# Millimeter-Wave Multi-Channel Backscatter Communication and Ranging with an FMCW Radar
## 7 Measurement Result and Discussion
### Single Tag Test  
The measurement setup for the single tag test is shown in Figure 14.  
Figure 12: The S parameter measurement result of the transition including two bonding wires and a U-slot.  
Figure 13: The measurement result of the conversion gain with different frequencies of the self-packaged tag.  
The LMF signal is generated from a signal generator (Agilent E8257D, Santa Clara, CA, USA) with a frequency between 12.533 GHz and 13.033 GHz. The chirp rate is 0.1 Hz. An E-band six times multiplier (AT Microwave AT-AM6-7186-12, Shanghai, China) is connected after the signal generator to generate the FMCW signal with a frequency from 75.2 GHz to 78.2 GHz. The FMCW signal is sent out through an antenna over the air. At the tag side, the FMCW is first amplified by a W-band power amplifier (QuinStar QPI-W01128, Torrance, CA, USA). An arbitrary wave generator (AWG, Keysight M8195A, Santa Rosa, CA, USA) is used to generate the baseband I/Q signal. The amplified FMCW signal is mixed with the baseband I/Q signal by the self-packaged RF tag. The RF signal is sent back to the radar side. On the radar side, an E-band receiver module (Gotmic gRSC0015, Gothenburg, Sweden) [27] is used to down-convert the RF signal. The module comprises a six times multiplier and a mixer. The FMCW signal is generated inside the receiver module by the multiplier and mixed with the received RF signal. The down-converted signal is sampled by an oscilloscope (Teledyne LabMater 10 Zi-A, Thousand Oaks, CA, USA) for further post-processing. The EVM and BER are estimated by the oscilloscope. The oscilloscope and the AWG are not synchronized and running freely. The measurement distance between antennas from the radar side to the tag side is around 65 cm.  
Signals with different modulation schemes have been transmitted over this backscatter communication system. The highest transmitted data rate is 8 Gbps with QPSK or 16-QAM. The highest tested modulation order signal is a 32-QAM signal with a data rate of 2.5 Gbps. Constellation diagrams of different modulation signals with their highest data rate are shown in Table 2. Their estimated EVM with different symbol rates are shown in Figure 15. Their BERs are all below 2 \(\times\) 10\({}^{-4}\). These measurement results show that the self-package tag is capable of transmitting a large bandwidth signal of 8 GHz and high modulation order signal of 32-QAM.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Constellation**} & \multirow{2}{*}{QPSK} & \multirow{2}{*}{8-PSK} & \multirow{2}{*}{16-QAM} & \multirow{2}{*}{32-QAM} \\  & & & & \\ \hline Modulation & QPSK & 8-PSK & 16-QAM & 32-QAM \\ Symbol rate & 4 Gbaud & 2 Gbaud & 2 Gbaud & 500 Mbaud \\ Bit rate & 8 Gbps & 6 Gbps & 8 Gbps & 2.5 Gbps \\ Bit error rate & 9.9 \(\times\) 10\({}^{-6}\) & 1.5 \(\times\) 10\({}^{-4}\) & 1.93 \(\times\) 10\({}^{-5}\) & 1.3 \(\times\) 10\({}^{-4}\) \\ EVM & 23.44\% & 12.51\% & 11.04\% & 8.43\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Constellation diagrams of different modulation order signals with their highest achieved data rate of single tag measurement.  
Figure 14: The measurement setup for a single tag test.