# Millimeter-Wave Multi-Channel Backscatter Communication and Ranging with an FMCW Radar
## 6 Tag Hardware and Test Result  
A package solution for the tag is presented in this section. A commercial E-band fundamental quadrature mixer MMIC (Gotmic gMR0012, Gothenburg, Sweden) [21] is packaged in a waveguide interfaced block as a tag in the proposed radar-communication system. The LO port is used as a tag input port, the RF port is used for modulated signal output, and baseband IQ ports are used to control theRF port phase-shifting regarding input LO signal. The photo of the packaged module is shown in Figure 10. On the back side of the module, two waveguide ports of WR-8 interfaces are used for signal input and output. 20 dBi waveguide interfaced antennas are connected to the tag during measurement. On the side of the module, differential quadrature IQ input port are provided by coaxial interfaces. On the front side of the module, a 200 um thick metal carrier board with etched slots are used for carrying MMIC and the U-shape slots are used for MMIC to waveguide transition. Also, a printed circuit board (PCB) is used for IQ signal connections to the MMIC.  
Figure 8: EVM VS. chip length with different SNR of 5 Gbps chip rate signal in two tags scenario.  
Figure 9: Frequency estimation performance with different lengths of signature code.  
Different packaging solutions have been proposed, and many of them require a PCB. PCB introduces big loss at high frequencies, which is desired to be avoided in high frequency packaging solutions [22; 23; 24; 25]. A novel substrate-less packaging solution is proposed in the design of the tag module. The LO and RF ports are directly wire bonded to the packaging block without using any dielectric substrate to reduce cost and avoid high electric loss at these frequencies. The packaging concept is illustrated in Figure 11. A ridge waveguide is used at the interface, whose upper wall is cut open with a U-shape slot. The upper wall (green part) is 200 um thick metal carrier which also holds MMIC on it. The ridge (yellow part) is manufactured by milling on a metal block (backside of the module, invisible in this figure). The MMIC is attached above this upper wall, and the signal PAD on the MMIC is connected to the upper wall with two bonding wires across the U-slot. These bond wires act as radiation probes, and the U-slot provides a coupling between the bond wire and the ridge waveguide. The length of this slot defines the coupling frequency. From the ridge waveguide to standard rectangular waveguide, a transition as described in [26] is used. This transition's S parameter measurement result is presented in Figure 12, where port 1 is the lumped port on the MMIC and port 2 is the wave port at the ridge waveguide. The measurement exhibits insertion-loss less than 2 dB and return-loss less than \(-10\) dB between 70-85 GHz.  
Figure 11: An illustration of the packaging concept.  
Figure 10: A picture of the packaged chip.  
The mixer used as a tag is a resistive mixer where the LO signal is split into quadrature paths, where different attenuation is applied on the path based on IQ input, and these paths are then combined into RF output. This operation principle implies an energy loss converting an LO signal to an RF signal. The packaged module is tested with \(-10\) dBm LO input power and 1 GHz input at four different baseband ports (I+, 1-, Q+, and Q-) independently. The conversion gain is plotted versus different LO frequencies in Figure 13. It can be seen that the mixer introduces a 35 dB conversion loss over the 70-90 GHz band.