Patent Application
20200295844
The present invention relates to an optical receiver, an optical transmission system, a submarine optical cable system, and an optical signal receiving method, and particularly to an optical receiver, an optical transmission system, a submarine optical cable system, and an optical signal receiving method that can improve error correction capability.
Although introduction of an error correction code has dramatically improved optical transmission performance of an optical transmission device, reception performance, meanwhile, depends on performance of the error correction code. For this reason, each vendor of optical transmission devices is dedicated to developing a coding method with high correction capability. When error correction capability becomes higher, an optical signal with a lower optical-signal-to-noise ratio (OSNR) can be received, and long distance transmission or a span of relaying by an optical amplifier can be extended. Examples of a method for improving performance of the error correction code include a method of using two kinds of codes, a method of increasing a redundant code, and the like. However, error correction capability is determined by a coding method, and a method of increasing a redundant code also requires expansion of a band of a receiving circuit, and thus there is a possibility that a noise is increased and reception characteristics are thereby deteriorated.
Thus, there is a limit to improvement of correction capability obtained by only improving an error correction code. In view of the above, as a technique for improving error correction capability without depending on improvement of an error correction code, there is a proposed technique in which an optical transmission system has a redundant configuration and a normal optical signal is output by majority-decision logic. For example, in PTL 1, on an optical transmission side, the same main signal is route-diversified by a plurality of carriers and transmitted to an opposite side. PTL 1 proposes a technique in which majority-decision determination is performed, on an optical reception side, on respective codes of electric signals acquired from a result of reproduction of the optical signals of a plurality of carriers, and thereby, a signal of a correct code is selected and output.
Further, in PTL 2, on an optical transmission side, the same main signals are transmitted as signals with different transmission speeds and transmission timings to an opposite side through a plurality of transmission lines. Alternatively, in PTL 2, the same main signals are time-division-multiplexed repeatedly three times, and repeatedly transmitted to one transmission line. PTL 2 proposes a technique in which majority-decision determination or matching determination is performed, on an optical reception side, on the received result, and thereby, a signal of a correct code is selected and output.
[PTL 1] Japanese Unexamined Patent Application Publication No. 2007-184860
[PTL 1] Japanese Unexamined Patent Application Publication No. H7-250050
However, the techniques of PTL 1 and PTL 2 differ in implementation means such as formation of a plurality of routes, a transmission speed difference, a transmission timing difference, and generation of a plurality of transmission signals, meanwhile, are techniques each of which aims at dispersing causes of errors occurring on an optical transmission line. Accordingly, in the techniques of PTL 1 and PTL 2, redundancy needs to be added to an entire optical transmission system including the optical transmission side and the optical transmission line. Thus, in the techniques of PTL 1 and PTL 2, there is a problem to be solved that installation cost and operation cost increase, or band crowding occurs due to the same signals transmitted to the optical transmission line a plurality of times.
In order to solve the problem as described above, an object of the present invention is to provide an optical receiver, an optical transmission system, a submarine optical cable system, and an optical signal receiving method that can economically obtain error correction capability equivalent to or greater than capability based on an error correction code.
In order to solve the above-described problem, an optical receiver, an optical transmission system, a submarine optical cable system, and an optical signal receiving method according to the present invention mainly adopt the following characteristic configuration.
(1) An optical receiver according to the present invention is an optical receiver that receives an optical signal including an error correction code, the optical receiver including:
an optical branching means for branching a received optical signal into three optical signals, and outputting the optical signals;
three optical signal processing means for inputting one of the three optical signals branched by the optical branching means, operating independently, and performing signal processing including an identification and reproduction process concerning the input optical signal and an error correction process based on the error correction code; and
a bit determining means for performing, in a bit unit, majority-decision determination on the three signals on which error correction processes have been performed and that are output respectively from the three optical signal processing means, and outputting a signal determined as being correct.
(2) An optical transmission system according to the present invention is an optical transmission system that includes an optical transmitter, an optical transmission line, and an optical receiver, wherein the optical receiver is configured by using the optical receiver according to the above (1).
(3) A submarine optical cable system according to the present invention is a submarine optical cable system that includes an optical transmitter, a submarine optical cable, a submarine optical repeater, and an optical receiver, wherein the optical receiver is configured by using the optical receiver according to the above (1).
(4) An optical signal receiving method according to the present invention is an optical signal receiving method for receiving an optical signal including an error correction code, the method including:
an optical branching step of branching a received optical signal into three optical signals, and outputting the optical signals;
an optical signal processing step of inputting each one of the three branched optical signals, and independently performing, on each of the input optical signals, signal processing including an identification and reproduction process concerning the input optical signal and an error correction process based on the error correction code; and
a bit determining step of performing, in a bit unit, majority-decision determination on the three signals that are generated in the optical signal processing step and on which error correction processes have been performed, and outputting a signal determined as being correct.
According to the optical receiver, the optical transmission system, the submarine optical cable system, and the optical signal receiving method of the present invention, the following advantageous effect can be achieved.
Specifically, in the present invention, it is possible to obtain an advantageous effect that correction capability equivalent to or greater than capability of an error correction code can be economically obtained. Even without redundant configurations of the optical receiver and an optical transmission line, this advantageous effect by the present invention is achieved by the reason that the redundant configuration is introduced only to the optical receiver, and three optical signal processing means installed in the optical receiver independently perform separate signal processing, respectively, resulting in that positions of errors remaining after correction based on respective error correction codes generally vary as well, whereby separately from an error correction code, additional error correction can be made by performing error determination by majority-decision determination.
A preferred example embodiment of an optical receiver, an optical transmission system, a submarine optical cable system, and an optical signal receiving method according to the present invention is described below with reference to the accompanying drawings. Note that the optical receiver and the optical signal receiving method according to the example embodiment of the present invention are described in the following description, it is certainly possible that such an optical receiver applies to an optical transmission system including an optical transmission line, an optical transmitter, and an optical receiver. Further, such an optical receiver is certainly applicable to not only a transmission system on land but also a submarine optical cable system.
Further, the drawing reference symbols attached to the respective drawings below are attached to the respective elements, for convenience, as one example for facilitating understanding, and it goes without saying that the present invention is not intended to be limited to the illustrated configuration.
Before the description of the example embodiment of the present invention, the outline of the feature of the present invention is first described.
In the present invention, on the optical receiver side, one received optical signal is branched into three optical signals, and these three optical signals are signal-processed by three optical receiving circuits, independently from one another. The present invention is characterized mainly by including a mechanism in which a bit determining circuit compares, in a unit of one bit, three signals output from the respective optical receiving circuits, and performs majority-decision determination from a result of which a signal determined as being correct is output. Thus, an economical system having error correction capability equivalent to or greater than the capability of an error correction code can be implemented not by an entire system of an optical transmission system or a submarine optical cable system, but by the redundant configuration only on the optical receiver side. Further, in the present invention, configuration change from the conventional optical transmission system is only on the optical receiver side, and its application to not only an optical transmission system on land but also even a submarine optical cable system can be easily made without depending on a newly installed system or an existing system.
Next, one example embodiment of an optical receiver according to the present invention is described in detail with reference to
In
The optical receiver 2 illustrated in
In other words, when receiving the optical signal that has been separated per wavelength and that includes an error correction code, the optical receiver 2 illustrated in
Here, the optical branching unit 12 is an optical branching means that branches, into three optical signals, an optical signal received via the optical transmission line, and that outputs the optical signals. The first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 each receive one optical signal of the three optical signals that have been branched at the optical branching unit 12. Each of the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 independently operates, and performs, on the input signal, signal processes including an identification and reproduction process and an error correction process based on an error correction code. Each of these three optical receiving circuits is the above-described optical signal processing means. The first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18 are bit synchronization means that synchronizes, in a bit unit, the three signals that are output respectively from the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 and on which the error correction processes have been performed. Further, the bit determining circuit 19 performs, in a bit unit, majority-decision determination on the three signals that are output respectively from the three of the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18 and on which the error correction processes have been performed, and outputs the signal determined as being correct. The bit determining circuit 19 is the above-described bit determining means.
In other words, in the optical receiver 2, the received optical signal is input to the optical branching unit 12 and branched into three optical signals. The optical branching unit 12 includes three optical output ports, and these three optical output ports are respectively connected to the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15. Each of the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 independently performs the signal process on the input optical signal that has been branched at the optical branching unit 12, converts the optical signal into the electric signal, performs the error correction process, and outputs the signal.
Outputs of the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 are connected respectively to the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18. The first synchronization circuit 16 outputs a synchronization timing signal to the second synchronization circuit 17 and the third synchronization circuit 18. This synchronization timing signal is supplied in common with the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18 in order to make synchronization for each bit. In other words, the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18 synchronize, for each bit, the signals respectively output from the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15, based on the synchronization timing signal, thereby synchronize the signals with the same timing, and output the signals.
Respective outputs of the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18 are input to the bit determining circuit 19. The bit determining circuit 19 compares, for each bit, the signals output respectively from the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18, and performs the majority-decision determination, and outputs the signal determined as being correct by the majority-decision determination.
As described above up to this point, the optical receiver 2 illustrated in
Next, one example of an operation of the optical receiver 2 illustrated in
In the optical transmitter 1 opposite to the optical receiver 2, the optical transmitting circuit 11 adds an error correction code to an input signal, then converts the signal into an optical signal suitable for optical wavelength multiplex transmission, and transmits the optical signal as the optical transmission signal to the optical receiver 2.
In the optical receiver 2, when receiving optical signals that have been separated per wavelength from the optical transmission signal transmitted from the optical transmitter 1, the optical branching unit 12 branches the received optical signal into three optical signals. The optical branching unit 12 outputs the three branched optical signals respectively to the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 three of which are arranged in parallel. Each of the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 independently operates, receives the optical signal inputted thereto, performs identification and reproduction, and converts the signal into an electric signal. Further, after that, an incorporated forward error correction circuit (FEC) performs, by an error correction code added by the optical transmitting circuit 11 on the side of the optical transmitter 1, error correction on the signal that has converted into the electric signal, and outputs the signal.
The signals output from the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 are input respectively to three of the first synchronization circuits 16, the second synchronization circuit 17, and the third synchronization circuit 18. The first synchronization circuit 16 outputs a synchronization timing signal to the second synchronization circuit 17 and the third synchronization circuit 18. In synchronization with the synchronization timing signal from the first synchronization circuit 16, the second synchronization circuit 17 and the third synchronization circuit 18 cue up the signals output respectively from the second optical receiving circuit 14 and the third optical receiving circuit 15. By the cuing-up in synchronization with the synchronization timing signal, the second synchronization circuit 17 and the third synchronization circuit 18 performs positioning (a synchronization process) in such a way that the signals output respectively from the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18 are synchronized with the same timing.
Here, the synchronization timing signal output by the first synchronization circuit 16 to the second synchronization circuit 17 and the third synchronization circuit 18 is a signal generated at a timing when a control signal is detected in the first synchronization circuit 16, the control signal being predetermined and included in an overhead part of an optical signal transmitted from the side of the optical transmitter 1. The control signal is a predetermined control signal that is any one among control signals such as a synchronization signal and an error correction signal, for example. Accordingly, the second synchronization circuit 17 and the third synchronization circuit 18 may operate in such a way as to synchronize, with the synchronization timing signal from the first synchronization circuit 16, a timing of outputting the predetermined control signal included in the signals output respectively from the second optical receiving circuit 14 and the third optical receiving circuit 15.
In one example of the synchronization process in the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18, for example, when an optical transport network (OTN) frame is applied to the optical signal, the synchronization can be made by detecting a frame synchronization signal (frame alignment sequence (FAS)) included in an overhead part of the OTN frame.
As illustrated in the time charts of the signal 1, the signal 2, and the signal 3 in (A) of
The timings of the frame synchronization signals (FAS) of the signal 1, the signal 2, and the signal 3 output respectively from the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18 synchronize with one another, and thereby, the respective signals of the signal 1, the signal 2, and the signal 3 are output at the same timing for each bit, and are input to the bit determining circuit 19. Thus, in the bit determining circuit 19, the data (codes) at the same position included in the three signals of the signal 1, the signal 2, and the signal 3 can be compared at the same timing. In other words, the bit determining circuit 19 compares, in a bit unit, three sets of data at the same timing (at the same time position) included in the signal 1, the signal 2, and the signal 3 output respectively from the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18, and performs a majority-decision determination process.
For example, at the timing t2 in (B) of
Next, a limit of error correction capability based on an error correction code is supplementarily described with reference to the explanatory diagram of
As indicated by the limit value G on the horizontal axis in
Meanwhile, when a bit error rate BER of the input signal is smaller than the limit value G, this corresponds to a region where error correction is possible in the forward error correction circuit (FEC). When a bit error rate BER is smaller than the limit value G, error correction is more reliably performed as a bit error rate BER of the input signal becomes smaller, and thus, as illustrated by the straight line L2, a bit error rate BER of the signal output from the forward error correction circuit (FEC) is in a state of being “0”.
However, when a bit error rate BER of the input signal approaches the limit value G and reaches the limit vicinity region R surrounded by the broken line in
For this reason, in the limit vicinity region R, it is effective that the bit determining circuit 19 performs majority-decision determination on three signals output from the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 respectively via three of the first synchronization circuit 16, the second synchronization circuit 17, and the third synchronization circuit 18. In the limit vicinity region R, the bit determining circuit 19 performs majority-decision determination on three signals output from the respective optical receiving circuits, and thereby, the configuration of the optical receiver 2 as illustrated in
Note that in the case of the region that is on the left side of the limit vicinity region R and in which a bit error rate BER is small, error correction is reliably performed as indicated by the straight line L2, and three signals output from the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 match with one another. Thus, the majority-decision determination in the bit determining circuit 19 becomes non-effective. Further, in the case of the region that is on the right side of the limit vicinity region R and in which a bit error rate BER exceeds the limit value G of the error correction capability and is high, the error correction cannot become effective as indicated by the straight line L1. In the case of the region in which a bit error rate BER exceeds the limit value G of the error correction capability, three signals output from the first optical receiving circuit 13, the second optical receiving circuit 14, and the third optical receiving circuit 15 vary, and thus, the majority-decision determination in the bit determining circuit 19 cannot become effective.
As described above in detail, in a receiving system of the optical transmission system including the optical transmitter 1, the optical transmission line, and the optical receiver 2 as illustrated in
Namely, in the present example embodiment, variations in error correction results of the error correction function due to characteristic variations of the optical portions (a local emission light source, an optical mixer (interference between signal light and local emission light), optical branching, and the like) of the optical receiver 2 are adjusted to a correct signal by using majority-decision determination. In other words, the majority-decision determination can improve determination accuracy in the limit vicinity region R that is the limit as to whether or not the error correction can be performed reliably by the majority-decision determination as illustrated in
The techniques described in PTL 1 and PTL 2 described above aim at dispersing causes of errors occurring on an optical transmission line. For this reason, the techniques disclosed in PTL 1 and PTL 2 described above repeatedly perform transmission or the like by formation of a plurality of routes, transmission speed differences among a plurality of signals, transmission timing differences among a plurality of signals, or time division multiplexing, and perform majority-decision determination. Thus, according to the techniques described in PTL 1 and PTL 2 described above, redundancy needs to be provided not only on the optical reception side but also on the transmitting side of optical signals and in the optical transmission line. Further, in the techniques described in PTL 1 and PTL 2 described above, there is a problem that a band of an optical transmission line is crowded by the same signals repeatedly transmitted to the optical transmission line.
Meanwhile, in the above-described example embodiment, since the redundant configuration is added only to and within the optical receiver 2, equipment investment cost and operation cost can be made small, and a band of the optical transmission line can be effectively used without being crowded. Thus, application to an extremely long-distance wavelength multiplex optical transmission system based on an optical amplification transmission method can be suitably made.
In the example embodiment described above, a land transmission system is assumed as an optical transmission system to which the optical receiver 2 is applied, but without limitation to this case, application can be made to a submarine optical cable system as illustrated in
Here, needless to say, even when the optical receiver 22 in the submarine optical cable system with the system configuration as illustrated in
As described above in detail, in the above-described example embodiment, it is possible to obtain an advantageous effect that correction capability equivalent to or greater than capability of an error correction code can be economically obtained. In the above-described example embodiment, as described above in detail, the redundant configuration is introduced only in the optical receiver 2, and three of the first, second, and third optical receiving circuits 13, 14, and 15 installed in the optical receiver 2 independently perform the separate signal processes, respectively. In such a case, since positions of errors remaining after correction by respective error correction codes differ from one another, the bit determining circuit 19 performs error determination by the majority-decision determination. As a result, even when the optical transmitter 1 and the optical transmission line do not have a redundant configuration, additional error correction can be performed separately from that of an error correction code, and thus, such an advantageous effect can be achieved.
While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.
This application is based upon and claims the benefit of priority from Japanese patent application No. 2016-70620, filed on Mar. 31, 2016, the disclosure of which is incorporated herein in its entirety by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-070620 | Mar 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/011772 | 3/23/2017 | WO | 00 |
