  In this paper, we propose a new cooperation model for discrete memoryless
multiple access channels. Unlike in prior cooperation models (e.g.,
conferencing encoders), where the transmitters cooperate directly, in this
model the transmitters cooperate through a larger network. We show that under
this indirect cooperation model, there exist channels for which the increase in
sum-capacity resulting from cooperation is significantly larger than the rate
shared by the transmitters to establish the cooperation. This result contrasts
both with results on the benefit of cooperation under prior models and results
in the network coding literature, where attempts to find examples in which
similar small network modifications yield large capacity benefits have to date
been unsuccessful.
