On Cooperative Communication in Ad-Hoc Networks: The Case for Uncoordinated Location-Aware Retransmission Strategies

Cooperative communication methods in wireless networks, ranging from relaying by a common neighbor over a single wireless hop to opportunistic routing at the network layer, have been shown in recent years to offer significant performance gains over traditional approaches that ignore the broadcast nature of the wireless medium, and are particularly valuable in environments prone to channel shadowing and fading, such as mobile ad-hoc networks. A common feature of various cooperative methods proposed in the literature is the coordination required to discover the available neighbors and determine the optimal one(s) to be involved in the cooperation. However, the overhead cost of such coordination may be prohibitive in networks with highly dynamic topology (e.g. due to high mobility), as the discovery and negotiation overheads may negate much of the cooperation gains. Accordingly, we present the case for uncoordinated cooperative retransmission, where a node overhearing a frame may retransmit it "blindly" without any prior coordination with the transmitter, intended receiver, or any other neighbors in the vicinity. We pose and solve the problem of finding the optimal retransmission probability as a function of location, and characterize the optimal uncoordinated cooperation region through the solution of an integral equation that depends only on the a priori node density and wireless propagation model. Through numerical evaluation, we demonstrate that uncoordinated cooperation provides a low-overhead viable alternative, with a frame delivery probability that can even exceed that of coordinated cooperation methods in situations with high noise (or low transmission power) and high node density.