## Materials and Methods
### Environment Model  
The RT model consists of the environment geometry description and the transmitter-receiver (Tx-Rx) parameters. A geometrical entity is represented by the coordinates of its boundary faces (polygons) and the dielectric parameters (relative permittivity \(\varepsilon_{r}\) and conductivity \(\sigma\)) of each face. A Tx (Rx) antenna is defined with its location, radiation pattern, and the carrier frequency. For every Tx antenna, _rays_ are launched from its location in directions (nearly) uniformly distributed on a surface of a sphere centered at the Tx [14]. A ray is an abstraction which represents a flat wavefront, described with its (complex) EMF amplitudes and the propagation direction. The rays are propagated (traced) through the environment and their interactions (reflections, diffractions and transmissions), as well as path loss (PL) and time-of-flight, are tracked by the ray-tracer. If a ray passes sufficiently close to an Rx location, it is considered to be received and its state is recorded. The output of the simulation is a set of received rays for every defined Tx-Rx pair. The RT output is site-specific, i.e., the channel between a fixed Tx-Rx pair depends on the surrounding geometry. To generalize the results a number of geometrical entities was generated stochastically, based on a few macroscopic parameters. Each realization of the environment geometry we call an _environment sample_[15]. Figure 1 presents one of the environment samples obtained from the model we describe below. We simulate an outdoor urban macrocell bounded by a fixed flat square area 100 m by 100 m in size. Building blocks are represented by cuboids, width and length of which are sampled from a uniform random distribution in range from 15 m to 25 m. The height of a building block is drawn from a uniform random distribution in range from 5 m to 20 m. The buildings are positioned on a rectilinear grid, such that any two neighboring blocks are separated by exactly one empty grid cell. Rows of building blocks and straight lanes form a Manhattan-like urban city landscape. The spacing (lane) width is set equal to 10 m, 15 m, or 20 m randomly with equal probability. The dielectric properties of the cuboids model concrete material with \(\varepsilon_{r}=7\), \(\sigma=1.5\times 10^{-2}\) S/m. The ground plane is assigned asphalt dielectric properties with \(\varepsilon_{r}=5.7\), \(\sigma=5\times 10^{-4}\) S/m. The locations and properties of the Tx and Rx antennas are fixed in the model. The simulations are performed at a single frequency of 3.5 GHz, foreseen to be heavily used in 5G networks. The BS (Tx) is a rectangular 10-by-10 element array of vertically polarized half-wave dipole antennas with a half-wavelength uniform inter-element spacing (\(\lambda\simeq 85\) mm at 3.5 GHz). The center of the Tx array is positioned at \(x=1\) m, \(y=50\) m and \(z=25\) m (Figure 1). The BS height of 25 m correspond to the macrocell scenario in the 3 GPP model [9]. In the following simulations, the BS coverage range spans from \(-60^{\circ}\) to \(+60^{\circ}\) in azimuthal (\(\varphi\)) and from \(105^{\circ}\) to \(135^{\circ}\) in polar angular directions (\(\theta\)) in coordinate system shown in Figure 1. This is in accordance to the model used in [6]. To include most of the simulated ground-plane area within its coverage range, the BS array is tilted down by \(30^{\circ}\) around the \(y\)-axis through its center. A UE (Rx) is modeled as a single-terminal device equipped with a vertically-oriented vertically-polarized half-wave dipole antenna. The UEs are arranged on a regular rectilinear grid with 2 m spacing in \(x\) and \(y\) directions, at a height of 1.5 m above the ground-plane (\(z\)-axis). Only the grid nodes that fall within the cell and are located outside the building block interiors are kept in the simulation, which, on average, results in around 600 UEs per simulation. These UEs are used as potential active receivers in the analysis described in Section 2.3.