This paper proposes a new TDMA-based cooperative MAC scheme for a two-hop relay network in which there are several helpers besides a default relay node. Here, a helper has lower obligation than the default relay to help packet forwarding. The proposed method is developed from the existing CR-TDMA scheme, and named as opportunistic CR-TDMA or OCR-TDMA. We derive the throughput performance of CR-TDMA and OCR-TDMA.
Both the theoretical analysis and simulation results demonstrate that OCR-TDMA provides superior throughput performance compared with CR-TDMA, even though OCR-TDMA is a limited modification of CR-TDMA. Amazingly, the OCR-TDMA gains remarkable cooperation benefit even when the helpers have low resource and capability to cooperate.