Signal processing device, signal processing method and program

Abstract

Provided is a signal processing device comprising a control unit that refers to, among digital electric signals obtained by converting optical signals that have been transmitted by first to N-th transmitters (N is an integer greater than or equal to 2), which convert electrical signals representing a bit sequence into optical signals representing the bit sequence and transmit the obtained optical signals, and have propagated trough a multi-core fiber, an n-th main digital signal (n is an integer greater than or equal to 1 and smaller than or equal to N) transmitted by the n-th transmitter and also refers to a symbol sequence represented by the result of decoding the optical signals transmitted by at least part of the first to N-th transmitters excluding the n-th transmitter, so as to remove crosstalk of the n-th main digital signal.

Description

[Technical Field]

[0001] The present invention relates to a signal processing device, a signal processing method, and a program. [Background technology]

[0002] With the recent launch of 5G (5th Generation) services, high-definition video service distribution, and the development of IoT (Internet of Things) services, the amount of communication traffic flowing through optical networks has been steadily increasing year by year. Measures to address the increasing demand for communication traffic in optical networks have included, for example, upgrading the functionality of optical communication system equipment installed in optical network terminal stations and introducing optical amplifiers or optical switches without changing the structure of the optical fiber used as the transmission path.

[0003] The optical fiber that forms the basis of current high-capacity optical networks is single-mode fiber (SMF), except for short-distance local networks such as LANs (Local Area Networks). Single-mode fiber has a single core within the cladding that serves as the pathway for optical signals, and is designed to allow only single-mode propagation in wavelength bands such as the C-band and L-band used in high-capacity long-distance optical networks. This has enabled the realization of high-capacity long-distance optical networks that can stably transfer information reaching several terabits per second over long distances.

[0004] In such optical networks, digital coherent transmission technology, which uses digital signal processing and coherent transmission and reception technologies, has been commercially introduced in 100-Gbps-class optical transmission equipment. Digital coherent transmission technology combines a coherent reception method with ultra-high-speed digital signal processing. The coherent reception method is a receiving method that detects the interference light between light and local oscillator light on the receiving side. Ultra-high-speed digital signal processing is a process that, after digitizing the signal, removes phase noise caused by frequency and phase fluctuations in the transmitting light source that generates the signal light and the receiving light source that generates the local oscillator light.

[0005] Digital coherent transmission technology has made it possible to realize small, inexpensive, and low-power-consumption optical transmitter / receiver modules and optical transceivers using such modules without using complex phase-locked loop circuits, etc. The advent of digital coherent transmission technology has made it possible not only to improve the receiving sensitivity during optical transmission in large-capacity optical networks, but also to dramatically improve information transmission efficiency by encoding information in the amplitude, phase, and polarization of optical carrier waves.

[0006] However, in recent years, with the improvement in information transmission efficiency, the transmission capacity is approaching the theoretical limit of the transmission capacity that SMF can provide. Therefore, attention has been focused on space division multiplexing transmission technology, which involves upgrading the transmission medium to optical fiber with a new structural form called space division multiplexing optical fiber, and carrying different independent information on the light propagating at each spatial degree of freedom in the optical fiber.

[0007] An example of a spatially multiplexed optical fiber is a multi-core optical fiber (MCF), which has multiple cores arranged in the cladding. If each core of the MCF is an independent transmission path in parallel, it is expected that the transmission capacity per optical fiber will be greatly improved. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] S. Luis, BJ Puttnam, G. Rademacher, Y. Awaji and N. Wada, "On the Use of High-Order MIMO for Long-Distance Homogeneous Single-Mode Multicore Fiber Transmission," 2017 European Conference on Optical Communication (ECOC), paper Th2.F2, 2017. Summary of the Invention [Problem to be solved by the invention]

[0009] However, optical transmission systems using MCFs have performance-limiting factors that were not observed in the existing transmission medium, SMF. For example, when optical signals are input to each core of an MCF, unintended environmental conditions such as bending and vibration can cause phase matching between adjacent cores, resulting in interference between optical signals propagating between different cores. In other words, optical transmission systems using MCFs can sometimes experience crosstalk between cores. This phenomenon is called inter-core crosstalk (IXT).

[0010] IXT statistically behaves as white noise between optical signals modulated at modulation rates of several tens of GBauds, and has cumulative characteristics over transmission distance. Therefore, IXT, along with noise from spontaneous emission generated during the optical amplification process, is a performance-limiting factor that degrades the signal-to-noise ratio of optical signals in optical transmission systems. Therefore, IXT limits the transmission capacity that can be provided by spatially multiplexed transmission lines.

[0011] It has been reported that IXT can be partially compensated for by applying MIMO (Multiple-input Multiple-output) technology, which is widely used in wireless systems (see Non-Patent Document 1).

[0012] However, the proposal assumes that the optical phases between optical transceivers for different cores are synchronized. The proposal also implicitly assumes an interface configuration for transferring the received optical signal between signal processing devices that process the receiving signal. Therefore, considering the throughput requirements for optical signals exceeding 100 Gbps, the proposed technology is not easy to implement.

[0013] In view of the above circumstances, an object of the present invention is to provide a technique for improving the transmission capacity in an optical transmission system. [Means for solving the problem]

[0014] One aspect of the present invention is a signal processing device comprising: a control unit that converts an electrical signal representing a bit string into an optical signal representing the bit string, and transmits the optical signal obtained by the conversion; and a control unit that removes crosstalk from the nth main digital signal, which is a signal transmitted from the nth transmitter (n is an integer between 1 and N) among signals obtained by converting optical signals transmitted from first to Nth transmitters (N is an integer between 2 and 3) that have propagated through a multicore fiber into digital electrical signals, based on a symbol string representing the decoding results of the optical signals transmitted from at least some of the first to Nth transmitters excluding the nth transmitter.

[0015] One aspect of the present invention is a signal processing method comprising a control step of converting an electrical signal representing a bit string into an optical signal representing the bit string, and transmitting the optical signal obtained by the conversion, and removing crosstalk from the n-th main digital signal, which is a signal transmitted from the n-th transmitter (n is an integer between 1 and N) among signals obtained by converting optical signals transmitted from first to N-th transmitters (N is an integer between 2 and 3) that have propagated through a multicore fiber into digital electrical signals, and based on a symbol string representing the decoding results of the optical signals transmitted from at least some of the first to N-th transmitters excluding the n-th transmitter.

[0016] One aspect of the present invention is a program for causing a computer to function as the signal processing device described above. [Effects of the Invention]

[0017] The present invention makes it possible to improve the transmission capacity in an optical transmission system. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is an explanatory diagram illustrating an outline of an optical transmission system according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of an optical transmitter according to the embodiment. [Figure 3] FIG. 2 is an explanatory diagram illustrating an example of the configuration of a signal processing unit according to the embodiment. [Figure 4] FIG. 1 is a diagram showing an example of a hardware configuration of a signal processing device according to an embodiment. [Figure 5] 4 is a flowchart showing an example of a flow of processing executed by a signal processing device according to an embodiment. [Figure 6] FIG. 1 is a first diagram showing an example of an experimental result using the signal processing device according to the embodiment. [Figure 7] FIG. 2 is a second diagram showing an example of experimental results using the signal processing device according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] (Embodiment) 1 is an explanatory diagram illustrating an overview of an optical transmission system 100 according to an embodiment. The optical transmission system 100 includes optical transmitters 1-1 to 1-N (N is an integer equal to or greater than 2), a spatial multiplexer 2, a multicore fiber (MCF) 3, a spatial multiplexer / demultiplexer 4, optical receivers 5-1 to 5-N, and a signal processing device 6. The optical transmission system 100 may be a system that performs wavelength multiplexing, or may not perform wavelength multiplexing.

[0020] Optical transmitter 1-n (n is an integer between 1 and N) converts an electrical signal representing a bit string into an optical signal representing the bit string, and transmits the optical signal obtained by the conversion. Thus, optical transmitter 1-n (n is an integer between 1 and N) transmits a bit string. Hereinafter, the bit string to be converted by optical transmitter 1-n will be referred to as bit string bn. Thus, for example, optical transmitter 1-1 converts an electrical signal representing bit string b-1 into an optical signal representing bit string b-1.

[0021] 2 is a diagram showing an example of the configuration of an optical transmitter 1-n according to an embodiment. The optical transmitter 1-n includes a signal processing unit 10 having a processor 91 such as a CPU (Central Processing Unit) and a memory 92 connected by a bus, and executes a program. By executing the program, the optical transmitter 1-n functions as a device including the signal processing unit 10, an interface unit 11, a storage unit 12, a DA converter 14, a light source 15, an optical amplifier unit 16, and an optical modulator unit 17.

[0022] More specifically, the processor 91 reads out a program stored in the storage unit 12 and stores the read out program in the memory 92. When the processor 91 executes the program stored in the memory 92, the optical transmitter 1-n functions as a device including a signal processing unit 10, an interface unit 11, a storage unit 12, a DA converter 14, a light source 15, an optical amplifier unit 16, and an optical modulator unit 17.

[0023] The signal processing unit 10 controls the operation of each functional unit of the optical transmitter 1-n and outputs an electrical signal representing a bit string to the DA converter 14.

[0024] The interface unit 11 includes an interface for connecting the optical transmitter 1-n to an external device. The interface unit 11 communicates with the external device via wired or wireless communication. The external device is, for example, a user terminal. In such a case, for example, information indicating a bit string to be transmitted by the optical transmitter 1-n is input from the user terminal to the interface unit 11. The signal processing unit 10 acquires the information indicating the bit string input to the interface unit 11 (hereinafter referred to as "bit string information"). The signal processing unit 10 outputs an electrical signal of the bit string indicated by the acquired bit string information.

[0025] A signal for synchronizing the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-N may be input to the interface unit 11. When such a signal is input to the interface unit 11, the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-N are synchronized.

[0026] A signal for synchronizing the clocks of the optical transmitters 1-1 to 1-N may be input to the interface unit 11. When such a signal is input to the interface unit 11, the clocks of the optical transmitters 1-1 to 1-N are synchronized. More specifically, synchronization of the clocks of the signal processing units 10 and DA converters 14 provided in the optical transmitters 1-1 to 1-N means synchronization of the clocks of the signal processing units 10 and DA converters 14 provided in the optical transmitters 1-1 to 1-N.

[0027] The storage unit 12 is configured using a computer-readable storage medium device (non-transitory computer-readable recording medium) such as a magnetic hard disk device, a semiconductor storage device, etc. The storage unit 12 stores various information related to the optical transmitters 1-n.

[0028] The DA converter 14 is a digital-to-analog converter that converts the digital electrical signal output by the signal processing unit 10 into an analog electrical signal. Note that the digital electrical signal means a signal that is an electrical signal and also a digital signal. Also, the analog electrical signal means a signal that is an electrical signal and also an analog signal. Specifically, the digital electrical signal output by the signal processing unit 10 is a bit string electrical signal.

[0029] The light source 15 is a light source that emits light. The light source 15 is, for example, a semiconductor laser. The light source 15 is independently controlled among the optical transmitters and emits continuous light in a free-running state.

[0030] The optical amplification unit 16 amplifies the light emitted by the light source 15. The optical modulation unit 17 modulates the light amplified by the optical amplification unit 16 based on the analog digital signal obtained by the DA converter 14, and generates an optical signal representing the bit string indicated by the bit string information input to the interface unit 11. The optical modulation unit 17 transmits the generated optical signal to the outside of the optical transmitter 1-n. Therefore, the optical signal transmitted by the optical modulation unit 17 is emitted from the optical transmitters 1-1 to 1-N.

[0031] Returning to the description of FIG. 1. The optical signals emitted from the optical transmitters 1-1 to 1-N enter the spatial multiplexer 2.

[0032] The spatial multiplexer 2 receives the optical signals emitted by each of the optical transmitters 1-1 to 1-N. The optical signals received by the spatial multiplexer 2 enter the multi-core fiber 3. That is, the spatial multiplexer 2 performs spatial multiplexing of the optical signals.

[0033] The multi-core fiber 3 is a multi-core fiber through which the incident optical signals propagate. The optical signals that have propagated through the multi-core fiber 3 enter the spatial demultiplexer 4. The multi-core fiber 3 is, for example, a spatial multiplex fiber capable of propagating N spatial channels. The multi-core fiber 3 may be, for example, a spatial multiplex fiber with an allowable number of spatial channels of M (M < N, M is an integer).

[0034] The spatial multiplexing demultiplexer 4 receives the optical signals output from the multi-core fiber 3. The optical signals received by the spatial multiplexing demultiplexer 4 are incident on the optical receivers 5-1 to 5-N. Therefore, the spatial multiplexing demultiplexer 4 splits the spatially multiplexed optical signals.

[0035] Each of the optical receivers 5-1 to 5-N receives an optical signal that has passed through the multi-core fiber 3. More specifically, the optical receiver 5-n receives a signal transmitted by the optical transmitter 1-n.

[0036] Incidentally, interference (coupling) occurs between spatial channels of optical signals propagating through the multi-core fiber 3 when a phase matching condition is satisfied during propagation through the multi-core fiber 3. For example, a transmission signal xn emitted from an optical transmitter 1-n is subject to crosstalk from an optical signal xp emitted from another optical transmitter 1-p (p is an integer between 1 and N, inclusive, and different from n). Therefore, the optical signal received by the optical receiver 5-n is an optical signal subject to crosstalk from a signal transmitted by an optical transmitter 1-p other than the optical transmitter 1-n.

[0037] In this way, in the optical transmission system 100, the optical signal transmitted by the optical transmitter 1-n and affected by crosstalk is received by the optical receiver 5-n.

[0038] Each of the optical receivers 5-1 to 5-N converts the received optical signal into an analog electrical signal and outputs it to the signal processing device 6.

[0039] Therefore, in the optical transmission system 100, the optical signal transmitted by the optical transmitter 1-n and affected by crosstalk is converted into an analog electrical signal by the optical receiver 5-n. Then, in the optical transmission system 100, the electrical signal obtained by the conversion by the optical receiver 5-n propagates to the signal processing device 6.

[0040] The signal processing device 6 includes a control unit and a storage unit, each of which includes a processor such as a CPU and a memory, connected by a bus, and executes a program. The signal processing device 6 acquires the electrical signals output by the optical receivers 5-1 to 5-N.

[0041] The signal processing device 6 includes signal processing units 60-1 to 60-N. Hereinafter, the signal processing units 60-1 to 60-N will be referred to as signal processing units 60 when they are not distinguished from one another.

[0042] 3 is an explanatory diagram illustrating an example of the configuration of the signal processing unit 60 in the embodiment. The signal processing unit 60 includes an AD (analog-digital) converter 610. The AD converter 610 acquires an analog electrical signal propagated to the signal processing device 6 and converts it into a digital signal. That is, the signal processing device 6 includes the AD converter 610 and converts the analog electrical signal propagated to the device itself into a digital signal. In the example of FIG. 3, four AD converters, XI, XQ, YI, and YQ, are used in one spatial channel. Therefore, in the example of FIG. 3, there are 4N AD converters 610.

[0043] The signal processing unit 60 includes a decoding unit 620. The decoding unit 620 is included in a control unit included in the signal processing device 6. The decoding unit 620 executes an estimated signal generation process. The estimated signal generation process is a process of estimating a bit string transmitted by the optical transmitter 1-n based on the n-th primary digital signal and generating an electrical signal representing the estimated bit string. In other words, the estimated signal generation process is a process of generating an electrical signal representing an estimation result of the bit string transmitted by the optical transmitter 1-n based on the n-th primary digital signal. Therefore, the estimated signal generation process is a process of decoding the optical signal transmitted by the optical transmitter 1-n and estimating the bit string represented by the optical signal.

[0044] The nth main digital signal is a signal resulting from the analog electrical signal obtained by conversion by the optical receiver 5-n being converted into a digital signal by the AD converter 610. Therefore, for example, the first main digital signal is a signal resulting from the analog electrical signal obtained by conversion by the optical receiver 5-1 being converted into a digital signal by the AD converter 610. Therefore, the nth digital signal is an electrical digital signal.

[0045] Hereinafter, an electrical signal representing the estimation result of the bit string transmitted by optical transmitter 1-n will be referred to as the n-th estimation signal. Therefore, for example, the first estimation signal is an electrical signal representing the estimation result of the bit string transmitted by optical transmitter 1-1. Since the n-th estimation signal is an electrical signal representing the estimation result of the bit string, it is an electrical signal representing a bit string. If the term n-th estimation signal is used to explain the estimation signal generation process, the estimation signal generation process is a process of generating the n-th estimation signal based on the n-th primary digital signal.

[0046] In Figure 3, the hat (^) is used as an accent mark for b1, b2, and b N are examples of the first estimated signal, the second estimated signal, and the Nth estimated signal, respectively.

[0047] The estimated signal generation process is a process for generating the nth estimated signal based on the nth primary digital signal, and therefore the nth estimated signal is the result of decoding the bit string transmitted by the optical transmitter 1-n.

[0048] The decoding unit 620 includes a signal detection unit 621, a mapping unit 624, and a demapping unit 625. The signal detection unit 621 includes an equalization unit 622 and an optical phase recovery unit 623.

[0049] The equalization unit 622 performs equalization processing. The equalization processing is processing for estimating the strength of interference between the optical signal transmitted by the optical transmitter 1-n and the optical signals transmitted by at least some of the optical transmitters 1-1 to 1-N excluding the optical transmitter 1-n, based on the optical signal transmitted by the optical transmitter 1-n and the auxiliary symbol sequence. The auxiliary symbol sequence is a symbol sequence represented by the result of decoding the optical signals transmitted by at least some of the optical transmitters 1-1 to 1-N excluding the optical transmitter 1-n. Therefore, the auxiliary symbol sequence may be a bit sequence generated by another signal processing unit 60 through an estimated signal generation process, for example.

[0050] Since waveform distortion or polarization rotation occurs due to interference between the optical signal transmitted by the optical transmitter 1-n and the optical signal transmitted by another optical transmitter 1-p, the magnitude of the interference between the optical signal transmitted by the optical transmitter 1-n and the optical signal transmitted by another optical transmitter 1-p is the magnitude of the waveform distortion or polarization rotation. The equalization process is, for example, the process described in Reference 1 below.

[0051] Reference 1: K. Shibahara et al., "Iterative Unreplicated Parallel Interference Canceler for MDL-Tolerant Dense SDM (12-Core × 3-Mode) Transmission Over 3000 km," in Journal of Lightwave Technology, vol. 37, no. 6, pp. 1560-1569, 2019.

[0052] The optical phase recovery unit 623 executes optical phase recovery processing. The optical phase recovery processing is processing for estimating phase noise contained in the nth primary digital signal to be processed, based on the optical signal transmitted by the optical transmitter 1-n and a symbol sequence represented by the result of decoding the optical signals transmitted by at least some of the optical transmitters 1-1 to 1-N excluding the optical transmitter 1-n. Specifically, the signal to be processed in the optical phase recovery processing is the nth primary digital signal. The optical phase recovery processing is processing described, for example, in Reference 2 below.

[0053] Reference 2: Kohki Shibahara, Takayuki Mizuno, and Yutaka Miyamoto, "MIMO carrier phase recovery for carrier-asynchronous SDM-MIMO reception based on the extended Kalman filter," Opt. Express 29, pp. 17111-17124, 2021.

[0054] Using an example in which the signal to be equalized is a first main digital signal, the equalization process described in Reference 1 and the optical phase recovery process described in Reference 2 will be explained using mathematical expressions. When the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-N are synchronized, the first main digital signal is, for example, y1 expressed by the following equation (1).

[0055]

number

[0056] h ij represents the channel matrix element corresponding to the path from the optical transmitter 1-i to the optical receiver 5-j, where i and j are integers between 1 and N. Note that the j at the shoulder of the exponential function represents the imaginary unit. t i represents the phase noise of the light source provided in the optical transmitter 1-i. r i represents the phase noise of the light source included in the optical receiver 5-i. i represents the noise superimposed on the received signal i. t i and φ r i The sum of these is an example of phase noise contained in the n-th primary digital signal.

[0057] The second term on the right side of equation (1) represents the crosstalk superimposed on the optical signal transmitted by the optical transmitter 1-1. Based on the p-th estimated signal, which is the output signal from the signal processor 60-p (p≠1), information for removing the crosstalk is obtained by equalization processing and optical phase recovery processing. More specifically, in the equalization processing, h 1i is estimated, and in the optical phase retrieval process, the shoulder is j(φ t i +φ r i ) is estimated. ij is a quantity that represents the magnitude of interference between the optical signal transmitted by the optical transmitter 1-i and the optical signal transmitted by another optical transmitter 1-j.

[0058] h estimated by equalization and optical phase recovery processing ij and the shoulder is j(φ t i +φ r i ) and the value of the exponential function, it is possible to remove crosstalk.

[0059] Therefore, the signal detection unit 621 detects h 1i and the shoulder is j(φ t i +φ r i ) and the crosstalk is removed from the nth primary digital signal to be processed. Hereinafter, the process of removing crosstalk from the nth primary digital signal based on the results of the equalization process and the optical phase recovery process will be referred to as the removal process.

[0060] In this way, the signal detection unit 621 executes equalization processing, optical phase recovery processing, and removal processing. As a result of the removal processing, an n-th primary digital signal from which crosstalk has been removed is obtained.

[0061] The optical phase recovery process may be, for example, the process described in Reference 3 below.

[0062] Reference 3: T. Pfau, S. Hoffmann, and R. Noe, “Hardware-Efficient Coherent Digital Receiver Concept With Feedforward Carrier Recovery for M-QAM Constellations,” Journal of Lightwave Technology vol. 27, no. 8, pp. 989-999, 2009.

[0063] Using an example in which the signal to be equalized is a first main digital signal, the equalization process described in Reference 1 and the optical phase recovery process described in Reference 3 will be explained using mathematical expressions. When the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-N are not synchronized, the first main digital signal is, for example, y1 expressed by the following equation (2).

[0064]

number

[0065] Here, φ1 is a quantity defined by the following equation (3): φ1 is given in a simple form by the phase synchronization of the light source provided in the optical transmitter.

[0066]

number

[0067] The second term on the right side of equation (3) represents the crosstalk superimposed on the optical signal transmitted by the optical transmitter 1-1. Based on the p-th estimated signal, which is the output signal from the signal processor 60-p (p ≠ 1), information for removing the crosstalk is obtained by equalization processing and optical phase recovery processing.

[0068] More specifically, the equalization process uses h 1i is estimated, and the optical phase recovery process estimates the value of an exponential function whose shoulder is jφ1. The signal detection unit 621 performs the removal process on the n-th primary digital signal to estimate h estimated by the equalization process.1i and the value of the exponential function with a shoulder jφ1 estimated by the optical phase recovery process, are used to obtain the n-th primary digital signal from which crosstalk has been removed.

[0069] The mapping unit 624 performs mapping processing on the processing target. The mapping processing is a processing of a bit string, and is a processing of converting the bit string to be processed into a symbol string. The symbol string obtained by the conversion is an example of an auxiliary symbol string. The symbol string input to the equalization unit 622 is the output of the mapping unit 624. Therefore, the mapping unit 624 receives as input another bit string that is the source of the symbol string used in the equalization processing and the optical phase recovery processing. The input bit string is the processing target of the mapping processing.

[0070] In Figure 3, the accent marks x2 and x N Both signals represented by x and x2 are examples of auxiliary symbol sequences. The x2 signal with a hat accent (^) is an auxiliary symbol sequence obtained as a result of the mapping process for the second estimated signal. N is an auxiliary symbol sequence obtained as a result of mapping processing on the Nth estimated signal. In the example of FIG. 3, the number of mapping units 624 is N-1.

[0071] The demapping unit 625 executes demapping processing. The demapping processing is processing for converting a processing target into a bit string. The processing target of the demapping processing is the nth primary digital signal from which crosstalk has been removed by the removal processing. The signal obtained by the demapping processing is a bit string represented by the signal from which crosstalk has been removed from the nth primary digital signal. Therefore, the signal obtained by the demapping processing is an electrical signal representing an estimation result of the bit string transmitted by the optical transmitter 1-n.

[0072] The equalization process, optical phase recovery process, removal process, and demapping process generate an electrical signal representing an estimation result of the bit string transmitted from the optical transmitter 1-n based on the n-th primary digital signal. Therefore, the equalization process, optical phase recovery process, removal process, and demapping process are included in the estimated signal generation process.

[0073] It should be noted that the mapping process does not necessarily have to be performed if the symbol sequence obtained by the mapping process can be used in the equalization process and the optical phase recovery process. When the mapping process is performed, the mapping process is also included in the estimated signal generation process.

[0074] In this way, the estimated signal generation process estimates the bit string converted from an electrical signal to an optical signal by the optical transmitter 1-n using an auxiliary symbol string, rather than a high-speed analog signal or a quantized digital signal. That is, the estimated signal generation process estimates the bit string converted from an electrical signal to an optical signal by the optical transmitter 1-n without needing to satisfy the first and second preconditions.

[0075] The first prerequisite is that the optical phases between the optical transceivers for different cores are synchronized. The second prerequisite is that an interface is configured between the signal processing devices that process the receiving side signals to transfer the received optical signals that have propagated through the different cores.

[0076] 4 is a diagram showing an example of the hardware configuration of a signal processing device 6 according to an embodiment. The signal processing device 6 includes a control unit 61 having a processor 93 such as a CPU and a memory 94 connected via a bus, and executes a program. The signal processing device 6 functions as a device including the control unit 61, a connection unit 62, and a storage unit 63 by executing the program.

[0077] More specifically, the processor 93 reads out the program stored in the storage unit 63 and stores the read out program in the memory 94. The processor 93 executes the program stored in the memory 94, whereby the signal processing device 6 functions as a device including the control unit 61, the connection unit 62, and the storage unit 63.

[0078] The control unit 61 controls the operations of various functional units included in the signal processing device 6. The control unit 61 includes a decoding unit 620. Therefore, the control unit 61 executes, for example, an estimated signal generation process. That is, the control unit 61 removes crosstalk from the n-th primary digital signal, which is obtained by converting the optical signals transmitted from the optical transmitters 1-1 to 1-N and propagating through the multi-core fiber 3 into digital electrical signals, based on the n-th primary digital signal and the auxiliary symbol sequence.

[0079] The connection unit 62 includes an AD converter 610. Therefore, the connection unit 62 is connected to the optical receivers 5-1 to 5-N and generates the n-th primary digital signal. The connection unit 62 includes an output circuit 630 which is a circuit connected to the output destination of the signal output by the estimated signal generation process. Therefore, the connection unit 62 is connected to the output destination of the signal output by the estimated signal generation process.

[0080] The storage unit 63 is configured using a computer-readable storage medium device (non-transitory computer-readable recording medium) such as a magnetic hard disk device or a semiconductor storage device. The storage unit 63 stores various information related to the signal processing device 6. The storage unit 63 stores, for example, a bit string representing the n-th estimated signal obtained by the estimated signal generation process.

[0081] 5 is a flowchart showing an example of the flow of processing executed by the signal processing device 6 in the embodiment. The connection unit 62 generates an n-th primary digital signal (step S101). Next, the control unit 61 executes an estimated signal generation process on the n-th primary digital signal (step S102). Next, the control unit 61 outputs the result of the estimated signal generation process in step S102 (step S103).

[0082] <Experimental Results> An example of an experimental result using the signal processing device 6 according to the embodiment will be described. In the experiment, a signal transmission simulation was performed. In the experiment, the optical transmission system 100 had two optical transmitters and two optical receivers. In the experiment, a 16QAM signal with a modulation rate of 10 GBaud was generated and received. The linewidth of the light source of the optical transmitter and the light source of the optical receiver were both 100 kHz.

[0083] In the experiment, the crosstalk between one signal and the other signal propagating through the multi-core fiber 3 was 15 dB. In the experiment, a simulation was performed in which the condition that the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-2 were not synchronized, and a simulation in which the condition that the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-2 were synchronized was satisfied.

[0084] Fig. 6 is a first diagram showing an example of an experimental result using the signal processing device 6 according to the embodiment. More specifically, the result in Fig. 6 is an example of a simulation result that satisfies the condition that the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-2 are not synchronized.

[0085] The vertical axis in Figure 6 shows the bit error rate of one of the signal sequences. The horizontal axis shows the optical signal to noise ratio (OSNR). The "Theory" results show theoretical results obtained under the assumption that there is no crosstalk. The "No XT Compensation" results show simulation results without crosstalk compensation. The "Technology 1" curve shows the results obtained when the optical phase recovery process is the process in Reference 2. The "Technology 2" curve shows the results obtained when the optical phase recovery process is the process in Reference 3.

[0086] In the simulation, a separation matrix in which the amount of crosstalk is assumed to be known is used as a matrix having components of weighting coefficients in equalization processing for crosstalk compensation.

[0087] The results in Figure 6 show that when crosstalk compensation is not performed, the amount of crosstalk remains constant even when the OSNR increases, so the characteristics asymptotically approach a line with a constant bit error rate.The results in Figure 6 also show that for the crosstalk compensation characteristics of "Technology 2," the equalization process increases the crosstalk component because the light phase of the light source on the transmitting side is not synchronized, resulting in a deterioration in characteristics compared to the curve without crosstalk compensation.

[0088] The results in Figure 6 show that the results when applying the crosstalk compensation method of "Technology 1" are close to the results of "Theory." This result indicates that crosstalk can be compensated even when the light source phase on the transmitting side is asynchronous.

[0089] 7 is a second diagram showing an example of an experimental result using the signal processing device 6 according to the embodiment. The result in FIG. 7 is an example of an experimental result that satisfies the condition that the initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-2 are synchronized.

[0090] The definitions of "Theory," "No XT compensation," "Technology 1," and "Technology 2" are the same as in Figure 6. Figure 7 shows that the results when applying the "Technology 1" crosstalk compensation method are close to the "Theory" results. Furthermore, Figure 7 shows that due to the synchronization condition of the light source phase on the transmitting side, the results when applying the "Technology 2" crosstalk compensation method show characteristics that are generally similar to those obtained by the "Technology 1" crosstalk compensation method.

[0091] The signal processing device 6 configured in this manner estimates the bit string converted from an electrical signal to an optical signal by the optical transmitter 1-n using an auxiliary symbol string, rather than a high-speed analog signal or a quantized digital signal. That is, the signal processing device 6 estimates the bit string converted from an electrical signal to an optical signal by the optical transmitter 1-n without needing to satisfy the first and second preconditions described above.

[0092] Therefore, the signal processing device 6 can reduce the time required to transmit and receive electrical signals within the signal processing device 6. Since the transmission capacity of an optical transmission system is also affected by the signal processing performance of the signal processing device 6, the signal processing device 6 that can reduce the time required to transmit and receive electrical signals within the signal processing device 6 enables communications that satisfy the required throughput characteristics of optical signals exceeding 100 gigabits per second. Therefore, the signal processing device 6 can improve the transmission capacity of the optical transmission system.

[0093] (Variation) The clocks of the optical transmitters 1-1 to 1-N may be synchronized.

[0094] The signal processing device 6 may further include a chromatic dispersion compensator, a clock recovery unit, or a frequency offset compensator. The signal processing unit 10 included in each of the optical transmitters 1-1 to 1-N may perform forward error correction coding on the electrical signal representing the bit string. In this case, the demapping unit 625 performs decoding processing on the forward error correction coding.

[0095] The processes in the signal processing units 60-1 to 60-N may be performed sequentially in any order, or may be performed in parallel. When the processes are performed in parallel, the bit decision error rate decreases as q (q is a natural number) of the output from the signal processing unit 60-n after the qth time (q is a natural number) increases. Specifically, the processes in the signal processing units 60-1 to 60-N are equalization, optical phase recovery, removal, mapping, and demapping.

[0096] The initial phases of the optical signals transmitted by the optical transmitters 1-1 to 1-N are synchronized by, for example, providing each optical transmitter with light branched from a single light source, or by optical phase locking using an optical injection-locked laser.

[0097] The signal processing device 6 may be implemented using a plurality of information processing devices communicably connected via a network, in which case the respective functional units of the signal processing device 6 may be distributed and implemented in the plurality of information processing devices.

[0098] The optical transmitter 1-n is an example of an n-th transmitter. Therefore, the optical transmitter 1-1 is an example of a first transmitter, and the optical transmitter 1-N is an example of an N-th transmitter.

[0099] All or part of the functions of the signal processing device 6 may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as flexible disks, magneto-optical disks, ROMs, and CD-ROMs, and storage devices such as hard disks built into computer systems. The program may be transmitted via a telecommunications line.

[0100] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Explanation of symbols]

[0101] 100...optical transmission system, 1-1 to 1-N, 1-n...optical transmitter, 10...signal processing unit, 11...interface unit, 12...storage unit, 14...DA converter, 15...light source, 16...optical amplifier unit, 17...optical modulation unit, 2...spatial multiplexer, 3...multicore fiber, 4...spatial multiplexer / demultiplexer, 5-1 to 5-N...optical receiver, 6...signal processing device, 60-1 to 60-N...signal processing unit, 610...AD converter, 620...decoding unit, 621...signal detection unit, 622...equalization unit, 623...optical phase recovery unit, 624...mapping unit, 625...demapping unit, 61...control unit, 62...connection unit, 630...output circuit, 63...storage unit, 91...processor, 92...memory, 93...processor, 94...memory

Claims

1. a control unit that converts an electrical signal representing a bit string into an optical signal representing the bit string and transmits the optical signal obtained by the conversion; a control unit that removes crosstalk from the n-th main digital signal, which is a signal transmitted from an n-th transmitter (n is an integer of 1 to N) among signals obtained by converting optical signals transmitted from first to N-th transmitters (N is an integer of 2 or more) that have propagated through a multicore fiber into digital electrical signals, and based on a symbol string that represents a result of decoding the optical signals transmitted from at least some of the first to N-th transmitters excluding the n-th transmitter; A signal processing device comprising:

2. the control unit estimates a magnitude of interference between an optical signal transmitted from an n-th optical transmitter and an optical signal transmitted from a p-th optical transmitter (p is an integer between 1 and N, and is different from n), and a phase noise included in the n-th primary digital signal, based on the n-th primary digital signal and the symbol sequence. The signal processing device according to claim 1 .

3. the initial phases of the optical signals output from the first to Nth transmitters are synchronized; The signal processing device according to claim 1 .

4. A control step in which a computer converts an electrical signal representing a bit string into an optical signal representing the bit string, and transmits the optical signal obtained by the conversion, the optical signal being transmitted from a first transmitter to an Nth transmitter (N is an integer of 2 or more) which transmit the optical signal obtained by the conversion and which has propagated through a multicore fiber, is converted into a digital electrical signal, the nth main digital signal being the signal transmitted from the nth transmitter (n is an integer of 1 to N inclusive), and the symbol string representing the result of decoding the optical signal transmitted from at least some of the first transmitter to the Nth transmitter, excluding the nth transmitter; A signal processing method comprising:

5. A program for causing a computer to function as the signal processing device according to any one of claims 1 to 3.

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