On the cost and benefit of cooperation

In cooperative communication, network nodes that would otherwise act independently instead coordinate their efforts with the aim of improving communication performance. To better understand cooperation, we consider communication over a multiple access channel using a 'cooperation facilitator', a node that receives rate-limited message descriptions from the transmitters and sends rate-limited message descriptions back. This model includes the conferencing encoders model and a prior model from the current authors as special cases. We characterize a class of multiple access channels for which there is no gain in sum-capacity under current or prior cooperation models. We then show that for all other multiple access channels, the gain in sum-capacity can be far greater than the capacity of the cooperation facilitator's output links. These channels violate the edge removal property. The Gaussian multiple access channel is an important special case for which we explicitly characterize the sum-rate cooperation gain.