# Communication Architecture in Mixed-Reality Simulations of Unmanned Systems
## 2 Related Work
However, since these approaches separate the mobility and network traffic simulation stages, they do not support a study of the interaction between these two stages, e.g., how the delays in communication influence the movement of the vehicles. [PERSON] et al. [39] designed a testbed architecture that couples both traffic and network simulators and allows for the interaction between them in real time. In their testbed, vehicles broadcast information beacons (short messages with own position and other relevant data)using the network simulator. However, the architecture does not allow directed unicast messaging between the entities. MR simulations, unlike the previously mentioned approaches, allow for simultaneous movement of entities and studying their networking capabilities on various levels of fidelity. In these simulations, parts of the hardware components as well as some entities can be simulated depending on the resources available and the required level of the fidelity. The first notion of the mixed-reality concept was presented in work by [PERSON] et al. [10], where the authors mainly focus on the visualization displays and overlays, e.g., for use in robot teleoperation. One of the first notions of the MR simulations used in the development of mobile robots is presented by [PERSON] et al. in [11]. The authors used the MR simulation tool based on the 3D robot simulator Gazebo in a single robot scenario to demonstrate advantages of the combination of real entity and simulated objects for the development of the robotic system. The most recent approach to deal with issues in the development of complex robotic systems was proposed by [PERSON] et al. in [14]. The authors came up with the concept of the incremental multi-level development of complex systems by the use of MR test-beds. Their concept was further extended by [PERSON] et al. [15] and used for the practical development of various types of multi-vehicle systems. Using the methodology of incremental development, MR simulations allow relatively simple transition between individual levels of system virtualization--from the utilization of only SWEs and emulation functionality to the utilization of a combination of HWEs and SWEs in field experiments. In the latter case, the system partially loses the advantages of pure emulation, such as the simplicity and repeatability of experiments; however, the simulation fidelity increases since information from the real world is used. MR simulations require both broadcast and directed message exchange between SWEs and HWEs while allowing for the movement of entities together with the wireless network simulation to preserve the communication characteristics in the virtual world. Although some of the mentioned work partially solves the problem, a communication architecture that would address all required aspects, to the best of our knowledge, has not yet been described. However, since some MR simulation issues arise from a combination of two different networks (i.e., the networks of SWEs and of HWEs), some solutions can be found in the literature considering multi-network systems [40]. These issues are e.g., problems with _network partitions_ (where two or more UAVs are out of the communication range of the others and/or the simulation platform, usually the ground control station (GCS)) and problems with _sharing a single address_ (when HWEs need to communicate with SWEs using a single address of the GCS). Furthermore, the entities in the architecture require information about the addresses of their neighbors. This problem is addressed by the IPv6 Neighbor Discovery protocol by sending advertisements, as described in [41], by broadcasting messages to create a proximity map of the vehicles' neighborhood [42; 43], or, if the available bandwidth is an issue, by using beacons on separate communication channels [44].