JP7829429B2 - Optical communication systems, base stations, and mobile devices - Google Patents

Description

This invention relates to an optical communication system, a base station, and a mobile device.

For example, in underwater communication, optical communication systems that use light (particularly visible light) as a transmission medium are known. Because light has high directivity, conventional optical communication systems generally assume that the transmitting and receiving optical communication devices are fixed, and that communication is conducted with the transmitting and receiving sides facing each other.

In the future, it is anticipated that optical communication systems will enable optical communication between multiple mobile devices and base stations. Under these assumptions, when using self-propelled mobile devices such as robots or drones, a situation may arise where the mobile device moves outside the base station's communication range (i.e., the mobile device becomes outside the communication area), resulting in a loss of optical communication between the mobile device and the base station. In such a situation, a method is needed to move the mobile device back into the base station's communication range and resume optical communication with the base station.

As a method for resuming communication when a mobile device goes out of range on the ground, a method has been proposed in which, when a mobile device equipped with a wireless communication unit that performs wireless communication using radio waves and a GPS (Global Positioning System) satellite radio wave receiving unit moves to a destination outside the communication range, the mobile device travels in the reverse direction of its travel route based on travel route information acquired in advance via wireless communication and current location information obtained via GPS (see Patent Document 1).

Patent No. 6680525

The method described in Patent Document 1 assumes that the mobile device can acquire its current location information via GPS, and therefore has the problem of not being applicable to cases where the mobile device cannot receive satellite signals. For example, in an optical communication system that performs optical communication underwater, satellite signals cannot reach the mobile device, so the method described in Patent Document 1 cannot be applied.

In particular, in optical communication systems that operate underwater, optical communication may be interrupted if the mobile device is carried out of range by ocean currents or becomes obscured by obstacles. In this case, it becomes impossible to determine the location of the affected mobile device, making it impossible to manage the device.

Therefore, the present invention aims to provide an optical communication system, base station, and mobile device that enable a mobile device that has gone out of range to resume optical communication with a base station, even in cases where the mobile device cannot utilize its current location information.

The optical communication system according to the first embodiment comprises a plurality of mobile devices and a base station that communicates optically with each of the plurality of mobile devices. Upon detecting a first mobile device among the plurality of mobile devices whose optical communication has been interrupted, the base station instructs a second mobile device, different from the first mobile device, to transmit a search optical signal for locating the first mobile device. The second mobile device transmits the search optical signal in response to the instruction from the base station.

The base station according to the second embodiment is a base station used in an optical communication system, comprising: an optical communication unit that performs optical communication with each of a plurality of mobile devices; and a control unit that controls the optical communication unit to instruct a second mobile device, different from the first mobile device, to transmit a search optical signal for searching for the first mobile device, in response to the detection of a first mobile device from the plurality of mobile devices whose optical communication has been interrupted.

The third embodiment of the mobile device is a mobile device that performs optical communication with a base station in an optical communication system, and comprises: an optical communication unit that receives a transmission instruction from the base station instructing the transmission of a search optical signal for searching for another mobile device whose optical communication with the base station has been interrupted; and a control unit that controls the optical communication unit to transmit the search optical signal in response to the receipt of the transmission instruction from the base station.

A mobile device according to the fourth embodiment is a mobile device that performs optical communication with a base station in an optical communication system, and comprises an optical communication unit that receives a search optical signal from another mobile device for searching for the mobile device after optical communication with the base station is interrupted, and a control unit that identifies the direction of arrival of the search optical signal in response to the receipt of the search optical signal from the other mobile device and performs movement control to move in the direction of arrival.

According to one aspect of the present invention, an optical communication system, base station, and mobile device can be provided that enable a mobile device that has gone out of range to resume optical communication with a base station, even in cases where the mobile device cannot utilize its current location information.

This is a diagram showing an example configuration of an optical communication system according to the embodiment. This figure shows an example of a base station configuration according to the embodiment. This figure shows an example of the external configuration of a base station according to the embodiment. This figure shows an example configuration of a mobile device according to the embodiment. This figure shows an example of the external configuration of a mobile device according to the embodiment. This figure shows DL communication as an example of optical communication according to the embodiment. This figure shows an example of the configuration of a communication frame used in the optical communication system according to the embodiment. This is a diagram illustrating the restart control of optical communication according to the embodiment. This is a diagram illustrating the restart control of optical communication according to the embodiment. This is a diagram illustrating the restart control of optical communication according to the embodiment. This is a diagram illustrating the restart control of optical communication according to the embodiment. This is a diagram illustrating the restart control of optical communication according to the embodiment. This figure shows the control flow in a base station according to the embodiment. This diagram shows the control flow in the mobile device (second mobile device) during communication according to the embodiment. This figure shows the control flow in the lost mobile machine (first mobile machine) according to the embodiment. This shows another embodiment of an optical communication system. This shows another embodiment of an optical communication system. This figure shows a base station and/or mobile device according to another embodiment. This figure shows a base station and/or mobile device according to another embodiment.

The optical communication system according to the embodiment will be described with reference to the drawings. In the drawings, identical or similar parts are denoted by the same or similar reference numerals.

The optical communication system according to this embodiment is a system that performs optical communication using visible light as an example of light. However, the optical communication system may also be a system that performs optical communication using light other than visible light, such as infrared light. Furthermore, the optical communication system according to this embodiment is a system that performs optical communication underwater. However, the 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.

(1) Example of Optical Communication System Configuration First, an example of the configuration of an optical communication system according to the embodiment will be described. Figure 1 is a diagram showing an example of the configuration of an optical communication system 1 according to the embodiment. The optical communication system 1 has a plurality of mobile devices 100 (100a, 100b) and a base station 200. However, the number of mobile devices 100 and the number of base stations 200 are not limited to the example in Figure 1.

Base station 200 is an example of an optical communication device. In the example shown in Figure 1, base station 200 is located on the water surface. For example, base station 200 is fixed to a buoy. Base station 200 is connected to the network via a backhaul line. The backhaul line may be a wireless line or a wired line. To efficiently secure a communication area underwater, base station 200 may be installed at a predetermined distance from other adjacent base stations. Base station 200 may be installed temporarily, for example, for the period during which underwater surveys are conducted using the mobile device 100.

Mobile device 100 is another example of an optical communication device. Each mobile device 100 is submerged in water. Each mobile device 100 is configured to move underwater. For example, each mobile device 100 is a self-propelled mobile device 100 such as an underwater robot or underwater drone. Each mobile device 100 communicates with a base station 200 using visible light (specifically, underwater visible light communication). That is, the base station 200 is the serving base station for each mobile device 100. Each mobile device 100 may be equipped with sensors such as image sensors and generate sensor data. For example, each mobile device 100 may transmit uplink (UL) data containing sensor data to the base station 200 via visible light communication. Each mobile device 100 may receive downlink (DL) data containing instruction data from the base station 200 via visible light communication. Each mobile device 100 may perform movement and sensing operations (such as photography) based on the instruction data.

In such an optical communication system 1, a situation may arise where the mobile device 100 moves out of the communication range of the base station 200 (i.e., the mobile device 100 goes outside the communication range), and optical communication between the mobile device 100 and the base station 200 is interrupted. For example, the mobile device 100 may be carried out of the communication range by ocean currents, or it may be hidden behind an obstacle, causing the optical communication to be interrupted. In this case, it becomes impossible to know where the mobile device 100 has gone, and it becomes impossible to manage the mobile device 100. In this embodiment, it is possible to move the mobile device 100 back into the communication range of the base station 200 and resume optical communication with the base station 200. In the following, the range in which communication with the base station 200 is possible will also be referred to as "within the range," and the range in which communication with the base station 200 is impossible will also be referred to as "outside the range."

(2) Example of base station configuration Next, an example of the configuration of the base station 200 according to the embodiment will be described.

(2.1) Example of base station block configuration Figure 2 is a diagram showing an example of the configuration of a base station 200 according to the embodiment. The base station 200 has an optical communication unit 201, a control unit 230, and a backhaul communication unit 240. The base station 200 may have a battery to supply the power necessary for the operation of the base station 200. The optical communication unit 201 performs visible light communication with the mobile device 100. The optical communication unit 201 has a light-emitting unit 210 and a light-receiving unit 220.

The light-emitting unit 210 transmits an optical signal to the mobile 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 or a light-emitting diode. Each light-emitting element 211 converts an electrical signal (transmission signal) output by the transmitter 212 for visible light communication into an optical signal and transmits the optical signal. 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 signal after signal processing, and outputs it to the light-emitting elements 211. In this embodiment, each of the plurality of light-emitting elements 211 has a different optical axis orientation. In other words, each of the multiple light-emitting elements 211 has a different directionality (transmission directionality).

The light-receiving unit 220 receives an optical signal from the mobile 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. 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 as 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. In this embodiment, the light-receiving elements 221 may also be provided in a one-to-one relationship with the light-emitting elements 211. Specifically, the light-receiving elements 221 have directivity (receiving directivity) in the same direction as the corresponding light-emitting elements 211. In other words, 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.

The control unit 230 controls the overall operation of the base station 200. For example, the control unit 230 controls the light-emitting unit 210 and the light-receiving unit 220. 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 in processing by the processor 231. The processor 231 may include a digital signal processor and a CPU. The digital signal processor performs modulation, demodulation, encoding, and decoding of digital signals. The CPU executes programs stored in memory and performs various processes.

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 include a network communication unit 241 for communication with a network (e.g., a core network) and an inter-base station communication unit 242 for inter-base station communication with adjacent base stations.

In the base station 200 configured in this way, the optical communication unit 201 communicates with each of the multiple mobile devices 100 using visible light. The control unit 230, upon detecting a first mobile device (hereinafter also referred to as the "lost mobile device") 100 from which visible light communication has been lost, controls the optical communication unit 201 to instruct a second mobile device (hereinafter also referred to as the "communicating mobile device") 100, which is different from the first mobile device 100, to transmit a search light signal to locate the first mobile device 100. This effectively extends the communication range of the base station 200 using the communicating mobile device 100, allowing the search light signal to reach a wider area and search for the lost mobile device 100. As a result, the lost mobile device 100, which has moved outside the communication range, can resume visible light communication with the base station 200.

(2.2) Example of the external configuration of a base station Figure 3 is a diagram showing an example of the external configuration of a base station 200 according to the embodiment.

The base station 200 has a hemispherical light-receiving unit 250 constituting the optical communication unit 201, and a main body 260 connected to the light-receiving unit 250. However, the base station 200 may be configured as a whole in a spherical shape. The light-receiving unit 250 has a plurality of dispersed light-receiving regions 251. Each light-receiving region 251 is provided with a pair of light-emitting elements 211 and light-receiving elements 221. This configuration makes it easy for the base station 200 to perform visible light communication with mobile devices 100 in various directions.

In the example shown in Figure 3, the hemispherical light-receiving and light-emitting section 250 has a total of 19 light-receiving and light-emitting regions 251#0 to 251#18. That is, the base station 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.

(3) Example of Mobile Device Configuration Next, an example of the configuration of the mobile device 100 according to the embodiment will be described.

(3.1) Example of Mobile Device Block Configuration Figure 4 is a diagram showing an example of the configuration of a mobile device 100 according to the embodiment. The mobile device 100 has an optical communication unit 101, a control unit 130, and a mobile mechanism 140. The mobile device 100 may have a battery to supply the power necessary for the operation of the mobile device 100. The optical communication unit 101 performs visible light communication with the base station 200. The optical communication unit 101 has a light-emitting unit 110 and a light-receiving unit 120.

The light-emitting unit 110 transmits an optical signal to the base station 200 under the control of the control unit 130. The light-emitting unit 110 comprises 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 or a light-emitting diode. Each light-emitting element 111 converts the electrical signal (transmission signal) output by the transmitter 112 for visible light communication into an optical signal and transmits the optical signal. The transmitter 112 may be configured as 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 elements 111. In this embodiment, each of the plurality of light-emitting elements 111 has a different optical axis orientation. That is, each of the plurality of light-emitting elements 111 has a different direction of directivity (transmission directivity).

The light-receiving unit 120 receives optical signals from the base station 200. The light-receiving unit 120 includes a plurality of light-receiving elements 121 (121#0, 121#1, ...) and a receiver 122. Each light-receiving element 121 may be a photodiode. 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. 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 light-receiving elements 121, performs signal processing on the converted received signal, and outputs it to the control unit 130.

In this embodiment, the light-receiving element 121 is provided in a one-to-one pair with the light-emitting element 111. Specifically, 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.

The control unit 130 controls the overall operation of the mobile device 100. For example, the control unit 130 controls the light-emitting unit 110 and the light-receiving unit 120. 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 in processing by the processor 131. The processor 131 may include a digital signal processor and a CPU. The digital signal processor performs modulation, demodulation, encoding, and decoding of digital signals. The CPU executes programs stored in memory and performs various processes.

The moving mechanism 140 moves the mobile machine 100 under the control of the control unit 130. The moving mechanism 140 includes, for example, a motor and a screw connected to the motor's rotating shaft.

The mobile device 100 configured in this way communicates with the base station 200 using visible light. When mobile device 100 is the second mobile device 100, the optical communication unit 101 receives a transmission instruction from the base station 200 to transmit a search optical signal to search for another mobile device (i.e., the first mobile device/lost mobile device) 100 whose visible light communication with the base station 200 has been interrupted. The control unit 130, in response to receiving the transmission instruction from the base station 200, controls the optical communication unit 101 to transmit the search optical signal. This effectively extends the communication range of the base station 200 by utilizing the mobile devices 100 within the communication range, allowing the search optical signal to reach a wider area and search for the lost mobile device 100.

On the other hand, when mobile device 100 is the first mobile device 100, the optical communication unit 101 receives a search optical signal from another mobile device (second mobile device) 100 to locate the mobile device 100 after visible light communication with the base station 200 is interrupted. The control unit 130, upon receiving the search optical signal from the second mobile device 100B, identifies the direction from which the search optical signal is coming and performs movement control to move in that direction. This allows the mobile device 100 to return to the communication range of the base station 200 and resume visible light communication with the base station 200.

(3.2) Example of the external configuration of the mobile device Figure 5 is a diagram showing an example of the external configuration of the mobile device 100 according to the embodiment.

The mobile device 100 has a hemispherical light-receiving/light-emitting section 150 that constitutes the optical communication unit 101, and a main body 160 connected to the light-receiving/light-emitting section 150. However, the mobile device 100 may be configured as a whole in a spherical shape. The light-receiving/light-emitting section 150 has a plurality of dispersed light-receiving/light-emitting regions 151. Each light-receiving/light-emitting region 151 is provided with a pair of light-emitting elements 111 and light-receiving elements 121. This configuration makes it easy for the mobile device 100 to perform visible light communication with base stations 200 in various directions.

In the example shown in Figure 5, 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 mobile 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.

(4) An example of optical communication Next, an example of optical communication according to the embodiment will be described. Figure 6 is a diagram showing DL communication as an example of optical communication according to the embodiment. In the example of Figure 6, a cross-section of the base station 200 is shown in a simplified manner.

In the base station 200, multiple light-emitting elements 211 are arranged such that as the distance between one light-emitting element 211 and another 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, which is not adjacent to light-emitting element 211#0, is larger than the angle between the optical axis of light-emitting element 211#0 and the optical axis of light-emitting element 211#1, which is adjacent to light-emitting element 211#0.

The base station 200 associates the mobile device 100 with a pair of light-emitting elements 211 and light-receiving elements 221 corresponding to the direction in which the mobile device 100 is located. In the example shown in Figure 6, the base station 200 associates the mobile device 100 with light-emitting element 211#7 and its paired light-receiving element 221#7 (not shown), and uses the pair of light-emitting element 211#7 and light-receiving element 221#7 to perform visible light communication with the mobile device 100. Specifically, the base station 200 uses the light-emitting element 211#7 for DL communication with the mobile device 100 and the light-receiving element 221#7 for UL communication with the mobile device 100.

Similarly, the mobile device 100 associates a pair of light-emitting elements 111 and light-receiving elements 121 corresponding to the direction in which the base station 200 is located with the base station 200, and uses this pair to perform visible light communication with the base station 200. Specifically, the mobile device 100 uses the light-emitting element 111 for UL communication with the base station 200 and the light-receiving element 121 for DL communication with the base station 200.

Figure 7 shows an example of the configuration of a communication frame used in the optical communication system 1 according to the embodiment. In the example in Figure 7, one communication frame is composed of 10 time slots, but the number of time slots constituting one communication frame is not limited to 10. Each time slot is composed of a predetermined number of symbol intervals.

In the frame configuration example shown in Figure 7, 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.

The synchronization slot (Sync.) is a time slot in which the base station 200 transmits a synchronization optical signal (and a base station-specific reference signal). The mobile device 100 identifies the base station 200 by the synchronization optical signal received from the base station 200 and establishes or maintains synchronization with the base station 200 using this signal. The control slot (Ctrl.) is a time slot in which the base station 200 transmits a control optical signal (for example, scheduling information indicating the slot assignments for DL and UL). Note that the base station-specific reference signal may be transmitted in all slots except the UL slot.

DL slots #0 to #3 constitute the DL communication period. The base station 200 assigns each of DL slots #0 to #3 to one or more mobile devices 100. The base station 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 a time-division multiplexer.

UL slots #0 to #3 constitute a UL communication period. The base station 200 assigns each of UL slots #0 to #3 to one or more mobile devices 100. The mobile devices 100 transmit UL data optical signals in their assigned UL slots.

The base station 200 can communicate simultaneously with multiple mobile devices 100 located in different directions. Specifically, the base station 200 can spatially multiplex multiple mobile devices 100 located in different directions. Therefore, the base station 200 may assign one DL slot to multiple mobile devices 100.

(5) Control of restarting optical communication Next, with reference to Figures 8 to 12, the control of restarting visible light communication according to the embodiment will be described.

As shown in Figure 8, the base station 200 is communicating with each of the multiple mobile devices 100 (100A, 100B1, 100B2) using visible light (specifically, DL communication and UL communication). Mobile device 100A is an example of a first mobile device. Mobile devices 100B1 and 100B2 are examples of second mobile devices.

Here, we assume that mobile device 100A has been carried out of the base station 200's communication range by ocean currents or other factors, as shown in Figure 9. Base station 200 detects mobile device 100A whose visible light communication has been interrupted. For example, if base station 200 does not receive an optical signal (e.g., a response optical signal or a data optical signal) from mobile device 100A for a certain period of time, it determines that visible light communication with mobile device 100A has been interrupted. Hereafter, mobile device 100A whose visible light communication has been interrupted will be referred to as "lost mobile device 100A," and mobile devices 100B (100B1 and 100B2) that are continuing visible light communication will be referred to as "mobile devices 100B that are communicating."

As shown in Figure 10, when base station 200 detects the lost mobile device 100A, it instructs the mobile devices 100B1 and 100B2 to transmit a search optical signal to locate the lost mobile device 100A. Specifically, base station 200 transmits a transmission instruction for the search optical signal to mobile devices 100B1 and 100B2. This transmission instruction may be a type of control optical signal transmitted by base station 200. Each of the mobile devices 100B1 and 100B2 transmits a search optical signal in response to the instruction from base station 200.

In the example shown in Figure 10, the base station 200 transmits a search light signal transmission instruction to all mobile devices 100B1 and 100B2 that are currently communicating. However, the base station 200 may also transmit the search light signal transmission instruction only to mobile devices 100B1 that are presumed to be close to the lost mobile device 100A. Specifically, the base station 200 may transmit a search light signal transmission instruction only to mobile devices 100B1 that are located within a predetermined directional range from the direction in which the lost mobile device 100A was located before visible light communication with the lost mobile device 100A was interrupted (i.e., the pair of light-emitting element 211 and light-receiving element 221 associated with the lost mobile device 100A).

Furthermore, the search light signal has a signal format that allows it to be identified as a search light signal. The search light signal may or may not include the identifier of the lost mobile device 100A.

Upon receiving an instruction to transmit a search light signal, the mobile device 100B in communication intermittently transmits the search light signal. Specifically, the mobile device 100B transmits the search light signal at a timing different from that of visible light communication (DL communication and UL communication) between the base station 200 and the mobile device 100B. This allows the mobile device 100B to transmit the search light signal while continuing visible light communication with the base station 200. For example, the mobile device 100B transmits the search light signal in time slots other than the DL slots and UL slots assigned by the base station 200. Here, the base station 200 performs scheduling control for the mobile device 100B so that the timing of visible light communication between the base station 200 and the mobile device 100B is different from the timing of the mobile device 100B transmitting the search light signal. This enables the mobile device 100B to efficiently transmit the search light signal.

Furthermore, when the mobile device 100B receives an instruction to transmit a search light signal, it transmits the search light signal in a direction different from the direction in which the base station 200 is located. Specifically, the mobile device 100B transmits the search light signal using a different light-emitting element 111 than the one used for visible light communication with the base station 200, particularly the light-emitting element 111 on the opposite side from the one used for visible light communication with the base station 200. Since it is assumed that the lost mobile device 100A does not exist in the direction in which the base station 200 is located, transmitting the search light signal in a direction different from that direction suppresses the transmission of unnecessary search light signals and reduces the power consumption of the mobile device 100B. In addition, interference caused by the search light signal to visible light communication between another mobile device 100B and the base station 200 can be suppressed.

As shown in Figure 11, the lost mobile device 100A, upon receiving a search light signal from the communicating mobile device 100B (in the illustrated example, the communicating mobile device 100B1), identifies the direction from which the search light signal is coming and performs movement control to move in that direction. Specifically, the lost mobile device 100A identifies the photodetector 121 with the highest received intensity of the search light signal and performs movement control to move in the direction corresponding to that photodetector 121. This allows the lost mobile device 100A to return to the communication range (i.e., within the communication range) of the base station 200.

Here, the lost mobile device 100A attempts to resume visible light communication with the base station 200 while moving in the direction from which the search light signal is coming. For example, the lost mobile device 100A attempts to receive the optical signal (synchronization signal and/or reference signal) from the base station 200 using all of its photodetectors 121, and resumes visible light communication with the base station 200 by transmitting a notification light signal or a communication request light signal to the base station 200 upon successful reception of the optical signal from the base station 200. Upon resuming visible light communication with the base station 200, the lost mobile device 100A stops its movement control, which is moving in the direction from which the search light signal is coming. For example, the lost mobile device 100A stops moving when it successfully receives the optical signal from the base station 200. This allows the lost mobile device 100A to resume visible light communication with the base station 200.

However, if the lost mobile device 100A moves in the direction from which the search light signal is coming, it may collide with the mobile device 100B1 that is in communication. Therefore, when the lost mobile device 100A moves in the direction from which the search light signal is coming, it may monitor the received intensity of the search light signal and stop its movement control if the received intensity exceeds a threshold. This makes it possible to stop the movement of the lost mobile device 100A when it is deemed to be approaching the mobile device 100B1 that is in communication.

When visible light communication with the lost mobile device 100A resumes, the base station 200 instructs the mobile device 100B, which is currently communicating, to intermittently stop transmitting the search light signal (or to continuously stop transmitting the search light signal as described later). The mobile device 100B stops transmitting the search light signal in response to the instruction from the base station 200. This suppresses the transmission of unnecessary search light signals and reduces the power consumption of the mobile device 100B.

If communication with the lost mobile device 100A is not resumed within a predetermined time from the start of the search for the lost mobile device 100A, the base station 200 instructs the mobile device 100B, which is currently communicating, to switch to a search mode that expands the direction in which the search light signal is transmitted, as shown in Figure 12. For example, the base station 200 measures this predetermined time by starting a timer when it instructs the mobile device 100B to transmit the search light signal. When the mobile device 100B is in search mode in response to the instruction from the base station 200, it expands the direction in which the search light signal is transmitted compared to when it is not in search mode. This makes it possible to increase the probability that the search light signal reaches the lost mobile device 100A. For example, when the mobile device 100B is in search mode, it transmits the search light signal in all directions from which it can transmit. Specifically, the mobile device 100B transmits the search light signal using all of its light-emitting elements 111.

During communication, mobile unit 100B increases the frequency of transmitting the search light signal compared to when not in search mode. This increases the likelihood that the search light signal reaches the lost mobile unit 100A. During search mode, mobile unit 100B may interrupt visible light communication with base station 200 and continuously transmit the search light signal. Continuous transmission of the search light signal may mean transmitting the search light signal in two or more consecutive time slots. For example, continuous transmission of the search light signal may mean transmitting the search light signal in all DL slots and all UL slots.

During communication, the mobile device 100B may increase the transmission output (i.e., light intensity) of the search light signal when in search mode compared to when not in search mode. This makes it possible to transmit the search light signal over a longer distance.

However, if the lost mobile device 100A becomes stuck on an obstacle or other object and is unable to move, even if the search mode is executed, the lost mobile device 100A cannot resume visible light communication with the base station 200. Therefore, the base station 200 may send alarm information to the network side (e.g., a server) if communication with the lost mobile device 100A is not resumed within a predetermined time after switching to search mode. For example, the base station 200 measures this predetermined time by starting a timer when it instructs the communicating mobile device 100B to switch to search mode. This allows the network side to be aware that there is a lost mobile device 100A that cannot resume visible light communication.

(6) Example of Control Flow for Restart Control of Optical Communication Next, with reference to Figures 13 to 15, the control flow for restart control of visible light communication according to the embodiment will be described.

(6.1) Example of control flow at a base station Figure 13 is a diagram showing the control flow at a base station 200 according to the embodiment. In Figure 13, the mobile unit (lost mobile unit) 100A is denoted as mobile unit A.

In step S101, the control unit 230 of the base station 200 controls the optical communication unit 201 to perform visible light communication with multiple mobile devices 100.

In step S102, the control unit 230 of the base station 200 detects that communication with the mobile device 100A has been lost.

In step S103, the control unit 230 of the base station 200 performs a calling process to the lost mobile device 100A. For example, the control unit 230 of the base station 200 controls the optical communication unit 201 to transmit a calling optical signal to summon the lost mobile device 100A, waits for a response from the lost mobile device 100A, and then attempts to resume visible light communication with the lost mobile device 100A.

In step S104, the control unit 230 of the base station 200 determines whether visible light communication with the lost mobile device 100A has resumed. If it determines that visible light communication with the lost mobile device 100A has resumed (step S104: Yes), this flow ends.

If it is determined that visible light communication with the lost mobile device 100A cannot be resumed (Step S104: No), in Step S105, the control unit 230 of the base station 200 determines whether a predetermined time has elapsed since the call processing to the lost mobile device 100A was initiated. If the predetermined time has not elapsed (Step S105: No), the process returns to Step S104.

If it is determined that a predetermined time has elapsed (Step S105: Yes), in Step S106, the control unit 230 of the base station 200 controls the optical communication unit 201 to instruct the mobile device 100B to transmit a search optical signal.

In step S107, the control unit 230 of the base station 200 determines whether visible light communication with the lost mobile device 100A has resumed.

If it is determined that visible light communication with the lost mobile device 100A has resumed (step S107: Yes), in step S108, the control unit 230 of the base station 200 controls the optical communication unit 201 to instruct the other mobile devices 100B that are currently communicating to stop transmitting the search light signal. Then, in step S109, the control unit 230 of the base station 200 resumes visible light communication with the lost mobile device 100A and controls the optical communication unit 201 to perform visible light communication with the lost mobile device 100A.

On the other hand, in step S110, the control unit 230 of the base station 200 determines whether a predetermined time has elapsed since instructing the mobile device 100B to transmit the search light signal. If the predetermined time has not elapsed (step S110: No), the process returns to step S107.

If it is determined that a predetermined time has elapsed (Step S110: Yes), in Step S111, the control unit 230 of the base station 200 controls the optical communication unit 201 to stop visible light communication with the mobile device 100B during communication, and also controls the optical communication unit 201 to instruct the mobile device 100B during communication to switch to search mode.

In step S112, the control unit 230 of the base station 200 determines whether visible light communication with the lost mobile device 100A has resumed. If it determines that visible light communication with the lost mobile device 100A has resumed (step S112: Yes), the control unit 230 of the base station 200 proceeds to step S108.

If it is determined that visible light communication with the lost mobile device 100A has not resumed (Step S112: No), in Step S113, the control unit 230 of the base station 200 determines whether a predetermined time has elapsed since transitioning to search mode. If the predetermined time has not elapsed (Step S113: No), the process returns to Step S112.

If it is determined that a predetermined time has elapsed (step S113: Yes), in step S114, the control unit 230 of the base station 200 controls the optical communication unit 201 to instruct the mobile device 100B to stop transmitting the search optical signal. Then, in step S115, the control unit 230 of the base station 200 resumes visible light communication with the mobile device 100B in response to the end of the search mode. In step S116, the control unit 230 of the base station 200 controls the backhaul communication unit 240 to send an alarm notification to the network side, and this flow ends.

(6.2) Example of control flow in a mobile device during communication Figure 14 is a diagram showing the control flow in a mobile device (second mobile device) 100B during communication according to the embodiment.

In step S201, the optical communication unit 101 of the mobile device 100B receives a transmission instruction for a search optical signal from the base station 200.

In step S202, the control unit 130 of the mobile device 100B controls the optical communication unit 101 to transmit the search optical signal at the transmission timing scheduled by the base station 200.

In step S203, the control unit 130 of the mobile device 100B determines whether the optical communication unit 101 has received an instruction from the base station 200 to stop transmitting the search optical signal.

If the optical communication unit 101 determines that it has received an instruction from the base station 200 to stop transmitting the search optical signal (step S203: Yes), in step S204, the control unit 130 of the mobile device 100B controls the optical communication unit 101 to stop transmitting the search optical signal, and this flow ends.

If the optical communication unit 101 determines that it has not received an instruction from the base station 200 to stop transmitting the search optical signal (step S203: No), in step S205, the control unit 130 of the mobile device 100B determines whether a predetermined time has elapsed since it was instructed to transmit the search optical signal. If the predetermined time has not elapsed (step S205: No), the process returns to step S203.

If it is determined that a predetermined time has elapsed (Step S205: Yes), in Step S206, the optical communication unit 101 of the mobile device 100B receives an instruction from the base station 200 to switch to search mode.

In step S207, the control unit 130 of the mobile device 100B, upon receiving an instruction to transition to search mode, controls the optical communication unit 101 to stop visible light communication with the base station 200, and also controls the optical communication unit 101 to continuously transmit search light signals in all directions.

In step S208, the control unit 130 of the mobile device 100B determines whether the optical communication unit 101 has received an instruction from the base station 200 to stop transmitting the search optical signal. If the optical communication unit 101 has received an instruction from the base station 200 to stop transmitting the search optical signal (step S208: Yes), in step S209, the control unit 130 of the mobile device 100B controls the optical communication unit 101 to stop transmitting the search optical signal, and this flow ends.

If the optical communication unit 101 determines that it has not received a command from the base station 200 to stop transmitting the search optical signal (step S208: No), in step S210, the control unit 130 of the mobile device 100B determines whether a predetermined time has elapsed since receiving the command to switch to search mode. If the predetermined time has not elapsed (step S210: No), the process returns to step S208.

If it is determined that a predetermined time has elapsed (Step S210: Yes), in Step S211, the control unit 130 of the mobile device 100B controls the optical communication unit 101 to stop transmitting the search optical signal. Then, in Step S212, the control unit 130 of the mobile device 100B controls the optical communication unit 101 to resume visible light communication with the base station 200, and this flow ends.

(6.3) Example of control flow in a lost mobile machine Figure 15 is a diagram showing the control flow in a lost mobile machine (first mobile machine) 100A according to the embodiment.

In step S301, the control unit 130 of the lost mobile device 100A detects the interruption of visible light communication with the base station 200.

In step S302, the control unit 130 of the lost mobile device 100A transitions to out-of-range mode.

In step S303, the control unit 130 of the lost mobile device 100A determines whether the optical communication unit 101 has received a search optical signal from the mobile device 100B that is currently communicating. If it is determined that the optical communication unit 101 has not received a search optical signal (step S303: No), the process returns to step S303.

If the optical communication unit 101 determines that it has received the search light signal (step S303: Yes), in step S304, the control unit 130 of the lost mobile device 100A identifies the direction from which the search light signal received by the optical communication unit 101 originated. Here, it is assumed that multiple light-receiving elements 121 in the optical communication unit 101 of the lost mobile device 100A have received the search light signal.

In step S305, the control unit 130 of the lost mobile device 100A selects the light-receiving element 121 with the highest received signal intensity among the multiple light-receiving elements 121 that received the search light signal, i.e., the direction from which the search light signal arrives with the highest received signal intensity.

In step S306, the control unit 130 of the lost mobile device 100A controls the mobile mechanism 140 to move in the direction of arrival selected in step S305, and also controls the optical communication unit 101 to switch to a continuous reception mode for continuously receiving (monitoring) optical signals (particularly optical signals from the base station 200).

In step S307, the control unit 130 of the lost mobile device 100A determines whether the received intensity of the search light signal exceeds a threshold.

If it is determined that the received intensity of the search light signal does not exceed the threshold (step S307: No), in step S308, the control unit 130 of the lost mobile device 100A determines whether visible light communication with the base station 200 has resumed. For example, the control unit 130 of the lost mobile device 100A determines whether the optical communication unit 101 has received an optical signal from the base station 200. If it is determined that visible light communication with the base station 200 has not resumed (step S308: No), the process returns to step S306. If it is determined that visible light communication with the base station 200 has resumed (step S308: Yes), this flow ends.

On the other hand, if it is determined that the received intensity of the search light signal exceeds a threshold (step S307: Yes), in step S309, the control unit 130 of the lost mobile device 100A controls the mobile mechanism 140 to stop moving in the direction from which the search light signal is coming. Then, in step S310, the control unit 130 of the lost mobile device 100A controls the optical communication unit 101 to resume (implement) communication with the base station 200, and this flow ends.

(7) Summary of Embodiments As described above, in the base station 200, the optical communication unit 201 performs visible light communication with each of the multiple mobile devices 100. When the control unit 230 detects a lost mobile device 100A from among the multiple mobile devices 100 whose visible light communication has been interrupted, it controls the optical communication unit 201 to instruct the mobile device 100B that is currently communicating to transmit a search optical signal to search for the lost mobile device 100A.

In the mobile device 100B during communication, the optical communication unit 101 receives a transmission instruction from the base station 200 to transmit a search optical signal for searching for the lost mobile device 100A. The control unit 130 controls the optical communication unit 101 to transmit the search optical signal in response to receiving the transmission instruction from the base station 200.

In the lost mobile unit 100A, the optical communication unit 101 receives a search light signal from the communicating mobile unit 100B after visible light communication with the base station 200 is interrupted. The control unit 130, upon receiving the search light signal from the communicating mobile unit 100B, identifies the direction from which the search light signal is coming and performs movement control to move in that direction.

This allows the lost mobile device 100A to resume visible light communication with the base station 200, even if it is unable to use its current location information.

(8) Other Embodiments In the embodiments described above, an example was given in which the base station 200 is installed on the water surface. However, the base station 200 may be installed on the bottom of the water, as shown in Figure 16. The mobile device 100 moving underwater performs visible light communication with the base station 200 located below (diagonally below) itself. Alternatively, the base station 200 may be installed on an underwater wall, as shown in Figure 17. The mobile device 100 performs visible light communication with the base station 200 while moving vertically underwater.

In the above-described embodiment, an example was explained in which the light-receiving and light-emitting unit 150 of the mobile device 100 and the light-receiving and light-emitting unit 250 of the base station 200 are configured in a hemispherical shape. However, the mobile device 100 and/or base station 200 may be configured in an overall spherical shape (or, from another viewpoint, a mirror ball shape), as shown in Figure 18. For example, the mobile device 100 and/or base station 200 may constitute a polyhedron, with each face of the polyhedron constituting a light-receiving and light-emitting region, and a pair of light-emitting and light-receiving elements arranged on each face. Alternatively, the mobile device 100 and/or base station 200 may be configured in an overall rod shape, as shown in Figure 19. For example, the mobile device 100 and/or base station 200 may constitute a rectangular prism, with the sides of the rectangular prism constituting light-receiving and light-emitting regions, and a pair of light-emitting and light-receiving elements arranged on each side.

A program may be provided that causes a computer to execute each process performed by the mobile device 100 or base station 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 for example, it may be a recording medium such as a CD-ROM or DVD-ROM. Furthermore, the circuits that execute each process performed by the mobile device 100 or base station 200 may be integrated, and at least a part of the mobile device 100 or base station 200 may be configured as a semiconductor integrated circuit (chipset, SoC).

The phrases “based on” and “depending on/in response to” used in this disclosure do not mean “based solely on” or “in response to” unless otherwise specified. “Based on” means both “based solely on” and “at least partially on.” Similarly, “depending” means both “at least partially on” and “at least partially on.” The terms “include,” “comprise,” and variations thereof do not mean that only the listed items are included; they mean that only the listed items may be included, or that additional items may be included in addition to the listed items. Furthermore, the term “or” used in this disclosure is not intended to mean exclusive OR. Additionally, any reference to elements using designations such as “first,” “second,” etc., used in this disclosure does not generally limit the quantity or order of those elements. These designations may be used in this disclosure as a convenient way to distinguish between two or more elements. Therefore, references to the first and second elements do not imply that only two elements may be adopted there, or that the first element must precede the second element in any way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall be plural unless it is clearly indicated otherwise from the context.

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

(9) Addendum The following addendum is provided regarding the features of the embodiment described above.

(Note 1)
Multiple mobile units,
The system comprises a base station that communicates optically with each of the aforementioned multiple mobile devices,
The base station
Upon detecting the first mobile device from among the plurality of mobile devices whose optical communication has been interrupted, a second mobile device, which is different from the first mobile device, is instructed to transmit a search optical signal for searching for the first mobile device.
The second mobile device is,
An optical communication system that transmits the search optical signal in response to instructions from the base station.

(Note 2)
The second mobile device is,
The optical communication system described in Appendix 1, wherein the search optical signal is transmitted at a timing different from the timing at which optical communication is performed between the base station and the second mobile device.

(Note 3)
The base station
The optical communication system according to Appendix 2, wherein scheduling control is performed on the second mobile device so that the timing of the optical communication between the base station and the second mobile device and the timing of the second mobile device transmitting the search optical signal are different.

(Note 4)
The second mobile device is,
An optical communication system according to any one of the appendices 1 to 3, which transmits the search light signal in a direction different from the direction in which the base station is located.

(Note 5)
The base station
An optical communication system according to any one of the appendices 1 to 4, wherein, based on the direction in which the first mobile device was located before the optical communication with the first mobile device was interrupted, the system instructs only the second mobile devices, among a plurality of second mobile devices that are continuing optical communication with the base station, to transmit the search optical signal to those second mobile devices located within a predetermined directional range from the aforementioned direction.

(Note 6)
The first mobile device is,
In response to receiving the search light signal from the second mobile device, the direction of arrival of the search light signal is determined.
An optical communication system according to any one of the appendices 1 to 5 that performs motion control to move toward the aforementioned direction of arrival.

(Note 7)
The first mobile device is,
While moving toward the aforementioned direction of arrival, attempt to resume optical communication with the base station.
The optical communication system described in Appendix 6, which stops the movement control in response to the resumption of optical communication with the base station.

(Note 8)
The first mobile device is,
When moving toward the aforementioned direction of arrival, the reception intensity of the search light signal received from the second mobile device is monitored.
The optical communication system according to Appendix 6 or 7, which stops the movement control when the received signal strength exceeds a threshold.

(Note 9)
The base station
An optical communication system according to any one of the appendices 1 to 8, which instructs the second mobile device to stop intermittently transmitting the search optical signal or to stop continuously transmitting the search optical signal in response to the resumption of optical communication with the first mobile device.

(Note 10)
The base station
If communication with the first mobile unit is not resumed within a predetermined time from the start of the search by the first mobile unit, the second mobile unit is instructed to switch to a search mode that expands the direction in which the search light signal is transmitted.
The second mobile device is,
An optical communication system according to any one of the appendices 1 to 9, wherein, in the search mode, the direction in which the search optical signal is transmitted is expanded compared to when the search mode is not in use.

(Note 11)
The second mobile device is,
The optical communication system described in Appendix 10, wherein, in the search mode, the second mobile device transmits the search optical signal in all directions from which it can transmit.

(Note 12)
The second mobile device is,
The optical communication system according to Appendix 10 or 11, wherein the frequency of transmitting the search optical signal is increased when in the search mode compared to when not in the search mode.

(Note 13)
The second mobile device is,
The optical communication system described in Appendix 12, which interrupts optical communication with the base station and continuously transmits the search optical signal during the search mode.

(Note 14)
The base station
An optical communication system according to any one of the appendices 10 to 13, which transmits alarm information to the network side in response to the failure to resume communication with the first mobile device within a predetermined time after transitioning to the search mode.

(Note 15)
Each of the aforementioned multiple mobile devices is
It has multiple pairs of light-emitting and light-receiving elements,
The aforementioned pairs are
An optical communication system as described in any one of the appendices 1 to 14, each having directionality in a different direction.

(Note 16)
The base station
It has multiple pairs of light-emitting and light-receiving elements,
The aforementioned pairs are
An optical communication system as described in any one of the appendices 1 to 15, each having directionality in a different direction.

(Note 17)
The optical communication system described in any one of the appendices 1 to 16, wherein the plurality of mobile devices and the base station perform optical communication underwater.

(Note 18)
A base station used in optical communication systems,
An optical communication unit that communicates optically with each of the multiple mobile devices,
A base station comprising: a control unit that controls the optical communication unit to instruct a second mobile device, which is different from the first mobile device among the plurality of mobile devices, to transmit a search optical signal for searching for the first mobile device, in response to the detection of a first mobile device among the plurality of mobile devices whose optical communication has been interrupted.

(Note 19)
A mobile device that performs optical communication with a base station in an optical communication system,
An optical communication unit that receives a transmission instruction from the base station to transmit a search optical signal for searching for another mobile device whose optical communication with the base station has been lost,
A mobile device comprising: a control unit that controls the optical communication unit to transmit the search optical signal in response to receiving the transmission instruction from the base station.

(Note 20)
A mobile device that performs optical communication with a base station in an optical communication system,
After optical communication with the aforementioned base station is interrupted, the optical communication unit receives a search optical signal from another mobile device to locate the mobile device,
A mobile device comprising: a control unit that, upon receiving the search light signal from the second mobile device, identifies the direction of arrival of the search light signal and performs movement control to move toward the direction of arrival.

1: Optical communication system 100: Mobile device 100A: Lost mobile device (first mobile device)
100B: Mobile device in communication (second mobile device)
101: Optical communication unit 110: Light-emitting unit 111: Light-emitting element 112: Transmitter 120: Light-receiving unit 121: Light-receiving element 122: Receiver 130: Control unit 131: Processor 132: Memory 140: Moving mechanism 150: Light-receiving/light-emitting unit 151: Light-receiving/light-emitting area 160: Main unit 200: Base station 201: Optical communication unit 210: Light-emitting unit 211: Light-emitting element 212: Transmitter 220: Light-receiving unit 221: Light-receiving element 222: Receiver 230: Control unit 231: Processor 232: Memory 240: Backhaul communication unit 241: Network communication unit 242: Inter-base station communication unit 250: Light-receiving/light-emitting unit 251: Light-receiving/light-emitting area 260: Main unit

Claims (20)

Multiple mobile units,
The system comprises a base station that communicates optically with each of the aforementioned multiple mobile devices,
The base station
Upon detecting the first mobile device from among the plurality of mobile devices whose optical communication has been interrupted, a second mobile device, which is different from the first mobile device, is instructed to transmit a search optical signal for searching for the first mobile device.
The second mobile device is,
An optical communication system that transmits the search optical signal in response to instructions from the base station. The second mobile device is,
The optical communication system according to claim 1, wherein the search optical signal is transmitted at a timing different from the timing at which the optical communication between the base station and the second mobile device is performed. The base station
The optical communication system according to claim 2, wherein scheduling control is performed on the second mobile device so that the timing of the optical communication between the base station and the second mobile device and the timing of the second mobile device transmitting the search optical signal are different. The second mobile device is,
The optical communication system according to claim 1, wherein the search light signal is transmitted in a direction different from the direction in which the base station is located. The base station
The optical communication system according to claim 1, wherein, based on the direction in which the first mobile device was located before the optical communication with the first mobile device was interrupted, the system instructs only the second mobile devices, among a plurality of second mobile devices that are continuing the optical communication with the base station, that are located within a predetermined directional range from the aforementioned direction, to transmit the search optical signal. The first mobile device is,
In response to receiving the search light signal from the second mobile device, the direction of arrival of the search light signal is determined.
The optical communication system according to claim 1, which performs motion control to move toward the direction of arrival. The first mobile device is,
While moving toward the aforementioned direction of arrival, attempt to resume optical communication with the base station.
The optical communication system according to claim 6, wherein the motion control is stopped in response to the resumption of optical communication with the base station. The first mobile device is,
When moving toward the aforementioned direction of arrival, the reception intensity of the search light signal received from the second mobile device is monitored.
The optical communication system according to claim 6, wherein the motion control is stopped when the received signal strength exceeds a threshold. The base station
The optical communication system according to claim 1, wherein, in response to the resumption of optical communication with the first mobile device, the second mobile device is instructed to stop intermittently transmitting the search optical signal or to stop continuously transmitting the search optical signal. The base station
If communication with the first mobile unit is not resumed within a predetermined time from the start of the search by the first mobile unit, the second mobile unit is instructed to switch to a search mode that expands the direction in which the search light signal is transmitted.
The second mobile device is,
The optical communication system according to claim 1, wherein, in the search mode, the direction in which the search light signal is transmitted is expanded compared to when not in the search mode. The second mobile device is,
The optical communication system according to claim 10, wherein, in the search mode, the second mobile device transmits the search optical signal in all directions from which it can transmit. The second mobile device is,
The optical communication system according to claim 10, wherein the frequency of transmitting the search optical signal is increased when in the search mode compared to when not in the search mode. The second mobile device is,
The optical communication system according to claim 12, wherein, in the search mode, optical communication with the base station is interrupted and the search optical signal is continuously transmitted. The base station
The optical communication system according to claim 10, wherein if communication with the first mobile device is not resumed within a predetermined time after transitioning to the search mode, an alarm information is transmitted to the network side. Each of the aforementioned multiple mobile devices is
It has multiple pairs of light-emitting and light-receiving elements,
The aforementioned pairs are
The optical communication system according to any one of claims 1 to 14, wherein each has directionality in a different direction. The base station
It has multiple pairs of light-emitting and light-receiving elements,
The aforementioned pairs are
The optical communication system according to any one of claims 1 to 14, wherein each has directionality in a different direction. The optical communication system according to any one of claims 1 to 14, wherein the plurality of mobile devices and the base station perform optical communication underwater. A base station used in optical communication systems,
An optical communication unit that communicates optically with each of the multiple mobile devices,
A base station comprising: a control unit that controls the optical communication unit to instruct a second mobile device, which is different from the first mobile device among the plurality of mobile devices, to transmit a search optical signal for searching for the first mobile device, in response to the detection of a first mobile device among the plurality of mobile devices whose optical communication has been interrupted. A mobile device that performs optical communication with a base station in an optical communication system,
An optical communication unit that receives a transmission instruction from the base station to transmit a search optical signal for searching for another mobile device whose optical communication with the base station has been lost,
A mobile device comprising: a control unit that controls the optical communication unit to transmit the search optical signal in response to receiving the transmission instruction from the base station. A mobile device that performs optical communication with a base station in an optical communication system,
After optical communication with the aforementioned base station is interrupted, the optical communication unit receives a search optical signal from another mobile device to locate the mobile device,
A mobile device comprising: a control unit that, in response to receiving the search light signal from another mobile device, identifies the direction from which the search light signal is coming and performs movement control to move in the direction from which the signal is coming.