# Radio Channel Scattering in a 28 GHz Small Cell at a Bus Stop: Characterization and Modelling
## 1 Introduction
At higher frequencies, like in [18], we find empirical results at 29.5 GHz. The literature includes a series of wideband models for 5G radio channels that take delay dispersion into account. Such models can be classified [19] as deterministic, map-based (like METIS [20; 21]) and stochastic. These can be further classified as geometry-based stochastic models, like COST2100 [22], WINNER+ [23] and 3 GPP [24], or non-geometry-based stochastic models, such as the Saleh-Valenzuela model [25]. However, this is not a clear-cut division, as some models incorporate both deterministic and stochastic characteristics, like IEEE 802.11 ay [26] and MiWEBA [27]. All of them are wideband models that provide an estimation of a channel's PDP. Regarding the DS, we should mention specific geometrical-based stochastic models (GBSMs), which were developed for mobile communication channels and assume a particular geometrical distribution of static scatterers around the propagation environment. The classic Jakes model is one of these, but more sophisticated models include the one-ring [28], disk [29] and elliptical [30] models. In the one-ring model, scatterers are uniformly distributed on a thin ring, whereas for the disk model, they are distributed on a disk, being centered on the mobile station in both cases. For the elliptical model, scatterers are distributed on an ellipse, and the mobile and base stations are located on its foci. It should be noted that propagation paths suffering single scattering on the ellipse will have the same delay. Ring and disk models may be more adequate for narrowband systems, while the ellipse model would perform better for wideband ones [31]. The presence of moving scatterers was considered in [32], where a general vehicle-to-vehicle model was developed, considering random scatterer velocities with Gaussian and mixed Gaussian distributions. In this paper, it is assumed that waves can propagate from transmitter to receiver after a single bounce on the moving scatterer. The F2F channel model is obtained by particularizing the vehicle-to-vehicle model to the case in which the vehicle velocities are equal to zero. The application of the ring, disk and elliptical models to the case of an F2F radio channel can be found in [33]. The models are compared with measurement results at 29.5 GHz from [18] and measurement results at 5.8 GHz from [34]. Models for F2F links with a single moving scatterer can be found in [33], where a model is also proposed for the case of a non-uniform scattering pattern. The corresponding DS shows good agreement with measurement results performed with a wideband MIMO sounder in an indoor environment at 5.8 GHz. In this paper, we consider the case of a small cell at 28 GHz deployed at a bus stop. We have measured the channel impulse response in this scenario using a wideband channel sounder. Despite using fixed antennas, the scenario cannot be considered static. The presence of moving scatterers, such as vehicles passing by the bus stop, could have a significant impact on channel behavior [35]. Particularly, moving objects will have an impact on the Doppler spread of the radio signals. It should be noted that this effect could be strong due to the linear dependency of the Doppler shift on the carrier frequency [36]. We analyze both the delay and the Doppler spread produced by the radio channel. Moreover, as the channel may have non-stationary behavior, we analyze the Doppler spectrum variation as vehicles pass by the bus stop. There are several novel aspects in the paper. First, we provide wideband results for an F2F outdoor environment at 28 GHz, including the PDP and DS. Second, we obtain the DS separately for the main delay contributions. Third, we analyze the temporal variation of the DS, showing that the channel exhibits non-stationary behavior. Finally, we determine the requirements a model of this radio channel should comply with. After this introductory section, the rest of the paper is organized as follows. In Section 2, we describe the measurement system and measurement environment.