Wireless system, communication device, and communication method

Abstract

A wireless system comprising a first communication device and at least one second communication device, said first communication device and said at least one second communication device communicating using optical signals that have the same wavelength in the uplink direction and the downlink direction, wherein the first communication device comprises at least one distribution unit that outputs, toward the at least one second communication device via an optical transmission path, an optical signal or signals outputted from at least one first photoelectric conversion unit, and outputs, to at least one second photoelectric conversion unit, an optical signal inputted via an optical transmission path, the at least one second communication device comprises a distribution unit that outputs, toward the first communication device via an optical transmission path, an optical signal outputted from a first photoelectric conversion unit, and outputs, to a second photoelectric conversion unit, an optical signal inputted via an optical transmission path, and the first communication device or the at least one second communication device comprises a polarized wave adjustment unit that adjusts the polarization plane of a converted optical signal so as to be different from the polarization plane of an optical signal transmitted by a communication partner.　

Description

Wireless system, communication device, and communication method

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

[0002] Traditionally, millimeter-wave bands have attracted attention due to their high-speed transmission capabilities, but the large propagation loss in millimeter-wave bands necessitates the dense deployment of wireless base stations. Studies are underway to achieve flexible and economical wireless area deployment by dividing wireless base station functions into central stations and base stations and deploying simple base stations using analog RoF (Radio over Fiber). Conventional RoF systems use separate wavelengths for upstream and downstream communications, but from the perspective of wavelength resource utilization efficiency, it is desirable to achieve both upstream and downstream communications using a single wavelength.

[0003] Therefore, the DDM (Directional Division Multiplexing) method is known as a bidirectional communication method using only one wavelength over one optical transmission line (see, for example, Non-Patent Document 1). Fig. 13 is a diagram showing an example of the configuration of a conventional wireless system 1 using the DDM method. The wireless system 1 includes a central station 2 and a base station 3. The central station 2 includes an E / O converter 4, an O / E converter 5, and a multiplexer / splitter 6. The E / O converter 4 uses the input signal to convert it into a signal of wavelength λ １ The O / E converter 5 converts the input wavelength λ １ The optical signal is converted into an electrical signal. The multiplexer / divider 6 is configured using, for example, an optical splitter or an optical coupler. The multiplexer / divider 6 branches or multiplexes the input optical signal. Usually, the distribution ratio of the optical splitter is assumed to be 1:1.

[0004] The base station 3 includes a multiplexer / splitter 7, an O / E converter 8, and an E / O converter 9. The multiplexer / splitter 7 branches or multiplexes the optical signal transmitted from the central station 2. The O / E converter 8 converts the wavelength λ 2 branched by the multiplexer / splitter 7 into an E / O converter 9. １ The E / O converter 9 converts the optical signal of wavelength λ into an electrical signal. １ This configuration enables communication using the same wavelength in both directions.

[0005] Ulrich Killat, “Access to B-ISDN via PONs: ATM Communication in Practice”, 1996. Masamichi Fujiwara, Jun-ichi Kani, Hiro Suzuki, and Katsumi Iwatsuki, “Impact of Backreflection on Upstream Transmission in WDM Single-Fiber Loopback Access Networks”, 2006.

[0006] However, conventional configurations can suffer from the following problems. Reflected waves (e.g., downstream signals reflected by a connector and partially mixed with upstream light) due to discontinuities in the optical transmission line (e.g., connector connections) can degrade receiver sensitivity. Furthermore, backscattered light due to Rayleigh scattering on the optical transmission line can also contribute as returned light (see, for example, Non-Patent Document 2). Because light is injected into an emitting laser from the opposite direction (called returned light), the laser's oscillation characteristics may fluctuate depending on the intensity of the returned light. Using an optical splitter results in large insertion loss (e.g., 3 dB or more). Thus, conventional configurations, in systems using the same wavelength for both the upstream and downstream directions, suffer from large optical power losses and degraded communication quality due to the effects of returned light.

[0007] In view of the above circumstances, the present invention aims to provide a technology that can suppress the deterioration of communication quality while suppressing the loss of optical power in a system that uses the same wavelength in the upstream and downstream directions.

[0008] One aspect of the present invention is a wireless system including a first communication device and one or more second communication devices, wherein the first communication device and the one or more second communication devices communicate with each other using optical signals of the same wavelength in an upstream direction and a downstream direction, the first communication device including one or more first opto-electrical converters that convert an input electrical signal into an optical signal, one or more second opto-electrical converters that convert the input optical signal into an electrical signal, and one or more first distributors that output the optical signals output from the one or more first opto-electrical converters to the one or more second communication devices via an optical transmission path, and output the optical signals input via the optical transmission path to the one or more second opto-electrical converters. and a unit, wherein the one or more second communication devices comprise a third opto-electrical conversion unit that converts an input electrical signal into an optical signal, a fourth opto-electrical conversion unit that converts the input optical signal into an electrical signal, and a second distribution unit that outputs the optical signal output from the third opto-electrical conversion unit to the first communication device via an optical transmission path, and outputs the optical signal input via the optical transmission path to the fourth opto-electrical conversion unit, and at least one of the first communication device or the one or more second communication devices comprises a polarization adjustment unit that adjusts the polarization plane of the converted optical signal so that it is different from the polarization plane of the optical signal transmitted by a communication partner.

[0009] One aspect of the present invention is a communication device having either a signal processing function or a communication function that a wireless communication device has, wherein the communication device and a communication device with which it communicates communicate using optical signals of the same wavelength in the upstream and downstream directions, and the communication device is equipped with a first opto-electrical conversion unit that converts an input electrical signal into an optical signal, a second opto-electrical conversion unit that converts the input optical signal into an electrical signal, a distribution unit that outputs the optical signal output from the first opto-electrical conversion unit to the communication device with which it communicates via an optical transmission path, and outputs the optical signal input via the optical transmission path to the second opto-electrical conversion unit, and a polarization adjustment unit that adjusts the polarization plane of the optical signal converted by the first opto-electrical conversion unit so that it is different from the polarization plane of the optical signal transmitted by the communication device with which it communicates.

[0010] One aspect of the present invention is a communication method performed by a communication device having either a signal processing function or a communication function provided in a wireless communication device, in which the communication device and a communication device with which it is communicating communicate using optical signals of the same wavelength in the upstream and downstream directions, a first opto-electrical conversion unit converts an input electrical signal into an optical signal, a second opto-electrical conversion unit converts the input optical signal into an electrical signal, the optical signal output from the first opto-electrical conversion unit is output to the communication device with which it is communicating via an optical transmission path, the optical signal input via the optical transmission path is output to the second opto-electrical conversion unit, and the polarization plane of the optical signal converted by the first opto-electrical conversion unit is adjusted to be different from the polarization plane of the optical signal transmitted by the communication device with which it is communicating.

[0011] According to the present invention, in a system using the same wavelength in the upstream and downstream directions, it is possible to suppress the loss of optical power and also suppress the deterioration of communication quality.

[0012] FIG. 1 is a diagram illustrating an example of the configuration of a wireless system S. FIG. 1 is a diagram illustrating an example of the configuration of a wireless system in a first embodiment. FIG. 2 is a sequence diagram illustrating the flow of processing in the wireless system in the first embodiment. FIG. 2 is a diagram illustrating an example of the configuration of a wireless system in a second embodiment. FIG. 3 is a diagram for explaining specific processing in the second embodiment. FIG. 4 is a diagram illustrating electrical spectra after detection when the crossing angle θ of the polarization planes is changed to 90 degrees, 85 degrees, and 80 degrees. FIG. 4 is a diagram illustrating an example of the configuration of a wireless system in a third embodiment. FIG. 5 is a diagram illustrating an example of the configuration of a conventional wireless system to which WDM-PON is applied. FIG. 6 is a diagram illustrating an example of the configuration of a wireless system in a fourth embodiment. FIG. 7 is a diagram illustrating an example of the configuration of a conventional wireless system. FIG. 8 is a diagram illustrating an example of the configuration of a wireless system in a fifth embodiment. FIG. 9 is a diagram illustrating an example of wavelength allocation. FIG. 10 is a diagram illustrating an example of the configuration of a conventional wireless system using the DDM method.

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] Before describing a specific configuration of the present invention, one possible configuration for solving the problems of the present invention will be described. FIG. 1 is a diagram showing an example configuration of a wireless system S. The wireless system S includes a central station 10 and one or more base stations 20. The central station 10 and the one or more base stations 20 are connected via an optical transmission path 30. The optical transmission path 30 is an optical fiber. Here, an example in which there is one base station 20 will be described. The central station 10 and the one or more base stations 20 form a single base station. Specifically, the central station 10 has a signal processing function that is a function obtained by separating the signal processing function and the communication function that a base station has. The one or more base stations 20 have a communication function that is a function obtained by separating the signal processing function and the communication function that a base station has.

[0015] Between the central station 10 and the base station 20, transmission is performed using analog RoF, which transmits radio signals using optical fiber. Here, the radio signals transmitted through the optical fiber may be at a radio frequency output by the radio base station 11, or may be converted from a radio frequency to an intermediate frequency lower than the radio frequency and then converted back to the original radio frequency at the base station 20. Between the central station 10 and one base station 20, the same wavelength is used in the uplink and downlink directions. In this way, bidirectional communication using the DDM method is performed between the central station 10 and one base station 20. This technology can be applied to a mobile fronthaul. In this case, the central station 10 is a CS (Central Station) and the base station 20 is an RRU (Remote Radio Unit).

[0016] In the following description, the uplink direction is the direction from the base station 20 to the central station 10, and the downlink direction is the direction from the central station 10 to the base station 20. In Fig. 1, the description will be given taking as an example a configuration in which the base station 20 performs wireless communication with subordinate wireless terminals by TDD (Time Division Duplex).

[0017] Next, we will explain the specific configuration of the central station 10 and the base station 20. First, we will explain the specific configuration of the central station 10. The central station 10 includes a radio base station 11, an E / O converter 12, an O / E converter 13, an intensity adjustment unit 14, and a circulator 15.

[0018] The wireless base station 11 has a transmission port and a reception port. The wireless base station 11 transmits and receives signals in accordance with the TDD timing. For example, the wireless base station 11 outputs a modulated signal modulated with downlink transmission data in accordance with the TDD transmission timing to the E / O converter 12. Furthermore, the wireless base station 11 outputs a TDD signal for controlling switching between transmission and reception to the intensity adjustment unit 14. The TDD signal alternates between "transmission" time slots indicating the timing of transmission and "reception" time slots indicating the timing of reception, with a gap time inserted between the "transmission" time slot and the "reception" time slot.

[0019] The E / O converter 12 receives a modulated signal output from the wireless base station 11. The E / O converter 12 uses the modulated signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby generating an optical signal of a certain wavelength. １ The E / O converter 12 generates an optical signal of the generated wavelength λ １ The optical signal is output to the intensity adjusting unit 14. The optical modulator is, for example, a Mach-Zehnder modulator.

[0020] The O / E converter 13 receives the optical signal output from the circulator 15. As described above, the same wavelength is used in the upstream and downstream directions between the central station 10 and the base station 20. Therefore, the O / E converter 13 receives the optical signal having the wavelength λ １ The O / E converter 13 converts the input optical signal of wavelength λ １ The O / E converter 13 converts the optical signal into an electrical signal, and outputs the electrical signal to the wireless base station 11.

[0021] The intensity adjuster 14 adjusts the intensity of the input optical signal based on the TDD signal output from the radio base station 11. Specifically, the intensity adjuster 14 adjusts the intensity so that the intensity of the input optical signal is high during the period indicated by the "transmit" time slot in the TDD signal. A high intensity state of the optical signal means adjusting the intensity so that the attenuation of the intensity of the input optical signal is reduced and the intensity is maintained high. On the other hand, the intensity adjuster 14 adjusts the intensity so that the intensity of the input optical signal is low during the period indicated by the "receive" time slot in the TDD signal. A low intensity state of the optical signal means adjusting the intensity so that the attenuation of the intensity of the input optical signal is increased and the intensity is reduced. In this way, the intensity adjuster 14 adjusts the input power to the circulator 15 so as to limit it at times other than the transmission timing of the downstream radio signal in the central station 10.

[0022] For example, during the period indicated by the "transmit" time slot in the TDD signal, the intensity of the optical signal is attenuated less than during the period indicated by the "receive" time slot in the TDD signal. It is assumed that TDD timing is synchronized between the central station 10 and the base station 20. The intensity adjuster 14 is, for example, a variable optical attenuator (VOA). The intensity adjuster 14 may control the optical intensity of the optical signal with an arbitrary waveform based on the TDD signal.

[0023] The synchronization of the TDD timing between the central station 10 and the base station 20 may be achieved using an external control line, by transmitting the main signal and the control signal at the same wavelength using subcarrier multiplexing (SCM), or by transmitting the main signal and the control signal at different wavelengths. The TDD signal output by the radio base station 11 may be used as information related to the TDD timing.

[0024] The circulator 15 has at least three or more ports. In the following description, it is assumed that the circulator 15 has three ports. The first port of the circulator 15 is connected to the intensity adjustment unit 14. The second port of the circulator 15 is connected to the optical transmission path 30. The third port of the circulator 15 is connected to the O / E converter 13. An optical signal input to the first port of the circulator 15 is output from the second port. An optical signal input to the second port of the circulator 15 is output from the third port. An optical signal input to the third port of the circulator 15 is output from the first port.

[0025] Next, we will explain the specific configuration of the base station 20. The base station 20 includes a circulator 21, an O / E converter 22, a TDD switch 23, an E / O converter 24, an intensity adjuster 25, and an antenna 26.

[0026] The circulator 21 has at least three or more ports. In the following description, it is assumed that the circulator 21 has three ports. The first port of the circulator 21 is connected to the O / E converter 22. The second port of the circulator 21 is connected to the intensity adjustment unit 25. The third port of the circulator 21 is connected to the optical transmission path 30. An optical signal input to the first port of the circulator 21 is output from the second port. An optical signal input to the second port of the circulator 21 is output from the third port. An optical signal input to the third port of the circulator 21 is output from the first port.

[0027] The O / E converter 22 converts the wavelength λ output from the circulator 21 into １ The O / E converter 22 converts the input optical signal of wavelength λ １ The O / E converter 22 converts the optical signal into an electrical signal. The O / E converter 22 outputs the electrical signal to the TDD switch 23. The electrical signal output from the O / E converter 22 is a downstream transmission signal.

[0028] The TDD switch 23 switches the connection based on a TDD signal obtained via an external control line. For example, the TDD switch 23 connects the O / E converter 22 to the antenna 26 during a period indicated by a "transmit" time slot in the TDD signal. This causes the electrical signal output from the O / E converter 22 to be transmitted as radio waves by the antenna 26. On the other hand, the TDD switch 23 connects the E / O converter 24 to the antenna 26 during a period indicated by a "receive" time slot in the TDD signal. This causes the electrical signal based on the radio waves received via the antenna 26 to be input to the E / O converter 24.

[0029] The E / O converter 24 receives an electrical signal output from the antenna 26. The E / O converter 24 uses the received electrical signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby generating an optical signal of a certain wavelength. １ The E / O converter 24 generates an optical signal of the generated wavelength λ １ The optical signal is output to the intensity adjusting unit 25. The optical modulator is, for example, a Mach-Zehnder modulator.

[0030] The intensity adjustment unit 25 adjusts the intensity of the input optical signal based on the TDD signal obtained via an external control line. Specifically, the intensity adjustment unit 25 controls the intensity of the input optical signal so that it is low during the period indicated by the "transmission" time slot in the TDD signal. On the other hand, the intensity adjustment unit 25 controls the intensity of the input optical signal so that it is high during the period indicated by the "reception" time slot in the TDD signal. In this way, the intensity adjustment unit 25 adjusts the input power to the circulator 21 so as to limit it except for the timing of transmitting the upstream radio signal from the base station 20. The intensity adjustment unit 25 is, for example, an optical variable attenuator.

[0031] By using a circulator as described above, it becomes possible to communicate using a single wavelength in both the upstream and downstream directions. Furthermore, by using a circulator instead of a conventional optical splitter, it is possible to suppress optical power loss and also to reduce the effects of optical feedback. Therefore, in a system that uses the same wavelength in both the upstream and downstream directions, it is possible to suppress optical power loss and to suppress degradation of communication quality.

[0032] The wireless system S shown in FIG. 1 is applicable only to systems that perform wireless communication using TDD, and is not applicable to systems that perform wireless communication using FDD (Frequency Division Duplex). Furthermore, in the wireless system S shown in FIG. 1, the optical intensity of the optical signal output from the E / O converter 12 is adjusted to match the TDD timing. Therefore, if the TDD timing is misaligned between the central station 10 and the base station 20 due to factors such as optical fiber delay, the system may be inapplicable (applicable only with short fiber lengths). In particular, in a configuration in which the main signal and control signal are transmitted via SCM, the need for extinction to match the TDD timing can result in the control signal being interrupted, leading to unstable control. Thus, there is room for improvement in the wireless system S shown in FIG. 1.

[0033] Therefore, the following describes a configuration that can be applied to a wireless system that performs two-way communication using the DDM method, not only a system that performs wireless communication using TDD, but also a system that performs wireless communication using FDD.

[0034] (First embodiment) Fig. 2 is a diagram showing an example of the configuration of a wireless system 100 in the first embodiment. The wireless system 100 includes a central station 10 and one or more base stations 20. The wireless system 100 is basically the same as that described in Fig. 1 except for some differences in the configuration of the central station 10 and the one or more base stations 20. The wireless system 100 may be a system that performs wireless communication using TDD or FDD. The following description will focus on the differences from the wireless system S.

[0035] For ease of explanation, the polarization state of the light used for upstream communication from the base station 20 to the central station 10 is referred to as E ｘ and the polarization state of the light used for downstream communication from the central station 10 to the base station 20 is E ｙ The polarization state of the light after upstream transmission is E ｘ ', and the polarization state of the light after downstream transmission is E ｙ Furthermore, the polarization state of the light returning to the base station 20 as backscattered light is expressed as E ｘ '', and the polarization state of the light returning to the central station 10 as backscattered light is E ｙ ´´Let's say.

[0036] In the first embodiment, it is assumed that there is no change in the polarization plane due to optical fiber transmission. ｃ The polarization plane of the optical signal of wavelength λ is the same before it is output from the central station 10 to the optical transmission line 30 and when it is received by the base station 20 via the optical transmission line 30. Similarly, the optical signal of wavelength λ ｃ The polarization plane of the optical signal is the same before it is output from the base station 20 to the optical transmission line 30 and when it is received by the central station 10 via the optical transmission line 30 .

[0037] (Configuration of central station 10) First, a description will be given of a specific configuration of the central station 10. The central station 10 includes a radio signal processing unit 115, an E / O converter 120, an O / E converter 130, a polarization adjustment unit 140, and a circulator 150. The central station 10 is one aspect of a first communication device.

[0038] The radio signal processing unit 115 has a transmission port and a reception port. The radio signal processing unit 115 outputs a modulated signal (transmission signal) modulated with downlink transmission data to the E / O converter 120. The radio signal processing unit 115 processes an uplink reception signal. If the radio system 100 is a system that performs wireless communication using TDD, the radio signal processing unit 115 performs the same processing as the radio base station 11 shown in FIG. 1. If the radio system 100 is a system that performs wireless communication using FDD, the radio signal processing unit 115 uses different frequencies for uplink and downlink.

[0039] The E / O converter 120 receives the modulated signal output from the radio signal processing unit 115. The E / O converter 120 uses the modulated signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby generating an optical signal of a certain wavelength. ｃ The E / O converter 120 generates an optical signal of the generated wavelength λ ｃ The E / O converter 120 outputs the optical signal to the polarization adjustment unit 140. The optical modulator is, for example, a Mach-Zehnder modulator. The E / O converter 120 is one aspect of a first opto-electrical conversion unit.

[0040] The O / E converter 130 receives the optical signal output from the circulator 150. As described above, the same wavelength is used in the upstream and downstream directions between the central station 10 and the base station 20. Therefore, the O / E converter 130 receives the optical signal having the wavelength λ ｃ The O / E converter 130 converts the input optical signal of wavelength λ ｃ The O / E converter 130 converts the optical signal into an electrical signal. The O / E converter 130 outputs the electrical signal to the wireless signal processing unit 115. The O / E converter 130 is one aspect of a second optical-electrical conversion unit.

[0041] The polarization adjustment unit 140 controls the polarization state of the optical signal output from the E / O converter 120. Specifically, the polarization adjustment unit 140 adjusts the polarization plane of the optical signal generated by the E / O converter 120 so that it is different from the polarization plane of the optical signal transmitted by the base station 20. For example, the polarization adjustment unit 140 adjusts the polarization plane of the optical signal generated by the E / O converter 120 so that it is orthogonal to the polarization plane of the optical signal transmitted by the base station 20. In other words, the polarization adjustment unit 140 adjusts the polarization state of the optical signal generated by the E / O converter 120 so that it is orthogonal to the polarization plane of the optical signal transmitted by the base station 20. ｙ Adjust so that:

[0042] The circulator 150 has at least three or more ports. In the following description, it is assumed that the circulator 150 has three ports. A first port of the circulator 150 is connected to the polarization adjustment unit 140. A second port of the circulator 150 is connected to the optical transmission path 30. A third port of the circulator 150 is connected to the O / E converter 130. An optical signal input to the first port of the circulator 150 is output from the second port. An optical signal input to the second port of the circulator 150 is output from the third port. An optical signal input to the third port of the circulator 150 is output from the first port. The circulator 150 is one aspect of a first distribution unit or a distribution unit.

[0043] (Configuration of base station 20) Next, a description will be given of a specific configuration of the base station 20. The base station 20 includes a circulator 210, an O / E converter 220, a radio signal processing unit 235, an E / O converter 240, a polarization adjustment unit 250, and an antenna 260.

[0044] The circulator 210 has at least three or more ports. In the following description, it is assumed that the circulator 210 has three ports. A first port of the circulator 210 is connected to the O / E converter 220. A second port of the circulator 210 is connected to the polarization adjustment unit 250. A third port of the circulator 210 is connected to the optical transmission path 30. An optical signal input to the first port of the circulator 210 is output from the second port. An optical signal input to the second port of the circulator 210 is output from the third port. An optical signal input to the third port of the circulator 210 is output from the first port.

[0045] The O / E converter 220 converts the wavelength λ output by the circulator 210 into ｃ The O / E converter 220 converts the input optical signal of wavelength λ ｃThe O / E converter 220 converts the optical signal into an electrical signal. The O / E converter 220 outputs the electrical signal to the wireless signal processing unit 235. The electrical signal output from the O / E converter 220 is a downstream transmission signal. The O / E converter 220 is one aspect of the first optical-electrical conversion unit or the third optical-electrical conversion unit.

[0046] The radio signal processing unit 235 outputs an electrical signal based on the radio waves received via the antenna 260 to the E / O converter 240. The radio signal processing unit 235 transmits the electrical signal output from the O / E converter 220 as radio waves from the antenna 260. When the radio system 100 is a system that performs radio communication using TDD, the radio signal processing unit 235 performs processing similar to that of the TDD switch 23 shown in Fig. 1. When the radio system 100 is a system that performs radio communication using FDD, the radio signal processing unit 235 performs communication using different frequencies for uplink and downlink.

[0047] The E / O converter 240 receives an electrical signal output from the antenna 260. The E / O converter 240 uses the received electrical signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby generating an optical signal of a certain wavelength. ｃ The E / O converter 240 generates an optical signal of the generated wavelength λ ｃ The E / O converter 240 outputs the optical signal to the polarization adjustment unit 250. The optical modulator is, for example, a Mach-Zehnder modulator. The E / O converter 240 is one aspect of the second opto-electrical conversion unit or the fourth opto-electrical conversion unit.

[0048] The polarization adjustment unit 250 controls the polarization state of the optical signal output from the E / O converter 240. Specifically, the polarization adjustment unit 250 adjusts the polarization plane of the optical signal generated by the E / O converter 240 so that it is different from the polarization plane of the optical signal transmitted by the central station 10. For example, the polarization adjustment unit 250 adjusts the polarization plane of the optical signal generated by the E / O converter 240 so that it is orthogonal to the polarization plane of the optical signal transmitted by the central station 10. In other words, the polarization adjustment unit 250 adjusts the polarization state of the optical signal generated by the E / O converter 240 so that it is orthogonal to the polarization plane of the optical signal transmitted by the central station 10. ｘ Adjust so that:

[0049] (Processing of the wireless system 100 in the first embodiment) FIG. 3 is a sequence diagram showing the flow of processing of the wireless system 100 in the first embodiment. Here, downlink communication will be described as an example. The wireless signal processing unit 115 accepts input of downlink transmission data. The wireless signal processing unit 115 generates a modulated signal based on the input downlink transmission data (step S101). The wireless signal processing unit 115 outputs the generated modulated signal to the E / O converter 120. Note that the wireless signal processing unit 115 may also output an unmodulated transmission signal to the E / O converter 120.

[0050] The E / O converter 120 modulates continuous light with an optical modulator (not shown) provided inside using the input modulation signal to produce a light beam with a wavelength λ ｃ The E / O converter 120 generates an optical signal of the generated wavelength λ ｃ The optical signal is output to the polarization adjuster 140 .

[0051] The polarization adjuster 140 adjusts the wavelength λ output from the E / O converter 120. ｃ The polarization adjuster 140 receives an optical signal of wavelength λ ｃ The polarization plane of the optical signal of wavelength λ is adjusted (step S103). ｃ The polarization plane of the optical signal of wavelength λ ｃ The polarization adjuster 140 adjusts the polarization plane of the optical signal of wavelength λ to be perpendicular to the polarization plane of the optical signal of wavelength λ. １ After adjusting the polarization plane of the optical signal, the wavelength λ after the polarization plane adjustment ｃ The optical signal of wavelength λ output from the polarization adjuster 140 is output to the circulator 150. ｃ The optical signal of wavelength λ is input to the first port of the circulator 150. The circulator 150 converts the optical signal of wavelength λ ｃ The second port of the circulator 150 is connected to the optical transmission line 30. Therefore, the optical signal having the wavelength λ 1 output from the second port of the circulator 150 is ｃ The optical signal is output to the optical transmission line 30 (step S104).

[0052] The wavelength λ output from the optical transmission line 30 ｃThe optical signal of wavelength λ is input to the third port of the circulator 210 of the base station 20 (step S105). ｃ The optical signal of wavelength λ is output from the first port. The first port of the circulator 210 is connected to the O / E converter 220. Therefore, the optical signal of wavelength λ is output from the first port of the circulator 210. ｃ The optical signal is input to the O / E converter 220 .

[0053] When the base station 20 transmits an optical signal to the central station 10, the wavelength λ ｃ 1, the communication light (wavelength λ 1 transmitted from the central station 10) input to the O / E converter 220 is also backscattered light generated by the optical signal. ｃ The polarization plane of the backscattered light is perpendicular to the polarization plane of the optical signal (E in FIG. 1). ｘ ´,E ｙ ´ ' set or E ｙ ´,E ｘ In the case of the pair (group '), multipath interference does not occur between the optical carrier of the main signal and the backscattered light. Therefore, it is possible to transmit the main signal without significant degradation in quality.

[0054] In the present invention, the polarization state is adjusted so that the polarization plane of the optical signal transmitted by the central station 10 is orthogonal to the polarization plane of the optical signal transmitted by the base station 20. Therefore, multipath interference does not occur between the optical carrier of the main signal and the backscattered light. This makes it possible to transmit the main signal without significantly degrading its quality. The O / E converter 220 converts the input wavelength λ ｃ The O / E converter 220 converts the optical signal into an electrical signal (step S106). The O / E converter 220 outputs the electrical signal to the wireless signal processing unit 235. The wireless signal processing unit 235 transmits the electrical signal output from the O / E converter 220 as a wireless radio wave via the antenna 260 (step S107).

[0055] According to the wireless system 100 configured as above, the central station 10 receives the wavelength λ 1 output from the E / O converter 120. ｃ The polarization plane of the optical signal of wavelength λｃ a polarization adjuster 140 for adjusting the polarization plane of the optical signal of wavelength λ 1 , which is adjusted by the polarization adjuster 140, to be different from the polarization plane of the optical signal of wavelength λ 1 . ｃ is output to the base station 20 via the optical transmission path 30, and the optical signal of wavelength λ ｃ The base station 20 further includes a circulator 150 that outputs the optical signal of wavelength λ output from the E / O converter 240. ｃ The polarization plane of the optical signal of wavelength λ ｃ a polarization adjuster 250 for adjusting the polarization plane of the optical signal of wavelength λ 1 , which is adjusted by the polarization adjuster 250, to be different from the polarization plane of the optical signal of wavelength λ 1 . ｃ The optical signal of wavelength λ is output to the central station 10 via the optical transmission line 30, and the optical signal of wavelength λ is input via the optical transmission line 30. ｃ and a circulator 210 that outputs the optical signal to an O / E converter 220.

[0056] In this way, the use of a circulator in the wireless system 100 enables communication using a single wavelength in both the upstream and downstream directions. Furthermore, by using a circulator instead of a conventional optical splitter, it is possible to suppress optical power loss and also to reduce the influence of optical feedback. Therefore, in a system that uses the same wavelength in both the upstream and downstream directions, it is possible to suppress optical power loss and also suppress deterioration of communication quality.

[0057] Furthermore, in the wireless system 100, the central station 10 transmits at a wavelength λ ｃ and the wavelength λ transmitted by the base station 20. ｃ The polarization plane of the optical signal is adjusted so that it is different from that of the optical signal of the other optical signal. Therefore, interference can be suppressed even when communications are performed using the same wavelength. Furthermore, multipath interference does not occur between the optical carrier of the main signal and the backscattered light. Therefore, it is possible to transmit the main signal without significantly degrading its quality.

[0058] Furthermore, in the wireless system 100, the polarization state of the optical signal can be adjusted by inputting the optical signal into the polarization adjustment unit, so that the system is applicable even when the TDD timing is shifted between the central station 10 and the base station 20 due to the influence of optical fiber delay or the like. Furthermore, the wireless system 100 is not limited to systems that perform wireless communication using TDD, but can also be applied to systems that perform wireless communication using FDD, thereby improving convenience.

[0059] (Variant 1 of the first embodiment) In the above-described configuration, both the central station 10 and the base station 20 are provided with a polarization adjustment unit, but at least one of the central station 10 or the base station 20 may be provided with a polarization adjustment unit.

[0060] (Second Modification of the First Embodiment) The central station 10 may include a polarized beam splitter between the O / E converter 130 and the circulator 150. In this way, the central station 10 splits the backscattered light (E in FIG. 2 ) by the polarized beam splitter before it is input to the O / E converter 130. ｙ Similarly, the base station 20 may include a polarized beam splitter between the O / E converter 220 and the circulator 210. In this way, the base station 20 can split the backscattered light (E in FIG. 2 ) by the polarized beam splitter before it is input to the O / E converter 220. ｘ ´´) polarization may be cut.

[0061] (Second embodiment) In the second embodiment, a configuration is described in which the polarization crossing angle between the polarization state of an optical signal received at a central station or a remote station and the returned light is determined, and the amount of adjustment of the polarization plane (e.g., the angle of the polarization plane) adjusted by the polarization adjustment unit of the device itself is controlled based on the determination result.

[0062] FIG. 4 is a diagram showing an example of the configuration of a wireless system 100a according to the second embodiment. The wireless system 100a includes a central station 10a and one or more base stations 20a. The central station 10a and the one or more base stations 20a are connected via an optical transmission path 30. Here, an example in which there is one base station 20a will be described. The central station 10a and the one or more base stations 20a form a single base station. Specifically, the central station 10a has a signal processing function that is a function obtained by separating the signal processing function and communication function provided in a base station. The one or more base stations 20a have a communication function that is a function obtained by separating the signal processing function and communication function provided in a base station. The wireless system 100a may be a system that performs wireless communication using TDD or FDD.

[0063] Between the central station 10a and the base station 20a, transmission is performed using analog RoF, which transmits radio signals using optical fiber. The same wavelength is used in the uplink and downlink directions between the central station 10a and one base station 20a. In this way, bidirectional communication using the DDM method is performed between the central station 10a and one base station 20a. This technology can be applied to mobile fronthaul. In this case, the central station 10a is a CS and the base station 20a is an RRU.

[0064] For ease of explanation, the polarization state of the light used for upstream communication from the base station 20a to the central station 10a is referred to as E ｘ and the polarization state of the light used for downstream communication from the central station 10a to the base station 20a is E ｙ The polarization state of the light after upstream transmission is E ｘ ', and the polarization state of the light after downstream transmission is E ｙ Furthermore, the polarization state of the light returning to the base station 20a as backscattered light is expressed as E ｘ '', and the polarization state of the light returning to the central station 10a as backscattered light is E ｙ ´´Let's say.

[0065] In the second embodiment, the polarization plane is rotated and changes over time due to optical fiber transmission.ｃ The polarization plane of the optical signal of wavelength λ 1 may be different before being output from the central station 10a to the optical transmission line 30 and when it is received by the base station 20a via the optical transmission line 30. ｃ The polarization plane of the optical signal may be different before it is output from the base station 20a to the optical transmission path 30a and when it is received by the central station 10a via the optical transmission path 30a.

[0066] (Configuration of central station 10a) First, the specific configuration of the central station 10a will be described. The central station 10a includes a radio signal processing unit 115, an E / O converter 120, an O / E converter 130, a polarization adjustment unit 140a, a circulator 150, a polarization state determination unit 160a, and a control unit 170a. The central station 10a is one aspect of the first communication device.

[0067] The central station 10a differs in configuration from the central station 10 in that it includes a polarization adjustment unit 140a instead of the polarization adjustment unit 140, and in that it newly includes a polarization state determination unit 160a and a control unit 170a. The following description will focus on the differences from the central station 10.

[0068] The polarization state determination unit 160a determines the polarization states of the optical signal input to the O / E converter 130 and the returned light (e.g., backscattered light) based on the electrical signal converted by the O / E converter 130. Specifically, the polarization state determination unit 160a determines the polarization crossing angle between the optical signal and the returned light based on the electrical signal converted. Specific processing will be described later.

[0069] The control unit 170a controls the angle adjusted by the polarization adjustment unit 140a in accordance with the polarization state determined by the polarization state determination unit 160a.

[0070] The polarization adjustment unit 140a controls the polarization state of the optical signal output from the E / O converter 120. Specifically, the polarization adjustment unit 140a adjusts the plane of polarization of the optical signal generated by the E / O converter 120 so that it is different from the plane of polarization of the optical signal transmitted by the base station 20a. For example, under the control of the control unit 170a, the polarization adjustment unit 140a adjusts the plane of polarization of the optical signal generated by the E / O converter 120 so that it is orthogonal to the plane of polarization of the backscattered light. The change in the plane of polarization caused by the control of the control unit 170a is minimal, and its effect on the received signal from the base station 20a is negligible. Through this process, the polarization adjustment unit 140a adjusts the plane of polarization of the optical signal generated by the E / O converter 120 so that it is orthogonal to the plane of polarization of the optical signal transmitted by the base station 20a.

[0071] (Configuration of base station 20a) Next, a specific configuration of the base station 20a will be described. The base station 20a includes a circulator 210, an O / E converter 220, a radio signal processing unit 235, an E / O converter 240, a polarization adjustment unit 250a, an antenna 260, a polarization state determination unit 270a, and a control unit 280a. The base station 20a is one aspect of the second communication device.

[0072] The base station 20a differs in configuration from the base station 20 in that it includes a polarization adjustment unit 250a instead of the polarization adjustment unit 250, and in that it newly includes a polarization state determination unit 270a and a control unit 280a. The following description will focus on the differences from the base station 20.

[0073] The polarization state determination unit 270a determines the polarization states of the optical signal input to the O / E converter 220 and the returned light (e.g., backscattered light) based on the electrical signal converted by the O / E converter 220. Specifically, the polarization state determination unit 270a determines the polarization crossing angle between the optical signal and the returned light based on the electrical signal converted. Specific processing will be described later.

[0074] The control unit 280a controls the angle adjusted by the polarization adjustment unit 250a in accordance with the polarization state determined by the polarization state determination unit 270a.

[0075] The polarization adjustment unit 250a controls the polarization state of the optical signal output from the E / O converter 240. Specifically, the polarization adjustment unit 250a adjusts the plane of polarization of the optical signal generated by the E / O converter 240 so that it is different from the plane of polarization of the optical signal transmitted by the central station 10a. For example, under the control of the control unit 280a, the polarization adjustment unit 250a adjusts the plane of polarization of the optical signal generated by the E / O converter 120 so that it is orthogonal to the plane of polarization of the backscattered light. The change in the plane of polarization due to the control of the control unit 280a is small, and its effect on the received signal from the central station 10a is negligible. Through this process, the polarization adjustment unit 250a adjusts the plane of polarization of the optical signal generated by the E / O converter 240 so that it is orthogonal to the plane of polarization of the optical signal transmitted by the central station 10a.

[0076] Next, the polarization state determination method and the polarization state adjustment will be described with reference to FIG. 5. FIG. 5 is a diagram for explaining specific processing in the second embodiment. Here, for the sake of simplicity, only the E ｘ is rotated by an angle θ, and E in the y-polarized state ｙ is input to the O / E converter. In this situation, the polarized wave E is expressed as the following equation (1).

[0077]

[0078] Furthermore, the photocurrent after square-law detection in the O / E converter is expressed by the following equation (2).

[0079]

[0080] FIG. 6 shows the electrical spectrum after detection when the polarization plane crossing angle θ is changed to 90 degrees, 85 degrees, and 80 degrees. Note that this is different from the θ in the theoretical formula. Referring to FIG. 6 , it can be seen that the beat noise near the DC component (near DC) increases as the polarization plane crossing angle deviates from the orthogonal state. That is, to maintain the polarization plane crossing angle in the orthogonal state, the polarization adjustment unit adjusts the polarization state so that the beat noise near the DC component is minimized. Therefore, the control units 170a and 280a instruct the polarization adjustment units 140 and 250 to adjust a control amount (e.g., an adjustment angle of the polarization plane) so that the beat noise near the DC component is minimized. The polarization adjustment units 140 and 250 adjust the polarization plane of the optical signal output from the E / O converters 120 and 240 according to the control amount instructed by the control units 170a and 280a.

[0081] In the wireless system 100a configured as described above, although the plane of polarization may rotate over time during fiber transmission in an actual optical transmission line 30, this configuration makes it possible to control the polarization state in accordance with the time fluctuations, thereby enabling stable one-wavelength bidirectional uplink and downlink communication.

[0082] (Variant 1 of the second embodiment) In the above-described configuration, both the central station 10a and the remote station 20a are shown to have a polarization adjustment unit, a polarization state determination unit, and a control unit, but at least one of the central station 10a or the remote station 20a may have a polarization adjustment unit, a polarization state determination unit, and a control unit.

[0083] (Second Modification of the Second Embodiment) The central station 10a may include a polarized beam splitter between the O / E converter 130 and the circulator 150. In this way, the central station 10a splits the backscattered light (E in FIG. 4 ) by the polarized beam splitter before it is input to the O / E converter 130. ｙ Similarly, the base station 20a may include a polarized beam splitter between the O / E converter 220 and the circulator 210. In this way, the base station 20a can split the backscattered light (E in FIG. 4 ) by the polarized beam splitter before it is input to the O / E converter 220. ｘ ´´) polarization may be cut.

[0084] (Third embodiment) In the third embodiment, a configuration is described in which a central station or a base station controls the adjustment amount of the polarization plane (e.g., the angle of the polarization plane) adjusted by the polarization adjustment unit of the device itself based on information indicating the polarization state notified from a communication partner (e.g., from the perspective of the central station, the communication partner is the base station, and from the perspective of the base station, the communication partner is the central station).

[0085] FIG. 7 is a diagram showing an example configuration of a wireless system 100b according to the third embodiment. The wireless system 100b includes a central station 10b and one or more base stations 20b. The central station 10b and the one or more base stations 20b are connected via an optical transmission path 30. Here, an example in which there is one base station 20b will be described. The central station 10b and the one or more base stations 20b form a single base station. Specifically, the central station 10b has a signal processing function that is a function obtained by separating the signal processing function and communication function provided in a base station. The one or more base stations 20b have a communication function that is a function obtained by separating the signal processing function and communication function provided in a base station. The wireless system 100b may be a system that performs wireless communication using TDD or FDD.

[0086] Between the central station 10b and the base station 20b, transmission is performed using analog RoF, which transmits radio signals using optical fiber. The same wavelength is used in the uplink and downlink directions between the central station 10b and one base station 20b. In this way, bidirectional communication using the DDM method is performed between the central station 10b and one base station 20b. This technology can be applied to mobile fronthaul. In this case, the central station 10b is a CS and the base station 20b is an RRU.

[0087] For ease of explanation, the polarization state of the light used for upstream communication from the base station 20b to the central station 10b is referred to as E ｘ and the polarization state of the light used for downstream communication from the central station 10b to the base station 20b is E ｙ The polarization state of the light after upstream transmission is E ｘ ', and the polarization state of the light after downstream transmission is E ｙFurthermore, the polarization state of the light returning to the base station 20b as backscattered light is expressed as E ｘ '', and the polarization state of the light returning to the central station 10b as backscattered light is E ｙ ´´Let's say.

[0088] In the third embodiment, the polarization plane is rotated and changes over time due to optical fiber transmission. ｃ The polarization plane of the optical signal of wavelength λ 1 may be different before being output from the central station 10b to the optical transmission line 30 and when it is received by the base station 20b via the optical transmission line 30. Similarly, the polarization plane of the optical signal of wavelength λ 1 may be different before being output from the central station 10b to the optical transmission line 30 and when it is received by the base station 20b via the optical transmission line 30. ｃ The polarization plane of the optical signal may be different before it is output from the base station 20 b to the optical transmission line 30 and when it is received by the central station 10 b via the optical transmission line 30 .

[0089] (Configuration of central station 10b) First, a specific configuration of central station 10b will be described. Central station 10b includes a radio signal processing unit 115, an E / O converter 120, an O / E converter 130, a polarization adjustment unit 140a, a circulator 150, a polarization state determination unit 160a, and a control unit 170b. Central station 10b is one aspect of the first communication device.

[0090] The central station 10b differs in configuration from the central station 10a in that it includes a control unit 170b instead of the control unit 170. The following description will focus on the differences from the central station 10a.

[0091] The control unit 170b controls the angle adjusted by the polarization adjustment unit 140a based on the polarization state determined by the polarization state determination unit 160a and information indicating the polarization state notified from the base station 20b. Here, the information indicating the polarization state notified from the base station 20b may be notified via a control line connecting the central station 10b and the base station 20b, or may be multiplexed into the main signal and notified via the optical transmission path 30. The control unit 170b may control the polarization adjustment unit 140a so that the polarization crossing angles of the downlink and uplink main signals are orthogonal at the timing of transmission of the main signal (assuming a TDD system, for example), or may control based on only one of them.

[0092] As shown in Figure 7, when the polarization adjustment unit 140a of the central station 10b changes the polarization state, it is possible to make either the polarization crossing angle of the light input to the O / E converter 130 of the central station 10b or the polarization crossing angle of the light input to the O / E converter 220 of the base station 20b orthogonal. For example, assuming a TDD system, the polarization crossing angle needs to be orthogonal in the O / E converter 220 of the base station 20b at the timing of downstream communication (central station 10b → base station 20b), and the polarization crossing angle needs to be orthogonal in the O / E converter 130 of the central station 10b at the timing of upstream communication (base station 20b → central station 10b). The control unit 170b of the central station 10b can know the status of both the polarization crossing angle of the light input to the O / E converter 130 of the central station 10b and the O / E converter 220 of the base station 20b. Therefore, the control unit 170b controls the polarization state by outputting to the polarization adjustment unit 140a an instruction to change the polarization state so as to satisfy the above conditions every moment in accordance with the timing of TDD.

[0093] (Configuration of base station 20b) Next, a specific configuration of the base station 20b will be described. The base station 20b includes a circulator 210, an O / E converter 220, a radio signal processing unit 235, an E / O converter 240, a polarization adjustment unit 250a, an antenna 260, a polarization state determination unit 270a, and a control unit 280b. The base station 20b is one aspect of the second communication device.

[0094] The base station 20b differs in configuration from the base station 20a in that it does not include the polarization adjustment unit 250 and includes a control unit 280b instead of the control unit 280a. The following description will focus on the differences from the base station 20a.

[0095] The control unit 280b notifies the central station 10b of information indicating the polarization state determined by the polarization state determination unit 270a. When notifying the central station 10b of information indicating the polarization state via the control line, the control unit 280b notifies the central station 10b via the control line of a control signal (electrical signal) including information indicating the polarization state. When multiplexing the information indicating the polarization state onto the main signal and notifying the central station 10b via the optical transmission path 30, the control unit 280b outputs the control signal (electrical signal) including information indicating the polarization state to the E / O converter 240. As a result, the E / O converter 240 multiplexes the control signal output from the control unit 280b and the main signal output from the radio signal processing unit 235.

[0096] In the third embodiment, the polarization adjustment unit is arranged in either the central station 10b or the base station 20b (in FIG. 7, it is arranged in the central station 10b). When the polarization adjustment unit is arranged in the base station 20b, the central station 10b and the base station 20b in the above description can be interchanged.

[0097] According to the wireless system 100b configured as above, it is possible to obtain the same effects as those of the second embodiment.

[0098] Furthermore, in the wireless system 100b, neither the central station 10b nor the base station 20b needs to be provided with a polarization adjustment unit. This allows for simplification of the device and reduction in costs. In particular, in a wireless system including multiple base stations 20b, a more significant effect can be achieved because polarization adjustment units are not required in some or all of the base stations 20b.

[0099] Fourth Embodiment In a fourth embodiment, a bidirectional communication configuration using the same wavelength in the upstream and downstream directions in a wireless system employing a WDM-PON (Wavelength Division Multiplexing - Passive Optical Network) will be described.

[0100] First, the configuration of a wireless system that applies a conventional WDM-PON will be described. Fig. 8 is a diagram showing an example of the configuration of a conventional wireless system 1a that applies a WDM-PON. The wireless system 1a comprises a central station 2a, multiple base stations 3a-1 to 3a-n (n is an integer of 2 or greater), and an optical splitter SP. The central station 2a and the multiple base stations 3a-1 to 3a-n are connected via an optical transmission path and the optical splitter SP.

[0101] Transmission is performed using analog RoF between the central station 2a and the base stations 3a-1 to 3a-n. Conventionally, different wavelengths are used for the uplink and downlink directions between the central station 2a and one base station 3. For example, in a conventional configuration, a total of two wavelengths are used, one for the uplink direction and one for the downlink direction.

[0102] The central station 2a includes a plurality of E / O converters 411-1 to 411-n, a plurality of O / E converters 412-1 to 412-n, and a wavelength multiplexer / demultiplexer 413. The E / O converter 411-1 modulates continuous light with an optical modulator (not shown) provided inside using an input modulation signal to generate a wavelength λ １ The E / O converter 411-n uses the input modulation signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby generating an optical signal of wavelength λ ３１ The E / O converters 411-1 to 411-n output the generated optical signals to the wavelength multiplexing / demultiplexing unit 413.

[0103] The O / E converter 412-1 converts the wavelength λ ２ The O / E converter 412-n converts the optical signal of wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 413 into an electrical signal. ３２ The wavelength multiplexing / demultiplexing unit 413 converts the optical signals input thereto into electrical signals. The wavelength multiplexing / demultiplexing unit 413 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexing / demultiplexing unit 413 multiplexes (multiplexes) the optical signals output from the E / O converters 411-1 to 411-n to generate a multiplexed signal. The wavelength multiplexing / demultiplexing unit 413 outputs the generated multiplexed signal to the optical transmission path. For example, the wavelength multiplexing / demultiplexing unit 413 demultiplexes the optical signals input thereto via the optical transmission path and outputs the demultiplexed signals to the O / E converters 412-1 to 412-n.

[0104] The optical splitter SP splits the multiplexed signal transmitted from the central station 2a and outputs the split signals to the base stations 3a-1 to 3a-n. The optical splitter SP outputs the optical signals transmitted from the base stations 3a-1 to 3a-n to the central station 2a.

[0105] The base stations 3a-1 to 3a-n have the same configuration. Here, the configuration of the base station 3a-1 will be described as an example. The base station 3a-1 has an O / E converter 32-1, an E / O converter 33-1, and a wavelength multiplexing / demultiplexing unit 34-1. The E / O converter 33-1 uses the input signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby generating an assigned transmission wavelength (for example, wavelength λ ２ The E / O converter 33-1 outputs the generated optical signal to the wavelength multiplexing / demultiplexing unit 34-1.

[0106] The O / E converter 32-1 converts the wavelength λ 1 demultiplexed by the wavelength multiplexer / demultiplexer 34-1 into １ The wavelength multiplexing / demultiplexing unit 34-1 converts the optical signals input thereto into electrical signals. The wavelength multiplexing / demultiplexing unit 34-1 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexing / demultiplexing unit 34-1 multiplexes (multiplexes) the optical signals output from the E / O converter 33-1 to generate a multiplexed signal. The wavelength multiplexing / demultiplexing unit 34-1 outputs the generated multiplexed signal to the optical transmission path. For example, the wavelength multiplexing / demultiplexing unit 34-1 demultiplexes the optical signals input thereto via the optical transmission path and outputs the demultiplexed signal to the O / E converter 32-1.

[0107] As shown in Figure 8, a passive double star configuration is an example of a configuration with multiple base stations 3a-1 to 3a-n. However, conventional methods require an optical splitter SP and wavelength multiplexing / demultiplexing units 413 and 34, and optical section loss increases as the number of base stations 3a increases. For example, if the optical splitter SP splits into 16 beams, an optical loss of 12 dB or more occurs in the optical splitter SP alone. The optical loss is calculated, for example, based on the following equation (3):

[0108]

[0109] In formula (3), L оｐｔ represents distance attenuation, and 2L ＭＵＸＤＥＭＵＸrepresents the insertion loss of the wavelength multiplexing / demultiplexing unit × 2, and 10 log N represents the loss of the optical splitter SP. In the conventional configuration, two wavelengths are used in the upstream and downstream directions, so there is a restriction that the base station 3a can only be deployed up to half the maximum number of available wavelengths.

[0110] A configuration of a wireless system 100c according to a second embodiment will be described, which makes it possible to increase the number of base stations while suppressing an increase in optical loss that accompanies an increase in the number of base stations compared to the configuration shown in Fig. 8. Fig. 9 is a diagram showing an example configuration of a wireless system 100c according to a fourth embodiment. The wireless system 100c includes a central station 10c, multiple base stations 20-1 to 20-n, and a wavelength multiplexing / demultiplexing unit 40. The central station 10c and the multiple base stations 20-1 to 20-n are connected via an optical transmission path 30 and the wavelength multiplexing / demultiplexing unit 40. In the following description, when there is no need to distinguish between the base stations 20-1 to 20-n, they will simply be referred to as base stations 20.

[0111] Next, the specific configurations of the central station 10c and the base station 20 will be described. First, the specific configuration of the central station 10c will be described. The central station 10c includes a plurality of E / O converters 120-1 to 120-n, a plurality of O / E converters 130-1 to 130-n, a plurality of polarization adjusters 140-1 to 140-n, a plurality of circulators 150-1 to 150-n, and a wavelength multiplexing / demultiplexing unit 180. The central station 10c differs in configuration from the central station 10 in that it includes a plurality of E / O converters 120, O / E converters 130, polarization adjusters 140, and circulators 150, and in that it newly includes a wavelength multiplexing / demultiplexing unit 180. Although omitted in FIG. 9, the central station 10c also includes a radio signal processing unit 115.

[0112] The central station 10c uses a different wavelength for each combination of the E / O converter 120 and the O / E converter 130. For example, the wavelength λ １ is used, and the wavelength λ is used for the combination of the E / O converter 120-n and the O / E converter 130-n. ｎ is used.

[0113] The E / O converters 120-1 to 120-n perform the same processing as the E / O converter 120 shown in the first embodiment except that they use different wavelengths. The O / E converters 130-1 to 130-n perform the same processing as the O / E converter 130 shown in the first embodiment except that they use different wavelengths.

[0114] The circulators 150-1 to 150-n have at least three or more ports. In the following description, it is assumed that the circulators 150-1 to 150-n have three ports. The first ports of the circulators 150-1 to 150-n are connected to the polarization adjustment units 140-1 to 140-n. The second ports of the circulators 150-1 to 150-n are connected to the wavelength multiplexing / demultiplexing unit 180. The third ports of the circulators 150-1 to 150-n are connected to the O / E converters 130-1 to 130-n. An optical signal input to the first port of the circulators 150-1 to 150-n is output from the second port. An optical signal input to the second port of the circulators 150-1 to 150-n is output from the third port. The optical signal input to the third port of each of the circulators 150-1 to 150-n is output from the first port.

[0115] The wavelength multiplexing / demultiplexing unit 180 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexing / demultiplexing unit 180 multiplexes (multiplexes) the optical signals output from the circulators 150-1 to 150-n (the optical signals output from the polarization adjustment units 140-1 to 140-n) to generate a multiplexed signal. The wavelength multiplexing / demultiplexing unit 180 outputs the generated multiplexed signal to the optical transmission path 30. For example, the wavelength multiplexing / demultiplexing unit 180 demultiplexes the optical signals input via the optical transmission path 30 and outputs them to the circulators 150-1 to 150-n.

[0116] The base stations 20-1 to 20-n have the same configuration as the base station 20 in the first embodiment. The base stations 20-1 to 20-n communicate with the central station 10c using different wavelengths. For example, the base station 20-1 uses a wavelength λ １ The base station 20-n communicates with the central station 10c using a wavelength λ ｎ The communication is performed between the central station 10c and the central station 10b using the above.

[0117] (Processing of the wireless system 100c in the fourth embodiment) Next, the flow of processing of the wireless system 100c in the fourth embodiment will be described. Here, the description will be made taking downstream communication as an example. The E / O converters 120-1 to 120-n use the input modulation signal to modulate continuous light with an optical modulator (not shown) provided inside to generate a wavelength λ １ ~λ ｎ Here, the central station 10c generates an optical signal of wavelength λ １ ~λ ｎ However, the central station 10c may transmit the transmission data to at least one base station 20.

[0118] The E / O converters 120-1 to 120-n convert the generated wavelength λ １ ~λ ｎ The polarization adjusters 140-1 to 140-n output the optical signals of wavelength λ output from the E / O converters 120-1 to 120-n. １ ~λ ｎ The specific processing performed by the polarization adjusters 140-1 to 140-n is the same as that in the first embodiment. １ ~λ ｎ After adjusting the polarization plane of the optical signal, the wavelength λ after the polarization plane adjustment １ ~λ ｎ The optical signals are output to the circulators 150-1 to 150-n.

[0119] The wavelength λ output from the polarization adjusters 140-1 to 140-n １ ~λ ｎ The optical signal of wavelength λ is input to the first port of each of the circulators 150-1 to 150-n. １ ~λ ｎ The second ports of the circulators 150-1 to 150-n are connected to the wavelength multiplexing / demultiplexing unit 180. Therefore, the optical signals of wavelength λ 1 output from the second ports of the circulators 150-1 to 150-n are １ ~λ ｎThe optical signal is output to the wavelength multiplexer / demultiplexer 180 .

[0120] The wavelength multiplexer / demultiplexer 180 multiplexes the wavelengths λ 1 output from the circulators 150-1 to 150-n. １ ~λ ｎ The wavelength multiplexing / demultiplexing unit 180 multiplexes the optical signals of wavelength λ contained in the multiplexed signal to generate a multiplexed signal. The wavelength multiplexing / demultiplexing unit 180 outputs the generated multiplexed signal to the optical transmission line 30. The multiplexed signal output from the wavelength multiplexing / demultiplexing unit 180 is input to the wavelength multiplexing / demultiplexing unit 40 via the optical transmission line 30. The wavelength multiplexing / demultiplexing unit 40 demultiplexes the input multiplexed signal ..., for example, １ to the base station 20-1, and outputs the optical signal of wavelength λ ｎ The optical signal is output to the base station 20-n.

[0121] Since the base stations 20-1 to 20a-n perform the same processing, the processing of the base station 20-1 will be described below as an example. １ The optical signal of wavelength λ is input to the third port of the circulator 210-1 of the base station 20-1. １ The optical signal of wavelength λ is output from the first port. The first port of the circulator 210-1 is connected to the O / E converter 220-1. Therefore, the optical signal of wavelength λ is output from the first port of the circulator 210-1. １ The optical signal of wavelength λ is input to the O / E converter 220-1. １ The O / E converter 220-1 converts the optical signal into an electrical signal. The O / E converter 220-1 outputs the electrical signal to the wireless signal processing unit 235-1. The wireless signal processing unit 235-1 transmits the electrical signal output from the O / E converter 220-1 as a wireless radio wave via the antenna 260-1.

[0122] In this way, the central station 10c includes a circulator 150 between the E / O converter 120 and the O / E converter 130 and the wavelength multiplexing / demultiplexing unit 180. This makes it possible for the central station 10c to transmit signals in both the upstream and downstream directions using a single wavelength in combination with one E / O converter 120 and one O / E converter 130.

[0123] The loss in the optical section of the wireless system 100c configured in this manner is calculated based on, for example, the following equation (4), including the insertion loss of the wavelength multiplexing / demultiplexing unit 180 in the central station 10c, the insertion loss of the wavelength multiplexing / demultiplexing unit 40 at the relay point, the circulator insertion loss in the central station 10c and the base station 20, the polarization controller (e.g., polarization adjustment units 140, 250) insertion loss, and the polarization beam splitter insertion loss (if provided), as well as the transmission loss.

[0124]

[0125] In formula (4), L оｐｔ represents distance attenuation, and 2L ＭＵＸＤＥＭＵＸ represents the insertion loss of the wavelength multiplexer / demultiplexer × 2, and 2L ｃｉｒ represents the insertion loss of the circulators 150 and 210 × 2. ＰＢＳ represents the insertion loss of the polarizing beam splitter. ＰＣ represents the insertion loss of the polarization controller.

[0126] The insertion loss of each device (e.g., circulator, polarization controller, and polarization beam splitter) is approximately 1 dB or less for commercially available products. As such, in the configuration of this embodiment, assuming that the optical transmission path length is the same as that of the conventional configuration shown in Figure 8, it is possible to suppress the increase in optical loss that occurs with an increase in the number of base stations 20, and it is possible to deploy base stations 20 equal to the maximum number of available wavelengths. In other words, it is possible to deploy twice as many base stations 20 as in the conventional configuration.

[0127] (Variation 1 of the Fourth Embodiment) The central station 10c may include a plurality of polarization adjustment units 140a, polarization state determination units 160a, and control units 170a in the second embodiment, or a plurality of polarization state determination units 160a and control units 170b in the third embodiment. The base station 20 may include a polarization adjustment unit 250a, polarization state determination unit 270a, and control unit 280a in the second embodiment, or a polarization state determination unit 270a and control unit 280b in the third embodiment.

[0128] Fifth Embodiment In the fifth embodiment, a configuration for bidirectional communication using the same wavelength in the upstream and downstream directions in a network topology in which a plurality of base stations are connected in a cascade configuration will be described.

[0129] First, a conventional configuration will be described. FIG. 10 is a diagram showing an example of the configuration of a conventional wireless system 1b. The wireless system 1b includes a central station 2b and multiple base stations 3b-1 to 3b-2. The central station 2b and the base station 3b-1, and the base station 3b-1 and the base station 3b-2 are connected via optical transmission paths. As shown in FIG. 10, the multiple base stations 3b form a cascade network topology. Note that while FIG. 10 shows an example in which there are two base stations 3b, even when there are three or more base stations 3b, the base stations 3b form a cascade network topology.

[0130] Transmission is performed between the central station 2b and the base station 3b-1 using analog RoF. Conventionally, different wavelengths are used for the uplink and downlink directions between the central station 2b and one base station 3b.

[0131] The central station 2b includes a plurality of E / O converters 411-1 to 411-n, a plurality of O / E converters 412-1 to 412-n, and a wavelength multiplexer / demultiplexer 413. The E / O converter 411-1 modulates continuous light with an optical modulator (not shown) provided inside using an input modulation signal to generate a wavelength λ ＤＬ１ The E / O converter 411-n uses the input modulation signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby generating an optical signal of wavelength λ ＤＬｎ The E / O converters 411-1 to 411-n output the generated optical signals to the wavelength multiplexing / demultiplexing unit 413.

[0132] The O / E converter 412-1 converts the wavelength λ ＵＰ１ The O / E converter 412-n converts the optical signal of wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 413 into an electrical signal. ＵＰｎThe wavelength multiplexing / demultiplexing unit 413 converts the optical signals input thereto into electrical signals. The wavelength multiplexing / demultiplexing unit 413 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexing / demultiplexing unit 413 multiplexes (multiplexes) the optical signals output from the E / O converters 411-1 to 411-n to generate a multiplexed signal. The wavelength multiplexing / demultiplexing unit 413 outputs the generated multiplexed signal to the optical transmission path. For example, the wavelength multiplexing / demultiplexing unit 413 demultiplexes the optical signals input thereto via the optical transmission path and outputs the demultiplexed signals to the O / E converters 412-1 to 412-n.

[0133] The base stations 3b-1 to 3b-n have the same configuration. Here, the configuration of the base station 3b-1 will be described as an example. The base station 3b-1 includes an O / E converter 32-1, an E / O converter 33-1, a circulator 37-1, a branching filter 38-1, an electric amplifier 39-1, an electric amplifier 41-1, a multiplexer 42-1, and a circulator 43-1. As described above, in the conventional configuration, each base station 3b includes two circulators.

[0134] The circulator 37-1 has at least three or more ports. In the following description, it is assumed that the circulator 37-1 has three ports. The first port of the circulator 37-1 is connected to the central station 2b via an optical transmission path. The second port of the circulator 37-1 is connected to the demultiplexer 38-1. The third port of the circulator 37-1 is connected to the multiplexer 42-1. An optical signal input to the first port of the circulator 37-1 is output from the second port. An optical signal input to the second port of the circulator 37-1 is output from the third port. An optical signal input to the third port of the circulator 37-1 is output from the first port.

[0135] The demultiplexer 38-1 demultiplexes the input multiplexed signal. For example, the demultiplexer 38-1 demultiplexes the wavelength λ ＤＬ１ The demultiplexer 38-1 demultiplexes the optical signal of the demultiplexed wavelength λ ＤＬ１ The demultiplexer 38-1 outputs the optical signal of wavelength λ included in the multiplexed signal to the O / E converter 32-1. ＤＬ２, … , λ ＤＬｎ The demultiplexer 38-1 outputs the optical signal to the circulator 43-1. The demultiplexer 38-1 is, for example, an optical thin film filter.

[0136] The O / E converter 32-1 converts the wavelength λ 1 demultiplexed by the demultiplexer 38-1 into ＤＬ１ The optical signal is converted by the O / E converter 32-1 into an electrical signal. The electrical amplifier 39-1 amplifies and outputs the electrical signal converted by the O / E converter 32-1. The electrical amplifier 41-1 amplifies and outputs the electrical signal to be transmitted in the upstream direction to the E / O converter 33-1.

[0137] The E / O converter 33-1 modulates continuous light with an optical modulator (not shown) provided inside using the input electrical signal, thereby converting the assigned transmission wavelength (for example, wavelength λ ＵＰ１ The E / O converter 33-1 outputs the generated optical signal to the multiplexer 42-1. The multiplexer 42-1 multiplexes the optical signal (for example, with a wavelength λ ＵＰ１ ) and the optical signal output from the circulator 43-1 (for example, the optical signal with wavelength λ ＵＰ２, … , λ ＵＰｎ The multiplexer 42-1 is, for example, an optical thin film filter.

[0138] The circulator 43-1 has at least three ports. In the following description, it is assumed that the circulator 43-1 has three ports. The first port of the circulator 43-1 is connected to the demultiplexer 38-1. The second port of the circulator 43-1 is connected to the base station 3b-2 via an optical transmission path. The third port of the circulator 43-1 is connected to the multiplexer 42-1. An optical signal input to the first port of the circulator 43-1 is output from the second port. An optical signal input to the second port of the circulator 43-1 is output from the third port. An optical signal input to the third port of the circulator 43-1 is output from the first port.

[0139] 10, in the configuration of the conventional wireless system 1b, all of the wavelengths used pass through the circulator at each base station 3b, but the passband of an actual circulator is narrow, and when considering the use of a method with a wide wavelength spacing such as CWDM (Coarse Wavelength Division Multiplexing), it is difficult to realize the bands from 1310 nm to 1550 nm. For example, when considering using 1530, 1550, 1570, 1590, and 1610, which are included in the 1550 nm band in CWDM, assuming that two wavelengths are used in total for the upstream and downstream directions, the practical limit is that two base stations 3b can be deployed.

[0140] A configuration of a wireless system 100d according to a fifth embodiment, which allows for a larger number of base stations than the configuration shown in Fig. 10, will be described. Fig. 11 is a diagram showing an example configuration of the wireless system 100d according to the fifth embodiment. The wireless system 100d includes a central station 10d and multiple base stations 20d-1 to 20d-2. The central station 10d and the base station 20d-1, and the base station 20d-1 and the base station 20d-2 are connected via optical transmission paths. As shown in Fig. 11, the multiple base stations 20d form a cascade network topology.

[0141] 11 shows an example in which there are two base stations 20d, but even when there are three or more base stations 20d, the base stations 20d form a cascade network topology. In the following description, when there is no need to distinguish between the base stations 20d-1 to 20d-n, they will simply be referred to as base stations 20d.

[0142] Next, the specific configurations of the central station 10d and the base station 20d will be described. First, the specific configuration of the central station 10d will be described. The central station 10d has the same configuration as the central station 10c.

[0143] Next, the configuration of the base station 20d will be described. Since each base station 20d has the same configuration, the base station 20d-1 will be used as an example. The base station 20d-1 includes a circulator 210-1, an O / E converter 220-1, an E / O converter 240-1, and a wavelength multiplexing / demultiplexing unit 290-1. Although not shown in FIG. 11, the base station 20d-1 also includes a radio signal processing unit 235-1 and an antenna 260-1.

[0144] The circulator 210-1 has at least three or more ports. In the following description, it is assumed that the circulator 210-1 has three ports. The first port of the circulator 210-1 is connected to the wavelength multiplexing / demultiplexing unit 290-1. The second port of the circulator 210-1 is connected to the O / E converter 220-1. The third port of the circulator 210-1 is connected to the E / O converter 240-1. An optical signal input to the first port of the circulator 210-1 is output from the second port. An optical signal input to the second port of the circulator 210-1 is output from the third port. An optical signal input to the third port of the circulator 210-1 is output from the first port.

[0145] The O / E converter 220-1 performs the same processing as the O / E converter 220 in the first embodiment. The E / O converter 240-1 performs the same processing as the E / O converter 240-1 in the first embodiment.

[0146] The wavelength multiplexer / demultiplexer 290-1 demultiplexes a multiplexed signal input via an optical transmission line. １ The wavelength multiplexer / demultiplexer 290-1 demultiplexes the optical signal of the demultiplexed wavelength λ １ The wavelength multiplexer / demultiplexer 290-1 outputs the optical signal of wavelength λ included in the multiplexed signal. ２, … , λ ｎ The wavelength multiplexer / demultiplexer 290-1 outputs the optical signal (for example, wavelength λ) output from the circulator 210-1 to the base station 20d-2 via the optical transmission path. １an optical signal (for example, an optical signal with a wavelength λ ２, … , λ ｎ The wavelength multiplexing / demultiplexing unit 290-1 multiplexes the first optical signal and the second optical signal (optical signal) to generate a multiplexed signal. The wavelength multiplexing / demultiplexing unit 290-1 outputs the generated multiplexed signal to the central station 10d via an optical transmission line. The wavelength multiplexing / demultiplexing unit 290-1 is, for example, an optical thin film filter.

[0147] (Processing of the wireless system 100d in the third embodiment) Next, the flow of processing of the wireless system 100d in the third embodiment will be described. Here, the description will be made taking downstream communication as an example. The E / O converters 120-1 to 120-n use the input modulation signal to modulate continuous light with an optical modulator (not shown) provided inside, thereby converting the wavelength λ １ ~λ ｎ Here, the central station 10d generates an optical signal of wavelength λ １ ~λ ｎ However, the central station 10d may transmit transmission data to at least one base station 20d.

[0148] The E / O converters 120-1 to 120-n convert the generated wavelength λ １ ~λ ｎ The polarization adjusters 140-1 to 140-n output the optical signals of wavelength λ output from the E / O converters 120-1 to 120-n. １ ~λ ｎ The specific processing performed by the polarization adjusters 140-1 to 140-n is the same as that in the first embodiment. １ ~λ ｎ After adjusting the polarization plane of the optical signal, the wavelength λ after the polarization plane adjustment １ ~λ ｎ The optical signals are output to the circulators 150-1 to 150-n.

[0149] The wavelength λ output from the polarization adjusters 140-1 to 140-n １ ~λ ｎThe optical signal of wavelength λ is input to the first port of each of the circulators 150-1 to 150-n. １ ~λ ｎ The second ports of the circulators 150-1 to 150-n are connected to the wavelength multiplexing / demultiplexing unit 180. Therefore, the optical signals of wavelength λ 1 output from the second ports of the circulators 150-1 to 150-n are １ ~λ ｎ The optical signal is output to the wavelength multiplexer / demultiplexer 180 .

[0150] The wavelength multiplexer / demultiplexer 180 multiplexes the wavelengths λ 1 output from the circulators 150-1 to 150-n. １ ~λ ｎ The wavelength multiplexing / demultiplexing unit 180 multiplexes the optical signals of wavelengths λ 1 and λ 2 included in the multiplexed signal to generate a multiplexed signal. The wavelength multiplexing / demultiplexing unit 180 outputs the generated multiplexed signal to the optical transmission path 30. The multiplexed signal output from the wavelength multiplexing / demultiplexing unit 180 is input to the base station 20d-1 via the optical transmission path 30. The wavelength multiplexing / demultiplexing unit 290-1 of the base station 20d-1 demultiplexes the input multiplexed signal. The wavelength multiplexing / demultiplexing unit 290-1 demultiplexes the input multiplexed signal, for example, １ to the circulator 210-1, and outputs the optical signal of wavelength λ ２ ,…,λ ｎ The optical signal is output to the base station 20d-2.

[0151] The wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 290-1 １ The optical signal of wavelength λ is input to the first port of the circulator 210-1 of the base station 20d-1. １ The optical signal of wavelength λ is output from the second port. The second port of the circulator 210-1 is connected to the O / E converter 220-1. Therefore, the optical signal of wavelength λ is output from the second port of the circulator 210-1. １ The optical signal of wavelength λ is input to the O / E converter 220-1. １The O / E converter 220-1 converts the optical signal into an electrical signal. The O / E converter 220-1 outputs the electrical signal to a wireless signal processing unit 235-1 (not shown). The wireless signal processing unit 235-1 transmits the electrical signal output from the O / E converter 220-1 as a wireless radio wave via an antenna 260-1.

[0152] The wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 290-1 ２ ,…,λ ｎ The optical signal of the wavelength λ 1 included in the multiplexed signal is input to the wavelength multiplexer / demultiplexer 290-2 of the base station 20d-2. ２ to the circulator 210-2, and outputs the optical signal of wavelength λ ３ ,…,λ ｎ The optical signal of wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 290-2 is output to the base station 20d-3. ２ The optical signal is input to the first port of the circulator 210-2 of the base station 20d-2.

[0153] The wavelength λ input to the first port of the circulator 210-2 ２ The optical signal of wavelength λ is output from the second port. The second port of the circulator 210-2 is connected to the O / E converter 220-2. Therefore, the optical signal of wavelength λ is output from the second port of the circulator 210-2. ２ The optical signal of wavelength λ is input to the O / E converter 220-2. ２ The O / E converter 220-2 converts the optical signal into an electrical signal. The O / E converter 220-2 outputs the electrical signal to a wireless signal processing unit 235-2 (not shown). The wireless signal processing unit 235-2 transmits the electrical signal output from the O / E converter 220-2 as a wireless radio wave via an antenna 260-2.

[0154] In this way, the base station 20d includes the circulator 210 between the O / E converter 220, the E / O converter 240, and the wavelength multiplexing / demultiplexing unit 290. As a result, by appropriately selecting the circulator 210 usable in each base station 20d, such as the 1310 nm band or the 1550 nm band, it becomes possible to deploy up to 18 base stations 20d, for example, which is the same as the number of wavelengths usable in CWDM.

[0155] (First Modification of Fifth Embodiment) A plurality of cascade configurations may be added by branching midway through a cascade-type optical transmission line configuration, or the optical transmission line may be configured to form a loop.

[0156] (Variation 2 of the Fifth Embodiment) The central station 10d may include a plurality of polarization adjustment units 140a, polarization state determination units 160a, and control units 170a in the second embodiment, or a plurality of polarization state determination units 160a and control units 170b in the third embodiment. The base station 20d may include a polarization adjustment unit 250a, polarization state determination unit 270a, and control unit 280a in the second embodiment, or a polarization state determination unit 270a and control unit 280b in the third embodiment.

[0157] (Variation 1 common to the first to fifth embodiments) In the optical connection sections between the central station 10, 10a, 10b, 10c, 10d and the base stations 20, 20a, 20b, 20d, and between the base stations 20, 20a, 20b, 20d, the type of connector end face may be any of flat, PC (Physical Contact), UPC (Ultra Physical Contact), and APC (Angled Physical Contact).

[0158] (Modification 2 common to the first to fifth embodiments) In the above embodiments, analog RoF has been described as an example, but the present invention can also be applied to analog IFoF (Intermediate Frequency over Fiber) by adding necessary components.

[0159] (Modification 3 common to the first to fifth embodiments) The base stations 20, 20a, 20b, and 20d may each include a frequency converter, an amplifier, and a TDD switch. Furthermore, the base stations 20, 20a, 20b, and 20d may each include a phased array antenna as the antenna 260, and may be capable of beam direction control. Regarding the control of the beam direction, the base stations 20, 20a, 20b, and 20d may each include a control unit. The control units of the base stations 20, 20a, 20b, and 20d may be controlled by a control signal from the central stations 10, 10a, 10b, 10c, and 10d.

[0160] (Variation 4 common to the first to fifth embodiments) The control signal (including the TDD signal when the wireless systems 100, 100a, 100b, 100c, and 100d are systems that perform wireless communication using TDD) may be polarization-multiplexed into an optical signal of one wavelength.

[0161] (Variant 6 common to the first to fifth embodiments) By adding an optical amplifier to the optical transmission path connecting the base stations 20, 20a, 20b, and 20d, it is possible to compensate for optical loss and increase the number of base stations that can be connected.

[0162] (Variation 7 common to the first to fifth embodiments) The wavelength allocation during transmission through an optical transmission line may be any of ODSB (Optical Double Side Band), OSSB (Optical Single Side Band), and OCS (Optical Carrier Suppression) as shown in Fig. 11 . Fig. 11 is a diagram showing an example of wavelength allocation. Fig. 11(A) is a diagram showing an example of ODSB, Fig. 11(B) is a diagram showing an example of OSSB, and Fig. 11(C) is a diagram showing an example of OCS.

[0163] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0164] The present invention is applicable to techniques using analog RoF.

[0165] 10, 10a, 10b, 10c, 10d... central station, 11... radio base station, 12, 24, 120, 120-1 to 120-n, 240, 240-1, 240-2... E / O converter, 13, 22, 130, 130-1 to 120-n, 220, 220-1, 220-2... O / E converter, 14, 25... intensity adjustment unit, 15, 21, 150, 150-1 to 150-n, 210, 210-1... circulator, 20, 20-1 to 20-n, 20a, 20b, 20d-1 to 20d-n... base station, 23... TDD switch, 26, 260, 260-1... antenna, 30... optical transmission path, 40, 180, 290-1 to 270-2... wavelength multiplexing / demultiplexing unit, 100, 100a, 100b, 100c, 100d... wireless system, 115, 235... wireless signal processing unit, 140, 140a, 250, 250-1 to 250-2, 250a... polarization adjustment unit, 160a, 270a... polarization state determination unit, 170a, 170b, 280a, 280b... control unit

Claims

1. A wireless system comprising a first communication device and one or more second communication devices, wherein the first communication device and the one or more second communication devices communicate using optical signals of the same wavelength in both upstream and downstream directions, wherein the first communication device comprises: one or more first opto-electrical conversion units that convert input electrical signals into optical signals; one or more second opto-electrical conversion units that convert input optical signals into electrical signals; one or more first distribution units that output the optical signals output from the one or more first opto-electrical conversion units to the one or more second communication devices via an optical transmission path, and output optical signals input via the optical transmission path to the one or more second opto-electrical conversion units; wherein the one or more second communication devices comprise: a third opto-electrical conversion unit that converts input electrical signals into optical signals; a fourth opto-electrical conversion unit that converts input optical signals into electrical signals; and a second distribution unit that outputs the optical signals output from the third opto-electrical conversion units to the first communication device via the optical transmission path, and output optical signals input via the optical transmission path to the fourth opto-electrical conversion unit. and at least one of the first communication device and the one or more second communication devices comprises a polarization adjustment unit that adjusts the polarization plane of the converted optical signal so that it differs from the polarization plane of the optical signal transmitted by a communication partner.

2. The wireless system of claim 1, wherein at least one of the first communication device and the one or more second communication devices further comprises: a polarization state determination unit that determines the polarization state of the optical signal before conversion and the returned light based on the converted electrical signal; and a control unit that controls the angle adjusted by the polarization adjustment unit according to the polarization state determined by the polarization state determination unit.

3. The wireless system according to claim 2, wherein the control unit controls the angle adjusted by the polarization adjustment unit based on both the polarization state of light determined by the polarization state determination unit and the polarization state notified by the communication partner.

4. A wireless system according to any one of claims 1 to 3, wherein the polarization adjustment unit adjusts the plane of polarization of the converted optical signal so as to minimize beat noise near a DC component.

5. The wireless system of claim 1, wherein the one or more first photoelectric conversion units, the one or more second photoelectric conversion units, and the one or more first distribution units provided in the first communication device are a plurality of first photoelectric conversion units, a plurality of second photoelectric conversion units, and a plurality of first distribution units; the one or more second communication devices are a plurality of second communication devices; the first communication device and each of the plurality of second communication devices use the same wavelength in the upstream and downstream directions, and communication is performed using different wavelengths between the plurality of second communication devices; an inter-device wavelength multiplexer / demultiplexer is provided between the first communication device and the plurality of second communication devices, which multiplexes or demultiplexes input optical signals; and the first communication device further comprises a wavelength multiplexer / demultiplexer that multiplexes optical signals of different wavelengths output from each of the plurality of first distribution units and demultiplexes optical signals output from the inter-device wavelength multiplexer / demultiplexer.

6. The wireless system according to claim 1, wherein the one or more first photoelectric conversion units, the one or more second photoelectric conversion units, and the one or more first distribution units provided in the first communication device are a plurality of first photoelectric conversion units, a plurality of second photoelectric conversion units, and a plurality of first distribution units; the one or more second communication devices are a plurality of second communication devices; the first communication device and each of the plurality of second communication devices use the same wavelength in the upstream and downstream directions, and communication is performed between the plurality of second communication devices using different wavelengths; the plurality of second communication devices form a cascade network topology; the first communication device further comprises a wavelength multiplexer / demultiplexer that multiplexes optical signals of different wavelengths output from each of the plurality of first distribution units and demultiplexes optical signals output from an external device; each of the plurality of second communication devices further comprises a wavelength multiplexer / demultiplexer that multiplexes or demultiplexes input optical signals; and the first distribution unit is provided between the wavelength multiplexer / demultiplexer and the first photoelectric conversion unit and the second photoelectric conversion unit.

7. A communication device having either a signal processing function or a communication function provided in a wireless communication device, wherein the communication device and a communication device as a communication partner communicate using optical signals of the same wavelength in the upstream and downstream directions, and comprising: a first opto-electrical conversion unit that converts an input electrical signal into an optical signal; a second opto-electrical conversion unit that converts the input optical signal into an electrical signal; a distribution unit that outputs the optical signal output from the first opto-electrical conversion unit to the communication device as the communication partner via an optical transmission path, and outputs the optical signal input via the optical transmission path to the second opto-electrical conversion unit; and a polarization adjustment unit that adjusts the polarization plane of the optical signal converted by the first opto-electrical conversion unit so that it differs from the polarization plane of the optical signal transmitted by the communication device as the communication partner.

8. A communication method performed by a communication device having either a signal processing function or a communication function provided in a wireless communication device, wherein the communication device and a communication device as a communication partner communicate using optical signals of the same wavelength in the upstream and downstream directions, a first photoelectric conversion unit converts an input electrical signal into an optical signal, a second photoelectric conversion unit converts the input optical signal into an electrical signal, the optical signal output from the first photoelectric conversion unit is output to the communication device as the communication partner via an optical transmission path, the optical signal input via the optical transmission path is output to the second photoelectric conversion unit, and the polarization plane of the optical signal converted by the first photoelectric conversion unit is adjusted to be different from the polarization plane of the optical signal transmitted by the communication device as the communication partner.

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