ADI’s RF transceiver chips provide strong support for ETC roadside module development
Latest update time：2021-07-06
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Influenced by government policies, the application of electronic road toll collection (ETC) has exploded in China. In 2019 alone, the number of cars equipped with on-board units (OBUs) soared from 80 million to 200 million. As the number of OBU installations surged, the application scope of ETC also expanded from highways to cities, such as parking charges and vehicle information collection. Currently, parking charges and vehicle information collection systems are based on cameras, so if ETC applications are to be promoted in urban environments, ETC roadside units (RSUs) need to be integrated into the camera system. This article introduces an ETC RSU module solution using the AD9361 radio frequency (RF) transceiver. The target application of this solution is China's ETC RSU, so it must comply with China's GB/T 20851-2019 "Electronic Toll Collection—Dedicated Short-Range Communication" standard. 1 The module is compact and small, only 11 cm × 6 cm, and can be easily integrated into the camera system. In addition, this ETC RSU module can also be configured as a simple RF instrument for testing ETC RSU modules in customer production lines. Customers do not need to use expensive RF instruments in the production line, which can greatly save costs.
Standard Summary
According to the GB/T 20851-2019 standard, the requirements for the ETC RSU physical layer are summarized as shown in Tables 1 and 2.
Table 1. Summary of key requirements for the physical layer of the ETC RSU downlink
* Far from the carrier frequency 2.5 times the carrier bandwidth
Table 2. Summary of key requirements for the ETC RSU uplink physical layer
ETC RSU Solution Based on AD9361
The block diagram of the AD9361-based ETC RSU module is shown in Figure 1. The front and rear views are shown in Figure 2.
Figure 1. ETC RSU module block diagram.
Figure 2. ETC RSU module front and rear views.
The AD9361 from Analog Devices is a highly integrated RF transceiver that can be configured to address a wide range of applications. It integrates the RF, mixed-signal, and digital blocks required to provide all transceiver functions in a single device.
It is worth noting that the AD9361 has several features that are well suited for ETC RSU applications.
First, the AD9361 integrates programmable multiphase FIR filters in both the transmit and receive paths, which means that all digital domain filtering can be done inside the AD9361 instead of in the FPGA. This saves a lot of FPGA resources and allows the use of lower-cost FPGAs. For example, to meet the receive bandwidth requirements, customers can use the Filter Wizard tool provided by ADI to design and adjust the FIR filter response, and then download the filter coefficients to the AD9361.
Secondly, the AD9361 has a DCXO function, which means that the capacitor integrated in the AD9361 can be used as an external crystal. The ability of the AD9361 to tune the capacitor means that the AD9361 can accurately control the frequency of the external crystal. In a general implementation, customers can meet the RF frequency tolerance requirements by adjusting the RF PLL N divider, but whether the bit rate accuracy requirements can be met depends on the performance of the crystal, which cannot be adjusted. For solutions based on the AD9361, customers can use the DCXO function to adjust the crystal frequency to meet both the bit rate accuracy and RF frequency tolerance requirements. We can make a lookup table to compensate for the crystal frequency temperature drift to ensure that the ETC RSU module can meet the frequency tolerance and bit rate accuracy requirements over the entire operating temperature range.
Third, the AD9361 implements the receiver AGC function. It provides two modes: slow AGC and fast AGC. It is fully automatic and the customer does not need to provide any receiver gain control function in the FPGA at all. The fast AGC mode is very useful in ETC RSU applications and has been tested so that the gain adjustment can be stabilized when the uplink pilot signal transmission starts.
In the transmit path, the transmitter digital baseband signal is first sent to the AD9361 by the FPGA. The digital baseband signal is then filtered and interpolated in the AD9361 (from 10.24 MSPS to 163.84 MSPS). The DAC then converts the digital baseband signal to an analog baseband signal and performs a low-pass filter. Finally, an RF signal of 5.83 GHz (channel 1) or 5.84 GHz (channel 2) is obtained by up-conversion. In the transmitter RF domain, the AD9361 integrates an attenuator that can control the transmitter output power within a range of more than 80 dB. The attenuator can be used to adjust the output power level of the transmitter and the gain temperature compensation of the entire transmitter chain. After that, the transmitter signal is fed into the power amplifier (PA) inside the front-end module (FEM), and after further amplification, it flows through a microstrip low-pass filter (LPF) to filter out harmonics to meet the spurious emission requirements of the transmitter, and finally, the signal is fed to the antenna. In our ETC RSU module design, the transmitter attenuation of AD9361 is set to 8 dB, and the output power at the antenna port can reach 29 dBm, which means that AD9361 has enough attenuation dynamic range to compensate for the high temperature gain decrease and low temperature gain increase.
In the receive path, the RF signal from the antenna first passes through a low-pass filter, then feeds into a low-noise amplifier (LNA) inside the FEM, and then passes through a band-pass filter (BPF) to filter out out-of-band interference signals. In the AD9361 receive path, the signal is further amplified and then down-converted to an analog baseband signal. The analog baseband signal is low-pass filtered and then converted to a digital baseband signal by the ADC. In the digital domain, the signal is filtered to meet the receive bandwidth requirements and then decimated (from 163.84 MSPS to 10.24 MSPS). The AD9361 then sends the signal to the FPGA.
For the power solution, the input voltage of the module is 5 V. The ADP5014 contains four high-performance, low-noise step-down regulators. It converts 5 V to 3.3 V, 2.5 V, 1.8 V, and 1.3 V. The 5 V input and the four output voltages of the ADP5014 provide all the voltage rails required for the FEM, AD9361, FPGA, and MCU.
Transmitter Test Results
We tested the AD9361-based ETC RSU module according to all transmitter test cases defined in the standard and all passed with good margins. Screenshots of several key test cases are shown in Figures 3 to 6.
Figure 3. 29 dBm output power
Figure 4. 90% modulation depth
Figure 5. –50 dBc ACLR
Figure 6. 3.4 MHz occupied bandwidth
Receiver Test Results
We tested the ETC RSU module based on AD9361 according to all the receiver test items defined in the standard, and finally passed all the tests with good margins. In order to perform the receiver sensitivity test, the FM0 encoded and ASK modulated signals were downloaded to the signal generator, and the demodulation algorithm was implemented in the FPGA.
The measured receiving sensitivity of our RSU module is –95 dBm, which is much better than the required –70 dBm. Figure 7 is the I/Q data FFT diagram and I/Q data amplitude diagram when the input signal is –95 dBm. The figure shows that when the input signal level is –95 dBm, the signal SNR is still very good.
Figure 7. I/Q data FFT plot (top) and I/Q data amplitude plot (bottom) when the input signal level is –95 dBm.
For other test cases such as maximum input power, receive bandwidth, in-channel interference rejection, adjacent channel interference rejection, and blocking rejection, the module passed all tests.
Simple RF Instrumentation Solution
This ETC RSU module can be configured as a simple RF instrument to test the ETC RSU modules and antenna modules on the customer's production line.
The AD9361 has two RF channels. One channel is used to implement the ETC RSU module and the other is an RF channel used with the on-board highly directive microstrip coupler to perform return loss testing.
When testing the module transmitter, we used the AD9361 received signal strength indicator (RSSI) function. The RSSI function of the AD9361 has a calibrated accuracy of 0.25 dB, which is sufficient to test the output power of the ETC RSU module.
For module receiver testing, the output power of the AD9361 can be calibrated at one or two power levels. The AD9361 internal attenuator can then be used to provide a variety of accurate output powers for testing the receiver.
in conclusion
A compact ETC RSU module can be designed using the AD9361, and ADI provides a complete reference design including hardware and firmware. The module can be easily integrated into a camera system or used alone as a standard ETC RSU module. The module meets the ETC RSU requirements defined in the GB/T 20851-2019 standard. In addition, it can be configured as a simple RF instrument for use in the customer's production line.
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