# Distributed MIMO Measurements for Integrated Communication and Sensing in an Industrial Environment
## 3 Measurement System  
The multilink channel sounder has been developed utilizing the NI-Universal Software Radio Peripheral (USRP) (National Instruments Corporation, Austin, TX, USA) and the software suite LabVIEW 2023. The sounding system is portable and scalable, facilitating various measurement scenarios ranging from indoor and outdoor industrial settings to dense urban environments. The components of our multilink channel sounder system are listed in Table 1 and conceptual overview of the RF parts are shown in Figure 1. \begin{table}
\begin{tabular}{l c l} \hline \hline
**Hardware** & **Amount** & **Description** \\ \hline NI-USRP 2953r 40 MHz (National Instruments Corporation, & 7 & USRP \\ Austin, TX, USA) & 3 & 10 MHz and 1 pulse per second (1 PPS) Rb \\ SRS FS725 (Stanford Research Systems Inc., Sunnyvale, CA, USA) & 3 & standard \\ SRS FS740 (Stanford Research Systems Inc., Sunnyvale, CA, USA) & 1 & 10 MHz and 1 PPS with GNSS \\ Host computers & 7 & Radio control and logging data \\ Hoverboard & 1 & Acting as mobile agent/mobile user (UE) \\ Joymax SAF-6571 RS3X antennas (Joymax Electronics Co., Ltd., Tao-yuan City, Taiwan) & 13 & 12 as infrastructure and 1 on the UE \\ Ouster OSDome (128 lines) (Ouster Inc., San Francisco, CA, USA) & 1 & The light detection and ranging (lidar) used for simultaneous localization and mapping (SLAM) \\ Microstrain 3 DM-GX5-25 (AHRS) (Microstrain by HBK, Williston, VT, USA) & 1 & 9-DoF IMU for SLAM \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hardware for the multilink measurement system. Figure 1: An illustration of a three-node, multilink setup. The dashed line between the two Rubidium clocks (Rb-clock 1 and Rb-clock 2) illustrates that if the two Rubidium clocks are well synchronized—over several hours—then they can be disconnected for some time without losing the synchronization of the radios. To the RF ports of the USRPs, one can either connect single antennas or switched arrays. Designed for multilink channel sounding, the channel sounder records and stores all conceivable link combinations between antennas. To avoid interference among links, a TDMA strategy is employed. Each antenna is assigned a unique time slot for signal transmission, during which the remaining antennas are set to receive mode. Figure 2 provides a visual representation of this TDMA structure. As a reference signal, the transmit unit uses a Zadoff-Chu sequence [28]. The signal \(\hat{s}(t)\) is configured as an orthogonal frequency-division multiplexing (OFDM) symbol with Zadoff-Chu samples assigned to subcarriers [15]. The sounding system also allows for the nullification of a specified number of carriers at the spectrum's edges, thus providing flexibility in bandwidth utilization. The channel sounder captures and streams the raw complex samples directly to the disk on the host computer for subsequent offline postprocessing, which may include symbol averaging. Tight synchronization is necessary to achieve the TDMA structure. A one-pulse-per-second (1 PPS) synchronization signal is distributed to all radios, as well as a stable 10 MHz reference clock. Depending on the scenario, either synchronized Rubidium clocks or a GPS can be used to discipline the onboard clock and synchronize the triggers. For high-accuracy sensing measurements, rubidium clocks are the preferred choice, we are using SRS FS725 and FS740 (Stanford Research Systems Inc., Sunnyvale, CA, USA). We record all links, even those that could be assumed to be reciprocal. By saving all channel transfer functions, we enable the possibility to evaluate OTA calibration algorithms. The sounder is equipped with several adjustable parameters for the TDMA structure, as illustrated in Figure 3. Initially, the reference symbol \(\hat{s}\), intended for transmission, is generated and stored in the field-programmable gate array (FPGA) memory of each USRP. Subsequently, the number of repetitions of the reference symbol, denoted with \(M\), is defined. \(R\geq 2\), with the first symbol effectively serving as a cyclic prefix.