Aggregate station

Abstract

The present invention provides an aggregate station in a wireless communication system comprising: a plurality of extension stations and an aggregate station that have a communication function and a signal processing function, respectively, into which a communication function and a signal processing function of a base station are separated, the plurality of extension stations being connected to the aggregate station via an optical transmission line. The aggregate station comprises: an arithmetic unit for estimating a transmission delay time in a wired section that is a section between the aggregate station and the plurality of extension stations; and a switching control unit for switching connection between the aggregate station and the plurality of extension stations on the basis of the transmission delay time estimated by the arithmetic unit and resource information allocated to one or more wireless terminals that wirelessly communicate with one of the plurality of extension stations.　

Description

Aggregation station

[0001] The present invention relates to an aggregation station.

[0002] Analog Radio over Fiber (RoF) systems have been proposed for wireless communication systems. Analog RoF systems separate wireless base stations into a central station (CS) and a remote radio unit (RRU), which are connected by optical fiber. In analog RoF systems, the signal processing function of a wireless base station is separated into the central station and the antenna function into the remote radio unit. By deploying simple remote stations, studies are underway to achieve flexible and economical wireless area deployment.

[0003] Fig. 9 is a diagram showing an example of a wireless communication system S to which the above-mentioned analog RoF system is applied. The wireless communication system S includes a central station 1 and N base stations 2, where N is an integer equal to or greater than 2. As shown in Fig. 9, multiple base stations 2 are connected to a single baseband processing unit (BBU: Baseband Unit) of the central station 1. Furthermore, the base station 2-2 communicates wirelessly with a wireless terminal 3. It is known that noise called ingress noise occurs in a configuration in which multiple base stations 2 are connected to a single baseband processing unit of the central station 1 in this way.

[0004] Ingress noise is noise that flows in from a base station 2 other than the base station 2 that is actually communicating in uplink communication (a base station 2 that is not actually communicating), and in this situation, the upstream signal transmitted contains ingress noise, which causes a deterioration in communication quality (SNR: Signal to Noise Ratio) (see, for example, Non-Patent Document 1). When such ingress noise occurs, it may become difficult for the receiver to demodulate the signal.

[0005] Y. Ito, Y. Inada, Y. Fuke, “RoF Equipment Developed for Coverage in Small Areas where Received Power is Low”, NTT DOCOMO Technical Journal, Vol.15, No.2, July 2013.

[0006] As shown in Non-Patent Document 1, when multiple antennas are deployed in an RoF device, the noise figure (NF) of the noise received by the central station increases with the number of antennas. Therefore, it was necessary to design the line assuming the maximum number of connected antennas. As described above, the technology in Non-Patent Document 1 did not take measures to remove ingress noise. Therefore, in a configuration in which multiple base stations 2 are connected to a single baseband processing unit in a central station 1, there was still the problem of degradation of communication quality due to ingress noise. This problem occurs in both digital RoF, which transmits and receives digital signals, and analog RoF, which transmits and receives analog signals, but it has a greater impact on analog signals.

[0007] In view of the above circumstances, an object of the present invention is to provide a technique capable of reducing the influence of ingress noise in a system performing RoF transmission.

[0008] One aspect of the present invention is a central station in a wireless communication system including a central station having a signal processing function that is a function obtained by separating the signal processing function and the communication function that a base station has, and a plurality of base stations connected to the central station via an optical transmission path and having the communication function, wherein the central station includes a calculation unit that estimates a transmission delay time in a wired section that is a section between the central station and the plurality of base stations, and a switching control unit that switches connections between the central station and the plurality of base stations based on the transmission delay time estimated by the calculation unit and resource information assigned to one or more wireless terminals that communicate wirelessly with any of the plurality of base stations.

[0009] According to the present invention, it is possible to reduce the influence of ingress noise in a system that performs RoF transmission.

[0010] FIG. 1 is a diagram illustrating an example of the configuration of a wireless communication system in a first embodiment. FIG. 2 is a diagram for explaining the flow of ingress noise suppression in the first embodiment. FIG. 3 is a sequence diagram illustrating the flow of processing performed by the wireless communication system in the first embodiment. FIG. 4 is a sequence diagram illustrating the flow of processing performed by the wireless communication system in the first embodiment. FIG. 5 is a diagram illustrating an example of FDMA resource allocation information in a second embodiment. FIG. 6 is a diagram illustrating an example of a switching list for each base station 20. FIG. 7 is a diagram illustrating an example of time-frequency resource allocation information in a third embodiment. FIG. 8 is a diagram illustrating an example of a switching list for each OFDM symbol unit. FIG. 9 is a diagram illustrating an example of a wireless communication system to which an analog RoF system is applied.

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0012] (Summary) Before describing the details of the present invention, an overview of the invention will be described. In a wireless communication system according to the present invention, the functions of a wireless base station are separated into an aggregate station and a base station, and the objective is to improve communication quality by suppressing the generation of unnecessary ingress noise. Therefore, in the present invention, the aggregate station identifies a connection path to a base station with which communication is not being performed among multiple base stations, and disconnects the connection to the identified connection path. This suppresses the generation of unnecessary ingress noise. Below, a specific configuration for realizing the above processing will be described.

[0013] (First Embodiment) FIG. 1 is a diagram showing an example of the configuration of a wireless communication system 100 according to the first embodiment. The wireless communication system 100 includes a central station 10, m base stations 20, and a higher-level device 25, where m is an integer equal to or greater than 1. The central station 10 and each base station 20 are connected via an optical transmission path 40. For example, the central station 10 and base station 20-1 are connected via an optical transmission path 40-1, and the central station 10 and base station 20-m are connected via an optical transmission path 40-m. The optical transmission path 40 is an optical fiber. In this way, the wireless communication system 100 is configured such that a plurality of base stations 20 are connected to a baseband processing unit included in the central station 10.

[0014] The optical fiber may be a single mode optical fiber (SMF), a multimode optical fiber (MMF), a single-core optical fiber, or a multi-core optical fiber.

[0015] The central station 10 and each base station 20 constitute one base station. Specifically, the central station 10 has a signal processing function, which is one of the functions that a base station has when the signal processing function and the communication function are separated. Each base station 20 has a communication function, which is one of the functions that a base station has when the signal processing function and the communication function are separated. Transmission between the central station 10 and each base station 20 is performed using analog RoF, which transmits wireless signals using optical fiber. One wireless terminal 30 is wirelessly connected to any of the m base stations 20.

[0016] In the following description, the section between the central station 10 and each base station 20 is referred to as an RoF section, and the section between each base station 20 and the wireless terminal 30 is referred to as an RF section. Note that the locations where the TDD timings match are the base stations 20. Also, the RoF sections or RF cable lengths must be matched. Furthermore, the first embodiment will be described assuming that a time division multiple access (TDMA) method such as a subcarrier (SC: Single Carrier) is used as the digital modulation method.

[0017] The central station 10 controls the uplink and downlink transmission timings at the base station 20. Furthermore, the central station 10 uses a timing advance function to acquire the value of the total transmission delay time, which is the transmission delay time of the RoF section and the transmission delay time of the RF section. The timing advance function adjusts the uplink transmission timing of the wireless terminal 30 according to the transmission delay of the RF section, and the wireless terminal 30 performs advance transmission of the uplink signal according to the transmission timing notified to the wireless terminal 30 by the central station 10. Advance transmission means transmitting a signal a predetermined time earlier. Furthermore, the central station 10 notifies the wireless terminal 30 of the transmission delay time value, which is obtained by subtracting the transmission delay time of the RoF section from the total transmission delay time, as UL advance information. The UL advance information represents the transmission delay time of the RF section.

[0018] The base station 20 performs wireless communication with one or more wireless terminals 30 under the control of the central station 10. Each base station 20 has a unique identifier. The unique identifier makes it possible to identify each base station 20.

[0019] The higher-level device 25 generates time resource allocation information (hereinafter referred to as "time resource allocation information") including identification information of each base station 20, and notifies the central station 10 of the generated time resource allocation information. The time resource allocation information is information that indicates resources allocated to each wireless terminal 30 by time. Furthermore, in the time resource allocation information, each resource is associated with identification information of the base station 20. This allows the central station 10 to know which base station 20 will be communicating at what time.

[0020] The wireless terminal 30 performs wireless communication with the base station 20. Furthermore, the wireless terminal 30 performs pre-transmission of the upstream signal, taking into account the value of the transmission delay time notified by the central station 10.

[0021] (Device Configuration) Next, a description will be given of the configurations of the central station 10, the base station 20, and the wireless terminal 30. The central station 10 includes an RoF master unit 11, an RoF delay measurement unit 12, a calculation unit 13, a resource notification unit 14, a switching control unit 15, a switching unit 16, and a baseband processing unit 17.

[0022] The RoF master unit 11 communicates with the base station 20. The RoF master unit 11 has a function of modulating an electrical signal output from the baseband processing unit 17 into an optical signal and a function of demodulating an optical signal transmitted from the base station 20 into an electrical signal. Here, the RoF master unit 11 modulates an electrical signal (RF signal) at a frequency used for wireless transmission into an optical signal. The RoF master unit 11 transmits the optical signal to the base station 20. The RoF master unit 11 functions as a communication unit that transmits and receives control signals and downlink signals to and from the base station 20.

[0023] The control signal includes a TDD control signal and UL advance information. The TDD control signal in the present invention is a signal used to switch between uplink signals and downlink signals at a timing when TDD synchronization is achieved in each base station 20. The UL advance information includes time information for advance transmission in the wireless terminal 30.

[0024] The RoF delay measurement unit 12 measures the transmission delay occurring in the optical transmission path in the RoF section. The RoF delay measurement unit 12 may synchronize the timing of the transmission delay in the RoF section using PTP (Precision Time Protocol), or may measure the delay at the central station 10 using RTT (Round Trip Time). Note that delay measurement is preferably performed periodically because the amount of delay varies depending on the temperature characteristics of the device and the optical transmission path. In particular, the temperature of the optical cable that constitutes the optical transmission path changes between morning, noon, evening, and night, so delay measurement must be performed during times of day when temperature fluctuations are large.

[0025] When measuring the transmission delay using the RTT, the RoF delay measurement unit 12 has a function of transmitting a delay measurement signal and a function of measuring the time at which the delay measurement signal is transmitted and the time at which the delay measurement signal returned by the base station 20 is received. The delay measurement signal is a signal used to measure the transmission delay in the RoF section.

[0026] The calculation unit 13 calculates the transmission delay time t ＲｏＦ Specifically, the calculation unit 13 estimates the transmission delay time t ＲｏＦ The statistical value is, for example, the mean or median.

[0027] The resource notification unit 14 receives time resource allocation information transmitted from the higher-level device 25. The resource notification unit 14 outputs the received time resource allocation information to the baseband processing unit 17 and the switching control unit 15.

[0028] The switching control unit 15 compares the time resource allocation information notified by the resource notification unit 14 with the transmission delay time t ＲｏＦThe switching control unit 15 obtains the obtained time resource allocation information and the transmission delay time t ＲｏＦ Based on the estimation result, the reception timing of the uplink signal, which is the timing at which the uplink signal arrives at the central station 10, is estimated. The switching control unit 15 notifies the switching unit 16 of switching information based on the estimation result.

[0029] The switching information is information for controlling the switching of each switch included in the switching unit 16. The switching information includes, for example, information that can identify which switch to turn ON or OFF. Turning a switch ON means that the RoF master unit 11 and the baseband processing unit 17 are electrically connected. Turning a switch OFF means that the RoF master unit 11 and the baseband processing unit 17 are not electrically connected. In other words, by turning a switch ON, a signal received by the RoF master unit 11 is output to the baseband processing unit 17 via the switch that is turned ON. On the other hand, by turning a switch OFF, the connection with the base station 20 corresponding to the switch that is turned OFF is cut off. As a result, noise does not flow in from the base station 20 corresponding to the switch that is turned OFF. This makes it possible to suppress inflow noise.

[0030] The switching unit 16 includes m switches 18. The number of switches 18 included in the switching unit 16 is the same as the number of base stations 20. Therefore, each switch 18 and each base station 20 are associated one-to-one. The switching unit 16 switches the state of each switch 18 based on switching information output from the switching control unit 15. For example, the switching unit 16 switches each switch 18 to either the ON state or the OFF state based on the switching information.

[0031] The switching unit 16 has 128 switches 18, and when an upstream signal arrives at switch 18-1, the switching unit 16 switches switch 18-1 to the ON state and switches the other 127 switches 18-2 to 18-128 to the OFF state.

[0032] The baseband processing unit 17 calculates the transmission delay time t ＲｏＦThat is, the baseband processing unit 17 transmits the downstream signal in advance by a transmission delay time t ＲｏＦ The baseband processing unit 17 transmits the downlink signal earlier by, for example, the transmission delay time t ＲＦ and TDD control signals are transmitted in the downlink signal.

[0033] Furthermore, the baseband processing unit 17 demodulates the uplink signal output from the switch 18 based on the time resource allocation information of each base station 20 notified by the resource notification unit 14. The baseband processing unit 17 also outputs a control signal and a modulated downlink signal to the RoF master unit 11. The baseband processing unit 17 has a function of linking the wireless terminal 30 to which base station 20 it is accommodated. Specifically, the baseband processing unit 17 can make this determination by allocating unique identification information to each base station 20.

[0034] The base station 20 includes an RoF slave unit 21 , an antenna unit 22 , and an RoF delay measurement unit 23 .

[0035] The RoF slave unit 21 communicates with the central station 10. The RoF slave unit 21 has a function of demodulating an optical signal transmitted from the central station 10 into an electrical signal, and a function of modulating an upstream signal (RF signal) received via the antenna unit 22 into an optical signal. The RoF slave unit 21 transmits the optical signal to the central station 10. In this way, the RoF slave unit 21 functions as a communication unit that transmits and receives optical signals to and from the central station 10.

[0036] The antenna unit 22 performs wireless communication with the wireless terminal 30. For example, the antenna unit 22 receives radio waves transmitted from the wireless terminal 30, converts them into electrical signals, and outputs them to the RoF slave unit 21. For example, the antenna unit 22 transmits the electrical signals output from the RoF slave unit 21 as radio waves to the wireless terminal 30. The antenna unit 22 may be a fixed directional antenna, or may be an antenna whose directivity can be changed like a phased array antenna.

[0037] The RoF delay measurement unit 23 is a dedicated device for performing a PTP sequence when timing synchronization is achieved by PTP. When delay measurement is performed by RTT, the RoF delay measurement unit 23 has a function of reflecting (looping back) a delay measurement signal transmitted from the central station 10.

[0038] The wireless terminal 30 includes an antenna unit 31 and a UL first-out transmitting unit 32 .

[0039] The antenna unit 31 performs wireless communication with the base station 20. For example, the antenna unit 31 receives radio waves transmitted from the base station 20, converts the radio waves into electrical signals, and outputs the signals to the UL advance transmission unit 32. For example, the antenna unit 31 transmits the electrical signals output from the UL advance transmission unit 32 as radio waves to the base station 20. The antenna unit 31 may be a fixed directional antenna, or may be an antenna whose direction of direction can be changed like a phased array antenna.

[0040] The UL advance transmitter 32 receives the transmission delay time t ＲＦ That is, the UL advance transmitting unit 32 advances the upstream signal by a transmission delay time t in the RF section from the timing at which the upstream signal is originally transmitted. ＲＦ Send the upstream signal a few minutes earlier.

[0041] As described above, ingress noise occurs when noise generated by each of the multiple base stations 20 is combined and input to a single baseband processing unit 17. Therefore, in a configuration in which multiple base stations 20 are extended from a single baseband processing unit 17, ingress noise will occur regardless of the connection method used between the central station 10 and each base station 20. For example, ingress noise will occur when the optical fiber connecting the multiple base stations 20 transmits light at different wavelengths, when different optical fiber cables (SMF, MMF) are mixed, when optical fiber and RF cables are mixed, etc. The present invention is applicable in such situations as well.

[0042] (Flow of ingress noise suppression) Next, the flow of ingress noise suppression by switching will be described with reference to Fig. 2. Fig. 2 is a diagram for explaining the flow of ingress noise suppression in the first embodiment. In Fig. 2, it is assumed that the base station 20-1 is communicating wirelessly with the wireless terminal 30. It is also assumed that the central station 10 has notified the wireless terminal 30 of UL advance information.

[0043] (1) The wireless terminal 30 receives the transmission delay time t ＲＦ (2) The base station 20-1 receives the upstream signal transmitted from the wireless terminal 30. The wireless terminal 30 transmits the upstream signal in advance with a transmission delay time t ＲＦ Since the upstream signal is transmitted in advance by the time resource allocation information, the timing at which the base station 20-1 receives the upstream signal coincides with the time indicated by the time resource allocation information.

[0044] (3) The transmission delay time t in the RoF section from the time when the base station 20-1 transmits the upstream signal ＲｏＦ The uplink signal arrives at the central station 10 later. The central station 10 knows the arrival time of the uplink signal at the base station 20-1 based on the time resource allocation information notified from the higher-level device 25. Therefore, the central station 10 calculates the transmission delay time t ＲｏＦ It can be estimated that the target uplink signal will arrive at the aggregate station 10 later. (4) The switching control unit 15 estimates the time at which the target uplink signal will arrive at the aggregate station 10, and based on the estimation result, notifies the switching unit 16 of switching information including a control instruction to turn on the switch 18-1 corresponding to the base station 20-1 and turn off the switch 18-m corresponding to the base station 20-m other than the base station 20-1. As a result, the switching unit 16 switches the states of the switch 18-1 and the switch 18-m on a symbol-by-symbol basis. This prevents noise from the base station 20-m corresponding to the switch 18-m in the OFF state from flowing into the aggregate station 10. As a result, ingress noise can be suppressed.

[0045] 3 and 4 are sequence diagrams showing the flow of processing performed by the wireless communication system 100 in the first embodiment. At the start of the processing in FIGS. 3 and 4, the transmission delay time t ＲｏＦ and the transmission delay time t ＲＦ 3 and 4, the case where there are two base stations 20 will be described as an example, but the same applies to the case where there are three or more base stations.

[0046] The resource notification unit 14 of the central station 10 acquires time resource allocation information transmitted from the higher-level device 25 (step S101). The resource notification unit 14 notifies the acquired time resource allocation information to the switching control unit 15 and the baseband processing unit 17. The RoF delay measurement unit 12 measures the transmission delay of the RoF section by exchanging signals with the base station 20 (step S102). An example of a method for measuring the transmission delay of the RoF section will be described. The RoF delay measurement unit 12 converts a delay measurement signal into an optical signal via the RoF master unit 11 and transmits it to the base station 20. The RoF slave unit 21 of the base station 20 demodulates the optical signal transmitted from the central station 10 and outputs the resulting delay measurement signal to the RoF delay measurement unit 23. The RoF delay measurement unit 23 returns the delay measurement signal output from the RoF slave unit 21 to the central station 10. For example, the RoF delay measurement unit 23 converts the delay measurement signal into an optical signal via the RoF slave unit 21 and transmits it to the central station 10 .

[0047] The RoF master unit 11 of the central station 10 demodulates the optical signal transmitted from the base station 20 and outputs the resulting delay measurement signal to the RoF delay measurement unit 12. The RoF delay measurement unit 12 measures the time required to transmit and receive the delay measurement signal as the transmission delay of the RoF section. The RoF delay measurement unit 12 executes this process multiple times. The RoF delay measurement unit 12 outputs the multiple measurement results obtained by executing the process multiple times to the calculation unit 13.

[0048] The calculation unit 13 calculates the transmission delay time t ＲｏＦ (Step S103). The calculation unit 13 estimates the estimated transmission delay time tＲｏＦ The baseband processing unit 17 calculates the transmission delay time t ＲｏＦ By subtracting the transmission delay time t ＲＦ (step S104). The baseband processing unit 17 estimates the estimated transmission delay time t ＲＦ Then, the baseband processing unit 17 generates a downlink signal including information indicating the transmission delay time t ＲｏＦ The RoF master unit 11 modulates the downstream signal output from the baseband processing unit 17 into an optical signal and transmits it (step S105). The central station 10 also transmits a TDD control signal to the base station 20.

[0049] The RoF slave unit 21-1 of the base station 20-1 acquires a downlink signal by demodulating the optical signal transmitted from the central station 10 (step S106). The RoF slave unit 21-1 outputs the acquired downlink signal to the antenna unit 22. The antenna unit 22-1 transmits the downlink signal output from the RoF slave unit 21-1 as a radio wave (step S107). The base station 20-1 holds the TDD control signal transmitted from the central station 10.

[0050] The antenna unit 31 of the wireless terminal 30 receives the radio wave transmitted from the base station 20-1 (step S108). The antenna unit 31 converts the received radio wave into an electrical signal and outputs it to the UL advance transmission unit 32. The UL advance transmission unit 32 converts the transmission delay time t ＲＦ As a result, the wireless terminal 30 acquires information indicating the transmission delay time t ＲＦ The UL advance transmitter 32 uses the timing advance function to transmit the generated uplink signal with a transmission delay time t ＲＦ The antenna unit 31 transmits the upstream signal output from the UL advance transmitting unit 32 as radio waves (step S109).

[0051] The switching control unit 15 of the central station 10 compares the time resource allocation information notified by the resource notification unit 14 with the transmission delay time t ＲｏＦ The switching control unit 15 acquires the time resource allocation information and the transmission delay time t ＲｏＦ Based on this, the base station 20 that transmits the uplink signal and the reception timing of the uplink signal are estimated (step S110).

[0052] Specifically, the switching control unit 15 first refers to the time resource allocation information and identifies the base station 20 that is the target for transmitting the uplink signal. Next, the switching control unit 15 refers to the time resource allocation information and identifies the arrival time of the uplink signal at the base station 20-1. Next, the switching control unit 15 calculates the transmission delay time t of the RoF section based on the identified arrival time of the uplink signal at the base station 20-1. ＲｏＦ Then, the switching control unit 15 adds the transmission delay time t ＲｏＦ (the time from the arrival time of the upstream signal to the transmission delay time t ＲｏＦ The time elapsed since the previous reception is estimated as the reception timing of the upstream signal.

[0053] The switching control unit 15 generates switching information indicating that, at the estimated timing of receiving the uplink signal, the switch 18 corresponding to the base station 20 receiving the uplink signal should be turned on and that the base stations 20 other than the base station 20 receiving the uplink signal should be turned off. For example, the switching control unit 15 generates switching information indicating that, at the estimated timing of receiving the uplink signal, the switch 18-1 corresponding to the base station 20-1 should be turned on and that the base stations 20-m other than the base station 20-1 should be turned off. The switching control unit 15 outputs the generated switching information to the switching unit 16 (step S111).

[0054] The antenna unit 22-1 of the base station 20-1 receives the radio waves transmitted from the wireless terminal 30 (step S112). The antenna unit 22-1 converts the received radio waves into electrical signals and outputs them to the RoF slave unit 21-1. The RoF slave unit 21-1 modulates the electrical signals output from the antenna unit 22-1 into optical signals and transmits them (step S113).

[0055] The switching unit 16 of the central station 10 switches the state of each switch at the timing indicated by the switching information output from the switching control unit 15. Specifically, the switching unit 16 switches the switch 18-1 corresponding to the base station 20-1 to the ON state and the switch 18-m corresponding to the base station 20-1 to the OFF state at the timing of receiving the upstream signal indicated by the switching information (step S114). As a result, the switch 18-1 is turned ON at the timing when the optical signal transmitted from the base station 20-1 arrives at the central station 10. On the other hand, since the switch 18-m is turned OFF at the timing when the optical signal transmitted from the base station 20-1 arrives at the central station 10, noise generated by the base station 20-m does not flow into the central station 10. As a result, the central station 10 can receive only the optical signal transmitted from the base station 20-1.

[0056] The RoF master unit 11 of the central station 10 receives the optical signal transmitted from the base station 20-1 (step S115). The RoF master unit 11 demodulates the received optical signal into an electrical signal and outputs it to the baseband processing unit 17 via the switch 18-1. The baseband processing unit 17 demodulates the electrical signal output from the RoF master unit 11 (step S116). As described above, the switching of the switching unit 16 disconnects the central station 10 from the base station 20 that is not communicating. Therefore, noise that causes ingress noise does not flow into the central station 10. Therefore, the central station 10 can demodulate the signal.

[0057] According to the wireless communication system 100 configured as described above, the connection between the base station 20 and the central station 10 that are not communicating is disconnected by switching. In the past, it was difficult to remove ingress noise, and line design was performed assuming the presence of ingress noise. However, in the wireless communication system 100 of the present embodiment, the above-described method can suppress ingress noise from entering the central station 10. Therefore, in a system that performs RoF transmission, it is possible to reduce the influence of ingress noise. As a result, the SNR in the wireless communication system 100 can be improved.

[0058] Second Embodiment In the first embodiment, a configuration when applied to a time division multiple access system has been described. In the second embodiment, a configuration when applied to frequency division multiple access (FDMA) will be described. Note that the system configuration and the functional units of each device in the second embodiment are the same as those in the first embodiment. The following description will focus on the differences from the first embodiment.

[0059] In the second embodiment, a frequency division multiple access method is applied as a digital modulation method. As shown in Fig. 5, the host device 25 generates FDMA resource allocation information for uplink (hereinafter referred to as "FDMA resource allocation information") including identification information of each base station 20, and notifies the central station 10 of the generated FDMA resource allocation information.

[0060] Fig. 5 is a diagram showing an example of FDMA resource allocation information in the second embodiment. As shown in Fig. 5, the FDMA resource allocation information is information indicating frequency resources allocated to each wireless terminal 30. In Fig. 5, "UE#1" represents wireless terminal 30-1, "UE#2" represents wireless terminal 30-2, "UE#3" represents wireless terminal 30-3, "UE#4" represents wireless terminal 30-4, and "UE#5" represents wireless terminal 30-5.

[0061] In the example shown in FIG. 5, the frequency f １ is assigned to the wireless terminal 30-2, and the frequency f ２ is assigned to the wireless terminals 30-4 and 30-5 according to time. ｎ It is shown that the resources are assigned to the base station 20. Each resource is associated with the identification information of the base station 20. Therefore, the central station 10 can know which base station 20 the wireless terminal 30 to which each resource is assigned is connected to.

[0062] The resource notification unit 14 of the central station 10 receives the FDMA resource allocation information transmitted from the higher-level device 25. The resource notification unit 14 outputs the received FDMA resource allocation information to the baseband processing unit 17 and the switching control unit 15.

[0063] The switching control unit 15 compares the FDMA resource allocation information notified by the resource notification unit 14 with the transmission delay time t ＲｏＦ The switching control unit 15 obtains the obtained FDMA resource allocation information and the transmission delay time t ＲｏＦ Based on this, a switching list for each base station 20 shown in Fig. 6 is generated. The switching list for each base station 20 corresponds to the FDMA request allocation. The switching list for each base station 20 shown in Fig. 6 is a "request allocation method" that allocates resources in accordance with the request of each wireless terminal 30. The switching control unit 15 controls the switching of each switch included in the switching unit 16 based on the time and frequency resources allocated to each wireless terminal 30 by the request allocation and the base station identification information linked to each wireless terminal 30.

[0064] FIG. 6 is a diagram showing an example of a switching list for each base station 20. In FIG. 6, information indicating the state of the switch 18 in time units (the time domain shown in FIG. 6) is shown for each base station 20. FIG. 6 shows a case where there are 128 base stations 20. The switching list for each base station 20 shown in FIG. 6 is a list for managing the time to which each wireless terminal 30 is assigned on the time axis, and the base station 20 identified by the base station identification information under which the wireless terminal 30, which has been assigned frequency resources divided on the base station identification information axis, is located. The numbers (1 to N) shown in the Time domain column indicate the minimum consecutive resolution units when allocating time. The minimum resolution is a parameter dependent on the system design.

[0065] In the example shown in Figure 6, the states of switch 18-1 corresponding to base station 20-1 identified by base station identification information "1" are shown as being ON at the timing indicated by time domain "1", OFF at the timing indicated by time domain "2", OFF at the timing indicated by time domain "3", ..., ON at the timing indicated by time domain "N".

[0066] The switching control unit 15 generates switching information in the same manner as in the first embodiment based on the generated switching list for each base station 20. The switching control unit 15 generates switching information including, for example, information indicated in the switching list for each base station 20. Then, the switching control unit 15 notifies the switching unit 16 of the generated switching information.

[0067] According to the wireless communication system 100 of the second embodiment configured as above, it is possible to obtain the same effects as those of the first embodiment even when applied to frequency division multiple access.

[0068] (Third Embodiment) In the first embodiment, a configuration when applied to a time division multiple access system has been described. In the third embodiment, a configuration when applied to orthogonal frequency division multiple access (OFDMA) will be described. Note that the system configuration and the functional units of each device in the third embodiment are the same as those in the first embodiment. The following description will focus on the differences from the first embodiment.

[0069] In the second embodiment, a multi-carrier (MC) scheme such as OFDMA is applied as the digital modulation scheme. As shown in Fig. 7 , the higher-level device 25 generates time-frequency resource allocation information (hereinafter referred to as "time-frequency resource allocation information") including identification information of each base station 20, and notifies the central station 10 of the generated time-frequency resource allocation information.

[0070] FIG. 7 is a diagram showing an example of time-frequency resource allocation information in the third embodiment. As shown in FIG. 7, the time-frequency resource allocation information is information indicating the relationship between the allocation time of resources allocated to each wireless terminal 30 and the allocated frequency. In FIG. 7, "UE#1" represents the wireless terminal 30-1, "UE#2" represents the wireless terminal 30-2, "UE#3" represents the wireless terminal 30-3, "UE#4" represents the wireless terminal 30-4, "UE#5" represents the wireless terminal 30-5, "UE#6" represents the wireless terminal 30-6, "UE#7" represents the wireless terminal 30-7, "UE#8" represents the wireless terminal 30-8, "UE#9" represents the wireless terminal 30-9, and "UE#10" represents the wireless terminal 30-10. It is assumed that each resource is associated with identification information of the base station 20. Therefore, the central station 10 can grasp to which base station 20 the wireless terminal 30 to which each resource is allocated is connected.

[0071] The resource notification unit 14 of the central station 10 receives the time-frequency resource allocation information transmitted from the higher-level device 25. The resource notification unit 14 outputs the received time-frequency resource allocation information to the baseband processing unit 17 and the switching control unit 15.

[0072] The switching control unit 15 compares the time-frequency resource allocation information notified by the resource notification unit 14 with the transmission delay time t ＲｏＦ The switching control unit 15 generates a switching list for each OFDM symbol shown in FIG. 8 based on the acquired time-frequency resource allocation information.

[0073] Fig. 8 is a diagram showing an example of a switching list for each OFDM symbol. Fig. 8 shows information indicating the state of the switch 18 for each base station 20 for each OFDM symbol. Fig. 8 shows a case where there are 128 base stations 20. The example shown in Fig. 8 shows that the switch 18-1 corresponding to the base station 20-1 identified by base station identification information "1" is in the ON state in slot "0", the OFF state in slot "1", the OFF state in slot "2", the ON state in slot "3", the ON state in slot "4", the OFF state in slot "5", the ON state in slot "6", the OFF state in slot "7", and the ON state in slots "8" to "13".

[0074] The switching control unit 15 generates switching information based on a switching list for each OFDM symbol, similar to the first embodiment. The switching control unit 15 generates switching information including, for example, information indicated in the switching list for each OFDM symbol. Then, the switching control unit 15 notifies the generated switching information to the switching unit 16.

[0075] According to the wireless communication system 100 of the third embodiment configured as above, it is possible to obtain the same effects as those of the first embodiment even when applied to orthogonal frequency division multiple access.

[0076] (Modification 1 common to the first to third embodiments) In the above-described embodiments, the RoF transmission method has been described assuming analog RoF, but the RoF transmission method may be digital RoF that transmits digital signals. In the case of digital RoF, each base station 20 further includes a modulator or demodulator and a frequency conversion device.

[0077] (Modification 2 common to the first to third embodiments) In the above-described embodiments, an RFoF (Radio Frequency over Fiber) configuration is used as an example to transmit an RF signal, which is one configuration of the A-RoF method that converts an electrical signal of a frequency used for wireless transmission into an optical signal. However, the present invention can also be applied to an IFoF (Intermediate Frequency over Fiber) configuration that converts an electrical signal to an intermediate frequency and then converts the electrical signal to an optical signal, or a BBoF (Base Band over Fiber) configuration that converts a baseband signal from an electrical signal to an optical signal. When the IFoF configuration is applied, the base station 20 is provided with a frequency conversion device that converts the baseband signal into a signal of a frequency used for wireless transmission.

[0078] Some or all of the functional units of the central station 10 and base station 20 in the above-described embodiments are realized as software by one or more processors, such as a central processing unit (CPU), executing a program stored in a storage device having a non-volatile storage medium (non-transitory storage medium) and memory. The program may be recorded on a computer-readable non-transitory storage medium. Examples of computer-readable non-transitory storage media include portable media such as flexible disks, magneto-optical disks, read-only memories (ROMs), and compact disc-ROMs (CD-ROMs), as well as storage devices such as hard disks built into computer systems.

[0079] Some or all of the functional units of the central station 10 and the base station 20 may be realized using hardware including electronic circuits (electronic circuits or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0080] 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.

[0081] The present invention can be applied to a system using RoF.

[0082] REFERENCE SIGNS LIST 10... aggregation station, 11... RoF base station unit, 12, 23, 23-1 to 23-m... RoF delay measurement unit, 13... calculation unit, 14... resource notification unit, 15... switching control unit, 16... switching unit, 17... baseband processing unit, 18, 18-1 = 18-m... switch, 20, 20-1 to 20-m... base station, 21, 21-1 to 21-m... RoF slave unit, 22, 22-1 to 22-m... antenna unit, 30... wireless terminal, 31... antenna unit, 32... UL first-out transmission unit, 40... optical transmission path, 100... wireless communication system

Claims

1. A central station in a wireless communication system comprising a central station having a signal processing function, which is a function obtained by separating the signal processing function and the communication function of a base station, and a plurality of base stations connected to the central station via an optical transmission path and having the communication function, the central station comprising: a calculation unit that estimates a transmission delay time in a wired section that is the section between the central station and the plurality of base stations; and a switching control unit that switches connections between the central station and the plurality of base stations based on the transmission delay time estimated by the calculation unit and resource information assigned to one or more wireless terminals that communicate wirelessly with any of the plurality of base stations.

2. The central station according to claim 1, wherein the switching control unit identifies one or more of the plurality of base stations that transmit uplink signals, and disconnects the central station from base stations other than the one or more identified base stations.

3. The central station according to claim 2, wherein the switching control unit determines the arrival time of an uplink signal transmitted from one or more wireless terminals to one or more base stations based on the resource information, and controls the central station to disconnect from base stations other than the one or more base stations at a time obtained by adding the transmission delay time to the determined arrival time of the uplink signal.

4. The central station according to any one of claims 1 to 3, wherein any one of the plurality of base stations and the one or more wireless terminals are connected by a predetermined multiple access method.

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