## 1 Introduction  
In recent years, there is a clear need to supply a sufficiently high data rate for areas with elevated user volume such as venues, hotels, and conference centers, etc., where a lack of signal is evident. Usually, in these situations the deployment of a new indoor radio cell is unnecessary considering outdoor radio cell availability. Under these conditions, outdoor-to-indoor (O2I) models become very relevant. These models characterize signal propagation inside buildings coming from external mobile radio base stations (BSs), which are mounted across a network of outdoor sites, occupying towers on hilltops, rooftops in built-up areas, and other promising outdoor structures. O2I radio propagation has become a challenging work, according to Small Cell Forum [1] some reasons for this are:  
* Due to the lower operating frequencies becoming exhausted, higher frequencies are being deployed. However, they are not as effective for range and building penetration. * O2I signal propagation is affected because the building fabric is more eco-friendly and noise-free, using low-emissivity glass which reflects the radiation from cellular antennas, and soundproof materials which attenuate radio waves. * There are large increases in demand for mobile cellular services which density available networks with more sites and an increase in the size of the spectrum that mobile network operators (MNOs) can deploy on those sites. This produces localized congestion; the service may be available, but it may not always be satisfactory. Therefore, the scientific community is encouraged to understand O2I radio wave propagation [2; 3; 4; 5] to help radio network engineers to achieve efficient radio coverage estimation, determine the optimal BS location, and perform interference feasibility studies. In seeking to understand those links, the applicability of standard urban microcell O2I path loss models such as COST231, WINNER+, ITU-R, and 3 GPP are empirically tested [6]. The COST 231 project is based on adjusted models such as the one-slope model (1 SM), the multi-wall model (MWM) and the linear attenuation model (LAM), that are based on propagation measurements [7]. The final O2I model proposed by the COST231 project is based on empirical data acquired for NLOS links, where the model relates the loss inside a room to the loss measured outside of it on the side nearest to the wall of interest, i.e., multi-floor propagation. The WINNER+ channel model followed a geometry-based stochastic channel modeling approach [8]. The channel parameters were determined stochastically, based on statistical distributions extracted from channel measurements. The ITU-R model [9] provided guidelines for both the procedure and the criteria (technical, spectrum and service) to be used in evaluating the proposed IMT-Advanced radio interface technologies (RTIs) or sets of RTIs (SRITs) for a number of test environments and deployment scenarios for the evaluation of the band of frequencies between 2 and 6 GHz. The NLOS model proposed by the 3 GPP is based on measurement campaigns, for O2I building penetration loss. This model considered the material penetration losses for two types of models: low loss and high loss. The composition of low and high loss is dependent on the use of metal-coated glass in buildings and the deployment scenario characteristics. The study reported for the 3 GPP project considered not only O2I building penetration losses but also O2I car penetration losses for the frequency range of 0.6 to 60 GHz [10]. According to [11], through an accurate channel modeling: the in-building radio propagation phenomena, complex by nature, can be characterized; the range of a wireless communication system can be calculated by assessing the expected coverage inside a building; signal strength/path loss can be predicted more accurately everywhere in a building; and channel performance predictions can be made quickly. There are essentially three approaches for propagation models: physical, empirical and hybrid.