Optical communication system and optical receiving device

Abstract

To provide a technique for suppressing deterioration of a signal-to-noise ratio in an optical receiving device in an optical communication system that transmits multiple pieces of information via a plurality of paths.SOLUTION: An optical transmitting device generates N (N is an integer of 2 or more) second signals by processing N first signals with a transfer function, inserts a training signal into each of the N second signals, and transmits the N second signals into each of which the training signal has been inserted to the optical receiving device via N paths. The optical receiving device includes: output means for removing the training signal from the signals received via the N paths and outputting N third signals; determination means for determining the signal-to-noise ratio of each of the N third signals based on the training signal of the signals received via the N paths; and second generation means for generating N fourth signals by processing a third signal having a signal-to-noise ratio higher than a threshold value among the N third signals with an inverse matrix of the transfer function.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to optical communication technology. [Background technology]

[0002] Free Space Optics (FSO) communication is a communication method that releases optical signals into free space instead of using fixed media such as optical fibers. However, in FSO communication, the received power at the optical receiving device may fluctuate due to the intrusion of an obstruction into the optical path from the optical transmitting device to the optical receiving device, and the signal-to-noise ratio (SNR) may deteriorate. For this reason, Non-Patent Document 1 discloses a configuration that reduces the fluctuation in received power by transmitting one signal via multiple paths (optical paths) and combining the signals received via each of the multiple paths to return to one signal.

[0003] Patent Document 1 also discloses a configuration in which a plurality of first signals are processed based on a transfer function to generate a plurality of second signals, and each of the plurality of second signals is transmitted through a different path. In an optical receiving device, each of the plurality of first signals is restored by processing a plurality of second signals received via each of the different paths based on an inverse matrix of the transfer function. In the configuration of Patent Document 1, even if the signal-to-noise ratio (SNR) of a certain path deteriorates, the decrease in SNR is distributed to each of the plurality of first signals, and it is possible to prevent a large deterioration in the SNR of one of the plurality of first signals. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2018-113566 A [Non-patent literature]

[0005] [Non-Patent Document 1] MAFernandes,et.al.,"400G MIMO-FSO Transmission with Enhanced Reliability Enabled by Joint LDPC Coding",2021 European Conference on Optical Communication(ECOC),2021,pp.1-4 Summary of the Invention [Problem to be solved by the invention]

[0006] In the configuration of Patent Document 1, the process in the optical receiving device adds components of a first signal of interest among multiple first signal components contained in multiple second signals received via multiple paths, and cancels out components of the first signal that are different from the first signal of interest, thereby restoring the first signal of interest. However, if a path is blocked and the signal from that path cannot be received, or if the signal-to-noise ratio of the signal from that path is significantly degraded, the noise components from that path will affect the restored signal.

[0007] The present invention provides a technique for suppressing degradation of the signal-to-noise ratio in an optical receiving device in an optical communication system that transmits a plurality of pieces of information via a plurality of paths. [Means for solving the problem]

[0008] According to one embodiment of the present invention, an optical communication system includes an optical transmitting device and an optical receiving device, wherein the optical transmitting device comprises a first generating means for generating N second signals by processing N (N is an integer equal to or greater than 2) first signals carrying data with a transfer function, an inserting means for inserting a training signal into each of the N second signals, and a transmitting means for transmitting the N second signals with the training signal inserted to the optical receiving device via N paths, wherein the optical receiving device comprises an outputting means for removing the training signal from each of the signals received via the N paths and outputting N third signals, a determining means for determining a signal-to-noise ratio of each of the N third signals based on the training signal for each of the signals received via the N paths, and a second generating means for generating N fourth signals by processing third signals among the N third signals whose signal-to-noise ratio is higher than a threshold with an inverse matrix of the transfer function. Effect of the Invention

[0009] According to the present invention, in an optical communication system that transmits a plurality of pieces of information via a plurality of paths, it is possible to suppress deterioration of the signal-to-noise ratio in an optical receiving device. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a configuration diagram of an optical communication system according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. In addition, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0012] 1 is a configuration diagram of an optical communication system according to this embodiment. In the following description, the optical communication system is assumed to be an FSO communication system. Signals X1(t) and X2(t) are signals carrying information (or data) #1 and information (or data) #2, respectively. An optical transmitter 1 transmits information #1 and information #2 to an optical receiver 2. For this purpose, a processor 10 processes signals X1(t) and X2(t) with a transfer function H to generate signals Y1(t) and Y2(t).

[0013] In this embodiment, the transfer function H is expressed as follows:

[0014]

number

[0015]

number

[0016] The inserter 11 periodically inserts a training signal into each of the signals Y1(t) and Y2(t). The training signals inserted into each of the signals Y1(t) and Y2(t) are known to the optical receiver 2. The optical transmitter 1 generates a first modulated light by modulating a continuous light with the signal Y1(t) into which the training signal has been inserted, and transmits the first modulated light to the optical receiver 2 via the path 31. Similarly, the optical transmitter 1 generates a second modulated light by modulating a continuous light with the signal Y2(t) into which the training signal has been inserted, and transmits the second modulated light to the optical receiver 2 via the path 32.

[0017] Noise N1(t) and noise N2(t) are superimposed on the first modulated light and the second modulated light during the process of propagation through space. The optical receiver 2 receives the first modulated light with noise superimposed thereon via the path 31, receives the second modulated light with noise superimposed thereon via the path 32, demodulates the first modulated light and the second modulated light, and outputs two electric signals. These two electric signals correspond to signals Y1(t) and Y2(t) with training signals inserted therein and with noise N1(t) and noise N2(t) superimposed thereon. The extractor 21 extracts the training signals inserted in these two electric signals, and outputs the extracted training signals to the controller 22. The extractor 21 also outputs the two electric signals after removing the training signals to the transmission / blocking unit 23. The two electrical signals after removing the training signal correspond to the signals Y1(t) and Y2(t) with noise N1(t) and N2(t) superimposed thereon, and are denoted as signals Y1'(t) and Y2'(t). Signals Y1'(t) and Y2'(t) are as follows:

[0018]

number

[0019] The transmission / blocking unit 23 normally outputs the signal Y1'(t) and the signal Y2'(t) to the processing unit 20 as is. The processing unit 20 calculates the inverse matrix H -1 The inverse matrix H -1 teeth,

[0020]

number

[0021]

number

[0022] By substituting equations (1) and (2) into equation (3), the signal X1′ becomes:

[0023]

number

[0024] The second and third terms of the above equation (5) are components of the same signal but in opposite phase to each other, so they cancel each other out. Therefore, the signal X1' becomes as follows:

[0025]

number

[0026] As shown in equation (6), signal X1'(t) is signal X1(t) superimposed with noise N1(t) from path 31 and noise N2(t) from path 32. Therefore, optical receiving device 2 can receive information #1 based on signal X1'(t). The same is true for signal X2'(t). Here, when the power of the signal component of the second modulated light becomes very weak and signal Y2'(t) consists only of noise N2(t) from path 32, equation (3) becomes as follows.

[0027]

number

[0028] By substituting equation (1) into equation (7), the signal X1'(t) becomes as follows.

[0029]

number

[0030] According to equation (8), signal X1'(t) is signal X1(t) with signal X2(t) superimposed thereon as interference, and with noise N1(t) from path 31 and noise N2(t) from path 32 superimposed thereon.

[0031] For this reason, in this embodiment, the control unit 22 determines the signal-to-noise ratios of the signals Y1'(t) and Y2'(t) based on the training signals extracted from the first modulated light and the second modulated light on which noise is superimposed. Then, the control unit 22 controls the transmission control unit 23 to output only the signals having the determined signal-to-noise ratios higher than a threshold value among the signals Y1'(t) and Y2'(t) to the processing unit 20, and to prevent the signals having the determined signal-to-noise ratios not higher than the threshold value from being input to the processing unit 20.

[0032] For example, when the signal-to-noise ratio of the signal Y2'(t) is deteriorated below a threshold, the signal Y2'(t) input to the processing unit 20 becomes 0. In this case, the formula (3) becomes as follows.

[0033]

number

[0034] By substituting equation (1) into equation (9), the signal X1'(t) is as follows:

[0035]

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[0036] Comparing equation (8) and equation (10), it can be seen that when the signal-to-noise ratio of signal Y2'(t) deteriorates due to an increase in noise N2(t) on path 32 or a weakening of the signal component of the second modulated light due to the influence of an obstruction on path 32, the noise component contained in signal X1'(t) can be reduced by blocking signal Y2'(t) using transmission blocking unit 23 to prevent it from being input to processing unit 20.

[0037] In addition, in the signal X1'(t), the signal X2(t) is superimposed as an interference signal on the signal of interest X1(t) (hereinafter, the signal of interest), but by appropriately selecting the modulation method, modulation rate, etc., the optical receiving device 2 can restore the information #1 carried by the signal of interest X1(t). In addition, in Fig. 1, for the sake of simplicity, the two pieces of information #1 and #2 are transmitted via two paths 31 and 32, but by increasing the number of paths N (N=2 in Fig. 1), the level of the signal of interest included in each signal output by the processing unit 20 can be increased.

[0038] In this embodiment, the output from the extraction unit 21 is described as an electrical signal, but it is also possible to provide an analog-to-digital converter (ADC) within the extraction unit 21 and output the waveform of the electrical signal expressed as numerical time-series data. In this case, the training signal extraction operation of the extraction unit 21 and the output operation of the electrical signal from which the training signal has been removed, the control unit 22, the equivalent cutoff unit 23, and the processing unit 20 can be implemented by digital signal processing on a computer.

[0039] In this embodiment, the optical communication system is an FSO communication system, but the present invention can also be applied to an optical communication system using optical fibers.

[0040] The above configuration makes it possible to suppress degradation of the signal-to-noise ratio in optical receiving devices in optical communication systems that transmit multiple pieces of information via multiple paths, thereby contributing to Goal 9 of the United Nations Sustainable Development Goals (SDGs) "Build resilient infrastructure, promote sustainable industrialization and foster innovation." [Explanation of symbols]

[0041] 10, 20: processing section, 11: insertion section, 21: extraction section, 22: control section, 23: transmission blocking section

Claims

1. An optical communication system including an optical transmitting device and an optical receiving device, The optical transmitter comprises: a first generating means for generating N second signals by processing N first signals (N being an integer equal to or greater than 1) carrying data with a transfer function; an insertion means for inserting a training signal into each of the N second signals; a transmitting means for transmitting the N second signals into which the training signal has been inserted to the optical receiving device via N paths; Equipped with The optical receiving device comprises: an output means for removing the training signal from each of the signals received via the N paths and outputting N third signals; a determining means for determining a signal-to-noise ratio of each of the N third signals based on the training signal of each of the signals received via the N paths; a second generating means for generating N fourth signals by processing, among the N third signals, a third signal having a signal-to-noise ratio higher than a threshold value using an inverse matrix of the transfer function; An optical communication system comprising:

2. The optical receiving device comprises:

2. The optical communication system according to claim 1, further comprising a pass-blocking means provided between said output means and said second generating means, for outputting to said second generating means those third signals among said N third signals whose signal-to-noise ratio is higher than said threshold, and for blocking those third signals among said N third signals whose signal-to-noise ratio is not higher than said threshold.

3. An optical receiving device that receives a signal having a training signal inserted therein from an optical transmitting device via N paths (N is an integer equal to or greater than 2), an output means for removing the training signal from each of the signals received via the N paths and outputting N first signals; determining means for determining a signal-to-noise ratio for each of the N first signals based on the training signal for each of the signals received via the N paths; a generating means for generating N second signals by processing first signals having a signal-to-noise ratio higher than a threshold value among the N first signals with an inverse matrix of a transfer function used for generating signals to be transmitted to the N paths by the optical transmitting device; An optical receiving device comprising:

4. 4. The optical receiving device according to claim 3, further comprising a pass-blocking means provided between the output means and the generating means, for outputting to the generating means the first signals among the N first signals whose signal-to-noise ratio is higher than the threshold value, and for blocking the first signals among the N first signals whose signal-to-noise ratio is not higher than the threshold value.

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