JP2016167655A - Optical transmission device and differential delay compensation method between adjacent channels applied to optical transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical transmission device capable of compensating for the interchannel interference at the time of demodulation, and a differential delay compensation method between adjacent channels applied to the optical transmission device.SOLUTION: An optical transmission device includes a transmission unit, and a reception unit having a demodulation unit for demodulating a received signal by performing wavelength separation on the reception side, and further provided with a frame synchronization unit (25) provided at a subsequent stage of the demodulation unit, and generating a frame pulse indicating the head of a frame, for each channel subjected to wavelength separation, and an interchannel delay detector (28) for generating the differential time delay information between adjacent channels by calculating the differential value of a frame pulse of the adjacent channels, and feeding back to the demodulation unit. The demodulation unit demodulates the signal obtained by performing delay compensation of a received signal, based on the differential time delay information.SELECTED DRAWING: Figure 3

Description

  The present invention is a multi-level modulation method applied to a digital communication apparatus such as an optical communication system, and accurately eliminates signal interference of adjacent channels, which is a problem when a client signal is divided and transmitted using a plurality of subcarriers. The present invention relates to an optical transmission apparatus and a delay difference compensation method between adjacent channels applied to the optical transmission apparatus.

  Examples of the optical modulation method employed in the conventional optical transmission / reception apparatus include On Off Keying (OOK) and two-phase phase modulation (Binary Phase Shift Keying: BPSK). In recent years, an increase in the capacity of an optical communication system has been demanded due to an increase in traffic on the Internet, and a method of handling a multi-level phase modulation signal using a digital signal processing technique has been studied.

  Here, for a multi-level modulation signal, quadrature phase shift keying (QPSK), differential quadrature phase modulation (Differential QPSK: DQPSK), eight phase modulation (Eight Quadrature Amplitude Modulation 8) and the like. is there.

  In contrast to the conventional method in which the on / off of the light intensity is assigned to the binary signal and directly detected, the digital coherent reception method extracts the light intensity and the phase information by the coherent reception method. The extracted intensity and phase information is quantized by an analog / digital converter and demodulated by a digital signal processing unit.

  One advantage of the digital coherent reception system is that a mechanism for synchronizing the frequency and phase of the received light with the frequency and phase of the local oscillation light source can be implemented as digital signal processing.

  This makes it possible to synchronize the frequency and phase of the received light and the frequency and phase of the local oscillation light source without mounting an optical PLL (Phase Locked Loop) that is difficult to implement.

  Further, the digital coherent reception system has a characteristic that it has an optical signal-to-noise ratio (OSNR) tolerance and a waveform distortion tolerance of the transmission line.

  Waveform distortion is a phenomenon that occurs when the refractive index of an optical fiber that is a transmission path is a function of wavelength, or a phenomenon that occurs when the refractive index changes with the intensity of light. The former is wavelength dispersion, and the latter is Also called non-linear effect. In the former case, in general, since a signal has a certain wavelength band, the waveform is distorted due to the difference in refractive index perceived by each wavelength. In the digital coherent reception system, waveform distortion correction is performed by using a digital filter (dispersion compensator) having characteristics inverse to the waveform distortion.

  At this time, the dispersion compensator needs to know the magnitude of the chromatic dispersion of the optical fiber that is the transmission path, and for this purpose, a training sequence that becomes a known signal is provided between the transceivers. The coefficient of the digital filter that is a dispersion compensator can be obtained from the waveform distortion state of the training sequence (see, for example, Patent Document 1).

  In the latter case, the waveform distortion due to the nonlinear effect can be corrected from the training sequence that becomes a known signal (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 11-313013 JP-A-5-160865

ITU-T Recommendation G. 709

However, the prior art has the following problems.
In the case of transmitting an ultrahigh-speed signal such as 400 Gbps or 1 Mbps, a method of dividing a client signal and using a plurality of subcarriers (carrier waves) has been proposed. That is, there is a method in which many carrier waves are prepared in one channel (frequency band) and divided client signals are transmitted in parallel by each carrier wave.

  According to this method, the channel spacing can be narrowed by the Nyquist filter in order to increase the frequency utilization efficiency. That is, it is possible to transmit the divided client signals in parallel by appropriately selecting the subcarrier frequency within the narrowed channel. However, in adopting such a method, it is important to compensate for inter-channel interference between adjacent channels caused by chromatic dispersion of optical fibers and nonlinear effects during transmission.

  In order to compensate for interference between adjacent channels between a plurality of subcarriers, it is necessary to exchange signals of adjacent channels with each other in an adaptive filter unit of the demodulation unit. At this time, on the receiving side, it is necessary to align the signal of the adjacent channel with the timing at the time of transmission from the transmitting side and compensate for the delay difference before demodulation.

  However, in the conventional method, the delay difference between subcarriers due to the influence of chromatic dispersion in the transmission path is compensated by the post-demodulation frame synchronization unit or the like. For this reason, inter-channel interference cannot be compensated at the time of demodulation.

  The present invention has been made to solve the above-described problems, and an optical transmission apparatus capable of compensating for a delay difference between adjacent channels during demodulation and a delay between adjacent channels applied to the optical transmission apparatus. The purpose is to obtain a difference compensation method.

  An optical transmission apparatus according to the present invention includes a transmission unit that arranges and transmits a client signal at a plurality of wavelengths via a transmission line, and a demodulation unit that performs demodulation processing on the received signal by performing wavelength separation on the reception side An optical transmission device including a receiving unit having a receiving unit that detects a frame from a demodulated signal output from the demodulating unit for each wavelength-separated channel provided in the subsequent stage of the demodulating unit. The time synchronization difference information between the adjacent channels is generated by calculating the difference value between the frame synchronization unit that generates the frame pulse indicating the head of the frame and the frame pulse of each adjacent channel generated by the frame synchronization unit. And an inter-channel delay detector that feeds back the generated time delay difference information to the demodulator. The demodulator is based on the fed back time delay difference information. There are, by moving the tap barycenter of the digital filter, and performs demodulation processing on the signal after the delay compensation to the received signal.

  In addition, a delay difference compensation method between adjacent channels applied to the optical transmission apparatus according to the present invention includes a transmission unit that arranges and transmits client signals at a plurality of wavelengths via a transmission line, and wavelength separation on the reception side. A delay difference compensation method between adjacent channels applied to an optical transmission device including a receiving unit that performs demodulation processing on a received signal by performing, for each channel that is wavelength-separated in the receiving unit By detecting the frame from the demodulated signal and generating a frame pulse indicating the beginning of the frame, and calculating a difference value between each frame pulse of the adjacent channel generated in the frame synchronization step, An interchannel delay detection step for generating time delay difference information between adjacent channels, and a digital filter tap based on the time delay difference information. By moving the heart, it performs delay compensation to the received signal, and has a demodulation processing step of performing demodulation processing on the signal after delay compensation.

  According to the present invention, the delay difference between adjacent channels detected by the interchannel delay detector can be fed back to the demodulator, so that the delay difference between adjacent channels can be accurately compensated before the demodulation process. An optical transmission apparatus and a delay difference compensation method between adjacent channels applied to the optical transmission apparatus can be obtained.

1 is a configuration diagram showing a digital communication system according to a first embodiment of the present invention. FIG. 2 is a configuration diagram showing details of the optical transmission device shown in FIG. 1 in Embodiment 1 of the present invention, and corresponds to a configuration diagram of a subcarrier multiplexing transmission device. FIG. 2 is a configuration diagram showing details of the optical transmission device shown in FIG. 1 in Embodiment 1 of the present invention, and corresponds to a configuration diagram of a subcarrier multiplex reception device. It is a block diagram which shows the detail of the optical reception front end shown in FIG. 3 in Embodiment 1 of this invention. It is the flowchart which showed the series of processes regarding the delay difference compensation method between the adjacent channels applied to the optical transmission apparatus in Embodiment 1 of this invention.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of an optical transmission device and a delay difference compensation method between adjacent channels applied to the optical transmission device of the invention will be described with reference to the drawings.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a digital communication system according to Embodiment 1 of the present invention. In the following description, such a digital communication system is simply referred to as an “optical communication system”. The optical communication system in FIG. 1 includes two optical transmission devices 1 a and 1 b and a communication path 2.

  In the following description, the optical transmission device 1a is a subcarrier multiplex transmission device that plays the role of an optical signal transmission side that receives a transmission signal from a client and outputs an optical signal. A case will be described as an example in which the subcarrier multiplex receiving apparatus serves as an optical signal receiving side that receives an optical signal through the network and outputs a signal to a client.

  FIG. 2 is a configuration diagram showing details of the optical transmission device 1a shown in FIG. 1 according to Embodiment 1 of the present invention, and corresponds to a configuration diagram of the subcarrier multiplexing transmission device. 2 includes an OTUk (k = 0, 1, 2, 3, 4...) (Optical channel Transport Unit-k) framer generation unit 11, error correction encoding units 12 (1), 12. (2), symbol mapping units 13 (1) and 13 (2), optical modulation units 14 (1) and 14 (2), and a wavelength multiplexing unit 15. Regarding the OTUk frame, Non-Patent Document 1 can be referred to, and details thereof are omitted.

  The OTUk framer generation unit 11 maps a client transmission signal to an OTUk frame as a data frame, adds information necessary for frame synchronization and maintenance control, and generates an optical transmission frame.

  The error correction encoders 12 (1) and 12 (2) generate redundant bits for error correction in the FEC (Forward Error Correction) part of the OTUk frame generated by the OTUk framer generator 11.

  The symbol mapping units 13 (1) and 13 (2) symbol-map the signals after encoding by the error correction encoding units 12 (1) and 12 (2) to generate a multilevel modulation signal. The optical modulation units 14 (1) and 14 (2) convert the multilevel modulation signals that are electrical signals generated by the symbol mapping units 13 (1) and 13 (2) into optical signals.

  Then, the wavelength multiplexing unit 15 multiplexes the optical signals converted by the optical modulation units 14 (1) and 14 (2) and outputs the multiplexed signals to the transmission path.

  Next, FIG. 3 is a configuration diagram illustrating details of the optical transmission device 1b illustrated in FIG. 1 according to Embodiment 1 of the present invention, and corresponds to a configuration diagram of a subcarrier multiplex reception device. The optical transmission device 1b in FIG. 3 includes a wavelength separation unit 21, optical reception front ends 22 (1) and 22 (2), A / D conversion units 23 (1) and 23 (2), a demodulation unit 24 (1), 24 (2), frame synchronizers 25 (1) and 25 (2), error correction decoders 26 (1) and 26 (2), an OTUk framer receiver 27, and an inter-channel delay detector 28. ing.

  The wavelength separation unit 21 separates the received optical signal for each wavelength channel. The optical reception front ends 22 (1) and 22 (2) convert the optical signal separated for each wavelength channel into an electrical signal.

  The A / D converters 23 (1) and 23 (2) convert the analog electrical signals output from the optical reception front ends 22 (1) and 22 (2) into digital signals.

  The demodulation units 24 (1) and 24 (2) demodulate the transmission signal by performing digital signal processing on the digital signals converted by the A / D conversion units 23 (1) and 23 (2).

  The frame synchronization units 25 (1) and 25 (2) detect the OTUk frame from the demodulated signals output from the demodulation units 24 (1) and 24 (2). The error correction decoding units 26 (1) and 26 (2) perform error correction using redundant bits added on the transmission side.

  The OTUk framer receiving unit 27 terminates information necessary for frame synchronization and maintenance control with respect to the OTUk frame after error correction by the error correction decoding units 26 (1) and 26 (2), and receives the client received signal. Demaps from the OTUk frame and outputs a client received signal.

  FIG. 4 is a configuration diagram showing details of the optical reception front end 22 shown in FIG. 3 according to Embodiment 1 of the present invention. The optical reception front end 22 in FIG. 4 converts the optical reception signal from the communication path 2 into an electrical analog signal. Specifically, the reception front end 22 includes a polarization beam splitter (PBS) 221, a local oscillator (LO) 222, a polarization beam splitter (PBS) 223, and a 90 ° optical hybrid 224 (1) and 224 (2). , Photoelectric converters (O / E) 225 (1) to 225 (4), and amplifiers (AMP) 226 (1) to 226 (4).

  The PBS 221 separates the optical signal received from the communication path 2 into an X polarization and a Y polarization. On the other hand, the PBS 223 separates the polarization of the LO output from the LO 222.

  The 90 ° optical hybrids 224 (1) and 224 (2) mix the optical signals separated by the PBS 223 and the X-polarized signals and the Y-polarized signals separated by the PBS 221, respectively. The O / E 225 (1) to 225 (4) convert the received optical signal into an electrical signal. Further, the AMPs 226 (1) to 226 (4) amplify the O / E converted signal.

  Note that the optical transmission device 1b shown in FIG. 3 according to the first embodiment has the frame synchronization units 25 (1) and 25 (25) across the frame synchronization units 25 (1) and 25 (2) for each wavelength channel. 2) having an inter-channel delay detection unit 28 that receives a frame synchronization signal from 2), detects a delay in frame synchronization timing of an adjacent wavelength channel, and feeds back to the demodulation units 24 (1) and 24 (2). This is a technical feature. The function of the inter-channel delay detection unit 28 will be described in detail below.

 The frame synchronization units 25 (1) and 25 (2) detect a known fixed pattern called a frame alignment signal arranged at the head of the OTUk frame, and generate a frame pulse indicating the head of the frame. .

  The inter-channel delay detection unit 28 calculates time delay difference information of the frame pulse between adjacent channels based on the frame pulses received from the frame synchronization units 25 (1) and 25 (2), and the frame synchronization unit 25 (1 ) And 25 (2) are transmitted to the demodulation units 24 (1) and 24 (2), which are the previous stage.

  The demodulating units 24 (1) and 24 (2) are configured as transversal filters (FIR filters) including a plurality of taps as shown in the upper right of FIG. Then, the demodulation units 24 (1) and 24 (2) having such a configuration perform signal weight calculation for each tap and perform demodulation. At this time, the demodulation units 24 (1) and 24 (2) perform timing adjustment by changing the position of the center of gravity obtained from the signal weight of the tap.

  Further, the demodulation units 24 (1) and 24 (2) exchange signals between adjacent wavelength channels in order to compensate for inter-channel interference in a state where the time delay difference is compensated. As a result, the optical transmission apparatus according to the first embodiment can compensate for signal interference with higher accuracy than the conventional method.

  FIG. 5 is a flowchart showing a series of processes relating to a delay difference compensation method between adjacent channels applied to the optical transmission apparatus according to Embodiment 1 of the present invention.

  First, in step S501, each of the frame synchronization units 25 (1) and 25 (2) provided after the receiving-side demodulation units 24 (1) and 24 (2) is arranged at the head of the OTUk frame. A known fixed pattern called a frame alignment signal is detected, and a frame pulse indicating the head of the frame is generated.

  Next, in step S502, the inter-channel delay detection unit 28 calculates time delay difference information of frame pulses between adjacent channels based on the frame pulses received from the frame synchronization units 25 (1) and 25 (2). . Furthermore, the inter-channel delay detection unit 28 feeds back the calculated time delay difference information to the demodulation units 24 (1) and 24 (2), which are the previous stage of the frame synchronization units 25 (1) and 25 (2). .

  Next, in step S503, the demodulation units 24 (1) and 24 (2) perform signal weight calculation for each tap and perform demodulation. At this time, the demodulation units 24 (1) and 24 (2) perform channel timing adjustment by changing the barycentric position obtained from the signal weight of the tap based on the fed back time delay difference information. Compensate for the time delay difference between.

  Next, in step S504, the demodulation units 24 (1) and 24 (2) exchange signals between adjacent wavelength channels in a state in which the time delay difference is compensated for, so The inter-adjacent channel interference is compensated using the signal and the own signal.

  As described above, according to the first embodiment, when a client signal is arranged at a plurality of wavelengths and transmitted, an adjacent signal detected by the frame synchronization unit is detected on the optical signal receiving side after error correction coding. The amount of delay is fed back to the demodulator, the delay is compensated by moving the tap center of the digital filter in the demodulator, and the inter-channel interference amount calculation and compensation is demodulated by exchanging the signals after delay compensation. The structure performed by the section is provided. As a result, it is possible to compensate for the delay difference between adjacent channels and accurately compensate for interference between adjacent channels during demodulation.

  In the above-described first embodiment, the case where the delay difference between adjacent channels and signal interference are compensated has been described. However, the present invention is not limited to such a system. The present invention can also be applied to a system that transmits using a plurality of cores or modes such as a multi-core fiber and a multi-mode fiber. According to the present invention, it is possible to compensate the delay difference between signals having different wavelengths, polarizations, modes, and cores by the demodulation unit.

  1a, 1b Optical transmission device, 2 communication path, 11 OTUk framer generator, 12 (1), 12 (2) Error correction encoder, 13 (1), 13 (2) Symbol mapping unit, 14 (1), 14 (2) Optical modulation unit, 15 Wavelength multiplexing unit, 21 Wavelength separation unit, 22 (1), 22 (2) Optical reception front end, 23 (1), 23 (2) A / D conversion unit, 24 (1 ), 24 (2) Demodulator, 25 (1), 25 (2) Frame synchronizer, 26 (1), 26 (2) Error correction decoder, 27 OTUk framer receiver, 28 Inter-channel delay detector, 221 Polarization beam splitter, 222 Local oscillator, 223 Polarization beam splitter, 224 (1), 224 (2) 90 ° optical hybrid, 225 (1) to 225 (4) photoelectric converters, 226 (1) to 226 ( ) Amplifier.

Claims (6)

A transmission unit that arranges and transmits client signals at a plurality of wavelengths via a transmission line;
An optical transmission apparatus comprising: a receiving unit having a demodulating unit that performs demodulation processing on a received signal by performing wavelength separation on a receiving side;
The receiving unit detects a frame from the demodulated signal output from the demodulating unit and generates a frame pulse indicating the head of the frame, for each channel that is provided after the demodulating unit and is wavelength-separated. A frame synchronization unit;
By calculating a difference value of each frame pulse of each adjacent channel generated by the frame synchronization unit, time delay difference information between the adjacent channels is generated, and the generated time delay difference information is sent to the demodulation unit. An inter-channel delay detector for feedback, and
The demodulator performs demodulation processing on the signal after performing delay compensation on the received signal by moving the tap centroid of the digital filter based on the fed back time delay difference information. Transmission equipment. The optical transmission apparatus according to claim 1, wherein the transmission unit transmits the client signal by wavelength multiplexing. The adjacent channel is an adjacent mode of a multicore fiber or a multimode fiber,
The optical transmission device according to claim 1, wherein the reception unit performs the delay compensation for signals having different wavelengths or polarizations. The demodulator compensates for interference between adjacent channels by exchanging signals between the adjacent channels after performing the delay compensation on the received signal, and compensates for the interference between adjacent channels. The optical transmission device according to any one of claims 1 to 3, wherein the demodulation processing is performed on a signal of. A transmission unit that arranges and transmits client signals at a plurality of wavelengths via a transmission line;
A delay difference compensation method between adjacent channels applied to an optical transmission device including a receiving unit that performs demodulation processing on a received signal by performing wavelength separation on a receiving side,
In the receiving unit,
A frame synchronization step for detecting a frame from the demodulated signal for each wavelength-separated channel and generating a frame pulse indicating the head of the frame;
An inter-channel delay detection step of generating time delay difference information between the adjacent channels by calculating a difference value of the frame pulses of each of the adjacent channels generated in the frame synchronization step;
A demodulation processing step of performing delay compensation on the received signal by moving the tap centroid of the digital filter based on the time delay difference information, and performing the demodulation processing on the signal after delay compensation. A delay difference compensation method between adjacent channels applied to an optical transmission apparatus. In the receiving unit,
The demodulation processing step compensates for interference between adjacent channels by exchanging signals between the adjacent channels after performing the delay compensation on the received signal, and compensates for the interference between adjacent channels. The delay difference compensation method between adjacent channels applied to the optical transmission device according to claim 5, wherein the demodulation processing is performed on a later signal.

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