Optimal Relaying in a Slotted Aloha Wireless Network With Energy Harvesting Nodes

In this paper, we derive optimal policies for cooperation of a wireless node in relaying the packets of a source node, in a random access environment. In our scenario, the source node sends its packets by its harvested energy and the relay node exploits its harvested energy in relaying the source packets not detected successfully at the destination. The relaying policies determine whether the relay node accepts or rejects the unsuccessfully transmitted source packets and how the relay node prioritizes the accepted source packets to its own packets. The optimization goal is to minimize the average transmission delay of source packets with and without a constraint on the average transmission delay of relay packets. We derive the optimal policies in static and dynamic formulations. In static one, optimum rejection and prioritization probabilities corresponding to the aforementioned aspects of relaying policies are derived. To this end, we model the status of the relay node by a quasi-birth-death process, derive the required parameters, and apply some numerical optimization methods. In dynamic formulation, the relay node optimally decides based on its status by employing a constrained Markov decision process. By numerical results, we show the efficiency of the optimal relaying policies in different conditions.