Short Range Wireless
Developing an electronic toll collection roadside unit module with an RF transceiver
By Tairan Sun, product applications engineer, Bill Wang, system applications engineer, Shawn Liu, applications engineer, and Aaron He, system applications engineer at Analog Devices (ADI)
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n 2019, electronic toll collection (ETC) applications saw an explosion of growth in China due to government policy. The number of cars with on- board units (OBUs) installed increased from 80million to 200million in just one year. Along with this OBU installation boom, the use of ETC applications was extended from expressway to city for things such as parking charges and vehicle information collection. Currently, the parking charge and vehicle information collection systems are camera based, so for ETC applications in city settings, it is desirable to integrate the ETC roadside unit (RSU) into the camera system. This article describes
an ETC RSU module implementation using the AD9361 radio frequency (RF) transceiver. Its target applications are China’s ETC RSUs, which means it must comply with the Chinese standard GB/T 20851-2019 “Electronic Toll Collection-Dedicated Short Range Communication.”The module is of quite compact size, 11cm × 6cm, so it is easy to integrate inside the camera system. What’s more, this ETC RSU module can also be configured as a simple RF instrument, which could be used to test the ETC RSU module
Table 1. ETC RSU Downlink Physical Layer Key Requirement Summary
in a customer’s production line. Customers would not need an expensive RF instrument in their production line, resulting in huge cost savings.
Standard summary
According to the GB/T 20851-2019 standard, the ETC RSU physical layer requirement is summarised in Table 1 and Table 2.
Table 2. ETC RSU Uplink Physical Layer Key Requirement Summary
AD9361-based ETC RSU solution A block diagram for an AD9361-based ETC RSU mod- ule is shown as Figure 1 (Top of next page). The front view and back view are shown as Figure 2.
Figure 2. ETC RSU module front view and back view.
Analog Devices’ AD9361 is a highly integrated RF transceiver capable of being configured for a wide range of applications. The device integrates all RF, mixed-signal, and digital blocks necessary to provide all transceiver functions in a single device. It is worth highlighting that the AD9361 has several features that are very useful in ETC RSU applications.
*2.5 times carrier bandwidth away from carrier frequency 42 November 2021 Components in Electronics
Firstly, in both the transmit path and the receive path, the AD9361 integrates programmable polyphase FIR filters, which means that all digital domain filtering can be done inside the AD9361, rather than in the FPGA. This saves a lot of FPGA resources, and a lower cost FPGA can be chosen. For example, to meet the receiving bandwidth requirement, a customer could use the Filter Wizard tool provided by Analog Devices to design and tune the FIR filter response, and then get the filter coefficient to download into the AD9361. Secondly, the AD9361 has the DCXO feature, which means that the AD9361 integrates the capacitor for the external crystal. The AD9361 is capable of tuning the capacitor, which means that the AD9361 can control the external crystal frequency very precisely. In general implementation, the customer adjusts the
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