JP6550018B2 - Multicarrier optical receiver and optical transmission system - Google Patents

Description

  The present invention relates to a multicarrier optical receiver and an optical transmission system technology.

  With the rapid increase of traffic in data centers, etc., standardization of 100 GbE (Gigabit Ethernet) and (Ethernet is a registered trademark) and development of optical modules are advanced as large-capacity optical transmission systems. For example, the main stream of 100 GbE optical modules is composed of 4 waves x 25 [Gbit / s] IM-DD (Intensity Modulation-Direct Detection) transceivers, and CFP 4 (C form-factor pluggable) as an optical interface in a data center 4) Development of smaller power-saving modules such as QSFP 28 (Quad Small Form-factor Pluggable 28) is in progress.

  On the other hand, in order to connect data centers directly with 100 GbE optical interface, standards for long distance transmission are being considered. For example, the commercialization of small modules corresponding to LR4 which is a standard for 10 km transmission has started, and the commercialization of small modules corresponding to ER4 which is a standard for 40 km transmission is also planned. In order to cover the loss for 40km in the optical module compatible with ER4, improvement of the transmission and reception characteristics by improvement of the extinction ratio by the Mach-Zehnder type external modulator and improvement of the minimum light receiving sensitivity by APD is studied (Non-patent document 1) .

In addition to optical fiber loss and transmission / reception characteristics, factors that limit the transmission distance of the optical interface include optical signal waveform distortion due to wavelength dispersion of the optical fiber and polarization mode dispersion (PMD).
With regard to chromatic dispersion, when a G.652 optical fiber having a zero dispersion wavelength in the 1.3 μm band is used, the influence of the chromatic dispersion at the signal light wavelength of the existing 100 GbE 1.3 μm band is scarce.
On the other hand, PMD is a phenomenon in which individual components of an optical pulse incident on a fiber reach the output end at different times due to changes in refraction in the fiber. As a measurement value of the laying optical fiber, values such as PMD coefficient of 4.79 ps / √km (Non-patent document 2) and 0.94 ps / √km (Non-patent document 3) have been reported. These values do not become a problem at the time of optical signal transmission of 10 Gbit / s, but they become non-negligible values in the transmission of 25 Gbit / s optical signal with a narrower bit interval.

The resistance to PMD is affected by the modulation scheme (detection scheme) of the optical transmission system. The detection method is roughly classified into a "direct detection (DD) method" of directly measuring the intensity of the received optical signal and a "coherent detection" system of measuring the phase of the received optical signal.
First, as an example of direct detection, there is an IM-DD method in which a digital code “1” is turned on with an optical intensity “on”, and a digital code “0” is modulated into light intensity “off”. Although direct detection is easy to miniaturize because its mechanism is simple, the resistance to PMD is low because it directly receives signal components that have spread in time due to the effect of PMD.

On the other hand, in coherent detection, for example, there is a PSK (Phase-Shift Keying) method or the like in which the light intensity for each bit is constant, and QPSK (Quadrature Phase-Shift Keying) using four points in the signal space diagram among them is Wave multiplexed DP (Dual polarization) -QPSK is mainly used (Non-patent Document 4). A digital coherent system using DP-QPSK is capable of PMD compensation by digital signal processing technology for the electric field component of the received signal, and has high tolerance to PMD.
In addition, EVM (Error Vector Magnitude) with respect to a received signal is known as a parameter | index which evaluates the transmission characteristic including the optical signal waveform distortion by PMD or wavelength dispersion (nonpatent literature 5).

Mengyuan Huang, et al., “25 Gb / s Normal Incident Ge / Si Avalanche Photodiode,” ECOC 2014, We. 2.4.4, (2014). D. Breuer, et al., “Measurements of PMD in the installed fiber plant of Deutsche Telekom,” LEOS Summer Topical Meetings, MB 2.1, pp. 5-6, 2003. Toshiya Matsuda, et al., “PMD Design for High-Speed WDM Backbone Network Systems Based on Field PMD Measurements”, IEICE Trans. Commun., Vol. E94-B, no. Kazuro Kikuchi, “Phase-Diversity Detection of Multilevel Optical Modulation with Digital Carrier Phase Estimation,” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, NO. 4, JULY / AUGUST 2006. W. Freude, et al., "Quality Metrics for Optical Signals: Eye Diagram, Q-factor, OSNR, EVM and BER," Proc. ICTON 2012, Mo. B1.5, 2012.

In order to reduce the cost and power consumption of the optical transmission system, the optical module connecting the optical fibers is miniaturized and the power consumption is reduced, and the transmission characteristic deterioration due to the characteristics of the optical fiber is suppressed and the transmission distance is extended. Need to be compatible.
However, the IM-DD system is excellent in miniaturization, but is weak in transmission characteristic deterioration due to the characteristics of the optical fiber. On the other hand, the existing 100 G digital coherent system using a DP-QPSK signal is excellent in the resistance to transmission characteristic deterioration due to the characteristics of the optical fiber, but its configuration is complicated, so application to a small optical module is difficult. Thus, both methods have advantages and disadvantages.

  Therefore, the main object of the present invention is to provide a multicarrier optical receiver and an optical transmission system that balances the resistance to transmission characteristic deterioration due to the characteristics of the optical fiber and the miniaturization of the module in a well-balanced manner.

In order to solve the above problems, the multicarrier optical receiver of the present invention has the following features.
That is, the multicarrier optical receiver comprises an optical receiving module that receives two optical signals independently through the optical fiber in a direct detection system;
An A / D converter for converting the two optical signals received by the optical receiving module into digital signals,
A pseudo signal circuit that pseudo generates a digital signal of a coherent detection method from two digital signals converted by the A / D converter;
And a digital signal processor for adaptively equalizing the signal waveform of the digital signal of the pseudo coherent detection method generated by the pseudo signal circuit.
The pseudo signal circuit is
A delay circuit that adjusts the delay so that the centers of the pulses coincide with each other for the two digital signals output from the A / D converter;
The amplitude is converted by normalizing each of the two digital signals output from the delay circuit, and one of the two digital signals after the normalization is combined on a two-dimensional complex plane. And an amplitude conversion circuit for converting it into a digital signal of the pseudo coherent detection method.

  As a result, by connecting the module of the direct detection system, which is easy to miniaturize, and the digital signal processor that corrects the transmission characteristic deterioration due to the characteristics of the optical fiber, through the pseudo signal circuit, It is possible to balance the resistance to the deterioration of the transmission characteristics and the miniaturization of the module due to the above.

In the present invention, the digital signal processor is
An adaptive equalization circuit that performs signal waveform shaping by adaptive equalization processing on a digital signal of a coherent detection method generated pseudo by said pseudo signal circuit;
A phase correction circuit for correcting a phase rotation component with respect to an output signal of the adaptive equalization circuit;
And a discrimination circuit for demodulating a data signal sequence with respect to the output signal of the phase correction circuit.

  This makes it possible to increase the resistance to transmission characteristic deterioration due to the characteristics of the optical fiber, even for the pseudo-generated digital signal of the coherent detection method.

In the present invention, the optical receiving module receives an optical signal using two sets of IM-DD systems as a direct detection system;
It is characterized in that the amplitude conversion circuit synthesizes a QPSK signal as the digital signal of the pseudo coherent detection system.

  As a result, it is possible to balance the resistance to the deterioration of the transmission characteristics due to the characteristics of the optical fiber and the miniaturization of the module by utilizing the inexpensive and widely used light modulation module of intensity modulation.

In the present invention, the light receiving module receives an optical signal using two sets of M value amplitude modulation direct detection methods as the direct detection method;
The amplitude conversion circuit, as a digital signal of the pseudo-coherent detection method, characterized by combining the signals of the ^{M 2 -QAM (Quadrature Amplitude Modulation)} .

  As a result, it is possible to balance the resistance to the deterioration of the transmission characteristics due to the characteristics of the optical fiber and the miniaturization of the module by utilizing the light modulation module of amplitude modulation with a simple mechanism.

The present invention provides a multicarrier optical receiver as described above,
A multi-carrier optical transmitter for transmitting an optical signal to the multi-carrier optical receiver via the optical fiber;
The optical transmission module of the multicarrier optical transmitter may transmit an optical signal by a direct detection method corresponding to the optical reception module.

  As a result, it is possible to introduce an optical transmitter and an optical receiver as an optical transmission system in which the resistance to transmission characteristic deterioration due to the characteristics of the optical fiber and the miniaturization of the module are balanced in a well-balanced manner.

The present invention transmits n sets of optical signals from the multicarrier optical transmitter to the multicarrier optical receiver as one set of two optical signals.
The pseudo signal circuit performs processing of combining two digital signals into one digital signal of the pseudo coherent detection scheme, thereby simulating n digital signals from 2 n digital signals. To generate.

  As a result, even when the number of signal channels is large, it is possible to introduce an optical transmission system in which the resistance to transmission characteristic deterioration due to the characteristics of the optical fiber and the miniaturization of the module are well balanced.

  According to the present invention, it is possible to provide a multicarrier optical receiver and an optical transmission system that balances the resistance to transmission characteristic deterioration due to the characteristics of the optical fiber and the miniaturization of the module in a well-balanced manner.

FIG. 1A is an overall configuration diagram of an optical transmission system. FIG. 1B is a detailed configuration diagram of a part of the optical transmission system. It is explanatory drawing of the processing content of the delay circuit concerning this embodiment. It is explanatory drawing of the processing content of the amplitude conversion circuit concerning this embodiment. FIG. 4A shows an experimental environment for evaluation. FIG. 4 (b) is the evaluation result in FIG. 4 (a). FIG. 5 (a) is an experimental environment for evaluation. FIG.5 (b) is an evaluation result in Fig.5 (a).

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of an optical transmission system. FIG. 1 (a) is a general configuration diagram, and FIG. 1 (b) is a partial detailed configuration diagram.
The optical transmission system of FIG. 1A transmits a signal from the optical transmission module 11 of the multicarrier optical transmitter 1 to the optical receiving module 21 of the multicarrier optical receiver 2 via the optical fiber 9.
The multicarrier optical transmitter 1 and the multicarrier optical receiver 2 may support one-way communication or may support two-way communication. When supporting bi-directional communication, the multicarrier optical transmitter 1 has the optical receiving module 21 and the multicarrier optical receiver 2 has the optical transmitting module 11.

Here, in the optical transmission system of FIG. 1A, as a detection method between the light transmission module 11 and the light reception module 21, a direct detection method (of multicarrier) that receives two wavelength signals independently is used. , Module miniaturization and power saving can be realized.
The direct detection method to be employed may be, for example, two sets of intensity modulation direct detection (IM-DD) or two sets of M value amplitude modulation direct detection.

  The multicarrier optical receiver 2 includes an A / D (Analog-to-Digital) converter 22, a pseudo signal circuit 23, and a DSP (Digital Signal Processor: digital signal processor) 24 in addition to the above-described optical receiving module 21. Have. The A / D converter 22 converts the two O / E (Optical / Electronic) converted received signals into digital signals in the light receiving module 21.

The DSP 24 is a circuit that performs adaptive equalization such as PMD compensation for a signal of a coherent detection method such as a QPSK signal and signal identification. However, the light transmitting module 11 and the light receiving module 21 transmit signals of the direct detection method, and phase information of the light signal is lost. Therefore, if the digital signal with lost phase information of the optical signal is input to the DSP 24 as it is, the DSP 24 can not execute adaptive equalization.
Therefore, the pseudo signal circuit 23 generates a pseudo QPSK signal to be input to the DSP 24 at the rear stage based on the digital signal output from the A / D converter 22 at the front stage. As a result, unlike the normal digital coherent system, the phase information of the optical signal in the received signal is lost, so that it is not possible to completely compensate for the transmission characteristic deterioration due to the characteristics of the optical fiber such as wavelength dispersion and PMD. However, in a relatively short transmission distance region such as a metro network, it is possible to sufficiently enhance the resistance to transmission characteristic deterioration due to the characteristics of the optical fiber.

The pseudo signal circuit 23 has a delay circuit 23a and an amplitude conversion circuit 23b.
The delay circuit 23 a controls the delay of the digital signal output from the A / D converter 22.
FIG. 2 is an explanatory diagram of processing contents of the delay circuit 23a.
First, digital signals before delay adjustment will be described with reference to reference numerals 101 and 102.
At 101, directly detected optical signals are input to the respective circuits in the delay circuit 23a (with τ ₁ on the upper side and τ ₂ on the lower side in FIG. 1B). The horizontal axis of the light signal is the time axis, and the vertical axis is the light intensity.
At 102, the light intensities of the two waveforms at the reference time indicated by the dashed vertical line 101 are each shown as the horizontal axis (I axis) of the graph. In the upper graph, since the heights of the two waves are located at the upper limit and the lower limit at the reference time, the size of the I axis of the code 102 also ranges from the upper limit (I = + 1) to the lower limit (I = 0) (2 Indicated by one point).

Next, the delay-adjusted digital signal will be described with reference to reference numerals 111 and 112.
At 111, the upper graph remains unchanged from the state of 101, but by adjusting the lower graph a little in time (shifting it to the left), both the upper and lower graphs will peak at the upper limit and lower limit at the reference time. Adjusted to
The reference numeral 112 is a graph corresponding to the reference numeral 111, and the upper and lower graphs are both shown in the range of the upper limit (I = + 1) to the lower limit (I = 0) at the reference time.

As described above with reference to FIG. 2, the delay circuit 23 a adjusts the delay with respect to the output signal of the A / D converter 22 so that the center position of the bit matches. Since the delay adjustment processing of the delay circuit 23a is signal processing for two uncorrelated signals, it is sufficient if the pulse centers coincide with each other, and the maximum delay amount is half of one bit. Therefore, the circuit of the delay circuit 23a is simple and contributes to the miniaturization.
On the other hand, in the prior art such as the optical transmission system described in JP-A-2015-65516, it is necessary to adjust the delay in units of bits so that the bit patterns match, for signal processing on two correlated signals. The delay amount is the delay difference between the transmission paths. Therefore, the delay adjustment process in the prior art becomes complicated, and miniaturization is difficult.

FIG. 3 is an explanatory diagram of processing contents of the amplitude conversion circuit 23b.
Reference numeral 121 is the same graph as reference numeral 112 in FIG. That is, the output signal of the delay circuit 23a becomes the input signal of the amplitude conversion circuit 23b.
The code | symbol 122 normalizes the signal which the amplitude conversion circuit 23b shows with the code | symbol 121. FIG. The lower limit (the point located on the left) of the horizontal axis (I axis) extends from I = 0 to I = -1. Hereinafter, for two signals of reference numeral 122, the signal shown in the upper graph is taken as a signal I ₁ (t), and the signal shown in the lower graph is taken as a signal I ₂ (t).

A signal space diagram 123 (signal point arrangement on a two-dimensional complex plane) 123 shows the result of combining the two signals 122 with the amplitude conversion circuit 23b. Reference numeral 123 is a signal space diagram composed of a horizontal axis (I axis) and a vertical axis (Q axis), and shows a polarization multiplexed QPSK pseudo signal using four points on the circumference. For example, a point on the first quadrant (both the I axis and the Q axis are positive) is associated with the digital value "11".
The amplitude conversion circuit 23 _b generates the pseudo signal Ex (t) based on the signal of the code 122 according to the formula “Ex (t) = I ₁ (t) + jI ₂ (t)”.

  Referring back to FIG. 1 (b), the DSP 24 comprises an adaptive equalization circuit 24a, a phase correction circuit 24b, and an identification circuit 24c. As described above, the DSP 24 is a circuit that performs adaptive equalization for a QPSK signal, but the pseudo signal Ex (t) output from the amplitude conversion circuit 23b also performs adaptive equalization in the same manner as the QPSK signal. be able to. As a circuit for performing adaptive equalization for a QPSK signal, for example, one described in Japanese Patent Laid-Open No. 2015-65516 may be used.

The adaptive equalization circuit 24 a corrects the waveform of the pseudo signal Ex (t) output from the amplitude conversion circuit 23 b (performs signal waveform shaping). Specifically, from the complex signal input Ex (t), the adaptive equalization circuit 24 a performs phase correction circuit 24 b on complex signal EX (t) that has been subjected to adaptive signal processing of FIR (Finite Impulse Response) filter by maximum likelihood estimation. Output to In addition, the realization algorithm of maximum likelihood estimation may use arbitrary algorithms, such as CMA (Constant Modulus Algorithm) and DD-LMS (Decision Directed Least Mean Square) described in JP-A-2015-65516, for example. .
Non-Patent Document 4 describes that the tap coefficient h ₁₁ of the FIR filter is updated using (Expression 2) with respect to the output (Expression 1) of the adaptive equalization circuit 24 a. That is, the "the Tap update" of the adaptive equalization circuit 24a of FIG. 1 (b), a mechanism for updating the tap coefficients h ₁₁ of the FIR filter.

Here, μ is a step size parameter. Unlike normal coherent reception, signal rotation does not occur due to frequency difference or phase difference between signal light and local light. However, since the phase rotation component associated with the adaptive equalization circuit 24a is generated, the phase correction circuit 24b corrects the phase rotation component.
Then, the discrimination circuit 24c demodulates the data signal sequence based on the complex signal output from the phase correction circuit 24b, and outputs it to the subsequent stage (another device).

  As described above, in the optical transmission system described with reference to FIGS. 1 to 3, an example in which two wavelength signals are independently received as the multicarrier direct detection method has been described. Here, since each wavelength signal is independent, by repeating the same signal processing as (Expression 1) and (Expression 2) of the adaptive equalization circuit 24a, not only two wavelength signals but also 2n (where n is The present invention can be extended to multicarrier signals of 2 or more integer waves. That is, the pseudo signal circuit 23 is expanded to generate n sets of pseudo QPSK signals from the 2 n digital signals output from the A / D converter 22.

Further, the pseudo signal generated by the pseudo signal circuit 23 is not limited to the QPSK signal, and may be an M ² QAM (Quadrature Amplitude Modulation) signal in which points are arranged in a grid shape in the signal space diagram. For example, a 4-QAM signal of M = 2 has one point in each quadrant and a total of four points, resulting in the same signal space diagram as the QPSK signal.

FIG. 4 shows the result of evaluating the EVM (Error Vector Magnitude) of the optical transmission system.
FIG. 4 (a) shows an experimental environment for evaluation, in which the optical transmitter module 11 transmits 4 x 25.8 Gbit / s NRZ signals arranged at 50 GHz intervals in the range of 1551.72 nm to 1552.92 nm. The multiplexer is configured to input the transmission signals from the two transmitters (Tx) into one optical fiber 9 at a signal light power of 0 dBm / channel. As this transmission path condition, a G. 652 optical fiber with a zero dispersion wavelength of 1310 nm is used, and the optical loss value in the optical signal band is 0.3 dB / km. Then, in the light receiving module 21, the demultiplexer outputs the signal transmitted through the optical fiber 9 to four receivers (25G Rx).

In the graph of FIG. 4B, “conventional IM-DD” in which the PMD compensation is not performed on the reception signal output from the A / D converter 22 (that is, the pseudo signal circuit 23 and the DSP 24 are not provided); It is a graph which compares EVM by "this invention" which compensates (that is, the pseudo signal circuit 23 and DSP24 are provided like Fig.4 (a)).
The “conventional IM-DD” and the “present invention” share the point of using the IM-DD as a modulation / demodulation system of the light transmission module 11 → the light reception module 21. Therefore, the modules can be easily miniaturized.
Furthermore, as shown in the graph of FIG. 4B, the “invention” has a good transmission characteristic because the value of EVM is low not only for short distances but also for medium distances (̃20 km). That is, the transmission distance satisfying the EVM 25% or less corresponding to the bit error rate 1E-4 correctable by Reed-Solomon [255, 239] which is the existing FEC is 15% in "conventional IM-DD", "invention It is extended to 20 km by using

FIG. 5A shows a configuration in which PC 31 and DGD (Differential Group Delay) 32 are added to the transmission path of the optical fiber 9 in the experimental environment of FIG. 4A.
The PC 31 is a polarization controller that adjusts the received signal to a polarization state that maximizes waveform degradation due to the DGD 32. The DGD 32 is a DGD emulator that is provided at the end of the transmission path and simulates PMD by giving DGD = 3 · PMD [ps] corresponding to the risk factor 1E-4.
In FIG. 5 (a), a 4 x 25.8 Gbit / s NRZ signal wavelength-arranged in the range of 1295.56 nm to 1309.14 nm, which is a specification of 100 GbE, is used as the transmission signal, and the transmission path is 0 dBm / channel. Enter in As transmission path conditions, a G.652 optical fiber with a zero dispersion wavelength of 1310 nm is used, the PMD coefficient is 1 ps / √km, and the optical loss value in the optical signal band is 0.4 dB / km.

  The graph of FIG. 5 (b) shows EVM with respect to the transmission distance, as in FIG. 4 (b). In "conventional IM-DD", the transmission distance satisfying EVM 25% or less is limited to about 27 km due to the influence of DGD, but DGD is compensated even in 40 km by using "present invention" I understand that.

As described above, in the optical transmission system according to the present embodiment, the light transmission module 11 and the light reception module 21 in the front stage use a direct detection method such as IM-DD widely spread as an inexpensive transmission / reception method. While applying to an optical module, signal waveform distortion is compensated for by the maximum likelihood approximation of the adaptive equalization circuit 24 a for the DSP 24 in the latter stage. Then, in order to input the signal of direct detection to the PMD compensation (adaptive equalization of signal waveform) mechanism of coherent detection, a pseudo signal circuit 23 is newly prepared. As a result, it is possible to provide an optical transmission system in which the extension of the transmission distance by the improvement of the resistance to the deterioration of the transmission characteristics due to the characteristics of the optical fiber and the miniaturization and power saving of the module can be well balanced.
Further, in addition to the fact that inexpensive optical modules can be used, it is also expected to reduce the development cost of the digital signal processing part by diverting existing ones to the mounting contents and design contents of the DSP 24.

1 multi-carrier optical transmitter 2 multi-carrier optical receiver 9 optical fiber 11 optical transmitter module 21 optical receiving module 22 A / D converter 23 pseudo signal circuit 23a delay circuit 23b amplitude conversion circuit 24 DSP (digital signal processor)
24a adaptive equalization circuit 24b phase correction circuit 24c identification circuit

Claims (6)

An optical receiving module that receives two optical signals independently via a direct detection method via an optical fiber;
An A / D converter for converting the two optical signals received by the optical receiving module into digital signals,
A pseudo signal circuit that pseudo generates a digital signal of a coherent detection method from two digital signals converted by the A / D converter;
And a digital signal processor for adaptively equalizing the signal waveform of the digital signal of the pseudo coherent detection method generated by the pseudo signal circuit.
The pseudo signal circuit is
A delay circuit that adjusts the delay so that the centers of the pulses coincide with each other for the two digital signals output from the A / D converter;
The amplitude is converted by normalizing each of the two digital signals output from the delay circuit, and one of the two digital signals after the normalization is combined on a two-dimensional complex plane. What is claimed is: 1. A multi-carrier optical receiver comprising: an amplitude conversion circuit that converts a digital signal of the pseudo coherent detection method. The digital signal processor is
An adaptive equalization circuit that performs signal waveform shaping by adaptive equalization processing on a digital signal of a coherent detection method generated pseudo by said pseudo signal circuit;
A phase correction circuit for correcting a phase rotation component with respect to an output signal of the adaptive equalization circuit;
The multicarrier optical receiver according to claim 1, further comprising: an identification circuit that demodulates a data signal sequence with respect to an output signal of the phase correction circuit. The optical receiving module receives an optical signal using two sets of IM-DD (Intensity Modulation-Direct Detection) systems as a direct detection system,
The multicarrier optical reception according to claim 1 or 2, wherein the amplitude conversion circuit combines the signal of QPSK (Quadrature Phase-Shift Keying) as the digital signal of the pseudo coherent detection system. Machine. The light receiving module receives an optical signal using two sets of M value amplitude modulation direct detection methods as the direct detection method,
The amplitude conversion circuit, a digital signal of the pseudo-coherent detection ^{scheme, M 2 -QAM (Quadrature Amplitude Modulation} ) multicarrier light according to claim 1 or claim 2, wherein the combining the signals Receiving machine. A multicarrier optical receiver according to claim 1 or 2;
A multi-carrier optical transmitter for transmitting an optical signal to the multi-carrier optical receiver via the optical fiber;
An optical transmission system characterized in that an optical transmission module of the multicarrier optical transmitter transmits an optical signal by a direct detection method corresponding to the optical reception module. The multicarrier optical transmitter transmits n sets of optical signals as one set of two optical signals to the multicarrier optical receiver,
The pseudo signal circuit performs a process of combining two digital signals into one digital signal of the pseudo coherent detection scheme, thereby simulating n digital signals from 2 n digital signals by performing n sets of processing. The optical transmission system according to claim 5, characterized in that:

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