CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Phase of International Application No. filed on Jan. 16, 2020, which claims priority to Japanese Application No. 2019-009434 filed on Jan. 23, 2019. The entire disclosures of the above applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a wireless communication system, an accommodation station apparatus, and a wireless communication method.

BACKGROUND ART

In wireless communication services, millimeter wave bands are attracting attention as frequency bands capable of high-speed transmission. However, there is a problem in which long-distance transmission is difficult with millimeter wave bands because their propagation loss is great.

A radio over fiber (RoF) system is known as a solution to this problem. In the RoF system, an accommodation station (a master station) modulates the intensity of an optical carrier with a radio frequency (RF) signal to be transmitted and transmits the modulated signal through an optical fiber. A base station (a slave station) reconverts the optical signal received through the optical fiber to the RF signal and radiates the reconverted RF signal through an antenna as a radio wave. Use of such an RoF system enables long-distance transmission of RF signals of millimeter wave bands.

However, when the RoF system is applied to millimeter wave bands, another issue arises involving expanding the coverage of the base station. One solution is beamforming using an array antenna. In the beamforming using an array antenna, the phases of RF signals incident on antenna elements of the array antenna are controlled such that radio waves radiated from the antenna elements interfere with each other. This controls an overall radiation direction of radio waves.

Regarding beamforming in the RoF system, in the related art, the phases of RF signals incident on antenna elements are controlled by controlling the wavelengths of optical carriers utilizing the fact that delay differences occur between optical signals of wavelengths due to chromatic dispersion during transmission through an optical fiber (see, for example, Patent Document 1).

FIG. 20is a block diagram of an RoF system900to which the technique of Patent Document 1 is applied. A multi-wavelength variable light source911in an accommodation station910outputs a plurality of optical signals. The wavelength intervals between the optical signals can be changed arbitrarily. An optical modulator912modulates an optical signal of each wavelength with an RF signal to be transmitted. Thus, the optical modulator912outputs a plurality of optically modulated signals. Each optically modulated signal is transmitted though the optical fiber920. Here, the optically modulated signals undergo different delay differences depending on the wavelengths due to the influence of chromatic dispersion. An optical demultiplexer931in a base station930splits a plurality of optically modulated signals transmitted through the optical fiber920into respective optically modulated signals of the wavelengths. A plurality of optical to electrical (O/E) converters932-1, . . . ,932-nconvert the split optically modulated signals of the wavelengths into electrical signals. Antenna elements933-1, . . . ,933-nradiate the converted electrical signals as RF signals. At this time, phase differences also occur between the RF signals because of the delay differences due to chromatic dispersion during transmission through the optical fiber920, so that directivity can be formed.

Also, there are methods for performing beamforming of an array antenna using optical signals without being limited to the RoF system. As one of the methods, in the related art, wavelengths are not controlled but fixed wavelengths are assigned to antenna elements and chromatic dispersion or path differences are used to cause delay differences between optical signals of the wavelengths (see, for example, Non Patent Document 1).

FIG. 21is a diagram of a wireless system905to which the technique of Non Patent Document 1 is applied. A multi-wavelength light source951outputs a plurality of optical signals of different wavelengths. An optical modulator952modulates an optical signal of each wavelength with an RF signal to be transmitted. Thus, the optical modulator952outputs a plurality of optically modulated signals. Each optically modulated signal is sent to a programmable dispersion matrix (PDM)953.

FIG. 22is a block diagram illustrating the configuration of the PDM953. The PDM953includes n+1 pieces of 2×2 optical switches961-1, . . . ,961-(n+1) and n dispersion elements962-1,962-2, . . . ,962-nwhich have dispersion values of D0, 2D0, . . . , 2n-1D0, respectively. The dispersion elements962-1, . . . ,962-nare each made of a dispersion fiber, a grating fiber, or the like. The PDM953adjusts the dispersion values as a whole by switching the 2×2 optical switches961-1, . . . ,961-(n+1). The optically modulated signals input to the PDM953undergo different delay differences depending on the dispersion values adjusted by the PDM953.

An optical demultiplexer954in the wireless system905illustrated inFIG. 21splits a plurality of optically modulated signals output from the PDM953into respective optically modulated signals of the wavelengths. This optical splitting is fixed because the wavelengths are preset corresponding to antenna elements956-1, . . . ,956-n. A plurality of optical to electrical (O/E) converters955-1, . . . ,955-nconvert the split optically modulated signal of the wavelengths into electrical signals. The antenna elements956-1, . . . ,956-nradiate the converted electrical signals as RF signals. At this time, phase differences also occur between the RF signals because of the delay differences due to the dispersion of the PDM953, so that directivity can be formed.

CITATION LIST

Patent Document

Patent Document 1: JP 4246724 B

Non Patent Document

Non Patent Document 1: Dennis TK Tong, Ming C. Wu, “A Novel Multiwavelength Optically Controlled Phased Array Antenna with a Programmable Dispersion Matrix”, IEEE Photonics Technology Letters, June 1996, VOL. 8, NO. 6, pp. 812-814

SUMMARY OF THE INVENTION

Technical Problem

The technique of Patent Document 1 makes wavelengths variable and dispersion fixed to cause delay differences between modulated optical signals as described above. Here, it is necessary to greatly adjust the wavelength interval between optically modulated signals depending on the direction in which the directivity is formed, the fiber length, and the frequency of the RF signal. This increases the width of a wavelength band to be used and thus the wavelength utilization efficiency seems to be lowered. Different wavelengths must be used for base stations particularly in a wavelength division multiplex-passive optical network (WDM-PON). If the technique of Patent Document 1 is applied to such a situation, a wavelength band for beamforming must also be preset in the WDM-PON, greatly expanding the wavelength band to be used.

Further, in the technique of Patent Document 1, the wavelengths are adjusted for directivity formation. Thus, it is also necessary to adjust wavelengths sent to the antenna elements of the base station. Therefore, the optical demultiplexer of the base station needs to change the wavelengths sent to the antenna elements of the base station every time the directivity is formed, and when the directivity is changed dynamically, splitting of the optical demultiplexer also needs to be changed dynamically. This means that it is necessary to control the optical demultiplexer of the base station.

One advantage of applying the RoF system is that the base station can be simplified by consolidating the functions into the accommodation station, in addition to the long-distance transmission of RF signals. However, the technique of Patent Document 1 requires control of the optical demultiplexer of the base station, limiting the simplification of the base station.

Furthermore, the technique of Patent Document 1 requires distance information of the optical fiber in the wavelength adjustment for adjusting the delay differences between optically modulated signals. In general, the distance information of the optical fiber from the accommodation station to the base station is considered to be unknown, or even if known, the exact length cannot be known. It is very difficult to measure the fiber length particularly when the optical fiber is in a passive optical network (PON) configuration. The applicable range of Patent Document 1 seems to be very limited because it requires accurate distance information of an optical fiber.

On the other hand, the technique of Non Patent Document 1 makes the wavelengths fixed and the dispersion variable to cause delay differences between modulated optical signals. Here, the wavelength utilization efficiency is better than that of Patent Document 1 because the wavelengths are fixed. It is also unnecessary to control the optical demultiplexer because the optical splitting is fixed. However, high accuracy seems to be required for the design and manufacture of the PDM for adjusting the dispersion. Thus, there is a risk of increasing the size and cost of apparatuses.

In addition, Non Patent Document 1 does not mention application to RoF. Therefore, when RoF is applied to Non Patent Document 1 for long-distance optical fiber transmission, the influence of chromatic dispersion during transmission through the optical fiber must be taken into consideration, in addition to the dispersion adjustment by the PDM. Further, both Patent Document 1 and Non Patent Document 1 mention only beamforming of transmitting antennas and do not mention beamforming of receiving antennas.

In view of the above circumstances, it is an object of the present invention to provide a wireless communication system, an accommodation station apparatus, and a wireless communication method which can perform beamforming of transmitting and receiving antennas of an RoF system while limiting deterioration of wavelength utilization efficiency and cost increase without requiring base station control and optical fiber distance information.

Means for Solving the Problem

An aspect of the present invention provides a wireless communication system including an accommodation station apparatus; a base station apparatus that is connected to the accommodation station apparatus through an optical transmission line and has n antenna elements where n is an integer of 2 or more; and a terminal configured to wirelessly communicate with the base station apparatus. The accommodation station apparatus includes: a transmission unit configured to adjust phases of optically modulated transmission signals, obtained respectively by modulating light of n different first wavelengths with a transmission signal, by first amounts of phase adjustment corresponding respectively to the n different first wavelengths and output a combined optically modulated transmission signal obtained by combining the phase-adjusted optically modulated transmission signals of the n different first wavelengths to the optical transmission line; a transmission phase control unit configured to, when transmitting p beacon signals where p is an integer of 2 or more as the transmission signal, control the transmission unit such that the transmission unit adjusts phases of the p beacons signals by the first amounts of phase adjustment which differ between the p beacon signals; a reception unit configured to demultiplex a combined optically modulated received signal transmitted through the optical transmission line into optically modulated received signals of n different second wavelengths and convert the demultiplexed optically modulated received signals into electrical signals which have been phase-adjusted by second amounts of phase adjustment corresponding respectively to the n different second wavelengths; a reception phase control unit configured to, when q combined optically modulated received signals where q is an integer of 2 or more indicating identification information of a beacon signal selected from the p beacon signals based on the terminal's received powers of the p beacon signals have been input from the optical transmission line, control the reception unit such that the reception unit adjusts phases of the q combined optically modulated received signals by the second amounts of phase adjustment which differ between the q combined optically modulated received signals; a transmission phase adjustment amount determination unit configured to control the transmission unit such that the transmission unit performs phase adjustment by the first amounts of phase adjustment that have been used for the beacon signal of the identification information indicated by the combined optically modulated received signals; and a reception phase adjustment amount determination unit configured to control the reception unit such that the reception unit performs phase adjustment by the second amounts of phase adjustment that have been used for a combined optically modulated received signal selected from the q combined optically modulated received signals based on received powers of electrical signals into which the q combined optically modulated received signals have been converted. The base station apparatus includes: a base station optical demultiplexing unit configured to demultiplex the combined optically modulated transmission signal transmitted through the optical transmission line into optically modulated transmission signals of the n different first wavelengths; an optical to electrical conversion unit configured to convert the optically modulated transmission signals of the n first wavelengths into electrical signals and radiate the electrical signals wirelessly from the antenna elements corresponding respectively to the n first wavelengths; an electrical to optical conversion unit configured to modulate optical signals of the n different second wavelengths corresponding respectively to the n antenna elements with wireless signals that the n antenna elements have received from the terminal to generate optically modulated received signals; and a base station optical combining unit configured to output a combined optically modulated received signal obtained by combining the optically modulated received signals of the n second wavelengths generated by the electrical to optical conversion unit to the optical transmission line. The terminal includes: a wireless reception unit configured to receive a wireless signal from the base station apparatus; a wireless transmission unit configured to transmit a wireless signal to the base station apparatus; and a beacon selection unit configured to, when the wireless reception unit has received the p beacon signals, cause the wireless transmission unit to transmit q wireless signals indicating the identification information of the beacon signal selected based on the received powers of the p beacon signals.

An aspect of the present invention provides the above wireless communication system, wherein the n antenna elements are arranged at predetermined intervals, the n first wavelengths are wavelengths at first wavelength intervals included in a range in which chromatic dispersion in the optical transmission line is capable of being regarded as constant, the n second wavelengths are wavelengths at second wavelength intervals included in a range in which chromatic dispersion in the optical transmission line is capable of being regarded as constant, the n first amounts of phase adjustment are amounts of phase adjustment which make phases of wireless signals spaced at first phase intervals, and the n second amounts of phase adjustment are amounts of phase adjustment which make phases of wireless signals spaced at second phase intervals.

An aspect of the present invention provides the above wireless communication system, wherein the transmission unit includes: an optical modulation unit configured to modulate light of the n different first wavelengths with the transmission signal to generate optically modulated transmission signals; a phase adjustment unit configured to adjust phases of the optically modulated transmission signals of the n first wavelengths generated by the optical modulation unit by the first amounts of phase adjustment corresponding respectively to the first wavelengths; and an optical combining unit configured to combine the optically modulated transmission signals of the n first wavelengths which have been phase-adjusted by the phase adjustment unit to generate a combined optically modulated transmission signal and output the combined optically modulated transmission signal to the optical transmission line.

An aspect of the present invention provides the above wireless communication system, wherein the transmission unit includes: a splitting unit configured to split the transmission signal into n transmission signals corresponding respectively to the n different first wavelengths; a phase adjustment unit configured to adjust phases of the n transmission signals split by the splitting unit by the first amounts of phase adjustment corresponding to the first wavelengths corresponding to the n transmission signals; an optical modulation unit configured to modulate light of the n different first wavelengths with the transmission signals which have been phase-adjusted by the first amounts of phase adjustment corresponding to the first wavelengths to generate optically modulated transmission signals; and an optical combining unit configured to combine the optically modulated transmission signals of the n first wavelengths generated by the optical modulation unit to generate a combined optically modulated transmission signal and output the combined optically modulated transmission signal to the optical transmission line.

An aspect of the present invention provides the above wireless communication system, wherein the reception unit includes: a demultiplexing unit configured to demultiplex the combined optically modulated received signal into optically modulated received signals of the n second wavelengths; a phase adjustment unit configured to adjust phases of the optically modulated received signals of the n second wavelengths demultiplexed by the demultiplexing unit by the second amounts of phase adjustment corresponding to the second wavelengths; a combining unit configured to combine the optically modulated received signals of the n second wavelengths which have been phase-adjusted by the phase adjustment unit; and a conversion unit configured to convert the optically modulated received signals combined by the combining unit into electrical signals.

An aspect of the present invention provides the above wireless communication system, wherein the reception unit includes: a demultiplexing unit configured to demultiplex the combined optically modulated received signal into optically modulated received signals of the n second wavelengths; a conversion unit configured to convert the optically modulated received signals of the n second wavelengths demultiplexed by the demultiplexing unit into electrical signals; and a phase adjustment unit configured to adjust phases of the n electrical signals obtained through conversion of the conversion unit by the second amounts of phase adjustment corresponding to the second wavelengths.

An aspect of the present invention provides an accommodation station apparatus connected to a base station apparatus through an optical transmission line, the base station apparatus being configured to perform wireless communication with a terminal through n antenna elements where n is an integer of 2 or more, the accommodation station apparatus including: a transmission unit configured to adjust phases of optically modulated transmission signals, obtained respectively by modulating light of n different first wavelengths corresponding to then antenna elements with a transmission signal, by first amounts of phase adjustment corresponding respectively to the n first wavelengths and output a combined optically modulated transmission signal obtained by combining the phase-adjusted optically modulated transmission signals of the n first wavelengths to the optical transmission line; a transmission phase control unit configured to, when transmitting p beacon signals where p is an integer of 2 or more as the transmission signals, control the transmission unit such that the transmission unit adjusts phases of the p beacons signals by the first amounts of phase adjustment which differ between the p beacon signals; a reception unit configured to demultiplex a combined optically modulated received signal transmitted through the optical transmission line into optically modulated received signals of n different second wavelengths corresponding to the n antenna elements and convert the demultiplexed optically modulated received signals into electrical signals which have been phase-adjusted by second amounts of phase adjustment corresponding respectively to the n second wavelengths; a reception phase control unit configured to, when q combined optically modulated received signals where q is an integer of 2 or more indicating identification information of a beacon signal selected from the p beacon signals based on the terminal's received powers of the p beacon signals have been input from the optical transmission line, control the reception unit such that the reception unit adjusts phases of the q combined optically modulated received signals by the second amounts of phase adjustment which differ between the q combined optically modulated received signals; a transmission phase adjustment amount determination unit configured to control the transmission unit such that the transmission unit performs phase adjustment by the first amounts of phase adjustment that have been used for the beacon signal of the identification information indicated by the combined optically modulated received signals; and a reception phase adjustment amount determination unit configured to control the reception unit such that the reception unit performs phase adjustment by the second amounts of phase adjustment that have been used for a combined optically modulated received signal selected from the q combined optically modulated received signals based on received powers of electrical signals into which the q combined optically modulated received signals have been converted.

An aspect of the present invention provides a wireless communication method for a wireless communication system including an accommodation station apparatus, a base station apparatus that is connected to the accommodation station apparatus through an optical transmission line and has n antenna elements where n is an integer of 2 or more, and a terminal configured to wirelessly communicate with the base station apparatus, the wireless communication method including: a transmission step in which the accommodation station apparatus adjusts phases of optically modulated transmission signals, obtained respectively by modulating light of n different first wavelengths with a transmission signal, by first amounts of phase adjustment corresponding respectively to the n different first wavelengths, and outputs a combined optically modulated transmission signal obtained by combining the phase-adjusted optically modulated transmission signals of the n different first wavelengths to the optical transmission line; a transmission phase control step in which the accommodation station apparatus, when transmitting p beacon signals where p is an integer of 2 or more as the transmission signals in the transmission step, performs control to adjust phases of the p beacons signals by the first amounts of phase adjustment which differ between the p beacon signals; a reception step in which the accommodation station apparatus demultiplexes a combined optically modulated received signal transmitted through the optical transmission line into optically modulated received signals of n different second wavelengths and converts the demultiplexed optically modulated received signals into electrical signals which have been phase-adjusted by second amounts of phase adjustment corresponding respectively to the n second wavelengths; a reception phase control step in which the accommodation station apparatus, when q combined optically modulated received signals where q is an integer of 2 or more indicating identification information of a beacon signal selected from the p beacon signals based on the terminal's received powers of the p beacon signals have been input from the optical transmission line, performs control to adjust phases of the q combined optically modulated received signals by the second amounts of phase adjustment which differ between the q combined optically modulated received signals in the reception step; a transmission phase adjustment amount determination unit in which the accommodation station apparatus performs control to perform phase adjustment by the first amounts of phase adjustment that have been used for the beacon signal of the identification information indicated by the combined optically modulated received signals in the transmission step; a reception phase adjustment amount determination step in which the accommodation station apparatus performs control to perform phase adjustment by the second amounts of phase adjustment that have been used for a combined optically modulated received signal selected from the q combined optically modulated received signals based on received powers of electrical signals into which the q combined optically modulated received signals have been converted in the reception step; an optical demultiplexing step in which the base station apparatus demultiplexes the combined optically modulated transmission signal transmitted through the optical transmission line into optically modulated transmission signals of the n different first wavelengths; an optical to electrical conversion step in which the base station apparatus converts the optically modulated transmission signals of the n first wavelengths into electrical signals and radiates the electrical signals wirelessly from the antenna elements corresponding respectively to the n first wavelengths; an electrical to optical conversion step in which the base station apparatus modulates optical signals of the n different second wavelengths corresponding respectively to the n antenna elements with wireless signals that the n antenna elements have received from the terminal to generate optically modulated received signals; an optical combining step in which the base station apparatus outputs a combined optically modulated received signal obtained by combining the optically modulated received signals of the n second wavelengths generated in the electrical to optical conversion step to the optical transmission line; a wireless reception step in which the terminal receives a wireless signal from the base station apparatus; a wireless transmission step in which the terminal transmits a wireless signal to the base station apparatus; and a beacon selection step in which the terminal, when the p beacon signals have been received in the wireless reception step, causes q wireless signals indicating the identification information of the beacon signal selected based on the received powers of the p beacon signals to be transmitted in the wireless transmission step.

Effects of the Invention

According to the present invention, it is possible to perform beamforming of transmitting and receiving antennas of an RoF system while limiting deterioration of wavelength utilization efficiency and cost increase without requiring base station control and optical fiber distance information.

