Multidimensionally modulated optical signal demodulator and optical signal demodulation method

Abstract

Current high-capacity optical communications, as typified by the IEEE 802.3av standard, have the problem that the optical transmission is completely cut off if the optical loss exceeds 29 dB. A frame is composed of a plurality of time slots, and the time positions of the time slots with symbols, and the X-polarized and Y-polarized waves of the optical symbols to be carried in the slots and the IQ-axis components of each polarization plane are stored in the following manner: an optical polarization compensation unit (RXe) that separates the X-polarized wave and the Y-polarized wave; The index decoding code (DE) based on the position of the symbol of the X polarization component and the Y polarization component Id an index demodulation unit (10) for demodulating the The symbol carried in the time slot is decoded by a symbol decoding code (DE AP An optical signal demodulating device having a symbol demodulating section (20) that demodulates the X-polarized component and the Y-polarized component of the optical signal demodulating device can demodulate the original signal.

Description

[Technical field]

[0001] The present invention relates to an optical signal demodulator that is applied to optical access networks, particularly to optical fiber transmission networks. The present invention is an optical signal demodulator that can receive optical signals based on a time domain index optical modulation method (Patent Document 1), which can maintain the connection of a communication line even with a low average optical signal power compared to conventional techniques, and realizes a multidimensional time domain index optical modulation method that performs modulation in a multidimensional space of the time axis and polarization axis, or the time axis, polarization axis, and IQ axis of a complex plane.

[0002] By using the optical signal demodulation device of the present invention, it is possible to achieve operation similar to that of a frequency offset compensation circuit, phase compensation circuit, and phase slip reduction circuit built into a conventional digital coherent optical signal receiver, even for optical symbols that arrive at irregular timing in accordance with the time-domain index modulation rules of a time-domain index optically modulated signal.

[0003] Furthermore, by using a polarization rotation compensation circuit controlled by a specified training optical signal, it is possible to receive a multidimensional time-domain index optical modulated signal that combines the time axis and the polarization axis of a separated polarization multiplexed signal, or a multidimensional time-domain index optical modulated signal that combines the time axis, polarization axis, and IQ axis of the complex plane. [Background technology]

[0004] Optical access networks have spread to more than 20 million households in Japan and have become an essential social infrastructure system in our highly information-driven society.

[0005] In particular, in recent years, optical access network services that realize a maximum transmission rate of 10 Gbit / s in each household are being developed in accordance with the IEEE 802.3av (10GE-PON) standard (Non-Patent Document 1). In many cases, this system adopts a configuration in which optical fiber lines are branched in a tree shape from an Optical Line Terminal (OLT) installed in a communication station in each local city, and connected to multiple (maximum 32) Optical Network Units (ONUs) installed in each household (Figure 29).

[0006] In this system, the optical loss budget between the OLT and the ONU is 29 dB, and the transmission distance is 20 km. If these conditions are no longer met, the communication line cannot be maintained and the 10 Gbit / s transmission rate will be completely cut off.

[0007] On the other hand, when the time domain index optical modulation method (Non-Patent Document 2) is adopted, it is possible to maintain the optical signal-to-noise ratio by increasing the optical signal peak power while keeping the average optical power constant by thinning out optical symbols in accordance with changes in the transmission path conditions between the OLT and the ONU. At this time, the absolute value of the mutual information of the communication path decreases, but it is possible to maintain the normalized mutual information at a value close to 1 bit / symbol, and the connectivity of the communication line can be maintained with the support of the error correction function (Patent Document 2). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent Application No. 2023-3440 [Patent Document 2] International Publication No. 2014 / 115840 (Patent No. 5886984) [Non-patent literature]

[0009] [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] K. Kikuchi, “Fundamentals of coherent optical fiber communications,” J.Lightw. Technol., vol.34, no.1, pp.157-179, Aug. 2016. [Non-Patent Document 4] 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 5] Senta Suzuki, Yutaka Miyamoto, Masato Tomizawa, Hisakazu Sakano, Koichi Murata, Shinji Mino, Mitsufumi Shibayama, Masamasa Shibuya, Kiyoshi Fukuchi, Hiroshi Onaka, Takeshi Hoshida, Kosuke Komaki, Takashi Mizuochi, Kazuo Kubo, Yoshikuni Miyata, and Toru Kamio, “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 6] K. Itoh, “Analysis of the phase unwrapping algorithm,” Applied Optics, vol. 21, no. 14, pp. 2470, July 1982. [Non-Patent Document 7] A. Alvarado, T. Fehenberger, B. Chen, and F. J. 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 8] 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. [Non-Patent Document 9] M. Ready and R. Gooch, “Blind equalization based on radius directed adaptation,” in Proceedings of Int. Conf. Acoust., Speech, Signal Process., 1990, vol. 3, pp. 1699-1702, (1990). Summary of the Invention [Problem to be solved by the invention]

[0010] As represented by the IEEE 802.3av (10GE-PON) standard (Non-Patent Document 1), phase diversity coherent optical communication is expected to become the basis for large-capacity optical communication. However, due to the fact that the communication capacity per communication is increasing due to the spread of video content, the social demand for extending the transmission distance is expected, and the number of accesses increases during disasters, etc., there may be cases in the future where the optical loss budget exceeds 29 dB.

[0011] Even in such cases, it is considered necessary to provide a best-effort optical access service according to the communication traffic state and the transmission line conditions. In other words, the object 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 lowered when the optical loss budget exceeds 29 dB. [Means for solving the problem]

[0012] In view of the above problems, the present invention provides a demodulation device and method for optical communications that provides a best-effort optical access service in accordance with the communication traffic state and transmission line conditions even when the optical loss budget exceeds 29 dB.

[0013] That is, the present invention provides a means for increasing the efficiency of communication capacity versus optical signal energy in a time domain index optical modulation method having the above-mentioned functions. As a means for this, a multidimensional time domain index optical modulation signal combining the polarization plane of a separated polarization multiplexed signal in addition to the time axis, or a multidimensional time domain index optical modulation signal combining the time axis, polarization axis, and IQ axis of a complex plane is transmitted and received. In this case, it is necessary to perform a polarization rotation compensation circuit, frequency offset compensation, phase compensation, etc. for optical symbols that arrive intermittently as in the case of a conventional coherent optical signal receiving device, and in addition, it is necessary to correctly grasp the time axis and polarization axis by receiving a training sequence inserted in the optical signal.

[0014] More specifically, the optical signal demodulation device according to the present invention comprises: A superframe is formed by a training section having a plurality of frames and to which a training sequence is added, and a payload section having a plurality of frames; The frame is composed of a number of time slots, At least one of the time slots is appended with a symbol; An optical signal demodulation device that receives an optical signal having an index code representing an original signal formed at a time position of the time slot to which the symbol is added in an X polarization component and a Y polarization component, and demodulates the original signal, a phase diversity detector (RXa) that performs phase diversity detection on the optical signal and converts it into a polarization-mixed complex digital signal (CE'); a polarization rotation compensation unit (RXe) that receives the polarization mixed complex digital signal (CE') and outputs a polarization separated polarization orthogonal complex digital signal (CE); The training sequence is recognized from the polarization orthogonal complex digital signal (CE) output from the polarization rotation compensation unit (RXe), and a start notification signal (S TOP a timing channel determination unit (RXf) that generates a complex digital signal (CS) consisting of an X-polarized component and a Y-polarized component by determining a polarization plane of the polarization-separated orthogonal complex digital signal; The complex digital signal (CS) is input, and the head notification signal (S TOP ) based on the position of the symbol of the X polarization component and the Y polarization component, Id an index demodulation process unit (10) for demodulating the The complex digital signal (CS) is input, and the symbol carried on the time slot is decoded into a symbol decoding code (DE AP a symbol demodulation process unit (20) for demodulating the X-polarized wave component and the Y-polarized wave component, When the index demodulation process unit (10) receives the time slot carrying the symbol, it transmits a notification signal (activation pattern signal APCS ) and The symbol demodulation process unit (20) receives the notification signal (activation pattern signal AP CS ) to obtain and decode the symbol.

[0015] Further, the optical signal demodulation method according to the present invention comprises the steps of: A superframe is formed by a training section having a plurality of frames and to which a training sequence is added, and a payload section having a plurality of frames; The frame is composed of a number of time slots, At least one of the time slots is appended with a symbol; An optical signal demodulation method for receiving an optical signal having an index code representing an original signal formed at a time position of the time slot to which the symbol is added, in an X polarization component and a Y polarization component, and demodulating the original signal, comprising: a phase diversity detection step (RXa) of detecting the optical signal by phase diversity and converting it into a polarization mixed complex digital signal (CE'); a polarization rotation compensation step (RXe) for receiving the polarization mixed complex digital signal (CE') and outputting a polarization separated polarization orthogonal complex digital signal (CE); The training sequence is recognized from the polarization orthogonal complex digital signal (CE), and a start notification signal (S TOP a timing channel determination step (RXf) of generating a polarization plane of the polarization-separated complex digital signal; For each of the X-polarized wave component and the Y-polarized wave component, the head notification signal (S TOP ) based on the position of the symbol of the X polarization component and the Y polarization component, Id (10) an index demodulation step of demodulating the The head notification signal (S TOP ) to receive a notification signal (activation pattern signal AP CS ) and The complex digital signal (CS) is input, and the notification signal (activation pattern signal AP CS The symbol carried in the time slot is decoded based on the symbol decoding code (DE AP ) for each of the X-polarized component and the Y-polarized component. Effect of the Invention

[0016] According to the optical signal demodulation device and method of the present invention, while the optical loss does not exceed 29 dB, the coherent optical modulation method is used for transmission, and when the optical loss exceeds 29 dB, the time domain index modulation method or the multidimensional time domain index modulation method that combines the polarization plane of the separated polarization multiplexed signal in addition to the time axis is also used, thereby making it possible to maintain communication.Furthermore, a means is provided for expanding the ratio of the communication capacity of the system to the optical signal energy.

[0017] Furthermore, the present invention realizes operations similar to those of the frequency offset compensation circuit, phase compensation circuit, and phase slip reduction circuit built into conventional digital coherent optical signal receivers for optical symbols arriving at irregular timings in accordance with the time-domain index modulation rules of a time-domain index optical modulation signal. This not only enables transmission and reception of codes in accordance with the time-domain index modulation rules, but also enables coherent detection of arriving optical symbols. In other words, like conventional digital coherent optical signal receivers, it is possible to transmit and receive codes in accordance with the M-phase quadrature phase amplitude optical modulation rules.

[0018] In addition, it is possible to transmit and receive multidimensional index optical modulated signals that combine not only the time axis but also the polarization planes of separated polarization multiplexed signals, or multidimensional index optical modulated signals that combine the time axis, polarization axis, and IQ axis of the complex plane.

[0019] As a result, the present invention can realize a plurality of operation modes, as described below. Specifically, in a normal operation mode, a system employing the present invention realizes data transmission using an M-phase quadrature amplitude modulation (M-QAM) optical modulation method, and ensures communication capacity (normal mode). On the other hand, when customer data traffic is low, the system employing the present invention switches to an index optical modulation operation mode, and reduces the average optical signal power while ensuring the peak power of the optical signal (ECO mode).

[0020] On the other hand, when the transmission line distance is long or the transmission line conditions are poor, the operation mode is switched to the index optical modulation mode, and the peak power of the optical signal is increased to ensure the 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 the transmission line conditions.

[0021] The present invention also provides a means for further increasing the ratio of communication capacity to optical signal energy when operating in the aforementioned ECO mode and Long Reach / Fault Tolerant mode. [Brief description of the drawings]

[0022] [Figure 1] An example of a modulation rule in a multi-dimensional index optical modulation (time / polarization) method is shown. [Diagram 2] An example of the modulation rule in the multi-dimensional index optical modulation (time, polarization, IQ axis) method is shown. [Diagram 3] 2 shows an example of a modulation rule in an amplitude optical modulation method. [Figure 4] FIG. 2 shows a logical model diagram of a time domain index optical modulation method to be realized by the present invention, or a multidimensional time domain index optical modulation method that combines the polarization planes of separated polarization multiplexed signals in addition to the time axis. [Diagram 5] FIG. 1 is a diagram for explaining the effects realized by a system to which the present invention is applied. [Figure 6] FIG. 2 is a diagram illustrating polarization rotation of an optical signal. [Figure 7] FIG. 13 is a diagram showing how a training sequence is periodically arranged in a frame-based data string. [Figure 8] FIG. 2 is a diagram illustrating an optical frequency difference between optical signals. [Figure 9] FIG. 2 illustrates a transmission mode in which the invention can be implemented. [Figure 10] 1 is a diagram showing a configuration of an optical communication receiving device according to the present invention; [Figure 11] 2 is a diagram showing a configuration of an optical polarization correction unit RXe of an optical communication receiving device according to the present invention. [Figure 12] 13 is a diagram showing the configuration of an error signal generating unit in an optical polarization correction unit RXe. FIG. [Figure 13] FIG. 1 is a diagram showing the frame and symbol configurations of a time-polarization two-dimensional index optical modulation method. [Figure 14] 2 is a diagram showing a configuration of a timing channel determination unit of the optical communication receiving device of the present invention. [Figure 15] FIG. 2 is a diagram showing the overall configuration and operation of a demodulation unit. [Figure 16] FIG. 2 is a diagram illustrating a configuration of a frequency offset compensation unit of a demodulation unit. [Figure 17] FIG. 13 is a diagram illustrating the effect of frequency offset compensation. [Figure 18] FIG. 2 is a diagram illustrating a configuration of a phase compensation unit of a demodulation unit. [Figure 19] 11A and 11B are diagrams illustrating the effect of a phase compensation unit. [Figure 20] FIG. 1 is a diagram showing the frame and symbol configuration of a time / polarization / IQ plane five-dimensional index optical modulation method. [Figure 21] FIG. 11 is a diagram illustrating the configuration of a receiving unit and a demodulating unit according to a second embodiment. [Figure 22] 13 is a diagram showing a configuration of an optical polarization compensation unit RXe according to the second embodiment. FIG. [Diagram 23] FIG. 11 is a diagram illustrating a configuration of an error signal generating unit according to a second embodiment. [Figure 24] FIG. 11 is a diagram illustrating a configuration of a demodulation unit of an optical receiving device according to a second embodiment. [Diagram 25]Fig. 10 is a diagram showing the configuration of a frequency offset compensating unit in the second embodiment Fig. 11 is a diagram showing the configuration and operation of a polarization rotation compensating unit. [Figure 26] FIG. 11 is a diagram illustrating a configuration of a phase compensation unit in the second embodiment. [Figure 27] FIG. 11 is a diagram illustrating a configuration of a provisional decision calculation unit used in a frequency offset compensation unit in the second embodiment. [Figure 28] FIG. 13 is a diagram illustrating a configuration of a provisional decision calculation unit used in a phase compensation unit in the second embodiment. [Figure 29] 1 is a diagram showing the relationship between an OLT and an ONU. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The configuration and operation of the optical signal demodulation device according to the present invention will be described below with reference to the drawings. Note that the following description is an example of one embodiment of the present invention and an example, and the present invention is not limited to the following description. The following description can be modified within the scope of the present invention. Furthermore, the parts referred to as "part" or "unit" in the following embodiments can be realized by either a hardware configuration or a software configuration. Furthermore, the parts referred to as "part" or "unit" may be referred to as "processes" having the functions to be performed. Furthermore, the addition unit, multiplication unit, and division unit are elements that perform complex calculations, even if they are not described as "complex."

[0024] The principle of the time-domain index optical modulation method, which is the premise of this invention, is as follows. In the time-domain index optical modulation method, an index optical modulation signal frame (hereinafter simply called a "frame") is composed of multiple time slots. Optical signals are transmitted and received using this frame as a single unit. A time slot has a predetermined time width. There is a time gap between one time slot and the next time slot. However, if the sending side and receiving side are completely synchronized, there is no need for a time cap. The time width of a time slot, including the time gap, is represented by "Ts".

[0025] Under these conditions, we 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 thinning out a fixed number of optical symbols in each frame, the amplitude of the optical symbols, and therefore the signal-to-noise ratio of the optical symbols, increases. In addition, it becomes possible to transmit and receive codes by changing the transmission and reception time slots of the thinned optical symbols according to the time-domain index modulation rules.

[0026] Here, a symbol refers to light (optical symbol carrier: hereinafter also simply referred to as "optical signal LS") on which the sending side carries information during a time slot. In the present invention, at least the X-polarized component and Y-polarized component of the optical signal LS and a signal in which information is added to the time position of the symbol in the frame are decoded. Furthermore, a signal in which information is added to the IQ axes of the X-polarized component and the Y-polarized component is decoded. The I-axis and Q-axis to which information is added may be called Ich (I channel), Qch (Q channel), or the I-axis component and Q-axis component.

[0027] Symbol thinning refers to a state in which there is no light or the intensity is low (dark) during a time slot. Alternatively, a time slot in which no light symbol exists is expressed as a time slot to which a "Null" symbol is assigned. Changing a time slot includes changing the time of the time slot or changing the intensity of the symbol in the time slot.

[0028] In Fig. 1, 16 types of frames are shown. Each frame is composed of 8 time slots. In the conventional time domain (N, K, M-QAM / M-PAM) method, the activation pattern is defined by the pattern of optical symbols assigned to each time slot for each polarization plane (Patent Document 1). On the other hand, in the present invention, it is possible to define 16 types of activation patterns by combining both the X polarization plane and the Y polarization plane. For example, this is an activation pattern in which an optical symbol is assigned only to the first symbol in the X polarization plane and symbols are thinned out from all time slots in the Y polarization plane.

[0029] In each time slot, the black time slots carry optical symbols. The optical symbols carry information in the M-QAM / M-PAM format, such as QPSK or 4PAM. For example, the third frame shows that an optical symbol is carried in the third time slot. White time slots are time slots that do not carry optical symbols. In this way, 16 types of activation patterns are possible depending on the combination of the position of the optical time slot carrying the optical symbol and the polarization plane, and by assigning an index bit to each activation pattern, 4 bits' worth of information (from "0000" to "1111") can be specified.

[0030] In addition, when transmitting information with the same optical signal power as the time domain (8,1,M-QAM / M-PAM) method, it is possible to select to insert optical symbols into two of 16 types of time slots, and a total of 120 types of activation patterns can be defined. In this case, the communication capacity C (bit / sec) is given by equation (1), and 6 bits of information can be specified.

[0031]

number

[0032] Furthermore, the number of definable activation patterns for the same time slot and number of optical symbols can be further increased by including the real and imaginary axes (IQ axes) of the optical symbol complex plane in addition to the time domain and polarization plane. Figure 2 shows 32 types of frames by adding Ich and Qch to Figure 1. Each frame consists of 8 time slots.

[0033] If the activation pattern is defined by combining the IQ axis of the optical symbol complex plane in addition to the X and Y polarization planes, 32 types of activation patterns can be considered depending on the combination of the position of the optical time slot on which the optical symbol is carried and the polarization plane. By assigning an index bit to each activation pattern, 5 bits of information can be specified.

[0034] In addition, when transmitting information with the same optical signal power as the time domain (8,1,M-QAM / M-PAM) method, it is possible to select to insert optical symbols into 4 of 32 types of time slots, and a total of 35,960 types of activation patterns can be defined. In this case, the communication capacity C (bit / sec) is given by equation (2), and 15 bits of information can be specified.

[0035]

number

[0036] Furthermore, by simultaneously performing coherent detection of the optical symbols, it becomes possible to transmit and receive codes using M-QAM / M-PAM optical modulation symbols in the same way as conventional digital coherent optical signal receivers. Figure 3 shows an example of signal points of optical symbols sent in a time slot. Figure 3(a) shows a QPSK (Quadrature Phase Shift Keying) signal, and (b) shows a 4PAM (4 Pulse Amplitude Modulation) signal.

[0037] The communication path realized by the multidimensional index optical modulation (time / polarization) method is logically a communication path consisting of a parallel index bit (hereinafter referred to as "INDBit") channel that transmits and receives codes by changing the symbol position within one frame of the optical symbol transmission / reception time slot in accordance with the time-domain index modulation rules, and an amplitude-phase modulation bit (hereinafter referred to as "APMBit") channel that transmits and receives codes using M-QAM optical modulation.

[0038] Figure 4 is a conceptual diagram of the overall configuration of a communication system using INDBit and APMBit. The physical configuration is basically the same as that of optical transmission systems commercially available today. However, the transmitter's MAP and the receiver's Demap function are the termination points, and logically, the INDBit channel (Index ch) and the APMBit channel (APM ch) are transmitted.

[0039] On the transmitting side (optical signal transmitter 1), the original information INF (original signal INF) is divided into INDBit and APMBit by the serial / parallel converter (S / P). Each signal has an error correction code added (FEC Enc.), is mapped (MAP), and then is transmitted from the transmitter (TX). In transmission path 2, logically, the model shows that Index ch and APM ch are transmitted, but physically, the optical signal LS with a phase modulation signal superimposed thereon is transmitted in the time slot shown in Figure 1.

[0040] On the receiving side (optical signal demodulator 3), the polarization multiplexed optical signal received by the receiving section (RX) is separated after polarization rotation compensation by the polarization rotation compensation section RXe, and the timing channel determination section RXf determines the training sequence from this output signal, and determines the frame and subframe start timing and the polarization plane of the received signal. In addition, a timing signal for determining the IQ axis of the received signal is also generated.

[0041] Next, the index demodulation process unit 10 of the demodulation unit 6 decodes the index modulated signal (DMAP) and performs error correction (FEC Dec.). In addition, in the symbol demodulation process unit 20, the amplitude phase modulated signal is frequency offset compensated (FC) and phase shift compensated (PC), decoded (DMAP), and performs error correction (FEC Dec.).

[0042] In the optical signal demodulation device 3 of the present invention, an activation pattern signal AP CSis sent. In this way, INDBit and APMBit are demodulated. These signals are converted back to serial signals by the parallel / serial converter 29, and decoded information DE(INF) corresponding to the original information INF (original signal INF) is obtained. The overall communication capacity C (bit / sec) of these is given by equation (3).

[0043]

number

[0044] Here, N is the number of symbols per frame of the time-domain index optical modulation signal, K is the number of symbols to insert optical symbols into, [ ] 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.

[0045] 5 shows the relationship between the ratio of the number of optical symbols per frame (K / N) (horizontal axis) and the transmission data rate C (bits / symbol) (vertical axis). The open marks indicate the case of the multidimensional (N,K,BPSK / QPSK) IM method (which also uses polarization and IQ axes as indexes) of the present invention, which combines time and polarization or IQ axes, and the filled marks indicate the case of the time domain (N,K,BPSK / QPSK) IM method.

[0046] In all the examples, the number of time slots in the frame is 9. Therefore, the black circles indicate the case of the time domain (9,K,BPSK) IM method. The open circles are an example of the method group according to the present invention, with the open circles indicating the time-polarization multidimensional (18,K,BPSK) IM method, the open squares indicating the time-polarization multidimensional (18,K,QPSK) IM method, and the open triangles indicating the time-polarization-IQ multidimensional (36,K,BPSK) IM method. In FIG. 5, "TD" indicates the time domain, "POL" indicates the polarization plane, and "IQ" indicates that the IQ axis is used for the index signal.

[0047] For example, in the time domain (9,1,BPSK) index optical modulation transmission method (in Fig. 5, black circles indicate K with K being 1: K / N=1 / 9), which transmits codes in a low average optical signal power state with the average number of optical symbols narrowed down to 1 / 9, a total of 4 bits can be transmitted: 3 bits on the INDBit channel, which transmits information in an activation pattern, and 2 bits on the APMBit channel, which transmits information in BPSK optical symbols. A total of 8 bits can be transmitted in both polarization planes.

[0048] On the other hand, in the time-polarization multidimensional (18,2,BPSK) IM method of the present invention, which provides the same signal power as the above method, a total of 9 bits can be transmitted: 7 bits on the INDBit channel and 2 bits on the APMBit channel, which are channels that transmit information using a combined activation pattern of both polarization planes.

[0049] In the case of the time-polarization multidimensional (36,2,QPSK) IM method, in which the APM channel is transmitted using QPSK optical symbols, the capacity of the APM Bit channel is doubled to 4 bits, allowing a total of 11 bits to be transmitted.

[0050] Furthermore, by adopting the time-polarization-IQ multidimensional (36,4,BPSK) IM method instead of the time-polarization multidimensional (36,2,QPSK) IM method, a total of 19 bits can be transmitted, 15 bits on the INDBit channel and 4 bits on the APM Bit channel. With this method, the communication capacity can be expanded by 19 / 2 times compared to the 1 / 9 rate polarization multiplexed BPSK modulation transmission method (1 / 9 bit / symbol x 2), which narrows the number of optical symbols to 1 / 9.

[0051] The present invention is an optical signal demodulation device and method for demodulating an optical signal in which one frame is composed of multiple time slots and symbols are carried in at least one of the time slots, as shown in Fig. 1. In other words, the present invention aims to apply an intradyne digital coherent detection method (Non-Patent Document 3) to an optical receiver in an optical access network that employs a time-domain index optical modulation method. In this optical signal demodulation device, in order to correctly decode the received optical symbols, it is necessary to mainly solve the following problems.

[0052] (1) Polarization rotation compensation can be stably performed under conditions where optical symbols arrive irregularly in accordance with the index optical modulation rule. (2) The frame start timing and polarization plane can be correctly determined from the training sequence. (3) Under the same conditions, it can stably perform frequency offset compensation and phase compensation, and can also decode conventional M-QAM modulated transmissions while correctly distinguishing between I and Q channels. (4) If the condition of the transmission line deteriorates, the transmission capacity is reduced to ensure communication quality and line connectivity.

[0053] The problem of (1) is also present in conventional optical modulation methods. However, in index optical modulation methods where the ratio of peak power to average power of the optical signal becomes large, it becomes difficult to ensure stable operation with conventional polarization rotation compensation circuits. In the present invention, the presence or absence of an optical symbol is determined in each time slot, and an error signal that gives a deviation from the control target of the polarization rotation compensation circuit is generated only in the time slot where an optical symbol exists, thereby achieving stable operation.

[0054] Fig. 6 shows an example of the polarization rotation of a polarization multiplexed optical signal and the operation of a polarization rotation compensator that compensates for this. In the optical signal demodulator 3, it is necessary to compensate for the polarization rotation that inevitably occurs due to changes in the state of the optical fiber that is the transmission path (asymmetry of the optical fiber shape, birefringence, temperature, tension, etc.). It is well known that the rotation speed of the polarization is relatively slow, generally 100 Hz or less.

[0055] Fig. 6(a) shows the polarization plane (X polarization plane, Y polarization plane) of the optical signal received by the optical signal demodulator 3. It shows that the polarization plane of the optical signal has rotated by θ with respect to the polarization axis (X' polarization axis, Y' polarization axis) that can be distinguished by the receiver due to changes in the state of the fiber received in the transmission path. Fig. 6(b) shows the state after compensation has been made and the signal has been returned to a state where it can be demodulated. The polarization rotation compensation unit RXe performs this operation.

[0056] The challenge of (2) is the need to correctly determine the frame start timing and polarization plane from the training sequence. As shown in Figure 7, in the index optical modulation method, the timing at which optical symbols are inserted becomes irregular. Under these conditions, it is necessary to grasp the frame start timing.

[0057] Figure 7 shows how training sequence optical symbols are arranged in an index optical modulated signal. Time progresses from left to right in Figure 7. As shown in Figure 7, the index optical modulated signal is arranged in a disorderly manner according to the information that the optical symbols are intended to transmit in a time series. Therefore, to make it possible to grasp the start timing of a frame, training sequence optical symbols are arranged at regular intervals, and the start timing of the optical symbols is grasped from the regularity of the arrival of the optical symbols.

[0058] In Fig. 7, a training sequence of four frames is transmitted after two frames of index optical modulation signal IND-α, followed by three frames of index optical modulation signal IND-β. The training sequence frame has optical symbols in the first and third slots.

[0059] The problem (3) is an issue that inevitably arises when a multidimensional (time-polarization or time-polarization-IQ axis) index optical modulation method is applied. In the conventional M-QAM optical modulation symbol, an optical symbol is always present in each time slot, and the frequency offset and phase difference between the received optical symbol and the local laser light are extracted by interfering with this optical symbol and the local laser light built into the receiver. However, in the present invention, it is necessary to extract the frequency offset and phase difference between the received optical symbol and the local laser light from the M-QAM optical modulation symbol that arrives irregularly at the optical receiver.

[0060] Fig. 8 shows the relationship between the optical signal LS and the frequency of the local light Lo used in the optical signal demodulator 3. The horizontal axis represents the optical frequency, and the vertical axis conceptually represents the optical 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 light Lo, but it is not easy to finely adjust the frequency of the local light Lo, and the two lights are shifted in frequency by the optical frequency difference Δf (hereinafter also referred to as the "offset frequency Δf"). It is necessary to compensate for this and to correctly grasp the IQ axes of the complex optical symbols obtained by demodulating the optical symbols.

[0061] The problem (4) is derived from the objective of making the transmission rate variable in the optical access network system that can be realized by the present invention. In the present invention, the transmission rate can be made variable by thinning out optical symbols in accordance with the change in the transmission path conditions between the OLT (Optical Line Terminal) and the ONU (Optical Line Unit) of the optical access network system to be realized.

[0062] Figure 9 shows one frame transmitted in the section from the OLT (see Figure 29) to the ONU. When the transmission line conditions are good, conventional M-QAM optical modulation symbols are transmitted (Figure 9(a): normal mode). On the other hand, when the transmission line conditions are poor and exceed the optical loss budget of 29 dB, operation is based on the time domain index optical modulation method, and optical symbols are thinned out (Figure 9(b): ECO mode). In an optical receiving device that realizes such operation, it is necessary to make it possible for the same optical receiver to demodulate both M-QAM optical modulation symbols and multidimensional index optical modulation symbols, and to increase the common parts of the receiving device to efficiently realize the desired operation.

[0063] Furthermore, when the transmission path distance is long or the transmission path conditions are poor, the operation mode is switched to index optical modulation and the peak power of the optical signal is increased to ensure connectivity of the communication line (Fig. 9(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 demodulator determines whether or not there is an optical symbol, and then generates an activation pattern, which is input to the APMBit demodulator. The APMBit demodulator demodulates the M-QAM optical modulation symbol according to this activation pattern only for time slots where an optical symbol exists.

[0064] In the ECO mode of Figure 9(b), under conditions where optical symbols arrive irregularly in accordance with the index optical modulation rules, it is necessary to decode the transmission code of the INDBit channel according to the index modulation rules while simultaneously performing frequency offset compensation and phase compensation to decode the transmission code of the APMBit channel.

[0065] The present invention provides a means for implementing an intradyne digital coherent detection optical signal decoding circuit that can adapt to such transmission path conditions, demodulate both M-QAM optical modulation symbols and multidimensional index optical modulation symbols, and is compatible with a multidimensional index optical modulation method that can solve these problems.

[0066] The present invention not only provides a means for solving the problems (1) to (3), but also provides a means for efficiently realizing the decoding of conventional M-QAM optical modulation symbols and the decoding of multidimensional (9,1,M-QAM) index optical modulation symbols while satisfying the requirements of the problem (4) and increasing the common use parts of the optical signal demodulation circuit. In this case, a training sequence optical symbol is used to determine the IQ axis in either operation mode.

[0067] The optical signal LS that can be demodulated by the optical signal demodulator 3 according to the present invention is a signal in which one frame is composed of N time slots, and M-QAM symbols are carried in K of the slots. In addition, the activation pattern is defined taking into account the dimensions of the polarization plane and the complex plane, and the optical signal LS modulated to utilize the multidimensional activation pattern is called a "multidimensional (N, K, M-QAM) index optical modulated signal."

[0068] (First embodiment) 10 shows a block diagram of the main components of the optical signal demodulator 3. The optical signal LS transmitted through the optical fiber is received by the phase diversity detector RXa and converted to a digital signal by the analog-to-digital converter RXb. After that, the dispersion compensation unit RXc compensates for the chromatic dispersion of the optical fiber transmission line, the sampling phase synchronization unit RXd extracts the sampling timing of the analog-to-digital converter RXb, and the polarization relationship is compensated for by the polarization rotation compensation unit RXe.

[0069] Before the polarization rotation compensation unit RXe, the X-polarized component and the Y-polarized component have not yet been polarization separated. Therefore, the complex digital signal at this point is called a mixed polarization complex digital signal (CE'). The polarization rotation compensation unit RXe performs polarization rotation compensation and polarization dispersion compensation using an adaptive filter with a multi-stage tap configuration. Therefore, the polarization rotation compensation unit RXe separates the mixed polarization complex digital signal (CE') into a polarization orthogonal complex digital signal (CE) having vertical polarization and horizontal polarization components whose polarization planes are orthogonal to each other.

[0070] The timing channel determination unit RXf correctly determines the frame start timing and polarization plane from the training sequence of the received signal, and generates a timing signal (Tpu=S TOP It also determines whether the x-component (CEx) and y-component (CEy) of the polarization orthogonal complex digital signal (CE) correspond to the polarization plane of the complex digital signal CSx, which is the X-polarized component, or the complex digital signal CSy, which is the Y-polarized component, during transmission of the original signal.

[0071] Here, the discrimination between the X-polarized component and the Y-polarized component is made possible by making a difference in the training sequence patterns of both polarization components. For example, the X-polarized component has an optical symbol placed in the first time slot of the last frame of the training sequence pattern, while the Y-polarized component has an optical symbol placed in the fourth time slot of the last frame of the same pattern, making it possible to discriminate by making a difference.

[0072] As described above, the phase diversity detector RXa, the analog-to-digital converter RXb, the dispersion compensation unit RXc, the polarization rotation compensation unit RXe, and the timing channel determination unit RXf constitute a receiver unit RX that performs phase diversity detection on the optical signal LS and converts it into a complex digital signal CS (CSx and CSy are collectively referred to as "CS"). The receiver unit RX outputs the complex digital signal CS. The complex digital signal CS still contains the optical frequency difference Δf (see FIG. 8) between the local light Lo and the optical signal LS, and the phase error of the carrier component of the M-QAM signal.

[0073] The complex digital signal CS output from the receiver RX is demodulated by the index demodulation process 10 of the demodulator 6 into the signal of the INDBit channel as a transmission code DE. Id The symbol demodulation process unit 20 performs frequency offset compensation and phase compensation to convert the signal of the APM Bit channel into the transmission code DE AP is decoded as

[0074] The symbol demodulation process unit 20 includes an X-polarized symbol demodulation process unit 20X that performs symbol demodulation of the X-polarized wave, and a Y-polarized symbol demodulation process unit 20Y that performs symbol demodulation of the Y-polarized wave. When these are collectively referred to, they are simply called the symbol demodulation process unit 20. In addition, the transmission code decoded by the X-polarized symbol demodulation process unit 20X is referred to as DEx AP and the decoded transmission code by the Y-polarized symbol demodulation process unit 20Y is DEy AP Let us assume that.

[0075] Also, error correction may be performed in the index demodulation process unit 10 or the symbol demodulation process unit 20. Furthermore, error correction may be performed in the final stage of decoding. Id and transmission code DE AP is converted back to serial data by a parallel / serial converter 29 (referred to as a "P / S converter"), and a decoded signal DE (INF) is obtained by decoded the original signal INF (original information INF). The optical signal demodulator 3 according to the present invention is sufficient if it has a receiver RX and a demodulator 6 located downstream of it. The demodulator 6 is sufficient if it has an index demodulation process unit 10 and a symbol demodulation process unit 20. The transmission code DE Id is the index decoded code DE Id This is called the transmission code DE AP The symbol decoded code DE AP It may also be called.

[0076] Fig. 11 shows the circuit configuration of the polarization rotation compensation unit RXe that can also support the index optical modulation method of the present invention. The basic configuration is based on the Least Mean Square (LMS) method shown in Non-Patent Document 4. Basically, the x-component CEx' and the y-component CEy' of the polarization mixed complex digital signal (CE') are back-calculated using a Jones Matrix to restore the correct polarization orthogonal complex digital signals CEx, CEy of each polarization plane. This can be expressed mathematically as in equation (4).

[0077]

number

[0078] Here, h xx , h xy , h yx , h yy are the elements of the Jones Matrix. The LMS method is an algorithm that updates the values ​​of each element of the Jones Matrix while comparing the output signals CEx and CEy with the control target value, and is known to have high convergence and stable operation. This convergence algorithm is expressed as a repetitive calculation such as equation (5).

[0079]

number

[0080] Here, n of each variable indicates the number of sampling times (effectively time) in the circuit. μ is called the convergence coefficient, and a value of about 0.001 to 0.01 is applied. Furthermore, d X (n), d Y (n) is the control target value for each of the x component and y component of the polarization orthogonal complex digital signal (CE). The optical polarization compensation unit RXe in FIG.

[0081] error signal e X , e Y is generated by the error signal generators RXe11 and RXe12 in Fig. 11, which are operated as follows so that they also operate when demodulating the index optical modulation signal. The error signal generator RXe11 generates the output signal CE from the polarization rotation compensation unit RXe. X The output signal CE X The amplitude value is calculated and a threshold value is determined.

[0082] FIG. 12 shows the configuration of the error signal generating unit RXe11 (RXe12 has the same configuration). X After calculating the amplitude value of the output signal CE, the output signal CE is compared with a threshold value in a threshold value determination unit RXe62. Xexceeds the threshold, the control target value of the X-polarized component is set by target value setting unit RXe66 to be 1, which is the original value of the X-polarized optical symbol of the index optical modulation signal. Then, the error from this control target value is calculated by multiplication unit RXe68 and output to coefficient update units RXe21 and RXe22.

[0083] On the other hand, if the threshold is not exceeded, the output of the error signal is set to 0, and the coefficient h xx , h xy Temporarily suspend the update of the index optical modulation signal, or set it to 0, which is the original value at the time of the arrival of the null symbol of the X-polarized optical symbol of the index optical modulation signal, and reduce the error e from this control target value. x is set to zero and output to the coefficient update units RXe21 and RXe22.

[0084] The error signal generator RXe12 outputs the output signal CE Y The output of the coefficient update unit RXe23 and the coefficient update unit RXe24 is the coefficient h yx , h yy Update the following.

[0085] Through the above operations, polarization demultiplexing of the x and y components of the polarization orthogonal decoded digital signal is achieved on the output side. That is, the decoded digital signal of the optical signal LS is polarization demultiplexed. Note that this polarization rotation compensation unit RXe does not have the ability to distinguish whether the output x and y components are X-polarized component signals or Y-polarized component signals, so there is a possibility that the X-polarized signal and the Y-polarized signal will be swapped on the output side. The X-polarized component and the Y-polarized component are identified by the timing channel determination unit RXf at the subsequent stage.

[0086] The timing channel determination unit RXf (see Fig. 10) operates based on the signal frame format including the training sequence shown in Fig. 13. That is, by determining the arrival timing of the training sequence portion inserted periodically, the timing of the beginning of the frame constituting the index optical modulated signal is determined, and also whether the received complex digital signal is an X-polarized component or a Y-polarized component.

[0087] For example, the training sequence shown in Fig. 13 is composed of 16 frames including the X-polarized symbol and the Y-polarized symbol, and a superframe is composed of 1024 frames including 1008 frames (payload section) that contain the subsequent transmission information. Furthermore, the 16 frames that make up the training sequence transmit signals of the same pattern in all superframes.

[0088] This pattern inserts an optical symbol with a complex value of 1+j on the X polarization side at the start of the frame, and an optical symbol with a complex value of 1+j on the Y polarization side at the third time slot from the start of the frame, as shown in Fig. 13. This allows the timing channel determination unit RXf to grasp the start timing of the frame that constitutes the index optical modulated signal, and to determine whether each complex digital signal is an X polarization component or a Y polarization component.

[0089] Figure 14 shows an example of the timing channel determination unit RXf (see Figure 10). Note that since the same processing is performed on the x and y components of the polarization orthogonal complex digital signal (CE), the components of the y component are shown in parentheses. The amplitude value of the polarization orthogonal complex digital signal CEx (CEy) is calculated in the amplitude calculation unit RXf11 (RXf21). The amplitude value is compared with a threshold value in the threshold determination unit RXf12 (RXf22), and if the amplitude value exceeds the threshold value, a logical "1" is output, and if not, a logical "0" is output. This output value is inserted into the multi-stage delay unit RXf13 (RXf23).

[0090] A register RXf15 (RXf25) is provided that stores logical values ​​that correspond to the training sequence pattern, separate from this multi-stage delay unit RXf13 (RXf23). For example, in response to the training sequence in Fig. 13, a register RXf15 is provided that stores the values ​​"100000010000001000000...1000000", a total of 144 (=1 frame 9 symbols x 16 frames).

[0091] The exclusive OR of these values ​​and the output value of each multi-stage delay unit RXf13 (RXf23) is calculated in the exclusive OR unit [XOR] RXf14 (RXf24), and the sum of the exclusive ORs is calculated in the summation unit [Σ] RXf16 (RXf26). When this summation becomes 0, the head notification signal generator RXf32 outputs the head notification signal S. TOP Note that the multi-stage delay unit RXf13 (RXf23), register RXf15 (RXf25), and exclusive OR unit [XOR] RXf14 (RXf24) are marked with a code only at the beginning, and the codes at the subsequent stages are omitted.

[0092] This operation is performed for each of the x and y components of the polarized waves, and the head notification signal S TOP The channel generated two symbol times later is determined to be the Y-polarized component. If it is determined that the X-polarized component and the Y-polarized component have been swapped, the switch unit RXf30 switches the output destination of the X-polarized component CSx and the Y-polarized component CSy, and outputs them to the demodulation unit 6. In other words, the timing channel determination unit RXf determines whether the x and y components of the polarization orthogonal complex digital signal (CE) are the X-polarized component CSx or the Y-polarized component CSy. The X-polarized component CSx and the Y-polarized component CSy are collectively referred to as the complex digital signal (CS). Note that the head notification signal S TOP is also transmitted to the demodulation unit 6 (see FIG. 10).

[0093] <Demodulation section> The optical signal demodulation performed by the demodulator 6 (FIG. 10) to realize the multidimensional (N, K, M-QAM) index optical modulation method will be described with reference to FIG. 15. In the following description, the configurations and signals related to the X-polarized component CSx and the Y-polarized component CSy will be distinguished by adding "x or X" and "y or Y". In addition, the description will mainly focus on the X-polarized component CSx, but even if not specifically stated, the configurations and signals related to the corresponding Y-polarized component CSy may exist.

[0094] The complex digital signal CS of each polarization component (a signal consisting of 2CH of I-axis component and Q-axis component: the X-polarized component and the Y-polarized component are denoted by the symbols CSx and CSy, respectively) output from the receiver RX shown in Fig. 10 is input to the demodulator 6. The complex digital signal CS is branched at a branching unit TIN (TINX, TINY) and input to an amplitude calculation unit 11 (11X, 11Y) of the index demodulation process unit 10 and a delay unit 21 (21X, 21Y) of the symbol demodulation process unit 20.

[0095] <Index demodulation process unit 10> The amplitude calculation unit 11 (11X, 11Y) of the index demodulation process unit 10 calculates the absolute value (or the square of the absolute value) of the complex digital signal CS (CSx, CSy), calculates the intensity of the complex digital signal CS (CSx, CSy) corresponding to the received optical symbol, and sets this as the output value. This output is called the symbol intensity P CS (P CSx , P CSy )

[0096] Next, the operation of the index demodulation process unit 10 will be described. The delay unit 13 outputs a head notification signal S in accordance with the frame head symbol when the timing channel determination unit RXf (see FIG. 10) is correctly driven. TOP (=Tpu) is sent and input to the IndexBit demapping unit 14, and the head symbol strength P CS The delay is adjusted so that the timing at which the signal is input to the IndexBit demapping unit 14 is synchronized with the timing at which the signal is input to the IndexBit demapping unit 14.

[0097] The IndexBit demapping unit 14 receives the head notification signal S from the timing channel determination unit RXf. TOP and the output signal of the delay unit 13 (absolute value of the complex digital signal CS (CSx, CSy)) P CS (P CSx , P CSy The IndexBit demapping unit 14 receives the head notification signal S from the timing channel determination unit RXf. TOP Based on this, the frame timing of the Index symbol is identified, and the symbol intensity P CS (P CSx , P CSy The IndexBit is demapped from the absolute value (or the square of the absolute value) of the complex digital signal CS calculated by the amplitude calculation unit 11, and the decoded code is the index decoded code DE Id Output.

[0098] The demapping rule may be based on the time domain index modulation rule of FIG. 1. In this case, the input signal from the delay unit 13 (symbol intensity P CS (P CSx , P CSy )) is the activation pattern signal AP CS The activation pattern signal AP is decoded by demapping the activation pattern to index bits. CS From this, it is possible to know the time slot in which a symbol exists in one frame.

[0099] <Register section 16> Referring again to FIG. 15, the decoded index decode code DE Id The register unit 16 receives the head notification signal S TOP The signal is driven based on the head notification signal S TOP Only when the index bit demapping unit 14 is ON, the index decoded code DE Id The register unit 16 receives and stores the error code and decoding process.

[0100] On the other hand, the IndexBit demapping unit 14 decompresses the input signal (symbol intensity P CS (P CSx , P CSy )) is determined, so the activation pattern signal AP CS This activation pattern signal AP CS are transmitted to the frequency offset compensation units 22 (22X, 22Y) and APMBit demapping units 25 (25X, 25Y) of the symbol demodulation process unit 20. The above is the operation of the index demodulation process unit 10.

[0101] <Symbol demodulation process unit 20> Next, the operation of the symbol demodulation process unit 20 will be described. The delay units 21 (21X, 21Y) receive the activation pattern signal AP CS arrives at the frequency offset compensation unit 22 (22X, 22Y) and the complex digital signal CS (CSx, CSy) branched by the branch unit TIN (TINX, TINY) arrives at the frequency offset compensation unit 22 (22X, 22Y). In other words, the frequency offset compensation unit 22 (22X, 22Y) can acquire a time slot with a symbol in the complex digital signal CS (CSx, CSy).

[0102] The purpose of the frequency offset compensator 22 (22X, 22Y) is to estimate the offset frequency Δf in the complex digital signal CS (CSx, CSy) and to remove the component. The offset frequency Δf in the complex digital signal CS (CSx, CSy) is generated in the phase diversity detector RXa (see FIG. 10) installed in the receiver RX of the present invention (see FIG. 8).

[0103] The phase diversity detector RXa tries to extract the phase information of the received optical symbol by interfering with the local laser light to detect the received optical symbol, but the optical frequencies of the received optical symbol and the local laser light do not necessarily match, and an optical frequency difference exists (Fig. 8). This optical frequency difference becomes the oscillation frequency of the electrical signal, which becomes the offset frequency Δf of the complex digital signal CS. Note that the offset occurs simultaneously in the X polarization and the Y polarization, so the offset frequency Δf is compensated for separately for the X polarization and the Y polarization.

[0104] <Frequency offset compensator 22> Next, the frequency offset compensator 22 will be described in detail with reference to Fig. 16. As shown in Fig. 15, the frequency offset compensator 22, phase compensator 23, and APMBit demapping unit 25 of the present invention include two systems of processing for X-polarized signals and Y-polarized signals. In the following explanation, the signal processing system for X-polarized waves will be explained, but the same processing is performed in a circuit with exactly the same configuration for Y-polarized waves. In addition, the compensation for frequency offset Δf and the like will be explained without the "x or X" indicating X-polarized waves. The frequency offset compensator 22 of the present invention removes the influence of modulation from the complex digital signal CSx by a method called the power method, and extracts the phase rotation of the received optical symbol carrier.

[0105] The frequency offset compensation unit 22X receives the complex digital signal CSx from the delay unit 21X (see FIG. 15). This complex digital signal CSx is branched, and one of the branched complex digital signals CSx is input to the power calculation unit 221. Here, the data modulation component contained in the complex digital signal CSx is removed.

[0106] For example, in the case of a complex digital signal CSx generated from a QPSK optically modulated optical symbol, 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 CSx is expressed as in equation (6).

[0107]

number

[0108] To remove the effect of the data modulation component, a fourth power operation is performed. In other words, by raising both sides of equation (6) to the fourth power, it is expressed as equation (7).

[0109]

number

[0110] The phase angles of the data modulation components, π / 4, 3π / 4, -π / 4, -3π / 4 (rad), are eliminated by raising them to the fourth power. Here, Δωt is the angular frequency expression of the frequency offset Δf. In other words, the influence of the data modulation components is eliminated, and the optical angular frequency offset Δω and optical phase difference Δθ (phase shift) between the carrier component of the optical symbol and the local light Lo ("carrier offset-phase component CC ωθ This carrier offset and phase component CC ωθ to the sample and hold units 222 and 225. ωθ If we write it explicitly, it becomes as equation (8).

[0111]

number

[0112] The sample and hold units 222, 224, and 225 generate a timing signal (or an activation pattern signal AP) generated at the timing when the IndexBit demapping unit 14 determines that an optical symbol exists. CS ) arrives, the carrier offset and phase components CC ωθ This holds the activation pattern signal AP CS The carrier offset and phase component CC corresponding to the optical symbol at the timing when ωθ can be held.

[0113] The output of the sample and hold unit 222 (carrier offset and phase components CC ωθ ) is input to a delay unit 223. This delay unit 223 gives a delay equivalent to the time slot time of the optical symbol, and the signal is input to a sample and hold unit 224. CS ) arrives twice in succession at equal intervals with respect to the optical symbol time slot, the complex digital signal held by the sample and hold unit 224 is the carrier offset and phase components CC ωθ In comparison, the past value for one optical symbol time slot is held.

[0114] Also, if the timing signal (or activation pattern signal AP CS ) is held in the sample and hold unit 224, the carrier offset and phase components CC ωθ is the carrier offset and phase components CC held in the sample and hold units 222 and 225. ωθ In comparison, the past values ​​for N optical symbol time slots are held.

[0115] The carrier offset and phase components CC held by the sample and hold unit 224 are ωθ "CC ωθ1 " and the carrier offset and phase components CC held by the sample and hold units 222 and 225 are ωθ and "CC ωθ2 " Carrier offset phase component CC ωθ1 is the carrier offset and phase component CC ωθ2 Carrier offset and phase components of the previous symbol CC ωθ It is.

[0116] In either case, the activation pattern signal AP CS The carrier offset and phase component CC corresponding to the optical symbol at the timing when ωθThe carrier offset and phase components CC held in the sample and hold units 224 and 225 are also stored. ωθ can be calculated from the number of optical symbol time slots from when the data is held in the sample & hold unit 224 until when the data is held in the sample & hold unit 225. This makes it possible to calculate the phase difference per unit time even in a situation where the arrival of optical symbols is intermittent due to the introduction of index optical modulation, and therefore to reproduce the optical angular frequency offset Δω (corresponding to the offset frequency Δf) between the optical symbol and the local light Lo.

[0117] The multiplier 226 multiplies the carrier offset and phase components CC ωθ1 Then, the complex conjugate of the carrier offset and phase component CC ωθ2 is multiplied by the multiplier 226 to obtain a signal from which the optical phase angle difference between the carrier component of the optical symbol and the local light (hereinafter, Δθ) has been removed (this is referred to as the carrier offset component CC ω That is, it can be obtained as shown in equation (9).

[0118]

number

[0119] This carrier offset component CC ω (t) is the carrier offset phase component CC, which is the input signal from the sample and hold unit 225. ωθ2 and the carrier offset and phase components CC, which are input signals from the sample and hold unit 224. ωθ1 Here, CC ωθ1 * (t) is CC ωθ1 The complex conjugate of (t), t 2 -t 1 is the time interval between each symbol.

[0120] In other words, if the hold values ​​of the sample and hold units 224 and 225 are complex digital signals sampled and held at a time interval of N optical symbol time slots, the carrier offset component CC in equation (9) is ω It can be seen that (t) has information on the amount of phase change of the complex digital signal (corresponding to the carrier component of the regenerated optical symbol) that has changed over the course of time equivalent to N optical symbol time slots.

[0121] The sample and hold unit 222, the delay unit 223, the sample and hold units 224 and 225, and the multiplication unit 226 are connected to the carrier offset and phase components CC ωθ and activation pattern signal AP CS is input and the carrier offset component CC ω 16, the carrier offset component extraction unit 22B1 is written as “22B1: 222, 223, 224, 225, 226” to mean that the carrier offset component extraction unit 22B1 includes the sample & hold unit 222, the delay unit 223, the sample & hold units 224, 225, and the multiplication unit 226.

[0122] The phase calculation unit 227 calculates the exponent part of the equation (9), Δω(t 2 -t 1 ), that is, the phase angle value is output. Here, the phase angle value is -π≦Δω(t 2 -t 1 Therefore, if the phase angle value suddenly jumps from a value near π to a value near -π, an error occurs. 1 =Δω(t 2 -t 1 ) is minimized. For the algorithm of this operation, for example, Non-Patent Document 6 may be adopted. The phase rotation value φΔf output from this unwrap unit 228 is input to a selector unit 229. Note that the phase rotation value φΔf is a phase rotation value due to the offset frequency Δf, and is therefore referred to as a phase rotation value φΔf due to offset.

[0123] The delay unit 22A1 receives the phase rotation value φΔf due to the offset input to the selector unit 229 and the activation pattern signal AP CS In this way, the selector unit 229 adjusts the delay so that the timing of the activation pattern signal AP CS At the timing when this signal is ON, the phase rotation value φΔf due to the offset input from the unwrap unit 228 can be appropriately output.

[0124] On the other hand, the selector unit 229 selects the activation pattern signal AP CS When the activation pattern signal AP is off, the register section 22A0 selects the value 0 stored in the register section 22A0 (shown as "0" in FIG. 16) and outputs it. CS When the phase rotation value φΔf due to the offset is output, the activation pattern signal AP CS When the output of the adder 22A2 is OFF, a value of 0 can be stably output. These output values ​​are then input to the adder 22A2.

[0125] The adder 22A2 calculates the cumulative value of the phase rotation value φΔf due to the offset. In order to calculate the cumulative value of the phase rotation value φΔf due to the offset, the output value of the adder 22A2 is branched to the delay unit 22A3 and delayed by a time (Ts) equivalent to one optical symbol time slot width. Then, the output value is input to the adder 22A2 together with the phase rotation value φΔf due to the offset output from the selector unit 229.

[0126] This makes it possible to calculate the cumulative value ΣφΔf of the phase rotation value φΔf due to the offset (also simply called the “offset cumulative value ΣφΔf”). That is, after n symbol time slots, n The cumulative offset value ΣφΔf(t n ) at time t 1 The amount of phase rotation in 1 Then, the offset cumulative value ΣφΔf is expressed as in equation (10).

[0127]

number

[0128] This offset cumulative value ΣφΔf(t) is nothing but the sum of the phase rotation amounts (offset-induced phase rotation values ​​φΔf) caused by frequency offsets occurring at the same time. Therefore, this offset cumulative value ΣφΔf(t) is input to the exponent calculation unit 22A4.

[0129] The exponent calculation unit 22A4 calculates the output value of the addition unit 22A2, ΣφΔf(t n ) is input, and exp{-jΣφΔf(t n )} and input it to the multiplication unit 22A6. This complex value is used as the complex offset compensation amount C ΣφΔf It is called.

[0130] The phase calculation unit 227, the unwrap unit 228, the selector unit 229, the register unit 22A0, the adder unit 22A2, the delay unit 22A3, and the exponent calculation unit 22A4 calculate the carrier offset component CC ω to complex offset compensation amount C ΣφΔf 16, the complex offset compensation amount calculation unit 22B2 is described as "22B2: 227, 228, 229, 22A0, 22A2, 22A3, 22A4" to mean that the complex offset compensation amount calculation unit 22B2 includes the phase calculation unit 227, the unwrap unit 228, the selector unit 229, the register unit 22A0, the addition unit 22A2, the delay unit 22A3, and the exponent calculation unit 22A4.

[0131] The multiplication unit 22A6 multiplies the complex digital signal CSx input to the frequency offset compensation unit 22 by the complex offset compensation amount C output from the exponential calculation unit 22A4. ΣφΔf This compensates for the frequency offset component Δf of the complex digital signal CSx described in equation (2) input to the frequency offset compensator 22, and obtains an output of a complex digital signal shown in equation (11). This complex digital signal is a signal whose offset frequency has been compensated for, and is the offset compensation signal CSx f It is called.

[0132]

number

[0133] Here, the delay unit 22A5 adds a time delay (t 1 From n The delay is then adjusted to the same value as the time it takes for the

[0134] The output of the multiplication unit 22A6 is input to the sample and hold unit 22A8 via the delay unit 22A7. By setting an appropriate delay amount in the delay unit 22A7, the sample and hold unit 22A8 outputs the activation pattern signal AP CS The offset compensation signal CSx, which is the output of the sample and hold unit 22A8, can be output when the sample and hold unit 22A8 is turned on. f is the overall output of the frequency offset compensator 22. Therefore, the frequency offset compensator 22 receives the complex digital signal CSx and outputs an offset compensated signal CSx in which the offset frequency Δf generated in the receiver RX is compensated for. f Output.

[0135] The effect of the overall operation of the frequency offset compensation unit 22 is shown in Fig. 17. In each photograph in Fig. 17, the horizontal axis corresponds to the real axis, and the vertical axis corresponds to the imaginary axis. The results show the effect of operating the frequency offset compensation unit 22X using a PN7 stage pseudorandom data pattern as a data modulation component. As shown in Fig. 17(a), the value of the complex digital signal CSx before being input to the frequency offset compensation unit 22X exhibits a rotating behavior on the complex plane over time.

[0136] On the other hand, after the compensation process in the frequency offset compensation unit 22X, the value of the complex digital signal CSx at the output of the frequency offset compensation unit 22X becomes stationary, as shown in FIG. f is input to the phase compensation unit 23X.

[0137] <Phase compensation unit 23> Next, the phase compensation unit 23X will be described with reference to FIG. 18. The phase compensation unit 23X, which is arranged immediately after the frequency offset compensation unit 22X as shown in FIG. 15, is configured as shown in FIG. 18. First, the input complex digital signal (offset compensation signal CSx f ) is branched into two. One of them is input to the power calculation unit 231. Here, the complex digital signal (offset compensation signal CSx f ) and recovers the phase difference information between the carrier phase of the received optical symbol and the local light phase. The operation of this power calculation unit 231 may be the same as that of the power calculation unit 221 of the frequency offset compensation unit 22. The output of the power calculation unit 221 is the carrier phase component CC θ The carrier phase component CC θ If we write it explicitly, it becomes as equation (12).

[0138]

number

[0139] The averaging unit 232 calculates the complex digital signal (carrier phase component CC θ ) for L symbols for each of the real and imaginary parts θ The phase calculation unit 233 calculates the average value Av(CC θ ) to calculate the optical phase difference Δθ. Here, the optical phase difference Δθ is the optical phase difference between the carrier component of the optical symbol and the local light.

[0140] The optical phase difference Δθ is -π≦Δθ(t 2 -t 1Therefore, if the optical phase difference Δθ suddenly jumps from a value near π to a value near -π, an error occurs. 1 =Δθ(t 2 -t 1 ) is minimized. The operation of this unwrapping unit 234 is the same as that of the unwrapping unit 228 of the frequency offset compensating unit 22. The optical phase difference Δθ output from this unwrapping unit 234 is multiplied by 1 / M in a dividing unit 235 to become a phase shift rotation value Φθ. Here, M is a value that depends on the modulation method of the optical symbol, and in the case of QPSK, for example, M=4.

[0141] The exponent calculation unit 236 receives the phase shift rotation value Φθ, which is the output value of the division unit 235, and outputs the complex value of exp(-jΦθ), which is input to the multiplication unit 238. This complex value is used as the complex phase shift compensation amount C Φθ The unwrap unit 234, the divider 235, and the exponent calculator 236 receive the optical phase difference Δθ and calculate the complex phase shift compensation amount C Φθ The above calculation may be called a complex phase shift compensation amount calculation unit 23B1.

[0142] The delay unit 237 delays the complex digital signal CSx input to the phase compensation unit 23X. f (Offset compensation signal CSx f The delay unit 237 receives the complex digital signal CSx f (Offset compensation signal CSx f ) is given a delay equal to the time it takes for it to pass through the exponent calculation unit 236 and be input to the multiplication unit 238, and then output to the multiplication unit 238.

[0143] The multiplier 238 multiplies the complex digital signal CSx input to the phase compensation unit 23X by f (Offset compensation signal CSx f ) is the complex phase shift compensation amount C output from the exponent calculation unit 236 ΦθMultiply. The complex digital signal output from the multiplication unit 238 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 Δθ between the carrier component of the optical symbol and the local light emission is compensated.

[0144] Since the offset frequency and phase difference of this signal are compensated, it is called the offset & phase compensation signal CSx fθ This is the output from the multiplication unit 238, the offset & phase compensation signal CSx fθ is used as the output of the entire phase compensation unit 23X and input to the APM Bit demapping unit 25X. The offset & phase compensation signal CSx fθ is expressed as in equation (13).

[0145]

Equation

[0146] The above is the overall configuration of the phase compensation unit 23X. The effect of the phase compensation unit 23X is shown in FIG. 19. In FIG. 19, for both photos (a) and (b), the horizontal axis is the real axis and the vertical axis is the imaginary axis. As shown in FIG. 19(a), the value of the complex digital signal CSx f (offset compensation signal CSx f ) before being input to the phase compensation unit 23X is shifted from the position of the signal point defined on the complex plane. On the other hand, as a result of the compensation process in the phase compensation unit 23X, for the output of the phase compensation unit 23X, the value of the complex digital signal (offset & phase compensation signal CSx fθ ) is, as shown in FIG. 19(b), near the defined signal point. This output value is input to the APM Bit demapping unit 25X (see FIG. 15).

[0147] <APMBit demapping unit 25> Referring to FIG. 15 again, the APM Bit demapping unit 25X will be described. The APM Bit demapping unit 25X operates in synchronization with the activation pattern signal AP CS Here, the activation pattern signal AP CSis the complex digital signal (offset & phase compensation signal CSx) detected at the timing when the optical symbol is ON. fθ ) is input to the APMBit demapping unit 25X via the delay unit 24 so as to be synchronized with the activation pattern signal AP CS The complex digital signal (offset & phase compensation signal CSx) input when fθ ) to digital code.

[0148] 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, the codes "11", "01", "10", and "00" are assigned to the signal points 1+j, -1+j, 1-j, and -1-j on the complex plane.

[0149] The APMBit demapping unit 25X converts the input complex digital signal (offset & phase compensation signal CSx fθ ), the 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 APMBit demapping unit 25X is decoded as the symbol decoded code DEx AP Note that error correction processing may be performed on the downstream side of the APMBit demapping unit 25X up to the register unit 27X.

[0150] In addition, the signal point constellation has a rotational symmetry of 90° in phase angle. Therefore, when decoding optical symbols, the interpretation of the I-axis and Q-axis of the complex plane may be reversed. To avoid this, the APM Bit demapping unit 25X has an IQ channel discrimination function.

[0151] To drive this IQ channel discrimination function, the transmitter periodically inserts a training optical symbol (complex value 1+j) to provide a phase reference for the received optical symbol. After discriminating the training optical symbol, the IQ channel discrimination function obtains the phase reference (complex value 1+j) of the optical symbol from this and correctly discriminates the phase rotation of the optical symbol, i.e., discriminates the IQ channel.

[0152] In addition, in an optical transmission system having the functions of the present invention, by adopting a differential encoding method, the IQ channel discrimination function of the APMBit demapping unit 25X can be omitted. In the APMBit demapping unit 25X compatible with the differential encoding method, the differentially encoded complex digital signal S N-1 and the differentially encoded complex digital signal S input at the timing of the Nth activation pattern input. N 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, etc.

[0153] <Register section 27> The register unit 27X receives the activation pattern signal AP CS With input, activation pattern signal AP CS When is ON, the decoded APMBit code string (symbol decoded code DEx AP ) and holds it. Here, the delay unit 26 acquires and holds the activation pattern signal AP CS and the corresponding APMBit symbol decoded by the demapping unit 25X, decoded code DEx AP The delay time is set so that the arrival times of the

[0154] The AMPBit data (symbol decoded code DEx AP ) is the code string of the IndexBit decoded by the index demodulation process unit 10 and stored in the register unit 16 (index decoded code DE Id) and the signal is read out while being synchronized with the timing of the symbol demodulation process unit 20.

[0155] <Parallel / serial conversion section 29> The parallel / serial conversion unit 29 converts the index decoded code DE of the IndexBit decoded by the index demodulation process unit 10 and stored in the register unit 16 into Id and the symbol decoded code DEx of APMBit decoded by the symbol demodulation process unit 20 and stored in the register unit 27. AP and the symbol decoded code DEy AP and converts these into a serial code string SDAT, and decodes the transmission code string (original information INF: original signal INF) to be transmitted by the optical signal transmitter 1, resulting in a decoded original signal (decoded information DE(INF)). Here, the parallel / serial converter 29 inputs a trigger signal whose timing has been adjusted by the delay unit 28, and reads out the codes stored in the register unit 16 and the register unit 27 (27X, 27Y) based on this signal.

[0156] The above is the first embodiment of the present invention. This embodiment has the function of receiving optical symbols generated based on a time-domain (N, K, M-QAM) index optical modulation scheme, but is also characterized in that it is possible to receive optical symbols generated based on an M-QAM optical modulation scheme.

[0157] When only M-QAM optical symbols are received, the demapping (conversion from a complex digital signal to a code) in the IndexBit demapping unit 14 is stopped, but the activation pattern signal AP CS The frequency offset compensators 22 (22X, 22Y) continuously output the optical symbols. This allows the frequency offset compensators 22 (22X, 22Y) to continuously operate in accordance with the arrival period of the optical symbols.

[0158] As a result, the frequency offset compensation operation can be realized even for optical symbols generated based on the M-QAM optical modulation method. In addition, the phase compensation unit 23 (23X, 23Y) and the APMBit demapping unit 25 (25X, 25Y) of the present invention are originally designed to operate continuously at the period of the optical symbol time slot regardless of the multidimensional (N, K, M-QAM) index optical modulation method or the M-QAM optical modulation method, so they operate in common regardless of either method. However, for discrimination of the training optical symbol, the circuit is switched according to the operation mode of the multidimensional (N, K, M-QAM) index optical modulation method and the M-QAM optical modulation method. Note that this mode switching and the stop of demapping in the IndexBit demapping unit 14 may be received as separate signals.

[0159] The parallel / serial conversion unit 29 changes the readout pattern from the register unit 16 and the register unit 27 (27X, 27Y) provided in the previous stage according to the operation mode of the multidimensional (N, K, M-QAM) index optical modulation method and the M-QAM optical modulation method. For example, in the case of the time-polarization (9, 2, QPSK) index optical modulation method, one frame is composed of a time slot for nine optical symbols, and only two optical symbols in total for both polarizations are ON during one frame. In that case, the symbol ON / Off pattern is 18 C 2 =153 types, and 7-bit transmission is possible with the Index Bit.

[0160] On the other hand, since only one QPSK optically modulated optical symbol arrives in the APMBit, 2-bit transmission is possible. When the parallel / serial converter 29 operates in the time-polarization (9,2,QPSK) index optical modulation method, the activation pattern signal AP CS At the timing when this signal is input, a 7-bit code is read out from the register unit 16 and a 4-bit code is read out from the register unit 27 (27X, 27Y).

[0161] On the other hand, when receiving QPSK optically modulated optical symbols, the parallel / serial conversion unit 29 converts the activation pattern signal AP CS At the timing when the input is made, the code is read out 4 bits at a time from the register unit 27 (27X, 27Y). By realizing such an operation, the operation mode switching between the time domain (N, K, M-QAM) index optical modulation method and the M-QAM optical modulation method is realized.

[0162] In this way, the optical signal transmission device 1 of the present invention has the advantage that it can support both the time-polarization multidimensional (N, K, M-QAM) index optical modulation method and the M-QAM optical modulation method with approximately the same circuit configuration.

[0163] Second embodiment The second embodiment is an embodiment of a system that transmits information using a time-polarization-IQ multidimensional (N, K, BPSK) index optical modulation method as shown in Figure 20, instead of the time-polarization multidimensional (N, K, M-QAM) index optical modulation method shown in the first embodiment.

[0164] In this embodiment, as in the first embodiment, the configuration shown in FIG. 21 is used, and the operation from the phase diversity detector RXa to the timing channel determination unit RXf is almost the same as that of the time-polarization multidimensional (N, K, BPSK) index optical modulation method of the first embodiment.

[0165] In this method, there are three amplitude values ​​for each polarization component: 0, 1, and √2. Note that "√2" represents the square root of "2." 0 is the value when a time slot has null optical symbols assigned to both Ich and Qch. 1 is the value when a time slot has optical symbols assigned to either Ich or Qch. And finally, √2 is the value when a time slot has optical symbols assigned to both Ich and Qch.

[0166] Fig. 22 shows the circuit configuration of the polarization rotation compensation unit RXe that can also be used with the index optical modulation method of the present invention. The basic configuration is the same as that of the first embodiment (Fig. 11) and is based on the Least Mean Square (LMS) method described in Non-Patent Document 4. The error signal e X , e Y is generated in the error signal generating unit (RXe71, RXe72) in FIG. 22, and the following operation is performed so that it also operates when demodulating a time-polarization-IQ (N, K, BPSK) index optical modulated signal.

[0167] The configuration of the error signal generation unit RXe71 (RXe12 has the same configuration) is shown in Figure 23. In the error signal generation unit RXe71, the amplitude value of the output signal CEx of the input polarization rotation compensation unit RXe is calculated in an amplitude calculation unit RXe74, and then compared with a threshold value in a threshold determination unit RXe75. A target value is set in a target value setting unit RXe76 according to the result of the threshold determination unit RXe75.

[0168] Here, unlike the first embodiment, there are three types of control target values ​​selected by threshold judgment. That is, when optical symbols are assigned to the same time slot in both Ich and Qch, the original value of the total amplitude value of Ich and Qch is √2. Therefore, when the amplitude value of the complex digital signal exceeds (1+√2) / 2, the control target value is selected to be √2.

[0169] On the other hand, when an optical symbol is assigned to either Ich or Qch, the amplitude value should be 1. Therefore, when the amplitude value of the complex digital signal is greater than 1 / 2 and less than (1 + √2) / 2, the control target value is selected to be 1.

[0170] Finally, if the amplitude of the complex digital signal is less than 1 / 2, the output of the error signal is set to 0, and the coefficient h xx , h xyor set it to 0, which is the original value when the null symbol of the X-polarized optical symbol of the index optical modulation signal arrives. The error signal generation circuit RXe71 outputs the error from these control target values.

[0171] Referring again to Fig. 22, the error signal generator RXe72 operates using the output signal CEy of the polarization rotation compensation unit RXe as input, and realizes an operation similar to that of the error signal generator RXe71. However, the output destination is the coefficient update units RXe23 and RXe24, and the coefficient h yx , h yy Update the following.

[0172] With the above operations, polarization demultiplexing (also simply called "polarization demultiplexing") is achieved on the output side even if IQch (meaning Ich and Qch collectively) is superimposed on the x and y components of the polarization orthogonal complex digital signal. However, since this polarization rotation compensation unit RXe does not have the ability to distinguish whether the output x and y components are X-polarized component signals or Y-polarized component signals, there is a possibility that the X-polarized signal and the Y-polarized signal will be swapped on the output side. The X-polarized component and the Y-polarized component are identified by the timing channel determination unit RXf at the subsequent stage.

[0173] Next, the timing channel determination unit RXf (see FIG. 21) extracts the frame start timing from the training sequence. The basic configuration of the timing channel determination unit RXf is the same as that of the first embodiment in FIG. 14. This operation is performed for each of the x component and y component of the polarization orthogonal complex digital signal.

[0174] Here, whether each of the x and y components is an X-polarized component or a Y-polarized component at the time of original signal transmission can be determined by providing a difference in the training sequence patterns for both polarization components, as in the first embodiment. For example, the X-polarized component can be determined by providing a difference such that the first and fourth time slots of the last frame of the training sequence pattern are respectively carrying optical symbols of Qch and Ich, and the fifth and last time slots of the last frame are respectively carrying optical symbols of Qch and Ich for the Y-polarized component.

[0175] Head notification signal S TOP The switch unit RXf30 determines that the channel generated two symbol times later is the Y-polarized component. If it determines that the X-polarized component and the Y-polarized component have been swapped, the switch unit RXf30 switches the output destination of the X-polarized component and the Y-polarized component, and outputs them to the demodulation unit 6a. This operation turns the polarization orthogonal complex digital signal (CE) into a complex digital signal CS with a definite X-polarized component and Y-polarized component. In other words, the switch unit RXf30 determines whether the x-component (CEx) and y-component (CEy) of the polarization orthogonal complex digital signal (CE) belong to the polarization plane of the complex digital signal CSx, which is the X-polarized component, or the complex digital signal CSy, which is the Y-polarized component.

[0176] FIG. 24 shows the configuration of the demodulation unit 6a of this embodiment. The demodulation unit 6a has almost the same configuration as the demodulation unit 6 (FIG. 15) of the first embodiment, but there are differences in the operation of the IndexBit demapping unit 14 and the configurations of the frequency offset compensation unit 22a (22Xa, 22Ya) and phase compensation unit 23a (23Xa, 23Ya) arranged in the symbol demodulation process unit 20. The APMBit demapping unit 25 and register unit 27 following them have the same configuration. In the following explanation, the processing sequence of the X-polarized component will be explained, but the exact same processing is performed for the Y-polarized component. In addition, the compensation for the frequency offset Δf and the like will be explained by omitting "x or X" indicating the X-polarized wave.

[0177] In this embodiment, the original signal (INF) also has symbols on Ich and Qch. The signal input to the demodulator 6a has already been determined by the preceding timing channel determination unit RXf as being the X-polarized component or the Y-polarized component of the complex digital signal CS. However, for example, it has not yet been possible to identify whether the separated signal of the X-polarized component is Ich or Qch.

[0178] Therefore, a symbol for identifying the IQch is used in the final frame of the training sequence. TOP Identify the last frame of the training sequence with and generate the activation pattern signal AP CS In addition, IQch judgment instruction signal IQ CS This IQch decision instruction signal IQ CS is sent to the phase compensation unit 23 at the timing when the received signal should be an Ich signal according to a predesigned training sequence pattern. Next, the frequency offset compensation unit 22a (22Xa, 22Ya) and the phase compensation unit 23a (23Xa, 23Ya) will be described. FIG. 25 is referenced. FIG. 25 shows the configuration of the frequency offset compensation unit 22Xa. The difference from the frequency offset compensation unit 22X (FIG. 15) shown in the first embodiment is that a tentative decision calculation unit 221a (described later) is applied instead of the power calculation unit 221 arranged in the frequency offset compensation unit 22X (FIG. 15). The tentative decision calculation unit 221a provides the same function as the power calculation unit 221 in the first embodiment in that it is applied as a means for removing the influence of the data modulation component from the complex digital signal CSx corresponding to the received optical symbol.

[0179] Please refer to Fig. 26. Fig. 26 shows the configuration of a phase compensation unit 23Xa. The phase compensation unit 23Xa differs from the phase compensation unit 23X in that a tentative decision calculation unit 231a is applied instead of the power calculation unit 231 of the phase compensation unit 23X.

[0180] The frequency offset compensating unit 22Xa and the phase compensating unit 23Xa perform the same operations as the frequency offset compensating unit 22X and the phase compensating unit 23X. Therefore, the operation of the symbol demodulating process unit 20 including the frequency offset compensating unit 22Xa and the phase compensating unit 23Xa is also the same as in the first embodiment.

[0181] Referring again to Fig. 25, the frequency offset compensation unit 22Xa differs from the frequency offset compensation unit 22X in Fig. 15 in that the power calculation unit 221 is replaced with a provisional decision calculation unit 221a and that the complex offset compensation amount C ΣφΔf is input to the provisional decision calculation unit 221a.

[0182] 27 shows the configuration of the tentative decision calculation unit 221a incorporated in the frequency offset compensation unit 22a. First, a complex digital signal CSx of Equation (7) corresponding to an optical symbol is input to the tentative decision calculation unit 221a. Here, it is assumed that the optical symbol is QPSK optically modulated, and the optical angular frequency offset of the carrier component of the optical symbol and the local light is Δω, and the optical phase difference is Δθ. When the complex digital signal CSx expressed by Equation (14) is input to this tentative decision calculation unit 221a, the complex digital signal CSx is branched into two, one of which is input to the multiplication unit 2211 and the other to the multiplication unit 2213.

[0183]

number

[0184] On the other hand, the exponential calculation unit 22A4 on the output side of the frequency offset compensation unit 22Xa outputs a signal for compensating for the optical angular frequency offset (complex offset compensation amount C ΣφΔf ) is fed back. However, this feedback signal is n time t earlier than m Complex digital signal CSx(t m ), it is expressed as in equation (15).

[0185]

number

[0186] The complex offset compensation amount C in Eq. (15) ΣφΔf is input to the multiplier 2211 and multiplied by the complex digital signal CSx of the equation (14). The result of the multiplication is the complex digital signal CSx of the equation (16). np It becomes.

[0187]

number

[0188] This signal can be said to be a signal in which the continuous phase rotation of the complex vector caused by the frequency offset Δf is suppressed. This signal is suppressed complex digital signal CSx np This signal is input to the provisional decision unit 2212. The provisional decision unit 2212 outputs the suppression complex digital signal CSx np Select the most likely signal point for the optical angular frequency offset and optical phase difference component ω(t n -t m ) + Δθ n -Δθ m The reciprocal value TCS of the provisional decision data modulation component expressed by equation (17) with -1 fθ and inputs it to the multiplication unit 2213.

[0189]

number

[0190] The multiplication unit 2213 multiplies the complex digital signal CSx input to the frequency offset compensation unit 22Xa and expressed by equation (11) by the complex digital signal expressed by equation (14), thereby removing the data modulation component from the complex digital signal CSx input to the frequency offset compensation unit 22Xa, and obtaining the carrier offset·phase component CC of the optical symbol expressed by the following equation (18): ωθThis signal is the carrier offset / phase component CC ωθ is the same as:

[0191]

number

[0192] In this way, the tentative judgment calculation unit 221a of this embodiment realizes the same operation as the power calculation unit 221 of the first embodiment.

[0193] Fig. 26 shows the configuration of phase compensation unit 23Xa, and Fig. 28 shows the configuration of provisional decision calculation unit 231a newly provided in this embodiment. This embodiment realizes the same operation as provisional decision calculation unit 221a (Fig. 27) provided in frequency offset compensation unit 22Xa (Fig. 25). However, the scope of the same operation relates to the procedure from equation (17) to equation (18).

[0194] 28, the temporary decision unit 2311 receives the offset compensation signal CSx f is input. Offset compensation signal CSx f was calculated as in equation (11). Equation (11) is shown again below.

[0195]

number

[0196] The temporary decision unit 2311 outputs the complex digital signal CS f (Offset compensation signal CSx f ), select the most likely signal point, and calculate the optical angular frequency offset and optical phase difference component Δθ n -Δθ m The reciprocal value TCS of the tentative decision data modulation component expressed by equation (19) with -1 θ and inputs it to the multiplication unit 2312.

[0197]

number

[0198] Signal of equation (19) and offset compensation signal CSx f By multiplying the carrier phase component CC expressed by equation (20), θ This is the same operation as that of the power calculation unit 231 in the phase compensation unit 23 in the first embodiment.

[0199]

number

[0200] The phase compensation unit 23 (see FIG. 26) also has an IQch adjustment unit 239 at the final stage. The IQch adjustment unit 239 adjusts the IQch decision instruction signal IQ CS The offset and phase compensation signal CSx is demodulated at the timing of receiving this signal. fθ is the Ich signal of the training sequence. The phase angle of this signal is then determined and compared with a reference phase angle corresponding to a predetermined training sequence Ich signal (for example, if the reference phase angle of the training sequence Ich signal is 0 (rad), the difference with this reference phase angle is compared).

[0201] If the phase angle of the training sequence Ich signal is Δθ TR If there is a deviation, the offset and phase compensation signal CSx is used to cancel this deviation. fθ Then, the offset and phase compensation signal CSx is generated again. fθ This outputs the offset and phase compensation signal CSx fθ Among these, a signal with a phase angle of 0 (rad) to π (rad) is Ich, and a signal with a phase angle of π / 2 (rad) to -π / 2 (rad) is Qch, so that the latter circuit can stably process the signal. CS and the offset & phase compensation signal CSx from the multiplication unit 238 fθThe offset and phase compensation signal CSx with IQch determined is input. fθ Output.

[0202] The second embodiment is compatible with not only the time-polarization-IQ (N, K, BPSK) index optical modulation scheme in which signal points arranged at equal phase intervals are defined, but also the time-polarization (N, K, M-QAM) index optical modulation scheme in which arbitrary signal points are defined.

[0203] The index demodulation process unit 10 and the symbol demodulation process unit 20 in the second embodiment have the same configuration as the configuration shown in Fig. 15 in the first embodiment. Therefore, the same signals are input and output to and from the frequency offset compensation unit 22a and the phase compensation unit 23a in Fig. 24. Therefore, also in Fig. 25, the sample and hold unit 222, the delay unit 223, and the sample and hold units 224 and 225 detect the carrier offset and phase components CC ωθ and activation pattern signal AP CS is input and the carrier offset component CC ω It can be said that the carrier offset component extraction unit 22B1 extracts the above-mentioned signal.

[0204] In addition, the phase calculation unit 227, the unwrap unit 228, the selector unit 229, the register unit 22A0, the adder unit 22A2, the delay unit 22A3, and the exponent calculation unit 22A4 calculate the carrier offset component CC ω to complex offset compensation amount C ΣφΔf The complex offset compensation amount calculation unit 22B2 calculates the following. [Industrial Applicability]

[0205] This invention is applied to optical access networks, which are essential social infrastructure systems in an 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 cannot be met, the communication line cannot be maintained, and the 10 Gbit / s transmission rate is completely cut off.

[0206] The present invention realizes a time domain index optical modulation method, and makes it possible to vary the transmission rate by thinning out optical symbols in accordance with changes in the transmission line conditions between the OLT and the ONU. In other words, even under conditions where the transmission line conditions are poor and exceed the optical loss budget of 29 dB, if the additional optical loss budget is up to about 8 dB, it is possible to avoid a complete disconnection of the communication line by gradually reducing the transmission rate. In other words, even for ONUs deployed in locations farther away from the OLT, the connectivity of the communication line is ensured, although the transmission rate is reduced, and this contributes to the expansion of the area where optical access network services are provided, and ultimately to the elimination of the digital divide. [Explanation of symbols]

[0207] 1 Optical signal transmitter 2 Transmission Path 3. Optical signal demodulator 6 Demodulation section 6a Demodulation section 10 Index demodulation process 11 Amplitude calculation section 13 Delay section 14 IndexBit Demapping 15 Delay section 16 Register section 20 Symbol demodulation process section 20X X-polarized symbol demodulation process section 20Y Y-polarized symbol demodulation process section 21 Delay section 22 Frequency offset compensation section 221 Power calculation unit 222 Sample and hold section 224 Sample and hold section 225 Sample and Hold Section 223 Delay Section 226 Multiplication section 227 Phase calculation section 228 Unwrap 229 Selector Section 22A0 Register section 22A1 Delay section 22A2 Addition section 22A3 Delay section 22A4 Exponential calculation section 22A6 Multiplication section 22A5 Delay section 22A7 Delay section 22A8 Sample and hold section 23 Phase compensation section 231 Power calculation unit 232 Averaging section 233 Phase calculation section 234 Unwrap Section 235 Division part 236 Exponential calculation section 238 Multiplication section 237 Delay Section 22B2 Complex offset compensation amount calculation unit 24 Delay section 25 APMBit Demap Section 26 Delay section 27 Register section 28 Delay section 29 Parallel / serial conversion section 221a Provisional decision calculation unit 231a Provisional decision calculation unit 22a Frequency offset compensation section 23a Phase compensation section 2211 Multiplication section 2213 Multiplication section 2212 Provisional Judgment Section 2311 Provisional Judgment Department 2312 Multiplication section 23b Phase compensation section 232b Averaging section 234 Unwrap Section 239 IQch adjustment section 23B1 Complex phase shift compensation amount calculation unit RX Receiver RXa Phase Diversity Detector RXb Analog-to-Digital Converter RXc dispersion compensation section RXd Sampling Phase Synchronization Unit RXe Polarization Rotation Compensation Unit RXe11 error signal generator RXe12 error signal generator RXe21 Coefficient update section RXe22 Coefficient Update Unit RXe23 Coefficient Update Unit RXe24 Coefficient update section RXe60 amplitude calculation section RXe62 Threshold judgment unit RXe66 Target value setting unit RXe68 Multiplication Unit RXf timing channel determination section RXf11(RXf21) Amplitude calculation section RXf12(RXf22) Threshold judgment section RXf13(RXf23) Multi-stage delay section RXf15(RXf25) Register RXf14 (RXf24) Exclusive OR section [XOR] RXf16(RXf26) Summation part [Σ] RXf30 Switch section RXf32 Head notification signal generator S TOP Leading signal RXe71 error signal generator RXe72 error signal generator (RXe71, RXe72) Error signal generation section RXe74 amplitude calculation section RXe75 Threshold judgment unit RXe76 Target value setting unit RXe23 Coefficient Update Unit LS optical signal Lo Local light AP CS Activation Pattern Signal INF original information DE(INF) Decryption information Δf (offset frequency Δf) Optical frequency difference (CE') Polarization-mixed complex digital signal (CE) Polarized orthogonal complex digital signal CS Complex digital signal CSx Complex digital signal CSy Complex digital signal DE Id INDBit channel signal transmission code DE AP The signal on the APMBit channel is DEx AP Symbol Decoding Code DEy AP Symbol Decoding Code DEx AP The decoded transmission code by the X-polarized symbol demodulation process unit 20X is DEy AP The decoded transmission code by the Y-polarized symbol demodulation process unit 20Y is DE Id Index Decoding Code DE AP Symbol Decoding Code TIN Branch P CS Symbol Intensity S TOP Leading signal Δf frequency offset Δω Optical angular frequency offset Δθ Optical phase difference CC ωθ Carrier offset and phase components CC ω Carrier offset component 22B1 Carrier offset component extraction unit φΔf Phase rotation value φΔf Phase rotation due to offset φΔf offset cumulative value Σ C ΣφΔf Complex offset compensation amount CS f Offset Compensation Signal CSx f Offset Compensation Signal CC θ Carrier Phase Component Av(CC θ ) Average Φθ Phase shift rotation value C ΦθComplex phase shift compensation amount CS fθ Offset & Phase Compensation Signal CSx fθ Offset & Phase Compensation Signal SDAT Serial code string CS np Suppression Complex Digital Signal TCS -1 fθ Reciprocal value of provisional decision data modulation component IQ CS IQch judgment instruction signal DE(INF) Decoded signal IQ CS IQch judgment instruction signal CSx np Suppression Complex Digital Signal

Claims

1. A superframe is formed by a training section having a plurality of frames and to which a training sequence is added, and a payload section having a plurality of frames; The frame is composed of a number of time slots, A symbol is added to at least one of the time slots; An optical signal demodulation device that receives an optical signal having an index code representing an original signal formed at a time position of the time slot to which the symbol is added, in an X polarization component and a Y polarization component, and demodulates the original signal, a phase diversity detector (RXa) for detecting the optical signal by phase diversity and converting it into a polarization mixed complex digital signal (CE'); a polarization rotation compensation unit (RXe) that receives the polarization mixed complex digital signal (CE') and outputs a polarization separated polarization orthogonal complex digital signal (CE); The training sequence is recognized from the polarization orthogonal complex digital signal (CE) output from the polarization rotation compensation unit (RXe), and a start notification signal (S ＴＯＰ a timing channel determination unit (RXf) that determines a polarization plane of the polarization-separated orthogonal complex digital signal and generates a complex digital signal (CS) consisting of an X-polarized component and a Y-polarized component; The complex digital signal (CS) is input, and the head notification signal (S ＴＯＰ ) based on the position of the symbol of the X polarization component and the Y polarization component, Ｉｄ an index demodulation process unit (10) for demodulating the The complex digital signal (CS) is input, and the symbols carried in the time slots are converted into a symbol decoding code (DE ＡＰ a symbol demodulation process unit (20) for demodulating the X-polarized wave component and the Y-polarized wave component of the same; When the index demodulation process unit (10) receives the time slot carrying the symbol, it transmits a notification signal (activation pattern signal AP) indicating the presence of the symbol to the symbol demodulation process unit (20). ＣＳ ) and The symbol demodulation process unit (20) receives the notification signal (activation pattern signal AP ＣＳ and acquiring and decoding the symbols based on the received signal.

2. The index demodulation process unit (10) The symbol intensity (P ＣＳｘ , P ＣＳｙ ) an amplitude calculation unit (11x, 11y) for calculating The symbol intensity (P ＣＳｘ , P ＣＳｙ ) and the head notification signal (S ＴＯＰ ) is input, and the notification signal (activation pattern signal AP ＣＳ ), and the index decoded code (DE Ｉｄ 2. The optical signal demodulation device according to claim 1, further comprising an IndexBit demapping unit (14) for outputting an IndexBit signal.

3. The symbol demodulation process unit (20) The complex digital signal (CS) is input, The offset compensation signal (CS) generated by the phase diversity detector (RXa) is compensated for the frequency offset. ｆ A frequency offset compensator (22) that outputs a The offset compensation signal (CS ｆ ) is input, and the phase shift generated in the phase diversity detector (RXa) is compensated for to generate an offset & phase compensation signal (CS ｆθ ), and a phase compensation unit (23) that outputs The offset & phase compensation signal (CS ｆθ ) is input, and a symbol decoded code (DE ＡＰ 3. The optical signal demodulation device according to claim 1, further comprising an APMBit demapping unit (25) for outputting an APMBit demapping signal for each of the X-polarized component and the Y-polarized component of the complex digital signal (CS).

4. the optical signal is formed with the frame having the symbol in an I-axis component and a Q-axis component of the X-polarized component and in an I-axis component and a Q-axis component of the Y-polarized component, 2. The optical signal demodulation device according to claim 1, wherein the index demodulation process section (10) and the symbol demodulation process section (20) distinguish between the I-axis component and the Q-axis component.

5. The index demodulation process unit (10) The symbol intensity (P ＣＳｘ , P ＣＳｙ ) an amplitude calculation unit (11x, 11y) for calculating The symbol intensity (P ＣＳｘ , P ＣＳｙ ) and the head notification signal (S ＴＯＰ ) is input, and the notification signal (activation pattern signal AP ＣＳ ), and an IQch determination indication signal IQ ＣＳ and the index decoded code (DE Ｉｄ 5. The optical signal demodulation device according to claim 4, further comprising an IndexBit demapping unit (14) for outputting an IndexBit signal.

6. A superframe is formed by a training section having a plurality of frames and to which a training sequence is added, and a payload section having a plurality of frames; The frame is composed of a number of time slots, A symbol is added to at least one of the time slots; An optical signal demodulation method for receiving an optical signal having an index code representing an original signal formed at a time position of the time slot to which the symbol is added, in an X polarization component and a Y polarization component, and demodulating the original signal, comprising: a phase diversity detection step (RXa) of detecting the optical signal by phase diversity and converting it into a polarization mixed complex digital signal (CE'); a polarization rotation compensation step (RXe) for receiving the polarization mixed complex digital signal (CE') and outputting a polarization separated polarization orthogonal complex digital signal (CE); The training sequence is recognized from the polarization orthogonal complex digital signal (CE), and a start notification signal (S ＴＯＰ a timing channel determination step (RXf) of generating a polarization plane of the polarization-separated complex digital signal; For each of the X-polarized wave component and the Y-polarized wave component, the head notification signal (S ＴＯＰ ) based on the position of the symbol of the X polarization component and the Y polarization component, Ｉｄ (10) an index demodulation step of demodulating the The head notification signal (S ＴＯＰ ) to receive a notification signal (activation pattern signal AP ＣＳ ) and The complex digital signal (CS) is input, and the notification signal (activation pattern signal AP ＣＳ The symbol carried in the time slot is decoded based on a symbol decoding code (DE ＡＰ a symbol demodulation step (20) of demodulating the X-polarized component and the Y-polarized component of the optical signal.

7. the optical signal is formed with the frame having the symbol in an I-axis component and a Q-axis component of the X-polarized component and in an I-axis component and a Q-axis component of the Y-polarized component, 7. The method of claim 6, wherein the index demodulation step (10) and the symbol demodulation step (20) distinguish between the I-axis component and the Q-axis component.

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