Hybrid Network Coding for Unbalanced Slotted ALOHA Relay Networks

In this paper, we investigate the throughput performance of the network coding (NC) schemes under the slotted ALOHA protocol. We consider the all-inclusive-interfering unbalanced network in which two client groups with different numbers of nodes communicate with each other through a relay node. We derive the closed-form expressions of the network throughput under the physical-layer network coding (PNC), traditional high-layer network coding (HNC), and non-network-coding (NNC), respectively. We also show the necessary and sufficient condition to make the relay node unsaturated. From the analytical results, we find that although PNC has better transmission efficiency in the two-way relay channel (TWRC); it does not always have better network throughput when the network has multiple client nodes. To further improve the network throughput, we propose the hybrid NC scheme, which allows the relay node to turn to HNC scheme if it fails to explore the PNC transmission. We further obtain the closed-form expression of the network throughput and the necessary and sufficient condition to make the relay node unsaturated in the hybrid NC scheme. Simulation results show that the hybrid NC scheme has better throughput performance than the PNC, HNC, and NNC schemes. Moreover, we optimize the network throughput of the hybrid NC scheme in terms of the transmission probability of the relay node. Last but not least, we evaluate the throughput performance of hybrid NC scheme through simulations.