# Wideband Channel Characterization for 6G Networks in Industrial Environments
## 2 Overview of Radio Wave Propagation for 6G Systems
### Indoor Channel Characteristics  
The work in [9; 27; 28; 29; 30; 31] conducted wideband measurements at 300 GHz in different indoor environments using a vector network analyzer (VNA)-based sounder. The path loss, root mean squared delay spread (RMSDS), and absorption attenuation were investigated at 300 GHz in short-range desktop and indoor office LoS and non-LoS (NLoS) scenarios in [9; 27]. These studies observed 3.9 dB misalignment losses for a 1.67 m LoS link. However, for NLoS links at a 2.12 m to 4.77 m distance, References [9; 27] reported additional losses of 12.9-30 dB and 29.7-52.7 dB for one and two reflections, respectively, depending on the involved surfaces of reflective objects. They also showed that about 86.7 dB and 65.5 dB absorption attenuations resulted at 300 GHz along a 0.1 m link from obstructions of the ray path by a 3.5 cm thick fiberboard door and a 2.5 cm thick window, respectively. In [28; 29], ultra-wideband measurements were carried out in a small indoor office at 300 GHz with approximately a 50 GHz bandwidth using virtual antenna arrays, and the path loss, reflection loss, K-factor, RMSDS, and capacity were investigated at different angles-of-arrival/angles-of-departure (AoAs/AoDs). The measurements showed about a 12-15 dB loss per reflection, and the RMSDS ranged from 1.6 to 3.7 ns, while the AOA angular spread ranged from 15.7\({}^{\circ}\) to 40.1\({}^{\circ}\) for different Tx locations [28]. A stochastic spatio-temporal indoor channel model at 300 GHz was introduced in [29] to provide complete channel characteristics in an indoor office scenario. [PERSON] and [PERSON] [30] presented an ultra-wideband statistical characterization at 300 to 320 GHz along a 0.7 m LoS link and a 0.3 m NLoS link. Three different materials were used to completely block the LoS signal from Tx to Rx on a desktop in a typical indoor environment. This study found that the path loss exponent (PLE) for the LoS link was around 1.9 and that the mean value of the RMSDS was 428.4 ps for the LoS link, while the RMSDS values for the NLoS link varied from 187 ps to 227 ps based on the obstacles' materials. The same value of the PLE was reported at 300-316 GHz for an indoor LoS channel in [31] based on wideband channel measurement along a 0.25 m LoS link. In [32], wideband LoS and NLoS indoor measurements were conducted using a VNA-based system in the 260-400 GHz frequency band to investigate the path loss, angle-of-arrival (AoA), and absorption coefficients of different scattering objects. Measurements at 140 GHz have been conducted in different indoor environments [8; 31; 33; 34]. The path loss, reflection, and RMSDS were investigated at 140 GHz in [33] based on ultra-wideband VNA-based system measurements of a 60 GHz bandwidth in LoS, obstructed LoS (OLoS), and reflected NLoS indoor environments over 0.4 to 0.9 m links. It was shown that the PLE is around 1.97 for the LoS and the PLEs are 1.89, 2.13, and 2.95 for the OLoS using plastic, glass, and ceramic obstructions between Tx and Rx, respectively. For reflected NLoS using an aluminum plate reflector, the results in [33] showed that the measured path loss was very similar to the free space path loss (FSPL). A similar PLE around 2.22 was reported for the LoS in an indoor shopping mall [34] based on wideband measurements of the 4 GHz bandwidth using a VNA system at 140 GHz along a 35 m link. The PLE in an indoor office environment at 140 GHz with a 1 GHz bandwidth was 2.02 using correlation-based sounding measurements [8]. Based on all mentioned radio propagation studies at 140 GHz, it can be noted that the PLE for the measured signal was close to a free space PLE of two. It was also found that the PLE for the 126-156 GHz measured band was 1.9 (close to a free space PLE of two) based on wideband measurements using a VNA in an indoor office along a 1.6 m LoS link [35].