JP2019009626A - Time synchronizing method for radio communication system, and data collection method - Google Patents

Abstract

To provide a time synchronizing method for a radio communication system that can be operated while reducing power consumption, and a data collection method.SOLUTION: A time synchronizing method for a radio communication system includes at least: a first step for acquiring sensor data by switching from a sleep mode to a transmission mode simultaneously when a slave unit is started based on its own internal clock; a second step for transmitting the sensor data from the slave unit to a master unit and then switching to a reception mode for a fixed time; a third step for transmitting a time correction signal based on an actual time to the slave unit in a case where the master unit receives the sensor data; a fourth step for correcting the internal clock and switching to the sleep mode in a case where the slave unit receives the time correction signal during the reception mode; and a fifth step for switching from the reception mode to the transmission mode in a case where the slave unit does not receive the time correction signal during the reception mode, re-transmitting the sensor data to the master unit, and switching to the reception mode.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a time synchronization method and a data collection method of a wireless communication system that can be operated with low power consumption.

In recent years, the aging of agricultural workers has progressed, and urgent knowledge and skills are required (Non-Patent Document 1). We have been developing rice action field servers to assist them and the applications that accompany them (Non-patent Document 2). Since field servers are installed in fields without power, it is desired to operate with mobile batteries for 6 months from rice planting to rice harvesting. In order to realize this, it is essential to reduce the power consumption of the field server.
Currently available products use a 3G line when storing sensor data and the like acquired from a field server in the cloud (Non-Patent Documents 3 and 4). However, a monthly usage fee for 3G lines is required for the number of field servers, which increases operational costs and can be a barrier to the introduction of field servers. Therefore, in order to reduce operational costs, we investigated the basic communication characteristics such as Wi-SUN, LoRa, etc. that do not require a line usage fee. As a result, it has been found that LoRa is suitable for rice fields (Non-Patent Documents 5 to 7).

Also, in order to operate for 6 months in an environment without a power source, it is necessary to turn off the power while sensor data acquisition and communication are not performed. Therefore, intermittent operation communication protocol and time synchronization technology are required. Research on time synchronization technology has been extensively conducted.
A time synchronization method using a radio clock has been proposed (Non-Patent Document 8). This method requires about 3 minutes to set the time. Therefore, the operation time becomes long and it is difficult to realize low power consumption. Also, it is necessary to convert the received signal (time code) into a format that can be handled in the field server, but a program that performs this processing cannot be written in a PIC microcomputer with little memory. Therefore, it is difficult to use for a rice action field server.
A method of performing time synchronization using GPS has been proposed (Non-Patent Document 9). In order to use this method, it is necessary to install a GPS receiving module in each field server, which leads to an increase in initial cost at the time of introduction. For this reason, it is difficult to use for rice action field servers where it is desired to lower the barrier to introduction.

RBS (Reference Broadcast Synchronization) has been proposed as a time synchronization method for wireless networks (Non-Patent Document 10). Time synchronization is performed by exchanging time information between nodes that have received an arbitrary broadcast packet. However, RBS increases the amount of information exchanged in proportion to the increase in the number of receiving nodes, and there is a problem of increasing power consumption. Therefore, it is difficult to apply to a rice field server.
As a method for improving RBS, TPSN (Timing-sync Protocol for Sensor Networks) has been proposed (Non-patent Document 11). The TPSN forms a tree structure with the parent device at the top, and the parent device (root node) performs time synchronization with the child node. The child node performs time synchronization with a lower child node. In this way, the time is synchronized throughout. Since this method requires a long time for time synchronization, it is not suitable for rice action field servers that require low power consumption operation.

Ministry of Agriculture, Forestry and Fisheries: Statistics on the agricultural labor force, Ministry of Agriculture, Forestry and Fisheries (online), source <http://www.maff.go.jp/j/tokei/sihyo/data/08.html> Shinji Toyoda, Keitaro Terada, Yuya Takada, Tadaaki Hirata, Mikiko Sode: Study of network construction method for field management using LoRa, IEICE technical report, NS2017-11, pp. 61-66 Fujitsu: FUJITSU Intelligent Society Solution Food and Agriculture Cloud Akisai (Akisai), Fujitsu (Online), Source <http://jp.fujitsu.com/solutions/cloud/agri/> Vegetalia Co., Ltd .: PaddyWatch, Vegetalia Co., Ltd. (online), where you can measure the water level and temperature of rice in smart agriculture <http://field-server.jp/paddywatch/> Wi-SUN Alliance: Home-Wi SUN Alliance, Wi-SUN Allian-ce (online), from <https://www.wi-sun.org/index.php/en/> LoRa Alliance: Home, LoRa Alliance (online), from <https://www.lora-alliance.org> Yuta Kawakami, Shinji Toyoda, Keitaro Terada, Mikiko Sode: Study of inter-field communication using Sub-GHz radio, IEICE technical report, NS2016-138, pp.107-112 Takafumi Watanabe, Takashi Morito, Masateru Minami, Hiroyuki Morikawa: Design and Implementation of Synchronous Batteryless Wireless Sensor Network Using Radio Clock, IPSJ Research Report Mobile Computing and Ubiquitous Communications, vol.2007, No.44, pp .113-118 (2007) Position Co., Ltd .: About GPS reception as a time reference, Position Co., Ltd. (online), source <http://www.posit.co.jp/mission/pdf/1998.6.pdf> J. Elson, L. Girod and D. Estrin: Fine-Grained Network Time Synchronization using Reference Broadcasts, Proceedings of the 5th Symposium on Operating Systems Design and Implementa-tion (OSDI ’02), Boston, Massachusetts (2002) S. Ganeriwal, R. Kumar and M. B. Srivastava: Timing-sync Protocol for Sensor Networks, Proceedings of the 1st ACM Conference on Embedded Network Sensor Systems (SenSys'03), Los Angeles, California (2003) Seiko Epson Corporation: RX-8025SA | RTC (I2C) Real Time Clock | Product Information | Epson Devices, Seiko Epson Corporation (Online), Where to Get .html>

  Therefore, we studied a network construction method using LoRa and intermittent communication protocol for rice field server (Non-patent Document 2). In order to perform the intermittent operation, it is necessary to simultaneously start a plurality of field servers and perform a cooperative operation to avoid simultaneous transmission. If the time management is performed by the microcomputer inside the field server without time synchronization, a maximum of 4,688 seconds will occur in one month. In addition, as a result of examining multiple real-time clock ICs that can be mounted on our field servers, it was found that even with ICs that have the smallest deviation, a deviation of approximately ± 13 seconds occurs in a month (non-patent literature). 12). A shift of more than 1 minute will occur after 6 months of operation. Since there are multiple field servers, it is difficult to operate for 6 months if a communication protocol is created in consideration of the difference.

  In view of the above problems, an object of the present invention is to provide a time synchronization method and a data collection method of a wireless communication system that can be operated with low power consumption.

(1) A time synchronization method for a wireless communication system according to the present invention is a time synchronization method for a wireless communication system including a parent device and a plurality of child devices connected to sensors, wherein the child device is based on its own internal clock. The first step of switching from the sleep mode to the transmission mode at the same time as starting and acquiring sensor data, the second step of transmitting the sensor data to the parent device, and then switching to the reception mode for a certain period of time, When the master unit receives the sensor data, a third step of transmitting a time correction signal based on real time to the slave unit, and when the slave unit receives the time correction signal during the reception mode A fourth step of correcting the internal clock and switching to the sleep mode; and when the slave unit does not receive the time correction signal during the reception mode, At least a fifth step of switching from a transmission mode to a transmission mode, retransmitting the sensor data to the parent device, and switching to a reception mode is provided.
(2) Further, in the second step, when the start time of another slave unit is reached during the reception mode of the slave unit, the slave unit in the reception mode is switched to the sleep mode.
(3) Further, in the fifth step, the slave unit that has not received the time correction signal retransmits the sensor data to the master unit after a lapse of a random time.
(4) When a slave unit is newly added to the wireless communication system, the added slave unit transmits a signal requesting the time correction signal to the master unit, and the master unit transmits the request. When the signal is received, the step of transmitting the time correction signal to the added slave unit, the added slave unit receives the time correction signal, corrects the internal clock, and enters the sleep mode. A step of switching is provided.

(5) A data collection method for a wireless communication system according to the present invention is a data collection method for a wireless communication system comprising a master unit and a plurality of slave units connected to sensors. At the same time as Step 1-1 and Step 1-1 where the slave unit transfers sensor data to the master unit and the slave unit after the sensor data transfer switches to sleep mode or power off, the slave unit closest to the master unit Establish a connection between the two slave units that are closest to each other among the multiple slave units other than the slave unit, and to the slave unit that is closer to the remote unit that is farther from the master unit among the two slave units that have established the connection The first and second steps when the sensor unit is transferred and the slave unit after the sensor data transfer is switched to the sleep mode or power off, and the slave unit closest to the master unit among the plurality of slave units that are not switched to the sleep mode or power off. The slave unit transfers sensor data to the master unit and the slave unit after the sensor data transfer switches to sleep mode or power off at the same time as step 2-1 and step 2-1 A connection is established between the two slave units that are closest to each other among the multiple slave units that are not connected, and the remote slave unit that is farther from the master unit among the two slave units that have established the connection is detected as a sensor. And the second and second steps in which the slave unit after transferring the sensor data is switched to the sleep mode or to turn off the power. After that, all the slave units transfer the sensor data to the master unit and enter the sleep mode. Alternatively, the 2-1 step and the 2-2 step are repeatedly performed until the power is turned off.
(6) A data collection method for a wireless communication system according to the present invention is a data collection method for a wireless communication system comprising a parent device and a plurality of child devices connected to sensors, wherein the plurality of child devices have their own internal clocks. At the same time as the start-up, the sleep mode or the power is switched from the power-off to the transmission mode, the first step of acquiring the sensor data, and the handset A closest to the base unit among the plurality of handset units transfers the sensor data to the base unit. Next, the master unit transmits a time correction signal based on real time to the slave unit A, and then the slave unit A that has received the time correction signal corrects its own internal clock and sleep mode or At the same time as Steps 2-1 and 2-1 when the power is switched off, a connection is established between the two slave units that are closest to each other among the slave units other than the slave unit A, and the connection is established. did Of the two slave units, the slave unit B1 farther from the master unit transfers the sensor data to the nearest slave unit B2, and then the nearest slave unit B2 sends a time correction signal based on its own internal clock. Step 2-2 for transmitting to the remote handset B1 and then receiving the time correction signal, the remote handset B1 corrects its own internal clock and switches to sleep mode or power off, Of the plurality of slave units that have not been switched to the sleep mode or power off, the slave unit C closest to the master unit transfers the sensor data to the master unit, and then the master unit is based on the real time for the slave unit C. At the same time as step 3-1 and step 3-1, the slave unit C that transmits a time correction signal and then receives the time correction signal corrects its own internal clock and switches to sleep mode or power off. , Switch to sleep mode or power off A connection is established between the two slave units that are closest to each other among a plurality of slave units, and a sensor is connected to the slave unit D2 that is closer to the slave unit D1 that is farther from the master unit among the two slave units that have established the connection. Transfer the data, and then the closer handset D2 transmits a time correction signal based on its own internal clock to the far handset D1, and then receives the time correction signal of the far handset The slave unit D1 has its own internal clock and has a step 3-2 for switching to the sleep mode or power off. Thereafter, all the slave units transfer the sensor data to the master unit to enter the sleep mode or the power source. Steps 3-1 and 3-2 are repeatedly performed until it is turned off.
(7) A data collection method for a wireless communication system according to the present invention is a data collection method for a wireless communication system comprising a parent device and a plurality of child devices connected to sensors, wherein the plurality of child devices have their own internal clocks. At the same time as the start-up, the sleep mode or the power is switched from the power-off to the transmission mode, the first step of acquiring the sensor data, and the handset A closest to the base unit among the plurality of handset units transfers the sensor data to the base unit. Next, the master unit transmits a time correction signal based on real time to the slave unit A, and then the slave unit A that has received the time correction signal corrects its own internal clock and sleep mode or At the same time as the 2-1 step and the 2-1 step when the power is turned off, a connection is established between two slave units among the plurality of slave units other than the slave unit A, and the two slave units that have established the connection are established. Machine One slave unit B1 transfers sensor data to the other slave unit B2, and then the slave unit B2 transmits a time correction signal based on its own internal clock to the slave unit B1, and then the time correction signal The slave unit B1 that has received the correction corrects its own internal clock and switches to sleep mode or power off, and is closest to the master unit among the plurality of slave units that have not switched to sleep mode or power off. The slave unit C transmits the sensor data to the master unit, and then the master unit transmits a time correction signal based on real time to the slave unit C, and then receives the time correction signal. The two slave units out of the plurality of slave units that are not switched to the sleep mode or the power-off at the same time as the third step 3-1 that switches the sleep mode or power-off while correcting its own internal clock Make connections with each other One of the two slave units D1 that established the connection transfers the sensor data to the other slave unit D2, and then the slave unit D2 sends a time correction signal based on its own internal clock The slave unit D1 that has been transmitted to the slave unit D1 and then received the time correction signal has a third and second steps in which the slave unit D1 corrects its own internal clock and switches to the sleep mode or the power off. The slave unit transfers sensor data to the master unit, and repeats steps 3-1 and 3-2 until the sleep mode or power is turned off.

In the invention of the time synchronization method of the wireless communication system of (1), after the slave unit transmits the sensor data to the master unit, the master unit that has received this transmits a time correction signal to the slave unit, and the master unit and the slave unit Perform time synchronization. The slave unit starts up based on its own internal clock, performs sensor data acquisition and communication, and then switches to the sleep mode so that power consumption can be suppressed, and the entire wireless communication system can be operated with low power consumption. .
Further, in the invention of (2), in the second step, when the start time of the other slave unit is reached during the slave unit reception mode, if the slave unit in the reception mode is switched to the sleep mode, It is possible to prevent a communication collision with the other slave unit.
In the invention of (3), in the fifth step, even when the slave unit decides to retransmit the sensor data to the master unit after the random time has elapsed, it is possible to prevent a communication collision with another slave unit.
In the invention of (4), since the time synchronization of the slave unit newly added to the wireless communication system can be performed, the system can be easily expanded.

In the invention of the data collection method of the wireless communication system according to (5), data collection is performed by performing merge processing and transmission of data between each slave unit holding the sensor data acquired from the sensor and between the slave unit and the master unit. It is possible to reduce the time required for That is, first, sensor data for one slave unit is sent to the master unit, then sensor data for two slave units is sent to the master unit, and then sensor data for four slave units is sent to the master unit. In this way, the number of data sent to the base unit increases by a factor of two each time. For this reason, data is collected rapidly. As a result of the comparison with the conventional method, it was confirmed that the method of the present invention has the shortest collection time for collecting the data of the child device in the parent device. Moreover, it has confirmed that power consumption was also small compared with the conventional method. Furthermore, it was confirmed that the rate of time required for data collection to the parent unit due to the increase in the number of slave units was small compared to other methods, and it was an effective method.
The invention of the data collection method of the wireless communication system according to (6) is the content of incorporating the invention of the time synchronization method of (1) into the invention of the data collection method of (5). Specifically, when the slave unit closest to the master unit transfers sensor data to the master unit, a time correction signal based on the real time is transmitted from the master unit to the slave unit, The clock is corrected and the sleep mode or the power is turned off. At the same time, a connection is established between the two child devices having the shortest distance among the other child devices, and the sensor data is transferred to the child device closer to the child device farther from the parent device. The closer handset transmits a time correction signal based on its own internal clock to the far handset. Then, the remote handset corrects its own internal clock, and enters the sleep mode or power off. By repeating such an operation, sensor data can be quickly collected while performing time synchronization of the slave units, and the wireless communication system can be accurately operated for a long time with low power consumption.
The invention of the data collection method of the wireless communication system according to (7) corresponds to an application example of the invention of the data collection method of (6). Specifically, when the slave unit closest to the master unit transfers sensor data to the master unit, a time correction signal based on the real time is transmitted from the master unit to the slave unit, The clock is corrected and the sleep mode or the power is turned off. At the same time, a connection is established between two appropriate slave units among the other slave units, and one slave unit transfers sensor data to the other slave unit. The other slave unit transmits a time correction signal based on its own internal clock to the one slave unit. Then, one slave unit corrects its own internal clock and goes into sleep mode or power off. By repeating such an operation, sensor data can be quickly collected while performing time synchronization of the slave units, and the wireless communication system can be accurately operated for a long time with low power consumption.

Overall image of the field management system Position relationship between each field and office Field server configuration Main unit configuration Communication protocol sequence diagram Sequence diagram at the time of sensor data collision Time synchronization mechanism between the base unit and the field server Proposed network topology Number of data transmission completion nodes 1 round uptime Maximum operating time comparison

An embodiment of a time synchronization method for a wireless communication system of the present invention will be described.
In the present embodiment, the wireless communication system is a field management system for acquiring and utilizing field environment information by various sensors.

[Overview of the entire system]
Fig. 1 shows the overall configuration of the field management system. This system consists of a field server (child device), a parent device, and a cloud service.
The field server is installed in the field, acquires sensor data, and then transmits the data to the parent device through a wireless network. The master unit aggregates the sensor data from the field servers installed in each field and transfers them to the cloud via a 3G line or Wi-Fi.
Cloud services are provided through applications such as smartphones and tablets, and web pages. Provide farmers with water level warnings, work plan proposals, work record preservation, and other functions.
For communication between the field server and the base unit, LoRa capable of long-distance communication is used. LoRa has a practical communication distance of 3,000 to 4,000 m, based on our basic communication characteristics survey [7]. In the Ishikawa Prefecture field, the distance between the main unit and the field server is about 3,000m. For this reason, we adopted LoRa, which allows direct communication between the field server and the base unit.
Figure 2 shows the positional relationship between each field and office of an assumed agricultural corporation in Ishikawa Prefecture. A, B, C, D, E, F, and G represent the position of each field, and a field server is installed at this point. P represents the location of the office, and the parent machine is installed in the office. The linear distances between the field servers A to G and the main unit P are 397 m between APs, 923 m between BPs, 943 m between CPs, 684 m between DPs, 1,150 m between EPs, 1,440 m between FPs, and 1,910 m between GPs.

[Overview of Field Server and Base Unit]
Details of the wireless network configured by the field server and the parent device will be described.
The configuration of the field server is shown in FIG. The field server consists of a mobile battery, a power ON / OFF circuit, an AVR microcomputer, a LoRa wireless module, various sensors, and an SD card module. Since the field server is installed on the field, it is difficult to supply power, so power is supplied by a mobile battery (lithium ion secondary battery). In order to realize low power consumption, the power supply ON / OFF circuit operates only for tens of seconds out of one hour. Wireless communication and sensor control are performed by an AVR microcomputer. Five types of sensors are installed to measure temperature, humidity, water level, soil temperature, and soil moisture content.
The sensor data is stored on the SD card along with a time stamp. This is a function for reliably storing the sensor data even when the sensor data cannot be transmitted to the parent device or when the time correction signal from the parent device cannot be received. If time synchronization with the master unit fails, 3,600 is added to the previous recording time stamp recorded on the SD card to obtain the current recording time stamp.

The power supply ON / OFF circuit is composed of a PIC microcomputer and FET, and controls the power supply to the module connected to the power supply ON / OFF circuit by the power line. PIC microcomputer controls FET by outputting HIGH / LOW at GPIO pin. The PIC microcomputer measures and controls the time required for power control using the timer interrupt of the internal clock.
Fig. 4 shows the configuration of the master unit. The main unit consists of a Raspberry Pi, a transmitting LoRa wireless module, a receiving LoRa wireless module, and a 3G dongle.
When the field server is turned on for the first time, it does not hold the time and starts without waiting. For this reason, it is desirable that the master unit always maintains the reception state and can cope with communication from the field server whenever the farmer installs the field server. In addition, the LoRa wireless module has a reception mode and a transmission mode, and switching the mode takes time. For this reason, the base unit is equipped with two reception-only and transmission-only LoRa wireless modules to reduce the waiting time for transmission and reception and maintain the reception state at all times.

[Communication protocol overview]
The field server starts when the time in the PIC is 3,600 (0) seconds, and acquires sensor data. The sensor data is transmitted to the parent device, and when the parent device has successfully received the sensor data, the parent device transmits the correction time to the field server. The field server receives the correction time, and the AVR microcomputer transfers the correction time to the PIC microcomputer. The PIC microcomputer corrects the time in the PIC based on this correction time. When transfer is completed, or when the operation time per hour described later has elapsed, power is turned off except for the power ON / OFF circuit such as AVR, and it waits until the next start-up.
Tables 1, 2, and 3 show frame formats used for communication.
Table 1 shows the common frame format. Consists of destination, source and payload. Table 2 shows the sensor data transmission format. This is a signal transmitted from the field server to the parent device. Since there are five types of sensors currently installed, a format that can store data of 2 bytes each for sensor data 1 to 5 is defined. If the number of sensor types increases, it can be handled by adding 2 bytes to the sensor data transmission format. Table 3 shows the format of the time correction signal. This is a signal transmitted from the master unit to the field server. The time stamp acquired by the master unit and the correction time for each field server calculated based on the time stamp are stored.

となる。センサ安定待機および取得時間は、電源投入後の電圧が安定し、センサ取得が可能になるまでの固定待機時間およびセンサデータを測定する時間である。センサデータ送信時間は親機にセンサデータを送信する時間である。モード切替時間は、LoRa通信モジュールを送信モードから受信モードへ切り替える固定時間である。受信時間は親機から時刻補正信号を受信する時間である。再送可能時間はセンサデータを親機に送信できない場合、再送処理を行える時間である。 A communication protocol sequence diagram is shown in FIG. FIG. 5 shows an example in which there are three field servers and no retransmission occurs in all communications. First, field server A (hereinafter referred to as FS-A) is started. After startup, wait for a fixed time until the sensor stabilizes. After that, the FS-A acquires sensor data, generates a transmission packet according to the sensor data transmission format shown in Table 2, and then transmits the sensor data to the parent device. Since the field server has only one LoRa wireless module, it switches from the transmission mode to the reception mode. This switching takes a few seconds. After switching to the reception mode, it waits for reception until the set timeout time, and receives a time correction signal sent from the parent device.
When the master unit receives sensor data from FS-A, it sends a time correction signal to FS-A. The FS-A corrects the time in the PIC based on the time correction signal. After the correction, even in the middle of the retransmittable time, the sleep state is entered. Since it is necessary to perform retransmission when the field server receives the time correction signal or when the master unit fails in transmission, a retransmittable time is provided. This series of operations is similarly performed by the field server B (FS-B) and the field server C (FS-C).
The operating time per hour is obtained from the following equation (Equation 1). This operating time is written in advance to the AVR microcomputer and sent to the PIC microcomputer every time the AVR microcomputer is powered on.
The operating time of each field server is

It becomes. The sensor stabilization standby time and acquisition time are a fixed standby time and a time for measuring sensor data until the voltage after power-on is stabilized and sensor acquisition becomes possible. The sensor data transmission time is a time for transmitting sensor data to the master unit. The mode switching time is a fixed time for switching the LoRa communication module from the transmission mode to the reception mode. The reception time is a time for receiving a time correction signal from the parent device. The retransmittable time is a time during which retransmission processing can be performed when sensor data cannot be transmitted to the parent device.

[Overview of operation when retransmission occurs]
The field server performs a time-out operation when the time correction signal cannot be received and the reception standby time has elapsed. After the timeout, the field server waits for a random number of seconds from 0.1 to 5.0 and then resends it. FIG. 6 shows the sequence operation in this case. In FIG. 6, the field server C (FS-C) has timed out and is performing a retransmission operation. The cause of timeout is
1) When the base unit cannot receive communications from the field server due to radio attenuation or noise that cannot be demodulated
2) There may be a case where there is a collision in transmission data due to overlapping of transmission times of multiple field servers. At this time, since the parent device does not send a time correction signal to the field server, the reception waiting of the field server times out. The field server that has timed out performs retransmission, but a waiting time of random seconds is provided in order to prevent re-collision with communication performed by the field server that is turned on for the first time.
After retransmitting, the field server switches from the transmission mode to the reception mode and waits for reception. This operation is continued until the field server receives the time correction signal from the parent device. However, in order to avoid communication collision with other field servers, the power is forcibly turned off when the operation start time of the other field server comes. Consider the time that can be retransmitted stably before the operation start time of other field servers, and perform one retransmission.

[Time correction signal]
Since the master unit is always waiting for reception, the field server can transmit sensor data at an arbitrary timing. The master unit synchronizes the time with the NTP server in advance and obtains the correct time. The parent device generates a time correction signal based on this time, and after receiving sensor data from the field server, transmits the time correction signal. As shown in the format of the time correction signal in Table 3, the time stamp has a UNIX (registered trademark) time of 4 bytes. This is used to write sensor data to EEPROM and SD card of AVR microcomputer. Since the correction time is used when correcting the time in the PIC, the time shifted from the current time for each field server is set to 2 to 3599 seconds. The CRC used for communication error detection and correction is not defined in the format because it is added by the LoRa communication module.
Fig. 7 shows the time synchronization mechanism between the field server and the base unit. The master unit sends the exact time obtained by the NTP server to the field server in the corrected time format. When the field server receives the correction time, it transfers it to the PIC microcomputer in the power ON / OFF circuit via the AVR microcomputer. The correction time is obtained by appropriately shifting the current time in order to prevent activation at the same time as other field servers. A calculation formula for shifting this time is shown in the following formula (Formula 2). With respect to the number 2, each field server has a uniquely assigned field server identifier (hereinafter FSID), by subtracting the FSID · 30 from the current time t _{_1,} correction time t _{_2} to be transmitted to the field server is required ing. Depending on the power-on time of the field server, the time _{t_2} is a negative value. In this case, 3,600 is added to _{t_2} .

FSID can be used in the range of 0x00 to 0x77, and start up in ascending order of FSID. By using FSID, the simultaneous start of each field server is prevented and the transmission signal of the field server is not collided.
The PIC that controls the power ON / OFF circuit always counts from 0 to 3,599 seconds with the internal clock. When the time in the PIC reaches 3,600 (0) seconds, power is supplied to the AVR microcomputer. When the field server receives the time correction signal from the parent device, it corrects the time in the PIC to the correction time transmitted from the parent device, and continues counting. By doing this, time synchronization is performed between the field server and the parent device. In the second and subsequent startups, power is supplied to the AVR microcomputer when the time in the PIC reaches 3,600 (0) seconds.
When the base unit cannot normally receive data from the field server due to a communication error or the like, the base unit does not transmit a time correction signal and maintains a sensor data reception standby state.

[Communication protocol verification method]
A 7-day operation test was conducted with the purpose of confirming whether the expected operation was performed between the main unit and the field server using LoRa and the designed communication protocol.
The verification content is
1) Check if the base unit can return the time correction signal to the field server within the reception waiting time of the field server
2) The field server that received the time correction signal confirms whether it will immediately go to sleep.
3) Check whether each field server changes the timing appropriately according to the time correction signal and starts at the specified time.
In the verification of the communication protocol, the number of field servers was the same as the field example we are targeting, with 7 units. The distance between the field server and the base unit was set around the base unit and all the field servers were installed within a radius of 1 m. The sensor data sent from the field server to the main unit was stored in the verification cloud.

In addition to the 7-day operation test, we will verify whether the assumed retransmission can be performed within the operating time assigned to each field server when communication between the field server and the base unit fails. In the verification method, when the field server transmits the first sensor data, the parent device does not wait for reception and times out reception standby of the field server. Thereafter, when the field server transmits sensor data for the second time, the parent device sends a reception standby and time correction signal to the field server. The field server receives this and confirms whether or not to enter the sleep state.
This protocol is intended for field management, and a time difference of several seconds is allowed in the time of sensor data collected from each field. However, when there is a large difference in the activation time of each field server, problems such as a collision of sensor data to be transmitted and a time correction signal from the parent device cannot be received. Therefore, we verified whether to start at a certain time specified for each field server. In the verification method, communication is performed in the order of 0x08 from the field server whose FSID of each field server is 0x02. Each field server is started every 30 seconds. The average error for 72 hours was calculated based on the time when the master device received the sensor data, and the average maximum error [s] and the average minimum error [s] were obtained.

[Verification result of communication protocol]
Table 4 shows the verification results of the communication protocol. The field server is operated for 7 days, and sensor data is acquired and transmitted every hour. The number of times that the field server has sent data to the base unit is 1,176 times because seven field servers and seven days at one hour intervals. In addition to this, the field server cannot receive the time correction signal from the base unit, and the number of retransmissions is nine. Therefore, the total sensor data transmission count is 1,185 times. Of the nine retransmissions, all nine completed time synchronization at the first retransmission.
Further, in the verification of the retransmission operation, after the field server retransmits the sensor data in the retransmission process, it receives a time correction signal from the parent device and shifts to the sleep state. It was confirmed that this operation was performed in sequence. Therefore, it can be judged that it is operating without problems.

Table 5 shows the results of verifying whether to start at a certain time specified for each field server.
In communication between all field servers performed at one-hour intervals, the difference between the assumed activation time given to each field server and the actual activation time was examined as an absolute value, and the average maximum / minimum error was obtained. As a result, the average maximum error was 5.277 seconds and the average minimum error was 4.041 seconds. From this, it can be determined that the start timing of each field server is shifted by approximately 5 to 6 seconds. Although the error is suppressed to be small by using time synchronization, there are not a few errors.
Causes of these errors include clock errors in the PIC microcomputer used in the power ON / OFF circuit, transmission time fluctuation due to individual differences in LoRa communication modules, error correction time due to CRC, errors in LoRa modulation and demodulation, etc. Is mentioned.

The error of the PIC microcomputer was verified because it can be measured. The PIC microcomputer was measured for 1 hour, and the difference from the actual time was regarded as the error. This measurement was measured for 12 hours with 7 PIC microcomputers (A to G), and the average error was calculated for each PIC microcomputer. The results are shown in Table 6 below. From the table, the maximum error is 6.512 seconds. From this, it can be understood that the error of each field server is mostly caused by the error of the PIC microcomputer.
However, in the field management communication protocol proposed this time, this error is within a range where collision with other field servers does not occur. Therefore, it was confirmed that the time synchronization was effective in this communication protocol, and the increase in time error proportional to the usage time could be reduced.

次式(数3)にシステム稼動時における電力量W_N[mW]を求める式を示す。ここで、V₁はフィールドサーバの定格電圧[V]、I_aおよびt₁はセンサ安定待機時とセンサ取得時の電流[mA]および時間[s]、I_bおよびt₂はセンサデータ送信時の電流[mA]および時間[s]、I_cおよびt₃はモード切り替え時の電流[mA]および時間[s]、I_dおよびt₄は受信待機時の電流[mA]および時間[s]、I_eおよびt₅は受信時の電流[mA]および時間[s]を示している。

[Evaluation of power consumption]
In addition to the verification of the communication protocol, an operation test was performed to verify the power consumption. The purpose of the verification is to confirm whether the system can be operated for the required period.
The verification contents are to measure whether the current consumption in each process is measured by a tester and theoretically calculate the power consumption to confirm whether the continuous operation in the required period is possible. Table 7 shows the measurement results.

The following equation (Equation 3) shows an equation for obtaining the electric energy W _N [mW] during system operation. Where V ₁ is the rated voltage [V] of the field server, I _a and t ₁ are the current [mA] and time [s] during sensor stabilization standby and sensor acquisition, and I _b and t ₂ are sensor data transmission Current [mA] and time [s], I _c and t ₃ are the current [mA] and time [s] during mode switching, and I _d and t ₄ are the current [mA] and time [s] during reception standby , I _e and t ₅ indicate current [mA] and time [s] at the time of reception.

次式(数5)に稼働時間T_h[h]を求める式を示す。ここで、V₂はモバイルバッテリーの電圧[V]、Qはモバイルバッテリーの電気量[mAh]を示している。分子のV₂・Qによってモバイルバッテリーの電力量[mWh]を求め、分母のW_NおよびW_sでフィールドサーバが消費する電力量[mW/h]を求めている。

The following equation (Equation 4) shows an equation for obtaining the electric energy W _s [mW] at the time of system sleep. Here, I _g represents the Sleep current [mA].

The following equation (Equation 5) shows an equation for obtaining the operating time T _h [h]. Here, V ₂ represents the voltage [V] of the mobile battery, and Q represents the amount of electricity [mAh] of the mobile battery. The power consumption [mWh] of the mobile battery is obtained from V ₂ and Q of the numerator, and the power consumption [mW / h] consumed by the field server is obtained from W _N and W _s of the denominator.

The theoretical value of the working days was obtained from Table 3 and Formulas 3-5. The power consumption per hour is 3.14 [mW / h], and it consumes 75.36 [mW] per day. Since the capacity of the mobile battery assumed this time is 75,000 [mWh], it was found that it can operate for about 995 days. Although this number of days is a theoretical value, it can be operated for 6 months, which is a requirement of agricultural corporations, even when natural discharge of mobile batteries is taken into account.
The theoretical value of the working days was obtained from Table 3 and Formulas 3-5. The power consumption per hour is 3.14 [mW / h], and it consumes 75.36 [mW] per day. Since the capacity of the mobile battery assumed this time is 75,000 [mWh], it was found that it can operate for about 995 days. Although this number of days is a theoretical value, it can be operated for 6 months, which is a requirement of agricultural corporations, even when natural discharge of mobile batteries is taken into account.

[Consideration of communication protocol and power consumption]
We consider the communication protocol devised this time, especially the effect of time synchronization. Conventionally, the field server we developed uses a communication method that does not synchronize time with the same hardware configuration. In this method, a broadcast signal serving as a data reception confirmation and a sleep command is sent from a master unit to all field servers at regular intervals. The time interval for sending the broadcast signal must be set in consideration of the time error in the field server. Therefore, it is necessary to set a wide interval. In addition, it is necessary to set a wide time interval in proportion to the increase in the number of field servers managed by the master unit. This is because all fluctuations in time due to the increase in the number of units must be allowed. In the case of seven units, which is the same number as the field example we are assuming, the operation time per field server is a minimum of 48 seconds and the power consumption is 6.10 [mW / h]. In the proposed communication protocol with time synchronization, the operation time was about 16 seconds and the power consumption was 3.14 [mW / h].
Compared to the conventional communication method, the communication protocol devised this time can synchronize the time to shorten the operating time per field server to a maximum of 30 seconds, and succeeded in realizing low power consumption with about twice the efficiency. did.

[Summary]
This paper describes the development of a field server that operates for 6 months from rice planting to rice harvesting and the communication protocol for field management using LoRa. In particular, the stability of the communication protocol for time synchronization was described. The network was constructed and verified using the proposed communication protocol. As a result, sensor data was transmitted 1,185 times, 9 of which succeeded in the first retransmission. Therefore, even if the first communication failed, it was confirmed that the sensor data was successfully received from all field servers.
The power consumption of the field server was 75.36 [mW] per day. Therefore, it was theoretically confirmed that 995 days of continuous operation was possible.
As the start timing of each field server, a deviation of approximately 5 to 6 seconds occurs. However, this deviation does not interfere with other field servers. Therefore, time synchronization is effective in this communication protocol, and an increase in error proportional to the operation time can be reduced.
From these facts, this field server can operate stably for a long period of time, which is effective for a field management system. Therefore, this system can be very expected to reduce the load of farm work.
However, the devised communication protocol has restrictions on the number of field servers that can be managed at the same frequency, and cannot connect more than 120 servers. Therefore, it is necessary to improve the upper limit of the number of connections by extending the devised communication protocol and supporting multi-hop.

Next, an embodiment of the data collection method of the wireless communication system of the present invention will be described.
In the present embodiment, the wireless communication system is a field management system for acquiring and utilizing field environment information by various sensors.

[Routing topology and communication procedure]
In the field management system for rice cultivation, it is necessary to operate for 6 months from rice planting to harvesting with batteries. In addition, it is necessary to provide the data acquired by the sensor to farmers without delay. Here, the goal is to minimize the time it takes to collect all data in one round and to set the power consumption so that each slave unit operates for 6 months or more as a constraint. It is assumed that the master unit and the slave unit are all synchronized in time, and each slave unit starts at the same time. At the time of data transmission / reception, the master unit and slave units can communicate with at most one slave unit or master unit in a single process, or broadcast from one slave unit or master unit to all nodes can be performed. . Therefore, in the master unit and each slave unit, broadcasting is performed at a prescribed timing while synchronizing, the master unit receives data from the slave unit, and the slave unit transmits data to the master unit or another slave unit. By repeating this, the data held by all the slave units is collected in the master unit.

即ち、1回の処理で必ず約1/2の子機の電源をoffできることとなる。これを繰り返すことにより、データ収集に必要な時間を最小限に抑えることが可能となる。また稼働時間も抑えることが出来るため、低電力化にも寄与する。 Explain the basic concept of the proposed method. Since it is desirable to present the data to the farmer as soon as possible, the parent device receives data from the child device without interruption and stores it in the cloud on the Internet. Further, for the purpose of reducing power consumption, it is desired that the slave unit transmits data as soon as possible, and turns off the power as soon as the data transmission is completed. Therefore, the slave unit always transmits data to the closest slave unit or master unit, and turns off the power when the transmission is completed.
The transmission process is repeated at the same interval while being synchronized. The slave units and master units that are operating in one transmission process send data after establishing a connection. The number of slave units that are operating at that time is N Assuming (T), the following equation (Equation 6) is obtained.

That is, it is possible to always turn off about 1/2 of the power of the slave unit in one process. By repeating this, the time required for data collection can be minimized. In addition, since the operation time can be suppressed, it contributes to low power consumption.

Figure 8 shows the network topology of the proposed method. The number attached to the branch represents the number of steps at which data is transmitted. In this example, there are 11 slave units and 1 master unit.
First, data is transferred from the slave unit closest to the master unit to the master unit. At the same time, a connection is established with the child device having the shortest distance between the child devices, and data is transferred from the child device farther to the parent device to the nearest child device. Next, the slave unit that has completed data transmission is turned off. This process is repeated until there are no nodes for which data transmission has not been completed. The routing protocol of the proposed method is as follows.
(1) Each slave unit is turned on and started
(2) Send a transmission request from the master unit to the slave unit by broadcast
(3) The slave unit that receives the transmission request acquires the data
(4) The slave unit closest to the master unit sends data to the master unit. The remaining slave units establish a connection between the slave units that are the closest to each other, and send data from the slave unit farther away from the master unit to the slave unit that is close to the master unit.
(5) Turn off the power of the node that has completed data transmission
(6) Repeat steps (4) and (5) until all slave units have transferred data and turned off.

[Routing network graph and data collection procedure]
Here, the routing network graph creation procedure will be described. The routing network graph is a table = graph describing the order, direction, and transmission timing of data transmission between slave units, between master units, slave units, and between slave units. The transmission process is performed according to this graph. A routing network is defined by an effective graph G = (V, E), where V is a set of nodes, and a parent device and a child device are nodes. E is a set of branches, and branch e = {I, j} indicates that data is transmitted from node i to node j. E_i represents a set of branches to which data is transmitted in i steps. Here, in order to simplify the following discussion, symbols are defined as follows.
V: A set of {N1, N2} nodes
N2: Set of slave nodes
N1: Set of master nodes
n_0: Master node
n_j: Slave node j where j = 1,2,3…
l: Number of master nodes
m: Number of slave nodes
e = {n_j, n_i}: Data transmission from node n_i to node n_j
E_i: Set of branches at step i
E = {E_1, E_2,…}
F: Set of transmission completion nodes
G: Set of incomplete transmission nodes
Neighbor (j, G): Node closest to node j in node set G for which transmission has not been completed
far (j, G): Node farthest from node j in node set G that has not been transmitted

The routing network graph creation program is shown below.
Procedure Routing (V, E)
1: F = {n_0};
2: G = N2;
3: t = 0;
4: While (nodes belonging to G exist) {
5: / * Send to master * /
6: t = t + 1;
7: i = Neighbor (n_0, G);
/ * Find the child node closest to the parent device * /
8: Create a branch {n_0, i} and put it in the set E_t representing the processing at t time steps
9: Delete node i from unsent set G.
10: Put node i into the transmission complete set F.
11: / * Data transfer between slave units * /
12: T = G;
13: While (nodes belonging to T exist) {
14: j = far (n_0, T);
15: Delete j from T.
16: i = Neighbor (j, T)
17: Delete i from T.
18: generate a branch {I, j} and put it in a set E_t representing processing at t time steps;
19: Delete node j from unsent set G.
20: Put node j into transmission complete set F.
twenty one: }
twenty two:}

The operation of the program will be described using the example of one master unit and 11 slave units shown in FIG.
In the first step, the child device 5 closest to the parent device is selected, and a branch representing transmission to the parent device is stretched. Next, the slave unit 11 farthest from the master unit is selected, and a branch to the slave unit 7 closest to the slave unit 11 is stretched. Similarly, a branch from handset 3 to handset 2, a branch from handset 10 to handset 9, a branch from handset 8 to handset 6, and a branch from handset 4 to handset 1 are created, The first step is complete. In the second step, as in the first step, a branch from the child device 9 closest to the parent device to the parent device is created, a branch from the child device 7 to the child device 6, and a branch from the child device 2 to the child device 4. Is created. In the third step, a branch from the child device 4 to the parent device is created, and in the fourth step, a branch from the child device 6 to the parent device is created.

即ち、N(T+1)=0 となるTがデータをすべて親機に送信するのに必要なステップとなる。例えば子機が10個の例の場合、

となり、4ステップですべてのデータを親機に送信できることとなる。 Next, the data collection communication procedure will be described. Data is collected in the parent machine according to the tree structure of the graph created by the routing network creation program. Here, the operation will be described using the example of FIG.
In the first step, transmissions corresponding to branches belonging to the branch set E_1 representing the processing in the first step are transmitted from the parent device to the child device 11, from the child device 11 to the child device 7, from the child device 3 to the child device 2, and to the child device 10. To the slave unit 9, the slave unit 8 to the slave unit 6, and the slave unit 4 to the slave unit 1 are transmitted. The slave unit that has received the transmission sets the data held by itself and the transmitted data as one packet, and prepares for the next transmission.
In the second step, transmissions corresponding to the branches belonging to the branch set E_2 representing the processing in the second step, transmission from the slave unit 9 to the master unit, slave unit 7 to the slave unit 6, and transmission from the slave unit 2 to the slave unit 4 are performed. Is called.
In the third step, transmission corresponding to the branches belonging to the branch set E_3 and transmission from the child device 4 to the parent device are performed.
In the fourth step, transmission corresponding to the branches belonging to the branch set E_4 and transmission from the child device 6 to the parent device are performed.
The number of steps required for data collection is given by the following equation (Equation 7), where N (T) is the number of active slave units in the T step.

That is, T in which N (T + 1) = 0 is a necessary step for transmitting all data to the parent device. For example, in the case of 10 slave units,

Thus, all data can be sent to the main unit in 4 steps.

[Performance evaluation by simulation]
A simulation was conducted to confirm the power consumption and the time required to collect all the handset data. The simulator was created in C language. First, a method for calculating power consumption will be described. Since the main unit is always supplied with power, its power consumption is excluded from the scope of consideration and only the sub unit is calculated. Each slave unit holds a battery and has a capacity of 75,000 (mWh). The slave unit has six modes (Table 8) according to the operating state. In addition, the current consumption based on the actual power consumption report using LoRa (Keitaro Terada, Shinji Toyoda, Tadaaki Hirata, Yuya Takada, Keiko Matsumoto, Mikiko Sode) Proposed communication protocol for field management using LoRa) Assuming that Here, there are a plurality of power values at the time of data transfer because the transmission power depends on the distance, and it is set to 53 mA, 62 mA, 69 mA, and 78 mA depending on the distance and the transmission power mode that can be set in the experimental equipment.

In addition, the time required for system startup and sensor data acquisition was 60 seconds from the measured values of experimental equipment, and the time required for data transmission / reception was 3.4 seconds. In each round, the system is first activated when each slave unit reaches the activation time, and data is acquired from the sensor. Next, the master unit transmits a data transmission request to all the slave units. When the slave unit receives the data transmission request, the slave unit proceeds to the data collection phase, and acquires data from the sensor and executes a procedure for transmitting the data to the master unit. When the data transmission is completed, each slave unit is turned off for the purpose of realizing low power consumption. In this simulation, it is assumed that time synchronization is complete. Also, there was no failure in data transmission / reception.
By the way, regarding the data combining time by the slave unit, the time should be measured and the power consumption should be derived based on the time measurement. However, the data to be transmitted is arranged in a continuous area for implementation. For this reason, it is so fine that it can be included in the power consumption during transmission, and is ignored in this simulation. In the future, it should be considered when the number of slave units to be merged increases, but this time it is included in the operation at the time of transmission.
Table 9 describes the arrangement of the created master unit and slave units. Here, a specified number of slave units are randomly arranged at a specified radius with the master unit as the center. DATA1 is the base unit 1 and handset 99, and the dispersion range of the base unit and the handset is a radius of 500m. DATA2 is the master unit 1 and the slave unit 200, and the dispersion range of the master unit and the slave unit is a radius of 500 m. DATA3 is set as the master unit 1 and the slave unit 300, and the dispersion range of the master unit and the slave unit is set to a radius of 3000 m. The number of slave units and the reason for selecting the distance are the size of the area that the farmer continuously monitors in the actual field, the number of slave units required when replacing it with the slave units, and the size of the entire field. Was determined in consideration of
Comparisons were made by simulation with Direct, PEGASIS, EPEGASIS, CHION and the proposed method. The items compared were the following three items. The reason for this is that the farmer can check the paddy field data in real time, that is, the data collection time is minimum, and the battery is operated for 6 months from rice planting to rice harvesting.
(1) Time required for data collection in one round
(2) Total power consumption in one round
(3) Maximum working days

The simulation results for DATA1 are shown in Table 10 and Fig. 9. It can be seen that the proposed method has the shortest time to collect data in the parent device compared to other methods. Moreover, it turns out that power consumption and a lifetime are the smallest. As a result, it can be seen that the theoretical value can be operated for 6 months from rice planting to rice harvesting.
FIG. 9 shows the number of transmission completion nodes in each step. Since Direct transmits data directly to the parent device, data transmission to the parent device is completed one node at a time. Therefore, it can be seen that the number of transmission completion nodes increases linearly. Since PEGASIS is a mechanism that sends data in a chain, when the last chain head sends data to the parent machine, all the child machine data is sent to the parent machine. Therefore, it is understood that all data is transmitted to the parent device at the last time and the processing is completed. EPEGASIS is a mechanism in which a plurality of chains are created, data is sent along the chain to the chain head, and then each chain head sends the data to the base unit. For this reason, the data is sent to the parent machine several times, and the data transmission completion nodes increase stepwise. CHIRON is a mechanism that divides the data into multiple small areas, forms a chain in each area, collects the data in the chain head, and transmits the data to the base unit. Therefore, it can be confirmed that small data for each cluster is collected and sent to the master unit.
On the other hand, in the proposed method, data for one slave unit is first sent to the master unit, then data for two slave units is sent to the master unit, and then data for four slave units is sent. In this way, the number of data sent to the base unit increases by a factor of two each time. For this reason, it can be confirmed that data is collected rapidly. As a result, it can be confirmed that the proposed method can collect data at the highest speed.

Table 11 shows the simulation results for DATA2. It can be seen that the proposed method has the shortest time to collect data in the parent device compared to other methods. Moreover, it turns out that power consumption and a lifetime are the smallest. In addition, the theoretical value shows that it can operate for 6 months from rice planting to rice harvesting. The proposed method has double the number of handsets compared to DATA1, but the increase in the time required for collection is as short as 3 seconds.
Table 12 shows the simulation results for DATA3. It can be seen that the proposed method has the shortest time to collect data in the parent device compared to other methods. Moreover, it turns out that power consumption and a lifetime are the smallest. In addition, the theoretical value shows that it can operate for 6 months from rice planting to rice harvesting. In addition, the number of handsets is three times that of DATA1, but the increase in time required for collection is as short as 6 seconds.

Fig. 10 shows the operating time in one round. With Direct and PEGASIS, as the number of data increases, the transmission time of data to the base unit increases and the operation time increases rapidly.
FIG. 11 shows EPEGASIS, CHIRON, and the proposed method extracted from FIG. From this figure, it can be seen that the proposed method has the smallest increase rate of time required for data collection with respect to the increase in the number of data, and can collect data in a short time even if the number of slave units increases. It can also be seen that the growth rate is 3% of the number of nodes. For this reason, it is considered the most suitable method for the assumed field management system.

[Summary]
Unlike other fields, paddy fields are difficult to supply with power, and it is also difficult to install large power generators that hinder farm work. Therefore, the field server has to operate with a battery or the like, and there is a problem in reducing power consumption. In addition, the line usage fee for communication must be reduced as much as possible in view of the total cost required for agriculture. We studied low power consumption and long distance transmission using LoRa using ISM band, which is a communication standard for IoT.
We proposed a routing method that enables data to be collected in a short time between the slave unit and the master unit, which are sensor nodes that operate for 6 months from rice planting to harvesting in a power-free environment. The proposed method has the feature of shortening the time required for data collection by performing merge processing and transmission of data between each slave unit that holds the data acquired from the sensor, and between the slave unit and the master unit. As a result of comparison with the method, it was confirmed that the collection time for collecting the data of the slave unit in the master unit was the shortest. Moreover, it has confirmed that power consumption was also small compared with the conventional method. Furthermore, it was confirmed that the rate of time required for data collection to the parent unit due to the increase in the number of slave units was small compared to other methods, and it was an effective method.
This proposal to enable timely transmission of the state of rice farming is a useful technique for reducing the burden of farm work. In the future, we plan to proceed with simulations and field experiments in order to obtain optimal solutions for the field by increasing / decreasing parameters such as the reception capability of the master unit, the data routing method between slave units, and the number of data aggregates. It is.

The present invention is a time synchronization method and a data collection method of a wireless communication system that can be operated with low power consumption, and has industrial applicability.

Claims (7)

In a time synchronization method of a wireless communication system including a parent device and a plurality of child devices connected to sensors,
The slave unit is activated based on its own internal clock and simultaneously switches from the sleep mode to the transmission mode, and obtains sensor data;
A second step in which the slave unit transmits the sensor data to the master unit and then switches to a reception mode for a certain period of time;
When the master unit receives the sensor data, a third step of transmitting a time correction signal based on real time to the slave unit;
When the slave unit receives the time correction signal during the reception mode, a fourth step of correcting the internal clock and switching to the sleep mode;
When the slave unit does not receive the time correction signal during the reception mode, the reception mode is switched to the transmission mode, the sensor data is retransmitted to the master unit, and at least a fifth step of switching to the reception mode is performed. A time synchronization method for a wireless communication system, comprising:   2. The radio according to claim 1, wherein, in the second step, when the start time of another slave unit is reached during the reception mode of the slave unit, the slave unit in the reception mode is switched to the sleep mode. A time synchronization method for a communication system.   The wireless communication system according to claim 1 or 2, wherein, in the fifth step, a slave unit that has not received the time correction signal retransmits the sensor data to the master unit after a lapse of a random time. Time synchronization method. When a slave unit is newly added to the wireless communication system,
A step of transmitting a signal for requesting the time correction signal from the added slave unit to the master unit;
When the master unit receives the request signal, transmitting the time correction signal to the added slave unit;
The wireless communication system according to any one of claims 1 to 3, further comprising a step in which the added slave unit receives the time correction signal, corrects the internal clock, and switches to a sleep mode. Time synchronization method. In a data collection method of a wireless communication system comprising a parent device and a plurality of child devices connected to sensors,
The first step in which the child device closest to the parent device among the plurality of child devices transfers the sensor data to the parent device and the child device after the sensor data transfer is switched to the sleep mode or the power off,
At the same time as Step 1-1, a connection is established between the two slave units that are closest to each other among the slave units other than the slave unit closest to the master unit, and the parent of the two slave units that have established the connection is established. The first and second steps in which the slave unit farther to the machine transfers the sensor data to the closer slave unit and the slave unit after the sensor data transfer is switched to sleep mode or power off,
The slave unit closest to the master unit among the plurality of slave units that have not been switched to the sleep mode or power off transfers the sensor data to the master unit, and the slave unit after the sensor data transfer is switched to the sleep mode or power off. −1 step,
Simultaneously with Step 2-1, a connection is established between the two slave units that are closest to each other among the plurality of slave units that are not switched to the sleep mode or power off, and the parent of the two slave units that have established the connection is established. The remote unit that is far from the machine transfers the sensor data to the nearby slave unit, and the slave unit after the sensor data transfer includes a step 2-2 for switching to the sleep mode or turning off the power.
Thereafter, the data collection method of the wireless communication system, wherein all the slave units transfer the sensor data to the master unit and repeat the steps 2-1 and 2-2 until the sleep mode or the power is turned off. . In a data collection method of a wireless communication system comprising a parent device and a plurality of child devices connected to sensors,
A first step of acquiring the sensor data by switching from the sleep mode or the power-off to the transmission mode at the same time when the plurality of slave units is activated based on its own internal clock;
Of the plurality of slave units, the slave unit A closest to the master unit transfers the sensor data to the master unit, and then the master unit transmits a time correction signal based on real time to the slave unit A, and then The slave unit A that has received the time correction signal corrects its own internal clock and is switched to the sleep mode or the power-off step 2-1;
At the same time as step 2-1, a connection is established between the two child devices closest to each other among the plurality of child devices other than the child device A, and the two child devices that have established the connection are far from the parent device. The remote handset B1 transfers the sensor data to the close handset B2, and then the close handset B2 transmits a time correction signal based on its internal clock to the far handset B1. Next, the far-end slave unit B1 that has received the time correction signal corrects its own internal clock and is switched to the sleep mode or the power-off step 2-2,
Of the plurality of slave units that have not been switched to the sleep mode or power off, the slave unit C closest to the master unit transfers the sensor data to the master unit, and then the master unit is based on the real time for the slave unit C. Step 3-1 for transmitting a time correction signal and then receiving the time correction signal, the slave unit C correcting its own internal clock and switching to sleep mode or power off;
Simultaneously with Step 3-1, a connection is established between the two slave units that are closest to each other among the plurality of slave units that are not switched to the sleep mode or power off, and the parent of the two slave units that have established the connection is established. The remote slave D1 transfers the sensor data to the remote slave D2, and then the remote slave D2 sends a time correction signal based on its internal clock to the remote slave D1. The remote handset D1 that has received the time correction signal and then corrected the internal clock of the remote handset D1 and is switched to the sleep mode or the power-off step 3-2.
Thereafter, the data collection method of the wireless communication system, wherein all the slave units transfer the sensor data to the master unit and repeat the steps 3-1 and 3-2 until the sleep mode or the power is turned off. . In a data collection method of a wireless communication system comprising a parent device and a plurality of child devices connected to sensors,
A first step of acquiring the sensor data by switching from the sleep mode or the power-off to the transmission mode at the same time when the plurality of slave units is activated based on its own internal clock;
Of the plurality of slave units, the slave unit A closest to the master unit transfers the sensor data to the master unit, and then the master unit transmits a time correction signal based on real time to the slave unit A, and then The slave unit A that has received the time correction signal corrects its own internal clock and is switched to the sleep mode or the power-off step 2-1;
At the same time as step 2-1, a connection is established between two child devices out of the plurality of child devices other than the child device A, and one child device B1 of the two child devices that have established the connection is connected to the other child device. The sensor data is transferred to the slave unit B2, then the slave unit B2 transmits a time correction signal based on its own internal clock to the slave unit B1, and then the slave unit B1 that has received the time correction signal 2-2 step of correcting its own internal clock and switching to sleep mode or power off;
Of the plurality of slave units that have not been switched to the sleep mode or power off, the slave unit C closest to the master unit transfers the sensor data to the master unit, and then the master unit is based on the real time for the slave unit C. Step 3-1 for transmitting a time correction signal and then receiving the time correction signal, the slave unit C correcting its own internal clock and switching to sleep mode or power off;
Simultaneously with Step 3-1, a connection is established between two slave units among a plurality of slave units that have not been switched to the sleep mode or power off, and one of the two slave units that have established the connection D1 Transfers the sensor data to the other slave unit D2, and then the slave unit D2 transmits a time correction signal based on its own internal clock to the slave unit D1, and then receives the time correction signal. The machine D1 has its own internal clock and has a third and second steps for switching to sleep mode or power off.
Thereafter, the data collection method of the wireless communication system, wherein all the slave units transfer the sensor data to the master unit and repeat the steps 3-1 and 3-2 until the sleep mode or the power is turned off. .

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