Communication system and time estimation method

Abstract

This communication system comprises: an aggregation station that transmits and receives a first training signal using an optical signal of a first wavelength, and transmits and receives a second training signal using an optical signal of a second wavelength; an optical propagation path that propagates the first training signal and the second training signal; and an extension station that transmits a first downlink wireless signal to a wireless terminal on the basis of the first training signal propagated from the optical propagation path, transmits a second downlink wireless signal to the wireless terminal on the basis of the second training signal propagated from the optical propagation path, and simultaneously transmits the first training signal and the second training signal to the optical propagation path on the basis of a first uplink wireless signal and a second uplink wireless signal simultaneously received from the wireless terminal.

Description

Communication system and time estimation method

[0001] The present invention relates to a communication system and a time estimation method.

[0002] With the increasing demand for wireless communication, high-frequency band communication has been attracting attention. However, since the propagation loss of wireless signals in free space increases in proportion to the square of the frequency, the transmission distance of wireless signals in high-frequency band communication is short. This limits the communication area of ​​high-frequency band communication.

[0003] One solution to this problem is a communication system using Radio over Fiber (RoF) (hereinafter referred to as "RoF system"). In an RoF system, the signal processing unit and antenna that make up a base station are separated using radio over fiber. Because the signal processing unit is provided in the central station and the antenna is provided in the base station, the RoF system is expected to bring about the advantages of a simplified base station configuration, power saving, and low cost.

[0004] In an RoF system, a communication area is expanded by installing base stations at locations distant from a central station (see Non-Patent Document 1). Furthermore, the communication area is further expanded by connecting multiple base stations to the central station. A double star network configuration is a network configuration that allows more base stations to be connected to the central station.

[0005] Hereinafter, the direction from the base station to the aggregation station will be referred to as "upstream," and the direction from the aggregation station to the base station will be referred to as "downstream."

[0006] In RoF systems, radio frequency (RF) signals are sometimes transmitted using time division duplex (TDD) (see Non-Patent Document 2). In time division duplex, the transmission of uplink signals and the transmission of downlink signals are divided in the time domain in a wireless communication section. Furthermore, when the frequency of the signal used for transmitting the uplink signal is the same as the frequency of the signal used for transmitting the downlink signal, the uplink signal and the downlink signal are transmitted exclusively based on a predetermined slot timing. If the uplink signal and the downlink signal are transmitted at a timing that does not match the slot timing, the uplink signal and the downlink signal will interfere with each other, making communication impossible.

[0007] Furthermore, when the signal processing unit and the antenna constituting the base station are separated from each other, the central station equipped with the signal processing unit controls the timing of time division duplexing. In a RoF system, due to a transmission delay of the optical signal in the RoF section, the timing at which the base station transmits or receives the signal may not match the slot timing, which may result in communication failure. To prevent this from happening, the central station needs to control the timing of time division duplexing by taking into account the transmission time (transmission delay) of the optical signal in the RoF section.

[0008] The transmission time of the optical signal in the RoF section is dominated by the transmission time of the optical signal in the optical transmission path. Therefore, in order for the central station to control the timing of time division duplexing in consideration of the transmission delay of the optical signal in the RoF section, information on the transmission time of the optical signal in the optical transmission path (optical fiber) in the RoF section is required.

[0009] Methods for estimating the transmission time of an optical signal in an optical transmission path include an estimation method using an optical time domain reflectometer (OTDR) (see Non-Patent Document 3) and an estimation method using point-to-multipoint (P2MP) discovery (see Non-Patent Document 4).

[0010] C. Lim, Y. Tian, ​​C. Ranaweera, TA Nirmalathas, E. Wong and KL Lee, “Evolution of Radio-Over-Fiber Technology,” IEEE Journal of Lightwave Technology, vol. 37, no. 6, Mar. 2019.3GPP(R) TS 38.101-2, “User Equipment (UE) radio transmission and reception; Part 2: Range 2 Standalone,” 2018.Recommendation ITU-R G.650.3:“Test methods for installed single-mode optical fiber cable links,” 13 August, 2017.“IEEE Std 802.3ah-2004,” IEEE Computer Society, 7 September 2004.

[0011] In a method for estimating transmission time (transmission delay) using an optical time domain reflectometer, the transmission time of an optical signal on an optical transmission path is estimated based on the time it takes for a central station to transmit a pulse signal, for the transmitted pulse signal to be reflected by a base station, and for the reflected pulse signal to return to the central station.

[0012] However, when the optical transmission path is configured as a double star and a single-wavelength optical signal is used to estimate the transmission time, the optical signals reflected from multiple base stations are mixed together, making it impossible to identify which base station the optical signal was reflected from. As a result, it is impossible to estimate the transmission time on each optical transmission path from the central station to each base station. When the optical transmission path is configured as a double star and a different wavelength is used for transmission time estimation at each base station, it is possible to identify which base station the optical signal was reflected from, but because the same number of wavelengths as the number of base stations must be allocated for transmission time estimation, the wavelength utilization efficiency is significantly reduced.

[0013] In a method of estimating transmission time using P2MP discovery, the transmission time on each optical transmission path from the central station to each base station is estimated based on the round trip time (RTT) of the optical signal used in the P2MP discovery sequence. In this method, each base station returns a signal with its own base station identifier embedded in it to the central station, so the round trip time (round trip time) of the optical signal between the central station and each base station can be obtained for each base station. However, since the base station needs the function to embed the identifier of its own base station in the signal, the configuration of the base station becomes complex.

[0014] Thus, there is a problem that the transmission time of an optical signal on an optical transmission path cannot be estimated unless the configuration of the base station is made complex or unless the wavelength utilization efficiency is sacrificed.

[0015] In view of the above circumstances, the present invention aims to provide a communication system and a time estimation method that are capable of estimating the transmission time of an optical signal in an optical transmission path without complicating the configuration of the base station and without sacrificing wavelength utilization efficiency.

[0016] One aspect of the present invention includes a central station that transmits and receives a first training signal using an optical signal of a first wavelength and transmits and receives a second training signal using an optical signal of a second wavelength; an optical transmission line that transmits the first training signal and the second training signal; and a base station that transmits a first downlink radio signal to a radio terminal based on the first training signal transmitted from the optical transmission line, transmits a second downlink radio signal to the radio terminal based on the second training signal transmitted from the optical transmission line, and simultaneously transmits the first training signal and the second training signal to the optical transmission line based on first uplink radio signal and second uplink radio signal simultaneously received from the radio terminal. the central station estimates the length of the optical transmission path based on the difference between the round-trip time of the first training signal traveling back and forth between itself and the wireless terminal and the round-trip time of the second training signal traveling back and forth between itself and the wireless terminal, the group velocity or group delay time of the optical signal of the first wavelength, and the group velocity or group delay time of the optical signal of the second wavelength; estimates the transmission time of the optical signal of the first wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the first wavelength and the length of the optical transmission path; and estimates the transmission time of the optical signal of the second wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the second wavelength and the length of the optical transmission path.

[0017] One aspect of the present invention includes a central station that transmits a control signal instructing a start of measurement, receives a first training signal using an optical signal of a first wavelength, and receives a second training signal using an optical signal of a second wavelength; an optical transmission path that transmits the control signal, the first training signal, and the second training signal; a synchronization device that synchronizes a time of the central station with a time of a wireless terminal; and a base station that transmits the control signal to the wireless terminal and simultaneously transmits the first training signal and the second training signal to the optical transmission path based on a first uplink wireless signal and a second uplink wireless signal that are simultaneously received from the wireless terminal, wherein the wireless terminal notifies the central station of a measurement start time that is a time at which the control signal is received at the wireless terminal, and transmits the first uplink wireless signal and the second uplink wireless signal to the base station, and the central station synchronizes the measurement start time, a length of time from the measurement start time to the time when the second training signal is received by the central station is defined as the transmission time of the first training signal; a length of time from the measurement start time to the time when the second training signal is received by the central station is defined as the transmission time of the second training signal; the length of the optical transmission path is estimated based on the difference between the transmission time of the first training signal and the transmission time of the second training signal, the group velocity or group delay time of the optical signal of the first wavelength, and the group velocity or group delay time of the optical signal of the second wavelength; the transmission time of the optical signal of the first wavelength in the optical transmission path is estimated based on the group velocity or group delay time of the optical signal of the first wavelength and the length of the optical transmission path; and the transmission time of the optical signal of the second wavelength in the optical transmission path is estimated based on the group velocity or group delay time of the optical signal of the second wavelength and the length of the optical transmission path.

[0018] One aspect of the present invention is a time estimation method executed by a communication system, the method comprising the steps of: transmitting and receiving a first training signal using an optical signal of a first wavelength, and transmitting and receiving a second training signal using an optical signal of a second wavelength; transmitting the first training signal and the second training signal; transmitting a first downlink radio signal to a radio terminal based on the first training signal transmitted from an optical transmission path; transmitting a second downlink radio signal to a radio terminal based on the second training signal transmitted from the optical transmission path; and simultaneously transmitting the first training signal and the second training signal to the optical transmission path based on first uplink radio signal and second uplink radio signal simultaneously received from the radio terminal. the transmitting and receiving steps include estimating the length of the optical transmission path based on a difference between a round-trip time of the first training signal traveling back and forth between a local aggregation station and the wireless terminal and a round-trip time of the second training signal traveling back and forth between a local aggregation station and the wireless terminal, a group velocity or a group delay time of the optical signal of the first wavelength, and a group velocity or a group delay time of the optical signal of the second wavelength; estimating a transmission time of the optical signal of the first wavelength in the optical transmission path based on the group velocity or the group delay time of the optical signal of the first wavelength and the length of the optical transmission path; and estimating the transmission time of the optical signal of the second wavelength in the optical transmission path based on the group velocity or the group delay time of the optical signal of the second wavelength and the length of the optical transmission path.

[0019] One aspect of the present invention is a time estimation method executed by the above-mentioned communication system, comprising the steps of: transmitting a control signal instructing a start of measurement; receiving a first training signal using an optical signal of a first wavelength; and receiving a second training signal using an optical signal of a second wavelength; transmitting the control signal, the first training signal, and the second training signal; synchronizing a time of a central station with a time of a wireless terminal; transmitting the control signal to the wireless terminal; and simultaneously transmitting the first training signal and the second training signal to an optical transmission path based on a first uplink wireless signal and a second uplink wireless signal simultaneously received from the wireless terminal, wherein the transferring step includes the wireless terminal notifying the central station of a measurement start time, which is the time when the control signal is received at the wireless terminal, and transmitting the first uplink wireless signal and the second uplink wireless signal to a base station; The steps of the time estimation method include defining a time length from the measurement start time to a time when the first training signal is received by the local aggregation station as a transmission time of the first training signal, defining a time length from the measurement start time to a time when the second training signal is received by the local aggregation station as a transmission time of the second training signal, estimating the length of the optical transmission path based on a difference between the transmission time of the first training signal and the transmission time of the second training signal, the group velocity or group delay time of the optical signal of the first wavelength, and the group velocity or group delay time of the optical signal of the second wavelength, estimating the transmission time of the optical signal of the first wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the first wavelength and the length of the optical transmission path, and estimating the transmission time of the optical signal of the second wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the second wavelength and the length of the optical transmission path.

[0020] According to the present invention, it is possible to estimate the transmission time of an optical signal in an optical transmission path without complicating the configuration of the base station and without sacrificing wavelength utilization efficiency.

[0021] Fig. 1 is a diagram showing an example of the configuration of an RoF system in a first embodiment; Fig. 2 is a diagram showing an example of the configuration of an RoF system in a fifth embodiment; Fig. 3 is a diagram showing an example of the configuration of an RoF system in a sixth embodiment; Fig. 4 is a diagram showing an example of the configuration of an RoF system in an eighth embodiment.

[0022] An embodiment of the present invention will be described in detail with reference to the drawings. (Summary) A RoF system (communication system) includes one or more base stations and a central station that communicates with each base station using optical signals. For example, in a double star RoF system, time division duplex (TDD) communication can be controlled by obtaining an estimate of the transmission time of an optical signal in an optical transmission path (optical fiber) between a base station that performs wireless communication with a wireless terminal (WT) under its control and a central station with which the base station communicates. In other words, time division duplex communication can be controlled based on an estimate of the transmission time of an optical signal in an optical transmission path used when the central station and the wireless terminal communicate in time division duplex communication.

[0023] Therefore, the central station downstream transmits a first training signal (first measurement signal) using an optical signal of a first wavelength in the first system (first signal system), and downstream transmits a second training signal (second measurement signal) using an optical signal of a second wavelength in the second system (second signal system).

[0024] The central station measures the round-trip time of a first training signal traveling back and forth between itself and the wireless terminal. The central station measures the round-trip time of a second training signal traveling back and forth between itself and the wireless terminal. The central station estimates the length of the optical transmission path based on the difference between the round-trip time of the first training signal and the round-trip time of the second training signal, the group velocities of the optical signal of the first wavelength, and the group velocities of the optical signal of the second wavelength.

[0025] The central station estimates a transmission time (transmission delay) of the optical signal of the first wavelength in the optical transmission path based on the group velocity of the optical signal of the first wavelength and the length of the optical transmission path, and estimates a transmission time (transmission delay) of the optical signal of the second wavelength in the optical transmission path based on the group velocity of the optical signal of the second wavelength and the length of the optical transmission path.

[0026] First Embodiment Fig. 1 is a diagram showing an example of the configuration of a RoF system 1a according to a first embodiment. The RoF system 1a (communication system) includes a central station 2a, an optical transmission path 3, and N (N is an integer equal to or greater than 1) base stations 4a. As an example, a passive double star network configuration is applied to the RoF system 1a. The base stations 4a are remote radio units (RRUs).

[0027] The central station 2a includes a signal processing unit 21, two converters 22, two optical circulators 23, an optical multiplexer / demultiplexer 24, and two converters 25. Here, the converter 22-1, the optical circulator 23-1, and the converter 25-1 are provided in a first system (first signal system) in the optical transmission line. The converter 22-2, the optical circulator 23-2, and the converter 25-2 are provided in a second system (second signal system) in the optical transmission line.

[0028] The signal processing unit 21 is realized as software by a processor such as a CPU (Central Processing Unit) executing a program stored in a storage device having a non-volatile recording medium (non-transitory recording medium) and memory. The program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and non-transitory recording media such as a hard disk or a solid-state drive (SSD) built into a computer system.

[0029] The signal processing unit 21 may be realized using hardware (accelerator) including an electronic circuit (electronic circuit 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).

[0030] The optical transmission path 3 includes an optical fiber and an optical splitter. The base station 4a includes an optical multiplexer / demultiplexer 41, two optical circulators 42, two converters 43, two circulators 44, two antennas 45, and two converters 46. As described above, the base station 4a does not need to be equipped with a signal processing unit, and therefore the configuration of the base station 4a is not complicated.

[0031] Here, the optical circulator 42-1, converter 43-1, circulator 44-1, antenna 45-1, and converter 46-1 are provided in a first system of the optical transmission line, while the optical circulator 42-2, converter 43-2, circulator 44-2, antenna 45-2, and converter 46-2 are provided in a second system of the optical transmission line.

[0032] First, downstream transmission (transmission of downstream signals) will be described. The central station 2a generates a wavelength-multiplexed optical signal (combined optical signal) using optical signals of wavelengths determined for each system in the optical transmission path. Hereinafter, the wavelength of the optical signal transmitted through the k-th system will be referred to as "λ ｋ The number of systems in the base station 4a is, for example, two.

[0033] Here, the signal processing unit 21 generates a downstream training signal (electrical signal) for each system of the optical transmission path. The training signal is used to estimate the transmission time of the optical signal in the optical transmission path. The training signals generated for each system may be the same signal or different signals. The signal processing unit 21 stores the transmission time of the downstream training signal as the measurement start time.

[0034] The converter 22-k converts the k-th training signal into a signal of wavelength "λ ｋ The optical circulator 23-k converts the wavelength of the optical signal into an optical signal of wavelength "λ ｋ ” to the optical multiplexer / demultiplexer 24 .

[0035] The optical multiplexer / demultiplexer 24 receives the wavelength "λ ｋ An optical signal of " is input from the optical circulator 23-k. The optical multiplexer / demultiplexer 24 multiplexes the input optical signals of multiple wavelengths. In other words, the optical multiplexer / demultiplexer 24 performs wavelength multiplexing on the optical signals of multiple wavelengths. The optical multiplexer / demultiplexer 24 transmits the wavelength-multiplexed optical signal to the optical transmission path 3. The optical transmission path 3 transmits the wavelength-multiplexed optical signal to each remote station 4a.

[0036] The base station 4a transmits a downstream radio signal to one or more subordinate radio terminals 5 based on the wavelength-multiplexed optical signal. Here, the optical multiplexer / demultiplexer 41 divides the wavelength-multiplexed optical signal into a signal of wavelength "λ ｋ That is, the optical multiplexer / demultiplexer 41 demultiplexes the optical signal of wavelength "λ ｋ The optical signal " is transmitted to the optical circulator 42-k.

[0037] The optical circulator 42-k is a light circulator for emitting light of wavelength "λ ｋ The converter 43-k outputs the optical signal of wavelength "λ ｋ The circulator 44-k converts the optical signal "" into an electrical signal. The circulator 44-k outputs the electrical signal to the antenna 45-k. The antenna 45-k transmits a wireless signal based on the electrical signal to the wireless terminal 5.

[0038] The wireless terminal 5 communicates with the central station 2a via the base station 4a and the optical transmission path 3. The wireless terminal 5 receives a wireless signal from the base station 4a using the antenna 51. The wireless terminal 5 converts the wireless signal into a training signal (electrical signal). The wireless terminal 5 performs a predetermined receiving process on the training signal.

[0039] Next, upstream transmission (transmission of upstream signals) will be described. When the wireless terminal 5 executes the reception process of the downstream training signal, it generates a response signal (upstream training signal) for each system of the optical transmission path. The response signals generated for each system may be the same signal or different signals. The wireless terminal 5 may include identification information of the wireless terminal 5 in each response signal. The antenna 51 of the wireless terminal 5 uses each wireless signal to simultaneously transmit the response signals for each system upstream to the base station 4a.

[0040] The base station 4a transmits upstream wavelength-multiplexed optical signals based on the response signals received for each system from the wireless terminals 5. Here, the antenna 45-k receives the response signals from the wireless terminals 5. In the first embodiment, the antennas 45-1 and 45-2 simultaneously receive the response signals from the wireless terminals 5. The antenna 45-k outputs an electrical signal based on the received response signals to the circulator 44-k.

[0041] The circulator 44-k outputs an electrical signal based on the received response signal to the converter 46-k. The converter 46-k outputs the electrical signal based on the received response signal at a wavelength "λ ｋ The optical circulator 42-k converts the wavelength of the optical signal into an optical signal of wavelength "λ ｋ ” to the optical multiplexer / demultiplexer 41.

[0042] The optical multiplexer / demultiplexer 41 receives a signal of wavelength "λ ｋ The optical signal of wavelength "λ" is input from the optical circulator 42-k. ｋ The optical multiplexer / demultiplexer 41 generates a wavelength-multiplexed optical signal (multiplexed optical signal) by multiplexing the optical signals " and ". The optical multiplexer / demultiplexer 41 transmits the wavelength-multiplexed optical signal to the optical transmission line 3.

[0043] The optical multiplexer / demultiplexer 24 demultiplexes the wavelength-multiplexed optical signal into optical signals of each wavelength. ｋ The converter 25-k outputs an optical signal of wavelength "λ ｋ The optical signal from the sensor is converted into a response signal (electrical signal).

[0044] The signal processing unit 21 stores the time when the response signal (uplink training signal) was received as the measurement end time. The signal processing unit 21 may perform predetermined signal processing on the received response signal. For example, the signal processing unit 21 stores in advance information associating the base stations 4a-n with the wireless terminals 5 subordinate to them. The signal processing unit 21 may identify the base station 4a-n that received the response signal based on the identification information of the wireless terminal 5 that transmitted the response signal uplink and the association between the base station 4a-n and the wireless terminal 5.

[0045] Next, a method for estimating the transmission time (transmission delay) of an optical signal will be described. The signal processing unit 21 measures the time length from the measurement start time to the measurement end time in the first system. That is, the signal processing unit 21 estimates the round-trip time "Δt １ The signal processing unit 21 measures the time length from the measurement start time to the measurement end time in the second system. That is, the signal processing unit 21 measures the round trip time "Δt ２ " is measured.

[0046] Here, the wireless terminal 5 simultaneously transmits response signals of each system (upstream training signals of each system) from the antenna 51 toward the antennas 45-1 and 45-2 of the base station 4a. For this reason, in the first embodiment, the round-trip time of the training signal in the first system, "Δt １ " and the round trip time of the training signal in the second system "Δt ２ The difference between " and " is equal to the difference between the transmission time of the upstream training signal in the first system and the transmission time of the upstream training signal in the second system.

[0047] More specifically, the upstream training signal received by the antenna 45-1 is transmitted through the first system and received by the signal processing unit 21. The upstream training signal received by the antenna 45-2 at the same time as the reception by the antenna 45-1 is transmitted through the second system and received by the signal processing unit 21.

[0048] Therefore, the round trip time of the first training signal "Δt １" and the round trip time of the second training signal "Δt ２ " and the difference "Δt １ -Δt ２ " is equal to the difference between the transmission time of the upstream training signal (upstream first training signal) in the first system of the optical transmission line 3 and the transmission time of the upstream training signal (upstream second training signal) in the second system of the optical transmission line 3. In this way, the difference in round-trip time is equal to the difference in upstream transmission time.

[0049] Round trip time "Δt １ " and round trip time "Δt ２ " and the difference "Δt １ -Δt ２ " is expressed as in equation (1).

[0050]

[0051] Here, "t ＷＴ " represents the length of time (processing time) from the time when the wireless terminal 5 generates a response signal to the time when the antenna 51 transmits a wireless signal based on the response signal.

[0052] "t １ａｒｅ " represents the spatial propagation time from the time when the antenna 51 transmits a radio signal based on the response signal to the time when the antenna 45-1 receives the radio signal. ２ａｒｅ " represents the spatial propagation time from the time when the antenna 51 transmits a radio signal based on the response signal to the time when the antenna 45-2 receives the radio signal. Here, when the distance between the antenna 45-1 and the antenna 45-2 is sufficiently short compared with the distance from the base station 4a to the wireless terminal 5, the spatial propagation time "t １ａｒｅ ” and the spatial propagation time “t ２ａｒｅ " are considered to be equivalent.

[0053] "t １ＲＲＵ " is the wavelength "λ １ The time length (processing time) until the time when the optical multiplexer / demultiplexer 41 transmits the wavelength-multiplexed optical signal including the optical signal "t ２ＲＲＵ " is the wavelength "λ ２The time length (processing time) until the time when the optical multiplexer / demultiplexer 41 transmits the wavelength-multiplexed optical signal including the optical signal "t" to the optical transmission line 3. Here, the signal processing in the first system and the signal processing in the second system are executed in parallel simultaneously, so the fluctuation of each processing time has almost no effect. For this reason, the processing time "t １ＲＲＵ " and processing time "t ２ＲＲＵ " are considered to be equivalent.

[0054] "t １ｏｐｔ " is the wavelength "λ " in the wavelength-multiplexed optical signal from the time when the optical multiplexer / demultiplexer 41 transmits the wavelength-multiplexed optical signal to the optical transmission line 3. １ The time length (wavelength "λ" in the optical transmission line 3) until the optical multiplexer / demultiplexer 20 receives the optical signal １ t １ｏｐｔ " represents the upstream transmission time of the training signal (first training signal) in the first system of the optical transmission line 3.

[0055] "t ２ｏｐｔ " is the wavelength "λ " in the wavelength-multiplexed optical signal from the time when the optical multiplexer / demultiplexer 41 transmits the wavelength-multiplexed optical signal to the optical transmission line 3. ２ The time length (wavelength "λ" in the optical transmission line 3) until the optical multiplexer / demultiplexer 20 receives the optical signal ２ t ２ｏｐｔ " represents the upstream transmission time of the training signal (second training signal) in the second system of the optical transmission line 3.

[0056] "t １ＣＳ " is the wavelength "λ １ represents the time length (processing time) from the time when the optical signal "t" is received by the optical multiplexer / demultiplexer 24 to the time when the signal processing unit 21 completes the reception processing of the response signal (upstream training signal). ２ＣＳ " is the wavelength "λ ２" represents the time length (processing time) from the time when the optical multiplexer / demultiplexer 24 receives the optical signal "t" to the time when the signal processing unit 21 completes the reception processing of the response signal (upstream training signal). Here, the signal processing in the first system and the signal processing in the second system are performed in the same manner, so fluctuations in the processing time have almost no effect. For this reason, the processing time "t １ＣＳ " and processing time "t ２ＣＳ " are considered to be equivalent.

[0057] Therefore, the round trip time "Δt １ " and round trip time "Δt ２ " and the difference "Δt １ -Δt ２ " is the difference in transmission time "t １ｏｐｔ -t ２ｏｐｔ " is equivalent to

[0058]

[0059] The transmission time of an optical signal in the optical transmission line 3 differs depending on the wavelength of the optical signal. The transmission speed (group velocity) of the optical signal for each wavelength is known.

[0060] The wavelength “λ” in the optical transmission line 3 １ The transmission time of the optical signal of " (the transmission time of the optical signal in the first system of the optical transmission line 3)" is expressed as in equation (3).

[0061]

[0062] Here, "L" represents the length of the optical transmission line 3. １ " is the wavelength "λ １ represents the group velocity of the optical signal.

[0063] The wavelength “λ” in the optical transmission line 3 ２ The transmission time of the optical signal of " (the transmission time of the optical signal in the second system of the optical transmission line 3)" is expressed as in equation (4).

[0064]

[0065] Here, "v ２ " is the wavelength "λ ２represents the group velocity of the optical signal. Therefore, the length "L" of the optical transmission line 3 is expressed as in equation (5).

[0066]

[0067] In this way, the signal processing unit 21 estimates the length "L" of the optical transmission line 3 based on the round-trip time of the training signal (a measurable value) and the group velocity (a known parameter).

[0068] The signal processing unit 21 calculates the length "L" of the optical transmission line 3 and the group velocity "v １ The transmission time (transmission delay) of the optical signal in the first system is estimated based on the wavelength "λ" in the optical transmission line 3. １ The transmission time of the optical signal (transmission time of the optical signal) is expressed as in equation (6).

[0069]

[0070] The signal processing unit 21 calculates the length "L" of the optical transmission line 3 and the group velocity "v ２ The transmission time (transmission delay) of the optical signal in the second system is estimated based on the wavelength "λ" in the optical transmission line 3. ２ " (transmission time of an optical signal) is expressed as in equation (7).

[0071]

[0072] As described above, the central station 2a transmits the first wavelength “λ １ The central station 2a transmits and receives the first training signal using an optical signal of the second wavelength "λ ２ The optical transmission line 3 transmits and receives the second training signal using an optical signal of the first wavelength "λ １ The optical transmission line 3 transmits the first training signal using an optical signal of the second wavelength "λ ２The base station 4a transmits a second training signal using an optical signal of "." The base station 4a transmits a first downlink radio signal to the wireless terminal 5 using the antenna 45-1 based on the first training signal transmitted from the optical transmission path 3. The base station 4a transmits a second downlink radio signal to the wireless terminal 5 using the antenna 45-2 based on the second training signal transmitted from the optical transmission path 3. The antenna 45-1 receives the first uplink radio signal from the wireless terminal 5. The antenna 45-2 receives the second uplink radio signal from the wireless terminal 5. The base station 4a simultaneously transmits the first training signal and the second training signal to the optical transmission path 3 based on the first uplink radio signal and the second uplink radio signal simultaneously received from the wireless terminal 5.

[0073] The central station 2a calculates the round-trip time "Δt １ ” and the round-trip time “Δt ２ " and the difference "Δt １ -Δt ２ ” and the group velocity of the optical signal of the first wavelength “v １ " and the second wavelength "λ ２ The group velocity of the optical signal "v ２ The central station 2a estimates the length "L" of the optical transmission line 3 based on the first wavelength "λ １ The group velocity of the optical signal "v １ " and the length "L" of the optical transmission line, the first wavelength "λ" in the optical transmission line 3 is calculated. １ The transmission time of the optical signal "t １ｏｐｔ The central station 2a estimates the second wavelength "λ ２ The group velocity of the optical signal "v ２ " and the length "L" of the optical transmission line 3, the second wavelength "λ ２ The transmission time of the optical signal " is estimated.

[0074] This makes it possible to estimate the transmission time of an optical signal on the optical transmission path 3 without complicating the configuration of the base station 4a and without sacrificing wavelength utilization efficiency.

[0075] Here, the wireless terminal 5 may include its own identification information in the first uplink wireless signal and the second uplink wireless signal. The central station 2a may identify the base station 4a-n that simultaneously transmitted the first training signal and the second training signal to the optical transmission path 3, based on the identification information included in the first uplink wireless signal and the second uplink wireless signal.

[0076] Second Embodiment The second embodiment is mainly different from the first embodiment in that the transmission time of an optical signal is estimated using a group delay time instead of a group velocity. The second embodiment will be described focusing on the differences from the first embodiment.

[0077] The transmission time of an optical signal may be estimated using group delay time instead of group velocity, which is the delay time of an optical signal per unit length of wavelength and is the reciprocal of group velocity (a known parameter).

[0078] The length "L" of the optical transmission line 3 is expressed as in equation (8).

[0079]

[0080] Here, "τ １ " is the wavelength "λ １ represents the group delay time of the optical signal. ２ " is the wavelength "λ ２ represents the group delay time of the optical signal of wavelength "λ" in the optical transmission line 3. １ The transmission time of the optical signal " is expressed as in equation (9).

[0081]

[0082] The wavelength “λ” in the optical transmission line 3 ２ The transmission time of the optical signal " is expressed as in equation (10).

[0083]

[0084] Furthermore, the signal processing unit 21 may use both the group velocity and the group delay time in the estimation process. For example, the signal processing unit 21 may use the group velocity to estimate the length "L" of the optical transmission line, and the group delay time to estimate the transmission time of the optical signal. For example, the signal processing unit 21 may use the group delay time to estimate the length "L" of the optical transmission line, and the group velocity to estimate the transmission time of the optical signal.

[0085] As described above, the transmission time of an optical signal is estimated using the group delay time instead of the group velocity. This makes it possible to estimate the transmission time of an optical signal on the optical transmission line 3 based on the group delay time without complicating the configuration of the base station 4a and without sacrificing wavelength utilization efficiency.

[0086] Third Embodiment The third embodiment differs from the first and second embodiments mainly in that the wireless terminal 5 includes a plurality of antennas 51. The third embodiment will be described focusing on the differences from the first and second embodiments.

[0087] The wireless terminal 5 may use a wireless signal to transmit a response signal upstream from one of the multiple antennas 51. The wireless terminal 5 may also use a wireless signal to transmit multiple response signals upstream simultaneously from the multiple antennas 51.

[0088] Furthermore, the wireless terminal 5 may perform synthesis or separation processing on a plurality of electrical signals based on a plurality of radio signals received by a plurality of antennas 51 .

[0089] (Fourth Embodiment) The fourth embodiment is mainly different from the first to third embodiments in that a predetermined type of antenna is used for transmitting and receiving a training signal. The fourth embodiment will be described mainly focusing on the differences from the first to third embodiments.

[0090] Antenna 45 is a predetermined type of antenna. Similarly, antenna 51 is a predetermined type of antenna. The type of antenna 45 and the type of antenna 51 may be the same or different. The predetermined type of antenna may be, for example, an antenna with a sub-array configuration or a dual-polarized antenna. The predetermined type of antenna may be, for example, an omni-antenna or an array antenna composed of multiple antenna elements.

[0091] If the antenna 45 is an array antenna, the base station 4a may perform beamforming, and if the antenna 51 is an array antenna, the wireless terminal 5 may perform beamforming.

[0092] Fifth Embodiment In the fifth embodiment, a different wavelength “λ” is used for each antenna element of an array antenna. ｋ The main difference between the fifth embodiment and the first to fourth embodiments is that " is assigned to the fifth embodiment. In the fifth embodiment, the differences from the first to fourth embodiments will be mainly described.

[0093] 2 is a diagram showing an example of the configuration of an RoF system 1b according to the fifth embodiment. The RoF system 1b (communication system) includes a central station 2b, an optical transmission line 3, and N base stations 4b. The base station 4b includes an optical multiplexer / demultiplexer 41, K optical circulators 42, K converters 43, K circulators 44, K converters 46, and an array antenna 47. The array antenna 47 is composed of K antennas 45.

[0094] Each antenna element (antenna 45) of the array antenna 47 has a different wavelength “λ ｋ The first antenna of the two antennas 45 selected from the K antennas 45 is assigned a wavelength "λ １ The second antenna of the two antennas 45 selected from the K antennas 45 transmits and receives a first system of radio signals (first training signals) based on the optical signal of wavelength "λ ２Based on the optical signal of ", a second system of radio signals (second training signals) is transmitted and received.

[0095] This makes it possible to estimate the transmission time of an optical signal on the optical transmission path 3 without unnecessarily complicating the configuration of the base station 4b having an array antenna and without sacrificing wavelength utilization efficiency.

[0096] Sixth Embodiment In the sixth embodiment, a different wavelength “λ 1 ” is outputted from each input / output port of a passive beam former. ｋ The sixth embodiment will be described mainly focusing on the differences from the first to fifth embodiments.

[0097] 3 is a diagram showing an example of the configuration of a RoF system 1c according to the sixth embodiment. The RoF system 1c (communication system) includes a central station 2c, an optical transmission line 3, and N base stations 4c. The base station 4c includes an optical multiplexer / demultiplexer 41, K optical circulators 42, K converters 43, K circulators 44, K converters 46, and a passive beamformer 48.

[0098] The passive beamformer 48 has K input / output ports. The passive beamformer 48 may include a beamforming circuit. Examples of the beamforming circuit include a Butler matrix, a Brass matrix, a Nolan matrix, and a Rotman lens. The passive beamformer 48 may include a reflector antenna such as a reflectarray. The passive beamformer 48 may include a lens antenna such as a transmit array.

[0099] The passive beam former 48 performs beam forming without using any active devices such as semiconductors, etc. Here, the passive beam former 48 controls the beam directions of the radio signals from the multiple antennas 45 by switching the input ports of the electrical signals.

[0100] This makes it possible to estimate the transmission time of an optical signal in the optical transmission path 3 without unnecessarily complicating the configuration of the base station 4c having a passive beamformer and without sacrificing wavelength utilization efficiency.

[0101] (Seventh Embodiment) In the seventh embodiment, the wireless terminal 5 does not perform signal processing on the training signals transmitted downlink in the first and second systems, but instead returns each training signal to the base station as a response signal, which is a main difference from the first to sixth embodiments. In the seventh embodiment, the differences from the first to sixth embodiments will be mainly described.

[0102] In the seventh embodiment, the base station includes two array antennas 47. The wireless terminal 5 includes two antennas (array antennas) 51. The two array antennas 47 face the two antennas 51 by analog beamforming.

[0103] In the seventh embodiment, the wireless terminal 5 assigns a wavelength "λ" to each wireless signal received by two antennas 51 (array antennas). １ ” optical signal (first system radio signal) and a wavelength “λ ２ The wireless terminal 5 performs a process of separating the optical signal of wavelength "λ" from the wireless signal (second system wireless signal) based on the optical signal of wavelength "λ" １ ” and radio signals based on optical signals of wavelength “λ ２ and a radio signal based on the optical signal of ", and transmitted to two array antennas 47 of the base station.

[0104] In the seventh embodiment, the difference in the round-trip time of the training signal “Δt １ -Δt ２ " is equal to the difference between the round-trip time (round-trip transmission time) of the optical signal of the first wavelength in the optical transmission line 3 and the round-trip time (round-trip transmission time) of the optical signal of the second wavelength in the optical transmission line 3. Therefore, the wavelength "λ １ The transmission time of the optical signal "t １ｏｐｔ " is expressed as equation (11) using the group velocity.

[0105]

[0106] The wavelength “λ” in the optical transmission line 3 ２ The transmission time of the optical signal "t ２ｏｐｔ " is expressed as equation (12) using the group velocity.

[0107]

[0108] The wavelength “λ” in the optical transmission line 3 １ The transmission time of the optical signal "t １ｏｐｔ " is expressed as equation (13) using the group delay time.

[0109]

[0110] The wavelength “λ” in the optical transmission line 3 ２ The transmission time of the optical signal "t ２ｏｐｔ " is expressed as equation (14) using the group delay time.

[0111]

[0112] This makes it possible to estimate the transmission time of an optical signal on the optical transmission path 3 without the wireless terminal 5 performing signal processing on the received training signal, without complicating the configuration of the base station, and without sacrificing wavelength utilization efficiency.

[0113] Eighth Embodiment The eighth embodiment is mainly different from the first to seventh embodiments in that the time at the central station and the time at the wireless terminal are synchronized. The eighth embodiment will be described focusing on the differences from the first to seventh embodiments.

[0114] 4 is a diagram showing an example of the configuration of an RoF system 1d according to the eighth embodiment. The RoF system 1d (communication system) includes a central station 2d, an optical transmission path 3, N base stations 4d, and a synchronizer 6.

[0115] The synchronizer 6 synchronizes the time of the signal processing unit 21 with the time of the wireless terminal 5 in advance.

[0116] The wireless terminal 5 receives a control signal instructing the start of measurement from the central station 2d via the optical transmission path 3 and the base station 4d. When the wireless terminal 5 receives the control signal instructing the start of measurement, it generates a response signal including information on the measurement start time (current time). The wireless terminal 5 wirelessly transmits the response signal including the information on the measurement start time to the base station 4d.

[0117] The signal processing unit 21 receives a response signal (upstream training signal) including information on the measurement start time from the wireless terminal 5 via the base station 4d and the optical transmission path 3. The signal processing unit 21 stores the time when the response signal including the information on the measurement start time was received as the measurement end time. The signal processing unit 21 calculates the time length (transmission time) "Δt" from the measurement start time to the measurement end time in the first system. １ " and the time length (transmission time) from the measurement start time to the measurement end time in the second system "Δt ２ The wireless terminal 5 may notify the signal processing unit 21 of the measurement start time using a signal other than the uplink training signal.

[0118] If the wireless terminal 5 can perform demultiplexing processing on multiple training signals based on each wireless signal received by the multiple antennas 51, the wireless terminal 5 may store the time at which the wireless terminal 5 received the training signal containing information about the measurement start time for each system as the measurement end time for each system. The wireless terminal 5 may notify the signal processing unit 21 of the stored measurement end time (the time at which the training signal was received). The signal processing unit 21 may estimate the transmission time of the optical signal in the optical transmission path 3 based on the measurement start time stored by the signal processing unit 21 and the measurement end time notified by the wireless terminal 5.

[0119] As described above, the central station 2d transmits a control signal instructing the start of measurement to the wireless terminal 5 via the optical transmission path 3 and the base station 4d. １ ” optical signal to receive the first training signal and the second training signal. ２The base station 4d receives the second training signal using the optical signal "." The optical transmission path 3 transmits the control signal, the first training signal, and the second training signal. The synchronizer 6 synchronizes the time of the central station 2d with the time of the wireless terminal 5. The wireless terminal 5 notifies the central station 2d of the measurement start time, which is the time of reception of the control signal instructing the wireless terminal 5 to start measurement. The wireless terminal 5 transmits the first uplink wireless signal and the second uplink wireless signal to the base station 4d. The base station 4d transmits the control signal to the wireless terminal 5. The base station 4d simultaneously transmits the first training signal and the second training signal to the optical transmission path 3 based on the first uplink wireless signal and the second uplink wireless signal simultaneously received from the wireless terminal 5.

[0120] The central station 2d determines the time length from the notified measurement start time to the time when the first training signal is received by the central station 2d (measurement end time) as the transmission time of the first training signal (=T ＷＴ +T １ａｒｅ +t １ＲＲＵ +t １ｏｐｔ +t １ＣＳ The central station 2d determines the time length from the notified measurement start time to the time when the second training signal is received by the central station (measurement end time) as the transmission time of the second training signal (=T ＷＴ +T ２ａｒｅ +t ２ＲＲＵ +t ２ｏｐｔ +t ２ＣＳ The central station 2d determines the difference between the transmission time of the first training signal and the transmission time of the second training signal as "Δt １ -Δt ２ " and the first wavelength "λ １ The group velocity of the optical signal "v １ " or group delay time "τ １ ” and the group velocity of the optical signal of the second wavelength “v ２ " or group delay time "τ ２ The central station 2d estimates the length of the optical transmission path based on the first wavelength "λ １ The group velocity of the optical signal "v １ " or group delay time "τ １ " and the length "L" of the optical transmission line 3, the first wavelength "λ １ The transmission time of the optical signal "t１ｏｐｔ The aggregation station 2d estimates the group velocity "v ２ " or group delay time "τ ２ " and the length "L" of the optical transmission line 3, the second wavelength "λ ２ The transmission time of the optical signal "t ２ｏｐｔ " is estimated.

[0121] This makes it possible to estimate the transmission time of an optical signal on the optical transmission path 3 without complicating the configuration of the base station 4d and without sacrificing wavelength utilization efficiency.

[0122] Ninth Embodiment The ninth embodiment is mainly different from the first embodiment in that the wavelength of the optical signal transmitted downstream is different from the wavelength of the optical signal transmitted upstream. The ninth embodiment will be described focusing on the differences from the first embodiment.

[0123] When the wavelength for downstream transmission differs from the wavelength for upstream transmission, the signal processing unit 21 estimates the length "L" of the optical transmission path 3 based on the group velocity or group delay time of the optical signal of the wavelength used to transmit the upstream training signal. The signal processing unit 21 estimates the transmission time of the optical signal in the optical transmission path 3 based on the group velocity or group delay time of the optical signal of the wavelength used to transmit the upstream training signal and the length "L" of the optical transmission path 3.

[0124] Furthermore, as in the seventh embodiment described above, in the case where the wireless terminal 5 does not perform signal processing on each training signal transmitted downstream in the first and second systems, but instead returns each training signal to the base station as a response signal, if the wavelength for downstream transmission and the wavelength for upstream transmission are different, the signal processing unit 21 may calculate the length "L" of the optical transmission path 3 as shown in equation (15).

[0125]

[0126] Here, "v １ＤＬ " represents the group velocity of the wavelength of the optical signal used to transmit the downstream signal of the first system. １ＵＬ " represents the group velocity of the wavelength of the optical signal used to transmit the upstream signal of the first system. ２ＤＬ" represents the group velocity of the wavelength of the optical signal used to transmit the downstream signal of the second system. ２ＵＬ " represents the group velocity of the wavelength of the optical signal used to transmit the upstream signal of the second system.

[0127] The signal processing unit 21 selects a first wavelength “λ 1 ” used for transmitting an upstream training signal. １ Based on the group velocity or group delay time of the optical signal "λ 1 " and the length "L" of the optical transmission line 3, the first wavelength "λ 1 " transmitted upstream in the optical transmission line 3 is １ The signal processing unit 21 estimates the transmission time of the optical signal using the first wavelength "λ" used for transmitting the downstream training signal. １ Based on the group velocity or group delay time of the optical signal "λ 1 " and the length "L" of the optical transmission line 3, the first wavelength "λ 1 " transmitted downstream in the optical transmission line 3 is calculated. １ " Estimate the transmission time of the optical signal.

[0128] The signal processing unit 21 selects a second wavelength "λ" used for transmitting an upstream training signal. ２ Based on the group velocity or group delay time of the optical signal "λ 1 " and the length "L" of the optical transmission line 3, the second wavelength "λ 1 " transmitted upstream in the optical transmission line 3 is ２ The signal processing unit 21 estimates the transmission time of the optical signal using the second wavelength "λ ２ Based on the group velocity or group delay time of the optical signal "λ 1 " and the length "L" of the optical transmission line 3, the second wavelength "λ 1 " transmitted downstream in the optical transmission line 3 is calculated. ２ The transmission time of the optical signal " is estimated.

[0129] (Tenth embodiment) In the tenth embodiment, the main difference from the first to ninth embodiments is that the frequency and polarization of the radio signal (radio signal) transmitted downstream are different from the frequency and polarization of the radio signal transmitted upstream.

[0130] In the transmission of a radio signal, the propagation speed of the radio signal remains the same even if the frequency and polarization of the radio signal are different, so each of the above formulas (1) to (15) can be applied to estimating the transmission time of an optical signal.

[0131] Eleventh Embodiment The network configuration of the RoF system may be a single star network configuration. The signal processing unit 21 may identify the base station that has received the response signal based on the identification information of the optical transmission line 3 connected to the base station.

[0132] Twelfth Embodiment In the first to eleventh embodiments, electrical-to-optical conversion and optical-to-electrical conversion are performed on radio frequency (RF) signals in the RoF system.

[0133] The twelfth embodiment is mainly different from the first to eleventh embodiments in that electrical-to-optical conversion and optical-to-electrical conversion are performed on a predetermined signal (e.g., an intermediate frequency signal). The twelfth embodiment will be described mainly focusing on the differences from the first embodiment.

[0134] In a system with an intermediate frequency over fiber (IFoF) configuration, electrical-to-optical conversion and optical-to-electrical conversion may be performed on an intermediate frequency signal converted from a high frequency signal.

[0135] In a baseband over fiber (BBoF) system, electrical-to-optical conversion and optical-to-electrical conversion are performed on a baseband signal, and after the optical-to-electrical conversion, the electrical signal may be frequency-converted to a high-frequency signal.

[0136] In the twelfth embodiment, the wavelength of the optical signal transmitted downstream may be different from the wavelength of the optical signal transmitted upstream.

[0137] 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.

[0138] The present invention is applicable to communication systems.

[0139] DESCRIPTION OF THE REFERENCE SIGNS 1a, 1b, 1c, 1d...RoF system, 2a, 2b, 2c, 2d...central station, 3...optical transmission path, 4a, 4b, 4c, 4d...base station, 5...wireless terminal, 6...synchronization device, 21...signal processing unit, 22...converter, 23...optical circulator, 24...optical multiplexer / demultiplexer, 41...optical multiplexer / demultiplexer, 42...optical circulator, 43...converter, 44...circulator, 45...antenna, 46...converter, 47...array antenna, 48...passive beam former, 51...antenna

Claims

1. A central station that transmits and receives a first training signal by using an optical signal of a first wavelength, and transmits and receives a second training signal by using an optical signal of a second wavelength; an optical transmission line that transmits the first training signal and the second training signal; and a base station that transmits a first downstream radio signal to a wireless terminal based on the first training signal transmitted from the optical transmission line, transmits a second downstream radio signal to the wireless terminal based on the second training signal transmitted from the optical transmission line, and simultaneously transmits the first training signal and the second training signal to the optical transmission line based on the first upstream radio signal and the second upstream radio signal simultaneously received from the wireless terminal, the central station estimates a length of the optical transmission path based on a difference between a round-trip time of the first training signal traveling back and forth between itself and the wireless terminal and a round-trip time of the second training signal traveling back and forth between itself and the wireless terminal, a group velocity or a group delay time of the optical signal of the first wavelength, and a group velocity or a group delay time of the optical signal of the second wavelength, estimates a transmission time of the optical signal of the first wavelength in the optical transmission path based on the group velocity or the group delay time of the optical signal of the first wavelength and the length of the optical transmission path, and estimates the transmission time of the optical signal of the second wavelength in the optical transmission path based on the group velocity or the group delay time of the optical signal of the second wavelength and the length of the optical transmission path.

2. A central station that transmits a control signal instructing a measurement start, receives a first training signal by using an optical signal of a first wavelength, and receives a second training signal by using an optical signal of a second wavelength; an optical transmission path that transmits the control signal, the first training signal, and the second training signal; a synchronization device that synchronizes a time of the central station with a time of a wireless terminal; and a base station that transmits the control signal to the wireless terminal and simultaneously transmits the first training signal and the second training signal to the optical transmission path based on a first uplink wireless signal and a second uplink wireless signal simultaneously received from the wireless terminal, wherein the wireless terminal notifies the central station of a measurement start time, which is the time when the control signal is received at the wireless terminal, and transmits the first uplink wireless signal and the second uplink wireless signal to the base station; the central station determines a time length from the measurement start time to a time when the first training signal is received by the central station as a transmission time of the first training signal, determines a time length from the measurement start time to a time when the second training signal is received by the central station as a transmission time of the second training signal, estimates a length of the optical transmission path based on a difference between the transmission time of the first training signal and the transmission time of the second training signal, a group velocity or a group delay time of the optical signal of the first wavelength, and a group velocity or a group delay time of the optical signal of the second wavelength, estimates a transmission time of the optical signal of the first wavelength in the optical transmission path based on the group velocity or the group delay time of the optical signal of the first wavelength and the length of the optical transmission path, and estimates a transmission time of the optical signal of the second wavelength in the optical transmission path based on the group velocity or the group delay time of the optical signal of the second wavelength and the length of the optical transmission path.

3. A communication system as described in claim 1 or claim 2, wherein the radio terminal includes identification information of the radio terminal itself in the first uplink radio signal and the second uplink radio signal, and the central station identifies the base station that simultaneously transmitted the first training signal and the second training signal to the optical transmission path based on the identification information included in the first uplink radio signal and the second uplink radio signal.

4. A communications system as described in claim 1 or claim 2, wherein, when the first wavelength in downstream transmission is different from the first wavelength in upstream transmission and the second wavelength in downstream transmission is different from the second wavelength in upstream transmission, the central station estimates the length of the optical transmission path based on the group velocity or group delay time of the optical signal of the first wavelength in upstream transmission, the group velocity or group delay time of the optical signal of the second wavelength in upstream transmission, and the difference.

5. A communications system as described in claim 1 or claim 2, wherein, when the first wavelength in downstream transmission is different from the first wavelength in upstream transmission and the second wavelength in downstream transmission is different from the second wavelength in upstream transmission, the central station estimates the length of the optical transmission path based on the difference between the group velocity of the optical signal of the first wavelength in downstream transmission, the group velocity of the optical signal of the first wavelength in upstream transmission, the group velocity of the optical signal of the second wavelength in downstream transmission, and the group velocity of the optical signal of the second wavelength in upstream transmission.

6. The communications system of claim 5, wherein the aggregation station estimates a transmission time of the optical signal of the first wavelength transmitted in the upstream direction through the optical transmission path based on a group velocity or group delay time of the optical signal of the first wavelength in the upstream transmission and the length of the optical transmission path, estimates a transmission time of the optical signal of the first wavelength transmitted in the downstream direction through the optical transmission path based on a group velocity or group delay time of the optical signal of the first wavelength in the downstream transmission and the length of the optical transmission path, estimates a transmission time of the optical signal of the second wavelength transmitted in the upstream direction through the optical transmission path based on a group velocity or group delay time of the optical signal of the second wavelength in the upstream transmission and the length of the optical transmission path, and estimates a transmission time of the optical signal of the second wavelength transmitted in the downstream direction through the optical transmission path based on a group velocity or group delay time of the optical signal of the second wavelength in the downstream transmission and the length of the optical transmission path.

7. A time estimation method executed by a communication system, comprising the steps of: transmitting and receiving a first training signal using an optical signal of a first wavelength, and transmitting and receiving a second training signal using an optical signal of a second wavelength; transmitting the first training signal and the second training signal; transmitting a first downlink radio signal to a radio terminal based on the first training signal transmitted from an optical transmission path, transmitting a second downlink radio signal to a radio terminal based on the second training signal transmitted from the optical transmission path, and simultaneously transmitting the first training signal and the second training signal to the optical transmission path based on the first uplink radio signal and the second uplink radio signal simultaneously received from the radio terminal, a group velocity or group delay time of the optical signal of the first wavelength, and a group velocity or group delay time of the optical signal of the second wavelength; estimating a transmission time of the optical signal of the first wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the first wavelength and the length of the optical transmission path; and estimating the transmission time of the optical signal of the second wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the second wavelength and the length of the optical transmission path. A time estimation method.

8. A time estimation method executed by a communication system, comprising the steps of: transmitting a control signal instructing a measurement start, receiving a first training signal using an optical signal of a first wavelength, and receiving a second training signal using an optical signal of a second wavelength; transmitting the control signal, the first training signal, and the second training signal; synchronizing a time of a central station with a time of a wireless terminal; transmitting the control signal to the wireless terminal, and simultaneously transmitting the first training signal and the second training signal to an optical transmission path based on a first uplink wireless signal and a second uplink wireless signal simultaneously received from the wireless terminal, wherein the transmitting step includes the wireless terminal notifying the central station of a measurement start time, which is the time when the control signal was received at the wireless terminal, and transmitting the first uplink wireless signal and the second uplink wireless signal to a base station, a group velocity or group delay time of the optical signal of the first wavelength and the group velocity or group delay time of the optical signal of the second wavelength; estimating a length of the optical transmission path based on a difference between the transmission time of the first training signal and the transmission time of the second training signal, a group velocity or group delay time of the optical signal of the first wavelength, and a group velocity or group delay time of the optical signal of the second wavelength; estimating a transmission time of the optical signal of the first wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the first wavelength and the length of the optical transmission path; and estimating the transmission time of the optical signal of the second wavelength in the optical transmission path based on the group velocity or group delay time of the optical signal of the second wavelength and the length of the optical transmission path.

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