## 3 Design Approach  
### Conceptual Design  
The hardware platform is made up of a UWB RF front end and a sub-GHz backbone (Figure 2). The RF front end consists of the DW1000, the necessary pheriperal circuits, and a power connection. For the sub-GHz backbone, an existing low-cost IoT board, the Zolertia RE-Mote [23], is used. The choice for the Zolertia RE-Mote for the backbone is based on several arguments. First, the board is an existing, of-the-shelf IoT platform with widespread software support from both Contiki OS and openWSN. The RE-Mote has on-chip support for two popular, low-power IoT transceivers: the CC1200 long-range, sub-GHz communication (820-950 MHz) and the CC2538 for low-power, short range communication in the 2.4 GHz bands. Furthermore, the TAISC framework is available for the RE-Mote, and it supports SPI communication with the DW11000. Although the layout of the UWB shield (Figure 3) has been designed for the RE-Mote, it also interfaces with other hardware platforms (Arduino, other IoT boards) by connecting the correct pins. Moreover, the hardware design is provided as open-source code, allowing easy modification towards other platforms. The full specifications of the UWB radio board can be found in Table 2. All pins of the Zolertia RE-Mote are connected to the Wi-PoSshield permitting future expansion of the system with additional sensor components. Optimization Techniques to Minimize Pulse Distortion  
As discussed earlier in the introduction of the paper, designing a board for such large bandwidths with minimal pulse distortion is challenging. An optimal board design is optimized for all six frequency bands supported by the DW1000 UWB transceiver. For this reason, the Wi-PoShardware includes the following optimizations: \begin{table}
\begin{tabular}{c c} \hline \hline Dimensions & 40.29 \(\times\) 70 \(\times\) 1.568 mm\({}^{3}\) \\ Stack & 4 layers \\ Printed circuit board (PCB) material & RO4350B and PR2116 \\ Antenna interfacing & SMA connector \\ UWB transceiver & DW1000 \\ Crystal oscillator & 402F38411 CAR (CTS-Frequency Controls) [\(\pm\)10 ppm] \\ Balun & HHM1595A1 \\ Power & USB-C (5.0 V) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specifications of the UWB shield. Figure 3: UWB shield and connected external omnidirectional antenna. Figure 2: High-level architecture of the hardware platform. * A 4-layer stack up is used for the PCB (Figure 4). The 2 nd and 3 rd layer are used as the ground plane and power plane, respectively. The use of a ground plane reduces voltage drops in potential of ground level throughout the board and creates short return paths for the currents from the different components, especially for the high sensitive signals on the top layer like the transmission lines. The ground layer shields the sensitive components from different noise sources on lower layers. A separate analog and digital ground is used, joined in the middle of the board. * The PCB has a thickness of 1.568 mm and is fabricated with high-quality Rogers 4350 (RO4350) material for optimal RF behavior between the top layer and layer 2 and between bottom layer and layer 3 (Figure 4). The dielectric constant for this material is well specified and dielectric losses are significantly smaller than with the frequently used FR4 material, especially for higher frequency use cases. Furthermore, the permittivity for RO4350 is more stable and the FR4 will have more fluctuations in permittivity between different batches. * The path from DW1000 towards the SMA connector is a straight line shielded by stitching vias to prevent electromagnetic interference and minimize pulse distortion. The widths of the microstrip lines are matched with the desired impedance: 100 \(\Omega\) for the differential pair from DW1000 to balun and 50 \(\Omega\) for the single transmission line from the balun to the single ended SMA connector. Care is taken that no obstacles are placed on the return path at the ground layer.