Wireless communication method, central station, and wireless communication system

Abstract

This wireless communication method is performed by a wireless communication system comprising: a central station; and one or more extension stations that are connected via an optical transmission path to the central station and that comprise a plurality of antenna elements. The wireless communication method comprises: causing a combination of a portion of the antenna elements of the plurality of antenna elements provided to the one or more extension stations to perform beam scanning; acquiring phase information which indicates a phase difference between the portion of the antenna elements and which is obtained by the beam scanning; estimating a phase difference between wavelengths allocated to the antenna elements on the basis of the phase difference between the portion of the antenna elements, the phase difference being indicated in the phase information; and performing remote beamforming on the basis of the estimated phase difference.　

Description

Wireless communication method, central station, and wireless communication system

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

[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] When using high-frequency bands for wireless communication, beamforming (BF) is required to ensure link budgets. Analog RoF systems require remote beamforming, in which the beamforming of base stations without signal processing functions is remotely controlled from a central station. A typical remote beamforming method for analog RoF systems involves switching the wavelength used for signal transmission depending on the beam (see, for example, Non-Patent Document 2). This method requires information on the optical fiber length and dispersion for beam control. However, it is difficult to obtain accurate information on the length and dispersion of installed optical fibers, making it difficult to apply the remote beamforming method described in Non-Patent Document 2 to the intended analog RoF system.

[0004] Therefore, a fixed wavelength beamforming method that does not require information on the length or dispersion of optical fiber has been proposed as a remote beamforming method for high-frequency band analog RoF systems (see, for example, Non-Patent Document 3). However, the remote beamforming method described in Non-Patent Document 3 has a problem in that when the number of elements in the array antenna increases and the bandwidth of the wavelength used becomes wider, the beamforming characteristics deteriorate and the desired beamforming gain cannot be obtained. Therefore, a pre-beamforming control method has been proposed that reduces the characteristic degradation of the fixed wavelength beamforming method in large-scale array antennas (see, for example, Patent Document 1).

[0005] International Publication No. 2022 / 172413

[0006] K. Ito, M. Suga, Y. Shirato, N. Kita, and T. Onizawa, “Efficiently Accommodating High-frequency-band Wireless Systems by Using Analog Radio-over-fiber”, NTT Technical Review, vol. 18, no. 5, pp. 19-23, May 2020.Shigeyuki Akiba1 , Masayuki Oishi1 , Yoshihiro Nishikawa2 , Kyo Minoguchi2 , Jiro Hirokawa and Makoto Ando, “Photonic Architecture for Beam Forming of RF Phased Array Antenna”, OFC2014, Mar. 2014.K. Ito, M. Suga, Y. Shirato, N. Kita, and T. Onizawa, “Remote Beamforming Scheme with Fixed Wavelength Allocation for Radio-Over-Fiber Systems Employing Single-Mode Fiber”, Journal of Lightwave Technology, vol. 40, no. 4, pp. 997-1006, Feb. 2022.

[0007] The control method described in Patent Document 1 is a method of performing a fixed-wavelength beamforming technique in advance using only a small number of antenna elements that cause little degradation in beamforming characteristics. In the control method described in Patent Document 1, in order to suppress beam degradation that occurs when performing remote beamforming in a large-scale array antenna, a radio base station first performs remote beamforming using a combination of some antenna elements and executes beam scanning. This operation is called pre-beamforming. The radio base station forms an optimal beam for a terminal device based on feedback on communication quality received from the terminal device with which it is communicating. Next, the radio base station selects a different combination of antenna elements and repeatedly performs the same operation.

[0008] The radio base station performs the above-described pre-beamforming operation for all combinations of antenna elements to obtain phase differences between the antenna elements, and calculates the phase of each antenna element from a reference antenna using the obtained phase differences. The radio base station obtains phases for controlling beamforming based on the calculated phases from the reference antenna, and performs remote beamforming using all antenna elements. While the control method described in Patent Document 1 improves beamforming characteristics, it requires pre-beam scanning for all combinations of antenna elements, which results in an increased overhead.

[0009] In view of the above circumstances, an object of the present invention is to provide a technique that can reduce overhead while suppressing beam degradation when remote beamforming is applied to a large-scale array antenna.

[0010] One aspect of the present invention is a wireless communication method performed by a wireless communication system including a central station and one or more remote stations connected to the central station via an optical transmission path and equipped with a plurality of antenna elements, the wireless communication method performing beam scanning on a combination of some of the multiple antenna elements equipped in the one or more remote stations, obtaining phase information indicating the phase difference between the some of the antenna elements obtained by the beam scanning, estimating the phase difference between the wavelengths assigned to each antenna element based on the phase difference between the some of the antenna elements indicated by the phase information, and performing remote beamforming based on the estimated phase difference.

[0011] One aspect of the present invention is an aggregate station having a signal processing function among the signal processing functions and communication functions of a wireless communication device, and comprising: a beamforming control unit that performs beam scanning using a combination of some of the antenna elements among a plurality of antenna elements provided in one or more base stations; a phase information acquisition unit that acquires phase information indicating the phase difference between the some of the antenna elements obtained by the beam scanning; and a phase estimation unit that estimates the phase difference between the wavelengths assigned to each antenna element based on the phase difference between the some of the antenna elements indicated by the phase information acquired by the phase information acquisition unit, wherein the beamforming control unit performs remote beamforming based on the phase difference estimated by the phase estimation unit.

[0012] One aspect of the present invention is a wireless communication system comprising a central station and one or more base stations connected to the central station via an optical transmission path, wherein the central station comprises: a beamforming control unit that performs beam scanning using a combination of some of the antenna elements among a plurality of antenna elements provided in the one or more base stations; a phase information acquisition unit that acquires phase information indicating the phase difference between the some of the antenna elements obtained by the beam scanning; and a phase estimation unit that estimates the phase difference between the wavelengths assigned to each antenna element based on the phase difference between the some of the antenna elements indicated by the phase information acquired by the phase information acquisition unit, wherein the beamforming control unit performs remote beamforming based on the phase difference estimated by the phase estimation unit, and the one or more base stations comprise: a plurality of antenna elements; and a transceiver unit that converts a plurality of optical signals of different wavelengths transmitted from the central station into electrical signals and outputs them to corresponding antenna elements, or converts an electrical signal output from any of the plurality of antenna elements into an optical signal and outputs it to the central station.

[0013] According to the present invention, when remote beamforming is applied to a large-scale array antenna, it is possible to reduce overhead while suppressing beam degradation.

[0014] FIG. 1 is a diagram for explaining an overview of the present invention. FIG. 2 is a diagram showing an example of the configuration of a wireless communication system in an embodiment. FIG. 3 is a diagram showing specific configurations of the RoF unit of a central station and the RoF unit of a base station in an embodiment. FIG. 4 is a diagram showing an example of an antenna element set used for pre-beamforming in an embodiment. FIG. 5 is a diagram showing an example of an antenna element set used for pre-beamforming in an embodiment. FIG. 6 is a sequence diagram showing the flow of processing performed by a wireless communication system in an embodiment. FIG. 7 is a flowchart showing the flow of a calculation process of a phase adjustment amount performed by a central station in an embodiment.

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0016] (Overview) Before describing the details of the present invention, an overview of the invention will be described using Fig. 1. Fig. 1 is a diagram for explaining the overview of the present invention. In a wireless communication system in a configuration in which wireless base stations are separated into a central station and a base station, the objective is to reduce the overhead of pre-beamforming, which is performed by the central station to acquire phase information used to perform remote beamforming on the base station. The phase information is, for example, the phase difference between adjacent antenna elements.

[0017] In this way, in the wireless communication system of the present invention, as shown in FIG. 1A, the phase difference between wavelengths corresponding to all antenna elements (curve L1 in FIG. 1A) is estimated. Note that i (i is an integer between 0 and n-1) shown in FIG. 1 is identification information for identifying the antenna element. Therefore, λ ｉ represents the wavelength assigned to the i-th antenna element, and n is the number of antenna elements and is an integer of 2 or greater.

[0018] In the present invention, in order to estimate the phase difference between wavelengths corresponding to all antenna elements, first, as shown in FIG. 1B, a pre-beamforming process is performed using a combination of some antenna elements (hereinafter referred to as an “antenna element set”) to estimate the phase difference between adjacent antenna elements (for example, λ ｉ and λ ｉ＋１ The phase difference (circle C1 in FIG. 1B) between the two signals is obtained.

[0019] In the present invention, the phase differences within the wavelength bands corresponding to all antenna elements are estimated from the phase differences between some adjacent antenna elements obtained by pre-beamforming. In this case, as shown in Fig. 1C, the phase differences within the wavelength bands corresponding to all antenna elements (square S1 in Fig. 1C) are estimated by interpolation, or as shown in Fig. 1D, the phase differences within the wavelength bands corresponding to all antenna elements (curve L2 in Fig. 1D) are estimated by approximation.

[0020] In the method of Patent Document 1, combinations of antenna elements to be used for pre-beamforming are set so as to include all antenna elements equipped at the base station, pre-beamforming is performed for all combinations, and the amount of phase adjustment required for beamforming using all antenna elements is determined based on the phase information obtained by pre-beamforming.

[0021] In contrast, in the present invention, as described in Fig. 1, the phase difference between wavelengths assigned to each antenna element is estimated by interpolation or approximation based on the phase difference between adjacent antenna elements obtained by some of the antenna element pairs of the pre-beamforming antenna element pairs shown in Patent Document 1. For example, in the present invention, the phase difference is estimated by interpolation or approximation by performing pre-beamforming only on two or more arbitrary antenna element pairs out of the antenna element pairs used for pre-beamforming. Then, in the present invention, the phase adjustment amount for each antenna element is estimated based on the acquired phase information.

[0022] This reduces the overhead due to beam scanning. For example, when pre-beamforming is performed using four elements on an 8x8 element planar array, there are 27 combinations of antenna elements that include all of the antenna elements. On the other hand, by performing pre-beamforming on, for example, only eight of the 27 combinations according to the present invention and using estimated values ​​for the phase adjustment amounts for the remaining antenna elements, the overhead due to beam scanning can be reduced by approximately 70%. A specific configuration for realizing the above processing will be described below.

[0023] (Details) Fig. 2 is a diagram showing an example of the configuration of a wireless communication system 100 according to an embodiment. The wireless communication system 100 includes a central station 10 and L (L is an integer equal to or greater than 1) base stations 20. The central station 10 and the L base stations 20 are connected via an optical transmission path 30. The optical transmission path 30 is an optical fiber. The central station 10 and the L base stations 20 form one base station. Specifically, the central station 10 has a signal processing function that is one of the separate functions provided in a base station, namely, a signal processing function and a communication function. One or more base stations 20 have a communication function that is one of the separate functions provided in a base station, namely, a signal processing function and a communication function.

[0024] Between the central station 10 and the base station 20, transmission is performed using analog RoF, which transmits radio signals using optical fiber.

[0025] Next, we will explain the specific configuration of the central station 10 and the base station 20. First, we will explain the specific configuration of the central station 10. The central station 10 includes a BF control unit 11, an RF unit 12, an RoF unit 13, a phase information acquisition unit 14, and a phase estimation unit 15.

[0026] The BF control unit 11 controls beamforming of each base station 20. To control beamforming, the BF control unit 11 performs beam scanning on some sets of antenna elements among a plurality of antenna elements included in one or more base stations 20. For example, the BF control unit 11 performs pre-beamforming.

[0027] The RF unit 12 generates a training signal for beam scanning under the control of the BF control unit 11. Furthermore, the RF unit 12 modulates the generated training signal or demodulates a signal received by the RoF unit 13.

[0028] The RoF unit 13 performs phase control of signals corresponding to each antenna element, for example, under the control of the BF control unit 11. The RoF unit 13 converts downstream signals from electrical signals to optical signals (hereinafter referred to as "electrical-to-optical conversion"), converts upstream signals from optical signals to electrical signals (hereinafter referred to as "optical-to-electrical conversion"), and performs multiplexing of downstream signals and demultiplexing of upstream signals.

[0029] The phase information acquisition unit 14 acquires phase information obtained by the beam manipulation executed by the BF control unit 11. Specifically, the phase information acquisition unit 14 acquires phase information indicating the phase difference between some antenna elements obtained by pre-beamforming.

[0030] The phase estimation unit 15 estimates the amount of phase adjustment between each antenna element based on the phase information acquired by the phase information acquisition unit 14. Specifically, the phase estimation unit 15 estimates the phase difference between the wavelengths assigned to each antenna element based on the phase difference between some of the antenna elements indicated by the phase information acquired by the phase information acquisition unit 14, and estimates the amount of phase adjustment between each antenna element based on the estimated phase difference.

[0031] Next, a description will be given of the configuration of the base station 20. Since the base stations 20-1 to 20-L have the same configuration, they will be described as the base station 20. The base station 20 includes an RoF unit 21 and an antenna 22.

[0032] The RoF unit 21 performs, for example, demultiplexing of downstream signals, multiplexing of upstream signals, optical-to-electrical conversion of downstream signals corresponding to each antenna element, and electrical-to-optical conversion of upstream signals.

[0033] The antenna 22 includes a plurality of antenna elements 23-0 to 23-(n-1). The antenna elements 23-0 to 23-(n-1) emit, as radio waves, electrical signals (e.g., training signals) output from the RoF unit 21. The antenna elements 23-0 to 23-(n-1) receive radio waves transmitted from a terminal device of a communication partner.

[0034] FIG. 3 is a diagram showing the specific configuration of the RoF unit 13 of the central station 10 and the RoF unit 21 of the base station 20 in this embodiment. As shown in FIG. 3, the RoF unit 13 of the central station 10 includes a phase shifter 131 and an optical multiplexing / demultiplexing unit 132. Although not shown in FIG. 3, the RoF unit 13 includes a converter on the path between the RF unit 12 and the phase shifter 131. The converter converts the electrical signal output from the RF unit 12 into an optical signal. The phase shifter 131 changes the phase of the signal. The phase shifter 131 in FIG. 3 changes the phase of the optical signal. Note that the signal whose phase is changed by the phase shifter 131 and the amount of phase change are controlled by the BF control unit 11. The optical multiplexing / demultiplexing unit 132 multiplexes or demultiplexes the input optical signal. The optical multiplexing / demultiplexing unit 132 multiplexes each optical signal whose phase has been changed by, for example, the phase shifter 131, and outputs the multiplexed signal to the optical transmission path. The optical multiplexer / demultiplexer 132 demultiplexes an optical signal input via an optical transmission line according to wavelength and outputs the demultiplexed signal.

[0035] 3, the RoF unit 21 of the base station 20 includes an optical multiplexing / demultiplexing unit 211 and a plurality of converters 212. The optical multiplexing / demultiplexing unit 211 multiplexes or demultiplexes input optical signals. For example, the optical multiplexing / demultiplexing unit 211 demultiplexes optical signals input via an optical transmission path according to wavelength and outputs the demultiplexed signals to the corresponding converters 212. For example, the optical multiplexing / demultiplexing unit 211 multiplexes the optical signals output from the converters 212 and outputs the multiplexed signals to the optical transmission path.

[0036] The converter 212 converts an input optical signal into an electrical signal, or converts an input electrical signal into an optical signal. The converter 212 converts an optical signal output from the optical multiplexing / demultiplexing unit 211 into an electrical signal and outputs it to the antenna 22. The converter 212 converts an electrical signal output from the antenna 22 into an optical signal and outputs it to the optical multiplexing / demultiplexing unit 211.

[0037] 4 and 5 are diagrams showing an example of an antenna element set used for pre-beamforming in the embodiment. Note that the antenna element sets shown in FIGS. 4 and 5 are merely examples, and other combinations may be used. ｉ is, for example, the antenna element 23-i provided in the base station 20 in FIG.

[0038] 4 shows a configuration in which the linear arrays in the first and last rows in the y-axis direction are selected as an antenna element set. ０ and λ １ and λ ２ and λ ３ and the first antenna element set, λ ２ and λ ３ and λ ４ and λ ５ the second antenna element set, λ ４ and λ ５ and λ ６ and λ ７ and the third antenna element set, λ ５６ and λ ５７ and λ ５８ and λ ５９ the fourth antenna element set, λ ５８ and λ ５９ and λ ６０ and λ ６１ and the fifth antenna element set, λ ６０ and λ ６１ and λ ６２ and λ ６３ The six antenna element pairs are selected as antenna element pairs to be used for pre-beamforming, with the sixth antenna element pair being selected as the sixth antenna element pair. Then, the central station 10 calculates the optimum phase difference α ｑ The phase difference between the antenna elements 23-0 to 23-(n-1) is estimated by interpolation or approximation based on the above.

[0039] 5 shows a configuration in which the first pair of antenna elements in each row in the y-axis direction is selected as the antenna element pair. ０ and λ １ and λ ２ and λ ３ and the first antenna element set, λ ８ and λ ９ and λ １０ and λ １１ the second antenna element set, ..., λ ５６ and λ ５７ and λ ５８ and λ ５９The eight antenna element pairs are selected as antenna element pairs to be used for pre-beamforming, with the eight antenna element pairs being the eighth antenna element pair. Then, the central station 10 calculates the optimum phase difference α ｑ The phase difference between the antenna elements 23-0 to 23-(n-1) is estimated by interpolation or approximation based on the above.

[0040] 6 is a sequence diagram showing the flow of processing performed by the wireless communication system 100 according to the embodiment. In FIG. 6, the flow of processing in which the central station 10 controls the base station 20 to perform pre-beamforming will be described.

[0041] The BF control unit 11 selects a pair of antenna elements to be used for pre-beamforming (step S101). For example, the BF control unit 11 selects one pair of antenna elements from among a plurality of pairs of antenna elements determined by the method shown in Fig. 4 or 5. The BF control unit 11 then outputs information for identifying the selected pair of antenna elements to the RoF unit 13. Furthermore, the BF control unit 11 outputs an instruction to the RF unit 12 to execute pre-beamforming.

[0042] The RF unit 12 generates a training signal for beam scanning in response to an instruction from the BF control unit 11 (step S102). The RF unit 12 outputs the generated training signal to the RoF unit 13. The RoF unit 13 converts the training signal output from the RF unit 12 into an optical signal using a converter. Specifically, the RoF unit 13 converts the training signal output from the RF unit 12 into an optical signal corresponding to the antenna elements 23-0 to 23-(n-1) used for pre-beamforming using a converter (step S103). The optical signal converted by the converter is output to the phase shifter 131.

[0043] The phase shifter 131 of the RoF unit 13 adds an equal-interval phase difference (additional phase interval) to the optical signal corresponding to each of the multiple antenna elements 23 used for pre-beamforming in accordance with an instruction from the BF control unit 11 (step S104). In this way, the phase shifter 131 controls the phase of the optical signal corresponding to the antenna elements 23 used for pre-beamforming. The optical signals to which the equal-interval phase difference has been added by the phase shifter 131 are multiplexed in the optical multiplexing / demultiplexing unit 132 (step S105). In this way, a multiplexed signal is generated. The multiplexed signal generated by the optical multiplexing / demultiplexing unit 132 is transmitted to the base station 20 via an optical transmission path.

[0044] The multiplexed signal transmitted through the optical transmission path is input to the RoF unit 21 of the base station 20. The multiplexed signal input to the RoF unit 21 is demultiplexed by the optical multiplexing / demultiplexing unit 211 (step S106). The optical signals demultiplexed by the optical multiplexing / demultiplexing unit 211 are converted into electrical signals by the converter 212 (step S107). The electrical signals converted by the converter 212 are radiated as radio waves by the antenna elements 23 (step S108). Because the signals input to each antenna element 23 have approximately equal phase differences, an analog beam is formed when radiated from the antenna elements 23. The radio waves (training signals) radiated from the antenna elements 23 are received by the communication partner (e.g., a wireless communication terminal).

[0045] In pre-beamforming, training signals are repeatedly transmitted while changing the interval of the phase added by the RoF unit 13 in accordance with instructions from the BF control unit 11. That is, the central station 10 executes the processes of steps S104 and S105 while changing the interval of the phase added by the RoF unit 13 in accordance with instructions from the BF control unit 11. After completing the transmission of training signals for all beam patterns, the central station 10 determines the optimal phase interval for the set of antenna elements used for pre-beamforming based on feedback from the communication partner.

[0046] 7 is a flowchart showing the flow of a calculation process of the phase adjustment amount performed by the central station 10 in the embodiment. In FIG. 7, a method for estimating the phase adjustment amount of all antenna elements 23 from the pre-beamforming results of several antenna element sets when the 0th antenna element 23-0 is used as a reference is described.

[0047] The central station 10 performs pre-beamforming for an arbitrary antenna element set q. The phase information acquisition unit 14 calculates the optimal phase interval α for the arbitrary antenna element set q based on information fed back from the communication partner. ｑ (Step S201). The phase information acquisition unit 14 acquires the optimum phase interval α ｑ The phase estimator 15 outputs information indicating the above.

[0048] The optimal phase interval obtained by the phase information acquisition unit 14 is an approximate average value of the phase difference between adjacent elements of the antenna elements 23 used for pre-beamforming. In other words, it is a value closest to the phase difference between elements to which wavelengths near the center are assigned among the antenna elements 23 used.

[0049] If the number of antenna elements 23 used for pre-beamforming is m, the antenna elements 23 used for pre-beamforming are the i-th to i+m-1-th elements. When pre-beamforming is performed using the i-th antenna element 23-i as a reference, the obtained optimum phase interval is the value closest to the optimum phase difference between the i+m / 2-1-th and i+m / 2-th antenna elements. Therefore, the phase difference between the wavelengths assigned to these two antenna elements is α ｑ Let's say.

[0050] The phase estimation unit 15 determines whether or not information on the optimal phase interval for a predetermined number of antenna element pairs has been acquired based on the information indicating the optimal phase interval output from the phase information acquisition unit 14 (step S202). If the phase estimation unit 15 determines that information on the optimal phase interval for a predetermined number of antenna element pairs has not been acquired (step S202-NO), the phase estimation unit 15 selects another antenna element pair. Then, the process of step S201 is executed.

[0051] On the other hand, if the phase estimator 15 determines that it has acquired information on the optimal phase intervals for the predetermined number of antenna element pairs (step S202-YES), the phase estimator 15 estimates the phase differences between wavelengths based on the acquired information indicating the optimal phase intervals (step S204).The phase estimator 15 estimates the phase differences α between the wavelengths assigned to all antenna elements 23 based on the acquired relationship between the wavelengths and the phase differences. ｊ (j=1, 2, . . . , n−1). The phase estimator 15 estimates the phase difference α ｊ When estimating, an interpolation or approximation method is used.

[0052] As the interpolation method, any of linear interpolation, cubic spline interpolation, Lagrange interpolation, and Radial Basis Function (RBF) interpolation may be used. As the approximation method, any of the least squares method and fitting may be used. Note that the phase estimation unit 15 may perform the interpolation or approximation using machine learning or deep learning. The phase estimation unit 15 calculates the estimated phase difference α between all the antenna elements 23. ｊ The phase adjustment amount ^α in the i-th antenna element 23-i from ｉ (^ is a superscript of α) is calculated based on the following formula (1) (step S205).

[0053]

[0054] The phase estimation unit 15 calculates the phase adjustment amount ^α for all of the antenna elements 23 based on the above equation (1). The phase estimation unit 15 outputs the calculated phase adjustment amount ^α for all of the antenna elements 23 to the BF control unit 11.

[0055] This allows the BF control unit 11 to acquire information on the phase adjustment amount ^α for all of the antenna elements 23. Therefore, the BF control unit 11 performs beamforming using the acquired information on the phase adjustment amount ^α for all of the antenna elements 23. Specifically, the BF control unit 11 uses the information on the phase adjustment amount ^α for all of the antenna elements 23 to cause the phase shifter 131 of the RoF unit 13 to control the phase of the signal corresponding to each antenna element 23 by the value indicated by the information on the phase adjustment amount ^α.

[0056] According to the wireless communication system 100 configured as described above, the central station 10 performs pre-beamforming, which executes beam scanning on a combination of some of the antenna elements 23 out of the multiple antenna elements 23 equipped in one or more base stations 20, acquires phase information indicating the phase difference between some of the antenna elements obtained by pre-beamforming, estimates the phase difference between the wavelengths assigned to each antenna element 23 based on the phase difference between some of the antenna elements 23 indicated by the acquired phase information, and performs remote beamforming based on the estimated phase difference.

[0057] As described above, in the present invention, instead of performing pre-beamforming for all antenna element pairs as in the conventional technique, the phase difference between wavelengths assigned to each antenna element 23 is estimated based on the phase difference between some of the antenna elements. Therefore, when remote beamforming is applied to a large-scale array antenna, it is possible to reduce overhead while suppressing beam degradation. As a result, it is possible to perform highly accurate remote beamforming and obtain a desired beamforming gain even in an analog RoF system equipped with a large-scale array antenna.

[0058] Furthermore, in the wireless communication system 100, the phase difference between the wavelengths assigned to each antenna element 23 is estimated by interpolation or approximation based on the phase difference between some of the antenna elements. This eliminates the need to perform pre-beamforming for all antenna element pairs. As a result, the overhead required for pre-beamforming can be reduced.

[0059] (Variation 1) In the above-described embodiment, a configuration has been shown in which pre-beamforming is performed using the BF of a downlink signal, but pre-beamforming may also be performed using the BF of an uplink signal. When pre-beamforming is performed using pre-beamforming of an uplink signal, the communication partner generates and transmits a training signal. Feedback from the communication partner is not required to determine the optimal phase interval.

[0060] (Variation 2) In the above-described embodiment, the configuration in which the phase of the training signal is controlled in the optical domain has been described, but the configuration in which the phase of the training signal is controlled in the electrical domain may also be used. In such a configuration, the RoF unit 13 includes a converter that converts an electrical signal into an optical signal on the path between the phase shifter 131 and the optical multiplexing / demultiplexing unit 132.

[0061] (Variation 3) In the above-described embodiment, optical fiber transmission is performed using the same wavelength for downstream and upstream signal transmission, but different wavelengths may be used for the upstream and downstream. When different wavelengths are used for the upstream and downstream, the optimal phase interval for each of the downstream and upstream signal transmissions can be obtained by performing pre-beamforming on both the downstream and upstream signals. Therefore, the phase adjustment amount for each wavelength is estimated using this. Alternatively, the difference in the amount of phase rotation in optical fiber transmission may be estimated from the difference in wavelengths used for downstream or upstream signal transmission, and the phase adjustment amount for each of the downstream or upstream signal transmissions may be estimated using this value to correct the optimal phase interval obtained by pre-beamforming for either the downstream or upstream signal transmission.

[0062] (Variation 4) In the above-described embodiment, wireless transmission is performed using the same frequency for downstream and upstream signal transmission. However, different frequencies may be used for upstream and downstream. When the frequencies in the wireless section are different, the amount of phase rotation caused by optical fiber transmission changes even if the optical wavelength is the same. Therefore, pre-beamforming is performed on both the downstream and upstream signals to estimate the phase adjustment amount. Alternatively, the difference in the amount of phase rotation caused by optical fiber transmission may be estimated from the difference in frequency in the wireless section, and the phase adjustment amount for each of the downstream and upstream signal transmissions may be estimated by correcting the optimal phase interval obtained by either the downstream or upstream signal transmission using this value. However, if the optical wavelengths used in the downstream and upstream signal transmissions are the same and the frequencies are close, and the degradation due to the frequency difference is tolerable, the phase adjustment amount estimated from either the downstream or upstream signal transmission may be used for both the downstream and upstream signal transmissions.

[0063] (Modification 5) The present invention is also applicable to a case where downstream and upstream signal transmissions are performed using optical fiber transmissions with different wavelengths and wireless transmissions are performed using different frequencies.

[0064] (Variation 6) In the above-described embodiment, an electrical signal at a frequency used for wireless transmission is converted into an optical signal, but the present invention is also applicable to an intermediate frequency over fiber (IFoF) configuration in which an electrical signal is converted into an optical signal after frequency conversion to an intermediate frequency, or a baseband over fiber (BBoF) configuration in which a baseband signal is converted into an optical signal. In these configurations, even if the frequency of the wireless section is different for downstream and upstream signal transmission, as long as the intermediate frequency or baseband is the same and the optical wavelength is the same, the phase adjustment amount estimated from the result of pre-beamforming for either downstream or upstream signal transmission can be applied to both downstream and upstream signal transmission.

[0065] (Variation 7) In the above-described embodiment, the central station 10 is configured to retain information about the optimal phase interval obtained by pre-beamforming in downlink signal transmission and estimate the amount of phase adjustment. However, the information about the optimal phase interval and the estimation of the amount of phase adjustment may be performed by the communication partner, and the amount of phase adjustment estimated by the communication partner may be returned to the central station 10 as feedback information.

[0066] (Variation 8) In the above-described embodiment, the phase difference between wavelengths is estimated by interpolation or approximation. The interpolation and approximation methods used for the estimation are, for example, the methods listed in the next section. When approximation is used, a regularization method may also be applied. As the regularization method to be applied, for example, any of the methods of lasso regression (L1 regularization), ridge regression (L2 regularization), or Elastic Net may be used. Furthermore, machine learning technology may be used for the estimation.

[0067] (Modification 9) In the above-described embodiment, only the minimum functions required for the central station 10 and the base station 20 are shown. However, the central station 10 and the base station 20 may be provided with functions such as an amplifier, a filter, and a switch.

[0068] (Modification 10) The present invention can be applied to either a linear array or a planar array. When applied to a planar array, it is desirable to perform pre-beamforming using a combination of antenna elements with narrow wavelength intervals.

[0069] (Modification 11) The present invention is applicable to wireless communication systems using either FDD (Frequency Division Duplex) or TDD (Time Division Duplex) duplex methods.

[0070] (Modification 12) The present invention is applicable to wireless communication systems using any multiple access method, such as FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), etc. The present invention is also applicable to communications using a single carrier.

[0071] (Modification 13) The present invention is applicable to wireless communication systems using any access method, such as packet access or random access.

[0072] (Modification 14) The present invention is applicable not only to single-mode fibers but also to RoF systems using multi-core fibers and multi-mode fibers.

[0073] Some or all of the functional units of the central station 10 and base station 20 in the above-described embodiments are realized as software by one or more processors, such as a central processing unit (CPU), executing a program stored in a storage device having a non-volatile storage medium (non-transitory storage medium) and memory. The program may be recorded on a computer-readable non-transitory storage medium. Examples of computer-readable non-transitory storage media include portable media such as flexible disks, magneto-optical disks, read-only memories (ROMs), and compact disc-ROMs (CD-ROMs), as well as storage devices such as hard disks built into computer systems.

[0074] Some or all of the functional units of the central station 10 and the base station 20 may be realized using hardware including electronic circuits (electronic circuits or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0075] 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.

[0076] The present invention is applicable to systems using analog RoF.

[0077] 10... Aggregation station, 20, 20-1 to 20-L... Outgoing station, 11... BF control unit, 12... RF unit, 13... RoF unit, 14... Phase information acquisition unit, 15... Phase estimation unit, 21... RoF unit, 22... Antenna, 23, 23-0 to 23-(n-1)... Antenna element, 131... Phase shifter, 132... Optical multiplexing / demultiplexing section, 211... Optical multiplexing / demultiplexing section, 212... Converter

Claims

1. A wireless communication method performed by a wireless communication system including an aggregation station and one or more remote stations connected to the aggregation station via an optical transmission line and including a plurality of antenna elements, wherein beam scanning is performed in a combination of some of the plurality of antenna elements included in the one or more remote stations, phase information indicating a phase difference between the some of the antenna elements obtained by the beam scanning is acquired, a phase difference between wavelengths assigned to each antenna element is estimated based on the phase difference between the some of the antenna elements indicated by the phase information, and remote beamforming is performed based on the estimated phase difference. Wireless communication method.

2. The wireless communication method according to claim 1, wherein the phase difference between wavelengths assigned to each antenna element is estimated by approximating or interpolating based on the phase difference between the some of the antenna elements indicated by the phase information.

3. An aggregation station having the signal processing function among the signal processing function and the communication function included in a wireless communication device, including a beamforming control unit that causes beam scanning to be performed in a combination of some of the plurality of antenna elements included in one or more remote stations, a phase information acquisition unit that acquires phase information indicating a phase difference between the some of the antenna elements obtained by the beam scanning, and a phase estimation unit that estimates a phase difference between wavelengths assigned to each antenna element based on the phase difference between the some of the antenna elements indicated by the phase information acquired by the phase information acquisition unit, wherein the beamforming control unit performs remote beamforming based on the phase difference estimated by the phase estimation unit. Aggregation station.

4. A wireless communication system comprising an aggregation station and one or more remote stations connected to the aggregation station via an optical transmission line, wherein: The aggregation station includes: A beamforming control unit that causes beam scanning in a combination of some of the plurality of antenna elements included in the one or more remote stations; A phase information acquisition unit that acquires phase information indicating a phase difference between the some of the antenna elements obtained by the beam scanning; A phase estimation unit that estimates a phase difference between wavelengths assigned to each antenna element based on the phase difference between the some of the antenna elements indicated by the phase information acquired by the phase information acquisition unit; The beamforming control unit performs remote beamforming based on the phase difference estimated by the phase estimation unit; The one or more remote stations include: A plurality of antenna elements; A transceiver that converts a plurality of optical signals having different wavelengths transmitted from the aggregation station into electrical signals and outputs them to corresponding antenna elements, or converts an electrical signal output from any of the plurality of antenna elements into an optical signal and outputs it to the aggregation station. A wireless communication system.

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