Communication device and communication system

Abstract

This communication device of a base station that applies analog optical fiber radio and is divided into a host device and an overhang station includes: a light source that generates a first optical carrier and a second optical carrier the have a frequency difference required for photo-mixing; an electro-optical conversion unit that generates optical signals of mutually different signals respectively carried on the first optical carrier and the second optical carrier; and a multiplexing unit that multiplexes a plurality of the optical signals generated by the electro-optical conversion unit.

Description

Communication device and communication system

[0001] The present invention relates to a communication device and a communication system.

[0002] There is a technology called analog RoF (Radio-over-Fiber) that modulates the intensity of an optical signal with a radio signal and transmits the radio-shaped optical signal over an optical fiber. With analog RoF, the original radio signal can be extracted simply by performing O / E (Optical-to-Electrical) conversion, which converts the transmitted optical signal into an electrical signal. Generally, a photodiode is used for O / E conversion.

[0003] By applying this analog RoF to a wireless base station, the functions of a conventional wireless base station can be separated into a central station (signal processing unit) and a base station (antenna unit) (see, for example, Non-Patent Document 1). Conventional wireless base stations without functional separation have functions such as an antenna, an amplifier, electro-optical conversion (converting electrical signals to optical signals), optical-to-electrical conversion, and signal processing. However, by applying analog RoF and consolidating the signal processing function in the central station, the functions of the base station can be simplified. This reduces the size and power consumption of the base station, improving installation ease and cost efficiency. Furthermore, by consolidating signal processing functions dependent on the wireless system in the central station, it is possible to leave only common functions independent of the wireless system in the base station. This allows the base station to be shared among multiple wireless systems as long as the frequency range supported by the antenna and amplifier is within the range. Furthermore, the establishment and renewal of wireless systems can be performed solely through operations on the central station side. This enables efficient deployment and operation of wireless systems.

[0004] However, electro-optical converters capable of intensity modulating electrical signals in high frequency bands such as the millimeter wave band are generally expensive because they require high processing speeds. This is one of the barriers to economically building high-frequency band wireless systems. In contrast, transmission methods known as BBoF (Baseband over Fiber) and IFoF (Intermediate Frequency over Fiber) can alleviate the processing speed requirements of electro-optical converters. These transmission methods involve intensity-modulating BB or IF electrical signals in a central station using a modulator and optically transmitting them, and then converting the optical signal to an electrical signal in a base station, followed by frequency conversion, to obtain a signal with the desired frequency. However, the BBoF and IFoF configurations require frequency conversion in the base station, which necessitates additional functionality in the base station.

[0005] Furthermore, one of the signal transmission technologies for analog RoF is photomixing (see, for example, Non-Patent Document 2). Photomixing enables optical transmission of high-frequency electrical signals using an easily available and relatively inexpensive electro-optical converter. Furthermore, photomixing does not require additional functionality at the base station, and is therefore considered to be an effective means for a configuration in which analog RoF is applied and radio base stations are separated.

[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.M. Che, K. Kondo, H. Kanaya and K. Kato, “Arrayed Photomixers for THz Beam-Combining and Beam-Steering,” IEEE JLT, vol. 40, no. 20, pp. 6657-6665, Sep. 2022.

[0007] However, heterodyne detection, which is performed during photomixing, requires an optical carrier with two frequencies: one for transmitting the signal and one for frequency conversion. In other words, photomixing requires an optical carrier with two frequencies to transmit one signal. This reduces wavelength utilization efficiency. Furthermore, the transmitter must provide twice as many light sources as the number of transmitted signals, which increases the equipment size.

[0008] In view of the above circumstances, an object of the present invention is to provide a technology that enables more efficient signal transmission by photomixing while suppressing an increase in the size of the equipment in a communication system in which base station functions are functionally separated using analog RoF.

[0009] One aspect of the present invention is a communication device for a base station that applies analog optical fiber radio and is separated into a base station and a local station, and is equipped with: a light source that generates a first optical carrier and a second optical carrier having the frequency difference required for photomixing; an electrical-to-optical conversion unit that generates optical signals carrying different signals for the first optical carrier and the second optical carrier; and a multiplexing unit that multiplexes the optical signals generated by the electrical-to-optical conversion unit.

[0010] Another aspect of the present invention is a communication device for a base station that applies analog fiber optic radio and is separated into an aggregation station and a self-device, and that includes: a transmission unit that extracts a sideband wave having a desired signal and the optical carrier having a frequency that is separated from the sideband wave by the frequency of the desired signal from an optical signal that is generated in the aggregation station by multiplexing optical signals that are generated by carrying different electrical signals on first and second optical carriers that have a frequency difference required for photomixing; and an optical-electrical conversion unit that converts the optical signal extracted by the transmission unit into an electrical signal.

[0011] Another aspect of the present invention is a communication system of a base station that applies analog fiber optic radio and is separated into a central station and a base station, wherein the central station comprises: a light source that generates a first optical carrier and a second optical carrier having a frequency difference required for photomixing; an electrical-to-optical conversion unit that generates optical signals carrying different electrical signals for the first optical carrier and the second optical carrier, respectively; and a multiplexing unit that multiplexes the optical signals generated by the electrical-to-optical conversion unit; and the base station comprises: a transmission unit that extracts, from the optical signal transmitted from the central station, a sideband wave having a desired signal and an optical carrier having a frequency separated from the sideband wave by the frequency of the desired signal; and an optical-to-electrical conversion unit that converts the optical signal extracted by the transmission unit into an electrical signal.

[0012] According to the present invention, in a communication system in which base station functions are separated using analog RoF, it is possible to perform signal transmission by photomixing more efficiently while suppressing an increase in the size of the equipment.

[0013] FIG. 1 is an overall configuration diagram of a communication system 1 in a first embodiment of the present invention. FIG. 2 is a flowchart showing an operation of a central station 10 in the first embodiment of the present invention. FIG. 3 is a flowchart showing an operation of a base station 20 in the first embodiment of the present invention. FIG. 4 is an overall configuration diagram of a communication system 1a in a second embodiment of the present invention. FIG. 5 is an overall configuration diagram of a communication system 1b in a third embodiment of the present invention.

[0014] Hereinafter, a communication device and a communication system according to an embodiment of the present invention will be described in detail with reference to the drawings.

[0015] <First Embodiment> The first embodiment will be described below. Fig. 1 is an overall configuration diagram of a communication system 1 in the first embodiment of the present invention. The communication system 1 is a system in which wireless base station functions are separated into an aggregate station and a base station, and analog RoF is used to configure a simplified wireless communication base station. As shown in Fig. 1, the communication system 1 includes an aggregate station 10, base stations 20-1 and 20-2, and an optical fiber 30. In the following description, when it is not necessary to distinguish between the base station 20-1 and the base station 20-2, they will simply be referred to as "base station 20." The aggregate station 10 and the base station 20 are connected by the optical fiber 30.

[0016] In the central station 10, a plurality of electrical signals output from a signal processing unit 11 (described later) are electro-optically converted using optical carriers each having a different frequency, and the resulting wavelength multiplexed signals are output to the optical fiber 30. The frequency of each optical carrier is determined so that the frequency difference between the sidebands of adjacent optical signals is equal to the frequency f of the desired radio signal. ＲＦ That is, the frequency interval of the optical carrier is set to f ＲＦ +f ＩＦ At the base station 20, f ＲＦ Photoelectric conversion is performed using an optical carrier and a sideband wave having a frequency difference of f ＲＦ The generated electric signal has a frequency of 100 kHz and is radiated as a radio wave from an antenna 23 described later.

[0017] As shown in FIG. 1, the central station 10 includes a signal processing unit 11, electrical-to-optical conversion units 12-1 and 12-2, a light source 13, and a multiplexer 14.

[0018] In the following description, when there is no need to distinguish between the electrical-optical conversion unit 12-1 and the electrical-optical conversion unit 12-2, they will simply be referred to as "electrical-optical conversion unit 12."

[0019] The signal processing unit 11 is ＩＦ (f ＩＦ <f ＲＦ The signal processing unit 11 outputs the generated radio signals to the electrical-to-optical conversion units 12-1 and 12-2.

[0020] The electrical-optical conversion unit 12 generates an optical signal by performing modulation using the optical carrier input from the light source 13 and the electrical signal input from the signal processing unit 11. The electrical-optical conversion unit 12 outputs the generated optical signal to the multiplexer 14.

[0021] The light source 13 generates a plurality of optical carriers having a frequency difference required for photomixing, and outputs the generated optical carriers to the electrical-to-optical conversion units 12-1 and 12-2, respectively.

[0022] The multiplexer 14 multiplexes the optical signals output from the electrical-optical conversion units 12-1 and 12-2, and outputs the wavelength-multiplexed optical signal to the optical fiber 30. The optical signal output from the multiplexer 14 is transmitted to the base station 20 via the optical fiber 30.

[0023] As shown in FIG. 1, the base station 20 includes an optical bandpass filter 21, an optical-electrical conversion unit 22, and an antenna 23.

[0024] The optical signal transmitted through the optical fiber 30 is input to the optical bandpass filter 21. The optical bandpass filter 21 separates the sidebands containing the desired signal from the sidebands f ＲＦ The optical carriers having different frequencies are extracted and output to the optical-electrical conversion unit 22 .

[0025] The optical-electrical conversion unit 22 detects the optical signal input via the optical bandpass filter 21 and outputs an electrical signal to the antenna 23. Note that the optical-electrical conversion unit 22 also receives sideband waves containing signals other than the desired signal (hereinafter referred to as "unwanted signals"), and therefore outputs the unwanted signals as well.

[0026] The antenna 23 radiates the desired electrical signal input from the optical-electrical conversion unit 22 into space as a radio wave. If the unwanted signal is outside the fractional bandwidth of the antenna 23, the antenna 23 also functions as a filter to remove the unwanted signal. If the unwanted signal is within the fractional bandwidth of the antenna 23, an electrical filter (not shown) can be inserted between the optical-electrical conversion unit 22 and the antenna 23 so that only the desired electrical signal is input to the antenna 23.

[0027] The following describes an example of the operation of the central station 10. Fig. 2 is a flowchart showing the operation of the central station 10 in the first embodiment of the present invention.

[0028] The signal processing unit 11 is ＩＦ (f ＩＦ <f ＲＦ ) and outputs the radio signals to the electrical-to-optical converters 12-1 and 12-2 (step S001).

[0029] Next, the light source 13 generates a plurality of optical carriers having a frequency difference required for photomixing, and outputs them to the electrical-to-optical conversion units 12-1 and 12-2, respectively (step S002).

[0030] Next, the electrical-optical conversion unit 12 performs modulation using the optical carrier input from the light source 13 and the electrical signal input from the signal processing unit 11 to generate an optical signal and outputs it to the multiplexer 14 (step S003).

[0031] Next, the multiplexer 14 multiplexes the optical signals output from the electrical-to-optical converters 12-1 and 12-2, and transmits the multiplexed optical signals to the base station 20 via the optical fiber 30 (step S004).

[0032] This completes the operation of the central station 10 shown in the flowchart of FIG.

[0033] An example of the operation of the base station 20 will now be described. Fig. 3 is a flowchart showing the operation of the base station 20 in the first embodiment of the present invention.

[0034] The optical bandpass filter 21 receives an input of an optical signal transmitted through an optical fiber 30. The optical bandpass filter 21 separates a sideband having a desired signal from the sideband and a component f ＲＦ Optical carriers having different frequencies are extracted and output to the optical-electrical conversion unit 22 (step S101).

[0035] Next, the optical-electrical conversion unit 22 detects the optical signal input via the optical bandpass filter 21 and outputs an electrical signal to the antenna 23 (step S102).

[0036] Next, the antenna 23 radiates the desired electrical signal input from the optical-electrical conversion unit 22 into space as a radio wave (step S103).

[0037] This completes the operation of the base station 20 shown in the flowchart of FIG.

[0038] As described above, the communication system 1 according to the first embodiment of the present invention is a communication system in which base station functions are separated using analog RoF. The communication system 1 transmits optical signals on two wavelengths, each carrying a different signal, which is used in heterodyne detection during photomixing. The transmitting side (central station 10) performs electrical-to-optical conversion at one wavelength per signal, and the converted optical signals are multiplexed and transmitted via an optical fiber 30. The receiving side (base station 20) performs optical-to-electrical conversion using the optical carrier wavelength used on the other side through an optical bandpass filter 21 to convert the signal back to an electrical signal.

[0039] With this configuration, the communication system 1 according to the first embodiment can transmit one signal over one wavelength while maintaining the advantages of photomixing. This allows the communication system 1 to reduce the decrease in frequency utilization efficiency while suppressing increases in the device scale and device costs, thereby enabling more efficient signal transmission by photomixing.

[0040] Second Embodiment A second embodiment will be described below. A communication system 1a in the second embodiment described below differs in configuration from the communication system 1 in the first embodiment described above in that different signals are transmitted using upper and lower sideband waves of an optical signal. By transmitting different signals using upper and lower sideband waves, the communication system 1a in the second embodiment can further improve wavelength utilization efficiency compared to the communication system 1 in the first embodiment described above.

[0041] Fig. 4 is an overall configuration diagram of a communication system 1a according to the second embodiment of the present invention. As shown in Fig. 4, the communication system 1a includes a central station 10a, base stations 20-1 and 20-2, and an optical fiber 30. The central station 10a and the base station 20 are connected by the optical fiber 30.

[0042] As shown in FIG. 4, the central station 10a includes a signal processing unit 11, electrical-to-optical conversion units 12a-1 to 12a-3, a light source 13, and a multiplexer .

[0043] In the following description, when it is not necessary to distinguish between the electro-optical conversion units 12a-1, 12a-2, and 12a-3, they will be simply referred to as "electro-optical conversion units 12a."

[0044] The signal processing unit 11 is ＩＦ (f ＩＦ <f ＲＦ ) and outputs it to the electrical-optical converters 12a-1 to 12a-3. ＲＦ is the frequency of the desired radio signal.

[0045] The electro-optical conversion unit 12a performs modulation using the optical carrier input from the light source 13 and the electrical signal input from the signal processing unit 11 to generate an optical signal having different signals in the upper and lower sidebands. The electro-optical conversion unit 12 outputs the generated optical signal to the multiplexer 14. The electro-optical conversion unit 12a also has a function to generate, for example, an analog ISB (Independent Sideband) signal.

[0046] The light source 13 generates a plurality of optical carriers having a frequency difference required for photomixing, and outputs the generated optical carriers to the electrical-to-optical conversion units 12-1 to 12-3, respectively.

[0047] The multiplexer 14 multiplexes the optical signals output from the electrical-optical conversion units 12a-1 to 12a-3, and outputs the wavelength-multiplexed optical signal to the optical fiber 30. The optical signal output from the multiplexer 14 is transmitted to the base station 20 via the optical fiber 30.

[0048] The configuration of the base station 20 is the same as that of the first embodiment, and therefore a description thereof will be omitted.

[0049] As described above, the communication system 1a according to the second embodiment of the present invention is a communication system in which base station functions are separated using analog RoF. The communication system 1a performs optical transmission by carrying different signals on two wavelengths used in heterodyne detection during photomixing. The transmitting side (central station 10a) performs electro-optical conversion at one wavelength per signal, and the converted optical signals are multiplexed and transmitted via the optical fiber 30. The receiving side (base station 20) performs photo-electrical conversion using the optical carrier wavelength used on the other side through the optical bandpass filter 21, and converts the signals back into electrical signals.

[0050] With this configuration, the communication system 1a according to the second embodiment can transmit two signals over one wavelength while maintaining the advantages of photomixing. As a result, the communication system 1a can reduce the decrease in frequency utilization efficiency while suppressing increases in the device size and device costs, and can more efficiently transmit signals by photomixing.

[0051] Furthermore, the communication system 1a in the second embodiment transmits different signals using upper and lower sidebands of an optical signal, which allows the communication system 1a to achieve higher wavelength utilization efficiency than the communication system 1 in the first embodiment.

[0052] <Third Embodiment> A third embodiment will be described below. A communication system 1b in the third embodiment described below transmits another signal using a wavelength between the two wavelengths used in heterodyne detection. When detecting by optical-electrical conversion, a desired sideband and optical carrier are extracted by an optical bandpass filter 21 preceding the optical-electrical conversion. However, in this case, because the optical carrier and sideband with a wavelength between the two wavelengths are also input to the optical-electrical conversion unit 22, the multiple signals output from the optical-electrical conversion unit 22 each have a different frequency.

[0053] At this time, depending on the wavelength used and the frequency of the input signal when modulating by electro-optical conversion, the frequency f of the desired radio signal may be ＲＦThere is a possibility that signals with frequencies close to the wavelengths of the optical signals may be generated, which may make it difficult to process them using filters in the electrical stage. In this case, the generation of unwanted signals can be suppressed by providing a filter (filter 24, described later) in the stage preceding the optical bandpass filter 21 that passes only the two wavelengths used in heterodyne detection (and the sidebands associated with the wavelengths).

[0054] In a communication system 1b according to a third embodiment described below, the frequency interval of the optical carrier is f ＲＦ +f ＩＦ The configuration differs from that of the communication system 1 in the first embodiment described above in that it is narrower.

[0055] Fig. 5 is an overall configuration diagram of a communication system 1b according to a third embodiment of the present invention. As shown in Fig. 5, the communication system 1b includes a central station 10b, base stations 20b-1 and 20b-2, and an optical fiber 30. In the following description, when it is not necessary to distinguish between the base station 20b-1 and the base station 20b-2, they will simply be referred to as "base station 20b." The central station 10b and the base station 20b are connected by the optical fiber 30.

[0056] As shown in FIG. 5, the central station 10b includes a signal processing unit 11, electrical-to-optical conversion units 12-1 to 12-3, a light source 13b, and a multiplexer .

[0057] The signal processing unit 11 is ＩＦ (f ＩＦ <f ＲＦ The signal processing unit 11 outputs the generated radio signals to the electrical-to-optical conversion units 12-1 to 12-3, respectively.

[0058] The electrical-optical conversion unit 12 generates an optical signal by performing modulation using the optical carrier input from the light source 13b and the electrical signal input from the signal processing unit 11. The electrical-optical conversion unit 12 outputs the generated optical signal to the multiplexer 14.

[0059] The light source 13b emits light having an arbitrary frequency difference Δf (Δf=(f ＲＦ +f ＩＦ) / n, where n is an integer equal to or greater than 2. The light source 13b outputs the generated optical carriers to the electrical-to-optical converters 12-1 to 12-3, respectively.

[0060] The multiplexer 14 multiplexes the optical signals output from the electrical-optical conversion units 12a-1 to 12a-3, and outputs the wavelength-multiplexed optical signal to the optical fiber 30. The optical signal output from the multiplexer 14 is transmitted to the base station 20 via the optical fiber 30.

[0061] As shown in FIG. 5, the base station 20 b includes an optical bandpass filter 21 , an optical-electrical converter 22 , an antenna 23 , and a filter 24 .

[0062] The optical signal transmitted through the optical fiber 30 is input to the filter 24. The filter 24 extracts only the optical signal having the optical carrier and sidebands used for photomixing. The filter 24 may be, for example, a combination of two optical bandpass filters corresponding to the two optical signals and a multiplexer. The filter 24 may be, for example, an arrayed waveguide grating. The filter 24 outputs the extracted optical signal to the optical bandpass filter 21.

[0063] The optical bandpass filter 21 separates the sidebands containing the desired signal from the optical signal input from the filter 24 and the sidebands containing the desired signal. ＲＦ The filter 24 extracts the optical carrier having a different frequency from the optical signal and outputs it to the optical-electrical conversion unit 22. If the filter 24 is configured to extract only the desired sideband and optical carrier, the optical bandpass filter 21 becomes unnecessary.

[0064] The optical-electrical conversion unit 22 detects the optical signal input via the filter 24 and the optical bandpass filter 21, and outputs an electrical signal to the antenna 23. Note that the optical-electrical conversion unit 22 also receives sideband waves containing unwanted signals (signals other than the desired signal), and therefore outputs the unwanted signals as well.

[0065] The antenna 23 radiates the desired electrical signal input from the optical-electrical conversion unit 22 into space as a radio wave. If the unwanted signal is outside the fractional bandwidth of the antenna 23, the antenna 23 also functions as a filter to remove the unwanted signal. If the unwanted signal is within the fractional bandwidth of the antenna 23, an electrical filter (not shown) can be inserted between the optical-electrical conversion unit 22 and the antenna 23 so that only the desired electrical signal is input to the antenna 23.

[0066] As described above, the communication system 1b according to the third embodiment of the present invention is a communication system in which base station functions are separated using analog RoF. The communication system 1b performs optical transmission by carrying different signals on two wavelengths used in heterodyne detection performed during photomixing. The transmitting side (central station 10b) performs electro-optical conversion at one wavelength per signal, and the converted optical signals are multiplexed and transmitted via optical fiber 30. The receiving side (base station 20b) performs photo-electrical conversion using the optical carrier wavelength used on the other side through filter 24 and optical bandpass filter 21, and converts the signals back into electrical signals.

[0067] With this configuration, the communication system 1b according to the third embodiment can transmit one signal over one wavelength while maintaining the advantages of photomixing. As a result, the communication system 1b can reduce the decrease in frequency utilization efficiency while suppressing increases in the equipment size and equipment costs, and can more efficiently transmit signals by photomixing.

[0068] In the above-described embodiments, a plurality of base stations 20 (or 20b) are connected to one central station 10 (or 10a, 10b), and each base station 20 (or 20b) transmits a different signal. However, this configuration is not limited to this. For example, a configuration may be adopted in which one base station 20 (or 20b) is connected to one central station 10 (or 10a, 10b), the base station 20 (or 20b) is provided with a plurality of antennas 23, and each of the plurality of antennas 23 transmits a different signal.

[0069] In each of the above-described embodiments, the number of signals to be multiplexed may be further increased by combining with other multiplexing techniques such as polarization multiplexing, mode multiplexing, and multi-core SDM (Space Division Multiplexing).

[0070] In each of the above-described embodiments, an optical comb (optical frequency comb) can be used as the light source 13 (or 13b) of the central station 10 (or 10a, 10b). However, the present invention is not limited to this configuration. For example, other light sources such as laser diodes having the same number as the electrical-optical conversion units 12 (or 12a) having the desired wavelengths may be used as the light source 13 (or 13b).

[0071] As the electrical-optical conversion unit 12 of the central station 10 in the first embodiment (or the central station 10b in the third embodiment), instead of using a general intensity modulator, a modulator that generates a carrier suppression signal, a carrier suppression signal and a single sideband suppression signal, or a single sideband signal only may be used.

[0072] In each of the above-described embodiments, a single-mode fiber (SMF) can be used as the optical fiber 30, but a multi-mode fiber (MMF), a multi-core fiber (MCF), a polarization-maintaining fiber, or other optical signal transmission cables may also be used.

[0073] In each of the above-described embodiments, it is assumed that the optical bandpass filter 21 provided in the base station 20 (or 20b) is configured to pass the optical carrier and sidebands used in heterodyne detection without loss. However, this configuration is not limited to this, and an optical bandpass filter designed to reduce the level of the optical carrier may also be used. In this case, since only the level of the optical carrier is reduced, the modulation depth of the optical signal can be relatively improved.

[0074] In each of the above-mentioned embodiments, the central station 10 (or 10a, 10b) may transmit multiple signals to one base station 20 (or 20b), or may perform MIMO transmission using the multiple signals transmitted to the base station 20 (or 20b).

[0075] In each of the above-described embodiments, the format of the signal output from the signal processing unit 11 of the central station 10 (or 10a, 10b) is not limited to a specific frequency or radio system.

[0076] In each of the above-described embodiments, the antenna 23 provided in the base station 20 (or 20b) may be a single antenna or an array antenna using multiple antenna elements. The array antenna may also have a sub-array configuration.

[0077] In each of the above-described embodiments, the central station 10 (or 10a, 10b) and the base station 20 (or 20b) are configured with the minimum functions required for the present invention, but may be further provided with functions such as filters, amplifiers, and switches as necessary.

[0078] In the above-described embodiments, the device referred to as a "central station" can be replaced with a device called a "CS (Central Station)," a "Base Station," a "CU (Central Unit)," a "DU (Distributed Unit)," a "RU (Radio Unit)," etc. On the other hand, the device referred to as a "base station" can be replaced with a device called an "RRU (Remote Radio Unit)," a "Distributed Antenna (DA)," an "antenna," etc.

[0079] According to the above-described embodiment, an analog radio-over-fiber communication device is applied, and the communication device of the base station, which is separated into the base station and the central station, includes a light source, an electrical-to-optical converter, and a multiplexer. For example, the base station is the central station 10, 10a, or 10b in the embodiment, the base station is the base station 20 or 20b in the embodiment, the light source is the light source 13 or 13b in the embodiment, the electrical-to-optical converter is the electrical-to-optical converter 12 or 12a in the embodiment, and the multiplexer is the multiplexer 14 in the embodiment.

[0080] The light source generates a first optical carrier and a second optical carrier having a frequency difference required for photomixing. The electro-optical conversion unit generates optical signals carrying different signals on the first optical carrier and the second optical carrier, respectively. The multiplexing unit multiplexes the multiple optical signals generated by the electro-optical conversion unit.

[0081] In the communication device, the electrical-optical converter may generate an optical signal having different signals in the upper and lower sidebands of the optical signal. In this case, for example, the electrical-optical converter is the electrical-optical converter 12a in the embodiment.

[0082] In the above communication device, the light source may further generate a third optical carrier having a frequency between the frequency of the first optical carrier and the frequency of the second optical carrier. In this case, for example, the light source is light source 13b in the embodiment. Then, the electro-optical conversion unit may generate an optical signal carrying a different signal on the third optical carrier.

[0083] Furthermore, according to the above-described embodiment, an analog radio-on-fiber communication device in a base station that is separated into a central station and a base station itself includes a transmission unit and an optical-electrical conversion unit. For example, the communication system is the communication system 1, 1a, or 1b in the embodiment. The transmission unit extracts a sideband having a desired signal and an optical carrier having a frequency separated from the sideband by the frequency of the desired signal from an optical signal that is generated in the central station by multiplexing optical signals generated by carrying different electrical signals on first and second optical carriers having a frequency difference required for photomixing. The optical-electrical conversion unit detects the optical signal extracted by the transmission unit and converts it into an electrical signal.

[0084] Furthermore, according to the above-described embodiment, the communication system is a communication system of base stations separated into a central station and a base station, employing analog radio over fiber optics. The central station includes a light source, an electrical-to-optical converter, and a multiplexer. The light source generates a first optical carrier and a second optical carrier having a frequency difference required for photomixing. The electrical-to-optical converter generates optical signals carrying different electrical signals on the first optical carrier and the second optical carrier, respectively. The multiplexer multiplexes the multiple optical signals generated by the electrical-to-optical converter. The base station includes a transmission unit and an optical-to-electrical converter. The transmission unit extracts, from the optical signal transmitted from the central station, a sideband having a desired signal and an optical carrier having a frequency separated from the sideband by the frequency of the desired signal. The optical-to-electrical converter detects the optical signal extracted by the transmission unit and converts it into an electrical signal.

[0085] In the above communication system, the electrical-optical converter may generate a signal having different electrical signals in the upper and lower sidebands of the optical signal.

[0086] In the above communication system, the light source may further generate a third optical carrier having a frequency between the frequency of the first optical carrier and the frequency of the second optical carrier, and the electro-optical conversion unit may generate an optical signal carrying a different signal on the third optical carrier.

[0087] In the above communication system, the base station may further include a filter in front of the transmission unit. For example, the filter is filter 24 in the embodiment. The filter extracts an optical signal having an optical carrier and sidebands used for photomixing from the optical signal transmitted from the central station.

[0088] In the above-described embodiment, some or all of the configuration of the central stations 10, 10a, and 10b and the base stations 20 and 20b may be implemented by a computer. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program may be loaded into a computer system and executed. Note that the term "computer system" herein includes hardware such as an OS and peripheral devices. Furthermore, the term "computer-readable recording medium" refers to portable media such as floppy disks, optical magnetic disks, ROMs, and CD-ROMs, as well as storage devices such as hard disks built into computer systems. Furthermore, the term "computer-readable recording medium" may also include media that dynamically store programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or telephone lines, and media that store programs for a fixed period of time, such as volatile memory within the computer systems that serve as servers or clients in such cases. Furthermore, the above program may be one that realizes part of the above-mentioned functions, or may be one that can realize the above-mentioned functions in combination with a program already recorded in a computer system, or may be one that is realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0089] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the present invention that do not deviate from the gist of the present invention.

[0090] 1, 1a, 1b Communication system 10, 10a, 10b Central station 11 Signal processing unit 12, 12a Electrical-to-optical conversion unit 13, 13b Light source 14 Multiplexer 20, 20b Base station 21 Optical bandpass filter 22 Optical-to-electrical conversion unit 23 Antenna 24 Filter 30 Optical fiber

Claims

1. A communications device for a base station that uses analog radio over fiber and is separated into a base station and a local station, comprising: a light source that generates a first optical carrier and a second optical carrier with the frequency difference required for photomixing; an electrical-to-optical converter that generates optical signals carrying different signals on the first optical carrier and the second optical carrier; and a multiplexer that multiplexes the optical signals generated by the electrical-to-optical converter.

2. The communication device according to claim 1, wherein the electrical-to-optical converter generates the optical signal having different signals in upper and lower sidebands of the optical signal.

3. The communication device according to claim 1, wherein the light source further generates a third optical carrier having a frequency between the frequency of the first optical carrier and the frequency of the second optical carrier, and the electrical-to-optical conversion unit generates the optical signal carrying a further different signal on the third optical carrier.

4. A communications device for a base station that uses analog radio-over-fiber technology and is separated into a central station and its own device, comprising: a transmission unit that extracts a sideband wave having a desired signal and an optical carrier having a frequency that is separated from the sideband wave by the frequency of the desired signal from an optical signal that is generated by multiplexing optical signals generated in the central station by carrying different electrical signals on first and second optical carriers that have a frequency difference required for photomixing; and an optical-electrical conversion unit that detects the optical signal extracted by the transmission unit and converts it into an electrical signal.

5. A communications system using analog radio-over-fiber technology and having a base station separated into a central station and a base station, wherein the central station comprises: a light source that generates a first optical carrier and a second optical carrier with a frequency difference required for photomixing; an electrical-to-optical converter that generates optical signals carrying different electrical signals for the first optical carrier and the second optical carrier; and a multiplexer that multiplexes the optical signals generated by the electrical-to-optical converter; and the base station comprises: a transmission unit that extracts, from the optical signal transmitted from the central station, a sideband wave having a desired signal and the optical carrier having a frequency separated from the sideband wave by the frequency of the desired signal; and an optical-to-electrical converter that detects the optical signal extracted by the transmission unit and converts it into an electrical signal.

6. The communication system according to claim 5, wherein the electrical-optical conversion unit generates the signal having the electrical signals different from each other in upper and lower sidebands of the optical signal.

7. The communication system according to claim 5, wherein the light source further generates a third optical carrier having a frequency between the frequency of the first optical carrier and the frequency of the second optical carrier, and the electrical-to-optical conversion unit generates the optical signal carrying a different signal on the third optical carrier.

8. A communication system as described in claim 7, wherein the base station further comprises a filter provided in front of the transmission section, for extracting an optical signal having the optical carrier and the sideband waves used for the photomixing from the optical signal transmitted from the central station.

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