# Communication Architecture in Mixed-Reality Simulations of Unmanned Systems
## 6 Addressing the Precision Aspects of the Proposed Architecture
### Bandwidth of a Single Relay Point  
Available transmission data rates of all entities are influenced by the use of a single message relay point, the simulation platform. The MR simulation is thus prone to three bandwidth modeling issues. The first issue is caused by the fact that SWEs do not use the RF modems directly. Thus, there are fewer modems than there are entities, so there is less RF noise than there would be in real scenarios. The second issue is caused by the communication among SWEs that is not affected by the limitations of the RF medium and modems. Hence, SWEs could theoretically send data at much higher data rates than entities in real scenarios. The third issue can arise from the fact that the platform's modem needs to transmit data for all SWEs, and this may require higher data rates than the modem is physically capable of achieving.  
The following proposed precautions should be taken to address the simulation precision issues caused by the single relay point in MR simulations.  
* Transmission delays caused by the medium access and back-off mechanisms need to be emulated. This is done by utilizing the medium access control (MAC) layer for SWEs as shown in Figure 5. Moreover, the arrival of HW messages at the simulation platform needs to be monitored and integrated into the MAC layer model to limit the available bandwidth of the SWEs. Obviously, this behavior is dependent on the MAC protocol used, whether they are contention-based, contention-free [22], or hybrid MAC protocols [23] suggested for VANETs. Section 5.3 describes the mechanism of the wireless network simulation that can be used to address this simulation aspect.
* The broadcasting nature of the wireless channel traffic needs to be emulated, and the bandwidth of HWEs needs to be limited. In the proposed architecture, this is achieved by injecting controlled RF signal interference. Specifically, all message transmissions between SWEs need to be re-broadcasted over the simulation platform's modems as stated in Section 5.3. This, however, results in the issues of undesired interferences discussed in Section 6.1.
* Transmission data rates of all SWEs must be limited not to collectively request to transmit more data per second than the modem used by the simulation platform is physically capable of transmitting. This is achieved by using a high data-rate modem for the simulation platform, limiting the number of simultaneous large data transmissions, or, if the previous cases are not applicable, by limiting the number of SWEs in the simulation. Another option to prevent large simultaneous data-rate requests is to ensure the distribution of SWE transmissions over time to comply with modem limitations. However, this last approach has not yet been implemented and is a subject of the future work.  
A comparison of the reached data rates in the case of hardware deployed entities and in the case of entities in an MR simulation that employs the proposed architecture and precautions is the main subject of the experiments presented in the following section.