Control method and aggregation station

Abstract

A control method performed by a wireless communication system including an aggregation station and at least one extension station connected to the aggregation station via an optical transmission line, the aggregation station and the extension station respectively having a signal processing function and communication function due to separation of the signal processing function and the communication function of a base station, wherein a transmission delay amount in a wired section, which is a section between the aggregation station and the at least one extension station, is estimated, the transmission delay amount in the wired section is subtracted from a total transmission delay amount in a section between the aggregation station and at least one wireless terminal connected wirelessly to the at least one extension station to thereby estimate a transmission delay amount in a wireless section, which is a section between the at least one extension station and the at least one wireless terminal, and the at least one wireless terminal is caused to perform advance transmission by a timing advance function on the basis of the estimated transmission delay amount in the wireless section.　

Description

Control method and aggregation station

[0001] The present invention relates to a control method and an aggregation station.

[0002] Conventionally, 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 remote radio units (RRUs), 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 station, and simple remote stations are deployed, enabling flexible and economical wireless area deployment (see, for example, Non-Patent Document 1).

[0003] As shown in Non-Patent Document 1, A-RoF transmission is performed between the central station and the base station. The base station includes an O / E converter that converts optical signals into electrical signals, an E / O converter that converts electrical signals into optical signals, and an antenna that communicates with wireless terminals. The base station may also include an amplifier that amplifies the optical signals. With digital RoF, there is a concern that the optical fiber bandwidth will increase when achieving high speeds and large capacities, but the application of analog RoF makes it possible to keep the optical fiber bandwidth at the same level as the wireless capacity.

[0004] A mobile network, which is one of the applications of the wireless communication system, employs a time division duplex (TDD) system (hereinafter referred to as the "TDD system") in which communication is performed by switching between an uplink (UL) and a downlink (DL) over time within the same communication frequency band. In the TDD system, interference occurs between the uplink and the downlink due to delays, such as propagation delays, that occur between transmitting and receiving stations.

[0005] A timing advance (TA) function has been proposed as a technique for avoiding interference between uplink and downlink (see, for example, Non-Patent Document 2). The timing advance function adjusts the UL transmission timing of a wireless terminal according to a transmission delay in a wireless section, and the wireless terminal transmits an UL signal in advance according to the transmission timing notified to the wireless terminal by a base station.

[0006] Kodai Ito, Mizuki Suga, Hirofumi Shirato, Naoki Kita, Takeshi Onisawa, Efficient accommodation of diverse high-frequency band wireless systems using analog RoF, NTT Technical Journal, 32(3), 15-17, 2020. “Physical channels and modulation (3GPP TS 38.211 version 16.7.0 Release 16)”, ETSI TS 138 211 V16.7.0 (2021-10).

[0007] When the timing advance function is applied to an analog RoF system, the wireless terminal performs advance transmission of the UL signal so that the TDD timing at the central station is synchronized with the sum of the transmission delay amount in the analog RoF section, which is the section between the central station and the base station, and the transmission delay amount in the wireless section, which is the section between the base station and the wireless terminal. This makes it difficult to achieve accurate TDD synchronization of the uplink at the base station. Thus, even when the timing advance function is applied to an analog RoF system, there are cases where it is impossible to avoid interference between the uplink and downlink with other TDD-synchronized wireless base stations due to the transmission delay occurring in the analog RoF section.

[0008] In view of the above circumstances, an object of the present invention is to provide a technology that can suppress the occurrence of interference between uplink and downlink with another TDD-synchronized radio base station in uplink transmission in a system that performs analog RoF transmission.

[0009] One aspect of the present invention is a control method performed by a wireless communication system that includes a central station having a signal processing function, which is a function that separates the signal processing function and the communication function of a base station, and one or more base stations that are connected to the central station via an optical transmission path and have the communication function, the control method comprising: estimating a transmission delay amount in a wired section that is the section between the central station and the one or more base stations; estimating a transmission delay amount in a wireless section that is the section between the one or more base stations and the one or more wireless terminals by subtracting the transmission delay amount in the wired section from a total transmission delay amount in the section between the central station and one or more wireless terminals that are wirelessly connected to the one or more base stations; and causing the one or more wireless terminals to perform advance transmission using a timing advance function based on the estimated transmission delay amount in the wireless section.

[0010] 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 separates the signal processing function and communication function of a base station, and one or more base stations connected to the central station via an optical transmission path and having the communication function, the central station including: a calculation unit that estimates a transmission delay amount in a wired section that is the section between the central station and the one or more base stations, and estimates a transmission delay amount in a wireless section that is the section between the one or more base stations and the one or more wireless terminals by subtracting the transmission delay amount in the wired section from a total transmission delay amount in the section between the central station and one or more wireless terminals that are wirelessly connected to the one or more base stations; and a communication unit that causes the one or more wireless terminals to perform advance transmission using a timing advance function by transmitting a signal including the estimated transmission delay amount in the wireless section to the one or more wireless terminals.

[0011] According to the present invention, in a system performing analog RoF transmission, it is possible to suppress the occurrence of interference between the uplink and downlink in uplink transmission with another TDD-synchronized radio base station.

[0012] Fig. 1 is a diagram illustrating a configuration example of a wireless communication system according to a first embodiment; Fig. 2 is a sequence diagram illustrating a flow of processing performed by the wireless communication system according to the first embodiment; Fig. 3 is a diagram illustrating a configuration example of a wireless communication system according to a second embodiment; Fig. 4 is a diagram illustrating a configuration example of a wireless communication system according to a third embodiment;

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

[0014] (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, in a configuration in which the functions of a wireless base station are separated between an aggregation station and a base station, the base station aims to suppress TDD timing errors in uplink transmissions and suppress interference between the uplink and downlink. Therefore, in the present invention, the aggregation station notifies a wireless terminal of a transmission delay amount obtained by subtracting the transmission delay amount of the analog RoF section known by the aggregation station from the transmission delay amount acquired by the timing advance function. Here, the transmission delay amount acquired by the timing advance function is the sum of the transmission delay amount of the analog RoF section and the transmission delay amount of the wireless section.

[0015] The wireless terminal performs advance transmission of the uplink signal, taking into account the value of the transmission delay amount notified by the central station (only the transmission delay amount in the wireless section). This enables the base station to receive a TDD-synchronized UL signal. The notification of the transmission delay amount value from the central station to the wireless terminal may be performed in a "Random Access Response," which is a response to a transmission delay request signal transmitted in the "Random Access Preamble" of Message 1 from the wireless terminal (UE: User Equipment) to the central station (gNB: g Node B) in the timing advance function. The timing advance function employs two methods, a contention-based method and a contention-free method, and either method may be used. A specific configuration for realizing the above processing will now be described.

[0016] First Embodiment Fig. 1 is a diagram showing an example of the configuration of a wireless communication system 100 according to a first embodiment. The wireless communication system 100 includes a central station 10 and one base station 20. The central station 10 and the base station 20 are connected via an optical transmission path 25. The optical transmission path 25 is an optical fiber. The optical fiber may be a single-mode optical fiber (SMF), a multi-mode optical fiber (MMF), a single-core optical fiber, or a multi-core optical fiber.

[0017] The central station 10 and the 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. The 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 the base station 20 is performed using analog RoF, which transmits wireless signals using optical fiber. One or more wireless terminals 30 are connected to the base station 20 wirelessly.

[0018] In the following description, the section between the central station 10 and the base station 20 is referred to as an analog RoF section, and the section between the base station 20 and the wireless terminal 30 is referred to as a wireless section.

[0019] 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 total transmission delay value of the transmission delay amount in the analog RoF section and the transmission delay amount in the wireless section. Furthermore, the central station 10 uses the timing advance function to notify the wireless terminal 30 of the transmission delay amount obtained by subtracting the transmission delay amount in the analog RoF section from the total transmission delay as the transmission delay amount in the wireless section.

[0020] The base station 20 performs wireless communication with one or more wireless terminals 30 under the control of the central station 10. The wireless terminals 30 perform wireless communication with the base station 20. Furthermore, the wireless terminals 30 perform pre-transmission of an uplink signal taking into account the value of the transmission delay amount 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 delay measurement unit 11, a calculation unit 12, a DL advance transmission unit 13, and an RoF master unit .

[0022] The RoF delay measurement unit 11 measures the transmission delay occurring in the optical transmission path in the RoF section. The RoF delay measurement unit 11 may synchronize the timing of the transmission delay in the RoF section using PTP (Precision Time Protocol) or may measure the delay 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.

[0023] The calculation unit 12 calculates the transmission delay amount t ＲｏＦ Specifically, the calculation unit 12 estimates the transmission delay t in the analog RoF section based on the statistical values ​​of the measurement results obtained multiple times by the RoF delay measurement unit 11 using PTP or RTT. ＲｏＦ The statistical value is, for example, the mean or median.

[0024] When the base station 20 receives a TDD-synchronized uplink signal, the calculation unit 12 calculates the transmission delay amount t ＲｏＦ and the transmission delay in the wireless section t ＲＦ The total transmission delay of the analog RoF section is calculated by subtracting t ＲｏＦ As a result, the calculation unit 12 performs a process of subtracting the transmission delay amount t ＲＦ The transmission delay amount t ＲｏＦ and the transmission delay in the wireless section t ＲＦ The total transmission delay between the time and the time is obtained by the timing advance function.

[0025] The DL advance transmission unit 13 calculates the transmission delay amount t ＲｏＦThe DL advance transmission unit 13 transmits the downstream signal early by a predetermined time. The advance transmission means transmitting the signal a predetermined time earlier. That is, the DL advance transmission unit 13 transmits the downstream signal early by a transmission delay amount t ＲｏＦ The DL advance transmission unit 13 transmits the downlink signal earlier by, for example, the transmission delay amount t ＲＦ The DL advance transmission unit 13 also transmits a TDD control signal in advance so that the base station 20 switches between the uplink signal and the downlink signal at a timing when TDD synchronization is achieved.

[0026] The RoF master unit 14 communicates with the base station 20. The RoF master unit 14 has a function of modulating the downlink signal (electrical signal) output from the DL advance transmitter 13 into an optical signal, and a function of demodulating the uplink signal (optical signal) transmitted from the base station 20 into an electrical signal. Here, it is assumed that the RoF master unit 14 modulates an electrical signal (RF signal) of a frequency used for wireless transmission into an optical signal. The RoF master unit 14 transmits the optical signal to the base station 20. In this way, the RoF master unit 14 functions as a communication unit that transmits and receives optical signals to and from the base station 20.

[0027] The base station 20 includes an RoF slave unit 21 , an antenna unit 22 , and an RoF delay measurement unit 23 .

[0028] 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.

[0029] 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.

[0030] 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. The delay measurement signal is a signal used to measure the transmission delay in the analog RoF section.

[0031] The wireless terminal 30 includes an antenna unit 31 and a UL first-out transmitting unit 32 .

[0032] 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.

[0033] The UL advance transmitter 32 calculates the wireless section transmission delay amount t included in the downlink signal transmitted from the central station 10. ＲＦ That is, the UL advance transmission unit 32 advances the uplink signal by a transmission delay amount t ＲＦ Send the upstream signal a few minutes earlier.

[0034] 2 is a sequence diagram showing the flow of processing performed by the wireless communication system 100 according to the first embodiment. At the start of the processing shown in FIG. 2, the transmission delay amount t ＲｏＦand the transmission delay in the wireless section t ＲＦ The total transmission delay between the

[0035] The RoF delay measurement unit 11 of the central station 10 measures the transmission delay of the analog RoF section by exchanging signals with the base station 20 (step S101). An example of a method for measuring the transmission delay of the analog RoF section will be described. The RoF delay measurement unit 11 converts a delay measurement signal into an optical signal via the RoF master unit 14 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.

[0036] The RoF master unit 14 of the central station 10 demodulates the optical signal transmitted from the base station 20 and outputs a delay measurement signal obtained by demodulating the optical signal transmitted from the base station 20 to the RoF delay measurement unit 11. The RoF delay measurement unit 11 measures the time required to transmit and receive the delay measurement signal as the transmission delay of the analog RoF section. The RoF delay measurement unit 11 executes this process multiple times. The RoF delay measurement unit 11 outputs multiple measurement results obtained by executing the process multiple times to the calculation unit 12. The calculation unit 12 calculates the transmission delay amount t ＲｏＦ is estimated (step S102).

[0037] Thereafter, the calculation unit 12 calculates the estimated transmission delay amount t of the analog RoF section from the value of the total transmission delay previously acquired by the timing advance function. ＲｏＦ By subtracting , the transmission delay amount t ＲＦ (Step S103). The calculation unit 12 estimates the estimated transmission delay amount t ＲＦ and the estimated transmission delay t ＲｏＦ The DL advance transmission unit 13 outputs information indicating the transmission delay amount t ＲＦThen, the DL advance transmitter 13 generates a downstream signal including information indicating the transmission delay amount t ＲｏＦ The RoF base unit 14 modulates the downstream signal output from the DL advance transmission unit 13 into an optical signal and transmits it (step S104).

[0038] The RoF slave unit 21 of the base station 20 demodulates the optical signal transmitted from the central station 10 to obtain a downstream signal (step S105). The RoF slave unit 21 outputs the obtained downstream signal to the antenna unit 22. The antenna unit 22 transmits the downstream signal output from the RoF slave unit 21 as a radio wave (step S106).

[0039] The antenna unit 31 of the wireless terminal 30 receives the radio wave transmitted from the base station 20 (step S107). 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 amount of transmission delay t ＲＦ As a result, the wireless terminal 30 acquires information indicating the transmission delay t ＲＦ The UL advance transmitter 32 uses the timing advance function to transmit the generated uplink signal with a transmission delay amount t ＲＦ The antenna unit 31 transmits the upstream signal output from the UL advance transmitting unit 32 as a radio wave.

[0040] The antenna unit 22 of the base station 20 receives the radio waves transmitted from the wireless terminal 30 (step S109). The antenna unit 22 converts the received radio waves into electrical signals and outputs them to the RoF slave unit 21. The RoF slave unit 21 modulates the electrical signals transmitted from the antenna unit 22 into optical signals and transmits them (step S110).

[0041] According to the wireless communication system 100 configured as described above, the central station 10 includes a calculation unit 12 that estimates the amount of transmission delay in the analog RoF section, which is the section between the central station 10 and one or more base stations 20, and estimates the amount of transmission delay in the wireless section by subtracting the amount of transmission delay in the analog RoF section from the total amount of transmission delay in the section between one or more wireless terminals 30 and the central station 10 (the amount of transmission delay in the analog RoF section + the amount of transmission delay in the wireless section), and a DL advance transmission unit 13 that causes the wireless terminal 30 to perform advance transmission using a timing advance function by transmitting a signal including the estimated amount of transmission delay in the wireless section to the wireless terminal 30.

[0042] This allows the radio terminal 30 to perform advance transmission according to the amount of transmission delay in the radio section notified by the central station 10. This makes it possible to synchronize the uplink TDD timing in the base station 20. As a result, in the radio communication system 100 in which there is a transmission delay between the central station 10 and the base station 20, it becomes possible to suppress the occurrence of interference in uplink transmission between the uplink and downlink of the base station 20 and another TDD-synchronized radio base station while maintaining the simplified configuration of the base station 20.

[0043] Second Embodiment In a second embodiment, a configuration in which a plurality of wireless terminals are connected to one base station will be described.

[0044] 3 is a diagram showing an example of the configuration of a wireless communication system 100a according to the second embodiment. The wireless communication system 100a includes a central station 10a and one base station 20. The central station 10a and the base station 20 are connected via an optical transmission path 25. The central station 10a and the base station 20 form one base station. Specifically, the central station 10a has a signal processing function that is obtained by separating the signal processing function and communication function of a base station. The base station 20 has a communication function that is obtained by separating the signal processing function and communication function of a base station. Transmission between the central station 10a and the base station 20 is performed using analog RoF, which transmits wireless signals using optical fiber. n wireless terminals 30 are connected to the base station 20 wirelessly, where n is an integer equal to or greater than 2.

[0045] As shown in FIG. 3, when a plurality of wireless terminals 30 are connected to one base station 20, the central station 10a calculates the transmission delay t ＲＦ１ and the transmission delay t in the wireless section between the base station 20 and the wireless terminal 30-n. ＲＦｎ This makes it possible to adjust the transmission and reception timings for both the downlink and the uplink.

[0046] (Device Configuration) Next, the configurations of the central station 10a, the base station 20, and the wireless terminal 30 will be described. Note that the base station 20 and the wireless terminal 30 perform the same processes as in the first embodiment, and therefore their description will be omitted. The central station 10a includes a RoF delay measurement unit 11, a calculation unit 12a, a DL advance transmission unit 13, and a RoF master unit 14. The central station 10a differs in configuration from the central station 10 in that it includes a calculation unit 12a instead of the calculation unit 12. The central station 10a is otherwise similar in configuration to the central station 10. The following description will focus on the differences from the central station 10.

[0047] The calculation unit 12a calculates the transmission delay amount t ＲｏＦ Furthermore, when the base station 20 receives a TDD-synchronized uplink signal, the calculation unit 12a estimates the transmission delay amount t ＲｏＦ and the transmission delay in the wireless section t ＲＦ１ The total transmission delay of the analog RoF section is calculated by subtracting t ＲｏＦ By performing the process of subtracting the transmission delay amount t ＲＦ１ Furthermore, when the base station 20 receives an uplink signal synchronized with TDD, the calculation unit 12a calculates the transmission delay amount t ＲｏＦ and the transmission delay in the wireless section t ＲＦｎ The total transmission delay of the analog RoF section is calculated by subtracting t ＲｏＦ By performing the process of subtracting the transmission delay t ＲＦｎ Calculate.

[0048] The calculation unit 12a holds information on the transmission delay in the section between the central station 10a and each wireless terminal 30 for each wireless terminal 30 connected to the base station 20. Then, as described above, the calculation unit 12a calculates the transmission delay amount t ＲＦ Calculate.

[0049] (Processing Flow) Next, a processing flow of the wireless communication system 100a according to the second embodiment will be described. At the start of processing, the transmission delay amount t ＲｏＦ and the transmission delay in the wireless section t ＲＦ It is assumed that the total transmission delay value between the first and second transmission delays is measured for each wireless terminal 30.

[0050] The RoF delay measurement unit 11 of the central station 10a measures the transmission delay in the analog RoF section by exchanging signals with the base station 20. The RoF delay measurement unit 11 outputs a plurality of measurement results to the calculation unit 12a. The calculation unit 12a calculates the transmission delay amount t ＲｏＦ Estimate.

[0051] Thereafter, the calculation unit 12a calculates the estimated transmission delay amount t of the analog RoF section from the value of the total transmission delay previously acquired by the timing advance function. ＲｏＦ By subtracting , the transmission delay amount t ＲＦ For example, the calculation unit 12a estimates the transmission delay amount t ＲｏＦ By subtracting , the transmission delay amount t ＲＦ１ For example, the calculation unit 12a calculates the estimated transmission delay amount t ＲｏＦ By subtracting the transmission delay t ＲＦｎ Calculate.

[0052] The calculation unit 12a calculates the estimated transmission delay amount t ＲＦ１ and the transmission delay amount t ＲＦｎ and the estimated transmission delay t ＲｏＦ The DL advance transmission unit 13 outputs information indicating the transmission delay amount t ＲＦ１ At this time, the DL advance transmitter 13 includes information indicating the radio terminal 30-1 as the destination of the first downlink signal in the first downlink signal. Then, the DL advance transmitter 13 transmits the generated first downlink signal with the transmission delay amount t ＲｏＦ The RoF base unit 14 modulates the downstream signal output from the DL advance transmission unit 13 into an optical signal and sends it out.

[0053] The RoF slave unit 21 of the base station 20 demodulates the optical signal transmitted from the central station 10a to obtain a downstream signal. The RoF slave unit 21 outputs the obtained downstream signal to the antenna unit 22. The antenna unit 22 transmits the downstream signal output from the RoF slave unit 21 as a radio wave.

[0054] The antenna unit 31-1 of the wireless terminal 30-1 receives the radio wave transmitted from the base station 20. The wireless terminal 30-n also receives the radio wave transmitted from the base station 20, but discards the signal because it is addressed to the wireless terminal 30-1. The antenna unit 31-1 of the wireless terminal 30-1 converts the received radio wave into an electrical signal and outputs it to the UL advance transmission unit 32-1. The UL advance transmission unit 32-1 converts the amount of transmission delay t in the wireless section contained in the electrical signal output from the antenna unit 31-1. ＲＦ１ As a result, the wireless terminal 30-1 acquires information indicating the transmission delay t ＲＦ１ The UL advance transmission unit 32-1 uses the timing advance function to transmit the generated uplink signal with a transmission delay amount t ＲＦ１ Send the specified number of minutes in advance.

[0055] The DL advance transmitter 13 calculates the transmission delay amount tＲＦｎ At this time, the DL advance transmitter 13 includes information indicating the radio terminal 30-n as the destination of the second downlink signal in the second downlink signal. Then, the DL advance transmitter 13 transmits the generated second downlink signal with the transmission delay amount t ＲｏＦ The RoF base unit 14 modulates the downstream signal output from the DL advance transmission unit 13 into an optical signal and sends it out.

[0056] The RoF slave unit 21 of the base station 20 demodulates the optical signal transmitted from the central station 10a to obtain a downstream signal. The RoF slave unit 21 outputs the obtained downstream signal to the antenna unit 22. The antenna unit 22 transmits the downstream signal output from the RoF slave unit 21 as a radio wave.

[0057] The antenna unit 31-n of the wireless terminal 30-n receives the radio waves transmitted from the base station 20. The wireless terminal 30-1 also receives the radio waves transmitted from the base station 20, but discards the signal because it is addressed to the wireless terminal 30-n. The antenna unit 31-n of the wireless terminal 30-n converts the received radio waves into an electrical signal and outputs it to the UL advance transmission unit 32-n. The UL advance transmission unit 32-n converts the amount of transmission delay t in the wireless section contained in the electrical signal output from the antenna unit 31-n. ＲＦｎ As a result, the wireless terminal 30-n acquires information indicating the transmission delay t ＲＦｎ The UL advance transmitter 32-n uses the timing advance function to transmit the generated uplink signal with the transmission delay amount t ＲＦｎ Send the specified number of minutes in advance.

[0058] In the wireless communication system 100a configured as described above, the central station 10a estimates the amount of transmission delay in the wireless section between the base station 20 and each wireless terminal 30, and notifies each wireless terminal 30 of information indicating the amount of transmission delay in the corresponding wireless section. This allows each wireless terminal 30 to perform advance transmission according to the amount of transmission delay in the wireless section. Therefore, even when multiple wireless terminals 30 are connected to one base station 20, the same effects as in the first embodiment can be obtained.

[0059] Third Embodiment In a third embodiment, a configuration in which a plurality of base stations are connected to a central station will be described.

[0060] 4 is a diagram showing an example of the configuration of a wireless communication system 100b according to the third embodiment. The wireless communication system 100b includes a central station 10b and m base stations 20, where m is an integer equal to or greater than 2. The central station 10b and each base station 20 are connected via an optical transmission path 25. For example, the central station 10b and base station 20-1 are connected via an optical transmission path 25-1, and the central station 10b and base station 20-m are connected via an optical transmission path 25-m.

[0061] The central station 10b and each base station 20 constitute one base station. Specifically, the central station 10b has a signal processing function, which is one of the functions that a base station has when the signal processing function and 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 communication function are separated. Transmission between the central station 10b 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.

[0062] As shown in Fig. 4, when multiple base stations 20 are connected to a central station 10b, the central station 10b uses the timing advance function to acquire all combinations of the transmission delay amount in the analog RoF section and the transmission delay amount in the wireless section. For example, in the configuration shown in Fig. 4, the central station 10b uses the timing advance function to acquire a first total transmission delay, which is the sum of the transmission delay amount in the analog RoF section between the central station 10b and the base station 20-1 and the transmission delay amount in the wireless section between the base station 20-1 and the wireless terminal 30, and acquires an mth total transmission delay, which is the sum of the transmission delay amount in the analog RoF section between the central station 10b and the base station 20-m and the transmission delay amount in the wireless section between the base station 20-m and the wireless terminal 30.

[0063] Furthermore, the central station 10b estimates the amount of transmission delay in the analog RoF section between each of the base stations 20. In the configuration shown in FIG. 4, the central station 10b estimates the amount of transmission delay t ＲｏＦ１ and the transmission delay t in the analog RoF section between the central station 10b and the base station 20-m. ＲｏＦｍ When the base station 20-1 receives a TDD-synchronized uplink signal, the central station 10b estimates the transmission delay amount t ＲｏＦ１ The value of the transmission delay amount obtained by subtracting the above is notified to the wireless terminal 30 as the transmission delay amount in the wireless section.

[0064] (Device Configuration) Next, the configurations of the central station 10b, the base station 20, and the wireless terminal 30 will be described. Note that the base station 20 and the wireless terminal 30 perform the same processing as in the first embodiment, and therefore their description will be omitted. The central station 10b includes an RoF delay measurement unit 11, a calculation unit 12b, m DL advance-out transmission units 13, an RoF master unit 14, and a switch 15. The central station 10b differs in configuration from the central station 10 in that it includes a calculation unit 12b instead of the calculation unit 12, that it includes multiple DL advance-out transmission units 13, and that it newly includes a switch 15. The central station 10b is otherwise similar in configuration to the central station 10. The following description will focus on the differences from the central station 10.

[0065] The switch 15 is provided between the RoF delay measurement unit 11 and the RoF master unit 14, and switches the connection destination of the central station 10b. For example, the switch 15 switches so as to measure the transmission delay of the analog RoF section between the central station 10b and the base station 20-m after completing the measurement of the transmission delay of the analog RoF section between the central station 10b and the base station 20-1.

[0066] The calculation unit 12b calculates the transmission delay t ＲｏＦ Furthermore, the calculation unit 12b estimates the transmission delay t ＲＦThe calculation unit 12b holds information on the transmission delay in the section between the central station 10b and the wireless terminal 30 for each wireless terminal 30 connected to each base station 20. Then, the calculation unit 12b calculates the transmission delay amount t ＲＦ Furthermore, the calculation unit 12b calculates the transmission delay amount t ＲｏＦ After recognizing which base station 20 the estimated result of transmission delay is between, the transmission delay amount t ＲｏＦ to the corresponding DL advance transmission unit 13.

[0067] The DL advance transmitter 13 advances a downlink signal by the amount of transmission delay of the analog RoF section in accordance with the amount of transmission delay of the analog RoF section for each base station 20 calculated by the calculation unit 12b. For example, the DL advance transmitter 13-1 advances a downlink signal to the base station 20-1, and the DL advance transmitter 13-m advances a downlink signal to the base station 20-m. The DL advance transmitter 13-1 advances a downlink signal by the amount of transmission delay t ＲｏＦ１ The DL advance transmitting unit 13-m transmits a downstream signal by the amount of transmission delay t ＲｏＦｍ The downstream signal is transmitted first for the duration of the signal.

[0068] (Processing Flow) Next, a processing flow of the wireless communication system 100b according to the third embodiment will be described. It is assumed that, at the start of processing, the first total transmission delay and the m total transmission delay have been measured by the timing advance function.

[0069] The RoF delay measurement unit 11 of the central station 10b measures the transmission delay of the analog RoF section for each base station 20 by exchanging signals with each base station 20. Specifically, first, the switch 15 switches the central station 10b to connect the central station 10b to the base station 20-1. Then, the RoF delay measurement unit 11 measures the transmission delay of the analog RoF section by exchanging signals with the base station 20-1. The RoF delay measurement unit 11 outputs a plurality of measurement results to the calculation unit 12b. The calculation unit 12b calculates the transmission delay amount t of the analog RoF section between the central station 10b and the base station 20-1 based on the plurality of measurement results output from the RoF delay measurement unit 11. ＲｏＦ１ Estimate.

[0070] Next, the switch 15 switches over to connect the central station 10b and the base station 20-m. Then, the RoF delay measurement unit 11 measures the transmission delay in the analog RoF section by exchanging signals with the base station 20-m. The RoF delay measurement unit 11 outputs a plurality of measurement results to the calculation unit 12b. The calculation unit 12b calculates the transmission delay amount t ＲｏＦｍ Estimate.

[0071] In the above example, the central station 10b is configured to estimate the amount of transmission delay in the analog RoF section between each base station 20. However, when the base station 20 to which the wireless terminal 30 is connected is the base station 20-1, the central station 10b estimates the amount of transmission delay t ＲｏＦ１ Only the above may be estimated.

[0072] Thereafter, the calculation unit 12b calculates the estimated transmission delay amount t ＲｏＦ By subtracting , the transmission delay amount t ＲＦ For example, when the radio terminal 30 receives a TDD-synchronized uplink signal at the base station 20-1 to which the radio terminal 30 is connected, the calculation unit 12b calculates the estimated transmission delay amount tＲｏＦ１ By subtracting , the transmission delay amount t ＲＦ１１ As described above, the first total transmission delay is the sum of the transmission delay amount in the analog RoF section between the central station 10b and the base station 20-1 and the transmission delay amount in the wireless section between the base station 20-1 and the wireless terminal 30.

[0073] For example, when the radio terminal 30 receives a TDD-synchronized uplink signal at the base station 20-m connected to the base station 20-m, the calculation unit 12b calculates the estimated transmission delay amount t of the analog RoF section from the value of the m-th total transmission delay acquired in advance by the timing advance function. ＲｏＦｍ By subtracting the transmission delay t ＲＦｍ１ As described above, the m-th total transmission delay is the sum of the transmission delay amount in the analog RoF section between the central station 10b and the base station 20-m and the transmission delay amount in the wireless section between the base station 20-m and the wireless terminal 30.

[0074] The calculation unit 12b calculates the estimated transmission delay amount t ＲＦ and the estimated transmission delay t ＲｏＦ to the corresponding DL advance-out transmitting unit 13. Here, it is assumed that the corresponding DL advance-out transmitting unit 13 is the DL advance-out transmitting unit 13-1. In this case, the calculation unit 12b calculates the transmission delay amount t ＲＦ１１ and the transmission delay amount t ＲｏＦ１ The DL advance transmission unit 13-1 outputs information indicating the transmission delay amount t ＲＦ１１ At this time, the DL advance transmission unit 13 includes information indicating the wireless terminal 30 as the destination of the first downlink signal in the first downlink signal. Then, the DL advance transmission unit 13-1 transmits the generated first downlink signal with the transmission delay amount t ＲｏＦ１ The RoF base unit 14 modulates the downstream signal output from the DL advance transmission unit 13-1 into an optical signal and sends it out.

[0075] The RoF slave unit 21-1 of the base station 20-1 demodulates the optical signal transmitted from the central station 10b to obtain a downstream signal. The RoF slave unit 21-1 outputs the obtained downstream signal to the antenna unit 22-1. The antenna unit 22-1 transmits the downstream signal output from the RoF slave unit 21-1 as a radio wave.

[0076] The antenna unit 31 of the wireless terminal 30 receives radio waves transmitted from the base station 20-1. The antenna unit 31 of the wireless terminal 30 converts the received radio waves into an electrical signal and outputs it to the UL advance transmission unit 32. The UL advance transmission unit 32 converts the amount of transmission delay t ＲＦ１１ As a result, the wireless terminal 30 acquires information indicating the transmission delay t ＲＦ１１ The UL advance transmitter 32 uses the timing advance function to transmit the generated uplink signal with a transmission delay amount t ＲＦ１１ Send the specified number of minutes in advance.

[0077] In the above example, the case where the radio terminal 30 receives a TDD synchronized uplink signal at the base station 20-1 to which it is connected has been described. However, when the radio terminal 30 receives a TDD synchronized uplink signal at the base station 20-m to which it is connected, the central station 10b receives the TDD synchronized uplink signal at the base station 20-m to which it is connected. ＲＦｍ１ The wireless terminal 30 transmits a downlink signal including information indicating the above to the wireless terminal 30 via the base station 20-m. In this way, the wireless terminal 30 performs advance transmission at different timings for each base station 20 to which it is connected.

[0078] According to the wireless communication system 100b configured as above, even when a plurality of base stations 20 exist, it is possible to obtain the same effects as those of the first embodiment.

[0079] Furthermore, in the wireless communication system 100b, the central station 10b includes the same number of DL advance transmission units 13 as the number of base stations 20. This makes it possible to synchronize the TDD timing in each base station 20 even if the transmission delay amount in the analog RoF section between the central station 10b and each base station 20 differs.

[0080] (Modification 1) The central station 10b may be configured to include the same number of RoF delay measurement units 11 and calculation units 12b as the number of base stations 20. That is, if there are m base stations 20, the central station 10b will include m RoF delay measurement units 11 and calculation units 12b. When the central station 10b is connected to the base station 20-1, the central station 10b uses the RoF delay measurement unit 11 and calculation unit 12b corresponding to the base station 20-1 to calculate the transmission delay t ＲｏＦ１ can be estimated.

[0081] (Modification 2) The central station 10b may measure the RTT by identifying each base station 20 using a different optical wavelength according to WDM (Wavelength Division Multiplexing).

[0082] (Variation 3) The central station 10b may perform RTT measurements by assigning delay measurement signals of different frequency bands to each base station 20. In this case, the delay measurement signals of different frequency bands may be transmitted by employing a subcarrier multiplexing (SCM) method in which the signals are multiplexed and transmitted at the same optical wavelength.

[0083] (Modification 4) In the above-described embodiment, a configuration in which one wireless terminal 30 is connected to the base station 20 has been described, but the present invention can also be applied to a configuration in which a plurality of wireless terminals 30 are connected to the base station 20. In this case, as described in the second embodiment, the central station 10b may calculate the amount of transmission delay in the wireless section for each wireless terminal 30.

[0084] (Modification 1 common to the first to third embodiments) The RoF delay measurement unit 11 included in the central station 10 may be implemented inside the RoF master unit 14 .

[0085] (Modification 2 common to the first to third embodiments) In the above-described embodiments, a configuration in which an electrical signal at a frequency used for wireless transmission is converted into an optical signal (A-RoF method for transmitting RF signals) has been described as an example, but the present invention can also be applied to an IFoF (Intermediate Frequency over Fiber) configuration in which frequency conversion is performed to an intermediate frequency and then electrical-to-optical conversion is performed, or a BBoF (Base Band over Fiber) configuration in which a baseband signal is converted from electrical to optical. Note that when the IFoF configuration is applied, the base station 20 is equipped with a frequency conversion device that converts the baseband signal into a signal at a frequency used for wireless transmission.

[0086] In the above-described embodiments, some or all of the functional units of the central stations 10, 10a, 10b and base station 20 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 a 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.

[0087] Some or all of the functional units of the central stations 10, 10a, 10b 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).

[0088] 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.

[0089] The present invention is applicable to systems using analog RoF.

[0090] 10, 10a, 10b... aggregation station, 11, 23, 23-1 to 23-m... RoF delay measurement unit, 12... calculation unit, 13, 13-1 to 13-m... DL first-out transmission unit, 14... RoF base unit, 15... switch, 20... base station, 30, 30-1 to 30-n... wireless terminal, 21, 21-1 to 21-m... RoF slave unit, 22, 22-1 to 22-m, 31, 31-1 to 31-n... antenna unit, 32, 32-1 to 32-n... UL first-out transmission unit, 100, 100a, 100b... wireless communication system

Claims

1. A control method performed by 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 communication function of a base station, and one or more base stations connected to the central station via an optical transmission path and having the communication function, the control method comprising: estimating a transmission delay amount in a wired section between the central station and the one or more base stations; estimating a transmission delay amount in a wireless section between the one or more base stations and the one or more wireless terminals by subtracting the transmission delay amount in the wired section from a total transmission delay amount in a section between the central station and one or more wireless terminals connected wirelessly to the one or more base stations; and causing the one or more wireless terminals to perform advance transmission using a timing advance function based on the estimated transmission delay amount in the wireless section.

2. The control method according to claim 1, wherein, when the one or more wireless terminals are a plurality of wireless terminals, a transmission delay amount in the wireless section between each of the plurality of wireless terminals and the one or more base stations is estimated.

3. A control method according to claim 1 or 2, wherein, when the one or more base stations are multiple base stations, a transmission delay amount in a wired section between the central station and each of the multiple base stations is estimated, and a transmission delay amount in a wireless section between the base station to which the one or more wireless terminals are connected and the one or more wireless terminals is estimated by subtracting the transmission delay amount in the wired section between the base station to which the one or more wireless terminals are connected and the central station from the total transmission delay amount in the section between the one or more wireless terminals and the central station.

4. 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 one or more 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 amount in a wired section, which is the section between the central station and the one or more base stations, and estimates a transmission delay amount in a wireless section, which is the section between the one or more base stations and the one or more wireless terminals, by subtracting the transmission delay amount in the wired section from a total transmission delay amount in the section between the central station and one or more wireless terminals connected to the one or more base stations by wireless, and a communication unit that transmits a signal including the estimated transmission delay amount in the wireless section to the one or more wireless terminals, thereby causing the one or more wireless terminals to perform advance transmission using a timing advance function.

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