JP5943891B2 - Optical transmission system - Google Patents

Description

  The present invention transmits the same optical signal from a redundant optical signal transmitter via different optical transmission lines, and collects and receives them at a redundant optical signal receiver, compensates for the transmission delay difference of the redundant optical signal, phase synchronization and waveform equalization The present invention relates to an optical transmission system that performs signal synthesis by performing.

  In an ultra-high-speed transmission system with a transmission rate per wavelength of 100 Gbit / s or more, a digital coherent technology combining a coherent optical communication technology and a digital signal processing technology has been widely used. The DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) method, which is standard as a modulation / demodulation method in 100 Gbit / s long-distance optical transmission systems, multiplexes 32 Gbit / s signals by using four-level phase modulation. Thus, a coherent optical signal is generated and further multiplexed by using two polarizations to generate a 128 Gbit / s coherent optical signal. On the receiving side, the signal coherently detected using local light having the same wavelength as that of the signal light is digitized using an A / D converter, and then clock extraction, frequency offset compensation, and transmission path are performed by digital signal processing using a DSP. Excellent transmission characteristics are realized by performing chromatic dispersion compensation, polarization dispersion compensation, separation of polarization signals, and the like.

  In addition, as a network using 100 Gbit / s signals, an optical signal output from an optical transmitter collides with other wavelengths in an arbitrary path (Directionless) at an arbitrary wavelength (Colorless) in one wavelength unit. A CDC-ROADM network using an optical cross-connect (XC) device that realizes a CDC function that can be switched without (Contentionless) has been studied (Non-Patent Document 1). In the CDC-ROADM network, in addition to the conventional protection using two paths, the optical signal is switched to the third backup route using the CDC function to ensure the reliability of the network even in the event of a catastrophic disaster. It becomes possible.

G. Prasanna, BS Kishore, GK Omprasad, KS Raju, R. Gowrishankar, K. Venkataramaniah, R. Johnson, and P. Voruganti, “Versatility of a colorless and directionless WSS based ROADM architecture,” Communication Systems and Networks and Workshops, Page (s): 1-8, Jan, 2009. H. Y. Choi, T. Tsuritani, and I. Morita, “BER-adaptive flexible format transmitter for elastic optical networks,” Optics Express, vol.20, no.17, pp.18652-18658, 2012.

  As a further large-capacity transmission system, combined use of multi-value modulation signals and multiplexing using a plurality of wavelengths such as 16QAM (Quadratuere Amplitude Modulation) has been studied. However, since the reception sensitivity is lowered due to the multi-level modulation signal, the transmission distance is limited (Non-Patent Document 2).

  Further, in the case of switching using the CDC function, it takes about several seconds to physically switch the optical switch constituting the optical XC apparatus, and the optical signal is interrupted during that time.

  The present invention roughly adjusts a transmission delay difference due to a difference in optical transmission path lengths of a plurality of optical signals transmitted by phase modulation with the same bit pattern on a transmission side, and uses a digital signal processing on a reception side to receive a received signal. An object of the present invention is to provide an optical transmission system capable of synthesizing signals by performing phase synchronization and waveform equalization.

  The present invention transmits the same optical signals 1 to N (N is an integer of 2 or more) transmitted from a redundant optical signal transmitter through optical transmission lines each having a predetermined transmission delay, and a redundant optical signal receiver. In the optical transmission system in which signals are received at once and signal synthesis is performed, when the transmission delay of the optical signal k (k is an integer of 1 to N) is the largest among the optical signals 1 to N, the redundant optical signal receiver The optical signals 1 to N transmitted through the transmission path are coherently detected, and further output from N optical signal receivers that output a digital signal after analog-digital conversion and N optical signal receivers, respectively. And a digital signal processing device that synthesizes and outputs a signal that has been subjected to phase synchronization and adaptive equalization processing of another digital signal based on the digital signal corresponding to the optical signal k. The optical signal transmitter A transmission delay compensator that compensates for the transmission delay of signals 1 to N is provided, the digital signal processing device monitors parameters used for adaptive equalization processing of optical signals 1 to N, and is used for adaptive equalization processing of optical signal k. The transmission delay difference with other optical signals is estimated based on the parameters, and the transmission delay difference is set in the transmission delay compensator corresponding to the optical signals 1 to N, and the transmission delay difference between the optical signals 1 to N is roughly calculated. Control means for adjusting is provided.

  In the optical transmission system of the present invention, the control means is used for adaptive equalization processing of the optical signals 1 to N while sequentially increasing the delay amount of the transmission delay compensator other than the transmission delay compensator corresponding to the optical signal k. In this configuration, the parameter is monitored, and the transmission delay difference with other optical signals is estimated based on the parameters used in the adaptive equalization processing of the optical signal k.

  In the optical transmission system of the present invention, the control means transmits the redundant optical signal from the redundant optical signal receiver to the parameters used for the adaptive equalization processing of the optical signals 1 to N by the digital signal processing apparatus via the predetermined transmission means. It is the structure which carries out feedback transmission to the machine.

  In the optical transmission system of the present invention, the parameter used for the adaptive equalization processing of the optical signals 1 to N in the digital signal processing apparatus is the tap coefficient of the adaptive equalizer, and the control means is the adaptive equalization of the optical signal k. A transmission delay difference between signals is estimated based on a peak difference of a distribution made up of absolute values of elements of tap coefficients used for the process and adaptive equalization processing of other optical signals.

  The present invention monitors transmission delay differences due to differences in optical transmission path lengths of a plurality of optical signals transmitted by phase modulation with the same bit pattern while monitoring parameters of adaptive equalization processing in digital signal processing on the receiving side. Coarse adjustment can be performed on the transmission side, and signal synthesis can be performed by performing phase synchronization and adaptive equalization processing between reception signals using digital signal processing on the reception side. As a result, transmission quality can be improved to facilitate large-capacity transmission, and optical signal disconnection when an optical network failure occurs can be prevented to improve stability.

It is a figure which shows the structure of Example 1 of the optical transmission system of this invention. It is a figure which shows the structural example of the adaptive equalizer 23-i of Example 1. FIG. It is a figure which shows the structural example of the digital signal processing apparatus 22 in Example 2 of the optical transmission system of this invention. It is a figure which shows the structural example of the adaptive equalizer 23x-i of Example 2. FIG. It is a figure which shows the structural example of the adaptive equalizer 23y-i of Example 2. FIG. It is a figure which shows the tap coefficient (before transmission delay compensation) at the time of digital signal processing of DP-16QAM signal. It is a figure which shows the tap coefficient (after transmission delay compensation) at the time of the digital signal processing of DP-16QAM signal. It is a figure which shows the constellation map after the digital signal processing of DP-16QAM signal.

FIG. 1 shows a configuration of a first embodiment of an optical transmission system according to the present invention.
In FIG. 1, the redundant optical signal transmitter 10 inputs the same electrical signal to the optical signal transmitters 11-1 to 11-N via the transmission delay compensators 12-1 to 12-N, respectively. Signals 1 to N (N is an integer of 2 or more) are converted and output in parallel to the optical transmission line. Transmission delay compensators 12-1 to 12 -N installed in front of the optical signal transmitters 11-1 to 11 -N are controlled using the controller 13.

  The optical transmission path for transmitting the same optical signals 1 to N is, for example, a configuration using N single-core optical fibers, or L × K (= N) parallel using K multi-core fibers with L cores. It may be configured to perform transmission.

  The redundant optical signal receiver 20 coherently detects the optical signals 1 to N transmitted through the optical transmission path, and performs AD conversion to output a digital signal, respectively. The digital signal processing device 22 is configured to receive digital signals output from the optical signal receivers 21-1 to 21-N and perform signal synchronization by performing phase synchronization and adaptive equalization processing.

The digital signal processing device 22 includes adaptive equalizers 23-1 to 23 -N, phase synchronizers 24-1 to 24 -N, and a signal synthesizer 25. The adaptive equalizers 23-1 to 23-N respectively receive the nth digital signals x _{in, 1} (n) to x _{in, N} (n) output from the optical signal receivers 21-1 to 21-N. The adaptive equalization signals x _{out, 1} (n) to x _{out, N} (n) are output to the phase synchronizers 24-1 to 24-N. The phase synchronizers 24-1 to 24-N receive the adaptive equalization signals x _{out, 1} (n) to x _{out, N} (n) output from the adaptive equalizers 23-1 to 23-N, respectively. , Phase differences φ ₁ (n) to φ _N (n) and phase synchronization signals x ′ _{out, 1} (n) to x ′ _{out, N} (n) are output. The signal synthesizer 25 inputs the phase synchronization signals x ′ _{out, 1} (n) to x ′ _{out, N} (n) output from the phase synchronizers 24-1 to 24-N, and equal gain synthesis or maximum ratio The combined signal X _out (n) is output. Further, the phase differences φ ₁ (n) to φ _N (n) output from the phase synchronizers 24-1 to 24-N and the phase synchronization signals x ′ _{out, 1} (n) to x ′ _{out, N} (n) And a phase synchronization signal x ′ _{out, k} (n) output from an arbitrary phase synchronizer 24-k (k = 1 to N) as a reference is an adaptive equalizer 23-1 to 23-. N is fed back to each.

In the phase synchronizer 24-i (i is an integer of 1 to N), a component out of phase synchronization caused by a frequency offset at the time of coherent detection in the optical signal receiver 21-i is estimated as the phase difference φ _i (n). And output. Further, using the estimated phase difference φ _i (n), the phase difference φ _i (n) is corrected from the adaptive equalization signal x _{out, i} (n), and the phase synchronization signal x ′ _{out, i} (n) = x _{out, i} (n) exp (−jφ _i (n)) is output (j is an imaginary unit). Then, the phase difference φ _i (n) and the phase synchronization signal x ′ _{out, i} (n) output from the phase synchronizer 24-i are input to the adaptive equalizer 23-i, and an arbitrary phase synchronizer 24 By inputting the phase synchronization signal x ′ _{out, k} (n) output from −k to the adaptive equalizer 23-i, phase synchronization and adaptive equalization processing are performed. The configuration and operation of the adaptive equalizer 23-i will be described later.

The value of the tap coefficient h _i of the adaptive equalizer 23-i is fed back to the controller 13 of the redundant optical signal transmitter 10. Here, the value of the tap coefficient is fed back by any means such as via a monitoring control network or a counter device.

The controller 13 of the redundant optical signal transmitter 10 has a transmission delay compensator 12 other than the transmission delay compensator 12-k corresponding to the kth optical transmission line k having the longest transmission delay, for example, the longest optical transmission line length. Increase the delay amount of -i sequentially. At the same time, the absolute value of the tap coefficient h _i (= [h _i (1), h _i (2),..., H _i (M)]) of the fed back adaptive equalizer 23-i is calculated, Monitor. M is an integer of 2 or more. The controller 13 increases the delay amount of the transmission delay compensator 12-i until the transmission delay difference can be compensated with the M tap coefficients of the adaptive equalizer 23-i, and then has the largest transmission delay. Distribution _hk (n) | consisting of absolute values of tap coefficient h _k and tap coefficient h _i with reference to tap coefficient h _k for waveform equalization of the signal transmitted through k-th optical transmission line k And | h _i (n) | are used to estimate the transmission delay difference between signals. n is an integer of 1 to M. The controller 13 compensates the transmission delay difference generated between the redundant optical signals by setting the delay amount corresponding to the estimated transmission delay difference in the transmission delay compensator 12-i.

The controller 13 includes a processing unit for estimating the transmission delay difference between the signals from the tap coefficient h _k and the tap coefficient h _i in the redundant optical signal receiver 20, and information on the transmission delay difference is stored in the redundant optical signal transmitter 10. The transmission delay difference generated between the redundant optical signals may be compensated by feeding back to the controller 13 and setting a delay amount corresponding to the transmission delay difference in each of the transmission delay compensators 12-i.

Further, in the configuration of the digital signal processing device 22 shown in FIG. 1, in order to compensate for signal degradation caused by chromatic dispersion occurring in the optical transmission line, chromatic dispersion compensation is provided before the adaptive equalizers 23-1 to 23-N. the vessel was placed, a digital signal x _{in, 1} (n) ~x _{in the} digital signal x and _n (n) is the chromatic dispersion compensation _{'in, 1 (n) ~x} ' in, converted to _n (n) It is good also as a structure which each inputs into the adaptive equalizers 23-1 to 23-N. Further, an arbitrary compensator such as compensation for waveform distortion due to a nonlinear optical effect can be incorporated into the digital signal processing device 22.

FIG. 2 shows a configuration example of the adaptive equalizer 23-i according to the first embodiment (i = 1 to N).
In FIG. 2, the adaptive equalizer 23-i includes (M−1) delay units 31 and M tap coefficients h _i (= [h _i (1), h _i (2),. _i (M)]) 32, an adder 33, a tap coefficient calculation unit 34, a convergence determination unit 35, and a reference signal generation unit 36.

The adder 33 includes the digital signal x _{in, i} (n) and the digital signals x _{in, i} (n−1),..., X _{in, i} (n−M + 1) delayed by the M−1 delay units 31. to) by adding the digital signals obtained by multiplying each of M tap coefficients h _i 32, the adaptive equalization signal _{x out, i (n) =} h i x T in, i (n) (x T in, i ( n) is x _{in, i} (n) transpose matrix)
Is output.

The reference signal generator 36 uses the phase synchronization signal x ′ _{out, k} (n) output from the arbitrary arbitrary phase synchronizer 24-k as a reference signal d _k according to a predetermined tap coefficient update algorithm. Output (n).

The tap coefficient calculation unit 34 includes the adaptive equalization signal x _{out, i} (n) output from the adder 33, the phase difference φ _i (n) output from the phase synchronizer 24-i, and the phase synchronization signal x ′. _The tap coefficient h _i is updated using _{out, i} (n) and the reference signal d _k (n) output from the reference signal generator 36, and the error signal ε _i (n) is output.

The convergence determination unit 35 performs convergence determination using the error signal ε _i (n) output from the tap coefficient calculation unit 34, and after convergence, the tap coefficient calculation unit 34 outputs a reference output from the reference signal generation unit 36. The tap coefficient h _i is updated using the signal d _k (n).

  A tap coefficient updating method will be described by taking a QPSK signal as an example. In the tap coefficient calculation unit 34, the tap coefficient before convergence can be updated by the following equation using, for example, a CMA (Constant Modulus Algorithm) which is a blind algorithm.

μはステップサイズパラメータ、ｘ^* _in,i(n)はｘ_in,i(n) の複素共役を示す。

μ represents a step size parameter, and x ^* _{in, i} (n) represents a complex conjugate of x _{in, i} (n).

  After convergence, for example, by using a decision-directed least mean square (DD-LMS) algorithm, which is a decision feedback algorithm, the tap coefficients are updated by the following equation to synchronize the phase between signals and perform waveform equalization. Can do.

At this time, the reference signal d _k (n) is generated by the reference signal generation unit 36 using the phase synchronization signal x ′ _{out, k} (n) output from the arbitrary phase synchronizer 24-k. In the case of a QPSK signal, the reference signal d _k (n) is determined by the following equation.

Further, after convergence, the phase of the adaptive equalization signal x _{out, 1} (n) to x _{out, N} (n) is synchronized by updating the tap coefficient by the following formula using CMA which is a blind algorithm. Waveform equalization can also be performed.

At this time, since the phase synchronization signal x ′ _{out, k} (n) is used as the reference signal d _k (n), the reference signal generator 36 uses the phase synchronization signal x ′ output from the arbitrary phase synchronizer 24-k. _{out, k} (n) is output as is.

  As described above, the case of the QPSK signal has been described as an example of updating the tap coefficient. However, the digital signal processing apparatus of the present invention can be applied to a signal modulated by an arbitrary modulation method. Further, although CMA is used before convergence and DD-LMS algorithm or CMA is used after convergence, any adaptive equalization algorithm can be used.

  FIG. 3 shows a configuration example of the digital signal processing apparatus 22 in the second embodiment of the optical transmission system of the present invention.

  The redundant optical signal transmitter (not shown) in the optical transmission system of this embodiment outputs the same optical signal modulated and polarization multiplexed by 256 Gbit / s DP-16QAM system to the optical transmission lines 1, 2, and 3. To do. Each optical transmission line is composed of five sections with one section between the optical amplifiers, and the optical fiber lengths of one section in the optical transmission paths 1, 2, and 3 are 80 km, 90 km, and 100 km, respectively. In relation to the first embodiment, a configuration example corresponding to the case where N = 3 and k = 3 is shown.

The optical signal receivers 21-1 to 21-3 of the redundant optical signal receivers separate the polarization-multiplexed optical signals via the optical transmission lines 1 to 3 into x-polarized waves and y-polarized waves, respectively. Coherent detection, AD conversion, and the n-th x-polarized digital signal x _{in, 1} (n) to x _{in, 3} (n) and y-polarized digital signal y _{in, 1} (n) to y _{in , 3} (n) is output. An x-polarized signal is indicated by a solid line, and a y-polarized signal is indicated by a broken line. The digital signal processing device 22 includes n-th x-polarized digital signals x _{in, 1} (n) to x _{in, 3} (n) output from the optical signal receivers 21-1 to 21-3, and y-polarization. Wave digital signals y _{in, 1} (n) to y _{in, 3} (n) are input, and the signals are synthesized by performing phase synchronization and adaptive equalization processing. In FIG. 3, the notation (n) indicating the nth digital signal is omitted.

  The digital signal processing device 22 includes chromatic dispersion compensators 26x-1 to 26x-3 corresponding to x polarization, adaptive equalizers 23x-1 to 23x-3, phase synchronizers 24x-1 to 24x-3, signal synthesis 25x, chromatic dispersion compensators 26y-1 to 26y-3 corresponding to y polarization, adaptive equalizers 23y-1 to 23y-3, phase synchronizers 24y-1 to 24y-3, and signal synthesizer 25y. Composed.

The chromatic dispersion compensators 26x-1 to 26x-3 corresponding to the x polarization input the digital signal xin _{, 1} (n) to xin _{, 3} (n) of the nth x polarization, and the chromatic dispersion is obtained. Compensated chromatic dispersion compensation signals x ′ _{in, 1} (n) to x ′ _{in, 3} (n) are output. The chromatic dispersion compensators 26y-1 to 26y-3 corresponding to the y polarization input digital signals y _{in, 1} (n) to y _{in, 3} (n) of the nth y polarization _{, and} perform chromatic dispersion. Compensated chromatic dispersion compensation signals y ′ _{in, 1} (n) to y ′ _{in, 3} (n) are output.

The adaptive equalizers 23x-1 to 23x-3 corresponding to x-polarizations are chromatic dispersion compensation signals x'in _{, 1} (n) to x'in _{, 3} (n) and y'in _{, 1} (n) to y ′ _{in, 3} (n) is input, and adaptive equalization signals x _{out, 1} (n) to x _{out, 3} (n) are output. The phase synchronizers 24x-1 to 24x-3 receive the adaptive equalization signals xout _{, 1} (n) to xout _{, 3} (n), respectively, and the phase differences φ _x1 (n) to φ _x3 (n) and The phase synchronization signals x ′ _{out, 1} (n) to x ′ _{out, 3} (n) are output. The signal synthesizer 25x receives the phase synchronization signals x ′ _{out, 1} (n) to x ′ _{out, 3} (n), and outputs the synthesized signal X _out (n) by performing equal gain synthesis or maximum ratio synthesis. Further, the phase differences φ _x1 (n) to φ _x3 (n) output from the phase synchronizers 24x-1 to 24x-3 and the phase synchronization signals x ′ _{out, 1} (n) to x ′ _{out, 3} (n) The phase synchronization signal x ′ _{out, 3} (n) output from the reference phase synchronizer 24-3 is fed back to the adaptive equalizers 23x-1 to 23x-3, respectively.

The adaptive equalizers 23y-1 to 23y-3 corresponding to the y-polarizations are chromatic dispersion compensation signals x'in _{, 1} (n) to x'in _{, 3} (n) and y'in _{, 1} (n) to y ′ _{in, 3} (n) is input, and adaptive equalization signals y _{out, 1} (n) to y _{out, 3} (n) are output. The phase synchronizers 24y-1 to 24y-3 receive the adaptive equalization signals y _{out, 1} (n) to y _{out, 3} (n), respectively, and the phase differences φ _y1 (n) to φ _y3 (n) and The phase synchronization signal y ′ _{out, 1} (n) to y ′ _{out, 3} (n) is output. The signal synthesizer 25y receives the phase synchronization signals y ′ _{out, 1} (n) to y ′ _{out, 3} (n) _{, and} performs equal gain synthesis or maximum ratio synthesis to output a synthesized signal Y _out (n). Further, the phase differences φ _y1 (n) to φ _y3 (n) and the phase synchronization signals y ′ _{out, 1} (n) to y ′ _{out, 3} (n) output from the phase synchronizers 24y-1 to 24y-3. The phase synchronization signal y ′ _{out, 3} (n) output from the reference phase synchronizer 24-3 is fed back to the adaptive equalizers 23y-1 to 23y-3, respectively.

  As described above, in the digital signal processing device 22 according to the second embodiment, adaptive equalizers 23x-1 to 23x-3 and 23y-1 are used to compensate for signal degradation caused by chromatic dispersion occurring in the optical transmission lines 1 to 3. To 23y-3, the chromatic dispersion compensators 26x-1 to 26x-3 and 26y-1 to 26y-3 are arranged, but the other phase synchronizers 24x-1 to 24x-3 and 24y-1 are arranged. To 24y-3 and the signal synthesizers 25x and 25y have the same functions as those in the first embodiment.

FIG. 4 illustrates a configuration example of the adaptive equalizer 23x-i according to the second embodiment (i = 1, 2, 3).
In FIG. 4, the adaptive equalizer 23x-i includes 2 (M-1) delay units 31 and M tap coefficients h _{xx, i} (= [h _{xx, i} (1), h _{xx, i} (2), ..., h _{xx, i} (M)]) and M tap coefficients h _{xy, i} (= [h _{xy, i} (1), h _{xy, i} (2), ..., h _{xy, i} (M)]) 32, an adder 33, a tap coefficient calculation unit 34, a convergence determination unit 35, and a reference signal generation unit 36.

The adder 33 receives the chromatic dispersion compensation signals x ′ _{in, i} (n) and y ′ _{in, i} (n) and the chromatic dispersion compensation signals x ′ _{in, i} (delayed by M−1 delay units 31). n-1), ..., x'in _{, i} (n-M + 1) and y'in _{, i} (n-1), ..., y'in _{, i} (n-M + 1), respectively. The chromatic dispersion compensation signals multiplied by the tap coefficients h _{xx, i} and h _{xy, i} 32 respectively are added, and the adaptive equalization signal x _{out, i} (n) = h _{xx, i} x ′ ^T _{in, i} (n) + H _{xy, i} y ' ^T _{in, i} (n)
Is output.

The reference signal generation unit 36 uses the phase synchronization signal x ′ _{out, 3} (n) as a reference output from the phase synchronizer 24-3 corresponding to the optical transmission line with the longest transmission delay, and uses the tap coefficient described above. A reference signal d ₃ (n) corresponding to the update algorithm is output.

The tap coefficient calculation unit 34 includes the adaptive equalization signal x _{out, i} (n) output from the adder 33, the phase difference φ _xi (n) output from the phase synchronizer 24-i, and the phase synchronization signal x ′. _The tap coefficients h _{xx, i} and h _{xy, i} are updated using _{out, i} (n) and the reference signal d _x3 (n) output from the reference signal generator 36 _, and the error signal ε _xi (n) is updated. Output.

The convergence determination unit 35 performs convergence determination using the error signal ε _xi (n) output from the tap coefficient calculation unit 34, and the reference is output from the reference signal generation unit 36 in the tap coefficient calculation unit 34 after the convergence. The tap coefficients h _{xx, i} and h _{xy, i} are updated using the signal d _xk (n).

FIG. 5 shows a configuration example of the adaptive equalizer 23y-i according to the second embodiment (i = 1, 2, 3).
In FIG. 5, the adaptive equalizer 23y-i includes 2 (M-1) delay units 31 and M tap coefficients h _{yx, i} (= [h _{yx, i} (1), h _{yx, i} (2), ..., h _{yx, i} (M)]) and M tap coefficients h _{yy, i} (= [h _{yy, i} (1), h _{yy, i} (2), ..., h _{yy, i} (M)]) 32, an adder 33, a tap coefficient calculation unit 34, a convergence determination unit 35, and a reference signal generation unit 36. Each unit has the same function as each unit of the adaptive equalizer 23x-i illustrated in FIG.

Tap coefficients h _{xx, i} , h _{xy, i} , h _{yx, i} , obtained by the respective tap coefficient calculators 34 of the adaptive equalizer 23x-i in FIG. 4 and the adaptive equalizer 23y-i in FIG. And h _{yy, i} are fed back to the controller 13 of the redundant optical signal transmitter 10 shown in FIG. Further, as in the first embodiment, the value of the tap coefficient is fed back by an arbitrary means such as via a monitoring control network or a counter device.

4 and 5, for example, as in the first embodiment, DD− using CMA before convergence and reference signals d _x3 (n) and d _y3 (n) after convergence. By switching to the LMS algorithm and updating, it is possible to perform waveform equalization in synchronization with the phase between signals.

In the tap coefficient calculation unit 34, the tap coefficient update formula by the CMA before convergence is as follows.

Further, the tap coefficient updating formula by the DD-LMS algorithm after convergence using the reference signals d _x3 (n) and d _y3 (n) is as follows.

Here, the reference signals d _x3 (n) and d _y3 (n) are output from the phase synchronization signals x ′ _{out, 3} (n) output from the phase synchronizers 24x-3 and 24y-3 in the reference signal generator 34. And y ′ _{out, 3} (n) are generated in the same manner as in Examples 1 and 2.

  As in the first embodiment, any adaptive equalization algorithm can be used as the tap coefficient update algorithm before and after convergence.

FIG. 6 shows an absolute value distribution | h _{xx, i} (n) | of each element of the tap coefficient at the time of digital signal processing of the DP-16QAM signal before transmission delay compensation (in the second embodiment, n is 1 to 21). integer).

In the digital signal processing device 22, since the third received signal transmitted through the optical transmission line 3 having the longest transmission delay is used as the reference signal, only the absolute value distribution | h _{xx, 3} (n) | of each element of the tap coefficient A peak shape can be seen. | H _{xx, 1} (n) | and | h _{xx, 2} (n) | are found in | h _{xx, 3} (n) | because transmission delay compensation using the transmission delay compensator has not started. It does not have such a peak shape. In this case, since a transmission delay occurs between the signals, the digital signal processing device 22 cannot perform signal synthesis by performing phase synchronization and adaptive equalization processing between the received signals.

FIG. 7 shows the absolute value distribution | h _{xx, i} (n) | of each element of the tap coefficient at the time of digital signal processing of the DP-16QAM signal when transmission delay compensation is performed using the transmission delay compensator.

Similarly to the first embodiment, the controller 13 is used to set the delay amounts of the transmission delay compensators 12-1 and 12-2 other than the transmission delay compensator 12-3 corresponding to the optical transmission line 3 having the largest transmission delay. As shown in FIG. 7, the absolute value distributions | h _{xx, 1} (n) | and | h _{xx, 2} (n) | of the tap coefficient elements are changed to | h _{xx, 3} ( n) The peak shape is the same as |. In this way, by compensating the transmission delay between signals using the transmission delay compensator, the digital signal processing device 22 can perform signal synchronization and adaptive equalization processing to synthesize signals.

In FIG. 7, the delay amount of the transmission delay compensator 12-1 is insufficient by 3 bits and the delay amount of the transmission delay compensator 12-2 is excessive by 3 bits due to the peak difference of the absolute value of the tap coefficient. I understand that there is. From the transmission delay of the controller 13 is estimated, | h _{xx, 3} | peak to the | h _{xx, 1} | and | h _{xx, 2} | as the peak of the match, transmission delay compensation by the controller 13 By adjusting the delay amounts of the counter 12-1 and the transmission delay compensator 12-2, the transmission delay between signals can be completely compensated.

In this embodiment, the controller 13 estimates the delay amount using the absolute value of the tap coefficient | h _{xx, i} |. However, | h _{xy, i} |, | h _{yx, i} |, or | h _{yy , i} | may be used to estimate the delay amount.

FIG. 8 shows a constellation map after digital signal processing of the DP-16QAM signal. The transmission characteristics when signals after transmission through the optical transmission lines 1 to 3 are received by ordinary digital coherent are BER (Bit Error Rate), 5.80 × 10 ⁻³ , 5.91 × 10 ⁻³ , 6.05 × 10 ^{−3, respectively.} Met.

Next, according to the configuration described in the second embodiment, after the transmission delay difference on the transmission side is compensated by the controller and the transmission delay compensator, the waveforms are equalized and synthesized by synchronizing the phase between the signals by digital signal processing. The BER after waveform synthesis was 4.58 × 10 ^-5 , confirming improvement in transmission characteristics.

In this example, assuming that a failure occurs in the optical transmission line, the transmission characteristic of the combined signal of the optical transmission lines 1 to 3 when the optical transmission line 1 is disconnected is 4.37 × 10 ^{−4 in} BER. . In the digital signal processing in the optical transmission system of the present invention, since the tap coefficient is updated using the reference signal, it is possible to suppress noise components generated when the optical transmission path is interrupted. For this reason, even when a failure occurs in the optical transmission line, the transmission characteristics are improved compared to the case of single transmission. For example, the transmission distance is determined by the transmission characteristic obtained by combining two optical transmission lines, and three transmissions are normally performed. By operating on the road, transmission can be continued without interruption even if a failure occurs in any of the paths.

DESCRIPTION OF SYMBOLS 10 Redundant optical signal transmitter 11-1 to 11-N Optical signal transmitter 12-1 to 12-N Transmission delay compensator 13 Controller 20 Redundant optical signal receiver 21-1 to 21-N Optical signal receiver 22 Digital Signal processing devices 23-1 to 23-N, 23x-1 to 23x-3, 23y-1 to 23y-3 Adaptive equalizers 24-1 to 24-N, 24x-1 to 24x-3, 24y-1 24y-3 phase synchronizer 25,25x, 25y signal combiner 26x-1~26x-3 chromatic dispersion compensator 26y-1~26y-3 chromatic dispersion compensator 31 delayer 32 tap coefficients h _i
33 Adder 34 Tap Coefficient Calculation Unit 35 Convergence Determination Unit 36 Reference Signal Generation Unit

Claims (4)

The same optical signals 1 to N (N is an integer of 2 or more) transmitted from the redundant optical signal transmitter are transmitted through optical transmission lines each having a predetermined transmission delay, and are collectively received by the redundant optical signal receiver. In an optical transmission system that combines signals,
When the transmission delay of the optical signal k (k is an integer of 1 to N) among the optical signals 1 to N is the largest,
The redundant optical signal receiver is:
N optical signal receivers that coherently detect the optical signals 1 to N transmitted through the optical transmission line, and further perform analog-digital conversion to output a digital signal;
The digital signal output from each of the N optical signal receivers is input, and a signal obtained by performing phase synchronization and adaptive equalization processing of another digital signal on the basis of the digital signal corresponding to the optical signal k. A digital signal processing device that synthesizes and outputs,
The redundant optical signal transmitter includes a transmission delay compensator that compensates for transmission delays of the optical signals 1 to N;
The digital signal processing device monitors the parameters used for the adaptive equalization processing of the optical signals 1 to N, and the transmission delay difference from other optical signals based on the parameters used for the adaptive equalization processing of the optical signal k And a control means for coarsely adjusting the transmission delay difference of the optical signals 1 to N, and setting the transmission delay difference in the transmission delay compensator corresponding to the optical signals 1 to N. Optical transmission system. The optical transmission system according to claim 1,
The control means monitors the parameters used for the adaptive equalization processing of the optical signals 1 to N while sequentially increasing the delay amount of the transmission delay compensator other than the transmission delay compensator corresponding to the optical signal k. An optical transmission system characterized in that a transmission delay difference with another optical signal is estimated based on a parameter used for adaptive equalization processing of the optical signal k. The optical transmission system according to claim 1,
The control means feeds back the parameters used for the adaptive equalization processing of the optical signals 1 to N by the digital signal processing device from the redundant optical signal receiver to the redundant optical signal transmitter via a predetermined transmission means. An optical transmission system characterized by being configured to transmit. In the optical transmission system according to any one of claims 1 to 3,
The parameter used for the adaptive equalization processing of the optical signals 1 to N in the digital signal processing device is a tap coefficient of the adaptive equalizer,
The control means generates a signal based on a peak difference of a distribution composed of absolute values of tap coefficients used for adaptive equalization processing of the optical signal k and tap coefficients used for adaptive equalization processing of other optical signals. An optical transmission system characterized by estimating a transmission delay difference between the two.

Images