WO2025052517A1 - Wireless communication system - Google Patents

Abstract

An embodiment of the present invention is a wireless communication system comprising an optical wireless communication device and a wireless communication terminal, wherein the optical wireless communication device comprises: an array antenna disposed in y rows (where y is an integer of 2 or more) * x columns (where x is an integer of 2 or more); a first pre-beam forming unit that performs first pre-beam forming on a transmission beam of the array antenna in a first direction of the array antenna; a second pre-beam forming unit that performs second pre-beam forming on the array antenna in a second direction on the basis of a result of the first pre-beam forming performed by the first pre-beam forming unit; and a beam forming unit that performs beam forming to be used when downlink transmission from the optical wireless communication device to the wireless communication terminal is performed on the basis of a result of the second pre-beam forming performed by the second pre-beam forming unit.

Description

Wireless communication system

The present invention relates to a wireless communication system.

In recent years, in order to meet the increasing demand for wireless communications, attention has been focused on the use of high-frequency bands, which have a wider bandwidth available than microwave bands. However, propagation loss in free space increases in proportion to the square of the frequency. As a result, high-frequency band communications have a shorter transmission distance and can only be carried out in a limited area.

A system that utilizes Radio over Fiber (RoF) (hereinafter referred to as the RoF system) is known as a means of solving this problem. In the RoF system, the central station converts radio frequency (RF (Radio Frequency)) signals to electrical/optical (E/O (Electrical-Optical)), transmits the modulated optical signal to the base station via optical fiber, and the base station converts optical/electrical (O/E (Optical-Electrical)) signals to extract the radio frequency signal, which is input to an antenna and radiated. By using the RoF system, it becomes possible to transmit high-frequency band radio frequency signals over long distances. In addition, a system configuration is being considered that separates the functions of the base station into the central station and the base station, and reduces the functions of the base station to reduce the size and power consumption, thereby improving ease of installation (Non-Patent Document 1).

In RoF systems that use high frequency bands, beamforming using an array antenna is required to expand the area that can be covered by the radio waves emitted from the base station. A typical beamforming method for securing a budget is to adjust the phase of the signals transmitted and received from each array antenna and control the radio waves so that they reinforce each other in the desired direction. In RoF systems, from the perspective of simplifying the base station, it is desirable to apply remote beamforming, in which the central station controls the beamforming of the base station.

Patent Document 1 is one of the methods for remote beamforming in an RoF system. Fig. 9 shows an example of the configuration of a first device related to the present invention, and is an example of the configuration of the device disclosed in Patent Document 1. The optical wireless communication device 300 shown in Fig. 9 includes a central station 310 and a base station 330. The central station 310 and the base station 330 communicate via an optical fiber 320. The central station 310 includes a phase control unit 311 and a multiplexing/demultiplexing unit 312. The base station 330 includes a multiplexing/demultiplexing unit 331, optical/electrical/electrical-optical conversion units 332-1 to 332-n, and antenna elements 333-1 to 333-n.

The remote beamforming method disclosed in Patent Document 1 (hereinafter referred to as fixed wavelength beamforming) is characterized in that wavelengths are assigned to each element at equal intervals with sufficiently narrow intervals. When wavelengths are assigned to the i-th antenna element (i=1, 2, ... n) among n antenna elements at Δλ intervals, the wavelength λ _i corresponding to the i-th antenna element when i=1 is the reference antenna is expressed by the following formula (1).

When Δλ is sufficiently small, the dispersion of each wavelength can be considered to be the same, and the delay (amount of phase rotation Δθ _{i — ideal} ) during optical fiber transmission for each wavelength can ideally be expressed as in the following equation (2).

Here, Δβ is the difference in the amount of phase rotation between wavelengths due to optical fiber transmission. Since the phase rotation amount Δθ _{i _ideal} is added to the signal of the i-th antenna element relative to the reference antenna due to optical fiber transmission, the signal radiated from each antenna element has a phase difference of Δβ intervals, thereby forming a beam. Furthermore, by providing a phase adjustment amount of α intervals to the signal corresponding to each antenna element in the central station, the phase difference between the elements changes, and the direction of the beam can be changed. When a phase adjustment amount of α intervals is provided in the central station, the phase θ _{i _ideal} of the signal radiated from the i-th antenna element is ideally expressed by the following formula (3).

Here, θ _I is the initial phase of the radio frequency signal. At this time, the phase difference with the adjacent antenna element i+1 is expressed as the following equation (4).

All antenna elements are equally spaced (α-Δβ), and the beam direction can be controlled by controlling the value of α at the central station.

However, since the actual dispersion differs depending on the wavelength, the phase rotation amount Δθ _i during optical fiber transmission includes a difference φ _i (hereinafter referred to as an error) according to the wavelength with respect to (i-1)Δβ. The actual phase rotation amount Δθ _i including the error φ _i is expressed by the following formula (5). This state is shown in FIG. 10, which is an example of the configuration of the second device related to the present invention. The optical wireless communication device 400 shown in FIG. 10 includes a central station 410 and a base station 430. The central station 410 and the base station 430 communicate with each other via an optical fiber 420. The central station 410 includes a phase control unit 411 and a multiplexing/demultiplexing unit 412. The base station 430 includes a multiplexing/demultiplexing unit 431, optical/electrical/electrical-optical conversion units 432-1 to 432-n, and antenna elements 433-1 to 433-n.

Moreover, the phase θ _i of the signal radiated from the i-th antenna element is given by the following equation (6).

As a result, the phase difference between antenna elements also contains an error that differs for each wavelength, as shown in equation (7) below.

For example, in the case of a commonly installed single mode fiber (SMF (Single Mode Fiber)), the longer the wavelength, the larger the dispersion, and therefore the larger the delay in optical fiber transmission. Therefore, the relationship Δθ _i+1 > Δθ _i and φ _i+1 > φ _i are established. When the wavelength interval is narrow and the number of antenna elements is small, the loss of equal spacing caused by the error φ _i is also small, so the effect on beamforming is slight. However, when the number of antenna elements increases and the difference in dispersion increases, the error φ _i becomes large, and the equal spacing of the phase difference of the signals radiated from the antenna elements is lost, causing the beam to collapse. When the beam collapses, the peak gain during beamforming deteriorates, and the desired gain cannot be obtained even if the number of antenna elements is increased.

As a solution to this problem, Non-Patent Document 2 proposes a method of performing fixed-wavelength beamforming in advance using a small number of antenna elements (hereinafter, advance beamforming using a small number of antenna elements is referred to as "pre-beamforming") and determining the beamforming phase based on the phase information obtained there.

FIG. 11 shows an example of the configuration of a third device related to the present invention, and is a diagram showing an example of the configuration of a device proposed in Non-Patent Document 2. The optical wireless communication device 500 shown in FIG. 11 includes a central station 510 and a base station 530. The central station 510 and the base station 530 communicate via an optical fiber 520. The central station 510 includes a phase control unit 511 and a multiplexing/demultiplexing unit 512. The base station 530 includes a multiplexing/demultiplexing unit 531, optical/electrical/electrical-optical conversion units 532-1 to 532-n, and antenna elements 533-1 to 533-n.

The optical wireless communication device 500 attempts pre-beamforming using m antenna elements (m≦n) for n antenna elements. In pre-beamforming, a beam sweep is performed according to the procedure of the remote beamforming method of Patent Document 1, feedback is received from the communication partner, and the optimal beam and the interval α of the phase adjustment amount for forming the optimal beam are determined. Pre-beamforming is attempted for all antenna elements while changing the combination of antenna elements. Pre-beamforming is performed to find the phase difference between antenna elements that will not cause beam collapse, so the combination of antenna elements used in pre-beamforming needs to be such that the phase difference between all antenna elements can be obtained.

For the qth pre-beamforming pattern (q = 1, 2, ..., p, where p is the number of combination patterns of antenna elements used in pre-beamforming), antenna element i that satisfies the following formula (8) is used.

Here, j (j = 1, 2, ... m) is the index of the antenna element in the antenna set used in pre-beamforming. In order to obtain information on the amount of phase rotation between wavelengths (between antenna elements) during optical fiber transmission, in pre-beamforming from the second pattern onwards, antenna elements are selected so that they include at least one antenna element used in the previous pre-beamforming.

Next, in the qth pattern of pre-beamforming, a beam sweep is performed while scanning the phase adjustment amount as described in Patent Document 1, and _αq for forming an optimal beam is determined based on feedback from the communication partner, and the determined _αq is stored. Using the interval _αq of the phase adjustment amount obtained by trialing the qth pattern of pre-beamforming, the phase adjustment amount α^ _i for each element during beamforming using all antenna elements is calculated by the following formula (9).

The interval _αq of the phase adjustment amount obtained by pre-beamforming includes an approximate average value of the phase error difference φ _i+1 -φ _i between the wavelengths of the wavelengths used in pre-beamforming. Therefore, when pre-beamforming is performed using antenna elements to which relatively short wavelengths are assigned among all antenna elements, the approximate average value of φ _i+1 -φ _i becomes small, so the value of α _q becomes small, and when pre-beamforming is performed using antenna elements to which relatively long wavelengths are assigned, the value of α _q becomes large. In this way, the interval _αq of the phase adjustment amount including the phase error difference φ _i+1 -φ _i according to the wavelength is obtained by pre-beamforming, so the phase adjustment amount α^ _i obtained using this according to equation (9) is a value that reflects the difference in phase error with adjacent wavelengths according to the wavelength, and is almost offset by the phase rotation amount Δθ _i during optical fiber transmission.

The pre-beamforming method for fixed-wavelength beamforming uses a small number of antenna elements so that beam collapse does not occur during pre-beamforming. However, in the case of a large-scale two-dimensional array antenna, one of the horizontal and vertical directions has a continuous wavelength arrangement, but the other has a wavelength arrangement with widely spaced wavelengths. As a result, the difference in phase error becomes large at the pre-beamforming stage, so beams are not formed properly and the desired phase information cannot be obtained. This makes beam formation difficult even with fixed-wavelength beamforming using all antenna elements.

JP 2020-120252 A

Hirofumi Shirato, Kodai Ito, Mizuki Suga, Kazuto Goto, Hidenori Toshinaga, Naoki Kita, "Proposal of millimeter wave FWA system using RoF," IEICE General Conference, B-5-112, 2019. Mizuki Suga, Kodai Ito, Takuto Arai, Hirofumi Shirato, and Naoki Kita, "Beam control method for improving wavelength-fixed beamforming performance," IEICE General Conference, B-5-123, 2021.

In the remote beamforming method of Patent Document 1, as the number of antenna elements increases and the wavelengths used expand, the phase error caused by the dispersion of the optical fiber increases, causing the beam to collapse. As a method to prevent this, there is a control method in Non-Patent Document 2 in which remote beamforming of Patent Document 1 is performed in advance using a small number of antenna elements, and the beamforming phase is determined based on the phase information obtained there. However, in the case of a large-scale two-dimensional array antenna, wavelengths are assigned to antenna elements arranged in two dimensions, so the wavelength difference between adjacent antenna elements in one of the two dimensions becomes large. Therefore, a beam cannot be formed at the stage of remote beamforming of Patent Document 1, which is performed in advance, making it difficult to apply the control method of Non-Patent Document 2. As a result, beam formation is difficult even in the remote beamforming of Patent Document 1, which uses all antenna elements.

In view of the above, the present invention aims to provide a wireless communication system that can generate optimal beams and prevent beam collapse in beamforming using two-dimensionally arranged array antennas.

One aspect of the present invention is a wireless communication system including an optical wireless communication device and a wireless communication terminal, the optical wireless communication device including an array antenna arranged in y rows (y is an integer of 2 or more) x columns (x is an integer of 2 or more), a first pre-beam forming unit that performs first pre-beam forming on the transmission beam of the array antenna in a first direction of the array antenna, a second pre-beam forming unit that performs second pre-beam forming on the array antenna in a second direction different from the first direction based on the result of the first pre-beam forming performed by the first pre-beam forming unit, and a beam forming unit that performs beam forming for use in downlink transmission from the optical wireless communication device to the wireless communication terminal based on the result of the second pre-beam forming performed by the second pre-beam forming unit.

Another aspect of the present invention is a wireless communication system including an optical wireless communication device and a wireless communication terminal, the optical wireless communication device including an array antenna arranged in y rows (y is an integer of 2 or more) x columns (x is an integer of 2 or more), a first pre-beam forming unit that performs a first pre-beam forming on the reception beam of the array antenna in a first direction of the array antenna, a second pre-beam forming unit that performs a second pre-beam forming on the array antenna in a second direction different from the first direction based on the result of the first pre-beam forming performed by the first pre-beam forming unit, and a beam forming unit that performs beam forming for use when performing uplink reception from the wireless communication terminal to the optical wireless communication device based on the result of the second pre-beam forming performed by the second pre-beam forming unit.

Another aspect of the present invention is a wireless communication system including an optical wireless communication device and a wireless communication terminal, the optical wireless communication device including an array antenna arranged in y rows (y is an integer equal to or greater than 2) x columns (x is an integer equal to or greater than 2), a first pre-beam forming unit that performs a first pre-beam forming on a transmission beam of the array antenna in a first direction of the array antenna, a second pre-beam forming unit that performs a second pre-beam forming on the array antenna in a second direction different from the first direction based on a result of the first pre-beam forming performed by the first pre-beam forming unit, and a second pre-beam forming unit that transmits a second pre-beam forming signal from the optical wireless communication device to the wireless communication terminal based on a result of the second pre-beam forming performed by the second pre-beam forming unit. The wireless communication system includes a first beamforming unit that performs beamforming used when performing downlink transmission to the terminal, a third prebeamforming unit that performs third prebeamforming on the reception beam of the array antenna in the first direction of the array antenna, a fourth prebeamforming unit that performs fourth prebeamforming on the array antenna in the second direction based on the result of the third prebeamforming performed by the third prebeamforming unit, and a second beamforming unit that performs beamforming used when performing uplink reception from the wireless communication terminal to the optical wireless communication device based on the result of the fourth prebeamforming performed by the fourth prebeamforming unit.

Another aspect of the present invention is a wireless communication method used in a wireless communication system including an optical wireless communication device and a wireless communication terminal, in which the optical wireless communication device performs first pre-beamforming in a first direction of the array antenna for a transmission beam of an array antenna arranged in y rows (y is an integer of 2 or more) * x columns (x is an integer of 2 or more) of the optical wireless communication device, and the optical wireless communication device performs second pre-beamforming in a second direction different from the first direction for the array antenna based on a result of the first pre-beamforming, and the optical wireless communication device performs beamforming for downlink transmission from the optical wireless communication device to the wireless communication terminal based on a result of the second pre-beamforming.

Another aspect of the present invention is a wireless communication method used in a wireless communication system including an optical wireless communication device and a wireless communication terminal, in which the optical wireless communication device performs a first pre-beamforming on a reception beam of an array antenna arranged in y rows (y is an integer of 2 or more) x columns (x is an integer of 2 or more) of the optical wireless communication device in a first direction of the array antenna, and based on a result of the first pre-beamforming, the optical wireless communication device performs a second pre-beamforming on the array antenna in a second direction different from the first direction, and based on a result of the second pre-beamforming, the optical wireless communication device performs beamforming for use when performing uplink reception from the wireless communication terminal to the optical wireless communication device.

Another aspect of the present invention is a wireless communication method used in a wireless communication system including an optical wireless communication device and a wireless communication terminal, the method comprising: performing a first pre-beamforming on a transmission beam of an array antenna arranged in y rows (y is an integer equal to or greater than 2)*x columns (x is an integer equal to or greater than 2) included in the optical wireless communication device, in a first direction of the array antenna; performing a second pre-beamforming on the array antenna in a second direction different from the first direction based on a result of performing the first pre-beamforming; performing a beamforming used when performing downlink transmission from the optical wireless communication device to the wireless communication terminal based on a result of performing the second pre-beamforming on the optical wireless communication device; performing a third pre-beamforming on a reception beam of the array antenna in the first direction of the array antenna; and performing a fourth pre-beamforming on the array antenna in the second direction based on a result of performing the third pre-beamforming on the optical wireless communication device.
This is a wireless communication method in which the optical wireless communication device performs beamforming based on a result of performing the fourth pre-beamforming, the beamforming being used when performing uplink reception from the wireless communication terminal to the optical wireless communication device.

The present invention makes it possible to provide a wireless communication system that can generate optimal beams and prevent beam collapse in beamforming using a two-dimensionally arranged array antenna.

1 is a schematic configuration diagram of a wireless communication system according to a first embodiment of the present invention. 1 is a schematic block diagram showing a configuration of an optical wireless communication device according to a first embodiment of the present invention. 1 is an explanatory diagram of an array antenna of an optical wireless communication device according to a first embodiment of the present invention. 5 is a flowchart showing a process of the optical wireless communication device according to the first embodiment of the present invention. FIG. 11 is a schematic block diagram showing a configuration of an optical wireless communication device according to a second embodiment of the present invention. 10 is a flowchart showing a process of the optical wireless communication device according to the second embodiment of the present invention. FIG. 11 is a schematic block diagram showing a configuration of an optical wireless communication device according to a third embodiment of the present invention. 10 is a flowchart showing a process of an optical wireless communication device according to a third embodiment of the present invention. 2 is a configuration example of a first device related to the present invention. 4 is a configuration example of a second device related to the present invention. 13 is a configuration example of a third device related to the present invention.

Below, an embodiment of the present invention will be described with reference to the drawings. The embodiment described below is merely an example, and the embodiment to which the present invention is applicable is not limited to the following embodiment.

[First embodiment]
First, a first embodiment of the present invention will be described.

FIG. 1 is a schematic diagram of a wireless communication system 1000 according to a first embodiment of the present invention. The wireless communication system 1000 includes an optical wireless communication device 100A and a wireless communication terminal 200. The optical wireless communication device 100A transmits and receives data by performing wireless communication with the wireless communication terminal 200. Note that FIG. 1 shows a case in which the wireless communication system 1000 includes one optical wireless communication device 100A and one wireless communication terminal 200, but is not limited to this. The wireless communication system 1000 may include two or more optical wireless communication devices and wireless communication terminals, and data may be transmitted and received by wireless communication between the two or more optical wireless communication devices and wireless communication terminals.

FIG. 2 is a schematic block diagram showing the configuration of an optical wireless communication device 100A according to a first embodiment of the present invention.

As shown in FIG. 2, the optical wireless communication device 100A includes an array antenna 110, a control unit 120, and a storage unit 130. The array antenna 110 is composed of multiple antenna elements arranged on a two-dimensional plane, and transmits and receives data via wireless communication with the wireless communication terminal 200. The control unit 120 is a CPU (Central Processing Unit) or the like, and controls each part of the optical wireless communication device 100A. The storage unit 130 is a RAM (Random Access Memory) or the like, and stores data transmitted, received, and generated by the optical wireless communication device 100A.

The control unit 120 (FIG. 2) of the optical wireless communication device 100A includes a first pre-beam forming unit 121, a second pre-beam forming unit 122, and a beam forming unit 123. The processes performed by the first pre-beam forming unit 121, the second pre-beam forming unit 122, and the beam forming unit 123 will be described in the flowcharts below.

3 is an explanatory diagram of the array antenna 110 of the optical wireless communication device 100A according to the first embodiment of the present invention. As shown in Fig. 3, the array antenna 110 is configured by y rows (y is an integer of 2 or more) x columns (x is an integer of 2 or more) of antenna elements arranged on a two-dimensional plane. Note that the symbol * is a multiplication symbol. Let i be the identification number of the antenna elements constituting array antenna 110, and the values of i = 1, 2, 3, ..., x-1, x, x+1, x+2, x+3, ..., 2x-1, 2x, ..., (y-1)*x+1, (y-1)*x+2, (y-1)*x+3, ..., y*x _{-1 ,} y _*x , and the wavelengths λ1, λ2, _λ3 , ..., λx _{-1 ,} λx, λx ₊₁ , λx+2, λx ₊ 3, ..., _{λ2x- 1} , λ2x, ..., λ _(y-1)*x+1 , λ _(y-1)*x+2 , λ _(y-1)*x+3 , ..., λy _*x-1 , λy _*x, will be described below.

4 is a flowchart showing the processing of the optical wireless communication device 100A according to the first embodiment of the present invention. First, the first pre-beam forming unit 121 of the optical wireless communication device 100A performs first pre-beam forming (step S11) on the transmission beam of the array antenna 110 in the row direction of the array antenna 110 (that is, the row direction of the antenna elements of y rows and x columns in FIG. 3, in which the wavelength λ increases continuously in the row direction (for example, λ ₁ →λ ₂ )).

Specifically, the first pre-beam forming unit 121 of the optical wireless communication device 100A performs the first pre-beam forming in the direction in which the continuous wavelengths are arranged (that is, the row direction in FIG. 3, also referred to as the first direction). At this time, the first pre-beam forming unit 121 performs the first pre-beam forming as a linear array for each two-dimensionally arranged row. The first pre-beam forming unit 121 uses the interval α ψ,q' of the phase adjustment amount obtained by the q'th pattern of the first pre-beam forming in the ψth (ψ=1, 2, ..., _y ) row of the first pre-beam forming to obtain the relative phase adjustment amount with respect to the reference antenna element in the ψth row by the following formula (10). Here, χ=1, 2, ..., x.

Next, the second pre-beam forming unit 122 of the optical wireless communication device 100A performs second pre-beam forming in the column direction (i.e., the column direction of the antenna elements of y rows and x columns in Figure 3, in which the wavelength λ increases stepwise in the column direction (e.g., λ1 → λX+1), also referred to as the second direction) for the array antenna 110 based on the result of the first pre-beam forming performed by the first pre-beam forming unit ₁₂₁ (i.e., information on the interval of _the phase adjustment amount and the phase adjustment amount obtained in the first pre-beam forming in step S11) (step S12), thereby obtaining a phase adjustment amount that can suppress beam collapse in the second pre-beam forming and generate an optimal beam.

Specifically, the second pre-beam forming unit 122 of the optical wireless communication device 100A obtains a phase difference between reference antenna elements in each row of antenna elements constituting the array antenna 110 by the second pre-beam forming. Therefore, in the first pre-beam forming, it is necessary to perform pre-beam forming for each row by using an antenna element on the same column as the reference antenna element. Here, a case will be described in which the first antenna element (χ=1) in each row is used as the reference antenna element in the first pre-beam forming, and the antenna element with i=1 is used as the reference antenna in the second pre-beam forming. In the second pre-beam forming, the second pre-beam forming unit 122 obtains candidates for the phase adjustment amount to be used in the second pre-beam forming from the interval α _ψ,q′ of the phase adjustment amount obtained in the first pre-beam forming and the phase adjustment amount α^ _ψ,χ . It can be estimated that the phase rotation amount α^ _ψ,1 of the (ψ-1)*x+1th antenna element from the reference antenna (i=1) will be within the range shown in equation (11) below, based on the interval αψ _-1,p' of the phase adjustment amounts obtained by pre-beamforming of the p'th pattern in the previous row (ψ-1th row), the phase adjustment amount α^ _ψ-1,x at the (ψ-1)*xth antenna element, and the interval αψ,1 of the phase adjustment amounts obtained by pre-beamforming of the first pattern in the ψ-th row in which the ( _ψ- 1)*x+1th antenna element is arranged.

Here, p' is the number of combination patterns of antenna elements used for pre-beamforming in the ψ-th row. The phase within this range is swept by Δα to list candidate phase adjustment amounts. This is obtained for all antenna elements in the first column, and the candidate phase adjustment amounts are combined to create a combination pattern of phase adjustment amounts to be used for the second pre-beamforming. The combination patterns of phase adjustment amounts created using all antenna elements in the first column are applied in order to perform the second pre-beamforming. The combination pattern (1, α^ _2,1 , ..., α^ _y,1 ) of phase adjustment amounts that forms the optimal beam in the first column is determined based on feedback from the communication partner.

Next, the beamforming unit 123 of the optical wireless communication device 100A performs beamforming to be used when performing downlink transmission from the optical wireless communication device 100A to the wireless communication terminal 200 based on the result of the second pre-beamforming performed by the second pre-beamforming unit 122 (i.e., the phase adjustment amount obtained in the second pre-beamforming in step S12) (step S13).

Specifically, the beamforming unit 123 of the optical wireless communication device 100A calculates the phase adjustment amount when performing beamforming using all antenna elements from the phase adjustment amounts obtained by the first pre-beamforming and the second pre-beamforming, using the following formula (12).

According to the first embodiment, by performing beamforming using the phase adjustment amount obtained by equation (12), it is possible to reduce beam collapse by utilizing the operation of fixed-wavelength beamforming even in a two-dimensional array antenna, while maintaining the advantages of fixed-wavelength beamforming, such as high wavelength utilization efficiency, beam control that does not require information on optical fiber length, no control required at the base station, and applicability to high frequency bands.

Furthermore, according to the first embodiment, by applying the phase adjustment amount based on the result of the first pre-beamforming and the result of the second pre-beamforming to beamforming using all antenna elements constituting the array antenna 110, it is possible to approximately cancel out the error φ _i during optical fiber transmission and make the phase differences between the antenna elements approximately equal. This makes it possible to perform beamforming using all antenna elements 110 in the two-dimensional array antenna 110.

Furthermore, according to the first embodiment, in remote beamforming in an RoF system, even if the number of antenna elements increases, it is possible to prevent beam collapse and ensure beamforming gain.

[Second embodiment]
Next, a second embodiment of the present invention will be described. In the first embodiment, the phase adjustment amount is determined by forming and sweeping a transmission beam from the optical wireless communication device 100A (e.g., a base station), but in the second embodiment, the phase adjustment amount is determined by forming a reception beam of the optical wireless communication device (e.g., a base station).

The schematic configuration diagram of the wireless communication system according to the second embodiment differs from the schematic configuration diagram of the wireless communication system 1000 according to the first embodiment (FIG. 1) in that an optical wireless communication device 100B is provided instead of the optical wireless communication device 100A. The optical wireless communication device 100B transmits and receives data by performing wireless communication with the wireless communication terminal 200 (FIG. 1).

FIG. 5 is a schematic block diagram showing the configuration of an optical wireless communication device 100B according to a second embodiment of the present invention. As shown in FIG. 5, the optical wireless communication device 100B includes an array antenna 140, a control unit 150, and a storage unit 160. The array antenna 140 is composed of multiple antenna elements arranged on a two-dimensional plane, and transmits and receives data via wireless communication with the wireless communication terminal 200 (FIG. 1). Note that the configuration of the array antenna 140 according to the second embodiment is similar to that of the array antenna 110 (FIG. 3) according to the first embodiment, and therefore a description thereof will be omitted. The control unit 150 is a CPU or the like, and controls each unit of the optical wireless communication device 100B. The storage unit 160 is a RAM or the like, and stores data transmitted, received, and generated by the optical wireless communication device 100B.

The control unit 150 (FIG. 5) of the optical wireless communication device 100B includes a first pre-beam forming unit 151, a second pre-beam forming unit 152, and a beam forming unit 153. The processes performed by the first pre-beam forming unit 151, the second pre-beam forming unit 152, and the beam forming unit 153 will be described in the flowchart below.

6 is a flowchart showing the processing of the optical wireless communication device 100B according to the second embodiment of the present invention. First, the first pre-beam forming unit 151 of the optical wireless communication device 100B performs the first pre-beam forming (step S21) on the reception beam of the array antenna 140 in the row direction of the array antenna 140 (that is, the row direction of the antenna elements of y rows and x columns in FIG. 3, in which the wavelength λ increases continuously in the row direction (for example, λ ₁ →λ ₂ )), thereby acquiring information on the interval of the phase adjustment amount and the phase adjustment amount. The processing of step S21 can be performed using the same method as the processing of step S11.

Next, the second pre-beam forming unit 152 of the optical wireless communication device 100B performs second pre-beam forming (step S22) in the column direction (i.e., the column direction of antenna elements of y rows and x columns in FIG. 3, in which the wavelength λ increases stepwise in the column direction (e.g., λ 1 → λ X+1 )) for the array antenna 140 based on the result of the first pre-beam forming performed by the first pre-beam forming unit ₁₅₁ (i.e., information on the interval of the phase adjustment _amount and the phase adjustment amount acquired in the first pre-beam forming in step S21), thereby acquiring a phase adjustment amount capable of suppressing beam collapse in the second pre-beam forming and generating an optimal beam. Note that the process of step S22 can be performed using a method similar to that of step S12.

Next, the beamforming unit 153 of the optical wireless communication device 100B performs beamforming to be used when performing uplink reception from the wireless communication terminal 200 (FIG. 1) to the optical wireless communication device 100B based on the result of the second pre-beamforming performed by the second pre-beamforming unit 152 (i.e., the phase adjustment amount acquired in the second pre-beamforming in step S22) (step S23). Note that the process of step S23 can be performed using a method similar to the process of step S13.

In the second embodiment, the same effect as in the first embodiment can be obtained. Furthermore, according to the second embodiment, two stages of pre-beamforming are performed using a beam sweep signal transmitted from the wireless communication terminal 200 of the communication partner, and the amount of phase adjustment for each antenna element is determined by the reception beam, so that feedback from the communication partner is not required in the case of the reception beam.

[Third embodiment]
Next, a third embodiment of the present invention will be described.

The schematic configuration diagram of the wireless communication system according to the third embodiment differs from the schematic configuration diagram of the wireless communication system 1000 according to the first embodiment (FIG. 1) in that it includes an optical wireless communication device 100C instead of the optical wireless communication device 100A. The optical wireless communication device 100C transmits and receives data by performing wireless communication with the wireless communication terminal 200 (FIG. 1).

FIG. 7 is a schematic block diagram showing the configuration of an optical wireless communication device 100C according to a third embodiment of the present invention. As shown in FIG. 7, the optical wireless communication device 100C includes an array antenna 170, a control unit 180, and a storage unit 190. The array antenna 170 is composed of multiple antenna elements arranged on a two-dimensional plane, and transmits and receives data via wireless communication with a wireless communication terminal 200 (FIG. 1). Note that the configuration of the array antenna 170 according to the third embodiment is similar to that of the array antenna 110 (FIG. 3) according to the first embodiment, and therefore a description thereof will be omitted. The control unit 180 is a CPU or the like, and controls each unit of the optical wireless communication device 100C. The storage unit 190 is a RAM or the like, and stores data transmitted, received, and generated by the optical wireless communication device 100C.

The control unit 180 (FIG. 7) of the optical wireless communication device 100C includes a first prebeam forming unit 181, a second prebeam forming unit 182, a first beam forming unit 183, a third prebeam forming unit 184, a fourth prebeam forming unit 185, and a second beam forming unit 186. The processing performed by the first prebeam forming unit 181, the second prebeam forming unit 182, the first beam forming unit 183, the third prebeam forming unit 184, the fourth prebeam forming unit 185, and the second beam forming unit 186 will be described in the flowchart below.

8 is a flowchart showing the processing of the optical wireless communication device 100C according to the third embodiment of the present invention. First, the first pre-beam forming unit 181 of the optical wireless communication device 100C performs the first pre-beam forming (step S31) on the transmission beam of the array antenna 170 in the row direction of the array antenna 170 (that is, the row direction of the antenna elements of y rows and x columns in FIG. 3, in which the wavelength λ increases continuously in the row direction (for example, λ ₁ →λ ₂ )), thereby acquiring information on the interval of the phase adjustment amount and the phase adjustment amount. The processing of step S31 can be performed using a method similar to that of step S11.

Next, the second pre-beam forming unit 182 of the optical wireless communication device 100C performs second pre-beam forming (step S32) on the array antenna 170 in the column direction (i.e., the column direction of the antenna elements of y rows and x columns in FIG. 3, in which the wavelength λ increases stepwise in the column direction (e.g., λ ₁ → λ _X+1 )) based on the result of the first pre-beam forming performed by the first pre-beam forming unit 181 (i.e., information on the interval of the phase adjustment amount and the phase adjustment amount acquired in the first pre-beam forming in step S31), thereby acquiring a phase adjustment amount capable of suppressing beam collapse in the second pre-beam forming and generating an optimal beam. Note that the process of step S32 can be performed using a method similar to that of step S12.

Next, the first beam forming unit 183 of the optical wireless communication device 100C performs beam forming to be used when performing downlink transmission from the wireless communication device 100C to the wireless communication terminal 200, based on the result of the second pre-beam forming performed by the second pre-beam forming unit 182 (i.e., the phase adjustment amount acquired in the second pre-beam forming in step S32) (step S33). Note that the process of step S33 can be performed using a method similar to the process of step S13.

Next, the third pre-beam forming unit 184 of the optical wireless communication device 100C performs third pre-beam forming on the reception beam of the array antenna 170 in the row direction of the array antenna (i.e., the row direction of the antenna elements of y rows and x columns in FIG. 3, in which the wavelength λ increases continuously in the row direction (e.g., _λ1 → _λ2 )) (step S34), thereby acquiring information on the interval between phase adjustment amounts and the phase adjustment amount. Note that the processing of step S34 can be performed using a method similar to that of step S11.

Next, the fourth pre-beam forming unit 185 of the optical wireless communication device 100C performs fourth pre-beam forming (step S35) in the column direction (i.e., the column direction of the antenna elements of y rows and x columns in FIG. 3, in which the wavelength λ increases stepwise in the column direction (e.g., λ 1 →λ X+ ₁ )) for the array antenna 170 based on the result of the third pre-beam forming performed by the third pre-beam forming unit ₁₈₄ (i.e., information on the interval of the phase adjustment amount and the phase adjustment amount acquired in the third pre-beam forming in step S34), thereby acquiring a phase adjustment amount capable of suppressing beam collapse in the fourth pre-beam forming and generating an optimal beam. Note that the process of step S35 can be performed using a method similar to that of step S12.

Next, the second beam forming unit 186 of the optical wireless communication device 100C performs beam forming to be used when performing uplink reception from the wireless communication terminal 200 (FIG. 1) to the optical wireless communication device 100C based on the result of the fourth pre-beam forming performed by the fourth pre-beam forming unit 185 (i.e., the phase adjustment amount acquired in the fourth pre-beam forming in step S35) (step S36). Note that the processing of step S36 can be performed using a method similar to that of step S13.

In the explanation of the flowchart of the optical wireless communication device 100C according to the third embodiment shown in FIG. 8, the case where the processes of steps S31 to S33 are performed and then the processes of steps S34 to S36 are performed is explained, but this is not limited thereto, and the processes of steps S31 to S33 may be performed after the processes of steps S34 to S36 are performed.

When the optical wavelengths used in the downlink and uplink are the same, the phase adjustment amount for both the downlink and uplink can be determined by using the method described in the first or second embodiment, but when the optical wavelengths used in the downlink and uplink are different, it is effective to use the third embodiment. When the optical wavelengths used in the downlink and uplink are different, the phase adjustment amount for each of the downlink and uplink can be obtained by implementing both the method using the transmission beam and the method using the reception beam described in the first and second embodiments.

In the third embodiment, the difference in the amount of phase rotation in optical fiber transmission may be estimated from the difference in wavelengths used in the downlink and uplink, and the phase adjustment amount obtained by either the method using the transmission beam or the method using the reception beam may be corrected using this value to obtain the phase adjustment amount for each of the downlink and uplink.

[Fourth embodiment]
A fourth embodiment of the present invention will now be described. The configurations of the wireless communication system and the optical wireless communication device according to the fourth embodiment are similar to those of the wireless communication system and the optical wireless communication device according to the third embodiment, and therefore description thereof will be omitted.

In the first embodiment, it is assumed that the frequency of the wireless section is the same for the downlink and uplink, but in the fourth embodiment, the frequency of the wireless section is different for the downlink and uplink. If the frequency of the radio frequency signal in the electro-optical conversion is different for the downlink and uplink, the phase error that occurs during optical fiber transmission will change even if the optical wavelength is the same, so in the fourth embodiment, as in the third embodiment, the phase adjustment amount corresponding to each of the downlink and uplink is obtained.

In the fourth embodiment, the difference in the amount of phase rotation in optical fiber transmission may be estimated from the difference in the frequencies of the radio frequency signals, and the phase adjustment amount obtained by either the method using the transmitting beam or the method using the receiving beam may be corrected using this value to obtain the phase adjustment amount for the downlink and the uplink.

However, if the optical wavelengths used in the downlink and uplink are the same and the frequencies are close, and the degradation due to the frequency difference is tolerable, the phase adjustment amount obtained in either the first or second embodiment may be used for both the downlink and uplink.

The third and fourth embodiments may be combined to use different optical wavelengths for the downlink and uplink, and different frequencies for the wireless section.

In the first to fourth embodiments, a configuration is assumed in which a radio frequency signal is transmitted over RoF without frequency conversion, but the first to fourth embodiments can also be applied to IFOF (Intermediate Frequency over Fiber), which converts the radio frequency signal to an intermediate frequency and then performs electrical-to-optical conversion. In the case of IFOF, the phase adjustment amount can be obtained according to formulas (8) to (12) in the same way as in RoF. In the case of IFOF, if the intermediate frequency and optical wavelength are the same in the downlink and uplink, the phase adjustment amount obtained in either the first or second embodiment can be used for both the downlink and uplink. In this case, the frequency of the wireless section may be different between the downlink and uplink. In addition, the first to fourth embodiments can also be applied to the case in which different optical wavelengths, intermediate frequencies, and frequencies in the wireless section are used in the downlink and uplink in IFOF. If either the optical wavelength or the intermediate frequency is different, it is desirable to determine the phase adjustment amount for the downlink and the uplink, respectively, as described in the third and fourth embodiments, or to estimate the difference in the amount of phase rotation in optical fiber transmission from the difference in wavelength or the difference in intermediate frequency, and use that value to correct the amount of phase adjustment.

Note that, in equations (8) to (12), it is assumed that the antenna elements to be used for pre-beamforming are selected so as to minimize the number of times pre-beamforming is performed. However, if it is not necessary to minimize the number of times pre-beamforming is performed, the number of times pre-beamforming is performed and the combination of antenna elements to be used for pre-beamforming do not need to follow equation (8).

In the first embodiment, it is assumed that each pre-beam forming sweeps all beam patterns, but if it is necessary to shorten the beam scanning time, a technology for reducing the beam scanning time (number of times) may be applied.

In the first embodiment, it is assumed that the central station holds the difference in the phase adjustment amount obtained by pre-beamforming in the downlink and calculates the phase adjustment amount, but the wireless communication terminal of the communication partner may hold the difference in the phase adjustment amount and calculate the phase adjustment amount, and return the obtained phase adjustment amount as feedback information to the optical wireless communication device including the central station.

In the wireless communication systems according to the first to fourth embodiments, as shown in FIG. 3, a case has been described in which wavelengths are assigned to antenna elements of an array antenna so that the wavelengths increase continuously in the row direction of the two-dimensionally arranged array antenna, and wavelengths are assigned to antenna elements of the array antenna so that the wavelengths increase stepwise in the column direction of the two-dimensionally arranged array antenna. In the wireless communication systems according to the first to fourth embodiments, a case has been described in which the first pre-beamforming is performed in the row direction, and then the second pre-beamforming is performed in the column direction. However, the present invention is not limited to this. For example, in FIG. 3, wavelengths may be assigned to antenna elements of an array antenna so that the wavelengths increase continuously in the column direction of the two-dimensionally arranged array antenna, and wavelengths may be assigned to antenna elements of an array antenna so that the wavelengths increase stepwise in the row direction of the two-dimensionally arranged array antenna. In this case, it is sufficient to perform the first pre-beamforming in the column direction, and then the second pre-beamforming in the row direction.

The optical wireless communication device according to the first to fourth embodiments includes a processor such as a CPU and a memory. The processor executes the functions of each part of the optical wireless communication device by executing a program stored in the memory. All or part of the functions of each part of the optical wireless communication device may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above 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, a CD-ROM, and a semiconductor storage device (e.g., an SSD: Solid State Drive), and storage devices such as a hard disk and a semiconductor storage device built into a computer system. The above program may be transmitted via a telecommunications line.

The present invention can be applied to wireless communication systems that can generate optimal beams and prevent beam collapse in beamforming using two-dimensionally arranged array antennas.

100A, 100B, 100C...optical wireless communication device,
110: Array antenna,
120: Control unit,
121: First pre-beam forming unit,
122: Second pre-beam forming unit,
123: Beam forming unit,
130...Storage unit,
140: Array antenna,
150: Control unit,
151: First pre-beam forming unit,
152: Second pre-beam forming unit,
153: Beam forming unit,
160...Storage unit,
170: Array antenna,
180: Control unit,
181: First pre-beam forming unit,
182: Second pre-beam forming unit,
183: First beam forming unit,
184: third pre-beam forming unit,
185...fourth pre-beam forming unit,
186: second beam forming unit,
190...Storage section,
200: wireless communication terminal,
300: Optical wireless communication device,
310: Aggregation station,
311: Phase control unit,
312...multiplexing/demultiplexing section,
320...optical fiber,
330... Expansion station,
331: Multiplexer/demultiplexer unit 332-1 to 332-n: photoelectric/electrical-optical converter unit,
333-1 to 333-n...antenna elements,
400...Optical wireless communication device,
410: Aggregation station,
411: Phase control unit,
412...multiplexing/demultiplexing section,
420...optical fiber,
430... Expansion station,
431: Multiplexing/Demultiplexing section 432-1 to 432-n: Photoelectric/electrical-optical conversion section,
433-1 to 433-n...antenna elements,
500...Optical wireless communication device,
510: Aggregation station,
511: Phase control unit,
512...multiplexing/demultiplexing section,
520...optical fiber,
530... Expansion station,
531: Multiplexer/demultiplexer unit 532-1 to 532-n: photoelectric/electrical-optical converter unit,
533-1 to 533-n...antenna elements, 1000...wireless communication system

Claims (7)

A wireless communication system including an optical wireless communication device and a wireless communication terminal,
The optical wireless communication device includes:
An array antenna arranged in y rows (y is an integer of 2 or more) x columns (x is an integer of 2 or more);
a first pre-beam forming unit that performs first pre-beam forming on a transmission beam of the array antenna in a first direction of the array antenna;
a second pre-beam forming unit that performs a second pre-beam forming on the array antenna in a second direction different from the first direction based on a result of the first pre-beam forming performed by the first pre-beam forming unit;
a beam forming unit that performs beam forming used when performing downlink transmission from the optical wireless communication device to the wireless communication terminal based on a result of the second pre-beam forming performed by the second pre-beam forming unit;
A wireless communication system comprising: A wireless communication system including an optical wireless communication device and a wireless communication terminal,
The optical wireless communication device includes:
An array antenna arranged in y rows (y is an integer of 2 or more) x columns (x is an integer of 2 or more);
a first pre-beam forming unit that performs first pre-beam forming on a reception beam of the array antenna in a first direction of the array antenna;
a second pre-beam forming unit that performs a second pre-beam forming on the array antenna in a second direction different from the first direction based on a result of the first pre-beam forming performed by the first pre-beam forming unit;
a beam forming unit that performs beam forming used when performing uplink reception from the wireless communication terminal to the optical wireless communication device based on a result of the second pre-beam forming performed by the second pre-beam forming unit;
A wireless communication system comprising: A wireless communication system including an optical wireless communication device and a wireless communication terminal,
The optical wireless communication device includes:
An array antenna arranged in y rows (y is an integer of 2 or more) x columns (x is an integer of 2 or more);
a first pre-beam forming unit that performs first pre-beam forming on a transmission beam of the array antenna in a first direction of the array antenna;
a second pre-beam forming unit that performs a second pre-beam forming on the array antenna in a second direction different from the first direction based on a result of the first pre-beam forming performed by the first pre-beam forming unit;
a first beam forming unit that performs beam forming used when performing downlink transmission from the optical wireless communication device to the wireless communication terminal based on a result of the second pre-beam forming performed by the second pre-beam forming unit;
a third pre-beam forming unit that performs third pre-beam forming on a reception beam of the array antenna in the first direction of the array antenna;
a fourth pre-beam forming unit configured to perform a fourth pre-beam forming in the second direction on the array antenna based on a result of the third pre-beam forming performed by the third pre-beam forming unit;
a second beam forming unit that performs beam forming used when performing uplink reception from the wireless communication terminal to the optical wireless communication device based on a result of the fourth pre-beam forming performed by the fourth pre-beam forming unit;
A wireless communication system comprising: The wireless communication system according to claim 3 , wherein a frequency used in the uplink is different from a frequency used in the downlink. A wireless communication method used in a wireless communication system including an optical wireless communication device and a wireless communication terminal,
performing a first pre-beamforming on a transmission beam of an array antenna arranged in y rows (y is an integer equal to or greater than 2)*x columns (x is an integer equal to or greater than 2) included in the optical wireless communication device, in a first direction of the array antenna;
the optical wireless communication device performs second pre-beamforming on the array antenna in a second direction different from the first direction based on a result of the first pre-beamforming;
the optical wireless communication device performs beamforming to be used when performing downlink transmission from the optical wireless communication device to the wireless communication terminal, based on a result of performing the second pre-beamforming. A wireless communication method used in a wireless communication system including an optical wireless communication device and a wireless communication terminal,
performing a first pre-beamforming on a reception beam of an array antenna arranged in y rows (y is an integer equal to or greater than 2)*x columns (x is an integer equal to or greater than 2) included in the optical wireless communication device, in a first direction of the array antenna;
the optical wireless communication device performs second pre-beamforming on the array antenna in a second direction different from the first direction based on a result of the first pre-beamforming;
the optical wireless communication device performs beamforming to be used when performing uplink reception from the wireless communication terminal to the optical wireless communication device, based on a result of performing the second pre-beamforming. A wireless communication method used in a wireless communication system including an optical wireless communication device and a wireless communication terminal,
performing a first pre-beamforming on a transmission beam of an array antenna arranged in y rows (y is an integer equal to or greater than 2)*x columns (x is an integer equal to or greater than 2) included in the optical wireless communication device, in a first direction of the array antenna;
the optical wireless communication device performs second pre-beamforming on the array antenna in a second direction different from the first direction based on a result of the first pre-beamforming;
the optical wireless communication device performs beamforming to be used when performing downlink transmission from the optical wireless communication device to the wireless communication terminal based on a result of performing the second pre-beamforming;
the optical wireless communication device performs third pre-beamforming on a reception beam of the array antenna in the first direction of the array antenna;
the optical wireless communication device performs fourth pre-beam forming on the array antenna in the second direction based on a result of the third pre-beam forming;
the optical wireless communication device performs beamforming to be used when performing uplink reception from the wireless communication terminal to the optical wireless communication device, based on a result of performing the fourth pre-beamforming.

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