Wireless synchronization device, wireless synchronization system, and wireless synchronization method

Abstract

It can synchronize with high precision in dense environments. [Solution] The wireless synchronization device is a wireless synchronization device that is connected to a communication target device, which is a wireless synchronization device to be communicated with, via a wireless transmission path, and transmits a first radio wave to the communication target device and receives a second radio wave from the communication target device, calculates a time error between the device and the communication target device based on first time information relating to the transmission time of the first radio wave of the device itself, second time information relating to the reception time of the first radio wave by the communication target device, third time information relating to the transmission time of the second radio wave by the communication target device, and fourth time information relating to the reception time of the second radio wave of the device itself, and performs time synchronization based on the time error.

Description

Technical Field

[0001] The present invention relates to a wireless synchronization device, a wireless synchronization system, and a wireless synchronization method.

Background Art

[0002] Conventionally, techniques for establishing time synchronization between a plurality of devices have been known. For example, there are basic wired network synchronization techniques such as NTP (Network Time Protocol) and PTP (Precision Time Protocol). Also, according to wireless communication such as Wi-Fi (Wireless Fidelity), although the accuracy deteriorates, the synchronization technique using NTP can be used for synchronization of a wireless network.

[0003] Patent Document 1 discloses a technique in which, in a plurality of wireless devices forming a mesh network, some wireless devices distribute the UTC (Universal Time Coordinated) time obtained from GPS (Global Positioning System) to other wireless devices to achieve synchronization between the plurality of wireless devices.

[0004] Also, Patent Document 2 discloses a technique in which a plurality of wireless devices performing TDD (Time Division Duplex) communication form a mesh network and perform time synchronization between wireless devices facing each other.

[0005] The techniques described in Patent Document 1 and Patent Document 2 are based on a method (Cristian’s algorithm) also used in NTP, PTP, etc., calculate the time difference and propagation delay time between a plurality of wireless devices, and perform time synchronization.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

[0007] However, the technologies described in Patent Documents 1 and 2 have problems in dense environments, such as having to reduce the frequency of communication required for synchronization due to bandwidth limitations of wireless communication, and a decrease in time accuracy due to inappropriate communication channel configuration.

[0008] Therefore, the present invention has been made in view of the above points, and aims to present a technology that enables high-precision synchronization in a dense environment. [Means for solving the problem]

[0009] One aspect of the present invention is a wireless synchronization device that is connected to a communication target device, which is a wireless synchronization device to be communicated with, via a wireless transmission path, and which transmits and receives radio waves bidirectionally with the communication target device, calculates a time error between itself and the communication target device based on a plurality of time information indicating the time during which the radio waves were transmitted and received, and performs time synchronization based on the time error.

[0010] Furthermore, in one aspect of the present invention, the radio wave transmission and reception are transmitted and received by time measurement communication, at least one of the plurality of time information is transmitted and received by time information communication, and the time information communication and the time measurement communication use different types of wireless communication channels.

[0011] Furthermore, one aspect of the present invention is a wireless synchronization device that is connected to a communication target device, which is a wireless synchronization device to be communicated with, via a wireless transmission path, and transmits a first radio wave to the communication target device, receives a second radio wave from the communication target device, calculates a time error between itself and the communication target device based on first time information relating to the transmission time of the first radio wave of itself, second time information relating to the reception time of the first radio wave by the communication target device, third time information relating to the transmission time of the second radio wave by the communication target device, and fourth time information relating to the reception time of the second radio wave of itself, and performs time synchronization based on the time error.

[0012] Furthermore, in one aspect of the present invention, the first radio wave and the second radio wave are transmitted and received by time measurement communication, and at least one of the first time information, second time information, third time information, and fourth time information is transmitted and received by time information communication, and the time information communication and the time measurement communication use different types of wireless communication channels.

[0013] Furthermore, in one aspect of the present invention, the amount of data per unit time in the time information communication is greater than the amount of data per unit time in the time measurement communication.

[0014] Furthermore, in one aspect of the present invention, the data rate of the time information communication is higher than the data rate of the time measurement communication.

[0015] Furthermore, in one aspect of the present invention, the bandwidth used for the time information communication is wider than the bandwidth used for the time measurement communication.

[0016] Furthermore, in one embodiment of the present invention, the frequency used for the time information communication is higher than the frequency used for the time measurement communication.

[0017] Also, in one aspect of the present invention, at least one of the first radio wave and the second radio wave is transmitted with a polarization wave in a predetermined direction and received with a polarization wave in substantially the same direction as the transmitted direction.

[0018] Also, in one aspect of the present invention, for the transmission of at least one of the first radio wave and the second radio wave, an antenna having different radiation characteristics between vertical polarization and horizontal polarization, or an antenna having different radiation characteristics between right-handed circular polarization and left-handed circular polarization is used.

[0019] Also, in one aspect of the present invention, the antenna is a slot array antenna.

[0020] Also, in one aspect of the present invention, the wireless synchronization device changes the expression method of time according to at least one of the first time information, the second time information, the third time information, and the fourth time information in accordance with the time error between the own device and the communication target device.

[0021] Also, in one aspect of the present invention, the expression method includes the resolution of the time indicated by the time information.

[0022] Also, in one aspect of the present invention, the expression method includes the data amount of the time information.

[0023] Also, in one aspect of the present invention, when the time error is less than a predetermined value, the wireless synchronization device reduces the data amount of at least one of the first time information, the second time information, the third time information, and the fourth time information.

[0024] Also, in one aspect of the present invention, when the time error is less than a predetermined value, the number of bits representing the first time information, the second time information, the third time information, and the fourth time information is made uniform and reduced.

[0025] Also, in one aspect of the present invention, the clock of the own device is time-synchronized with the clock of the communication target device, and the wireless synchronization device obtains the third time information based on the clock of the own device by sharing in advance the timing at which the second radio wave is transmitted between the own device and the communication target device.

[0026] Also, one aspect of the present invention is a wireless synchronization system in which a plurality of wireless synchronization devices including a first wireless synchronization device and a second wireless synchronization device are connected to each other via a wireless transmission path. The first wireless synchronization device transmits a first radio wave to the second wireless synchronization device, and the second wireless synchronization device transmits a second radio wave to the first wireless synchronization device. The first wireless synchronization device calculates a time error between the first wireless synchronization device and the second wireless synchronization device based on first time information regarding the transmission time of the first radio wave, second time information regarding the reception time of the first radio wave by the second wireless synchronization device, third time information regarding the transmission time of the second radio wave by the second wireless synchronization device, and fourth time information regarding the reception time of the second radio wave, and performs time synchronization based on the time error.

[0027] Also, in one aspect of the present invention, when a plurality of wireless synchronization devices including a first wireless synchronization device and a second wireless synchronization device are connected to each other via a wireless transmission path, the first wireless synchronization device transmits a first radio wave to the second wireless synchronization device, the second wireless synchronization device transmits a second radio wave to the first wireless synchronization device, the first wireless synchronization device calculates a time error between the first wireless synchronization device and the second wireless synchronization device based on first time information regarding the transmission time of the first radio wave, second time information regarding the reception time of the first radio wave by the second wireless synchronization device, third time information regarding the transmission time of the second radio wave by the second wireless synchronization device, and fourth time information regarding the reception time of the second radio wave, and the first wireless synchronization device performs time synchronization based on the time error.

Advantages of the Invention

[0028] According to the present invention, high-precision synchronization is possible in dense environments. [Brief explanation of the drawing]

[0029] [Figure 1] This is a block diagram showing an example configuration of a wireless synchronization system 1 comprising a plurality of wireless synchronization devices 10 according to the embodiment. [Figure 2] This is a sequence diagram showing an example of the flow of information communication between multiple wireless synchronization devices 10 in the first wireless synchronization method according to the embodiment. [Figure 3] This flowchart shows an example of the processing flow performed by each wireless synchronization device 10 in the first wireless synchronization method according to the embodiment. [Figure 4] This diagram illustrates the effects of channel separation. [Figure 5] This is a sequence diagram showing an example of the flow of information communication between multiple wireless synchronization devices 10 in a second wireless synchronization method according to the embodiment. [Figure 6] This flowchart shows an example of the processing flow performed by each wireless synchronization device 10 in the second wireless synchronization method according to the embodiment. [Figure 7] This is a sequence diagram showing an example of the flow of information communication between multiple wireless synchronization devices 10 in a third wireless synchronization method according to the embodiment. [Figure 8] This flowchart shows an example of the processing flow performed by each wireless synchronization device 10 in the third wireless synchronization method according to the embodiment. [Figure 9] This figure shows a schematic example of the hardware configuration of the information processing device 90 applied to this embodiment. [Figure 10] This is a diagram illustrating conventional time synchronization methods. [Modes for carrying out the invention]

[0030] [Traditional time synchronization method] Figure 10 is a diagram illustrating conventional time synchronization methods. Figure 10(A) shows an overview of NTP, and Figure 10(B) shows an overview of PTP. Conventionally, there have been methods of synchronizing clocks by building a wireless network, performing bidirectional communication, and measuring and sharing the time when the communication occurs. More obviously, multiple wireless stations that are not connected by wires can synchronize by exchanging protocols such as NTP over wireless communication such as Wi-Fi. Furthermore, in all of these methods, the basic mechanism of synchronization is the same as that of NTP and PTP performed over wired connections, although there are slight differences.

[0031] These synchronization methods are based on "Cristian's algorithm," which uses bidirectional communication and measures round-trip time (RTT) to cancel out delays caused by the communication path and determine the pure time difference between devices. Although NTP and PTP (IEEE 1588) differ in message format and process, the essential part of calculating the time difference is the same as shown in the diagram. Furthermore, the methods for performing time synchronization via wireless communication are also based on the same principles, with only minor differences in the messages and synchronization path determination in the protocols.

[0032] While NTP and PTP, two representative wired synchronization methods, share the same fundamental principle for calculating time differences, as described above, there are differences in how messages are exchanged. In NTP, the data included in the transmitted packet that is relevant to calculating the time difference are the Origin Timestamp (the transmission timestamp of the last packet received from the other party), the Receive Timestamp (the reception timestamp of the last packet received from the other party), and the Transmit Timestamp (the transmission timestamp of the packet being sent). Since the time at which this packet is received can be measured by the receiving device, the time difference can be calculated from a total of four time information points (timestamps): the three above plus the time of the last reception.

[0033] On the other hand, PTP incorporates a hardware timestamp mechanism and a mechanism to cancel out delays caused by routing processes, etc., in order to provide more accurate time synchronization. As a result, the message content is slightly modified, with the master (device A) sending t90 (the timestamp when the Sync packet was sent) in the outbound Sync packet to the slave (device B), completing time sampling in the return Delay Request, and then sending t93 (the timestamp when the Delay Request was received) again in the Delay Response. The slave obtains the two timestamps that can be measured by the master through communication and can calculate the time difference from the remaining two timestamps that the slave can measure itself. Note that since many devices have a configuration where the exact time a packet was sent can only be known after it has been sent, PTP allows the sending of Follow-up messages in addition to Sync, and t90 is often sent later in the Follow-up message.

[0034] Furthermore, wireless synchronization technologies such as those listed in Patent Documents 1 and 2 also fundamentally require the aggregation of some of the four timestamps necessary for calculating the time difference to one side via wireless communication. In addition, time synchronization can be performed from the master (e.g., PTP) or from the slave (e.g., NTP), but there is no essential difference between the two.

[0035] Many wireless time synchronization technologies are essentially the same as NTP and PTP, which use wired communication networks, but applied to wireless environments. In wireless environments, the aforementioned mechanisms can often be used as is, and efforts are made to extend them to mesh networks to take advantage of the many-to-many topology that is easily constructed in wireless environments. However, in environments where many such wireless synchronization devices are widespread and exist at high density (dense environments), new challenges specific to wireless transmission lines arise. Specifically, the mutual communication for time synchronization described above is highly likely to collide in narrow-bandwidth wireless communication lines (wireless transmission lines), resulting in frequent packet loss or the need to reduce the frequency of communication to avoid it. In the former case, it is unintentional; in the latter case, it is intentional. As a result, the opportunities for time synchronization (time correction) are reduced, which may lead to a decrease in synchronization accuracy.

[0036] Furthermore, another challenge unique to wireless communication is that the "time measurement (measuring the arrival time of packets, etc.)" and "transmission of time information (sharing of time information at the time of transmission or reception)" performed by such synchronization methods have different characteristics that are inherently required of the transmission path.

[0037] For time measurement, it is important that the packet you send reliably reaches the recipient and that its arrival time can be precisely measured; no special information transmission is necessary. Considering the characteristics of wireless transmission lines, it is necessary to deliver the packet itself, not the information, reliably to the recipient with sufficient receiving power (low path loss), and to devise ways to minimize the combination of multipath waves and direct waves, which degrades the accuracy of time measurement. If another multipath wave (reflected wave) with a time difference is combined with the direct wave used for time measurement, the edges of the symbols become ambiguous, and the accuracy of symbol timing measurement deteriorates. This reduces the accuracy of measuring the timing when the packet arrived.

[0038] On the other hand, for transmitting time information, it is sufficient that the information is transmitted accurately (i.e., that the data can be demodulated and restored). In fact, transmission lines configured to actively utilize multipath, or to incorporate retransmission mechanisms, redundancy, and error correction codes are more suitable. Conversely, in time measurement, multipath is counterproductive as it results in the loss of measurement opportunities, making retransmission impossible, and redundancy and error correction codes meaningless.

[0039] [Comparison with conventional time synchronization methods] Thus, in a wireless environment, bidirectional communication and time synchronization through time measurement should not be performed on a single transmission path. By utilizing different transmission paths according to each function, it may be possible to improve time accuracy, stability, and robustness. However, such problems do not arise in wired communication paths. In principle, multipath does not exist in wired connections, the bandwidth is wide enough that the impact of packet communication for time synchronization is negligible, and the effects of attenuation and packet loss due to polarization and distance are also negligible. Therefore, in this embodiment, we propose a method that enables stable and highly accurate synchronization even in dense environments. Specifically, by reducing the amount of message data required for time synchronization, the bandwidth of wireless communication can be used efficiently, and by measuring the time when wireless communication occurred and sharing that time information on a separate communication path, stable and highly accurate time synchronization becomes possible.

[0040] [Embodiment] Next, preferred embodiments of the wireless synchronization device, wireless synchronization system, and wireless synchronization method according to this embodiment will be described in detail below with reference to the attached drawings. In the drawings, identical or similar parts are denoted by the same or similar reference numerals. Note that this embodiment is not limited to these embodiments and includes various modifications or improvements. In other words, the components described below include those that can be easily imagined by a person skilled in the art, and those that are substantially the same, and the components described below can be combined as appropriate. Furthermore, this embodiment may include various omissions, substitutions, or modifications of components without departing from the spirit of the present invention.

[0041] In all the figures used to illustrate the embodiments, components with the same function are given the same reference numerals, and repeated explanations are omitted. Furthermore, in this application, "based on XX" means "based on at least XX," and includes cases where it is based on other elements in addition to XX. Also, "based on XX" is not limited to cases where XX is used directly, but also includes cases where it is based on XX after calculations or processing have been performed on it. "XX" is any element (for example, any information). Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0042] Figure 1 is a block diagram showing an example configuration of a wireless synchronization system 1 comprising a plurality of wireless synchronization devices 10 according to this embodiment. The wireless synchronization system 1 comprises a plurality of wireless synchronization devices 10. The first wireless synchronization device 11 and the second wireless synchronization device 12 are illustrative examples of the plurality of wireless synchronization devices 10. When the first wireless synchronization device 11 and the second wireless synchronization device 12 are not distinguished, they may simply be referred to as wireless synchronization device 10.

[0043] [Example of a wireless synchronization device configuration] The wireless synchronization device 10 comprises a wireless communication unit 21, a storage unit 22, a synchronization unit 23, and a clock 24 as its functional units. The wireless synchronization device 10 is configured using information processing equipment such as a smartphone, tablet, personal computer, radio station, or dedicated equipment.

[0044] All or part of the functions of the wireless synchronization device 10 may be implemented using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above program may be recorded on a computer-readable recording medium. Computer-readable recording media include, for example, portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, and semiconductor memory devices (e.g., SSDs: Solid State Drives), as well as storage devices such as hard disks and semiconductor memory devices built into computer systems. The above program may be transmitted via a telecommunications line.

[0045] The wireless synchronization device 10 is a device that operates in synchronization with other wireless synchronization devices 10. Synchronization includes synchronizing specific operations or processes, time synchronization, spatial synchronization, and spatiotemporal synchronization. Time synchronization means that the times of multiple wireless synchronization devices 10 are synchronized. Spatial synchronization means that multiple wireless synchronization devices 10 are aware of each other's positions (relative positions). Spatiotemporal synchronization is the combination of time synchronization and spatial synchronization. In the following description, the case where each wireless synchronization device 10 is independent is described as an example, but this embodiment is not limited to this example, and multiple wireless synchronization devices 10 may be provided in a single device. The wireless synchronization device 10 may have a functional unit (not shown) that performs specific operations or processes, or it may operate an external device by outputting a control signal.

[0046] Wireless communication standards include, for example, LPWA (Low Power Wide Area), Bluetooth®, BLE (Bluetooth Low Energy), and Wi-Fi. Radio frequencies are typically ISM (Industrial, Scientific, and Medical radio band) bands such as the 920 MHz band and the 2.4 GHz band. This allows for synchronization over long distances (km) to short distances (m).

[0047] The wireless communication unit 21 transmits and receives (hereinafter sometimes simply referred to as transmitting and receiving) communication information with the wireless communication unit 21 of another wireless synchronization device 10. The wireless communication unit 21 is, for example, a wireless device that transmits and receives communication information using radio waves. The wireless communication unit 21 may have one wireless device or multiple wireless devices. The wireless device may include an antenna as a component. Communication information is, for example, a code or a packet. The communication information includes synchronization information used to synchronize the clock 24. The wireless communication unit 21 receives communication information transmitted from the wireless communication unit 21 of another wireless synchronization device 10 and obtains synchronization information by decoding the received communication information.

[0048] The wireless communication unit 21 performs time measurement communication required for measuring the timing of radio wave transmission and reception using a time measurement communication channel, and performs time information communication required for transmitting and receiving synchronization information using a time information communication channel. The synchronization information includes, for example, one or more pieces of time information T indicating the timing of radio wave transmission and reception. The time measurement communication channel and the time information communication channel may be the same wireless communication channel or may be wireless communication channels of different types.

[0049] The storage unit 22 is composed of, for example, a hard disk drive or semiconductor memory (flash memory, RAM, ROM), and stores various types of information, such as programs and data read by the functional unit of the wireless synchronization device 10. In addition, synchronization information (time information T) is recorded in the storage unit 22 as needed.

[0050] The synchronization unit 23 adjusts the time of the clock 24 with other wireless synchronization devices 10 based on the synchronization information. The synchronization unit 23 calculates information to be used to adjust the clock 24 based on the synchronization information. The information used to adjust the clock 24 includes, for example, the time difference δ and the propagation delay time RTT / 2. Based on the calculated information, the synchronization unit 23 may adjust (correct) its own clock 24 to cancel out the time difference δ, or it may correct some timing control using that time difference δ. Correcting the timing control means delaying or advancing some operation timing, taking into account the time difference δ of the clock 24. As a result, the wireless synchronization device 10 can synchronize its time with other wireless synchronization devices 10 that are connected to it via a wireless transmission path.

[0051] The clock 24 may be a real-time clock for handling time, a counter for timing, an oscillator, an oscillator circuit, or an oscillator itself. If the clock 24 is a time or a counter, the synchronization unit 23 will operate to match its value. If the clock 24 is an oscillator or the like, the synchronization unit 23 will operate to match the rising or falling timing of the clock. The clock 24 may also include an oscillator and a real-time clock.

[0052] [First wireless synchronization method] Figure 2 is a sequence diagram showing an example of the flow of information communication between a plurality of wireless synchronization devices 10 in the first wireless synchronization system according to the embodiment. Figure 3 is a flowchart showing an example of the processing flow performed by each wireless synchronization device 10 in the first wireless synchronization system according to the embodiment. Figure 3(A) shows the processing flow of the first wireless synchronization device 11, and Figure 3(B) shows the processing flow of the second wireless synchronization device 12. In the following description, the functional parts of the first wireless synchronization device 11 are denoted with "A" at the end of their names, and the functional parts of the second wireless synchronization device 12 are denoted with "B" at the end of their names, thereby distinguishing between the functional parts of the first wireless synchronization device 11 and the second wireless synchronization device 12. For example, the wireless communication unit 21 of the first wireless synchronization device 11 is denoted as the wireless communication unit 21A. When these are not distinguished, "A" or "B" is not denoted at the end of their names.

[0053] (Steps S100, S200) The first wireless synchronous device 11 and the second wireless synchronous device 12 start their synchronous operation.

[0054] (Step S111) The wireless communication unit 21A transmits a first radio wave to the second wireless synchronization device 12 via the time measurement communication channel. The wireless communication unit 21A also measures the transmission time of the first radio wave and records the first time information T1 indicating the measured time in the storage unit 22A or the like. In the following description, the transmission of the first radio wave may be referred to as the first radio wave transmission, and the second radio wave, which will be described later, may also be referred to in the same way.

[0055] Time information T can be any measure of an indicator that can represent the passage of time, and measuring time can be any method such as using a counter, phase measurement, time interval counter, or DMTD. The time referred to here may be UTC time, TAI (temps atomique international; International Atomic Time), or any numerical value that can represent the passage of time. For example, it may be the elapsed time since the wireless synchronization device 10 was started up, or the CPU clock counter value. Furthermore, since time information T can be any indicator that indirectly represents time, as long as the time difference can be obtained using the same principle, it may be information that indicates the phase of a clock or the phase of a radio wave. In the following explanation, these will simply be referred to as time.

[0056] The time information T measured by the wireless communication unit 21 may be, for example, the time of the packet. The time of the packet does not have to be the time when the beginning of the packet was transmitted or received, but may be the timing of symbols, chips, etc., included in the packet, such as the preamble or the Start of Frame Delimiter (SFD).

[0057] (Step S211) The wireless communication unit 21B receives a first radio wave from the first wireless synchronization device 11 via the time measurement communication channel. The wireless communication unit 21B measures the time of reception of the first radio wave and records second time information T2 indicating the measured time in the storage unit 22B.

[0058] (Steps S112, S212) The wireless communication unit 21A transmits the first time information T1 measured in step S111 to the second wireless synchronization device 12 via the time information communication channel. The wireless communication unit 21B receives the first time information T1 from the first wireless synchronization device 11 via the time information communication channel and records it in the storage unit 22B. The wireless communication unit 21B also transmits the second time information T2 measured in step S211 to the first wireless synchronization device 11 via the time information communication channel. The wireless communication unit 21A receives the second time information T2 from the second wireless synchronization device 12 via the time information communication channel and records it in the storage unit 22A.

[0059] (Step S213) The wireless communication unit 21B transmits a second radio wave to the first wireless synchronization device 11 via the time measurement communication channel. The wireless communication unit 21B also measures the transmission time of the second radio wave and records third time information T3 indicating the measured time in the storage unit 22B.

[0060] (Step S113) The wireless communication unit 21A receives the second radio wave from the second wireless synchronization device 12 via the time measurement communication channel. The wireless communication unit 21A also measures the time of reception of the second radio wave and records the fourth time information T4 indicating the measured time in the storage unit 22A.

[0061] (Steps S224, S114) The wireless communication unit 21B transmits the third time information T3 measured in step S213 to the first wireless synchronization device 11 via the time information communication channel. The wireless communication unit 21A receives the third time information T3 from the second wireless synchronization device 12 via the time information communication channel and records it in the storage unit 22A. The wireless communication unit 21A also transmits the fourth time information T4 measured in step S113 to the second wireless synchronization device 12 via the time information communication channel. The wireless communication unit 21B receives the fourth time information T4 from the first wireless synchronization device 11 via the time information communication channel and records it in the storage unit 22B.

[0062] (Step S115) The synchronization unit 23A acquires the first time information T1, the second time information T2, the third time information T3, and the fourth time information T4. That is, the synchronization unit 23A acquires information regarding the time the first radio wave transmitted from the unit was transmitted and the time it was received by the communication target device, and information regarding the time the second radio wave transmitted from the communication target device was transmitted from the communication destination and the time it was received by the unit. Based on this information, the synchronization unit 23A calculates at least the time difference δ (time difference, time error) and the propagation delay time RTT / 2 from equation (1) below. The synchronization unit 23A corrects the clock 24A as necessary.

[0063]

number

[0064] (Step S215) The clock 24B acquires the first time information T1, the second time information T2, the third time information T3, and the fourth time information T4. That is, the synchronization unit 23B acquires information regarding the time the second radio wave transmitted from the unit was transmitted and the time it was received by the communication target device, and information regarding the time the first radio wave transmitted from the communication target device was transmitted from the communication destination and the time it was received by the unit. Based on this information, the synchronization unit 23B calculates at least the time difference δ from the time difference δ and the propagation delay time RTT / 2 using equation (1) above. The synchronization unit 23B corrects the clock 24B as necessary.

[0065] Multiple wireless synchronization devices 10 synchronize with each other by having the first wireless synchronization device 11 repeatedly execute the process from step S111 to step S115, and the second wireless synchronization device 12 repeatedly execute the process from step S211 to step S215.

[0066] The processing flow described above is merely an example, and the order of processing may differ from that shown in the diagram. Furthermore, in the case of the first wireless synchronization device 11 and the second wireless synchronization device 12, either wireless synchronization device 10 may act as the time reference (master), and either wireless synchronization device 10 may correct its own clock 24 (slave).

[0067] In Figures 2 and 3, the time measurement communication channel for transmitting and receiving the first and second radio waves, and the time information communication channel for transmitting and receiving one or more time information T, may be wireless communication channels of different types. That is, the transmission performed to obtain a timestamp and the communication performed to share that timestamp are separated.

[0068] The role of time information communication is to transmit information, not the transmission of radio waves itself. Therefore, time information communication should convey data containing time information T to the communication target device (communication partner) by making maximum use of redundancy through multiple packet transmission (multiple transmissions of the same time information T), stable communication through reception of multipath waves, ensuring antenna, time, and frequency diversity, and data recovery through error correction, etc.

[0069] On the other hand, in timekeeping communication, the timing of when the transmitted packet was sent and received is crucial, so it is important that the packet reaches the communication partner while maintaining an accurate timekeeping format. Therefore, other information accompanying timekeeping communication is not essential. Consequently, in timekeeping communication, it is desirable to lower the symbol rate, chip rate, or data rate to increase the minimum receiving sensitivity, avoid combination with multipath waves that could lead to timekeeping errors, and ensure that the signal reaches the communication target device while maintaining the highest possible field strength.

[0070] Sending multiple packets, which are considered useful in time information communication, in time measurement communication is equivalent to losing the opportunity for synchronization because packet identity cannot be ensured. In time measurement communication, the use of multipath reduces synchronization accuracy, error correction is meaningless, and since there is no need to exchange particularly large amounts of data, a wide bandwidth is not required.

[0071] Thus, since the conditions for appropriate communication channels differ between time information communication channels and time measurement communication channels, it is desirable to use different communication channels, each suited to its respective purpose, rather than a single wireless communication channel. Different methods include, for example, differences in communication method (unicast, broadcast, or multicast), frequency, frequency channel, polarization, coding rate, or coding method. For example, if the communication methods are different, two communications occurring simultaneously can be explicitly separated; if the frequency bands or frequency channels are different, they can be easily separated; and the same applies if the polarizations are configured to be orthogonal to each other. More specifically, time information communication may be conducted in the 2.4 MHz unlicensed band, and time measurement communication may be conducted in the sub-GHz unlicensed band. In Japan, the sub-GHz band is the 920 MHz band.

[0072] Furthermore, "different methods" can be any method that allows for the separation of time information communication and time measurement communication. Note that the two different wireless communication channels do not necessarily have to include two time-division-divided wireless communication channels. Using time-division-divided wireless communication channels may result in extra packets for the physical layer, potentially leading to inefficient use of bandwidth.

[0073] Figure 4 is a diagram illustrating the effects of channel separation. Figure 4(A) shows a case where time measurement communication used for time measurement and time information communication used for sharing time information T are performed on a single path (wireless communication path). Figure 4(B) shows a case where time measurement communication and time information communication are performed on different paths. In Figure 4, the first wireless synchronous device communicates information with the second wireless synchronous device, the third wireless synchronous device, and the fourth wireless synchronous device.

[0074] As mentioned above, time information communication requires transmitting information with one or two timestamps, so a large amount of data per unit of time is desirable. On the other hand, time measurement communication does not require the storage of information, so it is sufficient to observe the fact that radio waves were transmitted, and in dense environments, a small amount of data per unit of time is desirable. In other words, it is desirable that the amount of data per unit of time for time information communication be greater than the amount of data per unit of time for time measurement communication used for measurement. Note that the amount of data may be determined by a standard or it may be determined by the results of actual data communication. It is also desirable that time measurement communication include identification information (e.g., MAC (Media Access Control address) address) to identify which wireless synchronization device 10 transmitted the radio waves. Identification information may be, for example, a 6-byte short UUID (Universally Unique Identifier). Even in this case, the data size of time measurement communication is smaller than the data size of the two timestamps handled by time information communication.

[0075] From a similar perspective, the data rate, available bandwidth, and frequency should be configured to match the characteristics of time information communication, which transmits a large amount of data, and time measurement communication, which transmits a small amount of data.

[0076] In conventional methods, for example, when using the sub-GHz band for wireless synchronization to synchronize time over a distance of several hundred meters, the communication frequency needs to be reduced as the number of connected devices increases to prevent collisions due to synchronization errors, etc. Furthermore, considering that time information communication is also performed on the same channel, the radio wave transmission time becomes longer, increasing the probability of interference with other devices. In addition, some countries have radio wave laws that impose restrictions on transmission time, requiring a further reduction in communication frequency.

[0077] In the method according to this embodiment, time information communication containing a large amount of information (which substantially occupies most of the transmission time) may be performed, for example, in the 2.4 GHz band of a separate channel. As a result, the radio resources of the sub-GHz band, which have many bandwidth constraints, can be efficiently utilized, and time measurements can be performed at a higher frequency than conventional methods, thus improving synchronization accuracy, reducing the likelihood of collisions, and improving stability.

[0078] Time information communication can be transmitted over channels with higher data rates, thus suppressing interference and allowing for the sharing of measurement results from frequent time measurement communications at a sufficient speed. Although the propagation distance of radio waves decreases due to the higher frequency of time information communication, it is easy to extend the range to a similar distance of several hundred meters by utilizing redundancy and error correction codes. Naturally, packet loss will increase, but in time information communication, as long as data can be transmitted, packet loss does not affect the synchronization accuracy, so no problems arise.

[0079] Furthermore, in time measurement communication, while retaining the characteristics of the sub-GHz band, the diffraction phenomenon unique to low frequencies makes it possible to transmit packets while maintaining a high electric field strength even over long distances.

[0080] As described above, by using different communication channels for time information communication and time measurement communication, the synchronization frequency increases, allowing for repeated corrections while the deviation of each clock 24 is still small, thus reducing the average time difference. Furthermore, the characteristics of the crystal oscillator required to achieve the same accuracy can be reduced, enabling cost reduction and miniaturization. In addition, a high synchronization frequency allows for quicker feedback to the synchronized state during drift.

[0081] In time measurement communication, reflected waves (multipath) degrade the accuracy of time measurement, so it is desirable to use polarization in a specific direction for the transmission and reception of at least one of the first or second radio waves. For example, when the first radio synchronization device 11 transmits the first radio wave to the second radio synchronization device 12, the first radio wave may be transmitted with polarization in a predetermined direction and received with polarization in approximately the same direction as the transmitted wave. That is, the radio communication unit 21 receives with right-hand polarization if the polarization at the time of transmission is right-hand polarization, receives with left-hand polarization if the polarization at the time of transmission is left-hand polarization, receives with horizontal polarization if the polarization at the time of transmission is horizontal polarization, and receives with vertical polarization if the polarization at the time of transmission is vertical polarization. Polarization in approximately the same direction may be polarization in the same direction, or polarization with a slightly different angle (direction) from the polarization at the time of transmission, such that the angle difference is sufficient to reduce the effect of reflected waves.

[0082] Furthermore, in order to reduce the influence of reflected waves and extract only the direct wave, it is desirable that the antenna used by the wireless communication unit 21 for at least one of the first and second radio waves be an antenna that can achieve polarization isolation. Polarization isolation means that, in the case of linear polarization, the gains of horizontal polarization and vertical polarization are different, and in the case of circular polarization, the gains of right-hand and left-hand circular polarization are different. In other words, it is an antenna whose radiation characteristics differ depending on the type of polarization. For such an antenna, it is even more desirable to use a slot array antenna that can selectively transmit or receive only horizontally polarized radio waves. With this, it is possible to cut out some or all of the waves with different polarizations generated by reflection, and to observe only the main direct wave, and to prevent the synthesis of delayed reflected waves.

[0083] Furthermore, for time measurement communications where accuracy is crucial, an antenna with strong polarization characteristics may be used, while for time information communications where information can be transmitted even using multipath, an antenna with strong polarization characteristics may not be used. In this way, different devices may be used for transmitting and receiving. By functionally dividing the wireless communication unit 21 of the wireless synchronization device 10, time synchronization can be achieved more efficiently even in dense environments.

[0084] [Second wireless synchronization method] Figure 5 is a sequence diagram showing an example of the flow of information communication between a plurality of wireless synchronization devices 10 in the second wireless synchronization method according to the embodiment. Figure 6 is a flowchart showing an example of the processing flow performed by each wireless synchronization device 10 in the second wireless synchronization method according to the embodiment. Figure 6(A) shows the processing flow of the first wireless synchronization device 11, and Figure 6(B) shows the processing flow of the second wireless synchronization device 12. Matters already explained above may be omitted in the following explanation.

[0085] The first wireless synchronization method described above shows an example in which each wireless synchronization device 10 communicates time information and shares time information T each time it transmits radio waves. On the other hand, the second wireless synchronization method differs from the first wireless synchronization method in that it performs time information communication to send and receive any of the first time information T1 to the fourth time information T4 after the time measurement communication required for measuring the first time information T1 to the fourth time information T4 has been completed.

[0086] (Step S121) The wireless communication unit 21A transmits a first radio wave to the second wireless synchronization device 12 via the time measurement communication channel. The wireless communication unit 21A also measures the transmission time of the first radio wave and records the first time information T1 indicating the measured time in the storage unit 22A or the like.

[0087] (Step S221) The wireless communication unit 21B receives a first radio wave from the first wireless synchronization device 11 via the time measurement communication channel. The wireless communication unit 21B measures the time of reception of the first radio wave and records second time information T2 indicating the measured time in the storage unit 22B.

[0088] (Step S222) The wireless communication unit 21B transmits a second radio wave to the first wireless synchronization device 11 via the time measurement communication channel. The wireless communication unit 21B also measures the transmission time of the second radio wave and records third time information T3 indicating the measured time in the storage unit 22B.

[0089] (Step S122) The wireless communication unit 21A receives the second radio wave from the second wireless synchronization device 12 via the time measurement communication channel. The wireless communication unit 21A also measures the time of reception of the second radio wave and records the fourth time information T4 indicating the measured time in the storage unit 22A.

[0090] Through the above process, the measurement of the first time information T1 to the fourth time information T4 required for calculating the time difference δ is completed. Subsequently, the first wireless synchronization device 11 and the second wireless synchronization device 12 perform the processes of steps S112, S114, S212, and S214 together.

[0091] (Steps S223, S123) The wireless communication unit 21B transmits the second time information T2 and the third time information T3 to the first wireless synchronization device 11 via the time information communication channel. The wireless communication unit 21A receives the second time information T2 and the third time information T3 from the second wireless synchronization device 12 via the time information communication channel and records them in the storage unit 22A. The wireless communication unit 21A also transmits the first time information T1 and the fourth time information T4 to the second wireless synchronization device 12 via the time information communication channel. The wireless communication unit 21B receives the first time information T1 and the fourth time information T4 from the first wireless synchronization device 11 via the time information communication channel and records them in the storage unit 22B.

[0092] (Steps S125, S225) The synchronization units 23A and 23B calculate the time difference δ etc. based on the first time information T1 to the fourth time information T4 and perform synchronization.

[0093] [Third wireless synchronization method] Figure 7 is a sequence diagram showing an example of the flow of information communication between multiple wireless synchronization devices 10 in the third wireless synchronization method according to the embodiment. Figure 8 is a flowchart showing an example of the processing flow performed by each wireless synchronization device 10 in the third wireless synchronization method according to the embodiment. Figure 8(A) shows the processing flow of the first wireless synchronization device 11, and Figure 8(B) shows the processing flow of the second wireless synchronization device 12.

[0094] The third wireless synchronization method is similar to the second wireless synchronization method in that, after the time measurement communication required for measuring the first time information T1 to the fourth time information T4 is completed, it performs time information communication to send and receive any of the first time information T1 to the fourth time information T4. The third wireless synchronization method differs from the first and second wireless synchronization methods in that one wireless synchronization device 10 acts as a time synchronization source, and the other wireless synchronization device 10 adjusts its own clock 24, that is, they are in a master-slave relationship.

[0095] Steps S131, S132, S231, and S232, which perform time measurement communication, are the same processes as steps S121, S122, S221, and S222 in the second wireless synchronization method.

[0096] (Steps S133, S233) The wireless communication unit 21A transmits the first time information T1 and the fourth time information T4 to the second wireless synchronization device 12 via the time information communication channel. The wireless communication unit 21B receives the first time information T1 and the fourth time information T4 from the first wireless synchronization device 11 via the time information communication channel and records them in the storage unit 22B.

[0097] (Step S234) The synchronization unit 23B calculates the time difference δ etc. based on the first time information T1 to the fourth time information T4 and performs synchronization.

[0098] In the third wireless synchronization method, the first wireless synchronization device 11 is used as the reference time source, so the clock 24A of the first wireless synchronization device 11 does not need to be adjusted. Therefore, according to the third wireless synchronization method, the second time information T2 and the third time information T3 do not need to be shared with the first wireless synchronization device 11, and the amount of data per unit of time required for synchronization can be reduced.

[0099] [Fourth wireless synchronization method] As described above, conventional NTP and PTP perform time measurement communication and time information communication over a single path, while the first to third wireless synchronization methods perform time measurement communication and time information communication over paths of different methods. The fourth wireless synchronization method differs from the first to third wireless synchronization methods in that it changes the method of representing time using time information T according to the time difference δ. It should be noted that the fourth wireless synchronization method is applicable both when time measurement communication and time information communication are performed over a single path, and when time measurement communication and time information communication are performed over paths of different methods.

[0100] The appropriate time representation method differs depending on whether the time difference δ is large or small. By changing the time representation method using time information T according to the magnitude of the time difference δ, the time can be represented appropriately. The wireless synchronization device 10 may change the representation method to one defined for each predetermined value when the time difference δ between itself and the communication target device falls below one or more predetermined values ​​(thresholds) stored in the storage unit 22. Note that changing the representation method when the time difference δ becomes small may be managed and shared by, for example, the wireless synchronization device 10 operating as the master, or each wireless synchronization device 10 may share information that allows it to specify the representation method of changes such as the time difference δ and the representation method with other wireless synchronization devices 10.

[0101] The time representation method of time information T may include, for example, the resolution (precision) of the time indicated by time information T, the amount of data (number of bits) used when time information T represents time, or the time display format. Changing the representation method means changing only one of the resolution, amount of data, time display format, etc., or changing two or more of them. The time information T that changes the time representation method may be at least one of the first time information T1 to the fourth time information T4.

[0102] [Fourth wireless synchronization method: Resolution] When a significant time difference is expected between multiple wireless synchronization devices 10, such as during the initial stages of synchronization, it is desirable to make the time resolution represented by the time information T coarser (lower) and to make the time resolution represented by the time information T finer (higher) as the time difference δ decreases. This allows for rough time synchronization with low resolution in the initial stages of synchronization, and more precise time synchronization with high resolution towards the end of the synchronization period.

[0103] Furthermore, even if the number of bits is fixed, increasing the resolution reduces the dynamic range, but since a certain degree of synchronization is already achieved (the time difference is small) at the point when the resolution is increased, no problems occur during synchronization. By changing the resolution, it is possible to handle both the initial stages of synchronization, which require a high dynamic range even if the resolution is low, and the later stages of synchronization, which require a high resolution even if the dynamic range is low, while reducing the amount of data, thereby improving the synchronization frequency.

[0104] [Fourth wireless synchronization method: Data volume] For example, when representing time with a fixed resolution, it is desirable to reduce the amount of data by exchanging data in a 32-bit size during the initial stages of synchronization when the time difference δ is large, and then reducing it to 16 bits towards the end of synchronization when the time difference δ is small. Once synchronization is achieved, time information T can be communicated in a small size, so the data size per packet decreases, and the sampling frequency of the time difference δ can be increased. Furthermore, the change in data size leads to an increase in the sampling frequency of the time difference δ, which in turn leads to an improvement in synchronization accuracy. In addition, the smaller data size allows for an increase in the proportion of error correction codes and also increases redundancy.

[0105] Furthermore, when reducing the time difference δ, the wireless synchronization device 10 may reduce the number of bits required to represent all time information T, including the first time information T1, second time information T2, third time information T3, and fourth time information T4, by making them the same. That is, they can be reduced to the same number of bits. This allows for a simpler program configuration for processing each time information T.

[0106] [Fourth wireless synchronization method: Time display format] In the initial stages of synchronization, time can be represented in a highly flexible format based on the UTC epoch time (for example, the elapsed time from January 1, 1970, 00:00:00 AM). In the later stages of synchronization, the time display format may be changed to one that uses a common time reference for each wireless synchronization device 10 and can represent time with a smaller number of bits. For example, by sharing a reference time of 10 seconds prior to the present with other wireless synchronization devices 10, and using a time display format based on the elapsed time from the shared time, the time can be represented with a degree of freedom of at most 0 to a few tens of seconds, leading to a reduction in data volume and, consequently, an improvement in synchronization frequency.

[0107] [Fifth Wireless Synchronization Method] The fifth wireless synchronization method is similar to the fourth wireless synchronization method in that it can be applied when time measurement communication and time information communication are performed over a single path, and also when time measurement communication and time information communication are performed over paths using different methods. The fifth wireless synchronization method differs from the fourth wireless synchronization method in that it does not share some time information T when time synchronization is achieved.

[0108] The fifth wireless synchronization method may reduce the sharing of time information T by pre-setting and sharing the timing of radio wave transmission among multiple time-synchronized wireless synchronization devices 10. For example, if a time reference is shared between the first wireless synchronization device 11 and the second wireless synchronization device 12, and they both agree to transmit radio waves every 10 seconds from that reference, then the time 10 seconds later can be used as the time information T, and it is not necessary for the time information T to be shared. Specifically, if the first wireless synchronization device 11 has shared with the second wireless synchronization device 12 that it will transmit radio waves every 10 seconds from "2025 / 1 / 1 10:00:00", then the first time information T1 indicating the time when the first wireless synchronization device 11 transmits the first radio wave will be "2025 / 1 / 1 10:00:00 + 10N seconds (where N is an integer)", so it is not necessary for the first time information T1 to be shared from the first wireless synchronization device 11 to the second wireless synchronization device 12. In this case, the second wireless synchronization device 12 does not need to receive the first time information T1 from the first wireless synchronization device 11, as it acquires the first time information T1 based on the clock 24B. In this case, the first wireless synchronization device 11 does not transmit the first time information T1 to the second wireless synchronization device 12.

[0109] The same applies to the third time information T3; the first wireless synchronization device 11 acquires the second time information T2 from the clock 24A and does not need to receive it from the second wireless synchronization device 12. In this case, the second wireless synchronization device 12 does not transmit the third time information T3 to the first wireless synchronization device 11. The time information T that is reduced in the fifth wireless synchronization method may be either the first time information T1 or the third time information T3, or it may be the first time information T1 and the third time information T3.

[0110] This allows for an appropriate reduction in the data size of the time information T that needs to be shared, effectively increasing the frequency of time measurements in dense environments and improving synchronization accuracy. It also helps to reduce packet loss and other related issues.

[0111] A simple method for setting the transmission timing to realize the fifth wireless synchronization method is, for example, to set it as a constant multiple of the transmission frequency (e.g., 10 seconds). In this case, the wireless synchronization device 10 updates the timing of the next radio wave transmission to a future time by incrementing the aforementioned N.

[0112] Furthermore, the timing of radio wave transmission may be determined using the same pseudorandom number among multiple wireless synchronization devices 10, since the pseudorandom number seed is shared among the multiple wireless synchronization devices 10, or it may be determined using the said pseudorandom number and the aforementioned N. This makes it possible to avoid continuous collisions with other wireless synchronization devices 10.

[0113] Furthermore, the timing of radio wave transmission may be shared by setting a time reference and transmission interval in advance as described above, or it may be shared with other wireless synchronization devices 10 at the time of startup of the wireless synchronization device 10 or at predetermined intervals from startup.

[0114] Furthermore, in order to transmit radio waves accurately at a predetermined time, it is desirable that the wireless communication unit 21 that performs packet transmission be configured to determine the transmission timing in hardware. Also, when using standards that require carrier sensing, and when carrier sensing fails, it is common to retransmit, but since retransmission disrupts the time interval, it is desirable to wait until the next radio wave transmission timing without transmitting radio waves.

[0115] Furthermore, if the time difference δ falls below a predetermined value, the total amount of time information T data may be reduced. For example, if the transmission timing is determined by the magnitude of N as described above, and sufficient time synchronization is achieved, only the numerical value of N may be sent. Alternatively, if N can be estimated, at least one of the time information T components may be reduced. It is also effective to reduce a portion of the first time information T1 to the fourth time information T4, or to vary the number of bits unevenly.

[0116] [Hardware configuration of wireless synchronization device] Figure 9 is a schematic diagram of an example hardware configuration of an information processing device 90 applied to this embodiment. The information processing device 90 comprises a processor 91, main memory 92, communication interface 93, auxiliary storage device 94, input / output interface 95, and internal bus 96. The processor 91, main memory 92, communication interface 93, auxiliary storage device 94, and input / output interface 95 are connected to each other via the internal bus 96 so as to be able to communicate with each other. The information processing device 90 may be applied to, for example, a wireless synchronization device 10. In this case, for example, the wireless communication unit 21 may be configured using the communication interface 93. For example, the storage unit 22 may be configured using the auxiliary storage device 94. Also, the synchronization unit 23 may be configured using the processor 91 and the main memory 92.

[0117] [Summary of Embodiments] According to the embodiment described above, the first and second radio waves are transmitted and received by time measurement communication, and at least one of the first time information T1, second time information T2, third time information T3, and fourth time information T4 is transmitted and received by time information communication, and the time information communication and time measurement communication use different wireless communication channels. In cases where sufficient bandwidth cannot be secured in the wireless communication channel or sufficient communication time cannot be secured due to wireless standards or radio wave laws of each country, by using different communication channels or communication methods for measurement and time information exchange, the load of bidirectional communication necessary for time synchronization can be distributed and its frequency can be improved. Furthermore, since the radio wave characteristics suitable for measurement and the radio wave characteristics suitable for data communication are inherently different, by separating the channels for time measurement and time information sharing, it is possible to improve synchronization accuracy. Therefore, with the wireless synchronization device 10, stable and highly accurate synchronization can be achieved even in dense environments.

[0118] Furthermore, according to the above-described embodiment, the amount of data per unit time for time information communication is greater than the amount of data per unit time for time measurement communication. By allocating appropriate frequency bands according to the amount of data required for each communication—for measurement which does not require data storage and for the exchange of time information T which should be transmitted—it is possible to maximize the use of each bandwidth, increase the synchronization frequency, and improve synchronization accuracy.

[0119] Furthermore, according to the embodiment described above, the data rate of time information communication is higher than the data rate of time measurement communication. This suppresses interference and allows for the sharing of time measurement results at a high frequency. In addition, increasing the data rate, that is, increasing the carrier phase, leads to expanding the bandwidth of time information communication, which involves a large amount of data being transmitted and received.

[0120] Furthermore, according to the embodiment described above, the bandwidth used for time information communication is wider than the bandwidth used for time measurement communication. As mentioned above, it is desirable that time information communication, which involves a large amount of data, be allocated a high data rate, i.e., a wide bandwidth.

[0121] Furthermore, according to the above-described embodiment, the frequency used for time information communication is higher than the frequency used for time measurement communication. By allocating high frequencies that can propagate a large amount of information to time information communication, which involves a large amount of data being transmitted and received, and low frequencies that can propagate a small amount of information to time measurement communication, which involves a small amount of data being transmitted and received, communication can be performed efficiently.

[0122] Furthermore, according to the embodiment described above, at least one of the first and second radio waves is transmitted with polarization in a predetermined direction and received with polarization in substantially the same direction as the transmitted wave. In other words, it is received with the same polarization as at the time of transmission. This makes it possible to separate the direct wave from the reflected wave, which has a different polarization from the direct wave, and to suppress the decrease in accuracy that occurs when reflected waves are combined during time measurement.

[0123] Furthermore, according to the embodiments described above, for the transmission of at least one of the first and second radio waves, an antenna with different radiation characteristics for vertical and horizontal polarization, or an antenna with different radiation characteristics for right-hand and left-hand circular polarization, is used. By using antennas with significantly different polarization characteristics for orthogonal elements, that is, antennas with high polarization isolation, the effects of reflected waves can be reduced, and a decrease in time measurement accuracy can be suppressed.

[0124] Furthermore, according to the embodiment described above, the antenna provided in the wireless communication unit 21 is a slot array antenna. A slot array antenna can reduce the effects of reflected waves and suppress a decrease in time measurement accuracy.

[0125] Furthermore, according to the embodiment described above, the time representation method using at least one of the first time information T1, second time information T2, third time information T3, and fourth time information T4 is changed according to the time error between the device and the communication target device. The time information T required for synchronization differs depending on whether the time difference δ is large or small. For example, in the initial stages of synchronization, the time resolution can be low because the time needs to be adjusted significantly, while in the later stages of synchronization, a high time resolution is desirable because the time needs to be adjusted precisely. If the time representation method using time information T is fixed to a low resolution, it is difficult to achieve high-precision time synchronization, and if it is fixed to a high resolution, a wide dynamic range is required for the initial stages of synchronization when the time difference δ is large, which leads to a decrease in communication frequency due to an increase in the amount of data. By adaptively changing the representation method according to the time difference δ, high-precision time synchronization can be performed while keeping the amount of data down.

[0126] Furthermore, according to the embodiment described above, the method of representing time using time information T includes the resolution of the time indicated by the time information T. For example, by lowering the time resolution in order to make large time adjustments at the beginning of synchronization and increasing the time resolution in order to make precise time adjustments at the end of synchronization, it is possible to appropriately represent time while keeping the number of bits required to represent the time information T fixed. Therefore, by making the resolution of the transmitted time information T variable, the accuracy of the detectable time difference δ can be adaptively changed, and highly accurate synchronization can always be achieved with the minimum number of bits (data size).

[0127] Furthermore, according to the embodiment described above, the method of representing time using time information T includes the amount of data of time information T. By making the amount of data of the transmitted time information variable, that is, by adaptively changing the amount of data according to the synchronization status, the degree of freedom of the detectable clock time difference δ can be changed.

[0128] Furthermore, according to the embodiment described above, when the time error falls below a predetermined value, the amount of data of at least one of the first time information T1, second time information T2, third time information T3, and fourth time information T4 is reduced. By reducing the amount of data towards the end of synchronization when the time difference δ becomes small, the synchronization frequency can be increased, and the synchronization accuracy in a dense environment can be improved.

[0129] Furthermore, according to the above-described embodiment, when the time error falls below a predetermined value, the number of bits representing the first time information T1, second time information T2, third time information T3, and fourth time information T4 are made uniform and reduced. By reducing all time information T to the same number of bits, the calculation process is simplified, and the complexity and cost of the program can be reduced.

[0130] Furthermore, according to the embodiment described above, the clock 24 of the self-device (first wireless synchronization device 11) is synchronized with the clock 24 of the communication target device (second wireless synchronization device 12), and the timing of the transmission of the second radio wave is shared in advance between the self-device and the communication target device, thereby acquiring the third time information T3 based on the self-device's clock. If time synchronization is sufficiently achieved and there is a consensus among the multiple wireless synchronization devices 10 on the frequency of synchronization, there is no problem in transmitting radio waves at predetermined intervals. This makes it possible to determine the transmission and reception times without having the communication target device share the timing of radio wave transmission and reception through time information communication. Therefore, the amount of time information to be transmitted can be reduced, and the synchronization frequency can be increased in dense environments, improving synchronization accuracy.

[0131] Furthermore, the functions of all or part of the components of the wireless synchronization device 10 in the above-described embodiment may be realized by recording a program for realizing these functions on a computer-readable recording medium, having a computer system read the program recorded on this recording medium, and executing it. The term "computer system" here includes hardware such as an operating system and peripheral devices.

[0132] Furthermore, "computer-readable recording media" refers to portable media such as flexible disks, magneto-optical disks, ROMs, and CD-ROMs, as well as recording units such as hard disks built into computer systems. In addition, "computer-readable recording media" may also include those that dynamically hold programs for a short period of time, such as communication lines used when transmitting programs over networks such as the Internet or communication lines such as telephone lines, and those that hold programs for a certain period of time, such as volatile memory inside computer systems that act as servers or clients in such cases. Moreover, the above-mentioned program may be for the purpose of realizing some of the functions described above, and may also be able to realize the above-mentioned functions in combination with programs already recorded in the computer system.

[0133] Although one embodiment of this invention has been described in detail above with reference to the drawings, the specific configuration is not limited to that described above, and various design changes can be made without departing from the spirit of this invention. Furthermore, the configurations described in each embodiment and example above may be combined. [Explanation of Symbols]

[0134] 1...Wireless synchronization system, 10...Wireless synchronization device, 11...First wireless synchronization device, 12...Second wireless synchronization device, 21...Wireless communication unit, 22...Storage unit, 23...Synchronization unit, 24...Clock, T...Time information

Claims

1. A wireless synchronization device that is connected to a communication target device, which is a wireless synchronization device to be communicated with, via a wireless transmission path, The aforementioned communication target device transmits and receives radio waves bidirectionally. Based on multiple time information indicating the time when the aforementioned radio wave transmission and reception were performed, the time error between the device and the communication target device is calculated. Time synchronization is performed based on the aforementioned time error. Wireless synchronization device.

2. The aforementioned radio wave transmission and reception are performed by time measurement communication. At least one of the aforementioned multiple time information pieces is transmitted and received via time information communication. The aforementioned time information communication and the aforementioned time measurement communication use different types of wireless communication channels. The wireless synchronization device according to claim 1.

3. A wireless synchronization device that is connected to a communication target device, which is a wireless synchronization device to be communicated with, via a wireless transmission path, The first radio wave is transmitted to the aforementioned communication target device, The second radio wave is received from the aforementioned communication target device. Based on the first time information relating to the transmission time of the first radio wave by the device itself, the second time information relating to the reception time of the first radio wave by the communication target device, the third time information relating to the transmission time of the second radio wave by the communication target device, and the fourth time information relating to the reception time of the second radio wave by the device itself, the time error between the device and the communication target device is calculated. Time synchronization is performed based on the aforementioned time error. Wireless synchronization device.

4. The first and second radio waves are transmitted and received by time measurement communication. At least one of the first time information, second time information, third time information, and fourth time information is transmitted and received by time information communication. The aforementioned time information communication and the aforementioned time measurement communication use different types of wireless communication channels. The wireless synchronization device according to claim 3.

5. The amount of data per unit time in the aforementioned time information communication is greater than the amount of data per unit time in the aforementioned time measurement communication. The wireless synchronization device according to claim 2 or claim 4.

6. The data rate of the aforementioned time information communication is higher than the data rate of the aforementioned time measurement communication. The wireless synchronization device according to claim 2 or claim 4.

7. The bandwidth used for the aforementioned time information communication is wider than the bandwidth used for the aforementioned time measurement communication. The wireless synchronization device according to claim 2 or claim 4.

8. The frequency used for the aforementioned time information communication is higher than the frequency used for the aforementioned time measurement communication. The wireless synchronization device according to claim 2 or claim 4.

9. At least one of the first and second radio waves is transmitted with polarization in a predetermined direction and received with polarization in substantially the same direction as the transmitted wave. The wireless synchronization device according to claim 4.

10. For the transmission of at least one of the first and second radio waves, an antenna with different radiation characteristics for vertical polarization and horizontal polarization, or an antenna with different radiation characteristics for right-hand circular polarization and left-hand circular polarization, is used. The wireless synchronization device according to claim 4.

11. The aforementioned antenna is a slot array antenna. The wireless synchronization device according to claim 10.

12. Depending on the time error between the device and the communication target device, the method of representing time using at least one of the first time information, second time information, third time information, and fourth time information is changed. The wireless synchronization device according to claim 3.

13. The representation method includes the time resolution indicated by the time information. The wireless synchronization device according to claim 12.

14. The aforementioned representation method includes the amount of data for the time information. The wireless synchronization device according to claim 12.

15. If the aforementioned time error falls below a predetermined value, the amount of data of at least one of the first time information, second time information, third time information, and fourth time information is reduced. The wireless synchronization device according to claim 14.

16. If the aforementioned time error falls below a predetermined value, the number of bits representing the first time information, the second time information, the third time information, and the fourth time information are reduced to match the number of bits used. The wireless synchronization device according to claim 14.

17. The clock of the aircraft is synchronized with the clock of the communication target device. The timing of the transmission of the second radio wave is shared in advance between the device and the communication target device, thereby acquiring the third time information based on the device's clock. The wireless synchronization device according to claim 3.

18. A wireless synchronization system in which a plurality of wireless synchronization devices, including a first wireless synchronization device and a second wireless synchronization device, are connected to each other via a wireless transmission path, The first wireless synchronization device transmits a first radio wave to the second wireless synchronization device. The second wireless synchronization device transmits a second radio wave to the first wireless synchronization device. The first wireless synchronization device is Based on the first time information relating to the transmission time of the first radio wave, the second time information relating to the reception time of the first radio wave by the second wireless synchronization device, the third time information relating to the transmission time of the second radio wave by the second wireless synchronization device, and the fourth time information relating to the reception time of the second radio wave, the time error between the first wireless synchronization device and the second wireless synchronization device is calculated. Time synchronization is performed based on the aforementioned time error. Wireless synchronization system.

19. When a plurality of wireless synchronization devices, including a first wireless synchronization device and a second wireless synchronization device, are connected to each other via a wireless transmission path, The first wireless synchronization device transmits a first radio wave to the second wireless synchronization device, The second wireless synchronization device transmits a second radio wave to the first wireless synchronization device, The first wireless synchronization device calculates a time error between the first wireless synchronization device and the second wireless synchronization device based on first time information relating to the transmission time of the first radio wave, second time information relating to the reception time of the first radio wave by the second wireless synchronization device, third time information relating to the transmission time of the second radio wave by the second wireless synchronization device, and fourth time information relating to the reception time of the second radio wave. The first wireless synchronization device performs time synchronization based on the time error, A method for wirelessly synchronizing a handler.

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