# Millimeter-Wave Multi-Channel Backscatter Communication and Ranging with an FMCW Radar
## 2 Smart Traffic Infrastructure  
We consider a smart traffic system where the traffic infrastructure is equipped with passive tags, and vehicles are equipped with automotive radars. An intersection with traffic lights is a potential application scenario, as Figure 1 shows. Cameras attached to traffic lights are widely installed nowadays. Pictures and videos taken by cameras are currently mostly used for monitoring and can only be retrieved by the relevant government departments. With a radar-communication system, important traffic news, such as accident reports or temporary road obstacle locations, can be transmitted wirelessly directly to vehicles. In that case, drivers can re-plan their route in advance and avoid traffic congestion. Moreover, under extreme weather conditions, such as heavy rain and snow, the traffic signs and traffic lights are difficult to see. If the traffic information can be transmitted to vehicles wirelessly and displayed locally to the driver, fewer accidents occur. For these purposes, we require an information broadcast system. A backscatter communication system is suitable for this application. Traffic signs and lights carry certain information, which is perfect to use a tag to broadcast. In an intersection scenario, there are normally multiple signs, traffic lights, and cameras installed, so multiple tags are needed. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Single Tag** & \multirow{2}{*}{**Modulation**} & **Dual-Tag** & **RF Signal** & **RF Freq.** & **Radar** \\  & **Data Rate** & & **Data Rate** & **Type** & **(GHz)** & **Sensing** \\ \hline
[6] & 1 kbps & AM & – & CW & 60 & no \\
[7] & 1 kbps & OOK/FSK & – & CW & 0.867 & no \\
[8] & 336 kbps & Square wave & – & CW & 77 & no \\
[9] & 400 kbps & QPSK & – & CW & 0.915 & no \\
[10] & 96 Mbps & 16-QAM & – & CW & 0.915 & no \\
[11] & 2.5 Mbps & 32-QAM & – & CW & 5.8 & no \\
[14] & 37.5 Mbps & OOK & – & FMCW & 34.3–34.8 & yes \\
[16] & – & BPSK & 10 kbps & FMCW & 2.43–2.47 & yes \\ \multirow{2}{*}{This} & 2.5 Gbps/ & 32-QAM/ & 625 Mbps & \multirow{2}{*}{FMCW} & \multirow{2}{*}{75.2–78.2} & \multirow{2}{*}{yes} \\  & 8 Gbps & QPSK & (BPSK) & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Recently reported backscatter communications. Radars are prevalent in modern vehicles and are used to detect the surrounding obstacles, road users, or other vehicles. FMCW radars are the most commonly used vehicle radars for their ability to detect both the position and Doppler velocity of the surrounding target [19; 20]. If infrastructures are equipped with tags and vehicles have the FMCW radar installed, communication and radar sensing can be realized simultaneously. The FMCW radar chips can be reused in this radar-communication system. Only the signal processing requires an update. In this paper, a multi-channel backscatter communication with an FMCW radar system is presented. A novel package solution for the tag is proposed. The radar-communication system is tested with commercial E-band frontend modules. The single tag measurement demonstrates the highest transmission data rate of 8 Gbps and the highest modulation order signal of 32-QAM which shows the remarkable performance of the self-packaged tag. The frequency offset between the transmitter and the receiver is estimated for range measurement. In multi-tag measurement, two tags are used in the proposed radar-communication system. 625 Mbps BPSK signals are successfully transmitted with both tags. Table 1 compares the backscatter communication performance of related works. The paper is organized as follows. In Section 3, the principle of backscatter communication with FMCW is presented. Then, the multi-tag message separation is introduced in Section 4. In Section 5, the signal processing framework of the proposed system is introduced. The novel package solution for the tag is presented in Section 6, and its performance test results are also given in this section. The system measurement of the proposed system is presented in Section 7.