Wireless system, aggregation station, extension station, and communication method

Abstract

A wireless system according to the present invention comprises an aggregation station and one or more extension stations, wherein: the aggregation station and the one or more extension stations use the same wavelength to perform communication in the uplink direction and the downlink direction; the aggregation station includes one or more first photoelectric conversion units that convert an input electric signal into an optical signal having a wavelength which is used for communication between the one or more extension stations, one or more second photoelectric conversion units that convert the input optical signal into an electric signal, and one or more circulators that transmit, to the one or more extension stations via an optical transmission path, the optical signal output from the one or more first photoelectric conversion units and that output, to the one or more second photoelectric conversion units, an optical signal input from another port; and the one or more extension stations include a first photoelectric conversion unit that converts an input electric signal into an optical signal having a wavelength which is the same as the wavelength used by the aggregation station, a second photoelectric conversion unit that converts an input optical signal into an electric signal, and a circulator that transmits, to the aggregation station via the optical transmission path, the optical signal output from the first photoelectric conversion unit and that outputs, to one or more second photoelectric conversion units, the optical signal transmitted from the aggregation station.　

Description

Wireless system, central station, base station, and communication method

[0001] The present invention relates to a wireless system, a central station, a base station, 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. 8 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 21, an O / E converter 22, and a multiplexer / splitter 23. The E / O converter 21 uses an input signal to convert it into a signal of wavelength λ １ The O / E converter 22 converts the input wavelength λ １ The optical signal is converted into an electrical signal by the optical multiplexer / divider 23. The optical multiplexer / divider 23 is configured using, for example, an optical splitter or an optical coupler. The optical multiplexer / divider 23 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 31, an O / E converter 32, and an E / O converter 33. The multiplexer / splitter 31 branches or multiplexes the optical signal transmitted from the central station 2. The O / E converter 32 converts the wavelength λ 2 branched by the multiplexer / splitter 31 into an E / O signal. １ The E / O converter 33 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 central station and one or more base stations, wherein the central station and the one or more base stations communicate using the same wavelength in both the uplink and downlink directions, and the central station includes one or more first photoelectric conversion units that convert an input electrical signal into an optical signal of a wavelength used for communication with the one or more base stations, one or more second photoelectric conversion units that convert the input optical signal into an electrical signal, and one or more first photoelectric conversion units that transmit the optical signal output from the one or more first photoelectric conversion units to the one or more base stations via an optical transmission path, and transmit the optical signal to other ports. and one or more circulators that output optical signals input from the base station to the one or more second photoelectric conversion units, wherein the one or more base stations each include a first photoelectric conversion unit that converts the input electrical signal into an optical signal of the same wavelength as the wavelength used by the central station, a second photoelectric conversion unit that converts the input optical signal into an electrical signal, and a circulator that transmits the optical signal output from the first photoelectric conversion unit to the central station via an optical transmission path and outputs the optical signal transmitted from the central station to the one or more second photoelectric conversion units.

[0009] One aspect of the present invention is a central station having a signal processing function among the signal processing functions and communication functions of a wireless communication device, wherein the central station and the one or more base stations communicate using the same wavelength in the uplink and downlink directions, and the central station is equipped with one or more first opto-electrical conversion units that convert an input electrical signal into an optical signal of a wavelength used for communication with the one or more base stations, one or more second opto-electrical conversion units that convert the input optical signal into an electrical signal, and one or more circulators that transmit the optical signal output from the one or more first opto-electrical conversion units to the one or more base stations via an optical transmission path, and output an optical signal input from another port to the one or more second opto-electrical conversion units.

[0010] One aspect of the present invention is a base station that has a communication function among the signal processing function and communication function of a wireless communication device, wherein a central station and the base station communicate using the same wavelength in the uplink and downlink directions, and the base station is equipped with a first opto-electrical conversion unit that converts an input electrical signal into an optical signal of the same wavelength as the wavelength used by the central station, a second opto-electrical conversion unit that converts the input optical signal into an electrical signal, and a circulator that transmits the optical signal output from the first opto-electrical conversion unit to the central station via an optical transmission path and outputs the optical signal transmitted from the central station to the second opto-electrical conversion unit.

[0011] One aspect of the present invention is a communication method performed by a wireless system including a central station and one or more base stations, wherein the central station and the one or more base stations communicate using the same wavelength in both uplink and downlink directions, and the central station includes one or more first photoelectric conversion units that convert input electrical signals into optical signals with wavelengths used for communication with the one or more base stations, one or more second photoelectric conversion units that convert input optical signals into electrical signals, and transmit the optical signals output from the one or more first photoelectric conversion units to a front end via an optical transmission path. This is a communication method in which an optical signal input from another port is transmitted to one or more base stations, and an optical signal input from another port is output to the one or more second photoelectric conversion units, and in the one or more base stations, a first photoelectric conversion unit converts the input electrical signal into an optical signal with the same wavelength as the wavelength used by the central station, and a second photoelectric conversion unit converts the input optical signal into an electrical signal, and the optical signal output from the first photoelectric conversion unit is transmitted to the central station via an optical transmission path, and the optical signal transmitted from the central station is output to the one or more second photoelectric conversion units.

[0012] 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.

[0013] FIG. 1 is a diagram showing an example of the configuration of a wireless system in a first embodiment. FIG. 2 is a sequence diagram showing the flow of processing in the wireless system in the first embodiment. FIG. 3 is a diagram showing an example of the configuration of a conventional wireless system to which WDM-PON is applied. FIG. 4 is a diagram showing an example of the configuration of a wireless system in a second embodiment. FIG. 5 is a diagram showing an example of the configuration of a conventional wireless system. FIG. 6 is a diagram showing an example of the configuration of a wireless system in a third embodiment. FIG. 7 is a diagram showing an example of wavelength allocation. FIG. 8 is a diagram showing an example of the configuration of a conventional wireless system using the DDM method.

[0014] An embodiment of the present invention will be described below with reference to the drawings. (First Embodiment) Fig. 1 is a diagram showing an example of the configuration of a wireless system 100 according to the first embodiment. The wireless system 100 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. In the following description, 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 radio frequencies output by the radio base station 110, or may be radio frequencies that are first converted to an intermediate frequency lower than the radio frequency and then converted back to the original radio frequency at the base station 20. In the present invention, the same wavelength is used in the uplink and downlink directions between the central station 10 and one base station 20. As described above, in the present invention, bidirectional communication is performed between the central station 10 and one base station 20 using the DDM method. This technology can be applied to mobile fronthaul. 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 the following description (including other embodiments), a configuration will be described in which the base station 20 performs wireless communication with subordinate wireless terminals by TDD (Time Division Duplex) as an example.

[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 110, an E / O converter 130, an O / E converter 140, an intensity adjustment unit 150, and a circulator 160.

[0018] The radio base station 110 has a transmission port and a reception port. The radio base station 110 transmits and receives signals in accordance with TDD timing. For example, the radio base station 110 outputs a modulated signal modulated with downlink transmission data in accordance with the TDD transmission timing to the E / O converter 130. Furthermore, the radio base station 110 outputs a TDD signal for controlling switching between transmission and reception to the intensity adjustment unit 150. 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 130 receives a modulated signal output from the wireless base station 110. The E / O converter 130 uses the input 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 130 generates an optical signal of the generated wavelength λ １ The E / O converter 130 outputs the optical signal to the intensity adjusting unit 150. The optical modulator is, for example, a Mach-Zehnder modulator. The E / O converter 130 is one aspect of a first opto-electrical conversion unit.

[0020] The O / E converter 140 receives the optical signal output from the circulator 160. 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 140 receives the optical signal having the wavelength λ １ The O / E converter 140 converts the input optical signal of wavelength λ １ The O / E converter 140 converts the optical signal into an electrical signal. The O / E converter 140 outputs the electrical signal to the wireless base station 110. The O / E converter 140 is one aspect of a second optical-electrical conversion unit.

[0021] The intensity adjustment unit 150 adjusts the intensity of the input optical signal based on the TDD signal output from the radio base station 110. Specifically, the intensity adjustment unit 150 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 adjustment unit 150 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 adjustment unit 150 adjusts the input power to the circulator 160 so as to limit it except for the timing of transmitting 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 adjustment unit 150 is, for example, a variable optical attenuator (VOA). The intensity adjustment unit 150 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. In the following description, it is assumed that information regarding the TDD timing is notified to the base station 20 via the external control line. The TDD signal output by the radio base station 110 may be used as the information regarding the TDD timing.

[0024] The circulator 160 has at least three or more ports. In the following description, it is assumed that the circulator 160 has three ports. A first port of the circulator 160 is connected to the intensity adjustment unit 150. A second port of the circulator 160 is connected to the optical transmission path 30. A third port of the circulator 160 is connected to the O / E converter 140. An optical signal input to the first port of the circulator 160 is output from the second port. An optical signal input to the second port of the circulator 160 is output from the third port. An optical signal input to the third port of the circulator 160 is output from the first port.

[0025] 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 TDD switch 230, an E / O converter 240, an intensity adjustment unit 250, and an antenna 260.

[0026] 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 intensity 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.

[0027] The O / E converter 220 converts the wavelength λ output by the circulator 210 into １ The O / E converter 220 converts the input optical signal of wavelength λ １ The O / E converter 220 converts the optical signal into an electrical signal, and outputs the electrical signal to the TDD switch 230. The electrical signal output from the O / E converter 220 is a downstream transmission signal.

[0028] The TDD switch 230 switches the connection based on a TDD signal obtained via an external control line. For example, the TDD switch 230 connects the O / E converter 220 to the antenna 260 during a period indicated by a "transmit" time slot in the TDD signal. This causes the electrical signal output from the O / E converter 220 to be transmitted as radio waves by the antenna 260. On the other hand, the TDD switch 230 connects the E / O converter 240 to the antenna 260 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 260 to be input to the E / O converter 240.

[0029] 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. １The E / O converter 240 generates an optical signal of the generated wavelength λ １ The optical signal is output to the intensity adjusting unit 250. The optical modulator is, for example, a Mach-Zehnder modulator.

[0030] The intensity adjustment unit 250 adjusts the intensity of the input optical signal based on the TDD signal obtained via an external control line. Specifically, the intensity adjustment unit 250 controls the intensity of the input optical signal so that it is low during periods indicated by "transmission" time slots in the TDD signal. On the other hand, the intensity adjustment unit 250 controls the intensity of the input optical signal so that it is high during periods indicated by "reception" time slots in the TDD signal. In this way, the intensity adjustment unit 250 adjusts the input power to the circulator 210 so as to limit it except for the timing of transmitting the upstream radio signal from the base station 20. The intensity adjustment unit 250 is, for example, an optical variable attenuator.

[0031] (Processing of the Wireless System 100 in the First Embodiment) FIG. 2 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 base station 110 outputs a TDD signal to the intensity adjustment unit 150 (step S101). At this time, the wireless base station 110 transmits the TDD signal to the base station 20, for example, via a control line. This enables synchronization of the TDD timing between the central station 10 and the base station 20. The wireless base station 110 accepts input of downlink transmission data. The wireless base station 110 generates a modulated signal based on the input downlink transmission data (step S102). The wireless base station 110 outputs the generated modulated signal to the E / O converter 130. Note that the order of the processing of steps S101 and S102 may be reversed.

[0032] The E / O converter 130 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 １ The E / O converter 130 generates an optical signal of the generated wavelength λ １ The optical signal is output to the intensity adjusting unit 150.

[0033] The intensity adjuster 150 receives the TDD signal and the wavelength λ １ For convenience of explanation, the TDD signal and the optical signal of wavelength λ are used as inputs. １ The figure shows a configuration in which the TDD signal and the optical signal of wavelength λ are input at the same time. １ The intensity adjuster 150 adjusts the input wavelength λ 1 during the period indicated by the "transmit" time slot in the TDD signal. １ The intensity of the optical signal is adjusted so that it is high (step S104).

[0034] The intensity adjuster 150 adjusts the input wavelength λ 1 during the period indicated by the "receive" time slot in the TDD signal. １ This makes it possible to prevent the optical carriers used for transmission from entering due to reflection from the circulator 160 or backscattering in the optical transmission line 30, resulting in significant degradation of the wireless signal quality due to optical beat noise and interference between optical carriers when square-law detection is performed by the O / E converter 140.

[0035] The intensity adjusting unit 150 adjusts the wavelength λ １ After adjusting the intensity of the optical signal, the wavelength λ １ The optical signal of wavelength λ output from the intensity adjusting unit 150 is output to the circulator 160. １ The optical signal of wavelength λ is input to the first port of the circulator 160. The circulator 160 converts the optical signal of wavelength λ １ The second port of the circulator 160 is connected to the optical transmission line 30. Therefore, the optical signal of wavelength λ 1 output from the second port of the circulator 160 is １ The optical signal is output to the optical transmission line 30 (step S105).

[0036] The wavelength λ output from the optical transmission line 30 １ The optical signal of wavelength λ is input to the third port of the circulator 210 of the base station 20 (step S106). １The 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. １ The optical signal of wavelength λ is input to the O / E converter 220. １ The O / E converter 220 converts the optical signal into an electrical signal (step S107). The O / E converter 220 outputs the electrical signal to the TDD switch 230.

[0037] The TDD switch 230 acquires the TDD signal output from the wireless base station 110 via the control line. The TDD switch 230 switches the connection based on the acquired TDD signal (step S108). Here, the TDD switch 230 connects the O / E converter 220 to the antenna 260 during the period indicated by the "transmit" time slot in the TDD signal. As a result, the electrical signal output from the O / E converter 220 is input to the antenna 260 via the TDD switch 230. The antenna 260 transmits the input electrical signal as radio waves (step S109).

[0038] According to the wireless system 100 configured as described above, the central station 10 and the base station 20 communicate using the same wavelength in the uplink and downlink directions, and the central station 10 transmits the input electrical signal at a wavelength (for example, wavelength λ ) to be used for communication with the base station 20. １ ) optical signal, an O / E converter 140 converts the input optical signal into an electrical signal, and a circulator 160 transmits the optical signal output from the E / O converter 130 to the base station 20 via an optical transmission path and outputs an optical signal input from another port to the O / E converter 140. The base station 20 comprises an E / O converter 240 converts the input electrical signal into an optical signal with the same wavelength as the wavelength used by the central station 10, an O / E converter 220 converts the input optical signal into an electrical signal, and a circulator 210 transmits the optical signal output from the E / O converter 240 to the central station 10 via the optical transmission path and outputs an optical signal transmitted from the central station 10 to the O / E converter 220.

[0039] 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.

[0040] As described above, using a circulator can reduce the effects of returning light, but if the optical carriers used for transmission enter the receiving section due to reflection from the circulator or backscattering in the optical transmission path, there is a possibility that the quality of the wireless signal will be significantly degraded due to optical beat noise and interference between optical carriers when square-law detection is performed by O / E converter 140. Therefore, wireless system 100 is equipped with intensity adjusters 150 and 250 that adjust the optical intensity of the transmission signal after optical signal conversion in accordance with the TDD timing of the wireless signal. This makes it possible to suppress the degradation of wireless signal quality described above while enabling two-way communication using a single wavelength with a simple configuration.

[0041] Second Embodiment In a second 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.

[0042] First, the configuration of a wireless system that applies a conventional WDM-PON will be described. Fig. 3 is a diagram showing an example of the configuration of a conventional wireless system 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 more), and an optical splitter 4. 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 4.

[0043] 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.

[0044] The central station 2a includes a plurality of E / O converters 21-1 to 21-n, a plurality of O / E converters 22-1 to 22-n, and a wavelength multiplexer / demultiplexer 24. The E / O converter 21-1 uses an input modulation signal to modulate continuous light with an optical modulator (not shown) provided inside to generate a wavelength λ １ The E / O converter 21-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 21-1 to 21-n output the generated optical signals to the wavelength division multiplexer 24.

[0045] The O / E converter 22-1 converts the wavelength λ ２ The O / E converter 22-n converts the optical signal of wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 24 into an electrical signal. ３２ into electrical signals. The wavelength multiplexer / demultiplexer 24 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexer / demultiplexer 24 multiplexes (multiplexes) the optical signals output from the E / O converters 21-1 to 21-n to generate a multiplexed signal. The wavelength multiplexer / demultiplexer 24 outputs the generated multiplexed signal to the optical transmission path. For example, the wavelength multiplexer / demultiplexer 24 demultiplexes the optical signals input via the optical transmission path and outputs the demultiplexed signals to the O / E converters 22-1 to 22-n.

[0046] The optical splitter 4 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 4 outputs the optical signals transmitted from the base stations 3a-1 to 3a-n to the central station 2a.

[0047] 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 multiplexer / demultiplexer 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 division multiplexer / demultiplexer 34-1.

[0048] The O / E converter 32-1 converts the wavelength λ 1 demultiplexed by the wavelength multiplexer / demultiplexer 34-1 into １ The wavelength multiplexer / demultiplexer 34-1 converts the optical signals input thereto into electrical signals. The wavelength multiplexer / demultiplexer 34-1 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexer / demultiplexer 34-1 multiplexes (multiplexes) the optical signals output from the E / O converter 33-1 to generate a multiplexed signal. The wavelength multiplexer / demultiplexer 34-1 outputs the generated multiplexed signal to the optical transmission path. For example, the wavelength multiplexer / demultiplexer 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.

[0049] As shown in Figure 3, 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 4 and wavelength multiplexers / demultiplexers 24 and 34, and optical section loss increases as the number of base stations 3a increases. For example, if the optical splitter 4 splits into 16 beams, an optical loss of 12 dB or more will occur in the optical splitter 4 alone. The optical loss is calculated, for example, based on the following equation (1):

[0050]

[0051] In formula (1), L оｐｔ represents distance attenuation, and 2L ＭＵＸＤＥＭＵＸ represents the insertion loss of the wavelength multiplexer / demultiplexer × 2, and 10 log N represents the loss of the optical splitter 4. 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.

[0052] A configuration of a wireless system 100a 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. 3. Fig. 4 is a diagram showing an example configuration of the wireless system 100a according to the second embodiment. The wireless system 100a includes a central station 10a, multiple base stations 20a-1 to 20a-n, and a wavelength multiplexer / demultiplexer 40. The central station 10a and the multiple base stations 20a-1 to 20a-n are connected via an optical transmission path 30 and the wavelength multiplexer / demultiplexer 40. In the following description, when there is no need to distinguish between the base stations 20a-1 to 20a-n, they will simply be referred to as base stations 20a.

[0053] Next, the specific configurations of the central station 10a and the base station 20a will be described. First, the specific configuration of the central station 10a will be described. The central station 10a includes a plurality of E / O converters 130-1 to 130-n, a plurality of O / E converters 140-1 to 140-n, a plurality of circulators 160-1 to 160-n, and a wavelength multiplexer / demultiplexer 170. The central station 10a differs in configuration from the central station 10 in that it does not include an intensity adjustment unit 150, it includes a plurality of E / O converters 130, O / E converters 140, and circulators 160, and it newly includes a wavelength multiplexer / demultiplexer 170. Although omitted in FIG. 4, the central station 10a also includes a radio base station 110.

[0054] The central station 10a uses a different wavelength for each combination of the E / O converter 130 and the O / E converter 140. For example, the central station 10a uses a wavelength λ １ is used, and the wavelength λ is used for the combination of the E / O converter 130-n and the O / E converter 140-n. ｎ is used.

[0055] The E / O converters 130-1 to 130-n perform the same processing as the E / O converter 130 shown in the first embodiment except that they use different wavelengths. The O / E converters 140-1 to 140-n perform the same processing as the O / E converter 140 shown in the first embodiment except that they use different wavelengths.

[0056] The circulators 160-1 to 160-n have at least three or more ports. In the following description, it is assumed that the circulators 160-1 to 160-n have three ports. The first ports of the circulators 160-1 to 160-n are connected to the E / O converters 130-1 to 130-n. The second ports of the circulators 160-1 to 160-n are connected to the wavelength multiplexer / demultiplexer 170. The third ports of the circulators 160-1 to 160-n are connected to the O / E converters 140-1 to 140-n. An optical signal input to the first port of the circulators 160-1 to 160-n is output from the second port. An optical signal input to the second port of the circulators 160-1 to 160-n is output from the third port. The optical signal input to the third port of each of the circulators 160-1 to 160-n is output from the first port.

[0057] The wavelength multiplexer / demultiplexer 170 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexer / demultiplexer 170 multiplexes (multiplexes) the optical signals output from the circulators 160-1 to 160-n (the optical signals output from the E / O converters 130-1 to 130-n) to generate a multiplexed signal. The wavelength multiplexer / demultiplexer 170 outputs the generated multiplexed signal to the optical transmission path 30. For example, the wavelength multiplexer / demultiplexer 170 demultiplexes the optical signals input via the optical transmission path 30 and outputs the demultiplexed signals to the circulators 160-1 to 160-n.

[0058] The base stations 20a-1 to 20a-n have the same configuration as the base station 20 in the first embodiment. The base stations 20a-1 to 20a-n communicate with the central station 10a using different wavelengths. For example, the base station 20a-1 uses a wavelength λ １ The base stations 20a-n communicate with the central station 10a using a wavelength λ ｎ The communication is performed between the central station 10a and the central station 10b using the above.

[0059] (Processing of the Wireless System 100a in the Second Embodiment) Next, the flow of processing of the wireless system 100a in the second embodiment will be described. Here, the description will be made taking downstream communication as an example. The E / O converters 130-1 to 130-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 10a generates an optical signal of wavelength λ １ ~λ ｎ However, the central station 10a may transmit transmission data to at least one base station 20a.

[0060] The E / O converters 130-1 to 130-n convert the generated wavelength λ １ ~λ ｎ The optical signals of wavelength λ output from the E / O converters 130-1 to 130-n are output to the circulators 160-1 to 160-n. １ ~λ ｎ The optical signal of wavelength λ is input to the first port of each of the circulators 160-1 to 160-n. １ ~λ ｎ The second ports of the circulators 160-1 to 160-n are connected to the wavelength division multiplexer 170. Therefore, the optical signals of wavelength λ 1 output from the second ports of the circulators 160-1 to 160-n are １ ~λ ｎ The optical signal is output to the wavelength division multiplexer 170 .

[0061] The wavelength multiplexer / demultiplexer 170 divides the wavelengths λ 1 output from the circulators 160-1 to 160-n into １ ~λ ｎ The wavelength multiplexer / demultiplexer 170 multiplexes the optical signals of wavelength λ contained in the multiplexed signal to generate a multiplexed signal. The wavelength multiplexer / demultiplexer 170 outputs the generated multiplexed signal to the optical transmission line 30. The multiplexed signal output from the wavelength multiplexer / demultiplexer 170 is input to the wavelength multiplexer / demultiplexer 40 via the optical transmission line 30. The wavelength multiplexer / demultiplexer 40 demultiplexes the input multiplexed signal ..., for example, １to the base station 20a-1, and outputs the optical signal of wavelength λ ｎ The optical signal is output to the base station 20a-n.

[0062] Since the base stations 20a-1 to 20a-n perform the same processing, the processing of the base station 20a-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 20a-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, and outputs the electrical signal to the TDD switch 230-1.

[0063] The TDD switch 230-1 acquires the TDD signal output from the radio base station 110 via a control line. The TDD signal may be transmitted from the central station 10a at one wavelength by modulating the main signal and subcarrier. The TDD switch 230-1 switches the connection based on the acquired TDD signal. Here, the TDD switch 230-1 connects the O / E converter 220-1 and the antenna 260-1 during the period indicated by the "transmit" time slot in the TDD signal. As a result, the electrical signal output from the O / E converter 220-1 is input to the antenna 260-1 via the TDD switch 230-1. The antenna 260-1 transmits the input electrical signal as radio waves.

[0064] In this way, the central station 10a includes a circulator 160 between the E / O converter 130, the O / E converter 140, and the wavelength multiplexer / demultiplexer 170. This makes it possible for the central station 10a to transmit signals in both the upstream and downstream directions using a single wavelength in combination with one E / O converter 130 and one O / E converter 140.

[0065] The loss in the optical section of the wireless system 100a configured in this manner is calculated based on, for example, the following equation (2), including the insertion loss of the wavelength multiplexer / demultiplexer 170 in the central station 10a, the insertion loss of the wavelength multiplexer / demultiplexer 40 at the relay point, and the circulator insertion loss in the central station 10a and the base station 20a, as well as the transmission loss.

[0066]

[0067] In formula (2), 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 160 and 210 multiplied by 2. The insertion loss of commercially available circulators 160 and 210 is approximately 1 dB or less. As described above, in the configuration of this embodiment, assuming that the optical transmission path length is the same as that of the conventional configuration shown in FIG. 3, it is possible to suppress the increase in optical loss that occurs with an increase in the number of base stations 20a, and it is possible to deploy base stations 20a equivalent to the maximum number of available wavelengths. In other words, it is possible to deploy twice as many base stations 20a as in the conventional configuration.

[0068] (Modification of the Second Embodiment) In the above-described embodiment, the central station 10a and each base station 20a may be configured to communicate using wavelengths within the same wavelength grid (for example, a 100 GHz grid) of the wavelength multiplexer / demultiplexer 40. As a result, λ １ The frequency of the upstream or downstream light, denoted as "Frequency Shift", may be shifted by, for example, about 7 GHz (<100 GHz), thereby suppressing degradation of signal quality due to interference between the beat noise component with the return light and the upstream main signal.

[0069] Third Embodiment In a third 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.

[0070] First, a conventional configuration will be described. FIG. 5 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. 5, the multiple base stations 3b form a cascade network topology. Note that while FIG. 5 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.

[0071] 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.

[0072] The central station 2b includes a plurality of E / O converters 21-1 to 21-n, a plurality of O / E converters 22-1 to 22-n, and a wavelength multiplexer / demultiplexer 24. The E / O converter 21-1 modulates continuous light with an optical modulator (not shown) provided therein using an input modulation signal to generate a wavelength λ ＤＬ１ The E / O converter 21-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 21-1 to 21-n output the generated optical signals to the wavelength division multiplexer 24.

[0073] The O / E converter 22-1 converts the wavelength λ ＵＰ１ The O / E converter 22-n converts the optical signal of wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 24 into an electrical signal. ＵＰｎ into electrical signals. The wavelength multiplexer / demultiplexer 24 multiplexes or demultiplexes the input optical signals. For example, the wavelength multiplexer / demultiplexer 24 multiplexes (multiplexes) the optical signals output from the E / O converters 21-1 to 21-n to generate a multiplexed signal. The wavelength multiplexer / demultiplexer 24 outputs the generated multiplexed signal to the optical transmission path. For example, the wavelength multiplexer / demultiplexer 24 demultiplexes the optical signals input via the optical transmission path and outputs the demultiplexed signals to the O / E converters 22-1 to 22-n.

[0074] 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 demultiplexer 38-1, an optical amplifier 39-1, an optical 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.

[0075] 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.

[0076] 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.

[0077] The O / E converter 32-1 converts the wavelength λ 1 demultiplexed by the demultiplexer 38-1 into ＤＬ１ The optical amplifier 39-1 amplifies the electrical signal converted by the O / E converter 32-1 and outputs it. The optical amplifier 41-1 amplifies the electrical signal to be transmitted in the upstream direction and outputs it to the E / O converter 33-1.

[0078] 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.

[0079] 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.

[0080] 5, 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 two base stations 3b.

[0081] A configuration of a wireless system 100b according to a third embodiment, which allows for a greater number of base stations than the configuration shown in Fig. 5, will be described. Fig. 6 is a diagram showing an example of the configuration of the wireless system 100b according to the third embodiment. The wireless system 100b includes a central station 10b and multiple base stations 20b-1 to 20b-2. The central station 10b and the base station 20b-1, and the base station 20b-1 and the base station 20b-2 are connected via optical transmission paths. As shown in Fig. 6, the multiple base stations 20b form a cascade network topology.

[0082] 6 shows an example in which there are two base stations 20b, but even when there are three or more base stations 20b, the base stations 20b form a cascade network topology. In the following description, when there is no need to distinguish between the base stations 20b-1 to 20b-n, they will simply be referred to as base stations 20b.

[0083] Next, the specific configurations of the central station 10b and the base station 20b will be described. First, the specific configuration of the central station 10b will be described. The central station 10b has the same configuration as the central station 10a.

[0084] Next, the configuration of the base station 20b will be described. Since each base station 20b has the same configuration, the base station 20b-1 will be used as an example. The base station 20b-1 includes a circulator 210-1, an O / E converter 220-1, an E / O converter 240-1, and a wavelength multiplexer / demultiplexer 270-1. Although not shown in FIG. 6, the base station 20b-1 also includes a TDD switch 230-1 and an antenna 260-1.

[0085] 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 multiplexer / demultiplexer 270-1. 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.

[0086] 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.

[0087] The wavelength multiplexer / demultiplexer 270-1 demultiplexes a multiplexed signal input via an optical transmission line. １ The wavelength division multiplexer / demultiplexer 270-1 demultiplexes the optical signal of the demultiplexed wavelength λ １ The wavelength division multiplexer 270-1 outputs the optical signal of wavelength λ included in the multiplexed signal to the circulator 210-1. ２, … , λ ｎ The wavelength division multiplexer 270-1 outputs the optical signal (for example, the wavelength λ) output from the circulator 210 to the base station 20b-2 via the optical transmission path. １ an optical signal (for example, an optical signal with a wavelength λ ２, … , λ ｎ The wavelength multiplexer / demultiplexer 270-1 multiplexes the first optical signal and the second optical signal (optical signal 10b) to generate a multiplexed signal. The wavelength multiplexer / demultiplexer 270-1 outputs the generated multiplexed signal to the central station 10b via an optical transmission line. The wavelength multiplexer / demultiplexer 270-1 is, for example, an optical thin film filter.

[0088] (Processing of the wireless system 100b in the third embodiment) Next, the flow of processing of the wireless system 100b in the third embodiment will be described. Here, the description will be made taking downstream communication as an example. The E / O converters 130-1 to 130-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 10b generates an optical signal of wavelength λ １ ~λ ｎ However, the central station 10b may transmit transmission data to at least one base station 20b.

[0089] The E / O converters 130-1 to 130-n convert the generated wavelength λ １ ~λ ｎ The optical signals of wavelength λ output from the E / O converters 130-1 to 130-n are output to the circulators 160-1 to 160-n. １ ~λ ｎ The optical signal of wavelength λ is input to the first port of each of the circulators 160-1 to 160-n. １ ~λ ｎ The second ports of the circulators 160-1 to 160-n are connected to the wavelength division multiplexer 170. Therefore, the optical signals of wavelength λ 1 output from the second ports of the circulators 160-1 to 160-n are １ ~λ ｎ The optical signal is output to the wavelength division multiplexer 170 .

[0090] The wavelength multiplexer / demultiplexer 170 divides the wavelengths λ 1 output from the circulators 160-1 to 160-n into １ ~λ ｎ The wavelength multiplexer / demultiplexer 170 multiplexes the optical signals of wavelengths λ 1 and λ 2 included in the multiplexed signal to generate a multiplexed signal. The wavelength multiplexer / demultiplexer 170 outputs the generated multiplexed signal to the optical transmission path 30. The multiplexed signal output from the wavelength multiplexer / demultiplexer 170 is input to the base station 20b-1 via the optical transmission path 30. The wavelength multiplexer / demultiplexer 270-1 of the base station 20b-1 demultiplexes the input multiplexed signal. The wavelength multiplexer / demultiplexer 270-1 demultiplexes the input multiplexed signal, for example, １to the circulator 210-1, and outputs the optical signal of wavelength λ ２ ,…,λ ｎ The optical signal is output to the base station 20b-2.

[0091] The wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 270-1 １ The optical signal of wavelength λ is input to the first port of the circulator 210-1 of the base station 20b-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. １ The O / E converter 220-1 converts the optical signal into an electrical signal, and outputs the electrical signal to the TDD switch 230-1 (not shown).

[0092] The TDD switch 230-1 acquires the TDD signal output from the radio base station 110 via a control line. The TDD signal may be transmitted from the central station 10b at one wavelength by subcarrier modulation with a main signal. The TDD switch 230-1 switches the connection based on the acquired TDD signal. Here, the TDD switch 230-1 connects the O / E converter 220-1 to the antenna 260-1 during the period indicated by the "transmit" time slot in the TDD signal. As a result, the electrical signal output from the O / E converter 220-1 is input to the antenna 260-1 via the TDD switch 230-1. The antenna 260-1 transmits the input electrical signal as radio waves.

[0093] The wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 270-1 ２ ,…,λ ｎ The optical signal of the wavelength λ 1 included in the multiplexed signal is input to the wavelength multiplexer / demultiplexer 270-2 of the base station 20b-2. ２ to the circulator 210-2, and outputs the optical signal of wavelength λ ３ ,…,λ ｎThe optical signal of wavelength λ demultiplexed by the wavelength multiplexer / demultiplexer 270-2 is output to the base station 20b-3. ２ The optical signal is input to the first port of the circulator 210-2 of the base station 20b-2.

[0094] 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, and outputs the electrical signal to the TDD switch 230-2 (not shown).

[0095] The TDD switch 230-2 acquires the TDD signal output from the radio base station 110 via a control line. The TDD signal may be transmitted from the central station 10b at one wavelength by modulating the main signal and subcarrier. The TDD switch 230-2 switches the connection based on the acquired TDD signal. Here, the TDD switch 230-2 connects the O / E converter 220-2 to the antenna 260-2 during the period indicated by the "transmit" time slot in the TDD signal. As a result, the electrical signal output from the O / E converter 220-2 is input to the antenna 260-2 via the TDD switch 230-2. The antenna 260-2 transmits the input electrical signal as radio waves.

[0096] In this way, the base station 20b includes the circulator 210 between the O / E converter 220, the E / O converter 240, and the wavelength multiplexer / demultiplexer 270. As a result, by appropriately selecting the circulator 210 usable in each base station 20b, such as the 1310 nm band or the 1550 nm band, it becomes possible to deploy up to 18 base stations 20b, for example, which is the same as the number of wavelengths usable in CWDM.

[0097] (Modification of the Third 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 be a loop.

[0098] (Variation 1 common to the first to third embodiments) In each of the above-described embodiments, the basic configuration is that the light intensity adjustment is placed immediately after the E / O converter, but it is also possible to adjust the light intensity by adjusting the bias current (in the case of DML) or bias voltage (in the case of MZM and EML) of the E / O converter.

[0099] (Variation 2 common to the first to third embodiments) In the optical connection sections between the central station 10, 10a, 10b and the base stations 20, 20a, 20b, and between the base stations 20, 20a, 20b, the type of connector end face may be any of flat, PC (Physical Contact), UPC (Ultra Physical Contact), and APC (Angled Physical Contact).

[0100] (Modification 3 common to the first to third 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.

[0101] (Modification 4 common to the first to third embodiments) The base stations 20, 20a, and 20b may include a frequency converter and an amplifier. Furthermore, the base stations 20, 20a, and 20b may include a phased array antenna as the antenna 260, and may be capable of beam direction control. For the above-mentioned beam direction control, the base stations 20, 20a, and 20b may include a control unit. The control unit of the base stations 20, 20a, and 20b may be controlled by a control signal from the central stations 10, 10a, and 10b.

[0102] (Modification 5 common to the first to third embodiments) A control signal (including, for example, a TDD signal) may be polarization-multiplexed into an optical signal of one wavelength.

[0103] (Variant 6 common to the first to third embodiments) By adding an optical amplifier to the optical transmission path connecting the base stations 20, 20a, and 20b, it is possible to compensate for optical loss and increase the number of base stations that can be connected.

[0104] (Variation 7 common to the first to third 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. 7 . Fig. 7 is a diagram showing an example of wavelength allocation. Fig. 7A is a diagram showing an example of ODSB, Fig. 7B is a diagram showing an example of OSSB, and Fig. 7C is a diagram showing an example of OCS.

[0105] Although an embodiment of the present invention has been described above in detail 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.

[0106] The present invention is applicable to techniques using analog RoF.

[0107] 10, 10a, 10b... aggregation station, 20, 20a, 20b... base station, 30... optical transmission path, 40... wavelength multiplexer / demultiplexer, 110... radio base station, 230, 230-1... TDD switch, 130, 130-1 to 130-n, 240, 240-1, 240-2... E / O converter, 140, 140-1 to 140-n, 220, 220-1, 220-2... O / E converter, 150, 250... intensity adjustment unit, 160, 160-1 to 160-n, 210, 210-1... circulator, 170, 270-1, 270-2... wavelength multiplexer / demultiplexer, 260, 260-1... antenna

Claims

1. A wireless system comprising a central station and one or more base stations, wherein the central station and the one or more base stations communicate using the same wavelength in both the upstream and downstream directions, the central station comprising: one or more first photoelectric conversion units that convert an input electrical signal into an optical signal of a wavelength used for communication with the one or more base stations; one or more second photoelectric conversion units that convert the input optical signal into an electrical signal; and one or more circulators that transmit the optical signals output from the one or more first photoelectric conversion units to the one or more base stations via an optical transmission path and output optical signals input from other ports to the one or more second photoelectric conversion units; and the one or more base stations comprising: a first photoelectric conversion unit that converts the input electrical signal into an optical signal of the same wavelength as that used by the central station; and a second photoelectric conversion unit that converts the input optical signal into an electrical signal. a circulator that transmits the optical signal output from the first opto-electrical conversion unit to the central station via an optical transmission path and outputs an optical signal transmitted from the central station to the one or more second opto-electrical conversion units.

2. The wireless system described in claim 1, wherein the central station further comprises an intensity adjustment unit that adjusts the intensity of the optical signal so that the intensity of the optical signal input to the circulator is maintained high at the timing of transmission of the optical signal converted by the one or more first photoelectric conversion units and is low at times other than the timing of transmission of the optical signal converted by the one or more first photoelectric conversion units; and the one or more remote stations further comprise an intensity adjustment unit that adjusts the intensity of the optical signal so that the intensity of the optical signal input to the circulator is maintained high at the timing of transmission of the optical signal converted by the first photoelectric conversion unit and is low at times other than the timing of transmission of the optical signal converted by the first photoelectric conversion unit.

3. 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 circulators comprise a plurality of first photoelectric conversion units, a plurality of second photoelectric conversion units and a plurality of circulators, the one or more base stations comprise a plurality of base stations, the central station and each of the multiple base stations communicate using the same wavelength in the upstream and downstream directions, and different wavelengths are used between the multiple base stations, and an inter-device wavelength multiplexer / demultiplexer is provided between the central station and the multiple base stations for multiplexing or demultiplexing input optical signals, and the central station further comprises a wavelength multiplexer / demultiplexer for multiplexing optical signals of different wavelengths output from each of the multiple circulators and demultiplexing the optical signal output from the inter-device wavelength multiplexer / demultiplexer.

4. The wireless system described in claim 1, wherein the one or more first photoelectric conversion units, the one or more second photoelectric conversion units, and the one or more circulators provided in the central station are a plurality of first photoelectric conversion units, a plurality of second photoelectric conversion units, and a plurality of circulators; the one or more remote stations are a plurality of remote stations; the central station and each of the multiple remote stations use the same wavelength in the upstream and downstream directions, and communication is performed between the multiple remote stations using different wavelengths; the multiple remote stations form a cascade type network topology; the central station further comprises a wavelength multiplexer / demultiplexer that multiplexes optical signals of different wavelengths output from each of the multiple circulators and demultiplexes an optical signal output from outside; each of the multiple remote stations further comprises a wavelength multiplexer / demultiplexer that multiplexes or demultiplexes an input optical signal; and the circulator is provided between the wavelength multiplexer / demultiplexer and the first photoelectric conversion unit and the second photoelectric conversion unit.

5. A central station having a signal processing function among the signal processing function and communication function of a wireless communication device, wherein the central station and the one or more base stations communicate using the same wavelength in the upstream and downstream directions, and the central station is equipped with: one or more first photoelectric conversion units that convert an input electrical signal into an optical signal of a wavelength used for communication with the one or more base stations; one or more second photoelectric conversion units that convert an input optical signal into an electrical signal; and one or more circulators that transmit the optical signal output from the one or more first photoelectric conversion units to the one or more base stations via an optical transmission path, and output an optical signal input from another port to the one or more second photoelectric conversion units.

6. A base station having a communication function among the signal processing function and communication function of a wireless communication device, wherein a central station and the base station communicate using the same wavelength in the upstream and downstream directions, and the base station is equipped with: a first opto-electrical conversion unit that converts an input electrical signal into an optical signal having the same wavelength as the wavelength used by the central station; a second opto-electrical conversion unit that converts the input optical signal into an electrical signal; and a circulator that transmits the optical signal output from the first opto-electrical conversion unit to the central station via an optical transmission path and outputs the optical signal transmitted from the central station to the second opto-electrical conversion unit.

7. A communication method performed by a wireless system having a central station and one or more base stations, wherein the central station and the one or more base stations communicate using the same wavelength in both the upstream and downstream directions, and in the central station: one or more first photoelectric conversion units convert an input electrical signal into an optical signal of a wavelength used for communication with the one or more base stations; one or more second photoelectric conversion units convert the input optical signal into an electrical signal; transmit the optical signal output from the one or more first photoelectric conversion units to the one or more base stations via an optical transmission path; and output an optical signal input from another port to the one or more second photoelectric conversion units; and in the one or more base stations: a first photoelectric conversion unit converts the input electrical signal into an optical signal of the same wavelength as that used by the central station; a second photoelectric conversion unit converts the input optical signal into an electrical signal; and transmits the optical signal output from the first photoelectric conversion unit to the central station via the optical transmission path; outputting an optical signal transmitted from the central station to the one or more second optical-electrical converters.