# Wideband Channel Characterization for 6G Networks in Industrial Environments
## 3 Overview of Radio Propagation Measurements in Industrial Environments  
The radio channel propagation characteristics in industrial environments are quite different from typical indoor office environments due to the infrastructure of the industrial environments, which contain many electrical and mechanical machines [39]. The rich scattering in industrial environments due to the highly reflective structures produces heavy multipath propagation channels, especially at high frequencies such as millimeter-wave (mmWave) and THz [40]. Many radio propagation studies have been conducted in different indoor industrial environments in the sub-10 GHz band. Ultra-wideband measurements were carried out in two different industrial environments at 3.1-5.3 GHz and 3.1-10.6 GHz frequency bands [41]. It was found that the PLE was 1.1 along a 16 m point-to-point NLoS link, and the RMSDS values ranged from 28 to 38 ns and from 34 to 51 ns for LoS and NLoS, respectively. In [42], the path loss and K-factor characteristics at three different frequencies (0.9, 2.4, and 5.2 GHz) were presented based on narrowband measurements along 15 to 140 m Tx-Rx separation distances in four factories with different topographies. Different PLEs for each frequency were reported, ranging from 0.87 to 4.47 and from 1.68 to 4.49 for non-fixed (FI model) and fixed intercept (CI model) path loss models, respectively, in different facilities for wood and metal material processing. Detailed radio propagation characteristics for radio channels such as path loss, RMSDS, and K-factor at 2.2 and 5.4 GHz in various industrial environments were provided in [43]. A comprehensive review of radio channel measurements in different industrial environments in sub-6 GHz band was presented in [39]. Radio propagation measurements at the sub-THz band in indoor industrial environments are expected to attract more research interest [40]. For instance, a few mmWave radio propagation measurements have been conducted in various industrial environments at 28 GHz [44; 45; 46; 47; 48; 49; 50] and 60 GHz [44; 51; 52]. Narrowband measurements were conducted inside a factory environment at 28 GHz in [46] to study the path loss at a 5-100 m distance over 400 LoS and NLoS measured points, and it was shown that the PLE values were 2 and 4.1 for LoS and NLoS, respectively. Based on the conducted wideband measurements at 28 GHz in an engineering workshop, it was observed that the reflected and refracted mmWave propagation waves from metallic surfaces were significantly high [47]. In [49], based on wideband time domain measurements at 28 GHz with a 2 GHz bandwidth inside a circle-shaped machine hall, the path loss and RMSDS were investigated. It was found that the PLEs of the FI model were 1.9 and 0.9 for LoS and NLoS, respectively, and 1.8 and 2 based on the CI model for LoS and NLoS, respectively. It was shown that the FI model did not fit the measured path loss for the NLoS scenario. Using the same measurements system, the measurements were carried out at 3.7 and 28 GHz in an indoor factory with Tx-Rx separation distances of 4 to 26 m and 9 to 25 m for LoS and NLoS, respectively [50]. Using the ABG path loss model, the PLEs were 2.27 and 3.02 for LoS and NLoS, respectively. The RMSDS values at 28 GHz were 23.1 and 33.6 ns for LoS and NLoS, respectively. The path loss and RMSDS were studied in [51] based on the conducted wideband measurements using a VNA in a metal cabinet (to emulate the industrial machine) in the frequency band of 57-62 GHz. This study stated that the path loss was low as the PLEs varied from 0.004-0.021 along a Tx-Rx separation distance of 0.25-2.5 m. It was also indicated that the RMSDS values (up to 160 ns) were significantly larger than for typical indoor environments. In [44], the mmWave channel characteristics were investigated at 28 and 60 GHz based on wideband measurements using a VNA in two different factories representing light and heavy industry.