Multi-hop routing within TDMA slots for teams of cooperating robots

Small teams of cooperating robots have been shown to benefit from the increased reliability of synchronised message exchanges provided by TDMA-based schemes. However, such schemes may also impose a long propagation delay to communications between non neighbour robots. Such negative impact is further increased by long TDMA rounds, which favour reduced medium utilisation and energy consumption, and by decentralised mesh topologies, which increase the team coverage and layout flexibility. For small size teams, say up to 10 units, it is feasible to set-up a global TDMA framework as well as tracking the instantaneous network topology, making it available to each robot. In this work we use this knowledge to forward packets along their path within each TDMA slot. To that end, we present a novel communication protocol that combines global TDMA and multi-hop routing. It maintains the reliability benefits of the TDMA schemes while strongly reducing the end-to-end propagation delay of interactions between non neighbour robots. We validate our protocol with simulation results using OMNET++. In a worst-case topology scenario we achieved an end-to-end delay that can be as low as 35% that of a traditional TDMA implementation that forwards the packets to the immediate one-hop neighbours, only. In a concrete audio streaming application scenario reported in the literature for an alternative real-time token-passing protocol, our proposal achieves similar delays with significantly less management bandwidth.