The effect of cooperation at the network protocol level

There has been a great deal of attention on cooperative communication which exploits the spatial diversity among antennas belonging to multiple terminals. Most of the existing work focuses on the physical layer and shows how message relaying can improve the Shannon capacity region, outage probability, diversity order, etc. But it is possible to use relays in simple, innovative ways that depend on the protocol properties at the medium access control (MAC) and network layers. In this paper we build upon prior work on such relay use by considering sets of nodes in simple topology configurations in which reaching a common destination is accomplished through direct links as well as relayed transmissions. Each non-destination node generates its own traffic for the destination but the nodes that are closer to the destination have the capability and option to relay packets from nodes farther afield. Channel quality is modeled by a reception probability which injects the physical layer property into upper layer design and analysis. We consider bursty arrival processes and we characterize the stable throughput region and delay performance at each node. We show that a proposed cooperation strategy can lead to improved performance for both work-conserving and Time Division Multiple Access (TDMA) MAC protocols. The innovative elements in this work are the balance between own and relayed traffic at each node and the fact that the performance improvement is in part due to the concentration of the queues of failed packets into fewer virtual queues.