JP6480257B2 - Optical communication apparatus and optical communication system - Google Patents

Description

  The present invention relates to an optical communication technique for performing optical communication at an optimum coding rate according to the code error characteristic of a propagation path when performing optical communication using a forward error correction code.

  In a communication system in which an optical signal is transmitted and received via an optical transmission line, a signal waveform is degraded due to various state changes of a propagation path, and a code error occurs. Generally, an error correction code is adopted as a technique for correcting such a code error, and one of them is a technique called forward error correction (FEC). In forward error correction, redundant data is added to original data and transmitted on the transmission side, and error detection and correction are performed on the basis of the redundant data on the reception side.

  FIG. 19 is a block diagram showing a conventional optical communication system. Conventionally, when data communication is performed using an error correction code, in the optical communication apparatus 50 on the transmission side, a frame processing unit generates a transmission frame from input transmission data, and this is set in advance for forward error correction According to the encoding algorithm, redundant data is added and encoded by the encoding unit, and the obtained encoded data is converted into an optical signal by the optical transmission / reception unit and transmitted from the optical transmission path L. On the other hand, in the optical communication apparatus 60 on the reception side, the optical signal received from the optical transmission path L is converted to an electrical signal by the optical transmission / reception unit, and the encoding unit decodes it by the encoding algorithm corresponding to the transmission side. Errors are detected and corrected based on redundant data added to the decoded data, and the received data is extracted and output from the obtained received frame by the frame processing unit.

In general, code errors occurring in the propagation path are roughly classified into random errors due to white noise and burst errors occurring continuously due to sudden failure.
For example, in the 10G-EPON system, light is used as a signal medium, and a signal waveform has a burst error due to rapid temperature change, stress concentration, fiber breakage, etc. Solomon code RS (255, 223) is adopted.

Conventionally, when a Hamming code is adopted as a specific error correction code in a communication system as a coding technology using such forward error correction, 64B / 66B code is used in the physical layer (PCS: Physical Coding Sublayer). A coding technique that can be adapted when used is proposed (see, for example, Patent Document 1).
Also, in a wireless system, part of the data is divided into a plurality of parts, which are convoluted with different generator polynomials, and the obtained coded data is multiplexed with the remaining uncoded data, respectively, to obtain these coded data. There is also proposed a technique for protecting data from both burst errors and random errors by making them less susceptible to the same bursty code errors by communicating them temporally apart (for example, Patent Document 2) Etc.).

Patent No. 5106538 gazette Patent No. 3515325 gazette

  In the error correction code, a high correction capability can be obtained by increasing redundant data, but there is a trade-off that the communication bandwidth is reduced by the redundant data and the transfer efficiency is lowered. For this reason, it is necessary to use the coding rate according to the code error characteristic regarding the actual propagation path as the coding processing content for the error correction code, and in particular, higher transfer efficiency can be obtained with the minimum redundant data. It is important to identify the optimal coding rate.

  However, according to the prior art described above, although it is possible to cope with certain burst errors and random errors, as in the 10G-EPON system, there is return light generated on the propagation path such as the end face of the optical transmission line or branch optical transmission line In the case of the optical communication system, there is a problem that the coding algorithm corresponding to the random error of the white noise system can not cope with it because the return light becomes noise which does not follow the normal distribution.

  The present invention is intended to solve such a problem, and, even in the presence of return light generated on a propagation path, forward coding is performed at an optimal coding rate according to the code error characteristic regarding the propagation path. An object of the present invention is to provide a data communication technique capable of performing data communication by an error correction code.

  In order to achieve such an object, in the optical communication apparatus according to the present invention, transmission bit sequences constituting a transmission frame are sequentially encoded by an encoder based on a forward error correction encoding algorithm and a coding rate. An optical communication apparatus which electro-optically converts an optical signal into an optical signal by an optical transmitter and transmits the optical signal to the other optical communication apparatus through the optical transmission line, the light generated on a propagation path to the other optical communication apparatus A simulation unit for estimating a code error characteristic of the propagation path in a case where the transmission bit string is encoded and converted into an electrical signal and transmitted based on a return light parameter related to a return light to a transmitter, and the estimated code error characteristic And a coding setting unit configured to calculate the coding rate used in the forward error correction coding algorithm and to set the code rate in the encoder.

In the above SL optical communication device, wherein the simulation unit, the by applying the returning light parameters, to calculate the estimated light signal is estimated to reach the destination optical communication device relative rate equation which simulates the propagation path The code error characteristic of the propagation path is estimated by comparing the estimated bit string obtained by digital conversion and decoding of the obtained estimated optical signal with the transmission bit string.

  In one configuration example of the above-described optical communication apparatus according to the present invention, the simulation unit counts the number of symbol errors regarding a coding symbol by comparing the estimated bit string and the transmission bit string, and the encoding setting unit Identifies an error correction limit indicating the number of error bytes to be corrected by the encoding algorithm based on the number of symbol errors, and the code based on the error correction limit and the total number of bytes included in the transmission bit string The conversion rate is calculated.

  In one configuration example of the above-described optical communication apparatus according to the present invention, the simulation unit counts the number of bit errors and the number of symbol errors related to coding symbols by comparing the estimated bit string and the transmission bit string, The coding setting unit calculates a burst error index indicating a frequency at which a bit error occurrence interval is less than the coding symbol length based on the bit error number and the symbol error number, and a block error is calculated based on the burst error index. Alternatively, an encoding algorithm appropriate for burst errors may be selected and set in the encoder.

Further, one configuration example of the above-described optical communication device according to the present invention is a photon lifetime, a carrier lifetime, a first gain coefficient, a second gain coefficient, a transparent carrier density, a line width enhancement coefficient, and a natural with respect to the optical transmitter. The emitted light coefficients are τ _p , τ _c , G, G _s , N ₀ , α, and β _sp , and the return light intensity coefficient and return light delay for the return light _k are R _k and τ _k , respectively. Assuming that the density is N, the injection current density relating to the modulation signal of the transmission bit string is J, and the electric field of the estimated optical signal and its phase component are E and φ , respectively , the rate equation will be described later by Equation (1) and Equation It consists of (2) .

  Further, an optical communication system according to the present invention is an optical communication system including first and second optical communication devices connected via an optical transmission line, wherein the first optical communication device is any one of the above-described optical communication devices. The optical communication device of

  In one configuration example of the above-described optical communication system according to the present invention, a distribution related to the number of symbol errors per correction block based on a received bit string received by the second optical communication device from the first optical communication device. And the first optical communication apparatus is a block error or burst error corresponding to the observation result observed by the code error observation unit as a forward error correction coding algorithm. Using an appropriate forward error correction coding algorithm.

  According to the present invention, the code error characteristic in the case of considering the return light is estimated, and the coding rate calculated based on the obtained code error characteristic is used in the coding algorithm for forward error correction. . Therefore, even when return light is present on the propagation path, it is possible to perform data communication with the forward error correction code at the optimum coding rate according to the code error characteristic of the propagation path.

FIG. 1 is a block diagram showing a configuration of an optical communication system according to a first embodiment. It is a flowchart which shows a coding rate specific process. It is an example of various parameters used in a rate equation. It is an eye diagram of an estimated light signal waveform (without return light). It is an eye diagram of an estimated light signal waveform (near end return light only). It is an eye diagram of a presumed light signal waveform (far end return light only). It is an eye diagram of an presumed optical signal waveform (with near end and far end return light). It is a graph which shows the return light intensity dependence characteristic of the number of symbol errors. It is a graph which shows the return light intensity dependence characteristic of a symbol error index. It is a graph which shows the return optical delay dependence characteristic (near end return) of the number of symbol errors. It is a graph which shows the optical delay dependence characteristic (near end return) of symbol error index. It is a graph which shows the return optical delay dependence characteristic (far end return) of the number of symbol errors. It is a graph which shows the optical delay dependence characteristic (far end return) of symbol error index. It is a graph which shows the return optical delay (near end return) dependence of a coding rate. It is a block diagram which shows the structure of the optical communication system concerning 2nd Embodiment. It is a flowchart which shows code error observation processing. It is a graph which shows symbol error number distribution per error correction block. It is a principal part enlarged graph of FIG. It is a block diagram which shows the conventional optical communication system.

Next, embodiments of the present invention will be described with reference to the drawings.
First Embodiment
First, an optical communication system 1 according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the configuration of the optical communication system according to the first embodiment.
The optical communication system 1 includes an optical communication device 10 on the master station side and one or more optical communication devices 20 on the slave station side. , Are connected via the optical demultiplexer 30.

When the optical communication system 1 according to the present invention is applied to a 10G-EPON system, the optical communication device 10 corresponds to an OLT (Optical Line Terminal: station-side termination device), and the optical communication device 20 is an ONU (Optical Network Unit) End terminal device). The optical communication system 1 according to the present invention is not limited to this, and the present invention can be applied in the same manner as long as it is an optical communication system using a forward error correction code.
Further, in the present invention, in order to facilitate understanding, the case of transmitting from the optical communication device 10 to the optical communication device 20 will be described as an example, but the case of transmitting from the optical communication device 20 to the optical communication device 10 is also the same. May be applied.

[Optical communication device]
The optical communication device 10 is provided with a frame processing unit 11, an encoding unit 12, an optical transmission / reception unit 13, a simulation unit 14, and an encoding setting unit 15 as main functional units.

  The frame processing unit 11 has a function of generating a transmission frame (downlink frame) from transmission data (downlink data) input from a master station side communication terminal (not shown) externally connected, and from the encoding unit 12 It has a function of extracting reception data (uplink data) from the input reception frame (uplink frame) and outputting it to the transmitting communication terminal.

  The encoding unit 12 encodes the transmission bit sequence (downlink bit sequence) forming the transmission frame input from the frame processing unit 11 with the encoder 12 B based on the forward error correction encoding algorithm and the encoding rate. Function of outputting the transmission coded bit sequence (downlink coding bit sequence) to the optical transmission / reception unit 13 and the reception coding bit sequence (uplink coding bit sequence) input from the optical transmission / reception unit 13 using the coding algorithm for forward error correction And a function of outputting to the frame processing unit 11 a received frame composed of the received bit sequence (uplink bit sequence) obtained by decoding by the decoder 12A based on the coding rate.

  The light transmitting / receiving unit 13 has a function of electro-optically converting the transmission coded bit string (electric signal) input from the coding unit 12 into a light signal by the light transmitter 13B and transmitting the light signal from the light transmission path L1; The optical receiver 13A has a function of photoelectrically converting a received optical signal into a reception encoded bit string (electrical signal) and outputting the same to the encoding unit 12.

  The simulation unit 14 has a function of estimating the code error characteristic of the propagation path according to the transmission characteristic related to the propagation path with the optical communication device 20 based on the transmission bit string generated by the frame processing unit 11. ing. Here, the propagation path referred to here corresponds to a section from the optical transmitter 13B to the optical receiver 23A including the influence of the return light from the optical transmission path L3 in FIG.

  The coding setting unit 15 has a function of specifying an optimum coding rate for forward error correction based on the estimation result obtained by the simulation unit 14 and setting the coding rate in the encoder 12B.

  The optical communication device 20 is provided with a frame processing unit 21, an encoding unit 22, and an optical transmission / reception unit 23 as main functional units.

  The frame processing unit 21 has a function of generating a transmission frame (uplink frame) from transmission data (uplink data) input from a slave station side communication terminal (not shown) connected to the outside, and from the encoding unit 22 It has a function of extracting reception data (downlink data) from the input reception frame (downlink frame) and outputting it to the slave station communication terminal.

  The encoding unit 22 encodes the transmission bit sequence (uplink bit sequence) constituting the transmission frame input from the frame processing unit 21 by the encoder 22 B based on the forward error correction encoding algorithm and the encoding rate. And a function of outputting the transmission coded bit sequence (uplink coded bit sequence) to the optical transmission / reception unit 23, and a reception coded bit sequence (downlink coded bit sequence) input from the optical transmission / reception unit 23 using a coding algorithm for forward error correction. And a function of outputting to the frame processing unit 21 a received frame composed of the received bit string (downlink bit string) obtained by decoding by the decoder 22A based on the coding rate.

  The light transmitting / receiving unit 23 has a function of electro-optically converting the transmission coded bit string (electric signal) input from the coding unit 22 into a light signal by the light transmitter 23B and transmitting the light signal from the light transmission path L2. The optical receiver 23A has a function of photoelectrically converting a received optical signal into a reception encoded bit string (electrical signal) and outputting the same to the encoding unit 22.

[Operation of First Embodiment]
Next, with reference to FIG. 2, as an operation of the optical communication device 10 according to the present embodiment, a coding rate specification process in the optical communication device 10 will be described. FIG. 2 is a flowchart showing coding rate identification processing.

  First, the simulation unit 14 acquires from the frame processing unit 11 the transmission bit string of the transmission frame used for estimation of the code error characteristic (step 100), and after confirming the designated forward error correction coding algorithm (step 101) encoding based on a designated forward error correction encoding algorithm (step 102), and modulating the obtained encoded bit string with a modulation scheme for a physical layer such as NRZ (Non Return to Zero) A modulation signal is generated (step 103).

In this example, it is assumed that a frame used for estimation of code error characteristics is composed of a bit sequence based on PRBS (pseudo random bit sequence) 2 ⁹ -1, and a coding algorithm for forward error correction used for estimation of code error characteristics. Although the case where Reed Solomon code RS (255, 223) is used will be described as an example, another encoding algorithm may be used. Also, this forward error correction coding does not necessarily have to be applied at the time of estimation. This is far more disturbed by factors that contribute to noise that does not conform to the normal distribution, compared to the effects of coding application and differences in the coding algorithm to be applied to the estimation results. It is from.

When the estimation accuracy is further enhanced by applying encoding for forward error correction at the time of estimation, according to the above premise, on the expanded Galois field GF (2 ^m ) (m is an integer of 1 or more). The length of the bit string needs to be determined in consideration of the fact that the coding for forward error correction is performed. The present embodiment will be described on the assumption that an element consisting of 8 bits = 1 byte is one symbol (block), and forward error correction coding is performed on the expanded Galois field GF (2 ^m ). In this case, the length of the bit string is 2 ⁸ -1 = 255 bytes = 2040 bits, and this bit string is called an error correction block.

  Next, the simulation unit 14 generates a light emitting element parameter indicating the characteristic of the light emitting element used in the encoder 12B, a modulated element parameter indicating the characteristic of the modulated element used in the encoder 12B, and a return occurring on the propagation path. The modulation signal (electrical signal) obtained in step 103 is electro-optically converted by acquiring return light parameters for light (step 104) and applying these parameters to a rate equation which is a model simulating a propagation path. , And transmits an estimated optical signal estimated to arrive at the optical receiver 23A of the optical communication apparatus 20 (step 105). When the modulation unit 12 directly modulates and controls the light emitting element without using the modulated element, the modulated element parameter is unnecessary.

In the following, as one of systems in which the signal source is disturbed by factors that contribute to noise that does not follow the normal distribution, an isolatorless optical transmission / reception system (system in which return light exists) is formulated and numerically formulated A method of solving and estimating a code error will be described.
The isolator is a component that transmits only an electromagnetic wave propagating in one direction, and is disposed on the emission light side of the semiconductor laser in order to prevent the light source of the optical transmitter 13B from interfering with the return light. In this example, a distributed feedback laser (DFB-LD) is used as a light source.

FIG. 3 is an example of various parameters used in the rate equation. Among these, photon lifetime τ _p , carrier lifetime τ _c , and first and second gain coefficients G and G _s are light emitting device parameters, respectively, and transparent carrier density N ₀ , line width enhancement coefficient α, and spontaneous emission The light coefficient β _sp is a modulated device parameter. Also, the injection current density J indicates the modulation signal (electric signal) obtained in step 103, and becomes an input variable of the rate equation. In addition, the return light intensity coefficient R _k , the return light delay τ _k , and the return light intensity coefficient amplitude r are set for each of K (K is an integer of 1 or more) return lights k (k = 1 to K integer) _k and the return light intensity coefficient phase φ _k are return light parameters related to the propagation path.

Therefore, when there are K return lights, the electric field E of the estimated signal light obtained with the distributed feedback laser (DFB-LD) as a light source and the carrier density N of the semiconductor are expressed by the following formulas (1) and (2) The complex representation of the electric field E and the light intensity coefficient R _k is represented by the following equations (3) and (4).

In particular, the fourth term on the right side of Equation (1) indicates return light noise that causes noise that the signal source does not follow the normal distribution, and the delay components for the K return lights are summed and added as return light noise components It is done.
Further, based on the equation (1), the amplitude component A and the phase component φ of the electric field E are expressed by the following equations (5) and (6), and θ used in these equations (5) and (6) _k is equation (7).

  In this manner, the simulation unit 14 sets these equations in step 105 based on the parameters acquired in step 104 using the injection current density J of the modulation signal (electric signal) obtained in step 103 as an input variable. 1) The electric field E related to the estimated light signal is calculated by analyzing the equation (5) numerically.

FIG. 4 is an eye diagram of an estimated light signal waveform (without return light), which corresponds to the case of r ₁ = 0 and r ₂ = 0. FIG. 5 is an eye diagram of an estimated light signal waveform (near end return light only), which corresponds to the case of r ₁ = 10, r ₂ = 0, and τ ₁ = 0.1. FIG. 6 is an eye diagram of an estimated optical signal waveform (far-end return light only), which corresponds to the case of r ₁ = 0, r ₂ = 10, and τ ₂ = 10. FIG. 7 is an eye diagram of an estimated optical signal waveform (near end, with far end return light) corresponding to the case where r ₁ = 10, r ₂ = 10, τ ₁ = 0.1, and τ ₂ = 10. .

  Next, the simulation unit 14 sets, on the eye diagram, an identification point corresponding to a threshold value for digitally converting the estimated optical signal to an estimated bit string based on the obtained electric field E (step 106). Based on the identification point, the estimated optical signal is converted to an estimated encoded bit string (digital conversion), and then decoded by the same forward error correction encoding algorithm as that for encoding, and converted to an estimated bit string (step 107) ). Under the present circumstances, about an identification point, in the eye diagram of electric field E obtained when there is no return light based on a known method, the estimated light signal is set to a position where it can be identified as "0" and "1" without error. do it.

Thereafter, the simulation unit 14 compares the obtained estimated bit string with the original transmission bit string to count the number of bits in which an error has occurred as the bit error number N _{bit_err} , and corrects the obtained bit string as an error. The number of error correction blocks in which an error occurs is counted as the block error number N _{block_err} by comparing the original bit string with each block (step 108), and the bit error number N _{bit_err} and the block error number N _{block_err} are The bit string length N _{bit of the} entire bit string and the number of error correction blocks N _block are output to the coding setting unit 15 as the code error characteristics of the propagation path.

The coding setting unit 15 specifies an error correction limit N _{bit_limit} indicating the number of blocks to be _subjected to error correction based on the code error characteristics of the propagation path from the simulation unit 14 and specifies the _coding rate R _{coding according} to equation (8). Is calculated (step 109).

At this time, by using the number of block errors N _{block_err} as the error correction limit N _{bit_limit} , a _coding rate R _{coding that} is theoretically error free can be obtained. Also, the simulation is repeatedly executed to obtain the distribution of the block error number N _{block_err} , and based on this distribution, the error correction limit N _{bit_limit} corresponding to the allowable error rate set for the optical communication system 1 is _obtained . You may choose. As a result, it is possible to obtain a _coding rate R _coding with improved transfer efficiency as compared to the error free case.

Subsequently, the coding setting unit 15 calculates the bit error rate P _bit , the symbol error rate P _byte , and the random error symbol error rate P _{byte_rand according} to Equations (9), (10), and (11). After that, the burst error factor F is calculated by the equation (12) (step 110).

As shown in Equation (11), the random error symbol error rate P _{byte — rand} is a symbol error rate when it is assumed that the code errors generated in the propagation path are only random errors. Therefore, the burst error coefficient F is an index indicating the frequency at which the bit error interval is less than one byte (one symbol length). As described above, continuously occurring code errors are defined as burst errors. If the burst error factor F indicates a value larger than a certain reference value, it indicates that the code error occurrence interval is less than 1 byte. If the number of bit errors is sufficiently smaller than the total number of bits, the reference value is "1". In general, this reference value depends on the number of bit errors. As the number of bit errors increases, it is necessary to use a value smaller than "1" as the reference value.

Therefore, when the code error generated in the propagation path is only random error and there is no burst error, the burst error factor F becomes equal to "1". On the other hand, if a burst error is included, the burst error factor F deviates from "1". Thus, based on the burst error factor F, it can be determined whether or not a burst error is included.
Thereafter, the coding setting unit 15 specifies the coding algorithm for forward error correction to be used by the encoder 12B based on the burst error coefficient F (step 111), and calculates the coding rate and coding algorithm as The encoding processing content is set in the encoder 12B (step 112), and the series of coding rate identification processing is ended.

FIG. 8 is a graph showing the return light intensity dependency of the number of symbol errors, and FIG. 9 is a graph showing return light intensity dependency of the symbol error index. In both cases, the case where the transmission rate is 10 Gb / s and the correction block length is 255 bytes is shown, and plotted for each of the return optical delays τ = 0.1, 0.5, and 10 [ns].
FIG. 10 is a graph showing the return light delay dependence characteristic (near end return) of the number of symbol errors, and FIG. 11 is a graph showing the light delay dependence characteristic of the symbol error index (near end return). On the other hand, FIG. 12 is a graph showing the return light delay dependence characteristic (far end return) of the number of symbol errors, and FIG. 13 is a graph showing the light delay dependence characteristic of the symbol error index (far end return). In all cases, the return light intensity R = 10, 15, 20 [a. u. It is plotted for each of].

  The optical transmission / reception system is a system in which the optical communication device 10 and the optical communication device 20 are connected by the optical transmission paths L1 and L2 via the optical demultiplexer 30, as shown in FIG. When one end of the optical demultiplexer 30 is not terminated, return light is incident on the optical communication device. There is also light returned from the fiber end face of the optical transmitter 13B in the optical transmitter / receiver 13 of the optical communication device 10. A code error may occur in the transmission data of the optical communication device 10 due to the return light.

  The rate equation of equation (1) described above simulates a system in the presence of such return light. The code error estimation information is recorded on a recording medium. From the estimated information, the number of code errors of the optical transmission / reception system and the nature of whether the code errors are random or bursty are predicted, and the coding algorithm is determined.

  For example, the distance between the optical communication device 10 and the fiber break point is 100 mm, and the distance between the light source inside the optical transmitter 13B and the fiber end face is 1 mm. In addition, the intensity of the light source is 6 dBm, the intensity of return light from the reflection point inside the optical transmitter 13B (near end return light) is −8 dBm, and the intensity of return light from the optical communication device 20 (far end return light) is − It is 18 dBm. In this case, the near-end return light intensity is ten times the far-end return light intensity.

  Referring to FIG. 8, in the far-end return light region (in this embodiment, it is assumed that intensity <15 [au]), no symbol error occurs. On the other hand, referring to FIG. 10, the symbol error count indicates a value of 5 bytes or less in the presence of near-end return light (delay = 0.13 ns) in the present embodiment. Further, referring to FIG. 11, the burst error index may indicate 1 or more in the sweep range of the near end return optical delay time. It is judged from the values indicated by the symbol error number and the burst error index for the near-end return light area, and the correction limit is 5 bytes, and not only random errors but also burst errors, for example, Reed Solomon code RS (255, 245 Set coding algorithm such as BCH code and BCH code.

  FIG. 14 is a graph showing return optical delay (near end return) dependency of code rate, showing return optical delay dependency regarding code rate necessary to completely correct a byte code error. . According to this, if the near end return light delay is about 0.1 ns or more, the required coding rate is expected to be 0.7 or more and 1 or less.

[Effect of First Embodiment]
Thus, in the present embodiment, the simulation unit 14 transmits the transmission frame based on the return light parameter related to the return light to the optical transmitter 13 B generated on the propagation path to the optical communication device 20. The code error characteristic of the propagation path in the case is estimated, and the coding setting unit 15 calculates the coding rate used in the forward error correction coding algorithm based on the estimated code error characteristic, and the encoder 12 B It is set to.

  As a result, the code error characteristics in the case of considering the return light are estimated, and the coding rate calculated based on the obtained code error characteristics is used in the forward error correction coding algorithm. Therefore, even when return light is present on the propagation path, it is possible to perform data communication with the forward error correction code at the optimum coding rate according to the code error characteristic of the propagation path.

  Further, in the present embodiment, the simulation unit 14 calculates an estimated light signal estimated to reach the optical communication device 20 by applying the return light parameter to the rate equation simulating the propagation path. The code error characteristic of the propagation path may be estimated by comparing the estimated bit string obtained by digital conversion and decoding of the estimated light signal with the transmission bit string. Thereby, the code error characteristics can be estimated more accurately.

  Further, in the present embodiment, the simulation unit 14 compares the estimated bit sequence with the transmission bit sequence to count the number of symbol errors related to the encoded symbol, and the coding setting unit 15 determines the code based on the symbol error number. The coding rate may be calculated based on the error correction limit and the total number of bytes included in the transmission bit string by specifying an error correction limit indicating the number of error bytes to be corrected by the coding algorithm. This makes it possible to calculate the coding rate more accurately.

  Further, in the present embodiment, the simulation unit 14 compares the estimated bit sequence with the transmission bit sequence to count the number of bit errors and the number of symbol errors related to the encoded symbol, and the coding setting unit 15 calculates the number of bit errors and Based on the number of symbol errors, calculate a burst error index indicating the frequency at which the bit error occurrence interval is less than the coding symbol length, and select an appropriate coding algorithm for block error or burst error based on the burst error index. It may be set to the encoder 12B. This makes it possible to specify not only the coding rate but also the coding algorithm.

Further, in the present embodiment, the photon lifetime, the carrier lifetime, the first gain coefficient, the second gain coefficient, the transparent carrier density, the line width enhancement coefficient, and the spontaneous emission light coefficient for the optical transmitter 13B are respectively τ _p , _{τ c, G, G s,} N 0, α, and β and _sp, about the return light k returned light intensity coefficient and the returning light delay and R _k and tau _k, respectively, the carrier density of the semiconductor is N, the transmission bit sequence Assuming that the injection current density relating to the modulation signal is J and the electric field of the estimated optical signal and its phase component are E and φ , respectively , the equations (1) and (2) described above may be used as rate equations. This makes it possible to simulate the propagation path with extremely high accuracy, even in the presence of return light.

Second Embodiment
Next, an optical communication system 1 according to a second embodiment of the present invention will be described with reference to FIG. FIG. 15 is a block diagram showing the configuration of the optical communication system according to the second embodiment.
In the present embodiment, a code error related to a transmission frame transmitted from the optical communication apparatus 10 is observed by the optical communication apparatus 20, and a coding algorithm for forward error correction corresponding to the observation result, the optical communication apparatus 10 and the optical communication It is intended to be used in the device 20.

  In the present embodiment, the code error observation unit 24 of the optical communication device 20 receives the reception encoded bit string related to the transmission frame (downlink frame) from the optical communication device 10 received from the optical transmission line L2 by the optical transmission / reception unit 23 It has a function of observing the distribution regarding the number of symbol errors per error correction block based on (downstream encoded bit sequence).

The encoding unit 12 of the optical communication device 10 is a code for forward error correction appropriate to a block error or a burst error corresponding to the observation result observed by the code error observation unit 24 as a coding algorithm for forward error correction. It has a function of using an encoding algorithm in the encoder 12B.
The encoding unit 22 of the optical communication apparatus 20 is a code for forward error correction appropriate to a block error or a burst error corresponding to the observation result observed by the code error observation unit 24 as a coding algorithm for forward error correction. It has a function of using an encoding algorithm in the encoder 22B.

  At this time, the encoding error corresponding to the observation result may be identified by the code error observation unit 24 based on the observation result, and may be notified from the frame processing unit 21 to the encoding unit 12 or the encoding unit 22 or The encoding algorithm may be specified by the encoding unit 12 or the encoding unit 22 based on the observation result notified via the frame processing unit 21.

[Operation of Second Embodiment]
Next, code error observation processing in the code error observation unit 24 of the optical communication apparatus 20 will be described as an operation according to the present embodiment with reference to FIG. FIG. 16 is a flowchart showing code error observation processing. Here, the case where the code error observation unit 24 specifies the encoding algorithm for forward error correction corresponding to the observation result and notifies the encoding unit 12 and the encoding unit 22 from the frame processing unit 21 will be described as an example. Do.

  First, the code error observation unit 24 acquires a reception coded bit string (downlink coded bit string) related to a transmission frame (downlink frame) from the optical communication apparatus 10 received from the optical transmission line L2 by the optical transmission / reception unit 23 (Step 200) Based on the received coded bit sequence, the distribution regarding the number of symbol errors per error correction block is observed (step 201). With regard to the bit string used for this code error observation processing, the bit string for code error observation prepared in advance may be transmitted from the optical communication device 10, and the bit string used in the coding rate identification processing according to the first embodiment is used. It may be shared.

  FIG. 17 is a graph showing the symbol error number distribution per error correction block, where the horizontal axis is the number of symbol errors per correction block and the vertical axis is the frequency of the number of symbol errors generated. FIG. 18 is a main part enlarged graph of FIG. 17 in which the vicinity of frequency = 0 is enlarged.

  The number of symbol errors per error correction block is, for example, the number of symbols in which an error has occurred among 255 symbols (blocks) included in the 2040-bit error correction block. Since random errors occurring in the propagation path occur randomly, symbol errors per error correction block have a Poisson distribution as shown in FIG. On the other hand, since burst errors do not occur randomly but occur in a certain period, symbol errors per error correction block appear at a position outside the Poisson distribution as shown in FIG.

  The code error observing unit 24 confirms the presence or absence of the burst error based on the symbol error number distribution obtained as the observation result (step 202), and as shown in FIG. 202: YES), from the maximum value of the number of symbol errors corresponding to the burst error, specify the error correction limit regarding the propagation path to the optical communication device 10 (step 203), and handle burst errors based on the specified error correction limit The coding algorithm for forward error correction is identified (step 204), and the series of code error observation processing is ended.

  On the other hand, if there is no burst error (step 202: NO), the code error observing unit 24 specifies a forward error correction coding algorithm for random error (step 205), and a series of code error observations. End the process.

  Therefore, in the example of FIG. 18, since a symbol error occurs at a position out of the Poisson distribution, it is determined that there is a burst error. Further, if a burst error occurs once at a position where the number of symbol errors is about 70 at the maximum, and the error correction limit in the optical communication system 1 is 16 bytes, it is necessary to correct this burst error. The limit is specified as 70 bytes.

  As a result, redundant data increases by an amount corresponding to the error correction limit, and transfer efficiency decreases. Therefore, for example, by selecting an interleaving scheme based on Reed-Solomon code, and distributing and correcting code errors, the coding algorithm is improved such that the error correction limit is apparently lowered and the transfer efficiency is improved. May be set.

[Effect of Second Embodiment]
As described above, according to the present embodiment, the code error observing unit 24 of the optical communication device 20 observes the distribution regarding the number of symbol errors per correction block based on the received bit string received from the optical communication device 10 The encoder 12B of the apparatus 10, as a forward error correction coding algorithm, includes a forward error correction coding algorithm suitable for block errors or burst errors, corresponding to the observation result observed by the code error observation unit 24. It is intended to be used.

  As a result, optical communication can be performed using the optimal forward error correction encoding algorithm according to the code error characteristic regarding the propagation path between the optical communication device 10 and the optical communication device 20.

  Although the present embodiment has been described by way of example in which the coding algorithm for forward error correction used in actual optical communication is selected based on the observation result on the number of symbol errors per correction block, the present invention is limited thereto. It is not a thing. In the coding rate identification process described in the first embodiment, the forward error correction coding algorithm selected based on the observation result may be used as the forward error correction coding algorithm used by the simulation unit 14 . By this, when specifying the coding rate, it is possible to use the optimal forward error correction coding algorithm according to the code error characteristics regarding the actual propagation path, and as a result, specify the coding rate more accurately. It is possible to

[Extension of the embodiment]
As mentioned above, although this invention was demonstrated with reference to embodiment, this invention is not limited to the said embodiment. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention. Moreover, about each embodiment, it can combine arbitrarily and can be implemented in the not conflicting range.

  DESCRIPTION OF SYMBOLS 1 ... Optical communication system, 10 ... Optical communication apparatus (1st optical communication apparatus) 11, 11 ... Frame processing part, 12 ... Encoding part, 12A ... Decoder, 12B ... Encoder, 13 ... Optical transmission / reception part, 13A ... Optical receiver, 13 B: Optical transmitter, 14: Simulation unit, 15: Coding setting unit, 20: Optical communication device (second optical communication device), 21: Frame processing unit, 22: Coding unit, 22A ... Decoder, 22B: Coder, 23: Optical transceiver, 23A: Optical receiver, 23B: Optical transmitter, 30: Optical splitter, L1, L2, L3: Optical transmission path.

Claims (6)

Based on the forward error correction coding algorithm and the coding rate, the transmission bit sequence constituting the transmission frame is sequentially encoded by the encoder, then converted into an optical signal by the optical transmitter, and transmitted via the optical transmission path. An optical communication apparatus that transmits data to another optical communication apparatus,
The propagation path in the case where the transmission bit string is encoded and electro-optically converted and transmitted based on the return light parameter related to the return light to the optical transmitter generated on the propagation path to the other optical communication device A simulation unit that estimates code error characteristics;
A coding setting unit that calculates the coding rate used in a coding algorithm for forward error correction based on the estimated code error characteristic, and sets the coding rate in the encoder ;
The simulation unit calculates an estimated light signal estimated to reach the partner optical communication apparatus by applying the return light parameter to a rate equation simulating the propagation path, and obtaining the estimated light signal An optical communication apparatus characterized by comparing the estimated bit string obtained by converting and decoding D into a digital signal and the transmitted bit string to estimate the code error characteristic of the propagation path . In the optical communication device according to claim 1 ,
The simulation unit counts the number of symbol errors regarding a coding symbol by comparing the estimated bit sequence and the transmission bit sequence.
The coding setting unit specifies an error correction limit indicating the number of error bytes to be corrected by the coding algorithm based on the number of symbol errors, and the error correction limit and the total number of bytes included in the transmission bit string. The optical communication device, wherein the coding rate is calculated based on the above. In the optical communication device according to claim 1 ,
The simulation unit counts the number of bit errors and the number of symbol errors for a coded symbol by comparing the estimated bit sequence and the transmission bit sequence;
The coding setting unit calculates, based on the number of bit errors and the number of symbol errors, a burst error index indicating a frequency at which a bit error occurrence interval is less than a coding symbol length, and blocks based on the burst error index. An optical communication apparatus, wherein an encoding algorithm appropriate for an error or a burst error is selected and set in the encoder. In the optical communication device according to claim 1 ,
The photon lifetime, the carrier lifetime, the first gain coefficient, the second gain coefficient, the transparent carrier density, the line width enhancement coefficient, and the spontaneous emission coefficient relating to the optical transmitter are respectively τ _p , τ _c , G, G _s , Let N ₀ , α, and β _sp be the return light intensity coefficient and return light delay for the return light _k be R _k and τ _k respectively, let the semiconductor carrier density be N, and the injection current density for the modulation signal of the transmit bit string Letting J be the electric field of the estimated optical signal and its phase component be E and φ , respectively , the rate equation is

An optical communication device characterized by An optical communication system comprising first and second optical communication devices connected via an optical transmission line, wherein the first optical communication device comprises the optical communication according to any one of claims 1 to 4. An optical communication system comprising an apparatus. In the optical communication system according to claim 5 ,
The second optical communication device is
And a code error observation unit that observes a distribution related to the number of symbol errors per correction block based on the received bit sequence received from the first optical communication apparatus.
The first optical communication device is
A light is characterized by using, as a coding algorithm for forward error correction, a coding algorithm for forward error correction appropriate to block errors or burst errors, corresponding to the observation result observed by the code error observation unit. Communications system.

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