Cellular optical communication system, base station equipment, and terminal equipment

Abstract

Provided is a cellular optical communication system that performs optical communication, which is wireless communication by light, the cellular optical communication system comprising a base station device that manages a cell, and a terminal device that performs the optical communication with the base station device in the cell according to an OFDM scheme. The base station device and the terminal device perform optical communication based on an ACO-OFDM scheme when the terminal device is positioned in at least a cell-edge region.

Description

[Technical Field]

[0001] This disclosure relates to a cellular optical communication system, base station equipment, and terminal equipment. [Background technology]

[0002] For example, in underwater communication, optical communication systems are known that use light, particularly visible light, as the transmission medium for wireless communication (also simply called "optical communication"). Because light has high directivity, in conventional optical communication systems, it is common to perform one-to-one optical communication by placing the transmitting and receiving devices facing each other, under the assumption that the transmitting and receiving optical communication devices are fixed in place.

[0003] In such optical communications, intensity modulation direct detection (IM / DD) is widely considered. In the IM / DD method, data transmission is performed using the intensity of light emitted by light-emitting elements such as light-emitting diodes (LEDs). The transmitting side transmits an optical signal whose intensity is modulated according to the data to be transmitted. The receiving side acquires the transmitted data by directly detecting the intensity of the received optical signal with a photodetector such as a photodiode (PD).

[0004] One of the modulation methods used in IM / DD systems is subcarrier modulation. Subcarrier modulation is a method of transmitting data by controlling the intensity of light emitted from a light-emitting element in a sinusoidal manner and modulating the amplitude and / or phase of this sinusoidal wave (also called a "subcarrier"). In particular, the orthogonal frequency division multiplexing (OFDM) method, which uses multiple subcarriers, can achieve high transmission efficiency.

[0005] Radio communications, which use radio waves as a transmission medium, utilize OFDM signals with both positive and negative polarity. However, since optical communications do not have negative values ​​for light intensity, the OFDM method used in radio communications cannot be directly applied to IM / DD optical communications. Therefore, DCO (Direct Current biased Optical)-OFDM and ACO (Asymmetrically Clipped Optical)-OFDM methods have been proposed as optical wireless OFDM technologies that realize the OFDM method using the IM / DD method.

[0006] In the DCO-OFDM method, a DC bias (i.e., offset) is applied to the signal waveform to keep the signal within the range of zero or greater. On the other hand, in the ACO-OFDM method, only subcarriers with frequencies that are odd multiples of the fundamental frequency (also called "odd-numbered subcarriers") are used, and negative values ​​are clipped to zero at the transmitting end. Since odd-numbered subcarriers are odd functions, negative values ​​can be compensated for by inverting the waveform that is zero or greater at the receiving end.

[0007] The ACO-OFDM method does not add a DC bias like the DCO-OFDM method, resulting in superior power efficiency and higher noise immunity compared to the DCO-OFDM method. However, the ACO-OFDM method cannot use subcarriers with even multiples of the fundamental frequency (also called "even-numbered subcarriers"), resulting in lower throughput compared to the DCO-OFDM method. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Koji Ouchi, "Basic Technology of Optical Wireless OFDM Method," IEICE Fundamentals Review Vol.13 No.1 [Overview of the project]

[0009] The cellular optical communication system according to the first embodiment is a cellular optical communication system that performs optical communication, which is wireless communication using light, and comprises a base station device that manages a cell, and a terminal device that performs the optical communication with the base station device in the cell using the OFDM (Orthogonal Frequency Division Multiplexing) method. The base station device and the terminal device perform the optical communication using the ACO (Asymmetrically Clipped Optical)-OFDM method, at least when the terminal device is located in the cell edge region.

[0010] The base station device according to the second embodiment is a base station device for managing a cell in a cellular optical communication system that performs optical communication, which is wireless communication using light, and comprises an optical communication unit that performs the optical communication with a terminal device in the cell using the OFDM (Orthogonal Frequency Division Multiplexing) method, and a control unit that controls the optical communication unit to perform the optical communication with the terminal device using the ACO (Asymmetrically Clipped Optical)-OFDM method, at least when the terminal device is located in the cell edge region.

[0011] A terminal device according to the third embodiment is a terminal device used in a cellular optical communication system that performs optical communication, which is wireless communication using light, and comprises an optical communication unit that performs the optical communication with a base station device that manages a cell using the OFDM (Orthogonal Frequency Division Multiplexing) method, and a control unit that controls the optical communication unit to perform the optical communication with the base station device using the ACO (Asymmetrically Clipped Optical)-OFDM method, at least when the terminal device is located in the cell edge region. [Brief explanation of the drawing]

[0012] [Figure 1] This figure shows an example configuration of a cellular optical communication system according to one embodiment. [Figure 2]It is a diagram showing a configuration example of a communication frame used in a cellular optical communication system according to an embodiment. [Figure 3] It is a diagram showing a configuration example of a base station device according to an embodiment. [Figure 4] It is a diagram showing an external appearance configuration example of a base station device according to an embodiment. [Figure 5] It is a diagram showing a configuration example of a terminal device according to an embodiment. [Figure 6] It is a diagram showing an external appearance configuration example of a terminal device according to an embodiment. [Figure 7] It is a diagram for explaining spatial multiplexing transmission according to an embodiment. [Figure 8] It is a diagram for explaining combined transmission according to an embodiment. [Figure 9] It is a diagram for explaining the DCO-OFDM method, which is a type of optical wireless OFDM method. [Figure 10] It is a diagram showing a configuration example of a transmitter of a base station device and a receiver of a terminal device when the DCO-OFDM method is used in downlink transmission. [Figure 11] It is a diagram for explaining the ACO-OFDM method, which is a type of optical wireless OFDM method. [Figure 12] It is a diagram for explaining the ACO-OFDM method, which is a type of optical wireless OFDM method. [Figure 13] It is a diagram showing a configuration example of a transmitter of a base station device and a receiver of a terminal device when the ACO-OFDM method is used in downlink transmission. [Figure 14] It is a diagram for explaining the selection of an optical wireless OFDM method according to an embodiment. [Figure 15] It is a diagram for explaining the Layered-ACO-OFDM method. [Figure 16] It is a diagram for explaining problems when the Layered-ACO-OFDM method is applied to a cellular optical communication system. [Figure 17] It is a diagram for explaining problems when the Layered-ACO-OFDM method is applied to a cellular optical communication system. [Figure 18] This is a diagram for explaining spatial multiplexing transmission using the Layered-ACO-OFDM method according to an embodiment. [Figure 19] This is a diagram for explaining spatial multiplexing transmission using the Layered-ACO-OFDM method according to an embodiment. [Figure 20] This is a diagram for explaining combined transmission using the Layered-ACO-OFDM method according to an embodiment. [Figure 21] This is a diagram for explaining resource allocation according to an embodiment. [Figure 22] This is a diagram for explaining other embodiments. [Figure 23] This is a diagram for explaining other embodiments. [Figure 24] This is a diagram for explaining other embodiments.

Mode for Carrying Out the Invention

[0013] In the future, in an optical communication system, it is assumed that the coverage area will be divided into small areas called cells, and a base station device that manages the cells will perform optical communication with terminal devices within the cell, realizing cellular operation. Such an optical communication system is also referred to as a cellular optical communication system. However, the technology for performing optical communication using the OFDM method in a cellular optical communication system has not yet been established.

[0014] Therefore, an object of the present disclosure is to enable appropriate optical communication using the OFDM method.

[0015] The cellular optical communication system according to the embodiment will be described while referring to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0016] The cellular optical communication system according to the embodiment is a system that performs optical communication using visible light as an example of light. However, the cellular optical communication system may also perform optical communication using light other than visible light, such as infrared light. Furthermore, the cellular optical communication system according to the embodiment is a system that performs optical communication underwater. However, the cellular optical communication system is not limited to a system that performs optical communication underwater, and may also be a system that performs optical communication in space, for example.

[0017] (1) Example of a cellular optical communication system configuration First, an example configuration of a cellular optical communication system according to one embodiment will be described. Figure 1 is a diagram showing an example configuration of a cellular optical communication system 1 according to one embodiment.

[0018] In the cellular optical communication system 1, the coverage area is divided into small regions called cells, and a base station device 200 that manages the cells performs optical communication with terminal devices 100 within those cells. In the illustrated example, the cellular optical communication system 1 has multiple terminal devices 100 (100a and 100b) and a base station device 200. However, the number of terminal devices 100 and the number of base station devices 200 are not limited to the illustrated example.

[0019] Each terminal device 100 is an example of an optical communication device. Each terminal device 100 is located underwater. Each terminal device 100 is configured to be mobile underwater. For example, each terminal device 100 may be a self-propelled device such as an underwater robot or underwater drone. The terminal device 100 connects to a base station device 200 that manages the cell in which it is located (also referred to as a "serving cell") and performs optical communication with the base station device 200 to which it is connected (also referred to as a "serving base station").

[0020] Each terminal device 100 has multiple light-emitting and light-receiving units whose optical axes (or, in other words, the directivity of optical communication) are oriented in different directions. Each light-emitting and light-receiving unit includes at least one light-emitting element and at least one photodetector. This allows each terminal device 100 to perform optical communication in various ways using multiple light-emitting and light-receiving units while using light as the transmission medium.

[0021] Each terminal device 100 may be equipped with a sensor such as an image sensor and generate sensor data. For example, each terminal device 100 may transmit uplink (UL) data including sensor data to the base station device 200 via optical communication. Each terminal device 100 may receive downlink (DL) data including instruction data from the base station device 200 via optical communication. Based on the instruction data, the terminal device 100 may perform movement and sensing operations (such as taking pictures).

[0022] The base station device 200 is another example of an optical communication device. Multiple base station devices 200 may be arranged horizontally at intervals to create a wide coverage area underwater. The base station devices 200 may be installed temporarily, for example, during the period of underwater surveys using each terminal device 100. In Figure 1, the communication area cells of each base station device 200 are shown by dashed lines.

[0023] The base station device 200 has, for example, a floating member and is located near the water surface. The base station device 200 is communicably connected to the network 10 via a backhaul line. The backhaul line may be a wireless line. The backhaul line may be a wired line. The network 10 may include the Internet. The base station device 200 may perform inter-base station communication with other base station devices via the network 10.

[0024] The base station device 200 has multiple light-emitting and light-receiving units whose optical axes are oriented in different directions. Each light-emitting and light-receiving unit includes at least one light-emitting element and at least one photodetector. In the illustrated example, the base station device 200 has a hemispherical housing submerged in water, with multiple light-emitting and light-receiving units arranged in an array on the surface of the spherical housing. This allows the base station device 200 to perform optical communication in various ways using multiple light-emitting and light-receiving units while using light as the transmission medium.

[0025] The base station device 200 selects its own light-emitting / receiving unit corresponding to the direction of each terminal device 100 connected to it, and uses the selected unit to perform optical communication with the terminal device 100. Similarly, the terminal device 100 selects its own light-emitting / receiving unit corresponding to the direction of the base station device 200, which is its serving base station, and uses the selected unit to perform optical communication with the base station device 200.

[0026] The base station device 200 may also transmit a synchronization optical signal and / or a reference optical signal unique to its device from all light-emitting and receiving units in all directions. The terminal device 100 may identify the direction of the base station device 200 based on these optical signals, identify its own light-emitting and receiving unit corresponding to that direction, and perform optical communication with the base station device 200 using the identified light-emitting and receiving unit.

[0027] Thus, in the cellular optical communication system 1, the base station device 200 transmits optical signals from multiple light-emitting elements with different directivity to form a communication area (cell). The base station device 200 needs to appropriately accommodate multiple terminal devices 100 within the cell, including the cell edge region. In the cell edge region, the received power of the optical signal from the base station device 200 is low, and interference from adjacent cells may also occur. Therefore, the optical communication environment in the cell edge region is poor, and the signal-to-noise ratio (SNR) of the optical signal received by the terminal devices 100 in the cell edge region from the base station device 200 is generally low.

[0028] Figure 2 shows an example of the configuration of a communication frame used in a cellular optical communication system 1 according to one embodiment. In the illustrated example, one communication frame is composed of 10 time slots, but the number of time slots that make up one communication frame is not limited to 10. Each time slot is composed of a predetermined number of symbol intervals.

[0029] In this frame configuration example, the communication frame consists of one synchronization slot (Sync.), one control slot (Ctrl.), four DL slots (DL slot) #0 to #3, and four UL slots (UL slot) #0 to #3. However, in scenarios where the amount of data in UL communication is greater than that of DL communication, the number of UL slots in the communication frame may be greater than the number of DL slots.

[0030] The synchronization slot (Sync.) is a time slot in which the base station device 200 transmits a synchronization optical signal (and a reference optical signal specific to the base station device). The terminal device 100 identifies the base station device 200 by the synchronization optical signal received from the base station device 200 and establishes or maintains synchronization with the base station device 200 using the synchronization optical signal. Note that the reference optical signal specific to the base station device may be transmitted in all slots other than the UL slot. The reference optical signal is used by the terminal device 100 to measure the light intensity received from the base station device 200.

[0031] A control slot (Ctrl.) is a time slot on which the base station device 200 transmits a control optical signal. The control optical signal includes scheduling information indicating, for example, the resource allocation (e.g., time slot allocation) of DL and UL. The terminal device 100 determines its own time slot allocation by receiving the control optical signal from the base station device 200, for example.

[0032] DL slots #0 to #3 constitute a DL communication period. The base station device 200 assigns each of DL slots #0 to #3 to one or more terminal devices 100. The base station device 200 transmits DL data optical signals in each DL slot. Each DL slot may have a light-emitting element-specific reference signal (Ref.TxElement) and a data optical signal arranged in time division.

[0033] UL slots #0 to #3 constitute a UL communication period. The base station equipment 200 assigns each of UL slots #0 to #3 to one or more terminal devices 100. The terminal devices 100 transmit UL data optical signals in the assigned UL slots.

[0034] The base station device 200 can communicate simultaneously with multiple terminal devices 100 located in different directions from each other. Specifically, the base station device 200 can spatially multiplex multiple terminal devices 100 located in different directions from each other. Therefore, the base station device 200 may allocate one DL slot or one UL slot to multiple terminal devices 100.

[0035] In the cellular optical communication system 1, the IM / DD method is used for optical communication between the terminal device 100 and the base station device 200. In the IM / DD method, data transmission is performed using the intensity of light emitted by the light-emitting element on the transmitting side of the optical signal. The transmitting side transmits an optical signal whose intensity is modulated according to the data to be transmitted. The receiving side acquires the transmitted data by directly detecting the intensity of the received optical signal with a photodetector.

[0036] In the cellular optical communication system 1, at least in the DL (Digital Layer), the OFDM (Optical Fiber Modulation) method, a type of subcarrier modulation scheme, is used. In the following description of the embodiments, the DL will be described mainly, but control similar to that for DL ​​may also be applied to UL (Ultraviolet Layer).

[0037] The terminal device 100 performs optical communication with the base station device 200 in the base station device 200's cell using the OFDM method. The base station device 200 and the terminal device 100 perform ACO-OFDM optical communication at least when the terminal device 100 is located in the cell edge area. In other words, the base station device 200 adaptively uses ACO-OFDM at least when communicating with the terminal device 100 in a poor communication environment. This improves noise immunity, making it possible to perform OFDM optical communication appropriately.

[0038] (2) Example of base station equipment configuration Next, an example of the configuration of a base station device 200 according to one embodiment will be described.

[0039] (2.1) Example of a base station equipment block configuration Figure 3 shows an example configuration of a base station device 200 according to one embodiment. The base station device 200 includes a light-emitting unit 210, a light-receiving unit 220, a control unit 230, and a backhaul communication unit 240. The light-emitting unit 210 and the light-receiving unit 220 constitute an optical communication unit. The base station device 200 may have a battery to supply the power necessary for the operation of the base station device 200.

[0040] The light-emitting unit 210 transmits an optical signal to the terminal device 100 under the control of the control unit 230. The light-emitting unit 210 has a plurality of light-emitting elements 211 (211#0, 211#1, ...) and a transmitter 212. Each light-emitting element 211 may be a laser diode (LD) or a light-emitting diode (LED). In the following, an example in which each light-emitting element 211 is an LED will be mainly described. Each light-emitting element 211 converts the electrical signal (transmission signal) output by the transmitter 212 for optical communication into an optical signal and transmits the optical signal.

[0041] The transmitter 212 may be composed of an FPGA (Field Programmable Gate Array) and / or a SoC (System-on-a-chip). The transmitter 212 performs signal processing on the transmission signal output by the control unit 230, converts the processed signal, and outputs it to the light-emitting element 211. In one embodiment, each of the multiple light-emitting elements 211 has a different optical axis orientation. That is, each of the multiple light-emitting elements 211 has a different direction of directivity (transmission directivity).

[0042] The light-receiving unit 220 receives an optical signal from the terminal device 100. The light-receiving unit 220 has a plurality of light-receiving elements 221 (221#0, 221#1, ...) and a receiver 222. Each light-receiving element 221 may be a photodiode (PD). Each light-receiving element 221 receives an optical signal, converts the received optical signal into an electrical signal (received signal), and outputs the received signal to the receiver 222. The receiver 222 may be configured by an FPGA and / or SoC. At least a part of the receiver 222 may be configured integrally with the transmitter 212. The receiver 222 converts the received signal output by the light-receiving elements 221, performs signal processing on the converted received signal, and outputs it to the control unit 230.

[0043] For example, a light-receiving element 221 is provided in a one-to-one pair with a light-emitting element 211. Here, the light-receiving element 221 has the same directionality (receiving directionality) as its corresponding light-emitting element 211. That is, multiple pairs of light-emitting elements 211 and light-receiving elements 221 each transmit optical signals in different directions and receive optical signals from different directions.

[0044] The control unit 230 controls the overall operation of the base station device 200. For example, the control unit 230 controls the light-emitting unit 210 and the light-receiving unit 220. The operation of the base station device 200 described above and the operation of the base station device 200 described later may be controlled by the control unit 230. The control unit 230 includes at least one processor 231 and at least one memory 232. The memory 232 stores programs executed by the processor 231 and information used for processing by the processor 231. The processor 231 executes the programs stored in memory and performs various processes.

[0045] The backhaul communication unit 240 performs backhaul communication via the backhaul line under the control of the control unit 230. The backhaul communication unit 240 may have a network communication unit 241 that communicates with the network 10 (for example, the core network) and an inter-base station communication unit 242 that communicates with adjacent base stations.

[0046] The base station device 200 configured in this way manages the cell in the cellular optical communication system 1. The optical communication unit, composed of a light-emitting unit 210 and a light-receiving unit 220, performs optical communication with the terminal device 100 in the cell using the OFDM method (optical wireless OFDM method). The control unit 230 controls the optical communication unit to perform ACO-OFDM optical communication with the terminal device 100, at least when the terminal device 100 is located in the cell edge region. The ACO-OFDM optical communication may also be the Layered-ACO-OFDM optical communication method described later.

[0047] (2.2) Example of external configuration of base station equipment Figure 4 shows an example of the external configuration of a base station device 200 according to one embodiment.

[0048] The base station device 200 has a hemispherical light-receiving unit 250 and a main body 260 connected to the light-receiving unit 250. However, the base station device 200 may be configured to be spherical as a whole. The light-receiving unit 250 has a plurality of dispersed light-receiving regions 251. Each light-receiving region 251 is provided with at least one pair of light-emitting element 211 and light-receiving element 221. This configuration makes it easy for the base station device 200 to perform optical communication with terminal devices 100 in various directions.

[0049] In the illustrated example, the hemispherical light-receiving unit 250 has a total of 19 light-receiving regions 251, numbered 251#0 to 251#18. That is, the base station device 200 has a total of 19 light-emitting elements 211#0 to 211#18, and a total of 19 light-receiving elements 221#0 to 221#18.

[0050] (3) Example of terminal device configuration Next, an example of the configuration of a terminal device 100 according to one embodiment will be described.

[0051] (3.1) Example of terminal device block configuration Figure 5 shows an example configuration of a terminal device 100 according to one embodiment. The terminal device 100 includes a light-emitting unit 110, a light-receiving unit 120, and a control unit 130. The light-emitting unit 110 and the light-receiving unit 120 constitute an optical communication unit. The terminal device 100 may have a battery to supply the power necessary for the operation of the terminal device 100. The terminal device 100 may have a moving mechanism (for example, a motor and a screw) used to move the terminal device 100.

[0052] The light-emitting unit 110 transmits an optical signal to the base station device 200 under the control of the control unit 130. The light-emitting unit 110 has a plurality of light-emitting elements 111 (111#0, 111#1, ...) and a transmitter 112. Each light-emitting element 111 may be a laser diode (LD) or a light-emitting diode (LED). In the following, an example in which each light-emitting element 211 is an LED will be mainly described. Each light-emitting element 111 converts the electrical signal (transmission signal) output by the transmitter 112 for optical communication into an optical signal and transmits the optical signal.

[0053] The transmitter 112 may be configured with an FPGA and / or SoC. The transmitter 112 performs signal processing on the transmission signal output by the control unit 130, converts the processed signal, and outputs it to the light-emitting element 111. In one embodiment, each of the multiple light-emitting elements 111 has a different optical axis orientation. That is, each of the multiple light-emitting elements 111 has a different direction of directivity (transmission directivity).

[0054] The light-receiving unit 120 receives an optical signal from the base station device 200. The light-receiving unit 120 has a plurality of light-receiving elements 121 (121#0, 121#1, ...) and a receiver 122. Each light-receiving element 121 may be a photodiode (PD). Each light-receiving element 121 receives an optical signal, converts the received optical signal into an electrical signal (received signal), and outputs the received signal to the receiver 122.

[0055] The receiver 122 may be configured using an FPGA and / or SoC. At least a portion of the receiver 122 may be integrated with the transmitter 112. The receiver 122 converts the received signal output by the photodetector 121, performs signal processing on the converted received signal, and outputs it to the control unit 130.

[0056] For example, a light-receiving element 121 is provided in a one-to-one pair with a light-emitting element 111. Here, the light-receiving element 121 has the same directionality (receiving directionality) as its corresponding light-emitting element 111. That is, multiple pairs of light-emitting elements 111 and light-receiving elements 121 each transmit optical signals in different directions and receive optical signals from different directions.

[0057] The control unit 130 controls the overall operation of the terminal device 100. For example, the control unit 130 controls the light-emitting unit 110 and the light-receiving unit 120. The operation of the terminal device 100 described above and the operation of the terminal device 100 described later may be controlled by the control unit 130. The control unit 130 includes at least one processor 131 and at least one memory 132. The memory 132 stores programs executed by the processor 131 and information used for processing by the processor 131. The processor 131 executes the programs stored in the memory and performs various processes.

[0058] In the terminal device 100 configured in this way, the optical communication unit, which consists of a light-emitting unit 110 and a light-receiving unit 120, performs optical communication with the base station device 200 using the OFDM method (optical wireless OFDM method). The control unit 130 controls the optical communication unit to perform ACO-OFDM optical communication with the base station device 200, at least when the terminal device 100 is located in the cell edge region. The ACO-OFDM optical communication may also be the Layered-ACO-OFDM optical communication method described later.

[0059] (3.2) Example of external configuration of terminal device Figure 6 shows an example of the external configuration of a terminal device 100 according to one embodiment.

[0060] The terminal device 100 has a hemispherical light-receiving unit 150 and a main body 160 connected to the light-receiving unit 150. However, the terminal device 100 may be configured to be spherical as a whole. The light-receiving unit 150 has a plurality of dispersed light-receiving regions 151. Each light-receiving region 151 is provided with at least one pair of light-emitting element 111 and light-receiving element 121. This configuration makes it easy for the terminal device 100 to perform optical communication with base station devices 200 in various directions.

[0061] In the illustrated example, the hemispherical light-receiving and light-emitting section 150 has a total of seven light-receiving and light-receiving regions 151#0 to 151#6. That is, the terminal device 100 has a total of seven light-emitting elements 111#0 to 111#6, and a total of seven light-receiving elements 121#0 to 121#6.

[0062] (4) Downlink transmission Next, a downlink transmission according to one embodiment will be described.

[0063] The base station device 200 described above has multiple light-emitting elements 211 with different directivity. In downlink transmission, the base station device 200 uses the multiple light-emitting elements 211 to perform spatial multiplexing transmission, which transmits data to different terminal devices 100, or composite transmission, which transmits the same data to the same terminal device 100. In the cellular optical communication system 1, it is necessary to perform such downlink transmission appropriately for low-SNR terminals in the cell edge region.

[0064] (4.1) Spatial multiplexing Figure 7 is a diagram illustrating spatial multiplexing transmission according to one embodiment. Figure 7 shows a simplified cross-section of the base station equipment 200.

[0065] In the base station device 200, multiple light-emitting elements 211 are arranged such that as the distance between one light-emitting element 211 and another light-emitting element 211 increases, the angle between the optical axis of one light-emitting element 211 and the optical axis of the other light-emitting element 211 increases. The angle between the optical axis of light-emitting element 211#0 and the optical axis of light-emitting element 211#7 that is not adjacent to light-emitting element 211#0 is greater than the angle between the optical axis of light-emitting element 211#0 and the optical axis of light-emitting element 211#1 that is adjacent to light-emitting element 211#0.

[0066] In the illustrated example, the base station device 200 transmits an optical signal to terminal device 100a using light-emitting element 211#0, while simultaneously transmitting an optical signal to terminal device 100b using light-emitting element 211#7, whose optical axis is oriented in a different direction from that of light-emitting element 211#0. This spatially separates the transmission to terminal device 100a and the transmission to terminal device 100b, allowing simultaneous transmission while suppressing interference between them. This increases the number of terminal devices 100 that the base station device 200 can accommodate, thereby improving the system capacity of the cellular optical communication system 1.

[0067] (4.2) Composite transmission Figure 8 is a diagram illustrating a composite transmission according to one embodiment. Figure 8 shows a simplified cross-section of the base station equipment 200.

[0068] In the illustrated example, the base station device 200 transmits an optical signal to the terminal device 100 using the light-emitting element 211#1, while simultaneously transmitting the same optical signal to the same terminal device 100 using the light-emitting element 211#7, whose optical axis is oriented in a different direction than that of the light-emitting element 211#1. Since the optical signals transmitted by the light-emitting element 211#1 and the optical signals transmitted by the light-emitting element 211#7 are the same optical signals, they are partially combined and reinforce each other in water. As a result, it is possible to extend the communication range.

[0069] (5) Optical wireless OFDM method Next, the DCO-OFDM method and the ACO-OFDM method will be described as optical wireless OFDM methods according to one embodiment. Details of these methods are described in Non-Patent Document 1, but here we will give an overview of each method.

[0070] (5.1)DCO-OFDM method Figure 9 is a diagram illustrating the DCO-OFDM method, which is a type of optical wireless OFDM method.

[0071] In the DCO-OFDM method, a DC bias (i.e., offset) is applied to the signal waveform to keep the signal within the range of zero or greater. Specifically, the transmitting side applies a DC bias to the baseband OFDM signal to generate a non-negative polarity signal, and then transmits the non-negative polarity signal.

[0072] Figure 10 shows an example configuration of the transmitter 212 of the base station equipment 200 and the receiver 122 of the terminal equipment 100 when the DCO-OFDM method is used in downlink transmission.

[0073] The transmitter 212 of the base station device 200 includes a serial / parallel (S / P) conversion unit 212a, a symbol mapping unit 212b, an inverse fast Fourier transform (IFFT) unit 212c, a parallel / serial (P / S) conversion unit 212d, a digital / analog (D / A) conversion unit 212e, and an offset application unit 212f.

[0074] The S / P conversion unit 212a converts the transmission data output by the control unit 230 of the base station device 200 from serial data to parallel data and outputs the parallel data to the symbol mapping unit 212b. The symbol mapping unit 212b maps the parallel data to the symbol data of each subcarrier (first-order modulation) and outputs the frequency domain symbol data to the IFFT unit 212c. The IFFT unit 212c converts the symbol data into a time-domain waveform using IFFT and outputs a baseband OFDM signal. The frequencies of each subcarrier multiplexed in the OFDM method are integer multiples of the fundamental frequency f0. The P / S conversion unit 212d converts the baseband OFDM signal output by the IFFT unit 212c into serial data and outputs it. The D / A conversion unit 212e converts the data output by the P / S conversion unit 212d from a digital signal to an analog signal and outputs an analog signal. The offset application unit 212f adds a DC bias to the analog signal output by the D / A conversion unit 212e to generate a non-negative polarity signal, and outputs a non-negative polarity signal (electrical signal). The light-emitting element 211 then converts the electrical signal output by the D / A conversion unit 212e into an optical signal and transmits the optical signal.

[0075] This optical signal is received by the photodetector 121 of the terminal device 100 via an optical propagation path. The photodetector 121 converts the received optical signal into an electrical signal and outputs it to the receiver 122. The receiver 122 of the terminal device 100 includes an analog-to-digital (A / D) conversion unit 122a, an S / P conversion unit 122b, a fast Fourier transform (FFT) unit 122c, a symbol demapping unit 122d, and a P / S conversion unit 122e.

[0076] The A / D conversion unit 122a converts the electrical signal output by the photodetector 121 from an analog signal to a digital signal and outputs it. The S / P conversion unit 122b converts the digital signal output by the A / D conversion unit 122a into parallel data (time-domain waveform) and outputs it. The FFT unit 122c converts the time-domain waveform output by the S / P conversion unit 122b into frequency-domain symbol data using FFT and outputs it. The symbol demapping unit 122d demmaps the symbol data output by the FFT unit 122c and outputs it. The P / S conversion unit 122e converts the demapped data into serial data (i.e., the original transmission data) and outputs it to the control unit 130.

[0077] (5.2) ACO-OFDM method Figures 11 and 12 are diagrams illustrating the ACO-OFDM method, a type of optical wireless OFDM method.

[0078] As shown in Figure 11, the ACO-OFDM method generates OFDM signals using only subcarriers with frequencies that are odd multiples of the fundamental frequency f0 (also called "odd-numbered subcarriers"). In other words, the ACO-OFDM method does not use subcarriers with frequencies that are even multiples of the fundamental frequency f0 (also called "even-numbered subcarriers") to generate OFDM signals.

[0079] Furthermore, as shown in Figure 12, in the ACO-OFDM method, negative values ​​are clipped to zero at the transmitting end. Since odd-numbered subcarriers are odd functions, negative values ​​can be compensated for by inverting waveforms greater than or equal to zero at the receiving end. Thus, the ACO-OFDM method focuses on the fact that it can transmit signal waveforms of virtually both polarity using only non-negative polarity signals without adding a DC bias.

[0080] Figure 13 shows an example configuration of the transmitter 212 of the base station equipment 200 and the receiver 122 of the terminal equipment 100 when the ACO-OFDM method is used in downlink transmission. Here, we will explain the differences from the DCO-OFDM method.

[0081] In the transmitter 212 of the base station device 200, the symbol mapping unit 212b and the IFFT unit 212c do not use even-numbered subcarriers, so the IFFT is performed by setting the even-numbered subcarriers to 0. The subcarriers that make up the ACO-OFDM signal have the characteristic that the polarity is reversed between the first half of the signal and the second half of the signal on the time axis. This characteristic is not lost even with multiplexing. The negative value clipping unit 212g clips the negative values ​​of the analog signal output by the D / A conversion unit 212e to 0 to generate a non-negative polarity signal, and outputs a non-negative polarity signal (electrical signal). On the other hand, in the receiver 122 of the terminal device 100, the zero-clipped signal is replaced with a positive signal located at a position shifted by half the symbol length within the same symbol, with the polarity reversed, to demodulate the data as an OFDM signal.

[0082] (5.3) Selection of Optical Wireless OFDM Method As mentioned above, the ACO-OFDM method does not add a DC bias like the DCO-OFDM method, so it has better power efficiency and higher noise immunity compared to the DCO-OFDM method. However, the ACO-OFDM method cannot use even-numbered subcarriers, so its throughput is lower than that of the DCO-OFDM method.

[0083] Figure 14 is a diagram illustrating the selection of an optical wireless OFDM system according to one embodiment.

[0084] The base station equipment 200 and the terminal equipment 100 perform ACO-OFDM optical communication at least when the terminal equipment 100 is located in the cell edge region. The cell edge region refers to the edge region of the cell formed by the base station equipment 200, and is, for example, the region where the reception quality (e.g., reception intensity) of the reference optical signal received by the terminal equipment 100 from the base station equipment 200 is below a threshold.

[0085] In the illustrated example, terminal device 100a located in the cell edge region communicates with base station device 200 using the ACO-OFDM method. For example, base station device 200 transmits an optical signal to terminal device 100a using the ACO-OFDM method. This improves the reception quality of the optical signal received from base station device 200, even for terminal device 100a in a poor optical communication environment. Therefore, even when base station device 200 transmits optical signals from multiple light-emitting elements 211 with different directivity to form a communication area, multiple terminal devices 100 within the cell, including the cell edge region, can be appropriately accommodated.

[0086] The base station equipment 200 and the terminal equipment 100 may perform optical communication using an OFDM method selected from among multiple OFDM methods, including the ACO-OFDM method, based on the optical communication environment of the terminal equipment 100. This allows for the adaptive use of the OFDM method best suited to the optical communication environment of the terminal equipment 100.

[0087] For example, terminal device 100 reports to base station device 200 the reception quality (e.g., reception intensity) of the reference optical signal that terminal device 100 receives from base station device 200. If the reception quality reported by terminal device 100 is below a threshold, base station device 200 selects the ACO-OFDM method, configures terminal device 100 to use the ACO-OFDM method for optical communication, and then performs ACO-OFDM optical communication with terminal device 100.

[0088] Alternatively, if the reception quality (e.g., reception intensity) of the reference optical signal received by the terminal device 100 from the base station device 200 is below a threshold, the terminal device 100 selects the ACO-OFDM method and requests the base station device 200 to use the ACO-OFDM method for optical communication. In response to this request, the base station device 200 configures the terminal device 100 to use the ACO-OFDM method for optical communication and then performs ACO-OFDM optical communication with the terminal device 100.

[0089] Alternatively, the base station 200 or the terminal device 100 may select the OFDM method based on the distance between the terminal device 100 and the base station 200. For example, the base station 200 or the terminal device 100 may select the ACO-OFDM method if the distance is greater than or equal to a threshold. As a method for measuring (estimating) the distance between the terminal device 100 and the base station 200, the terminal device 100 and / or the base station 200 may have means for transmitting and receiving sound waves (acoustics), and the distance may be measured by detecting the position of the communication partner using sound waves (acoustics). The terminal device 100 and / or the base station 200 may have a camera, and the distance may be measured by detecting the position of the communication partner using the camera. If the transmission power (luminescence intensity) of the reference optical signal is known, the distance may be measured based on the path loss, which is the difference between the luminescence intensity and the received light intensity detected at the receiving end.

[0090] The base station equipment 200 and the terminal equipment 100 may perform DCO-OFDM optical communication when the terminal equipment 100 is located in the cell central region. center The region refers to the central region of the cell formed by the base station device 200 (i.e., the region surrounding the base station device 200), and is, for example, the region in which the reception quality (e.g., reception intensity) of the reference optical signal received by the terminal device 100 from the base station device 200 is above a threshold.

[0091] In the illustrated example, terminal device 100b located in the cell edge region communicates with base station device 200 using the DCO-OFDM method. For example, base station device 200 transmits an optical signal to terminal device 100a using the DCO-OFDM method. This improves throughput for terminal device 100b, which has a good optical communication environment.

[0092] For example, terminal device 100 reports to base station device 200 the reception quality (e.g., reception intensity) of the reference optical signal that terminal device 100 receives from base station device 200. If the reception quality reported by terminal device 100 is above a threshold, base station device 200 selects the DCO-OFDM method, configures terminal device 100 to use the DCO-OFDM method for optical communication, and then performs DCO-OFDM optical communication with terminal device 100.

[0093] Alternatively, if the reception quality (e.g., reception intensity) of the reference optical signal received by the terminal device 100 from the base station device 200 is above a threshold, the terminal device 100 selects the DCO-OFDM method and requests the base station device 200 to use the DCO-OFDM method for optical communication. In response to this request, the base station device 200 configures the terminal device 100 to use the DCO-OFDM method for optical communication and then performs DCO-OFDM optical communication with the terminal device 100.

[0094] Alternatively, the base station 200 or the terminal device 100 may select the DCO-OFDM method based on the distance between the terminal device 100 and the base station 200. For example, the base station 200 or the terminal device 100 may select the DCO-OFDM method if the distance is less than a threshold.

[0095] The base station equipment 200 and the terminal equipment 100 may switch the OFDM method used for optical communication from the ACO-OFDM method to the DCO-OFDM method when the terminal equipment 100 moves from the cell edge area to the cell center area. On the other hand, the base station equipment 200 and the terminal equipment 100 may switch the OFDM method used for optical communication from the DCO-OFDM method to the ACO-OFDM method when the terminal equipment 100 moves from the cell center area to the cell edge area. The base station equipment 200 or the terminal equipment 100 may detect the movement based on the latest reception quality (e.g., received light intensity) at the terminal equipment 100. The base station equipment 200 or the terminal equipment 100 may detect the movement based on the latest distance between the base station equipment 200 and the terminal equipment 100.

[0096] Furthermore, as described later, when using the Layered-ACO-OFDM method as the ACO-OFDM method, the base station equipment 200 and the terminal equipment 100 may perform ACO-OFDM (Layered-ACO-OFDM) optical communication when the terminal equipment 100 is located in the cell central region.

[0097] (6)Layered-ACO-OFDM method Next, a Layered-ACO-OFDM method according to one embodiment will be described.

[0098] The base station equipment 200 can transmit simultaneously to multiple terminal devices 100 on the same frequency using spatial multiplexing in the downlink. However, in situations where the terminal devices 100 are close together, it may be necessary to separate the frequencies to suppress interference between the terminal devices 100. In combined transmission, there are also situations where it is desirable to separate the frequencies in order to transmit simultaneously to multiple terminal devices 100.

[0099] In the OFDM method of radio communications, for example, frequencies can be divided by using a predetermined number of consecutive subcarriers as a single resource allocation unit for resource allocation. However, the ACO-OFDM method described above can only use odd-numbered subcarriers, and therefore cannot apply resource allocation like that used in the OFDM method of radio communications.

[0100] On the other hand, there is a technique called Layered-ACO-OFDM that improves upon the drawback of the ACO-OFDM method, which is that even-numbered subcarriers cannot be used. In the Layered-ACO-OFDM method, with respect to the fundamental frequency f0, 2 i Multiplexing an ACO-OFDM at the fundamental frequency of f0.

[0101] Figure 15 is a diagram illustrating the Layered-ACO-OFDM method.

[0102] In the Layered-ACO-OFDM method, f0, 2f0, 4f0, ... 2 iACO-OFDM with f0 as the fundamental frequency is multiplexed. In the illustrated example, the odd-numbered subcarriers of fundamental frequency f0, i.e., f0, 3f0, 5f0, ... constitute the first layer, Layer 1 (the lowest layer). The odd-numbered subcarriers of fundamental frequency 2f0, i.e., 2f0, 3(2f0)=6f0, 5(2f0)=10f0, ... constitute the second layer, Layer 2. The odd-numbered subcarriers of fundamental frequency 4f0, i.e., 4f0, 3(4f0)=12f0, 5(4f0)=20f0, ... constitute the third layer, Layer 3.

[0103] By multiplexing these layers, even-numbered subcarriers can be used in the ACO-OFDM scheme. It should be noted that the conventional Layered-ACO-OFDM scheme assumes one-to-one optical communication with the transmitter and receiver facing each other, and it is believed that throughput can be improved by multiplexing multiple layers using the Layered-ACO-OFDM scheme.

[0104] Figures 16 and 17 illustrate the problems that arise when applying the Layered-ACO-OFDM method to the cellular optical communication system 1.

[0105] As shown in Figure 16(1), in the ACO-OFDM method, out-of-band noise (clipping noise) associated with negative zero clipping occurs in even-numbered subcarriers. Therefore, as shown in Figure 16(2), in order to keep the total transmit power constant, including out-of-band noise, in Layered-ACO-OFDM, the transmit power of the desired signal must be reduced. In the illustrated example, out-of-band noise occurs in even-numbered subcarriers of fundamental frequency f0. Layer 2, consisting of odd-numbered subcarriers of fundamental frequency 2f0, and Layer 3, consisting of odd-numbered subcarriers of fundamental frequency 4f0, should have a high power density to account for this out-of-band noise, but the power density cannot be increased. Thus, in order to keep the total transmit power constant, including noise, in Layered-ACO-OFDM, the power density of the desired wave must be reduced, and Layered-ACO-OFDM is not suitable as is for application to terminal equipment 100 in the cell edge region.

[0106] Furthermore, in the Layered-ACO-OFDM method, i <jとして、2 i f0ACO-OFDM (lower layer) 2 j Interference occurs in the f0ACO-OFDM (upper layer). For example, as shown in Figure 17 (1), layer 1, which consists of odd-numbered subcarriers with fundamental frequency f0, has noise in its even-numbered subcarriers, and this noise interferes with layer 2, which consists of odd-numbered subcarriers with fundamental frequency 2f0. j f0ACO-OFDM (upper layer) 2 i No interference occurs to f0ACO-OFDM (lower layer) (see (2) in Figure 17). Therefore, on the receiving side, 2 i A replica of the clipping noise is reconstructed from the received signal of f0ACO-OFDM (lower layer), and 2 jWhen receiving the signal of f0 ACO-OFDM (upper layer), it is necessary to subtract the replica. That is, if interference cancellation is not performed in the upper layer using the interference replica generated using the decoding result in the lower layer, the signal in the upper layer cannot be decoded. Therefore, decoding and replica generation in the lower layer are essential when receiving the upper layer signal. As a result, the load on the receiving side increases, and different layers cannot be assigned to different terminal devices 100.

[0107] In one embodiment, on the premise that the base station device 200 transmits optical signals from a plurality of light emitting elements 211 having different directivities, different layers can be assigned to different terminal devices 100 by transmitting different layers with different directivities. That is, the base station device 200 assigns a plurality of ACO-OFDM resources (a plurality of layers) having different basic frequencies to a plurality of terminal devices 100 within its own cell. The i-th (i: an integer of 0 or more) ACO-OFDM resource among the plurality of ACO-OFDM resources has a basic frequency 2 i It is composed of odd subcarriers of f0.

[0108] (6.2) Spatial multiplexing transmission FIGS. 18 and 19 are diagrams for explaining spatial multiplexing transmission using the Layered-ACO-OFDM method according to one embodiment. [[ID=1,4]]

[0109] As shown in FIG. 18, when the base station device 200 performs spatial multiplexing transmission for different terminal devices 100, it assigns ACO-OFDM resources (that is, different layers) having different basic frequencies. In the illustrated example, the base station device 200 includes a light emitting element 211a and a light emitting element 211b whose optical axis direction is different from that of the light emitting element 211a. When the base station device 200 performs spatial multiplexing transmission in which transmission to the terminal device 100a using the light emitting element 211a and transmission to the terminal device 100b using the light emitting element 211b are performed simultaneously, it assigns the ACO-OFDM resource 1 (layer 1) to the terminal device 100a and assigns an ACO-OFDM resource 2 (layer 2) whose basic frequency is different from that of the ACO-OFDM resource 1 to the terminal device 100b.

[0110] The base station device 200 then transmits an optical signal from the light-emitting element 211a to the terminal device 100a using ACO-OFDM resource 1 (layer 1), and also transmits an optical signal from the light-emitting element 211b to the terminal device 100b using ACO-OFDM resource 2 (layer 2). The terminal device 100a receives the optical signal from the base station device 200 using ACO-OFDM resource 1 (layer 1) allocated to it by the base station device 200. The terminal device 100b receives the optical signal from the base station device 200 using ACO-OFDM resource 2 (layer 2) allocated to it by the base station device 200.

[0111] Here, terminal device 100a uses a low fundamental frequency (i.e., a lower layer), so it is not affected by interference from the optical signal intended for terminal device 100b. On the other hand, terminal device 100b uses a high fundamental frequency (i.e., a higher layer), but interference from the optical signal intended for terminal device 100a is reduced due to its different directivity. Therefore, terminal device 100b can ensure reception quality (e.g., SINR) without generating a replica of the signal intended for terminal device 100a. Thus, it becomes possible to use Layered-ACO-OFDM suitable for cellular optical communication system 1.

[0112] In this example, spatial multiplexing transmission to two terminal devices 100 has been described, but the base station device 200 may perform spatial multiplexing transmission to three or more terminal devices 100. In that case as well, the base station device 200 assigns different ACO-OFDM resources (different layers) to each terminal device 100 and transmits to each terminal device 100 with different directivity (different light-emitting elements 211).

[0113] As shown in Figure 19, the base station device 200 may increase the transmit power density of ACO-OFDM at a high fundamental frequency (i.e., a higher layer). Since ACO-OFDM at a high fundamental frequency has fewer subcarriers, the transmit power per subcarrier (transmit power density) can be increased while keeping the total transmit power constant. Therefore, it is possible to improve the noise immunity of terminal device 100b, including out-of-band noise from terminal device 100a. In the illustrated example, the fundamental frequency of ACO-OFDM resource 2 is higher than the fundamental frequency of ACO-OFDM resource 1. The base station device 200 uses the first transmit power density for transmission to terminal device 100a to which ACO-OFDM resource 1 is allocated, and uses a second transmit power density higher than the first transmit power density for transmission to terminal device 100b to which ACO-OFDM resource 2 is allocated.

[0114] Under these conditions, the base station equipment 200 may allocate a high-frequency ACO-OFDM resource to a terminal device 100 with a poor optical communication environment (for example, a terminal device 100 in the cell edge area). This allows for proper accommodation of terminal devices 100 in the cell edge area. In the illustrated example, terminal device 100b is further from the base station equipment 200 than terminal device 100a. That is, the optical communication environment of terminal device 100b is worse than that of terminal device 100a. In that case, the base station equipment 200 allocates ACO-OFDM resource 1 to terminal device 100a and ACO-OFDM resource 2, which has a higher fundamental frequency than ACO-OFDM resource 1, to terminal device 100b.

[0115] (6.3) Composite transmission Figure 20 is a diagram illustrating a composite transmission using a Layered-ACO-OFDM method according to one embodiment.

[0116] When the base station device 200 performs combined transmission from multiple light-emitting elements 211 to the same terminal device 100, it allocates ACO-OFDM resources of the same fundamental frequency to the terminal device 100. In the illustrated example, when the base station device 200 performs combined transmission to send the same data from light-emitting elements 211a and 211b with different optical axis directions to one terminal device 100, it controls the light-emitting elements 211a and 211b to perform combined transmission using the same ACO-OFDM resources allocated to that one terminal device 100. This enables appropriate combined transmission using the Layered-ACO-OFDM method.

[0117] (6.1) Resource allocation Figure 21 is a diagram illustrating resource allocation according to one embodiment.

[0118] The base station device 200 uses each of the multiple ACO-OFDM resources with different fundamental frequencies as a resource allocation unit to allocate resources (also referred to as "scheduling") to each terminal device 100 within its cell. Such resource allocation units may also be called resource blocks (RBs). In the illustrated example, the base station device 200 uses ACO-OFDM resource 1 (RB_0), consisting of odd-numbered subcarriers of fundamental frequency f0, ACO-OFDM resource 2 (RB_1), consisting of odd-numbered subcarriers of fundamental frequency 2f0, and ACO-OFDM resource 3 (RB_3), consisting of odd-numbered subcarriers of fundamental frequency 4f0, as resource allocation units to perform resource allocation.

[0119] Here, compared to the DCO-OFDM method, ACO-OFDM resource 1 (RB_0) has half the throughput, ACO-OFDM resource 2 (RB_1) has a quarter throughput, and ACO-OFDM resource 3 (RB_2) has an eighth throughput. Furthermore, as mentioned above, the higher the fundamental frequency of the ACO-OFDM used by the base station equipment 200, the higher the transmission power per subcarrier (transmission power density), and thus the better the reception quality can be.

[0120] For example, the base station equipment 200 may assign only RB_2 to a terminal device 100 with an extremely low SNR, scheduling it to increase the transmit power per subcarrier (transmit power density). The base station equipment 200 may assign all of RB_0, 1, and 2 to a terminal device 100 with a high SNR. In that case, the base station equipment 200 does not necessarily need to use DCO-OFDM. Furthermore, when the base station equipment 200 assigns multiple RBs to a single terminal device 100, it is not necessarily required to provide a power difference between the RBs.

[0121] (7) Other embodiments Although the above embodiments have mainly described downlink transmission, the operation of the above embodiments may also be applied to uplink transmission. For example, the terminal device 100 may communicate simultaneously with multiple base station devices 200. In that case, the terminal device 100 may perform spatial multiplexing to the multiple base station devices 200 using different ACO-OFDM resources with different directivity.

[0122] In the above embodiment, an example was described in which the base station device 200 is installed on the water surface. However, the base station device 200 may also be installed on the bottom of the water. The terminal device 100 moving underwater performs optical communication with the base station device 200 located below (diagonally below) it. Alternatively, the base station device 200 may be installed on the underwater wall. The terminal device 100 performs optical communication with the base station device 200 while moving vertically underwater.

[0123] As shown in Figure 22, multiple base station devices 200 may be arranged three-dimensionally in the water. In the illustrated example, each of the base station devices 200a and 200b is located near the water surface and is fixed to, for example, a buoy. Each of the base station devices 200a and 200b has a hemispherical housing, and multiple light-receiving units are arranged in an array on the surface of the hemispherical housing. Each of the base station devices 200a and 200b is communicably connected to the network 10 via a backhaul line. Base station device 200c is suspended from base station device 200a via ropes and / or cables (hereinafter referred to as "cables, etc."). Base station device 200e is suspended from base station device 200c via cables, etc. Similarly, base station device 200d is suspended from base station device 200b adjacent to base station device 200a via cables, etc. Base station device 200f is suspended from base station device 200d via cables, etc. Each of the base station devices 200c, 200d, 200e, and 200f has a spherical housing, and multiple optical communication units are arranged in an array on the surface of the spherical housing.

[0124] In the above-described embodiment, an example was given in which the light-receiving and light-emitting section 150 of the terminal device 100 and the light-receiving and light-emitting section 250 of the base station device 200 are configured in a hemispherical shape. However, the terminal device 100 and / or the base station device 200 may be configured as a sphere (or, from another viewpoint, a mirror ball shape) as shown in Figure 23. For example, the terminal device 100 and / or the base station device 200 may constitute a polyhedron, with each face of the polyhedron constituting a light-receiving and light-emitting region, and at least one pair of light-emitting and light-receiving elements placed on each face. Alternatively, the terminal device 100 and / or the base station device 200 may be configured as a rod shape as shown in Figure 24. For example, the terminal device 100 and / or the base station device 200 may constitute a rectangular prism, with the sides of the rectangular prism constituting light-receiving and light-emitting regions, and at least one pair of light-emitting and light-receiving elements placed on each side.

[0125] A program may be provided that causes a computer to execute each process performed by the terminal device 100 or the base station device 200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, it is possible to install the program on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transient recording medium. The non-transient recording medium is not particularly limited, but may be a recording medium such as a CD-ROM or DVD-ROM. Furthermore, the circuits that execute each process performed by the terminal device 100 or the base station device 200 may be integrated, and at least a part of the terminal device 100 or the base station device 200 may be configured as a semiconductor integrated circuit (chipset, SoC: System on a chip).

[0126] The phrases "based on" and "depending on / in response to" used in this disclosure do not mean "based solely on" or "depending solely on" unless otherwise specified. The phrase "based on" means both "based solely on" and "at least partially on." Similarly, the phrase "depending on" means both "at least partially on" and "at least partially on." The terms "include," "comprise," and their variations do not mean that only the listed items are included, but that they may include only the listed items or may include additional items in addition to the listed items. Furthermore, the term "or" used in this disclosure is not intended to mean exclusive OR. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles are intended to include multiple items unless it is clearly indicated in the context that this is not the case.

[0127] Although the embodiments have been described in detail above with reference to the drawings, the specific configuration is not limited to those described above, and various design changes can be made without departing from the gist of the invention.

[0128] This application claims priority to Japanese Patent Application No. 2022-187540 (filed November 24, 2022), and all of its contents are incorporated into the specification of this application.

[0129] (8) Note The following is an addendum regarding the features of the embodiment described above.

[0130] (Note 1) A cellular optical communication system (1) that performs optical communication, which is wireless communication using light, A base station device (200) that manages the cell, The cell comprises a terminal device (100) that performs optical communication with the base station device (200) using the OFDM (Orthogonal Frequency Division Multiplexing) method, The base station device (200) and the terminal device (100) perform the ACO (Asymmetrically Clipped Optical)-OFDM optical communication at least when the terminal device (100) is located in the cell edge region. Cellular optical communication system (1).

[0131] (Note 2) The base station device (200) and the terminal device (100) perform optical communication using an OFDM method selected from among a plurality of OFDM methods, including the ACO-OFDM method, based on the optical communication environment of the terminal device (100). Cellular optical communication system (1) as described in Appendix 1.

[0132] (Note 3) The aforementioned multiple OFDM methods further include the DCO (Direct Current Optical)-OFDM (Orthogonal Frequency Division Multiplexing) method, The base station device (200) and the terminal device (100) perform the DCO-OFDM optical communication when the terminal device (100) is located in the cell center region. Cellular optical communication system (1) as described in Appendix 2.

[0133] (Note 4) The base station device (200) and the terminal device (100) switch the OFDM method used for optical communication from the ACO-OFDM method to the DCO-OFDM method when the terminal device (100) moves from the cell edge region to the cell center region. Cellular optical communication system (1) as described in Appendix 3.

[0134] (Note 5) The base station device (200) and the terminal device (100) switch the OFDM method used for optical communication from the DCO-OFDM method to the ACO-OFDM method when the terminal device (100) moves from the cell center region to the cell edge region. Cellular optical communication system (1) as described in Appendix 3 or 4.

[0135] (Note 6) The base station device (200) has a control unit (230) that allocates multiple ACO-OFDM resources with different fundamental frequencies to multiple terminal devices (100) within the cell. Of the aforementioned multiple ACO-OFDM resources, the i-th (i: an integer greater than or equal to 0) ACO-OFDM resource has a fundamental frequency of 2 i It is composed of subcarriers having frequencies that are odd multiples of f0. A cellular optical communication system (1) as described in any of Appendix 1 to 5.

[0136] (Note 7) The control unit (230) uses each of the plurality of ACO-OFDM resources as a resource allocation unit to allocate resources to each terminal device (100) within the cell. Cellular optical communication system (1) as described in Appendix 6.

[0137] (Note 8) The base station device (200) further includes a plurality of light-emitting elements (211), including a first light-emitting element (211) and a second light-emitting element (211) whose optical axis direction is different from that of the first light-emitting element (211). When the control unit (230) performs spatial multiplexing transmission, which simultaneously transmits to a first terminal device (100) using the first light-emitting element (211) and to a second terminal device (100) using the second light-emitting element (211), it allocates the first ACO-OFDM resource to the first terminal device (100) and allocates the second ACO-OFDM resource, which has a different fundamental frequency from the first ACO-OFDM resource, to the second terminal device (100). Cellular optical communication system as described in Appendix 6 or 7 (1).

[0138] (Note 9) The control unit (230) uses a first transmission power density for transmission to the first terminal device (100) and a second transmission power density higher than the first transmission power density for transmission to the second terminal device (100) when the fundamental frequency of the second ACO-OFDM resource is higher than the fundamental frequency of the first ACO-OFDM resource. Cellular optical communication system (1) as described in Appendix 8.

[0139] (Note 10) The control unit (230) will, if the optical communication environment of the second terminal device (100) is worse than that of the first terminal device (100), allocate the first ACO-OFDM resource to the first terminal device (100) and also allocate the second ACO-OFDM resource, which has a higher fundamental frequency than the first ACO-OFDM resource, to the second terminal device (100). Cellular optical communication system (1) as described in Appendix 9.

[0140] (Note 11) The base station device (200) further has a plurality of light-emitting elements (211) with different optical axis directions, When the control unit (230) performs a combined transmission, sending the same data from the multiple light-emitting elements (211) to a single terminal device (100), it controls the multiple light-emitting elements (211) to perform the combined transmission using the same ACO-OFDM resource assigned to the single terminal device (100). Cellular optical communication system as described in Appendix 6 or 7 (1).

[0141] (Note 12) The terminal device (100) and the base station device (200) perform the optical communication underwater. A cellular optical communication system (1) as described in any of the appendices 1 to 11.

[0142] (Note 13) A base station device (200) that manages cells in a cellular optical communication system (1) that performs optical wireless communication, which is wireless communication using light, Optical communication units (210, 220) that perform optical communication with terminal devices (100) within the cell using the OFDM (Orthogonal Frequency Division Multiplexing) method, The system includes a control unit (230) that controls the optical communication unit to perform the ACO (Asymmetrically Clipped Optical)-OFDM optical communication with the terminal device (100) when the terminal device (100) is located in the cell edge region. Base station equipment (200).

[0143] (Note 14) A terminal device (100) used in a cellular optical communication system (1) that performs optical wireless communication, which is an optical wireless communication system, Optical communication units (110, 120) that perform optical communication with the base station equipment (200) that manages the cell using the OFDM (Orthogonal Frequency Division Multiplexing) method, The system includes a control unit (130) that controls the optical communication unit to perform the ACO (Asymmetrically Clipped Optical)-OFDM optical communication with the base station device (200) when at least the terminal device (100) is located in the cell edge region. Terminal device (100). [Explanation of Symbols]

[0144] 1: Cellular optical communication system 10: Network 100: Terminal device 110: Light-emitting part 111: Light-emitting element 112: Transmitter 120: Light receiving part 121: Photodetector 122: Receiver 122a: A / D conversion unit 122b: S / P conversion section 122c:FFT section 122d: Symbol Demapping Section 122e: P / S conversion unit 130: Control Unit 131: Processor 132: Memory 150: Light-receiving and light-emitting section 151: Light-receiving area 160: Main body 200:Base station equipment 210: Light-emitting part 211: Light-emitting element 212: Transmitter 212a: S / P conversion unit 212b: Symbol mapping section 212c:IFFT section 212d: P / S conversion unit 212e: D / A conversion unit 212f: Offset application section 212g: Negative value clip section 220: Light receiving section 221: Photodetector 222: Receiver 230: Control Unit 231: Processor 232: Memory 240: Backhaul Communications Department 241: Network Communications Department 242: Inter-base station communication unit 250: Light-receiving and light-emitting section 251: Light-receiving area 260: Main body

Claims

1. A cellular optical communication system that performs optical communication, which is wireless communication using light, A base station device that manages the cell, The cell comprises a terminal device that performs optical communication with the base station device using the OFDM (Orthogonal Frequency Division Multiplexing) method, The base station device and the terminal device perform the ACO (Asymmetrically Clipped Optical)-OFDM optical communication at least when the terminal device is located in the cell edge region. The base station device has a control unit that allocates multiple ACO-OFDM resources with different fundamental frequencies to multiple terminal devices within the cell, Of the aforementioned multiple ACO-OFDM resources, the i-th (i: an integer greater than or equal to 0) ACO-OFDM resource has a fundamental frequency of 2 ｉ It is composed of subcarriers having frequencies that are odd multiples of f0. Cellular optical communication system.

2. The base station device further includes a plurality of light-emitting elements, each including a first light-emitting element and a second light-emitting element whose optical axis direction is different from that of the first light-emitting element. When the control unit performs spatial multiplexing, which simultaneously transmits to a first terminal device using the first light-emitting element and to a second terminal device using the second light-emitting element, it allocates the first ACO-OFDM resource to the first terminal device and allocates the second ACO-OFDM resource, which has a different fundamental frequency from the first ACO-OFDM resource, to the second terminal device. The cellular optical communication system according to claim 1.

3. The base station device further comprises a plurality of light-emitting elements with different optical axis directions, When the control unit performs a combined transmission, which involves transmitting the same data from the multiple light-emitting elements to a single terminal device, it controls the multiple light-emitting elements to perform the combined transmission using the same ACO-OFDM resource assigned to the single terminal device. The cellular optical communication system according to claim 1.

4. A base station device for managing cells in a cellular optical communication system that performs optical wireless communication, which is wireless communication using light, An optical communication unit that performs optical communication with terminal devices within the cell using the OFDM (Orthogonal Frequency Division Multiplexing) method, The system includes a control unit that controls the optical communication unit to perform the ACO (Asymmetrically Clipped Optical)-OFDM optical communication with the terminal device when at least the terminal device is located in the cell edge region, and a control unit that allocates a plurality of ACO-OFDM resources with different fundamental frequencies to a plurality of terminal devices within the cell, Of the aforementioned multiple ACO-OFDM resources, the i-th (i: an integer greater than or equal to 0) ACO-OFDM resource has a fundamental frequency of 2 ｉ It is composed of subcarriers having frequencies that are odd multiples of f0. Base station equipment.

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