What is System in the loop and Hardware in the loop?

What is System in the loop and Hardware in the loop?

With new approaches for testing, the so called System in the loop (SITL) and Hardware in the loop (HITL), we will be able to connect directly to the Emulradio4Rail platform the real radio access equipment (prototypes), the radio emulators (physical systems) and the simulators and therefore be able to mimic the railway network and the railway channels behavior.