Optical wireless communication device

Abstract

This optical wireless communication device comprises: a light receiving unit that acquires, as received light, signal light transmitted through a free space; a coupling unit that couples the received light to an optical fiber; a demodulation unit that converts the received light coupled to the optical fiber into an electric signal and performs demodulation processing on the electric signal; a blocking unit that blocks the signal light or the received light for a given length of time at a stage prior to the demodulation unit with respect to the transmission direction of the received light, on the basis of a blocking instruction for the signal light or the received light; and a control unit that determines, in a predetermined cycle, whether or not the state of the demodulation unit is an abnormal state on the basis of the power of the received light, and outputs the blocking instruction to the blocking unit if the state of the demodulation unit is determined to be the abnormal state.

Description

Optical wireless communication device

[0001] The present invention relates to an optical wireless communication device.

[0002] In optical wireless communication, laser light is transmitted from an optical wireless communication device on the transmission side, and the laser light transmitted through free space is coupled to an optical fiber in the optical wireless communication device on the reception side. In free-space optical communication (FSO), absorption, scattering, and scintillation occur in the laser light due to atmospheric turbulence, so the coupling efficiency to the optical fiber fluctuates. As a result, the received optical power in the optical wireless communication device on the reception side fluctuates (see Non-Patent Documents 1 and 2).

[0003] It is possible to reduce the fluctuation of the received optical power by controlling the optical axis and wavefront of the laser light. However, particularly in an environment with strong atmospheric turbulence, it is not possible to completely remove the fluctuation of the received optical power. Therefore, it is necessary for the optical wireless communication device to have the ability to cope with the fluctuation of the received optical power.

[0004] In addition, an optical fiber transmission device including an optical transceiver (digital coherent transceiver) corresponding to digital coherent communication may be provided in the optical wireless communication device (see Non-Patent Document 3). Such an optical wireless communication device has a signal bit detection function using an analog circuit and a digital signal processor (DSP) as a function to cope with the fluctuation of the received optical power. Here, a signal error correction function using a digital signal processor strengthens the signal bit detection function.

[0005] Mohammed Elamassie and Murat Uysal, "Feedback-Free Adaptive Modulation Selection Algorithm for FSO Systems," IEEE Wireless Communications Letter, Vol.10, No.9, September 2021.Mohmed S. Salah, William G. Cowley and Khoa D. Nguyen, "Adaptive Transmission Schemes for Free-Space Optical Channel," 8th International Conference on Signal Processing and Communication Systems (ICSPCS), 2016.Anestis Tsakmalis, Symeon Chatzinotas and Bjorn Ottersten, "Automatic Modulation Classification for Adaptive Power Control in Cognitive Satellite Communications," 7th Advanced Satellite Multimedia Systems Conference and the 13th Signal Processing for Space Communications Workshop (ASMS / SPSC), 2014.

[0006] However, in devices that use optical fiber transmission equipment as a standalone unit, it is assumed that the optical transmission section consists only of optical fibers, and that free space is not included in the optical transmission section. In contrast, in optical wireless communication equipment equipped with optical fiber transmission equipment, depending on the fluctuation pattern of received optical power resulting from atmospheric turbulence in free space, the state of the demodulation unit (the digital signal processor of the optical fiber transmission equipment) that performs demodulation processing may enter an abnormal state. An abnormal state is, for example, a state in which the signal does not transmit even though the received optical power is sufficiently large (the state of the demodulation unit does not transition from link break). An abnormal state may also be, for example, a state in which signal error correction does not function even though the received optical power is sufficiently large.

[0007] In optical wireless communication devices equipped with optical fiber transmission equipment, the bit error rate decreases as the received optical power increases. In optical fiber transmission, the fluctuation range of the received optical power is small and the rate of fluctuation of the received optical power is slow, so the original bit error rate characteristics are realized as is.

[0008] In contrast, in free-space optical communication, the fluctuation range of the received optical power is large, and the rate of fluctuation in the received optical power is fast, which can prevent the original bit error rate characteristics from being fully realized. Thus, depending on the fluctuation pattern of the received optical power resulting from atmospheric turbulence, there is a problem in that the demodulation unit may not be able to recover from an abnormal state.

[0009] In view of the above circumstances, the present invention aims to provide an optical wireless communication device that can recover from an abnormal state in the demodulation unit regardless of the fluctuation pattern of the received optical power resulting from atmospheric turbulence.

[0010] One aspect of the present invention is an optical wireless communication device comprising: a light receiving unit that acquires signal light transmitted in free space as received light; a coupling unit that couples the received light to an optical fiber; a demodulation unit that converts the received light coupled to the optical fiber into an electrical signal and performs demodulation processing on the electrical signal; a blocking unit that, based on a blocking instruction for the signal light or the received light, blocks the signal light or the received light for a certain period of time in the transmission direction of the received light, prior to the demodulation unit; and a control unit that determines at predetermined intervals based on the power of the received light whether the state of the demodulation unit is abnormal, and outputs a blocking instruction to the blocking unit if it is determined that the state of the demodulation unit is abnormal.

[0011] The present invention makes it possible for the demodulation unit to recover from an abnormal state regardless of the fluctuation pattern of the received optical power resulting from atmospheric turbulence.

[0012] This figure shows an example configuration of the optical wireless communication system in the first embodiment. This figure shows an example of a state history table in the first embodiment. This flowchart shows an example of operation of the optical wireless communication device in the first embodiment. This figure shows an example configuration of the optical wireless communication system in the second embodiment. This figure shows an example configuration of the optical wireless communication system in the third embodiment. This figure shows an example of a state history table for blocking determination in the third embodiment. This figure shows an example of a state history table for change determination in the third embodiment. This flowchart shows an example of operation for optical blocking in the third embodiment. This flowchart shows an example of operation for parameter change in the third embodiment. This figure shows an example configuration of the optical wireless communication system in the fourth embodiment. This figure shows an example configuration of the optical wireless communication system in the fifth embodiment. This flowchart shows an example of operation for optical blocking in the fifth embodiment. This figure shows an example of hardware configuration of the optical wireless communication device in each embodiment.

[0013] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) Figure 1 is a diagram showing an example of the configuration of an optical wireless communication system 1a in the first embodiment. The optical wireless communication system 1a is a system that performs optical wireless communication (digital coherent communication) using signal light transmitted in free space. The signal light is, for example, laser light.

[0014] The optical wireless communication system 1a comprises an optical wireless communication device 2a and an optical wireless communication device 3a. The optical wireless communication device 3a comprises a light receiving unit 31, a coupling unit 32, a blocking unit 33, a demodulation unit 34, and a control unit 35a. The optical wireless communication device 3a includes a blocking unit 33 prior to the demodulation unit 34 with respect to the transmission direction of the received light. The control unit 35a comprises an acquisition unit 351, a history creation unit 352, a determination unit 353, a selection unit 354, and a modification unit 355.

[0015] In the following example, optical wireless communication device 2a (transmitting optical wireless communication device) transmits signal light to optical wireless communication device 3a (receiving optical wireless communication device) that matches the parameters used for demodulation processing in the demodulation unit 34. Optical wireless communication device 3a receives the signal light transmitted from optical wireless communication device 2a as received light.

[0016] In the following, as an example, the section from the optical wireless communication device 2a to the coupling unit 32 is the section in which signal light is transmitted through space (hereinafter referred to as the "space transmission section"). The section from the coupling unit 32 to the demodulation unit 34 is the section in which received light is transmitted through optical fiber (hereinafter referred to as the "optical fiber transmission section").

[0017] The light receiving unit 31 receives signal light arriving from the optical wireless communication device 2a in the spatial transmission section. That is, the light receiving unit 31 acquires the signal light arriving from the optical wireless communication device 2a as received light in the spatial transmission section. The light receiving unit 31 may change the transmission direction of the received light using, for example, a fast steering mirror (FSM) and a beam splitter. The light receiving unit 31 outputs the received light to the coupling unit 32.

[0018] The light-receiving unit 31 may also be equipped with a light-receiving sensor such as a photodiode. The light-receiving unit 31 may also be equipped with a quadrant detector (QD) sensor. The light-receiving unit 31 may also be equipped with optical elements such as a wavelength-separating optical filter and lens.

[0019] The coupling unit 32 couples the received light output from the light receiving unit 31 to the optical fiber connected to the blocking unit 33, for example, using a focusing lens. If no blocking instruction is input from the determination unit 353, the blocking unit 33 outputs the received light to the demodulation unit 34 by allowing the received light to pass through the optical fiber transmission section.

[0020] In order to restore the state of the demodulation unit 34 (DSP state) to a normal state, it is effective to forcibly cause a link break by temporarily blocking the received light before it is input to the demodulation unit 34. Therefore, when a blocking instruction is input from the determination unit 353, the blocking unit 33 blocks the received light transmitted to the demodulation unit 34 in the optical fiber transmission section for a certain period of time.

[0021] The demodulation unit 34 measures the received optical power of the received light coupled to the optical fiber at a predetermined period. This predetermined period may be, for example, a 1-second period or a 10-second period. The demodulation unit 34 (digital signal processor of the optical fiber transmission device) converts the received light coupled to the optical fiber into an electrical signal. The demodulation unit 34 performs demodulation processing on the converted electrical signal using the digital signal processor and parameters. The demodulation unit 34 obtains data transmitted from the optical wireless communication device 2a using signal light from the electrical signal through predetermined signal processing. The demodulation unit 34 calculates the bit error rate after error correction as an example of the bit error rate.

[0022] The control unit 35a controls the input of a cutoff instruction to the cutoff unit 33 and the change of parameters used by the demodulation unit 34 for demodulation processing.

[0023] If the acquisition unit 351 determines that the demodulation unit 34 is in a normal state (not in an abnormal state), it acquires status data of the received optical power and the bit error rate after error correction from the demodulation unit 34 at predetermined intervals. The acquisition unit 351 outputs each acquired status data to the history creation unit 352.

[0024] The acquisition unit 351 resumes acquiring status data when the state of the demodulation unit 34 is reset (when the interruption for a certain period of time is released). Furthermore, the acquisition unit 351 resumes acquiring status data when the parameters used by the demodulation unit 34 for demodulation processing are changed.

[0025] Figure 2 shows an example of a state history table (history of state data) in the first embodiment. The history creation unit 352 creates the history of state data, for example, in a table format. In the state history table illustrated in Figure 2, each state data, such as the received optical power and the bit error rate after error correction, is associated with the acquisition time of the state data. In the state history table, the history of the cutoff process and the history of the parameter change process may also be associated with the acquisition time of the state data.

[0026] Returning to Figure 1, we continue the explanation of the configuration example of the optical wireless communication system 1a. The determination unit 353 determines whether the received optical power is equal to or greater than the power reference value. If it is determined that the received optical power is equal to or greater than the power reference value, the determination unit 353 determines whether the bit error rate of the signal generated by the demodulation process is equal to or greater than the error rate reference value. If it is determined that the bit error rate is equal to or greater than the error rate reference value, the determination unit 353 determines that the state of the demodulation unit 34 is abnormal. If it is determined that the state of the demodulation unit 34 is abnormal, the determination unit 353 outputs a cutoff instruction to the cutoff unit 33.

[0027] Depending on the period in which the status data is acquired from the demodulation unit 34, it may be difficult to determine the status of the demodulation unit 34. In such cases, the determination unit 353 may determine whether the received optical power has been determined to be above the power reference value for a predetermined number of consecutive times (N times) or more. That is, the determination unit 353 may determine whether the number of consecutive times the received optical power has been determined to be above the power reference value is N times or more.

[0028] If the received optical power is determined to be equal to or greater than the power reference value for N consecutive times or more, the determination unit 353 may determine whether the bit error rate has been determined to be equal to or greater than the error rate reference value for N consecutive times or more. That is, the determination unit 353 may determine whether the number of times the bit error rate has been determined to be equal to or greater than the error rate reference value for N consecutive times or more is N or more.

[0029] If the bit error rate is determined to be above the error rate threshold for N consecutive times or more, the determination unit 353 determines that the state of the demodulation unit 34 is abnormal (not in a normal state). If the state of the demodulation unit 34 is determined to be abnormal, the determination unit 353 outputs a cutoff instruction to the cutoff unit 33.

[0030] The determination unit 353 determines whether the number of times the received light is blocked is equal to or greater than the threshold value. If the determination unit 353 determines that the number of times the received light is blocked is equal to or greater than the threshold value, it outputs a parameter selection instruction to the selection unit 354.

[0031] When the selection unit 354 receives a parameter selection instruction from the determination unit 353, it selects parameters to change the parameters used in the demodulation process. The parameters are, for example, parameters representing the modulation scheme or demodulation scheme. The parameters may also be, for example, parameters representing the transmission rate (modulation speed or demodulation speed).

[0032] In order to obtain the same bit error rate when the modulation speed is, for example, 200 Gbps and when the modulation speed is, for example, 100 Gbps, the received optical power when the modulation speed is 200 Gbps must be greater than the received optical power when the modulation speed is 100 Gbps.

[0033] Furthermore, when the modulation speed is 200 Gbps, the influence of fluctuations in received optical power is greater compared to when the modulation speed is 100 Gbps. Therefore, abnormal conditions are more likely to occur when the modulation speed is 200 Gbps than when the modulation speed is 100 Gbps.

[0034] Therefore, if the frequency of blocking the received light exceeds a certain threshold, the modification unit 355 changes, for example, the modulation speed and demodulation speed from 200 Gbps to 100 Gbps. The modification unit 355 outputs the selected parameters to the demodulation unit 34. This changes the parameters used for demodulation processing in the demodulation unit 34.

[0035] Next, an example of the operation of the optical wireless communication device 3a will be described. Figure 3 is a flowchart showing an example of the operation of the optical wireless communication device 3a in the first embodiment. For example, if the received optical power is equal to or greater than the power reference value, and the bit error rate of the signal generated by demodulation processing is equal to or greater than the error rate reference value, and this is determined to occur consecutively for a predetermined number of times (N times) or more, the control unit 35a determines that the state of the demodulation unit 34 is abnormal.

[0036] The acquisition unit 351 acquires status data of the received optical power and the bit error rate after error correction from the demodulation unit 34 at predetermined intervals (step S101). The determination unit 353 determines whether the received optical power has been determined to be equal to or greater than the power reference value for N consecutive times or more (step S102). If the number of times the received optical power has been determined to be equal to or greater than the power reference value for N consecutive times is less than N (step S102: NO), the determination unit 353 returns to step S101.

[0037] If the number of consecutive times the received optical power is determined to be equal to or greater than the power reference value is N or more (step S102: YES), the determination unit 353 determines whether or not the bit error rate of the signal has been determined to be equal to or greater than the error rate reference value for N or more consecutive times (step S103). If the number of consecutive times the number of times the bit error rate of the signal has been determined to be equal to or greater than the error rate reference value is less than N (step S103: NO), the determination unit 353 returns to step S101.

[0038] If the number of consecutive times the bit error rate of the signal is determined to be equal to or greater than the error rate threshold is N or more (step S103: YES), the determination unit 353 determines that the state of the demodulation unit 34 is abnormal. The determination unit 353 outputs a cutoff instruction to the cutoff unit 33 (step S104).

[0039] The determination unit 353 determines whether the number of times the received light is blocked is equal to or greater than the threshold value (step S105). If it is determined that the number of times the received light is blocked is less than the threshold value (step S105: NO), the determination unit 353 returns to step S101.

[0040] If it is determined that the number of times the received light is interrupted exceeds a certain threshold (meaning that parameters such as the demodulation method or demodulation speed are not optimal) (step S105: YES), the selection unit 354 selects parameters to change the parameters used in the demodulation process. The modification unit 355 outputs the selected parameters to the demodulation unit 34 (step S106). The determination unit 353 returns to step S101.

[0041] As described above, the optical wireless communication device 2a generates signal light that conforms to the parameters used for demodulation processing in the demodulation unit 34 through modulation processing. The optical wireless communication device 2a transmits the generated signal light to the optical wireless communication device 3a. The light receiving unit 31 acquires the signal light transmitted in free space as received light. The coupling unit 32 couples the received light to an optical fiber. The demodulation unit 34 converts the received light coupled to the optical fiber into an electrical signal. The demodulation unit 34 performs demodulation processing on the electrical signal according to the parameters. Based on the instruction to cut off the received light, the cutoff unit 33 cuts off the received light coupled to the optical fiber for a certain period of time in the transmission direction of the received light, prior to the demodulation unit 34. The control unit 35a determines at predetermined intervals whether the state of the demodulation unit 34 (DSP state) is in an abnormal state based on the power of the received light. If the control unit 35a determines that the state of the demodulation unit 34 is in an abnormal state, it outputs a cutoff instruction to the cutoff unit 33.

[0042] This allows the demodulation unit to recover from an abnormal state regardless of the fluctuation pattern of the received optical power resulting from atmospheric turbulence. Here, recovery from a predetermined abnormal state is possible based solely on the state data acquired from the demodulation unit 34.

[0043] (Second Embodiment) The main difference from the first embodiment is that the blocking unit 33 is provided in the spatial transmission section in the second embodiment. In the second embodiment, the description will focus on the differences from the first embodiment.

[0044] FIG. 4 is a diagram showing a configuration example of the optical wireless communication system 1b in the second embodiment. The optical wireless communication device 3b includes a light receiving unit 31, a coupling unit 32, a blocking unit 33, a demodulation unit 34, and a control unit 35b. The optical wireless communication device 3b includes the blocking unit 33 in front of the light receiving unit 31 with respect to the transmission direction of the received light.

[0045] When no blocking instruction is input from the determination unit 353, the blocking unit 33 outputs the signal light to the light receiving unit 31 by transmitting the signal light in the spatial transmission section. When a blocking instruction is input from the determination unit 353, the blocking unit 33 blocks the signal light for a certain period of time. The state of the demodulation unit 34 is reset by the blocking of the signal light (link disconnection).

[0046] When the state of the demodulation unit 34 is reset (when the blocking for a certain period of time is released), the acquisition unit 351 resumes the acquisition of the state data. Also, when the parameters used by the demodulation unit 34 for the demodulation process are changed, the acquisition unit 351 resumes the acquisition of the state data.

[0047] As described above, the light receiving unit 31 acquires the signal light transmitted through the free space as the received light. The coupling unit 32 couples the received light to the optical fiber. The demodulation unit 34 converts the received light coupled to the optical fiber into an electrical signal. The demodulation unit 34 performs a demodulation process according to the parameters on the electrical signal. The blocking unit 33 blocks the signal light transmitted through the free space for a certain period of time in front of the light receiving unit 31 and the demodulation unit 34 with respect to the transmission direction of the received light based on the blocking instruction of the signal light. The control unit 35a determines whether the state of the demodulation unit 34 is an abnormal state based on the received light power at a predetermined cycle. When it is determined that the state of the demodulation unit 34 is an abnormal state, the control unit 35a outputs a blocking instruction to the blocking unit 33.

[0048] As a result, regardless of the fluctuation pattern of the received optical power caused by atmospheric fluctuations, the state (DSP state) of the demodulation unit can return from an abnormal state.

[0049] (Third Embodiment) In the third embodiment, the main difference from the first and second embodiments is that the measurement unit measures the fluctuation of the received optical power (index value such as scintillation index) during a predetermined period. In the third embodiment, the description will focus on the differences from the first and second embodiments.

[0050] FIG. 5 is a diagram showing a configuration example of the optical wireless communication system 1c in the third embodiment. The optical wireless communication device 3c includes a light receiving unit 31, a coupling unit 32, a blocking unit 33, a demodulation unit 34, a control unit 35c, and a measurement unit 36. The optical wireless communication device 3c includes a blocking unit 33 at a stage prior to the light receiving unit 31 with respect to the transmission direction of the received light. The optical wireless communication device 3c includes a blocking unit 33 at a stage prior to the demodulation unit 34 with respect to the transmission direction of the received light.

[0051] FIG. 6 is a diagram showing an example of a state history table (history of state data) for blocking determination in the third embodiment. The history creation unit 352 creates a history of state data for blocking determination, for example, in a table format. In the state history table illustrated in FIG. 6, each state data of the received optical power and the bit error rate after error correction is associated with the acquisition time of the state data. In the state history table, the history of the blocking process may be associated with the acquisition time of the state data.

[0052] FIG. 7 is a diagram showing an example of a state history table (history of state data) for change determination in the third embodiment. The measurement unit 36 is, for example, an oscilloscope. The measurement unit 36 measures each state data of the received optical power and the fluctuation of the signal optical power or the received optical power. The history creation unit 352 creates a history of state data for change determination, for example, in a table format. In the state history table illustrated in FIG. 7, each state data of the average value of the received optical power during a predetermined period and the average value of the fluctuation of the received optical power during a predetermined period is associated with the measurement time of the state data. In the state history table, the history of the blocking process and the history of the parameter change process may be associated with the measurement time of the state data.

[0053] Next, an example of the operation of the optical wireless communication device 3c will be described. Figure 8 is a flowchart showing an example of the operation of light blocking in the third embodiment. Each step from step S201 to step S204 illustrated in Figure 8 is the same as each step from step S101 to step S104 illustrated in Figure 3. If the determination unit 353 executes step S204, it returns the process to step S201.

[0054] Figure 9 is a flowchart showing an example of parameter change operation in the third embodiment. For example, if, during a predetermined period, the average value of the received optical power is equal to or greater than the power reference value, the average value of the fluctuations is equal to or greater than the fluctuation reference value, and the number of times the received optical signal is interrupted is equal to or greater than the count reference value, the control unit 35c changes the parameters used for demodulation processing.

[0055] The acquisition unit 351 acquires state data of the received optical power and the signal optical power or fluctuations of the received optical power from the measurement unit 36 ​​at predetermined intervals (step S301). The determination unit 353 determines whether the average value of the received optical power over a predetermined period is equal to or greater than the power reference value (step S302). If it is determined that the average value of the received optical power over a predetermined period is less than the power reference value (step S302: NO), the determination unit 353 returns to step S301.

[0056] If it is determined that the average value of the received optical power over a predetermined period is equal to or greater than the power reference value (step S302: YES), the determination unit 353 determines whether the average value of the fluctuations in the signal optical power or received optical power over the predetermined period is equal to or greater than the fluctuation reference value (step S303). If it is determined that the average value of the fluctuations is less than the fluctuation reference value (step S303: NO), the determination unit 353 returns to step S301.

[0057] If it is determined that the average value of the fluctuations is equal to or greater than the fluctuation reference value (step S303: YES), the determination unit 353 determines whether the number of times the received light is blocked is equal to or greater than the count reference value (step S304). If it is determined that the number of times the received light is blocked is less than the count reference value (step S304: NO), the determination unit 353 returns to step S301.

[0058] If it is determined that the number of times the received light is interrupted is equal to or greater than the threshold value (step S304: YES), the selection unit 354 selects parameters to change the parameters used in the demodulation process. The modification unit 355 outputs the selected parameters to the demodulation unit 34 (step S305). The determination unit 353 returns to step S301.

[0059] As described above, the blocking unit 33, based on the instruction to block the received light, blocks the received light coupled to the optical fiber for a certain period of time, prior to the demodulation unit 34 in the transmission direction of the received light. The measurement unit 36 ​​measures the fluctuation of the received light power over a predetermined period. The measurement unit 36 ​​measures an index value, such as the scintillation index, as the fluctuation of the received light power. The scintillation index is an index value that indicates the degree of fluctuation of the light power. The larger the value of the scintillation index, the greater the fluctuation of the light power. The control unit 35c changes the parameters used for demodulation processing if, over a predetermined period, the average value of the received light power is equal to or greater than the power reference value, the average value of the fluctuation is equal to or greater than the fluctuation reference value, and the number of times the received light is blocked is equal to or greater than the count reference value.

[0060] This allows the demodulation unit to recover from an abnormal state, regardless of the fluctuation pattern of the received optical power resulting from atmospheric turbulence.

[0061] (Fourth Embodiment) In the fourth embodiment, the main difference from the third embodiment is that the blocking unit 33 is provided in the spatial transmission section. The fourth embodiment will be explained focusing on the differences from the third embodiment.

[0062] Figure 10 shows an example of the configuration of an optical wireless communication system 1d in the fourth embodiment. The optical wireless communication device 3d comprises a light receiving unit 31, a coupling unit 32, a blocking unit 33, a demodulation unit 34, a control unit 35d, and a measurement unit 36. The optical wireless communication device 3d includes a blocking unit 33 prior to the light receiving unit 31 with respect to the transmission direction of the received light.

[0063] If no blocking instruction is input from the determination unit 353, the blocking unit 33 outputs signal light to the light receiving unit 31 by allowing signal light to pass through the spatial transmission section. If a blocking instruction is input from the determination unit 353, the blocking unit 33 blocks the signal light transmitted in free space for a certain period of time. The state of the demodulation unit 34 is reset by the blocking of the signal light (link break).

[0064] As described above, the blocking unit 33, based on the signal light blocking instruction, blocks the signal light transmitted in free space for a certain period of time in relation to the transmission direction of the received light, prior to the light receiving unit 31 and the demodulation unit 34. The measurement unit 36 ​​measures the fluctuations in the power of the received light over a predetermined period. The control unit 35d changes the parameters used for demodulation processing if, during the predetermined period, the average value of the power of the received light is greater than or equal to the power reference value, the average value of the fluctuations is greater than or equal to the fluctuation reference value, and the number of times the received light is blocked is greater than or equal to the count reference value.

[0065] This allows the demodulation unit to recover from an abnormal state, regardless of the fluctuation pattern of the received optical power resulting from atmospheric turbulence.

[0066] (Fifth Embodiment) In the fifth embodiment, the main difference from the first to fourth embodiments is that the signal light cutoff instruction is generated based on the relationship between the index value of the fluctuation of the received optical power and the low level of the received optical power. The fifth embodiment will be explained focusing on the differences from the first to fourth embodiments.

[0067] Figure 11 shows an example of the configuration of an optical wireless communication system 1e in the fifth embodiment. The optical wireless communication device 3e includes a light receiving unit 31, a coupling unit 32, a blocking unit 33, a demodulation unit 34, a control unit 35e, and a measurement unit 36. The optical wireless communication device 3e includes a blocking unit 33 prior to the demodulation unit 34 with respect to the transmission direction of the received light. The control unit 35e includes an acquisition unit 351, a history creation unit 352, and a determination unit 353.

[0068] The blocking unit 33 may include a delay unit 331 (delay circuit). The delay unit 331 reduces the time difference between the timing of measuring fluctuations and the timing of blocking the received light by delaying the transmission of the received light.

[0069] Next, an example of the operation of the optical wireless communication device 3e will be described. Figure 12 is a flowchart showing an example of the operation of light blocking in the fifth embodiment. If the time during which the fluctuation of the received optical power is above the fluctuation reference value is above the reference time, and the received optical power is below the power reference value, there is a high possibility that the state of the demodulation unit 34 will be abnormal. In such a case, as a preventive measure to avoid the state of the demodulation unit 34 being abnormal, the blocking unit 33 blocks the received light based on the shielding instruction.

[0070] The acquisition unit 351 acquires state data of the received optical power and the signal optical power or fluctuation of the received optical power from the measurement unit 36 ​​at a predetermined period (step S401). The determination unit 353 determines whether the fluctuation of the received optical power is equal to or greater than the fluctuation reference value (step S402). If it is determined that the fluctuation is less than the fluctuation reference value (step S402: NO), the determination unit 353 returns to step S401.

[0071] If it is determined that the fluctuation is equal to or greater than the fluctuation reference value (step S402: YES), the determination unit 353 determines whether the time during which the fluctuation is equal to or greater than the fluctuation reference value is equal to or greater than the reference time (step S403). If it is determined that the time during which the fluctuation is equal to or greater than the fluctuation reference value is less than the reference time (step S403: NO), the determination unit 353 returns to step S401.

[0072] If it is determined that the time during which the fluctuation is above the fluctuation reference value is above the reference time (step S403: YES), the determination unit 353 determines whether or not the received optical power is below the power reference value (step S404). If it is determined that the received optical power is above the power reference value (step S404: NO), the determination unit 353 returns to step S401.

[0073] If the received optical power is determined to be below the power reference value (step S404: YES), the determination unit 353 determines that the state of the demodulation unit 34 is abnormal. The determination unit 353 outputs a cutoff instruction to the cutoff unit 33 (step S405). The determination unit 353 returns to step S401.

[0074] As described above, the measurement unit 36 ​​measures the fluctuations in the power of the received light (for example, an index value such as the scintillation index) over a predetermined period. The control unit 35e determines that the demodulation unit is in an abnormal state if, during the predetermined period, the average value of the fluctuations is greater than or equal to the fluctuation reference value, the time during which the average value of the fluctuations is greater than or equal to the fluctuation reference value is greater than or equal to the reference time, and the power of the received light is less than the power reference value.

[0075] This allows the demodulation unit to recover from an abnormal state, regardless of the fluctuation pattern of the received optical power resulting from atmospheric turbulence.

[0076] The control unit of the device of the present invention can also be implemented by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

[0077] (Hardware Configuration) Figure 13 shows an example of the hardware configuration of the optical wireless communication device 10 in each embodiment. The example of the hardware configuration of the optical wireless communication device 10 corresponds to the example of the hardware configuration of the optical wireless communication device in each embodiment.

[0078] The optical wireless communication device 10 is implemented as software by a processor 101 such as a CPU (Central Processing Unit) executing a program stored in a storage device 103 and a memory 102 having a non-volatile recording medium (non-temporary recording medium). The program may be recorded on a computer-readable recording medium. A computer-readable recording medium is a non-temporary recording medium such as a portable medium such as a flexible disk, magneto-optical disk, ROM (Read Only Memory), CD-ROM (Compact Disc Read Only Memory), or a storage device such as a hard disk or solid-state drive (SSD) built into a computer system. The communication device 104 performs predetermined communication processing (for example, modulation processing and transmission of signal light on the transmitting side, and reception and demodulation processing of signal light on the receiving side).

[0079] The optical wireless communication device 10 may be implemented using hardware that includes electronic circuits (electronic circuits or circuits) such as LSI (Large Scale Integrated Circuit), ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or FPGA (Field Programmable Gate Array).

[0080] Although embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention.

[0081] The present invention is applicable to optical wireless communication systems.

[0082] 1a, 1b, 1c, 1d, 1e... Optical wireless communication system, 2a, 2b, 2c, 2d, 2e... Optical wireless communication device, 3a, 3b, 3c, 3d, 3e... Optical wireless communication device, 10... Optical wireless communication device, 31... Light receiving unit, 32... Coupling unit, 33... Blocking unit, 34... Demodulation unit, 35a, 35b, 35c, 35d, 35e... Control unit, 36... Measurement unit, 101... Processor, 102... Memory, 103... Storage device, 104... Communication device, 331... Delay unit, 351... Acquisition unit, 352... History creation unit, 353... Judgment unit, 354... Selection unit, 355... Change unit

Claims

1. An optical wireless communication device comprising: a light receiving unit that acquires signal light transmitted in free space as received light; a coupling unit that couples the received light to an optical fiber; a demodulation unit that converts the received light coupled to the optical fiber into an electrical signal and performs demodulation processing on the electrical signal; a blocking unit that, based on a blocking instruction for the signal light or the received light, blocks the signal light or the received light for a certain period of time in the transmission direction of the received light, prior to the demodulation unit; and a control unit that determines at predetermined intervals based on the power of the received light whether the state of the demodulation unit is abnormal, and outputs a blocking instruction to the blocking unit if it is determined that the state of the demodulation unit is abnormal.

2. The optical wireless communication device according to claim 1, wherein the blocking unit blocks the received light coupled to the optical fiber based on the blocking instruction.

3. The optical wireless communication device according to claim 1, wherein the blocking unit blocks the signal light transmitted in the free space based on the blocking instruction.

4. The optical wireless communication device according to claim 1, wherein the control unit determines that the state of the demodulation unit is abnormal if it has determined for a predetermined number of consecutive times that the power of the received light is equal to or greater than a power reference value and the bit error rate of the signal generated by the demodulation process is equal to or greater than an error rate reference value.

5. The optical wireless communication device according to claim 1, further comprising a measuring unit for measuring fluctuations in the power of the received light over a predetermined period, wherein the control unit changes the parameters used for demodulation processing if, during the predetermined period, the average value of the power of the received light is equal to or greater than a power reference value, the average value of the fluctuations is equal to or greater than a fluctuation reference value, and the number of interruptions of the received light is equal to or greater than a count reference value.

6. The optical wireless communication device according to claim 1, further comprising a measuring unit for measuring fluctuations in the power of the received light over a predetermined period, wherein the control unit determines that the state of the demodulation unit is abnormal if, during the predetermined period, the average value of the fluctuations is equal to or greater than a fluctuation reference value, the time during which the average value of the fluctuations is equal to or greater than the fluctuation reference value is equal to or greater than a reference time, and the power of the received light is less than a power reference value.

Images