On the Tradeoff Between Collision and Cooperation in a Random Access Wireless Network With Energy Harvesting Nodes

In this paper, we investigate an energy harvesting cooperative network in which the cooperation is done in a random access environment. Although the cooperation provides an extra harvested energy supply for transmissions of the source, it causes probable collisions for the transmitted source packets. Thus, this kind of cooperation can improve or degrade the QoS of the source packets. We find the optimal policy in such a scenario to maximize the source throughput and derive the necessary and sufficient condition for no-cooperation policy to be throughput-optimal. Then, we prove that the maximum throughput is obtained by considering no-cooperation or full-cooperation policy depending on the network parameters. Also, we design optimal static and dynamic policies to minimize the average end-to-end delay for the source packets. In a static policy, the packets are accepted at the relay with a fixed probability while in a dynamic policy, the decision is made based on the status of data and energy buffers in the relay. We derive the necessary and sufficient condition under which the no-cooperation is the delay-optimal policy. When no-cooperation is not optimal, we model the scenario as a quasi-birth-death process and a constrained Markov decision process to derive static and dynamic delay-optimal policies, respectively. By several numerical results, we show the effect of tradeoff between cooperation and collision in the derived optimal policies in different network conditions.