Application of short-range wireless communication in automotive RFID system design

This system is a wireless identification system based on digital communication principles and built using an integrated single-chip narrowband UHF transceiver. The basic working principle and hardware design ideas of the wireless radio frequency identification system are explained, and a flowchart of the program design scheme is given. The radio frequency identification tag suitable for vehicle-mounted is designed from the perspective of low power consumption, efficient identification and practicality. The test results show that this system can achieve effective identification within a range of 300m under complex road conditions (busy roads) and can achieve effective identification within a range of 500m under line-of-sight conditions.

The Internet of Things refers to a huge network formed by combining various information sensing devices, such as sensors, radio frequency identification (RFID) technology, global positioning systems, infrared sensors, laser scanners, gas sensors, and other devices and technologies, to collect any objects or processes that need to be monitored, connected, and interacted in real time, and collect various required information such as sound, light, electricity, biology, and location, and combine with the Internet. Its purpose is to achieve the connection between objects and objects, objects and people, and all objects and networks, so as to facilitate identification, management and control. This project focuses on the key issues of data collection, transmission and application in the vehicle-mounted Internet of Things, and designs a new generation of vehicle-mounted radio frequency identification system based on short-range wireless radio frequency communication technology. The system consists of a short-range wireless communication on-board unit (On-Board Unit, OBU) and a base station system (Base Station System, BSS) to form a point-to-multipoint wireless identification system (Wireless identification system, WIS), which can be used for vehicle identification and intelligent guidance within the coverage of the base station.

1 System Hardware Design

The system hardware is mainly composed of the control part, the radio frequency part and the external expansion application part. With low-power MCU as the control unit, it integrates a single-chip narrowband UHF transceiver and a built-in optimized antenna. It uses advanced photovoltaic cells for power supply and implements a highly integrated short-range wireless identification radio frequency terminal (OBU). This terminal is small in size, low in power consumption, and has a wide range of application. It also establishes open protocols and standard interfaces, making it easy to connect with existing systems or other systems.

The system operation diagram is shown in Figure 1.

1.1 Control circuit design

The control unit uses the MSP430 series produced by TI, which has a relatively mature low-power application in the industry. This series is a 16-bit ultra-low-power mixed signal processor (Mired Signal Processor) that TI has launched on the market since 1996. It integrates many analog circuits, digital circuits and microprocessors on one chip to provide a "single-chip" solution for practical application needs. In the WIS system, the working principles of OBU and BSS are the same, so the design of the OBU part is mainly introduced, and the schematic diagram of its control part is shown in Figure 2.

The input voltage of MSP430F2274 is 1.8~3.6V. When running at 1 MHz clock, the power consumption of the chip is about 200~400μA, and the minimum power consumption in clock shutdown mode is only 0.1μA. Due to the different functional modules turned on when the system is running, three different working modes of standby, running and sleep are adopted, which effectively reduces the power consumption of the system.

The system uses two clock systems; the basic clock system and the digital oscillator clock system (Digitally Controlled Oscillator, DCO), which uses an external crystal oscillator (32 768Hz). After power-on reset, the MCU (Microprogrammed Control Unit) is first started by DCOCLK to ensure that the program starts executing from the correct position and that the crystal oscillator has enough start-up and stabilization time. Then the software can set the control bits of the appropriate registers to determine the final system clock frequency. If the crystal oscillator fails when used as the MCU clock MCLK, the DCO will automatically start to ensure the normal operation of the system; if the program runs away, it can be reset by the watchdog. This design uses the on-chip peripheral module watchdog (WDT), analog comparator A, timer A (Timer_A), timer B (Timer_B), serial port USART, hardware multiplier, 10-bit/12-bit ADC, SPI bus, etc.

1.2 RF Circuit

The RF part uses TI's CC1020 as the RF control unit. This chip is the industry's first true single-chip narrowband UHF transceiver, with three modulation modes: FSK/GFSK/OOK, and a minimum channel spacing of 50 kHz. It can meet the strict requirements of multi-channel narrowband applications (402-470 MHz and 804-940 MHz bands), and multiple working frequency bands can be switched freely. The working voltage is 2.3-3.6 V, which is very suitable for integration and expansion into mobile devices as wireless data transmission or electronic tags. The chip complies with EN300 220, ARIB STD-T67 and FCC CFR47 part15 specifications.

The carrier frequency of 430 MHz is selected as the working frequency band. This frequency band is the ISM frequency band, which meets the standards of the National Wireless Management Committee and does not require frequency application. The FSK modulation method is adopted, which has high anti-interference ability and low bit error rate. The forward error correction channel coding technology is adopted to improve the data's ability to resist burst interference and random interference. When the channel bit error rate is 10-2, the actual bit error rate can be 10-5~10-6. Under open line-of-sight conditions, a baud rate of 2A Kbs, and a large suction cup antenna (length 2m, gain 7.8 dB, 2m above the ground), the data transmission distance can reach 800m. The standard configuration of this RF chip can provide 8 channels to meet a variety of communication combinations. Due to the use of narrowband communication technology, communication stability and anti-interference are enhanced. The schematic diagram of the RF part is shown in Figure 3.

1.3 System power supply

The power supply of the system is a combination of photovoltaic cells as daily power supply and lithium-ion batteries as backup batteries. Under good light conditions, the energy storage battery is charged by solar energy. A certain amount of light time every day can basically meet the daily work needs of the OBU, greatly extending the service life of the backup battery and the working life of the OBU. It is suitable for vehicles that often operate outdoors and can collect sufficient sunlight for the photovoltaic battery to work.

1.4 System Development Environment

The system development environment is as follows: 1) IAR Embedded Workbench for MSP430 compiler; 2) PADS PCB Design Solutions 2007 Biss circuit board design tool.

2 System Programming

The program adopts modular design and is written in C language. It is mainly composed of 4 parts: main program module, communication program module, peripheral circuit processing module, interrupt and storage module. The main program mainly completes the initialization of the control unit, the configuration of various parameters, and the configuration and initialization of each peripheral module; the communication program module mainly handles the configuration of the RF chip and the 433 MHz transceiver processing; the peripheral circuit processing module mainly handles the system external LED indication, voltage detection, sound prompts, buttons and other processing; the interrupt and storage module mainly handles system interrupts and record storage. The main program flow is shown in Figure 4.

3 RF Communication Process

The communication process between OBU and BSS is divided into three steps: establishing link, exchanging information and releasing link, as shown in Figure 5.

Step 1: Establish a connection. The coordinate information of the OBU's location and its ID code are stored in the Flash of the control unit MCU through preset parameters and are stored for a long time. The BSS (base station system) uses the downlink to cyclically broadcast positioning (base station identification frame control) information to the OBU, determine the frame structure synchronization information and data link control information, and after the OBU entering the effective communication area is activated, it requests to establish a connection and confirm the validity and sends a response message to the corresponding OBU, otherwise it will not respond;

Step 2: Information exchange This design uses the method of detecting the strength of the RF signal to determine whether the OBU has entered the service area. When the detected signal strength is greater than 1/2 of the maximum signal, the sender and receiver implement a wireless handshake, and the OBU is considered to have entered the service area. In this stage, all frames must carry the private link identification of the OBU and implement error control. The ID number can be used to determine whether the OBU belongs to the same system. OBUs with ID numbers that are not in the same system are automatically deleted from the record. When the OBU reports information, the frequency hopping mechanism is used to randomly select a fixed channel in the service area for handshake communication to prevent channel congestion.

Step 3: Release the connection. When the detection signal strength is less than 1/2 of the maximum strength, the vehicle is considered to have left the station. After the RSU and OBU complete all applications, they delete the link identifier and issue a dedicated communication link release command. The connection release timer releases the connection based on the application service confirmation.

4 Development of OBU and BSS communication process

The communication protocol establishes a simple three-layer protocol structure based on the seven-layer protocol model of the Open Systems Interconnection architecture, namely the physical layer, data link layer and application layer.

1) Physical layer The physical layer is mainly a communication signal standard. Since there is no unified standard for 433 MHz short-range wireless communication in the world, the physical layers defined by various standards are also different, as shown in Table 1. Figure 6 shows the Manchester encoding method.

2) Data Link Layer The data link layer controls the information exchange process between the OBU and the BSS, and specifies the establishment and release of the data link connection, the definition and frame synchronization of data frames, the control of frame data transmission, fault tolerance control, data link layer control, and the parameter exchange of link connections. Data transmission is carried out in the form of data frame transmission, as shown in Figure 7.

3) Application layer The application layer formulates standard user function programs, defines the format of communication messages between applications, and provides an open message interface for other databases or applications to call.

5 Conclusion

The RFID system designed in this paper uses the MSP430 microcontroller of TI's low-power series, which is specially designed by TI for low-power battery-powered devices. The RF chip is also TI's CC1020, which has high integration, small size, low power consumption, and easy installation. It is suitable for the construction of vehicle parking-free monitoring and surveillance systems. The test results show that in complex road conditions (busy roads), effective recognition can be achieved within a range of 300 m, and in line-of-sight conditions, recognition can be achieved within a range of 500 m.