# MIMO Relaying UAVs Operating in Public Safety Scenarios
## 2 System Model  
The UAV-enabled network presented in Figure 3 where R = {r\({}_{1}\),..,r\({}_{m}\),...,r\({}_{M}\)}] relaying UAV (usually identified as a drone) distributed on the edges of several concentric circles with a radius of 15-80 m, and heights starting at 20 m-30 m respectively, to avoid collisions. The UAVs flew in a circular trajectory with an angular speed of 5 m/s. There were N = {1,..., n,..., N} users concentrated in a 350 \(\times\) 200 m\({}^{2}\) region who moved in random directions and at random speeds. The users' height was configured at z = 1.5 m. The evolved NodeBs (enB) were located at a height of 120 m and positioned at 300 m. This was the maximum height described in the European Union aircraft regulations [37]. The base station was connected to the remote host, which was a representation of the Evolved Packet Core (EPC) that included the Serving Gateway-SGW-Packet Data Network Gateway PGW, Mobility Management Entity-MME servers, and the futurist U-Space server. In this experiment, the eNB was not able to send packets directly to the users. The connection between the remote host, which is responsible for providing internet services to the users was established using three different links. The first one was a sidelink protocol link (L1) provided by the national institute of standards and technology--NIST-, the second used the LTE protocol-(L2), and the last one was simulated as a peer-to-peer connection named L3. Without loss of generality the Euclidean 3D equation to describe the distance between the base station and the drones as well as the distance between the drones and the users is presented in Equation (5). \[d_{rt} =\sqrt{\left(x_{r}-x_{u_{s}}\right)^{2}+\left(y_{r}-y_{u_{s}} \right)^{2}+\left(z_{r}-z_{u_{s}}\right)^{2}} \tag{5}\]  
where \(\left(x_{r},\ y_{r},z_{r}\right)\) and \(\left(x_{u},\ y_{u},z_{u}\right)\) are the user and relaying coordinates, respectively. To test the communication availability in the described scenario we adopted the network simulator 3.31 (NS 3.31) [38] for public safety operations which included the LTE protocol stack for communication and preconfigured sidelink protocols. According to the author [38], the LTE module had a PC5 interface that handled out-of-range scenarios that included data plane and control plane adjustments. In this configuration, we used the IPv6 network protocol. Connections were established according to users' requests and relay services availability using a relay selection algorithm. The search for possible relay services happened every discovery period. The S_RSRP measurements of possible candidates occurred every four discovery periods and the evaluation of candidates occurred every sixteen discovery periods. In the synchronization phase, both devices' exchange pools defined the frequency and bandwidth that were used. The packets were sent to the internet over a dedicated bearer using the LTE uplink configuration for transmission and reception. It used a half-duplex communication. In case of UAV replacement need, the  
Figure 3: Simulation Scenario. Four relaying drones with circular mobility and speed 5 m/s connecting people to LTE core. users were transferred to other available drones using the discovery, synchronization, and communication procedure available in PC5. The parameters used to configure the experiment are presented in Table 1. The frequency range chosen was a part of the 5G sub 6 GHz bands, which was band 22-35 GHz. According to the 3 GPP [39], the maximum number of connections using D2D communication is 16 due to resource management configurations. The implementation of MIMO features in the simulator was an abstraction of the perceived gain in the receiver when compared to SISO systems and it did not include the math needed to compute antenna correlation. Consequently, there was no degradation model related to the path correlation estimate in this experiment. This experiment used two MIMO techniques including Tx Diversity and the Spatial Multiplex Open Loop. The maximum drone speed limit was 160 km/h [40].