# A Passive RF Testbed for Human Posture Classification in FM Radio Bands
## 2 Experimental Testbed
### RF Sensing System  
The experimental setup used in this work consists of two similar Software-Defined Radio (SDR) devices and two identical directional antennas, two Low-Noise Amplifiers (LNAs), and computers to process the data. The SDR devices selected for this work were the _Nuand_ bladeRF 2.0 micro xA4 SDR boards that offer 2 \(\times\) 2 Multiple-Input Multiple-Output (MIMO) streaming, supporting the connection of 2 external receiving antennas (RX) and 2 external transmitting antennas (TX). The receiving RF chains of the SDRs are connected to a bias-tee, the _Nuand_ BT-200 wideband LNA, to boost the received signal. The bladeRF's complete specifications are presented in Table 1.  
In this work, the FM signal is broadcast by an outdoor commercial FM radio station, and the testbed is mainly focused on the acquisition of the FM signals through two different RF chains implementing the reference and surveillance channels that substantiate passive radar systems. The reference antenna is mainly aligned to receive the outdoor FM signal, while the surveillance antenna focuses on the area where the classification occurs. To minimize possible sample losses due to USB communication rate degradation, we implemented each one of the two RX chains in a different SDR board. Consequently, during the sample acquisition process, the two bladeRF boards need to be synchronized. For this purpose, we used a synchronization cable, illustrated in Figure 1, which is used for synchronizing multiple bladeRF devices in a multidevice setup. Synchronization is crucial in applications where precise timing across multiple SDRs is necessary, such as in distributed MIMO (Multiple-Input Multiple-Output) systems or other advanced radio communication setups. The cable is used to connect specific synchronization ports on the bladeRF devices, allowing them to share timing information and maintain synchronization. This is important in scenarios where multiple devices need to operate together seamlessly, exchanging data without timing discrepancies. In our work, we used a master clock signal to synchronize the two _Nuand_ bladeRF 2.0 micro xA4 boards. This is performed by selecting the internal clock of one of the boards, say board 1, as the master clock. The master clock signal is then transmitted through the synchronization cable to board 2, which instead of using its internal clock will use the clock signal provided \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**RF Specifications** & **Min** & **Value** & **Max** & **Unit** \\ \hline RF Frequency Range (RX) & 70 & - & 6000 & MHz \\ \hline RF Frequency Range (TX) & 47 & - & 6000 & MHz \\ \hline ADC/DAC Sample Rate & 0.521 & - & 61.44 & MS/s \\ \hline ADC/DAC Resolution & - & 12 & - & bits \\ \hline RF Bandwidth Filter & \textless{}0.2 & & 56 & MHz \\ \hline CW Output Power & - & +8 & - & dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: RF Specifications of the _Nuand_ bladeRF 2.0 micro xA4. Figure 1: Details of the two bladeRF 2.0 micro xA4 boards with their clocks connected. by board 1. Consequently, both boards use the same reference signal, and the samples gathered by each board are then synchronized. The boards allow this kind of operation by initially running a batch command that identifies which source clock is adopted by each card, and subsequently, they run a synchronization procedure that identifies the clock signal and regulates the synchronization parameters needed to successfully lock the synchronization loop. We connected the two SDR's clocks using the synchronization cable, and we configured them accordingly. The port CLK_OUT from the device connected to the reference antenna is connected to the port CLK_IN of the SDR connected to the surveillance antenna, so the clock signal of the reference device serves as the master clock for both SDR boards. The schematic depicted in Figure 2 represents the passive RF sensing system from a person's perspective in which relevant measurements are also provided.