# Radio Channel Scattering in a 28 GHz Small Cell at a Bus Stop: Characterization and Modelling
## 1 Introduction  
The fifth generation (5G) of mobile communication systems is expected to meet a set of requirements identified by the industry [1; 2; 3]:  
* 1-10 Gbps connections to end-points in the field;
* 1 ms end-to-end round-trip delay (latency);
* 1000 times increase of bandwidth per unit area;
* 10-100 times increase in the number of connected devices;
* (Perception of) 99.999% availability;
* (Perception of) 100% coverage;
* 90% reduction in network energy usage;
* Up to ten years of battery life for low power, machine-type devices. The number of requirements to be fulfilled simultaneously will depend on each application and service. Enhanced mobile broadband (eMBB) services, for example, will take advantage of the high transmission bandwidths, although they may not require 1 ms latency times. Due to the spectrum congestion below 6 GHz in Frequency Range 1 (FR1), the high bandwidths required for high-peed connections can only be found in the microwave and millimeter wave (mmWave) frequency bands of Frequency Range 2 (FR2) [4]. Among these candidate bands, the 28 GHz frequencyband is the one considered in our study. In this frequency region, the high free-space propagation losses, together with the high noise levels associated with large bandwidths, would yield low signal-to-noise ratios, unless directional antennas are used. The use of directional steerable antennas at both channel ends has been proposed [5; 6; 7; 8; 9] for 5G systems at mmWave frequencies. This represents an important change in mobile communication systems, where omnidirectional antennas have traditionally been used, at least at mobile terminals. As the radio channel characteristics depend on the antenna pattern [10], new measurements are required to properly determine the channel characteristics for directional antennas. However, smart antennas are complex and still expensive in terms of cost and energy consumption. Thus, despite the original premises of 5G mobile communication systems, current 5G rollouts are centered on FR1, while FR2 will eventually support fixed access [11]. In addition to high mmWave attenuation, the requirement to increase the available bandwidth per unit area has raised the need for ultra-dense small cell networks [12]. These are low-powered radio access points with coverage areas of around a 100 m radius and an inter-site distance of 200 m. This allows frequency reuse on an extremely dense basis. Small cells will be a crucial component of 5G networks because they increase network capacity, density and coverage [13]. The 28 GHz band has been proposed for these small cells [12]. Radio channel characterization and modeling are of the utmost importance for wireless communications, as communication systems have to be designed to reach the desired performance levels under the impairments introduced by the radio channel. Power delay profiles (PDP) and the Doppler spectrum (DS) are important parameters to estimate both time and frequency dispersion introduced by a radio channel. Delay dispersion, if not mitigated adequately, may give rise to inter-symbol interference that may limit the capacity of the channel, while the DS is important for analyzing the time variance of the radio channel. Despite most studies considering a narrowband characterization of the DS, since the first works on wideband propagation [14], it has been well known that the DS may be different for different delay contributions. The number of papers in the literature reporting fixed-to-fixed (F2F) outdoor radio channel measurements in the presence of moving scatterers is limited. For instance, in [15], the results of wideband measurements in a suburban environment at 2480 MHz are presented, but the Doppler analysis was not made separate for the different delayed components. More frequently, the DS is obtained from narrowband measurements, as in [16], where measurements in the presence of fast traffic on a motorway are reported, and in [17], where the moving scatterers are wind-blown leaves.