JP7751245B2 - Communication device and communication method - Google Patents

Description

The present invention relates to a communication device and a communication method.

A communication system has been proposed that uses analog Radio-over-Fiber (RoF) to divide the functions of a wireless base station into an aggregation station and a base station (see, for example, Non-Patent Document 1). The use of analog RoF simplifies the configuration of base stations, making the communication system more economical. Furthermore, the installation of multiple base stations in a wireless area makes the communication system more flexible.

The base station comprises an optical-electrical converter (O/E converter), an optical modulator (E/O converter), and an antenna. The base station may also comprise an amplifier. Analog RoF is applied to the section from the central station to the base station. If an intermediate frequency (IF) optical signal is transmitted in this section, the base station may also comprise a frequency converter.

As communications speeds and capacity increase, there is concern that the fronthaul (FH) band will increase, but by applying analog RoF to communications systems, it is possible to keep the FH band at the same level as the radio band.

Figure 11 shows an example of the relationship between the wireless area radius and the carrier-to-noise ratio (CNR) at the central station. A wireless signal transmitted upstream from a wireless terminal is received by the base station, and input power corresponding to the received power is input to an optical modulator installed in the base station. The optical modulator uses the input power to modulate the optical signal. If unwanted harmonics occur in the optical signal due to nonlinear distortion caused by the nonlinearity of the optical modulator, the signal-to-noise ratio, for example, the carrier-to-noise ratio (CNR) at the central station, deteriorates.

On the other hand, in order to widen the dynamic range of the input power to the optical modulator installed in the base station, a method has been proposed in which the base station performs distortion compensation using pre-distortion to improve the linearity of the optical signal transmitted from the optical modulator (see, for example, non-patent document 2).

This method achieves nonlinear digital pre-distortion (DPD) by combining linear and nonlinear optical modulator components. This model is, for example, the Wiener-Hammerstein (WH) model. The Wiener-Hammerstein model has the same basic structure as neural networks and multilayer perceptrons, and can utilize optimization techniques developed in the field of machine learning. Improving the linearity of the nonlinear block using optimization techniques makes it possible to expand the dynamic range of the input power.

Kodai Ito, Mizuki Suga, Hirofumi Shirato, Naoki Kita, and Takeshi Onisawa, "Efficient Accommodation of Diverse High-Frequency Band Wireless Systems Using Analog RoF," NTT Technical Review, 32(3), pp.15-17, 2020. T. Sasai, M. Nakamura, E. Yamazaki, A. Matsushita, S. Okamoto, K. Horikoshi, and Y. Kisaka: “Wiener-hammerstein model and its learning for nonlinear digital pre-distortion of optical transmitters,” Optics Express, Vol. 28, No. 21, pp. 30952-30963, Oct. 2020.

As mentioned above, if unwanted harmonics occur in the optical signal due to nonlinear distortion caused by the nonlinearity of the optical modulator, signal quality may be degraded. One way to prevent this degradation in signal quality is to widen the dynamic range of the input power to the optical modulator. To widen the dynamic range, it is possible to improve linearity by distortion compensation.

However, in order to improve linearity through distortion compensation such as predistortion, multiple linear devices (FIR (Finite Impulse Response) filters) are required. This increases the device cost of the base station.

Since the device cost of an optical modulator alone is not high, it is preferable to use an optical modulator alone for uplink communications from a base station without adding functions such as distortion compensation. However, it has been difficult to expand the radius of the wireless area by using an optical modulator in a base station, as the dynamic range of the input power is narrower than that of the wireless region.

In view of the above circumstances, the present invention aims to provide a communication device and a communication method that can expand the radius of the wireless area using an optical modulator whose input power dynamic range is narrower than that of the wireless area.

A communication device according to one embodiment of the present invention comprises an antenna unit that receives a radio signal and converts the received radio signal into an electrical signal; an electrical-to-optical conversion unit that generates an optical signal by intensity-modulating light using the signal power of the electrical signal converted by the antenna unit; and a control unit that controls the signal power of the electrical signal using a predetermined reception gain according to the beam formed by the antenna unit so that the signal power of the electrical signal input to the electrical-to-optical conversion unit does not exceed a threshold value.

A communication method according to one aspect of the present invention includes a receiving step in which an antenna unit receives a radio signal and converts the received radio signal into an electrical signal; a signal generating step in which an optical signal is generated by intensity-modulating light using the signal power of the electrical signal converted in the receiving step; and a control step in which the signal power of the electrical signal used to generate the optical signal in the signal generating step is controlled using a receiving gain predetermined according to the beam formed by the antenna unit so that the signal power of the electrical signal used to generate the optical signal does not exceed a threshold value.

This invention makes it possible to extend the wireless area radius using an optical modulator whose input power dynamic range is narrower than that of the wireless area.

FIG. 1 is a diagram illustrating an example of the configuration of a communication system according to a first embodiment. FIG. 10 is a diagram illustrating an example of the configuration of a base station according to a second embodiment. FIG. 11 is a diagram illustrating an example of the configuration of a base station according to a third embodiment. FIG. 13 is a diagram illustrating an example of the configuration of a base station according to a fourth embodiment. FIG. 13 is a diagram illustrating an example of the configuration of a base station according to a fifth embodiment. FIG. 13 is a diagram illustrating an example of the configuration of a base station in a modified example of the fifth embodiment. FIG. 20 is a diagram illustrating an example of the configuration of a base station according to a sixth embodiment. FIG. 13 is a diagram illustrating a configuration example of an antenna unit according to a sixth embodiment. FIG. 20 is a diagram illustrating an example of the configuration of a base station according to a seventh embodiment. FIG. 2 is a diagram illustrating a hardware configuration of a base station according to the first to seventh embodiments. FIG. 10 is a diagram illustrating an example of the relationship between the wireless area radius and the carrier power to noise power ratio in the central station.

An embodiment of the present invention will be described in detail with reference to the drawings. In this embodiment, the central station and base station communicate using analog RoF in the optical domain. In analog RoF, an optical signal intensity-modulated by an optical modulator using input power corresponding to the received power of the radio signal is transmitted through an optical fiber. In this embodiment, the reception gain for each antenna direction is set in advance so that the input power of the radio signal input to the optical modulator in uplink communication does not exceed the optical modulator's reception tolerance. By pre-designing the reception gain for each antenna direction in this way, nonlinear distortion occurring in the optical modulator can be suppressed without the need for additional schemes such as wireless terminal position estimation or reception gain measurement. Therefore, when analog RoF is applied, the base station can cover a wide area, from nearby to distant locations, without changing the device configuration of the optical modulator or expanding the dynamic range through additional signal processing. This enables efficient high-speed, high-capacity area deployment while keeping costs down.

(First embodiment)
Fig. 1 is a diagram showing an example of the configuration of a communication system 1 according to a first embodiment. The communication system 1 includes a wireless terminal 2, a base station 3, and a central station 4. Although Fig. 1 shows only one wireless terminal 2 and one base station 3, the number of each of the wireless terminals 2 and base stations 3 is arbitrary. The direction from the wireless terminal 2 to the central station 4 is referred to as an upward link (uplink), and the direction from the central station 4 to the wireless terminal 2 is referred to as a downward link (downlink).

The base station 3 comprises an antenna unit 311, a beam switching unit 312, a transmit/receive switch unit (hereinafter referred to as the "T/R SW unit") 313, a gain control unit 314, a receive signal amplifier unit 315, an electrical-to-optical conversion unit (hereinafter referred to as the "E/O conversion unit") 316, an optical-to-electrical conversion unit (hereinafter referred to as the "O/E conversion unit") 317, and a transmit signal amplifier unit 318.

The antenna unit 311 receives uplink signals radiated wirelessly from the wireless terminal 2, converts the received wireless signals into electrical received signals, and outputs them. The antenna unit 311 also radiates downlink transmission signals, which are electrical signals addressed to the wireless terminal 2, wirelessly. The antenna unit 311 may be composed of a single antenna element or multiple antenna elements. The antenna unit 311 forms beams instructed by the beam switching unit 312 and transmits and receives wireless signals to and from the wireless terminal 2. The beam switching unit 312 switches the beams formed by the antenna unit 311. For example, the beam switching unit 312 instructs the antenna unit 311 on each beam to be formed when performing a beam sweep.

The T/R SW unit 313 switches between transmission and reception in a time-division manner. During uplink communication, the T/R SW unit 313 outputs the upstream received signal received wirelessly by the antenna unit 311 to the received signal amplifier unit 315. During downlink communication, the T/R SW unit 313 outputs the downstream transmitted signal output by the transmitted signal amplifier unit 318 to the antenna unit 311.

The gain control unit 314 controls the signal power of the upstream signal received by the antenna unit 311 using a reception gain that is predetermined according to the beam formed by the antenna unit 311 so that the signal power (received power) of the upstream signal input to the E/O conversion unit 316 does not exceed a threshold. The threshold is the reception tolerance value of the optical region in the E/O conversion unit 316 or a value slightly lower than that reception tolerance value. The gain control unit 314 controls the reception gain of the upstream signal by instructing the reception signal amplification unit 315 on the gain.

The received signal amplifier 315 is, for example, an amplifier such as an LNA (Low Noise Amplifier). The received signal amplifier 315 may be composed of a single amplifier or multiple amplifiers. The received signal amplifier 315 amplifies the signal power of the uplink received signal output from the beam switching unit 312 according to the gain instructed by the gain control unit 314. The received signal amplifier 315 outputs the amplified uplink received signal to the E/O conversion unit 316.

The E/O conversion unit 316 is, for example, an E/O device such as an optical modulator. The E/O conversion unit 316 intensity-modulates light using the signal power of the received signal amplified by the received signal amplifier 315, and converts the upstream received signal into an analog RoF optical signal. The converted optical signal represents the signal waveform of the upstream signal received wirelessly. The E/O conversion unit 316 outputs the upstream optical signal to the optical fiber 5.

The O/E conversion unit 317 is, for example, an optical-electrical converter. The O/E conversion unit 317 receives an analog RoF downstream optical signal transmitted from the aggregation station 4 and transmitted through the optical fiber 6. The O/E conversion unit 317 converts the received downstream optical signal into an electrical transmission signal, and outputs the downstream transmission signal obtained by the conversion to the transmission signal amplifier 318. The transmission signal amplifier 318 amplifies the signal power of the downstream transmission signal received from the O/E conversion unit 317, and outputs the amplified transmission signal to the T/R SW unit 313.

The central station 4 comprises a baseband unit 41, an O/E converter 42, and an E/O converter 43. The baseband unit 41 performs digital signal processing of signals transmitted to and received from the wireless terminal 2. The O/E converter 42 is an optical-to-electrical converter. The O/E converter 42 receives the analog RoF upstream optical signal output from the base station 3 and transmitted through the optical fiber 5. The O/E converter 42 converts the received upstream optical signal into an electrical signal and outputs it to the baseband unit 41. The baseband unit 41 performs reception processing of the signal received from the O/E converter 42. The baseband unit 41 also outputs the downstream transmission signal to be transmitted to the wireless terminal 2 to the E/O converter 43. The E/O converter 43 is an optical modulator. The E/O converter 43 converts the downstream transmission signal received from the baseband unit 41 from an electrical signal to an analog RoF optical signal and outputs it to the optical fiber 6 between the base station 3. The downstream optical signal indicates waveform information of the downstream radio signal to be transmitted to the wireless terminal 2 .

Next, we will explain the gain control of the base station 3. The gain control unit 314 stores a reception gain table. The reception gain table is data linking a beam ID with an expected reception gain. The beam ID is information that uniquely identifies a beam formed by the antenna unit 311. The reception gain set in the reception gain table is high for beams formed in the far direction and low for beams formed in the near direction. The far direction is the direction toward an area where, when a beam with the maximum gain is directed toward the wireless terminal 2, the received power of the wireless signal from the wireless terminal 2 falls within the linear region of the E/O conversion unit 316. In this linear region, the relationship between the input power to the E/O conversion unit 316 and the optical intensity of the optical signal output from the E/O conversion unit 316 is linear. On the other hand, the near direction is the direction toward an area where, when a beam with the maximum gain is directed toward the wireless terminal 2, the received power of the wireless signal from the wireless terminal 2 falls within the nonlinear region (saturation region) of the E/O conversion unit 316. In this nonlinear region, the input power to the E/O conversion unit 316 is too strong, so differences in the strength of the input power are not easily reflected as differences in the strength of the optical signal intensity-modulated by the E/O conversion unit 316, and the relationship between the strength of the input power and the optical intensity of the optical signal becomes nonlinear.

The gain control unit 314 acquires a beam ID that identifies the beam currently being formed by the antenna unit 311. The gain control unit 314 may receive the beam ID from the beam switching unit 312, or may receive it directly from the baseband unit 41 of the central station 4 that is instructing the beam switching unit 312 to set the beam ID. The gain control unit 314 reads the reception gain associated with the acquired beam ID from the reception gain table. The gain control unit 314 instructs the reception signal amplifier 315 about the read reception gain. For example, the gain control unit 314 may change the amplification value of the reception signal amplifier 315 to change the amplification gain based on the read reception gain. The gain control unit 314 may also adjust the antenna gain of the antenna unit 311 instead of, or in addition to, instructing the reception gain to the reception signal amplifier 315. Note that if the antenna unit 311 is composed of a single antenna element, the gain control unit 314 controls the reception gain of the reception signal amplifier 315 without adjusting the antenna gain.

The received signal amplifier 315 amplifies the power of the upstream received signal received from the T/R SW unit 313 by the amplification value instructed by the gain control unit 314, and outputs the amplified received signal to the E/O conversion unit 316. The E/O conversion unit 316 generates an optical signal by intensity-modulating light from a light source (not shown) using the input power from the received signal amplifier 315, and outputs the generated optical signal to the optical fiber 5.

In Figure 1, the base station 3 is equipped with the gain control unit 314, but the gain control unit 314 may also be equipped in a device external to the base station 3, or the central station 4 may also be equipped with the gain control unit 314. According to the first embodiment, an E/O device (e.g., an optical modulator) with a narrower dynamic range than the wireless region can be used alone without adding a distortion compensation function to the base station, thereby achieving a simple and low-cost configuration.

Second Embodiment
The base station of the second embodiment amplifies radio signals received by a beam in the far direction using a high-gain amplifier, and reduces the gain of radio signals received by a beam in the near direction using an attenuator before amplifying them using the high-gain amplifier. The second embodiment will be described focusing on the differences from the first embodiment.

Figure 2 is a block diagram showing the configuration of a base station 3a according to the second embodiment. The communication system of the second embodiment is configured to include a base station 3a shown in Figure 2 instead of the base station 3 of the communication system 1 shown in Figure 1. In Figure 2, parts that are the same as those of the base station 3 according to the first embodiment shown in Figure 1 are given the same reference numerals, and their description will be omitted.

The base station 3a comprises an antenna unit 311, a beam switching unit 312, a T/R SW unit 321, a switch unit (hereinafter referred to as the "SW unit") 322, an attenuator 323, a gain control unit 324, a received signal amplifier unit 315, an E/O conversion unit 316, an O/E conversion unit 317, and a transmitted signal amplifier unit 318.

The T/R SW unit 321 switches between transmission and reception in a time-division manner. During uplink communication, the T/R SW unit 321 outputs the upstream signal received wirelessly by the antenna unit 311 to the SW unit 322. During downlink communication, the T/R SW unit 321 outputs the downstream signal output by the transmission signal amplifier unit 318 to the antenna unit 311.

The SW unit 322 switches whether to output the upstream received signal output from the T/R SW unit 321 to the attenuator 323 or to the received signal amplifier 315 in accordance with instructions from the gain control unit 324. The attenuator 323 may be a variable attenuator whose gain is changeable, or a fixed attenuator whose gain is not changeable. The attenuator 323 may be implemented integrally with the received signal amplifier 315, or may be implemented externally as shown in FIG. 2. The attenuator 323 attenuates the power of the upstream received signal received from the SW unit 322, and then outputs it to the received signal amplifier 315. As a result, the received signal amplifier 315 amplifies the upstream received signal received from the SW unit 322 or the attenuator 323, and outputs it to the E/O conversion unit 316. The gain control section 324 controls the output destination of the upstream signal in the SW section 322 so that the signal power of the upstream signal input to the E/O conversion section 316 does not exceed a threshold value.

The gain control of the base station 3a will now be described. The gain control unit 324 stores a reception gain table similar to that of the first embodiment. Like the gain control unit 314 of the first embodiment, the gain control unit 324 acquires a beam ID that identifies the beam currently being formed by the antenna unit 311 from the beam switching unit 312 or the baseband unit 41 of the central station 4. The gain control unit 324 reads the reception gain associated with the acquired beam ID from the reception gain table.

The gain control unit 324 controls the SW unit 322, the attenuator 323, and the received signal amplifier unit 315 so that the read reception gain is achieved. That is, when the beam is in the nearby direction and the reception gain requires attenuation of the signal power, the gain control unit 324 instructs the SW unit 322 to output the received signal to the attenuator 323. Furthermore, the gain control unit 324 may change the amplification gain of the received signal amplifier unit 315 if the received signal amplifier unit 315 is composed of a single amplifier, or may change the gain of the attenuator 323 if the attenuator 323 is a variable attenuator, or may do both. The attenuator 323 attenuates the power of the uplink received signal and then outputs it to the received signal amplifier unit 315.

On the other hand, if the beam is in the far direction and the reception gain does not require attenuation of the signal power, the gain control unit 324 instructs the SW unit 322 to output the reception signal to the reception signal amplifier unit 315. If the reception signal amplifier unit 315 is composed of a single amplifier, the gain control unit 324 may further change the amplification gain of the reception signal amplifier unit 315.

The received signal amplifier 315 amplifies the power of the upstream received signal received from the SW unit 322 or the attenuator 323, and outputs the amplified received signal to the E/O converter 316. The E/O converter 316 intensity-modulates the light using the input power from the received signal amplifier 315 to generate an optical signal, and outputs the generated optical signal to the optical fiber 5.

If the attenuator 323 is a fixed attenuator, the base station 3a may be equipped with multiple attenuators 323 with different attenuation values. The gain control unit 324 determines which of the multiple attenuators 323 to use, based on the reception gain read from the reception gain table. The gain control unit 324 instructs the SW unit 322 to output the received signal to the determined attenuator 323. The SW unit 322 switches the attenuator 323 to which the received signal is output, in accordance with the instructions from the gain control unit 324. The attenuator 323 to which the received signal is output attenuates the power of the received signal, and then outputs it to the received signal amplifier unit 315.

(Third embodiment)
The base station of the third embodiment amplifies the radio signals received by the beams in the far direction using a high-gain amplifier, and does not amplify the radio signals received by the beams in the near direction using an amplifier. The third embodiment will be described focusing on the differences from the second embodiment.

Figure 3 is a block diagram showing the configuration of a base station 3b according to the third embodiment. The communication system of the third embodiment is configured to include a base station 3b shown in Figure 3 instead of the base station 3 of the communication system 1 shown in Figure 1. In Figure 3, parts that are the same as those in the base station 3a according to the second embodiment shown in Figure 2 are given the same reference numerals, and their description will be omitted.

The base station 3b includes an antenna unit 311, a beam switching unit 312, a T/R SW unit 321, a SW unit 322, an attenuator 331, a gain control unit 332, a received signal amplifier 315, an E/O converter 316, an O/E converter 317, and a transmitted signal amplifier 318. The attenuator 331 has the same function as the attenuator 323 in the second embodiment. However, the attenuator 331 attenuates the power of the uplink received signal received from the SW unit 322 before outputting it to the E/O converter 316. The gain control unit 332 controls the output destination of the uplink signal in the SW unit 322 so that the received power of the uplink signal input to the E/O converter 316 does not exceed a threshold.

The gain control of the base station 3b will now be described. The gain control unit 332 stores a reception gain table similar to that of the first embodiment. Similar to the gain control unit 314 of the first embodiment, the gain control unit 332 acquires a beam ID that identifies the beam currently being formed by the antenna unit 311 from the beam switching unit 312 or the baseband unit 41 of the central station 4. The gain control unit 332 reads the reception gain associated with the acquired beam ID from the reception gain table.

The gain control unit 332 controls the SW unit 322, the attenuator 331, and the received signal amplifier unit 315 so that the read reception gain is achieved. That is, when the beam is in the nearby direction and the reception gain requires attenuation of the signal power, the gain control unit 332 instructs the SW unit 322 to output the received signal to the attenuator 331. Furthermore, when the attenuator 331 is a variable attenuator, the gain control unit 332 may change the gain of the attenuator 331.

On the other hand, if the beam is in the far direction and the reception gain does not require attenuation of the signal power, the gain control unit 332 instructs the SW unit 322 to output the reception signal to the reception signal amplifier unit 315. If the reception signal amplifier unit 315 is composed of a single amplifier, the gain control unit 332 may further change the amplification gain of the reception signal amplifier unit 315.

The received signal amplifier 315 amplifies the power of the upstream received signal received from the received signal amplifier 315 or the attenuator 331, and outputs the amplified received signal to the E/O converter 316. The E/O converter 316 intensity-modulates the light using the input power from the received signal amplifier 315 to generate an optical signal, and outputs the generated optical signal to the optical fiber 5.

If the attenuator 331 is a fixed attenuator, the base station 3b may be equipped with multiple attenuators 331 with different attenuation values. The gain control unit 332 determines which of the multiple attenuators 331 to use, based on the reception gain read from the reception gain table. The gain control unit 332 instructs the SW unit 322 to output the received signal to the determined attenuator 331. The SW unit 322 switches the attenuator 331 to which the received signal is output, in accordance with the instruction from the gain control unit 332. The attenuator 331 to which the received signal is output attenuates the power of the received signal, and then outputs it to the E/O conversion unit 316.

As described above, the gain control unit 332 of the base station 3b amplifies the signal power using the LNA when a beam is formed in the distant direction, and controls the signal power to be attenuated without using the LNA when a beam is formed in the nearby direction.

(Fourth embodiment)
The base station of the fourth embodiment performs switching so as to use a high-gain antenna in the far direction and a low-gain antenna in the near direction for wireless communication with the wireless terminal. The fourth embodiment will be described focusing on the differences from the above-mentioned embodiments.

Figure 4 is a block diagram showing the configuration of a base station 3c according to the fourth embodiment. The communication system of the fourth embodiment is configured to include a base station 3c shown in Figure 4 instead of the base station 3 of the communication system 1 shown in Figure 1. In Figure 4, parts that are the same as those of the base station 3 according to the first embodiment shown in Figure 1 are given the same reference numerals, and their description will be omitted.

The base station 3c comprises an antenna unit 341, a SW unit 344, a beam switching unit 345, a gain control unit 346, a T/R SW unit 313, a received signal amplifier unit 315, an E/O conversion unit 316, an O/E conversion unit 317, and a transmitted signal amplifier unit 318.

The antenna unit 341 includes a high-gain antenna 342 and a low-gain antenna 343. The high-gain antenna 342 has a high antenna gain, and the low-gain antenna 343 has a low antenna gain. The high-gain antenna 342 and the low-gain antenna 343 may each be composed of a single antenna element or multiple antenna elements. The high-gain antenna 342 and the low-gain antenna 343 convert the received radio signals into electrical signals and output them to the SW unit 344.

The SW unit 344 switches whether the antenna unit 341 uses the high-gain antenna 342 or the low-gain antenna 343 to transmit and receive radio signals to and from the wireless terminal 2. The beam switching unit 345 switches the beam formed by the antenna unit 341. For example, the beam switching unit 345 instructs the antenna unit 341 on each beam to be formed when performing a beam sweep. The gain control unit 346 controls whether to use the high-gain antenna 342 or the low-gain antenna 343 so that the received power of the uplink signal input to the E/O conversion unit 316 does not exceed a threshold value.

The gain control of the base station 3c will now be described. The gain control unit 346 stores a reception gain table similar to that of the first embodiment. The gain control unit 346 acquires a beam ID that identifies the beam currently being formed by the antenna unit 341 from the beam switching unit 345 or the baseband unit 41 of the central station 4. The gain control unit 346 reads the reception gain associated with the acquired beam ID from the reception gain table.

The gain control unit 346 determines whether to use the high-gain antenna 342 or the low-gain antenna 343 based on the read reception gain, and controls the SW unit 344 based on the determination. For example, if the read reception gain is higher than a predetermined value, the gain control unit 346 instructs the SW unit 344 to switch to the high-gain antenna 342, and if the read reception gain is lower than a predetermined value, the gain control unit 346 instructs the SW unit 344 to switch to the low-gain antenna 343. Note that information on whether to use the high-gain antenna 342 or the low-gain antenna 343 may be set in the reception gain table in association with the beam ID. Furthermore, the gain control unit 346 may instruct the antenna gain to be used to the high-gain antenna 342 or the low-gain antenna 343 based on the read reception gain, or may control the amplification gain of the reception signal amplifier 315, or may perform both.

The SW unit 344 switches between using the high-gain antenna 342 and the low-gain antenna 343 in accordance with instructions from the gain control unit 346. As a result, the SW unit 344 outputs the upstream received signal received by the high-gain antenna 342 or low-gain antenna 343 to the T/R SW unit 313 via the beam switching unit 345. The SW unit 344 also receives the downstream transmission signal output by the T/R SW unit 313 via the beam switching unit 345. The SW unit 344 wirelessly radiates the received downstream transmission signal from the high-gain antenna 342 or low-gain antenna 343 to be used.

In the fourth embodiment, the base station 3c controls the signal power of the received signal by switching between using a high-gain antenna or a low-gain antenna for communication with the wireless terminal 2 depending on the beam.

Fifth Embodiment
The base station of the fifth embodiment uses a massive array antenna in the far direction and a small number of array antennas in the near direction to communicate wirelessly with wireless terminals. As a result, the base station reduces the number of power feeds in the near direction to achieve low gain. The fifth embodiment will be described, focusing on the differences from the above-mentioned embodiments.

Figure 5 is a block diagram showing the configuration of a base station 3d according to the fifth embodiment. The communication system of the fifth embodiment is configured to include a base station 3d shown in Figure 5 instead of the base station 3 of the communication system 1 shown in Figure 1. In Figure 5, parts that are the same as those of the base station 3 according to the first embodiment shown in Figure 1 are given the same reference numerals, and their description will be omitted. The base station 3d includes an antenna unit 351, a SW unit 353, a beam switching unit 354, a gain control unit 355, a T/R SW unit 313, a received signal amplifier unit 315, an E/O conversion unit 316, an O/E conversion unit 317, and a transmitted signal amplifier unit 318.

The antenna unit 351 has N antennas 352 (N is an integer greater than or equal to 2). In Figure 5, the N antennas 352 are respectively referred to as antennas 352-1 to 352-N. The SW unit 353 connects the antenna 352 to be used among the N antennas 352 to the T/R SW unit 313 via the beam switching unit 354. The beam switching unit 354 switches the beams formed by the antennas 352-1 to 352-N. For example, the beam switching unit 354 instructs the antenna unit 351 on each beam to be formed when performing beam sweep. The gain control unit 355 controls the number of antennas 352 to be used so that the received power of the uplink signal input to the E/O conversion unit 316 does not exceed a threshold.

The following describes gain control of the base station 3d. The gain control unit 355 stores a beam table and an antenna usage table. The beam table is data that associates a beam ID, a reception gain, and a beam ID'. The beam ID identifies a beam formed using all N antennas 352. The beam ID' is information that identifies a beam formed using N or fewer antennas 352 for gain control. Note that the beam table does not need to include reception gain information. The antenna usage table is data that associates a beam ID' with antenna usage information. The antenna usage information indicates n antennas 352 (n is an integer between 1 and N) to be used for transmission and reception with the wireless terminal 2. The n antennas 352 to be used for transmission and reception are selected based on a reception gain that keeps the input power to the E/O conversion unit 316 below a threshold. Note that the antenna usage information may also indicate the number n of antennas 352 to be used.

The gain control unit 355 obtains the beam ID of the beam currently being formed by the antenna unit 351 from the beam switching unit 354 or the baseband unit 41 of the central station 4. The gain control unit 355 reads the beam ID' and reception gain corresponding to the obtained beam ID from the beam table. Furthermore, the gain control unit 355 reads the used antenna information corresponding to the read beam ID' from the used antenna table. The gain control unit 355 instructs the SW unit 353 to use the n antennas 352 indicated by the used antenna information. If the used antenna information indicates the number of antennas n, the gain control unit 355 selects the n antennas 352 to use and instructs the SW unit 353 to use the selected antennas 352. The SW unit 353 connects the n antennas 352 instructed by the gain control unit 355 to the T/R SW unit 313 via the beam switching unit 354, and controls the other (N-n) antennas 352 not to be connected to the T/R SW unit 313.

As described above, the SW unit 353 outputs the upstream received signals received by the n antennas 352 instructed by the gain control unit 355 to the T/R SW unit 313 via the beam switching unit 354. In addition, the SW unit 353 wirelessly radiates the downstream transmitted signals output by the T/R SW unit 313 from the instructed n antennas 352. When the gain control unit 355 reads the reception gain from the beam table, it controls the gain of the reception signal amplifier unit 315 based on the read reception gain.

Note that the beam table does not need to have a beam ID' corresponding to the beam ID for the far direction set. If the beam ID' is not read out, the gain control unit 355 does not issue an instruction to the SW unit 353.

As described above, the base station 3d, which has multiple antennas, uses a massive array antenna to achieve high gain for beams in the distant direction, and uses an array antenna composed of a small number of elements or a single antenna to achieve low gain for beams in the near direction.

(Modification of the fifth embodiment)
In the base station of the fifth embodiment, the number of antennas is reduced when the formed beam is in a nearby direction, but the base station may be configured in advance to form a beam in a nearby direction using a reduced number of antennas.

Figure 6 is a block diagram showing the configuration of a base station 3e according to a modified example of the fifth embodiment. In Figure 6, parts that are the same as those in the base station 3d shown in Figure 5 are given the same reference numerals, and their description will be omitted. The base station 3e comprises an antenna unit 351, a SW unit 353, a beam switching unit 356, a T/R SW unit 313, a received signal amplifier unit 315, an E/O conversion unit 316, an O/E conversion unit 317, and a transmitted signal amplifier unit 318.

The beam switching unit 356 stores a beam table and an antenna usage table. The beam table is data that associates beam IDs with beam IDs'. The antenna usage table is data that associates beam IDs' with antenna usage information. The antenna usage information indicates n antennas 352 (n is an integer such that 1≦n≦N) to be used for transmission and reception with the wireless terminal 2. The n antennas 352 to be used for transmission and reception are selected based on a reception gain that keeps the input power to the E/O conversion unit 316 below a threshold value.

The beam switching unit 356 reads from the beam table the beam ID' corresponding to the beam ID of the beam generated in the beam sweep. Furthermore, the beam switching unit 356 reads from the used antenna table the used antenna information corresponding to the read beam ID'. The beam switching unit 356 instructs the SW unit 353 to use the n antennas 352 indicated by the used antenna information. The SW unit 353 connects the n antennas 352 instructed by the beam switching unit 356 to the T/R SW unit 313 via the beam switching unit 356, and controls the other (N-n) antennas 352 not to be connected to the T/R SW unit 313.

Sixth Embodiment
The base station of the sixth embodiment has a beam selection antenna. In the case of a beam in a nearby direction, the base station reduces the gain of a received signal output from a port of the beam selection antenna using an attenuator or the like. The sixth embodiment will be described, focusing on the differences from the above-mentioned embodiments.

Figure 7 is a block diagram showing the configuration of a base station 3f according to the sixth embodiment. The communication system of the sixth embodiment is configured to include a base station 3f shown in Figure 7 instead of the base station 3 of the communication system 1 shown in Figure 1. In Figure 7, parts that are the same as those of the base station 3 according to the first embodiment shown in Figure 1 are given the same reference numerals, and their description will be omitted. The base station 3f includes an antenna unit 361, a SW unit 362, a beam switching unit 363, a T/R SW unit 313, a received signal amplifier unit 315, an E/O conversion unit 316, an O/E conversion unit 317, and a transmitted signal amplifier unit 318.

The antenna unit 361 has a beam selection antenna 3611. The beam selection antenna 3611 is, for example, a Butler matrix, a reflect array antenna, or a lens antenna. The beam selection antenna 3611 is connected to the SW unit 362 via N ports. The SW unit 362 switches which of the N ports of the beam selection antenna 3611 to use depending on the beam direction. The beam switching unit 363 switches the beam formed by the antenna unit 361. For example, the beam switching unit 363 instructs the antenna unit 361 on each beam to be formed when performing a beam sweep.

Figure 8 shows the detailed configuration of the antenna unit 361. The antenna unit 361 has a beam-selective antenna 3611. The beam-selective antenna 3611 has N antenna elements 3612 and N ports 3613. In Figure 8, the N antenna elements 3612 are respectively referred to as antenna elements 3612-1 to 3612-N, and the N ports 3613 are respectively referred to as ports 3613-1 to 3613-N. Each port 3613 corresponds to a different beam direction. The beam-selective antenna 3611 forms a beam using antenna elements 3612-1 to 3612-N in the direction corresponding to the port 3613 being used. The beam-selective antenna 3611 separates the wireless uplink signals received by antenna elements 3612-1 to 3612-N according to the receiving direction and outputs the signals from ports 3613-1 to 3613-N corresponding to the receiving direction. Each of the ports 3613-1 to 3613-N is connected to the SW unit 362 shown in Fig. 7. However, attenuators 3614 are provided between some of the ports 3613 corresponding to the nearby direction and the SW unit 362.

The gain control of base station 3f will now be described. The beam switching unit 363 of base station 3f shown in Figure 7 stores a port usage table. The port usage table is data that associates beam IDs with port usage information. The port usage information indicates one or more ports 3613 to be used for beam formation. The port usage information associated with the beam ID of the nearby direction is set to the port 3613 connected to the attenuator 3614. The port 3613 to be used for beam formation is selected based on a receiving gain that keeps the input power to the E/O conversion unit 316 below a threshold value.

The beam switching unit 363 reads port usage information corresponding to the beam ID to be formed from the port usage table. The beam switching unit 363 instructs the SW unit 362 to use the port 3613 indicated by the port usage information. The SW unit 362 connects the port 3613 instructed by the beam switching unit 363 to the T/R SW unit 313 via the beam switching unit 363, and controls the other ports 3613 so that they are not connected to the T/R SW unit 313.

As described above, when the antenna unit 361 receives a wireless signal via the antenna element 3612, the SW unit 362 outputs the upstream received signal output from the specified port 3613 to the T/R SW unit 313 via the beam switching unit 363. The SW unit 362 also outputs the downstream transmitted signal output by the T/R SW unit 313 to the port 3613 specified by the beam switching unit 363. The antenna unit 361 wirelessly radiates the transmitted signal input from the port 3613 from the antenna element 3612.

As a result of the above, base station 3f uses an attenuator to reduce the gain of the received signal when the beam is in a nearby direction.

Seventh Embodiment
The base station of the seventh embodiment directs a beam in a distant direction, but does not direct a beam in a nearby direction. The seventh embodiment will be described, focusing on the differences from the above-described embodiments.

Figure 9 is a block diagram showing the configuration of a base station 3g according to the seventh embodiment. The communication system of the seventh embodiment is configured to include the base station 3g shown in Figure 9 instead of the base station 3 of the communication system 1 shown in Figure 1. In Figure 9, parts that are the same as those of the base station 3 according to the first embodiment shown in Figure 1 are given the same reference numerals, and their description will be omitted. The base station 3g includes an antenna unit 311, a beam switching unit 371, a T/R SW unit 313, a received signal amplifier 315, an E/O converter 316, an O/E converter 317, and a transmitted signal amplifier 318. The beam switching unit 371 holds information on the beam ID of each beam used when performing beam sweeping. The beam switching unit 371 excludes the beam ID of a beam in the nearby direction from the beam ID of the target beam for beam sweeping. This prevents the beam switching unit 371 from forming a beam in the nearby direction during beam sweeping.

(Hardware configuration)
FIG. 10 is a diagram illustrating an example of the hardware configuration of the base station 3. The hardware configurations of the base stations 3a, 3b, 3c, 3d, 3e, 3f, and 3g are similar to those illustrated in FIG. 10. Some of the functions of the base stations 3, 3a, 3b, 3c, 3d, 3e, 3f, and 3g are implemented as software by a processor 101, such as a central processing unit (CPU), executing programs stored in a storage device 103 having a non-volatile storage medium (non-transitory storage medium) and a memory 102. The programs may be recorded on a computer-readable non-transitory storage medium. Examples of computer-readable non-transitory storage media include portable media such as flexible disks, magneto-optical disks, read-only memories (ROMs), and compact disc read-only memories (CD-ROMs), as well as storage devices such as hard disks built into computer systems. The communication unit 104 executes predetermined communication processing. The communication unit 104 communicates, for example, main signals, control signals, and programs.

Some or all of the functional units of base stations 3, 3a, 3b, 3c, 3d, 3e, 3f, and 3g may be realized using hardware including electronic circuits (electronic circuits or circuitry) using, for example, LSI (Large Scale Integrated circuit), ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or FPGA (Field Programmable Gate Array).

The gain control unit 314 of base station 3, the gain control unit 324 of base station 3a, the gain control unit 332 of base station 3b, the gain control unit 346 of base station 3c, the gain control unit 355 of base station 3d, the beam switching unit 356 of base station 3e, the beam switching unit 363 of base station 3f, and the beam switching unit 371 of base station 3g in the above-mentioned embodiments can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

According to the above-described embodiment, the communication device includes an antenna unit, an electrical-to-optical converter, and a control unit. For example, the communication device corresponds to base stations 3, 3a, 3b, 3c, 3d, 3e, 3f, and 3g in the embodiments; the electrical-to-optical converter corresponds to E/O converter 316 in the embodiments; and the control unit corresponds to gain controllers 314, 324, 332, 346, and 355 and beam switching units 356, 363, and 371. The antenna unit receives a radio signal and converts it into an electrical signal. The electrical-to-optical converter generates an optical signal by intensity-modulating light using the signal power of the electrical signal converted by the antenna unit. The control unit controls the signal power of the electrical signal using a predetermined reception gain depending on the beam formed by the antenna unit so that the signal power of the electrical signal input to the electrical-to-optical converter does not exceed a threshold. The reception gain is high when the beam formed by the antenna unit is in a far-field direction and low when the beam is in a near-field direction.

The control unit controls the signal power of the electrical signal converted by the antenna unit using one or both of an amplifier unit that amplifies the signal power and an attenuator unit that attenuates the signal power. For example, the amplifier unit corresponds to the received signal amplifier unit 315 in the embodiment, and the attenuator unit corresponds to the attenuators 323, 331, and 3614 in the embodiment.

The antenna unit may have a first high-gain antenna and a second low-gain antenna. For example, the first antenna corresponds to the high-gain antenna 342 of the embodiment, and the second antenna corresponds to the low-gain antenna 343 of the embodiment. The control unit controls the signal power of the electrical signal converted by the antenna unit depending on whether the first antenna or the second antenna is used to receive the radio signal.

The antenna unit may have multiple antenna elements. For example, the multiple antenna elements correspond to antennas 352-1 to 352-N in the embodiment. The control unit controls the signal power of the electrical signal converted by the antenna unit by controlling the antenna element used to receive the wireless signal or the number of antenna elements used to receive the wireless signal.

The control unit may control the signal power of the electrical signal converted by the antenna unit by not directing the beam in a nearby direction.

The communication device may also be a base station that transmits the optical signal generated by the electrical-to-optical conversion unit to the central station.

The above describes in detail an embodiment of the present invention with reference to the drawings, but the specific configuration is not limited to this embodiment and also includes designs that do not deviate from the gist of the present invention.

The present invention is applicable to communication systems that communicate using analog RoF (Radio-over-Fiber).

1 Communication system 2 Wireless terminal 3, 3a, 3b, 3c, 3d, 3e, 3f, 3g Base station 4 Central station 5, 6 Optical fiber 41 Baseband unit 42 O/E conversion unit 43 E/O conversion unit 101 Processor 102 Memory 103 Storage device 104 Communication unit 311, 341, 351, 361 Antenna unit 312, 345, 354, 356, 363, 371 Beam switching unit 313, 321 T/R SW unit 314, 324, 332, 346, 355 Gain control unit 315 Received signal amplifier unit 316 E/O conversion unit 317 O/E conversion unit 318 Transmitted signal amplifier unit 322, 344, 353, 362 SW unit 323, 331, 3614 Attenuator 342 High-gain antenna 343 Low-gain antennas 352-1 to 352-N Antenna 3611 Beam-selective antennas 3612-1 to 3612-N Antenna elements 3613-1 to 3613-N Port

Claims (8)

an antenna unit that receives a radio signal and converts the received radio signal into an electrical signal;
an electrical-to-optical conversion unit that generates an optical signal by intensity-modulating light using the signal power of the electrical signal converted by the antenna unit;
a control unit that controls the signal power of the electrical signal input to the electrical-optical conversion unit using a reception gain that is predetermined according to the beam formed by the antenna unit so that the signal power of the electrical signal does not exceed a threshold;
A communication device comprising: the control unit controls the signal power of the electrical signal using one or both of an amplifier unit that amplifies signal power and an attenuator unit that attenuates signal power.
The communication device according to claim 1 . the antenna unit has a first antenna with high gain and a second antenna with low gain,
the control unit controls the signal power of the electrical signal depending on whether the first antenna or the second antenna is to be used to receive the wireless signal.
The communication device according to claim 1 . the antenna unit has a plurality of antenna elements,
the control unit controls the signal power of the electrical signal by controlling the antenna element used to receive the wireless signal or the number of the antenna elements used to receive the wireless signal.
The communication device according to claim 1 . the control unit controls the signal power of the electrical signal by not directing the beam in a nearby direction.
The communication device according to claim 1 . The receiving gain is high when the beam formed by the antenna unit is in a far direction, and is low when the beam is in a near direction.
The communication device according to claim 1 . the communication device is a base station that transmits the optical signal to a central station;
The communication device according to any one of claims 1 to 6. a receiving step in which an antenna unit receives a radio signal and converts the received radio signal into an electrical signal;
a signal generating step of generating an optical signal by intensity-modulating light using the signal power of the electrical signal converted in the receiving step;
a control step of controlling the signal power of the electrical signal used to generate the optical signal in the signal generating step using a reception gain that is predetermined according to the beam formed by the antenna unit so that the signal power of the electrical signal does not exceed a threshold value;
A communication method comprising: