Communication device, communication system, and signal generation method

Abstract

This communication device comprises: a measurement unit that measures the polarization state of uplink laser light acquired from a remote station via an optical transmission path; and a control unit that uses a polarizer for each polarization component in accordance with the polarization state to correct at least one of the polarization of a downlink optical signal to be transmitted to the remote station via the optical transmission path and the polarization of an uplink optical signal acquired from the remote station via the optical transmission path. The communication device may further comprise a light source that generates downlink laser light. The measurement unit may acquire, as the uplink laser light, the downlink laser light reflected by the remote station.

Description

Communication device, communication system, and signal generation method

[0001] The present invention relates to a communication device, a communication system, and a signal generation method.

[0002] In wireless communication systems using analog RoF (Radio-over-Fiber), simple, extended stations are deployed in the wireless communication area. This enables a flexible and economical wireless communication system. Furthermore, when MIMO (Multiple Input Multiple Output) is implemented in wireless communication systems using analog RoF, multiple stream signals may be transmitted using wavelength division multiplexing (WDM), frequency division multiplexing (FDM), or multiple optical transmission lines (e.g., multicore fibers) (see Non-Patent Literature 1).

[0003] K. Kikuchi, “Fundamentals of coherent optical fiber communications,” J. Lightwave Technol., vol.34, no.1, pp.157-179, Jan. 2016.

[0004] In single-mode fiber (SMF), the state of polarization (SOP) of the optical signal changes in response to fluctuations in the optical transmission path. Therefore, the receiving communication device needs to measure and correct the polarization of the optical signal. However, since the receiving communication device needs to be equipped with a balanced photodiode and a digital signal processor (DSP), the configuration of the receiving communication device (the outbound station) becomes complex.

[0005] Thus, in wireless communication systems where analog RoF is applied, there is a problem in that the polarization cannot be corrected based on the polarization state measurement results unless the configuration of the extended station is made complex.

[0006] In view of the above circumstances, the present invention aims to provide a communication device, a communication system, and a signal generation method that can correct polarization based on the measurement results of the polarization state, even if the configuration of the extended station is simple, in a wireless communication system to which analog RoF is applied.

[0007] One aspect of the present invention is a communication device comprising: a measuring unit that measures the polarization state of uplink laser light acquired from an extended station via an optical transmission path; and a control unit that corrects at least one of the polarization of the downlink optical signal transmitted to the extended station via the optical transmission path and the polarization of the uplink optical signal acquired from the extended station via the optical transmission path, using polarizers for each polarization component according to the polarization state.

[0008] One aspect of the present invention is a communication system comprising a central station and an extended station, wherein the extended station has an output unit that outputs uplink laser light to an optical transmission path, and the central station has a measuring unit that measures the polarization state of the uplink laser light acquired from the extended station via the optical transmission path, and a control unit that corrects at least one of the polarization of the downlink optical signal transmitted to the extended station via the optical transmission path and the polarization of the uplink optical signal acquired from the extended station via the optical transmission path, using polarizers for each polarization component according to the polarization state.

[0009] One aspect of the present invention is a signal generation method performed by a communication device, comprising the steps of: measuring the polarization state of uplink laser light acquired from an extended station via an optical transmission path; and correcting at least one of the polarization of a downlink optical signal transmitted to the extended station via the optical transmission path and the polarization of an uplink optical signal acquired from the extended station via the optical transmission path, using polarizers for each polarization component according to the polarization state.

[0010] The present invention makes it possible to correct polarization based on the polarization state measurement results in a wireless communication system to which analog RoF is applied, even if the configuration of the extended station is simple.

[0011] This figure shows an example of the configuration of the communication system in the first embodiment. This figure shows an example of the correspondence between Stokes vectors and Jones vectors in the first embodiment, for each polarization state. This is a sequence diagram showing an example of the operation of the communication system in the first embodiment. This figure shows an example of the configuration of the communication system in the second embodiment. This is a sequence diagram showing an example of the operation of the communication system in the second embodiment. This figure shows an example of the configuration of the communication system in the third embodiment. This is a sequence diagram showing an example of the operation of the communication system in the third embodiment. This figure shows an example of the configuration of the communication system in the fourth embodiment. This is a sequence diagram showing an example of the operation of the communication system in the fourth embodiment. This figure shows an example of the hardware configuration of the communication device in each embodiment.

[0012] 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 a communication system 1a in the first embodiment. The communication system 1a is a wireless communication system to which analog RoF is applied. The communication system 1a comprises a central station 2a and one or more branch stations 3a. Hereinafter, the direction from the central station to the branch station will be referred to as "downlink". The direction from the branch station to the central station will be referred to as "uplink".

[0013] The aggregation station 2a is a communication device that serves as the master unit for analog RoF, and is, for example, a base station, CS (Central Station), CU (Central Unit), DU (Distributed Unit), or uplink RU (Radio Unit).

[0014] The aggregation station 2a comprises a control unit 21a, a light source 22a, a measurement unit 23a, two light-emitting elements 24, two polarizers 25, a polarization combiner 26, a splitter 27, two polarizers 28, and two photoelectric converters 29. Here, the aggregation station 2a is equipped with light-emitting element 24-1 and polarizer 25-1 for correcting V polarization in the downstream optical signal (pre-polarization correction). The aggregation station 2a is equipped with light-emitting element 24-2 and polarizer 25-2 for correcting H polarization in the downstream optical signal (pre-polarization correction). The aggregation station 2a is equipped with polarizer 28-1 and photoelectric converter 29-1, respectively, for correcting V polarization in the upstream optical signal (post-polarization correction). The aggregation station 2a is equipped with a polarizer 28-2 and a photoelectric converter 29-2, respectively, for correcting the H polarization in the upstream optical signal (for post-polarization correction).

[0015] The extension station 3a is a communication device acting as a slave unit of an analog RoF, and is, for example, a TRP (Transmission and Reception Point), RRU (Remote Radio Unit), RRH (Remote Radio Head), a downstream RU (Radio Unit), a distributed antenna (DA), an antenna, a panel, a TP (Transmission Point), or an RP (Reception Point). The extension station 3a is connected to the aggregation station 2a via an optical fiber 4. The optical fiber 4 is, for example, a single-mode fiber.

[0016] The projecting station 3a comprises a reflector 31, a polarization separator 32, two photoelectric converters 33, two light-emitting elements 34, and a polarization combiner 35. Here, the projecting station 3a includes a photoelectric converter 33-1 for photoelectric conversion of the downlink optical signal (V-polarized signal). The projecting station 3a includes a photoelectric converter 33-2 for photoelectric conversion of the downlink optical signal (H-polarized signal). The projecting station 3a includes a light-emitting element 34-1 for generating the uplink optical signal (V-polarized signal). The projecting station 3a includes a light-emitting element 34-2 for generating the uplink optical signal (H-polarized signal).

[0017] First, let's explain the measurement (observation) of the polarization state. The control unit 21a instructs the light source 22a to transmit a downstream laser beam (reference laser beam for polarization measurement) having, for example, two polarization components (V polarization component and H polarization component). The light source 22a is, for example, a laser diode. The light source 22a transmits the downstream laser beam to the reflector 31 via the optical transmission line 41-1. The wavelength of this laser beam may be the wavelength of an optical signal used for communication, or it may be the wavelength of an optical signal not used for communication.

[0018] The reflecting unit 31 (output unit) reflects the downstream laser light. That is, the reflecting unit 31 reflects the downstream laser light back to the aggregation station 2a. In this way, the reflecting unit 31 transmits (outputs) the downstream laser light to the aggregation station 2a as upstream laser light. Here, the reflecting unit 31 may convert the downstream laser light into an electrical signal (analog signal). The reflecting unit 31 may also reflect the downstream laser light back to the aggregation station 2a by converting that electrical signal into upstream laser light. Alternatively, the reflecting unit 31 may use a reflector such as a mirror to reflect the downstream laser light back to the aggregation station 2a as light.

[0019] The measurement unit 23a measures the polarization state (polarization characteristics) of the uplink laser light transmitted through the optical transmission path 41-1, for example, using a polarization array. For example, the measurement unit 23a measures the received power for each phase of the uplink laser light at 0 degrees, 45 degrees, 90 degrees, and 135 degrees. The measurement unit 23a outputs the polarization state measurement results to the control unit 21a.

[0020] Next, polarization correction will be explained. The control unit 21a estimates the Stokes parameter of the uplink laser beam based on the measurement results of the polarization state. The control unit 21a may also estimate the Stokes parameter for each polarization component based on the measurement results of the vertical linear polarization component and the horizontal linear polarization component. The control unit 21a estimates the Jones vector based on the Stokes parameter. Here, if the wavelength of the laser beam (reference laser beam for polarization measurement) and the wavelength of the optical signal used for communication are different, the Jones vector may be corrected to match the wavelength of the optical signal used for communication.

[0021] Figure 2 is a diagram showing an example of the correspondence between the Stokes vector and the Jones vector for each polarization state in the first embodiment. As illustrated in Figure 2, when the Stokes parameters "S1", "S2", and "S3" of the Stokes vector (S0, S1, S2, S3) are determined, the Jones vector is determined. Also, the amplitude of light is expressed as in Equation (1).

[0022]

[0023] Here, "A" ｘ represents the amplitude of light in the x-axis direction. "A" ｙ represents the amplitude of light in the y-axis direction. "ω" represents the angular frequency of light. "t" represents time. "k" represents the wave number of light. "z" represents the propagation direction of light (z-axis). "δ" ｘ represents the phase of light in the x-axis direction. "δ" ｙ represents the phase of light in the y-axis direction. When "ωt - kz" is eliminated from Equation (1), the Stokes vector is expressed as in Equation (2).

[0024]

[0025] Here, each of "A" ｘ０ ２ , "A" ｙ０ ２ , "S" １ , "S" ２ and "S" ３ is measurable. The phase difference "δ = δ ｘ - δ ｙ " is expressed as in Equation (3).

[0026]

[0027] Also, when light travels in the z direction, the electric field is represented as a two-component vector in the xy plane. The Jones vector "E" (electric field vector) is expressed as in Equation (4).

[0028]

[0029] Equation (4) is the measured value of the amplitude of light in the x-axis direction "|√A ｘ０ ２"|", and the measured value of the amplitude of light in the y-axis direction, "|√A ｙ０ ２ |", and the phase difference "δ" are used and expressed as in Equation (5).

[0030]

[0031] Here, since the laser light is reflected by the reflection part 31, the optical path length of the laser light is doubled (round trip). Therefore, "δ" is corrected to "δ / 2".

[0032] So far, the Jones vector "E" has been estimated on the premise that the phase of light in the X-axis direction is 0 degrees. In reality, since the phase of light in the X-axis direction rotates, the phase of light in the x-axis direction, "δ ｘ " needs to be estimated based on Equation (6).

[0033]

[0034] The phase of light in the x-axis direction, "δ ｘ " is expressed as in Equation (7).

[0035]

[0036] Here, since the laser light is reflected by the reflection part 31, the optical path length of the laser light is doubled (round trip). Therefore, "δ ｘ " is corrected to "δ ｘ / 2".

[0037] The Jones vector "^E" that reflects the phase of light in the x-axis direction, "δ ｘ " is expressed as in Equation (8).

[0038]

[0039] Here, the sign of the phase of light in the x-axis direction, "δ ｘ " is determined as in Equation (9).

[0040]

[0041] The number of polarization components (polarization multiplexing number) in the laser light (reference laser light for polarization measurement) may be one polarization or two polarizations. If the number of polarization components in the laser light is one polarization, the Jones vector of the other single-polarization component may be estimated based on the measurement result of one of the single-polarization components.

[0042] Returning to Figure 1, let's continue the explanation of the example configuration of communication system 1a. In downlink communication, a first analog signal (first electrical signal) is input to the light-emitting element 24-1. The light-emitting element 24-1 converts the first analog signal into a first optical signal. A second analog signal (second electrical signal) is input to the light-emitting element 24-2. The light-emitting element 24-2 converts the second analog signal into a second optical signal.

[0043] In downlink communication, the control unit 21a places a polarizer 25-1, which corresponds to a Jones matrix, after the light-emitting element 24-1. The control unit 21a places a polarizer 25-2, which corresponds to a Jones matrix, after the light-emitting element 24-2. Polarizer 25-1 corrects the phase of the first optical signal. Polarizer 25-2 corrects the phase of the second optical signal. Here, polarizer 25-1 corrects the phase of the first optical signal and polarizer 25-2 corrects the phase of the second optical signal so that the two polarization components of the downlink optical signal are orthogonal as a V-polarization component and an H-polarization component at the extension station 3a. Polarization combiner 26 combines the polarization components of the first optical signal and the polarization components of the second optical signal. Polarization combiner 26 transmits the downlink optical signal having predetermined polarization components (first polarization component and second polarization component) to the extension station 3a via the optical transmission line 41-2.

[0044] In downlink communication, the polarization separator 32 separates a downlink optical signal having predetermined polarization components (V-polarization component and H-polarization component) into a downlink optical signal having a V-polarization component (third optical signal) and a downlink optical signal having an H-polarization component (fourth optical signal). The photoelectric converter 33-1 converts the downlink optical signal having a V-polarization component into a third analog signal (third electrical signal). The photoelectric converter 33-2 converts the downlink optical signal having an H-polarization component into a fourth analog signal (fourth electrical signal). The third analog signal and the fourth analog signal are converted into radio signals. The converted radio signals are transmitted from an antenna (not shown).

[0045] In uplink communication, a fifth analog signal (fifth electrical signal) based on a radio signal received by an antenna (not shown) is input to the light-emitting element 34-1. The light-emitting element 34-1 converts the fifth analog signal into a fifth optical signal having a V-polarization component. A sixth analog signal (sixth electrical signal) based on a radio signal received by an antenna (not shown) is input to the light-emitting element 34-2. The light-emitting element 34-2 converts the sixth analog signal into a sixth optical signal having an H-polarization component. The polarization combiner 35 combines the polarization components of the fifth optical signal and the sixth optical signal. The polarization combiner 35 transmits the uplink optical signal with the combined polarization components to the aggregation station 2a via the optical transmission path 41-3.

[0046] In uplink communication, the brancher 27 branches the uplink optical signal having the combined polarization components. That is, the brancher 27 branches the uplink optical signal having predetermined third and fourth polarization components into an uplink optical signal having the third polarization component (seventh optical signal) and an uplink optical signal having the fourth polarization component (eighth optical signal). The brancher 27 outputs the uplink optical signal having the third polarization component to the polarizer 28-1. The brancher 27 outputs the uplink optical signal having the fourth polarization component to the polarizer 28-2.

[0047] In uplink communication, the control unit 21a places a polarizer 28-1, which corresponds to the Jones matrix, before the photoelectric converter 29-1. The control unit 21a places a polarizer 28-2, which corresponds to the Jones matrix, before the photoelectric converter 29-2. Polarizer 28-1 corrects the phase of the seventh optical signal. Polarizer 25-2 corrects the phase of the eighth optical signal. Here, polarizer 28-1 corrects the phase of the seventh optical signal and polarizer 25-2 corrects the phase of the eighth optical signal so that the two polarization components in the uplink optical signal are orthogonal as V-polarization component and H-polarization component at the aggregation station 2a. The photoelectric converter 29-1 converts the uplink optical signal with a V-polarization component into the seventh analog signal. The photoelectric converter 29-1 converts the uplink optical signal with an H-polarization component into the eighth analog signal.

[0048] Next, an example of the operation of the communication system 1a will be described. Figure 3 is a sequence diagram showing an example of the operation of the communication system 1a in the first embodiment. In the polarization measurement stage, the control unit 21a instructs the light source 22a to transmit the downstream laser light (step S101). The light source 22a transmits the downstream laser light to the reflection unit 31 via the optical transmission path 41-1 (step S102). The reflection unit 31 transmits the upstream laser light to the aggregation station 2a by reflecting the downstream laser light back to the aggregation station 2a (step S103). The measurement unit 23a measures the polarization state of the upstream laser light (step S104).

[0049] In the polarization correction stage, the control unit 21a estimates the Stokes parameter of the uplink laser beam based on the measurement results of the polarization state (step S105). Based on the Stokes parameter, the control unit 21a estimates the Jones vector (step S106).

[0050] In the polarization correction and downlink communication stage, the control unit 21a generates a downlink optical signal with predetermined polarization components using the light-emitting element 24 and polarizer 25 based on the Jones matrix corresponding to the Jones vector. Here, the control unit 21a controls the polarization of the downlink optical signal by placing the polarizer 25, which corresponds to the Jones matrix, after the light-emitting element 24. That is, the control unit 21a multiplies the phase data of the downlink optical signal by the Jones matrix so that the two polarization components in the downlink optical signal are orthogonal as V-polarization component and H-polarization component at the extension station 3a. The polarization combiner 26 transmits the downlink optical signal with predetermined polarization components to the extension station 3a via the optical transmission line 41-2 (step S107). The polarization separator 32 separates the downlink optical signal with predetermined polarization components (V-polarization component and H-polarization component) into a downlink optical signal with a V-polarization component and a downlink optical signal with an H-polarization component. The photoelectric converter 33-1 converts the downlink optical signal with a V-polarization component into a third analog signal. The photoelectric converter 33-2 converts the downstream optical signal having an H-polarization component into a fourth analog signal (step S108).

[0051] In the polarization correction and uplink communication stage, the light-emitting element 34-1 converts the fifth analog signal into an uplink optical signal having a V-polarization component. The light-emitting element 34-2 converts the sixth analog signal into an uplink optical signal having an H-polarization component. The polarization combiner 35 transmits the uplink optical signal with the combined polarization components to the aggregation station 2a via the optical transmission line 41-3 (step S109). The brancher 27 outputs the uplink optical signal having a third polarization component to the polarizer 28-1. The brancher 27 outputs the uplink optical signal having a fourth polarization component to the polarizer 28-2. The control unit 21a controls the polarization of the uplink optical signal by placing the polarizer 28, which corresponds to the Jones matrix, in front of the photoelectric converter 29. That is, the control unit 21a multiplies the phase data of the uplink optical signal by the Jones matrix so that the two polarization components in the uplink optical signal are orthogonal as a V-polarization component and an H-polarization component at the aggregation station 2a. Polarizer 28-1 converts an upstream optical signal having a predetermined third polarization component into an upstream optical signal having a V polarization component. Polarizer 28-2 converts an upstream optical signal having a predetermined fourth polarization component into an upstream optical signal having an H polarization component. Photoelectric converter 29-1 converts the upstream optical signal having a V polarization component into a seventh analog signal. Photoelectric converter 29-1 converts the upstream optical signal having an H polarization component into an eighth analog signal (step S110).

[0052] As described above, the light source 22a generates downstream laser light. The optical transmission line 41-1 transmits the downstream laser light to the extension station 3a. The measurement unit 23a acquires the downstream laser light reflected by the reflection unit 31 as upstream laser light. The measurement unit 23a measures the polarization state of the upstream laser light acquired from the extension station 3a via the optical transmission line 41-1. The control unit 21a may correct the polarization of the downstream optical signal transmitted to the extension station 3a via the optical transmission line 41-2 using polarizers 25 for each polarization component according to the polarization state. The control unit 21a may correct the polarization of the upstream optical signal acquired from the extension station 3a via the optical transmission line 41-3 using polarizers 28 for each polarization component according to the polarization state.

[0053] This makes it possible to correct polarization based on the polarization state measurement results, even if the configuration of the extended station 3a is simple, in a wireless communication system to which analog RoF is applied.

[0054] Here, the control unit 21a may estimate the Stokes parameters based on the polarization state. The control unit 21a may estimate the Jones vector based on the Stokes parameters. The control unit 21a may correct the polarization of the downstream optical signal by using polarizers 25 for each polarization component based on the Jones matrix. The control unit 21a may correct the polarization of the upstream optical signal by using polarizers 28 for each polarization component based on the Jones matrix.

[0055] (Second Embodiment) The main difference in the second embodiment from the first embodiment is that the projection station is equipped with a light source that generates the upward laser beam. The second embodiment will be explained focusing on the differences from the first embodiment.

[0056] Figure 4 shows an example of the configuration of the communication system 1b in the second embodiment. The aggregation station 2b comprises a control unit 21b, a measuring unit 23b, two light-emitting elements 24, two polarizers 25, a polarization combiner 26, a splitter 27, two polarizers 28, and two photoelectric converters 29. The extension station 3b comprises a polarization separator 32, two photoelectric converters 33, two light-emitting elements 34, a polarization combiner 35, and a light source 36.

[0057] The light source 36 transmits the uplink laser beam (reference laser beam for polarization measurement) to the aggregation station 2b via the optical transmission line 41-1. Here, the control unit 21b may instruct the light source 36 to transmit the uplink laser beam. The light source 36 may transmit the uplink laser beam to the aggregation station 2b based on the instruction from the control unit 21b.

[0058] In the first embodiment, the descending laser beam was reflected by the reflector 31, and the descending laser beam was folded back as the ascending laser beam. For this reason, in equation (5) described above, "δ" was corrected to "δ / 2". Also, in equation (7) described above, "δ ｘ " is "δ ｘIt was corrected to "δ / 2". In contrast, in the second embodiment, the laser light is not folded back and the light source 36 generates the upward laser light, so in the above equation (5), it is not necessary to correct "δ" to "δ / 2". Also, in the above equation (7), "δ ｘ " is "δ ｘ There is no need to correct it to " / 2".

[0059] Next, an example of the operation of the communication system 1b will be described. Figure 5 is a sequence diagram showing an example of the operation of the communication system 1b in the second embodiment. In the polarization measurement stage, the light source 36 transmits the uplink laser light to the aggregation station 2b via the optical transmission line 41-1 (step S201). Steps S202 to S208 are the same as steps S104 to S110 illustrated in Figure 3.

[0060] As described above, the light source 36 of the extension station 3b may transmit the uplink laser light to the aggregation station 2b via the optical transmission path 41-1. This makes it possible to correct the polarization based on the polarization state measurement results in a wireless communication system to which analog RoF is applied, even if the configuration of the extension station 3b is simple.

[0061] (Third Embodiment) In the third embodiment, the main difference from the first embodiment is that the wavelength used for measuring parameters other than polarization (e.g., delay) and the wavelength used for measuring polarization are the same. The third embodiment will be explained focusing on the differences from the first embodiment.

[0062] Figure 6 shows an example of the configuration of the communication system 1c in the third embodiment. The aggregation station 2c comprises a control unit 21c, a light source 22c, a measuring unit 23c, two light-emitting elements 24, two polarizers 25, a polarization combiner 26, a splitter 27, two polarizers 28, and two photoelectric converters 29.

[0063] The downstream laser light (reference laser light for polarization measurement) transmitted from the light source 22c may be shared for both the polarization measurement function and a predetermined function other than the polarization measurement function. Here, the wavelength of the downstream laser light may be the same as the wavelength of the laser light used for the predetermined function other than the polarization measurement function. The predetermined function is, for example, a function for measuring the delay of the laser light in the optical transmission line 41. The light source 22c may carry the signal used for that predetermined function on the downstream laser light (reference laser light for polarization measurement). When the reflection unit 31 reflects the downstream laser light as light to the aggregation station 2c, the light source 22c may transmit the downstream laser light for each function by time-division multiplexing.

[0064] Next, an example of the operation of the communication system 1c will be described. Figure 7 is a sequence diagram showing an example of the operation of the communication system 1c in the third embodiment. In the polarization measurement stage, the control unit 21c instructs the light source 22c to transmit the downstream laser light (step S301). The light source 22c transmits the downstream laser light of a wavelength used for a predetermined function to the reflection unit 31 via the optical transmission line 41-1 (step S302). The reflection unit 31 transmits the upstream laser light to the aggregation station 2c by folding the downstream laser light back to the aggregation station 2c (step S303). The measurement unit 23c measures the polarization state of the upstream laser light (step S304). Steps S305 to S310 are the same as steps S105 to S110 illustrated in Figure 3.

[0065] As described above, the downstream laser light transmitted from the light source 22c may be shared for both the polarization measurement function and other predetermined functions. This makes it possible to correct the polarization based on the polarization state measurement results in a wireless communication system to which analog RoF is applied, even if the configuration of the extension station 3c is simple.

[0066] (Fourth Embodiment) In the fourth embodiment, the main difference from the third embodiment is that the polarization state is estimated using a pre-trained machine learning model. The fourth embodiment will be explained focusing on the differences from the third embodiment.

[0067] Figure 8 shows an example configuration of the communication system 1d in the fourth embodiment. The aggregation station 2d comprises a control unit 21d, a light source 22d, a measurement unit 23d, two light-emitting elements 24, two polarizers 25, a polarization combiner 26, a splitter 27, two polarizers 28, and two photoelectric converters 29. The control unit 21d includes a learning model having a neural network. The neural network is, for example, a deep neural network.

[0068] During the machine learning training phase, the measurement unit 23d generates a trained model having a neural network from the trained model using a machine learning method. The machine learning method is not limited to a specific method, but is, for example, supervised learning. Training data, including training data and correct labels, is prepared in advance. The measurement unit 23d inputs, for example, the time-series waveform or spectrum of laser light transmitted through the optical transmission path 41-1 in the past as training data (explanatory variables) into the trained model. The waveform and spectrum may each be represented in the form of image data. The measurement unit 23d obtains the polarization state of the laser light in the training data from the trained model as an estimation result. Based on the comparison result between the estimation result and the correct label, the measurement unit 23d generates a trained model (a trained model with improved accuracy) from the trained model. The measurement unit 23d may, for example, generate the trained model from the trained model using backpropagation on the comparison result.

[0069] During the estimation phase, the measurement unit 23d inputs the time-series waveform or spectrum of the uplink laser beam as the data to be estimated (explanatory variable) into the trained model. The measurement unit 23d obtains the polarization state of the uplink laser beam from the trained model as the estimation result (dependent variable). The measurement unit 23d outputs the polarization state estimation result to the control unit 21a.

[0070] The control unit 21d determines the control parameters of polarizers 25 and 28 based on the polarization state estimation results. Here, the control unit 21d determines the control parameters of polarizer 25 based on the polarization state estimation results so that the two polarization components of the downstream optical signal are orthogonal as V-polarization component and H-polarization component at the extension station 3d. Similarly, the control unit 21d determines the control parameters of polarizer 28 based on the polarization state estimation results so that the two polarization components of the upstream optical signal are orthogonal as V-polarization component and H-polarization component at the aggregation station 2d.

[0071] In downlink communication, the control unit 21d places a polarizer 25-1 corresponding to a control parameter after the light-emitting element 24-1. The control unit 21d also places a polarizer 25-2 corresponding to a control parameter after the light-emitting element 24-2.

[0072] In uplink communication, the control unit 21d places a polarizer 28-1, which corresponds to a control parameter, in front of the photoelectric converter 29-1. The control unit 21d also places a polarizer 28-2, which corresponds to a control parameter, in front of the photoelectric converter 29-2.

[0073] Next, an example of the operation of the communication system 1d will be described. Figure 9 is a sequence diagram showing an example of the operation of the communication system 1d in the fourth embodiment. In the polarization measurement stage, the control unit 21d instructs the light source 22d to transmit the downstream laser light (step S401). The light source 22d transmits the downstream laser light of a wavelength used for a predetermined function to the reflector 31 via the optical transmission line 41-1 (step S402). The reflector 31 transmits the upstream laser light to the aggregation station 2d by folding the downstream laser light back to the aggregation station 2d (step S403). The measurement unit 23d measures the waveform or spectrum of the upstream laser light for each polarization component (step S404). The measurement unit 23d inputs the waveform or spectrum into the trained model. The measurement unit 23d obtains the polarization state estimation result from the trained model (step S405).

[0074] The control unit 21d determines control parameters based on the polarization state estimation result (step S406). The control unit 21d places a polarizer 25 corresponding to the control parameters downstream of the light-emitting element 24. The polarization combiner 26 transmits a downstream optical signal having predetermined polarization components to the extension station 3a via the optical transmission line 41-2 (step S407). The polarization separator 32 separates the downstream optical signal having predetermined polarization components (V-polarization component and H-polarization component) into a downstream optical signal having a V-polarization component and a downstream optical signal having an H-polarization component. Steps S408 and S409 are the same as steps S108 and S109 illustrated in Figure 3.

[0075] The control unit 21a places polarizers 28 corresponding to control parameters in front of the photoelectric converter 29. Polarizer 28-1 converts an upstream optical signal with a predetermined polarization component into an upstream optical signal with a V polarization component. Polarizer 28-2 converts an upstream optical signal with a predetermined polarization component into an upstream optical signal with an H polarization component (step S410).

[0076] As described above, the measurement unit 23d measures the waveform or spectrum of the uplink laser light for each polarization component. The measurement unit 23d inputs the waveform or spectrum of the uplink laser light as explanatory variables into the trained model. The measurement unit 23d obtains the polarization state of the uplink laser light from the trained model as the objective variable. That is, the measurement unit 23d estimates the polarization state of the uplink laser light based on the measurement results of the waveform or spectrum.

[0077] The control unit 21d corrects the polarization of the downlink optical signal transmitted to the outbound station 3d via the optical transmission line 41-1 using polarizers 25 for each polarization component according to the polarization state. The control unit 21d corrects the polarization of the uplink optical signal acquired from the outbound station 3d via the optical transmission line 41-1 using polarizers 28 for each polarization component according to the polarization state.

[0078] This makes it possible to correct polarization based on the polarization state measurement results, even if the configuration of the extended station 3d is simple, in a wireless communication system to which analog RoF is applied.

[0079] (Hardware Configuration) Figure 10 shows an example of the hardware configuration of the communication device 11 in each embodiment. The example of the hardware configuration of the communication device 11 corresponds to the example of the hardware configuration of the aggregation station in each embodiment. The example of the hardware configuration of the communication device 11 may also correspond to the example of the hardware configuration of the extension station in each embodiment.

[0080] The communication device 11 is implemented as software by a processor 12, such as a CPU (Central Processing Unit), executing a program stored in a storage device 13 and a memory 14 that have a non-volatile recording medium (non-temporary recording medium). The program may be recorded on a computer-readable recording medium. Computer-readable recording media include, for example, portable media such as flexible disks, magneto-optical disks, ROMs (Read Only Memory), CD-ROMs (Compact Disc Read Only Memory), and non-temporary recording media such as hard disks or solid-state drives (SSDs) built into computer systems. The communication unit 15 performs predetermined communication processing.

[0081] The communication device 11 may be implemented using hardware including 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).

[0082] While 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.

[0083] This invention is applicable to communication systems.

[0084] 1a, 1b, 1c, 1d... Communication system, 2a, 2b, 2c, 2d... Aggregation station, 3a, 3b, 3c, 3d... Extension station, 4... Optical fiber, 21a, 21b, 21c, 21d... Control unit, 22a, 22c, 22d... Light source, 23a, 23b, 23c, 23d... Measurement unit, 24... Light-emitting element, 25... Polarizer, 26... Polarization combiner, 27... Splitter, 28... Polarizer, 29... Photoelectric converter, 31... Reflector, 32... Polarization separator, 33... Photoelectric converter, 34... Light-emitting element, 35... Polarization combiner, 41... Optical transmission path

Claims

1. A communication device comprising: a measuring unit that measures the polarization state of uplink laser light acquired from an extended station via an optical transmission path; and a control unit that corrects at least one of the polarization of the downlink optical signal transmitted to the extended station via the optical transmission path and the polarization of the uplink optical signal acquired from the extended station via the optical transmission path, using polarizers for each polarization component according to the polarization state.

2. The communication device according to claim 1, further comprising a light source that generates a downstream laser beam, wherein the measuring unit acquires the downstream laser beam reflected by the extended station as the upstream laser beam.

3. The communication device according to claim 1, wherein the control unit estimates Stokes parameters based on the polarization state, estimates Jones vectors based on the Stokes parameters, and corrects at least one of the polarization of the downstream optical signal and the polarization of the upstream optical signal by using polarizers for each polarization component based on the Jones matrix corresponding to the Jones vectors.

4. The communication device according to claim 1, wherein the measuring unit measures the waveform or spectrum of the upward laser light and estimates the polarization state based on the waveform or spectrum.

5. A communication system comprising a central station and an extended station, wherein the extended station has an output unit that outputs uplink laser light to an optical transmission path, and the central station has a measuring unit that measures the polarization state of the uplink laser light acquired from the extended station via the optical transmission path, and a control unit that corrects at least one of the polarization of the downlink optical signal transmitted to the extended station via the optical transmission path and the polarization of the uplink optical signal acquired from the extended station via the optical transmission path, using polarizers for each polarization component according to the polarization state.

6. A signal generation method performed by a communication device, comprising the steps of: measuring the polarization state of uplink laser light acquired from an extended station via an optical transmission path; and correcting at least one of the polarization of a downlink optical signal transmitted to the extended station via the optical transmission path and the polarization of an uplink optical signal acquired from the extended station via the optical transmission path, using polarizers for each polarization component according to the polarization state.

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