Aerospace and Electronic Systems Magazine August 2016 - 6

Cellular for the Skies: Exploiting Mobile Network Infrastructure for Low Altitude Air-to-Ground Communications Table 1. Theoretical Data Rates and Latencies for Mobile Cellular [9] Technology Data rate (downlink)-Mbps Data rate (uplink)-Mbps EDGE 0.236 0.059 UMTS 2 0.384 HSPA+ 42 LTE LTE-A 11.5 300 75 3,000 1,500 frastructure can also enable the implementation of a system with multiple operators and multiple UAVs, and simplifies the introduction of redundancy into the communication link, through the use of two or more mobile frequencies. Another important advantage is the fact that it provides access to the internet, enabling the implementation of a system supported over multiple different access networks. Despite the potential advantages, cellular networks are not deployed with the aim of supporting A2G communications. For example, base station (BS) antennas are often tilted down, which can result in possible loss of radio coverage even at low altitudes. Furthermore, these networks may not be a solution, or at least the only solution for some specific applications like search and rescue where, in a disaster situation like an earthquake, the cellular network infrastructure can fail to operate. RADIO PROPAGATION Maintaining a reliable communication link between a BS and a UAV requires that the received power remains above a given threshold. Thus, in order to use cellular networks for A2G communications, it is important to analyse the conditions of signal propagation, which implies the development of propagation mod- els for the scenarios where the communications will occur. These models typically depend on Technologies several factors, e.g., frequency, distance, BS and terminal heights, antenna pattern, and obstacles. Most of the existing models for outdoor environments were created for terrestrial Latency-ms communication scenarios, which in the case of a UAS can correspond to the link established 150 between a GCS and a BS. However, these mod100 els are not adequate for the A2G link between a UAV and a BS, and so new ones have to be 50 developed. 10 Within the scope of the SAAS project, we 10 have proposed a new propagation model for computing the path loss in A2G links in outdoor urban scenarios [11]. The model was developed and validated for the frequency ranges of Global System for Mobile Communications (900 MHz), UMTS (1,800 MHz), and LTE (2,100 MHz) through several field trials and received power measurements in the vicinity of a BS using weather balloons. Figure 1 shows a batch of samples obtained in the 1,800 MHz band close to a BS located on the roof of a building with a height of 11 m. The altitude of the BS antenna was approximately 15 m and the downtilt angle was 10 degrees. Using multiple sets of measurements taken from different locations, a statistical analysis was performed. The results allowed us to extend a terrestrial propagation model through the adjustment of existing parameters and the addition of new ones, in order to fit it, as close as possible, to the behaviour of the real channel. This led to an empirical propagation model that can reproduce the average path loss between the transmitter and receiver as a function of distance, BS and terminal heights, frequency, tilt angle, elevation, azimuth, and sectorization (more details can be found in [11]). Based on the propagation model, Figure 2a shows the profile of the average received power (Prx) as a function of position (BS is located at the origin) for an altitude of 19 m and Figure 2b shows the profile as a function of the altitude of the UAV for a fixed horizontal distance to the BS of 8 m. Figure 1. Received power measurements in the 1,800 MHz band (a) obtained using weather balloons (b). 6 IEEE A&E SYSTEMS MAGAZINE AUGUST 2016

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2016