This paper considers wireless communication protocols implementing both cooperation and Hybrid Automatic Repeat reQuest (HARQ). Based on a simple example (one Source, one Relay and one Destination), we propose the use of proba-bilistic protocols as an alternative to classical, deterministic ones. Starting initially with the Finite State Markov Chain (FSMC) analysis of a deterministic protocol, the idea of probabilistic protocol comes as an association with the simplified FSMC of the deterministic protocol. This FSMC contains two parameters that can be optimized for finding the best performance. So far, probabilistic protocols have been proposed at higher layers of communication systems, while we consider here the physical and MAC layers. In our analysis, the Relay works in Demodulate-and-Forward (DMF) mode, and we demonstrate that (i) the FSMC analysis predicts accurately the performance of the actual system, and (ii) the performance of probabilistic protocol (when the two parameters are carefully tuned) outperforms the performance of a reference deterministic protocol. Analysis is checked upon Monte Carlo simulations.