unit 112 (see Figure 20) located on the output side of the common frequency offset compensation unit 22coma outputs a signal (complex frequency offset compensation amount C) that compensates for the optical angle frequency offset. ΣφΔf) is fed back. However, this feedback signal is at time t n at an earlier time t m the complex digital signal CS(t m ), since it is a signal generated based on this, it is expressed as shown in Equation (11).

[0149]

Math

[0150] The complex frequency offset compensation amount C in Equation (11) ΣφΔf is input to the multiplier 2211 and multiplied by the complex digital signal CS in Equation (10). As a result of the multiplication, the complex digital signal CS in Equation (12) np is obtained.

[0151]

Math

[0152] This signal can be described as a signal in which continuous phase rotation of the complex vector caused by the influence of frequency offset Δf is suppressed. This signal is defined as the suppressed complex digital signal CS np This signal is input to the tentative decision unit 2212. The tentative decision unit 2212 processes the suppressed complex digital signal CS of Equation (12) np by selecting the signal point with the highest likelihood, and processes ω(t n -t m )+Δθ n -Δθ m from which optical angular frequency offset and optical phase difference components have been removed, the inverse value TCS of the tentative decision data modulation component expressed by Equation (13) -1 fθ is output and input to the multiplier 2213.

[0153]

Math

[0154] The multiplier 2213 multiplies the complex digital signal CS, expressed by equation (7), input to the common frequency offset compensation unit 22coma, with the complex digital signal expressed by equation (10). By doing so, it removes the data modulation component from the complex digital signal CS input to the common frequency offset compensation unit 22coma, and obtains the carrier offset and phase component CC of the optical symbol, expressed by equation (14) below. ωθ The signal is reproduced. This signal is the carrier offset / phase component CC, which is the output of the power calculation unit 102 in the common frequency offset compensation unit 22com shown in the first embodiment. ωθ This is the same as above. The common frequency offset compensation unit 22coma calculates a provisional phase determination value for each input polarization complex digital signal CS and compares it with the provisional phase determination value of the next detected complex digital signal to determine the complex frequency offset amount C. ΣφΔf It can be said that this calculates and compensates for the frequency offset of each polarization complex digital signal CS.

[0155]

number

[0156] Thus, the provisional determination calculation unit 102a of this embodiment achieves the same operation as the power calculation unit 102 of the first embodiment.

[0157] Figure 22 shows the configuration of each polarization phase compensation unit 23a within each polarization demodulation unit 6a, and Figure 23 shows an example of each polarization provisional determination calculation unit 231a newly provided in this embodiment. Figure 22 illustrates the X polarization phase compensation unit 23xa of the X polarization demodulation unit 6Xa. This embodiment achieves the same operation as the provisional determination calculation unit 102a provided in the common frequency offset compensation unit 22coma. However, the scope of achieving the same operation is limited to the procedures from equation (13) to equation (14).

[0158] Referring to Figure 23, the provisional determination unit 2311x receives an offset compensation signal CS, which is the frequency offset compensated signal output from the common frequency offset compensation unit 22coma. f (Each polarization offset compensation signal CS) f) is input. Each polarization offset compensation signal CS f is obtained as shown in equation (7). Equation (7) is reproduced below.

[0159]

Math

[0160] The tentative decision unit 2311x obtains each polarization offset compensation signal CS of equation (7) f , selects the signal point with the highest likelihood, and outputs the reciprocal value TCS of the tentative decision data modulation component represented by equation (15) from which the optical angular frequency offset and Δθ of the optical phase difference component n -Δθ m have been removed, represented by equation (15) -1 θ , and inputs it to the multiplication unit 2312x.

[0161]

Math

[0162] By multiplying the signal of equation (15) by the X polarization offset compensation signal CSx f , the carrier phase component CCx represented by equation (16) can be obtained. This is the same operation as the exponentiation operation unit 231x (see FIG. 16) of the X polarization phase compensation unit 23x of the first embodiment. θ を得ることができる。これは第1の実施形態のX偏波位相補償部23xの累乗演算部231x(図16参照)と同じ動作となる。

[0163]

Math

[0164] The second embodiment is not limited to the time-domain (N,K,M-QAM) indexed optical modulation scheme in which signal points arranged at equal phase intervals are defined, and can also accommodate the time-domain (N,K,M-QAM) indexed optical modulation scheme in which arbitrary signal points are defined.

[0165] Furthermore, the X-polarization index demodulation unit 10x and the X-polarization symbol demodulation unit 20x in the second embodiment have the same configuration as those shown in Figure 11 of the first embodiment. Therefore, the input and output signals of the common frequency offset compensation unit 22coma and each polarization phase compensation unit 23a within each polarization demodulation unit 6a are the same signals. Accordingly, in the common frequency offset compensation unit 22coma in Figure 20, the sample and hold unit 103a, delay unit 104, sample and hold units 103c and 103b, and complex conjugate calculation unit 105 are the carrier offset phase component CC. ωθ and activation pattern signal AP CS The input is the carrier offset component CC. ω The carrier offset component extraction unit 22B1 can be described as the unit that extracts the carrier offset component CC. The angle calculation unit 107, unwrap unit 108, counter unit 109, division unit 110, averaging unit 111, and exponential calculation unit 112 extract the carrier offset component CC. ω From the complex frequency offset compensation amount C ΣφΔf This can be described as the complex frequency offset compensation amount calculation unit 22B2 that determines the value. [Industrial applicability]

[0166] This invention applies to optical access networks, which have become an indispensable social infrastructure system in our advanced information society. In conventional optical access networks, the optical loss budget between the OLT and the ONU is 29 dB, and the transmission distance is 20 km. If these conditions can no longer be met, it becomes impossible to maintain the communication line, and the 10 Gbit / s transmission rate is completely interrupted.

[0167] This invention realizes a time-domain index optical modulation scheme, making it possible to vary the transmission rate by decimating optical symbols in accordance with changes in the transmission path conditions between the OLT and the ONU. In other words, even under conditions where the transmission path conditions are poor and exceed the optical loss budget of 29 dB, if there is an additional optical loss budget of up to about 8 dB, it is possible to avoid a complete interruption of the communication line by gradually reducing the transmission rate. That is, even for ONUs deployed in more remote locations from the OLT, connectivity of the communication line can be ensured, albeit with a decrease in the transmission rate, thereby contributing to the expansion of the service area for optical access network services and ultimately to the elimination of the digital divide.

[0168] Furthermore, for time-domain (N,K,M-QAM) indexed optical modulation signals, the maximum allowable offset frequency can be expanded, reducing the cost of laser diodes applied to the signal source and local oscillator source of the system. [Explanation of symbols]

[0169] 1. Optical signal transmitting device INF original signal INFx X-polarized original signal INFy Y-polarized original signal 2. Transmission path 3. Optical signal demodulator 6 Demodulation Unit 6a Demodulation unit (having a provisional determination calculation unit) 6. Each polarization demodulation section 6X polarization demodulation section 6Y Y-polarization demodulation section 6a Each polarization demodulation section 6Xa (with provisional determination calculation unit) X polarization demodulation unit 6Ya (with provisional determination calculation unit) Y polarization demodulation unit 10 Polarization Index Demodulation Section 10x X-polarization index demodulation section 10y Y-polarization index demodulation section 11 Polarization amplitude calculation unit 11x X polarization amplitude calculation unit 12 Frame timing extraction unit 12x X polarization frame timing extraction unit 13x delay section 15x delay section 14 IndexBit Demap Section 14x X-polarization IndexBit Demap section 14y Y-polarization IndexBit Demap section 16. Each polarization register section 16x Register Section 20 Symbol demodulation section 20. Each polarization symbol demodulation section 20x X-polarization symbol demodulation section 20y Y-polarization symbol demodulation section 20a Each polarization symbol demodulation unit (having a provisional determination calculation unit) 20xa (with provisional determination calculation unit) X-polarization symbol demodulation unit 20ya (with provisional determination calculation unit) Y-polarization symbol demodulation unit 22com Common Frequency Offset Compensation Unit 22coma (Having a provisional determination calculation unit) Common frequency offset compensation unit 101 Multiplexer 102 Exponentiation Unit 103a Sample & Hold Section 103b Sample & Hold Section 103c Sample & Hold Section 103d Sample & Hold Section 103e Sample & Hold Section 104 Delay section 105 Complex Conjugate Calculation Unit 106 Multiplication section 107 Declination angle calculation section 108 Unwrapping section 109 Counter section 110 Division part 111 Averaging section 112 Exponential calculation section 22A6x Multiplication part 22A6y Multiplication section 22A6 Multiplication section 23 Phase Compensation Unit 23 Polarization Phase Compensation Sections 23a Each polarization phase compensation unit (having a provisional determination calculation unit) 231 Exponentiation Unit 232 Averaging section 233 Phase calculation section 234 Unwrapping section 235 Division part 236 Exponential calculation section 238 Multiplication part 237 Delay section 22B2 Complex frequency offset compensation amount calculation unit 24 Delay section 25 APMBit Demap Section 25x X polarization APMBit demapping section 26 Delay section 27 Each polarization register section 27x Register Section 28 Delay section 29 Parallel / Serial Conversion Unit 29 Polarization P / S conversion section 29x X polarization P / S conversion section 29y Y-polarization P / S conversion section 102a Provisional determination calculation unit 231a Polarization Preliminary Determination Calculation Unit 23x X polarization phase compensation section 23y Y polarization phase compensation section 23xa (with provisional determination calculation unit) X polarization phase compensation unit 23ya (with provisional determination calculation unit) Y polarization phase compensation unit 2211 Multiplication part 2213 Multiplication part 2212 Provisional Determination Unit 2311 Provisional Judgment Unit 2312 Multiplication part 232 Averaging section 234 Unwrapping section 23B1 Complex Phase Shift Compensation Amount Calculation Unit RX Receiver RXa Phase Diversity Detector RXb Analog-to-Digital Converter RXc Optical Polarization Compensation Unit LS optical signal Lo station light emission AP CS Activation pattern signal APx CS X-polarization activation pattern signal APy CS Y-polarization activation pattern signal DE(INF) Each polarization decoded original signal DE(INFx) X-polarized decoded original signal DE(INFy) Y-polarized decoded original signal Δf (offset frequency Δf) Optical frequency difference Δfmax Maximum allowable offset frequency CS complex digital signal CSx and CSi complex digital signals CSx complex digital signal CSy complex digital signal CSx f A complex digital signal with frequency offset compensation. CSy f A complex digital signal with frequency offset compensation. CS f Each polarization offset compensation signal CSx f X-polarization offset compensation signal CSy f Y-polarization offset compensation signal DE Id Index decryption code DEx Id X-polarized index decoding code DEY Id Y-polarization index decoding code DE AP Each polarization symbol decoding code DEx A X-polarized symbol decoding code DEY AP Y-polarization symbol decoding code P CS Symbol strength Px CS X-polarization symbol intensity S TOP Leading notification signal Sx TOP X-polarized leading notification signal Δω Optical angle frequency offset Δθ Optical phase difference xΔθ Optical phase difference CC ωθ Carrier wave offset and phase components CC ω Carrier offset component 22B1 Carrier wave offset component extraction unit φΔf Phase rotation value φΔf Phase rotation value due to offset ΣφΔf Offset cumulative value C ΣφΔf Complex frequency offset compensation CC θ Carrier phase component CCx θ Carrier phase component Av(CC θ ) Average value Av(CCx θ ) Average value Φθ Phase shift rotation value xΦθ Phase shift rotation value C Φθ Complex phase shift compensation Cx Φθ Complex phase shift compensation CS fθ Offset & Phase Compensated Signals CSx fθ Offset & Phase Compensated Signals SDAT serial code sequence SDATx serial code sequence CS np Suppressed complex digital signal TCS -1 fθ Reciprocal value of the modulation component of the provisional judgment data TCS -1 θ Reciprocal value of the modulation component of the provisional judgment data

Claims

1. An optical signal demodulator that receives an optical signal in which the original signal (INFx, INFy) is transmitted for each of the X-polarization and Y-polarization components, using symbols that are placed as complex digital signals (CSx, CSy) in at least one of the time slots, and index codes formed at the time positions of the time slots on which the symbols are placed, and demodulates the original signal (INFx, INFy). A receiving unit (RX) performs phase diversity detection on the optical signal and converts it into the complex digital signals (CSx, CSy) for each of the X-polarization and Y-polarization components, The system includes a demodulation unit (6) that receives the complex digital signals (CSx, CSy) for each of the X-polarization and Y-polarization components and demodulates the original signals of the X-polarization and Y-polarization components. The demodulation unit (6) consists of an X-polarization demodulation unit (6X), a Y-polarization demodulation unit (6Y), and a common frequency offset compensation unit (22com). The X-polarization demodulation unit (6X) is, The complex digital signal (CSx) of the X-polarization component output from the receiving unit (RX), A complex digital signal (CSxf) of the X-polarization component, which has been frequency offset compensated, is input from the common frequency offset compensation unit (22com). X-polarized activation pattern signal (APx) that indicates the position of the time slot on which the aforementioned symbol is placed. ＣＳ )and The X-polarized index decode (DEx) obtained by demodulating the aforementioned index code. Ｉｄ )and, The aforementioned X-polarization activation pattern signal (APx ＣＳ The X-polarization symbol decoding code (DEx) is obtained by decoding the symbols of the X-polarization component complex digital signal (CSxf) that has been frequency offset compensated in accordance with the above. ＡＰ ) generates, The aforementioned X-polarization index decoding code (DEx Ｉｄ ) and the X-polarization symbol decoding code (DEx ＡＰ ) generates and outputs the X-polarized decoded original signal (DE(INFx)), The aforementioned X-polarization activation pattern signal (APx ＣＳ ) is output to the common frequency offset compensation unit (22com), The Y-polarization demodulation unit (6Y) is, The complex digital signal (CSy) of the Y polarization component output from the receiving unit (RX), A complex digital signal (CSyf) of the frequency-offset-compensated Y-polarization component is input from the aforementioned common frequency offset compensation unit (22com). A Y-polarized activation pattern signal (APy) indicates the position of the time slot on which the symbol is placed. ＣＳ )and The Y-polarization index decoded code obtained by demodulating said index code (DEy Ｉｄ ), and The aforementioned Y-polarization activation pattern signal (APy ＣＳ ) The Y-polarization symbol decoding code (DEy) obtained by decoding the symbols of the Y-polarization complex digital signal (CSyf) of the frequency offset compensated Y-polarization component in accordance with ). ＡＰ ) generates, The aforementioned Y-polarization index decoding code (DEy Ｉｄ ) and the Y-polarization symbol decoding code (DEy ＡＰ ) generates and outputs a Y-polarized decoded original signal (DE(INFy)), The aforementioned Y-polarization activation pattern signal (APy ＣＳ ) is output to the common frequency offset compensation unit (22com), The aforementioned common frequency offset compensation unit (22com) is, The complex digital signal (CSx) of the X-polarization component, The complex digital signal (CSy) of the Y-polarization component, The X-polarization activation pattern signal (PAx) output from the X-polarization demodulation unit (6X) ＣＳ )and, The Y polarization activation pattern signal (PAy) output from the Y polarization demodulation unit (6Y) ＣＳ ) was entered, The complex digital signal (CSxf) of the X-polarization component with the frequency offset compensated, An optical signal demodulator characterized by outputting a complex digital signal (CSyf) of the Y-polarization component, with the frequency offset compensated, to the X-polarization demodulator (6X) and the Y-polarization demodulator (6Y), respectively.

2. The X-polarization demodulation unit (6X) is, X-polarization index demodulation unit (10x), X-polarization symbol demodulation unit (20x), It has an X-polarization parallel / serial conversion unit (29x), The X-polarization index demodulation unit (10x) is, The complex digital signal (CSx) of the X-polarization component is input. The aforementioned X-polarization activation pattern signal (PAx ＣＳ ) and the X-polarization index decoding code (DEx Ｉｄ ) outputs, The X-polarization symbol demodulation unit (20x) is, The complex digital signal (CSxf) of the X-polarization component with the frequency offset compensated, and the X-polarization activation pattern signal (PAx ＣＳ ) is input, and the X-polarization symbol decoding code (DEx ＡＰ ) outputs, The X-polarization parallel / serial conversion unit (29x) is, The aforementioned X-polarization index decoding code (DEx Ｉｄ ) and the X-polarization symbol decoding code (DEx ＡＰ ) is input and the X-polarized demodulated signal (DE(INFx)) is output. The Y-polarization demodulation unit (6Y) is, Y-polarization index demodulation unit (10y), Y-polarization symbol demodulation section (20y), It has a Y-polarization parallel / serial conversion unit (29y), The Y-polarization index demodulation unit (10y) is, The complex digital signal (CSy) of the Y polarization component is input. The aforementioned Y-polarization activation pattern signal (PAY ＣＳ ) and the Y-polarization index decoding code (DEy Ｉｄ ) outputs, The Y-polarization symbol demodulation unit (20y) is, A complex digital signal (CSyf) of the Y-polarization component with frequency offset compensation, and the Y-polarization activation pattern signal (PAy ＣＳ ) is input, and the Y polarization symbol decoding code (DEy ＡＰ ) outputs, The Y-polarization parallel / serial conversion unit (29y) is, The aforementioned Y-polarization index decoding code (DEy Ｉｄ ) and the Y-polarization symbol decoding code (DEy ＡＰ ) is input and the Y-polarized demodulated signal (DE(INFy)) is output. The aforementioned common frequency offset compensation unit (22com) is, The complex digital signal (CSxf) of the X-polarization component with the frequency offset compensated, The optical signal demodulation device according to claim 1, characterized in that it outputs the complex digital signal (CSyf) of the Y-polarization component with the frequency offset compensated to the X-polarization symbol demodulation unit (20x) of the X-polarization demodulation unit (6X) and to the Y-polarization symbol demodulation unit (20y) of the Y-polarization demodulation unit (6Y), respectively.

3. The aforementioned common frequency offset compensation unit (22com) is, The aforementioned X-polarization activation pattern signal (PAx ＣＳ ) and the Y polarization activation pattern signal (PAy ＣＳ When either of the above is input, the complex digital signal of the X-polarization component (CSx) or the complex digital signal of the Y-polarization component (CSy) obtained is raised to the power of M in accordance with the phase modulation symbol number M of the complex digital signal to obtain a complex frequency offset amount (C ΣφΔｆ ) calculate, The complex frequency offset amount (C ΣφΔｆ ) and the complex digital signal of the X-polarization component (CSx) and the complex digital signal of the Y-polarization component (CSy) are frequency offset compensated to obtain the frequency offset compensated complex digital signal of the X-polarization component (CSxf), The optical signal demodulator according to claim 2, which outputs a complex digital signal (CSyf) of the Y-polarization component with the frequency offset compensated.

4. The aforementioned common frequency offset compensation unit (22com) is, The aforementioned X-polarization activation pattern signal (PAx ＣＳ ) and the Y polarization activation pattern signal (PAy ＣＳ When either of the above is input, a provisional determination value of the phase of the complex digital signal of the X polarization component (CSx) or the complex digital signal of the Y polarization component (CSy) is calculated and compared with the provisional determination value of the phase of the next detected complex digital signal to determine the complex frequency offset amount (C ΣφΔｆ ) calculate, The complex digital signal of the X-polarization component (CSx) and the complex digital signal of the Y-polarization component (CSy) are frequency offset compensated by the aforementioned complex frequency offset amount (CΣφΔf), and the frequency offset compensated complex digital signal of the X-polarization component (CSxf) is obtained. The optical signal demodulator according to claim 2, which outputs a complex digital signal (CSyf) of the Y-polarized component with the frequency offset compensated.

5. An optical signal demodulation method comprising: receiving an optical signal on which the original signal (INFx, INFy) is transmitted for each of the X-polarization and Y-polarization components, using symbols that are placed as complex digital signals (CSx, CSy) in at least one of the time slots, and an index code formed at the time position of the time slot on which the symbols are placed, and demodulating the original signal (INFx, INFy), wherein one frame is composed of multiple time slots for each X-polarization component and Y-polarization component, and the original signal (INFx, INFy) is demodulated. A receiving process (RX) involves phase diversity detection of the optical signal and converting it into the complex digital signals (CSx, CSy) for each of the X-polarization and Y-polarization components. The system includes a demodulation step (6) for demodulating the original signals of the X-polarization component and the Y-polarization component from the complex digital signals (CSx, CSy) for each of the X-polarization component and the Y-polarization component. The demodulation process (6) consists of an X-polarization demodulation process (6X), a Y-polarization demodulation process (6Y), and a common frequency offset compensation process (22com). The X polarization demodulation process (6X) is as follows: The complex digital signal (CSx) of the X-polarization component obtained in the above receiving process (RX), From the frequency-offset-compensated X-polarization component complex digital signal (CSxf) obtained from the common frequency offset compensation process (22com), X-polarized activation pattern signal (APx) that indicates the position of the time slot on which the aforementioned symbol is placed. ＣＳ )and, The X-polarized index decode (DEx) obtained by demodulating the aforementioned index code. Ｉｄ The process of generating ) The aforementioned X-polarization activation pattern signal (APx ＣＳ The X-polarization symbol decoding code (DEx) is obtained by decoding the complex digital signal (CSxf) of the X-polarization component that has been frequency offset compensated in accordance with the above. ＡＰ The process of generating ) The aforementioned X-polarization index decoding code (DEx Ｉｄ ) and the X-polarization symbol decoding code (DEx ＡＰ The process includes generating the X-polarized decoded original signal DE(INFx) from ), The aforementioned Y polarization demodulation process (6Y) is: The complex digital signal (CSy) of the Y polarization component obtained by the receiving unit (RX), From the frequency-offset-compensated Y-polarization component complex digital signal (CSyf) obtained from the common frequency offset compensation unit (22com), A Y-polarized activation pattern signal (APy) indicates the position of the time slot on which the symbol is placed. ＣＳ )and The Y-polarized index decoding code (DEy) obtained by demodulating the aforementioned index code. Ｉｄ The process of generating ) The aforementioned Y-polarization activation pattern signal (APy ＣＳ The Y-polarization symbol decoding code (DEy) is obtained by decoding the Y-polarization complex digital signal (CSyf) of the Y-polarization component that has been frequency-off compensated in accordance with the above. ＡＰ The process of generating ) The aforementioned Y-polarization index decoding code (DEy Ｉｄ ) and the Y-polarization symbol decoding code (DEy ＡＰ The process includes generating the Y-polarized decoded original signal DE(INFy) from ). The aforementioned common frequency offset compensation process (22com) is, The complex digital signal (CSx) of the X-polarization component, The complex digital signal (CSy) of the Y-polarization component, The X-polarization activation pattern signal (PAx) generated in the X-polarization demodulation step (6X) ＣＳ )and, The Y polarization activation pattern signal (PAy) generated in the Y polarization demodulation step (6Y) ＣＳ ) and from, The complex digital signal (CSxf) of the X-polarization component with the frequency offset compensated, A method for demodulating an optical signal, characterized by generating a complex digital signal (CSyf) of the Y-polarization component with the frequency offset compensated, and outputting it to the X-polarization demodulation step (6X) and the Y-polarization demodulation step (6Y), respectively.

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