# Data-Driven Analysis of Outdoor-to-Indoor Propagation for 5G Mid-Band Operational Networks
## 2 Background, Related Work, and Contributions
### Relevant Experimental Studies  
There are several works discussing outdoor-to-indoor propagation in the 5G mid-band. Among others, notable examples can be found in [24; 38; 39] where, however, the focus was not on the evaluation of channel propagation parameters, as it is in the present paper. In particular, the work in [24] analyzed the electromagnetic wave propagation penetration within buildings, while [38] performed measurement campaigns to investigate the channel capacity using the Weichselberger channel matrix model. Cluster characteristics of the MPC of PDPs were instead analyzed in [39], based on outdoor-to-indoor Multiple Input Multiple Output (MIMO) measurements performed in an office building. Other works also investigated outdoor-to-indoor propagation at around 3.5 GHz, while also providing analyses of some of the channel propagation parameters defined in Section 2.1[22; 23; 40; 41; 42]. In [22], the authors presented a path loss analysis based on channel measurements at 3.5, 4.9, and 28 GHz. Results showed that the average path loss at 3.5 GHz was lower than the one experienced at 4.9 and 28 GHz, thus highlighting that sub-6 GHz signals could penetrate concrete walls and provide nearly seamless coverage. The work in [23] studied the path loss variation due to the height difference between the transmitter and receiver via outdoor-to-indoor channel measurements at 3.5 GHz. The authors observed path loss increase with the height difference between transmitter and receiver. The work in [40] investigated the outdoor-to-indoor propagation at 3, 10, 17, and 60 GHz and observed that the RMS delay spread was weakly correlated with frequency. It was also shown that the path loss was both material- and frequency-dependent. Some statistical characteristics of channel impulse responses at 3.5 and 2.35 GHz were also studied in [41]. Results showed that the RMS delay spread at 3.5 GHz was less than the one at 2.35 GHz. The work in [42] studied the applicability of COST231, WINNER+, ITU-R, and 3 GPP urban microcell outdoor-to-indoor path loss models in femtocell environments. It was shown that when the base station and the user equipment were on the same side of the street, the WINNER+ models predicted the received power with no more than 3.6 dB of R.M.S. error. Additionally, the 3 GPP, COST231, and WINNER+ models predicted the received power with no more than 3.6 dB of R.M.S. error when the maximum antenna gains were used; however, smaller values of antenna gains led to estimation errors up to 14.8 dB. In [43], empirical path loss modeling was studied in an indoor to outdoor scenario for femtocells networks at 0.9, 2, 2.5, and 3.5 GHz, and a Singular Value Decomposition (SVD) technique was used to derive the path loss model parameters. In parallel to the above works, channel characterization in the 5G mid-band was also studied in indoor-to-indoor scenarios [6; 10; 11; 12; 13; 25; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45;  
analyses were carried out to derive channel parameters, similarly to what the present paper aims to do for the outdoor-to-indoor case. In [25; 44], it was shown that the RMS delay spread depended on the environment size, while furniture had negligible impact. A similar analysis was carried out in [11; 12] for industrial indoor environments, with main focus on RMS delay spread in both Line of Sight (LOS) and NLOS conditions. Results showed that the RMS delay spread in NLOS locations was higher than in LOS locations. References [11; 26; 27] focused on path loss, showing that the path loss exponent is smaller in LOS than in NLOS. Other time-related parameters were also analyzed in [26; 28], showing that RMS delay spread and mean excess delay increase in presence of multiple paths. A characterization of the S-V model at 3.5 GHz and 28 GHz was carried out in [30] in an indoor environment, and in both indoor and outdoor environments in [10].