The Effect of Cooperation at the Network Protocol Level

There has been a great deal of attention on cooperative com- munication which exploits the spatial diversity among an- tennas 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 re- lays 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 con¯g- urations in which reaching a common destination is accom- plished through direct links as well as relayed transmissions. Each non-destination node generates its own tra±c for the destination but the nodes that are closer to the destina- tion have the capability and option to relay packets from nodes farther a¯eld. Channel quality is modeled by a re- ception 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 perfor- mance 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 tra±c at each node and the fact that the performance improve- ment is in part due to the concentration of the queues of failed packets into fewer virtual queues.