C-MAC:A MAC Protocol Supporting Cooperation in Wireless LANs

Cooperative diversity is a transmission technique, where multiple terminals forms a virtual antenna array that realizes spatial diversity gain in a distributed fashion. The concept of cooperation has already been introduced to MAC layer to design MAC protocol. However, it's much different with that at physical layer. In this paper, we present a new MAC protocol based on IEEE 802.11, called C-MAC, that can support the basic building block of cooperative system. That is, in C-MAC, source would invite a relay node into data transmission if there exits an available one. During data transmission, source sends the signal to destination at first. The relay node will retransmit the overheard information to the destination at the second time slot. The destination combines two signals from source and helper, thus creating spatial diversity and robustness against channel fading. The C-MAC is backward compatible with legacy IEEE 802.11 system. The performance of C-MAC mainly depends on the physical layer's performance as it just provides the support for cooperation at MAC layer. If the physical layer works well, C-MAC would outperform IEEE 802.11 considering packet error rate. We also do the simulation using ns-2 with assumptive physical parameters. The result shows C-MAC would outperform 802.11 if packet error rate is a little high, and C-MAC would lead to some unfairness to nodes without relay.